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Ocean bacteria may have shut off ancient global warming: mineral spikes in seafloor sediments coincide with halt in temperature rise.

Ocean 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.

The Paleocene-Eocene Thermal Maximum was a hot episode that occurred around 55.9 million years ago. During the roughly 170,000-year span, atmospheric carbon dioxide levels soared, temperatures rose by 5 degrees Celsius or more and ocean acidity spiked. The event ended in a relative hurry, 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 around the globe. 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 resulted from more phytoplankton falling from the ocean surface and being consumed by bacteria during the warm period. The phytoplankton that the microbes munched probably absorbed more carbon from the atmosphere as temperatures and carbon dioxide levels increased. The bacteria could have removed enough carbon from the atmosphere for long enough to reverse global warming, the authors conclude.

Using barite records is "a really creative, cool way to visualize these biogeochemical processes," says Aradhna Tripati, a geologist at UCLA. But she questions the researchers' assumption that bacteria in the much warmer ocean of 56 million years ago captured carbon at rates similar to today's.

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. "Humans can't wait for this."

Caption: 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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Title Annotation:EARTH & ENVIRONMENT
Author:Popkin, Gabriel
Publication:Science News
Date:May 31, 2014
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