Resolving for Mineralogy in the Marine CaCO3 Pump: The Contribution of Bony Fishes

The marine CaCO₃ pump is an important control on the distribution of alkalinity and DIC in the oceans, and the capacity of surface waters to absorb atmospheric CO₂. Accordingly, CaCO₃ production and dissolution is widely parameterised in ocean carbon cycle models, with calcite-producing planktonic organisms generally assumed to be the dominant driver. However, this approach overlooks an emerging recognition of greater complexity in the sources and mineralogies of CaCO₃: a recent example being bony fishes as a source of relatively soluble high-Mg calcite. Among other sources, the fish contribution is hypothesized to have potentially significant implications for how we understand spatial distributions of production, the extent and depths of dissolution, and ultimately how alkalinity is distributed throughout the oceans. Moreover, predictions that future climate and ocean acidification scenarios will stimulate higher rates of fish CaCO₃ production contrast with a growing consensus that most biogenic calcification will be inhibited under such conditions. This means that fish could become an increasingly important component of the CaCO₃ pump, prompting a pressing need for an improved understanding of their overall contribution. Here we present the latest fish CaCO₃ production data, with particular emphasis on mineralogical variability as a function of temperature and taxonomy. Our findings significantly advance our understanding of fish CaCO₃ production, but simultaneously invite further questions regarding their overall role in the CaCO₃ pump. These outstanding knowledge gaps—which continue to hinder formal accounts of fish CaCO₃ in ocean carbon cycling models—are also highlighted. For example, the products of mesopelagic fishes (a dominant component of global fish biomass) are completely unknown and their contributions are further complicated by daily vertical migrations—do they represent an upward or downward CaCO₃ export vector?