Carbon and oxygen isotopic composition of selected limestones and fossils

Abstract

Isotopic analyses are presented for 500 samples of marine and freshwater limestones and fossils. Mean carbon isotopic compositions, in the conventional δ-values, relative to the Chicago PDB standard, are +0.56 per mil (selected marine group) and −4.93 per mil (selected freshwater group); mean oxygen isotopic compositions are −5.25 per mil (marine) and −8.66 per mil (freshwater samples). The average carbon isotopic composition of marine limestones is nearly constant from the Cambrian age group to the Tertiary group and it is concluded that no great change has occurred in the carbon isotopic composition of marine bicarbonate over geologic time. Oxygen isotopic compositions of marine limestone exhibit a progressive increase in average oxygen-18 content from the oldest to the youngest age group, possibly due mainly to postdepositional recrystallization and oxygen exchange in the presence of waters different from those of the original environment. Mean δO 18 is −9.7 per mil for the selected Cambrian group, − 1.2 per mil for the Quaternary sample group. Average carbon and oxygen isotopic compositions of freshwater limestones show less regular changes with time of formation. Carbon isotope ratios are significantly different from those of marine limestones in sample groups as old as Carboniferous or Devonian, whereas oxygen isotope ratios differ in marine and freshwater sample groups as old as Jurassic. A graphic plot of δC 12 versus δO 13 gives environmental separation of more than 80 per cent of the samples, and the percentage of correct diagnosis is considerably improved in Jurassic and younger samples. It is concluded that carbon and oxygen isotopic compositions of limestones and calcareous fossils provide significant but not infallible criteria of depositional environment for samples of a wide range of geologic age. The difference in C 12 content between marine and fresh-water carbonates is attributed mainly to a variable contribution of C 13-deficient carbon from land plants and humus, which is added to freshwater systems in several stages, but has a relatively minor influence in most marine environments.