SCIENTIFIC OBJECTIVESThe primary scientific goal of paleoceanographers is to assess mechanisms of climate change as revealed by long-term changes in the ocean-climate system. In this regard, the role of the South Pacific is essentially unknown. The Leg 202 drilling experiment seeks to probe the climate system in three different, but compatible, ways. First, we seek to document the effect of tectonics on climate change, specifically the events associated with the opening of Drake Passage, the closing of the Panama Isthmus, and the uplift of the Andes Mountains. Second, we will document the Southern Hemisphere Eastern Boundary Current, eastern equatorial current, and subsurface water mass histories associated with the transition of climate regimes from one of polar warmth to one of glaciation in Antarctica, and eventually into the rhythmic Pleistocene ice ages of both the Northern and Southern Hemisphere. Third, we will seek a record of millennial-scale climate changes to assess the extent of rapid climate events as a test of two contrasting hypotheses. The first hypothesis is that rapid changes originate in the high North Atlantic Ocean and propagate elsewhere through the climate system, and the second hypothesis is that such changes originate in the low latitudes and propagate poleward.
Tectonic Impacts on Ocean Circulation and Climate
One of the most perplexing questions in climate is why global climate over the past 40 m.y. changed from very warm conditions (the "Greenhouse World") to conditions of unipolar (Southern Hemisphere) and later bipolar glaciation (the "Icehouse World"). Partial answers include plate tectonic processes, elevation and erosion of vast plateaus, opening and closing of oceanic gateways and changing concentrations of atmospheric greenhouse gases. The opening and closing of oceanic gateways are especially critical. The southeast Pacific Ocean is a key location to examine the response of the climate system to tectonic events, including the closure of the Isthmus of Panama, the opening of the Drake Passage, and the Neogene uplift of the Andes Mountains.
The opening of both the Australian-Antarctic gateway and the Drake Passage led to the development of the circumpolar current and consequently to a thermal isolation of Antarctica. This configuration is thought to be the ultimate cause for the initiation and continuation of Antarctic glaciation since about 35 Ma. This early cooling, including dramatic cooling of the deep sea (Kennett and Shackleton, 1976), and modified nutrient distributions within the global oceans (Brewster, 1980) are thought to reflect the slow drift of Australia away from Antarctica, which tended to isolate the southern continent (Kennett, 1974, 1977). However, considerable evidence exists for a much later opening of the Drake Passage, near 25 Ma (Barker and Burrell, 1977), which would contradict the hypothesis that the establishment of the Circum-Antarctic Current acted as final trigger for the Antarctic glaciation. Curiously, major expansion of ice in Antarctica is thought to have occurred (based on oxygen isotope and other data) near 15 Ma, 10 m.y. after the opening of Drake Passage. Thus, the climatic impact of Drake Passage opening, which created the Circumpolar Current, remains unknown. Numerical models predict a decrease of the Chile-Peru surface current and a distinct decrease in Antarctic deep and bottom water export to the north as water masses are entrained into a growing circumpolar circulation (Mikolajewicz et al., 1993). Site SEPAC-9A from the Chile Basin and Sites NAZCA-10A and 17A will provide an excellent opportunity to examine the timing and impact of the Drake Passage opening on the proposed changes in ocean circulation.
The Neogene tectonic closure of the Central American Isthmus from 13.0 to 2.7 Ma (Duque-Caro, 1990; Collins et al., 1996) resulted from the subduction of the Pacific, Cocos, and Nazca plates beneath the North and South American plates and, later, the Caribbean plate. The closure has always been an attractive candidate for the ultimate cause of the Pliocene intensification of the Northern Hemisphere glaciation since ~3.1 Ma (Mikolajewicz and Crowley, 1997). Closure-induced changes in global thermohaline circulation have been invoked to be the cause either for the onset (Berggren and Hollister, 1974) or for the delay (Berger and Wefer, 1996) or for setting the preconditions of the Northern Hemisphere glaciation (Haug and Tiedemann, 1998; Driscoll and Haug, 1998).
While the link between the isthmus closure and the Northern Hemisphere glaciation is still a matter of debate, recent studies clearly identify a close link between the formation of the Panama Isthmus and major oceanographic changes that occurred between 4.6 and 4.2 Ma, when the Panamanian sill shoaled to a water depth of <100 m (Haug and Tiedemann, 1998). The chain of evidence suggests the development of the modern Pacific-Atlantic salinity contrast of ~1 (Haug et al., 2001), a reorganization of the equatorial Pacific surface current system, the intensification of upper North Atlantic Deep Water formation, and the development of the modern chemical Atlantic-Pacific asymmetry, which is reflected in a strong increase in Caribbean/Atlantic carbonate preservation and a remaining strong carbonate dissolution in the Pacific (Haug and Tiedemann, 1998; Haug et al., 2001; Farrell et al., 1995; Cannariato and Ravelo, 1997). This major salinity contrast between ocean basins, driven in part by net freshwater transport as vapor across the Panama Isthmus, is likely responsible for maintaining the global thermohaline "conveyor belt" circulation, which is dominated by North Atlantic Deep Water (Gordon, 1986; Broecker, 1991). Changes in the equatorial surface current system will be monitored by sites from the Cocos and Carnegie Ridges. Changes in intermediate water chemistry that result from a restricted exchange of water masses through the Panamanian gateway because of late Miocene to early Pliocene shoaling of the sill depth will be registered by a comparison of sites from the Caribbean and Leg 202 (CAR-1C and NAZCA-10A).
The uplift of the Andes is expected to have caused significant changes in atmospheric circulation, wind-driven oceanic surface circulation, and hence, productivity. Marked changes in the uplift history of the Andes were detected during Leg 154 (Ceara Rise) by drilling the Atlantic side of South America. The increase in the Amazon River supply of terrigenous sediments and its change in clay mineralogy indicate major uplift phases from 12 to 8 Ma and since ~4.6 Ma (Curry, Shackleton, Richter, et al., 1995). This is consistent with paleobotanical reconstructions from the Central and Columbian Andes Mountains, assuming major phases of uplift since 10 Ma (Gregory Wodzicki, 2000) and 4.6 Ma (Hooghiemstra and Ran, 1994; van der Hammen et al., 1973). The early Pliocene phase is paralleled by the subduction of the Cocos Ridge at ~5 Ma, which formed during the passage of the Cocos plate over the Galapagos hotspot, and dramatically elevated the Central American volcanic arc and led to the final phase of the closure of the Isthmus of Panama (Dengo, 1985; Hoernle et al., pers. comm., 2001). If this hypothesis is true, analogous changes in clay mineralogy are expected in the hemipelagic sediment records of Leg 202. Unfortunately, the land record can not be used to assess the details of Neogene water mass changes that respond to mountain uplift, as the land record of marine sediments contains major hiatuses, for example, in Chile between 10 and 3.5 Ma (Martinez-Pardo, 1990). The hemipelagic sites contain significant terrigenous clay and will provide opportunities for understanding climatic and tectonic evolution on land as well as changes in marine environments.
Orbital and Polar Ice Sheet Impacts on Ocean Circulation and Climate.
Sites to be drilled during Leg 202 will provide an extraordinary opportunity to examine the details of regional climate responses to the onset and amplification of Pleistocene ice age cycles at orbital scales over the last 5 Ma. Evidence from the Southern Ocean (Imbrie et al., 1993) and from the equatorial Pacific Ocean (Pisias and Mix, 1997; Lea et al., 2000) suggest that near-surface changes in these areas precede those at high northern latitudes. Thus, climate changes here do not passively respond to Northern Hemisphere glaciation but could be part of the chain of responses that lead to Northern Hemisphere glaciation. Especially important for Leg 202 will be to assess the linkages between changes observed at higher southern latitudes (e.g., SEPAC-9A, 13A, and 14A) with those along the equator (CAR-1C, CAR-2C, and PAN-2). Site NAZCA-17A will provide a useful monitor of the advective link between the high and low latitudes, by monitoring the strength of the cool Humboldt Current.
Waters advected off the eastern boundary have a profound influence on the equatorial Pacific Ocean and have a significant role in maintaining the equatorial cold tongue (Bryden and Brady, 1985; Liu and Huang, 2000). The Peru-Chile Current and its extension into the equatorial Pacific Ocean are thought to have been stronger than at present during the last glacial maximum (LGM) (CLIMAP, 1976; Pisias and Mix, 1997; Mix et al., 1999b; Feldberg and Mix, in press). This current may have acted as a conduit for cold near-surface water from high latitudes to enter the cool tongue of the South Equatorial Current (SEC). Assessing the relative roles of advection of cold low-nutrient surface waters vs. upwelling of higher-nutrient subsurface waters into the equatorial Pacific Ocean are important because upwelled waters create very high surface-water pCO2. Advected surface water would be a weaker CO2 source. At present, the equatorial Pacific Ocean is a large net source of CO2 to the atmosphere (Tans et al., 1990), but the magnitude of this source has changed through time (Jasper et al., 1994), perhaps with global consequences. Understanding both the history of water masses and biological production (Mix, 1989) will be important in assessing the role of the eastern equatorial Pacific Ocean in the long-term balance of atmospheric CO2.
Significant changes in the SEC were inferred from ODP Site 846 (3°S, 91°W; 3307 m depth), which was drilled during Leg 138 to examine the interaction of eastern boundary waters with the equatorial currents. Quaternary planktonic foraminifers found here (Le et al., 1995) vary between a warm subtropical fauna (Globerinita. glutinata and Globigerinoides sacculifer), a cool-water fauna advected off the Peru margin (Globigerina bulloides), and higher-latitude fauna (Globorotalia inflata) that reflect northward advection in the Eastern Boundary Current. The obvious next step is to examine relationships between the tropical Pacific Ocean and the eastern boundary of the southeast Pacific Ocean in a latitudinal transect within the Peru-Chile Current, with targets in late Pleistocene ice ages and in older preglacial times.
Another climatic puzzle that has long interested paleoceanographers is the apparent regional stability of subtropical climates during the LGM (CLIMAP, 1981) and the apparent differences in the timing and amplitude of climate changes at the poles relative to those observed near the equator (e.g., Imbrie et al., 1989). These results have been questioned, as some data and models suggest that climate changes at the poles (especially the expansion of polar ice sheets and sea ice) and changes in greenhouse gases should influence global climate (Pinot et al., 1999). A legitimate concern regarding the low amplitude of change in the CLIMAP study of the LGM was that many samples from the subtropics had very low sedimentation rates, to the point that bioturbation could suppress real changes by mixing together glacial and interglacial fossil assemblages. Leg 202 may be able to address the controversy of the stable subtropics in a different way, by examining the response of the Southern Hemisphere subtropics to long-term growth of glaciation in Antarctica (with major expansion of ice near 15 Ma) and again to expansion of global glaciation within the past ~5 Ma. Sites SEPAC-9A, NAZCA-10A, and NAZCA-17A are especially well suited to this study, as they will span paleolatitudes from ~17° to 41°S over a broad time range.
Within Pleistocene time, the origin of the large 100-k.y. climate cycle at ~1 Ma remains puzzling. A number of mechanisms have been proposed, including a threshold response of high-latitude glaciers to gradual long-term cooling associated with uplift of mountain ranges (Ruddiman and Raymo, 1988) or reduction of greenhouse gases (Maasch and Saltzman, 1990), erosion of soft sediment below Northern Hemisphere glaciers to expose bedrock, allowing larger glaciers to grow by increasing basal friction (Clark and Pollard, 1998), atmospheric loading of cosmic dust to trigger a response to rhythmic changes in the plane of Earth's orbit (Muller and MacDonald, 1997), and long-term cooling of the deep sea at polar outcrops, which influenced sea-ice distributions (Gildor and Tziperman, 2001). A 100-k.y. cycle of climate may also originate independently of polar climate changes via a nonlinear response of tropical climate systems to orbital changes in seasonal insolation (Crowley et al., 1992). Evidence exists for rhythmic 100-k.y. cycles of sedimentation in the eastern tropical Pacific Ocean, which if climatically significant could have provided a "template" for a climate cycle that was later picked up by the global ice sheets (Mix et al., 1995). A range of evidence suggests that tropical climate changes at orbital scales preceded those of the Northern Hemisphere ice sheets and must vary independently of the high northern latitudes (Imbrie et al., 1989, McIntyre et al., 1989; Pisias and Mix, 1997; Harris and Mix, 1999; Lea et al., 2000).
Millennial-Scale Climate ChangesPolar or Tropical Causes?
A primary initiative for ODP is to understand the causes and consequences of millennial-scale climate change. Such changes are well documented in rapidly accumulating North Atlantic Ocean sediments (e.g., Labeyrie and Elliot, 1999; Clark et al., 1999). This has led to hypotheses that millennial-scale changes are driven either by instabilities in Northern Hemisphere ice sheets that surround the North Atlantic Ocean (MacAyeal, 1993) or by oscillations in the formation of North Atlantic Deep Water (Broecker et al., 1990) or, in some cases, perhaps to rhythmic solar forcing (Bond et al., 1997).
Similar millennial-scale climatic oscillations have been detected in the northeast Pacific Ocean (e.g., Hendy and Kennett, 1999, 2000), suggesting the transmission of rapid climate change events from their North Atlantic origins into the North Pacific Ocean (Mikolajewicz et al., 1997). An alternative hypothesis (though perhaps not mutually exclusive) is that rapid climate oscillations may emanate from the tropics, where they originate as an unstable response to insolation (McIntyre and Molfino, 1996), perhaps as oscillations in the frequency of El Niño Southern Oscillation (ENSO) events of the eastern tropical Pacific Ocean (Cane and Clement, 1999). This hypothesis is plausible given lake and ice core records from South America that suggest long-term changes in the mean state of ENSO events (Rodbell et al., 1999; Thompson et al., 1998), glacial-interglacial sea-surface temperature changes that mimic spatial patterns of change associated with modern La Niña events (Pisias and Mix, 1997; Mix et al., 1999b; Beaufort et al., 2001), and model results suggesting sensitivity of long-term average oceanic condition to changes in El Niño frequency, forced by orbital insolation (Clement et al., 1999). On an interannual scale, Liu and Huang (2000) argue based on modern heat budgets that warming in the region over the past 50 yr is related to reduction of the wind-driven advection of cool water off the eastern boundary and that such effects that dominate the eastern tropical Pacific extend even to the western Pacific Ocean and, perhaps, elsewhere. If this tropical hypothesis is true, we expect that millennial-scale climate events will be clearly recorded in sites with high sedimentation rates from the eastern equatorial Pacific Ocean, particularly near the Galapagos region, which is the area to be sampled at Site PAN-2A.
Denitrification associated with low-oxygen water masses in the eastern Pacific Ocean produces N2O, consuming nutrients, presenting a net source to the atmosphere. As this region is the largest center of denitrification in the global ocean it is likely to dominate the global budget. This may open a unique opportunity in high-resolution paleoclimatology. If variations in the strength of the oxygen minimum (recorded by geochemical tracers and benthic faunal assemblages) (e.g., Phleger and Soutar, 1973; Oberhänsli et al., 1990) and denitrification (detected via nitrogen isotope analyses of organic matter) (Ganeshram et al., 1995) in the eastern tropical Pacific Ocean prove to be responsible for a substantial portion of global variations in N2O observed in polar ice cores, the rich archives of paleoclimatic variations recorded in these two areas may be synchronized at high resolution.
Recent data, especially from the northeast Pacific Ocean (Behl and Kennett, 1996; Lund and Mix, 1998; Mix et al., 1999a) affirms the importance of understanding Pacific deep and intermediate water circulation on millennial timescales. Broecker (1998) points to the existence of a "bipolar seesaw" effect, in which millennial-scale changes in Antarctic temperature are out of phase with (leading) Northern Hemisphere events. This inference is buttressed by data from the Southern Ocean (Charles et al., 1996) and by comparison of ice-core 18O or D/H data from both hemispheres, synchronized with the methane record (Blunier et al., 1998). Because the patterns of change are significantly different in the Northern and Southern Hemispheres, we can use the pattern of ventilation events on the millennial scale as a "fingerprint" of their source.
ODP Site 893 from the Santa Barbara Basin provides compelling evidence of millennial-scale events of enriched oxygen content (i.e, unvarved sediments containing oxic benthic foraminiferal assemblages) that are approximately correlated with cold events in the North Atlantic Ocean (Kennett and Ingram, 1995; Behl and Kennett, 1996; Cannariato et al., 1999). The process driving such changes, however, remains uncertain. Variations in the bottom waters of the Santa Barbara Basin may reflect ventilation of intermediate water originating either in the North or South Pacific Ocean, changes in local productivity and its effect on oxygen content within the basin, or changes in regional productivity of the North Pacific Ocean that change the character of the oxygen minimum zone. Ventilation from the north is certainly possible and would be consistent with modeling studies that link NPIW ventilation to North Atlantic cooling during the Younger Dryas (Mikolajewicz et al., 1997). Other sites along the California margin and the Gulf of California show millennial-scale variations in intermediate and surface water properties, but limitations in dating so far preclude a definitive link to Younger Dryas cooling (van Geen et al., 1996; Keigwin and Jones, 1990).
Radiocarbon-dated evidence of organic carbon at ODP Site 1019 (980 m water depth off Northern California) indicates that regional productivity effects could contribute to variations in the oxygen minimum zone off California (Mix et al., 1999a). At this site, benthic foraminiferal 13C values suggest ventilation of intermediate water masses during the transition from warm (Bølling-Allerød) to cold (Younger Dryas) climate events (i.e., intermediate water ventilation leads to cooling). This apparent phasing between surface temperature and benthic 13C values within the same samples is important because it suggests that the ventilation event at intermediate depths of the North Pacific Ocean may lead the Younger Dryas cooling. Thus, evidence from ODP Site 1019 suggests that it is unlikely that intermediate water ventilation responds directly to North Atlantic cooling during the Younger Dryas event, as predicted for NPIW by some climate models (Mikolajewicz et al., 1997). The event with high benthic 13C values is from ~14.5 to 13 ka, similar to the timing of the Antarctic Cold Reversal as recorded in the Byrd ice core of Antarctica (Blunier et al., 1997). Flushing with AAIW, consistent with the LGM model circulation of Campin et al. (1999) might explain this event.
Leg 202 sites will contribute to our understanding of millennial-scale climate changes by providing records of change in high-sedimentation rate sites within the equatorial cold "tongue" (Site PAN 2A) and off central Chile (Sites SEPAC-13A, SEPAC-14A, and, if drilled, alternate Site SEPAC 19A). Site PAN-2A is ideally located to assess rapid variations in equatorial upwelling, perhaps related to long-term stability of ENSO events. The SEPAC sites are particularly well located for assessing variations in the strength of AAIW through time. Sedimentological evidence suggests that rapid climate changes are recorded off central Chile (Lamy et al., 1999) and are supported by evidence for millennial-scale oscillations in Andean glaciers (Lowell et al., 1995).
Leg 202 may provide materials for several studies not closely linked to its primary paleoceanographic objectives.
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