Isotopic signals in two morphotypes of Globigerinoides ruber (white) from the South China Sea: implications for monsoon climate change during the last glacial cycle

Based on the study of surface and downcore sediment samples from the South China Sea (SCS), two morphotypes of Globigerinoides ruber (white) have been differentiated by taxonomic criteria at species and/or subspecies level, a nd also by their stable isotopic compositions of oxygen and carbon. The two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu late (s.l.), represent two groups with different depth habitats, with G. ruber s.s. liv ing in the upper 30 m of the water column and G. ruber s.l. living at depth s below 30 m. The different depth habitats are indicated by the comparison between the present oceanographic data and isotopic signals of the two morp hotypes in the surface sediment from the SCS. Application of this informati on to a hemipelagic sediment core in the northern SCS shows that drastic ch anges existed in the morphotypes isotopic records during the last glacial c ycle. The isotope difference between the two morphotypes diminished during delta(18)O stage 2, especially during the last glacial maximum. It is infer red that this is the result of intensified monsoon winds, which caused mixi ng of the upper layer of the water column. During stage 2 the proportion of G. ruber s.l. increased, which is interpreted as a result of the increased productivity due to mixing of the subsurface nutrient-rich water into the surface layer. (C) 2000 Elsevier Science B.V. All rights reserved.