Molecular carbon isotopic (13C/12C) ratios of a selected marine organic biomarker compound (a C37 alka-dienone) from the Pigmy Basin (Deep Sea Drilling Program Hole 619) in the northern Gulf of Mexico varied by 5.3% in sychrony with the last ~100 ka glacial-interglacial cycle. This carbon isotopic (&dgr;13C) variation is likely due to the effect of climatically linked variations in concentrations of CO2(aq) on photosynthetic fractionation of carbon isotopes by phytoplankton. Measurement of the &dgr;13C of the alkadienone (&dgr;37:2) to reconstruct the carbon isotopic composition of total marine organic carbon permits improved estimates of the time-varying &dgr;13C of preserved marine organic carbon end members. With the time series record of marine end members, the concentrations and net accumulation rates of terrigenous Corg and marine Corg are calculated and their paleoceanographic significance is considered. Although this method may be improved by consideration of additional biomarkers representative of diverse marine and terrigenous inputs, it is the first application of molecular isotopic analysis to quantifying marine and terrigenous Corg concentrations. Further application of such techniques will be become valuable in reconstructing regional and global budgets of marine and terrigenous Corg accumulation.