Leg 202 set sail in 2002 with the goal of probing the climate system at three different scales: tectonic (millions of years), orbital (tens to hundreds of thousands of years, and millennial (thousands of years). In all three time frames, the expedition focused on three goals:
In 4 years of postcruise study, a large group of scientists has made substantial progress toward these goals. Significant results now, that set the stage for further efforts, include establishment and refinement of stratigraphies on all scales.
On the tectonic scales, it appears that the dominant regional response of southeast Pacific eastern boundary current postdates the rise of the Andes prior to 7 Ma. Instead, major cooling occurs roughly synchronously with expansion of polar ice sheets younger than ~3 Ma. The approximate hemispheric symmetry of such cooling implies a global mechanism, such as greenhouse forcing, rather than a purely regional response to Andean uplift. An exception to this finding is that terrigenous dust, most likely from the Atacama Desert, does appear to increase following uplift and then increase further as the ocean cooled.
The primary eastern Pacific response to tectonic closing of the Central American Isthmus is shoaling of the tropical thermocline between 5.3 and 4.0 Ma, followed by significant reduction of sea-surface salinity after 3.7 Ma. We infer that these sequential changes in the eastern Pacific system reflect sequential responses to a gradual (and perhaps not monotonic) restriction of the CAS. The thermocline rise most likely occurred in response to partial restriction, whereas the sea-surface salinity response awaited final closure, when surface waters were no longer able to flow eastward.
On orbital scales, Leg 202 provided dramatic evidence for the growth of 100-k.y. climate cycles during middle Miocene cooling (14.7–12.7 Ma), which in some ways mirrors the reestablishment of similar climate cycles during the MPT (1.2–0.7 Ma). As with the late Pleistocene cycles, greenhouse gas forcing may play a role in the onset and maintenance of these striking climate oscillations.
Within the Pliocene–Pleistocene interval, preliminary results provide some unresolved puzzles in which variability south of the Equator appears to be dominated by 41-k.y. climate cycles related to orbital obliquity, whereas variability north of the Equator tracks 23-k.y. cycles related to orbital precession. One possible scenario is that the SEC is sensitive to high-latitude climate effects, via regional subsurface circulation and upwelling (the so-called oceanic tunnel effect), whereas the intertropical convergence north of the Equator responds more directly to local insolation, with a precessional rhythm. Firm conclusions on these climate cycles await further data that will confirm the regional patterns of change around the Equator.
Leg 202 provided exciting new materials for study of millennial-scale climate change and provides an unambiguous result that climate oscillations of central Chile tracked changes in Antarctica. This finding refutes past results that suggested climate linkages of Chile to the North Atlantic. Direct correlation of land and marine systems, through terrigenous sediments, iron, and pollen at the ODP sites, shows that some of the terrigenous markers lag the regional marine and polar climate shifts, and this may explain why fragmentary land-based data failed to identify unambiguously the Antarctic climate pattern.
Nitrogen isotope data provide key insights into the biogeochemical system and argue for linkage to the high southern latitudes via subsurface circulation, which influences the oxygen minimum zone off Peru and controls the rate of denitrification in the southeast Pacific.
To be sure, this synthesis can only provide a progress report, rather than final conclusions. Although more than 30 publications have been produced so far, much more research is in progress, and the conclusions we make here may be amplified with greater detail, or modified by new results, as work progresses over the coming decade.