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C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31 21 Table 1 Chemolithoautotrophic pathways in oxic deep-sea hydrothermal environments and the corresponding changes in standard Gibbs free energy (ΔG°) Metabolic process Chemical reaction ΔG° 298K kJ mol −1 Methanotrophy (Jannasch, 1995) Sulfide oxidation (Jannasch, 1995) Iron (II) oxidation (Emerson et al., 2002; Edwards et al., 2003) Hydrogen oxidation (Jannasch, 1995; Takai et al., 2005) CH 4 +2O 2 →HCO 3 − +H + +H 2 O HS − +2O 2 →SO 2− 4 +H + HS − +1/2 O 2 +H + →S°+H 2 O Fe 2+ +1/4 O 2 +3/2 H 2 O→FeOOH+2H + H 2 +1/2 O 2 →H 2 O All thermodynamic constants were calculated from the PHREEQC database, except for iron oxidation 2-line ferrihydrite data reported in Majzlan et al. (2004) were used instead of the PHREEQC iron hydroxide constants. −803 −790 −182 −26 −263 Ltd) attached to the probe as described in Le Bris et al. (2005) (Fig. 1). In 2001, temperature measurements were performed in two shrimp swarms located in depressions on the side of two smoker groups at Rainbow (position markers Iris 7 and Iris 13). A complementary chemical characterization was performed for the swarm at Iris 13. In 2005, a swarm located at exactly the same spot was investigated. To characterize the spatial variation of temperature and dissolved chemical species within a shrimp assemblage, measurements were done in the center of the swarm and in bordering areas to unpopulated chimney walls (Fig. 2a). At TAG the chimneys were covered by huge shrimp swarms. Temperature measurements were done in distinct locations distributed within one of these swarm (Fig. 2b). Several discharges of black smoke were observed on the flank of the chimney complex. 2.2. In situ chemical analysis In 2001 and 2005, the submersible flow analyzer Alchimist (Le Bris et al., 2000) was installed on the Victor and used to determine in situ dissolved ferrous iron and sulfide concentrations by flow injection analysis (FIA). The FIA method for iron analysis is described in Sarradin et al. (2005). The fluid was drawn from the inlet positioned with the manipulator to the colorimetric detection unit installed on the ROV. The calibration of the device was performed in situ, before and after each sampling run. The analyses were done with a frequency of 60 s over a total sampling period of about 6 min for each position. The primary aim of these measurements was to correlate temperature and chemical parameters within the steep gradients encountered in these media. As described previously (Le Bris et al. 2001), the inlet was tightly attached to the temperature probe of the Victor. At TAG, combined Fe II and temperature measurements were obtained in different location of the swarm, as done in other vent fauna habitats (Le Bris et al., 2006). At Rainbow, one set of Fe II and temperature measurements could be obtained in 2001 but the inlet was accidentally detached from the temperature probe preventing the direct combination of the data sets. Successive measurements of temperature and iron at several points within the swarm enabled to establish a coarse correlation between these parameters. These measurements were repeated during the EXOMAR cruise at Rainbow. However, due to technical problems, measurements were obtained at only one point within the swarm. At both sites, Rainbow and TAG, the determination of the S − II /temperature correlation could not be achieved due to failure in the sulfide analyzer unit or the temperature probe. Simultaneous analyses of Fe II and S − II could solely be obtained in the surrounding of a swarm at Rainbow during the ATOS cruise. 2.3. Fluid sampling and analysis Discrete fluid samples were obtained in 2001 and 2005 using the VICTOR6000 pumping fluid multisampler consisting of 19 evacuated 200 ml titanium bottles, which were rinsed with 0.1N HCl and deionised water before operation. In 2005, the ROV temperature probe was coupled to the sampler inlet, enabling to measure temperature simultaneously to the fluid sampling. All samples were obtained within the swarms. In 2001, a few samples were additionally obtained with gas-tight titanium bottles. For these samples a combined autonomous probe enabled to set the average temperature at the sampling point. Since the ROV sampler inlet was not equipped with a temperature sensor in 2001, temperature data are therefore not available for these samples. In 2001, a punctual source of fluid in the immediate environment of the shrimp assemblage was sampled as well with titanium syringes. The pH was measured with a conventional pH electrode immediately after the sampling bottles were recovered on board. For oxygen measurements, sub-samples were carefully transferred in glass rod flasks avoiding the formation of gas bubbles. In order to fix oxygen, Winkler reagents were immediately added before subsequent titration (ATOS 2001).
22 C. Schmidt et al. / Marine Chemistry 108 (2008) 18–31 Table 2 Temperatures measured at different locations in a shrimp swarm at Rainbow (upper) and TAG (lower) as displayed in Fig. 2 (SD: standard deviation, N: number of data) Sampling point T min [°C] T mean [°C] T max [°C] SD [°C] N 1 5.3 6.2 6.7 0.4 12 2 5.3 6.2 7.1 0.4 27 3 6.4 7.7 9 0.6 29 4 9.7 11.9 14.5 1.3 33 5 6.8 8.5 11.3 1 26 6 7.3 10.9 15.8 2.2 29 7 7.2 12.3 18.2 2.2 28 8 9.3 10.6 14.6 1.2 30 9 7.4 9.3 12.7 1.2 32 10 11.4 16 17.3 1.3 28 1 5.3 7.2 11.9 1.2 97 2 2.8 5.3 7.9 1.2 94 3 3.3 5.9 9.1 1.3 97 4 4 7.1 13.3 1.7 86 5 5.1 6.4 8.2 0.7 97 6 4 7.5 12.6 1.6 85 7 3.6 6 8.5 1.1 96 8 4.1 9 17.4 2.5 90 9 4 10.2 16.6 3.2 96 10 3.6 4.6 5.4 0.4 97 Alternatively, oxygen was directly measured using an amperometric microelectrode (EXOMAR 2005). Fluid subsamples were preserved in gas-tight flasks under the addition of Hg 2 Cl 2 for CO 2 and CH 4 on-shore GC-analysis (ATOS samples). Applied analytical methods are described in Sarradin et al. (1998). Total iron was preserved in acidified sub-samples and analyzed on-shore using the ferrozine method after reduction of ferric iron with ascorbic acid. 2.4. Geochemical modeling Geochemical calculations were performed applying the computation code PHREEQC (version 2.8) (Parkhust and Appelo, 1999) in order to simulate the chemical composition of the mixing zone as function of temperature in the shrimp environment. The program is based on chemical equilibria in aqueous solutions interacting with minerals. The PHREEQC database accounts for temperature effects on thermodynamic constants but not for the influence of high pressure (about 23 MPa at 2300 m depth and 35 MPa at 3500 m depth). As hypothesized in Le Bris et al. (2003) the effect of pressure should be of minor importance for the investigated reactions in the temperature range (2–30 °C). The thermodynamic constants for the considered chemical species were defined from the PHREEQC database, except for aqueous ferrous iron-sulfide complexes. For these complexes, the database has been upgraded using data from Rickard and Morse (2005). According to these authors, only Fe(HS) + displays a certain congruency among authors with a determined log K close to 5.2. Additionally, soluble iron-sulfide molecular clusters (FeSaq) are expected to dominate labile iron-sulfide forms in hydrothermal fluids (Luther et al., 2001). According to Theberge and Luther (1997), these complexes will form at saturation of the medium with respect to precipitated ironmonosulfide FeS. The Ks used in our calculation for this precipitate corresponds to the value defined by Benning et al. (2000). Full equilibration between the dissolved species and the solid phase was not allowed, unless specified. The progressive mixing of the end-member fluid with seawater was modeled assuming that the chemistry of the hydrothermal fluid at Rainbow corresponds to the composition recorded in 1997 as presented in Charlou et al. (2002) and Douville et al. (2002). The end-member composition used for TAG refers to Edmonds et al. (1996). Theseendmembers are termed ‘reference end-members’ in the following sections. This definition does not infer a stable composition of the end-members over years, but rather provides a comparison basis from which the composition of the local source fluid could be estimated. Fitting the model outputs to empirical data enabled to quantify the depletion of Fe II and S − II in the local source fluids with respect to the conservative dilution of the reference end-members. The concentration of methane in the mixing gradient was estimated by similar adjustment to the field data. Furthermore, different scenarios were considered to account for oxygen consumption by electron donors which could spontaneously occur in the mixing zone or in the sampling bottles. For this purpose O 2 -pH and O 2 -T data-sets Table 3 Compilation of thermal data (mean values and ranges) in the immediate environment of the shrimp in different swarms at the Mid-Atlantic Ridge (RB: Rainbow, SP: Snake Pit, L: Logatchev) Site Location T [°C] Reference Diverse Swarm 10–40 Gebruk et al. (1993) SP Swarm 10–15 (5–37) Segonzac et al. (1993) L Swarm N20 Gebruk et al. (1993) RB Swarm 9–25 Desbruyères et al. (2000) RB Swarm 13.2 Desbruyères et al. (2001) RB Swarm 11 (4.7–25) Geret et al. (2002) RB Swarm 1 11.8 (3.9–16.6) this study (ATOS 2001) RB Swarm 2 8.7 (4.5–18.3) this study (ATOS 2001) RB Swarm 3 11.5 (3.2–18) this study (EXOMAR 2005) TAG Swarm 7 (2.8–17.4) this study (EXOMAR 2005)
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