16:15 〜 16:30
[AOS18-04] Characterization of water masses around the southern Ryukyu Islands based on
Nd isotope composition of benthic foraminifers and H and O isotope compositions of seawater
キーワード：Hydrogen isotopes, Oxygen isotopes, Neodymium isotopes, Kuroshio Current, Northwestern Pacific
The neodymium isotope composition (εNd) of recent benthic foraminifers and hydrogen (δD) and oxygen (δ18Ο) isotopes of seawater were analyzed along with salinity and temperature of seawater, to delineate the water mass structure on the Okinawa Trough (OT) and Pacific sides (Yonaguni, Iriomote, and Ishigaki sub-areas) of the southern Ryukyu Islands.
The εNd values on the OT side are lower than those on the Pacific side and show an eastward increase on both the OT and Pacific sides. The low εNd values extend down to the intermediate water domain. This is attributed to the strong influence of terrigenous-material inputs, characterized by low Nd values, from Eurasian and Taiwanese rivers into the East China Sea, including the OT. In addition, our study suggests that deeper intermediate water, which flows from the northern Philippine Sea into the OT via the Kerama Gap, may ventilate the southern OT.
The δD and δ18Ο values of seawater on the OT side indicate a more pronounced influence of riverine discharge over the Yonaguni sub-area and vertical mixing between subsurface-intermediate and intermediate-bottom water is stronger in this sub-subarea than in the Iriomote and Ishigaki sub-areas. The highest δD values in the northwest Pacific are found along the Kuroshio Current pathway. Therefore, the current is unique as it marks the boundary between less radiogenic Nd of coastal and epicontinental water and more radiogenic open-sea water. This indicates that, around the southern OT, the Nd composition is determined by the mixing of older rocks of the Eurasian continent/northwestern Pacific islands and younger rocks of the central Pacific.
The εNd values on the OT side are lower than those on the Pacific side and show an eastward increase on both the OT and Pacific sides. The low εNd values extend down to the intermediate water domain. This is attributed to the strong influence of terrigenous-material inputs, characterized by low Nd values, from Eurasian and Taiwanese rivers into the East China Sea, including the OT. In addition, our study suggests that deeper intermediate water, which flows from the northern Philippine Sea into the OT via the Kerama Gap, may ventilate the southern OT.
The δD and δ18Ο values of seawater on the OT side indicate a more pronounced influence of riverine discharge over the Yonaguni sub-area and vertical mixing between subsurface-intermediate and intermediate-bottom water is stronger in this sub-subarea than in the Iriomote and Ishigaki sub-areas. The highest δD values in the northwest Pacific are found along the Kuroshio Current pathway. Therefore, the current is unique as it marks the boundary between less radiogenic Nd of coastal and epicontinental water and more radiogenic open-sea water. This indicates that, around the southern OT, the Nd composition is determined by the mixing of older rocks of the Eurasian continent/northwestern Pacific islands and younger rocks of the central Pacific.