Paper No. 4
Presentation Time: 9:45 AM
USING ND ISOTOPES AS A PROXY FOR ORBITAL-SCALE CHANGES IN CONTINENTAL WEATHERING FLUX FROM CYCLIC MIDDLE PENNSYLVANIAN PALEOTROPICAL SUBTIDAL CARBONATE SUCCESSIONS OF THE SOUTHWEST U.S
Orbital-scale climate changes exert a primary control on marine and continental sedimentary patterns; however, understanding the relationships between coeval marine and continental deposits in deep time is hampered by post-depositional erosion/tectonic processes and biostratigraphic correlations between nonmarine and marine fossils. We utilize Nd isotopes from cyclic Middle Pennsylvanian marine carbonates as a proxy for evaluating climatically controlled changes in regional continental weathering fluxes (CWF) that are coincident with orbital-scale glacio-eustasy. Seventeen highstand-dominated (or interglacial-stage dominated) glacio-eustatic cycles were sampled from the Pedregosa Basin (southeastern Arizona) and Bird Springs basin (southern Nevada) with εNd values ranging from -12.3 to -5.95. εNd trends show clear orbital-scale variations with up to 3 epsilon unit shifts across individual cycles. The majority of cycles record higher εNd (low CWF) during sea-level highstands (interglacial stage), whereas the intervening minority of cycles record opposite trends of lower εNd values (high CWF) during highstands. Previous reports of widespread loess deposits accumulating throughout the U.S. Southwest during the Pennsylvanian and lack of interbedded fluvial deposits in the studied areas suggest that the main CWF to these carbonate-dominated marine basins was from eolian sources. If correct, this implies that the majority of cycles record upwind source areas that were wetter and/or less windy during interglacial stages. The alternation between high CWF and low CWF during interglacial stages at single locations and between coeval locations supports recent climate models of the late Paleozoic (Horton et al., 2012) which report ~10 ky-scale variations in low-latitude precipitation patterns in response to eccentricity amplification of precession driven insolation.