The Makgadikgadi Basin in Botswana developed from the Late Pleistocene onwards due to gradual shrinking of the giant Lake Palaeo-Makgadikgadi (LPM). This basin, in the central Kalahari, is now covered by one of the largest salt flats in the planet. Ostracod assemblages are dominated by the Limnocythere ssp., an opportunistic taxa colonizing the littoral areas of shallow evaporative, ephemeral lakes. The sediments from the pans show fluctuations in the Cl/K and Ca/Cl ratios, often in phase with the relative abundance of Limnocythere suggesting a cyclicity induced by changes of salinity and alkalinity in the water. Recent multi-proxy study documents the occurrence of a Late Pleistocene shallow, playa lake environment typified by strongly alkaline waters. Between ca. 16 and 2 ka BP, a prolonged drought with sustained aeolian conditions affected the study area. A subsequent, short humid period is followed by an overall desiccation trend that likely started with the Little Ice Age and continues until the present day. XRD and Raman analyses revealed the presence clastic quartz and evaporite minerals such as halite and gypsum and ubiquitous calcite both clastic, biogenic and authigenic in origin, from surface and shallow cores sediments. Raman analysis revealed the presence of unidentified clay minerals. Hence, some of the samples from the Makgadikgadi Salt Pans were processed for the preparation of oriented sections for clay minerals identification. Clay minerals identified were: illite, smectite, mixed-layer clays (probably chlorite), and kaolinite. No consistent clay mineralogical variation with depth was found. The authigenic clays present formed in the pans during periods of increased water alkalinity, high dissolved magnesium contents and relatively low detrital input, probably during dry winter periods. On the other end, detrital clay materials were brought in by the ephemeral rivers during the wet summer months. The identification of clay minerals in the Makgadikgadi Salt Pans has important implications for astrobiological studies as clays have been described by robotic mission on Mars in evaporitic environments similar to those described here. The clay mineral deposits on Mars are a primary target for astrobiological and habitability studies.
Authigenic clays and evaporites from the Makgadikgadi salt pans (Botswana): from Quaternary paleoenvironments to astrobiology perspectives
Fulvio Franchi;
2023-01-01
Abstract
The Makgadikgadi Basin in Botswana developed from the Late Pleistocene onwards due to gradual shrinking of the giant Lake Palaeo-Makgadikgadi (LPM). This basin, in the central Kalahari, is now covered by one of the largest salt flats in the planet. Ostracod assemblages are dominated by the Limnocythere ssp., an opportunistic taxa colonizing the littoral areas of shallow evaporative, ephemeral lakes. The sediments from the pans show fluctuations in the Cl/K and Ca/Cl ratios, often in phase with the relative abundance of Limnocythere suggesting a cyclicity induced by changes of salinity and alkalinity in the water. Recent multi-proxy study documents the occurrence of a Late Pleistocene shallow, playa lake environment typified by strongly alkaline waters. Between ca. 16 and 2 ka BP, a prolonged drought with sustained aeolian conditions affected the study area. A subsequent, short humid period is followed by an overall desiccation trend that likely started with the Little Ice Age and continues until the present day. XRD and Raman analyses revealed the presence clastic quartz and evaporite minerals such as halite and gypsum and ubiquitous calcite both clastic, biogenic and authigenic in origin, from surface and shallow cores sediments. Raman analysis revealed the presence of unidentified clay minerals. Hence, some of the samples from the Makgadikgadi Salt Pans were processed for the preparation of oriented sections for clay minerals identification. Clay minerals identified were: illite, smectite, mixed-layer clays (probably chlorite), and kaolinite. No consistent clay mineralogical variation with depth was found. The authigenic clays present formed in the pans during periods of increased water alkalinity, high dissolved magnesium contents and relatively low detrital input, probably during dry winter periods. On the other end, detrital clay materials were brought in by the ephemeral rivers during the wet summer months. The identification of clay minerals in the Makgadikgadi Salt Pans has important implications for astrobiological studies as clays have been described by robotic mission on Mars in evaporitic environments similar to those described here. The clay mineral deposits on Mars are a primary target for astrobiological and habitability studies.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
