The mass extinction of marine life in our oceans during prehistoric times is a warning that the same could happen again due to high levels of greenhouse gases, according to new research.

Professor Martin Kennedy from the University of Adelaide (School of Earth & Environmental Sciences) and Professor Thomas Wagner from Newcastle University, UK, (Civil Engineering and Geosciences) have been studying 'greenhouse oceans'—those that have been depleted of oxygen, suffering increases in carbon dioxide and temperature.

Using core samples drilled from the ocean bed off the coast of western Africa, the geologists studied layers of sediment from the Late Cretaceous Period (85 million years ago) across a 400,000-year timespan. They found a significant amount of organic material—marine life—buried within deoxygenated layers of the sediment.

Professor Wagner says the results of their research, published in the Proceedings of the National Academy of Sciences (PNAS), has relevance for our modern world: "We know that 'dead zones' are rapidly growing in size and number in seas and oceans across the globe," he said. "These are areas of water that are lacking in oxygen and are suffering from increases of CO2, rising temperatures, nutrient run-off from agriculture and other factors."

Their research points to a mass mortality in the oceans at a time when the Earth was going through a greenhouse effect. High levels of carbon dioxide in the atmosphere and rising temperatures led to a severe lack of oxygen (hypoxia) in the water that marine animals depend upon.

"What's alarming to us as scientists is that there were only very slight natural changes that resulted in the onset of hypoxia in the deep ocean," said Professor Kennedy. "This occurred relatively rapidly—in periods of hundreds of years, or possibly even less—not gradually over longer, geological time scales, suggesting that the Earth's oceans are in a much more delicate balance during greenhouse conditions than originally thought, and may respond in a more abrupt fashion to even subtle changes in temperature and CO2 levels."

Professor Kennedy said that the doubling of the amount of carbon dioxide in our atmosphere over the past 50 years is "like hitting our ecosystem with a sledge-hammer" compared to the very small changes in incoming solar energy (radiation) which was capable of triggering these events in the past.

"This could have a catastrophic, profound impact on the sustainability of life in our oceans, which in turn is likely to impact on the sustainability of life for many land-based species, including humankind," he added.

However, the geological record offers a glimmer of hope thanks to a naturally occurring response to greenhouse conditions. After a hypoxic phase, oxygen concentration in the ocean seems to improve, and marine life returns.

This research has shown that natural processes of carbon burial kick in and the land comes to the rescue, with soil-formed minerals collecting and burying excess dissolved organic matter in seawater. Burial of the excess carbon ultimately contributes to CO2 removal from the atmosphere, cooling the planet and the ocean.

"This is nature's solution to the greenhouse effect and it could offer a possible solution for us," said Professor Wagner. "If we are able to learn more about this effect and its feedbacks, we may be able to manage it, and reduce the present rate of warming threatening our oceans."

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