CLIMATE OF THE EAST AFRICAN HIGHLANDS IN THE EARLY NEOGENE (21.7 MA) ESTIMATED FROM FOSSIL LEAVES OF THE MUSH VALLEY, ETHIOPIA

The spring-fed Mush stream today cuts through organic-rich lacustrine shales preserving an abundance of Miocene plant materials including leaves, fruits, seeds, and pollen, most of which is exquisitely preserved. ²⁰⁶Pb/²³⁸Ur contained in zircons from interbedded ashes provide an age of 21.7 Ma, making this perhaps the earliest botanical assemblage to document Neogene environments in East Africa. More than 2500 fossil leaves were collected from five stratigraphic levels, each of which included age-equivalent sublocalities, within the paleolake deposits. Ultimately, independent proxies of regional climate, atmospheric chemistry, and local paleoecology will address questions of interest at local, regional, and global scales. The current presentation documents paleoclimate estimated from fossil leaf morphology, and provides a unique view of forest vegetation surrounding a volcanic crater lake in the northwestern highlands of Ethiopia, about 160 km ENE of Addis Ababa. Mean annual precipitation (MAP) at the time of deposition was approximately 1800 to 2000mm/yr, twice the modern rainfall (960 mm/ yr) and more than that reconstructed for the late Oligocene Chilga paleofloras 400 kms to the NW (1300 – 1400 mm/yr calculated using the same method). Paleotemperature estimates derived from tropical floras using fossil leaf morphology have a wide margin of error (up to +/- 5° C), but the same regression models used for both Oligocene and Miocene paleofloras indicate nearly the same conditions, 27 – 30°C MAT. Today, the Mush locality is approximately 800m higher than Chilga, and about 6° C cooler (MAT ~ 15°C). Thus, paleotemperature estimates indicate that either the Mush locality was not elevated relative to Chilga in the early Miocene, and MAT remained essentially the same between the two time intervals, or the Mush site had become elevated relative to Chilga by the early Miocene, but regional to global MAT was higher. As the Mush site is associated with a volcanic center resting on Oligocene flood basalts, the latter hypothesis is more likely to be correct.