Carbon stable isotope fractionation in marine systems: open ocean studies and laboratory studies
The stable carbon isotopic composition of surface water particulate organic matter (δ13CPOM) has been shown to correlate with the concentration of dissolved carbon dioxide [CO2(a,q)]. The potential of this proxy has been exploited mainly by palae-oceanographers for the reconstruction of past variations in the ocean and atmospheric CO2, derived from the analysis of the carbon isotopic composition of organic matter in marine sediments (δ13CSOM). Although these correlations have been shown to be useful, a demonstration of an apparent empirical relationship between two parameters is not necessarily indicative of a directly coupled process and it is obvious that we need to develop a sound theoretical basis for the interpretation of any relationship. The main mechanism suggested to account for the observed covariations in δ13CPOM and [CO2(aq)] J is not yet very well constrained. It is based on the assumption that in the majority of phytoplankton species, inorganic carbon acquisition and assimilation occurs solely by the diffusive entry of CO2(aq), and carboxylation by ribulose 1,5 bis-phosphate carboxylase/oxygenase (RUBISCO). Under these circumstances the difference in isotopic composition between CO2(aq) (δ13CCO2(aq)) in the surrounding sea water and the phytoplankton cell, ϵp, (δ13CCO2(aq) − δ13CPOM) is an expression of the fractionation during cellular fixation of carbon as modified by the degree to which carbon transport may be rate limiting with respect to carbon fixation. When this expression is used in higher plant stable isotope analysis the difference is termed Δ, ((δ13CCO2 − δ13CORG)/(1 +δ13CORG); the difference between ϵp and Δ at δ13CCO2 and δ13CORG values normally encountered tend to be negligible.