Soil Geochemistry along a Transect from Cedral, San Luis Potosí, to Tecpán, Guerrero, Mexico

The Servicio Geológico Mexicano (Mexican Geological Survey), U.S. Geological Survey, and the Geological Survey of Canada have completed pilot studies in preparation for a soil geochemical survey of North America. Pilot studies in Mexico consisted, in part, of collection and chemical analysis of soils along a 760-km transect extending from Cedral, San Luis Potosí, in central Mexico to the Pacific coast near Tecpán, Guerrero, in southern Mexico. A- and C-horizon soils were collected from 29 sites along the transect. The minus-2-mm fraction of these samples was pulverized and analyzed for 62 major and trace elements by ICP-MS and ICP-AES following a near-total four-acid extraction.

Observed variation in concentrations of K, Na, Ca, Mg, Ti, Fe, and Al is generally related to pedogenic processes occurring in the semi-arid landscapes along the transect. Elements such as Rb, Ba, and Sr are enriched in the C horizon and may be related to the presence of barite at some localities.

Data for Al, Ca, Na, and K have been used to determine a chemical alteration index (CAI) and a plagioclase alteration index (PAI). Plotting values of these indices along the transect enables us to distinguish different grades of weathering at each sampled locality.

This study showed the variation in soil maturity in different landscapes from the diverse physiographic and climatic settings found along the transect.