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On the forcing mechanism for the H2-driven deep biosphere

Published online by Cambridge University Press:  08 July 2008

Helge Hellevang
Helge Hellevang
Affiliation:
Center for Geobiology at Department of Earth Sciences, University of Bergen, Allègaten 41, 5007 Bergen, Norway e-mail: helge.hellevang@geo.uib.no
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Abstract

Heat produced in the mantle and core of the Earth by the decay of radioactive elements and mineral fusion results in large-scale mantle convection. The outer shell of the Earth that floats on the convective mantle is divided into rigid lithospheric plates. Subduction of dense cold plates into the mantle leads to plate tectonics. At divergent plate margins, heat is dissipated through hydrothermal convection cells. As ocean water is entrained into hydrothermal cells it interacts with fresh magmatic rocks and liberates ferrous iron. This iron reduces the ocean water to such an extent that it decomposes and forms hydrogen. Molecular hydrogen, as the most reduced component in the system, forms a basal component to a deep dark biosphere powered by metastable redox gradients. In this paper we review the driving force behind a hydrogen-driven deep biosphere. We present abundant observations of hydrogen produced at natural hydrothermal settings as well as in laboratory experiments. The key mineral reactions responsible for the bulk of this hydrogen production are then presented. A division of the reaction progression into an oxidized state and a reduced state is suggested. The amount of hydrogen produced is insignificant in the oxidized state whereas it becomes proportional to the amount of ferrous iron oxidized in the reduced state. The bulk of basalt-hosted aquifers are expected to reside in the oxidized state because of the low content of ferrous minerals, whereas abundant olivine in ultramafic-hosted systems is responsible for large-scale hydrogen production. Today the majority of the seafloor is basaltic. The Archean seafloor on the other hand consisted of fewer ultramafic exposures, but was dominated by ultramafic magnesium-rich lavas with a higher potential for hydrogen production than the present seafloor.

Keywords

Hydrogendeep biosphereserpentinizationOlivinehydrothermalblack smoker
Type
Research Article
Information
International Journal of Astrobiology , Volume 7 , Issue 2 , April 2008 , pp. 157 - 167
Copyright
Copyright © 2008 Cambridge University Press

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