Prediction of Holocene Mercury Accumulation Trends by Combining Palynological and Geochemical Records of Lake Sediments (Black Forest, Germany)

Abstract

Forest vegetation plays a key role in the cycling of mercury (Hg) and organic matter (OM) in terrestrial ecosystems. Litterfall has been indicated as the major transport vector of atmospheric Hg to forest soils, which is eventually transported and stored in the sediments of forest lakes. Hence, it is important to understand how changes in forest vegetation affect Hg in soil and its biogeochemical cycling in lake systems. We investigated the pollen records and the geochemical compositions of sediments from two lakes (Schurmsee and Glaswaldsee) in the Black Forest (Germany) to evaluate whether long-term shifts in forest vegetation induced by climate or land use influenced Hg accumulation in the lakes. We were particularly interested to determine whether coniferous forests were associated with a larger export of Hg to aquatic systems than deciduous forests. Principal components analysis followed by principal component regression enabled us to describe the evolution of the weight of the latent processes determining the accumulation of Hg over time. Our results emphasize that the in-lake uptake of Hg during warm climate periods, soil erosion after deforestation and emissions from mining and other human activities triggered changes in Hg accumulation during the Holocene stronger than the changes caused by forest vegetation alone. and trace element transport in several ways. First, the presence of vegetation stabilizes catchment soils and reduces the transport of particulate materials to surface waters [14]. Second, vegetation has an influence on the amount of litter that is produced, e.g., deciduous trees produce more litter than coniferous trees, which affects the trace element concentrations in the released dissolved organic matter (DOM) [15,16]. Third, vegetation and OM quality affect soil development, e.g., deciduous litter promotes the presence of invertebrates and soil mixing, which results in longer contact times between OM and soil mineral particles [17]. Fourth, differences in plant species compositions affect the interception area, which in turn affects the sequestration of atmospherically deposited trace elements [1,18]. Fifth, litter from different vegetation types decomposes at different rates, depending, e.g., on the litter chemical composition [19,20]. This affects the production of DOM and the amount of trace elements bound to each unit of OM so that more decomposed OM has higher Hg concentrations. Some studies have shown that Hg concentrations are higher in the litter of coniferous forests than deciduous forests [21,22]. In contrast, others studies have demonstrated that Hg concentrations in coniferous litter may be lower than those in deciduous litter [1,23,24], which might be due to varying conditions of atmospheric Hg concentrations or exposure. Jiskra et al.[25] indicated, with Hg isotope analyses in boreal soils, that 90% of Hg in forest soil is deposited by litter. While the vegetation acts as a short-term sink for atmospheric Hg, forest soils can be regarded as long-term sinks [26]. Drenner et a...

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