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The memory of soil charcoal assessed by Raman spectroscopy: A continuum of molecular signatures structured by production conditions and ageing (case study on boreal soils)
The soils of boreal landscapes are continuously shaped by fire and keep the memory of this essential ecosystem process. A part of the pyrogenic record is stored in the sequestered charred biomass, including wood charcoal, which is a long-living carrier of information about past vegetation and fire regimes. Raman spectroscopy of charcoal is still a developing tool used to decipher the chemical signature of the original heat impact and further diagenesis of charred wood in a depositional setting. Here, we examine the molecular structure of charcoal sequestered in Podzols and Arenosols of the European north during fires of the recent past and the Holocene paleofires. We combine Raman spectroscopy of charcoal with the extraction of a time factor estimate from the historical records of fire and radiocarbon dating of burned biomass. With these data, we further explore if changes in the molecular structure of charcoal are time-referenced. Over 950 Raman spectra were collected from macrocharcoal particles produced during 10 historically documented fires (2–160 years since fire) and those sequestered in 31 pyrogenic horizons and morphones of paleosols during the last ~ 9700 cal BP. Calculated Raman spectral metrics included D and G bands positions, their separation, and the four ratios: HD∕HG, HV/HG, H1710/HG, photoluminescence slope/HG. Obtained multivariate dataset suggests a continuum of diverse molecular signatures in charcoal associated with production conditions and ageing in soil. With the use of principal component analysis and hierarchical clustering, we assess the blocks of charcoal memory mostly associated with charring conditions and those attributed to such diagenetic processes as oxidative weathering and saturation by C–H-dominated organic matter. Pronounced changes in the molecular structure of charcoal happened rapidly and depended on soil conditions. While many charcoals became significantly altered within years and decades, some fragments may have better preserved the chemical signature of original charring conditions, even if produced in paleofires of the Early- and Mid-Holocene.