Disentangling the impact of vegetation and hydroclimate dynamics on plant wax stable isotopes since the Younger Dryas in Central Switzerland

The stable isotopic composition of plant waxes (δ¹³C and δ²H values) in lake sediments is a powerful tool for reconstructing past climates and environments, offering critical insights into hydroclimate variability and vegetation dynamics. Different plant types, such as grasses and woody vegetation, can exhibit distinct water sources and isotopic fractionations, directly influencing plant-wax isotopic signatures in sedimentary records. This influence is particularly pronounced in small basins, where local vegetation sources can dominate over regional signals, complicating the interpretation of δ²H records. Disentangling the effects of vegetation changes from hydroclimate variations in lipid biomarkers is paramount for robust interpretations yet remains quite challenging. 

Here, we aim to quantify the effects of climate, vegetation, and human activities on sedimentary records, improving our understanding of environmental changes in central Switzerland. We utilized δ¹³C and δ²H records of plant waxes (n-alkanes and fatty acids) from Rotsee, a small lake near Luzern. Our Bayesian-based ¹⁴C age model establishes a temporal framework for interpreting changes in vegetation and hydroclimate at Rotsee since the Younger Dryas period (12.9–11.7 ka). The extensive paleoclimate and archaeological research in the region offers a complementary and broader context for our findings. 

During the Younger Dryas, leaf wax δ¹³C and δ²H values were very low — consistent with cold and dry conditions. δ²H values of long-chain fatty acids increase rapidly during the transition to the Early Holocene, in what appears to be a double-step change: an initial ~20 ‰ increase driven by hydroclimate changes (δ¹³C remained stable) followed by ~30 ‰ driven by changes in regional vegetation (accompanied by a ~10‰ δ¹³C increase). This overall ~50 ‰ increase in δ²H aligns with a major and well-known regional warming trend.

During the Holocene Thermal Maximum, around 9.5 to 6 ka, δ¹³C values peaked, likely reflecting the dominance of woody plants and increased input of aquatic plants, while δ²H values remained stable. However, after 6 ka, there were significant shifts in δ¹³C values, possibly reflecting local vegetation source changes at Rotsee such as a decrease in aquatic plants. 

Around 2 ka, during the Roman Period, the plant wax isotopic records at Rotsee show a major shift, with δ²H values declining by ~40‰ and δ¹³C by ~5‰ in long-chain fatty acids. This substantial change in δ²H values exceeds what would be expected based on plausible hydroclimate variations during this period, suggesting a vegetation source-driven change. The intensification of forest clearance and cereal cultivation, as indicated by pollen records, suggests that human-driven land use changes likely strongly influenced the δ²H signal. After medieval times (ca. 0.5 ka), plant wax δ¹³C and δ²H values increased significantly, likely due to forest regrowth and Rotsee’s eutrophication.

Overall, this study highlights the potential of plant-wax isotopes to reveal complex interactions among climate, vegetation, and human activity. Our findings demonstrate how sedimentary records from small basins similar to Rotsee can provide valuable insights into local and regional environmental dynamics.