Abstract
BackgroundThe speciation of dissolved sulfide in the water immediately surrounding deep-ocean hydrothermal vents is critical to chemoautotrophic organisms that are the primary producers of these ecosystems. The objective of this research was to identify the role of Zn and Fe for controlling the speciation of sulfide in the hydrothermal vent fields at the Eastern Lau Spreading Center (ELSC) in the southern Pacific Ocean. Compared to other well-studied hydrothermal systems in the Pacific, the ELSC is notable for unique ridge characteristics and gradients over short distances along the north-south ridge axis.ResultsIn June 2005, diffuse-flow (< 50°C) and high-temperature (> 250°C) vent fluids were collected from four field sites along the ELSC ridge axis. Total and filtered Zn and Fe concentrations were quantified in the vent fluid samples using voltammetric and spectrometric analyses. The results indicated north-to-south variability in vent fluid composition. In the high temperature vent fluids, the ratio of total Fe to total Zn varied from 39 at Kilo Moana, the most northern site, to less than 7 at the other three sites. The concentrations of total Zn, Fe, and acid-volatile sulfide indicated that oversaturation and precipitation of sphalerite (ZnS(s)) and pyrite (FeS2(s)) were possible during cooling of the vent fluids as they mixed with the surrounding seawater. In contrast, most samples were undersaturated with respect to mackinawite (FeS(s)). The reactivity of Zn(II) in the filtered samples was tested by adding Cu(II) to the samples to induce metal-exchange reactions. In a portion of the samples, the concentration of labile Zn2+ increased after the addition of Cu(II), indicating the presence of strongly-bound Zn(II) species such as ZnS clusters and nanoparticles.ConclusionResults of this study suggest that Zn is important to sulfide speciation at ELSC vent habitats, particularly at the southern sites where Zn concentrations increase relative to Fe. As the hydrothermal fluids mix with the ambient seawater, Zn-sulfide clusters and nanoparticles are likely preventing sulfide oxidation by O2 and reducing bioavailability of S(-II) to organisms.
Highlights
The speciation of dissolved sulfide in the water immediately surrounding deepocean hydrothermal vents is critical to chemoautotrophic organisms that are the primary producers of these ecosystems
High temperature vent fluids were between pH 3.7 and 5.8; in contrast, the pH of the diffuse-flow samples ranged between pH 6.1 and 7.0 (Figure 2a)
Samples collected from high temperature/low pH sources contained the highest concentrations of acid-volatile sulfide, Fe and Zn (Figures 2b, c and 2d)
Summary
The speciation of dissolved sulfide in the water immediately surrounding deepocean hydrothermal vents is critical to chemoautotrophic organisms that are the primary producers of these ecosystems. Deep-sea hydrothermal vents in the Pacific Ocean support productive ecosystems that rely on symbiotic relationships between chemoautotrophic microorganisms and macroinvertebrates [1,2]. Many of these chemoautotrophs utilize inorganic sulfide as the electron source for carbon fixation. The concentration, speciation, and subsequent bioavailability of sulfide in the hydrothermal fluids change rapidly as warm, highly reducing vent fluids mix with cold, oxygenated seawater. These chemical gradients can govern the structure of the macroinvertebrate communities. Macrofauna that did not rely on these chemosynthetic symbionts thrived in habitats where excess Fe(II) reduced the bioavailability (and toxicity) of S(-II) through the formation of FeS clusters and particles [3]
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