Using Nd Isotopes to Evaluate Continental Weathering Flux in Carbonate-Dominated Marine Systems of the Southwestern U.S.

Mixed carbonate-siliciclastic marine successions provide a unique window to evaluate input from two different source areas—siliciclastics record continental weathering flux and marine currents whereas interbedded carbonates record changes in ambient seawater conditions and paleoecology. In marine successions composed entirely of carbonates, the terrestrial signal is lacking?? making the record of continental weathering flux difficult to evaluate.

We are using Nd-isotope ratios in cyclic marine carbonates as a proxy for continental weathering flux in deposits lacking significant marine siliciclastics, as well as a proxy for biolimiting nutrient flux which ultimately controls phytoplankton (and potential hydrocarbon) production. 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.

This study focuses on 3 separate, but coeval cyclic Middle Pennsylvanian successions in Arizona and Nevada. We sampled for Nd isotopes within a glacial-interglacial cycle context to understand the relationships between orbital-scale glacio-eustasy and continental weathering flux. These U.S. Southwest successions were chosen because orbital cycles are well developed and the siliciclastics were sourced from the Ancestral Rocky Mountains which have relatively well constrained and uniformly low Nd-isotope ratio values. The study sites represent deposition across ~20° of paleolatitude which likely captures a range of paleoclimate regimes. Preliminary Nd-isotope data from the cycles indicates primary Pennsylvanian marine values are preserved in the deposits and record measurable shifts that fluctuate with sea level. This relationship could provide insights into the timing and degree of siliciclastics into a marine environment within a cyclostratigraphic framework.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California