9.3.1: Introduction

The oceanic biological carbon pump (BCP) regulates the Earth carbon cycle by transporting part of the photosynthetically fixed CO₂ into the deep ocean. Suppressing this mechanism would result in an important increase of atmospheric CO₂ level. The BCP occurs mainly in the form of (1) organic carbon (OC) particles sinking out the surface ocean, of (2) neutrally buoyant OC (dissolved or particulate) entrained by downward water masses movements and/or mixing, and of (3) active transport of OC by migrating animals such as zooplankton and fishes. These various pools of OC differ in size since their sinking, production and decomposition rates vary spatially and temporally. Moreover, the OC transported to depths via these various export pathways as well as their decomposition pathways all have different ecological origins and therefore may response differently to climate changes. Currently, most ocean biogeochemical models do not resolve these various of OC pathways explicitly; rather, they imply that OC is therein created and destroyed equally. In addition, the organic composition of these various pools is largely unknown, especially at depths below 500 m.

Figure \(\PageIndex{1}\): Components of the "biological pump" are responsible for transforming Dissolved Inorganic Carbon (DIC) into organic biomass and pumping it in particulate or dissolved form into the deep ocean. Inorganic nutrients and Carbon Dioxide (CO₂) are fixed during photosynthesis by phytoplankton, which both release Dissolved Organic Matter (DOM) and are consumed by herbivorous zooplankton. Larger zooplankton-such as copepods, egest fecal pellets - which can be reingested, and sink or collect with other organic detritus into larger, more-rapidly-sinking aggregates. DOM is partially consumed by bacteria (black dots) and respired; the remaining refractory DOM is advected and mixed into the deep sea. DOM and aggregates exported into the deep water are consumed and respired, thus returning organic carbon into the enormous deep ocean reservoir of DIC. About 1% of the particles leaving the surface ocean reach the seabed and are consumed, respired, or buried in the sediments. There, carbon is stored for millions of years. The net effect of these processes is to remove carbon in organic form from the surface and return it to DIC at greater depths, maintaining the surface-to-deep ocean gradient of DIC (inset graph at lower right). Thermohaline circulation returns deep-ocean DIC to the atmosphere on millennial timescales.. (Copyright; Ducklow, Steinberg and Busseler, Upper Ocean Carbon Export and the Biological Pump, Oceanography, https://tos.org/oceanography/article...iological-pump )

Excerpted from:

Le Moigne, F. A. (2019). Pathways of organic carbon downward transport by the oceanic biological carbon pump. Frontiers in Marine Science, 6, 634.