ACTINIDE ENRICHMENT IN PHOSPHATE BONE FOSSILS OF THE GREEN RIVER FORMATION

Fossils from Laney and Wilkins Peak members of the Eocene Green River Formation (GRF) were analyzed using high energy XRF spectroscopy at Brookhaven National Labs National Synchrotron Light Source. Most phosphatic fossils (bones, scales, etc) contain elevated U and Th concentrations whereas the host rocks do not have anomalously high actinide concentrations. REE patterns for whole rocks show little difference relative to the SGR-1 (Green River Shale) standard. Lowest REE concentrations were found in nearly pure limestone samples, but the patterns are similar to the SGR-1 standard indicating that calcite dilutes the concentrations. This suggests that the REE’s reflect the detrital input. Phosphatic gar scales and dolomitic stromatolite samples are highly elevated in the HREEs. Volcanic ashes are common in the GRF and reactive glasses from these ashes are a likely source of elevated trace elements in phosphate and dolomite. There is a general positive correlation between Th and REE concentrations consistent with the observation that Th and REEs behave similarly. Fossilization has been shown to occur at a rapid rate and laboratory studies have shown that the process begins within seconds of death. Though the sorption rate of most metals decreases with higher alkalinity, organic acids and biogenic processes from bacteria may provide a mechanism for lowering the pH near fossils thus increasing the potential for the aqueous species to sorb during fossilization. Lake stratification likely plays a significant role in the actinide accumulation. A plausible sequence of events of fossilization could be: 1. Fish (or other bony animal) dies and begins descent to the lake bottom at which fossilization begins. 2. Soft tissue decays and produces locally lower pH. Organic acids and phosphate released during the decay of the soft tissue may play a role in complexing some of the incorporated trace elements. 3. Aqueous carbonate, phosphate and organic species with complexed trace elements enter into bone pore spaces and become incorporated during permineralization.