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Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts
POSTER PRESENTATION
The Relation Between the Geochemistry of Surficial Materials and Desert Tortoise Mortality in Selected Study Sites, southeastern California--a Progress Report
Maurice A. Chaffee,1 Kristin H. Berry,2 and Brenda B. Houser3
U. S. Geological Survey
1Geologic Division, Federal Center, MS 973, Denver, CO 80225
2Biological Resources Division, 6221 Box Springs Blvd., Riverside, CA 92507
3Geologic Division, 520 N. Park Avenue, Suite 355,Tucson, AZ 85719
Since 1989, several dozen ill, dying, or recently dead tortoises have been salvaged from numerous sites in the Mojave and Colorado deserts and necropsied to determine causes of death. Some tortoises were found have elevated levels of one or more elements in the kidneys and (or) livers as compared with control animals (see Jacobson et al., 1991, and Homer et al., 1994; 1996). These elements included Ba, Cd, Cr, Hg, Ni, Pb, and (or) V. Our study is focusing on the geochemical environment where the necropsied tortoises lived. We collected samples of rock, soil, and unconsolidated stream alluvium from 13 study sites and analyzed them for 48 elements (Ag, Al, As, Au, Ba, Be, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Eu, Fe, Hf, Hg, Ir, K, La, Lu, Mg, Mn, Mo, Na, Nd, Ni, P, Pb, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Th, Ti, U, V, W, Y, Yb, and Zn). In comparison to expected abundances for these elements based on the rock chemistry in the study areas, we found locally elevated concentrations of one or more of 25 elements (Ag, As, Au, Ba, Be, Br, Cd, Ce, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Rb, Sb, Sr, Th, U, V, W, and (or) Zn). The relatively weak enrichments of most of these elements can be related (1) to the presence of trace elements in alluvial magnetite (a common iron-oxide mineral) or (2) to trace elements in coatings of manganese- and or iron-oxide coatings on sediment grains. We did not find anomalous levels of any one element in all areas containing tortoises having the same disease, indicating that no one element seems to be responsible for a given disease. Additionally, with the possible exception of mercury, we found no significant correlation between elevated levels of a given element in tortoise organs and elevated levels of that element in the surficial environment of the tortoise’s habitat. Because the measured element concentrations for most of the elements studied generally represent long-term enrichment over thousands of years, it seems unlikely that recent increases in tortoise mortality are related to enrichments of these elements in surficial materials.
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