T234 Greenhouse to Icehouse Transition: Global Events of the Devonian, Carboniferous, and Early Permian I

Wednesday, 7 November 2018
8:00 AM - 12:00 PM
Room 140

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

Paper number: T234-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-324906

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