Holocene fire history of black colluvial soils revealed by pyrolysis-GC/MS: a case study from Campo Lameiro (NW Spain)

Abstract

The colluvial soils of Galicia (NW Spain) are records of Holocene environmental change. In spite of the omnipresence of charcoal fragments in these soils, the effect of fires on the development of the Galician landscape and the potential role of past societies are poorly understood. We isolated macroscopic charcoal (>2 mm) and NaOH-extractable soil organic matter (SOM) from two Galician soils. Molecular characterisation of the extractable SOM using pyrolysis-GC/MS showed that fire residues (black carbon; BC) were abundant in the extractable SOM, even in the horizons containing few macroscopic charcoals. This indicates that the macroscopic charcoal record often used in geoarchaeological studies gives an incomplete image of the fire history. Polycyclic aromatic hydrocarbons and other presumably BC-derived pyrolysis products in samples from 8500 to 7000 cal years BP are evidence of Mesolithic fires. Molecular indicators of BC are abundant also in the Neolithic ( c. 6000 BP), but the degree of thermal modification to the organic matter decreases, which could mirror a change in fuel type. This change coincides with the beginning of accelerated soil erosion in the area, and is followed by accumulation of relatively homogeneous BC-rich material up to at least the beginning of the Iron Age. This pattern is interpreted as deforestation around 6000 BP followed by periodic and presumably human-induced burning of poorly-developed vegetation (slash-and-burn). The open landscape of present-day Galicia would therefore be the outcome of prolonged anthropogenic burning, analogous to the open cultural landscapes of central Europe.