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Opportunities and Priorities in Arctic Geoscience 3 Role of the Arctic in Global Solid-Earth Geoscience GEOLOGIC FRAMEWORK AND TECTONIC EVOLUTION The Arctic Ocean Basin is a virtually closed basin, surrounded by the largest continents (Africa-Eurasia and Greenland-North America). A full understanding of its tectonic history is vital to the development of a plate tectonic model for the Northern Hemisphere. The study of the tectonic development of the Arctic has two main themes, both with broad implications for global tectonics. One is the kinematics of the processes by which the small but complex Arctic Ocean Basin was created by two systems of rifting, one Late Mesozoic and one Cenozoic, within the margins of the supercontinent of Laurasia. The second is the interaction, within the arctic region, of the extensional stresses that have entered it from the Atlantic and the convergent stresses that have influenced it from the Pacific. These stresses, acting since Late Mesozoic time, must have been resolved in the arctic region, perhaps through transformation along major but presently unrecognized faults within the Arctic Ocean Basin. Seismic reflection data collected from the Beaufort Sea suggest that the oceanic arctic basin was initiated by poorly understood rifting events within the periphery of the supercontinent of Laurasia (the Canadian Shield plus Eurasia) beginning in Jurassic time (see Figure 3). For example, east-west striking rift structures have been identified beneath parts of the Beaufort Sea shelf and slope, but the Jurassic rifting did not culminate in continental breakup and seafloor spreading (Grantz and May, 1983; Grantz et al., 1990b). It was premonitory, however, to renewed rifting and seafloor spreading which created the proto-Arctic Ocean at the site of the present Amerasia Basin by mid-Cretaceous time. The early deep-water connection of the new basin to the world ocean was with the paleo-Pacific, though the geometry of this connection is not yet known. Connection with the Atlantic came much later. In the absence of definitive data, the geometry, kinematics, and timing of rifting and the nature of the requisite involvement of the surrounding continents are in dispute; many alternative models have been proposed (see Lawyer and Scotese, 1990b). Northward migration of lithotectonic terranes of southern origin along the Pacific margins of nuclear Eurasia and North America is thought by some to have created a broad isthmus between Alaska and Eurasia in Late Jurassic and Early Cretaceous time that isolated the deep circulation of the oceanic proto-Arctic Basin from the paleo-Pacific (Churkin and Trexler, 1981; Fujita and Newberry, 1983). The geometry and timing of the closure are uncertain. Shallow seaways connected the Arctic Ocean Basin with the ancestral Gulf of Mexico in Late Jurassic and Cretaceous time, with the North Pacific until Albian time, and with the Tethyan realm in Late Jurassic to Paleogene time. However, detailed knowledge of their history and location and possibly other
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Opportunities and Priorities in Arctic Geoscience FIGURE 3 Major structural features of the arctic region (from Grantz et al., 1990a). connections between the Arctic and more southerly oceans is lacking. The history, location, and geometry of these seaways are critical to a number of paleoceanographic, paleontologic, and tectonic studies. Connection of the Arctic and Atlantic Oceans began with Early Jurassic intracontinental rifting in Northwest Europe (Ziegler, 1988). Rifting progressively separated Greenland and Eurasia from North America and then in Paleocene time split Greenland from Europe. Paleocene rifting along the Arctic Mid-Ocean Ridge also initiated separation of the Lomonosov Ridge from the Barents Shelf to create the proto-Eurasia Basin and a shallow seaway between the
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Opportunities and Priorities in Arctic Geoscience Arctic and Atlantic Oceans (Wilson, 1963; Kristoffersen, 1990a, b). This seaway, which developed into an oceanic rift that separated Svalbard from Greenland, permitted the interchange of deep water between the Arctic and the Atlantic by Middle Miocene time. Growth of the Eurasia Basin by spreading along the Arctic Mid-Ocean Ridge almost doubled the area of the Arctic Ocean Basin in Cenozoic time. The precise timing and geometry of the opening to the Atlantic and the tectonic history of the Lomonosov Ridge are questions with important implications for arctic paleoclimate, paleogeography, and paleobiogeography. The structures along which a southerly-trending extension of the Arctic Mid-Ocean Ridge entered the Eurasian continent in the Laptev Sea and was transformed to southwesterly-trending transpression in Northwestern Siberia tie mid-Atlantic extension to circum-Pacific compression. A detailed understanding of their geometry and history would provide a type example for such transitions elsewhere and in the geological record. Postrifting tectonic events were significant in the development of the Arctic Ocean Basin. For example, geophysical data and a single dredge sample of alkaline basalt from the Alpha Ridge suggest that the ridge may be the trace of a Late Cretaceous hot spot that now underlies Iceland (Forsyth et al., 1986). It is important to test this hypothesis because knowledge of the age and migration path of such a hot spot would help to define the tectonic motion of the crust with respect to the mantle in the Arctic during Cretaceous and Early Tertiary time. A broad zone of latest Cretaceous or earliest Tertiary extensional faulting on the Chukchi Shelf extends into the Chukchi Borderland (Grantz et al., 1990b), where extensional features of similar trend occur. These extensional features perhaps join the generally coeval rift structures of Baffin Bay via an as yet-unrecognized route across the Amerasia Basin. Surprisingly, Pacific-North American convergence also appears to reach the Arctic Ocean Basin. In the eastern Beaufort Sea, compressional structures of Eocene to Holocene age extend from the northeastern Brooks Range to the continental rise as far as 170 km north of the coastline (Grantz et al., 1990a, b). These northward-vergent geologic structures may connect, via a transcurrent shear zone and a band of earthquakes, with the north end of the modern Aleutian Benioff Zone in southern Alaska. The apparent propagation of Cenozoic convergent forces across Alaska from the Pacific Rim to the Arctic Ocean Basin raises questions about the character and geometry of the structures (e.g., transcurrent and detachment faults) that could transmit the required stresses through the continental crust for distances exceeding 1,000 km. MINERAL RESOURCES Although the deep Arctic Ocean Basin offers slim prospects for economic deposits of minerals or hydrocarbons, major deposits of oil, gas, and coal have been found beneath its extensive continental shelves (Hale, 1990; Haimila et al., 1990). They include the cluster of giant and supergiant oil and gas fields on the North Slope and Beaufort Shelf near Prudhoe Bay, Alaska; probable supergiant gas fields in the southeastern Barents and southern Kara Seas; and large deposits of bituminous coal beneath the eastern Chukchi Shelf. A comprehensive understanding of the plate tectonic history and kinematics of the Arctic would provide important background to the search for additional deposits on arctic continental shelves and coastal plains. For example, the oil and gas deposits near Prudhoe Bay owe their accumulation and trapping to complex structural and sedimentational processes along the mid-Cretaceous rift that created the Amerasia Basin, and many of the potentially significant oil and gas discoveries of the Canadian Beaufort Shelf occur in progradational sedimentary prisms deposited in the rifts that created the Amerasia Basin. The broad continental shelves of the Arctic also contain lithotectonic terranes that have been displaced large distances from their places of origin. Tectonic reconstructions of the arctic region may suggest places where continuations of these terranes, and of the large petroleum deposits, coal fields, and mineral districts that some of them contain, may be sought in other parts of the Arctic. Seismic reflection data suggest that large amounts of solid gas hydrate (clathrate) underlie the continental margin of the Beaufort Sea. Most of the hydrate gas is probably methane. The
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Opportunities and Priorities in Arctic Geoscience seismic data also indicate that free gas is trapped beneath the hydrate deposit over large areas. The hydrate deposits occur within the upper 300 to 700 m or more of soft sediment on the continental slope and rise. By extrapolating to the entire Arctic, one can estimate speculatively that more than 1015 m3 (35,000 Tcf) of methane at standard temperature and pressure may be tied up in hydrate deposits beneath the margins of the Arctic Ocean Basin (Kvenvolden and Grantz, 1990). Smaller deposits of hydrates are associated locally with permafrost beneath the inner arctic shelves. There appear to be no present or foreseeable technologies by which the hydrate-bound methane deposits of the Arctic Ocean Basin can be commercially developed, but the possible size of the resource, more than 1,300 times the volume of natural gas in the Prudhoe Bay field, makes acquisition of definitive information on the gas hydrate deposits of the Arctic Ocean Basin an important goal of arctic geoscience. Development of even a small part of the speculatively estimated resource would be an energy event of global significance. ENVIRONMENTAL HISTORY AND GLOBAL CHANGE The north magnetic dipole and, according to paleomagnetic theory, the north geographic pole have remained within or near the Arctic Ocean Basin since its inception in Early Jurassic time (Gordon et al., 1984). As a result, the basin contains a sedimentary record of up to 200 million years (Jurassic to Holocene) of north polar climate and oceanography. Because climatic conditions at the poles are perhaps the most sensitive indicators of world climate, this record can make a fundamental contribution to the understanding of global climate and oceanography for a major interval of geologic time. This interval encompasses dramatic changes in global climate, including the transition from widespread temperate climates in the Late Cretaceous to colder, more strongly zoned climates in the Paleogene, and the onset of the ice ages in the Late Neogene. Especially if correlated with events in Antarctica (which are also poorly known), this history would provide a data base critical to evaluating worrisome climatic and oceanographic changes, possibly in part anthropogenic, now affecting our planet. It would also significantly improve the usefulness of the geologic record for testing global models of climatic change. Tectonic reconstructions of the Arctic Ocean Basin show that it was landlocked, or nearly so, during much of its existence (Lawver et al., 1990a). The environmental conditions that this paleogeography imposed on the Arctic Ocean Basin are only beginning to be understood (Thiede et al., 1990). An isolated or semi-isolated arctic mediterranean sea with a temperate or polar climate surrounded by major continents probably received a greater inflow of nutrient-and sediment-rich river water, relative to its volume, than other oceans and probably had lower evaporation rates. Analogy with the Black Sea, which also receives major rivers and has only a tenuous connection with the world ocean, suggests that an isolated Arctic Ocean would have lower surface-layer salinity than normal seawater. The isolation may have allowed other deviations from world norms in salinity, temperature, nutrient levels, and stratification in the Arctic Ocean to reach stages of development not attainable in the world ocean, where extreme conditions tend to be precluded by mixing. Isolation may thus have maximized the amplitude of some kinds of environmental signals in the arctic sedimentologic record. On the other hand, during periods of improved connection between the deep waters of the arctic and world oceans, extreme conditions were probably modulated. Comparisons between paleoenvironmental conditions recorded in Arctic Ocean Basin sediments with coeval conditions in other oceans should provide valuable insights into past oceanic circulation. The utility of arctic stratigraphy for understanding modern changes and predicting future changes in world climate and sea level is greatest in the Neogene and Quaternary marine record. This record, especially if correlated with that in arctic ice caps and nonmarine sediments, can provide a detailed history of Late Cenozoic climate, oceanography, glaciation, and sea ice in the Arctic. This history—and improved knowledge of the processes involved, such as the amplitude of the environmental changes and the rates of onset and collapse of continental glaciation and other extreme conditions—would provide insight into the dynamics of past and present world climate. They would improve the understanding of Quaternary climate gained from the Climap,
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Opportunities and Priorities in Arctic Geoscience Comap, and other programs, but much more information is needed, and is available, in the arctic stratigraphic record to elucidate the processes involved. Pilot studies have demonstrated the potential of arctic paleoecology and paleontology for documenting the Cenozoic migration history of some circum-arctic terrestrial organisms between Eurasia and North America and of some marine taxa between the Atlantic and Pacific Basins via the Arctic (Marincovich et al., 1990). More details on these migrations would improve understanding of marine benthic faunal relationships and correlations between the eastern and western hemispheres. Arctic terrestrial taxa were adapted to long periods of winter darkness and summer daylight, and as the climate changed, the successful terrestrial and marine organisms adapted to cold. Study of the adaptations that permitted these organisms to flourish under polar conditions and comparison of their taxonomic and paleobiogeographic relations to kindred forms at lower latitudes should contribute significantly to interpretation of the fossil records elsewhere, and possibly of the mechanisms and rates of evolution and of the adaptations of organisms to extreme environments. As noted earlier, a large volume of the greenhouse gas methane is apparently held in gas hydrate deposits beneath the Arctic Ocean. Although not unique to the Arctic, the volume of methane estimated to reside in arctic hydrates makes its stability a significant concern. Either warming of arctic bottom waters during interglacial ages (such as the present time) or lowering of sea level during ice ages, acting alone, could cause partial decomposition of methane hydrates and foster the release of methane to the environment. Because the release of large volumes of methane to the atmosphere could have severe and sudden climatic consequences and because decomposition of methane hydrates may provide positive feedback to processes of climate change, the distribution, volume, composition, and stability of the large gas hydrate deposits of the Arctic Ocean Basin are of global concern. Modeling the effect of change in sea level and bottom water temperature on the stability of hydrate deposits in the Arctic would enable assessment of the role of gas hydrate in global climate change. More speculative opportunities for polar geoscience research with global significance involve the high magnetic flux and steep magnetic lines of force that occur near earth's magnetic poles. Because of this configuration of the lines of force, the modest shielding from charged particles that is provided by earth's magnetic field is weakest in the polar regions. As a result, these regions are characterized by a relatively high flux of cosmic radiation and extraterrestrial particulate matter that possibly has imprinted the fossil and sedimentary record in the Arctic Ocean Basin. It is important to know whether Arctic Ocean Basin sediments contain elements, such as beryllium, osmium, and iridium, in amounts significantly exceeding their crustal abundance and how such anomalous amounts, if found, vary with time. Such variations might provide a geologic-scale record of solar-terrestrial interaction and perhaps augment other observations of variations in the strength of earth's magnetic field through time. ARCTIC GEOLOGIC PROCESSES Many geologic processes and biological adaptations are best developed and therefore can be best studied in the Arctic. In this report, we focus on those geologic processes and environments that are most apt to have left a record in the solid earth that is useful for interpreting ancient environments. Examples include the character and volume of clastic and organic sediment incorporated in sea ice and the mechanisms by which this sediment is entrained and expelled from the sea ice to rain on the seabed, minerals that record ancient physical conditions in marine sediments, and organic compounds in fossil organisms that record ancient oceanographic conditions. This understanding would be most useful when applied to the Late Neogene and Quaternary stratigraphic record in the Arctic, which was affected by periodic episodes of continental glaciation and perennial sea ice. It would also contribute to our ability to interpret glacial and periglacial deposits in more ancient strata throughout the world. Many arctic geologic processes are important because they affect human economic activities in the region, and some of these economic activities could have adverse impact on the arctic environment.
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