Hydrocarbons in the crust, such as the oil and natural gas that we tap to meet our energy needs, primarily come from ancient life. In some unusual locations, hydrocarbons also appear to form abiotically, through geochemical reactions that are completely independent of life. One tool scientists have used to gain hints to the hydrocarbons’ origin is by determining the concentrations of carbon and hydrogen isotopes, which are atoms from the same element with different numbers of neutrons in the nucleus. The extra neutrons make the atoms slightly heavier and their presence creates a signature that can be traced as carbon and hydrogen move through an environment.
In a new paper  in Geochemical Perspectives Letters, researchers find that isotopic signatures are not always a reliable way to determine the origin of hydrocarbons at volcanic hydrothermal sites. DCO members Jens Fiebig (Goethe University Frankfurt, Germany), Andrea Ricci (University of Bologna, Italy), Franco Tassi (University of Florence, Italy), Fatima Viveiros, Catarina Silva (both at Universidade dos Açores, Portugal), Taryn Lopez (Alaska Volcano Observatory, USA) and colleague Andri Stefánsson (University of Iceland), performed the first global investigation of carbon and hydrogen isotopes in samples from a variety of volcanic hydrothermal sites worldwide. Several sites appeared to produce abiotic hydrocarbons based on isotopic signatures. But when the group looked at the global isotopic patterns and the organic matter carried in by surface water, they determined the hydrocarbons were actually “thermogenic,” and came from the high-temperature breakdown, called “cracking,” of organic compounds as they came in contact with the volcanic systems. The new findings emphasize the importance of considering carbon contributions from ubiquitous groundwater sources when interpreting the origins of hydrocarbons.
“I’m looking at hydrocarbons in volcanic systems because potentially, you have all the ingredients that are necessary for abiotic hydrocarbon production,” said Fiebig. These ingredients include inorganic carbon in the form of carbon dioxide, water (which can be a source of hydrogen), catalysts, and high temperatures. “When I was starting, I thought there might be potential for abiotic methane in these systems.”
To investigate further, Fiebig and his colleagues collected fluids from geothermal wells and gases from vents near volcanoes from 28 sites in eight countries. The hydrothermal systems are associated with different types of volcanism and range in temperature from 200 to 450 degrees Celsius. Back in the lab, the researchers measured the concentrations of carbon and hydrogen isotopes from the methane, ethane, propane, and n-butane in the samples. They compared these isotopic compositions to the isotopic signature of local organic carbon in the seawater and rainwater that feed the systems.
Several of the sites had an isotopic signature called a “carbon isotope reversal” that in the past scientists have taken as evidence that hydrocarbons formed abiotically. A reversal describes when the methane at a site has a higher concentration of heavy carbon isotopes than the ethane and other longer hydrocarbons in the sample. This is the opposite of what scientists see in thermogenic hydrocarbon samples, where methane tends to be isotopically lighter than the other hydrocarbons.
When the researchers took into account the organic matter in the surface water that percolates through the system, however, they found they could explain the reversal. Cracking of the surface organic matter inside the hydrothermal system created thermogenic hydrocarbons. But volcanic and geothermal systems are open to the atmosphere, and so those gases can escape. Isotopically light gases tend to escape more easily, leaving behind heavier isotopes in the original organic matter. Over time, this process skews the isotope concentration toward heavier and heavier methane, producing the carbon isotope reversal, and giving the appearance of abiotic hydrocarbon formation.
“Our results strongly imply that the source of the organic matter is the surface fluids,” said Fiebig. “This carbon isotope reversal can be obtained simply from open system cracking of organic matter.”
Fiebig emphasizes the importance of tracking the organic matter transported in fluids that move through the crust when looking for abiotic hydrocarbons. Volcanic systems and deep sea hydrothermal vents circulate large quantities of water, which can supply the system with organic carbon. Theses compounds must be ruled out as a source of thermogenic hydrocarbons if scientists hope to identify sites with abiotic hydrocarbons with confidence.
“Based on our results we cannot exclude that there are minor contributions from abiotic sources in the investigated systems. For more detailed insights, we need to determine mass fluxes of in- and outgoing organic carbon, the organic carbon content of the rocks hosting the hydrothermal systems at depth, and thermogenic hydrocarbon production rates,” said Fiebig.
Despite the results of his latest research, Fiebig is still searching for sources of abiotic hydrocarbons in Earth’s crust. He hopes that if we can find out if and where abiotic hydrocarbons form in Earth’s crust, then we can understand their role in the origin of life on Earth, and potentially explain how methane formed on Mars and other planets.
Main image: Members of the research team sample a geothermal well in Iceland. Credit: Shuhei Ono