The TAG hydrothermal mound on the Mid‐Atlantic Ridge (26°08′N, 44°50′W) was revisited in August 1994 with the submersible Shinkai 6500 in order to characterize time‐series fluid chemistry prior to the ODP drilling. Fluid samples were taken from both black smokers and white smokers. Si, pH, alkalinity, H 2 S, major cations (Na + , K + , Ca 2+ , Mg 2+ ), major anions (Cl − , SO 4 2− ), and minor elements (Li, Sr, B, Fe, Mn, Cu, Zn, Br) as well as Sr isotope ratios were measured. We report the first Br/Cl ratios for the TAG hydrothermal fluids, showing no fractionation between Br and Cl during the fluid‐rock interaction. This study shows small changes in composition of the black smoker fluids from the 1990 data (Edmond et al., 1995). Changes of pH, alkalinity, Fe, K, and 87 Sr/ 86 Sr values are suggestive of subsurface FeS precipitation and a decrease of water/rock ratio at a deeper reaction zone. Differences in chemical characteristics between the black and white smoker fluids were similarly observed as in 1990.
Abstract We collected free‐gas and in situ fluid samples up to a depth of 200.6 m from the Sagara oil field, central Japan (34°44′N, 138°15′E), during the Sagara Drilling Program (SDP) and measured the concentrations and stable carbon isotopic compositions of CH 4 and C 2 H 6 in the samples. A combination of the CH 4 /C 2 H 6 ratios with the carbon isotope ratios of methane indicates that the hydrocarbon gases are predominantly of thermogenic origin at all depths. The isotope signature of hydrocarbon gases of δ 13 < δ 13 suggests that these gases in the Sagara oil field are not generated by polymerization, but by the decomposition of organic materials.
The concentration of total metal sulfide throughout a water column over a submarine hydrothermal vent in Kikai Caldera south of Kyushu Island, Japan, at ~350 pmol/kg, was higher than that reported in previous studies, at <50 pmol/kg below the halocline. Seawater filtered at 0.2 μm pore size and unfiltered seawater exhibited almost identical metal sulfide concentrations throughout the water column, indicating that most metal sulfide existed in dissolved and particulate forms with diameters <0.2 μm. By using a mass balance calculation with the observed sulfide species of free and metal sulfides and carbonyl sulfide, we showed that ~70% of the metal sulfide supplied from hydrothermal vents were contained in the water column beyond the halocline without undergoing oxidative loss even after mixing into overlying oxic seawater. Our findings clearly indicate that sulfide and trace metals emitted from hydrothermal vents form a stable metal-sulfide complex with diameters <0.2 μm. These results also strongly support the recently proposed theory such that metal-sulfide complexation/nanoparticles play an important role in the long-distance transportation of trace metals in the ocean.
Abstract We provide an estimation of the heat output necessary to generate the neutrally buoyant plume above the TAG hydrothermal mound, Mid-Atlantic Ridge, located at 26°N, using a model of plume rise in a density-stratified environment with crossflow. The estimated heat output is 460 ± 40 MW. Previous studies have estimated that the heat outputs from high-temperature hydrothermal discharge and low-temperature diffuse flow at the TAG hydrothermal mound are 90 ± 20 MW and at least 780 MW, respectively. Consequently, the contribution of diffuse flow to make the neutrally buoyant plume is 370 ± 60 MW, which accounts for approximately 80% of the heat output to the neutrally buoyant plume. As this contribution is less than 50% of the total heat output from the diffuse flow, it is likely that more than 50% of the heat output from the diffuse flow dissipates in the ambient current.
