Isotopic evidence for Plio^Pleistocene environmental change at Gona, Ethiopia

A 4.5 Ma record of fluvial and lacustrine deposits is well exposed at Gona, in the Afar Depression of Ethiopia. We use isotopic values of pedogenic carbonate and fossil teeth to reconstruct Plio^Pleistocene environmental change at Gona. An increase in N13C values of pedogenic carbonates since 4.5 Ma points to a shift from woodlands to grassy woodlands in the early Pliocene, 310.4 to 33.9x (VPDB), to more open but still mixed environments in the late Pleistocene, 33.0 to 31.4x (VPDB). This pattern is also seen in isotopic records elsewhere in East Africa. However, at 1.5 Ma the higher proportion of C4 grasses at Gona is largely a result of a local facies shift to more water-limited environments. The wide range of N13C values of pedogenic carbonate within single stratigraphic levels indicates a mosaic of vegetation for all time intervals at Gona that depends on depositional environment. Elements of this mosaic are reflected in N13C values of both modern plants and soil organic matter and Plio^Pleistocene soil carbonate, indicating higher amounts of C4 grasses with greater distance from a river channel in both the modern and ancient Awash River systems. N18O values of pedogenic carbonates increase up-section from 311.9x in the early Pliocene to 36.4x (VPDB) in the late Pleistocene. The wide range of N18O values in paleovertisol carbonates from all stratigraphic levels probably reflects short-term climate changes and periods of strong evaporation throughout the record. Based on the comparison between N18O values of Plio^Pleistocene pedogenic carbonates and modern waters, we estimate that there has been a 6.5x increase in mean annual N18O values of meteoric water since 4.5 Ma. N18O values of pedogenic carbonate from other East African records indicate a similar shift. Increasing aridity and fluctuations in the timing and source of rainfall are likely responsible for the changes in N18O values of East African pedogenic carbonates through the Plio^Pleistocene. > 2004 Elsevier B.V. All rights reserved. 0012-821X / 04 / $ ^ see front matter > 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00707-6 * Corresponding author. Present address: Department of Geology and Geophysics, University of Utah, 135 S. 1460 East, Salt Lake City, UT 84112-0111, USA. Tel.: +1-801-403-9070; Fax: +1-801-581-7065. E-mail address: [email protected] (N.E. Levin). § Supplementary data associated with this article can be found at doi:10.1016/S0012821X03007076 EPSL 6970 6-2-04 Earth and Planetary Science Letters 219 (2004) 93^110 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl