In general, there are three currents (water masses) in the study area (Fig. 6) (Jan and Chao, 2003 and Jan et al., 2002): the ECSCC along the Fujian coast, the mostly northward TWC, and the upwelling Kuroshio subsurface water through the southwest of Taiwan. Because the ECSCC is mostly sourced from the Changjiang freshwater, the δ15N-Nitrate of ECSCC should inherit that of the CDW. The δ15N of the dissolved nitrate in the Changjiang Estuary varied with seasons and geographic regions: higher values (4.4 ± 0.5‰) in autumn and lower values (2.1 ± 0.8‰) in winter ( Liu et al., 2009). Although the historical changes in land use may have disturbed the δ15N values of the riverine nitrate ( Chang et al., 2002, Kendall, 1998 and Kendall et al., 2001), this
Tenovin-1 behavior has not been documented in the Changjiang Basin. Recent studies highlight that enhanced N2 fixation in the tropical river plumes (e.g., Amazon, Cogon, Mekong Rivers) substantially contribute to oceanic N2 fixation ( Foster et al., 2009, Grosse et al., 2010 and Subramaniam et al., 200
cool . However, N2 fixation in the ECS in summer is strongly influenced by the Changjiang river plume and is much lower than either the Kuroshio subsurface water upwelling or the Changjiang riverine input; it is likely hindered by the nutrient conditions ( Zhang et al., 2012). Lastly, the TWC is a mixture of the Kuroshio Branch Current and the South China Sea Warm Current (SCSWC) ( Hu et al., 2010). The upwelled nitrate δ15N of Kuroshio in the southern ECS is 5.5 to 6.1‰ ( Liu et al., 1996), and two time series
sediment traps (M1S and M2S, Kao et al., 2012) along the SCSWC have flux-weighted mean δ15N values of 5.2–5.6‰. The sedimentary δ15N of the Okinawa Trough ( Kao et al., 200
cool and the South China Sea ( Higginson et al., 2003, Jia and Li, 2011 and Kienast, 2000) exhibit fewer changes (from ~ 4‰ to ~ 6‰) during the late Holocene. Consequently, changes in the nitrate source and/or its isotopic composition are unlikely to dominate the regulation of the downcore δ15NMOM variations for the THB-2 core. We proposed that the downcore variations of δ15NMOM record the surface relative nitrate utilization, which will be discussed in detail in the next section.
