5:15 PM - 6:45 PM
[SCG48-P31] Recent deep-sea benthic foraminifera in southern part of Okinawa Trough, East China Sea
Keywords:Okinawa Trough, benthic foraminiferal assemblages, Scientific Drilling, East China Sea
Since benthic foraminifera distribution in the ocean is generally considered to be controlled by the surrounding marine environment, calcareous shells preserved as fossils in sediments are used as materials for paleoenvironmental reconstruction. There are various methods of paleoenvironmental reconstruction, with the analysis of benthic foraminiferal assemblages and geochemical analysis of individual fossils being the main methods. On the other hand, it remains not fully understood the distribution of benthic foraminifera species and the adaptation of each species to its environment. In particular, the distribution of benthic foraminifera is considered to be highly regional, so it is necessary to clarify the distribution of recent benthic foraminifera around Japan and its relationship with environmental parameters in order to reconstruct the former marine environment using benthic foraminifera fossils around Japan.
The southern Okinawa Trough, located at the southernmost tip of Japan, is a site where benthic foraminifera distributed at low latitudes are expected to be found around Japan and where the influence of the Kuroshio Current, a surface current, can be examined on their distribution. In the southern Okinawa Trough, water temperature and salinity are almost constant at depths greater than 1000 m (Katayama et al., 2020), which is useful for considering the relationship with other environmental factors such as organic matter content and dissolved oxygen when considering what changes in benthic foraminifera assemblages. However, the benthic foraminiferal assemblages in the southern Okinawa Trough have not been well studied in the trough axis, where the vertical change in water quality is considered to be particularly stable, although it has been studied around the Nansei Islands, on East China Sea continental shelf and its continental slope. In this study, we analyzed benthic foraminiferal assemblages in sea bottom surface sediments obtained from the KH-23-11 cruise conducted from December 28, 2023 to January 11, 2024. During the cruise, surface sediments were obtained by multiple corers at five stations between 1572 and 2268 m depth in the southern Okinawa Trough, and CTD observations were conducted at two stations. This study reports a summary of the recent benthic foraminiferal assemblages from the upper 10 cm of the multiple cores at each site.
The multiple cores were sliced into 1 cm pieces on board the ship and stored in sealed containers with rose bengal ethanol to distinguish and preserve the living benthic foraminifera in the sediment.
At the laboratory, the sediments were processed according to Kuwano (1956) and examined for individuals larger than 63 µm. As a result, living individuals of agglutinated and calcareous species were identified at all five sites.
For the uppermost (0-2 cm) layer of the sediments, the agglutinated species such as Rhabdammina spp., Reophax spp., Saccorhiza ramosa, and Lagenammina spp. are dominant in all sites. The calcareous species associated with these agglutinated species differ in composition on the eastern, southern, and western sides of the trough axis; Cassidulina spp. and Globocassidulina spp. on the southern side, Uvigerina spp. on the western side, and Pullenia bulloides predominate on the eastern side. Martinottiella communis (agglutinated species) was also characteristic on the west side. Additionally, the foraminiferal diversity was very high on the south side. For the sub-surface (>4 cm) samples, Chilostomella oolina and Globobulimina spp. which are few in the uppermost layer, were abundant. Both species tended to burrow deeper into the sediment on the east side than on the west side.
Water depths at each site were deeper on the south, west, and east sides of the trough in that order, and the CTD observations at two sites showed that both water temperature and salinity were almost constant at depths greater than about 1000 m. Here, the number of individuals and species of calcareous species, both living individuals and remains, tended to decrease with increasing water depth, suggesting that the depth of the lysocline was the main factor in the horizontal assemblages change at the uppermost surface. In addition, the high foraminiferal species diversity on south side is suggested to be related to nutrients from Iriomote Island, located just south of this site. Differences of the vertical distribution of the two sub-surface species among sites may be attributed to the amount of dissolved oxygen in the sediments.
References:
Katayama et al. (2020) GSJ Interim Report (80), 87-93.
Kuwano, Y. (1956) Contributions of Research Institute for Natural Resources (41-42) ,67-75.
The southern Okinawa Trough, located at the southernmost tip of Japan, is a site where benthic foraminifera distributed at low latitudes are expected to be found around Japan and where the influence of the Kuroshio Current, a surface current, can be examined on their distribution. In the southern Okinawa Trough, water temperature and salinity are almost constant at depths greater than 1000 m (Katayama et al., 2020), which is useful for considering the relationship with other environmental factors such as organic matter content and dissolved oxygen when considering what changes in benthic foraminifera assemblages. However, the benthic foraminiferal assemblages in the southern Okinawa Trough have not been well studied in the trough axis, where the vertical change in water quality is considered to be particularly stable, although it has been studied around the Nansei Islands, on East China Sea continental shelf and its continental slope. In this study, we analyzed benthic foraminiferal assemblages in sea bottom surface sediments obtained from the KH-23-11 cruise conducted from December 28, 2023 to January 11, 2024. During the cruise, surface sediments were obtained by multiple corers at five stations between 1572 and 2268 m depth in the southern Okinawa Trough, and CTD observations were conducted at two stations. This study reports a summary of the recent benthic foraminiferal assemblages from the upper 10 cm of the multiple cores at each site.
The multiple cores were sliced into 1 cm pieces on board the ship and stored in sealed containers with rose bengal ethanol to distinguish and preserve the living benthic foraminifera in the sediment.
At the laboratory, the sediments were processed according to Kuwano (1956) and examined for individuals larger than 63 µm. As a result, living individuals of agglutinated and calcareous species were identified at all five sites.
For the uppermost (0-2 cm) layer of the sediments, the agglutinated species such as Rhabdammina spp., Reophax spp., Saccorhiza ramosa, and Lagenammina spp. are dominant in all sites. The calcareous species associated with these agglutinated species differ in composition on the eastern, southern, and western sides of the trough axis; Cassidulina spp. and Globocassidulina spp. on the southern side, Uvigerina spp. on the western side, and Pullenia bulloides predominate on the eastern side. Martinottiella communis (agglutinated species) was also characteristic on the west side. Additionally, the foraminiferal diversity was very high on the south side. For the sub-surface (>4 cm) samples, Chilostomella oolina and Globobulimina spp. which are few in the uppermost layer, were abundant. Both species tended to burrow deeper into the sediment on the east side than on the west side.
Water depths at each site were deeper on the south, west, and east sides of the trough in that order, and the CTD observations at two sites showed that both water temperature and salinity were almost constant at depths greater than about 1000 m. Here, the number of individuals and species of calcareous species, both living individuals and remains, tended to decrease with increasing water depth, suggesting that the depth of the lysocline was the main factor in the horizontal assemblages change at the uppermost surface. In addition, the high foraminiferal species diversity on south side is suggested to be related to nutrients from Iriomote Island, located just south of this site. Differences of the vertical distribution of the two sub-surface species among sites may be attributed to the amount of dissolved oxygen in the sediments.
References:
Katayama et al. (2020) GSJ Interim Report (80), 87-93.
Kuwano, Y. (1956) Contributions of Research Institute for Natural Resources (41-42) ,67-75.