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 CO ₂ 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.

This study presents a new estimate of the oceanic anthropogenic CO₂(C_ant) sink over the industrial era (1780 to present), from assimilation of potential temperature, salinity, radiocarbon, and CFC‐11 observations in a global steady state ocean circulation inverse model (OCIM). This study differs from previous data‐based estimates of the oceanic C_ant sink in that dynamical constraints on ocean circulation are accounted for, and the ocean circulation is explicitly modeled, allowing the calculation of oceanic C_ant storage, air‐sea fluxes, and transports in a consistent manner. The resulting uncertainty of the OCIM‐estimated C_ant uptake, transport, and storage is significantly smaller than the comparable uncertainty from purely data‐based or model‐based estimates. The OCIM‐estimated oceanic C_ant storage is 160–166 PgC in 2012, and the oceanic C_ant uptake rate averaged over the period 2000–2010 is 2.6 PgC yr⁻¹ or about 30% of current anthropogenic CO₂ emissions. This result implies a residual (primarily terrestrial) C_ant sink of about 1.6 PgC yr⁻¹ for the same period. The Southern Ocean is the primary conduit for C_ant entering the ocean, taking up about 1.1 PgC yr⁻¹ in 2012, which represents about 40% of the contemporary oceanic C_ant uptake. It is suggested that the most significant source of remaining uncertainty in the oceanic C_ant sink is due to potential variability in the ocean circulation over the industrial era.

A data-constrained ocean circulation model is used to characterize the distribution of water masses and their ages in the global ocean. The model is constrained by the time-averaged temperature, salinity, and radiocarbon distributions in the ocean, as well as independent estimates of the mean sea surface height and sea surface heat and freshwater fluxes. The data-constrained model suggests that the interior ocean is ventilated primarily by water masses forming in the Southern Ocean. Southern Ocean waters, including those waters forming in the Antarctic and subantarctic regions, make up about 55% of the interior ocean volume and an even larger percentage of the deep-ocean volume. In the deep North Pacific, the ratio of Southern Ocean to North Atlantic waters is almost 3:1. Approximately 65% of interior ocean waters make first contact with the atmosphere in the Southern Ocean, further emphasizing the central role played by the Southern Ocean in the regulation of the earth’s climate. Results of the age analysis suggest that the mean ventilation age of deep waters is greater than 1000 yr throughout most of the Indian and Pacific Oceans, reaching a maximum of about 1400–1500 yr in the middepth North Pacific. The mean time for deep waters to be reexposed at the surface also reaches a maximum of about 1400–1500 yr in the deep North Pacific. Together these findings suggest that the deep North Pacific can be characterized as a “holding pen” of stagnant and recirculating waters.

The conversion of dissolved nutrients and carbon to organic matter by phytoplankton in the surface ocean, and its downward transport by sinking particles, produces a “biological pump” that reduces the concentration of atmospheric CO₂. Global rates of organic matter export are a poor indicator of biological carbon storage however, because organic matter gets distributed across water masses with diverse pathways and timescales of return to the surface. Here we show that organic matter export and carbon storage can be related through a sequestration efficiency, which measures how long regenerated nutrients and carbon will be stored in the interior ocean before being returned to the surface. For the first time, we derive global maps of the sequestration efficiency of the biological pump at different residence time horizons. These maps reveal how regional patterns of organic matter export contribute to the biological pump, and how the biological pump responds to changes in biological productivity driven by climate change.