Dissolved organic carbon (DOC) constitutes the majority of marine organic carbon. Understanding its source and sink processes is of great importance to the global carbon cycle and will provide information on achieving carbon neutrality. How do the different physical and biogeochemical processes interact to contribute to DOC and particulate organic carbon (POC) balances? Are there unique dynamics in different regions? The newspaper Science China Earth Sciences published online a study of the carbon cycle in the South China Sea led by Dr Peng Xiu (South China Sea Institute of Oceanology, Chinese Academy of Sciences) and Dr Wentao Ma (Second Institute of Oceanography, Ministry of Natural Resources). The aim of this study is to quantitatively assess the binding, sequestration and interaction processes between the biological carbon pump and the microbial carbon pump.

“The South China Sea (SCS) is the largest semi-enclosed marginal sea in the Western Pacific. We know that the alternation of the northeast winter monsoon and the southwest summer monsoon causes the distribution of the chlorophyll concentration of the phytoplankton to show a clear seasonal pattern depending on the satellite products ”, explains Dr. Ma. However, the organic carbon balance in SCS is less studied.

The team used a Foucault-resolution marine physico-biogeochemical model to analyze seasonal changes in phytoplankton photosynthesis and the storage of this fixed carbon in the SCS.

“Our research has focused on two main pathways of carbon sequestration, one is the deep sea storage by gravitational sinking and remineralization of the POC, known as the biological carbon pump (BCP), and the other is the microbial carbon pump (MCP). ), which transforms DOC from labile forms into refractory forms through microbial activities. Dr Xiu presents.

Numerical simulations reported the fluxes of carbon uptake by phytoplankton, POC export by gravity, and DOC production and transformation by microbes. “The results of the model can be validated by observations from satellite or on-board data sets. Says Dr. Ma. The production of refractory DOC (CDOC) reaches 26% of the carbon sequestration rate of the biological carbon pump, and its contribution to carbon storage cannot be ignored. In addition, this study also revealed that the SCS has three typical areas with distinct DOC production dynamics on the north coast, off the Luzon Strait and off the southeast coast of Vietnam.