Local variation of 13C and 18O content of mollusk shells and the relatively minor temperature effect in marginal marine environments

Abstract

Oxygen isotopic data for modern shells from known environments are interpreted as indicating that δ 18O of fossil shells can not be used as a basis for conclusions regarding palaeotemperatures in marginal marine environments of ancient seas, because the temperature effect is masked by larger effects due to river water additions and to evaporation and restricted mixing, which cause local variations in the oxygen isotopic composition of the water. The carbon isotopic data can be more clearly and unambiguously related to environment. Shell samples from marginal and intermediate salinity environments exhibit a progressive increase of 13 C 12 C ratio in the direction of increasing marine influence and decreasing influence of continental organic material and derived CO 2. There is a general positive correlation between salinity of the water and the 13C-content of shell carbonate, but that correlation does not extend to all of the sample groups from marginal lagoons, where ambiguous carbon isotopic compositions may result from the effects of abundant aquatic vegetation combined with restricted mixing with open marine waters. In attempting to apply carbon isotopic analyses of fossil carbonate to palaeogeography and to the interpretation of ancient near-shore environments, it probably will be more generally meaningful to relate the carbon isotope ratio of fossils to proximity to sources of continental carbon, rather than to palaeosalinity.