NEODYMIUM ISOTOPE RATIOS AS A PROXY FOR WATER MASS MIXING: USES IN TRACKING OCEANOGRAPHIC CHANGES NEAR THE M4-M5 SEQUENCE BOUNDARY OF THE LATE ORDOVICIAN

Paleoceanography is a powerful tool in reconstructing past climates and their changes over time. Neodymium isotopes (¹⁴³Nd/¹⁴⁴Nd; εNd) are used in paleoceanography to trace water mass mixing as neodymium’s low residence time prevents extensive mixing and allows for isotopically distinct bodies of water, or aquafacies, to be tracked through time and space. We present data from across the M4-M5 sequence boundary of the Late Ordovician Mohawkian Sea, which was a shallow water, tropical, carbonate sea. In the Upper Mississippi Valley the M4-M5 is in the lower Decorah Formation. Here, a transition from “warm to cool-water carbonates”, changes in terrigenous influx, and a large, positive δ¹³C excursion in the Guttenberg Member of the Decorah Formation known as the “GICE,” all have been used to suggest a cooling event predating the end Ordovician mass extinction by millions of years.

Previous work at this boundary contradicts a glacial explanation for cooling, suggesting that cooling may not be representative of a global phenomenon. Alternatively, changes in water masses interpreted from εNd changes can be used to explain cooling signals at the M4-M5. Neodymium data was collected from conodont microfossils, the phosphatic feeding apparatuses of small eel-like organisms. Data from the Drepanoistodus suberectus was used to eliminate the possibility of biologic variability in εNd fractionation. Conodonts leach Nd near the sediment water interface, reliably preserving isotopic signals from their aquafacies in diagenetically resistant apatite. Comparing εNd data found in conodonts to changes in lithology and the positive δ¹³C excursion across the M4-M5 boundary allows us to determine if water mass mixing played a role in cooling signals. Preliminary data has been collected on conodonts faunas spanning the Upper Platteville and Decorah formations. The positive δ¹³C isotope excursion and lithology changes are well documented by previous work. This work seeks to connect previous information suggesting cooler conditions in the Mohawkian to new εNd data. We predict that εNd data will shift from more negative to less negative across the M4-M5 boundary if water mass mixing played a role in previously documented cooling signals.