PP023-0009
Stable isotopes of authigenic clay minerals: Water balance signals in the Late Pleistocene Chew Bahir Basin, Ethiopia

Thursday, 10 December 2020
Poster
Daniel Deocampo1, Daniel Gebregiorgis2, Verena E Foerster3, Fred J Longstaffe4, Frank Schaebitz3, Asfawossen Asrat5, Christina Günter6, Annett Junginger7, Hauke Kraemer8, Nicole Stroncik9 and Martin H Trauth10, (1)Georgia State University, Department of Geosciences, Atlanta, GA, United States, (2)Georgia State University, Department of Geosciencese, Atlanta, GA, United States, (3)University of Cologne, Institute of Geography Education, Cologne, Germany, (4)University of Western Ontario, London, ON, Canada, (5)Addis Ababa University, School of Earth Sciences, Addis Ababa, Ethiopia, (6)University of Potsdam, Institute of Geosciences, Potsdam, Germany, (7)University of Tübingen, Department of Geosciences, Tübingen, Germany, (8)Potsdam Institute for Climate Impact Research, Potsdam, Germany, (9)Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany, (10)University of Potsdam, Potsdam-Golm, Germany
Abstract:
Authigenic clay minerals have previously been used in a number of African paleolake deposits as indicators of elevated paleo-salinity. Their formation generally requires elevated salinity, high pH and alkalinity, aluminous detritus, and readily available silica. Enhanced octahedral Mg substitution, K-uptake, low-temperature illitization, and isotopic equilibration with 18O-enriched lake waters are all hallmarks of the process.

Here we report on new stable isotopic data that complement earlier observations of incipient Mg enrichment and low-temperature illitization (Foerster et al., 2018) in Late Pleistocene lacustrine muds of the Chew Bahir rift basin in Ethiopia. Deposits from several short cores have been correlated with known regional episodes of aridity (e.g. Last Glacial Maximum, Younger Dryas) and humidity (i.e. African Humid Period). Muds deposited during the African Humid Period have whole-rock MgO/Al2O3 <0.5 and octahedral layer Mg/(Al+Fe) in <0.1µm clays of <0.7, whereas those of the LGM and YD have whole-rock MgO/Al2O3 >0.5 and submicron octahedral Mg/(Al+Fe) >0.8. The δ18O of structural oxygen within authigenic clay minerals were +22.5 to +23.0‰ (VSMOW) from the African Humid Period, and +24.2 to +25.2‰ from the LGM and YD. The elemental and isotopic geochemistries therefore covary and independently indicate episodes of relative freshness and salinity in the Chew Bahir paleolake. Enhanced Mg uptake and illitization indicate evaporative concentration of aqueous solutes, whereas enrichment in 18O indicates enhanced evaporative fractionation.

Most systems in which authigenic clay mineral neoformation is significant were much more saline than was the Chew Bahir paleolake. For example, well known examples at Olduvai Gorge and Lake Natron, Tanzania, are associated with true evaporite deposits and extreme abundances of authigenic silicates. These results suggest that, in closed-basin lake deposits that are not overly diluted by detrital or biogenic sediments, subtle changes in water balance can be determined by both elemental and isotopic analysis of submicron authigenic clay minerals, even without hypersaline conditions.

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