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

Abstract

We present here sulfate oxygen isotopic data (72 samples with both δ 18O and δ 17O) 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 (Δ 17O= δ 17O−0.52 δ 18O) 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 δ 18O and Δ 17O, 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 δ 18O and Δ 17O, or between the depth of soil horizon and Δ 17O 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 Δ 17O does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions.