Minerals formed at great depths beneath the surface suggest life may be operating further underground than ever demonstrated before. This has implications on the search for life on other worlds.
Although its name may seem to come from Middle Earth, aragonite is one of the two common forms of calcium carbonate (CaCO3). It is formed by biological processes, such as those found in the creation of corals and mollusk shells, but also by inorganic forces.
The origins of aragonite can be determined based on the carbon incorporated within. Biologically formed crystals incorporate lighter isotopes.
At the Geological Society of America's annual meeting, Yale undergraduate student Philippa Stoddard reported on the discovery of aragonite in the San Juan Islands, Washington State. The islands experience a “structural burial and rapid exhumation between 100 and 84 million years ago” at low temperatures and very high pressures, Stoddard notes.
In a fault zone at the southern end of Lopez Island, Stoddard found an outcrop where aragonite veins have anomalously light concentrations of carbon isotopes, with up to 50 fewer heavy isotopes per million atoms. This suggests the aragonite formed from methane released by microorganisms, rather than inorganic processes.
The claim is remarkable because the rocks in which the aragonite sit are thought to have been formed far deeper than even the dreams of Durin, 20km beneath the Earth's dark keel. Temperatures at such depths would be expected to be above the point where DNA becomes unstable, but Stoddard believes life could have survived, arguing, “at low surface pressures, bacterial life is known to remain active at temperatures of ~122° C. Biomolecules are stabilized by pressure, so bacterial life should extend to higher temperatures within the Earth's interior.”
The islands were formed in a subduction zone that preceded the nearby Cascadia subduction zone. In both cases, these zones arise when one tectonic plate overrides the other and forms a trench in the process.
"We reason that you could have life deeper in subduction zones, because you have a lot of water embedded in those rocks, and the rocks stay cold longer as the [plate] comes down," Stoddard told LiveScience.