ROLE OF SEAFLOOR PRODUCTION VERSUS CONTINENTAL BASALT WEATHERING IN MIDDLE TO LATE ORDOVICIAN SEAWATER 87SR/86SR AND CLIMATE

The Ordovician Period (486.9 to 443.1 Ma) provides a valuable case study for the various systems that drive and respond to global climate change. During the Ordovician, average sea surface temperatures dropped roughly 20ºC from the extreme warmth of the Early Ordovician (~42º C) to the Hirnantian glaciation (~22º C). This cooling to mild, modern-like sea surface temperatures potentially triggered the Great Ordovician Biodiversification Event (GOBE) and eventually may have led to the first of the “big five” extinctions: the end-Ordovician mass extinction. One potential driver of this cooling is increased weathering of calcium-bearing silicates, specifically those of basaltic composition. An important proxy for basaltic weathering in the geologic record is seawater ⁸⁷Sr/⁸⁶Sr. Marine ⁸⁷Sr/⁸⁶Sr decreases throughout the Ordovician, with an inflection in the Middle to Late Ordovician (Darriwilian to Sandbian stages) to a greater rate of decrease. Previous studies have used the ⁸⁷Sr /⁸⁶Sr curve and carbon cycle modeling to characterize the role of continental silicate weathering in driving Ordovician cooling (e.g., the Taconic Orogeny). However, these models have not accounted for an apparent increase in sea level and seafloor production in the Middle Ordovician (Darriwilian), which could increase the hydrothermal Sr flux as well as the CO₂ degassing flux along continental volcanic arcs.

Here, we present new paired ⁸⁷Sr /⁸⁶Sr and δ¹⁸O data from conodont apatite and integrate these new data with a modeling approach to show that increased hydrothermal circulation could have played a role in driving marine ⁸⁷Sr /⁸⁶Sr, specifically an inflection occurring c.a. 460.9 Ma ± 1 My in the serra conodont zone of the mid-Darriwilian Stage. This ⁸⁷Sr /⁸⁶Sr inflection is accompanied by an increase in δ¹⁸O, consistent with cooling. We can model this cooling alongside increased degassing if continental silicate weathering increased by >40% (due to an increase in weatherability). Including the role of seafloor production in seawater ⁸⁷Sr /⁸⁶Sr and Ordovician cooling allows for a more nuanced understanding of the many factors that drive multi-million-year shifts in climate.