Chemical compositions of mussels and clams from the Tangyin and Yonaguni Knoll IV hydrothermal fields in the southwestern Okinawa Trough

Studies of the chemical characteristics of mussels and clams in seafloor hydrothermal fields are important for understanding mass fluxes and elemental partitioning from hydrothermal vents into the biosphere, metal bioaccumulation of seafloor hydrothermal ecosystems, and the sources and sinks of biogeochemical and fluid cycles. We are the first to measure the mineral, major, trace and rare earth element, and carbon and oxygen isotope compositions of mussels (Bathymodiolus platifrons) and clams (Conchocele bisecta) from the Tangyin and Yonaguni Knoll IV hydrothermal fields in the southwestern Okinawa Trough. Mineralogical analysis shows that the carbonate shells of the mussel and clam samples are mainly composed of calcite and aragonite. Metal elements exhibit linear correlations in the shells (e.g., V and U) and tissues (e.g., Li and Rb) of the mussels and clams, suggesting that not all positive correlations of elements in tissues are inherited by the shells. V/As, Ca/Sr, and Fe/Cr ratios in the mussels and clams are close to those in the seawater, indicating that element ratios of seawater might be inherited by the mussels and clams. In addition, the Fe/Cr ratio of the shells of both mussels and clams can be used to trace the local seawater composition. The total LREE concentrations of mussel and clam tissue samples are higher than those of the mussel and clam shell samples, are similar to the hydrothermal fluids, exhibit LREE enrichment (La-CN/Nd-CN ratios = 1.86-32.1), and no or only slightly negative Eu anomalies, indicating that benthic animals are a sink of LREEs from hydrothermal fluids, and that the Eu/Eu* ratios of fluids change when fluids are incorporated into the tissues of the mussels and clams. In addition, the delta C-13 values of mussel shell samples are heavier than those of the clam shell samples in the hydrothermal field, indicating that more than one carbon source may be involved in defining the delta C-13 compositions of the shells. The majority of the 8180 values of clam shell samples fall in the range of delta O-18 values of the mussel shell samples, and are close to the hydrothermal fluid delta O-18(H20) values, implying that the delta O-18 values of mussel and clam shell carbonate is influenced by the hydrothermal environment (magmatic water and fluid dilution with seawater). (C) 2016 Elsevier B.V. All rights reserved.