The Role of Isotopically Different Terrestrial Plants in Controlling Sedimentary n-Alkane 2H/1H Composition

Abstract:

Recent studies of modern terrestrial plants growing at a single location reveal that ²H/¹H fractionation between source water and leaf wax n-alkyl lipids can vary by up to 100‰. This provides a challenge for interpreting sedimentary n-alkyl δ²H records, because the contribution of isotopically different plants can shift through time. Palaeohydrological studies often either ignore this factor or try accounting for it by identifying the presence/absence of vegetation change using n-alkane molecular distributions or δ¹³C values.

To evaluate potential problems associated with those approaches, we applied two different methods for reconstructing palaeoprecipitation δ²H from n-alkanes extracted from 1 m core from Stiffkey saltmarsh, Norfolk, UK. We found that sediment n-alkane d²H values shifted by up to 70‰ since the 1500’s, with the main positive excursion between the 1720’s and 1820’s. Over the same period, however, δ¹³C values varied by only 1-3‰. We used the IsoConc mixing model to predict down-core vegetation change. We identified three time intervals where plant contributions shifted. We then calculated palaeoprecipitation δ²H using: (a) previously published ε_wax/water, and (b) ε_wax/water calculated for the Stiffkey plants. The palaeoprecipitation δ²H determined using the first approach showed 30-45‰ shifts from the 1620’s to 1870’s. In contrast, calculations using ε_wax/water derived specifically for the Stiffkey plants showed only small fluctuations of <14‰.

This study demonstrates that the assumption of a minimal role of shifts in isotopically different palaeovegetation may lead to erroneous conclusions regarding δ²H values of paleoprecipitation. In addition, selection of appropriate ε_wax/water values is critical for accurate reconstruction of this parameter. New methods are required to identify, quantify, and correct for vegetation shifts in sedimentary sequences to ensure the influence of plant community change can be evaluated when interpreting biomarker δ²H records.