New insights into the local redox landscape of the Middle Ordovician Baltoscandian carbonate shelf from I/Ca records

The Ordovician contained one of the most significant radiations in marine biodiversity of the Phanerozoic in what is referred to as the Great Ordovician Biodiversification Event (GOBE) or Ordovician Radiation(s). While the precise driving mechanisms of such profound biotic changes remain debated, many hypotheses invoke oxygen, along with other variables such as plate tectonics and climate change, as one of the possible key factors in helping fuel biodiversification. Numerous studies document the fundamental role of oxygen in driving increases in biodiversity throughout the Phanerozoic, and similar hypotheses have been put forth for the GOBE; however, this postulation requires a variety of geochemical data at high spatiotemporal resolution to elucidate the evolution of marine oxygen over this time.
Here, we employ the iodine-to-calcium ratio (I/Ca) redox proxy as a tool to investigate the local redox landscape of several carbonate-dominated Middle Ordovician sections spanning the Baltoscandian paleoshelf. Iodine is one of the first elements to respond to changes in water column oxygenation, making this proxy especially useful for evaluating local shifts in marine oxygen content relative to the timings of Ordovician biodiversification pulses. The findings of this study expand upon and are consistent with previously published high-resolution I/Ca records from Baltica by Lindskog et al. (2023), suggesting these regional marine environments underwent shifts to comparatively more oxygenated conditions that are coincident with increases in diversity (i.e., carbonate skeletal variety and abundance). Though we cannot unambiguously connect biodiversity trends to redox conditions on global scales based on our I/Ca data alone, this regional study at least further implicates local marine oxygenation as one of the drivers of increased biodiversity in Baltica and may provide insight into connections between habitability, oxygenation, and climate during the GOBE.