Shells 'thinning due to fossil fuels'
The shells of tiny single-celled organisms in the Southern Oceans are thinning due to acid-forming carbon dioxide being absorbed by oceans, new research has found.
Dr Will Howard of the Antarctic Climate & Ecosystems Cooperative Research Centre and colleagues report their findings today in the journal Nature Geoscience.
"The ocean is currently taking up somewhere in the neighbourhood of a third of our fossil fuel emissions," says Howard, a palaeo-climatologist.
Scientists have predicted this absorption of carbon dioxide would boost the acidity of oceans and interfere with the ability of some marine species to build their shells.
"It was predicted and now we're seeing it," says Howard. "This is not an issue for the future any more. This is an issue for now."
The researchers studied the shells of a protozoa called foraminifera (Globigerina bulloides), in the Southern Ocean south of Tasmania.
They collected current day foraminifera as they fell to the bottom of the ocean and compared them to foraminifera dating back in time, taken from cores taken from the sea bed.
"[The method] gives us access to times when the carbon dioxide chemistry in the atmosphere and the ocean has changed measurably and noticeably, and we can see how the organisms responded," says Howard.
He and colleagues linked changes in shell formation with the amount of carbon dioxide around at the time.
"[The foraminifera] have more heavily calcified shells during intervals when carbon dioxide content was lower and thus the pH of the ocean was higher," says Howard.
By contrast, the researchers found modern shells were 30 to 35% lighter than those formed prior to the industrial period.
"When you're looking at the modern counterparts, they're making thinner shells than anything in the cores," say Howard.
He says the team has evidence that modern shells are thinner that at any time in the last 200,000 years.
Fossil fuel signature
Howard says previous research has confirmed that the carbon dioxide affecting the shells is from fossil fuels, which has a unique isotopic signature.
"We know independently that the carbon dioxide that they're picking up is at least partly anthropogenic," he says.
Howard says calcium carbonate shells of foraminifera, which fall to the bottom of the deep sea when they die, are a major vehicle for removing carbon from atmospheric circulation.
"One of the things we're concerned about is that if their shell-making ability is compromised that we might be reducing the efficiency of the ocean carbon pump," he says.
Howard says that over time, the ocean may be able to counteract acidity by dissolving accumulated shells of dead marine organisms on the ocean floor, thus raising ocean pH and its ability to take up CO2.
But, he says this will take a long time and come at the cost of living marine organisms.
"The buffering mechanisms in the ocean are quite slow compared to the rate at which we are putting fossil fuel carbon into the atmosphere and into the ocean."
The research was funded by the Australian Government Department of Climate Change.