Using 14C dating of stable humin fractions to assess upbuilding pedogenesis of a buried Holocene humic soil horizon, Towada volcano, Japan

The genesis of a 40-cm-thick buried Holocene humic soil horizon in a 6-m-high sequence near Towada volcano, Japan, was examined by dating stable, plant-derived humin particles by AMS-based radiocarbon (14C), and by analyzing stable carbon isotope ratios (δ13C), phytolith composition, and soil chemical properties. The AMS dating yielded calibrated ages ranging from 7570–7679 to 6180−6313 cal yr BP. We obtained a strong correlation (r = 0.99, ***p < 0.001) between sample depth and the mean age. The (now buried) humic soil was formed by the incremental accumulation of tephric loess and aeolian dust at a constant rate of ~ 30 mm per century for at least 1200 years. Whilst the soil surface was slowly rising, graminaceous vegetation was supplied at a constant rate so that developmental upbuilding pedogenesis ensued. The soil was buried and isolated by the fall of the Towada-Chuseri tephra about 6200 cal yr BP. Thus the tephra-soil sequence near Towada volcano represents both developmental and retardant upbuilding pedogenesis.

Research Highlights
► A strong correlation (r = 0.99, ***p < 0.001) occurs between depth and 14C ages of humin fractions in a buried Holocene humic soil horizon.
► Graminaceous plant residues were supplied constantly for 1,200 years whilst inorganic materials accumulated at ~ 30 mm per century.
► The humic soil horizon reflects developmental upbuilding pedogenesis.