5
The Next Stage of Arctic Solid-Earth Geoscience Research

GEOLOGIC FRAMEWORK AND TECTONIC EVOLUTION

Major Problems and Research Questions

The kinematics and history of the opening of the Arctic Ocean Basin within the margin of the supercontinent of Laurasia, the history and kinematics of the interaction in the Arctic of relatively orderly North Atlantic extension and highly mobile and complex Pacific Basin convergence, and the history of the interchange of seawater and biota between the Arctic and more southerly seas are first-order scientific problems awaiting solution in the Arctic. Our knowledge of the geologic framework of the ridges and subbasins of the Arctic Ocean, and of the age of its large areas of oceanic crust, is insufficient for solving these problems. However, this knowledge does provide some guidance in identifying the most productive regions for focusing our limited resources for support of research and in designing appropriate investigations.

A meaningful analysis of the geologic framework and tectonic evolution of the Arctic Ocean Basin would require that all the larger basins and ridges that comprise it be individually investigated and that the character of the geologic structures that bound these first-order features and of the continental margins be determined. Because the polar ice pack precludes acquisition of the required data from conventional research ships, more specialized and expensive platforms or logistic procedures would be required. Partly offsetting this difficulty is the circumstance that the Arctic is a small, almost landlocked basin most of which is no more than 700 km from bedrock outcrops on the circum-arctic landmasses or continental shelf islands. Extrapolation of geologic features from the surrounding continents could thus provide significant insight into the tectonic character of the basin. The proximity of the surrounding landmasses would also facilitate the testing of plate tectonic reconstructions by matching of geologic terranes and structures of known character and age across the basin. Paleomagnetic studies of the surrounding landmasses would provide important insights into past plate motions in the Arctic, and seismic studies based on a modern, standardized network of broad-band circum-arctic seismograph stations would tell us much about modern plate geometry and motions.

Tectonic Problems in the Amerasia Basin

The key to understanding the tectonic history of the Arctic lies in learning the geologic framework and tectonic history of the bathymetrically complex Amerasia Basin, one of the most inaccessible and poorly known areas on Earth. There are many quite divergent hypotheses but



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Opportunities and Priorities in Arctic Geoscience 5 The Next Stage of Arctic Solid-Earth Geoscience Research GEOLOGIC FRAMEWORK AND TECTONIC EVOLUTION Major Problems and Research Questions The kinematics and history of the opening of the Arctic Ocean Basin within the margin of the supercontinent of Laurasia, the history and kinematics of the interaction in the Arctic of relatively orderly North Atlantic extension and highly mobile and complex Pacific Basin convergence, and the history of the interchange of seawater and biota between the Arctic and more southerly seas are first-order scientific problems awaiting solution in the Arctic. Our knowledge of the geologic framework of the ridges and subbasins of the Arctic Ocean, and of the age of its large areas of oceanic crust, is insufficient for solving these problems. However, this knowledge does provide some guidance in identifying the most productive regions for focusing our limited resources for support of research and in designing appropriate investigations. A meaningful analysis of the geologic framework and tectonic evolution of the Arctic Ocean Basin would require that all the larger basins and ridges that comprise it be individually investigated and that the character of the geologic structures that bound these first-order features and of the continental margins be determined. Because the polar ice pack precludes acquisition of the required data from conventional research ships, more specialized and expensive platforms or logistic procedures would be required. Partly offsetting this difficulty is the circumstance that the Arctic is a small, almost landlocked basin most of which is no more than 700 km from bedrock outcrops on the circum-arctic landmasses or continental shelf islands. Extrapolation of geologic features from the surrounding continents could thus provide significant insight into the tectonic character of the basin. The proximity of the surrounding landmasses would also facilitate the testing of plate tectonic reconstructions by matching of geologic terranes and structures of known character and age across the basin. Paleomagnetic studies of the surrounding landmasses would provide important insights into past plate motions in the Arctic, and seismic studies based on a modern, standardized network of broad-band circum-arctic seismograph stations would tell us much about modern plate geometry and motions. Tectonic Problems in the Amerasia Basin The key to understanding the tectonic history of the Arctic lies in learning the geologic framework and tectonic history of the bathymetrically complex Amerasia Basin, one of the most inaccessible and poorly known areas on Earth. There are many quite divergent hypotheses but

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Opportunities and Priorities in Arctic Geoscience no general agreement as to its origin and age (Lawver and Scotese, 1990b). In contrast, it is widely accepted that the origin of the smaller, structurally and bathymetrically simpler, and more accessible Eurasia Basin formed by seafloor spreading along the Arctic Mid-Ocean Ridge beginning between chrons 29 and 25 (65 to 59 million years) on the geomagnetic polarity time scale (Kristoffersen, 1990a, b). The committee therefore recommends that the predominant focus of geologic framework and tectonic studies in the Arctic Ocean Basin be the Amerasia Basin, but selected studies in the Eurasia Basin are also recommended. Ridge Systems A three-phase program is recommended for study of the geologic framework of the four large ridge systems of the Amerasia Basin (see Figure 2). All four ridges meet the margins of the surrounding continents abruptly at linear or curvilinear continental slopes that have the morphologic appearance of major structural boundaries, and the ridges are therefore thought to be structurally isolated from the surrounding continents. Data on the geologic character and structural history of these ridges is required before well-constrained plate tectonic reconstructions of the Arctic Ocean Basin and its environs can be made. Phase one would acquire regional physiographic and geophysical data. They would consist of systematic digital seabed side-scan sonar imagery and bathymetry of the entire Amerasia Basin, uniform mapping of its magnetic and gravity fields, establishment of a modern circum-arctic seismograph network, and a program of circum-arctic paleomagnetic studies. Phase two, guided in part by the regional data generated by phase one, would focus on the lithology and shallow geologic structure of the ridges. It would consist of extensive sampling of the ridges by coring, dredging, and shallow drilling; coarse grids of seismic reflection and sonobuoy refraction profiles; and paleomagnetic study of core samples. Phase three would seek to determine the deep crustal structure of the ridges through the acquisition of deep (crustal) seismic refraction and heat flow data and seismic surface wave propagation studies. The proposed investigations appear feasible with present or foreseeable logistical platforms and technology, as discussed in Chapter 6. For all the ridges or ridge systems, seismic profiling across their boundaries with the adjacent continental margins would be required to establish their tectonic character and history. Determination of the stratigraphy and environmental history of the sedimentary sections that cap or drape over the ridges would also be required. This history would establish the age of the ridges as independent tectonic entities, the history of their subsequent vertical movements, and their influence on the oceanography of the Arctic Ocean Basin through geologic time. The record of vertical movements of the ridges with respect to sea level through time would provide important insights into their tectonic character and development. Lomonosov Ridge: Based on physiographic and gravity data and on crustal velocity structure, it has been inferred that the Lomonosov Ridge is a sliver of continental crust separated from the Barents-Kara Shelf by rifting in the Eurasia Basin (Weber and Sweeney, 1990). A series of bedrock samples along the ridge would be needed to test the rifting hypothesis for the origin of the Eurasia Basin and to determine the direction of opening with respect to the basin axis by seeking specific lithologic ties with the outer Barents-Kara Shelf. This shelf is relatively well known from reconnaissance geologic mapping of the many islands that rise above the outer continental shelf between Svalbard and the Taymyr Peninsula. A series of transverse seismic reflection profiles across the Lomonosov Ridge may resolve the character of its bounding fault systems and thereby provide crucial data on the origin of the Amerasia Basin. If the boundary between Lomonosov Ridge and Makarov Basin consists of extensional faults, it would support suggestions that the Amerasia flank of the ridge is a passive margin and that the Makarov Basin formed by seafloor spreading away from the Lomonosov Ridge. If the boundary consists of transcurrent faults, it would support suggestions that the basin formed by spreading parallel to the Lomonosov Ridge, which carried Arctic Alaska and Chukotka away from Arctic Canada. Understanding the character and age of this feature is basic to constructing an adequate tectonic model for the Arctic Ocean Basin.

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Opportunities and Priorities in Arctic Geoscience Alpha Ridge: Seismic refraction and reflection data, gravity modeling, a large positive magsat anomaly, aeromagnetic anomalies with amplitudes exceeding 1,500 nT, and a single sample of alkaline basalt collected by the Canadian CESAR expedition in 1983 suggest that the Alpha Ridge may be near a volcanic pile as much as 40 km thick (Forsyth et al., 1986; Weber and Sweeney, 1990). Other hypotheses of origin include a spreading center subduction zone, a transcurrent fault, a hot spot track, or a displaced fragment of continental crust. The inferred volcanic basement is overlain by Campanian and younger biosiliceous pelagic oozes, but sampling is insufficient to demonstrate that they are the oldest sediments that rest on the volcanic rocks. Its bathymetry suggests that the Alpha Ridge has been extensively block faulted. Regional sampling on the numerous fault scarps that characterize Alpha Ridge would be needed to verify the petrographic character and age of its acoustic basement and to document the age of the oldest sediments that overlie this basement. Seismic reflection profiles would be needed to record the thickness and seismic stratigraphy of the locally thick and faulted sediments and to identify sites where the oldest sedimentary strata and complete stratigraphic sections can be sampled. Seismic reflection profiles would also be needed to document the geologic structure and tectonic style of the ridge and the character of its boundaries with the flanking basins and the magnetically contrasting Mendeleev Ridge. Mendeleev Ridge: Mendeleev Ridge resembles Alpha Ridge in having rough bathymetry and extensively faulted and tilted sedimentary rocks draped over basement of high acoustic impedance at moderate depths (Hall, 1973, 1990), but even less is known about it, at least in the western literature. The two ridges meet at about a 135° angle near Cooperation Gap, near the center of the Arctic Ocean Basin. Of particular significance is the observation that the large positive Magsat anomaly that coincides with Alpha Ridge begins to drop in amplitude near Cooperation Gap and dies out along the Mendeleev Ridge, suggesting that these ridges are underlain by different rock types (Haines, 1985). Side-scan imagery and seismic reflection profiling should serve to locate sample sites at which basement lithology and the character and environmental history of the overlying sediments can be determined. A major objective of the sampling would be to learn why the large amplitude magnetic anomaly of Alpha Ridge dies out over the Mendeleev Ridge. Suites of samples and seismic reflection data across the ridge would be needed to establish its geologic framework and environmental history and allow assessment of its tectonic character and its place in arctic tectonics. Chukchi Borderland: The fourth ridge province in the Amerasia Basin is the cluster of north-trending flat-topped ridges and plateaus, the Chukchi Borderland, which lies between the Chukchi Shelf and the center of the Amerasia Basin (Hall, 1973, 1990). Seismic reflection profiles indicate that flat-lying to gently dipping well-bedded sedimentary rocks, in places 1 km or more thick, cap the ridges. These low-dipping strata overlie more strongly deformed beds of higher acoustic impedance that, in one area of Northwind Ridge, consist of marine foraminifer-bearing lutite of Middle Albian age (Grantz et al., 1990b). Elsewhere, they overlie rocks that generate high-amplitude magnetic anomalies. The high-standing flat-topped physiography of the ridges and the presence, at least locally, of moderately to highly magnetic rocks at high structural levels within them suggest that they are underlain by wave-truncated continental or volcanic rocks. A few bathymetric and seismic reflection profiles indicate that the borderland is broken by northerly-trending normal faults that are geologically young. A plate tectonic model for the Amerasia Basin must accommodate the Chukchi Borderland, which projects into the basin in a manner that has no obvious tectonic explanation. Tomographic inversion of trans-arctic seismic surface wave velocities suggests that the ridges of the borderland are of continental thickness (Jih et al., 1988), but additional surface wave studies and seismic refraction measurements are needed to confirm this point. Reconnaissance seismic reflection profiles indicate that the sediments capping the flat-topped ridges of the borderland are less intensely faulted than those of the Alpha and Mendeleev Ridges (Hall, 1973), suggesting significant differences in tectonic origin and deformational history. The primary research goal in

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Opportunities and Priorities in Arctic Geoscience the borderland would be to determine the age, lithologic character, and stratigraphy of acoustic basement in the ridges and plateaus of the borderland and of the capping sedimentary section by a comprehensive program of subseabed sampling, seismic reflection, and seismic refraction profiling. Seismic profiling would also be needed to delineate the northerly-trending normal faults that, at least locally, traverse the borderland and perhaps form major structural boundaries between its ridges and basins. Acquisition of three piston core samples of Albian marine sedimentary rock from the east flank of Northwind Ridge from an icebreaker in 1988 suggest that suitable sampling sites for coring and shallow drilling pre-Quaternary rocks can be found in the borderland. Subbasins Satisfactory plate tectonic reconstructions of the arctic region will require understanding of the character and age of the seafloor beneath the subbasins of the Amerasia Basin. Acquisition of the required data should therefore be a primary objective of any program of solid-earth geoscience research in the Arctic. The program would have to be conducted in several parts of the Amerasia Basin, however, because it consists of two large subbasins of irregular shape and perhaps of compound origin and two small isolated subbasins. The large subbasins are the elongated Makarov Basin, which lies between the Lomonosov and Alpha-Mendeleev Ridges, and the broader Canada Basin, which is bordered by Canada, Alaska, the Chukchi Borderland, and Alpha Ridge. These subbasins are nearly 4,000 m deep, even though they contain thick sedimentary fills, suggesting that they are underlain by oceanic crust. In the Canada Basin, the thickness of the fill exceeds 12 km, and in the Makarov Basin, 6 or 8 km. Seismic refraction data have revealed a velocity-depth curve similar to that of oceanic crust beneath the southern Canada Basin and part of the Makarov Basin (Weber and Sweeney, 1990; Forsyth et al., 1986), but whether linear seafloor magnetic anomalies are present is controversial. Available aeromagnetic profiles are too widely spaced to confirm unequivocally the presence of magnetic anomalies that can be ascribed to seafloor-spreading in the southern Canada Basin on the basis of the low amplitude anomalies that have been found there; according to some investigators, the magnetic anomalies near the North Pole in the Makarov Basin are the product of structural relief in magnetic basement (Taylor et al., 1981). The small Chukchi and Northwind subbasins in the Chukchi Borderland are also underlain by thick sections of young flat-lying sediment, but the thickness of these fills is not known. In the absence of definitive seismic refraction and aeromagnetic data, we can only speculate that these small heavily sedimented subbasins rest on either oceanic or tectonically thinned continental crust. A three-phase program of geoscience studies is proposed for the subbasins of the Amerasia Basin. The first phase would supplement existing deep seismic refraction data in the subbasins with modeling of aerogravity and aeromagnetic data, seismic surface wave studies, and seismic reflection profiling. A primary objective of phase one would be to determine, with submarine or airborne surveys, whether the subbasins contain magnetic anomalies indicative of seafloor spreading. The geometry of such anomalies, if they exist and can be mapped, is basic to plate tectonic reconstructions in the Arctic Ocean Basin. Delineation of these anomalies has proven difficult because the subbasins are relatively small and because they contain terrigenous sedimentary fills as much as 12 km thick that have depressed the subjacent oceanic crust and significantly reduced, and perhaps obscured, the magnetic signal. The amplitude of the presently mapped aeromagnetic anomalies in the southern Canada Basin, which some suggest originated in seafloor spreading, averages about 200 nT (Taylor et al., 1981). Continental margin geology suggests that much of the Canada Basin formed during the Cretaceous magnetic quiet interval (Albian to Santonian) and may lack seafloor-spreading anomalies that are mappable (Grantz and May, 1983). Magnetic and gravity surveys across the southern Canada Basin should therefore be spaced as closely as would be commensurate with our present estimate of the depth of the subjacent oceanic crust, about 10 km. Second-phase studies would obtain additional refraction and heat flow data in the subbasins at sites selected after the analysis of phase one data would be complete. Determination of the

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Opportunities and Priorities in Arctic Geoscience thickness of sedimentary fill in the subbasins, the character and thickness of the underlying crust, and the relation of heat flow, basement elevation, and age for the various subbasins of the Amerasia Basin would be major objectives of phase two. Heat flow measurements collected from Ice Island T-3 from the mid-1960s to the early 1970s are consistent with the interpretation that parts of the Canada Basin and Alpha and Mendeleev Ridges are 108 years old or older (Langseth et al., 1990). However, the northwest part of the Canada Subbasin and part of the Makarov Subbasin exhibit heat flow that is higher than would be expected from Mesozoic oceanic lithosphere. Did these areas form later than other subbasins in the Amerasian Basin, or did they and possibly the Alpha Ridge near the North American margin undergo magmatic or tectonic rejuvenation in Early Tertiary time? Additional heat flow data from these subbasins, especially when interpreted in conjunction with side-scan sonar imagery and seismic reflection data, would reveal whether the heat flow anomalies are related to Early Tertiary volcanism or tectonism and address the question of whether the Canada and Makarov Subbasins contain major subdivisions of disparate ages. Third-phase studies, based on deep scientific drilling of the subbasin fills, would be a major scientific achievement. With the possible exception of a few sites near their margins, however, there appears to be little prospect that the technological and financial resources to achieve significant stratigraphic penetration of the thick sedimentary fill in the subbasins could be mustered until well into the 21st century. Stratigraphic coring and shallow drilling at dispersed sites on the crests and flanks of the numerous ridges of the Amerasia Basin would offer a more favorable current prospect for piecing together a stratigraphic section, albeit noncontinuous, representing the entire history of the Amerasia Basin. Such a dispersed section could be obtained in conjunction with other data sets and at considerably less cost than by deep drilling. Selected Tectonic Problems in the Eurasia Basin Fram Strait Fram Strait is the major gateway for deep-water communication between the Arctic and the water masses of lower latitudes. Seafloor magnetic anomalies suggest that the initial plate boundary in this area was a passive margin that was also the site of Late Eocene volcanism (Soper et al., 1982). The volcanism formed the Morris Jesup Rise and at least part of Yermak Plateau during shear or transpressive motion between Greenland and Svalbard. The gateway appears to have opened after an Early Oligocene change to oblique rifting and the thermal subsidence of Yermak Plateau, and it became an effective channel for deep-ocean circulation in Middle Miocene time (Kristoffersen, 1990b). The geometry, kinematics, and timing of the early evolution of the Fram Strait gateway need to be documented in detail by seismic reflection profiles and sampling, because interconnection of the deep Arctic Ocean Basin with the world ocean was an event of far-reaching oceanographic and climatic influence. Associated problems are the timing and geometry of the volcanic construction of Morris Jesup Rise, the possible dual crustal origin of Yermak Plateau, and the significance of the high heat flow along the western margin of the plateau (Crane et al., 1982). These problems could be addressed by over-ice seismic surveys, geological sampling, and heat flow measurements across Morris Jesup Rise. The southern half of Yermak Plateau is accessible to conventional marine multichannel surveys in good ice years (Sundvor et al., 1982). Basement sampling by several shallow (less than 100 m) drill holes is also recommended. The Slow-Spreading Arctic Mid-Ocean Ridge The evolution of the Eurasia Basin and the Arctic Mid-Ocean Ridge are relatively well understood, but it remains necessary to characterize the lithosphere of this very slowly-spreading young ocean basin in terms of heat flow and basement elevation versus age (Vogt et al., 1979b). These relations would be valuable for comparison with similar measurements on other Cenozoic basins in an effort to determine the global variability of parameters that control lithospheric evolution.

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Opportunities and Priorities in Arctic Geoscience Seafloor at the Arctic Mid-Ocean Ridge is formed at a rate of 0.8-1.5 cm/yr, which is the slowest of any present part of the mid-ocean ridge system. East of 120°E, the ridge is progressively buried by sediments, but water depths in the axial valley of the western part of the ridge reach 5,000 m and are the deepest in the Arctic Ocean. Scattered bathymetric profiles obtained by submarines across the western part show a morphology similar to other slow-spreading ridges with indications that the width of the axial valley decreases with spreading rate. Another unique feature of the Arctic Mid-Ocean Ridge is the thin oceanic crust (2 km) that was observed on its northern flank, which may also be characteristic of slow-spreading ridges (Jackson et al., 1982). The low rate of spreading and volcanism at the Arctic Mid-Ocean Ridge makes it an end-member representation of magmatic and tectonic processes at spreading ridges. Important aspects of such slow-spreading ridges are large heat loss and the role of oceanic crustal strength in maintaining a steady-state ridge morphology (Sleep and Rosendahl, 1979). These problems would best be addressed by a concentrated effort in a restricted area analogous to the French-American Mid-Ocean Undersea Survey (FAMOUS) of the Mid-Atlantic Ridge. Multichannel seismic reflection and refraction data; gravity, magnetic, and heat flow measurements; and detailed bathymetry mapping and geological sampling would be needed to provide the necessary constraints for a better understanding of slow-spreading ridges. Recent studies have demonstrated that active spreading centers release large amounts of heat into the oceans, but the possible effects of the heat released by the active spreading system in the Eurasia Basin have received little attention. Although it is difficult to assign good numerical estimates to the thermal energies involved, it is reasonable to assume that the Arctic Mid-Ocean Ridge was (and is) releasing energy at a rate of about 1018 to 1019 joules per year. The effects of releasing this amount of energy into the Eurasia Basin prior to the opening of the deep-water connection with the Atlantic about 15 million years ago could have significantly altered its thermal stratification, salinity, and chemistry, and perhaps its biota (Thiede, 1979). The climatic effects of slightly elevated surface water temperatures in a closed polar sea might also be significant. Continental Margins A diversity of first-order tectonic features that in large part shaped the physiography and geologic framework of the arctic region underlie the continental margins of the Arctic Ocean Basin, and an understanding of their structural character and age is an essential prerequisite for understanding arctic tectonics. The arctic margins include passive margins of Cretaceous and Tertiary age, transform margins of Tertiary and perhaps Cretaceous age, and in the eastern Beaufort Sea, a convergent margin of Cenozoic age. The arctic margin is also one of the few places on Earth where an active spreading ridge, the Arctic Mid-Ocean Ridge, trends into a continent (Kristoffersen, 1990a). It is suspected that some of these margins may be of compound origin. An example is the continental margin that borders Greenland and the Canadian Arctic Islands, which is thought to be a transform fault of Cenozoic age between Svalbard and Greenland and a passive or transtensional margin of Cretaceous age opposite the Canadian Arctic Islands (Vogt et al., 1979a; Sweeney et al., 1990). Continental margin structures in the Beaufort and Laptev Seas have been studied during favorable ice years with multisensor marine geophysical surveys. The continental margin in the Beaufort Sea north of Alaska is of the Atlantic type and appears to have experienced premonitory rifting in Jurassic time and breakup in Hauterivian (Early Cretaceous) time. Recent Soviet seismic reflection and gravity measurements in the Laptev Sea found extensional faults subparallel to the Arctic Mid-Ocean Ridge on the broad Laptev Shelf (Kogan, 1974; Nalivkin, 1983; and Kristoffersen, 1990a), but apparently, their surveys did not reach the continental margin. Data on the continental margins are lacking elsewhere in the Arctic, where they would have to be studied in ice-covered seas using logistically difficult field methods. The tested methods include aeromagnetic profiling, seismic refraction and reflection profiling on pack ice in spring, and coring and shallow drilling from icebreakers. Partially tested methodologies include aerogravity profiling and seismic reflection profiling in multiyear sea ice from icebreakers.

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Opportunities and Priorities in Arctic Geoscience Because of the broad area of scientific interest along the arctic continental margins and the logistic difficulty and expense of acquiring geoscience data on sea ice, a realistic program to study the arctic margins must focus on key areas with well-defined structural targets. Long-range side-scan imaging surveys and detailed bathymetry would greatly assist the site selection process. Existing data and anticipated geophysical work by industry and governments would provide guidance in the Beaufort and eastern Chukchi Seas, but basic data are inadequate or lacking for other arctic margins. Compilation of a definitive list of key study areas is therefore not feasible, but suggestions based on current understanding follow. For each of the key study areas, the committee recommends acquisition of geophysical and geologic data along transects normal to the continental margin. The transects should be anchored at tie points near the mainland, outer shelf islands, or existing geophysical surveys, and they should extend at least 100 km basinward of the shelf break. The general location of the suggested transects is shown in Figure 8. Each transect would consist of two or more parallel geophysical lines along which geological samples and both low-frequency and high-resolution seismic reflection, seismic refraction, gravity field, heat flow, and bathymetric data would be obtained. Canada Basin Onshore-offshore transects across the continental margins on opposite sides of the southeast Canada Basin would test various hypotheses that have been proposed for their origin, including formation by rotational rifting about a pivot in the Mackenzie Delta region in mid-Cretaceous time. The transects proposed for the Canada Basin should extend completely across its southeastern corner to search for multistage rifting and transform faulting along the opposing, possibly conjugate margins. Determining whether the margin at the Canadian Arctic Islands was the site of multiple deformations would be a major objective. The study would provide new data on the structural deformation that Cenozoic convergence, originating at the distant Pacific Rim, has imposed on the sedimentary column in the southeastern Canada Basin. Tromso-Mackenzie Lineament An alignment of continental margins and fracture zones, the Tromso-Mackenzie lineament, extends for almost 4,000 km along the western Barents, northern Greenland, and Canadian Arctic Islands continental slopes from near Tromso, Norway, to the Mackenzie Delta in Canada. The lineament is the locus of Cenozoic transcurrent transform faulting on the Spitzbergen fracture zone in Fram Strait and probable Jurassic and Cretaceous transcurrent and extensional faulting off the Queen Elizabeth Islands of Arctic Canada (Vogt et al., 1979b). Better understanding of this interregional tectonic feature is needed for plate tectonic reconstruction of the arctic region, including the surrounding northern continents. The Tromso-Mackenzie lineament could be better understood by concentrating research along six transects (see Figure 8). Such transects would also provide important new information on the geology of the Arctic Mid-Ocean Ridge and the Spitzbergen fracture zone in Fram Strait, the Morris Jessup Plateau, and the Lomonosov and Alpha Ridges near their junctions with North America. Sea-ice conditions along the lineament from the Queen Elizabeth Islands to Greenland appear to be especially unfavorable for icebreaker operations (Hibler, 1980). Over-ice surveys would thus be more favorable than icebreaker surveys in the area. Laptev-Mackenzie Margin Some tectonic models suggest that the long curvilinear continental margin that lies north of Alaska and Eurasia between the Mackenzie Delta and the Laptev Sea is a passive margin of Jurassic and Cretaceous age that is conjugate to the Tromso-Mackenzie lineament (Lawver and Scotese, 1990b), but this interpretation is supported by seismic reflection data only in the Beaufort and eastern Chukchi Seas. The absence of data west of the eastern Chukchi Sea and

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Opportunities and Priorities in Arctic Geoscience FIGURE 8 Location of proposed continental margin transects in the Arctic Ocean region. TM=TromsoMackenzie Lineament, CB=Canada Basin, EC=East Siberian-Chukchi Seas, LS=Laptev Sea. the right-angle junctions of the ridges of the Chukchi Borderland and Mendeleev Ridge with the Laptev-Mackenzie margin require that a high priority be given to determining the tectonic character and history of this margin. For this purpose, we suggest establishing six transects of the type proposed for the Tromso-Mackenzie lineament. These transects would also address the tectonic character of the junctions with the ridges of the Chukchi Borderland and Mendeleev Ridge and provide data for a critical comparison of the Laptev-Mackenzie margin with the Tromso-Mackenzie lineament. They may help to determine whether the western part of the margin has been affected by transform faulting from the Eurasia Basin. All these questions are critical to understanding the plate tectonic development of the Arctic Ocean Basin. Laptev Sea A transition from rifting by seafloor spreading to rifting by crustal extension in continental rocks can be studied at the junction of the slow-spreading Arctic Mid-Ocean Ridge and the

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Opportunities and Priorities in Arctic Geoscience continental margin in the Laptev Sea. Aeromagnetic data and plate tectonic considerations suggest that more than 300 km of the 400 km of extension represented by Cenozoic oceanic crust in the eastern Eurasia Basin were imposed on the Laptev Shelf, where the crust has a continental thickness of more than 30 km (Kogan, 1974). How extensional strain is accommodated across the Laptev transition from oceanic to continental crust and the nature and stability of the thermal regime along the extensional axis across the transition are important tectonic problems. These studies could also provide information on the metamorphic effects and changes in physical properties of the ocean crust that may have resulted from thermal blanketing by thick sediments in the axial valley of the Arctic Mid-Ocean Ridge near the Laptev Shelf. A long seismic refraction profile colinear with the axis of the Arctic Mid-Ocean Ridge and three perpendicular lines across the Laptev Shelf and the upper and lower Laptev slopes are proposed. Additional required data are multichannel seismic reflection profiles and gravity, magnetic, and heat flow measurements along the refraction lines. Combined use of passive and active seismic monitoring using ocean bottom seismometers and land-or ice-based arrays would provide important additional information. A transect from the continental shelf north of the New Siberian Islands to the axis of the Eurasia end of the Lomonosov Ridge is proposed to be undertaken in conjunction with the Laptev margin studies. A rigorous understanding of the kinematics of rifting in the Laptev transition requires knowledge of the geometry and amount of transform motion, if any, between the Lomonosov Ridge and the east Siberian-Laptev Shelf. Special Studies Comparative Studies of Trans-Arctic Geologic Structure and Stratigraphy The character, distribution, and structure of the geologic formations and lithotectonic terranes that encircle the Arctic Ocean Basin provide important constraints on regional plate tectonic models. Because most of the circum-arctic continents and islands have been mapped at reconnaissance or larger scales, it would appear to be a simple matter to compile the information, as many investigators are presently doing, and test proposed models against this body of data. In practice, however, this test has been difficult to apply. Barriers of language, lack of awareness of or access to the pertinent scientific investigators or literature, and national differences in scientific perspective and terminology commonly obscure or make unavailable structural and stratigraphic data critical to useful tectonic and geologic comparisons. Small-scale map compilations of circum-arctic geology or tectonics (e.g., Okulitch et al., 1989) are also of limited usefulness for trans-arctic correlation and tectonic interpretation. Such maps provide useful overviews of the distribution of chronostratigraphic units around the Arctic, but they tend to be compilations of conventional wisdom. In addition, because the primary data sources for these compilations are often not provided, it is difficult to separate observations from interpretations. An example of the need for comparisons at larger scales is found in Alaska and northeast Siberia, which appear to be amalgamations of numerous exotic lithotectonic terranes with extensive accretionary histories (Fujita and Newberry, 1983; Zonenshain et al., 1990). Careful large-scale mapping and interpretation is a prerequisite for meaningful trans-arctic comparisons within these geologically complex terranes. Until recently, national policy and logistics considerations in many of the arctic nations severely restricted working visits by scientists of other nations to their territories, and those that were allowed were commonly structured as formal scientific exchanges or field excursions with planned itineraries. Such programs have been productive, but until access to the relevant terranes of the arctic rim is permitted, the key questions of trans-arctic geologic correlation and arctic tectonic evolution will not be answered. The committee places a high priority on comparative trans-arctic geological, geophysical, and tectonic field studies by individuals or small groups of investigators. Such studies will be most productive when conducted as collaborative studies among investigators of the regions under comparison.

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Opportunities and Priorities in Arctic Geoscience Paleomagnetic Analysis of Arctic Tectonic Problems Although there are abundant paleomagnetic data for the circum-arctic cratons, there are few data, at least in the open literature, from rocks at the immediate margins of the Arctic Ocean. Most of the published work comes from Spitzbergen and surrounding islands, Ellesmere and nearby islands in Arctic Canada (Wynne et al., 1983), and from the Brooks Range and North Slope in Alaska (Harbert et al., 1990). Some summary paleomagnetic data are also available from the Soviet Union (Kharmov, 1989a, b), but with a few notable exceptions, insufficient supporting data are provided to assess the likelihood of magnetic overprinting. Such assessment is needed because pervasive overprinting has been found in the Alaskan Arctic. A common problem in making paleomagnetic studies in both modern and ancient high arctic rocks is the steep inclination of the earth's magnetic field, which degrades declination measurements. The steep inclination, and the fact that many key localities have complex reformational histories that introduce uncertainties in restoring paleohorizontal indicators to modern horizontal, have led many investigators to rely more on paleolatitude than on paleomagnetic pole positions (Stone, 1989). Fortunately, most tectonic reconstructions of the Arctic imply paleolatitude changes well within the resolution of the paleomagnetic technique. The movement of the major circum-arctic cratons has been defined fairly well from paleomagnetic measurements and seafloor magnetic anomalies, but the motions of the smaller crustal fragments nearer the margins of the Arctic Ocean Basin are largely unconstrained. Because many arctic tectonic models make different predictions concerning plate translations and rotations, they are especially amenable to paleomagnetic testing. For example, the displacements of the paleomagnetic poles that would have occurred had large parts of Chukotka and Alaska rotated about a pivot in the Mackenzie Delta area are quite different from those required by translation of Chukotka-Alaska from a location adjacent to the Lomonosov Ridge (Halgedahl and Jarrard, 1987). A paleomagnetic sampling program in the circum-arctic rim to test major tectonic hypotheses for the arctic region and to provide critical benchmarks for constructing new or revised hypotheses is therefore proposed. Initial investigations should also include the Mesozoic rocks of the New Siberian Islands that are key to rotational models for the Arctic and the assorted lithotectonic terranes and ancient island are sequences that lie between the Siberian shield and nuclear North America. The latter samples would test and help to resolve the timing of the postulated isolation of the Arctic from the Pacific Ocean in Late Mesozoic time. Seismologic Investigations Earthquake Seismology Teleseismic studies offer considerable potential for study of the tectonics and deep crustal structure of the Arctic Ocean Basin and can provide continuous remote monitoring of current tectonic activity (e.g., Fujita et al., 1990). The acquisition of high-quality seismological data from the Arctic is hampered, however, by a lack of seismic stations near the coast and by the low quality of the few existing stations. In addition, few accessible standardized seismic data are available from the large Soviet sector of the Arctic. These factors have resulted in a relatively high detection threshold, about mb > 4.5 for earthquakes in the central Arctic Ocean and have limited our ability to conduct detailed tectonic and structural studies of this seismogenic region. This handicap would be removed if a permanent network of modern broad-band digital stations were installed in boreholes at coastal stations and islands around the Arctic Rim. Borehole instruments are needed in the Arctic to reduce noise from seasonal melting and freezing of permafrost. The network should feature standardized formats to facilitate the transfer of earthquake data to researchers worldwide. The continuous record of seismicity that such a network would compile would be especially useful for studying the central Arctic Ocean Basin, where seismogenic structures appear to generate low-to moderate-magnitude earthquakes with long recurrence intervals. The network would greatly improve understanding of the structure and rheology of the crust and upper

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Opportunities and Priorities in Arctic Geoscience mantle, the nature of the continent-ocean transition zones, plate interactions, and neotectonics in the Arctic. The broad-band stations should be sited to create favorable source-receiver paths for surface wave and tomographic inversions across the Arctic Ocean Basin. Existing seismic stations with long operating histories should be upgraded and ice stations should operate seismographs where feasible. The broad-band stations should be supplemented with local networks in areas of tectonically significant seismicity such as northeastern Alaska, the Mackenzie Delta, and the southwestern Canadian Arctic Islands. Such local networks provided important supplementary data in northern Alaska and the Canadian Arctic from 1975 to 1982. Structure and Rheology of the Crust and Upper Mantle A primary objective of seismological research in the Arctic Ocean Basin is to determine the structure of the crust and upper mantle beneath the basins, ridges, and continental margins of the Amerasia Basin. Tomographic inversion of surface-wave phase velocities can provide information on the structure of the crust and upper mantle at wavelengths of 100 km or less with resolution in depth of 10 km (Yu and Mitchell, 1979). Because regional body wave phases are also well excited along most earthquake paths that cross the Arctic Ocean, three-dimensional elastic and anelastic modeling of these phases would provide insights into the structure beneath the large bathymetric features of the Arctic Ocean Basin, information that would be especially useful when interpreted in conjunction with improved potential field and long-offset seismic refraction data. Reflected body wave phases could also be used to constrain further the elastic and anelastic parameters of near-surface features. The present paucity of high-quality seismograph stations in the Arctic, particularly in the Soviet sector, limits the number of ray paths that can be studied. The lack of stations on arctic continental margins also makes it difficult to separate the contributions of ocean and continental crust to the dispersion curves. High-quality stations can also serve as anchors for regional arrays, similar to those envisioned by PASCAL, which can better constrain near-source and near-station structure through the calculation of source and receiver functions. High-priority targets for seismic crustal structure studies include the Chukchi Borderland, Mendeleev Ridge, the Canada Plain, and the East Siberian and Barents Shelves. Improved earth structure models would also enhance moment tensor inversion and other source process studies. Further, more numerous and improved seismograph stations, possibly including ocean-bottom seismometers, would aid in the study of ocean-floor anisotropy and the details of regional surface wave dispersion. Continent-Ocean Transition Some segments of the arctic continental margin display high seismic activity. Along the Canadian Arctic Islands, for example, clusters of earthquakes with magnitudes approaching 6 are associated with positive gravity anomalies. Both sediment loading (Hasegawa et al., 1979) and deglaciation stresses (Stein et al., 1979) have been proposed as causative factors. More precise location of the low-to moderate-magnitude events would provide significant insight into the structural character and geodynamics of these margins. It is curious that earthquake clusters with events as large as magnitude 7 lie landward of the continental slope near some aseismic arctic continental margins. These clusters occur within continental shelf basins or along subbasin structures, and their study may provide important insights into the development of structurally controlled sedimentary basins within passive continental margins. Plate Interactions Delineation of microseismicity and transform faults along the Arctic Mid-Ocean Ridge and its junction with the Laptev Shelf and identification of volcanic earthquakes along the ridge axis would provide important insights into the mechanisms that operate at slow-spreading mid-ocean

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Opportunities and Priorities in Arctic Geoscience Late Cenozoic sea level change, regional glaciation, shelf aggradation, and paleoclimate that would strongly complement the well-documented record of eustatic high stands of sea level on the Alaskan Arctic Coastal Plain (Blasco et al., 1990; Dinter et al., 1990). An important advance in understanding of these events would be achieved by coring the 100+ m Late Cenozoic sedimentary sequence at several additional sites on both the Alaskan and the sectors of the Beaufort Shelf. A major stumbling block in paleoclimatic research is our inability to model adequately the effects of future temperature changes in the Arctic. The Quaternary stratigraphic record on arctic shelves and coastal plains may be useful for creating such models because it is located in the zone of glacial-eustatic transgressions and regressions of sea level and encompasses times when conditions were warmer than at present. This record may reveal correlations between Northern Hemisphere climatic and oceanographic changes and waxing and waning of the Greenland Ice Sheet and arctic sea ice that may suggest causes and rates of climate change. Arctic coastal deposits show that such changes occurred as recently as 1.5 to 1 million years ago and perhaps as recently as 125,000 years ago. A series of 30-m cores across the Beaufort Shelf would be an effective way to study these changes at moderate cost. Lack of sea ice and large ice sheets in the Arctic 1.5 to 1 million years ago and perhaps 125,000 years ago produced major differences in albedo and heat budget with respect to the present Arctic. Documenting and dating of these intervals of low albedo would address the susceptibility of arctic sea ice to modification by orbitally forced (Milankovitch-driven) climatic change, orographic influences on air masses from lower latitudes, or more random causes of climate change. Such paleoclimatic studies may be useful for modeling the future impacts of warmer climate. Determination of the role of perennial and seasonal sea ice in glacial/interglacial cycles of the past 900,000 years is needed to understand the processes that drove climatic change during the Quaternary. These cycles occurred about every 100,000 years (Berger, 1988), but glacial intensity and interglacial warming were not uniform from cycle to cycle. Evidence on the lead time between the onset of forcing conditions in the ocean-atmosphere system and the onset of glaciation or deglaciation is especially needed. Continuous drill cores from the Arctic Ocean Basin and from shelves and coastal plains that escaped Quaternary glaciation would provide key data on past and future global climatic change. High-latitude lakes with thick sedimentary sections beyond the limits of Quaternary glaciation would also provide promising drilling targets (Andrews and Brubaker, in press). A special focus of research should be the most recent periods of major transition, in particular, the late glacial to Holocene transition, when dating control would be best, and the last glacial/interglacial/glacial cycle (150,000 to 80,000 years ago). The latter could be compared with the forthcoming data from the Greenland Ice Sheet Project II. Coordinated interpretation of arctic sediment and Greenland ice cores would provide an especially powerful data base for climate modeling. Useful assessments of past and future climate change in the Arctic will also come from detailed studies of Holocene terrestrial and continental shelf deposits. It is now known that the warmest postglacial period in the Arctic occurred during the mid-Holocene, when insolation was high and the thickness of the active layer in areas of permafrost increased (Bradley, 1990). The recognition of this mid-Holocene warm interval in cores and outcrops from the Arctic Ocean Basin and from the arctic shelf and coastal plain, and in ice cores from Greenland and the Canadian Arctic Islands, would establish an important environmental datum. This might establish criteria by which similar events could be recognized in pre-Holocene sections. In summary, our knowledge of the environmental evolution of the Arctic will be greatly advanced by a focused effort to recover a detailed stratigraphic record of the last 65 million years from several areas of the Arctic. The last 3 million years, and especially the last 900,000 years of this record, are critical to understanding the nature and causes of alternating glacial and interglacial climates in high latitudes and the sensitivity of the Arctic to future anthropogenic global change.

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Opportunities and Priorities in Arctic Geoscience The Record in the Arctic Ocean Basin Cretaceous and Tertiary Sediments Seven piston cores—six Cretaceous and one Early Cenozoic—constitute the entire existing stratigraphic record of pre-Late Cenozoic deep-ocean sediments in the Arctic Ocean Basin (see Figure 9) (Clark, 1988; Phillips et al., 1990). Three cores from Northwind Ridge consist of Lower Cretaceous (Albian) bioturbated siltstone, and one core from Alpha Ridge consists of Upper Cretaceous (Campanian) organic-rich mud composed of abundant plant material in a matrix of marine phytoplankton. Two cores of Maastrichtian age and one of Eocene age from Alpha Ridge consist largely of siliceous plankton skeletons. The pre-Pliocene arctic piston cores contain important paleoenvironmental information, but they are far too few to constitute a history of its arctic environmental evolution. The paucity of the record does not enable resolution of some basic controversies about the significance of the few samples that are in hand. For example, Upper Cretaceous and Eocene sediments in four of the cores contain species of dinoflagellates, diatoms, and silicoflagellates known also from coeval sediments at lower latitudes. Mutual occurrence of so many species in widely separated water masses suggests to some investigators that there was substantial interocean circulation of relatively warm water between the Arctic and more southerly oceans at these times (Clark and Kitchell, 1979; Kitchell and Clark, 1982). Other investigators, however, on the basis of shallow-water marine invertebrate faunas near the Cretaceous-Tertiary boundary, suggest that the Arctic Ocean was isolated during at least part of this interval (Gartner and Keany, 1978). The presence in the Maastrichtian and Eocene cores of laminations consisting of layers rich in diatom resting spores in alternation with layers rich in their vegetative cells suggests strong Late Cretaceous and Paleogene seasonality in the Arctic Ocean (Kitchell et al., 1987). There is a significant gap in the Arctic Ocean Basin sedimentary record between the Eocene and the Pliocene. The gap may result from the same Late Miocene unconformity that is widely recognized on arctic shelves, but it is more likely a result of inadequate sampling. In contrast, many samples of Plio-Pleistocene glacial-marine sediment have been obtained from the ridges and slopes of the Arctic Ocean Basin; many samples of Quaternary turbidites have also been obtained from the continental rises and abyssal plains (Clark et al., 1980; Campbell and Clark, 1977). These younger sediments contrast strongly in lithologic character with the older sediments of the basin. Evidently, the Late Cretaceous-Early Cenozoic earth had a much warmer Arctic Ocean than has existed at any time during or since the Pliocene or perhaps in earlier Neogene time. This change in arctic climate, its correlative oceanographic conditions in the arctic and other oceans, and its significance for modern world climate need to be documented by detailed studies of the stratigraphic record that is available in the Arctic Ocean Basin. More samples would be needed to document the changes in sea level, climatic cycles, circulation patterns, and current regimes in the Arctic Ocean Basin through time; chemical cycles of silica, carbon, and phosphorous; patterns of interaction between sediment and fauna; diagenesis; and many other aspects of the lithologic and biotic record in ancient Arctic Ocean sediments. Such data are necessary to understand the paleoclimatic and paleooceanographic history of the Arctic Ocean and the mechanisms of global climatic change. An important ancillary question is whether the Arctic Ocean Basin was a site for evolution and adaption of marine life to cooler conditions and, ultimately, sea ice. Arguably, the unique characteristic of the Arctic Ocean is its perennial cover of sea ice (Clark, 1990a). The geographic extent, distribution, and thickness of the ice cover exert a major influence on global climate today, as they undoubtedly did in the past. We do not know when the ice cover first formed, the detailed chronology of its waxing and waning during the Late Cenozoic, and the environmental conditions that caused it to form, melt, and reform periodically. The known effect of the size of the polar ice pack on modern climate suggests that its history would be of relevance to understanding past and predicting future global climatic change. This record would be particularly useful when discrepancies between interpretations based on deep-sea sediments and those based on sections from the continental margin, the circum-arctic, and the ice cap are resolved and unified interpretations become available (Clark, 1990b).

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Opportunities and Priorities in Arctic Geoscience Accumulating field experience suggests that a composite stratigraphic section representing all the major time units and the principal Late Mesozoic and Cenozoic paleoenvironments of the Arctic Ocean Basin could be obtained from long piston cores and shallow rotary drill holes sited on the basis of seismic reflection profiles and bathymetry. The committee recommends that a coring program be initiated. Sample sites would have to be on the crests and flanks of ridges and from continental slopes that are isolated from the deltas of major river systems. Existing cores and seismic reflection records collected from ice island T-3, the LOREX, CESAR, and FRAM I–IV experiments, and the U.S. Coast Guard cutter Polar Star provide an adequate basis for initiating a coring program, but additional seismic reflection data would be needed to achieve optimum stratigraphic coverage. The cores would provide answers to many questions concerning the paleoenvironment and tectonic history of the Arctic and its role in world climate. For example, they would fill in the gap in sediment samples between the Eocene and Pliocene in the Arctic Ocean Basin, when the major transformation from an ice-free to an ice-covered Arctic Ocean occurred. They would also provide data on oceanic circulation in the Cenozoic Arctic, faunal interchange between the Arctic and more southerly oceans, and the evolution of the arctic ecosystem from a warmer ocean in mid-Cretaceous time to modern conditions. Sampling of pre-Pliocene sediments would significantly increase the observational data base for hindcasting and testing climatic models and improve understanding of the natural history of the Arctic Ocean and its relation to global climatic change. For example, what conditions enhanced the thermal stratification in the Arctic Ocean that fostered the development of sea ice, and how often were these conditions repeated during the past 3 million years of earth history? How did changes in bottom water formation in the Norwegian and Greenland Seas during glacial/interglacial cycles affect the physical and chemical oceanography of the Arctic Ocean Basin? Ongoing research in the more southerly ocean basins is attempting to address these and similar issues, but these studies would benefit from closer integration with studies of the sedimentary record in the Arctic Ocean Basin and its continental shelves and coastal plains. Quaternary Sediments Our present understanding of the Arctic Ocean is comparable to our understanding of oceans at lower latitudes 20 years ago. Fundamental questions concerning the Arctic Ocean during the Quaternary need answers before we can fully evaluate its role in global climate. The Quaternary record in the Arctic Ocean Basin contains a wealth of information on the character, detailed chronology, and possible causes of the glacial/interglacial cycles that have dominated global climate during the Late Cenozoic. Better understanding of this record would improve evaluation of the environmental record contained in older sediments in the Arctic Ocean Basin. A central question for understanding the Quaternary environment in the Arctic Ocean Basin is how and why it alternated between ice-free and ice-covered states. A well-constrained and detailed chronology from arctic subsea sediment cores would greatly assist in this task, but techniques used widely in lower latitude ocean basins (stable isotopic measurements, biostratigraphic indices, and radiometric dating) have proven difficult to apply. Recent experience suggests that better prospects for dating arctic cores may lie in paleomagnetic stratigraphy derived with sensitive cryogenic magnetometers that permit extensive demagnetization of samples without sacrificing sensitivity and in radiocarbon 14C dating by accelerator mass spectrometry (AMS). Radiocarbon dating allows individual components of the sediment to be dated, thus eliminating the spuriously old ages caused by detrital limestone grains or reworked fossils in bulk sediment samples dated by conventional methods. A second major issue is how Arctic Ocean Quaternary sedimentation rates and sediment composition differ between glacial and interglacial times. What can the differences tell us about glaciological processes? Although there has been much controversy regarding the rates of sedimentation in the Amerasia Basin (Clark et al., 1980; Sejrup et al., 1984; and Macko and Aksu, 1986), it is now widely accepted that rates were low (2 to 3 mm/1,000 years) throughout the Late Quaternary and that the lithostratigraphy of Clark and others (1980) is largely supported by new studies of the paleomagnetic chronostratigraphy (Jones, 1987; Witte and Kent, 1988).

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Opportunities and Priorities in Arctic Geoscience Sedimentation rates in the Eurasia Basin, on the other hand, are found to be nearly an order of magnitude larger (Mienert et al., 1990). Realistic interpretation of the Quaternary and older stratigraphic record and reconstruction of the Late Quaternary climate and oceanography of the Arctic Ocean Basin require precise understanding of the sedimentary processes by which modern and Holocene sediments were transported to the basin and deposited and altered within it. Many of these questions can be addressed with sediment traps; buoys for measuring ongoing physical, chemical, and biological processes; and new box cores from a wide range of sedimentary environments in the Arctic Ocean Basin. Existing samples in core repositories are, with few exceptions, inadequate. Box cores enable recovery of undisturbed samples of a large cross section that include the sediment-water interface and as much as 50 cm of the underlying sedimentary column. In the Arctic Ocean Basin, cores 50 cm long can recover sediments of both the Holocene and the last glacial maximum and contain sufficient material for study and intercomparison by a variety of analytical techniques. Giant gravity and large-diameter piston cores can recover samples of the entire Late Quaternary sequence in sufficient volume for analysis by multiple techniques. Such analyses would clarify the role of the Arctic in the earth's carbon cycle and measure other environmentally significant components such as nitrogen, phosphorus, and silica. Even though overall sedimentation rates in the Amerasia Basin appear to have been low, little is known about their variations in time and space. Radiocarbon dating by AMS allows for the first time to direct measurement of sedimentation rates during the Holocene interglacial and the last glacial maximum. Preliminary results show that Holocene sedimentation rates in the Arctic Ocean Basin are approximately 10 to 20 times greater (1 to 2 cm/1,000 yrs) than those of the last glacial maximum (0.05 to 0.1 cm/1,000 yrs) (Mienert et al., 1990). What environmental conditions and processes were responsible for these variations? Did they exist during earlier glacial and interglacial intervals? Are they compatible with overall Quaternary sedimentation rates for the Arctic Ocean Basin? A detailed chronology of variation in Arctic Ocean sedimentation rates, especially when closely correlated with temporal changes in lithology, fossils, chemistry, and other physical and paleobiologic features of the sediments through time, should be a major objective of solid-earth geoscience in the Arctic. The lithologic component of arctic sediments offers opportunities for identifying and dating major arctic climatic events. For example, paleomagnetic chronostratigraphy suggests that prior to approximately 1.5 million years ago, little ice-rafted detritus and virtually no detrital carbonate entered the Arctic Ocean Basin (Jones 1987). Preliminary data suggest that since that time floods of lithologically distinctive sediment from the Queen Elizabeth Islands entered the Amerasia Basin approximately every 400,000 years. Were these islands glaciated less frequently than the approximate 100,000-year cycle of ice sheet growth and decay observed elsewhere? Or did the outwash go elsewhere during some glaciations? Since the last glaciation has removed most of the traces of prior glacial events in the circum-arctic landmasses, the arctic marine detrital record is essential for the study of Quaternary glaciations. Insights into the mechanism of global change may result from such correlations because they would provide close correlations between accelerated erosion and glaciation events in the circum-arctic landmasses with paleoceanographic conditions in the Arctic Ocean Basin. Preliminary studies suggest that it may be possible to identify the provenance of the sediments that periodically flooded the central Arctic Ocean Basin by comparing their mineralogy, petrography, and perhaps isotopic character with lithologically unique terranes in the circum-arctic land-masses. If successful, such studies would enable interpretation of when specific circum-arctic terranes were glaciated, thereby providing insights into the chronology and dynamics of Quaternary continental glaciation. The lateral variability in thickness and lithology of the biogenic and nonbiogenic Late Quaternary sediments of the central Arctic has important implications for sedimentary processes in the Arctic Ocean Basin. In the past, it was assumed that virtually the entire basin, particularly the Canada Basin, contained the same sequence of synchronous sediments, as demonstrated by the core-to-core correlations for the Canada Basin illustrated by Clark and others (1980). It is now possible to test this hypothesis with AMS 14C dating. Time slices of the last 40,000 years

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Opportunities and Priorities in Arctic Geoscience can now be constructed with unprecedented resolution by this method to test whether Late Quaternary sedimentary units are synchronous or time transgressive across the Arctic Ocean Basin. Such studies would provide data concerning supply and transport in the basin and the relative strength and duration of sediment sourcelands during latest Quaternary time. The stratigraphic record of deglaciation in the Arctic Ocean Basin and its comparison with the record at lower latitudes provide useful information for both Quaternary studies and climate modeling. Numerous competing theories of glaciation/deglaciation exist, and many predict explicit configurations of ice masses through time in the Arctic Ocean Basin and its environs. Unfortunately, the age of the successive deglaciations in the Arctic Ocean Basin is poorly known (e.g., Denton and Hughes, 1981). Detailed mineralogical and stable isotopic analyses of the sediments deposited during each deglaciation, combined with AMS 14C dating, would reveal how this region moved from glacial to interglacial conditions and the chronology of that transition. Combining these data with the better-understood deglaciation sequences of the Norwegian-Greenland Sea and the North Atlantic would show when the Arctic was oceanographically reconnected with the world ocean and whether deglaciation in the Arctic was earlier, later, or coeval with deglaciation in other ocean basins. Insights into the dynamics of ice-sheet decay and ocean circulation would be gained from such studies. The Record in Arctic Ice Cores Drill cores from the massive ice caps of Greenland and the Canadian Arctic Islands provide a record of the last 130,000 years of the earth's climatic and environmental history (Robin, 1983; NRC, 1984a, b). According to the Committee on the Role of the Polar Regions in Climatic Change of the Polar Research Board (NRC, 1984b, p. 47), ice at the base of the ice sheet in south-central Greenland could be as old as 1 million years. An ice core to bedrock would provide an invaluable complement to the record preserved in the terrestrial and marine sedimentary rocks of the Arctic. Some of the ice core data, such as its detailed record of variation in atmospheric composition, including CO2 content, atmospheric turbidity, and micrometeorite flux, may be obtainable by no other method. The climatic and environmental history embedded in ice cores will be most fully understood when it is supplemented and complemented by evaluation of the sedimentary record preserved in the Arctic Ocean Basin and its continental shelves and bordering lowlands. ARCTIC GEOLOGIC PROCESSES AND ENVIRONMENTAL INDICATORS Knowledge of the influence of glacial and interglacial climates on the chemistry, mineralogy, lithology, and texture of arctic biogenic and clastic sediments is meager, but recent observations suggest that certain features of the arctic sedimentary record may enable interpretation, and in some cases quantification, of past environmental conditions and processes in the arctic sedimentary record (Thiede et al., 1990). Quantification, if achievable to a significant degree, would provide the ability to test climatic and oceanographic models by hindcasting from present conditions to numerical data points at specific times in the geologic past. Paleoenvironmental Indicators Detailed analyses of both organic and inorganic materials plus isotopic measurements of a few light elements can provide the basis for estimation of environmental conditions in ancient seas and their shallow sediments. Bottom water temperature, for example, can be inferred from the presence of calcite pseudomorphs after ikaite (CaCO36 H2O) which forms where organic-rich sediment accumulates rapidly in normal seawater near and below O°C (Suess et al., 1982; Jansen et al., 1987). Paleoenvironmental conditions in modern and Late Cenozoic sediments can also be estimated from analyses of certain organic compounds (biomarkers) in sediments and fossils. Long-chain alkanes in certain species of algae and dinoflagellates, for example, have been used to estimate paleotemperature in subsea cores from mid-latitude oceans (Eganhouse and

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Opportunities and Priorities in Arctic Geoscience Kaplan, 1988) and an antarctic lake (Volkman et al., 1988), and the method may be applicable to Arctic Ocean Basin sediments. The ratio of 18O/16O in biogenic carbonate in marine sediments that have not experienced diagenesis by burial deeper than 200 m reflects primarily variations in the volume of the continental ice sheets, which contain more of the lighter 16O isotope than seawater (Hoefs, 1987). In the absence of salinity anomalies, water temperatures during sedimentation in ancient oceans can be interpreted from this ratio. Ratios of 18O/16O in calcareous marine organisms, particularly foraminifera, can therefore serve as an index for glacial versus interglacial climatic conditions in Late Cenozoic marine sediments (Shackleton and Opdyke, 1973; Aksu, 1985). Increases in 13C/12C ratios in fossil planktonic marine organisms and organic matter may reflect variations in biological productivity or sedimentary carbon flux from the continents (Aksu, 1985; Darby et al., 1989). Although carbon and oxygen isotopic ratios and their interpretation are most reliable in diagenetically unaltered rocks, interpretations of these ratios have been made from strata as old as Early Cretaceous (Douglas and Savin, 1975) and Devonian (Brand, 1989). If useful interpretations can be made from rocks as old as Early Cretaceous, they would enable interpretation of important aspects of arctic paleooceanography and climatic history. Stratigraphic variations in oxygen and carbon ratios in the arctic sedimentary record are difficult to establish because of sampling problems resulting from bioturbation and because of the dominance of foraminifera-poor zones representing glacial intervals. Relatively large differences in salinity between near surface and deeper water in the Arctic Ocean also complicate interpretation of the data. As a result, mainly interglacial events are available for isotopic study in arctic cores. Gaps in the arctic isotopic record may be represented by foraminifera-bearing strata in the adjacent Greenland or Norwegian Seas (Henrich et al., 1989), where sea ice may have been thinner or intermittent and biological productivity higher than in the Arctic Ocean during the ice ages. Correlation of variations in oxgen and carbon isotope ratios with changes in biostratigraphy and lithology in Late Cenozoic arctic cores would provide insights into the onset, timing, and character of glacial/interglacial cycles and oceanographic conditions in the Arctic during the Quaternary. Sedimentation Several aspects of sedimentation of the Arctic Ocean Basin are unusual, and a basic question is how, and to what degree, the Late Cenozoic climate imprinted the arctic sedimentary record. Clastic deposition rates in the arctic marine environment are largely dependent on continental denudation rates, involving glacial outwash, river runoff, eolian supply, and coastal erosion. However, the flux of such materials in density currents, sea ice, and winds from the continents and continental shelves to the Arctic Ocean Basin is poorly known. Aerosols, for example, are estimated to contribute between 1 and 10 percent of the yearly accumulation of arctic sediment (Mullen et al., 1972; Darby et al., 1974), but quantitative data to constrain this range are lacking. Further, the sediment load of not a single arctic stream has been gauged in Alaska, and the proportion of sediment in the sediment/ground ice mix that is eroded annually from the arctic coastal plains and shallow shelves by marine abrasion and transferred to the shelf is too poorly known to allow adequate estimation of the sediment budget on arctic shelves (Reimnitz et al., 1988). Only a start has been made in assessing the mineralogy, sedimentation rate, and provenance of arctic clastic and biogenic sediments from cores and in mapping the areal variations in thickness and character of the youngest sedimentation units on seismic reflection profiles. Characterization and quantification of the clastic and biogenic sediment now being contributed to the Arctic Ocean Basin by winds, sea ice, icebergs, and turbidity currents, and by chemical, biological, or biochemical processes in the water column and in the shallow sediments would significantly improve interpretation of the lithostratigraphic record in arctic cores. Sampling of the water column and ice canopy for the clastic and biogenic components by sediment traps, ice coring in different parts of the Arctic Ocean Basin (Barnes et al., 1982; Honjo et al., 1988), and knowledge of overall water and ice circulation patterns would also be required. Quantification of the flux of fine-grained biogenic and clastic particles from the

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Opportunities and Priorities in Arctic Geoscience seabed to the sea-ice canopy in frazil ice, their residence time in the canopy, and their dispersal to the Arctic and North Atlantic Basins would help to establish a sediment budget for these materials. Modern atmospheric circulation patterns in the Arctic are understood in sufficient detail to identify the sources of particulate matter. With modern conditions as a guide, some insight into arctic paleoatmospheric circulations might be gained if the eolian component in arctic sediment cores was to be identified and quantified. Such data would provide a means for recognizing and dating glacial/interglacial cycles in the marine sedimentary record. Viewed globally, arctic continental denudation rates are low, and consequently, sediment supply to the sea by rivers is also low (Milliman and Meade, 1983). Greater amounts of mobile sediment are presently being supplied to the seafloor and water column by coastal and inner shelf erosion, driven largely by sea-ice bulldozing, ice rafting, and current scour and resuspension of sediment around grounded ice (Barnes et al., 1988). The balance in sediment supply between marine inner shelf and upland sources must have been different in the past, when glaciers were more extensive and large alluvial fan systems on arctic coastal plains delivered large volumes of sediment to the sea. When in the glacial/interglacial cycle, and under what climatic conditions, were these remarkable fans active? If we could distinguish sediment deposited when inner marine shelf erosion was dominant from sediment deposited when alluvial outwash systems were dominant, we could improve our ability to distinguish glacial/interglacial cycles, and perhaps even stages in these cycles, in the lithostratigraphic record. A detailed understanding of the lithology and flux of arctic shelf sediments during the transition from full glacial to full interglacial conditions may permit better interpretation and dating of Late Cenozoic glacial/interglacial events in the arctic sedimentary record than can be achieved from the terrestrial record alone. A core drilling program along several inner-shelf to upper-slope transects would characterize the hydraulic cross-shelf and along-shelf sediment transport systems that operate on ice-covered arctic shelves, which are now poorly understood, and assess the character, geologic history, and economic potential of the gravel deposits that have been found beneath the outer Beaufort Shelf. Associated with sea-ice formation on arctic shelves is the formation of dense, cold brine rejected from the growing and recrystallizing of nearly salt-free sea ice (Aagaard et al., 1983). These dense brines flow down submarine valleys on the shelves (Garrison and Becker, 1976) into the deep Arctic Ocean Basin in winter outbursts (Honjo et al., 1988) that may imprint the sedimentary record. Thus, some of the well-laminated sediment in Barrow Sea Valley and the Arctic Ocean Basin may be deposits from brine-charged density currents rather than from turbidity currents. If brine density current deposits could be recognized (e.g., from the presence of ikaite) in the lithostratigraphic record, they might identify former periods of sea-ice formation on arctic shelves. Their efficacy for transporting fine-grained clastic and biogenic sediment may also warrant attention. Role of Sea Ice in Arctic Sedimentation The Pleistocene lithostratigraphic record in Arctic Ocean cores shows repeated alternations of intervals with and without ice-rafted materials and thus yields information on the sea-ice cover (Henrich, 1990; Henrich et al., 1989). Sediment with ice-rafted clasts and abundant foraminifera is commonly held to indicate interglacial conditions, and fine-grained sediment without these materials to indicate glacial conditions. There is general agreement that the coarse detritus was deposited from glacial ice or sea ice, but the strata that lack coarse ice-rafted elastics are controversial. The lack of foraminifera in such intervals has been attributed, for example, to low surface water salinity and decreased nutrient levels (Herman et al., 1989), dissolution of foraminifera tests, high sedimentation rates, and thicker-than-present ice cover (Aksu, 1985). A research goal would be to determine the origin of the foraminifer-poor, fine-grained intervals which become thicker and muddier toward the basin margins. How, and under what oceanographic conditions, did the intervals form? Were they deposited during the initial phases of deglaciation, when melting glacial ice released large quantities of silt-and clay-size sediment into the basin that may have masked biologic productivity, or were foraminifera preferentially

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Opportunities and Priorities in Arctic Geoscience dissolved from these beds (Henrich et al., 1989)? Under what conditions could dissolution of foraminifera occur at the relatively shallow depths at which some of these beds occur? The foregoing speculations recognize the importance of ice-rafted deposits in the Arctic Ocean Basin but are based on meager data on sediment in modern sea ice. The significance of sediment in sea ice is indicated by recent investigations suggesting that turbid, sediment-bearing sea ice on the Alaskan Beaufort Shelf may, in some years, transport 16 times more sediment than the yearly input from rivers feeding the same region (Kempema et al., 1989). Turbid ice forms from rising frazil ice that is suspended in the water column, where it was formed under turbulent, super-cooled, open-water conditions. The rising frazil ice appears to collect fine particulate matter and plankton from the water column and possibly directly from the seafloor and carry them to the sea surface. Upon reaching the surface, the frazil ice forms a layer of sediment-laden slush ice that soon congeals into sea ice. On sandy, gravelly, or shelly substrates shallower than 30 m, anchor ice forms when frazil is present. The anchor ice, sometimes carrying with it seabed sediment and biota, also rises to the sea surface to be incorporated as patchy inclusions in sea ice (Reimnitz et al., 1987). Recrystallization in multiyear sea ice aggregates the initially dispersed fine-grained sediment into cohesive mud pellets that are indistinguishable in texture, microfossil content, and composition from pellets previously thought to record rafting and deposition by glacial ice (Goldschmidt et al., in press). The presence of dropstones and sand-size sediment was also thought to record exclusively rafting by glacial ice (Clark and Hanson, 1983). Clearly, more needs to be learned about the character, provenance, quantity, and dispersal of sediment in arctic sea ice and about criteria by which such sediment can be recognized in the lithostratigraphic record. Meeting these objectives would require additional work on the conditions under which sediment is entrained in sea ice, on how sediment in sea ice affects its albedo and melting, and on patterns of sea-ice sediment release to the seabed. What is the role of wind in carrying sediment to the sea ice surface? What is the effect of sea ice on the production of those biologic organisms that constitute the paleontologic and organic geochemical record in Arctic Ocean sediments, and how does this organic productivity contrast with that at times when the Arctic Ocean was ice free? Paleobiogeography and Paleoecology The Arctic Ocean is the last of the world's oceans whose marine faunal history is poorly known. Recent studies nevertheless suggest that its paleobiogeographic and paleoecologic histories are closely related to Mesozoic and Cenozoic tectonic events and can therefore provide an independent set of observations for testing and dating plate tectonic reconstructions (Marincovich et al., 1990). These events are manifested in changing paleoecologic tolerances of successive faunas in the Arctic and in shifting paleobiogeographic relationships with faunas of adjacent ocean basins and epicontinental seaways. Because present knowledge of Arctic Ocean faunas is limited, the prospects for major increases of knowledge from paleontological studies are bright. Triassic mollusks show that an ancestral Arctic Ocean was a northern gulf of the Pacific Ocean at the site of the present Amerasian Basin (Westermann, 1973). Strong genus-level molluscan ties were maintained between the Pacific and Arctic Oceans throughout the Jurassic and very Early Cretaceous, although with fewer shared species than during the Triassic. No broad Pacific-Arctic opening existed after the Neocomian, about 125 million years ago (Fujita and Newberry, 1983). The developing Mesozoic faunal provincialism in the Arctic reflected an increasingly restricted marine connection between the Arctic and Pacific Basins produced by plate tectonic movements. The co-occurrence of North Pacific and Arctic Ocean ammonites in Late Early Albian faunas of southern Alaska demonstrates that limited north-south seaways were at least intermittently present until about 110 to 105 million years ago (Williams and Stelck, 1975). By Albian time, however, Arctic Ocean faunas were of cold-temperate aspect, recording markedly cooler waters than those in the North Pacific and the epicontinental seaways of North America and western Siberia. Study of several known but almost unstudied Jurassic and Lower Cretaceous marine molluscan faunas in Alaska and adjacent parts of Canada would help to refine

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Opportunities and Priorities in Arctic Geoscience these paleoecological inferences and to date the tectonic events that controlled the marine connections between the Arctic and the world ocean. Late Mesozoic mollusks show strong biogeographic ties from northern Alaska through the Western Interior Seaway as far south as the Dakotas and down the eastern margin of North America to Nûgssuaq, West Greenland (Balkwill et al., 1983). These faunas define a cold-temperate northern faunal province centered on the Arctic Ocean. Refinement of the species-level identifications of these faunas would more precisely delineate the paleobiogeography of this Late Cretaceous arctic faunal province and document paleoecological (i.e., paleoclimatic) changes with latitude. The Arctic Ocean may have been geographically isolated during the Cretaceous/Tertiary transition because there is no obvious faunal break in marine mollusks across this boundary in the Ocean Point beds of the North Slope (Marincovich et al., 1985, 1990). Isolation is also suggested by unrealistically high paleotemperatures calculated from preliminary 18O measurements of mollusks from Ocean Point and generally coeval Paleocene beds on Ellesmere Island; the 18O values might indicate that the Arctic Ocean was hyposaline, rather than warm. Therefore, the measurements need to be repeated and additional ones obtained from around the Arctic, and the systematics evaluated further. If the Arctic Ocean was isolated during the Cretaceous/Tertiary transition, it may have permitted some taxa to survive longer there than elsewhere and other taxa to evolve which appeared in other oceans only later. The first appearance of previously isolated Arctic Ocean taxa in North Atlantic faunas would effectively date the earliest shallow-water connections between the two ocean basins. Integration of the mammalian and marine faunal evidence for the openings and closings of Arctic-North Atlantic seaways would require additional detailed and extensive fieldwork. Opening of the Bering Strait allowed North Pacific mollusks and microfossils to enter the Arctic, where they rapidly displaced most of the Atlantic-Arctic holdover marine fauna evolving (Durham and MacNeil, 1967). For all its importance, the date of this onslaught is unresolved. It evidently occurred during the Early Pliocene warm interval and prior to the first appearance of Pacific mollusks below a paleomagnetically dated 3 to 3.5 million year old basalt bed on Iceland (Gladenkov, 1981). Unpublished Soviet data place the first appearance of an Atlantic-Arctic mollusk in Early Pliocene faunas on eastern Kamchatka about 4 million years ago. Resolving the time of opening of the Bering Strait would require coordinated Soviet, U.S., and Canadian biostratigraphic and taxonomic studies. The sequence of multiple Pliocene and Pleistocene marine transgressions in northwest Canada, western and northern Alaska, and adjacent parts of the USSR show clear alternations between relatively cold and relatively warm marine faunas (Hopkins, 1967). However, the precise ages and magnitudes of the climatic fluctuations are not known, and they constitute an important subject for additional study. Possible Record of Solar-Terrestrial Interactions Ice caps in the polar regions collect particulate matter settling out of the atmosphere. On a macro scale, this process has been demonstrated by the large numbers of meteorites obtained from the antarctic ice sheet. Deep arctic lakes may also provide a good environment for recording the influx of exotic material because their cold bottom waters commonly produce anoxic conditions that significantly reduce bioturbation of the bottom sediments. The annual accretion rate of extraterrestrial material onto the Earth is 10,000 tons/yr. Most of this mass occurs as particles in the 0.1 to 0.4-mm range. The flux of 0.01-mm particles is 1/m2/day and of 0.1-mm diameter particles about 1/m2/yr. The concentration of the particles in polar ice is normally quite low, but the particles can be recovered from melted fractions and positively identified because of unique properties of typical meteoritic materials. The majority of particles have undifferentiated elemental compositions that match the abundances found in the sun and primitive meteorites for the majority of condensable elements. Typical particles are composed of Fe, Mg, Si, C, Al, Ca, and Ni, in descending order of abundance. The particles also contain

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Opportunities and Priorities in Arctic Geoscience undepleted levels of trace siderophiles such as Ir. Typical particles also contain high amounts of solar wind He (with near solar 3He/4He ratios), they contain tracks caused by irradiation by solar cosmic rays, and they contain detectable amounts of 26Al and 10Be produced by irradiation in space. If variations in 10Be and other marginally reactive elements could be detected in arctic sediment cores, they would permit the study of solar variability through time. Although they have not been reported or searched for, it is possible that temporal layers enriched in extraterrestrial components may occur in detectable concentrations in lake or deep-sea sediment cores. A meteor shower, a large impact, or other disturbance in the background flux of cosmic debris could produce layers enriched in extraterrestrial dust particles in the micron to 100-micron range. If submicron particles are channeled along magnetic field lines during accretion, it is possible that the abundance of small particles could vary with magnetic reversals and field strength and be of geophysical interest. Gas Hydrates and Offshore Permafrost Unknown but apparently large quantities of natural gas are trapped within and beneath solid ice-like substances, commonly called gas hydrates or clathrates, that lie within the shallow sediments of large areas of the Arctic Ocean and adjacent coastal plains (Kvenvolden and Grantz, 1990). The hydrates, crystalline three-dimensional cage structures of water molecules, are initiated and stabilized by included molecules of natural gas under the conditions of low temperature and high pressure found in three arctic environments: (1) offshore within shallow sediment of the continental margin where water depths exceed 400 m and cold bottom water and hydrostatic pressure establish the necessary stability conditions, (2) onshore within and below cold, continuous permafrost more than 250 m thick, where the mean surface temperature is <-5°C, and (3) beneath the inner continental shelf where relict low temperature and deep permafrost formed under preexisting subaerial conditions persist locally after rapid transgression following the Holocene rise in sea level. Gas hydrates are of economic interest because they lie close to the earth's surface and can be inferred to contain a major resource of natural gas. Methane hydrate, probably the most common naturally occurring variety, contains about 160 times as much methane as an equal volume of the free gas under standard conditions of temperature and pressure (Davidson et al., 1978). Thus, relatively small volumes of methane hydrate could constitute valuable energy resources if ways could be found to extract the gas safely and economically. Gas hydrates are also of environmental interest because the inadvertent melting of methane hydrate can cause profound natural and engineering disruptions through loss of strength and the uncontrolled release of free gas (Carpenter, 1981). The distribution of free gas, probably methane, in shallow sediment suggests that the large gas hydrate deposits of the continental margin in the Beaufort Sea may leak gas to the surface. Where relict nearshore permafrost-bearing gas hydrate is being destabilized by coastal erosion and warming bottom waters, methane may also be reaching the environment (Kvenvolden, 1988). The flux of methane from the gas hydrate deposits to the environment is of concern because methane, like carbon dioxide, is a greenhouse gas that affects the radiation balance of the atmosphere and therefore climate. Atmospheric concentrations of methane are increasing at a rate of about 1 percent per year; at this rate, although future levels are difficult to predict accurately, the amount of methane in the atmosphere is expected to double in 40 or 50 years. Definitive study of gas hydrate deposits in the Arctic is now technologically feasible, at least in areas that are seasonally ice free. Seismic reflection and refraction methods can be used to map the distribution and thickness of the continental margin hydrate deposits and to define some aspects of their internal structure. Water column and shallow sediment geochemical sampling may be able to detect and identify gases leaking from hydrate deposits in shallow sediments, and the hydrates are within reach of shallow drilling. Drilling the large hydrate deposits at the continental margin, however, presents possible safety problems of arctic gas hydrates (Sloan, 1990). But overall, current technology allows for estimation of the three-dimensional distribu-

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Opportunities and Priorities in Arctic Geoscience tion, composition, internal structure, and stability features that are necessary for estimating the flux of hydrate gases to the atmosphere. Study of the smaller gas hydrate deposits associated with permafrost beneath the inner continental shelves and coastal plains of the Arctic would be less costly but more difficult than study of the larger continental margin deposits. Seismic surveys, geochemical sampling, and drilling would be less expensive on the inner shelf and coastal plain, and comparatively inexpensive seismic surveys may be able to identify gas hydrates beneath permafrost. However, well logs, especially mud logs, and direct sampling with pressure core barrels are the only methods now available for reliably identifying and studying gas hydrate deposits within permafrost, although equilibrium borehole temperatures can identify the zones of hydrate stability in the subsurface (Lachenbruch et al., 1987).