DID A TECTONIC UPLIFT AND BASALTIC WEATHERING ENHANCE NUTRIENT DELIVERY TO THE MIDDLE ORDOVICIAN OCEANS?

The Ordovician Darriwilian Stage records a ~2‰ carbon isotope excursion known as the MDICE (Mid-Darriwilian Carbon Isotope Excursion) that broadly coincides with the most rapid diversification of the Ordovician radiations. While a growing number of studies have shown the significance of the MDICE as a chronostratigraphic marker for global correlations, the cause of this carbon isotope excursion and its potential links to climatic, biotic, and tectonic events that took place during the Darriwilian remains an open question. Here we use a basin-wide high-resolution chemostratigraphy that integrates carbon cycle (δ¹³C) and weathering (εNd, and ⁸⁷Sr/⁸⁶Sr) proxies with physical stratigraphy from three mixed siliciclastic-carbonate sections in the central Appalachian Basin (Rocky Gap - VA, Collierstown - VA, and Clear Spring - MD) to explore potential causes of the MDICE.

In the Appalachian basin, the MDICE at Clear Spring occurs within shallow tidal flat facies with peak δ¹³C values (+1‰) that span a lithologic shift from peritidal carbonates of the Pinesburg Station Dolomite (‘Knox-Beekmantown Group’) to gastropod-bearing fenestral limestones of the overlying Row Park Formation (St. Paul Group). The Row Park transitions into restricted lagoonal facies of the New Market Formation, and the entire Clear Spring MDICE section appears conformable based on conodont-apatite based ⁸⁷Sr/⁸⁶Sr and facies succession. Further south, an erosional surface known as the Knox Unconformity is observed at the Collierstown and Rocky Gap sections and the MDICE is not present. Missing time is indicated not only by δ¹³C and physical stratigraphy but also by rapid shifts in the ε_Nd and ⁸⁷Sr/⁸⁶Sr curves. Furthermore, our ε_Nd and ⁸⁷Sr/⁸⁶Sr data show a correlation between the MDICE and a significant continental weathering shift toward ‘juvenile’/basaltic values. These correlations suggest a potential link between the MDICE, change in relative sea level, and increase in basaltic weathering, possibly due to the Taconic uplift. We aim to use a carbon cycle box model to further test the hypothesis that weathering of basaltic rocks with high phosphorus content could invigorate primary productivity during Ordovician biodiversification events.