[S-EM20] [JJ] 地磁気・古地磁気・岩石磁気

[SEM20-13] Exploration of pedogenic nanoscale particles causing magnetic enhancement in Chinese loess deposits

*佐野 拓郎¹、兵頭 政幸^1,2、松本 恵³、瀬戸 雄介¹ (1.神戸大学 理学研究科 惑星学専攻、2.神戸大学 内海域環境教育研究センター、3.神戸大学 研究基盤センター)

キーワード：中国レス、土壌化、磁気増大

Based on magnetic enhancecment in paleosols, magnetic susceptibility (χ) and its frequency dependence (χ_fd) are widely accepted as reliable proxies of paleorainfall, and thus often used for estimation of the East Asian Summer Monsoon intensity in the Chinese Loess Plateau (CLP). In spite of its paleoclimatic importance, pedogenic particles causing magnetic enhancement have never been detected. We challenged this problem, using three sets of loess and paleosol samples from Lingtai and Xifeng in the central CLP. Each bulk sample was divided into three subsamples with different detrital grain size ranges (D1: > 10 μm, D2: 10–1 μm, D3: < 1 μm), all of which were subjected to rock magnetic experiments including χ and χ_fd measurements, isothermal remanent magnetization (IRM) composition analyses and thermomagnetic analyses. The paleosol bulk samples show high χ values ranging from 116–177 × 10^-8 m³/kg, which are 4.0–6.1 times higher than loess bulk samples, and 49.6–66.2 % of the magnetic enhancement is contributed by grains in D2 subsamples. Besides, values of χ_fd are also increased by 4.0–5.7 times for all paleosol bulk samples, and the enhanced χ_fd is dominantly contributed by D2 subsamples (61.0–77.0 %). The rock magnetic experiments reveal that pedogenic particles causing magnetic enhancement are magnetite and maghemite. Based on these results, scanning electron microscope (SEM) and trans electron microscope (TEM) observations were conducted on magnetic extracts from D2 subsamples. As a result, we found nanoscale magnetite inclusions in weathered muscovite particles. These magnetite inclusions are secondarily produced through elution/oxidation of Fe compounds in muscovite during pedogenesis. The inclusions can explain the facts that the magnetic enhancement partly caused by superparamagnetic grains is mostly contributed by D2 size grains, and the main magnetic extracts are not detrital magnetite/maghemite grains but silicate minerals. Hence, we conclude the magnetite inclusions in weathered muscovite particles are the most likely candidate for pedogenic particles causing magnetic enhancement.