Abstract:

As the largest marginal sea in the western Pacific, the South China Sea (SCS) plays an important role in the global ocean and the global oceanographic research. In the past 40 years, chemical oceanographic research in the SCS achieved systematic new discoveries and new understandings; researchers put forward many new theoretical viewpoints, making important contributions to the development of oceanography. A subsurface layer was revealed in the SCS, maximum values of ecological environmental parameters represented by nitrite are present; and the depth ranges of water layers for different parameters are different, forming a thermocline ecosystem that has significantly different characteristics from the other ecosystems. The carbon cycle process in the SCS is very complex, and changes of the biological pump controlled by biological activities, and the regional and seasonal changes of carbon sources and sinks, are all great. The characteristics and intensity of carbon sources and sinks in the SCS are unique in different regions at different times. Annually, the SCS is a weak source of atmospheric carbon dioxide. The characteristics of ecological environment and the cycling process of chemical material in the Pearl River Estuary and deep-sea basins were found not only closely related to but also significantly different from that in shelf marginal seas and coral reefs, as the dissolved oxygen concentration is low in the bottom, and the Pearl River Estuary is basically an ecologically fragile area characterized by hypoxia. Based on the systematic understanding of the rapid material circulation and vertical transfer of chemical substances controlled by biological processes in the Nansha coral reef ecosystem, a new mechanism - “resembling drift-net theory” - was proposed to explain how the coral reef ecosystem could maintain high productivity. Systematic studies on the sedimentary chemistry of the SCS show that there is a close coupling between the sediments and chemical cycling of water bodies. The distribution of chemical substances in coral reefs or in sediment cores of the SCS can be used to retrieve historical environment changes. The paleo productivity of surface seawater in the SCS during the glacial period was 1.6 times higher than that of the interglacial period. A "biological explosion event" occurred in the southern SCS in the late Miocene. The productivity in that period was mainly affected by the monsoon and terrigenous nutrients input, while the influences of northeast monsoon and southwest monsoon differed in different regions. These new discoveries and new understandings in chemical oceanography of the SCS in the past 40 years have laid a strong foundation for further systematic and in-depth understanding of the oceanographic processes in the region. In the future, chemical oceanographic research will definitely provide scientific support for the sustainable utilization of resources and environment in the SCS.

Key words: new understanding, the thermocline ecosystem, hypoxia and fragile ecological zone, carbon source and sink, resembling drift-net theory, ecological environment evolution, chemical oceanography, past 40 years, the South China Sea