CHEMICAL VARIATION IN A- AND C-HORIZON SOIL ALONG TWO NORTH AMERICAN CONTINENTAL TRANSECTS

Concentrations of 45 elements were determined for multiple soil horizons at 265 sites along two transects, one along the 38th parallel from Maryland to California, the other from northern Manitoba to the Texas-Mexico border. C-horizon compositions exhibit prominent discontinuities at many geological boundaries, such as between the Rocky Mountains and Basin and Range Provinces and between carbonate and clastic sedimentary sequences in the midwestern U.S. A-horizons show many of these same variations but differ significantly from C-horizons in that ten elements (Hg, S, Bi, Cd, Mn, Nb, P, Sb, Se, and organic C) are significantly enriched in A-horizon soil, whereas 12 elements (Al, Ca, Fe, Mg, Co, Cr, Ga, Li, Pb, Sc, V, carbonate C) are depleted compared to C-horizons. Compositional variations of both A- and C-horizon soil also show varying degrees of correspondence to other mappable landscape features such as lithology of parent material, physiography, climate, land use and land cover, soil order, and ecological divisions. Although each of these variables provides some capability to predict soil compositions, all are imperfect, presumably because of the complexities of interactions between these factors during soil formation. Of those variables that we have tested, ecological divisions appear to be the single most powerful predictor, perhaps because their definition integrates other variables such as geology, pedology, physiography, vegetation, and climate. The continental transects are a pilot phase for an anticipated multi-agency, multi-national North American soil geochemical survey. Results of the pilot study indicate that samples at the density of the transects (about one sample per 40 km) will define regional trends in soil composition and allow interpretations of fundamental causes of soil compositional variability at continental and regional scales. Details of the planned continental survey are available at: http://gswiki.usgs.gov/Soils/.