Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates

A global interval of elevated carbon isotope ratios (δ13C) during the Upper Ordovician, linked to glaciation, is not solely a record of global ocean chemistry but is influenced by sea-level changes and early marine diagenesis. Research on carbonate strata from Anticosti Island, Canada, and the Great Basin, USA, reveals that variations in δ44Ca and Sr/Ca ratios reflect primary aragonite mineralogy and diagenetic processes, which modulate δ13C records. Deep-ramp settings preserve elevated δ13C due to increased aragonitic preservation during sea-level lowstands, while mid- and inner ramp settings show muted shifts due to extensive early marine diagenesis under seawater-buffered conditions. This challenges the assumption that shallow-water carbonate proxies always reflect global ocean changes.

Jones, D. S., Brothers, R. W., Crüger Ahm, A.-S., Slater, N., Higgins, J. A., & Fike, D. A. (2019). Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates. Geology, 48(2), 194–199. https://doi.org/10.1130/g46861.1