Solution to corrosive ocean mystery reveals future climate

Around 55 million years ago, an abrupt global warming event triggered a highly corrosive deep-water current to flow through the North Atlantic Ocean. The origin of this corrosive water has puzzled scientists for a decade.

Now, researchers have discovered this current and how it formed. The findings, published today in Nature Geoscience, also have profound implications for the sensitivity of our current climate to carbon dioxide emissions.

The researchers explored the acidification of the ocean that occurred during a period known as the Paleocene Eocene Thermal Maximum (PETM) when the Earth warmed 5°C in response to a rapid rise in CO₂ in the atmosphere and one of the largest mass extinctions occurred in the deep ocean.

This period is considered to closely resemble the scenario of global warming we are experiencing today.

"There has been a longstanding mystery about why ocean acidification caused by rising atmospheric CO₂ during the PETM was so much worse in the Atlantic compared to the rest of the world's oceans," said lead author from the ARC Centre of Excellence for Climate System Science, Ms Kaitlin Alexander.

"Our research suggests the shape of the ocean basins and changes to ocean currents played a key role in this difference. Understanding how this event occurred may help other researchers to better estimate the sensitivity of our climate to increasing CO₂."

To get their results the researchers recreated the ocean basins and land masses of 55 million years ago in a global climate model.

At this time there was a ridge on the ocean floor between the North and South Atlantic that separated the deep water in the North Atlantic from the rest of the world's oceans. It has been described as being like a giant bathtub on the ocean floor.

The simulations showed this "bathtub" in the North Atlantic became filled with extremely corrosive water that came from the Arctic Ocean, mixed with dense salty water from the Tethys Ocean and sank to the North Atlantic seafloor where it accumulated. The sediment in this area indicates the water was so corrosive that it dissolved all the calcium carbonate produced by organisms that settled on the ocean floor.

When the Earth warmed as a result of a rapid increase in atmospheric CO₂, it eventually warmed this corrosive bottom water. As this corrosive water warmed it became less dense and was replaced by denser water sinking from above.

The corrosive deep water was pushed up and spilled over the edge of the giant "bathtub" and flowed down into the South Atlantic.

"That corrosive deep water spread south through the Atlantic, then east into the Southern Ocean and eventually made its way to the Pacific," said co-author Prof Tim Bralower from the Department of Geosciences at Pennsylvania State University.

"The pattern of the event corresponds very closely to what the sediment records tell us, which show almost 100% dissolution of calcium carbonate in the South Atlantic sediment."

Determining how the event occurred also has important implications for today's climate and how it might warm in response to increases in atmospheric CO₂.

This is because if the high amount of acidification in the Atlantic Ocean was an indication of global acidification, then it would suggest enormous amounts of CO₂ are necessary to increase temperatures by 5°C.

However, these latest findings suggest that other factors made the Atlantic bottom water more corrosive than in other ocean basins.

"We now understand why the dissolution of sediments in the Atlantic Ocean was different from records in other ocean basins," said fellow author of the paper Associate Professor Katrin Meissner from the Climate Change Research Centre at the University of New South Wales.

"Using all sediments combined we can now estimate that the amount of greenhouse gases released into the atmosphere causing a temperature rise of 5°C was around the CO₂ equivalent of 7,000 - 10,000 gigatons of carbon. This is similar to the amount of carbon available in fossil fuel reservoirs today."

The big difference between the PETM and the alteration to the current climate is the speed of the change, said lead author Kaitlin Alexander.

"Today we are emitting CO₂ into the atmosphere ten times faster than the rate of natural CO₂ emissions during the PETM," Ms Alexander said.

"If we continue as we are, we will see the same temperature increase that took a few thousand years during the PETM occur in just a few hundred years. This is an order of magnitude faster and likely to have profound impacts on the climate system."

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This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government.

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