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4 Scientiﬁc Accomplishments: Earth’s Climate History Scientiﬁc ocean drilling has revolutionized studies of history of ice sheets (Zachos et al., 1992; Ehrmann, 1998; Earth’s climate system and has produced the most important Backman and Moran, 2009); sea surface temperature (Huber, geological archives of global climate history. Combining sci- 2008; Bijl et al., 2009); and ocean bottom temperatures entiﬁc ocean drilling results from a wide array of geological (Triapati and Elderﬁeld, 2005). Cores from scientiﬁc ocean settings and geographical regions has transformed scientiﬁc drilling have tied together marine and continental records, understanding of the patterns and processes of past climate further constraining the timing of signiﬁcant events in global change, providing records of natural variability against climate history. which present and future climate change can be assessed. As their length and quality have improved, scien- Deep Sea Drilling Project (DSDP), Ocean Drilling Program tiﬁc ocean drilling records have strongly contributed to the (ODP), and Integrated Ocean Drilling Program (IODP) understanding of dramatic and continuous change in Earth’s expeditions have focused on many aspects of environmental climate system over the past ~100 myr, from extremes of processes and change, including global-scale orbital climate expansive warmth with ice-free poles to massive continental forcing; processes and thresholds of Northern Hemisphere ice sheets and polar ice caps. Signiﬁcantly, these records and Antarctic glaciations and global sea level change; abrupt of millions to tens of millions of years ago provide critical millennial-scale climate change; and past global warmth and insights into environmental changes when atmospheric CO2 extreme climate events. levels were similar to or higher than today. The identiﬁcation Although paleoclimate data from scientiﬁc ocean drill- of orbital cycles that drive repeated cycles of polar ice sheet ing have lower resolution and sometimes lower precision growth and collapse and global sea level ﬂuctuations of up to than modern meteorological data, they extend over longer 120 m (e.g., Chappell et al., 1996) remains one of the most periods of time and provide information regarding various fundamental discoveries of scientiﬁc ocean drilling. climate states, their stability, and impacts on other Earth sys- The geological record below the seafloor extends tems. Innovations in piston coring technology during DSDP beyond instrumental and ice core records for tens of millions and ODP led to recovery of high-quality and high-resolution of years, when Earth’s climate was warmer than the present. sediment cores. One of these innovations was double- and Because of this, scientiﬁc ocean drilling records provide triple-coring sediments at the same site, which allowed for important context for assessing future climate change. Over splicing cores together through biostratigraphy and match- the past decade, there have been moves toward the integration ing physical properties, creating longer, continuous records. of ocean core proxy data that act as tracers of past climate These composite records have created a more complete and (Box 4.1; Figure 4.1) and the use of numerical ice sheet and detailed account of past changes in the ocean environment climate models to predict future climates. Past warm periods on annual to orbital (10 to 100 kyr) time scales, providing such as the Pliocene (5.3-2.6 Ma) and Eocene epochs (55- new insights into ocean-atmosphere, ocean-cryosphere, and 33.9 Ma) offer more realistic future climate analogs, which ocean-biosphere interactions (ODP, 2007). Progressively scientists can use to improve model performance, gaining higher resolution, better dated sediment records have led better understanding of Earth system responses to elevated to reconstructions of atmospheric carbon dioxide (CO2) greenhouse gas levels. concentrations for the past 60 myr (Royer, 2006); Cenozoic 39
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40 SCIENTIFIC OCEAN DRILLING CO2 concentrations have been among the most important Box 4.1 contributions of scientiﬁc ocean drilling to paleoclimate Proxy Records in studies. Climates prior to 3 myr (particularly the past 65 Scientiﬁc Ocean Drilling myr) were generally warmer than today, and were associated with higher pCO2 levels. Reconstructions show a long-term Over the past 40 years, a wide range of climate decrease in global average temperatures, from a maximum proxies that measure different components of of about 26 °C in the early Eocene Epoch (~ 50 Ma) to a marine sediments have been developed for use pre-industrial Holocene average value of 14 °C. This pattern as tracers of past changes in climate and ocean of global cooling is associated with declining pCO2 levels, circulation. These proxies include variations in plant from 2,000-4,000 ppm range in the Paleocene and Eocene to and animal species abundances, which track past less than 400 ppm by ~24 myr (Pearson and Palmer, 2000; changes in environmental conditions at a specific Pagani et al., 2005b; Beerling and Royer, 2011). Studying location, as well as evolutionary distributions of warm climate extremes recorded in ocean sediments enable fossils, which provide important age control and new insights into Earth system responses to elevated green- stratigraphic markers for correlation. Isotopic and house gas levels. geochemical measurements of fossil shell mate- rial provide information on past oceanographic conditions (e.g., temperature, salinity), past ocean Scientiﬁc Accomplishments and Signiﬁcance chemistry (e.g., pH, carbonate ion concentration, deepwater mass circulation), and the concentra- Scientiﬁc ocean drilling has signiﬁcantly contributed tion of paleo-atmospheric CO2 (boron isotopes to the recognition and quantiﬁcation of latitudinal differ- and boron-calcium ratios). More recently, chemical ences in temperature in response to pCO2 (Figure 4.1) and measurements of fossil organic compounds have other high-latitude Earth system feedbacks (e.g., sea ice been developed to reconstruct sea surface tempera- albedo) that can lead to polar temperature ampliﬁcation (e.g., tures (e.g., alkenone saturation ratios, long-chain Dowsett, 2007; Huber, 2008; Bijl, 2009). Sea surface temp- tetraethers), partial pressure of CO2 (pCO2; e.g., eratures reconstructed from globally distributed drill cores alkenone isotopic chemistry), and hydrology (e.g., have demonstrated that the early Eocene (55-48 Ma) had the leaf wax biomarkers such as compound specific warmest climates of the past 65 myr, depicting a world that deuterium measurements on alkanes). A number of proxies are now well established was ~10-12 °C warmer and with greatly reduced latitudinal and routinely applied (e.g., stable oxygen and car- temperature gradients compared with the present day (Bijl bon isotopic ratios of foraminiferal calcite), while et al., 2009; see Figure 4.1). others are still in a more developmental state. All Cores recovered from the Arctic (ODP Legs 151, 163; proxies must deal with various levels of associated IODP Leg 302; Figure 4.2a) and the Antarctic (ODP Legs uncertainty due to a lack of knowledge regarding 113, 119, 120, 188, 189; IODP Leg 318; Figure 4.2b) indi- precise relationships between the proxy and the cate that polar regions of the greenhouse world could support environmental characteristic being measured (e.g., only small terrestrial ice sheets, or had limited perennial sea some pCO2 and sea surface temperature prox- ice (Moran et al., 2006; Stickley et al., 2009). This ﬁnding ies), particularly when applied to older sediments. implies global sea levels more than 60 m higher than the Nevertheless, as laboratory studies continue and present, when atmospheric CO2 levels may have been as high calibrations improve, there has been significant con- vergence between different proxies used to estimate as 2,000-4,000 ppm (Pagani et al., 2005b; also discussed in temperature and pCO2 (e.g., Beerling and Royer, the following section). 2011). This explosive growth in the number, type, Observations of past warm extremes are important for and utility of proxies has led to a significantly better evaluating the performance of climate models in response understanding of past global environmental condi- to higher levels of pCO2 (Huber and Caballero, 2011, and tions. The combination of physical measurements references therein). Although ampliﬁcation of polar warming of past temperatures with chemical measurements during the past warm periods appears to be underestimated indicating past atmospheric CO2 concentrations has by the current generation of climate models, the sensitiv- been particularly valuable for understanding the ity of past tropical temperatures is generally overestimated sensitivity of the climate system to CO2 forcing. relative to proxy-based temperatures from scientiﬁc ocean drilling. Comparisons between models and paleoenviron- mental observations from drill cores play an important role PAST WARM CLIMATE EXTREMES AND THE in evaluating the performance of Intergovernmental Panel GREENHOUSE WORLD on Climate Change (IPCC) climate models that simulate warmer global climates. The reconstruction of Cenozoic surface temperature A key discovery of the Paleocene-Eocene greenhouse distributions and their relationship to changes in atmospheric world (55-50 myr) was the potential of the climate system
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41 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY Observations such as these, from the last time global atmo- spheric pCO2 levels approached ~400 ppm, may provide an analog for assessing the range of future equatorial climate changes due to anthropogenic warming. Fields of Inquiry Enabled Scientiﬁc ocean drilling has enabled scientists to extend the relationship between atmospheric pCO2 and global sur- face temperature by millions to tens of millions of years, con- ﬁrming signiﬁcantly warmer than present climate extremes that are increasingly relevant to future climate projections. The importance of past climate information was acknowl- edged in the IPCC’s Fourth Assessment Report (IPCC, 2007), when it introduced a chapter on paleoclimate archives. As the quality and global coverage of pCO2 and temperature FIGURE 4.1 Latitudinal variations in climate sensitivity, derived proxies from ocean sediments steadily improve, the IPCC’s from scientiﬁc ocean drilling results. These variations consist- Fifth Assessment Report (IPCC, in preparation) will place ently show that the effect of higher carbon dioxide on sea surface increased emphasis on these observations to verify the per- temperature, and thus air temperature, increases towards the poles. formance of climate models during warm extreme intervals. SOURCE: Adapted from Bijl et al. (2009), with additional data Cores recovered from scientiﬁc ocean drilling have from Paul and Shafer-Neth (2003) and Dowsett (2007). enabled improved estimates of Earth’s climate sensitivity to sustained higher levels of greenhouse gases and to dramatic transient perturbations to the carbon cycle (including ocean acidiﬁcation). These data have also determined the sensitiv- to experience abrupt and transient temperature excursions ity of ice sheets to elevated greenhouse gas concentrations occurring within 1 to 10 kyr, termed “hyperthermals” (discussed in more detail in the following section), including (Bohaty and Zachos, 2003; Zachos et al., 2005). These greater insight into the processes that lead to temperature hyperthermals had warming of several degrees C, indicated ampliﬁcation in polar regions. Finally, the integration of by changes in oxygen isotope (δ18O) and Mg/Ca records observations of physical and chemical processes elucidated (Kennett and Stott, 1991; Zachos et al., 2003; Tripati and by drilling records is critical for the next generation of cli- Elderﬁeld, 2005). The ﬁrst and largest of the hyperthermals mate models. was the Paleocene-Eocene Thermal Maximum (PETM; Box 4.2) at 55.8 Ma, which lasted for approximately 100 kyr. One of the most important high CO2 analogs studied Goals Not Yet Accomplished in ocean sediment cores is the Early Pliocene Epoch (5.3 to In a prior review of ODP, the NRC (1992) recommended 2.6 Ma), when continental and ocean conﬁgurations, eco- that the understanding of past climates, especially of rates systems, and ice sheet extent were similar to today. Proxy and magnitudes of climate variability, should be improved. estimates from sediment cores in a range of ocean basins This recommendation was translated into a scientiﬁc priority indicate peak Pliocene values that are comparable to pres- for the IODP Initial Science Plan (IODP, 2001), and expec- ent day values of 379 ppm (IPCC, 2007). Although the high tations in this ﬁeld have largely been met. In some aspects, latitudes were signiﬁcantly warmer, tropical sea surface and such as resolution of past climate extremes, the outcome has air temperatures were similar to the present (e.g., Dowsett, possibly exceeded the goals set forth by the plan. However, 2007). Drill core data (Raymo et al., 2006; Naish et al., 2009) there is still progress to be made. and ice sheet simulations (Pollard and DeConto, 2009) show Spatial coverage of ocean records of past extreme warm complete deglaciation of the Greenland and West Antarctic intervals is biased toward the North Atlantic and East Paciﬁc, ice sheets and the low elevation margins of the East Antarctic leaving large swaths of the ocean ﬂoor to be sampled. Con- ice sheet, with global sea levels up to 20 m higher than the sequently, sea surface temperature datasets for these times present (Miller et al., 2011; Raymo et al., 2011). (e.g., Pliocene, Eocene) are inadequate for the robust data- Other regional phenomena, such as a permanent El model comparison needed to better constrain future climate Niño-like state in the tropical Paciﬁc during the Pliocene, projections and understand regional climate variability. The can be inferred from sediment core proxy data. In conjunc- polar regions presently experience temperature increases that tion with climate models, they imply drought and a potential are two to three times greater than the global average (Hol- collapse of the Asian Monsoon, increased eastern Paciﬁc land and Bitz, 2003; Bijl et al., 2009; Miller et al., 2010), precipitation, and increased cyclonic activity (e.g., Brierley yet the mechanisms and feedbacks are poorly understood, as and Fedorov, 2010; Fedorov et al., 2010; Ravelo et al., 2010).
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42 SCIENTIFIC OCEAN DRILLING a) 180˚ 15 0˚ 0˚ 15 0˚ 12 12 0˚ 90˚W 90˚E ˚ 60 60 ˚ ODP Leg 151 Leg 163 30 ˚ 30 ˚ IODP Exp 302 0˚ b) 40˚ DSDP N Leg 28 ODP 20˚ Leg 113 Leg 119 Leg 120 0˚ Leg 188 Leg 189 -20˚ IODP Exp 318 -40˚ -60˚ -80˚ 30˚ 60˚ 90˚E 120˚ 150˚ 180˚ 150˚ 120˚ 90˚W 60˚ 30˚ 0˚ 30˚ FIGURE 4.2 Location maps of DSDP, ODP, and IODP expeditions in polar regions that were related to past climate extremes. (a) Illustrates the Arctic using a stereographic projection. (b) Illustrates the Antarctic using a Mercator projection. Both have a color range of –9,000 to 9,000 m, with white marking the 0 m depth. SOURCE: IODP-USIO.
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43 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY Box 4.2 The Paleocene-Eocene Thermal Maximum core from ODP Hole 690B. A 35-50 percent species reduction of benthic foraminiferal taxa was associ- Observations of an extreme change in the carbon ated with the enrichment of light carbon and a δ18O chemistry of fossils at 55.8 myr suggest that Earth excursion interpreted as reflecting a > 4-8 °C abrupt experienced a sudden release of carbon into the increase in surface water temperatures. This event atmosphere, followed by a rapid 4 to 8 °C increase was termed the Paleocene-Eocene Thermal Maxi- in global temperature. This is the best past analog of mum (PETM). Study of these cores established the rapid changes in atmospheric CO2 so far observed onset of the warming event as taking on the order in the geologic record. Kennett and Stott (1991) of 1 kyr and lasting over ~130-190 kyr (Kelly et al., discovered a large Cenozic carbon isotopic (δ13C) 1996; Bralower et al., 1997; Roehl et al., 2000). excursion at the Paleocene-Eocene boundary, in Analysis of other scientific ocean drilling cores, such as those from ODP Sites 525, 527, and 865, a 3.0 ODP Leg 208, and IODP Expedition 302 to the Arctic Ocean (Sluijs et al., 2006) showed these 2.0 excursions were global. Arctic cores recovered by 1.0 δ13C ( ‰) the IODP Arctic Coring Expedition (ACEX) in 2004 reveal that surface temperatures increased from 18 0 to 23 °C, synchronous with other PETM records. –1.0 Southern Ocean The sudden disruption in the carbon cycle—nearly 690 Central Paci c equivalent to burning modern fossil fuel reserves— –2.0 865 b –1.0 14 South Atlantic produced significant ocean acidification, disrupted 525 527 the deep ocean ecosystem, and caused significant Temperature (°C) –0.5 12 evolutionary turnover in benthic dwelling foramin- δ18O ( ‰) ifera. Advanced piston coring in 2003 at the Walvis 0 10 Ridge in the South Atlantic recovered a set of cores that recorded the climate and chemistry changes as- 0.5 8 sociated with this event as well as the subsequent, several hundred thousand year recovery of ocean 1.0 c 100 chemistry following the carbon disruption. For the first time it was possible to fully document the size 80 of the carbon perturbation (an initial pulse of 3,000 60 CaCO 3 (%) GT in less than a few thousand years), the response 40 of the surface warming, and the role of the oceans South Atlantic in removing the carbon from the atmosphere and (water depth) 20 1262 (4.8 km) neutralizing the increased pH of the deep sea. 1263 (2.6 km) One of the most striking possible explanations for 0 54.0 54.5 55.0 55.5 56.0 the event is catastrophic and massive ocean floor Age (millions of years ago) methane hydrate dissociation triggered by otherwise incremental warming (Dickens et al., 1997b), which Paleocene-Eocene Thermal Maximum, as recorded in oce- anic benthic isotopic records from Antarctic, south Atlantic, could produce abrupt global warming and then later and Pacific Ocean drill sites. The rapid decrease in carbon oxidize CH4 to CO2. However, methane derived from isotope ratios (top panel) indicates a large increase in at- heating of organic-rich shales by intrusions of the mospheric methane and carbon dioxide. This is coincident North Atlantic large igneous province (LIP) provides with 5 °C of global warming (middle panel, presented with a plausible alternative (Svensen et al., 2004; see oxygen isotope values). Subsequent ocean acidification is Chapter 2 for a discussion of LIPs). indicated by a rapid decrease in the abundance of calcium carbonate (lower panel). SOURCE: Zachos et al., 2008.
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44 SCIENTIFIC OCEAN DRILLING are implications for ice sheet stability. Polar regions remain (within a few tens of thousands of years) and caused at least woefully undersampled, with only one drilling expedition a 60 m global sea level fall (Zachos et al., 1996). Coring in the high Arctic Ocean and only a few expeditions in of thick, continuous Paleogene sediments in the Weddell the Antarctic, yet these sparse data points are the basis for Sea (ODP Leg 113) led to the idea that thermal isolation many current models of climate responses in polar regions. due to the separation of South America and Australia from Improved understanding of the role of the Southern Ocean Antarctica initiated ice sheet development. However, more in the carbon cycle is another priority. Recovering sediment recent numerical model simulations imply that a threshold cores from high latitudes presents one of the most important in declining pCO2 was the ﬁrst-order control on Antarctic technological challenges for future scientiﬁc ocean drilling glaciations (DeConto and Pollard, 2003; Huber et al., 2004). High-resolution δ18O records from the Southern Ocean and will need the innovative use of both mission-speciﬁc platforms and the JOIDES Resolution to drill strategic tran- (ODP Site 1090) and the equatorial Paciﬁc (ODP Site 1218) sects. In addition to increased spatial coverage, continued illustrate Antarctic ice sheet behavior during the Oligocene advances in paleoclimatological observations from climate and early Miocene early icehouse world (33-15 Ma; e.g., proxies will be needed to provide robust veriﬁcation of cli- Pälike et al., 2006). Glacial-interglacial ice volume changes mate models. Achieving this goal will entail close coopera- equivalent to 10-40 m of global sea level change were driven tion between the scientiﬁc ocean drilling and Earth system by a pervasive 40,000-year orbital forcing, with major glacial modeling communities. events occurring every 1-2 million years. The ﬁrst physical evidence for orbitally paced variability in the East Antarctic Ice Sheet during the Oligocene and Miocene came from CENOZOIC ICE SHEET EVOLUTION AND sea ice–based drill cores (e.g., the Antarctic Geological GLOBAL SEA LEVEL CHANGE Drilling program [ANDRILL]1; Naish et al., 2001), which Changes in global sea level over the past 40 myr reﬂect conﬁrmed climatic patterns observed in global ice volume the evolution of polar ice sheets from ephemeral, small- proxy records from scientiﬁc ocean drilling oxygen isotope medium Antarctic ice sheets (prior to 33.5 myr) to a large records (e.g., ODP Leg 120, Zachos et al., 1996; ODP Leg Antarctic ice sheet and variably sized Northern Hemisphere 154, Zachos et al., 2001b; ODP Leg 199, Pälike et al., 2006). continental ice sheets for the past 2.7 myr. Marine sedimen- Integrating data from ice-based and ODP cores demonstrated tary archives provided by scientiﬁc ocean drilling have revo- that under warmer climates the Antarctic ice sheets were less lutionized understanding of Earth’s Cenozoic climate system stable than today. and have imparted new insights into the pattern of behavior Unlike the Antarctic, a detailed, relatively continuous of polar ice sheets and their inﬂuence on global sea level ocean sediment record of the Arctic’s glacial history was (Figure 4.3). These studies also have important implications unavailable until the mid-2000s, when an astute strategy for assessing future sea level rise in a warming world, where combining icebreakers and drillships succeeded in recover- uncertainties in sea level projections are large because ice ing the ﬁrst direct evidence for Cenozoic climate change sheet dynamics and climate system behavior during steadily from this region (ACEX; e.g., Moran et al., 2006). The warming conditions are still poorly understood. ACEX cores captured a 55 million year long history of the central Arctic Ocean, including the transition from a warm greenhouse world during the late Paleocene and early Scientiﬁc Accomplishments and Signiﬁcance Eocene (Brinkhuis et al., 2006; Slujis et al., 2006) to a colder Scientiﬁc ocean drilling has played an integral role in icehouse world inﬂuenced by sea ice (Stickley et al., 2009) understanding the transition from a greenhouse to “icehouse” and apparent sparse icebergs (Eldrett et al., 2007) from the climate system with the onset of Antarctic glaciation 33 myr middle Eocene to the present. ago, at the Eocene-Oligocene boundary. In 1973, DSDP Leg In the Northern Hemisphere, the interval from 3.0 to 28 drilled on the Antarctic continental shelf in the Ross Sea, 2.5 myr ago is marked by the progressive expansion of con- providing the ﬁrst physical evidence of continental glacia- tinental ice and global cooling, which initiated a pattern of tion extending back into the Oligocene (Hayes et al., 1975) glacial-interglacial cycles controlled by long-term periodic and dispelling the then-prevailing hypothesis that Antarctica variations in Earth’s orbit. A major increase in understanding had only been extensively glaciated since the beginning of these variations came from DSDP Leg 81, which recovered the Quaternary (2.588 myr). Drilling of continental shelf an almost continuous sediment record from Site 552 in the sites in Prydz Bay (ODP Leg 119) provided the ﬁrst direct high-latitude Atlantic Ocean using the newly employed evidence of continental-scale ice sheets calving at the Ant- hydraulic piston corer. At this site, Shackleton et al. (1984) were able to show that positive excursions in δ18O correlated arctic coastline (Hambrey et al., 1991), and ice-rafted debris collected at the Kerguelen Plateau (ODP Leg 120) offered with the inﬂux of ice-rafted debris—indisputable evidence further conﬁrmation of glaciation at the Eocene-Oligocene for nearby continental ice sheets. They established the ﬁrst boundary (Wise et al., 1991; Zachos et al., 1992). These same cores indicated that the Antarctic ice sheet grew quickly 1 See http://www.andrill.org.
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45 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY FIGURE 4.3 Illustration of three major contributions to Cenozoic climate studies from scientiﬁc ocean drilling. The composite datasets used in the ﬁgure were generated from analysis of scientiﬁc ocean drilling sediment cores. (A) Global sea level curve from continental margin cores, which represent changes in sea level in response to polar ice volume ﬂuctuation (e.g., Miller et al., 2005; Kominz et al., 2008). (B) Atmospheric CO2 concentrations reconstructed from organic biomarkers and foraminifera preserved in ocean sediments (Pearson and Palmer, 2000; Pagani et al., 2005b). (C) Global atmospheric temperature curve (bold red line) adapted from Crowley and Kim (1995) overlaid on compiled benthic δ18O data representing global ice volume and deep ocean temperature (Zachos et al., 2001a). Major periods of warmth and transitions to cooler climate are also presented. SOURCE: Modiﬁed from R. Levy, GNS Science.
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46 SCIENTIFIC OCEAN DRILLING age for onset of major continental glaciations (~2.5 myr ago), understanding of thresholds for both Antarctic and Northern based on observations that only small amounts of ice-rafted Hemisphere glaciation and deglaciation (e.g., DeConto et debris were found in the cores before this time. Subsequently, al., 2008). Consequently, the plausible range of changes in drill cores from the equatorial Atlantic (DSDP Leg 94; ODP global sea level can be better constrained. In addition, well- Leg 108) and North Paciﬁc (ODP Leg 145) reﬁned this date dated reconstructions of global sea level rise following the to 2.7 myr ago (Ruddiman et al., 1986; Haug et al., 1999, last glaciation, derived from drilling corals, are increasing 2005). the ability to identify meltwater sources and rates of sea Drilling of passive continental margins has provided level rise. a detailed 100 myr long history of global sea level change (e.g., Miller et al., 1996, 1998, 2005; Kominz et al., 2008). As Goals Not Yet Accomplished part of an integrated study of the passive continental margin, ODP drilled a transect across New Jersey that extended from Extracting physical records of past polar ice sheet vari- offshore to onshore. ODP Legs 150,174A, and 150X/174AX ability and sea level changes will remain a challenge, because sampled the slope, outer shelf, and onshore, respectively; it requires integrated onshore and offshore drilling transects dated unconformities produced during sea level fall; and cor- on continental margins and core retrieval over multiple time- related them to increases in δ18O values indicative of periods frames and depositional settings, including difﬁcult drilling of polar ice volume growth. More than 30 oscillations in environments such as sea ice and unconsolidated sediments. global sea level during the Oligocene and Miocene (33-6 Ma) Although IODP mission-speciﬁc platforms have begun to were identiﬁed, proving the validity of the oxygen isotope address recovery issues, challenges still remain and lead curve as a proxy for changes in global ice volume (Miller times are long. Increasing the use of logging-while-drilling et al., 1998). Stratigraphic patterns (e.g., unconformities, technology could ﬁll in some of the gaps related to poor bedding geometries) in the New Jersey siliciclastic basins core recovery. correspond to scientiﬁc ocean drilling cores recovered in Opportunities exist for scientiﬁc ocean drilling to build carbonate platforms off the margins of Australia (ODP Legs on cooperation with other programs that specialize in drilling 133, 182, and 194) and the Bahamas (ODP Leg 166), imply- on land and in shallow waters (e.g., the International Con- ing a global origin driven by sea level change. tinental Scientiﬁc Drilling Program [ICDP]) and glaciated Although early studies of Late Quaternary sea level continental margins (e.g., ANDRILL), especially to address changes using corals were not done under the auspices of sci- the role of high latitudes in Earth’s climate system. For entiﬁc ocean drilling, IODP has recently successfully drilled example, the evidence for ice-rafted debris in ACEX cores reefs and shallow water carbonate sequences with mission- has sparked a debate about the existence of continental-scale speciﬁc platforms in Tahiti (IODP Expedition 310; Camoin ice sheets in the Northern Hemisphere prior to 2.7 myr ago et al., 2007) and the Great Barrier Reef (IODP Expedition (e.g., Eldrett et al., 2007; Tripati et al., 2008; Stickley et al., 325). Four transects of the Great Barrier Reef were drilled, 2009). Although coupled ice sheet and climate models do with good core recovery of the last glacial cycle. The Tahiti not favor signiﬁcant Northern Hemisphere ice at atmospheric expedition recovered excellent records of the last interglacial CO2 concentrations above pre-industrial levels (~300 ppm; global sea level high stand (125 kyr ago) and of the rapid DeConto et al., 2008), additional long paleoclimate records rise in sea level during deglaciation since the last ice age, are critically needed to address these key questions and to providing critical constraints on past sea level high stands, provide a better understanding of the climate history of the the rate of sea level rise (up to 4 m per century; Deschamps et Arctic. al., 2008), and the potential to ﬁngerprint meltwater sources. Understanding the spatial heterogeneity of sea level rise in response to ice mass changes will also be critical for assessing potential regional impacts of rising sea level. Fields of Inquiry Enabled Geodynamical models and overlapping sea level records Scientiﬁc ocean drilling has contributed signiﬁcantly to recovered from a range of latitudes in different tectonic and understanding the growth of polar ice sheets and the timing sedimentary settings will be needed to identify the relative of glacial and interglacial cycles in the Northern Hemi- contributions of different processes that create a global pat- sphere, as well as their inﬂuence on ﬂuctuations in global tern of sea level change. Scientiﬁc ocean drilling will play a sea level over the past 100 myr. Long-term projections of critical role in further development of proxies for hydrologic sea level rise remain highly uncertain, primarily because of cycles, sea ice coverage, and continental ice volumes. poor understanding of the dynamic behavior of ice sheets during sustained warming. Physical records of past ice ORBITAL FORCING sheet behavior recovered through scientiﬁc ocean drilling have enabled scientists to evaluate the relationship between The study of climate variability due to changes in Earth’s surface temperature and greenhouse gas concentrations orbit provides one of the best examples of an emerging ﬁeld over the full spectrum of climate states, leading to better of scientiﬁc inquiry that blossomed because of scientiﬁc
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47 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY ocean drilling. The earliest observations that glacial-inter- al., 1986), despite the fact that there were no changes in the glacial climate changes at 23, 42, and 100 kyr were paced characteristics of the orbital variability, and that eccentricity by changes in Earth’s orbital geometry related to preces- plays only a small role in the amount of energy Earth receives sion, obliquity, and eccentricity (“Milankovitch cycles”) from the sun. Sediments collected by the drilling programs (Shackleton and Opdyke, 1973; Hays et al., 1976) relied on have been used to develop and test models of orbital forc- the analysis of conventional short piston cores in relatively ing (Imbrie et al., 1992, 1993; Raymo, 1997; Huybers and low sedimentation rate locations. The advent of hydraulic Wunsch, 2004; Huybers, 2006) and how the growth of the piston coring (DSDP Leg 64 in 1978) and its ﬁrst deployment large ice sheets may have changed the response to the forcing for paleoceanographic studies (DSDP Leg 68 in 1979) pro- (Raymo and Huybers, 2008). On longer time scales, studies duced the ﬁrst long, undisturbed records of marine sediment of orbital variability have linked eccentricity forcing at 400- from which researchers were able to derive high-resolution kyr periods with changes in Antarctic ice sheet growth and records of oxygen isotopic chemistry in a well-dated, inde- decay, marine productivity, and carbon burial in the earlier pendent chronology based on paleomagnetic reversal stratig- Cenozoic (Pälike et al., 2006). As a result, all known peri- raphy (see also Chapter 2). The initial records from DSDP ods of orbital forcing have been documented in the marine Sites 502 and 503 extended the history of marine oxygen records, a feat which would not have been possible without isotope variations back to approximately 3.5 myr ago; prev- scientiﬁc ocean drilling and the development of hydraulic ious records using traditional piston cores had been limited piston coring (more information on piston coring can be to observations of the past 1 myr or so. DSDP Leg 81 fol- found in Box 2.2). lowed with the striking observation of a signiﬁcant increase The late Pleistocene records of climate change have also in glacial sediment delivery to the North Atlantic at about 2.5 provided important constraints on climate sensitivity—the myr, marking the initiation of Northern Hemisphere glacia- magnitude of climate change expected from a doubling tion (Shackleton and Hall, 1984). DSDP Leg 94 coring in of atmospheric CO2 concentration (Hansen et al., 2006, North Atlantic Sites 607 and 609 quantiﬁed the changing 2007)—thus making these records among the most soci- nature of the climate system response to orbital forcing, from etally relevant accomplishments of scientiﬁc ocean drilling the evolution of the obliquity-dominated response in the late and conventional piston coring. Ice core CO2 variations Pliocene and early Pleistocene to the eccentricity-dominated from Vostok and EPICA (European Project for Ice Coring response in the late Pleistocene. The nature of this change in Antarctica) combined with sea surface temperature vari- has been well documented elsewhere, but the reasons for the ability observed in marine and continental locations provide change remain an area of active research. independent estimates of the sensitivity of climate to changes Based on these early successes, ODP embarked on a in atmospheric CO2 concentration. Earlier Cenozoic recon- global-scale effort (Figure 4.4) to observe and study orbitally structions of climate have also been used to constrain climate forced climate throughout tropical (ODP Leg 108 in the sensitivity (the Paleocene-Eocene Thermal Maximum, for eastern equatorial Atlantic; ODP Leg 117 in the Arabian Sea; instance, described in Box 4.2). A low equilibrium sensitivity ODP Leg 130 in the western equatorial Paciﬁc; ODP Leg of warming to greenhouse gas increase is ruled out based on 138 in the eastern equatorial Paciﬁc; and ODP Leg 154 in the relationship of glacial-interglacial changes in CO2, the the western equatorial Atlantic) and high-latitude locations calculated changes in Earth’s energy budget due to orbital of all ocean basins (ODP Leg 145 in the North Paciﬁc; ODP variability and albedo changes, and the observed magnitude Legs 151, 162, and 172 in the North Atlantic; ODP Leg 177 of climate and ice volume changes. This conclusion would in the Southern Ocean; ODP Leg 181 in the western South not be possible without the ability to link marine, land-based, Paciﬁc; ODP Leg 188 in Prydz Bay; and ODP Leg 202 in and ice core records of climate and CO2. the eastern South Paciﬁc). This major effort and its successes The development of this understanding of orbital cli- are easily among the most signiﬁcant for the scientiﬁc ocean mate variability and its causes has provided a time scale drilling community. and framework for interpreting scientiﬁc results from a wide range of research disciplines. Scientiﬁc ocean drilling played a signiﬁcant role in producing the long time series of Scientiﬁc Accomplishments and Signiﬁcance marine oxygen isotope variability that was used to modify The sediments collected by scientiﬁc ocean drilling and constrain the paleomagnetic reversal ages and to develop have played a major role in advancing the understanding of a revised Astronomical Polarity Time Scale for the Cenozoic orbitally forced climate changes. The very long and highly (Kent, 1999), thus linking continental- and marine-based resolved records provided a means to document the chang- research on a common and accurate time scale (see Box 2.2). ing effects of orbital forcing as ice sheets grew from modest In the most recent example, Lisiecki and Raymo (2005) cor- sizes in the early Pliocene to large, continental-scale glaciers related a global set of long, marine oxygen isotope records, in the late Pliocene (~3 Ma). The response to forcing shifted dominated by the set of advanced piston core sites recovered from obliquity-dominated (41kyr period) to eccentricity- by ODP in the 1980s and 1990s, to develop a continuous, dominated (100-kyr period) about 800 kyr ago (Ruddiman et highly resolved record of oxygen isotope variability and
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48 SCIENTIFIC OCEAN DRILLING ODP 80˚ Leg 108 N Leg 117 Leg 130 Leg 138 Leg 145 Leg 151 Leg 154 Leg 162 Leg 172 Leg 177 60˚ Leg 181 Leg 188 Leg 202 40˚ 20˚ 0˚ -20˚ -40˚ -60˚ 120˚ 150˚E 180˚ 150˚W 120˚ 90˚ 60˚ 30˚W 0˚ 30˚E 60˚ 90˚ FIGURE 4.4 Location map of ODP legs related to orbital forcing. This is a Mercator projection with a color range of –9,000 to 9,000 m, with white marking the 0 m depth. SOURCE: IODP-USIO. paleomagnetic reversals (Box 4.3; Figure 4.5). On longer and boron) properties have provided new target datasets for time scales, marine oxygen isotope records and magnetic testing a variety of coupled climate and biogeochemistry reversals have been used to reﬁne the Cenozoic chronology models in ways that cannot be accomplished using the very for at least the past 40 myr, with reﬁnements to earlier stages short records of climate variability in the historical record. still under way. The broad impact of the development of this The fundamental understanding of how Cenozoic climate chronology can be observed in terrestrial-based studies of evolved has also provided a framework to evaluate the effects archeology, anthropology, and climate, including the com- of changing climate on evolution, including hominins. parison of major human evolutionary events with changes in Scientiﬁc ocean drilling did not provide the ﬁrst evi- climate based on marine oxygen isotope records from ODP dence for orbital forcing of climate, but without the develop- sites (e.g., deMenocal, 2011; see section on “Co-evolution ment of long, continuous, undisturbed sedimentary sections, of life and the planet” at the end of this chapter). it is unlikely that the ﬁeld would have progressed so far in such a short time. Since the late 1970s when orbital forcing was ﬁrst being observed and quantiﬁed using a small number Fields of Inquiry Enabled of conventional piston cores, the ﬁeld has progressed to a The development of the Astronomical Polarity Time much broader understanding and a high degree of conﬁdence Scale for the Cenozoic has had widespread impact through- in the scale of the forcing and changes in the climate system out the geosciences, inﬂuencing research in paleoclimate response. The successes have largely been based on high- studies, archeology, anthropology, and astronomy. The resolution data collected from long, continuous hydraulic combined sets of proxy records from terrestrial and marine piston cores. A major outcome of scientiﬁc ocean drilling is sections of physical (temperature, rainfall) and biogeochemi- the understanding of the pervasiveness of orbital forcing on cal (carbon isotopes, carbonate system proxies like barium climate change. The study of orbitally forced changes in cli-
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49 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY mate using marine sediments also provides a great example Box 4.3 of what can occur when a ﬁeld that is ready for explosive Developments in Coring Technology growth meets up with a tool (the hydraulic piston corer) that and Core Recovery is nearly perfectly suited for the task. The past four decades of scientific ocean drilling Goals Not Yet Accomplished have led to great contributions in riserless deep- water drilling technology, which have significantly There are still unresolved issues about how small, improved core quality and extended the amount of orbitally controlled changes in the total amount of energy core that can be recovered during drilling. Early cor- received from the sun are ampliﬁed by feedbacks within the ing with the Glomar Challenger during DSDP mainly climate system to cause the large Earth system responses used a four cone commercial industry bit. With this seen during ice ages. In the most recent deglaciation, varia- bit, core recoveries were low to moderate, and tions in Earth’s orbit led to a rapid increase in global average many of the cores were highly disturbed. The more temperature, a sharp rise in atmospheric CO2, polar ice sheet modern JOIDES Resolution helped to increase the collapse, and a rise in global sea level. A better understand- overall core recovery and revolutionized deepwater ing of how these systems interacted will provide important coring practices (see white papers from Dennis Kent and Ted Moore, Appendix C). Innovations in piston insight into coupling between the atmosphere, ocean, and ice coring technology during DSDP, later advanced by sheets. Scientiﬁc ocean drilling will continue to play a major ODP, led to recovery of high-quality cores in soft to role in furthering these research activities. The progress dur- medium-soft formations (Larson et al., 1980; Gelfgat ing the past several decades in this ﬁeld of inquiry has been et al., 1994). remarkable and highly inﬂuential, with many major new A major technological advance in core recovery insights still on the horizon. occurred when DSDP Site 607 in the North Atlantic was double-cored with the newly developed hy- draulic piston corer (see Box 2.2). The cores were ABRUPT CLIMATE CHANGE then correlated to fill in gaps caused by loss of core At ODP’s advent in the early 1980s, the main focus of material from ship heaving (Ruddiman et al., 1986). its climate research was orbital variability because so little Piston cores are now routinely double- and triple- cored and spliced together on the basis of matching was known about millennial-scale climate variability. In the continuous logs of physical properties recorded on mid-1980s, the observation that Greenland ice cores recorded board the ship to produce a composite depth record. rapid, abrupt changes in air temperature on millennial time ODP, and later IODP, also evolved wireline coring scales, also known as Dansgaard-Oeschger events, led to techniques that permitted deeper penetrations interest in determining the causes of these changes and in into medium to hard formations (Storms, 1990). ﬁnding similar records in marine sediments and continental The scientific ocean drilling programs worked with climate archives. Because there was no known external forc- industry to innovate better coring bits that would ing on these time scales, the principal hypothesis to explain have longer life and provide less disturbed cores. the abrupt climate changes centered on coupled ocean- One breakthrough was the extended core barrel for atmospheric interactions (Broecker et al., 1992), and rapid drilling harder sediments, which combined piston changes in North Atlantic overturning circulation became coring with a follow-up rotary coring bit (Brewer et al., 2005). In conjunction with Schlumberger, IODP a major research focus for the paleoceanographic research also developed logging-while-coring systems that community. Sampling of legacy cores from prior scientiﬁc measure gamma rays, resistivity, and full bore re- ocean drilling expeditions greatly facilitated the understand- sistivity images (Goldberg et al., 2004). ing of changes in the North Atlantic region and associated DSDP, ODP, and IODP achieved these techno- far ﬁeld climate effects. Although not included in earlier logical advancements with limited development bud- scientiﬁc ocean drilling planning documents, short period gets, especially when compared to the research and climate was listed as a research priority in the ODP Long development budgets of the commercial offshore Range Plan (ODP, 1990); the ﬁrst legs dedicated to climate drilling industry. variability on millennial or shorter time scales began in 1995. Later expeditions included locations around the globe (e.g., ODP Leg 162: North Atlantic-Arctic Gateways; ODP Leg 167: California Margin; ODP Leg 169: Saanich Inlet; ODP Scientiﬁc Accomplishments and Signiﬁcance Leg 172: Northwest Atlantic Sediment Drifts; ODP Leg 202: In a series of important contributions, Bond et al. Southeast Paciﬁc Paleoceanographic transects; IODP Legs (1992, 1993) used legacy cores from DSDP Site 609 to 303 and 306: North Atlantic Climate I and II; and IODP Leg develop a comprehensive record of ice-rafted debris for the 323: Bering Sea Paleoceanography). North Atlantic, identifying major ice-rafted debris events
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50 SCIENTIFIC OCEAN DRILLING 3 5 9 11 47 49 25 31 37 63 1 7 13 15 17 19 21 43 45 55 35 39 51 53 57 59 61 3.5 29 33 41 27 23 δ O (°/ ) °° 4 56 3 18 28 4.5 4 8 14 18 20 10 2 6 12 16 5 Brunhes Matuyama Jaramillo 0 200 400 600 800 1000 1200 1400 1600 1800 MG7 KM3 MG11 MG5 G17 K1 M1 3 G3 G7 G11 MG1 G19 77 81 87 91 97 99 101 G1 G15 75 83 89 93 95 79 73 85 MG6 K2 KM6 δ O (°/ ) MG8 °° 67 G5 G8 G22 3.5 71 MG4 KM2 G20 18 94 102 80 G10 M2 4 66 104 70 Olduvai Gauss Kaena Mammoth 78 4.5 1800 2000 2200 2400 2600 2800 3000 3200 3400 2.6 T5 T7 T3 CN5 ST1 T1 Gi17 Co1 CN1 N1 N5 NS1 Si3 TG1 TG3 TG5 Gi7 2.8 Gi13 Gi21 Gi25 Gi27 CN7 ST3 Gi15 Gi19 Gi1 Gi5 Co3 N9 NS3 Si1 Si5 Gi9 N7 Gi3 Gi23 3 T2 TG4 δ O (°/ ) Co4 °° Co2 CN6 3.2 Gi10 Gi26 N6 NS6 Gi22 18 Gi12 Gi20 3.4 3.6 Gilbert Cochiti Nunivak Sidufjall Thvera 3.8 3600 3800 4000 4200 4400 4600 4800 5000 5200 Time (ka) FIGURE 4.5 The marine oxygen isotope and paleomagnetic record for the past 5 myr. SOURCE: Lisiecki and Raymo, 2005. Reproduced by permission of American Geophysical Union. (Heinrich events) as well-higher frequency changes that cor- site, proving that millennial climate ﬂuctuations are found related with the abrupt air temperature swings observed in not only in the eccentricity-dominated interval of the late Greenland. The close coupling between air temperature and Pleistocene but also in the obliquity-dominated interval of ice-rafted debris strongly suggested that changes in North the Pliocene and early Pleistocene. Atlantic Ocean overturning circulation were related to rapid Scientiﬁc ocean drilling also played a major role in changes in Greenland air temperatures. McManus et al. understanding the far-ﬁeld effects of North Atlantic changes (1994) extended the ice-rafted debris record at the same site at this time by acquiring cores from sites with high sedi- (DSDP Site 609) through the interglacial period 120 kyr ago, mentation rates. The Santa Barbara Basin (ODP Site 893) demonstrating that millennial variability was not limited to provided evidence that interstadial-stadial ﬂuctuations also the glacial climate. At ODP Site 980, McManus et al. (1999) occurred in the eastern Paciﬁc (Hendy and Kennett, 1999), documented pervasive millennial-scale delivery of ice-rafted with colder intervals (stadials) associated with increased debris over the last 500 kyr, commencing when Northern ventilation of the intermediate-depth eastern Paciﬁc (Behl Hemisphere continental glaciers reached approximately 50 and Kennett, 1996). Cariaco Basin (Site 1002) cores recorded percent of their maximum size. Raymo et al. (1998) observed abrupt changes in sediment chemistry and lithology, which, millennial-scale ﬂuctuations in the early Pleistocene at ODP in parallel with the Greenland air temperature record (Figure Site 983, while McIntyre et al. (2001) found 2 to 5 kyr spac- 4.6), reﬂected past changes in evaporation and precipitation ing of ice-rafted debris events in the late Pliocene at the same over northern South America (Peterson et al., 2000a). These
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51 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY observed changes have been attributed to the migration of Coupled ocean-atmosphere models demonstrate that the Intertropical Convergence Zone. Combined with other abrupt reductions in the salinity of the North Atlantic reduce observations of millennial-scale climate ﬂuctuations in the the meridional overturning circulation and cool North Atlan- Mediterranean region (ODP Site 977: Martrat et al., 2004) tic air temperatures (Manabe and Stouffer, 1997). Tempera- and other continental locations (e.g., Hulu Cave speleothem ture and ice rafting patterns in the North Atlantic exhibit this record [Wang et al., 2001]), the synchroneity of millennial- same variability. During periods of higher ice-rafted debris scale climate changes implies that ocean-atmosphere reor- input and greater freshwater delivery into the North Atlantic, ganizations happen quickly and have widespread impact on colder air temperatures prevailed over Greenland and colder temperature and moisture patterns in and beyond much of sea surface temperatures were found in the high-latitude the Northern Hemisphere. North Atlantic. Benthic foraminiferal records also showed that ventilation of the deep North Atlantic Ocean was sig- niﬁcantly reduced (Oppo and Lehman, 1995), helping to Fields of Inquiry Enabled establish strong coupling between ice sheets, atmospheric Rapid advances in understanding the coupled nature of circulation, and ocean overturning. atmosphere-ocean circulation occurred through comparison These combined model-data investigations have been of the observational data of abrupt climate change patterns instrumental in showing strong coupling of freshwater input (many derived from scientiﬁc ocean drilling records) with and reduced meridional overturning in the North Atlantic, the results of high-resolution numerical model simulations widespread cooling in the circum-North Atlantic region, of the coupled ocean-atmosphere system. Observations of and perturbation of atmospheric circulation in the tropics rapid climate changes in Greenland ice cores and in North and monsoonal regions of southern Asia. The close match Atlantic sediments were quickly conﬁrmed in other conti- between numerical simulations and observations from drill nental records, and the patterns were reproduced by coupled cores provide some of the best independent conﬁrmation of ocean-atmosphere simulations of North Atlantic overturning climate model reliability. and its response to variations in freshwater forcing. Marine and continental records from tropical locations documented Goals Not Yet Accomplished latitudinal shifts in the position of the Intertropical Conver- gence Zone (Peterson et al., 2000b) forced by changes in Although this area of inquiry has grown quickly, many North Atlantic surface temperature gradients (Vellinga and unanswered research questions remain. The origin of climate Wood, 2002). variability on millennial scales remains elusive, and there is FIGURE 4.6 Comparison of measured color reﬂectance (550 nm) of Cariaco Basin sediments from ODP Hole 1002C to oxygen isotope composition (δ18O) from the Greenland Ice Sheet Project (GISP) II ice core (Stuiver and Grootes, 2000). Laminated sediments with benthic microfauna (along top) indicate that deposition occurred under anoxic conditions. Deposition of dark sediments occured during warm in- terglacial/interstadial times, and deposition of light-colored bioturbated sediments occurred during colder stadial intervals. Visual tiepoints (denoted by a dashed line) show correlations between cores. SOURCE: Modiﬁed from Peterson et al., 2000b.
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52 SCIENTIFIC OCEAN DRILLING still signiﬁcant debate about potential causes. Although there the role of giant meteorite impacts on the extinction and is a close correlation between rapid climate change and North evolution of life; the signature of stable isotopic anomalies in Atlantic overturning, it is not yet known if the North Atlantic relation to global warming events and biological adaptation; is the cause or a response to external drivers (Broecker et al., and climate change and hominin evolution. 1990; Kleiven et al., 2010; Billups et al., 2011). Photosynthesis by marine phytoplankton accounts for The selection of sites with appropriate sedimentation about one-half of global primary productivity. As shown in rates has lacked geographic coverage and is strongly biased Late Triassic (228 myr ago) to recent records in marine sedi- toward the North Atlantic. Many of the legacy cores are from ments preserved on land and recovered in scientiﬁc ocean locations that were originally chosen for study of orbital- drilling cores, the nature of the phytoplankton has changed scale climate variability and thus often have sedimentation substantially, primarily with major evolutionary radiations rates that are too low for abrupt climate change studies. High- and ecological expansions of dinoﬂagellates, diatoms, and resolution studies of millennial-scale climate variability are coccolithophores. These changes have directly affected likely to remain an important priority for scientiﬁc ocean the composition of ocean ﬂoor sediments. In one speciﬁc drilling for another decade or more. example, diatoms alone account for ~40 percent of marine net primary productivity, ~50 percent of carbon export to marine sediment, and about ~20 percent of CO2 drawdown; CO-EVOLUTION OF LIFE AND THE PLANET although their marine appearance has been documented in A fundamental distinguishing feature of Earth is the the Early Cretaceous record at ODP Site 693 (Gersonde presence of life that modiﬁes planetary processes, including and Harwood, 1990), their expansion as a major ecologi- the composition and properties of the atmosphere, hydro- cal and biogeochemical force occurred during the early to mid-Cenozoic as documented in ODP and DSDP cores2 sphere, and lithosphere. The ~70 percent of the planet that is covered with oceans is both a living reactor of Earth system (Spencer-Cervato, 1999; Rabosky and Sorhannus, 2009). processes and a repository for the ocean ﬂoor sediments that The temporally parallel rise in diatom productivity and the record changes in oceanic life. Scientiﬁc ocean drilling is spread of grasslands have led to a controversial suggestion the best way to access this record in its most pristine form, of a causal link via the silica cycle, which could lower global where it is accessible with minimal alteration and provides CO2 as part of a positive feedback system (Johansson, 1996; the potential to obtain a full history of ocean sediments and Conley, 2002; Falkowski et al., 2004). The record of this is the processes active in and on them. carried in marine organisms via alkenone and boron isotopes Scientiﬁc ocean drilling results, integrated with onshore and other chemical proxies in scientiﬁc ocean drilling cores efforts, have led to radically new concepts of the relation- (DSDP Sites 511, 513, 516, 588, 608, 612, 730, and 803; ships between evolution and extinction in the context of ODP 865, 871, and 872 [e.g., Pearson and Palmer, 2000; climate forcing (such as the PETM), many of which have Pagani et al., 2005b]). direct societal relevance. Others are scientiﬁcally compel- The discovery of an iridium anomaly by Alvarez et al. ling, such as the Chixulub impact and its timing relative (1980), shocked quartz and glass spherules (Bohor et al., to the Cretaceous-Tertiary boundary. For the most part, the 1984), and anomalous fern spore concentrations (Tschudy ocean ﬂoor record extends back well into the Jurassic, with et al., 1984) in terrestrially exposed marine and continental progressively larger areas covered by younger sediments that deposits at the Cretaceous-Paleogene boundary (K-T bound- have been proportionally more densely sampled. Scientiﬁc ary; 65.5 Ma) led to the meteorite impact hypothesis of mass ocean drilling to advance the knowledge of co-evolution of extinction, the ﬁrst testable hypothesis for that event. The life and the planet has been highlighted as a priority in ODP Alvarez discovery led to a concerted effort in exploring ocean and IODP planning documents (e.g., IODP, 2001) and past cores to document the global geographic anomaly distribu- achievements and future needs have been described in recent tion, temporal distribution of similar anomalies, effects on NRC reports such as The Geological Record of Ecological marine ecosystems, and location of the impact. By the 1990s, Dynamics (NRC, 2005) and Understanding Climate’s Inﬂu- many scientiﬁc ocean drilling sites had been found with these ence on Human Evolution (NRC, 2010). phenomena clearly expressed (e.g., ODP Site 1049), not only demonstrating the global distribution of the anomalies (Smit, 1999) and the abrupt nature of the extinctions (e.g., Scientiﬁc Accomplishments and Signiﬁcance DSDP Sites 356 and 384; Thierstein, 1981), but also hinting A large proportion of scientific ocean drilling has at the location of the impact site (e.g., Bohor, 1990). The involved biostratigraphy, as well as organisms that serve impact site at Chicxulub was discovered by geophysics and as ecological proxies or carriers of chemical proxies of coring by Pemex (Penﬁeld and Camargo, 1981), but was environmental change, or as intrinsically important to basic not conﬁrmed until 1991 through analysis of oil drill cores understanding of life on the planet. Major scientiﬁc advance- (Hildebrand et al., 1991), and later ocean and continental cor- ments have been realized in understanding co-evolution of phytoplankton, the atmosphere, and terrestrial ecosystems; 2 See http://services.chronos.org/databases/neptune/index.html.
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53 SCIENTIFIC ACCOMPLISHMENTS: EARTH’S CLIMATE HISTORY ing by the IODP, ICDP, and DOSECC (Drilling, Observation Africa, while DSDP Site 231 and ODP Sites 721 and 722 and Sampling of the Earths Continental Crust) programs in record dust derived from East Africa and Arabia, allowing the early 2000s (see review by Schulte et al., 2010). for the construction of complete composite sequences for the Many additional processes involving the K-T boundary two areas (deMenocal, 1995, 2004). These records show that have been investigated by examination of scientiﬁc ocean North Africa’s continental aridity tracked cold North Atlantic drilling cores, notably the δ13C anomaly (DSDP Site 524; sea surface temperatures associated with Northern Hemi- ODP Leg 207; DSDP Sites 528 and 577; ODP Site 1001A; sphere glaciations, while East Africa’s aridity was inﬂuenced Hsü et al., 1982; Hsü and McKenzie, 1985; D’Hondt et more by Indian Ocean sea surface temperature (NRC, 2010). al., 1998; D’Hondt, 2005; Schulte et al., 2010); impact- Dust records also show similar changes in the frequency of climatic (ice) oscillations seen in the δ18O record. Informa- generated tsunamis (DSDP Sites 536 and 540, Alvarez et al., 1992; ODP Leg 174AX, Olsson, 1997), mass-ﬂow deposits tion from these cores suggests that prior to 2.8 myr ago, the (DSDP Sites 387 and 386; ODP Site 1001; Smit, 1999; African climate was regulated by low-latitude precessional Norris et al., 2000), proximal ejecta (Claeys et al., 2002), (26 kyr) forcing of monsoonal climate. Evolutionary steps of and rhenium-osmium systems and their relation to the Dec- African hominins and other vertebrates occurred with more can Traps (DSDP Sites 245, 525, 577, and 245; ODP Site arid, open conditions near 2.8, 1.7, and 1.0 myr; these times 690; Ravizza and Peucker-Ehrenbrink, 2003; Robinson et are coincident with the changes in the frequency modes and al., 2009). climate shifts. Stable carbon isotopic anomalies have proven to be In addition, freshwater diatom records from equatorial associated with extinction and biotic turnover events. Studies Atlantic core V30-40 (Pokras and Mix, 1987) suggested of the PETM extreme warming event (Box 4.2) demonstrate that hemi-precessional cycles (approximately 10- and 5-kyr that excursions and extinctions were coincident with a shal- cycles) were important to African tropical aridity, which lowing of the carbonate compensation depth due to ocean was conﬁrmed by cores in Lake Malawi (Cohen et al., 2007; acidiﬁcation (Zachos et al., 2005) and an intensiﬁcation of Lyons et al., 2009) that suggest human migrations were the hydrological cycle involving shifts in the distribution tied to orbitally controlled megadroughts. The correlation and intensity of precipitation (Schmitz and Pujalte, 2007). of speciation events with the climate and vegetation shifts Complementary work on the continents showed that there seen in ocean drilling cores has transformed thinking on were latitudinal and intercontinental migrations for both the origins of humans. These hypotheses are guiding the terrestrial plants and mammals at the PETM, including the selection of ocean and continental drilling cores, as well as widespread dispersal of modern mammalian orders (see methodologies to test the hypotheses themselves (e.g., Potts, Bowen et al., 2002; Wing et al., 2003). Although not involv- 2006; Ravelo et al., 2010). ing extinctions of the magnitude of the K-T boundary, the PETM event did involve a massive reorganization of marine Fields of Inquiry Enabled and terrestrial biota with permanent effects and had an inferred forcing (CO2) similar to that of anthropogenic global More than perhaps any single achievement, the culture change (Zachos et al., 2008). of scientiﬁc ocean drilling has changed the way the his- The continuous and detailed records of Cenozoic cli- tory of life has been studied. Organisms are examined fully matic and biotic change recorded in marine sediments and integrated in their environmental and geochemical context, recovered by scientiﬁc ocean drilling have provided envi- sometimes as carriers of chemical environmental proxies, ronmental context for explanations of biotic events on the sometimes as parts of communities, and always as part of an continents, particularly the evolution of humans in Africa integrated stratigraphy in which superposition is unequivo- (NRC, 2010; Ravelo et al., 2010; deMenocal, 2011). While cal. Without scientiﬁc ocean drilling, the impact hypothesis aspects of ocean records integrate global processes such as likely would not have become as forceful a paradigm for oxygen isotope anomalies due to ice volume, others capture extinction processes and certainly not a current mainstay of more regional processes involving dust, freshwater diatoms, modern Earth science education (see Chapter 5). phytoliths, and sporomophs blown from adjacent continents. Understanding the co-evolution of life and Earth was not In particular, Indian and South Atlantic Ocean drill cores an explicit goal in the DSDP and ODP eras but has come to record processes occurring on the African continent, where the forefront with more recent IODP expeditions and recent humans evolved, within a global framework. ODP coring in community workshops (Ravelo et al., 2010), in which life the Mediterranean (e.g., at Site 967) has also given rise to plays a leading role. Another important outcome of this excellent dust records that provide important evidence for research has been the ability to combine the strength of data African continental climate conditions (Larrasoaña et al., from new, speciﬁcally tailored drilling expeditions with the 2003). Marine sediment cores from ODP Sites 659, 661, 662, great value of the ocean drill core repository for comparative 663, and 664 record dust from plumes originating in West analysis and increased global coverage.
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54 SCIENTIFIC OCEAN DRILLING Goals Not Yet Accomplished example, very little new core-based progress has occurred in understanding the roles of LIPs in biotic change or the overall The integrated approach to understanding the Earth-life structure of the Chixulub crater. Scientiﬁc ocean drilling may system exempliﬁed by scientiﬁc ocean drilling has resulted also continue to contribute to understanding the processes in a spectacular understanding of some of the largest biotic that link climatic and evolutionary events in hominin evolu- changes the planet has seen in the past 200 myr. Although tion. Finally, the effects of the evolution of new life forms some initial discoveries, such as the K-T impact, occurred and new physiological modalities on biogeochemical cycles on land, deeper understanding was achieved by the contex- has not been examined in scientiﬁc ocean drilling studies; tual approach provided by sediments preserved in the ocean organisms and their physiology are a ﬁrst-order control on basins. However, a number of scientiﬁc ocean drilling-related processes such as oxygenation, terrestrialization, agronomic goals for the Earth-life system have yet to be realized. For revolution, human culture, and technology.