STRATIGRAPHY, GEOCHEMISTRY, AND PALEOCLIMATIC IMPLICATIONS FROM ENDOGENIC CARBONATE MARL SEDIMENTS OF THE TULE VALLEY SUB-BASIN OF PLEISTOCENE LAKE BONNEVILLE

Lacustrine marl deposits of the Pleistocene Lake Bonneville system contain critical records of hydrological, temperature, and depositional variability during the last glacial and deglacial period. This study investigates the geochemical and physical sedimentary characteristics of a ~2m marl sequence from the Tule Valley Basin, a small southern sub-basin of the larger lake complex. This likely climatically sensitive basin contains well-preserved endogenic carbonate sediments that were deposited following the initial influx of alkaline lakewaters during the Tule Valley Flood at 19.3 ka. Geochemical and physical analyses of these deposits lead to a more comprehensive understanding of depositional mechanisms and the nature of the paleoclimate record of these chemically sensitive sediments.

Radiocarbon dating of bulk marl and gastropod samples confirms the pelagic nature of these deposits and suggests a freshwater reservoir offset (FRO) of 700 ¹⁴C years assuming the base of the deposit represents the Tule Valley Flood at 19.3 ka. This may have implications for radiocarbon measurements from other basins which do not account for any FRO. Sedimentary structures such as alternating lamina of lineated diatoms and carbonate indicate a seasonal or current-driven depositional mechanism. Aragonite:calcite ratios vary widely throughout the section (up to 40:1) and indicate five possible periods of lake level drop as inferred from periods of heightened aragonite precipitation and thus more evaporative lake conditions. A large spike in nitrogen and hydrogen and a concurrent drop in total carbon and aragonite:calcite values from approximately 17.6-16.5 ka provides evidence for a large freshening event or other prolonged influx of organic-rich and Mg-depleted waters to the sub-basin. The correlation of stable isotope data, carbonate mineralogy, and elemental composition of marl sediments infers highly evaporative conditions in the Tule Valley as well as heightened sensitivity to climatic variability during the Late Pleistocene when compared to other larger basins in the Lake Bonneville system. Clumped isotope measurements, when coupled with oxygen isotope data, will help to delineate the relative effects of temperature and water balance on the paleoclimate record contained in marl sediments.