What controls planktic foraminiferal calcification?

Abstract. Planktic foraminifera are key producers of pelagic carbonate, and their shell weight is suggested to have been influenced by the environment in which they calcify. However, there is debate about the use of size-normalised weight (SNW) as a proxy, as some authors invoke a carbonate system control on calcification (and by extension SNW as a pCO2 proxy), while others suggest that species optimum conditions, nutrient concentration, or temperature drive shell weight. To better understand this proxy, we investigate what drives SNW and whether discrepancies in the proposed control on weight are due to differing data collection methodologies and/or regionally different drivers. We integrate new and published SNW data with environmental hindcast data from the CMIP6 modelling suite. Using Bayesian regression modelling, we find that the environment alone does not explain the variability in SNW across species. Although physiology likely modulates the response to the environment, we find little evidence of a unifying driver at the ecogroup level. Instead, we identify species-specific responses associated with drivers including (but not limited to) the carbonate system, which are likely different between ocean basins. We hypothesise that this is partly influenced by cryptic species and regional phenotypic plasticity in changes to shell weight that are not well understood, such as the thickness of calcite deposited during some species' reproductive phases. Consequently, which species to use as a pCO2 proxy or whether multiple species should be used in parallel to reduce uncertainty should be carefully considered. We strongly encourage the regional testing and calibration of pCO2–SNW relationships.