Dynamic redox–promoted iron and nutrient cycling drove graptolite evolution across the Ordovician-Silurian transition

Graptolites were abundant and cosmopolitan zooplankton in Early Paleozoic oceans, but a prominent change in species occurred across the Late Ordovician mass extinction. We use ocean redox, iron isotope (δ⁵⁶Fe), and phosphorus phase partitioning records from shelf and deep-ocean settings to evaluate the drivers behind this major reshaping of the pelagic marine ecosystem. A marked decrease in mesopelagic graptolites coincided with a stepwise negative δ⁵⁶Fe shift on the shelf, driven by partial seawater Fe drawdown resulting from episodic intensification of mid-depth euxinia. Subsequently, a positive δ⁵⁶Fe shift in both deep-ocean and shelf sediments reflects extensive seawater Fe removal during the development of more widespread euxinia. This led to enhanced sedimentary phosphorus recycling from sediments, which ultimately fueled the radiation of epipelagic graptolites. Thus, wide-scale changes in Fe cycling, linking the global oceanic redox state to phosphorus cycling, were ultimately responsible for the initial demise and subsequent radiation of select graptolite species.