ABSTRACT

The stable oxygen isotope (δ¹⁸O) values of biogenic calcium carbonates (CaCO₃) present in hard tissues of marine organisms vary with the δ¹⁸O of the surrounding seawater (δ¹⁸O_sw) and water temperature. Consequently, the ambient water temperature when marine organisms existed can be estimated using δ¹⁸O of CaCO₃ with δ¹⁸O_sw. Thus, the aims of this study were to reveal the spatiotemporal variations in δ¹⁸O_sw and their correlations with salinity in the East Asian marginal seas. We collected seawater samples (n = 2222) at 1394 stations around Japan, primarily from the surface layer of the East China Sea (ECS) and Sea of Japan (SOJ), from 2015 to 2021. We analyzed δ¹⁸O_sw and the stable hydrogen isotope of seawater (δD_sw) along with the measured water temperature and salinity. The δ¹⁸O_sw and δD_sw values ranged from –3.48‰ to +0.45‰, and –21.5‰ to +2.3‰, respectively. In our full data, both δ¹⁸O_sw and δD_sw had positive linear relationships with salinity as follows: δ¹⁸O_sw = 0.235 × salinity – 7.94 (r² = 0.85) and δD_sw = 1.56 × salinity – 52.9 (r² = 0.85). Furthermore, the relationship between δ¹⁸O_sw and δD_sw for full data as follows: δD_sw = 6.44 × δ¹⁸O_sw – 0.18 (r² = 0.96). These relationships varied across seasons, areas (the ECS or SOJ), and water depths. In particular, δ¹⁸O_sw and δD_sw of less-saline water were different in the ECS and SOJ. These fine-scale, wide-range, and high-precision δ¹⁸O_sw and δD_sw datasets can contribute to paleoceanography, environmental analysis, oceanography, and fisheries science.