Abstract

The Shale Hills catchment is a recently established Critical Zone Observatory in central PA used to investigate regolith evolution and element cycling in shale terrains. We observe excess Mn in the surface soils at Shale Hills CZO. Atmospheric deposition best explains the Mn enrichment. This study seeks to model atmospheric input and biogeochemical Mn cycles in order to explain the observed addition front. Mn enrichment in surface soils is a largely uncharacterized and potentially widespread phenomenon. Soils in Shale Hills exhibit net Mn addition relative to bedrock. Our studies indicate a strong Mn addition signal in ridge soils relative to the basin floor, suggesting physiographic differences in deposition. Current estimates of Mn in wet and dry deposition are insufficient to account for the quantified magnitude of Mn enrichment. Therefore, we are examining sequestration of long-term mineral deposition as a potential input source. Initial characterization indicates Mn is present in a mixed (II/IV) oxidation state in both amorphous and oxide-associated forms. Surface chemistry derived from X-ray photoelectron spectroscopy shows the strongest Mn signals for ridge soils and upper soil horizons. Sand and silt-sized particles show no differences in Mn chemistry, suggesting Mn is uniformly distributed in the soil as a surface coating. Continuing work involves analyzing Mn distribution in soil profiles across the region and nation for comparable Mn enrichment signals. Preliminary data indicates that Mn addition may be prevalent outside of Shale Hills. Spatially diverse characterization of Mn profiles will aid understanding of Mn sources, atmospheric deposition, and controls on regolith cycling.

Citation

Herndon E.M., Jin L., and Brantley S.L., (2008): Mn enrichment in surface soils: a signal for dust?. American Geophysical Union fall meeting, San Francisco, CA..