2

Scientific Accomplishments: Solid Earth Cycles

Data from scientific ocean drilling have offered important insights through which we have begun to better understand solid Earth cycles, including the interaction and evolution of Earth’s crust, mantle, and core. Changes in Earth’s magnetic field, the processes of continental rifting, the subduction of oceanic lithosphere, and voluminous outpourings of magma onto the crust, for example, are significant manifestations of solid Earth cycles and are recorded in rocks on the ocean floor. The following sections review the scientific accomplishments and new fields of inquiry that scientific ocean drilling results have fostered in our knowledge of the solid Earth. Some of the remaining challenges about the solid Earth to which scientific ocean drilling can contribute are also discussed.

GEOMAGNETISM

Earth’s magnetic dipole field, generated in the liquid outer core, undergoes occasional reversals that are recorded in rocks containing iron-bearing magnetic (ferromagnetic, sensu lato) minerals. Because of the globally contemporaneous nature of Earth’s magnetic field reversals, rocks preserving these magnetic signatures can be precisely dated and can serve as markers for the frequency and duration of magnetic reversals through geologic time. These kinds of magnetic minerals are typically abundant in basaltic rocks (such as ocean crust) and in some fine-grained sedimentary rocks overlying the ocean crust. Researchers involved in scientific ocean drilling have been able to take advantage of the magnetic properties of these rocks for more than four decades, spurring numerous fundamental discoveries, for example, about the age of the ocean crust, the way in which ocean crust is generated and destroyed, the timing of climatic oscillations, and the development of accurate geologic time scales.

Scientific Accomplishments and Significance

The most widely known scientific accomplishment for which scientific ocean drilling was essential is the verification of the seafloor spreading hypothesis (Box 2.1), a lynch-pin for establishing the paradigm of plate tectonics in the early 1970s. Scientific ocean drilling focused on obtaining ages of the seafloor magnetic reversal stratigraphy and corresponding biostratigraphic ages of sediments at successive distances from the Mid-Atlantic Ridge axis; these data confirmed the increasing age of ocean crust away from seafloor spreading centers. The landmark verification of the seafloor spreading hypothesis occurred as a result of the earliest Deep Sea Drilling Project (DSDP) cruises and particularly with data from DSDP Leg 3, which drilled to the top of the oceanic basement in the South Atlantic (e.g., Maxwell et al., 1970). Biostratigraphic, magnetostratigraphic, and isotope geochronologic analyses of seafloor samples from subsequent DSDP cruises (e.g., DSDP Legs 5, 6, 12) combined with similar data from continental sequences led to development of precise geologic time scales, specifically the Geomagnetic Polarity Time Scale (GPTS), which included astronomical, geomagnetic, and biostratigraphic calibrations (e.g., Berggren et al., 1985; Cande and Kent, 1995; Gradstein et al., 2004; Box 2.2). Another significant achieving of the scientific ocean drilling program was modeling the magnetic contributions of submarine source layers (basalt and gabbro), which matched the observed marine magnetic anomalies.

Beyond these first-order contributions, drill cores led to the discovery of variations in spreading rates between ocean basins and the age of the oldest ocean crust (e.g., DSDP Legs 9 and 69 and Ocean Drilling Program [ODP] Leg 138; Hayes et al., 1972; Wilson et al., 2003). A new way to confirm lateral seafloor spreading rates and the veracity of the GPTS came from analysis of vertical magnetic reversal stratigraphy



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 13
2 Scientific Accomplishments: Solid Earth Cycles Scientific Accomplishments and Significance Data from scientific ocean drilling have offered impor- tant insights through which we have begun to better under- The most widely known scientific accomplishment for stand solid Earth cycles, including the interaction and evolu- which scientific ocean drilling was essential is the verifica- tion of Earth’s crust, mantle, and core. Changes in Earth’s tion of the seafloor spreading hypothesis (Box 2.1), a lynch- magnetic field, the processes of continental rifting, the sub- pin for establishing the paradigm of plate tectonics in the duction of oceanic lithosphere, and voluminous outpourings early 1970s. Scientific ocean drilling focused on obtaining of magma onto the crust, for example, are significant mani- ages of the seafloor magnetic reversal stratigraphy and cor- festations of solid Earth cycles and are recorded in rocks on responding biostratigraphic ages of sediments at successive the ocean floor. The following sections review the scientific distances from the Mid-Atlantic Ridge axis; these data con- accomplishments and new fields of inquiry that scientific firmed the increasing age of ocean crust away from seafloor ocean drilling results have fostered in our knowledge of the spreading centers. The landmark verification of the seafloor solid Earth. Some of the remaining challenges about the spreading hypothesis occurred as a result of the earliest solid Earth to which scientific ocean drilling can contribute Deep Sea Drilling Project (DSDP) cruises and particularly are also discussed. with data from DSDP Leg 3, which drilled to the top of the oceanic basement in the South Atlantic (e.g., Maxwell GEOMAGNETISM et al., 1970). Biostratigraphic, magnetostratigraphic, and isotope geochronologic analyses of seafloor samples from Earth’s magnetic dipole field, generated in the liquid subsequent DSDP cruises (e.g., DSDP Legs 5, 6, 12) com- outer core, undergoes occasional reversals that are recorded bined with similar data from continental sequences led to in rocks containing iron-bearing magnetic (ferromagnetic, development of precise geologic time scales, specifically the sensu lato) minerals. Because of the globally contemporane- Geomagnetic Polarity Time Scale (GPTS), which included ous nature of Earth’s magnetic field reversals, rocks preserv- astronomical, geomagnetic, and biostratigraphic calibrations ing these magnetic signatures can be precisely dated and can (e.g., Berggren et al., 1985; Cande and Kent, 1995; Gradstein serve as markers for the frequency and duration of magnetic et al., 2004; Box 2.2). Another significant achieving of the reversals through geologic time. These kinds of magnetic scientific ocean drilling program was modeling the magnetic minerals are typically abundant in basaltic rocks (such as contributions of submarine source layers (basalt and gabbro), ocean crust) and in some fine-grained sedimentary rocks which matched the observed marine magnetic anomalies. overlying the ocean crust. Researchers involved in scientific Beyond these first-order contributions, drill cores led to ocean drilling have been able to take advantage of the mag- the discovery of variations in spreading rates between ocean netic properties of these rocks for more than four decades, basins and the age of the oldest ocean crust (e.g., DSDP Legs spurring numerous fundamental discoveries, for example, 9 and 69 and Ocean Drilling Program [ODP] Leg 138; Hayes about the age of the ocean crust, the way in which ocean crust et al., 1972; Wilson et al., 2003). A new way to confirm is generated and destroyed, the timing of climatic oscilla- lateral seafloor spreading rates and the veracity of the GPTS tions, and the development of accurate geologic time scales. came from analysis of vertical magnetic reversal stratigra- 13

OCR for page 13
14 SCIENTIFIC OCEAN DRILLING basis for such an approach was first proposed by Shackleton and Opdyke (1973) when they dated high-resolution δ18O Box 2.1 Confirmation of Seafloor Spreading stratigraphy with fixed-point magnetic reversal ages to yield multi-millennial-scale paleoclimate (i.e., global ice volume) One of the earliest and most significant ac- records. When Hilgen (1991) extended his earlier cyclostrati- complishments of scientific ocean drilling was to graphic work to the Miocene-Pliocene boundary, it became provide data that confirmed seafloor spreading. possible to obtain three-way cross-validation of dated Data and samples retrieved by the drillship Glomar paleoclimate (biostratigraphy and lithostratigraphy based) Challenger in the early 1970s were used to con- records whose ultimate fixed-point dating scheme rested on firm that new ocean crust was being generated at the GPTS, synthesized from sediments of the world’s oceans. mid-ocean ridges, supporting the complete theory A global approach to developing a precise astronomical time of plate tectonics, the paradigm for solid Earth sci- scale used many DSDP and ODP cores and enabled oxygen ence. Data from survey ships that provided geo- isotope excursions to be precisely dated (e.g., Lisiecki and physical evidence for the existence of symmetric, Raymo, 2005; Box 2.2; also see Chapter 4). alternating patterns of magnetic polarity (“magnetic Another method to correlate oxygen isotope changes anomalies”) on either side of the world’s mid-ocean ridges led to suggestions by various scientists in the with age and geomagnetic reversals or excursions uses the early 1960s that new ocean crust was being created relative paleointensity of sediment cores (e.g., Channell et there. However, these interpretations could not be al., 2009; Figure 2.1). This method has provided independent conclusively substantiated in the absence of data age calibrations to which climate changes can be compared directly obtained from the seafloor. In 1970, Glo- and is also valuable for understanding the behavior of Earth’s mar Challenger drilled a series of boreholes across magnetic field through time. Measurements of continuous the Mid-Atlantic Ridge in the South Atlantic and relative geomagnetic paleointensity from long sediment retrieved basal sediments that had been deposited cores have confirmed occurrence of intensity minima dur- on ocean crust. The biostratigraphic ages of those ing dipole reversals and helped discover short-term dipole samples increased nearly linearly with distance excursions. from the ridge crest, in close agreement with the Another accomplishment in the area of global plate ages predicted by analysis of the magnetic anoma- lies on the seafloor. Confirmation of the theory of motions and reconstructions came when cores collected on seafloor spreading had direct consequences for the DSDP Leg 55 led to the initial recognition that the Hawaiian development of new fields of scientific inquiry and hotspot had not always been fixed at the latitude of Hawaii cross-disciplinary geoscientific research. (Kono, 1980). In 1992, ODP Leg 145 cored a thickness of ocean floor basalt opposite Detroit Seamount, one of the SOURCES: Dietz, 1961; Hess, 1962; Vine and Matthews, oldest seamounts in the Hawaiian-Emperor chain, and deter- 1963; Heirtzler et al., 1968; Maxwell et al., 1970. mined that its paleomagnetic latitude of formation was well north of Hawaii (Tarduno et al., 2003). ODP Leg 197 pro- vided compelling paleomagnetic evidence that from about 76 to 45 myr the Hawaiian hotspot was rapidly migrating phy recovered from ocean sediments at ODP Sites 677 and southward to reach its present position, a finding that held 846, where benthic and planktonic foraminiferal dates were implications for reconstructing past Pacific plate motions and correlated with Earth’s orbital variations (e.g., Shackleton also for the concepts of plume stability and mantle dynamics et al., 1990; also see Chapter 4). These results, in turn, have and circulation. had implications for understanding of the composition and behavior of oceanic lithosphere, processes occurring at sub- Fields of Inquiry Enabled duction zones, and the formation of large igneous provinces (LIPs) (see later sections of this chapter). Research derived from scientific ocean drilling has The development of hydraulic piston coring (HPC) and fundamentally influenced and advanced the fields of plate advanced piston coring (APC; see Box 2.2) allowed sampling tectonics, paleomagnetism and geomagnetism, and geo- of continuous undisturbed sediment cores from below the logic time scales. The combination of high-resolution sediment-water interface to the crystalline bedrock (Prell magnetostratigraphy and relative paleointensity; oxygen et al., 1980; Ruddiman et al., 1986; also see Box 4.3). This isotope records as a proxy for changes in climate; and bio-, unique achievement allowed the creation of a high-fidelity cyclo-, and lithostratigraphy have helped further the study Cenozoic paleoceanographic/paleoclimatic time series by of paleoclimate and paleoceanography (discussed in further cross-validating magnetic dipole reversal stratigraphy from detail in Chapter 4), which attests to the importance of cross- ocean core samples with higher resolution biostratigraphy disciplinary approaches in advancing research of the ocean and astronomically forced climate variations recorded in floor and Earth processes. finely laminated ocean sediments (e.g., Hilgen, 1991). The

OCR for page 13
15 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES Box 2.2 The Development and Evolution of Geological Time Scales Scientific ocean drilling has contributed significantly to the development of an accurate geological time scale for the past 150 million years (myr). The verification of seafloor spreading by dating basal age microfossils (Maxwell et al., 1970) tied together paleomagnetic reversal stratigraphy with seafloor magnetic anomalies and provided the first accu- rate time scales linking the age of ocean crust to the nannofossil and foraminiferal biostratigraphy of European uplifted marine sections (Berggren, 1969; Berggren et al., 1985). Building on this early success, the development of hydraulic and advanced piston corers (HPC and APC) in the late 1970s produced undisturbed marine sections that recorded paleomagnetic reversals, which allowed researchers to use changes in microfossil oxygen isotope chemistry controlled by long-term cycles in Earth’s orbit (Milankovitch climate cycles) to date the paleomagnetic reversal stratigraphy. Pre- viously these sections could only be recovered using traditional piston coring (Shackleton and Opdyke, 1973, 1976) in relatively recent marine sediments with slow sedimentation rates. The APC provided the first long, high-resolution records of oxygen isotope variations and paleomagnetic reversals, leading to the confirmation of the orbital theory of ice ages (Hays et al., 1976) and the observation of the per vasive fingerprint of orbital forcing on the marine record of climate change (further discussed in Chapter 4). Shackleton et al. (1990) extended the orbitally calibrated oxygen isotope chronology into the Pliocene in ODP Site 677 and noted that the dates of the magnetic reversals previously established by the radiometric dating of seafloor basalts were 5 to 7 percent younger than their predicted ages based on astronomical calibration, suggesting that the radiometric dates constraining the reversals were in error. Hilgen (1991) confirmed the Shackleton et al. (1990) observations through orbital tuning of uplifted Mediterranean marine sections and, in conjunction with the high-resolution oxygen isotope record from ODP Site 846 (Shackleton, 1995), further extended the time scale into the Miocene. The combined results of Shackleton et al. (1990, 1995) and Hilgen (1991) produced the first Astronomical Polarity Time Scale (Kent, 1999). Subsequent research showed that the K-Ar ages of the original time scales were indeed too young (Tauxe et al., 1992), and new 40Ar/39Ar radiometric results showed the same ages as the orbitally tuned dates. Deeper and older recoveries with the APC and extended core barrel systems extended the orbital chronology continuously back to 5 myr: Lisiecki and Raymo (2005) combined 36 DSDP and ODP sites to produce an astronomical time scale for the marine oxygen isotope record of the past 5 myr. This work was subsequently extended back into the Cenozoic (Billups et al., 2004; Pälike et al., 2006; Westerhold et al., 2008) with an accuracy of several tens of thousands of years. Goals Not Yet Accomplished mid-ocean ridge environments, with ~85 percent of this volume as plutonic rocks (predominantly gabbros) and the As one of the earliest scientific ocean drilling fields, the rest erupted on the ocean floor in the form of basaltic lava field of geomagnetism has seen many of its goals accom- (Crisp, 1984). The oceanic lithosphere forms the outermost plished. However, fundamental questions still remain in the layer of the solid Earth and consists of this mafic crust (gab- mechanisms behind magnetic field reversals and extend- bros and basalts) and rigid ultramafic upper mantle (typically ing temporal resolution of magnetic polarity stratigraphy. of peridotite composition); the crustal layer is about 5-10 km Continued work on paleomagnetism of deeper ocean crust thick, and the upper mantle section can extend for another (gabbros and ultramafic rocks) may address the sources of ~90 km below the crust. marine magnetic anomalies, providing observational bench- Preliminary observations from results of scientific ocean marks for geodynamo models of motions in Earth’s liquid drilling suggest that oceanic lithosphere compositions dif- iron core. Unanswered questions related to high-resolution fer depending upon the spreading rate of the nearby ridge past climate change may also draw upon the combination of system (Box 2.3). The only way to obtain direct measure- paleomagnetic, biostratigraphic, isotopic, and astronomical ments of the properties of oceanic lithosphere to understand work on ocean cores for resolution. its composition, genesis, and structure, and the causes and consequences of different ridge spreading rates, is through STRUCTURE, COMPOSITION, AND the type of sampling enabled by scientific ocean drilling. FORMATION OF OCEANIC LITHOSPHERE Scientific Accomplishments and Significance Earth’s lithosphere consists of the crust and the non- convecting portion of the upper mantle, formed by repeated One motivation for early drilling into ocean crust was to magmatic activity at mid-ocean ridges. This magmatism assess whether oceanic lithosphere has a layered lithological is the dominant process helping to transfer mass and heat structure from top to bottom of pillow basalts, sheeted dikes, from Earth’s interior. Approximately 62-75 percent of the gabbros, and ultramafic rocks (Box 2.3) in a manner similar global magma production rate of ~30 km3/yr comes from

OCR for page 13
16 SCIENTIFIC OCEAN DRILLING it was also the first time a full, intact section was drilled, providing insight into oceanic crust formation (Teagle et al., 2006; Wilson et al., 2006). These technological impediments to deeper drilling encouraged innovations: (1) improvements in drilling capa- bilities enabled by both ODP and IODP and (2) development of the concept of “tectonic windows,” i.e., regions where strata that are usually deeply buried have been brought closer to or exposed on the seafloor through tectonic processes such as low-angle detachment faults (e.g., Ildefonse et al., 2007; Escartin et al., 2008). The innovation to sample these tectonic windows through scientific ocean drilling has made it pos- sible to reach gabbroic and, in some cases, mantle sections of oceanic lithosphere (e.g., Cannat et al., 1997, 2006; Kelemen et al., 2004, 2007). ODP Hole 735B on Atlantis Bank in the Indian Ocean, for example, penetrated 1,508 m into gabbro (Dick et al., 1999); two Mid-Atlantic Ridge drill holes, 920D at 23° N and 1275 at 16° N, each penetrated ~200 m into mantle peridotite (Cannat et al., 1995; Kelemen et al., 2004); and ODP Hole 895D penetrated 94 m into peridotite exposed through rifted Pacific crust (Gillis et al., 1993). The presence of ultramafic rocks at or near the surface of the ocean floor at a number of slow and ultraslow spreading centers (e.g., Kelemen et al., 2004; Blackman et al., 2006) has resurrected the idea that the Hess model for oceanic lithosphere (i.e., serpentinized peridotite is overlain in igneous or tectonic FIGURE 2.1 Oxygen isotope (δ18O) and relative paleointensity contact by a thin layer of volcanic material; Hess, 1962) (RPI). Half-width of the error envelope in both cases is 2σ (2 × could apply to large sections of the oceanic lithosphere (Dick standard error). The oxygen isotope stack (red) is compared with et al., 2003; Michael et al., 2003). Although most of the the LR04 benthic isotope stack (blue) (Lisiecki and Raymo, 2005). erupted lavas are focused on or are close to the ridge axial Paleointensity minima in the stack correspond to established ages summit, mapping efforts combined with isotopic analyses of of magnetic excursions and chron/subschron boundaries: LA- drilled samples and seismic surveys have shown that lateral Laschamp, BL-Blake, IB-Iceland Basin, PR-Pringle Falls, BLT- magma transport may also occur through dikes in the shallow Big Lost, LP-La Palma, ST17-Stage 17, B/M-Brunhes-Matuyama boundary, KA-Kamikatsutra, SR-Santa Rosa, JA-Jaramillo Sub- crust that erupt off-axis to form seamounts (Zou et al., 2002; chron, PU-Punaruu, CB-Cobb Mountain Subchron, BJ-Bjorn, Durant and Toomey, 2009). GA-Gardar. SOURCE: Channell et al., 2009. Drilling innovations, such as the use of a re-entry cone guide to begin drilling hard rock formations and a drilling fluid-powered hammer to begin holes, have allowed re-entry to hard rock formations on the seafloor. Other innovations to the structure of ophiolites on land (called the “Penrose include testing of a mining-type continuous coring operation model” after a 1972 Penrose Field Conference consensus from a compensated platform, and mud motor-driven core definition of ophiolite structure; Anonymous, 1972). Penetra- barrels (Miller and Huey, 1992; Brewer et al., 2005). These tion into ocean crust was inaugurated by DSDP Leg 37 in improvements led to increases in the depths of holes drilled 1974 at 37° N along the Mid-Atlantic Ridge. Although the during IODP legs and in the amount of recovered core. For total number of holes drilled into ocean crust between 1974 example, IODP Hole 1309D on the Atlantis Massif along the and the end of ODP in 2004 is about 50, these early efforts Mid-Atlantic Ridge penetrated to a depth of 1,415 m, and were hampered by technical difficulties in both penetrating Hole 1256D in the equatorial east Pacific penetrated to 1,507 the very dense crystalline rock and recovering samples. Only m (e.g., Blackman et al., 2006; Teagle et al., 2006; Wilson et 17 of those 50 holes penetrated depths greater than 200 m al., 2006; Drouin et al., 2009). into the crust, and only 7 holes reached depths greater than Drilling on ODP Legs 147 and 209 confirmed the pres- 500 m within crystalline rocks (Dick et al., 2006; Figure 1.3). ence of abundant impregnated periodotites formed by partial One of these holes, 1256D, is notable for penetrating 1,507 crystallization of migrating mantle melt beneath the base of m into superfast-spreading lithosphere produced at the East the crust and ODP Leg 209 and IODP Expeditions 304-305 Pacific Rise. Not only did drilling reveal a classic, Penrose- substantiated that the lower crust was composed of many type crust of pillow basalt, sheeted dikes, and gabbros, but small sills rather than one large magma chamber (Mével

OCR for page 13
17 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES Box 2.3 Structure of Oceanic Lithosphere The structure and thickness of oceanic lithosphere depend a great deal on spreading rates. An idealized (“Penrose model”) struc- ture of oceanic lithosphere includes pelagic sediments (layer 1), pillow basalts (layer 2A), sheeted dikes (layer 2B), isotropic gabbros (layer 3A), layered gabbros (layer 3B), and ultramafics (layer 4). However, the complete sequence is ubiquitous only in lithosphere produced at fast and intermediate spreading ridges, such as the Cocos-Nazca Ridge (Figure A above). Lithosphere produced at slow and ultraslow spreading centers may have ultramafic rocks exposed on the seafloor or ultramafic rocks in tectonic contact with a carapace of mafic materials, most likely pillow basalts without sheeted dikes or gabbros. This type of oceanic lithosphere has been observed at the slow Southwest Indian and Mid-Atlantic Ridges, respectively (Figures B and C), and in Arctic Ocean lithosphere produced at the ultraslow Gakkel Ridge (Figure D). SOURCES: Perfit et al., 1994; figure from Dick et al., 2006. et al., 1996; Kelemen et al., 2004; Blackman et al., 2006). contributed to growth in the study of different types of ocean These results have important implications for understanding basalts and have allowed for better connections between the the composition of the lower ocean crust. study of intact ocean crust and opholites exposed on land. Fields of Inquiry Enabled Goals Not Yet Accomplished Rocks recovered by DSDP, ODP, and IODP have Although considerably more is now known about fueled studies and advancements in geochronology, experi- oceanic lithosphere than before scientific ocean drilling mental petrology, geochemistry, geodynamics, seismology, began, major questions remain because so few holes have submersible-aided outcrop mapping and sampling, and struc- penetrated more than 500 m into the lithosphere. Answering tural geology, and have supplied a more complete picture of key questions requires more drilling of deeper sections of the formation and evolution of oceanic lithosphere in time the lower crust and shallow mantle in a variety of tectonic and space. Petrologic samples collected by drilling have also settings, which will be dependent on technological advances

OCR for page 13
18 SCIENTIFIC OCEAN DRILLING to achieve high recovery of cores in deeper drill holes. Basic and the geometry of initial continental fragmentation (Figure questions that have yet to be answered involve the structure 2.3). The Vøring and Rockall Margin drilling expeditions of the oceanic lithosphere, including further testing of the penetrated seaward dipping reflectors (SDRs) thought to Penrose and Hess models and the nature of the Mohorovičić mark the location of the continent-ocean boundary, revealing discontinuity (Moho), a refracted arrival in seismic profiles that the SDRs were associated with basaltic volcanic rocks that is thought to originate at the crust-mantle boundary. erupted subaerially (on land) instead of under water. Drilling The nature of the boundary—whether an alteration front or on the Galicia Bank demonstrated that the early sediment a phase change—remains controversial because it has not record was accessible and provided some of the first evidence been sampled in situ. There has yet to be a full penetration for subsidence history (Sibuet and Ryan, 1979). of the lower ocean crust to determine its vertical stratigraphy Nearly coincident with the initial DSDP rifted-margin and composition with depth, the processes by which it forms, legs, dredging on the flank of the Galicia Bank recovered and how these shape the composition of mid-ocean ridge mantle peridotites and set the stage for ODP Leg 103, which basalt, the most abundant magma type on Earth. Understand- was designed to sample the early rift history and determine ing the composition and evolution of oceanic lithosphere is the nature of the underlying crust (Boillot et al., 1980). ODP also intimately linked with understanding the exchange of Leg 103 yielded dates for sediments deposited during and elements between the lithosphere and seawater during hydro- after rifting and demonstrated that one of the prominent thermal alteration, with implications for understanding the seismic reflectors was likely a low-angle extensional fault diversity of microbial life forms and their role in the global near the base of the crust and that peridotite exposed on the carbon cycle (discussed further in Chapter 3). Galicia Bank ascended from a depth of 30 km (Boillot et al., 1988). Both observations suggested the need to re-evaluate prevailing models for continental rifting. CONTINENTAL BREAKUP AND Subsequent recommendations for additional drilling SEDIMENTARY BASIN FORMATION related to continental rifting suggested that ODP focus on Early models for the evolution of rifted margins pro- the Newfoundland-Iberia and southeast Greenland-Vøring vided a simple, quantitative framework for predicting the margins as representatives of end-member margins (“mag- subsidence of sedimentary basins (e.g., McKenzie, 1978; ma-rich” and “magma-poor”). Eight drilling legs were rec- Wernicke, 1985). These models generally assumed single- ommended, and between 1994 and 2004 five of those drilling phase rifting followed by instantaneous continental breakup, expeditions were conducted. leading to the direct juxtaposition of continental and ocean ODP Legs 149, 173, and 210 explored the geometries of crust (e.g., Figure 2.2a). Challenges to the models arose magma-poor continental margins offshore Iberia and New- almost as soon as they were published (e.g., Royden and foundland. ODP Leg 149 drilled a transect across the Iberia Keen, 1980), but the Galicia Margin discovery of mantle Abyssal Plain and sampled upper mantle rocks that separated rocks by dredging in the late 1970s (Boillot et al., 1980) extended continental crust from ocean crust (Whitmarsh and and drilling on ODP Leg 103 in 1985 (Boillot et al., 1987) Sawyer, 1996). ODP Leg 173 demonstrated that continental prompted a 30-year effort to develop more realistic alterna- crust was thinned to less than 5 km and that upper mantle tives. The general outcome of this effort is a fundamentally peridotites were brought to within a few hundred meters of new paradigm regarding the nature of continental breakup, the seafloor (Whitmarsh and Wallace, 2001; Whitmarsh et in which polyphase deformation leads to a complex transi- al., 2001). ODP Leg 210 added information from the conju- tion from continent to ocean crust (Figure 2.2b). The new gate margin off Newfoundland (Tucholke et al., 2007). model has significant implications for basic understanding As a complement to the drilling of the magma-poor mar- of continental breakup processes, plate reconstructions, and gins, ODP Legs 104, 152, and 163 explored the geometries of prediction of hydrocarbon distributions. The paradigm shift the magma-rich continental margins off northeast Greenland was brought about by a combination of approaches, in which and the northeast Atlantic (part of the North Atlantic Volcanic scientific ocean drilling played a significant part (DSDP Legs Province). These drilling legs established that SDRs were 38, 47, and 81; ODP Legs 103, 104, 149, 152, 163, 173, emplaced during initial development of the ocean basins, but and 210; Figure 2.3). Equally important have been marine well after the onset of rifting. ODP Leg 104 drilled 900 m of geophysical investigations and onshore studies, which have subaerial basaltic flows at the Vøring Margin and character- underscored the importance of several phases of lithosphere ized the initial breakup of the margin (Eldholm et al., 1989). deformation (e.g., Karner et al., 2007). ODP Leg 152 located the seaward extent of rifted continental crust along the Southeast Greenland Margin (Larsen and Saunders, 1998), but a later leg (ODP Leg 163) intended to Scientific Accomplishments and Significance shed light on the tectonic development of southeast Green- The first rifted margin expeditions, DSDP Legs 38 land failed to achieve its objectives because of ship damage (Vøring Margin), 47 (Galicia Bank), and 81 (Rockall Mar- sustained during extreme storm conditions. gin) examined the timing of initiation of seafloor spreading Other ODP expeditions that added to observations

OCR for page 13
19 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES FIGURE 2.2 Block diagram of magma-poor rifted margins. (a) The top diagram illustrates the classical representation of pre-, syn-, and postrift sediments over uniformly stretched continental crust, with high-angle normal faults in the upper crust and ductile deformation in the lower crust. Sedimentary basins formed during the deformation are all of the same age. (b) The bottom diagram illustrates a modern repre- sentation of the complex architecture at the ocean-continent transition. This architecture is acquired during multiple phases of deformation that begin with high-angle faulting, followed by extreme crustal thinning, mantle exhumation, and finally, seafloor spreading. Sedimentary basins formed during polyphase deformation are distinctly different in age. SOURCE: Modified from Péron-Pinvidic and Manatschal, 2009. of continental breakup included single expeditions in the 1990). ODP Leg 180 sought to drill an active, low-angle nor- Tyrrhenian Sea (ODP Leg 107), Broken Ridge (ODP Leg mal fault thought to play a major role in the thinning of con- 120), and Woodlark Basin (ODP Leg 180). Three of the seven tinental crust, the formation of the Moresby Seamount, and sites drilled in ODP Leg 107 reached basalt-floored basins; the formation of the Woodlark Basin (Taylor and Huchon, varying age dates of those basalts supported the hypothesis 2002). ODP Leg 120 was designed to study the origin and that they were emplaced during extension (Kastens et al., tectonic history of the Kerguelen Plateau, a large igneous 1987, 1988). One of the sites (ODP Site 651A) drilled man- province (Schlich et al., 1989). Preliminary drilling results tle-derived, serpentinized peridotite (Kastens and Mascle, reported no evidence for a continental origin of the plateau

OCR for page 13
20 SCIENTIFIC OCEAN DRILLING 90°N DSDP ODP Leg 38 Leg 103 80° 80° Leg 47 Leg 104 Leg 81 Leg 107 Leg 149 Leg 152 Leg 163 70° 70° Leg 173 Leg 210 60° 60° 50° 50° 40° 40° 30° 30° 20° 20° 60° 50° 40° 30° 20° 10°W 0° 10°E 20° 30° FIGURE 2.3 Location map of DSDP and ODP legs related to rifting margins. This is a Mollweide (equal area) projection with a color range of -9,000 to 9,000 m, with white marking the 0 m depth. SOURCE: IODP-USIO. (Schlich and Wise, 1992); however, subsequent geochemical to provide regional mapping context for the drilled horizons and geophysical studies have suggested that the Kerguelen (Péron-Pinvidic et al., 2007). Those new interpretations were Plateau is indeed composed of remnants of Indian-Antarctic subsequently coupled with geodynamic models, shallow continental crust that interacted with the Kerguelen Plume drilling by European research groups, and a synthesis of ana- (e.g., Bénard et al., 2010). The evolving interpretation of logs from the Alps and relevant mid-ocean ridges to evolve this feature is typical of the integrated, multidisciplinary the new paradigms for continental breakup (Lavier and studies that have allowed new paradigms for continental Manatschal, 2006; Péron-Pinvidic and Manatschal, 2009; breakup to progress. In summary, all of these expeditions Unternehr et al., 2010). The integration of these multidis- added elements to the evolving understanding of continental ciplinary studies over several decades provides an excellent breakup processes, but none had sufficient coverage to fully example of a case where scientific ocean drilling was part illuminate the story at any single margin. of an iterative process that also required both onshore and offshore geophysical and geologic calibration and validation, and geodynamic modeling. Fields of Inquiry Enabled The formation of magma-rich margins and associated The discovery of exhumed mantle during drilling LIPs has also recently been linked to biotic events and dipole expeditions along the Iberian Margin prompted extensive reversal frequency, especially in the case of the North Atlan- reinterpretation of existing seismic data and acquisition of tic and associated SDRs (Eldholm and Thomas, 1993) and several additional multichannel seismic profiles, designed the Paleocene-Eocene Thermal Maximum event (ODP Hole

OCR for page 13
21 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES 690B; Kennett and Stott, 1991; Bains et al., 1999; Svensen through forearc mud volcanoes. In 2008, a new IODP pro- et al., 2004). Though controversial, this example serves as a gram (SEIsmogenic Zone Experiment [SEIZE]) was initiated model for the initiation of other LIPs (see last section in this to drill through the plate boundary at a seismogenic depth, chapter for additional detail). thus contributing to the understanding of mechanisms that control whether a plate boundary fault slides slowly or becomes locked, episodically generating great earthquakes Goals Not Yet Accomplished (magnitude 8.0 or larger). The study of rifted margins is critical to understanding continental breakup processes, plate kinematic reconstruc- Scientific Accomplishments and Significance tions, and prediction of hydrocarbon resource distribution. In addition, the formation of volcanic margins may be inti- Determining the nature of the crust, sediments, and mately related to climatic events, biotic events, and dipole pore water entering a subduction zone has been the primary reversal frequency. When the IODP Initial Science Plan objective at several DSDP, ODP, and IODP sites, because the (2001) was written, key elements of the new paradigm for distinctive composition of material that is subducted acts as continental rifting were in place, but the number of margins a tracer of the processes at depth that lead to arc volcanism along which key sedimentary sequences were accessible (Plank and Langmuir, 1993, 1998). Studies have evolved by riserless drilling was limited. The Continental Breakup from identification of elements that can be used as tracers for and Sedimentary Basin Formation initiative was aimed at modeling and constructing mass-balance budgets of inputs answering remaining fundamental science questions via riser and outputs of material across a subduction zone. drilling using Chikyu. However, riser drilling of rifted mar- Sampling from a number of different subduction zones gins was not accomplished in IODP for a variety of reasons. revealed a correlation between the composition of sediments One of these was the two- to three-year delay in the delivery being subducted and the composition of the extruded basalts. of Chikyu, which decreased the amount of time available for Analysis of lithium isotopes from ODP Site 1039 suggests expeditions. IODP scientists cite this technical area as one that half of the lithium in the down-going plate offshore where the objectives of the IODP Initial Science Plan were Costa Rica is transferred to the arc, one-quarter returns to not achieved as planned (Keir Becker, PowerPoint presenta- the ocean through fluid expulsion along the decollement, tion, June 2010). and one-quarter may be recycled into the mantle (Chan and Future improvements to the understanding of continen- Kastner, 2000). However, in other subduction zones (e.g., tal rifted margins will depend not only on scientific ocean Kamchatka), a lack of correlation between sediment input drilling, but also on high-quality seismic reflection profiles to the subduction zone and the arc output in some regions and companion long-offset seismic surveys, which creates implies that at least in some subduction zones none of the an opportunity for collaboration between academia and subducted sediment and crust reaches the deep interior to be industry. U.S. scientists have recently finalized plans for a reworked and incorporated in the zone of melt generation new initiative on rifted margins as part of the NSF-funded (Kersting and Arculus, 1995). GeoPRISMS (Geodynamic Processes at Rifting and Sub- In some arcs, most notably in the Marianas, material ducting Margins) Program (GeoPRISMS, 2011). extruded into the forearc through mud volcanoes provides a direct window into hidden processes that operate deep within the subduction zone. ODP Legs 125 and 195 pro- SUBDUCTION ZONE PROCESSES AND THE vided samples of unusual minerals and freshened pore fluids SEISMOGENIC ZONE derived from interaction of subducted sediments and crust Subduction zones, where one tectonic plate plunges with the overlying forearc mantle (Maekawa et al., 1993; beneath another, are the source of some of Earth’s greatest Fryer et al., 1995, 1999). During ODP Leg 195, a long-term natural hazards. These areas are also where new continental borehole observatory was installed in a serpentine seamount crust is formed. Scientific ocean drilling plays a key role in in order to examine mass transport, geochemical cycling, a range of multidisciplinary studies of volcanic processes, and physical, chemical, and microbial fluid characteristics. particularly with regard to formation of new continental Drilling also played a critical role in the realization that crust in volcanic arcs. The knowledge derived from these crustal erosion, as well as accretion, occurs in forearcs. Sub- studies is essential for developing practical predictive models duction erosion results in forearc subsidence and formation and warning systems to protect the inhabitants of volcanic of large basins and deep terraces, subduction of crystalline regions from eruptions and associated hazards, such as forearc crust to depths at which arc magmas form, and trench mudflows and tsunamis. Past targets have included drill retreat and arc migration. Although previous studies noted sites devoted to determining the composition of subducted that a considerable amount of forearc material was miss- material, the manner in which material is accreted to the ing from the Japan and Peru-Chile trenches and attributed overriding plate and incorporated into volcanic arcs, and the it to the subduction of forearc crust (e.g., Miller, 1970a,b; chemistry of material that is subducted and then extruded Murauchi, 1971; Rutland, 1971), it was not until DSDP

OCR for page 13
22 SCIENTIFIC OCEAN DRILLING Legs 56 and 57, when drilling results were combined with the ash sequences provide an intriguing look at evolution a network of site survey seismic reflection and refraction of arc volcanism over time. Other ash studies from cores on profiles, that a detailed model for tectonic erosion and its the Caribbean Plate (ODP Leg 165) document the episodic- impact on forearc subsidence was developed (von Huene et ity of explosive volcanism in Central America and indicate al., 1980). Evidence of erosion of the upper plate has also that some eruptions in this region rivaled the largest super- been documented by scientific ocean drilling off Guatemala eruptions known in the geologic record. (DSDP Legs 67 and 84), Peru (ODP Leg 112), Tonga (ODP Recent accomplishments in understanding subduction Leg 135), and Costa Rica (ODP Leg 170 and IODP Expe- zones have been under the aegis of the SEIZE program, dition 334). It is likely that most arcs experience episodes which is designed to understand the processes that result in of subduction erosion as well as accretion (von Huene and great earthquakes at subduction zones. IODP efforts to date Scholl, 1991), with implications for megathrust earthquake have been focused on the Nankai Trough, offshore south- hazard and continental evolution (Stern, 2011). east Japan, which has a long history of earthquakes with magnitude 8.0 or larger.1 Reaching NanTroSEIZE’s ultimate Drilling has also provided some unique information on arc histories through recovery of well-preserved ash objective of sampling the plate boundary at seismogenic sequences (Figure 2.4). For example, cores from several depth (Box 2.4) requires the riser drilling capability of the DSDP and ODP legs (e.g., ODP Legs 125 and 126) yield Chikyu and has been the major component of the Japanese a nearly complete 45 myr record of arc volcanism in the contribution to IODP in recent years. One important result Mariana-Izu arc (e.g., Arculus et al., 1995). These studies thus far is the identification of a branching fault that reaches demonstrated that the Mariana and Izu arc systems shared the seafloor at a steeper angle than the main thrust fault of similar chemical behavior from 30 to 15 myr ago, diverging the subduction zone system and may be responsible for the about 12 myr ago (Plank, 2002). Although the reason for the large, historic tsunamis that have affected this region (Moore initial similarity and later change are not yet well known, et al., 2007; Bangs et al., 2009). Others include confirmation of recent activity on the fault from Pliocene sediments thrust over Pleistocene sediments, and indications of frictional heating from vitrinite (a component of coal) reflectance, related to high-velocity slip along a narrow fault plane Ash Accumulation Geochemical Evolution (Sakaguchi et al., 2011). Record in the of the Izu and Marianas Caribbean Sea Volcanic Arcs 0 0 Site 999 Fields of Inquiry Enabled Marianas Documenting the materials that enter subduction zones 10 Miocene is critical for a wide range of studies. Scientific ocean drilling 10 Age (Ma) is the only way to determine the composition of sediments 20 and uppermost crust entering the trench and to calibrate seis- mically based estimates of upper plate erosion. Some of this Izu Age (Ma) 20 Oligocene sediment is accreted or underplated to the forearc, and some 30 is subducted to great depth, providing volatiles that stimulate magmatic production in the arc, in proportions that are likely 40 30 controlled by both sediment thickness and composition. a) Understanding this partitioning has implications for studies Eocene of arc magmatism, forearc structure and geohazards, and the 50 0 0.5 1 formation of new continental crust. Documenting the occur- Th/Yb rence of subduction erosion transformed estimates of crustal 60 mass balance along active continental margins and provided a mechanism to explain marine observations of forearc basin b) 70 subsidence and terrestrial observations of continent-ward 200 100 migration of active volcanic arcs. Sediment subduction and Ash accumulation rate subduction erosion may also affect the frictional properties (cm/my) of the plate boundary. Although most knowledge about faulting processes is FIGURE 2.4 Arc history in the Southwest Pacific (a) and Carib- Figure 3. obtained from seismological and geodetic instruments and bean (b) as recorded by cored ash layers. Changes in isotopic ratios with time in the southwest Pacific provide fingerprints of changes in deep magmatic systems. In the Caribbean, changes in ash accu- mulation rate indicate temporal changes in volcanic vigor through 1 The Nankai Trough NanTroSEIZE site is about 500 km away from the epicenter of the magnitude 9.0 Tōhoku earthquake of 2011, which was the Cenozoic. SOURCE: Plank, 2002. located on the Japan Trench.

OCR for page 13
23 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES modeling, scientific ocean drilling is required to sample fault Lava flows in oceanic LIPs encompass vast areas, reaching plane materials at the depths at which earthquakes nucleate tens of thousands of square kilometers with thicknesses of and to measure temporal changes in physical properties a few to hundreds of meters. Oceanic LIPs are among the associated with a changing state of stress. Observatory instal- largest outpourings of materials from Earth’s interior during lations at these depths (see Box 3.2), coupled with real-time a short-lived event (e.g., Tolan et al., 1989), with overall volumes that fall in the range of 103 to 106 km3, yielding data delivery systems, have the potential to support practical earthquake early-warning systems as well as to provide new abnormally thick ocean crust of up to ~40 km (Gladczenko scientific insights into earthquake processes. Such observa- et al., 1997; Richardson et al., 2000; Miura et al., 2004; tories can measure in situ temperature, pore pressure, and Kerr and Mahoney, 2007). They represent about 10 percent strain, and could include other parameters as new tools are of the mass and energy flux from Earth’s deep interior (e.g., developed. Sleep, 1992) and have a large impact on ocean chemistry, climatic conditions, and even the trajectory of evolution and extinction of life forms. However, precisely how LIPs form Goals Not Yet Accomplished remains a subject of great debate, with possibilities of origin Some of the major unknowns about subduction zone including plume activity, bolide impacts, massive upwelling earthquakes (and plate-bounding faults in general) are the of eclogite, and lithospheric delamination. characteristics controlling locked faults with infrequent large earthquakes vs. those with slow, aseismic slip. Deep drilling Scientific Accomplishments and Significance in forearc mud volcanoes promises to provide the most direct link to geochemical processes at depth in the forearc. Obtain- Sampling oceanic LIPs remains more difficult than ing in situ, unaltered samples will provide scientists informa- accessing their continental counterparts, because of their tion on physical properties of the material, which is needed inaccessibility and burial beneath marine sediments. Only to test models of subduction zone processes and earthquake four submarine LIPs have been drilled thus far: the 55 Ma genesis. Installation of instruments to measure temperature, North Atlantic Volcanic Province on ODP Leg 104 (e.g., Sto- seismic waves, strain, pressure, fluid flow, and pore water rey et al., 2007), the ~120 Ma Ontong Java Plateau on DSDP chemistry at these depths are planned for the future. Leg 30 (e.g., Packham and Andrews, 1975) and ODP Leg 192 Achieving the ambitious goals of SEIZE remains a chal- (e.g., Tejada et al., 2004), the 120-95 Ma Kerguelen Plateau/ lenge, with questions about the feasibility of deep drilling Broken Ridge on ODP Leg 183 (e.g., Frey et al., 2000), and because of strong currents in the area, tool and instrument the ~145-130 Ma Shatsky Rise on ODP Leg 198. In some performances at high temperature, and the time and cost to cases where sediments completely blanket the underlying drill to such great depth. The delay in being able to use the igneous rock, scientific ocean drilling has provided the only Chikyu also impacted the project timeline. The broad spec- direct samples that have ever been collected (e.g., Neal et al., trum of onshore and offshore geophysical and geochemical 2008). Although LIPs are tens of kilometers thick, the cores observations required to further understand subduction zones recovered thus far have been limited to only a few tens to and their associated geohazards, makes this area of research hundreds of meters because of the difficulty of penetrating ideal for coordination between scientific ocean drilling and through thick sequences of crystalline rock. other programs (for example, GeoPRISMS, EarthScope, DSDP drilling of the North Atlantic volcanic province U.S. Geological Survey [USGS] hazards initiatives). Tech- for seaward-dipping reflectors (previously discussed in the nological advances are also needed to improve sediment continental breakup section) provided evidence that SDRs recovery in sandy sediments, and thus provide longer paleo- were subaerially erupted basalts that subsided below sea seismic records. Deeper and more complete turbidite and level. Dating of cores correlated the eruption of the North ash records from additional arcs will increase understanding Atlantic Igneous Province with the Paleocene-Eocene Ther- of arc histories, which will contribute to a predictive under- mal Maximum (PETM), a period of rapid global climate standing of why some arcs are very explosive and generate warming. This led to hypotheses that the climatic and volca- mega-eruptions with global impact. nic events are causally related and confirmed that submarine LIPs are formed in geologically short timespans. ODP drill cores from the Kerguelen/Broken Ridge LIP LARGE IGNEOUS PROVINCES revealed pieces of wood and sediment, confirming that some Large igneous provinces are areas on Earth’s surface areas reached above the sea surface as islands before being that are covered with large volumes of volcanic and plutonic submerged (Coffin et al., 2000). Drilling on ODP Leg 192 rocks, widely recognized as being related to mantle plumes suggested that the Ontong Java Plateau was formed almost or hotspots (see global distributions in Figure 2.5). In most entirely beneath the sea surface, unlike the Kerguelan Plateau places, they formed from large accumulations of mafic (Mahoney et al., 2001). Sampling 149 m into the basement at magma produced by large-volume melting over short time ODP Site 807 found basalt geochemistry indicative of high intervals (often no more than 2-3 myr; Coffin et al., 2006). degrees of melting (Frey et al., 2000), with fairly uniform

OCR for page 13
24 SCIENTIFIC OCEAN DRILLING Box 2.4 NanTroSEIZE Eight NanTroSEIZE expeditions were completed between 2008 and 2010.1 Stage 1, completed in February 2008, was a transect of eight sites that targeted the shallow part of the accretionary complex, providing information on stresses, pore water geochemistry, and sediment age, lithology, and physical properties (see white paper by Casey Moore, Ap- pendix C). Results from the three Stage 1 expeditions were reviewed by Kinoshita et al. (2009). Stage 2, completed in October 2009, characterized the sediments on the subducted slab and prepared the stage for drilling a deep-riser hole through the megasplay fault and decollement. Stage 3 began in August 2010 and entails riser drilling to a depth of 6,000-7,000 m beneath the seafloor to penetrate several active fault zones and the crust of the subducting plate (see Figure below). Stage 4 is the planned installation of a long-term observatory to measure fluid pressure, seismicity, strain, tilt, and temperature within the ultra-deep boreholes drilled during Stage 3 and to transmit the data in real time. a) 35° 0 50 N km Tokai Izu Eurasian 40° Plate N 30° Philippine Sea Plate Kii Peninsula 130°E 140° 150° 34° 1944 Site C0009 Site C0002 Site C0001 Sites C0003 and C0004 Site C0010 Site C0006 33° Site C0011 135°N 136° 137° 138° 139° b) (a) Map of the NanTroSEIZE transect. The epicenter and contours of seismic slip for the magnitude 8.1 tsunamigenic earthquake of 1944 are also shown (from McNeil et al., 2010). (b) Schematic illustration of the NanTroSEIZE transect (SOURCE: © JAMSTEC). 1 See http://www.jamstec.go.jp/chikyu/eng/Expedition/NantroSEIZE/.

OCR for page 13
25 SCIENTIFIC ACCOMPLISHMENTS: SOLID EARTH CYCLES FIGURE 2.5 Phanerozoic global LIP distribution. Red areas are LIPs (or portions thereof) generated by a transient “plume head”; blue areas are LIPs (or portions thereof) generated by a persistent “plume tail.” SOURCE: Modified from Coffin et al., 2006. compositions (Mahoney et al., 2001). Although oceanic recognized to be accompanied by the deposition of organic- LIPs are composed mainly of tholeiitic basalts similar to rich black shales. LIP eruptions could also release massive mid-ocean ridge basalts, they have higher concentrations of amounts of aerosols and greenhouse gases, especially car- large ion lithophile elements, which likely reflect not only bon dioxide, methane, and sulfur dioxide. Research into the the compositional source of the basalt but also differentiation cause and effect relationships between this type of massive processes in the magma prior to eruption. Unlike other LIPs, volcanism and changes in climate has grown significantly the submarine emplacement of the Ontong Java Plateau may with the emergence of data from both continental and oce- have lessened its impact on the climate. anic LIPs. LIPs may have also played an important role in The Shatsky Rise expedition (ODP Leg 198) recovered massive extinctions. an excellent series of sedimentary cores, with evidence for ocean anoxic events, the PETM, and the K-T boundary, but Goals Not Yet Accomplished was less successful penetrating the basement of the LIP itself (Bralower et al., 2002). Sampling LIPs by drilling has been likened to “pin- pricking an elephant,” as LIPs are considerably thicker than the deepest cores recovered thus far. Those cores, in the 800- Fields of Inquiry Enabled 900 m range from the Kerguelen Plateau, only cover a frac- LIPs provide a context to understand mantle variability tion of the estimated 30 km thickness of the LIP (Operto and and plume dynamics, the origin of mass extinction, and Charvis, 1995). Deeper drilling into the basement of these continental breakup events. Scientific ocean drilling provides provinces would offer further opportunity to understand their opportunities to study how the geochemical composition of emplacement, age, and geochemical variability. magmas changes over time, relationships to melting anoma- More thorough drilling of submarine LIPs could improve lies that cause LIPs, and the timing and rate of eruptions. understanding of LIP genesis and emplacement, because However, it also provides an opportunity to understand the there is no single model that currently satisfies all observa- complex interplay between volcanic, climatic, and biotic tions. In addition, thorough dating could provide stronger processes. evidence of potential linkages between LIP eruptions and Two other fields of inquiry that have benefited from the mass extinctions. Other issues that scientific ocean drilling study of massive submarine LIPs are ocean anoxia events could address include the initiation and duration of oceanic and volcanic eruption-related climate change. LIPs have LIP melting events, the impact on ocean chemistry and sea- been identified as potential triggers of oceanic anoxia, now floor geochemistry, and the temporal and spatial relation- ships between LIPs and ocean anoxic events.

OCR for page 13