THE HADAR BASIN LACUSTRINE RECORD: TECTONIC AND CLIMATIC INFLUENCES ON AUSTRALOPITHECUS AFARENSIS HABITATS

The Pliocene Hadar Formation (Ethiopia) preserves a rich geological and paleontological record relevant to early hominin evolution from <3.6 to 2.9 Ma. Following initial basin subsidence, lacustrine transgressions were a regular occurrence, with at least seven clearly recorded at the Hadar project area and several more minor or ephemeral events possible. Lacustrine deposits suggest a low-gradient shoreline composed mostly of mud with only localized evidence of beach deposits. Hadar lakes were most likely relatively shallow (<10m deep) and not very long-lasting as most deposits show evidence of periodic subaerial exposure and pedogenesis. The ubiquitous presence of Cyprideis, and fossilized plant stems also strongly suggest vegetated, marshy conditions.

Several recent publications (e.g., Trauth et al., 2005, 2007, 2009) have proposed that key junctures in hominin evolutionary history occur during episodes of high lake levels across East Africa at 400 kyr eccentricity maxima. Lacustrine records from the Hadar Basin have been incorporated into these publications to support this model. However, a closer look at the data from the region show much greater complexity, and a pattern inconsistent with a 400 kyr periodicity, with the only significantly persistent lacustrine episodes associated with eccentricity minimum (ca. 3.3-3.2 Ma; a pattern recorded elsewhere in Africa; e.g., Scholz et al., 2007), or initial basin subsidence (ca. <3.6-3.45 Ma). The final transgression at Hadar (ca. 2.95) occurs at the end of a period of eccentricity maxima, but is not coincident with other lacustrine phases in the East African records. Placed alongside marine oxygen isotope and terrigenous dust records, it is possible that the major Hadar transgression ca. 3.3 Ma was due to cooler temperatures and/or decreased evaporation rather than increased precipitation, while that at 3.6 Ma is likely due to tectonics. A significant faunal turnover and a size-related change in Australopithecus afarensis do occur during a period of eccentricity maxima (~3.05 Ma), but they occur during a phase of lacustrine regression and increase in arid-adapted taxa. Thus, basin scale tectonics and threshold responses of lake level to available moisture may have been more influential on Hadar habitats than continental scale paleoprecipitation patterns.