LOCATION MATTERS, OR DOES IT? TESTING CARBON AND OXYGEN ISOTOPIC VARIABILITY FROM ORDOVICIAN CARBONATE LITHOLOGIES: IMPLICATIONS FOR DIAGENETIC OVERPRINTS OF PALEOENVIRONMENTAL RECONSTRUCTIONS

Stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes are measured from bulk carbonate rocks to reconstruct ocean isotopic compositions, conduct chemostratigraphic correlation, and to provide insights into potential paleoenvironmental conditions. Recent work on modern carbonate systems suggests that >1‰ inter-grain variability exists, however, challenging whether interpretations made of ancient environments reflect primary conditions. Few studies have been conducted on whether such variability exists in deep time, and those that do include some facies that are non-ideal for chemostratigraphy. In this study, we test this hypothesis of inter-grain variability by resampling a previously studied Lower Ordovician section to document isotopic variability of multiple sampled spots from a rock sample (n>4). Study samples are from the Ibex area, western Utah, where previous work shows a >2‰ positive δ¹³C excursion is preserved in lime mudstone facies. Resampling at a higher resolution (<1m, n = 62) collected facies that range from lime mudstone to packstone, and many hand samples have multiple facies present. Isotopic data were measured using the PrecisION mass spectrometer coupled to an IsoFLOW device at Appalachian State University. Preliminary results indicate that lime mudstone facies of the same hand sample have average δ¹³C values between 0.075‰ (n=4), within analytical error (<0.08‰). However, samples with mixed lithologies can have δ¹³C values that can differ by as much as 0.23‰. d¹⁸O variations are larger where lime mudstone facies vary by 0.10‰ (n=4), but still within analytical error. Samples of wackestone- and packstone facies have δ¹⁸O values that vary by 0.17‰ and 0.30‰, respectively. Comparison of these new and published data indicate that mean lime mudstone δ¹³C values differ by an average of 0.01‰ and non-lime mudstone facies by 0.02‰. Mean lime mudstone δ¹⁸O values have no statistical difference, but non-lime mudstone facies are statistically different by up to 0.12‰. This study suggests that these ancient limestones can preserve a global signal but with some overprinting to explain observed inter-grain variability. However, this variability is much less than modern systems. Future work will consist of documenting this variability further and in connection to petrographic evidence of alteration.