INFLUENCES OF LIMNOLOGIC REGIME AND ANTHROPOGENIC SO42- LOADING ON MODES OF δ13CDIC VARIATION IN CACO3 - FORMING LAKES

Biogeochemical processes occurring in the bottom waters of stratified lakes (hypolimnia) contribute to the mixed-layer (epilimnetic) and whole-lake behavior of dissolved inorganic carbon stable isotopic values (d¹³C_DIC). The resulting isotopic effects are poorly characterized, especially in small lakes and those lakes that have the complication of precipitating/dissolving carbonate minerals in addition to effects of microbial metabolism and redox reactions. We present new d¹³C_DIC data from seasonally and permanently stratified, carbonate-producing lakes such as are commonly chosen for paleolimnological studies. Various oxic and anoxic hypolimnetic transformations evidently contribute to d¹³C_DIC values in the water column, and it may be possible to distinguish the relative dominance of these processes by relying upon seasonal geochemical characterization of each lake (water-column structure, pH, redox potential, and evolution of ionic composition) along with d¹³C data. Coupled with solute and fluid balance considerations (e.g., ionic gradients, carbonate system buffering, hypolimnetic vs. epilimnetic volumes), this information allows disparate lakes to be grouped into a small number of coherent modes, with distinct spatiotemporal relations between [DIC] and d¹³C_DIC. Recognition of these modes provides an improved framework for the interpretation of d¹³C paleorecords extracted from both endogenic carbonate minerals and organic matter.

We use water-column and sediment trap data from an anthropogenically meromictic, recently manipulated urban lake to test our prior interpretations based on (primarily) naturally functioning lake systems. In 2004 Lake McCarrons was treated with alum (aluminum sulfate) to help combat internal lake phosphorous loading and consequent lake eutrophication, resulting in a > 5-fold increase in [SO₄^2-]. Following this manipulation of lake solutes, d¹³C_DIC-[DIC] relationships changed qualitatively, suggesting that methanogenesis has been supplanted by sulfate reduction as a dominant process of hypolimnetic organic matter degradation. Consequent effects on d¹³C_DIC realigned McCarrons relative to other lakes, and highlight the role of solute loading and microbial community dynamics on lake d¹³C_DIC evolution.