水圏微量金属の化学量論比と安定同位体比に基づく地球化学的研究

抄録

Since 1983 I have developed analytical methods for trace metals in the hydrosphere, such as multielemental determination, stable isotope ratio analysis, and speciation, and applied them to marine geochemical study. Recently, I have advanced discussion by using stoichiometry and stable isotope ratios of trace metals. Here I describe two examples.

We have observed full depth sectional distributions of Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in the North Pacific Ocean. Although Ni, Cu, Zn, and Cd were classified as nutrient type elements based on vertical profiles in textbooks, they have distinct sectional distributions. We analyzed the dissolved metal to phosphate ratio (dM/PO₄) and the enrichment factor of dM (EF(dM)=(dM/dAl)/(M/Al)_crust), concluding that the distinct distributions are caused by scavenging. The effect of scavenging increases in the order of Cd<Ni, Zn<Cu. In addition, we found that δCu in deep water increases with the apparent oxygen utilization, while δCd in deep water is almost constant. These results suggest that the light isotope of Cu is preferentially scavenged from deep water as deep water becomes old. Thus, the independent data of stoichiometry and stable isotope ratios support the scavenging hypothesis.

We found the dMo/dW ratio in seawater is approximately 2000, although Mo and W have similar crustal abundances. This is attributed to the difference in distribution ratios of Mo and W to Fe–Mn oxides from seawater. Recently, we determined δMo and δW in seawater. The differences in δMo and δW between seawater and Fe–Mn oxides are consistent with those observed in laboratory experiments. Again, the independent data of stoichiometry and stable isotope ratios support that the distribution onto Fe–Mn oxides has dominant effects on dMo and dW in seawater.