FROM LAKE TO RIVERS– PALEOENVIRONMENTAL RECONSTRUCTIONS FROM PALEOSOLS OF LOPEROT, KENYA

The Early Miocene hominoid-bearing site of Loperot, Turkana Basin, Kenya, provides insight into the divergence of Hominoidea and Cercopithecoidea (Old World Monkeys) in eastern Africa. We expanded the previously published stratigraphy to include a 5 m lacustrine interval overlain by ~40 m of fluvial deposition characterized by floodplain paleosols. We analyze four paleosols from Loperot with the purpose of refining existing reconstructions of paleoenvironmental and paleoclimatic change. Using bulk geochemistry from these Loperot samples, coupled with stratigraphic data for the LpM4 Loperot column (Liutkus-Pierce et al., 2019), we trace the evolution of depositional and fossil-weathering systems in the early Miocene. The parent material proxy (TiO2/Zr) changes between the bottom and upper sections, which we link to the evolution in the depositional environment from a lacustrine to fluvial environment. The grain-size proxy (Al2O3/SiO) reveals a shift from finer-grained paleosols at the bottom to coarser-grained paleosols at the top of the LpM4 section, which we interpret as an overall increase in energy in the fluvial system as it transitions from a lacustrine to braided fluvial environment. Thus far, catarrhine fossils at Loperot have only been found in facies/paleosols inferred to be riparian woodland along a meandering river. Continued sampling of fossils at Loperot and adjacent localities will allow us to evaluate this further. Within the fining upward cycles characterizing the upper fluvial section, there is a progressive increase in grain size, coupled with the presence of cross-bedded sand and gravel deposits. This change in fluvial facies stems from an increase in energy, potentially recording a transition from a meandering to braided river system with a coarser-grained bedload. Moving upsection the sodium (Na) mass-transfer (tau) demonstrates >Na loss. The CALMAG and RFMAP proxies show an increasing subhumid to humid MAP trend upsection, consistent with more Na loss. This refined analysis of paleoenvironmental and paleoclimatic conditions at Loperot suggests that our Early Miocene ancestors evolved in a highly dynamic environment with changing hydrologic systems, sedimentary deposition styles, and variable precipitation and climatic conditions.