Oxygen-17 excesses of the Central Namib gypcretes: Spatial distribution

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We present here sulfate oxygen isotopic data (72 samples with both δ18O and δ17O) systematically collected from the Central Namib Desert. Surface soils from two shore-inland (west-east) transects exhibit a gradual increase in the sulfate oxygen-17 excess (Δ17O = δ17O-0.52 δ18O) until at ca. 70 km inland, where no continuous gypcrete deposit is observed further east (inland). The oxygen isotopic compositions for water-soluble sulfates extracted from soils and gypcretes range from 8.3 to 13.3‰ and 0.06 to 1.11‰ for δ18O and Δ17O, respectively. The lateral pattern is similar to what has been seen in the cold deserts of the Antarctic dry valleys. However, unlike the dry valleys, no discernible correlation is found between δ18O and Δ17O, or between the depth of soil horizon and Δ17O in the Namib. Possible explanations include a relatively smaller component of dimethylsulfide (DMS)-derived sulfate in the total gypsum deposits and/or more active surface processes (e.g., flooding and leaching) in the Central Namib Desert than in the Antarctic cold deserts. Although current state of knowledge is insufficient to delineate quantitatively the sulfate contributions from different sources and reactions, the measurement of sulfate Δ17O does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions. © 2001 Elsevier Science B.V. All rights reserved.

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Earth and Planetary Science Letters

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