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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 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.

Questions:

1. A whale releases how many tons of iron a year?

2. How many sperm whales inhabit the Southern Ocean?

3. Without whaling how many sperm whales could there have been?

Answers:

1. 50 tons.

2. 12,000.

3. 120,000.

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（中国日报网英语点津 Helen 编辑）

About the broadcaster:

Lee Hannon is Chief Editor at China Daily with 15-years experience in print and broadcast journalism. Born in England, Lee has traveled extensively around the world as a journalist including four years as a senior editor in Los Angeles. He now lives in Beijing and is happy to move to China and join the China Daily team.