Shock

The Potential for Abiotic Organic Synthesis and Biosynthesis at Seafloor Hydrothermal Systems

E. Shock¹’²* & P. Canovas¹

Corresponding author: eshock@asu.edu
¹GEOPIG, School of Earth & Space Exploration, Arizona State University, Tempe, AZ, 85287
²Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ, 85287

Abstract:
Abiotic hydrothermal organic synthesis is extremely unlikely if thermodynamic drives oppose the stabilization of aqueous organic compounds. These drives can be quantified via the chemical affinity (A) given by A = RT ln(K/Q) where R stands for the gas constant, T for temperature, and K and Q represent the equilibrium constant and activity product for a reaction, respectively. Positive values of A indicate that the progress of a reaction is thermodynamically favorable. In an analogous fashion, the energetic costs of biosynthesis of organic compounds are minimized when positive affinities prevail. In this study we compared the potential for abiotic organic synthesis and biosynthesis for five submarine hydrothermal fluids (from 9°N East Pacific Rise, Rainbow, Endeavor, Lau Basin, and Guaymas). As an example, the reaction leading to toluene is given by: 7CO2,aq + 18H2,aq ==> C6H5CH3,aq + 5H2O. The elevated concentrations of CO2,aq and H2,aq in hydrothermal fluids, together with low concentrations of organic compounds like toluene, lead to positive affinities for abiotic organic synthesis. However, affinities are not generally positive at the highest temperatures. Instead, affinities can become enormously positive for reactions of this type when hydrothermal fluids mix with seawater and cool. The decreasing temperature allows equilibrium constants to shift by orders of magnitude as activity products undergo much smaller changes. In effect, cooling allows the equilibrium constants to drive affinities positive at much higher temperatures than those where activity products drive the reactions negative again owing to the overwhelming influence of oxidized seawater. Mixing of Rainbow fluids and seawater generally yield the most positive affinities for the set of fluids selected for this study. Affinities during mixing of the other fluids generally decrease in the order: Guaymas > Endeavor > Lau > 9°N. Affinities for organic synthesis and biosynthesis reactions are often most positive for more complex organic compounds. Note that affinities never go positive for formaldehyde (CHOH), but can exceed 400 kJ mol-1 for dodecanoic acid (C11H23COOH). Also, affinities are much more positive for dodecanoic acid than for its much shorter counterpart acetic acid (CH3COOH).

Keywords:
abiotic organic synthesis, biosynthesis, Rainbow, Endeavor, Lau, Guaymas, EPR 9°N

Contributions to Integration and Synthesis:
The work described above is part of a theoretical synthesis of the potential for habitat generation, organic transformations and rock alteration using all available ridge hydrothermal fluid compositions, and independent simulations of water/rock/organic/microbe interactions. In particular, we are focusing on identifying tipping points in the behavior of ridge hydrothermal systems that enhance or inhibit these processes. These tipping points can be in temperature, pressure, composition or extent of reaction progress.