Quantitative and mechanistic understanding of the open ocean carbonate pump - perspectives for remote sensing and autonomous in situ observation

The open ocean carbonate pump represents the production and downward flux of particulate inorganic carbon (PIC) in the form of calcium carbonate synthesized by calcifying plankton. This pump operates alongside the organic carbon pump, which concerns the production and downward flux of organic carbon, mostly in the form of particles (POC). While the organic carbon pump draws down atmospheric carbon dioxide, the carbonate pump causes an increase in surface ocean carbon dioxide (CO₂), thereby counteracting the organic carbon pump. However, PIC produced by the carbonate pump is of high-density and has been hypothesized to enhance the downward flux of organic carbon, increasing the efficiency of the organic carbon pump. Here, we review our current quantitative and mechanistic understanding of the contemporary open ocean carbonate pump, its counter- and ballast effects. We first examine the relative contributions of the various calcifying plankton groups (coccolithophores, foraminifera, and pteropods) to PIC production and flux based on a global compilation of PIC flux observations. Next, we compare spatial patterns in calcification rates from remote sensing with observations of PIC flux at depth obtained from sediment traps and radiochemical tracers. We then review estimates of the counter effect of the carbonate pump on the partial pressure of CO₂, pCO₂, in surface waters based on remote sensing studies and estimates of the rain ratio of exported carbon and the amount of CO₂ released per PIC precipitated, Ψ. Next, we review our understanding of the PIC ballast effect and implementations in biogeochemical models. Lastly, we discuss observations of the organic carbon pump with autonomous BioGeoChemical-Argo (BGC-Argo) profiling floats and perspectives for extending observations to the carbonate pump.