Geochemical Investigation of the Distribution of Arabis macdonaldiana
in the Josephine Ophiolite, Six Rivers, National Forest, Del Norte County, California
By Cheryl Smith1 and Michael F. Diggles1
U.S. Geological Survey 0pen-File Report [in review]
U.S. Department of the Interior
DIRK KEMPTHORNE, Secretary
U.S. Geological Survey
Mark D. Myers, Director
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Arabis macdonaldiana, a federally listed endangered species, is now on the brink of extinction. The only known occurrences are in Del Norte, Siskiyou, and Mendocino Counties, California where it grows in soils developed on mafic and ultramafic rocks. Within the Six Rivers National Forest, in Del Norte County, it exhibits very sparse and patchy distribution in areas where it appears to be well suited for growth. In order to assess the influence of local geochemistry on Arabis macdonaldiana distribution, this study compares the soil and rock chemistry and mineralogy of sites both occupied and unoccupied by the Arabis macdonaldiana. Study area sites are underlain by serpentinized mafic and ultramafic rocks of the Josephine ophiolite complex, which provides a suitable habitat for Arabis macdonaldiana. Soils developed from such bedrock are relatively enriched in various toxic metals, including nickel, magnesium, barium, and chromium; they are also lacking in important nutrients. The range of nickel concentration within soil samples range from 1850 to 5980 parts per million (ppm). Sites with low plant densities have high nickel, suggesting the likelihood of nickel being a growth inhibitor. In addition, barium range from 0.497 to 7.55 micrograms per gram and is also a growth inhibitor.
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