ABSTRACT

Ten core samples and 78 dredge samples from Lake Mashu, a remote oligotrophic caldera lake in Hokkaido, Japan, were analyzed to reconstruct the history of hot spring activities on the lake bottom. The sedimentation rate was estimated using common tephra layers found both in Lake Mashu and in adjacent Lake Kussharo. From the depositional age of the upper common tephra (250 B.P.) obtained by ²¹⁰Pb dating of the Lake Kussharo core, the sedimentation rate of Lake Mashu was calculated to range 6.4 to 7.4 mg/cm² y. Iron and Mn that originated from hot springs on the lake bottom are oxidized in the oxic lake water to form Fe and Mn-rich layers. Iron is accumulated in the central parts of basins, whereas Mn is accumulated in relatively peripheral areas. The horizontal separation of these elements in the sediment is attributed to the difference in oxidation rate. Manganese content in pore water increases with depth at a site of Mn accumulation. This indicates that Mn is mobilized and deposited forming Mn concentrated layer just above Fe concentrated layer. X-Ray photoelectron spectroscopic examination revealed that Fe was in the form of ferric oxyhydroxide, and that Fe and Mn were concentrated on the surface of sediment particles. Annual depositional rates of Fe and Mn correspond approximately to the reported rates of supply from lake-bottom springs. Therefore Fe and Mn contents in the sediment indicate the variations in hot spring activity at the lake bottom. This activity has fluctuated in intensity over the last 700 years, but now appears to be diminishing.