Paleoenvironment inferred from a sediment core of Lake Shirakoma

NOZUE Yasuhiro, FUKUSHIMA Kazuo, KUMON Fujio

doi:10.3739/rikusui.66.81

This study attempts to reconstruct the paleoenvironment of Lake Shirakoma, an oligotrophic-dystrophic lake located at high altitude (2115 m) in central Japan, based on the chemical analyses of a sediment core. A 94 cm core was taken in the center of L. Shirakoma at a water depth of 8m. The radiocarbon dating indicates that the average sedimentation rate is 0.21 mm y^-1. According to the observation of sediment facies, 0-47 cm depth is regarded as organic-rich mud, 47-70 cm as peat mud and below 70 cm as peat. The bottom peat layers containing much plant debris indicates that the former stage of this lake should be wetland, which was far shallower than the present state. The fact means that L. Shirakoma was formed 2200 years ago from a wetland. In the transition stage from a wetland to a lake, the water depth might be shallow and the in-lake productivity in water might be high, because of lower C/N atomic ratio and higher 4α, 23, 24-trimethyl-5α-cholest-22-en-3β-ol concentration at the layers. On the other hand, there are a few molecules indicative of land-plant inputs during the past 2200 years. Dehydroabietane and 18-norabieta-8, 11, 13-triene are detected both in an extant inflow stream and in the sediment from L. Shirakoma. Those abietane compounds may provide a tool to estimate the paleovegetation in the catchments.

Paleoenvironment inferred from a sediment core of Lake Shirakoma

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