Increased continental weathering flux during orbital-scale sea-level highstands: Evidence from Nd and O isotope trends in Middle Pennsylvanian cyclic carbonates

Despite the common occurrence of orbital-scale (104–105 yr) sedimentary cycles in a wide range of Proterozoic through Neogene marine and non-marine depositional systems, understanding the effects and phase relationships of orbital-scale climate drivers on time-equivalent marine and non-marine deposits is difficult mainly due to correlation limitations between the geographically isolated deposition settings. Results from this study assess the relationships between orbital-scale continental weathering flux and glacial–interglacial marine cycles using Nd isotopes (from whole rock limestones) and δ18O values (from conodont apatite) from Middle Pennsylvanian cyclic marine carbonates in the U.S. Southwest.Conodont δ18O trends from 2 of 4 sampled glacial–interglacial carbonate cycles support previous interpretations that observed water-depth changes were controlled by glacio-eustasy (30–50 m magnitudes) combined with < 1° seawater temperature changes. Two additional sampled cycles show initially increasing, then decreasing δ18O trends. Based on these results, we suggest that δ18O better defines a eustatic sea-level curve, rather than a facies-derived curve.εNd trends in 5 of 8 sampled cycles are higher during regressive intervals (early glacial phase) and lower during sea-level highstands (interglacial phase), supporting the hypothesis that increases in precipitation and/or air temperatures during interglacial intervals result in increased continental weathering rates and/or increased flux of weathered solutes to the Middle Pennsylvanian marine basin. This hypothesis is in contrast to traditional sequence stratigraphic interpretations (increased siliciclastic shedding into marine basins during falling sea level/lowstands) and suggests that climatically-controlled precipitation and/or air temperature fluctuations influenced continental weathering flux more than sea level-controlled shoreline or baseline position in this paleotropical location. These results highlight the use of combined εNd and δ18O analyses as a tool for evaluating the response of marine and coeval non-marine systems to orbital-scale climate changes, particularly in deep-time depositional systems.

► O and Nd isotopes describe relationships between weathering and sea-level change.
► A eustatic sea-level curve best describes trends in δ18O over cycle development.
► Weathering flux increases during interglacials and decreases during glacials.
► Climate-controlled precipitation and/or temperature drives weathering flux.