Foraminiferal isotopic evidence for monsoonal activity in the South China Sea: a present-LGM comparison

The relationship between planktonic and benthic forairtimferal stable-isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modem surface and Last Glacial Maximum (LGM) samples from the South China Sea (SCS). The harmonic variation of benthic delta(18)O in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic delta(18)O variations. Planktonic delta(18)O fluctuates with sea surface temperature (SST) and salinity (SSS). The N-S temperature gradient results in planktonic delta(18)O decreasing from the northeast to the south. Cool, saline waters driven by the winter monsoon are interpreted to have been responsible for the high delta(18)O values in the northeast SCS. The East Asian monsoons not only brim nutrients into the South China Sea and maintain high nutrient concentration levels at the southwestern and northeastern ends. which cause depleted delta(13)C both in planktonic (surface) and benthic (bottom) samples but also reduce planktonic/benthic delta(18)O differences. The distribution of delta(18)O and delta(13)C in the surface and LGM samples are strikingly similar. indicating that the impact of SST and SSS has been maintained, and nutrient inputs, mainly from the northeastern and southwestern ends, have been controlled by monsoons since the LGM. Comparisons of the modem and LGM delta(18)O indicate a difference of about 3.6degreesC in bottom-water temperature and a large surface-to-bottom temperature gradient during the LGM as compared to today (C) 2004 Elsevier BY. All rigths reserved.