Geophysical and geological exploration of the Aurus clay pan as an archive of the long-term climatic and environmental history of the Namib Desert

Abstract

The Namib Desert is one of the oldest deserts on Earth. Whilst the history of the desert during the last glacial-interglacial cycle is relatively well understood, information on the older Pleistocene and Neogene history is very scarce, because longer-term records such as those in lakes and caves have not been formed due to the aridity. Here, we investigate the potential of the sedimentary record in the Aurus clay pan in the Sperrgebiet Domain in the southern Namib Desert, Namibia, for longer-term paleoclimate research. For this purpose, we conducted a multi-disciplinary geophysical site survey along with initial sedimentological investigations. We used a combination of transient electromagnetic (TEM), seismic and magnetic data, that allows us to derive information with respect to tectonic features as well as the deeper sedimentary architecture of the Aurus clay pan. TEM and seismics were applied along two perpendicular transects. The data obtained reveal a three- to four-layered subsurface structure with a maximum sedimentary thickness between 31 and 65 m overlaying resistive bedrock. The aeromagnetic data show a fault zone cutting the southwestern corner of the clay pan that correlates with deep conductive anomalies visible in the TEM data. Radiocarbon ages of plant remains from the sediment surface evidence modern deflation, probably due to a particularly dry climate. The composition of a 4-m-long sediment core, which probably reflects the depositional history from 3400 to 23,000 cal. yr BP, however, suggests that most of the Holocene was characterized by a relatively high humidity, whereas the latest Pleistocene again was much dryer. According to an extrapolation of the sedimentation rates in the upper 4 m to the base of the sedimentary infill the record in Aurus clay pan penetrates in the order of 340,000 cal. yr BP. This would be longer than most other paleoclimate records in the Namib, but much less then recently recovered in the Atacama Desert in Chile. A comparison of the geophysical and geological data from Aurus with comparable data and two drill cores from clay pans in the Atacama furthermore shows that the physical parameter in all cases allow to decipher the sediment thickness and general architecture, but alone provides limited information on the sediment composition.