Leaf Wax Hydrogen Isotopes as a Hydroclimate Proxy in the Tropical Pacific
S. Nemiah Ladd, Ashley E. Maloney, Daniel B. Nelson, Matthew Prebble, Giorgia Camperio, David Sear, Jonathan Hassall, Peter G. Langdon, Julian P. Sachs, Nathalie Dubois
Abstract
The hydrogen isotope composition (²H/¹H ratios) of leaf waxes preserved in sediments is increasingly used to reconstruct past hydroclimate. Here, we extend the global calibration of leaf wax ²H/¹H ratios to include surface sediments from 23 lakes and swamps on 15 tropical Pacific islands. Leaf wax ²H/¹H ratios from this new data set are not correlated with regional estimates of mean annual precipitation ²H/¹H ratios derived from isoscapes or from isotope-enabled general circulation models. Nevertheless, the new data fall within the predicted range of values based on a global calibration compiled from published surface sediments. In our global compilation, we find a strong positive linear correlation between ²H/¹H ratios of mean annual precipitation and the common leaf waxes n-C₂₉-alkane (R² = 0.73, n = 581) and n-C₂₈-acid (R² = 0.74, n = 242). In the tropical Pacific, the largest residuals are no greater than those observed elsewhere, and are likely due to (1) uncertainty in the ²H/¹H ratios of local precipitation and (2) variability in net fractionation for different plant types. Palynological analyses from the same samples suggest that there is no systematic relationship between any particular type of pollen distribution and deviations from the global calibration line. Overall, our results support the use of leaf wax ²H/¹H ratios in tropical Pacific lake sediments as proxies for large hydrological changes, especially when paired with ²H/¹H ratios of source-specific biomarkers. However, the interpretation of such records needs to be informed by careful consideration of local drivers of precipitation isotope variability.