Modern Sedimentation and Authigenic Mineral Formation in the Chew Bahir Basin, Southern Ethiopia: Implications for Interpretation of Late Quaternary Paleoclimate Records

Abstract

We present new mineralogical and geochemical data from modern sediments in the Chew Bahir basin and catchment, Ethiopia. Our goal is to better understand the role of modern sedimentary processes in chemical proxy formation in the Chew Bahir paleolake, a newly investigated paleoclimatic archive, to provide environmental context for human evolution and dispersal. Modern sediment outside the currently dry playa lake floor have higher SiO2 and Al2O3 (50–70 wt.%) content compared to mudflat samples. On average, mudflat sediment samples are enriched in elements such as Mg, Ca, Ce, Nd, and Na, indicating possible enrichment during chemical weathering (e.g., clay formation). Thermodynamic modeling of evaporating water in upstream Lake Chamo is shown to produce an authigenic mineral assemblage of calcite, analcime, and Mg-enriched authigenic illitic clay minerals, consistent with the prevalence of environments of enhanced evaporative concentration in the Chew Bahir basin. A comparison with samples from the sediment cores of Chew Bahir based on whole-rock MgO/Al2O3, Ba/Sr and authigenic clay mineral δ18O values shows the following: modern sediments deposited in the saline mudflats of the Chew Bahir dried out lake bed resemble paleosediments deposited during dry periods, such as during times of the Last Glacial Maximum and Younger Dryas stadial. Sediments from modern detrital upstream sources are more similar to sediments deposited during wetter periods, such as the early Holocene African Humid Period.

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“…During the deposition of these sediments, Lake Chew Bahir was generally shallow or seasonally dried out, and evaporation was exceeding precipitation. When vegetation cover on the adjacent slopes of rift shoulders and alluvial fans was reduced, material could be eroded more easily 7,55 .…”

“…Granulometric information can be used to provide insights into erosional and transport mechanisms in the CHB catchment, sedi mentation dynamics of detrital material and provenance of detritus (Extended Data Fig. 2) 13,55,58,59 . Grainsize distributions in the CHB record have been used to infer sediment transport processes.…”

“…Sediment colour may be the result of the primary mineralogical con stituents and their diagenetic changes reflecting oxic versus anoxic conditions at sediment deposition 13,61,62 . The ratio of haematite versus Timagnetite, for example, would be a typical example of minerals represented in the CHB catchment 16,53,55 responding to oxidation of the detrital Timagnetite. Dissolved iron (Fe) and ironhydroxides may originate from the volcanic bedrocks in the higher eastern bound ary of the CHB basin, the Teltele Plateau and from the northeastern part of the catchment, where Cenozoic volcanic rocks are exposed to weathering (Fig.…”

“…For the CHB basin, the best environmental proxy indicating aridity is the hydrochemically controlled potassium content (K) and the K/Zr ratio (Fig. 2) 7,16,18,55 . Authigenic processes such as illitization and zeolitic alteration with increasing alkalinity and salinity have been described for CHB to increasingly alter weathering silicates such as smectites into authigenic phases, at the expense of smectite abundances 16,60,67 .…”

“…Comparing this proxy with the total inorganic carbon and total organic carbon content of the sediment helped to correct this ratio for the presence of calcium carbonate 18 . Ti and Fe are typically found in Timagnetite and the volcanic bedrock in the CHB catchment (Sediment colour and colour reflectance data) 16,53,55 . Mn/Fe reflects oxic versus anoxic conditions and is closely correlated to variations in the colour reflectance values 16,18 .…”