Terrestrial vegetation reconstructions spanning the Paleogene-Neogene boundary in the Ethiopian highlands.

摘要

Global temperature reconstructions spanning the Late Oligocene to Early Miocene (28-22 m.y.a.) document an increase in global temperatures, however different proxies provide conflicting data on the magnitude of warming. A significant increase in global temperatures between about 26 and 25 m.y.a., derived from delta18O of benthic foraminifera, would have altered the Earth's hydrological cycle, resulting in a longer dry season and more xeric vegetation in tropical Africa. Whereas a minor warming, documented by reconstructions of Antarctic ice sheet volume, may not have had major influences on tropical vegetation. Very little work has been conducted in the African tropics, particularly during the Paleogene and earliest Neogene, due to a combination of regional under sampling and lack of preserved and accurately dateable exposures. Recently, plant and vertebrate fossils, and paleosols from Chilga (NW Ethiopia) documented flora, fauna and landscapes 28-27 Ma, filling temporal a spatial paleoenvironmental gap between localities from the earliest Oligocene of Egypt and middle Miocene of Kenya. New work in the Mush Valley (SE of Chilga) documents Early Miocene terrestrial floras dated radiometrically from 21.73+/-0.01 Ma to 22.63+/-0.03 Ma. Aside from a small 1974 report on plant fossils, Mush Valley paleoenvironments are undocumented.;A total of 43 localities were analyzed for their palynological makeup; 17 localities, 8 from the Late Oligocene Chilga area and 9 from the Early Miocene Mush Valley, yielded pollen sums sufficient for paleoenvironmental analysis. A total of 49 taxa were identified to the family or generic level based on preserved morphological characteristics. Comparison with modern ecological habitats of Alchornea, Celtis, Olea, Pycnanthus, Oligocodon, and Macaranga are consistent with tropical wet forests. Brachystegia, Isoberlinea, and taxa often associated with dry forests and woodlands today are interpreted to be wet forest species here due to their co-occurrence with strictly wet forest taxa.;The compositional make-up of each locality is statistically different based on cluster analysis and correspondence analysis; Chilga had a higher diversity of taxa while the Mush Valley had higher percentages of ecologically important taxa. The Mush Valley also documents the oldest occurrence of the Myristicaceae family from Africa by the presence of Pycnanthus pollen grains. The presence of wet, tropical forests from the terrestrial low latitudes of Africa is not consistent with an expected drying in response to hypothesized increase in global temperature between the Late Oligocene and Early Miocene, as documented by marine oxygen isotopes.