ABSTRACT

Distributions of trace metals (Mn, Fe, Cu, and Zn) and Zn chemical speciation were determined in Tachibana Bay to investigate the role of organic complexing ligands and sulfide on Zn in a coastal hydrothermal system. At the coastal area near Obama Hot Springs, dissolved sulfide was only detected in two bottom water samples in Stn. 6 (29.2 and 59.4 nM). At this coastal area, dissolved Mn and Fe concentrations were high and increased toward the bottom, whereas dissolved Cu and Zn concentrations generally decreased toward the bottom. The Zn complexing organic ligands (C_L) and conditional stability constants (log Kʹ_ZnL,Zn2+) in Tachibana Bay ranged from 0.9 nM to 1.6 nM and from 9.9 to 10.3, respectively. The free sulfide concentration calculated from Zn chemical speciation was much lower than the measured dissolved sulfide concentrations. Although dissolved Zn mainly precipitate as sulfide in shallow hydrothermally active area, the organic complexation of Zn is important in the remaining dissolved Zn.