STABLE HYDROGEN AND OXYGEN ISOTOPE COMPOSITIONS OF PEDOGENIC MINERALS FROM OLIGOCENE PALEOSOLS OF THE NORTHWESTERN ETHIOPIAN PLATEAU: IMPLICATIONS FOR PALEOCLIMATE AND PALEOELEVATION

We present the clay mineralogy, chemistry, and stable oxygen and hydrogen-isotope compositions measured from 20 phyllosilicate samples representing Oligocene-age (32.4 to 26.8 Ma) paleosol profiles from the Chilga basin, northwestern Ethiopian Plateau with excellent geochronometric constraints from intercalated volcanic ash-fall tuffs. These data are used to provide high-fidelity Oligocene-age paleotemperatures, meteoric rainfall patterns and stable isotope compositions, and Oligocene paleo-elevation of this region that currently occupies an elevation 1800-2000m above sea level in tropical Africa. The fine clay fraction samples consist of kaolinite or kaolinite-dominated mineral mixtures. Chemical and mineral data are used to calculate unique hydrogen and oxygen isotope fractionation factors for each sample in order to estimate paleotemperatures of crystallization from measured phyllosilicate dD and d¹⁸O values.

Phyllosilicate δD and δ¹⁸O values range from -62‰ to -95‰ and 17.7‰ to 21.8‰, respectively. Assuming the phyllosilicates preserve a record of isotopic equilibrium with Oligocene meteoric waters, the measured hydrogen and oxygen-isotope values correspond to phyllosilicate crystallization temperatures ranging from 12 ± 3°C to 32 ± 3 °C. Rainfall δ¹⁸O values range from -4.6‰ to -6.6‰. The basal parts of the Oligocene record preserve evidence for consistent warm (26-32°C) and humid (>1500 MAP). Modern tropical sites with similar ranges of mean annual temperature and δ¹⁸O rainfall values of precipitation are limited to tropical lowland sites in southeast Asia characterized by intense monsoons and an extensive wet season. Conversely, the Chilga region currently has a lower range of mean annual temperature (16-18°C) and more positive δ¹⁸O values of precipitation (-2.0 to 3.0‰) due to its high-elevation. This suggests the observed Oligocene-age Chilga paleosols formed at much lower paleo-elevation and substantial vertical uplift of the Ethiopian plateau has occurred since that time. Furthermore, significant shifts in temperature (~10°C) occur roughly on the order of 40kyr and 100kyr intervals, suggesting Chilga basin tropical paleoclimate was substantially impacted by obliquity and eccentricity cycles between ~28 and 26 Ma before present. These results provide the first evidence for significant tropical Paleogene climate change driven by Milankovitch-scale mechanisms.