PALEOCLIMATIC AND TAPHONOMIC IMPLICATIONS OF LATE OLIGOCENE LEAF ASSEMBLAGES FROM NORTHWESTERN ETHIOPIA

Abundant 28-27 million year old plant fossils from Chilga Woreda (“Chilga”), northwestern Ethiopia, include exquisitely preserved leaf compressions – sometimes in leaf mats- in massive or finely laminated overbank deposits. Thus, they were transported a very short distance from their point of origin, or not at all. These assemblages sample the taxonomic richness and diversity of a single plant community with minimal time averaging, making them ideal for quantitative paleoclimate estimates.

In this study, paleoprecipitation estimates are provided for two contemporaneous floras preserved under similar circumstances, located approximately 1.5 km apart, but with different floral compositions. The paleoprecipitation reconstructions are based upon regression formulae derived from calibration data consisting of leaf area measurements and precipitation for modern tropical plant communities. As the method relies upon species' leaf area rather than taxonomy, the two sites will serve as the first field test of the method, which should produce statistically indistinguishable estimates. Two formulae produce estimates of 1413 (+359, -287) and 1342 (+420, -320) for the Guang River flora, which are consistent with floral composition and high lateral heterogeneity of composition along a single outcrop, typical of tropical moist forest. Preliminary results of work on the Bull's Bellow flora indicate that estimates are likely to be comparable. The results of this study will have implications for all morphology-based paleoprecipitation estimates derived from leaf floras – regardless of the formulae applied.