Formation of modern dolomite in hypersaline pans of the Western Cape, South Africa



Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean-type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km2 in size. The δ18OPDB values of carbonate minerals in the pan sediments range from −2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ13CPDB values of pan carbonate minerals (−8·85 to −1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate-reducing bacteria in the black sulphidic mud zone found in the brine-type hypersaline pans. The knobbly, sub-spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The 87Sr/86Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca2+) ions with relatively minor amounts of Ca2+ derived from the chemical weathering of bedrock.