Are Holocene wood-charcoal fragments stratified in alpine and subalpine soils? Evidence from the Alps based on AMS 14C dates

The possible stratification of charcoal particles buried in well-drained soil in the subalpine and alpine belts in an inner valley of the northwestern Alps (Savoy, France) is examined. Radiocarbon dating and quantification of charcoal fragments more than 400 μm in diameter were used to analyse the soil-depth distribution of charred wood particles between 1700 and 2600 m a.s.l. in two study areas 10 km apart in the same valley. Above 2100 m a.s.l. in both study areas, charcoal concentrations are low and mainly contained in the upper horizons, and all dated charcoal fragments are older than 2300 cal. yr BP. Below 2100 m, the greatest charcoal concentration is located in the topmost 30 cm of the soil profiles, and the ¹⁴C dates yield late-Holocene ages (1600 to 75 cal. yr BP). When profiles have an elevated charcoal concentration in deeper horizons the charcoal dates are mid-Holocene (6360–2770 cal. yr BP). Radiocarbon dates of charcoal fragments do not reveal an age/depth relationship, although charcoal mass concentrations suggest a coarse stratification of charcoal particles in the soils below 2100 m at forested sites. This difference may be due to the charcoal dating on a single fragment which is not representative of the charcoal assemblage containing a few tens to thousands of fragments. The lack of stratification probably results from soil bioturbation by soil fauna, soil reworking by uprooted trees, and freeze-thaw processes. At higher elevation in the alpine belt, charcoal is concentrated in the topmost soil due to a low bioturbation rate. From a pedological point of view, the ¹⁴C dating and the quantification of charcoal concentration per soil level provides a time range estimate for soil-particle reworking by anecic earthworms, dung beetles and other biota. This has application in pedogenesis studies to estimate the rapidity of processes of soils dynamics. These data suggest that soils in the Alps have limited potential for palaeoecological studies based on a stratigraphic assumption but have more applications in precise spatial studies.