A wet basin during dry times: a new Pliocene lake record from the Afar region, Ethiopia

Abstract:

Lake records are critical archives for studying the climate and ecological history of Africa, but our ability to use them depends on their identification in the geologic record. We investigated a newly identified paleolake system exposed in the Godaya Valley in the Woranso-Mille study area, in the Afar region of Ethiopia. The > 55m-thick lacustrine-deltaic sequence, ≥ 3.804 ± 0.013 to 3.777 ± 0.014 Ma, extends from the base of local exposures up to a sequence of pyroclastic rocks, which is then capped by the basalts of the Am-Ado Plateau. There is remarkable lithologic continuity of the lake deposits across > 4 km of exposures; aerial extent of outcrop indicate that the lake was at least 9 km² and > 6-8 m deep. Delta lobes entered the lake from the northwest, similar to the current orientation of the modern Godaya River that flows east from the Ethiopian highlands. High sedimentation rates, Gilbert-type delta foresets, and the presence of debris flows suggest a rapidly filling basin surrounded by steep, nearby topography that is distinct from other Pliocene lake systems identified in the Afar. δ¹³C_28acid values of n-alkanoic acids derived from plant wax and pollen markers for Somalia-Masai shrubland/grassland indicate that vegetation in the catchment was dry C₃-dominated shrubland with some C₄ vegetation, broadly consistent with botanical signals from marine sediments in the Gulf of Aden at this time. A single sample from deltaic sands with enough pollen for a rigorous plant community analysis indicate a catchment with vegetation typical of dry, open Acacia woodland, but with some riparian forest and evergreen bushland. Despite poor preservation of biogenic silica suggesting alkaline conditions, identifiable silica remains from a single sample near the top of the lacustrine sequence include 17 diatom species, some with indications of a saline environment. The combination of data indicative of a dry catchment suggests that the lake and delta environments reflect local conditions and are not necessarily indications of a regional wet climatic interval. This work provides an initial view of a Pliocene lake system and demonstrates the potential of using it to learn more about both the tectonic and climate history of the Afar.