Middle Ordovician paleoenvironmental evolution of the western Laurentian carbonate platform: Evidence for persistent oxygenation of the shallow ocean and implications for biodiversification

The Great Ordovician Biodiversification Event (GOBE) encompasses a series of rapid and sustained diversification pulses unparalleled in Earth history. Despite the uniqueness and magnitude of the GOBE, a singular driving force for this critical interval has not been identified. Multiple hypotheses have been presented, some pairing the GOBE as cause and effect with extrinsic factors such as changing global temperatures, ocean chemistry, or paleogeography, while others have pointed to factors intrinsic to biology itself, such as the proliferation of skeletonization, predator-prey escalation, or the development of complex tiered seafloor communities. Sufficient data are not available at present to ascertain what combination of these factors was the key to generating the GOBE.

Here, we present new paired carbon and sulfur isotope data (δ¹³C_carb, δ¹³C_org, and δ³⁴S_CAS), along with iodine concentration data from a Darriwilian carbonate sequence exposed at Meiklejohn Peak in southwestern Nevada, USA. Within a detailed lithologic and biostratigraphic framework, these new data provide a robust correlation to chemostratigraphic zonation established in Baltica and eastern Laurentia along with insights into the evolution of Middle Ordovician biogeochemical cycling. Carbon isotope data indicate a slight (2 ‰) increase in the fractionation between carbonate carbon and organic matter (Δ¹³C) during the onset of the middle Darriwilian carbon isotope excursion (MDICE), and new carbonate-associated sulfate (CAS) sulfur isotope data are consistent with a decrease in global pyrite burial during this time. Both of these observations support previous work arguing for a synchronous increase in the [O₂] of the global oceans. New iodine concentration data from Meiklejohn Peak also suggest a progressive and sustained increase in the [O₂] of regional waters throughout the Darriwilian during a period of relative sea-level highstand. This expansion of stable and well‑oxygenated ecospace could have laid the foundation for rapid diversification during one of the largest pulses of the GOBE.