HR: 17:45h
AN: PP34A-08    [Abstracts]
TI: Two Million Years of Desert Aerosols: Evidence for Non-Biological Isotopic Fractionation of Ca in Hyperarid Soils of the Atacama Desert, Chile
AU: * Ewing, S A
EM: saewing@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California at Berkeley, 137 Mulford Hall, Berkeley, CA 94702 United States
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Division of Ecosystem Sciences, University of California at Berkeley, 137 Mulford Hall, Berkeley, CA 94702 United States
AU: Yang, W
EM: wenbo@eps.berkeley.edu
AF: Department of Earth and Planetary Sciences, University of California at Berkeley, McCone Hall, Berkeley, CA 94701 United States
AU: Kendall, C
EM: ckendall@usgs.org
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Stewart, B W
EM: bstewart@pitt.edu
AF: Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Department of Earth and Planetary Sciences, University of California at Berkeley, McCone Hall, Berkeley, CA 94701 United States
AB: Isotopes of Ca offer an opportunity to formulate a global Ca budget through geologic time, but their systematics are not yet well understood. In particular, there is disagreement with respect to relevant fractionation factors in natural systems, and there is a need for measurements that capture the natural range of δ⁴⁴Ca in settings relevant to global scale processes and reservoirs. We present Ca isotope results for a hyperarid soil (<2 mm rain y^-1) in the Atacama Desert. The soils of this region contain large amounts of gypsum and anhydrite, as well as relatively low amounts of calcite, and are subject to minimal weathering and biological activity. The soluble Ca they contain is the result of long-term deposition of dust and aerosols. Salt distribution and stable isotope trends in this soil reflect both atmospheric inputs and internal soil transport processes over the 2.1 My exposure time of the landform. In aqueous extracts designed to capture variably hydrated calcium sulfate minerals, δ⁴⁴Ca values increase with depth (2 m) from a minimum of -1.8 to a maximum of +0.7 (vs. bulk Earth). Preliminary analysis of carbonate-Ca indicates that it is less isotopically variable with depth than sulfate-Ca. Sulfate δ³⁴S and δ¹⁸O decrease with depth. Linear correlation among δ⁴⁴Ca, δ³⁴S and δ¹⁸O values could indicate (a) a change in atmospheric input values for all three elements over time, or (b) isotopic fractionation during downward transport. We present evidence that the latter is the primary driver of the isotopic variation that we observe. Sulfate-Δ¹⁷O values are invariant with depth (0.52 ± 0.06‰), suggesting a constant atmospheric source over time. Modeled δ³⁴S and δ¹⁸O values resemble observed values when a modified Rayleigh approach is used, assuming equilibrium isotopic fractionation during partial dissolution, with subsequent downward transport and reprecipitation. A comparison of contemporary inputs (from atmospheric and soil surface values) and long-term inputs (from depth-integrated soil values) indicates a long-term shift in atmospheric sources of Ca but not sulfate. However, Sr isotope results suggest that variation in the source of Ca inputs is minor. We conclude that both source values and transport processes must be considered to correctly interpret the Ca, S, and O isotope composition of sulfate minerals in arid environments. In this hyperarid soil, transport and mixing processes produce a depth-dependent trend in sulfate δ⁴⁴Ca that spans virtually the entire range of values previously observed on Earth. When interpreted in terms of mainly downward Ca transport within the soils, the results suggest that partial dissolution leaves residual sulfate-Ca that is depleted in heavy isotopes and can approach values previously thought to indicate a biogenic origin. The relationship that we observe between δ⁴⁴Ca and δ³⁴S suggests an effective fractionation factor of 0.5 between dissolved and precipitated Ca in the sulfate minerals present in this soil.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0486 Soils/pedology (1865)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005