Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 47-
Introduction Trace metals such as Fe, Cu, and Zn are essential micronutrients for bacteria and phytoplankton in the ocean. These trace metals interact with organic ligands and form organic-metal complexes, which reduces the bioavailable fractions (Bruland et al., 1991). It is therefore important to clarify the chemical species of the trace metals and their biogeochemical cycles in the ocean. However, little information is available on the distributions of dissolved trace metals and those chemical speciation in the hydrothermal system, which show quite different chemical compositions with ambient seawater (Von Damm et al., 1985). In this study, distributions of trace metals (Mn, Fe, Cu and Zn) and Zn chemical speciation were determined to investigate the geochemical processes of trace metals in the Tachibana Bay, where active hydrothermal activity was reported at the coastal area.Methods Seawater samples were collected in the Tachibana Bay, Nagasaki Prefecture by using acid-cleaned X-type Niskin samplers during TS Kakuyo-maru cruise (May 2012). All of seawater samples were filtered with capsule filters (<0.2μm, Acropak). After fltration, sulfide in seawater was determined onboard by using CSV. Samples for the dissolved trace metal concentration were acidified to pH<1.8 with ultra pure HCl and samples for Zn speciation were frozen immediately onboard the ship. On a land-based laboratory, dissolved trace metal concentrations were determined by using ICP-MS. Additionally, dissolved Cu and Zn concentrations were determined by using CSV and compared with those by ICP-MS. For the Zn speciation, ligand concentrations (CL) and conditional stability constants (K′ZnL) were obtained from a titration using competitive ligand equilibrium / adsorptive cathodic stripping voltammetry (van den Berg, 1985).Results and Discussion In the Tachibana Bay, high sulfide concentration (60 nM) was observed at the bottom of the coastal area, reflecting active hydrothermal system. Dissolved Cu and Zn were sharply decreased at the bottom of the coastal area, whereas dissolved Mn and Fe were increased. Also, relatively high total ligand concentration for Zn (CL) was calculated near the bottom. We found relatively high dissolved Zn concentrations with the ICP-MS compared with CSV, while dissolved Cu concentrations were generally similar. Difference in dissolved Zn concentrations between ICP-MS and CSV methods may suggest the presence of nanoparticulate Zn sulfide in the dissolved phase, which is electrochemically inert. The biogeochemical cycling of trace metals in the Tachibana Bay is considerably affected by hydrothermal system.
