THE GLACIAL TO HOLOCENE TRANSITION IN SEDIMENTS FROM BEAR LAKE, UTAH/IDAHO

Bear Lake, in northeastern Utah and southern Idaho, lies at an elevation of 1800 m, covers an area of 280 km², and has a maximum depth of 63 m. Throughout most of the Holocene, the Bear River did not flow into Bear Lake and Holocene sediments are comprised largely of endogenic carbonate. Glacial-age sediments, however, are comprised of organic-poor siliciclastic material, much of which was delivered to the lake by the Bear River. AMS ¹⁴C dates were obtained on a variety of materials including pollen concentrates, ostracodes, and total organic carbon. Beginning at some time prior to 32 cal ka, a significant amount of glacial flour derived from the Uinta Mountains was contributed to the siliciclastic sediments. In comparison to other clastic material, the glacial flour is finer-grained, hematite-rich (as determined by magnetic properties), and characterized by high Al relative to Ti. Taking the ¹⁴C-based chronology at face value, the content of glacial flour displays a number of well-defined millennial scale fluctuations before peaking at 25-24 cal ka. This suggests that maximum glacial extent in the Uintas occurred significantly earlier than in many areas. Alternatively, the ¹⁴C ages for this part of the record may be too old. For these glacial flour-rich sediments, all ¹⁴C ages were obtained from pollen concentrates that contained sparse degraded pollen, small pieces of charcoal, and other refractory organic material. Such materials may have been sequestered in the catchment. Younger sediments contain much more abundant, well preserved pollen. From 22 to 17 cal ka content of hematite-rich material from the Uintas declined while the proportion of more magnetite-rich material (probably from the local Bear Lake catchment) increased. Beginning at 17 cal ka, shortly after the catastrophic draining of Lake Bonneville, the Bear River gradually abandoned Bear Lake over a period of about 3,000 years, and clastic sediments were replaced by endogenic carbonate. Draining of Lake Bonneville may have contributed to the loss of the Bear River from Bear Lake by reducing lake-effect precipitation and thereby the volume of Bear River flow.