PALEOENVIRONMENTAL INTERPRETATIONS OF AN EARLY MIOCENE PRIMATE SITE USING SIZE AND STABLE ISOTOPIC SIGNATURES OF RHIZOLITHS; LOPEROT, WEST TURKANA (KENYA)

Loperot, Kenya (West Turkana), contains a unique assemblage of Early Miocene ape and monkey fossils. Here we use rhizolith size and isotopic signature to reconstruct the paleoenvironment.

Sedimentological analyses on ~28m of section indicate a large, perennial river (ripple-marked sandstones) and floodplain paleosols (Fluvisols). We measured rhizoliths from three important fossil-bearing units: crocodile skull [CS], monkey mandible [MM], and ape tooth [AT]. Rhizolith width measurements reveal distinct average widths (5.2mm for AT [n=58], 7.9mm for MM [n=56], and 10.2mm for CS [n=33]), with unit CS having a higher range in widths (1-σ st dev = 3.9) than units MM and AT (1-σ st dev = 2.4).

Rhizoliths from each unit were microdrilled across the short axis (from edge to center to edge). δ¹⁸O and δ¹³C values in 80% of the samples show a consistent U-shaped pattern (high at the edge, decreasing towards the center). Samples with multiple U’s represent conjoined rhizoliths and those that do not exhibit the U-pattern are weathered. The rhizoliths formed from the outside in, beginning with evaporative conditions (high δ¹⁸O on edges) and then infilling during inundation of the floodplain (low δ¹⁸O). Decaying plants enriched the local soil with ¹²C, causing δ¹³C values to decrease towards the rhizolith center.

Average δ¹³C values are consistent within each unit regardless of rhizolith size. Units CS and AT have similar average δ¹³C values (-10.51‰ and -10.58‰; pure C₃ forest) while unit MM records more open shrubland (-8.23‰). δ ¹³C values for unit CS rhizoliths vary by only ~1‰ (-11.2 to -10.1‰), indicating that the higher average width and range in widths of CS samples are more likely a product of plant development (young and mature plants together) rather than plant type (C₃ vs. C₄).

Rhizolith size variation and stable isotope values are important indicators of paleoenvironment and primate paleohabitats. When compared to δ¹³C values, rhizolith size can provide important information about local plant communities (type of plant vs. plant maturity). Isotopic patterns across individual rhizoliths record short-term environmental changes during formation (months?), whereas average δ¹³C values record longer-term changes in paleoecology (woodland vs. shrubland) and help broaden our understanding of the Early Miocene ape habitats.