PALYNOLOGICAL INVESTIGATION OF THE MATTO SUBMERGED FOREST ON THE CONTINENTAL SHELF IN THE JAPAN SEA OFF THE TEDORI-GAWA ALLUVIAL FAN, THE HOKURIKU REGION, CENTRAL JAPAN
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Abstract:
The Matto submerged forest was found in spring of 1998 as stumps, roots and erect trunks from the continental shelf at depth between 20 and 30 m, and in 2 to 3 km off the recent marginal area of the Tedori-gawa alluvial fan at Hama-sogo and Kurabe, Matto City, the Hokuriku region, Central Japan. This discovery proves the development of ancient forest in the early Holocene. Main tree remnants of the forest are Alnus, Quercus (Lepidobalanus), Morus and Fraxinus, the most commonly preserved genera. These trees had grown on the now submerged seaward margin of the old Tedori-gawa alluvial fan. An age of the submerged forest determined by the C-14 method is about 10 000 to 8 000 years B.P. According to pollen analyses, such marshy plants as Alnus, Salix, Cryptomeria, Gramineae and Nymphaceae are abundant, Lepidobalanus, Castanea and Fraxinus are common, and Fagus cf. crenata and Pinus are rare. Diatom remains give some indication of fresh water such as a bog and/ or small lake at the age of the forest development. Therefore, it is inferred that the forest had grown in back marshy area on the margin of the old Tedori-gawa alluvial fan. On the basis of tree composition in the submerged forest, it is estimated that Alnus japonica community of Sasa-Fagetum crenate association in the cool temperate climatic zone was fundamentally widely distributed, and an annual mean temperature at that age was a few degrees in centigrade lower than that of the Recent. The submerged forest shows the rising of sea-level coinciding with the warming of climate during the early Postglacial epoch. Remnants of an ancient forest were probably overwhelmed by a rise of the water table caused by a rise of sea-level, and preserved in the overlying flood debris derived from the upper stream of the ancient River Tedori. The present exhumation of the remnants is the result of a submarine erosion by the present coastal current along the coast of Japan Sea. This and the Nyuzen submerged forests are the only two existing submerged forests on the continental shelf throughout the world, where was once land area.
