[MIS20-P13] A finding of fossil trees buried in dammed-lake deposits formed by the Dondokosawa rock avalanche of the late 9th century in the Akaishi Mountains, and its significance
Keywords:Akaishi Mountains, rock avalanche , dammed-lake, fossil trees, paleoenvironment
The sampling sites were named LLD and ULD, respectively. LLD is located at the confluence of the Dondokosawa and Ohdanasawa rivers, and has a drainage area of 8.81 km2 with altitudes ranging from 1220 to 2780 m a.s.l. ULD is located at the outlet of the right tributary of the Dondokosawa River, and has a drainage area of 1.17 km2 with altitudes ranging from 1470 to 2630 m a.s.l. We collected 41 samples (24 samples in LLD and 17 samples in ULD) between April 2012 and October 2017 and identified species or genus of each sample. The 24 samples in LLD include eight of Hinoki cypress (Chamaecyparis obtusa), seven of Hemlock tree (Tsuga), three of Sawara cypress (Chamaecyparis pisifera), two of Birch tree (Betula), and one each of Japanese larch (Larix kaempferi), Fir tree (Abies), Spruce tree (Picea) and Japanese Wingnut (Pterocarya rhoifolia). The other 17 samples in ULD include eight of Tsuga, four of Abies, two each of L. kaempferi and Willow tree (Salix), and one of Azalea tree (Rhododendron). The rates of Tsuga in LLD and ULD account for approx. 29% and 47%, respectively. Besides, the rate of cypress species (C. obtusa and C. pisifera) in LLD accounts for approx. 46%, while both of the two species are absent in ULD.
Although the above results are based on a limited number of samples, it is estimated as vegetation characteristics in the late 9th century, when the DRA occurred, as follows. In the Ohdanasawa River catchment, an old-growth forest dominated by Hinoki cypresses of >350 years old and Hemlock trees was established, while another old-growth forest dominated by Hemlock trees of >300 years old was established in the right tributary catchment of the Dondokosawa River. The remarkable difference in the abundance of cypress species in the two adjacent catchments suggests a possibility that the distribution of the two species has been limited within the Ohdanasawa River catchment and has not extended to the upper part of the Dondokozawa River from that time.
References:Kariya (2012) Trans. Jpn. Geomorphol. Union 33, 297-313 [in Japanese with English abstract]; Kimura et al. (2018a) J. Jpn. Landslide Soc. 55, 42-52 [in Japanese with English abstract]; Kimura et al. (2018b) JpGU2018 MGI25-P09 [in Japanese with English abstract]; Kimura et al. (2019) Quat. Res. 58, 65-72 [in Japanese]; Yamada et al. (2018) Quat. Geochr. 44, 47-54.