The origin of sulphur in gypsum and dissolved sulphate in the Central Namib Desert, Namibia

This study investigates the sulphur source of gypsum sulphate and dissolved groundwater sulphate in the Central Namib Desert, home to one of Africa's most extensive gypsum (CaSO4. 2H(2)O) accumulations. It investigates previo usly suggested sulphate precursors such as bedrock sulphides and decomposit ional marine biogenic H2S and studies the importance of other potential sou rces in order to determine the origin of gypsum and dissolved sulphate in t he region. An attempt has been made to sample all possible sulphur sources, pathways and types of gypsum accumulations in the Central Namib Desert. We have subjected those samples to sulphur isotopic analyses and have compile d existing results. In addition, ionic ratios of Cl/SO4 are used to determi ne the presence of non-sea-salt (NSS) sulphur in groundwater and to investi gate processes affecting groundwater sulphate. In contrast to previous work , this study proposes that the sulphur cycle, and the formation of gypsum, in the Namib Desert appears to be dominated by the deposition of atmospheri c sulphates of phytoplanktonic origin, part of the primary marine productio n of the Benguela upwelling cells. The aerosol sulphates are subjected to t errestrial storage within the gypsum deposits on the hyper-arid gravel plai n and are traceable in groundwater including coastal sabkhas. The hypothesi s of decompositional marine biogenic HIS or bedrock sulphide sources, as co nsidered previously for the Namib Desert, cannot account for the widespread accumulation of gypsum in the region. The study area in the Central Namib Desert, between the Kuiseb and Omaruru rivers, features extensive gypsum ac cumulations in a ca. 50-70 km wide band, parallel to the shore. They consis t of surficial or shallow pedogenic gypsum crusts in the desert pavement, h ydromorphic playa or sabkha gypsum, as thin isolated pockets on bedrock rid ges and as discrete masses of gypsum selenite along some faults. The sulphu r isotopic values (delta(34)S parts per thousand CDT) of these occurrences are between delta(34)S +13.0 and +18.8 parts per thousand, with lower value s in proximity to sulphuric ore bodies (delta(34)S +3.1 and +3.4 parts per thousand). Damaran bedrock sulphides have a wide range from delta(34)S -4.1 to +13.8 parts per thousand but seem to be significant sources on a local scale at the most. Dissolved sulphate at playas, sabkhas, springs, borehole s and ephemeral rivers have an overall range between delta(34)S +9.8 and +2 0.8 parts per thousand. However, they do not show a systematic geographical trend. The Kalahari waters have lower values, between delta(34)S +5.9 and +12.3 parts per thousand. Authigenic gypsum from submarine sediments in the upwelling zone of the Benguela Current between Oranjemund and Walvis Bay r anges between delta(34)S -34.6 to -4.6 parts per thousand. A single dry atm ospheric deposition sample produced a value of delta(34)S +15.9 parts per t housand. These sulphur isotopic results, complemented by meteorological, hy drological and geological information, suggest that sulphate in the Namib D esert is mainly derived from NSS sulphur, in particular oxidation products of marine dimethyl sulphide CH3SCH3 (DMS). The hyper-arid conditions prevai ling along the Namibian coast since Miocene times favour the overall preser vation of the sulphate minerals. However, sporadic and relatively wetter pe riods have promoted gypsum formation: the segregation of sulphates from the more soluble halite, and the gradual seaward redistribution of sulphate. This study suggests that the extreme productivity of the Benguela Current c ontributes towards the sulphur budget in the adjacent Namib Desert. (C) 199 9 Elsevier Science B.V. All rights reserved.