A new study warns that the current stability of marine ecosystems is likely more fragile than it appears. Ocean deoxygenation is expected to significantly increase in response to global warming and increased nutrient runoff from continents.

Scientists have made a surprising discovery that sheds new light on the role low-oxygen conditions in the oceans, known as anoxia, played in one of the most devastating mass extinction events in Earth’s history. Their findings have implications for modern ecosystems and serve as a warning that marine environments may be more fragile than believed.

Published in the prestigious journal Nature Geoscience, new research suggests anoxia played an important part in disrupting ecosystems and driving extinctions in marine environments during the Triassic-Jurassic mass extinction around 200 million years ago. Surprisingly, the study shows that the global extent of extreme low-oxygen conditions known as euxinia was similar to levels seen today.

Throughout Earth’s history, a handful of major mass extinctions have occurred where global ecosystems collapsed and species went extinct, all seeming to coincide with climate and environmental disruptions causing ocean deoxygenation.

So ocean anoxia has been proposed as a likely culprit for marine extinctions during these periods, based on the idea more widespread deoxygenation would cause greater extinction.

Using chemical data from ancient mudstone deposits sampled via cores drilled in Northern Ireland and Germany, an international research team led by scientists from Royal Holloway, UK including scientists from Trinity College Dublin and the University of Utrecht discovered two key factors associated with the Triassic-Jurassic extinction.

They found pulses of deoxygenation in shallow marine environments along European continental margins at that time directly coincided with increased extinction levels in those areas.

More importantly, the team also discovered the global extent of extreme deoxygenation was actually quite limited, similar to today’s levels. Micha Ruhl of Trinity College Dublin commented Scientists have long suspected ocean deoxygenation plays an important role in disrupting marine ecosystems leading to species extinction in seas. Studying time periods of past extreme environmental change teaches us about potential tipping points in local and global ecosystems in response to climate forces.

Most significantly, results show even when global deoxygenation extent is like today, localized development of anoxic conditions and subsequent increased local extinction rates can trigger widespread or global ecosystem collapse and extinctions even in areas without deoxygenation.

This demonstrates global marine ecosystems become vulnerable even when just local environments along continental edges are perturbed. Understanding such processes is vitally important for assessing current ecosystem stability and associated food supplies, especially in a world where ocean deoxygenation is expected to greatly increase due to global warming and increased nutrient runoff.

Studying past global changes like the Triassic-Jurassic transition helps scientists unravel consequences of global climate and environmental shifts and identify fundamental Earth system processes controlling tipping points in our planet’s ecosystems.


Trinity College Dublin (University of Dublin) | Bond, A.D., Dickson, A.J., Ruhl, M. et al. Globally limited but severe shallow-shelf euxinia during the end-Triassic extinction. Nat. Geosci. (2023). doi.org/10.1038/s41561-023-01303-2

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