CONSTRAINED ISOTOPE MODELING IN A LATE MIOCENE LAKE FROM CENTRAL EUROPE

Lake Pannon in the Late Miocene Pannonian Basin System provides an excellent opportunity to investigate the complex interactions between terrestrial and aquatic environments through an interval of profound climate change. The evolution of Lake Pannon consisted of an initial low stand in which the basin was underfilled, a rapid (~1 myr) transgressive phase likely driven by increased precipitation and runoff, and a longer (~5 myr) regressive phase driven by the advance of the paleo-Danube delta. I compiled paleogeographic, paleoclimatic, and stable isotope data for these intervals, and used the data to constrain two mass balance models. The "kinetic model" estimated the oxygen isotope composition of lake water under varying conditions of evaporative loss and relative humidity. These estimates were compared to an independent estimate of lake water composition derived from the composition of lacustrine bivalves. The "equilibrium model" assumed that all isotope fractionation associated with evaporative loss occurred under temperature-dependent equilibrium conditions, and that lake composition was equal to the estimate derived from bivalves. These assumptions allowed me to investigate the temperature and evaporative water loss necessary to balance inflow with the observed composition of bivalve shells. The results of each model have major implications for environmental change in the Pannonian Basin System. Whereas previous work suggested that Lake Pannon was largely evaporative, my models indicate that a major outflow was needed to balance the lake volume and surface area lost to deltaic sedimentation. The initial phase of Lake Pannon could have been largely evaporative, particularly if summer were the dry season. However, a large outflow in the later stages would have provided a means of removing salt from the basin and produced freshwater conditions.