The origin of carbon isotope vital effects in coccolith calcite

摘要

For over half a century, calcite microfossils have been used to study Earth’spalaeoceanography and palaeoclimate. Coccoliths, readily fossilized calciteplates produced by a group of single-celled surface-ocean dwellingalgae called coccolithophores, have composed a significant fraction of thecarbonate component of pelagic sediments since the Late Triassic. However,coccoliths remain underused in palaeo-reconstructions, due largely toa lack of understanding of what controls their isotopic composition. Precipitatedin an intracellular chemical and isotopic microenvironment, coccolithsexhibit large and enigmatic departures from the isotopic composition ofabiogenic calcite termed vital effects. Here we show that carbon isotopicvital effects in coccolith calcite are largest when cellular carbon utilisationis high, and disappear when utilisation is low. The so-called isotopicallyheavy and light group coccoliths, characterised by respectively positive andnegative vital effects, are produced by coccolithophores with respectivelyhigh and low calcification to photosynthesis ratios, and are dominantly theresult of competing Rayleigh-type fractionation processes within the cell.We present a formalised, biologically grounded framework for accountingfor previously overlooked influences on the carbon isotopic vital effect incoccolith-calcite, which is compatible with trends in oxygen isotopes. Withthis mechanistic insight, we discuss the theoretical potential for, and currentlimitations of, a coccolith-based CO2 paleobarometer that may eventuallyfacilitate use of the ubiquitous and geologically extensive sedimentaryarchive of this material for application in palaeoclimate research.