[H-TT21] Development and applications of environmental traceability methods

[HTT21-16] Spatial variation of neodymium and strontium isotope ratios of shellfish soft bodies in the coastal sea of eastern Tohoku District

*Saitoh Yu¹, Takanori Nakano¹, Ki-Cheol Shin¹, Katsuyuki Yamashita², Hiroshi Amakawa⁴, Chikage Yoshimizu¹, Jun Matsubayashi¹, Yoshikazu Kato¹, Hiroyuki Togashi³, Yosuke Amano³, Yutaka Kurita³, Noboru Okuda¹, Ichiro Tayasu¹ (1.Research Institute for Humanity and Nature, 2.Graduate School of Natural Science and Technology, Okayama University, 3.Tohoku National Fisheries Research Institute,Fisheries Research Agency, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:Nd isotope ratio of organisms, soft bodies of shellfish, marine ecosystem

Sr isotope ratio (⁸⁷Sr/⁸⁶Sr) has been used as a geographical index of water and vegetation and a tracer of animal migration in terrestrial ecosystem. However, it has little power in marine ecosystem because of extremely homogeneous ⁸⁷Sr/⁸⁶Sr ratio in seawater. In contrast, the Nd isotope ratio (¹⁴³Nd/¹⁴⁴Nd) has a potential to become effective tracer of marine organisms because the ¹⁴³Nd/¹⁴⁴Nd ratio of seawater is known to show a variation in the area and depth in the ocean (Amakawa et al., 2004). Nonetheless, there is little report on the Nd isotope ratio for marine and even terrestrial organisms, largely due to the extremely low content of Nd in organisms, and the resultant poor awareness of the element in ecological fields.
In order to explore the potential of Nd isotope as a biogeographical tracer in marine ecosystem, we determined the isotope ratios of Nd and Sr in soft bodies of shellfishes, mainly oysters and mussels in the coastal sea of eastern Tohoku district. Our results show that the ¹⁴³Nd/¹⁴⁴Nd ratio of the shellfish has a wide variation (0.5123-0.5128), whereas the ⁸⁷Sr/⁸⁶Sr ratio lies in the narrow range (0.70912-0.70921), which is almost equivalent to the value of seawater. Although the shellfish has lower ¹⁴³Nd/¹⁴⁴Nd ratio than the exchangeable fraction of river sediments in the watershed of the coast, both ratios show a strong positive relationship (r² = 0.75). Further, the shellfishes have higher ¹⁴³Nd/¹⁴⁴Nd ratios than seawater in the northern Pacific, and the exchangeable ¹⁴³Nd/¹⁴⁴Nd ratio of river sediments has a weak negative relationship with the ⁸⁷Sr/⁸⁶Sr ratio of the associated river water (r² = 0.37). These results demonstrate that the shellfish contains Nd derived from the watershed rocks of the adjoining river and Nd from oceanic water. It is concluded that the Nd isotope ratio has promise as a geographical index of habitat and foraging site of marine organisms and an effective tracer of marine animal migration at least in coastal sea zones.