USING PALEOSOL CHROMA AND MINERALOGY TO TRACK CLIMATE AND ENVIRONMENTAL CHANGES AT LOPEROT, KENYA DURING THE EARLY MIOCENE

Soil chroma values (Munsell) are proposed to track paleoclimate during the time of soil formation—low chroma values indicate humid conditions and higher chroma values indicate aridity. We examine the color and mineralogy of 13 paleosols preserved in a 28-meter thick stratigraphic section from the early Miocene site of Loperot (western Turkana, Kenya) to better understand the environmental conditions during soil formation. Found in the region are catarrhine fossils of Cercopithecoidea and Hominoidea, two primate groups that diversified during the early Miocene (23-16 Ma) making this region important for understanding what environment our ancestors inhabited. Soil chroma values of the Loperot paleosols decrease upsection (from 4 - 6, to 1 - 3), suggesting an increase in humidity over time. If soil chroma is accurately tracking climate, we propose that we should also see a systematic change in mineralogy within the paleosols upsection. X-ray diffraction analysis of paleosols formed under arid conditions (lower in the section as suggested by soil chroma) should indicate the presence of feldspars and evaporite minerals. Those paleosols that formed under more humid conditions (higher in the section) should show a decrease in feldspar abundance at the expense of clay production and fewer evaporite minerals. Preliminary results indicate that feldspars and illite are abundant in Units 30, 26, 24, and 22 low in the section; consequently, these minerals decrease in abundance upsection. The decrease in feldspars coincides with an increase in montmorillonite in Units 12, 7, and 5. Evaporite minerals were present throughout the stratigraphic section and could form at anytime during soil formation. Therefore, we conclude that the mineralogy results confirm our predictions from the soil chroma data, and suggest that climate did indeed change from arid to humid during deposition of the 28-meter exposure at Loperot. There is limited literature about the environmental conditions under which Cercopithecoidea and Hominoidea diversified during the early Miocene; adding our information will allow us to better understand the environmental conditions that potentially influenced the evolution of our ancestors.