Exploring the mechanisms of Devonian oceanic anoxia: impact of ocean dynamics, palaeogeography and orbital forcing

Abstract. The Devonian is a warmer-than-present geological period spanning from 419 to 359 million years ago (Ma) characterized by multiple identified ocean anoxic/hypoxic events. Despite decades of extensive investigation, no consensus has been reached regarding the drivers of these anoxic events. While growing geological evidence has demonstrated a temporal correlation between astronomical forcing and anoxia during this period, underlying physical mechanisms remain unknown, hence questioning causality. Here, we perform multiple sensitivity experiments, using an Earth system model of intermediate complexity (cGENIE), to isolate the influences of specific Devonian climate and palaeogeography components on ocean oxygen levels, contributing to the better understanding of the intricate interplay of factors preconditioning the ocean to anoxia. We quantify the impact of continental configuration, ocean-atmosphere biogeochemistry (global mean oceanic PO₄ concentration and atmospheric pO₂), climatic forcing (pCO₂) and astronomical forcing on background oceanic circulation and oxygenation during the Devonian. Our results indicate that continental configuration is crucial for Devonian ocean anoxia, significantly influencing ocean circulation and oxygen levels while consistently modulating the effects of other Devonian climate components such as oceanic PO₄ concentration, atmospheric pO₂ and pCO₂, and orbital forcing. The evolution of continental configuration provides a plausible explanation for the increased frequency of ocean anoxic events identified during the Middle and Late Devonian periods, as it contributed to the expansion of oxygen-depleted zones. Our simulations also show that both the decreased atmospheric pO₂ and increased oceanic PO₂ concentration exacerbate ocean anoxia, consistent with established knowledge. The variation of pCO₂ reveals a wide range of ocean dynamics patterns, including stable oscillations, multiple convection cells, multistability and hysteresis; all leading to significant variations of the ocean oxygen levels, therefore strongly impacting the preconditioning of the ocean to anoxia. Furthermore, multistability and important hysteresis (particularly slow ocean time response) offer different mechanisms to account for the prolonged duration of some ocean anoxic events. Finally, we found that astronomical forcing substantially impacts ocean anoxia by altering ocean circulation and oxygen solubility, with obliquity consistently emerging as the primary orbital parameter driving ocean oxygen variations.