Recent history of sediment dynamics in Lake Toro and applicability of <SUP>210</SUP>Pb dating in a highly disturbed catchment in northern Japan

초록

Sediment dynamics for the last 300 years in Lake Toro and the applicability of Pb-210 dating (CIC and CRS models) are evaluated. The lake sedimentation and sediment yield were determined by Cs-137 (in 1963) and two tephra layers (Ko-c2 in 1694 and Ta-a in 1739). The average sediment yields for initial development periods of catchments were 13.0-14.1 t km(-2) yr(-1) until 1963 and increased to 30.5 t km(-2) yr(-1) after 1963 because of the conversion of floodplain and upland forests to cultivated lands. In particular, the western zone of the lake near the conjunction with the Kushiro River had a high sedimentation rate, which is attributable to sediment inflow back from the Kushiro River during floods. The CIC model was not applicable because of fluctuations in the Pb-210 profiles since 1963. The CRS model agrees with the Cs-137 dating in all sites except for one site close to the Kushiro River inflow. Pb-210 dating may contain considerable errors where the Pb-210 concentration profile and flux are perturbed by floodwater from the Kushiro River, which contains low levels of Pb-210. (C) 2009 Elsevier B.V. All rights reserved.

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