Glacial-Holocene calcareous nannofossils and paleoceanography in the northern South China Sea

Abstract

Last glacial to Holocene paleoceanography of the northern South China Sea was inferred from nannofossil variations and several hydrographic proxies from a piston core, SCS90-36 (17°59.70′N, 111°29.64′E, water depth 2050 m). The upper part of the sedimentary sequence (dated 15.5 to 1.2 ka) provided a high-resolution record whereas part of the sediments older than 15.5 ka was lost due to erosion. A correspondence analysis of the nannofossil succession suggests that the paleoceanography developed in four stages. The first stage (26−13.3 ka) has a fairly well-preserved diverse nannoflora dominated by Gephyrocapsa and Florisphaera profunda. The floral composition together with high concentration of ketones (C 37) and organic carbon indicates high surface-water fertility. The second stage, the deglacial period (13.3−10.7 ka), had an increased surface-water turbidity and a stronger influence of Pacific open-ocean waters as evidenced by the decrease of Florisphaera profunda and increase of Emiliania huxleyi, respectively. A preservation peak of calcareous microfossils centered at 12 ka correlates to the global preservation event of Termination I. The third stage, early Holocene (10.7−4.4 ka), is marked by a gradual increase of F. profunda and small placolith taxa at the expense of E. huxleyi. The floral composition indicates that conditions were more oligotrophic compared to the pre-Holocene. The preservation of nannofossils became progressively worse, indicating a rise of the nannofossil lysocline. In sediments deposited at 5.5 and 4 ka, nannofossil preservation improves, probably reflecting a local cooling event. During the last stage, from 4.4 to 1.2 ka, E. huxleyi, Umbilicosphaera and large Reticulofenestra increased their relative abundance to replace small placoliths. Further stratification of the surface water column may have been responsible for this floral succession.