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 CO₂ 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-CO₂ 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 CO₂(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO₂(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 CO₂(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^K₃₇ alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones e_p, and may therefore improve past pCO₂ estimates. [ABSTRACT FROM AUTHOR]