In 2000, researchers on an expedition for the US National Science Foundation discovered a field of hydrothermal vents on the Atlantis Massif. These featured a series of extraordinary limestone chimneys. The researchers named it "Lost City" after the mythical lost city of Atlantis.

Gretchen Bernasconi-Green. Credit: ETH Zurich

This past December, a team of scientists returned from an International Ocean Discovery Program (IODP) and announced they had discovered signs of life in rock samples taken from a series of mantle rocks harvested from this same area.

Yes, you read that right. Signs of life found in rocks from the ocean floor—rocks that come from our planet's mantle, no less.

The expedition's co-leader, ETH Zurich Professor Gretchen Bernasconi-Green recently discussed what this find means and whether the discovery should be considered "a scientific sensation."

Looking for an Explanation

The original goal of the expedition was to find out how mantle rock ended up on the sea floor and how it reacts to seawater. The team also hoped to find more out about the carbonization process that led to the formation of vents at Lost City. 

This beehive-like vent dismisses a steamingly hot and highly alkaline fluid. Credit: Univ. of Washington, IFE, URI-IAO and NOAA

"The chimneys are made of calcium, or rather calcium carbonate, which precipitates from the alkaline fluids. This is a natural form of carbon dioxide fixation," explains Bernasconi-Green.

"We wanted to better understand how much carbon is stored as carbonate in these rocks and the potential of this sequestration, especially in terms of an artificial CO2 sequestration on the sea floor or on land, aided by the serpentinisation reaction that led to the formation of alkaline hydrothermal vents."

Near the Lost City, the tectonic plates are drifting apart and large active faults have formed that carry mantle materials towards the surface. Bernasconi-Green compares the process to a conveyor belt. The team took advantage of the shifting and obtained samples of mantle rock through boreholes.

This is when they found something they hadn't expected: Signs of life in rock that originated from the earth's mantle (notably, the life comes from the rocks that originated there, not Earth's mantle itself).

Discovery Implications

A microbiologist on the research ship was able to isolate cells from a few of the rock samples. The next step will be examining the cells more closely in the laboratory. The core samples were taken to the Center for Marine Environmental Sciences (MARUM) in Bremen.

All the team knows right now is that they are microbial cells. However, it's too soon to say whether they are bacteria or archaea. The team does know that the cells differ considerably from cells found in ocean sediment. Therefore, they most likely originated in the mantle rock and not the seawater.

Remarkably, Bernasconi-Green does believe it's possible for life to actually arise in rock.

"Mantle rock contains a mineral called olivine, which turns into serpentine when it comes into contact with water in low temperatures. This produces hydrogen and methane. Both these gases provide a source of energy for micro-organisms that have to manage without sunlight."

Bernasconi-Green suggests that, perhaps, the cells the team found were capable of using these gases, which are present inside the rock, to carry out their metabolic processes. However, life in a rock isn't easy and the microbes are exposed to some very hostile conditions, such as extremely alkaline fluids that seep out of the vents at Lost City.

"The conversion of olivine to serpentine may occur on other planets too – on Mars, for example, where the presence of methane and hydrogen has already been established. Some scientists believe that would be sufficient in itself to allow the emergence of basic life forms."

Bernasconi-Green stresses that the team hasn't finished analyzing the core samples yet, so no official conclusions can be drawn. However, she sees the potential as "quite limited."

"It may be feasible, but the effort required to fix significant quantities of CO2 in places like the mid-ocean ridges would be very great – too great for it to be successful."

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