According to a study published in the journal Geophysical Research Letters, rocks formed beneath the ocean floor by fast-spreading tectonic plates may be a large, overlooked source of free hydrogen gas.
Deposits of serpentinized rock such as this could be a previously overlooked source of free hydrogen gas. Image credit: NOAA Ocean Explorer.
Recent discoveries of free hydrogen gas, which was once thought to be very rare, have been made near slow-spreading tectonic plates deep beneath Earth’s continents and under the sea.
“Our model, however, predicts that large quantities of hydrogen gas may also be forming within faster-spreading tectonic plates — regions that collectively underlie roughly half of the Mid-Ocean Ridge,” said lead author Dr. Stacey Worman of the University of Texas at Austin.
“Total hydrogen gas production occurring beneath the oceans is at least an order of magnitude larger than production occurring under continents, the model suggests.”
“A major benefit of this work is that it provides a testable, tectonic-based model for not only identifying where free hydrogen gas may be forming beneath the seafloor, but also at what rate, and what the total scale of this formation may be, which on a global basis is massive,” added co-author Prof. Lincoln Pratson of Duke University.
The new model calculates the amount of free hydrogen gas produced and stored beneath the seafloor based on a range of parameters — including the ratio of a site’s tectonic spreading rate to the thickness of serpentinized rocks that might be found there.
Serpentinized rocks — so called because they often have a scaly, greenish-brown-patterned surface that resembles snakeskin — are rocks that have been chemically altered by water as they are lifted up by the spreading tectonic plates in Earth’s crust.
Molecules of free hydrogen gas are produced as a by-product of the serpentinization process.
“Most scientists previously thought all hydrogen production occurs only at slow-spreading lithosphere, because this is where most serpentinized rocks are found,” Dr. Worman said.
“Although faster-spreading lithosphere contains smaller quantities of this rock, our analysis suggests the amount of hydrogen gas produced there might still be large.”
“Right now, the only way to get hydrogen gas — to use in fuel cells, for example — is through secondary processes,” Dr. Worman said.
“You start with water, add energy to split the oxygen and hydrogen molecules apart, and get hydrogen gas. You can then burn the gas, but you had to use energy to get energy, so it’s not very efficient.”
Mining free hydrogen gas as a primary fuel source could change that, but first scientists need to understand where the gas goes after it’s produced.
“Maybe microbes are eating it, or maybe it’s accumulating in reservoirs under the seafloor. We still don’t know,” Dr. Worman said.
“Of course, such accumulations would have to be quite significant to make hydrogen gas produced by serpentinization a viable fuel source.”
If further research confirms the model’s accuracy, it could also open new avenues for exploring the origin of life on Earth, and for understanding the role hydrogen gas might play in supporting life in a wide range of extreme environments, from the sunless deep-sea floor to other planets.
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Stacey L. Worman et al. 2016. Global rate and distribution of H2 gas produced by serpentinization within oceanic lithosphere. Geophysical Research Letters 43 (12): 6435-6443; doi: 10.1002/2016GL069066
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