ADVANCES IN QUANTITATIVE CLIMATE RECONSTRUCTIONS USING HYPERSALINE LAKE EVAPORITES

Hypersaline lakes form in depressions with no outlet, and where loss of water is through evaporation. Hypersaline lakes are simpler than freshwater lake systems because they can be viewed as closed systems with respect to most dissolved species. Climatic and hydrologic variables drive water, salt, and heat equilibria in these closed systems, manifested as changes in lake level, ionic composition and temperature. Integrated reconstruction of past lake levels, brine compositions, and brine temperatures would enable quantitative interpretations of past climate history. However, limited proxy data for hypersaline lakes has hampered this endeavour. Here, we present new methods for exploiting the potential of evaporites as robust archives of past lake conditions. Brillouin spectroscopy, nucleation-assisted microthermometry and laser ablation inductively coupled mass spectrometry on fluid inclusions trapped in halite, now allow characterization of the temperature, density, composition, and volume of ancient hypersaline lake waters. The high sedimentation rates of halite, up to 10 cm/yr, permit these reconstructions to be done at up to subannual resolution. We illustrate the effectiveness of these new methods with case studies in modern, Pleistocene and Eocene lakes including the Dead Sea in the eastern Mediterranean, Searles Lake, California, and the Green River Basin, Wyoming. In the Dead Sea, reconstructed water temperature is notably colder than today at the peak of the present and the last interglacials, and has an increased temperature seasonality during the last interglacial. Brine density increases from the start to the end of each interglacial period, as a response to a ~50% evaporative reduction of the lake volume. Minor elements respond markedly to variations in inflow, with ~100% and ~50% increases in the barium and strontium inventory of the lake, respectively, during the early Holocene humid period. In Searles lake, water temperature reconstructions from halite fluid inclusions and from biomarkers show comparable ranges, with a large variability during the last glacial from 8.3 °C below to 3.8 °C above modern values. In the Eocene Green River basin, water temperatures up to ~30 °C indicate a particularly warm climate.