REVISITING LONG-TERM DUSTFALL RATES AND CHEMISTRY, COLORADO FRONT RANGE

For many decades, geomorphologists and soil scientists have noted the influence of dustfall (clay + silt) on the morphology, texture and chemistry of late Pleistocene and Holocene soils in the Front Range and other upland areas of the S. Rocky Mountains. Contemporary measurements demonstrate the influence of dustfall P and other elements on ecosystem function, but deposition rates over longer time periods are also significant. In addition, recent advances in applying meteoric ¹⁰Be to analyze soil age and evolution argue that dust may be an important component of ¹⁰Be delivery. Measurements from sites in the Boulder Creek catchment and published values from nearby areas suggest that the long-term, net clay accumulation rate is about 0.04 gm cm^-2 kyr^-1 in soils on stable, dated surfaces such as Neoglacial, Pinedale and Bull Lake moraines. Clay concentration in parent material is the principal source of measurement uncertainty. Clay+ silt accumulation rates are ~0.1 gm cm^-2 kyr^-1. Because measured accumulation rates integrate fines generated by weathering and dustfall, they provide an upper limit for dustfall rates. If our values are correct, dustfall should not significantly influence the total amount of met¹⁰Be delivered to soil surfaces in the upland Front Range. The chemistry of soil fine fractions does not permit clear separation of pedogenic vs. dustfall origin nor provide simple clues about dust provenance. Trace components such as Zr, Nb and P show strong size fractionation in crushed fresh-rock samples of Boulder Creek granodiorite and other local rock types, complicating elemental-ratio approaches for assessing the origin of fine material. Contemporary dustfall measurements in the upland Front Range suggest that modern rates are much greater than long-term values, perhaps reflecting changes in land use or climate.