Quantifying the Carbon Export and Sequestration Pathways of the Ocean's Biological Carbon Pump

The ocean's biological carbon pump transfers carbon from the surface ocean to the deep ocean by several distinct pathways, including gravitational settling of organic particles, mixing and advection of suspended organic carbon, and active transport by vertically migrating metazoans. Carbon exported by these pathways can be sequestered as respired CO2 in the deep ocean for years to centuries. However, the contribution of each pathway to carbon export and sequestration remains highly uncertain. Here, satellite and in situ ocean biogeochemical observations are assimilated in an ensemble numerical model of the biological pump to quantify global and regional carbon export and sequestration. The ensemble mean global carbon export is 10.2 Pg C yr−1 and the total amount of carbon sequestered via the biological pump is 1,300 Pg C. The gravitational pump is responsible for 70% of the total global carbon export, 85% of which is zooplankton fecal pellets and 15% is sinking phytoplankton aggregates, while migrating zooplankton account for 10% of total export and physical mixing is responsible for the remaining 20%. These pathways have different sequestration times, with an average of 140 years for the gravitational pump, 150 years for the migrant pump, and only 50 years for the mixing pump. Regionally, the largest sequestration inventories and longest sequestration times are found in the northern high latitudes, while the shortest sequestration times are found in the subtropical gyres. These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.