Productivity Induced Changes in the Carbon-13 Isotopes of Organic Matter and Carbonates, Lake Greifen: A Possible Paleoproductivity and Paleo-CO₂ Indicator

David J. Hollander, Judith A. McKenzie, Kenneth J. Hsu

During productivity, organic matter preferentially incorporates the lighter carbon isotopes (¹²C), leaving behind a dissolved inorganic carbonate (DIC) surface water reservoir enriched in the heavier carbon isotope (¹³C). After the annual winter turnover and renutrientification of the surface waters, Lake Greifen provided us with an opportunity to study productivity as an isolated process and its effect on carbon-13 cycling in the organic matter-carbonate-CO₂ system. From May to August, Lake Greifen surface waters underwent a positive shift (^Dgr^dgr¹³C_{(DIC and calcite) May-August} = +2.5 ^pmil) in the carbon-13 ratios of the DIC and the inorganically precipitated calcite, while the particulate organic carbon (POC) und rwent larger shifts (^Dgr^dgr¹³C_{(DIC and calcite) May-August} = +8 ^pmil), also toward positive carbon-13 values. The isotopic composition of the DIC and calcite reflect the cumulative removal of carbon-12-enriched organic matter from the surface water reservoir. The large variations in the carbon-13 ratios of the POC indicate that, besides the surface reservoir carbon-13 effect, another fractionation process must be simultaneously operating to account for the anomalous positive carbon-13 excursion. The carbon-13 isotopic fractionation between the DIC and POC varied from ^Dgr^dgr¹³C_(DIC-POC) = 24.2 ^pmil in May to ^Dgr^dgr¹³C_(DIC-POC) = 17.78 ^pmil in August. It is correlated with the decreasing concentrations of CO_{2</ UB> in the surface waters and provides evidence that the CO2 concentration/fractionation accounts for the additional positive carbon-13 isotopic shift observed in the POC. A model for the carbon-13 cycling with increasing productivity in the organic matter-carbonate-CO2 system during CO2 reflux and basin stagnation (eutrophication) is proposed.}

Over the past 100 years the progressive eutrophication of Lake Greifen was the result of increasing rates of productivity as a consequence of higher nutrient supply rates. The transition from chalk to sapropelic sediments in Lake Greifen is associated with an initial shift to negative carbon-13 ratios in organic matter (^Dgr^dgr¹³C_(org) = -2.5 ^pmil) and calcites (^Dgr^dgr¹³C_(calcite) = -0.8 ^pmil) and increasing carbon-13 fractionation between the two components (^Dgr^dgr¹³C_{(calcite-organic)} = 28.2 ^pmil). These trends, when placed in the framework of the carbon-13 lake model, suggest that initially a CO₂ reflux circulation system was active where excess organic matter (enriched in carbon-12) was oxidized an the carbon-13 depleted CO₂ was recycled in the lakes, thereby decreasing the carbon-13 surface water reservoir and increasing the concentration of CO₂ in the surface waters. A second, stratigraphically younger trend in the carbon-13 ratios occurs within the sapropelic (eutrophic) sediments and is characterized by positive shifts both in the organic matter (^Dgr^dgr¹³C_(org) = +6 ^pmil) and the calcite (^Dgr^dgr¹³C_(calcite) = +2.5 ^pmil) and a decreasing fractionation between the calcite and the organic matter (^Dgr^dgr¹³C_{(calcite-organic)} = 24.75 ^pmil). These trends, relative to the lake model, suggest that with an extreme productivity increase and the subsequent eutrophication of the bottom waters, xcess organic matter was removed from the surface waters, thereby enriching the surface reservoir in carbon-13 (responsible for the positive carbon-13 shifts) and decreasing the concentration of CO₂ in the surface waters (responsible for the decreasing carbon-13 fractionation between calcite and organic matter). The use of the carbon-13 isotope lake model as a stratigraphic tool aiding in the interpretations of paleoproductivity and paleo-CO₂ concentrations is viable for lacustrine deposits.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.