More than 7 km of long and relatively continuous sediment sequences from 11 sites in the southeast and equatorial Pacific were recovered during Leg 202 for the study of the Earth's climate and biogeochemical systems on scales that range from tectonic (millions of years) to orbital (tens to hundreds of thousands of years) and centennial to millennial (hundreds to thousands of years). These materials will be used to test a broad set of hypotheses on (1) the response of the South Pacific Ocean to major tectonic and climatic events, such as the opening of the Drake Passage, uplift of the Andes Mountains, closure of the Isthmus of Panama, 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 well-known glacial events of the Northern Hemisphere; and (3) global and regional changes in climate, biota, and ocean chemistry on scales of centuries to millennia.
Three sites (1236, 1237, and 1241) targeted sequences with relatively low sedimentation rates of <30 m/m.y. to obtain long records of climate change representing the Neogene and, in some cases, the late Paleogene that are not subject to severe burial diagenesis. Two sites (1238 and 1239) targeted moderate sedimentation rates of 3080 m/m.y. to assess orbital-scale climate oscillations at a resolution suitable for the tuning of timescales and examination of changing responses to orbital forcing during the late Neogene. Six sites (1232 through 1235, 1240, and 1242) recovered sediments that accumulated rapidly, at rates of 802000 m/m.y., near the equator and in the southern latitudes to assess equator-to-pole climate linkages at both millennial and orbital scales.
Drilling strategy and near real-time stratigraphic correlation played a significant role in the successful recovery of these sequences. Drilling multiple holes at each site and extensive use of overdrilling with the advanced hydraulic piston corer (APC) provided long records with continuous recovery. Innovative use of rapid core logging allowed for real-time optimization of drilling strategies that maximized recovery and minimized redundant coring. Analysis of core expansion, as well as core-log integration and double-extended core barrel (XCB) coring at some sites facilitated the assembly of cores into a depth framework that will improve the quantitative analyses of sediment accumulation rates.
On Nazca Ridge, Site 1237 provides a continuous sediment sequence, recovered in overlapping APC cores, that spans >30 m.y. (modern to middle Oligocene). Exceptional preservation of the flora and fauna in this long, continuous record indicates that this site will provide a much-needed stratigraphic reference in the southeast Pacific. Abrupt changes in the presence of volcanic ash layers here document an increase in tectonic activity during the late Miocene, while at the same time an increase in biogenic components associated with productive upwelling systems, such as diatoms, are associated with late Cenozoic cooling. Site 1236 provides an equally good record from shallower water depths that, when paired with Site 1237, will document variations of deep and surface water masses in the subtropical South Pacific.
Near the equator, Sites 12381241 provide evidence for rhythmic oscillations of pelagic and hemipelagic sediments on the scale of Earth's orbital cycles, which will help to test the hypothesis on tropical vs. polar origins of the well-known 100-k.y. climate cycle that characterizes the late Pleistocene, as well as the response of the equatorial Pacific to closure of the Isthmus of Panama. Again, complete recovery of long and well-preserved sediment sequences will provide unprecedented resolution of biotic and environmental changes.
Century- to millennial-scale climate changes can be addressed with the records from rapidly accumulating (40200 cm/k.y.) sediments recovered at Sites 12331235 from the central Chile margin. These sites will provide important data related to the southern westerlies and Antarctic Intermediate Water variability. A detailed record of paleomagnetic intensity and secular variability will link these records into a global chronological framework. At Site 1232, in the Chile Basin, we also recovered a rapidly accumulating sequence of Pleistocene sediments, which documents terrigenous sediments eroded from the southern Andes and transported to the deep via turbidity currents. Near the equator, Sites 1240 and 1242 have moderately high sedimentation rates (~813 cm/k.y.), which will help to test linkages of millennial-scale climate changes between low and high latitudes.
Together, the array of sites recovered during Leg 202 provides a new view of Southern Hemisphere and tropical climate variability and biogeochemical systems across a broad range of spatial and temporal scales in a region of the ocean that has received relatively little study in the past.
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