Relict and Modern Sediments on the Continental Shelf of the Northern South China Sea: A Reconsideration

Understanding the temporal-spatial patterns of modern and relict sediments is of importance for assessing changes in the Quaternary environment and sea-level. Sedimentological and geochemical data is presented, along with in situ shell-based accelerator mass spectrometry C-14 ages of 30 samples from the surface sediments on the northern shelf of the South China Sea (NSSCS). The authors' data show that the NSSCS surface sediments exhibit considerable diversity in composition. Modern sediments are primarily constrained to the NW inner shelf, which is fed by fluvial sands sourced from Coastal South China river systems and dominated by the Pearl River Estuary delivery. The transport and discharge of the terrestrial sediments to the NSSCS is highly influenced by the Guangdong Longshore Current and its secondary cyclonic eddies. Relict sediments dominated by well-sorted, medium- and coarse-grained sands were identified in the NW Shenhu and NW Dongsha areas of the outer NSSCS. The sedimentology and geochemistry of the relict sediments imply that they were deposited in a dry and cold environment either during the low sea levels of the late Pleistocene (similar to 40 ka) or the early Holocene (similar to 10 ka). To the east, the Taiwan Shoal and vicinity are dominated by a sand mixture, at which the relict sediments were reworked by terrigenous supply and modern hydrodynamic environment due to the compound action of the Guangdong Longshore Current, seasonal cyclones, and Kuroshio Intrusion. The present isobaths of similar to-90 m in the NSSCS might be the reflective of the early Holocene coastal delta or the last glacial maximum shoreline. Plain Language Summary Reconstructing the temporal-spatial pattern for relict sediments in shelf environments is an important way of determining the shoreline or coastal delta position during the late glacial period or the early Holocene. The largest and longest zone of the relict sediments has been traditionally suggested to be along the outer shelf of the East Asian continental margin from Malaya to Korea across the NSSCS. However, less attention has been paid to the NSSCS relict sediments despite the fact that much research has documented the sediment supply, Holocene environment, and marine resource surveys. The authors' data revealed the development of the modern sediments in the NW inner shelf and the relict sediments in the NW Shenhu and NW Dongsha areas of the NSSCS outer shelf. The relict sediments were sourced from Coastal South China and SW Taiwan, and have a distinct depositional environment in comparison with that of the modern sediments. The temporal-spatial pattern of the relict sediments suggests that the current ca. -90 m isobaths were close to the shoreline or coastal delta at the last glacial period or the early Holocene.