PLIOCENE WARMTH AND ECOSYSTEM CHANGE IN EASTERN AFRICA

The Pliocene is a potential analog for a future warm world as it is the most recent geologic epoch with little ice in the northern hemisphere and when atmospheric pCO₂ levels exceeded 280 ppm. Proxy records indicate that maximum sea surface temperatures (SST) in the early Pliocene were similar to those of today but that SST gradients were reduced and the climate structure was fundamentally different. Testing the effects of these changes on terrestrial ecosystems requires proxy datasets that can distinguish local changes from regional responses to global phenomena. Terrestrial proxy data have traditionally provided information on vegetation and hydroclimate, but recently clumped-isotope thermometry has made temperature reconstruction possible in many terrestrial archives. Here we present clumped-isotope data from pedogenic carbonates from the Awash Basin, Ethiopia, that span the last 4.5 myr. Clumped-isotope derived soil temperatures average 35˚C and range from 28 to 45˚C (n = 42). There are no major temporal trends in the data, although temperatures in excess of 35˚C are more common before 2 Ma. Soil temperatures in the early Pliocene are similar to present-day carbonate clumped-isotope derived soil temperatures and warm season mean air temperatures in the Awash Basin. They also overlap with coeval, clumped-isotope derived soil temperatures from the Omo-Turkana Basin (>800 km southwest of the Awash Basin) which range from 30 to 41˚C (n = 27). When soil temperatures from the Awash and Omo-Turkana Basins are paired with pedogenic carbonate δ¹³C and δ¹⁸O values from the same strata, it is clear that significant changes in both hydroclimate and vegetation have occurred in both basins since 4.5 Ma. Reconstructed soil water δ¹⁸O values indicate that the δ¹⁸O of rainfall in both basins was lower in the Pliocene than today, which might reflect wetter conditions and differences in circulation patterns. Pedogenic carbonate δ¹³C data indicate an increase in C₄ vegetation over the last 4.5 myr, despite an interval of high woody cover during the middle Pliocene. Together, these data support conclusions from the marine proxy data that temperatures in the tropics were not substantially different in the early Pliocene than today but that changes in the climate system might have had substantial effects on terrestrial ecosystems.