Arsenic speciation in varved sediments of landslide-dammed paleolake along Ane River

Abstract

Landslide-dammed paleolake existed along upstream of Ane River, eastern Shiga Prefecture, which was formed by a collapse in Mt. Ibuki before about 5000 years ago. The lamination of the sediments comprises annually deposited structures (known as a varve). The µ-XRF mapping with a Horiba XGT-5000 X-ray analytical microscope shows that arsenic (As) is concentrated along the laminations. The sequential extractions indicate that amorphous sulfide phase occupies the second amount of As in the sediments (16%) after silicate phase (69%). In addition, synchrotron µ-XRF mapping and XANES analyses were investigated for As distribution and speciation in the laminations. As a result, on the one hand, a siderite (FeCO₃) lamina in summer is sandwiched by As₂S₃(III) rich layers (about 1 mm thick), suggesting the Fe(III) oxyhydroxide adsorbs arsenite (H₂AsO₃^-(III)) and arsenate (H₂AsO₄^-(V)) in water column during a vertical mixing in spring and autumn, followed by the deposition to lake bottom and then the sulfate reduction to As₂S₃(III) in the pore water. On the other hand, coarse particles (about 0.3 mm diameter) with AsS(II) and As₂S₃(III) are dispersed in the sediments. Their coexistence suggests that AsS(II) resulted from partial reduction of As₂S₃(III) by biogenic H₂S.