Spanish mackerel schooling in shallow water.
Credit: Veer Images
ADELAIDE: The mass extinction of marine life during prehistoric times could be a warning of things to come, as high levels of greenhouse gases may bring on a similar extinction.
That's according to two geologists who have been studying 'greenhouse oceans' - oceans that were depleted of oxygen and suffered from increases in carbon dioxide and temperature.
Using core samples drilled from the ocean bed off the coast of western Africa, Martin Kennedy from the University of Adelaide in South Australia and Thomas Wagner from Newcastle University in the U.K 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.
"Our research points to a mass mortality in the oceans at a time when the Earth was going through a greenhouse effect, with high levels of carbon dioxide in the atmosphere, and rising temperatures, leading to a severe lack of oxygen (hypoxia) in the water that marine animals are dependent on," Kennedy said.
'Dead zones' growing rapidly
"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 Kennedy.
"This occurred relatively rapidly - in periods of hundreds of years, or possibly even less - not gradually over longer, geological time scales, which suggests 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 than previously thought."
Wagner said the results of their research, published in the Proceedings of the National Academy of Sciences (PNAS), had relevance for our environment now.
"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."
Hitting ecosystem with slede-hammer
"If you consider that the amount of carbon dioxide in our atmosphere could double over the next 50 years, this will be 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," said Kennedy.
"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 that the geological record offered 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. Our results show that natural processes of carbon burial kick in. Importantly, this rescue comes from the land, with soil-formed minerals acting to collect and bury excess dissolved organic matter in seawater. Burial of that excess carbon ultimately contributes to CO2 removal from the atmosphere, cooling the planet and the ocean," said Kennedy.
"This is nature's solution to the greenhouse effect and it could offer a possible solution for us. 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."
