Influence of eolian deposition and rainfall amounts on the U-isotopic composition of soil water and soil minerals

Abstract

230Th–U dating of secondary deposits, such as speleothems and pedogenic carbonate and opal, provides information about the original (234U/238U) composition of the waters the minerals precipitated from. As a result, these types of deposits often display temporal variations in initial (234U/238U) that could reflect environmental variability. Past variations in soil water (234U/238U) may reflect changes in the flux of water through the soil, weathering rates, or dust inputs. In order to evaluate these potential influences, we analyzed modern soil water chemistry and U isotopic compositions of fluids and solids from three soil sequences in Nevada. Modern soil pore waters from Diamond Valley, Newark Valley, and Fish Lake Valley, Nevada show significant (234U/238U) differences between sites as a result of differing soil parent material. Newark Valley pore waters are more heavily influenced by carbonate weathering and display much more enriched (234U/238U) values than Diamond and Fish Lake Valley pore waters, which are dominated by silicate weathering. Newark Valley and Diamond Valley, although located very near to each other, have significantly different pore water (234U/238U) values, suggesting that deposition of distantly sourced dust does not exert a dominant influence on pore water (234U/238U) at these sites. Pore water (234U/238U) values also vary systematically along a rainfall gradient at Fish Lake Valley, where sites with lower rainfall display higher soil water (234U/238U) values. The results of this modern soil water chemistry study were used to parameterize two models that investigate the relative influence of dust content, α-recoil, and weathering on pore water (234U/238U) in the upper layer of the soil and at depth under changing infiltration rates. These models suggest that, while the composition of soluble dust entering a soil profile is an important consideration, the pore water (234U/238U) at depth, and thus the initial (234U/238U) of minerals that precipitate from pore waters, should be highly sensitive to changes in the infiltration flux through the soils. As infiltration flux is closely tied to rainfall, the initial (234U/238U) of soil minerals at these sites should provide information about past rainfall variability.