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.
Highlights
The field of sedimentary geochemistry is at the forefront of both outcrop- and core-based paleoclimate research
The mineralogical assemblage of the WS samples is composed primarily of sand- and silt-size particles containing a higher fraction of non-clay mineral silicate phases than the MF samples, TABLE 2 | Chemical composition of Lake Chamo water used for geochemical modeling
Analcime content increases in the MF samples that are transitional to the desiccated lake floor of the Chew Bahir basin
Summary
The field of sedimentary geochemistry is at the forefront of both outcrop- and core-based paleoclimate research. Authigenic mineral formation can control lake sediment geochemistry, in saline lakes (Deocampo and Jones, 2014). Large discrepancies between lake records remain unresolved, leading to difficulties in interpreting proxy records of climate (e.g., Trauth et al, 2003). These challenges are in large part due to an incomplete understanding of proxy formation and are relevant in paleolimnology, where down-core variability of selected elements in the sediment may have little direct relation with the rate and intensity of weathering of the surrounding catchment, but might rather reflect post-depositional hydrochemical conditions
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