[A-CG23] Coastal Ecosystems ? 1. Water Cycle and Land-Ocean Interactions

[ACG23-P02] Spatial distribution of oxygen and hydrogen isotope ratios of seawater in the nascent Kuroshio of Lamon Bay

Atsushi Watanabe¹, *Toshihiro Miyajima², Cesar Villanoy³, Maria Lourdes San-Diego McGlone³, Arnold L. Gordon⁴ (1.School of Environment and Society, Tokyo Institute of Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Marine Science Institute, University of the Philippines, 4.Lamont-Doherty Earth Observatory, Columbia University)

Keywords:oxygen isotope ratio of seawater, vertical structure, freshwater input, pH, Lamon Bay

The vertical and horizontal variations of oxygen (δ¹⁸O) and hydrogen (δ²H) isotopic composition of seawater were investigated in Lamon Bay and offshore waters, east of Luzon Island (14°-19°N, 122°-126°E; Fig. 1a) during a research cruise of the R/V Roger Revelle in April/May 2012. The δ¹⁸O increased with depth from the surface, reaching a maximum of +1.5‰ to +2.0‰ at the salinity maximum of North Pacific Tropical Water between 100 m and 200 m. Then, the δ¹⁸O decreased toward the salinity minimum of North Pacific Intermediate Water near a depth of 500 m. Below this salinity minimum, the δ¹⁸O remained within a narrow range between -1‰ and 0‰ to 4,000 m. The δ¹⁸O of surface water showed a spatial gradient from the nearshore (southwest) area of the Bay (-0.3‰) to the offshore (northeast) waters (+0.6‰) and significantly correlated to the salinity (p < 0.0001; Fig. 1b), reflecting influence of freshwater input from the Island. By linear regression, the δ¹⁸O of freshwaters supplied to Lamon Bay could be estimated to be around -5.3‰, which is consistent with the riverwater δ¹⁸O found in the central Philippines. Variation of the δ²H followed similar patterns as observed for δ¹⁸O, although relatively high δ²H values compared to δ¹⁸O were recorded in surface waters of nearshore area, reflecting the deuterium excess of freshwater supplied from the land. The δ¹⁸O and the salinity were distinctly lower in the cyclonic dipole (see Gordon et al., 2014), corresponding to the part of Lamon Bay between the nascent Kuroshio and Luzon Island. The average pH in this area (8.114±0.007) was also significantly (p < 0.003) lower than the other parts of the observed area (8.125–8.134; Fig. 1c). This spatial trend implies that the extent of freshwater inflow including lowered pH on Lamon Bay and offshore waters is constrained by the flow path of the nascent Kuroshio and its feeder current. On the other hand, the average pH of the nascent Kuroshio was slightly higher than the feeder current and the anticyclonic dipole waters (8.134 vs. 8.126, p = 0.0329), which suggests unexplored biogeochemical process that keeps pH in the Kuroshio high relative to that of the freshwater input.