5:15 PM - 7:15 PM
[HQR05-P02] Reconstruction of fluvial terrace formation in the Tokachi Plain using luminescence ages
Keywords:Luminescence dating, Fluvial terrace, Pleistocene
Along the middle reaches of Japanese rivers, where the impacts of sea-level fluctuations are minimal, fluvial aggradation has been demonstrated to have occurred during glacial periods as a result of decreased water discharge due to decreased precipitation and/or increased sediment supply due to active periglacial processes under cooler temperatures. Although previous studies have assumed that these terraces were formed in association with the 100,000-year glacial-interglacial cycle, few studies have examined the influence of climate and sea-level changes on fluvial terrace formation based on luminescence dating of fluvial terrace deposits. In this study, we conducted post-IR IRSL dating of fluvial terrace deposits in the Tokachi Plain to investigate the impact of climate change and sea-level changes on the formation of fluvial terraces.
The lowest part of the Ko I deposits yielded an age of ca. 120 ka, suggesting that the riverbed was at its lowest during the early MIS 5. The Ko I tread was formed around 75 ka in the downstream area, whereas in the upstream area, it was formed around 60 ka. In the downstream region, significant incision occurred earlier due to the influence of sea-level lowering, leading to terrace formation. In contrast, in the upstream region, where the impact of sea-level fluctuations was minimal, terrace formation occurred around 60 ka due to an increase in precipitation associated with the intensification of the summer monsoon, which reduced the ratio of sediment supply to river discharge. The incision during the formation of the Ko I tread was minor, less than 5 meters in the upstream region, and the riverbed elevation remained stable until the formation of the Ko II tread. The Ko II tread was formed around 20 ka and is presumed to have been formed by an increase in precipitation due to the strengthening of the summer monsoon. However, significant incision is estimated to have occurred only after approximately 9 ka. These formation processes align with the terrace formation processes described by Hirakawa and Ono (1974).
The Ks III tread is considered to have been formed around 180 ka in the downstream area and around 130 ka in the upstream area. In the downstream region, terrace formation occurred due to sea-level lowering associated with the transition from MIS 7 to MIS 6. In contrast, in the upstream region, the riverbed gradually rose during MIS 6, and incision is thought to have occurred during the transition from MIS 6 to MIS 5. During interglacial periods, the reduction in sediment supply due to vegetation cover on mountain slopes, along with increased precipitation, contributed to river incision (Hirakawa & Ono, 1974). In the upstream region, where the impact of sea-level fluctuations was minimal, rapid incision occurred during the transition from a glacial period to an interglacial period. Therefore, we conclude that the Ks III and Ko II treads were formed in association with the 100,000-year glacial-interglacial cycle.
The lowest part of the Ko I deposits yielded an age of ca. 120 ka, suggesting that the riverbed was at its lowest during the early MIS 5. The Ko I tread was formed around 75 ka in the downstream area, whereas in the upstream area, it was formed around 60 ka. In the downstream region, significant incision occurred earlier due to the influence of sea-level lowering, leading to terrace formation. In contrast, in the upstream region, where the impact of sea-level fluctuations was minimal, terrace formation occurred around 60 ka due to an increase in precipitation associated with the intensification of the summer monsoon, which reduced the ratio of sediment supply to river discharge. The incision during the formation of the Ko I tread was minor, less than 5 meters in the upstream region, and the riverbed elevation remained stable until the formation of the Ko II tread. The Ko II tread was formed around 20 ka and is presumed to have been formed by an increase in precipitation due to the strengthening of the summer monsoon. However, significant incision is estimated to have occurred only after approximately 9 ka. These formation processes align with the terrace formation processes described by Hirakawa and Ono (1974).
The Ks III tread is considered to have been formed around 180 ka in the downstream area and around 130 ka in the upstream area. In the downstream region, terrace formation occurred due to sea-level lowering associated with the transition from MIS 7 to MIS 6. In contrast, in the upstream region, the riverbed gradually rose during MIS 6, and incision is thought to have occurred during the transition from MIS 6 to MIS 5. During interglacial periods, the reduction in sediment supply due to vegetation cover on mountain slopes, along with increased precipitation, contributed to river incision (Hirakawa & Ono, 1974). In the upstream region, where the impact of sea-level fluctuations was minimal, rapid incision occurred during the transition from a glacial period to an interglacial period. Therefore, we conclude that the Ks III and Ko II treads were formed in association with the 100,000-year glacial-interglacial cycle.