Around 420 million years ago, a devastating mass extinction event, known as Lau/Kozlowskii extinction, wiped around 23 per cent of all marine animals from the Earth in the late prehistoric Silurian Period. While scientists have long struggled to find the reason for this mass extinction, they have finally found an answer and this time no asteroid impact or volcanic eruption is to blame.
Researchers from Florida State University have found out that the Lau/Kozlowskii extinction, which is considered as one of the ten most dramatic extinctions in the history of our planet, was triggered by “rapid and widespread depletion of oxygen in the global oceans”.
The study, published in the journal Geology and conducted by researchers from Florida State University (FSU), resolves a longstanding paleoclimate mystery, but at the same time, raises urgent concerns about our modern oceans.
“This makes it one of the few extinction events that is comparable to the large-scale declines in biodiversity currently happening today, and a valuable window into future climate scenarios,” said study co-author Seth Young, an assistant professor in the Department of Earth, Ocean and Atmospheric Science.
Scientists have long been aware of the Lau/Kozlowskii extinction, as well as a related disruption in Earth’s carbon cycle responsible for the burial of enormous amounts of organic matter that caused significant climate and environmental changes. However, the link and timing between these two associated events remained stubbornly opaque.
“It’s never been clearly understood how this timing of events could be linked to a climate perturbation, or whether there was direct evidence linking widespread low-oxygen conditions to the extinction,” said FSU doctoral student Chelsie Bowman, who led the study.
Researchers used advanced geochemical methods that include thallium isotope, manganese concentration, and sulfur isotope measurements from important sites in Latvia and Sweden. It enabled them 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 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.
“For the first time, this research provides a mechanism to drive the observed step-wise extinction event, which first coincided with ocean deoxygenation and was followed by more severe and toxic ocean conditions with sulfide in the water column,” Bowman said.