Multiple oxygen and sulfur isotopic analyses on water-soluble sulfate in bulk atmospheric deposition from the southwestern United States

Sulfate is a major component of bulk atmospheric deposition (including dust, aerosol, fog, and rain). We analyzed sulfur and oxygen isotopic compositions of water-soluble sulfate from 40 sites where year-round dust traps collect bulk atmospheric deposition in the southwestern United States. Average sulfur and oxygen isotopic compositions (δ³⁴S and δ¹⁸O) are 5.8 ± 1.4 (CDT) and 11.2 ± 1.9 (SMOW) (n = 47), respectively. Samples have an oxygen 17 anomaly (Δ¹⁷O), with an average value of 1.0 ± 0.6‰. Except for a weak positive correlation between δ¹⁸O and Δ¹⁷O values (r² ≈ 0.4), no correlation exists for δ¹⁸O versus δ³⁴S, Δ¹⁷O versus δ³⁴S, or any of the three isotopic compositions versus elevation of the sample site. Exceptional positive Δ¹⁷O values (up to 4.23‰) are found in samples from sites in the vicinity of large cities or major highways, and near-zero Δ¹⁷O values are found in samples close to dry lakes. Comparison of isotopic values of dust trap sulfate and desert varnish sulfate from the region reveals that varnish sulfate has average isotopic values that are ∼4.8‰ lower for δ¹⁸O, ∼2.1‰ higher for δ³⁴S, and ∼0.3‰ lower for Δ¹⁷O than those of the present-day bulk deposition sulfate. Although other factors could cause the disparity, this observation suggests a possibility that varnish sulfate may have recorded a long-term atmospheric sulfate deposition during the Holocene or Pleistocene, as well as the differences between sulfur and oxygen isotopic compositions of the preindustrial bulk deposition sulfate and those of the industrial era.