EVOLUTION OF THE PRETORIA SALTPAN - A DIATOM RECORD SPANNING A FULL GLACIAL-INTERGLACIAL CYCLE

In 1988-89 a 200 m core was drilled down to granite bedrock in the Pre toria Saltpan as part of a project to ascertain the origin of this unu sual feature. The top 90 m of the core consisted of lacustrine sedimen ts. Based on available dating, this lacustrine sequence probably cover s ca. 180,000 years, thus providing the longest sequence of its kind f rom southern Africa. A multi-disciplinary investigation of the lake se diments is being undertaken of which the study of diatom assemblages i s one part. Diatoms are generally present throughout the core and reco rd the long-term evolution of the Saltpan through gradual evaporative concentration. The lowest part of the core is dominated by Aulacoseira granulata var. angustissima; between ca. 50-38 m Pesudostaurosira bre vistriata predominates. Above this, the diatom record becomes patchy, especially in the top 30m. Nitzschia pusilla is the most abundant spec ies in much of the top of the core. Between 17.5-6.5 m (thought to be Late Pleistocene) valves and cysts of Chaetoceros muelleri become abun dant. The general increase in salinity and alkalinity through time ind icated by the diatoms is parallelled by the mineralogy of the sediment s with an up-core sequence of calcite-halite-trona-gaylussite. Within this overall development, the impact of climatic changes may be discer ned. Diatoms in the part of the core believed to cover the last interg lacial reflect a well mixed lake of moderate pH and alkalinity. At the end of the period (ca. 70 000 yr BP?) rapid drying occurred. During t he last glacial, the lake was generally saline and alkaline although i t may have been reasonably deep until ca. 24 000 yr BP. In common with other parts of tropical Africa, the last glacial maximum was dry. The lack of either a pollen or diatom record in the Holocene portion of t he core probably reflects dry conditions of extreme evaporative concen tration and oxidation.