Southern Ocean sperm whales offset their carbon footprint by defecating, scientists said on Wednesday, releasing tons of iron a year, stimulating the growth of phytoplankton which in turn absorb carbon dioxide.

Each whale releases about 50 tons of iron a year, their natural fertilization stimulating the process of photosynthesis.

An estimated 12,000 sperm whales that inhabit the Southern Ocean absorb about 400,000 tons of carbon each year, twice the amount they release by breathing, said scientists from Flinders University in South Australia.

"They eat their diet, mainly squid, in the deep ocean, and defecate in the upper waters where phytoplankton can grow, having access to sunlight," marine biologist Trish Lavery, who led the scientific study at Flinders University, said in a statement.

"Sperm whale poo is rich in iron, which stimulates phytoplankton to grow and trap carbon. When the phytoplankton die, the trapped carbon sinks to the deep ocean," Lavery said.

"They've well and truly bypassed being carbon neutral. They've actually gone one step further," she said. The whale study was published in the Royal Society's biological research journal, Proceedings of the Royal Society, Biological Sciences.

Lavery said that without whaling there may have been 120,000 sperm whales in the Southern Ocean and, according to her calculations, some 2 million tons of carbon may have been removed from the atmosphere each year through this process.