Late Pleistocene Variability of the Carbon Isotopic Composition of Organic Matter in the Eastern Mediterranean: Monitor of Changes in Carbon Sources and Atmospheric CO 2 Concentrations

The organic carbon isotopic record of the sapropels (S1 and S3–S10) and intercalated marl oozes has been determined in a 12‐m piston core from the eastern Mediterranean. The δ 13 C organic values are systematically lighter (mean=−21.0±0.82 ‰) in all sapropels and heavier (mean=−18.8±1.07‰) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the C organic /N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in δ 13 C organic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichment in 12 C, implying that all values were even heavier originally, and there is no secular trend in the δ 13 C organic record. The observed differences in δ 13 C organic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO 2 . First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13 C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO 2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO 2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO 2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO 2 to the mixed layer by upwelling, and this could have further lightened the δ 13 C organic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO 2 levels over longer time periods than can be obtained from ice cores.