Image caption Sperm whales may put a gentle (and unwitting) brake on climate change

Sperm whale faeces may help oceans absorb carbon dioxide from the air, scientists say.

Australian researchers calculate that Southern Ocean sperm whales release about 50 tonnes of iron every year.

This stimulates the growth of tiny marine plants - phytoplankton - which absorb CO2 during photosynthesis.

The process results in the absorption of about 400,000 tonnes of carbon - more than twice as much as the whales release by breathing, the study says.

The researchers note in the Royal Society journal Proceedings B that the process also provides more food for the whales, estimated to number about 12,000.

Phytoplankton are the basis of the marine food web in this part of the world, and the growth of these tiny plants is limited by the amount of nutrients available, including iron.

Faecal attraction

Over the last decade or so, many groups of scientists have experimented with putting iron into the oceans deliberately as a "fix" for climate change.

Not all of these experiments have proved successful; the biggest, the German Lohafex expedition, put six tonnes of iron into the Southern Ocean in 2008, but saw no sustained increase in carbon uptake.

Image caption The Lohafex expedition was the latest to probe iron fertilisation

Although 400,000 tonnes of carbon is less than one-ten-thousandth of the annual emissions from burning fossil fuels, the researchers note that the global total could be more substantial.

There are estimated to be several hundred thousand sperm whales in the oceans, though they are notoriously difficult to count; and lack of iron limits phytoplankton growth in many regions besides the Southern Ocean.

So it could be that whale faeces are fertilising plants in several parts of the world.

Crucial to the idea is that sperm whales are not eating and defecating in the same place - if they were, they could just be absorbing and releasing the same amounts of iron.

Instead, they eat their diet - mainly squid - in the deep ocean, and defecate in the upper waters where phytoplankton can grow, having access to sunlight.

Releasing the iron here is ultimately good for the whales as well, say the researchers - led by Trish Lavery from Flinders University in Adelaide.

Phytoplankton are eaten by tiny marine animals - zooplankton - which in turn are consumed by larger creatures that the whales might then eat.

The scientists suggest a similar mechanism could underpin the "krill paradox" - the finding that the abundance of krill in Antarctic waters apparently diminished during the era when baleen whales that eat krill were being hunted to the tune of tens of thousands per year.