5:15 PM - 6:45 PM
[MIS12-P15] Yanguardian Drias Period in the Northern Tohoku Region
Tashiro Moor on Mt. Hakkouda
Keywords:Younger Dryas, Pollen analysis
1. Research objectives
It has been reported that the Younger Dryas in Japan is not significantly colder than that in Europe and North America. These studies have not yet fully elucidated the geomorphology of the area, since some of them have not been analyzed with sufficient temporal resolution and the number of sites is not large. This study analyzes pollen at a short time resolution of about 20 to 30 years and examines the climate of the northern Tohoku region at that time.
2. Research Methods
Peat samples (300-390 cm depth) collected from the Tashiro Moor (40°41' 52" N, 140°55' 11") in the Hakkouda Mountains, Aomori Prefecture, were used in this study. The age of the analyzed strata was determined from tephra identified by Kito et al. (2017) and three AMS 14C dating requests.
3. Results and Discussion
The pollen of tall trees was divided into three pollen zones, TS7-I through TS7-III. The analytical stratigraphic levels were approximately 13,550 ~11,000 cal BP. The lowest age zone, TS7-I, is underlain by the Towada-Hachinohe pyroclastic flow deposit (15,000 years ago).
The slight increase of spruce and fir species in zone TS7-II is thought to correspond to the colder climate during Younger Dryas. However, subgenus Quercus decreased by only about 10% from the previous zone and remained stable at around 14%, suggesting that the cooler climate was not as pronounced as around the Tashiro Moor, where cool-temperate trees could grow at lower elevations in the mountain zone. The decrease in Betula and increase in Quercus subgenus from the central to upper part of zone TS7-II suggests that the transition from the Betula ermanii zone to a cool-temperate forest began, as the climate became intermediate between the cold and cool-temperate zones at lower elevations in the mountains than Tashiro Moor due to the weakening of the cold climate. The high rate of Myrica pollen and the high rate of Ilex pollen in the lower part of zone TS7-III suggest that the marshland was dry in the lower part of zone TS7-III immediately after the onset of warming at the Pleistocene/Holocene boundary from the Younger Dryas to the Holocene. Tashiro Moor is considered to have become wetter due to the increase of the Sphagnum and the decrease of Myrica in the upper TS7-III. This suggests that the wetting of the marshland occurred later than the warming at the beginning of the Holocene.
Based on the tephra stratigraphy, carbon dates, and pollen analysis, TS7-I (13,550~12,300 cal BP) is the Bølling-Allerød, TS7-II (12,300~11,300 cal BP) is Younger Dryas, and TS7-III (11,300~11,000 cal BP) is Pleistocene. TS7-III (11,300~11,000 cal BP) is considered to correspond to warming at the Pleistocene/Holocene boundary. The number of radiocarbon dates obtained in this study is small(three), and the validity of the dates has not been fully discussed; therefore, it is possible that the measured ages are generally shifted to younger.
During the Last Glacial Maximum, the climate was such that cold-temperate coniferous forests were established in Lake Nojiri and Ayame Moor, and cold-temperate mixed coniferous forests were established in Lake Suigetsu. The stable production and increase of cool-temperate trees in Lake Nojiri and Ayame Moor during the Younger Dryas suggest that cool-temperate deciduous broad-leaved forests were established in Lake Suigetsu and that the Younger Dryas was not a reversion to the LGM in Lake Tashiro Moor as well as in Lake Suigetsu, Lake Nojiri and Ayame Moor, and that the coldening of Lake Tashiro Moor was not a reversion to the LGM, but a moderate cooling.
The effect of cold weather during the period varied from region to region. Although it is thought that the cold weather weakened during the Younger Dryas period in Tashiro Moor and Ayame Moor, the mean annual temperature reconstructed at Suigetsu Lake declined throughout the Younger Dryas period, indicating no such weakening of the cold weather.
It has been reported that the Younger Dryas in Japan is not significantly colder than that in Europe and North America. These studies have not yet fully elucidated the geomorphology of the area, since some of them have not been analyzed with sufficient temporal resolution and the number of sites is not large. This study analyzes pollen at a short time resolution of about 20 to 30 years and examines the climate of the northern Tohoku region at that time.
2. Research Methods
Peat samples (300-390 cm depth) collected from the Tashiro Moor (40°41' 52" N, 140°55' 11") in the Hakkouda Mountains, Aomori Prefecture, were used in this study. The age of the analyzed strata was determined from tephra identified by Kito et al. (2017) and three AMS 14C dating requests.
3. Results and Discussion
The pollen of tall trees was divided into three pollen zones, TS7-I through TS7-III. The analytical stratigraphic levels were approximately 13,550 ~11,000 cal BP. The lowest age zone, TS7-I, is underlain by the Towada-Hachinohe pyroclastic flow deposit (15,000 years ago).
The slight increase of spruce and fir species in zone TS7-II is thought to correspond to the colder climate during Younger Dryas. However, subgenus Quercus decreased by only about 10% from the previous zone and remained stable at around 14%, suggesting that the cooler climate was not as pronounced as around the Tashiro Moor, where cool-temperate trees could grow at lower elevations in the mountain zone. The decrease in Betula and increase in Quercus subgenus from the central to upper part of zone TS7-II suggests that the transition from the Betula ermanii zone to a cool-temperate forest began, as the climate became intermediate between the cold and cool-temperate zones at lower elevations in the mountains than Tashiro Moor due to the weakening of the cold climate. The high rate of Myrica pollen and the high rate of Ilex pollen in the lower part of zone TS7-III suggest that the marshland was dry in the lower part of zone TS7-III immediately after the onset of warming at the Pleistocene/Holocene boundary from the Younger Dryas to the Holocene. Tashiro Moor is considered to have become wetter due to the increase of the Sphagnum and the decrease of Myrica in the upper TS7-III. This suggests that the wetting of the marshland occurred later than the warming at the beginning of the Holocene.
Based on the tephra stratigraphy, carbon dates, and pollen analysis, TS7-I (13,550~12,300 cal BP) is the Bølling-Allerød, TS7-II (12,300~11,300 cal BP) is Younger Dryas, and TS7-III (11,300~11,000 cal BP) is Pleistocene. TS7-III (11,300~11,000 cal BP) is considered to correspond to warming at the Pleistocene/Holocene boundary. The number of radiocarbon dates obtained in this study is small(three), and the validity of the dates has not been fully discussed; therefore, it is possible that the measured ages are generally shifted to younger.
During the Last Glacial Maximum, the climate was such that cold-temperate coniferous forests were established in Lake Nojiri and Ayame Moor, and cold-temperate mixed coniferous forests were established in Lake Suigetsu. The stable production and increase of cool-temperate trees in Lake Nojiri and Ayame Moor during the Younger Dryas suggest that cool-temperate deciduous broad-leaved forests were established in Lake Suigetsu and that the Younger Dryas was not a reversion to the LGM in Lake Tashiro Moor as well as in Lake Suigetsu, Lake Nojiri and Ayame Moor, and that the coldening of Lake Tashiro Moor was not a reversion to the LGM, but a moderate cooling.
The effect of cold weather during the period varied from region to region. Although it is thought that the cold weather weakened during the Younger Dryas period in Tashiro Moor and Ayame Moor, the mean annual temperature reconstructed at Suigetsu Lake declined throughout the Younger Dryas period, indicating no such weakening of the cold weather.