Oxygen Depletion in Ancient Oceans Caused Major Mass Extinction

Assistant Professor Seth Young, graduate student Chelsie Bowman, and Assistant Professor Jeremy Owens studied the link between oxygen depletion and a mass extinction event. (Image: Stephen Bilenky)

Late in the prehistoric Silurian Period, around 420 million years ago, a devastating mass extinction event wiped 23 percent of all marine animals from the face of the planet. For years, scientists struggled to connect a mechanism to this mass extinction, one of the 10 most dramatic ever recorded in Earth’s history.

Now, researchers from Florida State University have confirmed that this event, referred to by scientists as the Lau/Kozlowskii extinction, was triggered by an all-too-familiar culprit: rapid and widespread depletion of oxygen in the global oceans.

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Their study, published in the journal Geology, resolves a longstanding paleoclimate mystery, and raises urgent concerns about the ruinous fate that could befall our modern oceans if well-established trends of deoxygenation persist and accelerate.

Unlike other famous mass extinctions that can be tidily linked to discrete, apocalyptic calamities like meteor impacts or volcanic eruptions, there was no known, spectacularly destructive event responsible for the Lau/Kozlowskii extinction. Study co-author Seth Young, an assistant professor in the Department of Earth, Ocean and Atmospheric Science, said:

Scientists have long been aware of the Lau/Kozlowskii extinction, as well as a related disruption in Earth’s carbon cycle during which the burial of enormous amounts of organic matter caused significant climate and environmental changes.

But the link and timing between these two associated events — the extinction preceded the carbon cycle disruption by more than a hundred thousand years — remained stubbornly opaque. FSU doctoral student Chelsie Bowman, who led the study, said:

To crack this difficult case, the team employed a pioneering research strategy. Using advanced geochemical methods including thallium isotope, manganese concentration, and sulfur isotope measurements from important sites in Latvia and Sweden, the FSU scientists were able to reconstruct a timeline of ocean deoxygenation with relation to the Lau/Kozlowskii extinction and subsequent changes to the global carbon cycle.

The team’s new and surprising findings confirmed their original hypothesis that the extinction record might be driven by a decline of ocean oxygenation. Their multiproxy measurements established a clear connection between the steady creep of deoxygenated waters and the step-wise nature of the extinction event — its start in communities of deep-water organisms and eventual spread to shallow-water organisms.

Their investigations also revealed that the extinction was likely driven in part by the proliferation of sulfidic ocean conditions, Bowman said:

With the oxygen-starved oceans of the Lau/Kozlowskii extinction serving as an unnerving precursor to the increasingly deoxygenated waters observed around the world today, study co-author Jeremy Owens, an assistant professor in the Department of Earth, Ocean and Atmospheric Science, said that there are still important lessons to be learned from ecological crises of the distant past, adding:

Provided by: Zachary Boehm,  [Note: Materials may be edited for content and length.]

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