Distribution of dissolved Ni, Cu, and Zn and their isotopes in the Southern Ocean and the South Pacific Ocean

Nickel, copper, and zinc play important roles as micronutrients for organisms and as geochemical tracers in the ocean. To elucidate the biogeochemical cycling of these metals, their isotope ratios will be promising. We present distribution of concentrations and isotope ratios for dissolved Ni, Cu, and Zn in a meridional section along 170°W from 64°S in the Southern Ocean to 0°S in the equatorial Pacific (GEOTRACES section, GP19). Along this section, concentrations for Ni, Cu, and Zn show nutrient-type distribution. In the South Pacific, the maximal concentrations of Ni, Cu, and Zn occur at depths of 1000−4000 m where Pacific Deep Water (PDW) and Upper Circumpolar Deep Water (UCDW) are the dominant water masses. The isotope ratios of Ni (δ⁶⁰Ni) are almost constant in deep water at 1.3 ‰ and increase up to 1.8 ‰ in surface water. Isotope ratios of Zn (δ⁶⁶Zn) are lower in surface water than those of 0.5 ‰ in deep water. Isotope ratios of Cu (δ⁶⁵Cu) are ~0.4 ‰ in surface water and increase with depth. Unlike δ⁶⁰Ni and δ⁶⁶Zn, δ⁶⁵Cu is not constant in deep water. δ⁶⁵Cu shows maxima of ~0.7 ‰ at depths of 1000−4000 m, and decreases to ~0.5 ‰ in further depths.

For Ni, the isotope ratio negatively correlates with the logarithmic concentration of Ni in surface water. This suggests that phytoplankton preferentially take up light isotopes of Ni. For Zn and Cu, the isotope ratios are lighter in surface water than in deep water, although the biological uptake prefers light isotopes in a similar manner to Ni. Therefore, the isotope ratios of Zn and Cu in surface water would be controlled by other processes, such as adsorption onto particles and/or aerosol deposition. δ⁶⁵Cu in deep water correlates with AOU, suggesting removal of light Cu from the water column by scavenging or supply of heavy Cu from sediments in the North Pacific.