Utility of Nd isotope ratio as a tracer of marine animals: regional variation in coastal seas and causal factors

Isotopic compositions of animal tissue are an intrinsic marker commonly used to trace animal origins and migrations; however, few isotopes are effective for this purpose in marine environments, especially on a local scale. The isotope ratio of the lanthanoid element neodymium (Nd) is a promising tracer for coastal animal migrations. Neodymium derives from the same geologic materials as strontium, well known as an isotopic tracer (⁸⁷Sr/⁸⁶Sr) for terrestrial and anadromous animals. The advantage of the Nd isotope ratio (¹⁴³Nd/¹⁴⁴Nd, expressed as ε_Nd) is that it varies greatly in the ocean according to the geology of the neighboring continents, whereas oceanic ⁸⁷Sr/⁸⁶Sr is highly uniform. This study explored the utility of the Nd isotope ratio as a marine tracer by investigating the variation of ε_Nd preserved in tissues of coastal species, and the causes of that variation, in a region of northeastern Japan where the bedrock geology is highly variable. We measured ε_Nd and ⁸⁷Sr/⁸⁶Sr in seawater, river water, and soft tissues of sedentary suspension feeders: the mussels Mytilus galloprovincialis and Mytilus coruscus and the oyster Crassostrea gigas. We also measured concentrations of three lanthanoids (La, Ce, and Pr) in shellfish bodies to determine whether the Nd in shellfish tissue was derived from solution in seawater or from suspended particulates. The ε_Nd values in shellfish tissue varied regionally (−6 to +1), matching the ambient seawater, whereas all ⁸⁷Sr/⁸⁶Sr values were homogeneous and typical of seawater (0.7091–0.7092). The seawater ε_Nd values were in turn correlated with those in the adjacent rivers, linking shellfish ε_Nd to the geology of river catchments. The depletion of Ce compared to La and Pr (negative Ce anomaly) suggested that the Nd in shellfish was derived from the dissolved phase in seawater. Our results indicate that the distinct Nd isotope ratio derived from local geology is imprinted, through seawater, on the soft tissues of shellfish. This result underscores the potential of ε_Nd as a tracer of coastal marine animals.