5:15 PM - 6:45 PM
[MIS12-P19] Alkenone-based sea surface temperature variations over the past 20,000 years from sediment core at the Tokara Strait
Keywords:The Kuroshio Current, Alkenone SST, TOC, Holocene
The Kuroshio Current (KC) belongs to the subtropical circulation system and is a western boundary current that plays an important role in transporting large amounts of heat, salt, and water from low latitudes to mid-latitudes. The KC enters the East China Sea (ECS) from the east of Taiwan Island, flows northeastward, and then flows through the Tokara Strait to the south of the Japanese Islands. However, during the glacial period when sea level was low, the KC did not enter the ECS, and it has been pointed out that it may have taken a different flow path from that of today. On the other hand, geoscience and modeling studies suggest that the KC flowed through the ECS during the glacial period, although it weakened. These arguments are still ongoing, and previous studies have mostly documented the ECS, while the Tokara Strait has not received much attention. The Tokara Strait is a point where the KC is thought to have flowed continuously even at low sea level.
In this study, primary production and surface water temperatures are reconstructed based on elemental and alkenone analyses using sediments from the Tokara Strait, which is under the KC. Furthermore, by comparing the results with existing paleoenvironmental data from the Okinawa Trough and off Shikoku Island, we aimed to clarify the paleoenvironmental changes in the Tokara Strait over the past 20 kyr and their factors. A piston core TSK - 1PC (29°38.01'N, 131°04.32'E) was used in this study. This core was collected from the east of the Tokara Strait during the KH-02-1 cruise of the Hakuho-maru. The total length of the core is 14.1 m. Two tephra layers were identified at approximately 1.6 m and 7.6 m from the top of the core. The lithology of the core is nearly homogeneous silty clay, although bioturbation is observed in the upper tephra layer. The age model was established by a calendar age based on AMS 14C ages of planktonic foraminifera in 12 levels and 2 volcanic ash layers, indicating that the entire core records approximately 40 kyr. The sedimentation rate averaged 28 cm/kyr, which is high during glacial periods. In addition, sedimentation rates were found to be as high as 40-50 cm/kyr during the short periods of 13 and 10 kyr. Total carbon (TC), Total Nitrogen (TN), and Total Organic Carbon (TOC) in the sediments were determined by elemental analysis and inorganic carbon content was determined from the difference between TC and TOC, and CaCO3 concentration was calculated. TOC and CaCO3 varied between 0.48~1.22 % and 3.9~17.6 %, respectively. TOC was high during the deglacial to Bølling-Allerød (B/A) warm period and decreased during the Holocene. Although TOC fluxes were dependent on sedimentation rate changes, the results suggest that they may be influenced by changes in sea level. The Alkenone sea surface temperature (SST) fluctuated between 21.1-27.8 ℃, lowering 21.1-22.7 ℃ in the early deglacial period, increasing once during the B/A warm period, and then decreasing by about 1 ℃ during the Younger Dryas cold period. The mean SST over the past 10 kyr is about 25 ℃, but it has shown an increasing trend over the last 2,000 years. These SSTs show an increasing trend during the period of strong East Asian summer monsoon.
In this study, primary production and surface water temperatures are reconstructed based on elemental and alkenone analyses using sediments from the Tokara Strait, which is under the KC. Furthermore, by comparing the results with existing paleoenvironmental data from the Okinawa Trough and off Shikoku Island, we aimed to clarify the paleoenvironmental changes in the Tokara Strait over the past 20 kyr and their factors. A piston core TSK - 1PC (29°38.01'N, 131°04.32'E) was used in this study. This core was collected from the east of the Tokara Strait during the KH-02-1 cruise of the Hakuho-maru. The total length of the core is 14.1 m. Two tephra layers were identified at approximately 1.6 m and 7.6 m from the top of the core. The lithology of the core is nearly homogeneous silty clay, although bioturbation is observed in the upper tephra layer. The age model was established by a calendar age based on AMS 14C ages of planktonic foraminifera in 12 levels and 2 volcanic ash layers, indicating that the entire core records approximately 40 kyr. The sedimentation rate averaged 28 cm/kyr, which is high during glacial periods. In addition, sedimentation rates were found to be as high as 40-50 cm/kyr during the short periods of 13 and 10 kyr. Total carbon (TC), Total Nitrogen (TN), and Total Organic Carbon (TOC) in the sediments were determined by elemental analysis and inorganic carbon content was determined from the difference between TC and TOC, and CaCO3 concentration was calculated. TOC and CaCO3 varied between 0.48~1.22 % and 3.9~17.6 %, respectively. TOC was high during the deglacial to Bølling-Allerød (B/A) warm period and decreased during the Holocene. Although TOC fluxes were dependent on sedimentation rate changes, the results suggest that they may be influenced by changes in sea level. The Alkenone sea surface temperature (SST) fluctuated between 21.1-27.8 ℃, lowering 21.1-22.7 ℃ in the early deglacial period, increasing once during the B/A warm period, and then decreasing by about 1 ℃ during the Younger Dryas cold period. The mean SST over the past 10 kyr is about 25 ℃, but it has shown an increasing trend over the last 2,000 years. These SSTs show an increasing trend during the period of strong East Asian summer monsoon.