Recent research has revealed that deoxygenation levels in the oceans, similar to those seen today, played a critical role in marine extinctions during a major climate change event. This finding serves as a stark warning about the fragility of current marine ecosystems. Oceanic deoxygenation, which is expected to increase with global warming and nutrient runoff, poses a significant threat to marine life today, just as it did millions of years ago.

Published in Nature Geoscience, the study focuses on the Triassic–Jurassic mass extinction, which occurred around 200 million years ago. This extinction event was one of the largest in Earth’s history, with many species going extinct due to global environmental upheaval, including ocean deoxygenation (anoxia). Surprisingly, the research shows that while local anoxic conditions in shallow marine environments had devastating effects, the overall global extent of extreme oxygen depletion, known as euxinia, was similar to modern-day levels.

Throughout Earth’s history, mass extinctions have often coincided with periods of significant environmental disruption, and ocean deoxygenation has long been considered a potential cause of these extinctions. The assumption has been that more widespread deoxygenation would correspond to larger extinction events. However, this research, which draws on chemical data from ancient mudstone deposits obtained through drill cores in Northern Ireland and Germany, challenges that assumption.

An international team, led by scientists from Royal Holloway (UK) and including researchers from Trinity College Dublin and Utrecht University, linked key aspects of the Triassic–Jurassic extinction event. Their findings, based on samples from the Carnduff drill cores taken in the Larne Basin of Northern Ireland, revealed that pulses of deoxygenation in shallow waters along the margins of the European continent were directly correlated with increased extinction rates in those areas.

More notably, the research demonstrates that the global extent of severe oceanic deoxygenation was relatively limited, akin to what we see today. Despite this, local anoxic conditions still contributed to widespread ecosystem collapses and extinctions, even in areas where deoxygenation did not occur.

Dr Micha Ruhl, Assistant Professor in Trinity’s School of Natural Sciences and a member of the research team, emphasised the significance of these findings:
“Scientists have long suspected that ocean deoxygenation plays an important role in disrupting marine ecosystems, leading to the extinction of species. By studying periods of extreme environmental change in Earth’s history, we can see how this plays out. These findings offer critical insights into potential tipping points in local and global ecosystems, as they respond to climatic forces.

“What is crucial here is that even when the overall extent of deoxygenation is similar to what we are seeing today, localised anoxic conditions and associated extinction rates can trigger cascading effects that lead to widespread or even global ecosystem collapse. This vulnerability is concerning because it highlights how even local disturbances can threaten global marine ecosystem stability.”

Ruhl further stressed the importance of understanding these processes for evaluating the current state of marine ecosystems, especially as marine deoxygenation is predicted to increase due to global warming and nutrient runoff.
“Understanding such processes is of paramount importance for assessing present-day ecosystem stability and food supply, particularly in a world where marine deoxygenation is projected to increase in response to global warming and increased nutrient runoff from continents.”

A core sample obtained from the Carnduff-2 drill site in Northern Ireland illustrates the scale of this ancient environmental change. The sediment sample, which dates to around 201 million years ago, contains the shell of a seabed animal that lived shortly after the Triassic–Jurassic mass extinction, offering a glimpse into life in the immediate aftermath of one of Earth’s greatest extinction events.

Above: Samples of the Carnduff cores (here studied), which were drilled in the Larne Basin, Northern Ireland. 

The study of past global change events, such as the transition between the Triassic and Jurassic periods, enables scientists to better understand the consequences of large-scale environmental and climatic disruptions. This research provides important insights into the fundamental processes of Earth’s systems that drive ecosystem tipping points, offering a framework for evaluating present-day environmental challenges.

Above: A core sample of ~201 million year old sediments obtained from the Carnduff-2 core, drilled in the Larne Basin (Northern Ireland), showing the shell of an animal that lived on the seabed shortly after the Triassic–Jurassic global mass extinction.

This research was funded by a Natural Environment Research Council Doctoral Training Partnership award and the National Natural Science Foundation of China, contributing to a deeper understanding of the role of ocean deoxygenation in shaping Earth’s history and future.