EVAPORATIVE EVOLUTION OF OXYGEN AND HYDROGEN ISOTOPES IN SMALL LAKES: MODELING THE INFLUENCE OF GROUNDWATER EXCHANGE

In evaporative climates, isotopic composition of lake water is related to fluid balance and hydraulic residence time, as well as source composition. Values of d¹⁸O and dD generally increase with lake evaporation, hence paleorecord interpretations of these isotopes typically equate increased d values with periods of aridity and vice versa. In sediments from some small evaporative lakes, trends in d¹⁸O may diverge in apparent magnitude or direction of signal from other indicators of hydroclimatic change. Such apparent inconsistency in proxy records can in some cases be attributed to the effects of groundwater exchange on the isotopic trajectory of the evaporating lake. Here we present results from mass-balance simulations of lake isotopic evolution, designed to test the sensitivity of lake composition to groundwater fluxes. The vehicle for examining these processes is an Excel™ VisualBasic model structured around field examples from western Montana. The model incorporates lake hypsometry, precipitation and groundwater composition, climatology, and evaporative isotope fractionation into a fluid and isotope balance scheme with a monthly time step. Our results show that plausible groundwater inflow and outflow states can combine with simple climatic forcing to produce complex and nonintuitive lake compositional evolution.