ABSTRACT
The amount and seasonal distribution of paleo-rainfall is a major concern of paleoanthropology because they determine the nature of the vegetation and the structure of the ecosystem, particularly in eastern Africa. The delta(18)O and delta(13)C of paleosol carbonates are quantitative proxies of these critical features of the paleoenvironment. The Afar region of Ethiopia lies between the African and Indian summer monsoons, and is prone to profound climate change. In the western Afar, the dominant paleoenvironment of the Hadar Formation during the late Pliocene was a major meandering river's distal low, flat floodplain, on which muds accreted that were continuously transformed into vegetated soils with Bk horizons rich in CaCO(3). The mean delta(13)C of paleosols throughout the Hadar Formation translates to an average vegetative cover across the extensive floodplain of about 30% of the C(4) grasses and 70% of unspecified C(3) plants. The character of the paleosols, such as the one at Locality 333, and their delta(18)O(Carbonate) argue for a highly seasonal rainfall of about twice today's amount, implying that the C(3) plants were mostly sizeable trees and that the biome for Australopithecus afarensis was a grassy woodland. The amount of grasses abruptly increased in the lower Busidima Formation with its early Homo and artifacts to a more open grassy woodland of ca. 50% grasses. However, this transition in delta(13)C is not mirrored in the delta(18)O, which persists at a quite negative average value of -6.4 per thousand over the entire >2-Myr duration of both formations. This value for the carbonate means that the paleosoil water was a quite negative -4.1 per thousand, a significant 5 per thousand more negative than our estimate of modern rain at Hadar. We put the negative delta(18)O of paleo-Hadar's rainfall into an isotopic framework of the dynamic history of climate change in sub-Saharan northern Africa. There have been two end-member climate regimes: (1) an earlier persistently pluvial Pliocene regime, with its strong summer monsoon, as registered in the Hadar Formation; and (2) the modern cyclical, mostly arid regime that began ca. 1Myr ago, which has been punctuated by about ten cyclically predictable brief millennia-long pluvial episodes. The best known pluvial of the latter regime is the latest one, the African Humid Period (AHP), just 9.0-6.5kyr ago, whose delta(18)O(Rainfall) matches that for paleo-Hadar. The known climatological factors that brought on the AHP are probably the same ones that were persistently present for the Afar of the Pliocene. This dynamic rainfall history undoubtedly has influenced hominid occupation of the keystone Afar area at the gateway out of, and into, Africa.
