[H-TT18] 環境トレーサビリティ手法の開発と適用

[HTT18-08] Zinc stable isotopes in trace metal fractions of sediment cores from metal polluted areas

*Kai Nils Nitzsche¹、Toshihiro Yoshimura¹、Naoto F Ishikawa¹、Nanako O Ogawa¹、Katsuhiko Suzuki²、Naohiko Ohkouchi¹ (1.Biogeochemistry Research Center, Japan Agency for Marine-Earth Science and Technology、2.Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology)

キーワード：Zinc isotope, Sediment, Sequential extraction, Metal pollution, Trace metal

Zinc stable isotopes (δ⁶⁶Zn) of dated sediment cores have become popular tracers for Zn contamination throughout the time. Yet, the δ⁶⁶Zn of bulk sediments represents a complex mixture of Zn adsorbed on environmental surfaces, e.g., oxyhydroxides and carbonates, Zn as part of organic matter or Zn in lattice structures of minerals. Here, we have analyzed the Zn stable isotopes of metal fractions obtained by the optimised BCR sequential extraction procedure in sediment cores from historically metal polluted Lake Biwa, Osaka Bay and Tokyo Bay. We found the anthropogenic Zn was primarily hosted in the acid-labile (carbonates) and reducible (Fe-Mn oxyhydroxides) fractions. We expect to find the highest δ⁶⁶Zn values in the acid-labile fraction in agreement with the preferential adsorption and incorporation of ⁶⁶Zn in carbonates. The δ⁶⁶Zn values in the residual fraction (silicates) should agree with those found for the bulk silicate Earth. We expect to find the lowest δ⁶⁶Zn values in the reducible fraction as sinking Mn oxyhydroxides formed in the water column scavenge dissolved Zn released from anthropogenic sources with no or little isotopic fractionation. Thus, we conclude that δ⁶⁶Zn of the reducible fraction should represent a better indicator of the δ⁶⁶Zn of the anthropogenic Zn compared to the bulk sediment.