Title: Formation of ethane and propane via abiotic reductive conversion of acetic acid in hydrothermal sediments
Authors:  Min Song, Florence Schubotz, Mathia Kellermann, Christian Hansen, Wolfgang Bach, Andreas Teske, and Kai-Uwe Hinrichs
Journal: Proceedings of the National Academies of Sciences

Every day you use oil and natural gas. They drive your cars; they make your plastics; and they heat your home. It may come as a surprise then that even leading geoscientists don’t fully understand how oil and gas form. They know that oil and gas are made when organic matter (the ancient remains of plants and other lifeforms) is buried and heated, but they don’t yet know what exact molecular reactions occur to make these global commodities. For example, natural gas is full of short-chain hydrocarbons (SCH) known as methane (C₁), ethane (C₂), and propane (C₃), but there are many sources of SCH’s, including biological synthesis by microbes, thermal “cracking” of buried organic matter, and abiotic polymerization of C₁-compounds. Geoscientists study environments where oil and gas are actively being created to understand which of these sources is most important. One of those locations is at the bottom of the ocean at a hydrothermal vent in the Gulf of California called Guaymas Basin. The authors of this study in PNAS went to Guaymas Basin and discovered a totally new way of forming SCHs that could have implications all over the world.

Their discovery began with a curious pattern in the carbon isotope composition of methane, ethane, and propane that couldn’t be explained using current models of SCH formation. (For a more in-depth discussion of carbon isotopes, see my previous article). Usually methane has more ¹³C than ethane or propane leading to a higher ¹³C value for methane than the other SCHs. In this case, methane was the most ¹³C depleted. Another unusual feature of the Guaymas Basin samples was that the SCHs had decreasing ¹³C as carbon number increased (Figure 1), a pattern that couldn’t been reconciled with the low ¹³C value for methane. The answer to this conundrum came by looking at the short-chain fatty acids (SCFA), which showed a similar pattern in the ¹³C of acetate (C₂), propionate (C₃), and butyrate (C₄). If the SCFA ¹³C values matched those of the SCHs, could the SCHs come from the SCFAs?

Song et al. designed two experiments to answer this question. In both experiments, they put Guaymas Basin sediments into a high temperature reactor that mimics environmental conditions. To the first experiment, they added CO₂ that was labelled with ¹³C, making ¹³CO₂. The ¹³C-label acts as an inert tracer, like putting a red dot on the back of a playing card to track where it goes after shuffling the deck. ¹³CO₂ reacts similarly to normal CO₂ but can be easily distinguished and quantified. When Song et al. ran the high-temperature reaction (shuffled the deck) with ¹³CO₂, they found the ¹³C-label in acetate, the C₂ SCFA. Somehow CO₂ had inserted itself into the acetate molecule.

To test this further, they added ¹³C-labelled acetate to the samples and found that not only were ethane and propane produced, but they both had the label attached. Ethane and propane clearly were being generated from SCFAs. This model can explain the enigmatic ¹³C distributions in SCHs, since CO₂ has a characteristically high ¹³C value. If CO₂ makes its way into acetate and subsequently into the SCHs, the SCHs will have a higher ¹³C value than methane, as is observed in Guaymas Basin (Figure 2).

These experiments revealed that in Guaymas Basin, acetate is fueling the creation of ethane and propane, two important components of natural gas. This represents an entirely new way of creating oil and gas. This process could be quantitatively important all over the world at hydrothermal vents, because SCFAs are a common feature of these environments. Through their experiments with carbon isotopes, Song et al. solved one piece of the gas formation puzzle that has riddled geoscientists for decades.