Hydrogen Isotope Exchange in Hydrocarbons

Hydrogen isotope (2H and 1H) exchange between hydrocarbons and water under hydrothermal conditions was investigated by Eoghan Reeves (MIT/WHOI) and others at Woods Hole Oceanographic Institution [1].

Hydrogen isotope (2H and 1H) exchange between hydrocarbons and water under hydrothermal conditions was investigated by Eoghan Reeves (MIT/WHOI) and others at Woods Hole Oceanographic Institution [1]. Their experiments involved heating linear chains of carbon and hydrogen atoms  specifically C1 to C5 n-alkanes  in aqueous solutions of varying hydrogen isotope ratios (2H/1H) in the presence of a pyrite-pyrrhotite-magnetite redox buffer at 323oC and 35 to 36 MPa. Extensive and reversible incorporation of water-derived hydrogen into C2 to C5 n-alkanes was observed on timescales of months. In contrast, comparatively minor exchange was observed for methane (CH4).

Isotopic exchange is facilitated by reversible equilibration of n-alkanes and their corresponding n-alkenes with H2 derived from the disproportionation of water. Rates of hydrogen isotope variation in n-alkanes decreased with time, a trend that is consistent with an asymptotic approach to steady state isotopic compositions regulated by alkane-water isotopic equilibrium. Substantially slower H isotope variation was observed for ethane relative to C3 to  C5 n-alkanes, suggesting that the greater stability of C3+ alkenes and isomerization reactions may dramatically enhance rates of 2H/1H exchange in C3 to C5 n-alkanes.

Thus, in reducing acqueous environments, reversible reaction of alkanes and their corresponding alkenes facilitates rapid 2H/1H exchange between water and alkyl-bound hydrogen on relatively short geological timescales at elevated temperatures and pressures. The proximity of some thermogenic and purported abiogenic alkane H isotopic values to those predicted for equilibrium 2H/1H fractionation with ambient water suggest that this process may regulate the H isotopic signatures of some naturally occurring hydrocarbons.

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