Climate Record in a 240 kyr Sediment Core From Bear Lake, Utah/Idaho

Bear Lake, with an area of 380 km² and a maximum depth of 55 m, lies in a half graben at an elevation of about 2000 m asl. Coring from the GLAD800 platform (now the RV Kerry Kelts) penetrated the upper 120 m of sediment. Prior to this coring, three ¹⁴C-dated cores (4-5 m in length) provided a composite record extending back into the last glaciation (>20 ka) and established a basis for interpreting the long GLAD800 record. Sediments deposited during OIS 2 are reddish silty clays that were transported to the lake by the Bear River and contain glacial flour derived from the Uinta Mountains. These siliceous sediments are relatively magnetic, with magnetic properties dominated by detrital Fe oxides, and contain an ostracode (Cytherissa lacustris) indicative of cold climates and extremely dilute water. Toward the end of OIS 2, the Bear River abandoned Bear Lake and sedimentation became progressively dominated by carbonate precipitation. OIS 1 sediments are composed of endogenic carbonate (≈75%) with a small detrital component. Most of the carbonate is aragonite, and magnetization is extremely low, probably due in large part to the post-depositional destruction of detrital Fe oxides. Preliminary age control for the GLAD800 core (provided by tephra, a U/Th age at 66.4 m on fine-grained endogenic carbonate , and correlation to short ¹⁴C-dated cores) indicates that the cored section extends back to about 240 kyrs. We tentatively interpret two pre-Holocene intervals (61-67 m, and 99-105 m) to represent interglacial periods because of similarities to sediments in OIS 1. Carbonate-rich sediments in both intervals contain large amounts of aragonite. The upper interval is also similar to Holocene sediments in having low magnetization and a pollen assemblage dominated by juniper (Juniperus) and saltbush (Chenopodiaceae). Juniper and saltbush are also important, although less dominant, in the lower interval. Chronologic control, including a U/Th date of 127.7 +/- 3.9 kyrs from the upper interval, indicates that these intervals are probably OIS 5 and 7. Two intervals that contain predominantly siliciclastic material and the ostracode Cytherissa lacustris were probably deposited during glacial intervals OIS 4 and 6. Pollen assemblages in both intervals are dominated by spruce (Picea) and pine (Pinus). In contrast to the red sediments in OIS 2, these sediments are various shades of gray and have highly variable magnetic susceptibility. These differences reflect variable destruction of detrital Fe-oxides and formation of Fe-sufides (including the authigenic magnetic sulfide, greigite) in more reducing bottom-water conditions.