MG/CA INFERRED PALEOTEMPERATURE DURING THE MID- TO LATE HOLOCENE FROM THE SUNDA SHELF, SOUTH CHINA SEA

Two hundred foraminiferal samples from two cores collected on the Sunda Shelf approximately 80 km northwest of Bintulu, Sarawak, Malaysia were analyzed for Mg/Ca ratios in order to provide paleotemperatures for the last ca. 7200 years. The chronology was derived from twenty-nine AMS radiocarbon analyses of carbonates (foraminifera) combined with Bchron modeling. Mg/Ca ratios of the planktonic foraminifera Globigerinoides ruber and Globigerinoides sacculifer were analyzed. Temperatures were calculated from Mg/Ca values using a linear trend based on modern analogues covering the Mg/Ca range of the analyzed samples and temperatures similar to those of the region studied. Both cores provided averaged values of 27.5°C and agree for the interval where they overlap. The longest record was compared with high- and low-resolution studies from the surrounding region, which showed good agreement considering the differences between techniques and water masses. The correlation with SST derived from alkenones for the last ca. 2000 years shows a slope of 0.9, although correlation is weak. The overall record of averaged values increases from 27.3°C ca. 7200–2500 cal. years BP to 28.3°C after 2500 cal. years BP. This increase is accentuated over the last 600 years when SST reaches ca. 29°C. These values need to be placed in context of the potential errors in Mg/Ca-derived temperatures and chronologies. At the same time, they provide valuable information for understanding past behavior of the East Asian Monsoon (EAM), since EAM strength and duration is a major driver of precipitation, among other environmental variables. Shifts in climate are of significance to the heavily populated and agriculturally dependent regions in Southeast Asia. These results indicate a tendency towards a stronger EAM over the last 2500 years, especially during the last 600 years.