姉川上流域におけるヒ素の堆積過程と起源 : 上位段丘面の湖成層の化学分析

Lacustrine sediments are deposited on two terraces of the upstream in Ane River, central Japan. The depositional ages are estimated as ca. 5.0 ka for the lower terrace (younger lacustrine sediment) and ca. 35 ka for the upper terrace (older lacustrine sediment). These sediments are characterized by high concentrations of arsenic (As) more than the average of continental crust (2 μg/g). In this study, we analyzed chemical and stable isotopic compositions of the older lacustrine sediment samples to elucidate the sedimentary process and origin of As. The As content of older lacustrine sediment is 33.2 ± 10.9 μgig, which is 0.4 times lower than that of younger lacustrine sediment (76.7 ± 27.3 μgig). There is no difference in grain sizes between older and younger lacustrine sediments (11.9 ± 2.9 μm and 10.5 ± 6.8 μm), indicating no dilution effect of the As contents by the sedimentation rates. On the other hand, the As contents of the older and younger lacustrine sediments show weak positive correlations with the TOC contents (R=0.43-0.72). In addition, the TOC contents of older lacustrine sediment are 0.5 times lower than those of younger lacustrine sediment because the older and younger lacustrine sediments were deposited during the late Pleistocene cold and the mid-Holocene warm period, respectively. The C/N ratio and σ13CTOC values of two lacustrine sediments indicate that the organic matter is manly originated from land-derived C3 plants. These results suggest that the difference in the As contents of two sediments was caused by river inflow of the organic matter into lake. We consider that enhanced decomposition of organic matter after deposition promote reduction of As in the sediments, followed by diffusion and then reprecipitation of As sulfide. The inverse correlation between the As contents and σ34STS of the older lacustrine sediments suggests that the arsenic sulfide dissolution increased with enhanced weathering of the Mesozoic sedimentary rocks in the catchment.