Evolution of the Pretoria Saltpan — a diatom record spanning a full glacial-interglacial cycle

Abstract

In 1988–89 a 200 m core was drilled down to granite bedrock in the Pretoria Saltpan as part of a project to ascertain the origin of this unusual feature. The top 90 m of the core consisted of lacustrine sediments. Based on available dating, this lacustrine sequence probably covers ca. 180,000 years, thus providing the longest sequence of its kind from southern Africa. A multi-disciplinary investigation of the lake sediments is being undertaken of which the study of diatom assemblages is one part.

Diatoms are generally present throughout the core and record 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 Pseudostaurosira brevistriata predominates. Above this, the diatom record becomes patchy, especially in the top 30m. Nitzschia pusilla is the most abundant species 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 abundant.

The general increase in salinity and alkalinity through time indicated by the diatoms is parallelled by the mineralogy of the sediments with an up-core sequence of calcite-halite-trona-gaylussite. Within this overall development, the impact of climatic changes may be discerned. Diatoms in the part of the core believed to cover the last interglacial reflect a well mixed lake of moderate pH and alkalinity. At the end of the period (ca. 70000 yr BP?) rapid drying occurred. During the last glacial, the lake was generally saline and alkaline although it may have been reasonably deep until ca. 24000 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 the core probably reflects dry conditions of extreme evaporative concentration and oxidation.