Quantifying Global and Regional Carbon Export and Sequestration Pathways with a Data Assimilation Model of the Biological Pump

The ocean’s biological pump controls atmospheric CO₂ levels by capturing carbon from the atmosphere via photosynthesis and sequestering it at depth. This process is controlled by several mechanisms or “pathways”, including the passive sinking of zooplankton fecal pellets and phytoplankton aggregates, active transport by zooplankton diurnal vertical migration (DVM), and the physical mixing of dissolved organic carbon (DOC). We quantified the contributions of these pathways to export and sequestration using an inverse numerical model of the biological pump that assimilates various satellite and in situ oceanographic tracer data.

On global scales, the total carbon export is ~10 Pg C yr^-1, with roughly 70% of that from passive sinking of particulate organic carbon (POC), ~20% from DOC transport, and ~10% from zooplankton DVM. Total global sequestration is roughly 1,200 Pg C, with ~80% due to passive POC flux, ~5% to DOC transport, and ~15% to zooplankton DVM.

Carbon export and sequestration are greatest in the northern high latitudes, which account for ~50% of both quantities, and lowest in the subtropics, which account for only ~15% of global C export and ~10% of the total C sequestration. The higher contribution of the subtropics to C export compared to C sequestration is due to the large amounts of DOC exported in this region. DOC has a global average sequestration efficiency of only ~50 years, compared to a global average of ~125-200 years for the other mechanisms. This makes DOC the least important pathway for ocean carbon storage. These results suggest that, as the oceans warm and the subtropical gyres expand, carbon sequestration by the biological pump will decline.