The plants -- phytoplankton -- take up carbon from the ocean as they grow. Through the entire life and death cycle of these plants, the carbon then stays "trapped" for centuries to millennium.
Published estimates suggest that 12,000 sperm whales currently inhabit the Southern Ocean. Lavery and her team estimated the amount of prey consumed by each whale, along with the iron content of that prey. Iron is a critical phytoplankton fertilizer component.
Assuming that 75 percent of defecated iron persists in the photic -- or light receiving zone -- of the ocean, Southern Ocean sperm whales contribute 40 tons of iron to this region each year.
Humans driving cars, burning coal and engaging in other activities pump enormous amounts of carbon into the atmosphere, something that whales could never entirely offset.
"However, most whales are currently at 1 to 10 percent of their historical population sizes, so in the past, whales may have made a substantial contribution to carbon drawdown," Lavery said, adding that other marine mammals probably beneficially redistribute carbon just as whales do. These may include seals, sea lions and other types of whales, such as fin whales.