Temperature-dependent carbon isotope fractionation in coccolithophores

Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO 2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones. Methods: To clarify the non-CO 2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica , was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO 2 (aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO 2 (aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system. Results and Discussion: We find ε p in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects ε p more strongly than CO 2 (aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the U K’ 37 alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones ε p , and may therefore improve past pCO 2 estimates.