Chronology and properties of macrocharcoal sequestered in boreal forest soils since deglaciation (southeast of the Kola Peninsula)

Abstract

Fire relentlessly modifies the belowground pool of pyrogenic carbon (PyC) in boreal ecosystems. However, only a few soils offer a consistent record of PyC accumulation. Here, we report on the Holocene-scale record of biomass burning and PyC sequestration in the paleosoils of dry topographic hollows at the Keiva ice-marginal landform. We combine soil stratigraphy, radiocarbon dating, and chemical analyses to explore the chronology and properties of macrocharcoal that accumulated in soils for millennia. The most ancient pyrogenic horizons refer to 10,700–10,200 cal BP. They contain charred remnants of well-pronounced root systems as a part of the paleosoil, suggesting enough biomass for intensive burning and significant PyC sink into soils soon after deglaciation. The major phase of woody biomass burning occurred during 7000–5000 cal BP and resulted in the abundant deposition of charcoal in topographic hollows. Soil macrocharcoal maintained a remarkably consistent concentration of total organic carbon (70.70 ± 2.96 %) over the millennial timescale. Its conservation was strengthened by burial in dry sandy soils soon after the fire. Raman spectra of the least weathered charcoal fragments of various ages indicate that intensive biomass burning occurred regularly in the Holocene. Capacity to form pyrogenic archives of Keiva II was predetermined by glacial-associated features like kettle topography and their postglacial modification. Rich PyC record of Keiva II highlights the significance of this landform for soil-based paleofire studies in eastern Fennoscandia.