EXPLORING POST-DEPOSITIONAL PROCESSES IN LATE MIOCENE PALEOSOLS AT LOTHAGAM, TURKANA BASIN, KENYA

Climatic change is proposed as a key driver of primate evolution during the Miocene. However, due to limited outcrop exposure, questions remain about the paleoclimatic and paleoenvironmental conditions of the Late Miocene in the Turkana Basin, Kenya. Lothagam’s locality is key to our understanding of this critical time period as the sequence here spans the Miocene-Pliocene boundary. This study integrates descriptions of four paleosols with their major element geochemistry to parse the impacts of post-depositional processes from paleoclimatic and environment conditions shaping pedogenesis. Paleosols were described and sampled by horizon. Bulk sediment samples were analysed using WD XRF and we applied geochemical proxies for grain size (Al₂O₃/SiO₂), parent material (TiO₂/Zr), and estimates of paleoprecipitation (CALMAG). From the geochemical data and field descriptions it was determined that pedogenic carbonate was present with the exception of one paleosol. This latter paleosol weathered a Ca- and Mg-rich mafic parent material and thus precludes the use of CALMAG-derived MAP estimates. Additionally, geochemistry showed unusually high levels (>6 wt %) of Na2O in some paleosols. Previous work at Lothagam records the presence of Na-rich zeolites (e.g., analcime), suggesting rainfall models that utilise Na2O concentrations (e.g., CIA-K, PPM. RFMAP) are not suitable. The CALMAG modelled MAP yields values ranging between 1090 ± 108 mm/yr and 760 ± 108 mm/yr, indicating that Lothagam was characterised by subhumid to humid precipitation regimes. Because modern pedogenic carbonates in near-surface soil horizons are rarely found in soil weathering in MAP regimes >1000 mm/yr, we suggest that the lower-end MAP estimates are more realistic for Lothagam (760 mm/yr). Ongoing mineralogical analysis (XRD and petrography) will better constrain the impacts of pedogenesis vs zeolite diagenesis. These findings suggest that the geochemical proxies conventionally applied to analyse precipitation patterns need to be carefully evaluated where post depositional processes result in the authigenic formation of secondary minerals.