RECONSTRUCTING LATE PLEISTOCENE-HOLOCENE CLIMATIC LAND CONDITIONS OF CHILE

Changes in terrestrial climate and sediment source can be discerned from marine sediment cores in coastal settings. ODP Site 1234 is located at 36°S on a flat bench on the continental slope, 65 km off the Chilean coastline. This is a climatically sensitive latitude, with generally wet climates to the south and dry climates to the north. Shifting position of the onshore westerlies and ENSO events control rainfall. Additionally due to the orographic effect, the high Andes are typically wetter than the lower Coast Range. Sediments transport to Site 1234 is fluvially-controlled, so variations in composition and grain size through time could indicate changes in precipitation patterns affecting input from the Coastal Range and the Andes. The purpose of our course-based research was to reconstruct the regional land and climatic conditions from Hole 1234A Core 1-3. This 23-m record extends to 41 ka, thus spanning glacial and interglacial times. We used XRF to determine elemental compositions, XRD to determine clay mineral compositions, and LDPSA to determine grain size of 47 samples.

Our results show sediment input from both the Coast Range (illite and chlorite) and Andes (smectite) source rocks, but with an overall greater abundance of illite at Site 1234 compared to sites further north, suggesting more Coast Range influence at 36°S. MIS 3 and 2 are characterized by variability (~500 to 2000 years) in all proxies, which we interpret as millennial oscillations affecting regional rainfall and weathering conditions. In contrast, MIS 1 displayed broader trends, however this may be an artifact of sample spacing. In general, MIS 3 and 2 were wetter than MIS 1 (~14 to 0 ka), as indicated by input of fine sediments and high sedimentation rates. Changes in Fe are consistent with Ti, affirming that Fe is primarily a terrigenous input proxy. Their greatest change is at the MIS 2 to 1 transition. MIS 1 shows an increase in grain size, a slight increase in smectite abundance, and a decrease in smectite crystallinity. These data suggest less precipitation over the Coastal Range and thus greater relative influence of Andean sources. Drier conditions during MIS 1 are consistent with lower sedimentation rates. A wet and variable climate during MIS 3 and 2, and drier climate during MIS 1 are also consistent with Site 1234 pollen results (Heusser et al 2006).