NATURAL RADIOACTIVITY AND THE NATURE OF VERTEBRATE FOSSILS AT HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, HAGERMAN, ID

Hagerman Fossil Beds National Monument is located along the eastern edge of the western Snake River Plain in south central Idaho and possesses a unique and well preserved terrestrial vertebrate fossil assemblage from the Pliocene Epoch at 3.5 m.y.a.. Fossils are hosted by a series of lacustrine and fluvial floodplain silts and sands (Malde, 1972). X-ray diffraction (XRD) scans of fossil vertebrate specimens indicate a uniform mineral composition for the fossil specimens matching carbonate-fluorapatite [Ca5 (PO4, CO3)3F] with secondary calcite and quartz incorporated into the fossil matrix. SEM/EDS microbeam chemical analyses of selected specimens confirm the presence of a high-fluorine calcium phosphate phase. Trace element composition of the carbonate-fluorapatite is diverse with detectable concentrations (i.e. 100s to 1000s of ppm) of Cl, Al, Mn, Fe, Ti, S, Y, Ba, Cs, La, U, and Th. Reconnaissance X-ray fluorescence analysis (XRF) revealed the presence of I and nearly all lanthanide series elements. Natural radioactivity has been identified in both fossil and sedimentary deposits at Hagerman Fossil Beds, National Monument. The radioactivity is variable with specimens having no detectable radioactivity above background levels while other specimens are highly radioactive. SEM/EDS analysis indicates that U and Th concentrations in carbonate-fluorapatite range up to 5000 ppm. 238U and 230Th are naturally occurring radioactive isotopes and may be the primary source of radiation at Hagerman. Radioactive emission levels in some fossil specimens (e.g. cat. # 256) has been measured at 21,000 cpm. Oxidized mineral deposits have also exhibited radioactivity above background levels at about 400 cpm. Background levels range from near zero to 50 cpm. The presence of radioactivity has affected handling, preparation, and storage of radioactive specimens. For example, peak concentrations of radon in the fossil/collection storage room were measured at 25 piC/L range or 6 times the EPA recommended limit. The benefits of the radioactivity include using the presence, absence, or level of radioactivity as a tracer in mapping spatial distribution of geologic deposits and fossils on the Monument. Also, this assists with resource management, interpreting the geologic history, and the safety of field personnel.