2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 7
Presentation Time: 9:30 AM


DEAN, W.E.1, BISCHOFF, J.L.2, FORESTER, R.M.1, ROSENBAUM, J.G.1, SIMMONS, K.R.1 and SKIPP, G.L.1, (1)U.S. Geol Survey, MS 980 Federal Center, Denver, CO 80225, (2)U.S. Geol Survey, MS 470, 345 Middlefield Road, Menlo Park, CA 94025, dean@usgs.gov

The Bear River was artificially connected to Bear Lake during the early 1900s, but apparently did not flow into the lake throughout most of the Holocene. Sediments deposited over the last 26,000 years were recovered in three overlapping piston cores. The sediments deposited during the last glacial interval consist of carbonate- and organic-poor, red, silty clay. These glacial sediments probably indicate a more active Bear River flowing directly into Bear Lake. The Bear River left Bear Lake during the Holocene, and carbonate began to precipitate, first as calcite, then aragonite as the salinity of the lake increased. Aragonite precipitation was interrupted briefly about 9000 years ago during a period of freshening, probably caused by re-entry of Bear River.

Sediments deposited over the last 250,000 years were recovered in two parallel holes with a maximum length of 120 m using the new Global Lake Drilling (GLAD800) platform (now officially the R/V Kerry Kelts). The dominant sediment in most of the 120-m section is calcareous, gray to black, silty clay with calcite as the dominant carbonate mineral. The dominance of siliciclastic sediment indicates that the Bear River commonly was connected to Bear Lake, but either no red sediment was deposited or the red color was reduced to black and gray in the organic-rich sediments. XRD results indicate that two aragonitic intervals occurred prior to the Holocene aragonite. Preliminary volcanic-ash and U/Th dating indicate that these two intervals were deposited during the last two interglacials equivalent to Oxygen Isotope Stages 5 and 7. These high carbonate, aragonitic interglacial intervals coincide with warm continental climates, warm SSTs, and increased coastal upwelling recorded in recent ODP cores along the Pacific margin.