Geologic processes deep inside Earth: New light on genetic makeup of Earth's deep microbial life and geochemistry of lavas

Their research is being presented this week at the fall meeting of the American Geophysical Union in San Francisco.

Oceanography Professor Steven D'Hondt, along with postdoctoral researcher John Kirkpatrick and graduate student Emily Walsh, are working with other scientists to census microbes that live deep below Earth's surface. By conducting DNA analyses of microbes from sediments deep beneath the ocean floor, deep continental aquifers in South Africa, North America and Europe and elsewhere, they are discovering a fascinating network of subterranean microbial life.

"There is widespread interest in learning whether there are unique organisms living down there, or whether it is dominated by organisms that are common at the surface," said D'Hondt. "Are the same microorganisms found everywhere or is every subsurface ecosystem different? We're mapping the geographic diversity of the subsurface world."

The URI team is working to understand the evolution and natural selection of subsurface microbes. By sampling microbial communities from different depths and comparing samples from beneath the Indian Ocean, the Bering Sea, the South Pacific and elsewhere, they have found that very few types of microbes last very long.

"There doesn't appear to be any single trait or characteristic that is key to survival in these challenging environments," Kirkpatrick said. "If there were, there would be consistent winners and consistent losers, but the winners aren't consistent and almost everything seems to be losing and getting wiped out. It's remarkable that anything at all can survive under those conditions."

Meanwhile, Katherine Kelley, associate professor of oceanography, and colleagues at the Smithsonian Institution will report at the conference on their discovery of unsuspected linkages between the oxidation state of iron in volcanic rocks and variations in the chemistry of the deep Earth. Their findings suggest that carbon plays a more significant role in the circulation of the deep Earth than had previously been predicted.

"The relationships we've observed in basalts at mid-ocean ridges come about from the melting of Earth's upper mantle, and it tells us about the chemistry and composition of Earth beneath ocean basins," Kelley said. "We found a surprising relationship between the composition of lavas and the oxidation state of iron in the lavas."

The researchers used a microanalytical method called X-ray Absorption Near Edge Structure to analyze lava samples from mid-ocean ridges and found that carbon provides the means for exchanging oxygen and electrons with iron in the mantle, which is contrary to many years of previous research.

"Carbon in Earth's interior is impossible to measure directly, but it's important that we know how much carbon there is because the volcanic flux of carbon out of Earth's interior is a big variable in understanding how atmospheric CO₂ is cycled through our planet," Kelley said. "And that ties into our climate and the evolution of Earth."