The Pliocene environment of Hadar, Ethiopia: A comparative isotopic study of paleosol carbonates and lacustrine mollusk shells of the Hadar Formation and of modern analogs.

摘要

The oxygen and carbon isotopic compositions of paleosol carbonates and mollusk shells from Ethiopia indicate that the Pliocene climate at Hadar, and Holocene and late Pleistocene climates in most parts of the Afar Depression were significantly different, and more habitable, than the sterile climatic conditions of the modern Afar Depression.; Interpretation of the Pliocene data was guided by relations observed between the geochemistry of Modern, Holocene and late Pleistocene soil carbonates and mollusk shells and the environments in which they formed.; The 18O/16O ratios of Pliocene mollusks from Hadar indicate a climate change between 3.34 mya (mean δ 18O_carbonate + 2.1‰), when the lowermost shell bed of the Hadar Fm. was deposited, and 3.28 mya (mean δ18O _carbonate − 6.8‰), the age of the middle gastropod bed. This suggests evolution of a lake from one that was intensively evaporated at 3.34 mya to a fresher one, probably fed by local precipitation and/or heavy runoff from the nearby Western Plateau and escarpment around 3.28 mya. The large variation in δ18O_carbonate of microsamples of several individual shells from the upper gastropod bed (3.22 mya), suggests strong seasonal variation in the shallower lake environment of that time.; Of all the soil carbonates analyzed, the δ18O values of those from the Hadar Fm. are the most negative. This indicates that Pliocene conditions were significantly wetter and cooler than today's, probably analogous to the pluvial Holocene intervals in the Afar Depression.; The δ13C values of Pliocene soil carbonates from Hadar suggest that the vegetation was a mixture of C3 and C4 plants. The proportion of C4 plants increased from 35% above the Main Disconformity Surface ( ca 2.9 mya) to about 60% at the top of the formation (ca 2.3 mya).; Consistent with the results from the mollusk shells, the δ 18O values of Holocene and late Pleistocene soil carbonates indicate that the climate in the Afar during that interval was nearly as wet as that at Pliocene Hadar. In contrast, the δ18O values of Holocene/Pleistocene soil carbonates indicate different conditions on the Western Plateau and escarpment than in either the Afar or in Pliocene Hadar.