USING ND-ISOTOPE RATIOS IN MIDDLE PENNSYLVANIAN (DESMOINESIAN) MARINE CARBONATES AS A PROXY FOR CHANGES IN CONTINENTAL WEATHERING FLUX ON GLACIAL-INTERGLACIAL TIME SCALES

We are using Nd-isotope ratios in cyclic Middle Pennsylvanian marine carbonates as a proxy for glacial-interglacial changes in continental weathering flux. Typical glacial-interglacial cycles in central New Mexico (Mesa Sarca), Arizona (Whetstone Mnt and Gunnison Hills), and southern Nevada (Arrow Canyon) are composed of deeper subtidal argillaceous lime mudstones to wackestones overlain by shallow subtidal wackestones to grainstones; New Mexico and Arizona cycles are commonly capped by calcretes indicating prolonged subaerial exposure during sea-level falls/lowstands.Results from New Mexico (Mesa Sarca; Theiling et al., in review) and Arizona (Gunnison Hills) show average ε_Nd values of -9 to -11, which are consistent with a Proterozoic bedrock source (Ancestral Rocky Mountains), and record shifts of up to ~3 epsilon units suggesting significant changes in continental weathering flux on glacial-interglacial time scales.The New Mexico trends record the highest continental weathering influx during sea-level highstands, while the Arizona trends (Gunnison Hills) record the highest influx during falling sea levels. These trends could have been generated by glacial-interglacial scale variations in 1) rainfall/runoff, 2) wind intensity/direction, and/or 3) proximity of study areas to migrating shorelines. We are analyzing 2 additional coeval successions in Arizona (Whetstone Mnt) and Nevada (Arrow Canyon) to distinguish between these potential causes.

Theiling, B., Elrick, M., and Asmerom, Y., in review, Increased continental weathering flux during orbital-scale sea-level highstands: Evidence from Nd and O isotope trends in Middle Pennsylvanian cyclic carbonates, Palaeogeography, Palaeoclimatology, Palaeoecology.