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Charcoal Evidence That Rising Atmospheric Oxygen Terminated Early Jurassic Ocean Anoxia

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Charcoal Evidence That Rising Atmospheric Oxygen Terminated Early Jurassic Ocean Anoxia

Sarah J Baker et al. Nat Commun.

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) was characterized by a major disturbance to the global carbon(C)-cycle, and depleted oxygen in Earth's oceans resulting in marine mass extinction. Numerical models predict that increased organic carbon burial should drive a rise in atmospheric oxygen (pO2) leading to termination of an OAE after ∼1 Myr. Wildfire is highly responsive to changes in pO2 implying that fire-activity should vary across OAEs. Here we test this hypothesis by tracing variations in the abundance of fossil charcoal across the T-OAE. We report a sustained ∼800 kyr enhancement of fire-activity beginning ∼1 Myr after the onset of the T-OAE and peaking during its termination. This major enhancement of fire occurred across the timescale of predicted pO2 variations, and we argue this was primarily driven by increased pO2. Our study provides the first fossil-based evidence suggesting that fire-feedbacks to rising pO2 may have aided in terminating the T-OAE.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Summary of published data collected across the early Toarcian oceanic anoxic event.
(a) Carbonate carbon isotope profile from Peniche and the organic carbon isotope profile from Yorkshire illustrating the step-wise nature of the negative excursion, and postulated pulses of light carbon release from Kemp et al. Plotted alongside are the osmium isotope profile from Mochras and total organic carbon content from the Yorkshire section. (b) Handoh and Lenton's hypothesized model changes in atmospheric oxygen and wildfire frequency across an oceanic anoxic event, with period of modelled peak oxygen and wildfire frequency highlighted.
Figure 2
Figure 2. Charcoal and Phytoclast abundances across the T-OAE.
Charcoal and phytoclast data collected in this study, plotted against published wood and carbonate carbon isotope profiles from Peniche and Mochras, and calculated cyclostratigraphic timescale from the Peniche section by ref. , . δ13Cwood and δ13Ccarb profiles for Peniche are from ref. . The δ13C carb for Mochras is from ref. and correlated to the Peniche sequence using ref. .
Figure 3
Figure 3. Jurassic Palaeo-map of the sample localities and relative site stratigraphies for the Mochras and Peniche sections.
Palaeo-map and Peniche log were edited from ref. . Mochras log edited from ref.
Figure 4
Figure 4. SEM images of charcoal fragments from the Peniche section.
The fragments show the preservation of cellular anatomy and fused/homogenized cell walls—a key feature in charcoal identification.

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