Peat-core analysis for tracing the paleoclimate-related changes in aeolian dust deposition in Japan

Aeolian dust coming from arid regions of inland China supplies mineral resources to Japanese Islands. While annual deposition rate is as small as 5~10 g m^-2 yr^-1, total amount of deposition since the last glacial period can form soil layers with several meters depths. We focused on the mineral fertilization effect of aeolian dust on terrestrial ecosystems in Japan. The deposition and incorporation of dust-derived micaceous mineral into soil through time is our special interest because this mineral has an exceptionally higher ability to retain radiocesium than the others, thereby contributing to reduce soil-to-plant transfer of radiocesium after the Fukushima-Daiichi nuclear power Plant accident. However, little study has focused on aeolian dust as a controlling factor of radiocesium dynamics in soils. In this study, peat-core samples collected from the Kamiyoshi (KMY) basin in Kyoto, Japan was used to obtain the aeolian dust that had been deposited on land during the last glacial period. The KMY core with depth from 9.40 to 8.20 m (corresponding to 75 ka to 68 ka) was horizontally divided into 21 portions. The subdivided samples were treated with H₂O₂ to decompose organic matters and then size-fractionated to obtain <20 μm particles, i.e., typical size range of the aeolian dust. The <20 μm particles was used to determine mineral composition by X-ray diffraction and selective dissolution analysis. Furthermore, fine-quartz obtained after the selective dissolution analysis was used for oxygen isotopic analysis to determine δ¹⁸O value. We found that the <20 μm particles occupy more than 90% of total mineral fraction, composed mainly of quartz and micaceous mineral. The fine-quartz in this fraction showed δ¹⁸O values of 16.8±0.3‰, quite similar to those of loess or aeolian dust previously reported. All these properties strongly indicate the origin of the <20 μm particles is the aeolian dust. Interestingly, the aeolian dust flux between 68 ka and 75 ka ranged widely from 1.9 g m^-2 yr^-1 to 15.7 g m^-2 yr^-1 and highly associated with paleoclimate estimated by the pollen record. Namely, the higher aeolian dust flux was observed at colder and drier paleoclimate as indicated by the higher proportion of pinaceous conifer pollen. We conclude that associative analysis of mineral and pollen record in the peat-core can be a promising method to trace the long term deposition pattern of aeolian dust in terrestrial ecosystems in Japan.