Unusual Modern and Holocene Carbonate sedimentation in Bear Lake, Utah-Idaho
Abstract
Bear Lake (Utah-Idaho) is a modern carbonate factory, producing CaCO3 at a rate of up to 600 g/m2/yr. Although high, this rate is not particularly unusual. What is unusual is the carbonate mineralogy and changes in mineralogy that occur in the water column. In summer, near-surface sediments contain almost pure high-Mg calcite, whereas bottom traps contain predominantly aragonite with variable amounts of quartz, and minor dolomite. The bottom sediments also contain calcite that is about half high-Mg calcite and half low-Mg calcite. The transformation from high-Mg calcite to low-Mg calcite continues in the near-surface sediments, and high-Mg calcite is gone by about 10 cm below lake floor. Older sediments (last 7 kyr) consist of aragonite (50-60%), roughly equal amounts of quartz and low-Mg calcite (20-30% each), and minor dolomite (<10%). During summer thermal stratification, both the epilimnion and hypolimnion contain about 30 ppm Ca and 50 ppm Mg. HCl-leach-chemistry results are consistent with the mineralogy. The high-Mg-calcite-rich sediments in surface traps contain about twice as much Mg as the aragonite-rich sediments in bottom traps, whereas sediments in bottom traps contain about twice as much Sr and Ba. There is little difference in values of \delta18O in sediments from surface and bottom traps from three locations in the lake; all have values of about -8 to -9 per mil. Values of 87Sr/86Sr also are the same in bulk carbonate in surface and bottom traps, and are close to those for Bear Lake water (0.7092). These data pose many questions, but two key questions are: 1) Where is the aragonite formed? and 2. Why does high-Mg calcite form in the epilimnion and then transform to low-Mg calcite in the water column and near-surface sediments? Perhaps a key to the above questions is the major change in water chemistry of Bear Lake that occurred in 1912 when the Bear River was artificially diverted into Bear Lake to support irrigation and power production. A 1912 analysis of lake water showed concentrations of total dissolved solids (TDS), Mg, and Ca of 1060, 152, and 4.1 mg/L, respectively, whereas today they are 500, 53, and 31, respectively. In spite of these large changes in hydrochemistry, there is little difference in carbonate content or mineralogy in sediments deposited before and after 1912, but the Mg concentration in post-1912 sediments is about half the concentration in pre-1912 sediments when the lake had a much higher Mg/Ca ratio. Values of \delta18 O in bulk carbonate decreased markedly in post-1912 sediments, reflecting the decrease in TDS, but the concentrations of Sr and Ba increased markedly in post-1912 sediments.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMPP22A0499S
- Keywords:
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- 1040 Isotopic composition/chemistry;
- 1045 Low-temperature geochemistry;
- 1806 Chemistry of fresh water