Paleoseasonality through serial stable isotope analysis of Pleistocene to recent Etheria elliptica shells from the middle Atbara River, eastern Sudan

Rainfall magnitude and seasonality are primary determinants of African savannas today, governing essential aspects of biomass production, such as primary productivity and the amount of tree cover. It remains unclear to what extent past seasonality differed from today, potentially accounting for differences in habitats and faunal diversity. Stable isotopes along with trace elements of the growth layers in bivalve shells provide valuable insights into paleoenvironmental conditions, including rainfall, river productivity, and water temperature. In this study, we analyzed stable oxygen and carbon isotopes in 11 fossil and three modern Etheria elliptica shells collected from the middle Atbara River in eastern Sudan. The fossil specimens are from sediments dated by optically stimulated luminescence (OSL) to between ∼ 224 and ∼ 17 ka in age. The δ 18 O values of fossil and modern shells are broadly similar, although the fossils are more enriched (−2.4 ‰ to 5.9 ‰ for fossils vs. −6.4 ‰ to 1.5 ‰ for modern shells), possibly indicating more evaporative and drier conditions during the Pleistocene. However, the Atbara River is today altered by dams, which might also explain the more depleted isotopic values in modern shells. δ 13 C values are broadly similar in fossil and modern shells, ranging from −14.1 ‰ to 4.5 ‰ in fossils and −16.9 ‰ to −9.6 ‰ in modern shells but show significant changes over time. Enriched δ 13 C values in Pleistocene shells may reflect lower atmospheric CO 2 levels, while depleted values in modern shells align with post-industrial increases in CO 2 , although changes in catchment vegetation, weathering processes, or biological factors might also play a role. We also reconstructed the Atbara local meteoric water line (ALMWL) from seven river water samples. Results suggested that the Atbara River experiences low relative humidity compared to the global MWL. Moisture sources inferred from the ALMWL likely include the Indian Ocean, the Atlantic Ocean, and recycled moisture from continental input. These may have also supplied moisture during the Pleistocene, as fossil and modern shells exhibit comparable isotopic ranges. Our results overall indicate that a strong precipitation seasonality pattern has likely persisted in eastern Sudan since at least ∼ 200 ka.