Keywords:peat, cellulose, oxygen isotopes
Hokkaido Island is located at the northern margin where the East Asian monsoon influences on climate. Stronger summer monsoon derives a moist air mass from the Pacific Ocean to Hokkaido Island. In this study, we applied the δ18O values of sphagnum and plant cellulose in a 4-m long peat core from the Bekanbeushi high mire to the reconstruction of the East Asian summer monsoon. The δ18O values of sphagnum were always lower than those of higher plants, i.e., Vaccinium oxycoccos and Calamagrostis neglecta var. aculeolata. While the δ18O values of sphagnum reflects more directly those of precipitated water, those of higher plants should be enriched by transpiration. The difference of sphagnum and higher plants, thus, is a potential proxy of relative humidity. Temporal variation in the δ18O of sphagnum indicated that the value was lower around 1500 years ago and higher around 1100 years ago, corresponding to Dark Age Cold Period and Medieval Warm Period, respectively. This suggests that the summer monsoon rainfall was lower around 1500 years ago and higher around 1100 years ago. The difference of the δ18O values between plant tisuues and sphagnum has a negative correlation with the δ18O values of sphagnum. This suggests that relative humidity was higher in the periods when summer rainfall was more active, which is analogous to a long summer rainfall condition cased by the Baiu front activity in Hokkaido, “Ezo-tsuyu”. Ezo-tsuyu is a typical phenomenon when the East Asian summer monsoon strengthenes. We, thus, suggest that a warm and moist climate around 1100 years ago reflects frequent Ezo-tsuyu summers caused by stronger East Asian summer monsoon activity.