Applying Brillouin thermometry as a novel tool for reconstructing temperatures, depths, and seasonal biases of Holocene/Pleistocene Searles Lake, California

Abstract

Paleoclimate records from lakes of the southwestern USA have been limited by a lack of independent paleothermometers, resulting in conflicting characterizations of millennial-scale variability in temperature and moisture. Here a novel method called Brillouin thermometry is applied to halite-bearing dry intervals of the late Pleistocene/Holocene (45–0 ka) core record of Searles Lake, California. Halite from the sediment-water interface records lake bottom temperatures during dry, high salinity periods. Analysis of modern saline lakes of various chemistries, depths, climate zones, and mixing regimes shows that: 1) average bottom water temperature is approximately equal to mean annual air temperature, and 2) annual range of bottom water temperature is inversely proportional to lake depth. Brillouin temperatures for eight halite intervals 30.6 ka to 8.5 ka range from 11.8 ± 3.6 to 22.4 ± 3.2 °C. Bottom water temperature variability indicates paleolake depths of ~10 m during halite precipitation. Brillouin thermometric results are then assessed in comparison with two additional temperature proxy records from the same Searles Lake sediment core: 1) branched glycerol dialkyl glycerol tetraethers (brGDGTs) extracted from wet mud intervals, and 2) thermodynamic constraints from evaporite minerals and mineral sequences. Temperatures from brGDGTs for mud intervals 44.7 ka to 3.6 ka range from 13.4 ± 2.8 to 23.9 ± 3.0 °C. Comparisons of Brillouin/brGDGT temperatures with predicted equilibrium temperatures of salt crystallization indicate intervals where seasonal temperature variability forced the dissolution and/or recrystallization of existing temperature-sensitive evaporites. The multiproxy temperature record of Searles Lake agrees with other regional records at glacial/interglacial timescales but displays a wider degree of millennial-scale variability, with temperatures during the last glacial ranging from 8.3 °C below modern mean annual temperatures to 3.8 °C above.