46 / 2023-07-24 15:08:04

Nutricline displacement regulates spatiotemporal carbon export in the South China Sea

South China Sea, Nutricline, Particulate Organic Carbon Export

The export of phytoplankton-synthesized particulate organic carbon (POC) from the upper ocean to the deep sea removes CO2 from the atmosphere, playing a critical role in global climate. However, limited spatial and temporal coverage of field observations do not allow resolving the factors driving the POC export at seasonal and basin scale. Using an optimized one-dimensional biogeochemical model and a high-quality nutrient dataset, we examined the climatology of POC fluxes in the South China Sea (SCS). The POC fluxes (at 100 m deep) range from 0.24 to 12.7 mmol C m-2 d-1 and exhibit evident spatial and seasonal variations. During the summer, the POC fluxes in the northern and northwestern SCS regions are higher compared to the central and southern SCS regions. In winter, the POC fluxes in the northern SCS are significantly enhanced. Overall, the northern and western basins exhibit higher POC fluxes compared to the central and southeastern basins in all four seasons. Seasonally, both spring and winter exhibit higher POC fluxes compared to summer and fall. At the regions where NO3- concentrations > 0.1 mol L-1, surface NO3- concentrations emerged to be the primary factor influencing the distribution of POC fluxes, particularly in winter and in the northern SCS. In contrast, within the vast oligotrophic environment, the vertical displacement of the nutricline, driven by upwelling/downwelling induced by horizontal convergences/divergences at both meso- and basin scales, played a pivotal role in determining the distribution of POC fluxes. Our findings underscore the significance of subsurface nutricline displacement in driving carbon export in the oligotrophic ocean.