OXYGEN ISOTOPIC COMPOSITION OF SEMI-AQUATIC TURTLE PHOSPHATES FROM THE EARLY CRETACEOUS HOLLY CREEK FORMATION, SOUTHERN ARKANSAS

The Cretaceous Holly Creek Formation in southwest Arkansas contains the first macrovertebrate assemblage found in the state. Isotope analysis on the assemblage’s fauna can reveal paleoclimate information vital to the reconstruction of the Early Cretaceous environment and climate in southeastern North America.

This project focuses on the oxygen-isotopic composition of phosphate from turtle remains we attribute to Naomichyles speciosa, a turtle common to correlative sedimentary formations ranging from Texas to Montana and specifically from the Antlers Formation of Texas. 300-500 μg powder samples were drilled from over a dozen turtle carapace fragments and then converted to silver phosphate to be analyzed on a Thermo Advantage IRMS. Relative to VSMOW, the preliminary values of oxygen-isotopic composition of phosphate (δ¹⁸O_p) range from 16.79‰ to 20.32‰ with an average of 17.78‰ and standard deviation of 0.99‰. By converting these values to δ¹⁸O_water using Coulson et al. (2008) we are able to determine meteoric water δ¹⁸O_w compositions, averaging -3.99‰ ± 1.07‰. These compositions are consistent with those of meteoric water for the paleolatitude of southwest Arkansas (29.5°N). This provides an independent confirmation of the isotopic meteoric water gradient values initially estimated from pedogenic carbonates (Suarez et al. 2011).

If additional analyses are consistent with current data, it would imply a freshwater environment with occasional saltwater incursion from the nearby gulf coast. This study provides the groundwork for future study of other taxa from the only known Cretaceous vertebrate assemblage in Arkansas.