University of Bremen: Explanation for unusual isotope patterns

MARUM researchers have found traces of hydrocarbon gases in the Guaymas Basin in the Gulf of California that were not produced conventionally. In the laboratory, they were able to simulate the formation of hydrocarbons. Your study has now been published.
Under pressure and high temperature, hydrocarbons, which are also an essential component of oil and natural gas, are formed in the deep ocean floor. In the study, which has now been published in the journal Proceedings of the National Academy of Sciences (PNAS), the scientists trace a new pathway for the gases ethane and propane, two main components of natural gas, to be formed by reducing acetic acid. The team, in which researchers from the MARUM – Center for Marine Environmental Sciences at the University of Bremen and the University of North Carolina (USA) are involved, examined the isotope signatures and simulated the formation of hydrocarbons in an experimental set-up in the laboratory.

Isotopes of hydrocarbon compounds are like a fingerprint. They clearly indicate the way in which chemical compounds were formed – this also applies to simple hydrocarbons such as ethane, propane, butane and pentane. If hydrocarbons are burned, water and carbon dioxide are produced, and energy is released in the process. Hydrocarbons, including crude oil and natural gas, are formed over long periods of time at high pressure and high temperatures – researchers can then demonstrate this using appropriate isotope patterns.

Area in the Gulf of California as a model region
While researchers were examining samples from the Guaymas Basin, their measurements, however, came across isotope patterns that did not match the known educational pathways. The samples were taken with the manned submersible Alvin on an expedition with the research vessel ATLANTIS, so they could be precisely localized and the temperature in the seabed measured at the same time. “In the Guaymas basin, we can observe oil formation as if in time lapse,” explains co-author Dr. Florence Schubotz from MARUM. A lot of organic material collects in this basin and is deposited as sediment, which in turn heats up quickly due to the hydrothermal activities and thus forms oil – and above all very quickly compared to the millions of years in which oil is normally exposed to high and high pressure Temperatures is formed. In contrast, the combination of geological activity on the continental margin and the amount of sedimented biological material ensure that hydrocarbons are formed – the basin acts and reacts like a pressure cooker. Due to this peculiarity, researchers consider the Guaymas Basin as a model region for a comparatively young ocean floor. The basin lies on a ridge on which the movement of the tectonic plates creates a new sea floor.

Acetic acid is converted into gas in the laboratory
“These data surprised us because we couldn’t explain them with known mechanisms. We thought about alternative explanations and thought about what makes this system so special, which processes take place deep underground and could be responsible for the unusual isotope pattern of the hydrocarbon gases in our samples, ”explains first author Dr. Min song. To do this, the researchers examined the role of volatile fatty acids, which are abundant in the Guaymas Basin, and carried out simulation experiments. For the first time, they were able to show an alternative way of how the gases can be formed and, above all, to explain the isotope signatures. In the laboratory of MARUM researcher Prof. Wolfgang Bach, temperature and pressure were simulated as they exist in hydrothermal systems.

Results help to better understand processes in the ocean floor
This new explanation for an alternative formation of hydrocarbons in the ocean floor, emphasizes Schubotz, could now also be examined in other geothermal and hydrothermally heated sediment systems. For this reason, the results are an important contribution to research at MARUM. Colleagues from Min Song in the context of the Cluster of Excellence “The Ocean Floor – Unexplored Interface of the Earth” and with Prof. Andreas Teske from the University of North Carolina are involved in the studyinternational partners also involved. “Our results provide a conclusive explanation for unusual isotope patterns in hydrocarbon gases,” says co-cluster spokesman and study director Prof. Kai-Uwe Hinrichs. The hydrocarbons are not broken down into smaller components from longer compounds, but are built up from smaller components. No microorganisms are involved in this type of formation, which is why it is one of the abiotic pathways. In the future, the results will contribute to a better understanding of the processes in the ocean floor and, in particular, of material flows.

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