Our oceans once became breathless, scientists say, and the cause was something our planet faces again today: climate change. Photo / 123RF
Our oceans once became breathless, scientists say, and the cause was something our planet faces again today: climate change. Photo / 123RF
Our oceans once became breathless, scientists say, and the cause was something our planet faces again today: climate change.
A new study led by Otago University has investigated how ancient climate change events have driven oxygen depletion, where dissolved oxygen in the water becomes reduced to the point that it threatens life within it.
The famous period in Earth's geological history, which occurred 94 million years ago and is known as an Oceanic Anoxic Event, was more severe and on much longer timescales than the current changes.
But it has given the scientists studying this period an extreme case-study to help understand how the oceans are affected by high atmospheric CO2 emissions.
Research Fellow Dr Matthew Clarkson and Professor Claudine Stirling, of Otago's Chemistry Department, applied a revolutionary new tool to examine how the oceans responded to climate change in the past.
The scientists used a novel technique that measures naturally-occurring uranium isotopes from ancient sediments, which could be used to estimate ocean oxygen content, thus identifying an ancient geochemical record of how much of the ocean was deoxygenated these many millions of years ago.
They applied this technique to geological sediments that were once deposited in the ocean and are today preserved on land at the white cliffs in the south of England, and also in Italy.
They found that the likely driving mechanism of this anoxic, or deoxygenation, event was nutrient run-off, itself driven by high CO2 emissions and warmer temperatures; and that when CO2 emissions reduced, along with nutrient levels, global oceans recovered for a period.
Stirling said the ability to predict what could happen, thanks to the combination of uranium isotopes and modelling, is a significant breakthrough.
Cliffs in Eastbourne in the UK contain ancient sediments that scientists used to investigate ancient episodes of ocean oxygen depletion. Photo / Professor Hugh Jenkyns, Dept Earth Sciences, University of Oxford
"It helps us understand the missing piece of the puzzle, what happens to oxygen levels in our oceans when they are effected by global warning. CO2 levels in the atmosphere were much higher than they are now, so we won't see this level of change for a long time, but we will see the same sequence of events" she said.
Areas of ocean deoxygenation, known as "dead zones", could be found currently in a number of oceans around the world including in the eastern parts of the tropical Pacific, Atlantic and Indian Oceans.
These "dead zones" occurred because it is harder to dissolve oxygen in water when the oceans are warm, and also more oxygen is used up during the breakdown of biological material.
This material was produced following a rise in nutrient levels during global warming. Some of these nutrients come from run-off in rivers, and some from upwelling of deep ocean water.
"From studies like this scientists can describe the link between increased global temperatures and increased global weathering rates, which drive a high input of nutrients into the ocean," Clarkson said.
"This leads to high primary productivity in the oceans and eventually the loss of oxygen as the organic matter degrades by aerobic respiration."
The process was similar to eutrophication, which happened in many lakes and rivers due to the input of fertilisers, but in this case it occurred on a global oceanic scale, he said.
"Through comparison to other geochemical data, and simulating the event with a new biogeochemical model, we present strong evidence for the nutrient input hypothesis as a driving mechanism for anoxia [deoxygenation]."
Marine fauna suffered heavily during this event, although it is not considered one the major mass extinctions of Earth's history.
"Another significance of this study is that we are able to put a new estimate on the area of the seafloor that became anoxic, at around 8-15 per cent, compared to only 0.3 per cent in the modern ocean," Clarkson said.
Importantly, a number of completely independent studies, with very different methods, were finding consistent results for the Oceanic Anoxic Event.
Sediments preserved in rock in Italy. Photo / Professor Hugh Jenkyns, Dept. Earth Sciences, University of Oxford
"This helps gives scientists much greater confidence when trying to understand the legacy of modern human activity."
This particular Oceanic Anoxic Event was also thought to have lasted for around 1 million years, but the new data also shows for the first time that the global oceans briefly recovered in the middle of the event, before returning to widespread anoxia again.
"This recovery was the result of declining CO2 emissions from volcanic sources, and the removal of carbon from the atmosphere by weathering and the burial of organic matter.
"These two processes are known to help regulate the global climate, acting as negative feedback mechanisms similar to a thermostat, but they take a very long time."
The new study - the culmination of a major collaboration between Otago, Oxford, Exeter, and London Universities - has been published today in the journal Proceedings of the National Academy of Sciences.
