Non-equilibrium fractionation of the stable isotopes of carbon and oxygen during precipitation of calcium carbonate by marine phytoplankton

Field: Earth and Planetary Sciences (Physical Sciences)

The chemistry of the stable isotopes of carbon and oxygen during the precipitation of calcium carbonate by marine phytoplankton (cocco- 1ithophorids) was investigated. Samples studied include cocco1ithophorids grown in laboratory culture and calcareous nannofossils from Recent and ancient sediments. The stable isotope ratios of carbon (δ13C) of the calcium carbonate coccoliths of three species of cultured coccolithophorids and the δ13C of the dissolved inorganic carbon of the culturing medium were measured. The carbon of the cocco1ithophorid calcite was found not to be in isotopic equilibrium with the dissolved inorganic carbon of the culturing medium. The δ13c values could not be correlated with temperature, light intensity or aeration. Dudley (1976) had previously shown that the oxygen of the calcite (for the same set of samples) was not in isotopic equilibrium with the water in the growth medium. The biological fractionation model of Dudley (1976) can qualitatively explain the variation of δ13C of the coccoliths. The δ13C may or may not be a steady-state value depending on the intracellular reaction rates. Anomalously high δ13C values for coccoliths of Cricosphaera carterae grown under conditions of high temperature and high illumination are evidence for the direct utilization of HCOˉ3; for photosynthesis. The δ13C and δ180 of Recent nannofossils and foraminifera from ocean sediments were determined. The oxygen isotopic composition of the foraminifera indicated that the calcite was precipitated at or near oxygen isotopic equilibrium with the inferred isotopic composition of the seawater as reported by other investigators. The nannofossil calcite was not in oxygen isotopic equilibrium with seawater. There was, however, a covariance between δ180 of nannofossils and δ180 of foraminifera. An isotopic temperature equation for Recent nannofossils was derived using this covariance [t(°C) = 25.6 - 3.6(δC - δW)]. Temperatures calculated using this equation are significantly different from those calculated from the commonly used isotopic equilibrium equation of Epstein et al. (1953). Neither the carbon of the foraminifera calcite nor the carbon of the nannofossil calcite was in isotopic equilibrium with the total dissolved inorganic carbon of seawater. The δ13C and δ180 of nannofossils from DSDP sites 277, 279, 281 and 284 were determined and compared with published isotopic compositions of foraminifera from the same cores. The δ180 values of the nannofossils were strongly correlated with those of the foraminifera. While the nannofossils were not necessarily in oxygen isotopic equilibrium with their environment, the correlation implied that they responded to the same environmental parameters as the foraminifera. A nannofossil isotopic temperature equation was derived using this correlation [t(°C) = 17.6 - 2.9(δC - δW)]. Temperatures calculated using this equation are similar to those calculated using the equilibrium equation but different from those calculated using the equation derived above. The carbon of the nannofossils was in some cases closer to isotopic equilibrium with the total dissolved inorganic carbon of seawater than that of the planktonic foraminifera.