Large Contribution of Pteropods to Shallow CaCO3 Export

The literature on the relative contributions of pelagic calcifying taxa to the global ocean export of CaCO3 is divided. Studies based on deep sediment trap data tend to argue that either foraminifers or coccolithophores, both calcite producers, dominate export. However, the compilations of biomass observations for pteropods, coccolithophores, and foraminifers instead show that pteropods dominate the global ocean calcifier biomass and therefore likely also carbonate export. Here we present a new global ocean biogeochemical model that explicitly represents these three groups of pelagic calcifiers. We synthesize databases of the physiology of the three groups to parameterize the model and then tune the unconstrained parameters to reproduce the observations of calcifier biomass and CaCO3 export. The model can reproduce both these observational databases; however, substantial dissolution of aragonite above the aragonite saturation horizon is required to do so. We estimate a contribution of pteropods to shallow (100 m) export of CaCO3 of at least 33% and to pelagic calcification of up to 89%. The high production-high dissolution configuration that shows closest agreement with all the observations has a CaCO3 production of 4.7 Pg C/year but CaCO(3 )export at 100 m of only 0.6 Pg C/year. Plain Language Summary We show that pteropods contribute at least 33% to export of CaCO3 at 100m and up to 89% to pelagic calcification. This is in line with results by Betzer et al., 1984 and Byrne et al., 1984, and contradicts most of the work that has been published since then, which has tended to argue for the dominance of either coccolithophores or foraminifers. Pteropods precipitate CaCO3 in the crystal form of aragonite. This is more soluble than calcite, which is produced by coccolithophores and pelagic foraminifers. Thus, the ocean alkalinity cycle and associated buffer capacity for CO2 could be more sensitive to rising CO2 than has been suggested by existing Earth System Models, which only represent calcite.