Sulfur accumulation in Melanudands during development by upbuilding pedogenesis since 14-15 cal. ka

- We investigated distribution of S in Melanudands formed by upbuilding pedogenesis.
- S K-edge XANES spectra showed that the predominant S species is highly oxidized.
- Reduced/intermediate S appeared episodically in the profiles.
- Ester sulfate-S might be Al-associated forms in the Melanudands.
- Organic S compounds might be decayed as the topdown pedogenic processes in the soils.

To understand the contribution of upbuilding pedogenesis to the accumulation of sulfur (S) in Melanudands, we investigated the vertical distribution of S species using K-edge X-ray absorption near-edge structure (XANES) spectroscopy of S species and vertical variation of δ34S values of total S in two Japanese Melanudands developed above tephra Nt-S, dated at 14-15 cal. ka. These soils have been developed by upbuilding pedogenesis, in which episodic accumulations of volcanic ash and eolian dust transported from the Asian continent are added incrementally to the soil surface while topdown pedogenesis occurs concomitantly forming soil horizons. The oxidation states of S compounds determined by XANES analysis varied widely in the soil profiles. The predominant S species was highly oxidized S in ester sulfates and inorganic sulfates (+ 6 oxidation state). Proportions of S with reduced and intermediate oxidation states increased episodically to as much as 66%. Ester sulfate-S was found in the soil horizons with estimated ages less than 10 cal ka. A close correlation was found between ester sulfate-S concentrations and Al-associated organic S concentrations (p < 0.05); indeed, the concentrations themselves were roughly similar. The δ34S values of total S increased with depth in the two soil profiles. These results suggest that episodic deposits of volcanic ash and eolian dust trapped detritus and humus beneath them during the buildup of the soil surface. The organic S compounds in this material might be decayed as topdown pedogenic processes in the soils. Production of ester sulfate-S may be related to the warm and humid climate of the Holocene, which may have accelerated microbial activity responsible for transformation of reduced to highly oxidized S. Ester sulfate-S might have become Al-associated forms and accumulated in the soils due to their high pedogenic mineral contents.