電子スピン共鳴(ESR)による中国・韓国・日本の風成塵起源石英の同定 Identification of Eolian Dust Quartz in China, Korea and Japan by Electron Spin Resonance
風成塵の給源地を明らかにするために,電子スピン共鳴(Electron Spin Resonance, ESR)による微細石英(≤20μm)の酸素空格子信号量を測定した.分析試料は中国黄土高原～北京の馬蘭黄土,韓国の低位段丘上の土壌,山地斜面土壌,USAのPeoriaレス,日本の福井県黒田のボーリングコア,網野・鳥取両砂丘地の古土壌で,いずれも最終氷期の風成塵堆積層と完新世土壌から採取したものである.分析結果は,馬蘭黄土の酸素空格子信号量(任意単位)が5.8～8.3であり,韓国の最終氷期・完新世両土壌が6.0～7.7であった.両国の先カンブリア紀基盤岩の赤色風化土壌は11.2～12.4であり,カナダの先カンブリア紀岩石分布地域に由来するisotope stage2のPeoriaレスの分析値も11.0～14.0であった.黒田低地では,最終氷期に古生層山地から流水によって運ばれて堆積した>63μm石英は3.6～4.0であり,同じく20～63μm石英,≤20μm石英も3.3～4.7であった.いっぽう,広域風成塵起源の微細石英(≤20μm)は5.8～8.5であり,中国黄土および韓国土壌の数値と一致した.網野・鳥取両砂丘地の古土壌中の微細石英は3.7～4.8と低く,アジア大陸起源だけではなく,最終氷期に陸化した海底からの風成塵が多くを占めるためと考えられた.鳥取砂丘地のisotope stage 4に相当する層準の微細石英には,中国黄土の数値域に属する5.8を示すものや,AT上の古土壌層のように1.9と低い数値を示すものがある.それはisotope stage 4が風成塵堆積量の多い時期にあたっており,同層準中に大陸起源の広域風成塵が多く混入したためであり,逆に数値が低いのは大山新期火山灰起源の石英が多量に混入したためと考えた.
Electron spin resonance (ESR) spectra of quartz grains less than 20μm have been measured to identify the source areas of eolian dust. Thirty-seven samples of loesses, soils, paleosols and bore cores were measured: five Upper Malan loess from China, two loessderived soils on the lower terraces and one Holocene soil from Korea, nine Peoria loesses from the USA, two red soils on weathered Precambrian rocks from China and Korea, and twelve samples from Kurota bore-cores and six paleosols from the Tottori and Amino coastal sand dune fields along the Japan Sea coast in Japan. Upper Malan loess, Korean loessderived soils and Peoria loess were deposited during isotope stage 2. Kurota bore cores and the paleosols of Tottori and Amino were correlated with the last Glacial age.<br>Chinese loess quartz showed measurement values ranging from 5.8 to 8.3 (in arbitrary units), and red soil on the weathered Precambrian rock was 12.4. Korean soil quartz showed values ranging from 6.0 to 7.7, and red soil was 11.2. Peoria loess quartz originating in the Canadian Precambrian Shield showed values of ranging from 11.0 to 14.0. Kurota eolian dust quartz showed values of ranging from 5.8 to 8.5. Kurota fluvial quartz showed a range from 3.3 to 4.7 originating in the Paleosoic rocks of the surrounding area. Eolian dust quartz contained within paleosols at Tottori and Amino showed lower values, ranging from 3.7 to 4.8. Paleosol quartz of isotope stage 4 horizon in Tottori was 5.9, coinciding with the values of Chinese loess quartz. These values indicate that eolian dust from the Asian continental areas was deposited in large quantities into the paleosols during isotope stage 4 as well as stage 2. Paleosol quartz mixed with much of the Daisen tephra quartz showd the low value of 1.9.<br>To Summarize the results of the experiment: Precambrian quartzes from three countries have a similar range of intense signals, from 11.0 to 14.0, while fluvial quartzes originating in the surrounding Paleosoic area in Japan vary in the range from 3.3 to 4.7. The quartzes of Chinese loess and Korean soils are remarkably similar to ranges of ESR intense signal. This similarity is attributed to their common eolian origin in the arid and semi-arid Asian continental areas during isotope stage 2. Japanese eolian dust quartz varies over a wide range, from 3.7 to 8.5. It is concluded that the origins of one group of Japanese eolian dust, with ESR signal intensity ranging from 5.8 to 8.5, may be the Asian and and semi-arid areas such as the Tibetan Plateau, Taklamakan, and the Gobi desert. Another group, having ESR signal intensity ranging from 3.7 to 4.8, such as paleosols in the dune fields, may come from the dried sea floor during the last Glacial age.<br>Eolian dust quartz isolated from loess, soils, paleosols, and bore cores in East Asia is remarkably uniform in ESR signal intensity, and is a useful tracer for the provenance of the quartz.
第四紀研究 35(1), 25-34, 1996-02-29
Japan Association for Quaternary Research