A vast network of under-sea volcanoes pumping out nutrient-rich water in the Southern Ocean plays a key role in soaking up large amounts of carbon dioxide, acting as a brake on climate change, scientists say.
A group of Australian and French scientists have shown for the first time that the volcanoes are a major source of iron that single-celled plants called phytoplankton need to bloom and in the process soak up CO2, the main greenhouse gas.
Oceans absorb about a quarter of mankind's CO2 from burning fossil fuels and deforestation, with the Southern Ocean between Australia and Antarctica among the largest ocean "carbon sinks".
Phytoplankton underpin the ocean's food chain. When they die or are eaten, they carry large amounts of carbon that they absorb to the bottom of the ocean, locking up the carbon for centuries.
There have been a number of studies showing iron is released from deep-sea volcanoes, said Andrew Bowie, a senior research scientist with the Antarctic Climate & Ecosystems Cooperative Research Centre in Hobart, Tasmania.
"But no study has considered that on a global level and considered its importance on Southern Ocean carbon storage," Bowie, one of the authors, told Reuters.
The volcanoes are dotted along deep ocean ridges that mark major plate boundaries of the earth's crust and the study is based in part on measurements of how much iron there is in the Southern Ocean at depths of up to four kilometres.
The study is published in the latest issue of Nature Geoscience.
Iron is found in only small amounts across most of the Southern Ocean, curbing the growth of phytoplankton.
Scientists have known that large amounts of iron can come from wind-borne dust, for example from a big dust storm in Australia, or from iron-rich sediments from coastal waters, triggering phytoplankton blooms.
These sources are variable but could increase if some countries, such as Australia, become more arid.
The latest study shows the amount of iron from deep-sea volcanoes is relatively constant over long timescales and is responsible for between 5 and 15% of the total Southern Ocean carbon storage, and in some regions up to 30%.
That means the nutrients from the volcanoes can act as a buffer when other sources, such as dust, vary.
The scientists found that much of the deep iron-rich water reached the surface closer to Antarctica, leading to blooms of phytoplankton there.
Bowie said it was still unclear how climate change will affect the total amount of iron that makes it to the surface of the Southern Ocean.
Some studies show that stronger westerly winds blowing on the ocean's surface near Antarctica will draw greater amounts of iron-rich water to the top, fuelling more phytoplankton growth and therefore more CO2 captured from the air.
But Bowie said other research has also pointed to the possibility of greater stratification of the ocean's different layers, cutting the amount of iron-rich water reaching the surface.
"Right now we don't have enough reliable knowledge to say which of these factors would be more important than the other," he said.