Chronology of anthropedogenesis in the Omiya tableland, Japan, based on a 14C age profile of humic acid

Volcanic ash soils along the western edge of the Omiya tableland, Japan, are covered with thick anthropogenic soil horizons. The formation of anthropogenic soil horizons occurs because of the soil dressing practice known as Dorotsuke, where alluvial soil materials are deposited on fields and mixed with volcanic ash topsoil by tillage over the years. To clarify the chronology of this anthropedogenesis, carbon-14 (14C) age profiles were estimated using humic acid fractions from three pedons: an anthropogenic soil, an undressed Andosol, and a Fluvisol. Soil charcoal fragments were also dated to estimate maximum burial age. Charcoal fragments displayed vertically random age distributions, indicating that the fragments may have had multiple origins. However, the age of charcoal in the lower part of the anthropogenic soil horizons indicated that the initiation of anthropedogenesis occurred later than the late 13th century. The 14C age profile of humic acid in the Andosol exhibited little variation in age with depth in the subsoil. The 14C age profile of humic acid in the Fluvisol suggested that the humic acid fraction included allochthonous old carbon (C), although the soil itself had been formed from recent sediments. The 14C age profile of humic acid in the anthropogenic soil showed features of its two component soils. The 14C ages in the volcanic ash subsoil matched with those in the Andosol, whereas the ages increased in the anthropogenic soil horizons because of supplementation with old C from alluvial soil materials. However, the peak 14C ages occurred in the lower part of the anthropogenic horizons, whereas the middle part on the peak position displayed a gradual age-depth gradient. This feature was interpreted as a sign of 14C activity equilibrium throughout anthropedogenesis. On the basis of this postulated 14C activity equilibrium, the linear age-depth gradient at the peak position was derived from differences in burial time, and burial ages were calculated by estimating steady-state 14C. The calculated ages were lower than the charcoal ages. These age estimates suggest that anthropedogenesis was initiated in the Middle Ages and reached an intermediate stage before or during the first half of the Edo period.