Oxygen Isotopic Ratios of Quartz from Wind‐Erosive Soils of Southwestern United States in Relation to Aerosol Dust

Abstract

AbstractThe oxygen isotopic ratios (expressed as parts per thousand relative to mean ocean water, SMOW, δ18O) of the quartz from 13 soils undergoing much wind erosion during the study period of 1972–1975 in four southwestern states and from comparison areas were determined. The δ18O for quartz from eight Texas (TX) and Arizona (AZ) soils range from 13.0 to 15.9 ‰. The quartz of the sands and silts coarser than 20 µm from three of the soils had δ18O values ranging from 13.1 to 15.1 ‰, characteristic of an ultimate igneousmetamorphic origin. The δ18O values increase greatly with decreasing particle size of quartz from three soils ranging from loamy fine sand to loam to clay in texture. The δ18O of the 1–10 µm quartz fraction (aerosol size) ranged from 19.2 to 20.2 ‰ (19.55 ± 0.28 ‰; ± sigma) for the thirteen soils most affected by dust storms. The higher values are suggestive of an admixture of some quartz crystallized at low temperature to quartz of high temperature origin. These results are not significantly different from the mean of 20.73 ± 1.82 ‰ found earlier for the same fraction from 30 shales of the midcontinental United States, but are 4 to 7 ‰ higher than those reported for New Zealand, Australian, and southern African soils. The 1–10 µm quartz from the comparison areas of TX and AZ soils in Fluvial materials further southeast and southwest has a δ18O mean of 17.7 ± 0.6 ‰, reflecting the influx of a slightly higher proportion of quartz from igneous sources than in the soils most affected by wind erosion. The oxygen isotopic ratios of 1–10 µm quartz from three Hawaiian soils and two sediments from Lake Waiau occurring at 3,970 m altitude on the Mauna Kea summit on the Island of Hawaii give a δ18O mean of 18.3 ± 0.2 ‰. Quartz of the above soils, including sandy, loamy, and clayey soils of southwestern United States, in addition to shales reported earlier, are source reservoirs of quartz having intermediate oxygen isotopic ratios characteristic of mixtures of two sources of quartz, one crystallized at high (e.g., δ18O = 11 ‰) and one at low temperature (e.g., δ18O = 30 ‰).