A New Source of Hydrocarbons in the Deep Carbon Cycle

Acetate is a simple two-carbon molecule found almost everywhere in the surface environment. It can be found dissolved in oceans, lakes, and oilfield brines, and is part of numerous biochemical reactions in cells; fermentative bacteria especially, excrete acetate in the form of vinegar.

Due to its ubiquity on the surface, acetate could end up in the deep subsurface through subduction, a process where at the boundaries of tectonic plates, one edge sinks beneath the other into the mantle. New research suggests that acetate may also play an important role in the deep carbon cycle by contributing to the formation of hydrocarbons.

A new study finds that acetate dissolved in water can transform into the four-carbon hydrocarbon isobutane at the high temperatures and pressures of subduction zones. Isobutane does not mix well with water, and instead forms an oily liquid that might migrate independently in the subduction zone environment. The discovery represents a novel source of deep hydrocarbons and a new way for carbon to move through the subsurface. New experimental results in this study are by Fang Huang and Dimitri Sverjensky (Johns Hopkins University, USA) and Chair of the Deep Energy Community, Isabelle Daniel, with her lab members Hervé Cardon and Gilles Montagnac (all at Université de Lyon, France). The researchers report their findings in a new paper in Nature Communications [1].

Previous studies have shown that at elevated temperatures and low pressures, acetate decomposes into methane and carbon dioxide. But few researchers have looked at acetate’s behavior at high temperatures and pressures. In a 2014 modeling study [2], Sverjensky and Huang predicted that acetate would be stable under these conditions, so to follow up on this study, they collaborated with the Lyon group to analyze acetate’s behavior in the lab.

Huang and Daniel placed a solution of sodium acetate dissolved in water into a diamond anvil cell, an apparatus that can exert extreme pressures on a sample. They pressurized the solution up to 3.5 GPa, which is about 35,000 times the pressure on Earth’s surface, and heated the cell to 300º C for up to 60 hours. Using a technique called Raman spectroscopy, which involves shining a laser on a sample and then using the light scattered by the chemical bonds in the sample to identify the molecules present, the researchers identified the new compounds formed during the experiment.

After just a few hours, hydrocarbon droplets formed, which contained mainly isobutane and small amounts of methane, ethane, and propane. While some acetate remained, 45% of the mixture turned into liquid isobutane. Unlike acetate, isobutane does not dissolve well in water and thus has the potential to migrate through the subsurface as a hydrocarbon fluid, separate from aqueous fluids.

“It’s a surprise finding,” said Huang. “We suggest that the isobutane can form from aqueous fluids after subduction, providing a new carbon species in the deep earth, which will facilitate carbon transfer in the deep carbon cycle.”

The diamond anvil cell experiments represent a highly simplified version of the natural system, so Huang and Sverjensky used theoretical modeling to consider more realistic conditions in Earth’s subsurface. They modeled how the presence of three types of silicate rocks would impact acetate’s behavior. The model predicted smaller, but still significant, amounts of isobutane in the presence of different rock types, compared to the diamond anvil cell results, but found that the hydrocarbons should be stable at the higher pressures of subduction zones.

The researchers speculate that once the isobutane forms, it may move into the upper crust, where it likely decays into carbon dioxide and methane, possibly providing a source of natural gas and food for subsurface microbes. If the hydrocarbons and acetate travel deeper into the subsurface, these compounds may contribute to the formation of diamonds [3].

“Previously, it was widely thought that carbon in the deep earth is in C-O-H fluids,” said Huang, referring to the traditional view that fluids in the subsurface, called C-O-H fluids, are composed of a mix of water and molecular species, like methane and carbon dioxide. “Our study and similar studies recently published [4-5] suggest that hydrocarbons can form in the deep Earth.”

 

First author Fang Huang, a DCO member and PhD candidate at Johns Hopkins University. Photo courtesy of Fang Huang.
 
Researchers used Raman spectroscopy to detect Na2CO3 crystals that formed from a solution of sodium acetate in the gasket recovered from diamond anvil cell. Credit: Fang Huang
 
A membrane-type diamond anvil cell (disassembled) provided the pressure to simulate conditions in a subduction zone. Credit: Fang Huang

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