From lake to bog: A 15 kyr record of interplay between landscape changes and mercury accumulation (Réserve Naturelle du Luitel, 1250 m a. s. l., western Alps)

Abstract

Lakes and peatlands are valuable archives for reconstructing past environmental dynamics. Multiproxy studies strive to unravel the complexity of interactions between factors influencing the past evolution of landscapes, including ecosystem dynamics, geomorphological trajectories, palaeoclimatic variability and past human activities.

This study presents a unique 15,000-year multiproxy record detailing the transition of an alpine landscape from a lacustrine environment to a peatland ecosystem. The combination of high-resolution pollen analysis, inorganic trace elements measurements, and organic matter (OM) characterization by FTIR-ATR enables the connection of vegetation dynamics over time with its role in regulating soil (in)organic matter fluxes, including elements such as mercury (Hg).

The results indicate that wet and warm climate episodes, such as the mid Holocene climate optimum, significantly enhance mercury uptake by vegetation and lake primary producers. This process is followed by Hg accumulation in the lacustrine environment, closely associated with fresh organic matter. In contrast, during cold and arid climatic periods, such as the Late Glacial, Hg uptake decreases in both the catchment and lake ecosystems. This reduction, coupled with dilution by minerogenic inputs, leads to lower Hg accumulation in the lake. Throughout the Holocene, the lake gradually transitions into a peatland. From the early to mid-Holocene, Hg accumulation is driven by a combination of lacustrine biological pumping, foliar uptake, and atmospheric deposition. Around ca 4500 cal a BP, the disappearance of catchment contribution to the archive drastically changes the interpretation of the mercury signal, which becomes primarily driven by the uptake of atmospheric Hg by bog vegetation. Mercury accumulation is therefore mainly controlled by changes in Hg deposition or re-emissions, lake and peat development and OM humification. Finally, during the Late Holocene, local forest ecosystems decline, as pasturelands develop in the area. The natural Hg signal is also partially obscured by the effects of regional mining activities.