Geomicrobiology of the concretions of the Makgadikgadi pans (Botswana): An astrobiological perspective

The Makgadikgadi Basin of central Botswana hosts one of the largest salt lakes (pans) system on Earth. These pans are characterized by strong evaporation rate, high salinity and pH and high UV radiation. For this reason, they are considered ‘extreme environments’ and analogues to playa environments on Mars. Playa environments on Mars, such as Meridiani Planum, present layered sedimentary deposits and hydrated minerals and opaline silica. These Martian deposits, for their direct link with liquid water, became targets for ongoing and future astrobiological missions. Three types of concretions were identified in the Makgadikgadi pans: silica concretions -Silcrete, with opal as primary cement; Carbonate concretion –calcrete, with calcite as primary cement; Gypcrete with gypsum as cementing material. Here the focus is on the role of microbial organisms (e.g. extremophiles) in the precipitation of minerals within the silica concretions in the Makgadikgadi pans. The aim is to decipher microbe-mineral interactions and constraints the biomineralization processes in this extreme environment. Most of the silcretes are quartzarenites and/or quarzitic breccias cemented by a complex paragenesis of quartz and calcite. Lithoclasts of flint, sand-sized quartz grains and ostracods are cemented by microcrystalline quartz (brown opal) and pore-filling spatic (coarse crystals) quartz cement. The fractures and larger pore space are filled by spherulitic chalcedony: fibrous microcrystalline quartz with botryoidal habit and characteristic wavy-extinction. The remaining pore space and the spherules are cemented by microcrystalline, isopachous carbonates (probably calcite) as a late product of pan diagenesis. The silcretes are formed through direct precipitation of silica from groundwater within the pan system resulting in distinct silcrete morphologies e.g. nodules, massive, sub-outcrop, “scatter” or ellipsoidal. The complex paragenesis reflects fluctuation in the pH and water chemistry. It remains to be proven if the formation process was purely abiogenic or if there was also a biogenic component. In order to achieve this goal SEM analysis, looking for microbial fossils, and Raman Spectroscopy, for identification of organic matter and biomarkers, will integrate the petrographic analysis of the Makgadikgadi silcretes. This work, although at its preliminary stage, is bound to teach us how microbial communities of extremophiles interact with diagenetic products and what is the astrobiological potential of the Makgadikgadi silcretes.