Water-Rock Reactions Produce Hydrogen Gas at Temperatures within the Limits of Life

Hydrogen gas is produced in chemical reactions between anoxic water and iron-rich rocks at temperatures above 200°C – conditions too hot to support life. However, at hydrothermal vents or hot springs, where hydrogen-rich fluids mix with cooler waters and temperatures have dropped sufficiently, hydrogen-consuming organisms can survive. Less is known about the amount and mechanism of hydrogen production from water-rock reactions occurring at temperatures within the temperature limits of life (≤122°C) and the potential for these reactions to support in situ microbial life. Given the large expanse of the Earth’s crust that likely interacts with water under these cooler conditions, it is possible that these low-temperature reactions could sustain extensive subsurface microbial communities away from Earth’s hot spots.

In a study published in the journal Nature Geoscience, researchers from the University of Colorado – Boulder and the University of Alaska – Fairbanks, led by Dr. Alexis Templeton, demonstrated significant production of hydrogen gas when they submerged powdered rocks (peridotite) and minerals (olivine, pyroxene, magnetite) in anoxic water at 55°C and 100°C for a period of 100 days. The general mechanism of hydrogen production is well known: Reduced iron species (Fe²⁺) transform into oxidized species (Fe³⁺) and release electrons that split water into hydrogen gas. But what happens in detail at low temperatures? To address this question, the scientists analyzed the products of their low-temperature reactions using SSRL’s microXAS imaging instrument at Beam Line 2-3.

The researchers found that Fe³⁺-containing reaction products were primarily located on what are known as spinel minerals – minor components of the reaction substrates with a cubic structure and the general formula M²⁺M³⁺₂O₄, where M is a metal such as iron or chromium. The scientists also observed that reaction substrates containing a larger volume percent of spinels generally produced larger amounts of hydrogen than reaction substrates with smaller amounts of spinels. Spinel minerals are conductive and previous work by others has shown that Fe²⁺ will adsorb to spinel surfaces and electrons can be transferred from Fe²⁺ to the spinel. The researchers therefore proposed a model for hydrogen production in which the surfaces of spinel minerals are key mediators of the electron transfer between Fe²⁺ and water and thus play an important role in hydrogen production during low-temperature reactions. Interestingly, spinel-bearing rocks such as basalts are also common on Mars’ surface, suggesting that low-temperature hydrogen production from water-rock reactions may support potential microbial habitats on other planets as well.

 

Primary Citation

L. E. Mayhew, E. T. Ellison, T. M. McCollom, T. P. Trainor, and A. S. Templeton, "Hydrogen Generation from Low Temperature Water-Rock Reactions", Nat. Geosci. 6, 478 (2013) doi:10.1038/ngeo1825

Related Links

• Science Highlight – HTML  /  PDF
• SSRL Science Highlights Archive
• SSRL Beam Lines
• University of Colorado Boulder News Release

Contacts

Lisa Mayhew, University of Colorado - Boulder
Alexis Templeton, University of Colorado - Boulder