Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO2 levels

Abstract

Reports of the 13 C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO 2 in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO 2 is found to be significantly correlated with concentrations of dissolved CO 2. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO 2 developed here can be employed to estimate concentrations of CO 2 dissolved in ancient oceans and, in turn, partial pressures of CO 2 in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved‐inorganic‐carbon system and of air‐sea equilibria. Paleoenvironmental temperatures for each sample are estimated from re‐constructions of paleogeography, latitudinal temper‐ ature gradients, and secular changes in low‐latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO 2 were over 1000 μatm 160–100 Ma ago, then declined to values near 300 μatm during the next 100 Ma. Analysis of a high‐resolution record of carbon isotopic fractionation at the Cenomanian‐Turonian boundary suggests that the partial pressure of CO 2 in the atmosphere was drawn down from values near 840 μatm to values near 700 μatm during the anoxic event.