5:15 PM - 7:15 PM
[SGL23-P06] Sedimentary environment inferred from benthic foraminiferal assemblages in the southern Yatsushiro Sea
Keywords:Yatsushiro Sea, Hakuho-maru Expedition KH-18-3, Foraminifer, Diatom
The southern extension of the Hinagu Fault Zone, one of the epicenter faults of the 2008 Kumamoto Earthquake, constitutes a submarine fault group as the Yatsushiro Sea section. While sea-level change since the last glacial period and paleoenvironmental estimation have been conducted in the surrounding East China Sea and Ariake Sea, the Yatsushiro Sea is a unique sedimentary field because it is a closed sea and also rich in active structures. In this study, we examine the sedimentary environment of the southern part of the Yatsushiro Sea based on foraminiferal fossils, taking advantage of the mud sampling survey conducted after the Kumamoto Earthquake.
The Expedition KH-18-3 research cruise by the academic research vessel Hakuho Maru was conducted from July 27, 2018 to July 30, 2018, and mud sampling using a piston corer and multiple corers and stratigraphic exploration using a sam-bottom profiler were carried out in the southern Yatsushiro Sea. In the previous study, various analyses were performed from piston core samples (eg. Hori, 2019MS).
Samples were taken from piston cores in Sites PC06 and PC07 at roughly 50 cm intervals. The DI samples (DI055-DI064 from Site PC06 and DI065-DI073 from Site PC07) that were subsampled in 2019 is now significantly dry and so the preservation of foraminifera is poor. Accordingly, we re-subsampled at the same stratigraphic level as the DI samples and nemed them as the KS samples. The samples were rinsed on a sieve to remove mud components, and the residue was transferred to filter paper and dried in a constant-temperature dryer set at 50°C for approximately 24 hours. For observation on a slotted petri dish, the sample was divided into 1/2n pieces using a divider until the amount of particles was sufficient for observation. From the divided samples, more than 200 foraminifera per layer per minute were picked up using a nail brush under a microscope. If the number of foraminifera found did not reach the 200 foraminifera threshold, all foraminifera found in that stratum were picked up. Foraminifera with a dominance rate greater than 5% were identified as the dominant species.
Only KS055 (2-4 cm below the seafloor) in Site PC06 and KS065 (2-4 cm below the seafloor) in Site PC07 produced more than 200 individuals of benthic foraminifera, which were not compared with the DI samples. Both samples are considered to be almost the same benthic foraminiferal community, corresponding to Cluster Ia found at St. 59 in Rifardi et al. (1998), where the sampling sites are almost the same, and there seems to be no significant difference between the benthic foraminiferal communities in KS055 and KS065. In Site PC06 and Site PC07, which are located in the 0.5 to 1 ppm zone in the isopycnic curve of maximum mercury content in the Yatsushiro Sea (Rifardi et al., 1998), Bulimina denudata is the most dominant species in both samples, and the frequency of B. denudata at sites with abnormal mercury content reported by Oki et al. Nonionoides grateloupi is usually found on the outer shelf at depths of up to 180 m (Pascual et al., 2009) and is classified as an opportunistic species that dominates at the boundary between areas of high and low surface productivity (Nemoto et al., 2008 ; Olugbenga T. Fajemila et al., 2015), suggesting an environment with an intermittent supply of organic matter. The above two species are the common dominant species in KS055 and KS065, and Cibicides cf. lobatulus in KS055 and Ammonia tepida and Hopkinsina glabra in KS065. H. glabra is found in the central part of Kagoshima Bay at depths of 28-150 m, and in the coastal and bay areas of Matsushima Bay. The fact that foraminifera were rarely found in other stratigraphic units suggests that the environment at the time of deposition was unsuitable for the survival of foraminifera or for the preservation of their remains.
The present benthic foraminiferal assemblages at Site PC06 and Site PC07 were revealed, and were not significantly different from those at KS055 and KS065, suggesting that the earlier environment was not suitable for foraminifera.
The results of this study are based on benthic foraminiferal fossils from only two core samples. It is necessary to collect foraminifera from cores at other sites in the future to examine the environment on a broader scale. Quantitative evaluation of the piston core samples used in this study using samples other than foraminifera, such as diatoms, is expected to elucidate the detailed paleoenvironment of the strata from which foraminifera were not collected in this study.
The Expedition KH-18-3 research cruise by the academic research vessel Hakuho Maru was conducted from July 27, 2018 to July 30, 2018, and mud sampling using a piston corer and multiple corers and stratigraphic exploration using a sam-bottom profiler were carried out in the southern Yatsushiro Sea. In the previous study, various analyses were performed from piston core samples (eg. Hori, 2019MS).
Samples were taken from piston cores in Sites PC06 and PC07 at roughly 50 cm intervals. The DI samples (DI055-DI064 from Site PC06 and DI065-DI073 from Site PC07) that were subsampled in 2019 is now significantly dry and so the preservation of foraminifera is poor. Accordingly, we re-subsampled at the same stratigraphic level as the DI samples and nemed them as the KS samples. The samples were rinsed on a sieve to remove mud components, and the residue was transferred to filter paper and dried in a constant-temperature dryer set at 50°C for approximately 24 hours. For observation on a slotted petri dish, the sample was divided into 1/2n pieces using a divider until the amount of particles was sufficient for observation. From the divided samples, more than 200 foraminifera per layer per minute were picked up using a nail brush under a microscope. If the number of foraminifera found did not reach the 200 foraminifera threshold, all foraminifera found in that stratum were picked up. Foraminifera with a dominance rate greater than 5% were identified as the dominant species.
Only KS055 (2-4 cm below the seafloor) in Site PC06 and KS065 (2-4 cm below the seafloor) in Site PC07 produced more than 200 individuals of benthic foraminifera, which were not compared with the DI samples. Both samples are considered to be almost the same benthic foraminiferal community, corresponding to Cluster Ia found at St. 59 in Rifardi et al. (1998), where the sampling sites are almost the same, and there seems to be no significant difference between the benthic foraminiferal communities in KS055 and KS065. In Site PC06 and Site PC07, which are located in the 0.5 to 1 ppm zone in the isopycnic curve of maximum mercury content in the Yatsushiro Sea (Rifardi et al., 1998), Bulimina denudata is the most dominant species in both samples, and the frequency of B. denudata at sites with abnormal mercury content reported by Oki et al. Nonionoides grateloupi is usually found on the outer shelf at depths of up to 180 m (Pascual et al., 2009) and is classified as an opportunistic species that dominates at the boundary between areas of high and low surface productivity (Nemoto et al., 2008 ; Olugbenga T. Fajemila et al., 2015), suggesting an environment with an intermittent supply of organic matter. The above two species are the common dominant species in KS055 and KS065, and Cibicides cf. lobatulus in KS055 and Ammonia tepida and Hopkinsina glabra in KS065. H. glabra is found in the central part of Kagoshima Bay at depths of 28-150 m, and in the coastal and bay areas of Matsushima Bay. The fact that foraminifera were rarely found in other stratigraphic units suggests that the environment at the time of deposition was unsuitable for the survival of foraminifera or for the preservation of their remains.
The present benthic foraminiferal assemblages at Site PC06 and Site PC07 were revealed, and were not significantly different from those at KS055 and KS065, suggesting that the earlier environment was not suitable for foraminifera.
The results of this study are based on benthic foraminiferal fossils from only two core samples. It is necessary to collect foraminifera from cores at other sites in the future to examine the environment on a broader scale. Quantitative evaluation of the piston core samples used in this study using samples other than foraminifera, such as diatoms, is expected to elucidate the detailed paleoenvironment of the strata from which foraminifera were not collected in this study.