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

We present here sulfate oxygen isotopic data (72 samples with both delta O-18 and delta O-17) 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 (Delta O-17 = delta O-17 -0.52 delta O-18) until at ca. 70 kin 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 parts per thousand, and 0.06 to 1.11 parts per thousand for delta O-18 and Delta O-17, 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 delta O-18 and Delta O-17, or between the depth of soil horizon and Delta O-17 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 A170 does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions. (C) 2001 Elsevier Science BY. All rights reserved.