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, and also by their stable isotopic compositions of oxygen and carbon. The two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.), represent two groups with different depth habitats, with G. ruber s.s. living in the upper 30 m of the water column and G. ruber s.l. living at depths below 30 m. The different depth habitats are indicated by the comparison between the present oceanographic data and isotopic signals of the two morphotypes in the surface sediment from the SCS. Application of this information to a hemipelagic sediment core in the northern SCS shows that drastic changes existed in the morphotypes isotopic records during the last glacial cycle. The isotope difference between the two morphotypes diminished during δ18O stage 2, especially during the last glacial maximum. It is inferred that this is the result of intensified monsoon winds, which caused mixing 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.