Past Sea Surface Temperatures in the Tropical South China Sea Based on a New Foraminiferal Magnesium Calibration

Annemarie N. Whitko, David W. Hastings, Benjamin P. Flower

Foraminiferal Mg was used as a proxy for sea surface temperature (SST) change in the South China Sea. Twenty-five core-tops from this reagion were picked through for G. ruber, cleaned of contaminating phases such as clays and organic matter, and analyzed for Mg and Ca content using an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES). Mg/Ca values were calculated and compared to the known overlying sea surgace temperatures from which the core-top samples were collected to create calibrations specific to the depth confines of the South China Sea. Post-depositional dissolution of the foraminferal test proved to greatly effect these samples, especially those deeper than 2000 m, as illustrated by Mg/Ca and average foraminiferal weight verses depth profiles. The calibration for G. ruber, determined from core-tops extracted from shallower water depths and thus much less affected by dissolution, was applied to a downcore record, core 18287-3 from the South China Sea. This record indicates a Last Glacial Maximum (LGM) to present SST change of approximately 3 C. Two other major trends were identified in the record: the Bolling-Allerod warming interval and the Younger-Dryas interval, where near-glacial conditions were achieved. The timing of these events for the Mg/Ca-based record (core 18287-3) occurs at the same time as in the Greenland Ice Sheet Project Ice Core 2 (GISP2), indicating synchronous warming in the higher latitudes and tropics. The ability of the foraminiferal Mg proxy to detect temperature changes at the same time as in other proxies, combined with its ability to estimate temperature within a reasonable range, illustrates its promise as a paleo-thermometer.