GEOCHEMICAL EVIDENCE OF HOLOCENE EAST ASIAN MONSOON VARIABILITY FROM SUNDA SHELF, SOUTH CHINA SEA SEDIMENTS

The East Asian Monsoon (EAM) delivers vital rainfall to heavily populated, agriculturally dependent regions in Southeast Asia. Changes in EAM strength and duration influence precipitation and wind patterns, continental runoff, sea circulation, and nutrient transport in the South China Sea (SCS) and surrounding terrain. The SCS has been the focus of many paleoclimatological studies due to its central location to the EAM system. Most research has been conducted in the deep, northern SCS because of the high sedimentation rates there. The southern SCS, specifically the Sunda Shelf, has yet to yield a monsoon record due to its very thin sedimentary cover. However, flooded and in-filled fluvial paleochannels on the Sunda Shelf in the southern SCS provide an extended record of EAM-driven climate change.

In 2008, mud-dominated piston cores D42 (3.88 m long, 52.33 m water depth) and D45 (2.68 m long, 62.51 m water depth) were collected in fluvial paleochannels on the Sunda Shelf approximately 80 km northwest of Bintulu, Sarawak, Malaysia. Twenty-nine AMS radiocarbon age estimates of the benthic foraminifer Cavarotalia annectens provide age control. Core D45, collected on the edge of a v-shaped paleochannel, yields a ca. 7,000 year record, while core D42, collected in a shallower paleochannel, reflects ca. 3,000 years of accumulation. Mg/Ca ratios of planktonic foraminifera Globigerinoides ruber and Globigerinoides sacculifer, in combination with δ¹⁸O of G. ruber, allow for the analysis of sea surface temperature and salinity trends that may be related to EAM variability. Temperatures were calculated from Mg/Ca values using a warm water, multi-species approach. δ¹⁸O_seawater was calculated from temperature and δ¹⁸O_calcite using a high light equation. Finally, these values were transformed into salinities. Additionally, stable carbon isotopic analysis of both G. ruber and C. annectens provides information regarding continental runoff as well as surface and bottom water mixing.