Late quaternary benthic foraminifera and deep-water paleoceanography in the South China Sea

Abstract

Recent and late Quaternary deep-sea benthic foraminifera in the South China Sea (SCS) have been quantitatively studied, in order to approach some disputable questions about the glacial deep circulation of the Pacific from the paleoceanographic records of a marginal sea, for example, whether or not there existed the North Pacific Deep Water (NPDW) during glacials. Based on the quantitative data of deep-sea benthic foraminiferal faunas from sixty-six surface sediment samples of the SCS, a Q-mode factor analysis indicates that the Globocassidulina subglobosa assemblage, the Astrononion novozealandicum and Bulimina aculeata assemblages and the Eggerella bradyi assemblage are associated with the Intermediate Water Mass (IWM), Deep Water Mass (DWM) and the deep water below CCD in the SCS, respectively. The factor assemblages resulted from surface sample data were then applied to the quantitative analysis of late Quaternary deep-sea benthic foraminifera in eight cores from the SCS and western Pacific. The results show a generally increased contribution of the DWM assemblage relative to that of the IWM assemblage during glacials, which implies that the influence of the DWM increased and that of the IWM decreased in the SCS and western Pacific. However, there were two major deep-water paleoceanographic changes during the late Quaternary: (1) The deep-sea carbonate dissolution of the SCS and western Pacific was intensified during δ 180 stages 4–5, evidenced by the higher arenaceous/calcareous ratios of benthic foraminifera and relative abundance of Eggerella bradyi. (2) The influence of the Pacific DWM above 2500 m greatly increased during the last glacial maximum (LGM), inferred from the increased abundances of the Pacific DWM benthic foraminiferal assemblage as compared to those of the Pacific IWM and Pacific Bottom Water Mass assemblages in the western Pacific. Particularly, the water in which Favocassidulina favus was distributed could not enter the SCS since the LGM. Meanwhile, the great changes had taken place in the DWM of the SCS during the LGM, reflected in the occurrence of a clearly cooled and ventilated DWM and the replacement of A. novozealandicum and B. aculeata assemblages by B. aculeata-Melonis barleeanum assemblage at water depths of 1000–2500 m. These significant changes during the LGM may be related to the formation of the NPDW