Ocean Drilling Program, United States

The scientific objectives of Leg 202 are paleoceanographic: to assess climate and oceanographic changes in the southeast Pacific over Neogene time. The drilling experiment contains three major elements, to probe the climate system at three different but compatible scales: tectonic (millions of years), orbital (tens to hundreds of thousands of years), and millennial (thousands of years). On these three timescales, drilling will test a broad set of hypotheses on the following: 1. Linkages between high- and low-latitude climate changes in the Southern Hemisphere via the Eastern Boundary Current on orbital and millennial scales; 2. The response of the South Pacific Ocean to major tectonic and climatic events, such as the opening of the Drake Passage (creating a circumpolar current), closing of the Panama Isthmus (separating the Atlantic and Pacific Oceans), the uplift of the Andes Mountains (modifying wind systems), and major expansion of ice sheets in the high latitudes of the Southern and Northern Hemispheres at different times; and 3. The role of biological productivity and physical ventilation of water masses in modifying subsurface water masses near the eastern boundary, as reflected in biotic and geochemical composition of nutrients and oxygen in subsurface water masses. Our operational goals are to maximize the depth range of sites to sample different parts of the water column, to place the sites reasonably close to South and Central America to maximize sedimentation rates, and to assure good Neogene coverage of the Eastern Boundary Currents in a latitudinal transect, while avoiding as much as possible the influence of local tectonic deformation and input of turbidites. Some sites target low sedimentation rates to obtain long sequences of climate change in early Neogene time that are not subject to severe burial diagenesis. Others target higher sedimentation rates to assess orbital-scale climate oscillations at a resolution suitable for tuning of timescales and examination of changing responses to orbital forcing in late Neogene time. Finally, a few sites target rapidly accumulating sediments near the equator and in the midlatitudes to assess millennial-scale climate oscillations of the Southern Hemisphere. In all cases, we plan to recover and verify continuous sedimentary sections by drilling multiple advanced piston coring (APC) holes at each site. We expect to find a good record of biogenic sediment components, geochemical tracers of surface and subsurface water masses, and in some sites a record of terrigenous sediment components in hemipelagic and pelagic sediments.