INSIGHTS FROM ~17 MA SOIL CARBONATE STABLE ISOTOPES AND GCM RESULTS IN THE TURKANA BASIN, KENYA

Soil carbonates provide records of terrestrial climate conditions in arid to sub-humid regions and enable reconstruction of the past environment through interpretation of δ¹³C, δ¹⁸O, and Δ₄₇. New stable isotope measurements from samples collected at three late early Miocene fossil localities in the Turkana Basin — Buluk (~16.7 Ma), Loperot (~17 Ma), and Locherangan (~17.5 Ma) — reflect heterogeneous pedogenic carbonate formation and diagenetic overprinting that complicate our interpretation of the paleoclimate signals preserved in the primary carbonate. We propose a new soil carbonate sub-sampling approach that accounts for alteration after nodule formation by “un-mixing” the micrite precipitated in equilibrium with Miocene soil conditions from the crystalline calcite with high (50-100°C) formation temperatures. Analysis of three soil carbonate nodules from Loperot and five from Buluk show distinct calcite components in each sample, as measured in multiple sub-samples and as seen in petrographic and SEM imagery. We present these data along with results from paleoclimate simulations using the water isotope-enabled Community Earth System Model (iCESM1.2) with Miocene boundary conditions for geography, elevation, vegetation and soil properties, and atmospheric composition. Both soil carbonate clumped isotope temperatures (T(Δ₄₇)) and the model indicate hotter (~2°C) soil temperatures on the west side of the basin (Locherangan and Loperot) than the east side (Buluk), and the least negative δ¹⁸O of soil water estimated from calcite δ¹⁸O and T(Δ₄₇) are similar to model predictions for the dry season at both sites. Today, the isotope ratios of precipitation in Turkana are the product of extraordinarily hot and dry conditions, and mean annual precipitation (MAP) is far lower (200–300 mm/yr) than what the model predicts for the mid-Miocene (1100–1300 mm/yr). Dramatic changes have occurred on the landscape in Turkana over the past 17 Ma— uplift of the East African Plateau, aridification, and the subsequent spread of grasslands. Our data show δ¹³C values consistent with C3 ecosystems (-10.49 – -5.69‰ VPDB), a reflection of the environment as it was just prior to the MMCO. Comparisons between Miocene soils and soil carbonates, climate modeling, and present day conditions improve the paleoecological reconstructions that refine our understanding of this critical time.