Genesis of Banded Sulfur Sediments at the Jigokudani Valley, Tateyama Volcano, Japan

Abstract

Banded sulfur sediments, characterized by beautiful alternation of yellowish-white layers of fine-grained sulfur and light to dark gray clay layers, occur in the Jigokudani valley of Tateyama volcano, Japan. The sediments were deposited on the bottom of a now extinct crater lake that had been produced by phreatic explosions of the Tateyama volcano in Holocene age. Sulfur isotopic analyses imply that hydrogen sulfide of volcanic gas origin was oxidized to form colloidal sulfur in the presence of dissolved oxygen. Variation in contents of fossilized fresh water diatoms (dominated mostly by benthic species Pinnularia braunii (GRUN.) CL. var. amphicephala (A. MAYER) HUST.) and of native sulfur in the sediments suggests that lamination was caused by seasonally repetitive precipitation of the colloidal sulfur plus diatoms during circulating-oxidative periods of the lake and that of detrital materials during stagnant-reducing periods. Stratigraphic variation in the sulfur and diatom contents of the sediments may be interpreted to reflect changes in fumarolic activity and environmental conditions of the lake for a longer time scale. A sedimentation period of 3000 to 4000 years is calculated from the total thickness of the banded sulfur sediments coupled with sedimentation rates deduced from the above model. This period is in good harmony with the age of phreatic explosions (3000 to 6000 years) estimated by tephra chronology.