Minerals not needed to form energy-rich methane at hydrothermal vents

Microorganisms living around deep-sea vents - on ‘black smoker’ chimneys, as symbionts within animals, as well as in the deep sea plumes above them, are all known to consume this energy-rich ‘geo-fuel’ molecule present in vent fluids. For a long time, however, there has been no clear consensus on exactly how methane forms from carbon dioxide – a tough reaction to perform - beneath the mid-ocean ridge seafloor. A record-breaking high-temperature experiment conducted over 5 years now shows that crustal minerals are not needed to chemically catalyze carbon dioxide conversion to methane, only time.

“Our results show now there is actually very little convincing evidence, after nearly 30 years of debate and countless publications on this topic, that these minerals are increasing the speed of this reaction”, says Reeves. The study reacted simple dissolved carbon dioxide and hydrogen gas for 5 years (a geologic blink of an eye) at realistic ‘black smoker’ conditions without any added minerals or catalysts, monitoring the speed of conversion to methane with an isotope label. Reeves thought the result was interesting, but surely the speeds could not compete with mineral-catalyzed experiments - the dogmatic ‘Fischer-Tropsch’ process so accepted in the literature? When he compared the rates in his experiment with those claiming minerals were involved, he was astonished: “Speeds in my experiment were similar to some mineral-bearing experiments, so I went digging into older publications to confirm the same similarity – no one had tested this yet”. After painstakingly reconstructing the ‘speedometry’ of methane formation from countless published experiments claiming minerals were critical to making this geo-fuel, it was clear, “our speeds were remarkably similar, and if anything, some minerals, such as the common mantle mineral olivine, even appeared to slow it down”, says Reeves.

“We’ve known for a long time where the other major energy sources for life in black smoker fluids come from - dissolved hydrogen, hydrogen sulfide, iron and so on, but methane was for a long time the hardest to pin down, and debates in the literature have been ‘heated’ (pun intended)”, says Reeves. In the last decade, there has been a strong shift in our understanding, however, and we now know carbon dioxide must become trapped in pockets of fluid within newly formed igneous minerals, where it likely has enough time to form methane before being picked up by fluids again. “Until now we just didn’t know exactly how this happened or how fast”, Reeves says, “but now our data show that all you need is hydrogen, and a little patience..”. The findings have implications for any hot fluids residing within Earth’s crust, as well as for Solar System ‘ocean worlds’ like Enceladus where hydrogen-rich hydrothermal vents are known to exist.

The study (find it here) was funded by the U.S. National Science Foundation, Woods Hole Oceanographic Institution and the Research Council of Norway FRINATEK project HyPOD.