Leg 202 set out to study the oceanic, climatic, and biogeochemical systems in the southeast Pacific. Drilling probed these systems on scales that range from tectonic (millions of years) to orbital (tens of hundreds of thousands of years), and centennial to millennial (hundreds of thousands of years). The experiment addressed hypotheses on (1) the response of the South Pacific to major tectonic and climatic events, such as the opening of the Drake Passage, uplift of the Andes Mountains, closure of the Panama Isthmus, and major expansion of polar ice sheets; (2) linkage between climate changes in the high latitudes and the equatorial Pacific related to rhythmic changes in Earth's orbit and the relationship of such changes to known glacial events of the Northern Hemisphere; and (3) regional changes in climate, biota, and ocean chemistry on scales of centuries to millennia. The southeast Pacific is a relatively unstudied area, so Leg 202 opened study of new regional elements of the global climate system.
A total of 7081 m of sediment was recovered at 11 sites that form a latitudinal transect from 41°S to 8°N as well as a transect of intermediate to deep water masses at water depths of 490-4070 m. Sediments range in age from early Oligocene (~31.5 Ma) to Holocene (Tables T1, T2; Fig. F2). Operational successes during Leg 202 include the ability to make real-time drilling decisions by using a rapid-scanning core logging track. Based on the availability of these logging data, efficient planning of operations made time for extensive drilling over of APC cores that improved recovery. As a result, at all sites, we documented essentially complete recovery in some of the longest APC-cored intervals in ODP history (six holes >250 m and three holes >300 m). In some instances, recovery was nearly complete in double XCB-cored intervals at even greater depth. Other innovative operations included the use of a nonmagnetic core barrel, which significantly improved the quality of shipboard paleomagnetic data. The combination of detailed magnetic stratigraphies with excellent biostratigraphies based on all major microfossil groups provided a unified chronologic framework at sites that range from cool subtropical to warm tropical settings.
The diversity of sites as a function of latitude, water depth, sedimentation rate, and biological production of overlying waters yielded a unique opportunity to understand the role of organic matter oxidation on interstitial water chemistry. These findings provide significant geochemical constraints on the interpretation of climate and biogeochemical changes from sedimentary records. For example, sites with complete sulfate reduction (1232-1235, 1239, and 1242) are likely to be influenced more heavily by authigenic carbonate remineralization. Sites with little or no sulfate reduction (1236, 1237, and 1241) have very little remineralization of carbonates and offer opportunities for development of exceptionally long peleoceanographic records. Two shallower sites on the Chile margin (1233 and 1235) gave clear evidence of methane hydrates based on chloride decreases in interstitial waters, whereas one site (1240) revealed active fluid flow in the underlying basement.
The sedimentary imprint of the gradual plate tectonic drift of sites relative to the continental margin is clear, but superimposed on these long-term trends are substantial variations in local environments that drive changes in the production and preservation of biogenic sediment components and modify the supply of terrigenous sediments. For example, the slow drift of Site 1237 (Nazca Ridge) toward South America resulted in increasing terrigenous and opaline sediments at younger ages. Noncarbonate sediments also generally increased at younger ages at Sites 1238 and 1239 (Carnegie Ridge) as they approached South America, but gradual changes were punctuated by high sedimentation rates ~2-5 Ma. In contrast, Site 1241 (Cocos Ridge) recorded a long-term decrease in noncarbonate sedimentation at younger ages, as this site drifted to the northeast away from the productive equator.
An interval of low carbonate accumulation from 11 to 9 Ma has been referred to in the equatorial Pacific as a carbonate-crash event of poor preservation, perhaps related to deep-sea circulation. Our finding of low carbonate accumulation within this interval at Site 1236, which occupied relatively shallow paleo-water depths (<1000 m), suggests that low production may also have contributed to the carbonate-crash event.
A stepwise increase in terrigenous dust flux (Site 1237) and hematite contents (Site 1236) since 8 Ma may reflect a critical threshold in the history of Andean uplift, which strengthened the equatorward trade wind circulation and enhanced coastal upwelling and productivity. At approximately the same time, discrete ash layers began to appear at Site 1237, perhaps recording the onset of intense volcanism that accompanied the tectonic uplift of the Andes. More than 200 volcaniclastic horizons were deposited during the last ~9 m.y. at this site. Maxima in ash layer frequency occurred at ~8-6 Ma and during the last 3 m.y.
An interval of distinctly high carbonate accumulation rates, with an essentially synchronous onset near 7 Ma at Sites 1236-1239 and 1241 and at other equatorial Pacific sites (Legs 130 and 138), is thought to represent a widespread Miocene interval of high production and is often referred to as a biogenic bloom. Our finding of this carbonate bloom event at all water depths confirms production as the cause. Furthermore, our discovery of the event in the southeast Pacific proves that the event is not uniquely associated with equatorial upwelling as some previous hypotheses suggested, but instead may be linked to substantial changes in the eastern boundary current system or regional nutrient budgets.
The termination of the bloom event appears to be diachronous, as early as 5 Ma at the deepwater sites (e.g., Sites 849 and 850 drilled during Leg 138) but as late as 3 Ma at Site 1238 and, perhaps, even 2 Ma at Site 1239, two relatively shallow sites on Carnegie Ridge. Such diachroneity associated with water depth points to the influence of decreasing carbonate preservation through Pliocene time, perhaps a response to shoaling and final closure of the Panama Isthmus and the development of the global "conveyor belt" deep circulation between the Atlantic and Pacific.
Most of the sequences recovered during Leg 202 reveal clear lithologic changes in meter and decimeter scales that are most likely related to orbitally induced changes in precession and obliquity. If so, these cycles provide a basis for (1) developing orbitally tuned age models and (2) testing the phase relationships between major climate and oceanographic components to assess the role of South Pacific oceanography and biogeochemistry within a chain of climate forcing mechanisms related to orbital changes in the capture of insolation. An apparent ~400-k.y. cycle of lithologic change at several sites, especially prior to 1 Ma, is consistent with low-latitude climate oscillations driven by orbital precession, which can induce such long-period variations from responses to shorter-period orbital changes.
An especially exciting result of Leg 202 was the successful recovery of unprecedented ultra-high-resolution records from the Chile continental margin between 41° and 36°S at water depths between 490 and 1115 m (Sites 1233-1235). Shipboard stratigraphic data indicate that these records have exceptionally high sedimentation rates of up to 160 cm/k.y., apparently driven by extremely high terrigenous sediment supply in response to heavy continental rainfall. Dominance of siliciclastic sediments here also yielded a highly refined record of paleomagnetic variations. The Laschamp magnetic event at ~41 ka is particularly pronounced (e.g., covering an interval of ~2 m at Site 1233). These extraordinary paleomagnetic records provide opportunities for high-resolution regional and global correlation of marine and terrestrial records using paleomagnetic secular variation and paleointensity variation. This synchronization strategy will help to establish the relative phasing of millennial-scale climate changes in the Southern and Northern Hemispheres and offer the potential for fundamental advances in understanding the dynamics of Earth's magnetic field. Together with a rich array of well-preserved biogenic and mineralogic tracers of paleoclimate utility, these sites provide a unique chance for understanding the role of South Pacific oceanography and Southern Hemisphere climate in a global context on scales from millennia to centuries, and perhaps even decades.
In the tropics, rapid climate change and oceanographic changes on the scale of centuries and millennia are recorded at Site 1240 in the Panama Basin and at Site 1242 on Cocos Ridge. Both sites provide a complete stratigraphic sequence of the last ~2.6 m.y. with sedimentation rates varying in the range of at least 5-17 cm/k.y. Persistent decimeter- to meter-scale variability in core logging data (bulk density and magnetic susceptibility) are mainly related to changes in the relative supplies of carbonate, biosiliceous, and siliciclastic material and are tentatively interpreted to reflect millennial-scale changes in equatorial productivity and/or climate.
As with most ODP legs that focus on paleoceanographic objectives, rigorous tests of hypotheses must await detailed shore-based studies. Isotopic, geochemical, magnetic, faunal, and floral studies are planned. These studies will refine chronologies based on improved biostratigraphies, detailed tuning of lithologic and isotopic data to known changes in Earth's orbit, and linkage to radiometric dates via the paleomagnetic record. With these high-resolution data, refined reconstructions of sediment mass accumulation rates will lead to better understanding of variations in production and preservation of biogenic sediments. Changes in the input of terrigenous sediment components to the ocean will reveal new information about climatic and tectonic influences on continental erosion and explosive volcanism. Detailed studies or organic components, and their nitrogen and carbon isotopic character, along with biotic tracers of productive upwelling systems such as diatom species, will yield insights into nutrient cycling in the region. Analysis of subsurface water masses using isotopic, geochemical, and faunal analyses will help us to understand the role of the South Pacific in the global circuit of matter masses that is thought to influence global climate and biogeochemistry.
Leg 202 has opened a new window into understanding global environmental change, by providing high-quality sediment sequences from a previously unsampled region, by targeting sites that record variations on timescales ranging from decades to tens of millions of years, and by analyzing transects of both depth and latitude. We expect that this strategy will establish the linkages between a broad range of systems and will reveal the role of the southeast Pacific in global climate.