| Sumari: | For over half a century, calcite microfossils have been used to study Earth’s palaeoceanography and palaeoclimate. Coccoliths, readily fossilized calcite plates produced by a group of single-celled surface-ocean dwelling algae called coccolithophores, have composed a significant fraction of the carbonate component of pelagic sediments since the Late Triassic. However, coccoliths remain underused in palaeo-reconstructions, due largely to a lack of understanding of what controls their isotopic composition. Precipitated in an intracellular chemical and isotopic microenvironment, coccoliths exhibit large and enigmatic departures from the isotopic composition of abiogenic calcite termed vital effects. Here we show that carbon isotopic vital effects in coccolith calcite are largest when cellular carbon utilisation is high, and disappear when utilisation is low. The so-called isotopically heavy and light group coccoliths, characterised by respectively positive and negative vital effects, are produced by coccolithophores with respectively high and low calcification to photosynthesis ratios, and are dominantly the result of competing Rayleigh-type fractionation processes within the cell. We present a formalised, biologically grounded framework for accounting for previously overlooked influences on the carbon isotopic vital effect in coccolith-calcite, which is compatible with trends in oxygen isotopes. With this mechanistic insight, we discuss the theoretical potential for, and current limitations of, a coccolith-based CO2 paleobarometer that may eventually facilitate use of the ubiquitous and geologically extensive sedimentary archive of this material for application in palaeoclimate research.
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