Oxygen-depleted toxic oceans behind mass extinction 200 mn yrs ago

A new study has revealed that oxygen-depleted toxic oceans had key role in mass extinction over 200 million years ago.

Changes in the biochemical balance of the ocean were a crucial factor in the end-Triassic mass extinction, during which half of all plant, animal and marine life on Earth perished, as per the University of Southampton study.

The study reveals that a condition called "marine photic zone euxinia," which took place in the Panathalassic Ocean- the larger of the two oceans surrounding the supercontinent of Pangaea, occurs when the sun-lit surface waters of the ocean become devoid of oxygen and are poisoned by hydrogen sulphide, a by-product of microorganisms that live without oxygen that is extremely toxic to most other lifeforms.

The international team of researchers studied fossilised organic molecules extracted from sedimentary rocks and found molecules derived from photosynthesising brown-pigmented green sulphur bacteria, microorganisms that only exist under severely anoxic conditions, proving severe oxygen depletion and hydrogen sulphide poisoning of the upper ocean at the end of Triassic, 201 million years ago.

The researchers also documented marked changes in the nitrogen composition of organic matter, indicating that disruptions in marine nutrient cycles coincided with the development of low oxygen conditions.

Co-author Jessica Whiteside said that as tectonic plates shifted to break up Pangaea, huge volcanic rifts would have spewed carbon dioxide into the atmosphere, leading to rising temperatures from the greenhouse effect. The rapid rises in CO2 would have triggered changes in ocean circulation, acidification and deoxygenation.

Whiteside added that these changes have the potential to disrupt nutrient cycles and alter food chains essential for the survival of marine ecosystems and the data now provides direct evidence that anoxic, and ultimately euxinic, conditions severely affected food chains.

She noted that the same CO2 rise that led to the oxygen depleted oceans also led to a mass extinction on land, and ultimately to the ecological take-over by dinosaurs, although the mechanisms are still under study.

The study is published in the upcoming edition of Geology.