Characterization of water masses around the southern Ryukyu Islands based on isotopic compositions

Abstract

We investigated the water-mass structure on the Okinawa Trough and Pacific sides of the southern Ryukyu Island Arc (Yonaguni, Iriomote, and Ishigaki subareas) using the Nd isotope composition (143Nd/144Nd ratios; expressed as εNd values) of benthic foraminiferal tests in surface sediments, which reflect bottom-water composition, along with hydrogen and oxygen isotope compositions (δD and δ18O values, respectively) and physical properties (temperature and salinity) of seawater. The Okinawa Trough side has lower εNd values than the Pacific side due to continental/island material inputs characterized by relatively low εNd values. Moreover, within the Okinawa Trough, other processes control the Nd behavior of seawater and primarily affect the Yonaguni and Iriomote subareas, as follows. (1) Surface and subsurface waters are influenced by Taiwanese river discharge combined with temporospatial variations in oceanographic conditions including Kuroshio Current meandering. (2) Intermediate water is characterized by low εNd values (down to − 8.2), possibly attributable to sediment plumes and turbiditic fluxes. (3) The εNd values of bottom water indicate upwelling and vertical mixing, with composition therefore being similar to those of intermediate water. The εNd profiles are better defined on the Pacific side. High εNd values occur in surface and subsurface (< 300 m depth, potential density < 25.0 kg m−3) waters, and low values (down to − 7.0) occur in subsurface–core-intermediate water (400–600 m depth, 26–27 kg m−3). εNd values increase slightly to − 4.0 below 750 m depth and remain constant down to about 2000 m depth, below which deep water shows a slight decrease in εNd values. Intermediate and bottom/deep waters are distinguished from upper layers by their lower δD and δ18O values.

Origins of Low‐Oxygen Bottom Water Influenced by Tide and ENSO on the Outer‐Edge Shelf of East China Sea: Multi‐Chemical Tracer Approaches

“…To provide better and tighter controls over the mixing model results, Nd isotopic composition (εNd) was introduced to verify our result. The εNd data of mid‐shelf water (−8.7 ± 0.6, Che & Zhang, 2018), surface water (−10.2 ± 0.2, Che & Zhang, 2018), KSSW (−6.2 ± 0.26, Cruz Salmeron et al., 2022), and KIW (−3.3 ± 0.5, Che & Zhang, 2018) were selected from available literature. Due to the limited volume/amount of the pore water sample, we chose the εNd data of the surface sediments (εNd: −12.0 ± 0.36, Bi et al., 2017) as the pore water endmember.…”

Spatiotemporal variations of seawater <i>δ</i>¹⁸O and <i>δ</i>D in the Western North Pacific marginal seas near Japan

“…The spatially different equations of these studies (Horikawa et al, 2015;Kodaira et al, 2016) can cause difficulties in calculating δ 18 Osw using salinity in this area. Vertically, both δ 18 Osw and δDsw are high in the surface layer (δ 18 Osw and δDsw: +0.5 and +4.5‰, respectively), but low in the middle and deep layers (δ 18 Osw and δDsw: 0.0 and -0.4‰, respectively, below 755 m depth) of the Okinawa Trough in the ECS (Cruz Salmeron et al, 2022), but the values were not compared to salinity in Cruz Salmeron et al (2022). Therefore, the aims of this study were to evaluate the spatiotemporal variations in δ 18 O and δD values and create isoscapes of δ 18 Osw and δDsw in the ECS and SOJ based on big datasets (n = 2222).…”

Hydrogen and oxygen isotope compositions of bottom waters collected around off southwestern Tokara Islands

“…List of sampling site, temperature, salinity, pH, hydrogen and oxygen isotope compositions, and d-excess in the southwestern Tokara Islands. Temperature, salinity, and pH were fromItaki et al (2022).…”