The Ca isotopic composition of dust-producing regions: Measurements of surface sediments in the Black Rock Desert, Nevada

Dust is a relatively unconstrained flux in the geochemical cycle of Ca. The isotopic composition of dust-derived Ca has not been studied, though it is an important part of any attempt at deciphering Ca isotope-based proxy records. Accordingly, this study reports the elemental and calcium isotope geochemistry of 30 surface (upper similar to 0.5 cm) sediments from an arid dust producer, the Black Rock Desert in northwestern Nevada. Geochemical data for sequential water and 0.5 N HCl leaches, meant to sample mobile Ca, and selected leached residues are presented, along with X-ray diffraction (XRD) determinations of major mineralogy. Bulk playa sediments have Ca concentrations between 0.28 and 40 wt.% (median: 6.8 wt.%) and calcite concentrations of 2-32%. Isotopically, Ca sampled by water leaches (<1% of total Ca) are, on average, +0.33 +/- 0.16 parts per thousand (1 SD) heavier than acid leaches (>60% of total Ca), though the degree of fractionation (Delta(w-a)) varies between 0 parts per thousand and 0.6 parts per thousand. Acid leaches, which are the primary component of mobile Ca in the sediments, have delta Ca-44(SRM-915a) values of 0.78 +/- 0.08 parts per thousand, similar to the delta Ca-44 of modern nannofossil ooze and modern rivers. This means that dust produced in closed continental basins likely has little isotopic leverage to change the ocean's isotopic composition, and suggests that the Ca isotopic composition of dust is tied to rivers/weathering. In addition, while the Ca concentration data in the water leach suggest that evaporative evolution controls the amount of Ca in this reservoir, the isotope data are inconsistent with this conclusion. Instead, we hypothesize that adsorption of Ca on clays controls the Ca isotope systematics in the water leach. This hypothesis requires that there is no significant isotopic fractionation during evaporite mineral precipitation and may suggest that sorption at rates appropriate for natural systems might fractionate differently than sorption at laboratory rates. (C) 2012 Elsevier Ltd. All rights reserved.