17:15 〜 18:45
[MGI25-P01] Sedimentary record of paleo-coastal hazards and environment from coastal sinkhole in the Bonin Islands, Japan
キーワード：ストーム堆積物、熱帯低気圧、古気候、自然災害、極端波浪イベント
Tropical cyclone (TC)-induced hazard risk in global coastal areas is expected to be compounded by anthropogenic global warming. However, detecting long-term variabilities in TC activity solely from relatively short observational records poses a challenge. In contrast, geological records offer a valuable avenue for reconstructing the long-term history of TCs on centennial to millennial scales. The scarcity of both observational and geological records related to extreme wave events, including TCs and tsunamis, in remote areas far from the continent highlights the significance of oceanic islands as potential repositories for coastal hazard research.
The western North Pacific is the most active basin of TC activity on Earth. Some sedimentary records related to past extreme wave events have been reported from coastal regions such as the Marshall Islands and the South China Sea in the western North Pacific. However, there is no long-term record related to past TC events around the Bonin Islands, Japan. Reconstructing the record of past TCs in the Bonin Islands should contribute to enhancing our understanding of the spatial distribution of long-term TC activity in the western North Pacific.
This study aims to present a new record of past extreme wave events in the Bonin Islands and provide the preliminary results of field survey and laboratory analyses. The study area is Minamijima Island, one of the Bonin Islands. Minamijima Island and surrounding shallow coastal areas are characterized by karst landforms. Inyo-Ike Pond, a sinkhole located in the center of the island, is sheltered by high limestone cliffs on its northern, western, and eastern sides while featuring a sandy beach on its southern side. An arch-shaped topography adjacent to the beach allows ocean waves to enter the beach. Seawater and sediment may inundate Inyo-Ike Pond due to overwashing during TC approaching.
In this study, excavation using a peat sampler was undertaken inside and around Inyo-Ike Pond. X-ray CT imaging and high-resolution elemental analysis with XRF core scanning were conducted on the sediment cores. Radiocarbon dating was also performed on some samples obtained from the sediment cores. The sediment of Inyo-Ike Pond exhibits two distinct types. The primary composition of the sediment is dark-colored organic mud to silt layers, relatively enriched in Titanium (Ti), interpreted as having been deposited under fair-weather conditions. The other sediment type consists of sand layers abundant in Calcium (Ca) and Strontium (Sr), indicative of event deposits transported from the beach face by extreme wave events. A total of 186 cm of sediment core from Inyo-Ike Pond revealed over 20 event layers potentially linked to past extreme wave events. Furthermore, changes in the elemental composition and color of fair-weather sediments may reflect non-event changes in climatic and environmental conditions. Consequently, Inyo-Ike Pond sediments may serve as records of past environmental and climate changes and extreme wave events. In this study, we compare our results with other paleostorm and paleoclimate records and discuss the relationship between climate change and variability in TC activity in the western North Pacific.
The western North Pacific is the most active basin of TC activity on Earth. Some sedimentary records related to past extreme wave events have been reported from coastal regions such as the Marshall Islands and the South China Sea in the western North Pacific. However, there is no long-term record related to past TC events around the Bonin Islands, Japan. Reconstructing the record of past TCs in the Bonin Islands should contribute to enhancing our understanding of the spatial distribution of long-term TC activity in the western North Pacific.
This study aims to present a new record of past extreme wave events in the Bonin Islands and provide the preliminary results of field survey and laboratory analyses. The study area is Minamijima Island, one of the Bonin Islands. Minamijima Island and surrounding shallow coastal areas are characterized by karst landforms. Inyo-Ike Pond, a sinkhole located in the center of the island, is sheltered by high limestone cliffs on its northern, western, and eastern sides while featuring a sandy beach on its southern side. An arch-shaped topography adjacent to the beach allows ocean waves to enter the beach. Seawater and sediment may inundate Inyo-Ike Pond due to overwashing during TC approaching.
In this study, excavation using a peat sampler was undertaken inside and around Inyo-Ike Pond. X-ray CT imaging and high-resolution elemental analysis with XRF core scanning were conducted on the sediment cores. Radiocarbon dating was also performed on some samples obtained from the sediment cores. The sediment of Inyo-Ike Pond exhibits two distinct types. The primary composition of the sediment is dark-colored organic mud to silt layers, relatively enriched in Titanium (Ti), interpreted as having been deposited under fair-weather conditions. The other sediment type consists of sand layers abundant in Calcium (Ca) and Strontium (Sr), indicative of event deposits transported from the beach face by extreme wave events. A total of 186 cm of sediment core from Inyo-Ike Pond revealed over 20 event layers potentially linked to past extreme wave events. Furthermore, changes in the elemental composition and color of fair-weather sediments may reflect non-event changes in climatic and environmental conditions. Consequently, Inyo-Ike Pond sediments may serve as records of past environmental and climate changes and extreme wave events. In this study, we compare our results with other paleostorm and paleoclimate records and discuss the relationship between climate change and variability in TC activity in the western North Pacific.