Processes of Hillslope Denudation and Environmental Changes During the Holocene Around Sendai, Northeast Japan

Abstract

In this paper we discuss processes of slope denudation during the Holocene in the hilly area near Sendai.
We gave it our general consideration of the history of local geomorphic environments and hillslope development based on the analyses of sediments located in isolated depressions, including pollen analyses, radiocarbon datings and plant communitical data. The main results are as follows.
1. Three periods of hillslope instability are identified: 1) before 9, 000y.B.P., 2) between 6, 000 and 2, 500y.B.P., with a prominent period between 6, 000 and 5, 000y.B.P., and 3) after 1, 000y.B.P. The unstable period between 6, 000 and 5, 000y.B.P. is contemporaneous with the Hypsithermal epoch (Fig. 4).
2. It is not yet certain that the unstable period after 1, 000y.B.P. can be wholly attributed to soil erosion caused by human impacts on natural vegetation.
3. Palynological analysis is useful for the clarification of slope conditions at various in time, for increases and decreases of pollen grains of certain plants which reflect geomorphological conditions, (especially Zelkova, Ulmus, Carpinus, Fraxinus and Fagus), enable one to determine micro-topographical changes on hillslopes.
4. Different kind of denudation process on hillslope, i.e. soil creep on crest slope in the stable period (Fig. 7a) and small surface failure of slump type on side slope in the unstable period (Fig. 7b), has characterized present hillslope morphology.