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. Plain Language Summary Tiny organisms called phytoplankton help to reduce atmospheric CO2 levels by absorbing large amounts of carbon in the surface ocean during photosynthesis. Various biological and physical processes, or pathways, then work together to transfer some of this carbon to the deep ocean where it can be stored for potentially hundreds of years. In this study, we examined these pathways using computer models that are consistent with data from satellite-based sensors and ocean observations. Our model shows that the most important pathways for carbon storage are transport by sinking particles and swimming zooplankton, while transport by ocean currents is the least important pathway. The North Atlantic and North Pacific oceans have the greatest carbon storage power, while the subtropical oceans have the least. These results imply that ocean carbon storage may decrease in the future, because subtropical regions will expand in a warming climate.

Automated summary

The ocean's biological carbon pump transfers carbon from the surface ocean to the deep ocean through various pathways, and quantifying these pathways is crucial for understanding global carbon export and sequestration. The gravitational pump, zooplankton fecal pellets, sinking phytoplankton aggregates, migrating zooplankton, and physical mixing are all important contributors. The North Atlantic and North Pacific oceans have the highest carbon storage capacity, while subtropical gyres have the lowest. As the oceans stratify and subtropical gyres expand due to climate change, ocean carbon storage may weaken.