MULTI-PROXY EVIDENCE FOR ENVIRONMENTAL CHANGE DURING THE LAST TWO GLACIAL-INTERGLACIAL CYCLES FROM CORE BL00-1, BEAR LAKE, UTAH-IDAHO

A continuous, 120-m-long core (BL00-1) from Bear Lake, Utah-Idaho, contains evidence of hydrologic and environmental change over the last two glacial-interglacial cycles. The core was taken from 41.95N, 111.31W, near the depocenter of the 60-m-deep, spring-fed, mesosaline-alkaline lake, where lacustrine sediment has accumulated continuously. Chronological control is provided by ¹⁴C, paleomagnetic excursions, U-series, and correlations with Devils Hole. Analyses have been completed at multi-centennial to millennial scale, including (with decreasing temporal resolution): rock-magnetic properties; O and C isotopes on bulk-sediment carbonate; organic and inorganic C contents; mineralogy (XRD); Sr isotopes on bulk carbonate; ostracode taxonomy; O and C isotopes on ostracodes; pollen and diatom assemblages. Most of the core comprises massive silty clay and marl, with an endemic ostracode fauna, and fluctuating carbonate content (20 ± 9%), isotopic values (δ¹⁸O = -9.8 ± 1.1‰ for bulk carbonate), rock-magnetic properties, and forest pollen assemblages. The variability reflects changes in water and sediment discharge from the glaciated headwaters of the dominant tributary, Bear River, and the processes that influenced sediment delivery to the core site, including lake-level changes. Although its impact has varied, Bear River has remained tributary to Bear Lake during most of the last 200 ka. The lake disconnected from the river and, except for a few brief excursions, retracted into a topographically closed basin during global interglaciations (OIS 7, 5, 1). These intervals are dominated by endogenic aragonite (60-80%) with high δ¹⁸O values (-5.8 ± 1.7‰), indicative of strongly evaporitic conditions, and contain increased desert shrub pollen. The ⁸⁷Sr/⁸⁶Sr values (< 0.7100) also increase, quartz/dolomite ratios decrease, and Fe-oxide mineral concentrations decrease as expected in absence of Bear River inflow. The diversity of paleoenvironmental conditions inferred from BL00-1 reflects the influence of catchment-scale processes superposed on the overriding effect of open- or closed-basin status driven by glacial-interglacial changes in the delivery of Pacific moisture.