Nutricline heaving regulates carbon export in the South China Sea

Abstract

The export of phytoplankton-synthesized particulate organic carbon (POC) from the upper ocean to the depth removes CO₂ from the atmosphere, playing a critical role in the global climate system. However, substantial data and knowledge gaps in the spatial and temporal variability of the POC export hamper a full spectrum of understanding to the controls of POC export at both seasonal and basin scales. Here, we developed an optimized one-dimensional biogeochemical model, which well simulates the seasonal variations of biogeochemical parameters at the South-East Asia Time-series Study station in the South China Sea (SCS). Applying this model, we further examined the climatology of POC fluxes in the SCS using a high-quality nutrient dataset derived from an algorithm based on a relationship between field observed nutrients and temperature/salinity with high spatiotemporal coverage. 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 (from June to August), the POC fluxes in the northern and northwestern SCS regions are higher compared to the central and southern SCS regions. In winter (from December to February), 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, the basin averaged POC fluxes in spring and winter exhibit higher values than that in summer and fall. At the relatively eutrophic regions where surface nitrate (NO₃⁻) concentrations > 0.1 μmol L⁻¹, the surface NO₃⁻ concentrations are 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 subsurface nutricline heaving driven by upwelling/downwelling induced by horizontal convergences/divergences at both meso- and basin scales, plays a pivotal role in determining the distribution of POC fluxes. Our findings underscore the significance of subsurface nutricline heaving in driving carbon export in the oligotrophic ocean.