ONSET OF MAJOR OCEAN OXYGENATION DURING THE EARLY MIDDLE DEVONIAN PERIOD RECORDED BY URANIUM ISOTOPES IN MARINE LIMESTONES FROM THE WESTERN U.S.A

The Devonian Period (419 to 359 Ma) marks a critical transitional interval in Earth history that was characterized by the diversification of vascular land plants and a marked increase in root-depth penetration. It has been hypothesized that these events affected the redox balance of the ocean-atmosphere system by changing the intensity of terrestrial weathering, organic carbon burial rates, and ultimately, atmospheric pO₂. Specifically, there is some evidence to suggest that a major pulse of global oxygenation occurred around 400 Ma, based on both geochemical evidence and modelling of long-term biogeochemical cycles. To test this hypothesis and pinpoint the exact timing and pattern of oxygenation, we present a new, high-resolution uranium isotope (δ²³⁸U) dataset from limestones spanning most of the Devonian Period in the Great Basin, Nevada, USA. δ²³⁸U of marine limestones represents a powerful new proxy for constraining the global extent of seafloor anoxia.

In our composite dataset, δ²³⁸U values are consistently less than -0.5 ‰ (and as low as -0.9 ‰) through the Emsian Stage, suggesting extensive marine anoxia during this interval. This is followed by a significant, progressive shift towards higher δ²³⁸U values (consistently greater than -0.3 ‰) in the earliest Eifelian Stage, suggesting that a major pulse of ocean oxygenation occurred around 395 Ma. These relatively high δ²³⁸U values generally persist through the Givetian and Frasnian stages, although shorter-term redox events may become apparent with increasing resolution from our on-going analyses. This new high-resolution, seawater-derived δ²³⁸U profile identifies the hypothesized middle Paleozoic oxygenation event to the early Eifelian Stage, providing important constraints on the temporal feedbacks among early land plant evolution, global biogeochemical cycling, and ocean/atmosphere oxygenation.