Fresh to Hypersaline Transitions Across Two Glacial Cycles at Great Salt Lake, Utah

Abstract

Great Salt Lake (GSL), Utah, USA, is a large hypersaline lake, but during short‐lived (∼10–20 kyr) pluvials, it was an expansive freshwater lake. We update the age model of GLAD1‐GSL00‐4, a 120‐m sediment core recovered by the Global Lakes Drilling (GLAD) project, with new U‐series measurements dating evaporites representing the driest times in lake history and providing a new basal age of 236.3 ka (2 σ uncertainty = 6.7 kyr) during the penultimate interglacial. We measure microbial membrane lipid abundances, dialkyl glycerol diethers and glycerol dialkyl glycerol tetraethers (GDGT), to reconstruct salinity over the last two glacial cycles. High and variable salinity precludes use of GDGTs as a proxy for temperature. The freshwater, deep, expanded lake phases “Bonneville” and “Little Valley,” here dated to 30.3–16.1 ka (2 σ  = 1.0–1.4 kyr) and 140.4–134.6 ka (2 σ  = 5.5–5.7 kyr) respectively, occur during deglacial pluvials and briefly interrupt the hypersaline conditions which dominate the record. The salinity rise following the last pluvial agrees with water balance reconstructions from well‐dated lacustrine carbonates. However, evaporite (thenardite) deposition dated to 16.1–12.2 ka (2 σ  = 0.8–1.0 kyr) contrasts with Holocene timing elsewhere in the basin. After the Little Valley highstand, a salinity increase culminates in halite precipitation. Despite four disparate halite dates spanning 90–160 ka, age model assignment agrees with U‐series dating of Little Valley shoreline tufa. This continuous salinity record provides context for lake expansion/contraction followed by evaporite deposition across two glacial terminations, consistent with regional pluvials.