5:15 PM - 7:15 PM
[BPT03-P01] Temporal variation of lake environment over the past 24,000 years in the Lake Suwa, central Japan, based on diatom assemblages
Keywords:Lake Suwa, Diatom, Holocene, younger dryas event
The past 20,000 years since the conclusion of the most recent glacial period have been predominantly marked by a warming trend, subsequently followed by a period of climatic stability. However, documented evidence indicates the occurrence of abrupt cold events, such as the Younger Dryas and the 8.2 ka event. The occurrence of these cold events can be attributed to a series of atmospheric processes. The North Atlantic experienced a warming, leading to an enhancement of the Siberian High. Concurrently, the Intertropical Convergence Zone (ITCZ) underwent a southward migration, while the Asian summer monsoon exhibited signs of weakening. This shift in weather patterns resulted in a decline in precipitation and an increase in regional aridity. However, the factors driving these changes are still unknown in many respects, and there are only a few cases where records of these changes have been detected in Japan. Therefore, it is necessary to accumulate information on climate change in more regions and to clarify the spatiotemporal changes in order to elucidate these climate changes and their systems. The Suwa Basin is distinguished by substantial temperature variations, an extensive catchment area, and relatively high concentration. These characteristics suggest that the lake sediments of Lake Suwa document the climatic changes in the surrounding region with high temporal resolution.
The diatom assemblages indicate that from 24 to 16.5 ka, corresponding to diatom zone Ia, the environment was characterized by a eutrophic wetland. The lake's depth increased rapidly at approximately 16.5 ka, corresponding to diatom zone Ib, and it is estimated that the lake attained a certain depth by 15.7 ka. The increase in water depth from about 16.5 ka is considered to reflect a warming-mediated precipitation increase in Japan and East Asia, suggesting that this warming occurred several hundred years earlier than in the North Atlantic region. From approximately 16.0 to 15.5 ka, there was a significant transition from an environment dominated by planktonic species to one dominated by benthic species. This transition suggests that it occurred over a shorter period of time than the depth reduction period indicated by the stratigraphic analysis of the ST2020 core. The dominant diatom community in the area from approximately 15.5 to 13.7 ka, corresponding to diatom zone Ic, was predominantly composed of Staurosidae, and the community composition remained relatively stable, indicating that the water depth was maintained at a certain level conducive to the proliferation of the Staurosidae. The diatom valves produced at approximately 13.5~12.6 ka, corresponding to diatom categories II to VII, may have been transported from the catchment area of Lake Suwa by rivers and subsequently deposited in the lake. It is hypothesized that the lake experienced a temporary shallow marsh environment during approximately 12.6 to 11.3 ka, corresponding to diatom zone VIII, when water depths increased significantly, and the lake remained oligotrophic. The increase in diatom valves content during this interval is suggestive of global warming during the Holocene epoch, and the subsequent temporary increase in genus Eunotia may reflect the colder Younger Dryas event. The lake is hypothesized to have been eutrophic during the period between 11.3 and 10.0 ka, corresponding to the early stage of Diatom zone IX. The compositional changes of genus Aulacoseira. This progression, if substantiated, would indicate a gradual shift towards higher nutrient levels. The diatom assemblages from 10.0 ka to the present, corresponding to the latter half of diatom zone IX to Ⅺ, exhibits an alternation between periods of dominance by planktonic species and benthic species at intervals of approximately 1000 years.
The diatom assemblages indicate that from 24 to 16.5 ka, corresponding to diatom zone Ia, the environment was characterized by a eutrophic wetland. The lake's depth increased rapidly at approximately 16.5 ka, corresponding to diatom zone Ib, and it is estimated that the lake attained a certain depth by 15.7 ka. The increase in water depth from about 16.5 ka is considered to reflect a warming-mediated precipitation increase in Japan and East Asia, suggesting that this warming occurred several hundred years earlier than in the North Atlantic region. From approximately 16.0 to 15.5 ka, there was a significant transition from an environment dominated by planktonic species to one dominated by benthic species. This transition suggests that it occurred over a shorter period of time than the depth reduction period indicated by the stratigraphic analysis of the ST2020 core. The dominant diatom community in the area from approximately 15.5 to 13.7 ka, corresponding to diatom zone Ic, was predominantly composed of Staurosidae, and the community composition remained relatively stable, indicating that the water depth was maintained at a certain level conducive to the proliferation of the Staurosidae. The diatom valves produced at approximately 13.5~12.6 ka, corresponding to diatom categories II to VII, may have been transported from the catchment area of Lake Suwa by rivers and subsequently deposited in the lake. It is hypothesized that the lake experienced a temporary shallow marsh environment during approximately 12.6 to 11.3 ka, corresponding to diatom zone VIII, when water depths increased significantly, and the lake remained oligotrophic. The increase in diatom valves content during this interval is suggestive of global warming during the Holocene epoch, and the subsequent temporary increase in genus Eunotia may reflect the colder Younger Dryas event. The lake is hypothesized to have been eutrophic during the period between 11.3 and 10.0 ka, corresponding to the early stage of Diatom zone IX. The compositional changes of genus Aulacoseira. This progression, if substantiated, would indicate a gradual shift towards higher nutrient levels. The diatom assemblages from 10.0 ka to the present, corresponding to the latter half of diatom zone IX to Ⅺ, exhibits an alternation between periods of dominance by planktonic species and benthic species at intervals of approximately 1000 years.