Geochemical and Isotopic Estimates of Eolian Dust in Soils of the San Juan Mountains, USA.

Eolian dust deposition in the San Juan Mountain Range in southern Colorado has increased 5-7 fold in the past two centuries. This dust deposition contributes an exogenous supply of biologically relevant elements such as Ca, K, Mg, and P to these alpine ecosystems in the form of fine textured mineral particulates. The deposition of eolian dust may be an underestimated factor of soil formation and soil chemistry in these alpine settings. The importance of eolian dust relative to the weathering of local bedrock likely varies across bedrock types. The San Juan Range is geologically diverse with distinct regions of Meso-proterozic crystalline granites in the Weminuche Wilderness, Mesozoic sedimentary layers near Molas Pass in the San Juan National Forest, and Tertiary volcanic geology found on Red Mountain Pass in the Uncompahgre National Forest. Principle component analysis of element chemistry shows that bedrock and soils from these sites cluster by geology. In addition, these groups are chemically distinct from eolian dust collected from snow in the San Juan Range. Several elements seem to drive the difference of dust from soils and bedrock including Ca, Sr, Cu and Cd. The purpose of this research was to estimate the relative contribution of eolian dust to alpine soil element pools in the San Juan Mountains across a range of local geologic parent material. A calculation of element mass- balance shows that Cu and Cd are enriched in the surface soils of both volcanic and sedimentary soils relative to concentrations in local bedrock. However, Ca is enriched only in volcanic soils. These observations support the notion that eolian dust contributes to soil formation and that the relative contribution of dust across the landscape varies with geology. In addition to element mass-balance estimates we utilize Sr and Nd isotope measurements of soil, bedrock, and dust to further constrain the importance of eolian dust to these alpine soils.