Reconstructing Hydroclimate Changes During the Holocene using Leaf Wax Hydrogen Isotope Records from the Norwegian Arctic

Abstract:

Leaf wax hydrogen isotopes can provide important information about Holocene hydroclimate variability in the Arctic. However, when using isotopes to infer past climate it can be difficult to distinguish regional changes from local environmental influences or uncertainties in isotopic fractionation factors. Here we present two leaf wax hydrogen isotope (δD) records from sites that span a 10° latitudinal range in the Norwegian Arctic. We generated records from a peat core from northern Norway (Tønsnes; 69.74°N, 19.12°E), and a lake sediment core from Svalbard (Amsterdamøya; 79.77°N, 10.76°E). At both locations, δD values were measured on long-chain n-alkanes (n-C₂₉, n-C₃₁) at centennial-scale resolution. Over the last 8 ka the records reveal similar trends and ranges of values. δD of n-C₂₉ data from Tønsnes and Amsterdamøya range from -173 to -197‰ and -176 to -194‰, respectively. Both sites show a ~20‰ increase during the mid-Holocene (c. 8.0-2.8). This was followed by a ~10‰ decline during the late Holocene at both sites. The similarity in the absolute values and the trends of these spatially disparate δD records provides evidence that they are not strongly controlled by local environmental factors or changes in apparent fractionation between n-alkanes and source water. Therefore, we interpret the trends as a result of regional changes in precipitation isotopes. Most interesting is the disparity between regional temperature trends and our δD records during the mid-Holocene. In certain Arctic regions, temperature appears to be the primary control on precipitation isotopes. If the trends in our data were interpreted as a response to temperature, the records would suggest increasing temperatures through the mid-Holocene (c. 8.0-2.8 ka). This is at odds with regional air and sea-surface temperature reconstructions, which indicate decreasing temperatures during this period as a result of the reduction in insolation. Therefore, the increase in δD at these sites more likely reflects changes in the seasonality of precipitation, resulting from a progressive increase in the proportion of D-enriched summertime precipitation, or a change in the trajectory of air masses delivering precipitation to the region, an interpretation supported by historical measurements of precipitation isotopes from the region.