10:45 AM - 11:00 AM
[HTT18-07] Spatial distribution of Nd isotope ratio of seawater and its seasonality in Sendai Bay, Northeast Japan
Keywords:Nd isotope ratio, seawater, marine animals, marine products, Kuroshio Current, Oyashio Current
The results show that Nd isotope ratio of seawater in Sendai Bay varies widely from nearshore to offshore, and from surface to deeper layer, reflecting values of terrestrial water (εNd = -2 – +0.5), Kuroshio Current (εNd = ~-6) and Oyashio Current (εNd = ~-3). Common trend among sampling periods is that, at the 10 m deep level, εNd generally decreases from nearshore to offshore although it slightly increases at the most offshore site C22, at which seawater εNd is close to that of Oyashio Current (εNd = ~-3). In the vertical 2D profile, εNd is high at nearshore surface layer and low at offshore deeper layer both in July and December of 2017 (Fig. B).
The range of spatial variation is wider in summer than in winter. In summer, εNd of seawater ranges from -6 at the 100 m depth of C16 to -2 at the surface of C07, while it ranges from -5 at the 100 m depth of C16 to -3 at the surface of C07. Negative correlation of εNd to salinity suggests that the high εNd value in nearshore water derives from land water, while the low value in offshore deep water is from the Kuroshio Current in summer (Fig. C). On the other hand, in winter, except for seawater from 10 m depth of C01, salinity of all the other samples is high (>33.7) despite the generally high value of εNd (-4 – -2) (Fig. C). Oyashio Current is suggested to be prevailed in winter. In contrast with εNd and salinity, δ18O shows little variation spatially and temporally ranging from -0.6 to 0.1. Prominently slack mixing of Nd between different water masses compared to molecular of water is suggested to be the cause of wide variation in εNd values.