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New constraints on water temperature at Lake Bonneville from carbonate clumped isotopes


In the Great Basin, paleoshoreline reconstructions for closed basin lakes indicate that highstands correspond with Pleistocene glacial maxima. Lacustrine deposits at these sites are physically and chemically sensitive to changes in the balance between precipitation and evaporation, which drive lake level fluctuations. However, uncertainties remain regarding the magnitude of temperature change in the region between glacial and interglacial periods. In this thesis, carbonate clumped isotope thermometry is applied to define the parameters of temperature, evaporation, and precipitation at Lake Bonneville. Bonneville was the most expansive lake in the Great Basin during the Last Glacial Maximum (LGM). Multiple phases of carbonate were evaluated, including aragonitic shells of two taxa of lacustrine gastropod, marls, and tufa. Carbonate clumped isotope results of ancient material were calibrated by comparison to measurements of modern lacustrine carbonates at sites, where water temperatures are constrained. Summer water temperatures were estimated from gastropod and marl samples during the LGM, from 23 to 19 ka BP. Tufa and gastropod samples were also analyzed during the Stansbury Oscillation phase of lake history, from 25.8 to 24.5 ka, and the Bonneville Highstand and Provo phases of the lake, from 18 to 14.5 ka. Reconstructed water paleotemperatures were applied to estimate summertime and mean annual air temperatures using lake-atmospheric transfer functions. Warm season (May through September) water temperatures are estimated to be 7.1±0.6ºC lower than present-day values of 18.1 to 19.0ºC in the Bonneville Basin, and 3.1±0.9ºC lower than present-day values of 19.7ºC in the Sevier Subbasin. Annual air temperatures are estimated to be 9.2±0.8ºC lower than present-day values of 8.7 to 9.5ºC in the Bonneville Basin, and 3.3±1.1ºC lower than present-day values of 10.1ºC in the Sevier Subbasin. Clumped isotope temperature reconstructions exceed the PMIP climate model ensemble mean in the Bonneville Basin, but suggest less cooling than the ensemble mean in the Sevier Subbasin. Clumped isotope temperatures are also combined with carbonate oxygen isotope (δ18O) ratios to determine water δ18O associated with mineral precipitation, which tracks evaporative enrichment in lakes. Results indicate that lake water δ18O was above modern precipitation values, but that enrichment of water was less than at the modern Great Salt Lake. Reconstructed evaporation, derived from clumped isotope temperatures, varied from 60 to 83 percent of modern values, while precipitation over the lake was determined to be 75 percent of the modern rate. These results are lower than the PMIP3 ensemble mean, and suggest that Lake Bonneville transgressed, at least initially, as a result of lower evaporation, and not increased precipitation.

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