Bear Lake is a natural carbonate factory and has been for the past 13,500 radiocarbon years. The lake initially produced the mineral calcite (one of two forms of calcium carbonate, CaCO3), and then started depositing aragonite (the other form of CaCO3).
Before 13,000 years ago, the sediments deposited in Bear Lake consisted of red, silty clay containing small amounts of carbonate (less than 10% CaCO3) and even less organic carbon (less than 1%). Carbonate formation began about 13,500 years ago and is probably related to isolation of the lake from the Bear River. Between 13,500 and 12,500 years ago, the calcite content of the sediments tripled, and the organic carbon content quadrupled, due to increased productivity (algal growth) in the lake.
Without the fresh water supplied by the Bear River, the lake salinity steadily increased, reaching the threshold for aragonite formation about 10,000 years ago. The amount of CaCO3 in the sediments doubled as aragonite production began, reaching 60-70%. Oxygen isotope results indicate that the salinity of the lake increased markedly between 10,500 and 9000 years ago.
Between 8000 and 6500 years ago a sudden influx of fresh, non-saline water (called a salinity reversal) stopped aragonite formation and switched the lake system back into a calcite formation mode. This 1500-year salinity reversal might have come from re-newed flow of Bear River water into Bear Lake. For the past 6500 years, aragonite has been formed and deposited in Bear Lake sediments.
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