Vegetation history and the impact of tephra deposition during 7000 years based on pollen and tephra analysis of a Barasantou Bog sediment core, eastern Hokkaido, northern Japan

摘要

To estimate the impact of a volcanic eruption on vegetation, we conducted a field survey and collected peaty sediments and made pollen and tephra analysis from Barasantou Bog, in eastern Hokkaido, northern Japan. Using an electron probe micro-analyzer, we identified five tephra layers in the core: Ma-g (7.6 cal ka BP), Ma-e (5.5 cal ka BP), Ma-d (4.0 cal ka BP), Ta-c (2.5 cal ka BP), and Ko-c2 (AD 1694). When the peat began to accumulate again after deposition of the Ma-g tephra, the groundwater level of this area was very low, and Alnus, Poaceae, Thalictrum, Artemisia, and ferns dominated. Gradually, the groundwater level increased, and the condition of this wetland approached the present conditions, about 5.0 to 4.5 cal ka BP. Pinaceae conifers increased 2.5 cal ka BP. It is presumed that Picea glehnii and Abiesspecies formed wetland forests. Myrica gale and Alnus increased again, and grew to the edge of the wetland. At the same time, Quercus subgen. Lepidobalanus decreased and Betula increased. This trend might be due to the influence of human activity. The wetland vegetation changed from Thalictrum and Artemisia to Sanguisorba, and then from Sanguisorba to Menyanthes after the deposition of Ma-g tephra or after 2.5 ka BP. It was thought that the wetland changed from low moor to middle or high moor. A tephra layer tens of cm thick is not expected to affect pollen composition, and may have no clear impact on the surrounding vegetation. Sediment with a thickness of a few cm requires a deposition time of approximately 10 years; pollen analysis results are on the scale of the average vegetation change during that period. We conclude that short-term vegetation changes were not detected.