From the 8.2 ka event to the Little Ice Age: Holocene cold periods and human impact recorded in alpine glaciofluvial peatlands (Silvretta Mountains, Switzerland)
Abstract. High alpine peatlands are naturally impacted by extreme climatic conditions and heterogeneous topography. In the Alps, humans have been influencing their development for millennia, and accelerating climate change puts them under additional pressure. In the Swiss part of the Fimba Valley (or Val Fenga; > 2350 m a.s.l. (metres above sea level)), small-scale peatlands have been investigated to gain knowledge on climatic and anthropogenic impacts on alpine landscapes using quantitative and semi-quantitative geochemical parameters derived from inter-calibrated portable X-ray fluorescence spectrometry (pXRF) and XRF core scanning, sedimentology, and radiocarbon dating. The onset of peat formation, after the retreat of the Fimba Valley glacier, has been dated to a time window between 10 450 and 9000 cal BP at the lower (northern) end and to 6600 cal BP at the upper (southern) end of a 470 m long transect. Holocene cold episodes appear to have increased erosive glacial activity in the western side of the valley, resulting in high minerogenic sediment loads being deposited on the peatlands by a meltwater channel, interrupting peat accumulation repeatedly. In the early Holocene, distinct minerogenic layers suggest glacier growth and cold and potentially wet conditions around 9200, 8200, and 6300 cal BP. With the impact and extent of the 8.2 ka cold event still being under discussion for this region of the Alps, a coarse gravel layer is strong evidence for a marked glacial response in the Silvretta Mountains to a particularly cold and wet episode. Cooler climate conditions seem to have prevailed around 5400, 5000, 4500, and 3600 cal BP. Afterwards, the proportion of anthropogenic forcing in erosional processes and other disturbances increased. During and since the Middle Ages, soil or sediment erosion and decreasing peat accumulation were and have continued to be consequences of at least one of the following factors: deforestation; livestock grazing and traffic (trade, tourism); and, temporarily, the Little Ice Age. These impacts and their potential effects on carbon accumulation and flood risk mitigation in the valley should be considered in land management practice. Despite their strong minerotrophic character and a likely post-depositional release due to erosion and decomposition in recent layers, the peatlands have preserved clear signals of atmospheric lead (Pb) pollution: one correlated with the Roman period and another around 1450 cal BP. Rapidly changing sedimentation and hydrology in small mountain peatlands are a challenge for radiocarbon chronologies, high-resolution sampling, and the detection of atmospheric geochemical signals. Yet, our study demonstrates that dynamic glaciofluvial stream-bank mires are valuable for the reconstruction of the impact of climate and humans on alpine environments – from prehistory to the present.
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