11:30 〜 11:45
[MIS02-03] Reconstruction of temperature variations using long-chain alkenones in sediments from the Miocene Nina Formation distributed in Hidaka area (Hokkaido, Japan)
キーワード：日本海、中新世、バイオマーカー、アルケノン
1.Introduction
During the Miocene age, foreland basins were formed in central Hokkaido by an arc-arc collision event. The Hidaka sedimentary basin developed in the southern part of the basin is deposited from the early Miocene to late Miocene (Kawakami, 2013).
It has been known that less exposure of sedimentary rocks in the land section deposited after 9 Ma. Submarine hiatuses have been reported in various locations around the Japan Sea during the late Miocene (Watanabe. 1994). However, the sedimentary successions deposited during the late Miocene are distributed in the Hidaka sedimentary basin area such as Biratori and Hobetsu in central Hokkaido, Japan. In the Hae River section, where is the field of this study, the Miocene sedimentary rocks are exposed, and these are continuous muddy sediments and no submarine hiatus, contrasted with the Nina formation . The area where Hidaka Basin existed is a point where the Japan Sea and the Pacific Ocean are connected, thus it is an important field to analyze the marine linkage between the Japan Sea and the Pacific Ocean and the paleoenvironment of northern Japan in middle to late Miocene. In the present study, we report on the detection of alkenone in the muddy rocks of the Nina formation and reconstruction SST and paleovegetation of late Miocene in central Hokkaido.
2. Sample and methods
Hae river is located in southern part of Hidaka town. In the River, Late Miocene Nina Formation is exposed for 4 km, In upstream of the river, Nibutani formation is distributed due to unconformity caused by the Hiratori fault. The age of Nina Formation in Hae river is estimated to be 9.7-3.5 Ma based on diatom community composition (Maruyama et al. 2018).
The Nina Formation exposed in Hae River is mainly mudstone to sandy mudstone, occasionally interrupted by turbidite beds, the proportion of which increases towards the upper Nina formation. In this study, biomarker analysis was carried out by extracting free form components from the muddy sediments of Nina Formation, which were measured and analyzed by GC-MS and GC-FID.
3.Result and Discussion
Alkenones could be detected in all muddy sedimentary rocks except turbiditic sandstone, and it continues to be detected even at older ages, especially from 9.3 Ma. The UK'37 shows a consistent decrease from 9.3-8.7 Ma to the end of the Miocene, with minor increases and decreases from the end of the Miocene. The SST is calculated to average 26.0℃(4 samples) from 9.3 to 7.7 Ma, 22.3℃ (3 samples) from 7.7 to 6.5 Ma, and 18.9℃ (12 samples) after 6.5 Ma. This suggests that the SST in Hidaka Basin decreased rapidly from around 8.0 Ma.
In previous studies, Miocene SST variations around Japan suggest a decrease in SST due to winter cooling from about 7.7 Ma to 6.6 Ma in Japan sea(IODP Exp.346 U1425), and a cooling due to summer cooling caused by the weaking of the Kuroshio Current from 6.5 Ma in the Pacific Ocean (ODP site 1208) (Matsuzaki et al., 2020).
In the Hidaka Basin, a cooling trend is observed over a long period of time, suggesting that the temperature decrease period may have been prolonged due to the influence of both the Japan Sea and Pacific. The calculated SST tended to be higher than the SST calculated from the community composition of radiolarians in U1425. This trend may indicate the inflow of the Kuroshio Current into the Hidaka Basin. The possibility of an inflow of the Kuroshio Current is supported by the synchronous decrease in temperature between the North Pacific and the Hidaka Basin, and the SST after 6.5 Ma is similar to that of other Japanese waters.
The terrestrial vegetation composition calculated from the ACL shows a woody trend between 9.3 and 8.7 Ma, and a more herbaceous trend after that. However, this trend is weaker than the SST variability, which shows large changes. The ACL and alkenone-based SST values indicates weak negative correlation, suggests an increase in herbaceous vegetation with colder temperatures. Especially, after 6.5 Ma, when the SST average is 18.9℃. These results suggest that the transition from woody to herbaceous vegetation may occur actively during the cold season, and that the decrease in SST due to the decrease in the Kuroshio inflow may have caused significant changes in the terrestrial vegetation, especially in the Hidaka Basin.
During the Miocene age, foreland basins were formed in central Hokkaido by an arc-arc collision event. The Hidaka sedimentary basin developed in the southern part of the basin is deposited from the early Miocene to late Miocene (Kawakami, 2013).
It has been known that less exposure of sedimentary rocks in the land section deposited after 9 Ma. Submarine hiatuses have been reported in various locations around the Japan Sea during the late Miocene (Watanabe. 1994). However, the sedimentary successions deposited during the late Miocene are distributed in the Hidaka sedimentary basin area such as Biratori and Hobetsu in central Hokkaido, Japan. In the Hae River section, where is the field of this study, the Miocene sedimentary rocks are exposed, and these are continuous muddy sediments and no submarine hiatus, contrasted with the Nina formation . The area where Hidaka Basin existed is a point where the Japan Sea and the Pacific Ocean are connected, thus it is an important field to analyze the marine linkage between the Japan Sea and the Pacific Ocean and the paleoenvironment of northern Japan in middle to late Miocene. In the present study, we report on the detection of alkenone in the muddy rocks of the Nina formation and reconstruction SST and paleovegetation of late Miocene in central Hokkaido.
2. Sample and methods
Hae river is located in southern part of Hidaka town. In the River, Late Miocene Nina Formation is exposed for 4 km, In upstream of the river, Nibutani formation is distributed due to unconformity caused by the Hiratori fault. The age of Nina Formation in Hae river is estimated to be 9.7-3.5 Ma based on diatom community composition (Maruyama et al. 2018).
The Nina Formation exposed in Hae River is mainly mudstone to sandy mudstone, occasionally interrupted by turbidite beds, the proportion of which increases towards the upper Nina formation. In this study, biomarker analysis was carried out by extracting free form components from the muddy sediments of Nina Formation, which were measured and analyzed by GC-MS and GC-FID.
3.Result and Discussion
Alkenones could be detected in all muddy sedimentary rocks except turbiditic sandstone, and it continues to be detected even at older ages, especially from 9.3 Ma. The UK'37 shows a consistent decrease from 9.3-8.7 Ma to the end of the Miocene, with minor increases and decreases from the end of the Miocene. The SST is calculated to average 26.0℃(4 samples) from 9.3 to 7.7 Ma, 22.3℃ (3 samples) from 7.7 to 6.5 Ma, and 18.9℃ (12 samples) after 6.5 Ma. This suggests that the SST in Hidaka Basin decreased rapidly from around 8.0 Ma.
In previous studies, Miocene SST variations around Japan suggest a decrease in SST due to winter cooling from about 7.7 Ma to 6.6 Ma in Japan sea(IODP Exp.346 U1425), and a cooling due to summer cooling caused by the weaking of the Kuroshio Current from 6.5 Ma in the Pacific Ocean (ODP site 1208) (Matsuzaki et al., 2020).
In the Hidaka Basin, a cooling trend is observed over a long period of time, suggesting that the temperature decrease period may have been prolonged due to the influence of both the Japan Sea and Pacific. The calculated SST tended to be higher than the SST calculated from the community composition of radiolarians in U1425. This trend may indicate the inflow of the Kuroshio Current into the Hidaka Basin. The possibility of an inflow of the Kuroshio Current is supported by the synchronous decrease in temperature between the North Pacific and the Hidaka Basin, and the SST after 6.5 Ma is similar to that of other Japanese waters.
The terrestrial vegetation composition calculated from the ACL shows a woody trend between 9.3 and 8.7 Ma, and a more herbaceous trend after that. However, this trend is weaker than the SST variability, which shows large changes. The ACL and alkenone-based SST values indicates weak negative correlation, suggests an increase in herbaceous vegetation with colder temperatures. Especially, after 6.5 Ma, when the SST average is 18.9℃. These results suggest that the transition from woody to herbaceous vegetation may occur actively during the cold season, and that the decrease in SST due to the decrease in the Kuroshio inflow may have caused significant changes in the terrestrial vegetation, especially in the Hidaka Basin.