Preprints
https://doi.org/10.5194/egusphere-2025-4238
https://doi.org/10.5194/egusphere-2025-4238
02 Sep 2025
 | 02 Sep 2025
Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Spatially contrasted response of Devonian anoxia to astronomical forcing

Justin Gérard, Alexandre Pohl, Loïc Sablon, Jarno Huygh, Anne-Christine Da Silva, and Michel Crucifix

Abstract. The Devonian period, spanning from 419 to 359 million years ago, was marked by a warmer-than-present climate and recurring ocean anoxic events, with evidence increasingly suggesting a link between these events and astronomical forcing. Here, we explore how astronomical forcing influences ocean oxygenation by modulating the continental weathering flux of phosphate within a Late Devonian climate framework. To investigate this, we performed transient simulations spanning 1.1 Myr, crossing a 2.4 Myr eccentricity node using the cGENIE Earth system model. These simulations were driven by spatially resolved fluxes of reactive phosphorus from continents, computed using the emulator developed by Sablon et al. (2025), trained on GEOCLIM and HadSM3 outputs. Our results provide new evidence supporting eccentricity maxima as a driver of Late Devonian anoxic events. Additionally, global analysis reveals that obliquity variations can imprint a distinct signal on global ocean oxygen levels via their influence on biological productivity, offering a plausible mechanism for obliquity-driven anoxia under greenhouse conditions. Regional analysis revealed pronounced spatial heterogeneity in the biogeochemical response to astronomical forcing. Local ocean circulation emerged as a critical factor in shaping these patterns. The simulations indicate that astronomical forcing can, through its impact on continental weathering fluxes, exert a dominant influence on ocean oxygenation, with regional oxygen concentrations varying by up to 35 % and driving changes in regional anoxic volume of up to 19 %. Finally, these findings help explain why proxy records from different locations may show divergent expressions of astronomical signals, potentially leading to contrasting interpretations of their role in driving ocean anoxia.

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Justin Gérard, Alexandre Pohl, Loïc Sablon, Jarno Huygh, Anne-Christine Da Silva, and Michel Crucifix

Status: open (until 28 Oct 2025)

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Justin Gérard, Alexandre Pohl, Loïc Sablon, Jarno Huygh, Anne-Christine Da Silva, and Michel Crucifix
Justin Gérard, Alexandre Pohl, Loïc Sablon, Jarno Huygh, Anne-Christine Da Silva, and Michel Crucifix

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Short summary
We studied how changes in Earth’s orbit affected ocean oxygen during the Devonian, a time of repeated environmental crises and extinctions. Using computer simulations, we show that certain orbital cycles, especially eccentricity maxima, exacerbate oxygen loss in the oceans, while obliquity also played a key role at high latitudes. The results also help explain why records from different places show contrasting signals and provide new insight into how natural climate cycles can affect ocean life.
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