Using Nd isotopes to evaluate relationships between continental weathering flux and glacio-eustasy in cyclic Pennsylvanian deposits of the Southwestern U.S

Understanding the effects of orbitally controlled climate change on coeval marine and continental deposits in deep time (pre-Cenozoic) has been limited by the difficulties inherent in high-resolution biostratigraphic correlation between marine and non-marine depositional systems and poor preservation of continental deposits. We use Nd-isotope ratios as a proxy for continental weathering flux (includes weathering and transport of weathered material) in Middle Pennsylvanian cyclic marine carbonates to determine the relationship between orbital-scale glacial eustasy and continental weathering flux. We use Nd-isotope ratios because the residence time of Nd in the ocean is less than the mixing time of the ocean, the marine Nd budget is dominated by continental flux, and once incorporated into the sediment Nd is diagenetically stable. Preliminary tests on bulk limestone samples dissolved in acids of varying concentrations and limestone samples collected ~20 m apart generate similar Nd-isotope values (ɛNd = ~-10) indicating minimal Nd-isotope contamination from dissolved clay mineral phases within the limestone and lateral consistency of isotopic signatures. The calculated ɛNd = ~-10 at 300 my is similar to the value expected of the local continental crust at 300 my. We are analyzing samples from multiple, orbital-scale, Middle Pennsylvanian cycles from southeastern Arizona to compare the relationships between glacial-eustasy (derived from facies analysis) and continental weathering flux (derived from Nd-isotopes). Of particular interest are the potential relationships between orbital-scale wet/dry or wind intensity variations and glacially controlled sea-level rises and falls.