New research provides strong evidence that acidification is affecting marine organisms, leaving traces even in the remote reaches of the deep sea. Lab-based studies have previously shown that increasingly acid waters impair the shell-building of marine organisms such as corals and some plankton. Now, measurements direct from the ocean floor show that the shells of one species of planktonic foraminifera – calcium carbonate-encrusted creatures about the size of a grain of sand – have thinned by one-third since the pre-industrial era. The study’s authors suggest that anthropogenic ocean acidification is to blame – and that the thinning shells may be reducing a major global carbon sink.
A team led by Andrew Moy and William Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre in Tasmania used shells found in seabed sediment in the Southern Ocean near Tasmania to follow the fortunes of the forams. Marine sediment cores, valued for their records of past climate, also contain some naturally captured carbon dioxide: while living near the sea surface, the foraminifera encase themselves in minerals made from carbon that originated in the atmosphere, and upon their deaths they sink to the ocean bottom, where that carbon gets buried away.
Comparing shells skimmed from the top of the sediment since 1997 with pre-industrial-era shells of nearly the same size interred in sediment cores below, the researchers find that the recent shells are 30-35% lighter. Looking back 50,000 years using an even deeper core, they can also see shell weight loss tracking the release of carbon dioxide during the warming at the end of the last ice age. The results were published in Nature Geoscience this week (subscription).
The authors point out that in the subarctic Southern Ocean, anthropogenic carbon dioxide dissolving into the water – which has led to a decline in ocean pH of about 0.1 units globally and 0.14 units in this region – should be the main factor affecting the ability of marine creatures to shell themselves. In other places, changes in other variables such as temperature might confound the results.
“We don’t yet know what impact this will have on the organisms’ health or survival,” Howard told Scientific American. But the carbon-cycle impact of shell loss in and of itself could be considerable. The world’s planktonic foraminifera are responsible for about a quarter to a half of all the calcium carbonate that ends up on the deep-sea floor, according to an estimate quoted in the paper. Howard added, “If the shell-making is reduced, the storage of carbon in the ocean might be, as well.”
Anna Barnett
Image: Microscopic photo of shells found in seafloor sediment / NOAA, via Wikimedia
(This post has been corrected with the words “leaving traces” added to the first sentence.)