2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 14
Presentation Time: 8:00 AM-12:00 PM


LASCU, Ioan, Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, MYRBO, Amy, Limnological Research Center, University of Minnesota, Minneapolis, MN 55455 and SHAPLEY, Mark D., Institute of Marine Science, University of Alaska Fairbanks, 333 Irving II, Fairbanks, AK 99775, amyrbo@umn.edu

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 (d13CDIC). 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 d13CDIC 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 d13CDIC 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 d13CDIC. 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 [SO42-]. Following this manipulation of lake solutes, d13CDIC-[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 d13CDIC realigned McCarrons relative to other lakes, and highlight the role of solute loading and microbial community dynamics on lake d13CDIC evolution.