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

The relationship between planktonic and benthic foraminiferal stable-isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modern surface and Last Glacial Maximum (LGM) samples from the South China Sea (SCS). The harmonic variation of benthic δ18O in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic δ18O variations. Planktonic δ18O fluctuates with sea surface temperature (SST) and salinity (SSS). The N-S temperature gradient results in planktonic δ18O decreasing from the northeast to the south. Cool, saline waters driven by the winter monsoon are interpreted to have been responsible for the high δ18O values in the northeast SCS. The East Asian monsoons not only bring nutrients into the South China Sea and maintain high nutrient concentration levels at the southwestern and northeastern ends, which cause depleted δ13C both in planktonic (surface) and benthic (bottom) samples but also reduce planktonic/benthic δ18O differences. The distribution of δ18O and δ13C 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 modern and LGM δ18O indicate a difference of about 3.6 °C in bottom-water temperature and a large surface-to-bottom temperature gradient during the LGM as compared to today.