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Human activities are leading to severe environmental impacts, such as large-scale deforestation and increased erosion rates, thereby threatening the sustainability of the Earth’s Critical Zone. A research team headed by Xu Zhang, with colleagues from the Institut de Physique du Globe de Paris, Université Savoie Mont Blanc, and Centre national de la recherche scientifique, conducted research at the La Thuile lake in the Northern French Alps and found that the isotope composition of the element Lithium found in lake sediments can be used as a powerful proxy to reconstruct past weathering processes – which allowed the researchers to look into La Thuile’s geochemical past and explore soil formation and soil erosion processes throughout long timescales.

The Earth’s Critical Zone (CZ) refers to the thin layer between the bedrock and the land surface, which includes the hydrosphere and the biosphere. Since the onset of the Holocene, human activities have become its most prominent disturbing factor. In order to make accurate predictions about how the CZ will respond to current and future climate change impacts, it is essential to know how diverse climatic and human-induced factors actually affect the CZ.

Lake La Thuile is quite a small lake with a maximum depth of about eight meters. Back in 2010, a sediment sequence was extracted from the deepest part of the lake. Together with two depth profiles from nearby soils, sediment sequence was analyzed by Zhang et al. The element Lithium (Li) can be found primarily in silicate minerals and its isotope composition can be used as a proxy for reconstructing past weathering processes. The lighter ⁶Li isotope is preferably incorporated into soil-forming phases such as clay. This results in the heavier ⁷Li isotope showing higher concentrations in soil and river solutions­. Zhang et al. use this isotope signature to gain information about the extent of secondary mineral formation and soil formation processes.

Specifically, they found that the variation in the ⁷Li signature within the lake sediment record is a powerful proxy for elucidating the evolution of the CZ at the catchment scale. At Lake Thuile, clay minerals are mobilized from shallower soil horizons, accumulated in deeper layers of the catchment and preferentially transported to the lake. This can be seen both in the decreased ⁷Li isotope signature and in microscopy analyses conducted at Utrecht ľ¹Ï¸£ÀûÓ°ÊÓ's Electron Microscopy Centre. Zhang et al. were able to reconstruct La Thuile’s geochemical past. From the Last Glacial Period to the onset of the Holocene, ⁷Li levels remained stable, as soil development and erosion was limited. Between 8000 and 3000 years before present, ⁷Li values decreased, meaning there was substantial soil development through forest densification across the catchment. Between 3000 and the Medieval Warm Period, ⁷Li values have increased, showing the onset of human settlement and increased erosion rates. In the last millennium, physical erosion rates have been peaking.

To conclude, Zhang et al. show that lake sediments can potentially show the history of soil formation of a lake, thereby showing the complex interactions between climate, lithosphere, ecosystems, and human impacts.

The research, published in  was conducted in close co-operation with the Institut de physique du globe de Paris (IPGP), Université Savoie Mont Blanc, Centre National de la Recherche Scientifique (CNRS), and other research institutes in France and Norway.