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Theodore Them (left) and Jeremy Owens (right). The researchers used the samples to study the global record of oxygenation. Photo: Stephen Bilenky

The Toarcian Oceanic Anoxic Event occurred 183 million years ago, a collapse of traditional ancient ecosystems during the Early Jurassic.

For decades, scientists analyzing the Earth’s natural evidentiary trail picked up traces of an increase in ocean temperature and consequent acidification that fed off one another, leading to prehistoric mass extinction.

But a new study of the thallium isotopes present in the sedimentary layers shows that, instead, there were hundreds of thousands of years oxygen loss leading to the catastrophic cycle traditional identified with the Toarcian Oceanic Anoxic Event.

What emerges from the record, the researchers indicate, is a long-running trend toward hardship and struggle for survival that began at the bottom of the sea, the scientists write in the latest Proceedings of the National Academy of Sciences

“This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time,” according to the paper’s conclusion. “Thus, the TI data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth’s past and highlight the role of oxygen depletion on past biological evolution.”

Thallium is a long-term environmental marker since it is introduced through various natural cycles by rivers, high-temperature hydrothermal fluids, volcanic emission, mineral aerosols and pore-water fluxes from continental margin sediments, according to the team led by personnel from Florida State University.

The sites selected were centrally located in the Toarcian paleogeography of the Earth at that time: the Western Canada Sedimentary Basin, outside Alberta, and the Dotternhausen Quarry in Germany.

The researchers noticed the dynamic in the ancient layers, especially in the thallium combined with its byproduct in the environment, manganese oxide. Together, it shows a notable initial deoxygenation, the scientists said.

“In all likelihood, the decline in marine Mn oxide burial was linked to an expansion of bottom water anoxia, which restricted the area of oxic sediment deposition,” the paper said.

The contemporary implications of oceanic oxygen loss and temperature rise are clear, the scientists said in a statement through Florida State University.

“Over the past 50 years, we’ve seen that a significant amount of oxygen has been lost from our modern oceans,” said Theodore Them, a researcher at FSU and one of the authors. “While the timescales are different, past volcanism and carbon dioxide increases could very well be an analog for present events.”

 

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