ISOTOPES OF OXYGEN, CARBON, AND STRONTIUM TRACK CHANGES IN UNUSUAL ENDOGENIC CARBONATE SEDIMENTATION IN BEAR LAKE, UTAH AND IDAHO, OVER THE LAST 16,000 YEARS

Bear Lake, an oligotrophic, mesosaline lake in northeastern Utah and adjacent Idaho, has been producing large quantities of various endogenic carbonate minerals for the past 16,000 years. Prior to that, small amounts of calcite and dolomite in the sediments probably are detrital, derived from Pleozoic carbonate rocks in the Bear River Range to the west of the lake. The history of sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, percent organic carbon, percent CaCO₃, and X-ray diffraction mineralogy on samples from sediment traps, gravity cores, and piston cores. Historically, the Bear River, the largest river in the Great Basin and the main source of water for Great Salt Lake, did not enter Bear Lake until it was artificially diverted into the lake at the beginning of the 20^th century. This diversion profoundly changed the chemistry of the lake; most imporantly it greatly decreased the Mg/Ca ratio. The diversion of the river into the lake is well documented by the mineralogy of endogenic CaCO₃ and isotope ratios. During the last glacial interval, when increased Pacific moisture filled the large lakes of the Great Basin, the Bear River apparently did enter Bear Lake depositing red, calcareous, silty clay. About 16 cal. ka, the Bear River became disconnected from Bear Lake. A combination of warmer water, increased evaporation, and increased organic productivity triggered the precipitation of carbonate, first as low-Mg calcite. The isotopic evidence shows that the salinity of the lake increased due to evaporation and aragonite began to precipitate about 11 cal. ka. A return to calcite deposition between 8.5 and 7.5 cal. ka, accompanied by lower O-, C- and Sr-isotope ratios, indicates a brief re-entry of Bear River into the lake. However, aragonite is the dominant mineral that accumulated in bottom sediments of the lake during the last 7 ky, comprising an average of about 75% of the sediments. However, after diversion of the river into the lake the mineralogy of endogenic CaCO₃ changed to high-Mg calcite collected in sediment traps. However, high-Mg calcite is only a minor component (ca. 5%) of surface sediments due to a predominance of reworked aragonite that is at least 100 years old.