Observing the biological carbon pump of the twilight zone in the South China Sea

The biological carbon pump (BCP) is a key mechanism sustaining ocean carbon sequestration and thus significantly influences atmospheric CO₂ concentration. However, most of the key processes of the BCP, particularly in the twilight zone, remain poorly constrained. In this study, we use multiple approaches to constrain the key BCP processes throughout the water column in the South China Sea (SCS), including carbon export, remineralization and sequestration. Firstly, we calculated the small particulate organic carbon (POC) flux exported via the mixed layer pump (MLP) by biogeochemical profiling floats (BGC-float), which are typically ignored in low-latitude regions. We further combined three independent approaches, including BGC-float observation, in vivo reduction of the tetrazolium salt by the cellular electron transport system (in vivo INT), and the synthesis of prokaryotic respiration (PR) determined by radiolabeled leucine incorporation and zooplankton respiration (ZR) empirically estimated from the biomass (PR+ZR), to constrained the twilight zone remineralization (TZR) in the SCS. To reconcile methodological discrepancies, we estimated the possible range of carbon supply by integrating comprehensive carbon sources, including sinking POC flux, dissolved organic carbon input, lateral transport, dark carbon fixation, and active carbon transport by zooplankton migration. We find the in vivo INT approach may overestimate the TZR, while the TZR measured by BGC-float and PR+ZR approaches can be balanced with the total carbon sources. Finally, we further calculate the time-series POC flux at 1000 m by using the optical sediment trap equipped on the BGC-float, which indicates the real carbon sequestration flux and can be isolated from the atmosphere at the time scale of centuries to millennia. Our study provides new insights of the BCP and highlights the importance of inter-disciplinary and integrative process studies for constraining biogeochemical processes.