Fidelity of leaf-wax n-alkane and n-alkanoic acid D/H ratios in space and time

Abstract:

There is great potential for reconstructing past changes in the hydrologic cycle using the hydrogen isotopic composition of plant-wax biomarkers. However, empirical relationships relating plant-wax hydrogen isotope compositions (δD_wax) to source water are almost exclusively based upon modern plants, soils and sediments and a single compound class such as n-alkanes or n-alkanoic acids. Relatively little is known about the relationship between these compound classes and differences in how they record the hydrogen isotopic composition of source water. Here we present new hydrogen isotopic measurements from a suite of modern and late Quaternary lake sediment samples to test the fidelity of δD_waxto source water δD in space and time.

We find that within compound class the shared variance between C₂₉ and C₃₁ n-alkane and C₂₈ and C₃₀ n-alkanoic acid δD values is 87% and 86%. Between compound classes there is 53% shared variance between C₂₉ n-alkane and C₂₈ n-acid δD values. The apparent isotopic fractionation between source water and n-alkane δD values is consistent with prior studies from these regions that show the influence of vegetation type and climate. However, the n-alkanoic acid apparent fractionation values cannot be explained by these factors alone. Rather, the data require that for a large proportion of the samples there is a substantial contribution of long-chain n-acids from vegetation that uses lake water as the hydrogen source for lipid synthesis. This is consistent with overlapping n-acid compound distributions in lake sediment, soils, and aquatic and terrestrial plants from many of these regions.

Our results emphasize the utility of plant-wax δD for reconstructing water δD values and highlight the potential importance of non-terrestrial sources of long-chain n-acids in lake systems.