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Ocean bacteria may have shut off ancient global warming

Mineral spikes in sea-floor sediments coincide with halt in temperature rise

1:48pm, April 14, 2014

WARMER WORLD  The amount of carbon-containing matter falling to the deep ocean increased during an extraordinarily warm period around 56 million years ago, researchers argue. In this computer simulation of the oceans at the time, red indicates high levels of falling carbon-rich material and green indicates low amounts. 

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Ocean-dwelling bacteria may have vacuumed up carbon and halted a period of extreme warmth some 56 million years ago, according to a study published April 13 in Nature Geoscience.

The finding suggests how Earth might once have rapidly reversed a runaway greenhouse effect. However, rapidity is relative: The bacteria would be far too slow to head off today’s human-caused climate impacts.

Scientists seeking to understand the present-day climate often study the Paleocene-Eocene Thermal Maximum, a hot period that occurred around 55.9 million years ago. During this roughly 170,000-year period, atmospheric carbon dioxide levels soared, temperatures rose by 5 degrees Celsius or more and ocean acidity spiked. The period ended in a hurry, geologically speaking, over the course of 30,000 to 40,000 years. Scientists are unsure what stopped the warming; possibilities include uptake of carbon by organisms or by rock.

To investigate organisms’ role, University of California, Santa Cruz marine scientist Adina Paytan and colleagues measured how much of the mineral barite, or barium sulfate, was present in 12 seafloor sediment cores from the Atlantic, Indian, Pacific and Southern oceans. Oceanic bacteria produce barite when they break down dead phytoplankton that has fallen from surface waters; the barite then accumulates in sediment.

Paytan’s team found that barite spiked globally during the Paleocene-Eocene Thermal Maximum. The researchers think the elevated barite levels resulted from more phytoplankton falling from the oceans’ surfaces and being consumed by bacteria during the warm period. The phytoplankton that the microbes munched on probably absorbed more carbon from the atmosphere and transported it to the deep ocean faster as temperatures and carbon dioxide levels increased. 

Present-day oceanic bacteria convert around 0.4 percent of the carbon they digest to forms unusable by life. These compounds can then remain out of contact with the ocean surface for thousands of years. Assuming microbes converted carbon at a similar rate during the ancient warm period, bacteria could have removed enough carbon from the atmosphere for long enough to reverse global warming, the authors conclude.

Using barite records to draw conclusions about ancient bacteria is “a really creative, cool way to visualize these biogeochemical processes,” says Aradhna Tripati, a geologist at UCLA. But she questions whether bacteria in the much warmer ocean of 56 million years ago captured carbon at rates similar to today’s. Paytan’s team, she says, “may be stretching it a little bit with their interpretation.”

Richard Norris of the Scripps Institution of Oceanography applauds the research team for “solid data” showing that organisms can thrive even in much warmer oceans. Some scientists have argued that sea life decreased dramatically during the Paleocene-Eocene Thermal Maximum. “I think it’s encouraging that the future world may not be so quite so catastrophically terrible,” he says.

While the finding suggests oceanic bacteria could play a role in stopping human-caused climate warming, the microbes would take thousands of years, says team member and Wesleyan University environmental scientist Ellen Thomas. And, she adds, “Humans can’t wait for this.”

Editor's Note: This story was updated May 6, 2014, to remove incorrect assertion that some scientists argue that major die-offs occurred during the Paleocene-Eocene Thermal Maximum, and to remove a highly uncertain figure for carbon dioxide levels in that time period.

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