A Multiple-Isotope Study from the Loki's Castle Vent Field
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Geobiology (2014), 12, 308–321 DOI: 10.1111/gbi.12086 Barite in hydrothermal environments as a recorder of subseafloor processes: a multiple-isotope study from the Loki’s Castle vent field B. EICKMANN,1,2 I. H. THORSETH,1 M. PETERS,3,4 H. STRAUSS,3 M. BROC€ KER 5 AND R. B. PEDERSEN1 1Department of Earth Science, Centre for Geobiology, University of Bergen, Bergen, Norway 2Department of Geology, University of Johannesburg, Johannesburg, South Africa 3Institut fur€ Geologie und Palaontologie,€ Westfalische€ Wilhelms-Universitat€ Munster,€ Munster,€ Germany 4Centre for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China 5Institut fur€ Mineralogie, Westfalische€ Wilhelms-Universitat€ Munster,€ Munster,€ Germany ABSTRACT Barite chimneys are known to form in hydrothermal systems where barium-enriched fluids generated by leaching of the oceanic basement are discharged and react with seawater sulfate. They also form at cold seeps along continental margins, where marine (or pelagic) barite in the sediments is remobilized because of subseafloor microbial sulfate reduction. We test the possibility of using multiple sulfur isotopes (d34S, D33S, Δ36S) of barite to identify microbial sulfate reduction in a hydrothermal system. In addition to multiple sulfur isotopes, we present oxygen (d18O) and strontium (87Sr/86Sr) isotopes for one of numerous barite chimneys in a low-temperature (~20 °C) venting area of the Loki’s Castle black smoker field at the ultra- slow-spreading Arctic Mid-Ocean Ridge (AMOR). The chemistry of the venting fluids in the barite field identifies a contribution of at least 10% of high-temperature black smoker fluid, which is corroborated by 87Sr/86Sr ratios in the barite chimney that are less radiogenic than in seawater. In contrast, oxygen and mul- tiple sulfur isotopes indicate that the fluid from which the barite precipitated contained residual sulfate that was affected by microbial sulfate reduction. A sulfate reduction zone at this site is further supported by the multiple sulfur isotopic composition of framboidal pyrite in the flow channel of the barite chimney and in the hydrothermal sediments in the barite field, as well as by low SO4 and elevated H2S concentrations in the venting fluids compared with conservative mixing values. We suggest that the mixing of ascending H2- and CH4-rich high-temperature fluids with percolating seawater fuels microbial sulfate reduction, which is subsequently recorded by barite formed at the seafloor in areas where the flow rate is sufficient. Thus, low- temperature precipitates in hydrothermal systems are promising sites to explore the interactions between the geosphere and biosphere in order to evaluate the microbial impact on these systems. Received 7 October 2013; accepted 5 March 2014 Corresponding author: B. Eickmann. Tel.: +27 011 559 2308; fax: +27 011 559 4702; e-mail: [email protected] hydrothermal systems a suitable target site to study interac- INTRODUCTION tions between the geosphere and biosphere (Haymon, Seawater circulation and subsequent mixing with high-tem- 1983; Woodruff & Shanks, 1988; Schultz & Elderfield, perature fluids in deep-sea hydrothermal systems result in 1997; Martin et al., 2008). Black smokers mostly precipi- the formation of distinct black smoker structures with typi- tate at temperatures inappropriately high for micro-organ- cal sulfide-rich mineralogical composition and in microbial isms, but steep temperature gradients allow for the communities driven by geochemical energy, making colonization of the black smoker walls (Jannasch, 1985; 308 © 2014 John Wiley & Sons Ltd Loki’s Castle barite chimney 309 Schrenk et al., 2003; Jaeschke et al., 2012). For tracing pursued in this study provides the potential for a deeper these fluid–rock interactions, for example, changes in the insight into the barite formation. mineralogical composition of black smokers/or associated Recent studies expanded our knowledge about barite hydrothermal sediments, and to identify microbial sulfur formation and indicated that barite in hydrothermal sys- cycling, the measurement of 32S and 34S isotopes has been tems did not necessarily precipitate from a mixture of used for decades (Woodruff & Shanks, 1988; Shanks, ambient seawater and a hydrothermal fluid. There are three 2001; Rouxel et al., 2008). However, 32S and 34S do not different oceanic settings in which barite has been observed necessarily allow discrimination between abiological and to precipitate (i) above the seafloor, either as chimneys or biological reactions (Rouxel et al., 2008). In contrast, crusts (Kusakabe et al., 1990; Klugel€ et al., 2011), (ii) in recent developments in measuring the minor sulfur iso- stockwork mineralizations (Luders€ et al., 2001), and (iii) topes 33S and 36S provide deeper insights into the sulfur disseminated in hydrothermal sediments (Peters et al., cycling (Farquhar et al., 2000; Ono et al., 2007, 2012; 2011). Barite in stockwork mineralizations from the JADE Peters et al., 2010). Multiple sulfur isotope measurements hydrothermal field (Central Okinawa Trough) and massive on modern hydrothermal systems are capable of docu- barite from the Henry seamount (Canary archipelago) menting mass-dependent sulfur isotope fractionation along shows d34S and d18O values higher than those for seawater 34 18 the fluid pathways shedding new light into abiogenic and sulfate (d SSO4 ≥ +21.5&; d OSO4 ≥ +9.7&), indicating biogenic processes (Ono et al., 2007, 2012). that microbial sulfate reduction is also taking place in Depending on the chemistry of the vent fluids, sulfate hydrothermal environments (Luders€ et al., 2001; Klugel€ minerals, such as barite (BaSO4), are also commonly et al., 2011). formed at the seafloor. Two processes are known to create Microbial sulfate reduction is also thought to play a such Ba-rich fluids in marine habitats: (i) hydrothermal major role in the low-temperature alteration (≤110 °C) of leaching of the volcanic crust, and (ii) remobilization of the ocean crust (Rouxel et al., 2008; Lever et al., 2013). barite in sediments because of microbial sulfate reduction. Several profiles through oceanic crustal rocks show the pres- Barite precipitation induced by hydrothermal activity yields ence of 34S-depleted sulfides in the volcanic matrix and of d34S and d18O values that are similar or slightly lower than secondary sulfides in late-stage carbonate veins (Rouxel those of contemporaneous seawater sulfate (Kusakabe et al., 2008; Alford et al., 2011; Alt & Shanks, 2011; Ono et al., 1990; Paytan et al., 2002; De Ronde et al., 2003). et al., 2012). However, in cases where abiogenic and bio- In contrast, barite that precipitates from fluids modified by genic sulfides exhibit similar d34S values and therefore do microbial sulfate reduction exhibits d34S and d18O values not allow a distinction, combining d34S with D33S values that are higher than those of contemporaneous seawater allows this distinction (Johnston et al., 2007; Ono et al., sulfate (Fritz et al., 1989; Greinert et al., 2002; Torres 2007). Recent studies demonstrated that multiple sulfur et al., 2003; Brunner et al., 2005; Wortmann et al., 2007; isotopes can be used to identify biogenic sulfides that were Feng & Roberts, 2011; Griffith & Paytan, 2012). The produced by in situ microbial sulfate reduction, allowing strontium isotope (87Sr/86Sr) ratio of barite strongly for deeper insights into the sulfur cycling within the ocean depends on the fluid from which the barite precipitates crust (Ono et al., 2007, 2012; Peters et al., 2010). More- and is therefore ideal for tracing the nature of fluid–rock over, in contrast to the high-temperature vent sites where interaction or the time-dependent evolution of seawater abiogenic processes dominate, low-temperature venting (Paytan et al., 1993; McArthur et al., 2001). Hydrother- areas (≤110 °C) in hydrothermal systems, such as the pres- mal barite is characterized by 87Sr/86Sr ratios between the ently studied barite field, are promising target sites for modern seawater value (87Sr/86Sr = 0.70917) and those of exploring the impact of biogenic processes. Sulfur is a key end-member hydrothermal fluids (87Sr/86Sr = 0.70305), element in many biogenic processes, and its utilization by indicating that these barites precipitate from fluids influ- autotrophic or heterotrophic micro-organisms plays a major enced by hydrothermal activity (Albarede et al., 1981; role in the sulfur cycle in these habitats (McCollom & Griffith & Paytan, 2012). Non-hydrothermal barite that Shock, 1997). Barite chimneys are in particular interesting yields a less radiogenic Sr isotope signature than seawater as they represent low-temperature environments where bio- is indicative of the interaction with older marine sediments logical processes are important. (Griffith & Paytan, 2012). In contrast, barite that exhibits Here, we report combined d34S, D33S, D36S, d18O, and more 87Sr relative to contemporaneous seawater reflects 87Sr/86Sr data for a barite chimney from the recently dis- interaction with fluids or sediments that exhibit a more covered Loki’s Castle hydrothermal vent field at the Arctic radiogenic Sr isotope signature (Dia et al., 1993; Paytan Mid-Ocean Ridge (AMOR) in the Norwegian–Greenland et al., 2002; Torres et al., 2003). The Sr isotope signature Sea and their implications for barite formation and subsur- of barite is not completely unique for a certain setting, and face microbial processes in hydrothermal systems. Loki’s an interpretation that