2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 218-4
Presentation Time: 9:45 AM


JOHNSON, Michael R., Department of Geoscience, University of Wisconsin - Madison, Weeks Hall, 1215 W Dayton St, Madison, WI 53706 and GEARY, Dana H., Dept. of Geoscience, Univ of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706

Patterns in stable carbon and oxygen isotopes of lake carbonates can serve as records of environmental and hydrological change. Over its 8 million-year history, the late Miocene Lake Pannon experienced repeated regressions and transgressions driven by sea level fall, flooding, evaporation, and delta progradation. Decreasing oxygen isotope values observed in mollusc shells have been alternately interpreted as an indicator of freshening lake water, or as indicators of changing humidity. Carbon isotope values have been used to suggest hyperalkaline conditions. However, both of these proxies were doubtless influenced by the terrestrial environment, and other climatic factors. Freshwater input carries organic terrestrial carbon, constraining the composition of that carbon is important to accurately assessing fluctuations in lake carbon. Additionally, the oxygen compositions of precipitation and inflowing streams and rivers can deviate from modeled values in regions of high relief, but oxygen isotopes in mammalian tooth enamel can serve as a proxy for surface water. We investigated the possible controls on lake carbon and oxygen through modeling and stable isotope analyses on fossil material. A mass-balance model was used to assess the relative importance of evaporation, precipitation, and riverine input at different stages of lake evolution. Stable isotope profiles from endemic molluscs were used to calibrate the model, and to investigate short-term changes in lake environment. Terrestrial conditions were reconstructed from the stable isotope composition of equid tooth enamel.