PALEOPEDOLOGY AND PALEOCLIMATE SIGNIFICANCE OF EARLY MIOCENE FLUVIO-LACUSTRINE PALEOSOLS FROM MOROTO II SITE IN UGANDA

Fluvio-lacustrine paleosols associated with the early Miocene Moroto II vertebrate site in Uganda indicate pedogenesis in a semi-arid to subhumid paleoclimate in which precipitation was strongly seasonal. Paleo-Vertisols formed in moderately to poorly drained, clay-rich floodplain overbank and ephemeral pond deposits, include Vertisol profiles overthickened by sedimentation contemporaneous with pedogenesis, and are characterized by fine to medium, subangular to angular blocky peds, pedogenic slickensides, stage II pedogenic carbonate nodules and rhizoliths, and redoximorphic mottles associated with fine to medium root traces. Paleo-Inceptisols, as well as Alfisol-like paleosols with illuviated clay (Bt) and pedogenic carbonate (Bk) horizons, suggest a higher degree of landscape stability than for the Vertisols, good soil drainage during pedogenesis, and formed in well-drained quartzo-feldspathic and feldspatho-lithic channel-fill and abandoned channel fine to medium sand deposits. Feldspars, lithic fragments, and Fe-Mg silicates show weak to moderate amounts of weathering to smectite clays and Fe-oxides/oxyhydroxides, respectively. The paleosol succession is capped by an amygdaloidal basalt flow dated to ~21.4 Ma; new magnetostratigraphic results indicate that the fossil localities at Moroto were deposited in C6Aar.1r within, at most, a 244 kyr interval from 21.159 and 21.403 Ma. MAP estimates derived from bulk geochemical proxies range from as high as 940 mm yr^-1 (PPM1.0), to 752 mm yr^-1 (CIA-K) to 643 mm yr^-1 (CALMAG). Stable isotope values of pedogenic carbonates range from -1.5 to -12.5‰ VPDB for δ¹³C (mean -8.5‰, 3‰ SD), and, accounting for water stress, suggest a mixture of C3 and C4 plants in the ecosystem ranging from as little as 0% to as high as 50-60%; δ¹⁸O values range from -3.2 to -6.4 ‰ VPDB and co-vary along a simple linear mixing line, suggesting possible evaporative processes for ¹⁸O enrichment. Constraints on paleoclimate are important in refining our understanding of catarrhine primate environments and habitats, an important goal of the REACHE (Research on East African Catarrhine and Hominoid Evolution) project.