QUANTIFYING DEPOSITIONAL ENVIRONMENTS USING ULTRA HIGH-RESOLUTION GRAIN SIZE ANALYSES

Grain size analyses of sediments can reveal important information about the environmental context of sedimentary sequences. The Turkana Basin in northern Kenya contains a robust record of hominin fossils, including Nariokotome Boy (NK3 site), the most complete H. erectus specimen found to date. Coupling the fossil record to paleoenvironmental proxy records, including grain size analyses, the Turkana Basin provides an important testing ground for evolutionary hypotheses about hominin evolution. Here we present grain size data from a 216 m drill core (WTK13) from the Turkana Basin. Recovered as part of the Hominin Sites and Paleolakes Drilling Project in 2013, this core was recently resampled continuously at 0.5 cm intervals (~7 yrs/sample) to document the paleoenvironment at ~1.4 Ma, which correlates to the NK3, providing unprecedented resolution for this important hominin fossil. Previous facies analyses reveal Turkana’s dynamic lake level history, and grain size analyses complement this record by quantifying this highly variable environment. Samples were pre-treated to remove carbonate, biogenic silica, and other organic material from detrital material and size distribution was analyzed on a Malvern Mastersizer 3000. The section was dominated by clay, which was further investigated using end-member modeling. A four end-member solution explained on average 99% of the population variability. The bottom of the section was more coarse-grained with an abrupt fining transition at 38.83 mbsf, which corresponds with the transition out of a tuffaceous interval (Natoo tuff) and into a pedogenically modified interval. This is significant as the top of this tuff is the surface upon which Nariokotome Boy was recovered. Our 0.5 cm sampling resolution now enables us to quantify depositional changes on a scale comparable with previous descriptive facies analyses, enabling us to further refine transitions between paleosols, fluvial deposits, and lacustrine deposition at the interface of these three paleoenvironments.