Hydrogen isotope fractionation in algae: III. Theoretical interpretations

Abstract

Hydrogen isotope measurements of lipid biomarkers preserved in sediments are most commonly interpreted as qualitative, rather than quantitative indicators of paleoprecipitation owing to an imperfect knowledge of all factors controlling the isotopic fractionation occurring during biosynthesis. Here, we first offer a brief review of appropriate procedures for preparing enriched isotope substrates for use in tracer studies and outline the approximate δD threshold at which this transition occurs. We then present new interpretations to explain deviations from common stable isotope effects observed in our previous culture experiments and other studies.We draw particular attention to the disagreement between intercept and slope for product–substrate relationships from those predicted for isotope systems, even when R2 values are high, and attribute it to kinetic isotope fractionation. We demonstrate that reconstructing paleoenvironmental water δD values by simply adding a Δ to measured biomarkers δD values will result in a bias toward deuterium enriched values. This applies even to implicit reconstructions in the form of qualitative interpretations of measured lipid δD values as indicators of past hydroclimate. We therefore recommend reconstructing water δD values from lipid δD values using fractionation factor (α).We also discuss the apparently contradictory increase in D/H fractionation observed at elevated temperature and suggest that this may be the result of the unique wave-particle duality of hydrogen isotopes, which permits isotopologues to avoid surmounting the activation energy barrier that is necessary in traditional kinetic reactions.