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 87
Sr. Marine 87
Sr 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 87
Sr 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.