EXCHANGE OF CARBON-BOUND HYDROGEN OVER GEOLOGIC TIMESCALES

Hydrogen bound directly to carbon in organic molecules – usually considered “nonexchangeable”– likely does exchange over geologic timescales. How can we recognize when this has been important, and what are the specific impacts of exchange on D/H ratios? Equilibrium isotope effects will lead to organic hydrogen being depleted in D by 75 permil or more relative to water. The observation of large isotopic differences between organic molecules and coexisting water is not, therefore, clear evidence against exchange. The preservation of biosynthetically introduced intermolecular contrasts, such as those between n-alkyl and isoprenoid molecules, is a better but still insensitive assay for exchange. Stereochemical inversions at chiral centers in organic molecules, which proceed by mechanisms similar to those of many exchange reactions, may provide a much better proxy for the occurrence of hydrogen exchange.

Using previously published estimates of equilibrium fractionation factors, we calculate dD values for the following organic molecules in equilibrium with water (dD=0‰) at 30°C: icosane, -75‰; phytane, -115‰; cholestane, -165‰. For sterols, D/H ratios are affected by a wide array of processes during diagenesis. These include dehydration, double-bond migration, hydrogenation of double bonds, methyl shifts, stereochemical inversion, and simple exchange. Estimates of fractionation factors indicate that many of these effects counteract each other, so that changes in dD values of sterols may be relatively small. Downcore comparison of dD values for steranes and diasteranes might provide a particular opportunity for directly observing the isotopic consequences of hydrogen exchange.