HR: 1340h
AN: H43C-1502    [Abstracts]
TI: Naturally Occurring Cr and Ni in the Sacramento Valley: I. Geologic and Geomorphic Controls on the Spatial Distribution of Ultramafic Source Elements
AU: * Morrison, J M
EM: jmorrison@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AU: Goldhaber, M B
EM: mgold@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AU: Mills, C T
EM: cmills@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AU: Holloway, J M
EM: jholloway@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AU: Wolf, R E
EM: rwolf@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AU: Wanty, R B
EM: rwanty@usgs.gov
AF: U.S. Geological Survey, DFC, MS 964, Denver, CO 80225, United States
AB: We present results of a soil geochemical survey conducted along an east-west transect in northern California. The study area includes the Sierra Nevada, Sacramento Valley, and northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is primarily controlled by the transport and weathering of parent material from the Coast Range and Sierra Nevada. Ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in both ranges. These rocks and the soils forming from them have elevated concentrations of Cr and Ni. Surface soil samples (n=37) derived from UM rocks of the Sierra Nevada and Coast Range contain 1700 to 10,000 ppm Cr (mean=4345) and 1300 to 3900 ppm Ni (mean=2744). These high concentrations spatially correlate with mapped UM rocks. Valley soils west of the Sacramento River (n=47) contain 80 to 1420 ppm Cr (mean=435) and 65 to 224 ppm Ni (mean=137), reflecting a significant contribution from UM sources in the Coast Range. Valley soils on the east side (n=41) contain 30 to 370 ppm Cr (mean=169) and 16 to 110 ppm Ni (mean=55). The lower Cr and Ni concentrations on the east side reflect the dominance of granitic sources in the Sierra Nevada. Chromium naturally occurs as Cr(III), a non-toxic micronutrient, or Cr(VI) which is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils from UM parent show Cr primarily occurs as chromite and other mixed-composition spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory. We compared results of a 4-acid digestion (HNO3, HCl, HF, HClO4), which only partially dissolves chromite, and lithium metaborate fusion (LiBO3), which gives total Cr content. The ratio of acid digestion:LiBO3 fusion is low (0.2-0.48, mean=0.32, n=13) in Coast Range UM soils relative to valley soils (0.2-0.99, mean=0.50, n=50), which indicates a lower proportion of chromite in valley soils relative to UM soils. Elevated Cr concentrations (up to 50 ug L-1) in some Sacramento Valley groundwaters suggest that a redox mechanism exists in the weathering of Cr-bearing minerals. We hypothesize that regional-scale transport and weathering of UM material has resulted in enrichment of Cr and Ni in the Sacramento Valley and a change in the residence of Cr.
DE: 1065 Major and trace element geochemistry
DE: 1865 Soils (0486)
SC: Hydrology [H]
MN: 2007 Fall Meeting