Jill M. McDermott, Jeffrey S. Seewald, Christopher R. German, and Sean P. Sylva (2015). Pathways for abiotic organic synthesis at submarine hydrothermal fields. PNAS. 112 (25) 7668-7672; doi:10.1073/pnas.1506295112. published ahead of print June 8, 2015
Spreading centers are a source of dissolved inorganic carbon (DIC) and hydrogen (H2). Together these have potential to produce organic molecules (methane and hydrocarbons) through abiotic reduction of DIC. The organic molecules produced are ingredients that can explain the origin of life. Answers to what mechanisms facilitate the origination of methane and hydrocarbons in vent fluid are still being sought, specifically, at the Von Damm vent field the question is: is the abiotic production of these organic molecules dependent on serpentization- or not?
Previous research has suggested that methane and heavy hydrocarbons in vent fluid are dependent on H2 generation during active serpentization in hydrothermal systems during which seawater and ultramafic rocks react. A more recent study conducted by McDermott et al. challenges previous findings by concluding that the abiotic production of methane and organic compounds at the Von Damm vent feild is not dependent on serpentization. Instead they suggest that the production is related to abundant H2 in fluid inclusions and mixing in the shallow subsurface.
The study area is near the Cayman Trench, a spreading center between North and South America (figure 1). Samples were taken from the Von Damm hydrothermal vent field (Figure 2) above the Mount Dent core complex, which has ultramafic, gabbroic, and basaltic rocks. The vent field is 2,350 metres deep and fluids reach temperatures up to 226 degrees Celsius.
In the field Jason, a remotely operated vehicle, was used to collect fluid using leak tight samplers. Three types of fluid samples were taken: vent fluid, fluid near the vent, and seawater. Shipboard measurements were made within 24 hours of collection. Preliminary shipboard measurements revealed the fluids most unique from seawater came from the East Summit; these have elevated H2 (18.2 mmol/L), CH4 (2.81 mmol/L), hydrocarbons, dissolved metals, a slightly acidic pH of 5.6, and near zero magnesium. Samples collected for on land analyses were frozen for preservation.
The characteristics of fluid collected from the field are summarized in Table 1. Mixing plots of the fluids are presented in Figure 3. The dark points represent the East Summit fluids- the ones most unique from sea water. Seawater is represented with a yellow star. The other fluids collected are plotted in green; particularly in plots A and D, it is evident that they are mixes of East Summit fluid and seawater. Also noted, the fluids are similar to other vent systems with respect to chloride, H2, and hydrocarbon concentrations.
McDermott et al. determined that the abundance and isotopic composition of carbon species are constraints on methane production. They found that the carbon isotopic signature of methane is homogenous in the vent field (-15.4 ppt) and differs from what is expected from biogenic production (-30 to -70 ppt) and thermogenic production (-25 to -50 ppt) of methane. They attribute the homogenous d13C to abiotic methane production.
They also determine that DIC is preserved during deep water mixing, suggesting that the reduction of inorganic sources during active fluid circulation is not a source for methane. The similarity of the seawater DIC d13C and the end member DIC d13C support this conclusion. Additional support against production during deep water mixing stems from the correction for chloride enrichment that occurs during water-rock interactions. Once the correction is applied all chloride discrepancies can be explained leading researchers to conclude that there is no significant change to the concentration of DIC during deep mixing.
Methane is associated with alkane and hydrocarbon formation so researchers concluded that their formation is not occurring during deep water mixing either. This is confirmed with radiocarbon: the CH4 and DIC d14C are not even close; it is expected that they would be if they two were related.
McDermott et al. provide an alternative explanation for the methane production. They suggest that it is formed from the leaching of carbon rich fluid inclusions at depth. When the inclusions cool they form methane rich, DIC poor vapor and are eventually liberated from the active core complex. This would explain the d13C values, which are similar to values at other vent field where this alternative mechanism is believed to be occurring. It is also supported be helium isotopic compositions, which are what is expected if there is a mantle source. A secondary source of methane may be methanogenesis during shallow subsurface mixing; this source also provides a mechanism for the presence of formate, a salt of the organic acid formic acid, because DIC reduces to formate. McDermott et al. conclusions are further supported by thermodynamic models and the idea that the reduction of DIC by H2 rich fluids would form formate and not methane during active serpentization.
Research that indulges our minds in possible and logical mechanisms for organic molecules forming from inorganic ones is always interesting. The ideas proposed by McDermott et al. not only expand our understanding of how life may have originated, but also provide the unique opportunity to understand how life is sustained in extreme environments in present day.