Vanishing coccolith vital effects with alleviated CO2 limitation.

By recreating a range of geologically relevant concentrations of dissolved inorganic carbon (DIC) in the laboratory, we demonstrate that the magnitude of the vital effects in both carbon and oxygen isotopes of coccolith calcite of multiple species relates to ambient DIC concentration. Under high DIC levels, all the examined coccoliths lacked any offset from inorganic calcite, whereas in low (present-day) DIC concentrations, these vital effects and interspecies differences become substantial. These laboratory observations support the recent hypothesis from field observations that the appearance of interspecific vital effect in coccolithophores coincides with the long-term Neogene decline of atmospheric CO₂ concentrations. The present study brings further valuable constraints on coccolith isotopic compositions by demonstrating the threshold for the absence of vital effects under high DIC regimes. From a mechanistic viewpoint, we show that the vital effect is determined by physiology; growth rate, cell size and relative rates of photosynthesis and calcification, and a modulation of these parameters with ambient carbon availability. This study provides palaeoceanographers with a biogeochemical framework that can be utilised to further develop the use of calcareous nannofossils in palaeoceanography to derive sea surface temperature and pCO₂ levels. [ABSTRACT FROM AUTHOR]