Continental-Scale Patterns in Soil Geochemistry and Mineralogy: Results from Two Transects across the United States and Canada

In 2004, the U.S. Geological Survey and the Geological Survey of Canada collected soil samples from 221 sites along two continental-wide transects across Canada and the United States as a pilot study to test and refine sampling and analytical protocols for a planned soil geochemical survey of North America. A north-south transect extended from northern Manitoba to the US-Mexico border near El Paso, Texas. A west-east transect followed the 38th parallel from just north of San Francisco, California to the Virginia shore. The two transects crossed a wide diversity of soil parent materials, soil ages, climatic conditions, landforms, land covers, and land uses. Sample sites were selected randomly at approximately 40-km intervals. At each site, soils representing O, A, and C horizons, if present, were collected. A separate soil sample from a depth of 0 to 5 cm was collected regardless of soil horizon. All samples were analyzed for their near-total content of over 40 major and trace elements. The data reveal coherent, continental- to subcontinental-scale geochemical and mineralogical patterns controlled by underlying soil parent material, soil age, and climate. The data also demonstrate that at the continental-scale the dominance of any of these major soil-forming factors can change quickly across the landscape. Along the N-S transect, soil chemistry changes abruptly across the southern limit of glacial extent because of the change in soil parent from calcareous till to loess. Along the W-E transect, the distributions of some elements (e.g., Cr, Ni) are controlled by soil parent materials whereas for others (e.g., Ca, Al) the influence of parent composition can be obscured by climate effects. Anthropogenic effects on soil chemistry are recognized for single sites. At larger scales it is difficult to distinguish between natural and anthropogenic influences that may both contribute to element distribution