Seasonality in Marine Organic Carbon Export and Sequestration Pathways

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Abstract The ocean's biological carbon pump transports organic carbon from the surface to depth via three main pathways: the gravitational sinking of particles, active transport by vertically migrating zooplankton, and mixing and advection of suspended and dissolved organic carbon. Here, we use a global data‐assimilated ocean biogeochemical model to diagnose the seasonal variability of carbon export and sequestration by these gravitational, migrant, and mixing pumps. The total carbon export and sequestration are 10.2 ± 0.8 PgC yr −1 and 1,339 ± 17 PgC, respectively, similar to previous estimates that did not consider seasonality. However, the seasonality of the export and sequestration pathways is highly variable, especially in the high latitudes. In subpolar regions, the seasonal amplitude of the pumps is ∼40%–60% of the annual mean: export and sequestration by the gravitational and migrant pumps peak in the summer, while the mixing pump strongly opposes this seasonality, reaching a maximum during the winter. The sequestration time of exported carbon is generally higher during winter than summer in the subpolar regions, helping to augment carbon sequestration during the less productive winter months. The gravitational “e‐ratio,” or ratio of gravitational carbon export to net primary production, has a seasonal variability of ∼0.1 at high latitudes, with higher values in the summer compared to winter. Resolving seasonality reduces the inferred geographic variability of the e‐ratio compared with annual‐mean models, demonstrating the importance of seasonal observations and models to understand and quantify the processes regulating carbon export and sequestration.

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