The earth is a dissipative system where energy from the planet's interior is converted to heat which in turn drives plate tectonics and supply fresh reduced material to the earth's surface. Heat sources at magmatic active regions produce groundwater convection cells which effectively transport the heat to the surface. The reduced fresh material has a high free energy with respect to the groundwater and is rapidly oxidized, which results in the reduction of water to hydrogen.
The reduced H2-containing aqueous solution is transported by advection in hydrothermal cells and mix rapidly with ambient cold water in focused flows. Steep electrochemical gradients may form where mixing occur through permeable structures or membranes in chimneys or sediments.
The discovery of hydrogen and methane produced by groundwater-rock interactions implies that H2/CH4-based chemosynthetic ecosystems may be widespread. There is also new compelling evidence for abiotic synthesis of organic compounds in the subsurface hydrothermal environments where rocks rich in reduced elements occur. Knowledge of the magnitude and extent of hydrogen production is a key to understand the emergence of chemosynthetic life.
This lecture gives an overview of what is presently known about hydrogen
production with a main focus on hydrothermal systems along mid-ocean
ridges. The role of hydrogen as a substrate for chemosynthetic ecosystems
and as a key component in the inorganic synthesis of building blocks
for chemosynthetic life is discussed.
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