Sulphur isotopes in the central Namib Desert ecosystem

The Namib Desert is hyper-arid in terms of rainfall, but its ecology is influenced by frequent fog events. Fog utilisation by Namib biota has been well studied, but its role in nutrient deposition and cycling, particularly with respect to soil processes, still has open questions. Given its potential for distinguishing between various ecosystem components and fluxes, sulphur isotopic composition (δ34S) is evaluated here as a passive tracer of aerosol deposition and plant water sources in the Namib. Measurements of δ34S in Namib fog, groundwater, soils, plants and aerosols are presented and are consistent with the previously described system of sulphur cycling: primary marine sulphur accumulates as gypsum in the gravel plains and is redistributed by wind. Kuiseb River sediments had a wide range of δ34S values, with several samples that were quite depleted relative to soils, plants, groundwater and gypsum of the gravel plains. This depleted signal appears more commonly in the fine (0.5, 1.0 µm) rather than in the coarse (1.5, 7.6 µm) aerosol size fractions. Fog and aerosol δ34S values are consistent with local dust as a major sulphur source, limiting the utility of δ34S as a unique tracer of fog deposition. It can still provide useful information in certain situations. For example, the 16.5‰ δ34S value for the brackish groundwater at Hope Mine is distinct from the 10.2‰ value in Welwitschia mirabilis stem material at that site. This type of comparison could be one useful line of evidence in evaluating plant water sources.