Clumps of Organic Carbon Found Within Serpentinites

A new analysis of ancient rocks that once lay on the floor of the Tethys Sea finds clusters of organic compounds associated with minerals from serpentinites. Thermodynamic calculations had predicted that these compounds exist, but this is the first study to find and identify them.

In theory, the process of serpentinization, where seawater infiltrates and reacts with the mantle rock peridotite, should generate clumps of organic carbon called condensed carbonaceous matter (CCM). Scientists have predicted that CCM forms during serpentinization in the absence of life, based on thermodynamic calculations. But until recently, researchers had found only smaller organic compounds, like methane, short hydrocarbons, and organic acids, spewing from serpentinizing sites.

In a new study of ancient rocks that were once at the bottom of the Tethys Sea, a group of DCO Deep Energy Community members has identified three types of CCM that form during serpentinization. Each type is associated with a different assemblage of low-temperature minerals. Marie Catherine Sforna (formerly at the University of Modena e Reggio Emilia, Italy, now at University of Liège, Belgium), Daniele Brunelli (University of Modena e Reggio Emilia, Italy), Bénédicte Ménez, Céline Pisapia, (both at Institut de Physique du Globe de Paris, and Université Paris Diderot, France), and Valerio Pasini (Institut de Physique du Globe de Paris, France, and University of Modena e Reggio Emilia, Italy) report the discovery in a new paper in Nature Communications [1]. 

CCM in serpentinite
This microscopic image shows the accumulation of condensed carbonaceous material (CCM) associated with one low-temperature mineralogical assemblage in serpentinites from the Apennines. Credit: Unimore/IPGP

“We knew from hydrothermal vent fluids that serpentinization was producing small organics, like methane. Also, we know from experiments and from what nature tells us that methane likely only forms at high temperatures. So, something metastable should form at low temperatures and it’s likely this CCM,” said Sforna. “For the first time, we saw this kind of abiogenic condensed carbon within oceanic rocks.”

Sforna used multiple microscopy techniques to examine rocks collected from a piece of ancient seafloor tectonically-displaced onto a continent, called an ophiolite, which is now part of the Northern Apennine Mountains, south of Reggio Emilia in Italy. She sliced and polished pieces of rock in a carbon-free environment and used imaging and spectroscopy techniques to visualize and identify carbon compounds and their associated minerals in the rock. 

“We found three low-temperature mineral assemblages where we very frequently find organics,” said Sforna. Each group of minerals is associated with a different type of CCM. One type was made up of hydrocarbon chains that coat a type of hydrogarnet. The second consists of small aggregates of carbon attached to the alteration minerals of spinel and plagioclase, while the third type was composed of larger clumps of short hydrocarbon chains, and rings of aromatic carbon compounds, affixed to the minerals saponite and hematite. All of these minerals, and their associated CCM, formed during serpentinization reactions in oxygen-free environments, at temperatures below 200 degrees Celsius.

Further analysis confirmed that the CCM formed abiogenically, and were not simply remnants of ancient microbes. Studies showed no nitrogen compounds that could have come from proteins. The uniformity of the molecules making up each type of CCM also point to an origin independent of life, because the remains of old cells likely would contain a more diverse collection of carbon compounds.

The CCM within the serpentinites represents a long-term reservoir of organic carbon in the subsurface. These compounds likely formed in the subseafloor of the Tethys Sea during the Mesozoic Era, 252 to 66 million years ago. “It’s a way to store carbon for a very long time in a solid form that is not easily removable,” said Sforna. “It can release carbon to deep ecosystems and feed deep microbial life.”

Sforna expects that once scientists start looking for CCM at other serpentinizing sites they’ll find it everywhere. “It is there, we just need to look for it,” said Sforna. 
 

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