A Multiple-Isotope Study from the Loki's Castle Vent Field

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 ultraslow-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 multiple 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;

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 is solely based on 87Sr/86Sr ratios Castle is a basalt-hosted, sediment-influenced black smoker will remain ambiguous, but a multiple-isotope approach as system emanating vent fluids that in addition to H2S are

high in CH4,H2, and NH4 (Pedersen et al., 2010a). The into a composite mound. The size of the Loki’s Castle chemical load of these vent fluids provides various energy hydrothermal field is comparable with the Trans-Atlantic sources utilized by diverse microbial communities in and Geotraverse mound (TAG) on the Mid-Atlantic Ridge (Pe- on black smokers (Dahle et al., 2012; Jaeschke et al., dersen et al., 2010a). At the eastern flank of this composite 2012). mound, about 50 m from one of the black smoker chimneys, a dense field of numerous up to 1-m-tall active and extinct barite chimneys have been found. The barite chim- GEOLOGICAL SETTING AND SAMPLING neys are characterized by several branches, and active venting The AMOR in the Norwegian–Greenland Sea is part of the chimneys are typically covered by white microbial mats global ridge system that is characterized by ultraslow-spread- (Fig. 1C). Microbial mats also occur directly on the hydro- ing rates (below 20 mm a 1). The Loki’s Castle vent field is thermal sediment surface and on siboglinid tube worms, located at 73°300N and 8°E where the Mohns Ridge bends which are abundant in the barite field (Fig. 1D). The tem- northward into the Knipovich Ridge (Fig. 1A; Pedersen perature of clear shimmering water locally observed above et al., 2010a). Here, an approximately 30-km-long axial vol- microbial mats has been measured to be approximately canic ridge (AVR) hosts the Loki’s Castle vent field 20 °C. The barite chimneys and microbial mats frequently (Fig. 1B). At a water depth of ~2400 m, four black smoker occur along lines, which likely reflect the pattern and rate of chimneys emanate high-temperature (~320 °C) vent fluids. the fluid flow (Fig. 1D). The black smoker fluids have a

The up to 13-m-tall black smoker chimneys formed two pH of 5.5 and are characterized by H2S concentrations in 1 hydrothermal mounds (150 m apart) that are 20–30 m high the range of 2.6–4.7 mmol kg , as well as high CH4 1 1 and about 150–200 m across at the base where they coalesce (12.5–15.5 mmol kg ), H2 (4.7–5.5 mmol kg ), NH4,

Fig. 1 (A) Simplified map of the southern part of the Arctic Mid-Ocean Ridge (AMOR). The black box marks the location of the Loki’s Castle vent field. (B) Location of the Loki’s Castle vent field. (C) Numerous active (white) and extinct (brown) barite chimneys are associated with microbial mats and siboglinid tube worms in a low-temperature venting area. (D) Close-up of barite chimneys occurring in lines.

1 1 (4.7–6.1 mmol kg ), and CO2 (22.3–26.0 mmol kg ) Stable oxygen and multiple sulfur isotope measurements concentrations (Pedersen et al., 2010a). We sampled a total of twelve subsamples from the exterior For this study, we subsampled a barite chimney (GS09- and interior of the barite chimney (GS09-ROV7-1) to ROV7-1), hydrothermal surface sediment (GS10-ROV4) determine temporal changes in the oxygen (d18O), multi- typically containing abundant framboidal pyrite, as well as ple sulfur (d34S, D33S, D36S), and strontium (87Sr/86Sr) shimmering fluids above mat-covered chimneys (GS09- isotopic composition during chimney growth (Table 1). ROV7-BS and GS09-ROV8-BS). All samples were col- The subsamples were crushed to fine powder in an agate lected during R/V G.O. Sars cruises in 2009 and 2010 mortar. For traditional sulfur isotope measurements using a Bathysaurus XL remotely operated vehicle (ROV). (34S/32S), an aliquot of 250 lg of barite powder was In addition, ambient deep seawater samples (GS09-CTD7- homogeneously mixed with an equal amount of vanadium 2 and GS09-CTD8-4) were collected using a CTD rosette. pentoxide (V O ) in a tin capsule. Sulfur was liberated as A ROV-mounted fluid sampling device was used to sample 2 5 SO with a Carlo Erba elemental analyzer and measured the diffuse venting fluids. 2 for its d34S using a ThermoFinnigan Delta Plus mass spec- 18 trometer (EA-IRMS). For d Obarite, 200 lg of barite ANALYTICAL METHODS powder was weighed into silver capsules. Sulfate oxygen was liberated, although pyrolysis as carbon monoxide at Petrography 1450 °C in a temperature conversion/elemental analyzer (TC/EA) interfaced with a ThermoFinnigan Delta Plus XL Transmitted light microscopy of petrographic thin sections (cf., Kornexl et al., 1999; Turchyn & Schrag, 2006). was performed on a Nikon Eclipse LV1000POL that was Results are reported in the common d notation as permil equipped with a Nikon DS-FIL camera. Pictures were difference to the Vienna Standard Mean Ocean Water ref- taken using the NIS Elements BR (Nikon, Amsterdam, the erence (V-SMOW) for d18O and to the Vienna Canyon Netherlands) software package. Scanning electron micros- Diablo Troilite reference (V-CDT) for d34S: copy was carried out by a Zeiss Supra 55VP (Oslo,

Norway) ﬁeld emission scanning electron microscope 18 18 16 18 16 d O ¼ð O= OsampleÞð O= OreferenceÞ (FE-SEM), equipped with a Thermo Noran System SIX ð1Þ =ð18 =16 Þ energy dispersive spectrometer (EDS) system. SEM/BSE O Oreference 1000 images were acquired using carbon-coated petrographic d34 ¼ð34 =32 Þð34 =32 Þ thin sections, chimney fragments, and sediment. For min- S S Ssample S Sreference ð2Þ 34 32 eral identiﬁcation, a Bruker D8 Advance Series 2 powder =ð S= SreferenceÞ1000 X-ray diffractometer (XRD) operated at 40 kV and 40 mA with monochromatic Cu-Ka1 radiation was used. All min- Accuracy of mass spectrometric measurements was moni- eralogical and petrographical methods were performed at tored using the international reference materials IAEA-S1 the University of Bergen. (0.3&), IAEA-S2 (+22.4&), and IAEA-S3 (32.5&)

Table 1 Compilation of isotope data (d34S, D33S, D36S, d18O, 87Sr/86Sr) for the barite chimney, framboidal pyrite, and seawater samples

Sample ID Material d18O(&) d34S(&) D33S(&) D36S(&) 87Sr/86Sr

† GS09 ROV 7-1-1 Barite chimney 10.9 23.0 0.035 0.545 0.708379 † GS09 ROV 7-1-2 Barite chimney 10.7 22.2 0.031 0.737 0.708426 † GS09 ROV 7-1-3 Barite chimney 10.5 23.6 0.046 0.698 0.708394 † GS09 ROV 7-1-4 Barite chimney 10.6 22.2 0.041 0.707 0.708382 † GS09 ROV 7-1-5 Barite chimney 12.9 30.8 0.103 1.273 0.708392 † GS09 ROV 7-1-6 Barite chimney 11.5 24.3 0.059 0.851 0.708344 ‡ GS09 ROV 7-1-7 Barite chimney 14.4 30.8 0.095 1.111 0.708345 ‡ GS09 ROV 7-1-8 Barite chimney 15.6 33.2 0.125 1.483 0.708334 ‡ GS09 ROV 7-1-9 Barite chimney 15.4 33.9 0.133 1.078 0.708332 ‡ GS09 ROV 7-1-10 Barite chimney 15.3 34.2 0.126 2.615 0.708329 ‡ GS09 ROV 7-1-11 Barite chimney 15.9 36.1 0.159 1.679 0.708343 ‡ GS09 ROV 7-1-12 Barite chimney 15.1 32.7 0.135 0.965 0.708332 § GS09 ROV 7-1 Framboidal pyrite n.d. 9.9 0.016 0.475 n.d. ¶ GS10 ROV 4 Framboidal pyrite n.d. 16.4 0.115 0.370 n.d. GS09 CTD 7-2 Seawater 8.2 21.3 0.020 0.457 n.d. GS09 CTD 84 Seawater 8.3 21.2 0.040 1.760* n.d. n.d., not determined.*Analytical problems.†Barite chimney outside.‡Barite chimney inside.§Framboidal pyrite in the ﬂow channel of the barite chimney.¶Framboidal pyrite in the sediment from the low-temperature barite ﬁeld.

© 2014 John Wiley & Sons Ltd 312 B. EICKMANN et al. for sulfur and NBS 127 (+8.6&), IAEA-SO5 (12.1&), from barite powder and further puriﬁed by standard ion- and IAEA-SO6 (11.3&) for oxygen isotopes. Analytical exchange procedures (AG 50W-X8 resin) on quartz glass reproducibility as determined from replicate analyses was columns using 2.5 N HCl as eluent. For mass spectromet- better than 0.3& for sulfate sulfur and better than ric analysis, Sr was loaded with TaF5 on W ﬁlaments. 0.5& for sulfate oxygen isotope measurements. Isotopic ratios were determined in static mode on a Finni- For multiple sulfur isotope measurements, barite was con- gan-MAT Triton multicollector mass spectrometer. Cor- 86 88 verted to Ag2S using Thode solution (HI + HCl + H3PO2; rection for mass fractionation is based on a Sr/ Sr

Thode et al., 1961). Subsequently, Ag2S was converted to ratio of 0.1194. Total procedural blanks were below sulfur hexafluoride (SF6) via fluorination in nickel tubes (cf. 0.1 ng. Analysis of NBS standard 987 yielded a long-term Ono et al., 2006a). The cryogenically and chromatographi- average 87Sr/86Sr ratio of 0.710255 0.000034 (2r, cally purified SF6 was introduced into a ThermoScientific n = 30). MAT 253 mass spectrometer via a dual-inlet system, and the 32S, 33S, 34S and 36S isotopes were measured simulta- Low-temperature vent fluid analyses neously. D33S and D36S values were calculated from d33S, d34S, and d36S values: The low-temperature fluids (GS09-ROV7-BS and GS09-

ROV8-BS) were analyzed for H2S and NH4 using the : D33S ¼ d33S 1000 ðð1 þ d34S=1000Þ0 515Þ1 ð3Þ photometric methylene blue (Cline, 1969) and indophenol (Solorzano, 1969) methods, respectively, on-board imme- : D36S ¼ d36S 1000 ðð1 þ d34S=1000Þ1 90Þ1 ð4Þ diately after retrieval. Aliquots for later shore-based analy-

ses of SO4 by ion chromatography, and of K, Na, Mg, and 33 36 Precision for D S and D S (i.e., including ﬂuorination) Ba by inductively coupled plasma—optical emission spec- was 0.009& and 0.28& (1r), respectively. For isotopic trometry (ICP-OES), were ﬁltered (0.2 lm) and collected measurements of the dissolved sulfate in seawater samples, in HDPE bottles. The samples for ICP-OES analyses were

1.5 mL of barium chloride (BaCl2) solution (8.5%) was acidified with ultrapure HNO3 and stored in acid-cleaned added to 10 mL of acidified (pH 2) seawater. The precipi- bottles. The samples were stored at 4 °C until analyzed. tated barium sulfate (BaSO4) was centrifuged and dried. Framboidal pyrite from the barite chimney flow channel and the surface sediment was extracted by a wet chemical extrac- RESULTS tion process, using 25% hydrochloric acid (HCl) and a 1 M chromous(II)chloride solution to liberate any acid-volatile Morphology and mineralogy of the chimney sulfur (AVS, i.e., mono-sulfides) and the chromium-reduc- The sampled barite chimney was characterized by several ible sulfur (CRS; cf. Canfield et al., 1986), that is, disulfides branches that were covered by microbial mats indicating as hydrogen sulfide (H2S). The H2S was precipitated in a 3% active venting (Fig. 1D). We selected one major branch zinc acetate (ZnAc) solution and further converted to Ag2S (GS09-ROV7-1) with a distinct gray flow channel sur- by adding 10 mL of a 0.1 M silver nitrate (AgNO3) solution. rounded by a white, highly porous, and friable chimney All stable isotope measurements were taken at the Stable Iso- wall. Microscopic investigations indicated that the white tope Laboratory at the Institut fur€ Geologie und Pal€aontolo- chimney material consisted exclusively of barite, while the gie, Westf€alische Wilhelms-Universit€at Munster€ that is part darker flow channel was partially lined by framboidal pyr- of the Munster€ Isotope Research Center (MIRC). ite. XRD analysis confirmed that barite dominated the To calculate the exponent that characterizes the mass- chimney wall, and only trace amounts of pyrite were pres- dependent fractionation in a system, we used the definition ent. However, microscopy and XRD analysis revealed that by Johnston et al. (2005), which describes the fraction- framboidal pyrite was abundant in the hydrothermal sur- ation between a pair of samples, here sulfate and sulfide. face sediment in this barite field.

33 33 33 k ¼ lnð1 þ d Ssulfide=1000Þlnð1 þ d Ssulfate=1000Þ= 34 34 Chemical composition of fluid samples lnð1 þ d Ssulfide=1000Þlnð1 þ d Ssulfate=1000Þ ð5Þ The chemical composition of the low-temperature vent fluids (GS09-ROV7-BS, GS09-ROV8-BS) and the ambient deep seawater samples (GS09-CTD7-2, GS09-CTD8-4) is shown in Table 2 and illustrated in Fig. 2 together with Strontium isotope measurements the end-member composition of the high-temperature Strontium isotope measurements were taken at the black smoker fluids. The low-temperature hydrothermal € € € Institut fur Mineralogie, Westfalische Wilhelms-Universitat fluids are significantly lower in Mg, Na, and SO4, and € Munster. For this purpose, Sr was leached with 6 N HCl higher in NH4, K, Ba, and H2S, compared with the

Table 2 Chemical composition of end-member high-temperature vent fluid (Pedersen et al., 2010a; Baumberger, 2011), diffuse low-temperature fluids from the barite field, and background deep seawater samples (this study)

Na (mM)K(mM)Mg(mM)Ca(mM)SO4 (mM)H2S(mM)NH4 (mM)Ba(lM)

EM high-T ﬂuid 397 34.5 0.0 28.1 0.7 3.59 5.4 40.5 GS09 ROV 7-BS 442 11.9 46.9 11.5 24.1 0.1 0.6 5.1 GS09 ROV 8-BS 440 11.8 46.8 11.4 24.8 0.1 0.6 1.7 GS09 CTD 7-2 445 9.6 51.6 10.1 29.2 0.0 0.0 0.0 GS09 CTD 8-4 440 9.5 51.6 10.2 29.2 0.0 0.0 0.1

EM, end-member high-temperature vent fluid; ROV, remotely operated vehicle. seawater, indicating fluid mixing and microbial sulfate (Fig. 3A), whereas D33S values (0.031 to 0.103&) are reduction in the subseafloor of the barite field. lower than respective values for ambient seawater sulfate (Fig. 4A). Δ36S values (0.545–1.273&) are higher than ambient seawater sulfate (Fig. 4B). The 87Sr/86Sr ratios of Isotopic composition of barite, seawater sulfate, and the outer part range from 0.70834 to 0.70843 (Fig. 3B). framboidal pyrite The inner part of the chimney wall shows even higher Both the d18O and d34S values of the barite chimney d18O(+14.4 to +15.9&) and d34S values (+30.8 to (GS09 ROV-7) are higher in the inner part of the chimney +36.1&), as well as the most negative D33S(0.159 to wall than in the outer part (Table 1) and correlate posi- 0.095&) and the highest Δ36S values (0.965–2.615&). tively (R2 = 0.95). The d18O(+10.5 to +12.9&) and d34S Although we report Δ36S for the Loki’s Castle vent field to (+22.2 to +30.8&) values of the outermost parts of the expand the Δ36S data set for modern hydrothermal sys- chimney are higher than contemporaneous seawater sulfate tems, its discussion is currently limited due to very limited literature data so far. Thus, our interpretation will not be based on Δ36S. The 87Sr/86Sr ratios of the inner part of the chimney (0.70833–0.70835) are lower than of the outer part, and all are slightly less radiogenic than contemporaneous seawater (0.70917; Fig. 3B). d34S values for the two deep seawater samples are +21.2& (GS09-CTD8-4) and +21.3& (GS09-CTD7-2), whereas the d18O values are +8.3& and +8.2&, respectively. D33S values for the seawater sulfate are +0.040& (GS09-CTD8-4) and +0.020& (GS09-CTD7-2). The d34S value for framboidal pyrite in the hydrothermal sediment sample (GS10-ROV4) is 16.4&, and the D33S value is +0.115&, whereas framboidal pyrite in the flow channel of the barite chimney (GS09-ROV7-1) yields a d34S value of +9.9& and a D33S value of +0.016&.

DISCUSSION

Origin of the low-temperature fluids and barium in the barite field The barite chimneys, microbial mats, and sibiglinoid tube worms in this area show a heterogeneous distribution (Fig. 1C,D), which most likely reflects differences in the flow rate and pattern of the ascending low-temperature fluids. The orientation of barite chimneys along distinct lines requires a relatively high and constant flow rate that is most likely controlled by faults in this area, along which the Fig. 2 Cross-plots of Mg, Na, K, Ca, and Ba concentrations of ambient seawater, low-temperature fluids from the barite field (this study), and ascending hydrothermal fluids can migrate and discharge. high-temperature vent fluids at Loki’s Castle (Pedersen et al., 2010a; The chemical composition of this low-temperature fluid is Baumberger, 2011). indicative for mixing of a hydrothermal fluid with ambient

A 40 Fig. 3 Cross-plots for oxygen, sulfur, and strontium isotope ratios. (A) d34S vs. d18O values for the Loki’s Castle barite chimney and two seawater samples. The light gray area marks the sulfur and oxygen isotope range for barite chimneys along continental margins, referred to as digenetic barite (Greinert et al., 2002; Paytan et al., 2002; Feng & Roberts, 2011), while 35 the dark gray area marks the same range for barite chimneys in hydrother- Diagenetic barite mal systems (hydrothermal barite) at the Kermadec arc (De Ronde et al., et al. ) 2003) and the Mariana back arc (Kusakabe , 1990). The arrow indi- T d34 d18 d34 D cates S/ O ratios of pore ﬂuids from seep sediments ( S/

C 18 - d O = 2.2; Aharon & Fu, 2000) and for barite from the Gulf of Mexico

, 30 (d34S/d18O = 2.7; Feng & Roberts, 2011), as well as for barite from the

‰ ( Sea of Okhotsk (d34S/d18O = 2.4; Greinert et al., 2002). Literature oxygen

S 18 4 d + &

3 isotope data have been recalculated to fit a O value of 8.6 for NBS 127 following Brand et al. (2009). Errors for d34S(0.3&) and d18O (0.5&) are approximately equivalent to the size of the symbols. (B) d34S 25 vs. 87Sr/86Sr isotope ratios. The barite chimneys plot in the field for barite Chimney outside chimneys from continental margins (diagenetic barite) and are clearly dis- Chimney inside tinctive from known hydrothermal barite chimneys. The arrow indicates the Hydrothermal Seawater 87Sr/86Sr isotope ratio of the high-temperature vent fluids at Loki’s Castle barite (Baumberger, 2011). Errors for 87Sr/86Sr (0.000009) encompass the size 20 of the symbols. 5 10 15 20 18O (‰, V-SMOW) To identify the origin of the Ba in the barite chimneys B 90 40 and to further verify a hydrothermal contribution, we determined the Sr isotopic (87Sr/86Sr) composition. Sr 80 concentrations in barite reach up to 3 mol%, making the 30 Sr isotope signature of barite a reliable tracer of the Ba 70 source and the fluid interaction with various rocks and sed-

20 60 0.7083 0.70834 0.70838 0.70842 iments before discharging at the seaﬂoor (Grifﬁth & Pay-

) 87 86

T tan, 2012). The Sr/ Sr ratios of the Loki’s Castle barite

C 50 - chimney are slightly lower than contemporaneous seawater

V Diagenetic barite 87 86 , (0.70917), but are in sharp contrast to Sr/ Sr ratios for

‰ ( 40 barite chimneys from other hydrothermal systems (Fig. 3B;

4 3 Kusakabe et al., 1990; Paytan et al., 2002; Torres et al., 30 2003). The significant change in the 87Sr/86Sr ratio across Hydrothermal barite the chimney transect (insert in Fig. 3B) is corroborated by 20 changes in d18O, d34S, Δ33S, and Δ36S values, which most Modern likely reflect changes in the fluid composition and the mix- 10 marine barite ing ratio between a hydrothermal fluid and seawater, as

0 well as interaction with various rock types. This supports 0.703 0.704 0.705 0.706 0.707 0.708 0.709 0.710 0.711 our conclusion that the inner part of the chimney precipi- 87Sr/ 86Sr tated from the most evolved fluid (e.g., least influence by seawater). Thus, a high-temperature hydrothermal fluid signature is preserved in the barite chimney revealing subsurface mixing of a hydrothermal fluid with seawater perco- seawater (Fig. 2). Simple mixing calculations reveal that a lating in the mound. The high-temperature black smoker mixture of 90% seawater and 10% end-member black smo- fluids yield a homogenous 87Sr/86Sr ratio of 0.7081, con- ker fluid can explain the concentration of Mg, K, and NH4 tradicting ratios typical for bare basalt-hosted hydrothermal 87 86 in the low-temperature fluids, but fail to explain the H2S systems (Baumberger, 2011). Instead, the Sr/ Sr ratios

(100 lmol) and SO4 (24.8 mM) concentrations. Instead, of the Loki’s Castle vent fluids are shifted toward more an additional mechanism that consumes SO4 and produces radiogenic values, indicating fluid interaction with subsea- 87 86 H2S is required. The most likely process in this setting that floor sediments. The Sr/ Sr ratio of 0.7207 for rift val- can also be linked to the formation of barite chimneys is ley sediments in the vicinity to Loki’s Castle differs from microbial sulfate reduction in the subseafloor of the hydro- terrigenous sediments (87Sr/86Sr = 0.711), but confirms thermal mound, rather than simple mixing processes the conclusion of a sedimentary influence (T. Baumberger, between a hydrothermal fluid and seawater. personal communication). This is further supported by the

A 0.4 rift valley ~1.5 million years ago (Bruvoll et al., 2009). Thus, the entire axial volcanic ridge hosting the Loki’s Castle vent ﬁeld may be underlain by sediments (Pedersen 0.3 2 C Sediment biogenic et al., 2010b). Based on the 87Sr/86Sr ratios of the barite chimney, we conclude that the major Ba source for these 0.2 40 C chimneys, and therefore also for the fallout material and

) In-situ biogenic

the debris, is the high-temperature vent ﬂuid that yields a

‰ ( 0.1 150 C lM

S Ba concentration of up to 62 (Baumberger, 2011). We

3 Hydrothermal 3 further suggest that an additional Ba source could be

0 remobilization of barite in fallout material and chimney debris, induced by low sulfate concentrations in the mound due to the existence of a sulfate reduction zone. –0.1

Barite formation in the Loki’s Castle vent ﬁeld –0.2 –40 –20 0 20 40 Barite in hydrothermal systems is reported from the Mari- 34 S (‰, V-CDT) ana forearc (Kusakabe et al., 1990), the Kermadec (Herzig B 3 et al., 1998; De Ronde et al., 2003), the Tonga arc (Stof- fers et al., 2006), the Guaymas basin (Lonsdale & Becker, 2 1985; Paytan et al., 2002), the JADE hydrothermal ﬁeld (Luders€ et al., 2001), and the Henry seamount (Klugel€ 1 et al., 2011). Whether barite precipitates above the sea-

) ﬂoor as chimneys or below the seaﬂoor either as crusts or

‰ (

0 ﬁnely dispersed in the sediments is dependent on the ﬂuid

S 6 3 seepage rate (Aloisi et al., 2004). This model emphasizes –1 that continued upward fluid flow and seepage rates of Barite chimney outside more than 110 cm year 1 are required to form a barite Barite chimney inside –2 Seawater chimney in the oxic water column. Although this model Framboidal pyrite in sediment has been calculated for seepage conditions at cold seep Framboidal pyrite in chimney –3 sites, it provides a minimum estimate for the seepage rates –0.2 –0.1 0 0.1 0.2 0.3 0.4 in the low-temperature barite field at Loki’s Castle. The 33 S (‰) observation of emanating clear shimmering fluids above

34 33 some chimneys indicates that the flow rate at these sites is Fig. 4 (A) d S vs. D S cross-plot. The multiple sulfur isotope signature of 1 the framboidal pyrite from the uppermost sediments of the hydrothermal much higher than 110 cm year . 34 mound clearly separates from those of hydrothermal pyrites (Ono et al., Most barite found in hydrothermal systems yields d S 2007), but plot within the range for microbial sulfate reduction at elevated and d18O values between seawater sulfate (d34S =+21.5&; et al. temperatures (Ono , 2012) that supports the existence of a sulfate d18O =+9.7&) and a hydrothermal fluid (d34S =~0&; reduction zone at Loki’s Castle. The gray oval represents measurements of d18 = & seawater sulfate. The open circles indicate the chromium-reducible sulfur O 0 ), indicating that this phase precipitated from a (CRS) fraction from two sediment cores in the Loki’s Castle vent field. mixture between these two end-member fluids (Kusakabe Errors for D33S(0.009&) are approximately equivalent to the size of the et al., 1990; Paytan et al., 2002; De Ronde et al., 2003). symbols. (B) D33S vs. D36S cross-plot for the barite chimney, framboidal The hydrothermal barite can be used to trace mixing pro- pyrite, and ambient seawater from Loki’s Castle in comparison with hydro- cesses in these settings. However, even though the barite thermal (black circles; Ono et al., 2007) and biogenic pyrite (white circles; 36 chimneys formed in a hydrothermal environment and fluid Ono et al., 2006a,b). Errors for D S(0.28&) are approximately the same as the symbol size. characteristics indicate an input of approximately 10% high-temperature fluid (Fig. 2), the multiple S isotopic composition of the barite chimney suggests additional con- d18 high CH4 and NH4 concentrations in the high-tempera- tributing processes (Fig. 4). Obarite values higher than ture fluids, identifying the Loki’s Castle vent field as a sedi- seawater sulfate contradict a purely hydrothermal contribu- 34 mentary-influenced black smoker hydrothermal system tion. Similarly, d Sbarite values higher than that of seawater 33 (Pedersen et al., 2010a). The most likely sediment source sulfate in concert with Δ Sbarite values significantly lower in this area is the distal part of the Bear Island Fan, which than that of seawater sulfate (Δ33S = 0.008&) cannot be is deposited approximately 5 km southeast of the study explained by simple two-component mixing or solely by area. It has been suggested that the formation of the Bear precipitation from a hydrothermal fluid at high tempera- Island Fan initiated ~3 million years ago and reached the tures (Ono et al., 2007; Peters et al., 2010). If the barite

© 2014 John Wiley & Sons Ltd 316 B. EICKMANN et al. precipitated from a mixture between a hydrothermal vent less radiogenic strontium source, such as the ocean crust fluid and seawater, both the oxygen and the multiple sulfur (Paytan et al., 2002; Griffith & Paytan, 2012). A hydro- isotopes would clearly reflect such mixing processes. thermal fluid exhibits a 87Sr/86Sr ratio of 0.70350, and Instead, there are two major processes that control the varying mixing proportions between such a hydrothermal oxygen and sulfur isotopic composition of seawater sulfate fluid and present-day seawater (87Sr/86Sr = 0.70917) can and those are (i) the isotopic equilibrium between sulfate result in such a trend as observed in the barite chimney and water (Claypool et al., 1980; Zeebe, 2010), and (ii) (Fig. 3B; Albarede et al., 1981). Thus, 87Sr/86Sr ratios of microbial sulfate reduction (Fritz et al., 1989; Aharon & barite chimneys are capable of tracing changes in the mix- Fu, 2000; Brunner et al., 2005; Wortmann et al., 2007). ing ratios between a hydrothermal fluid and seawater, and Temperature changes alone are associated with oxygen iso- they can be used to identify a sedimentary influence in tope fractionation over a wide temperature range, but will hydrothermal systems. not result in a concomitant fractionation of sulfur isotopes Microbial sulfate reduction is the only process that frac- (Claypool et al., 1980; Fritz et al., 1989). Thus, thermal tionates both the oxygen and sulfur isotopes in the ratio isotope equilibrium between sulfate and seawater alone observed in the Loki’s Castle barite chimney and produces cannot explain the synchronous trend in d34S and d18O a residual sulfate pool with negative Δ33S values as shown values. Instead, d34S/d18O ratios vary between 2.1 and 2.4 by natural and experimental studies (Fritz et al., 1989; for the Loki’s Castle barite chimney (Fig. 3A), which is Aharon & Fu, 2000; Greinert et al., 2002; Johnston et al., comparable to ratios of microbial reduced seawater sulfate 2005; Zerkle et al., 2009; Feng & Roberts, 2011). at hydrocarbon seeps in the Gulf of Mexico (Aharon & Furthermore, negative Δ33S values are indicative for the Fu, 2000) and of barite chimneys formed by subseafloor activity of sulfate-reducing bacteria rather than sulfur-dis- microbial sulfate reduction in the Gulf of Mexico and the proportionating bacteria (Farquhar et al., 2003; Johnston Sea of Okhotsk (Greinert et al., 2002; Feng & Roberts, et al., 2005). We therefore infer that the negative Δ33S val- 2011). Typical d34S/d18O ratios for microbial sulfate ues of the barite chimney indicate a significant activity of reduction in natural systems vary between 1.4 and 4 sulfate-reducing bacteria in the subseafloor biosphere com- (Aharon & Fu, 2000; Brunner et al., 2005; Wortmann munity in this low-temperature venting habitat (Fig. 5). et al., 2007). A sulfate reduction zone at the Loki’s Castle vent field is To further explore and test the microbial sulfate reduc- further supported by the multiple sulfur isotopic composition hypothesis, we measured the minor sulfur isotope 33S tion of framboidal pyrite in the surface sediment of the that is reported in the respective Δ33S notation. barite field and in the flow channel of the barite chimney Recent studies have shown that the minor sulfur isotopes (Table 1; Fig. 4). The sulfur isotopic composition of provide additional information on the two main sulfur- the framboidal pyrite in the sediments (d34S = 16.4&, based metabolisms, the microbial sulfate reduction D33S =+0.115&) clearly points to a biogenic origin, and 2 ! 33k (SO4 H2S) and the disproportionation of intermediate its corresponding value of 0.5132 is indicative of 0= 2= ! 2 þ Δ36 sulfur compounds (S SO3 S2O3 SO4 H2S; microbial sulfate reduction. Furthermore, a S value of Farquhar et al., 2003; Johnston et al., 2005; Zerkle et al., 0.370& for this framboidal pyrite is in accordance with 2009). This is particularly important because it allows dis- biogenic sulfides (Ono et al., 2006a,b; Fig. 4B). Addi- tinguishing between both metabolic pathways (Farquhar tional evidence for sulfate reduction in the Loki’s Castle et al., 2003; Johnston et al., 2005), but for an alternative vent field is provided by the multiple sulfur isotopic com- view, see Sim et al. (2011). Thus, we interpret the oxygen position of the CRS fraction from two sediment cores, and multiple sulfur isotope data and their observed trend which were obtained in close proximity to the barite field in the barite chimney as the result of a progressive change (Eickmann et al., unpublished data; Fig. 4A). We com- in the fluid chemistry (Figs 3 and 4), from seawater to a pared those data to a recent model by Ono et al. (2012) fluid that was modified by microbial sulfate reduction and and most of the sediment data plot in the field for sulfate mixing with a hydrothermal fluid (Figs 2 and 3B). This reduction between 40 °C and 150 °C (Fig. 4A), suggest- conclusion is based on the fact that the outer barite chim- ing that microbial sulfate reduction is a common process ney exhibits a seawater-like oxygen and multiple sulfur iso- recorded by multiple sulfur isotopes in the barite chimney. topic composition, while this isotopic composition changes A d34S value of +9.9& and a D33S value of +0.016& for toward the inner part of the barite chimney (Figs 3 and 4). the framboidal pyrite in the flow channel of the barite In addition to the oxygen and sulfur isotope data, the chimney identify this pyrite also as microbial in origin. same change is observed for the 87Sr/86Sr ratio across the However, according to the model by Ono et al. (2012), barite chimney (Fig. 3B), starting with a 87Sr/86Sr this pyrite has been formed above 150 °C, which excludes ratio close to that for present-day seawater and diminishing a microbial origin. The formation of framboidal pyrite in toward the inner part. The observed change in the the chimney suggests anaerobic conditions in the flow 87Sr/86Sr ratio can be explained by the interaction with a channel. However, we cannot exclude the possibility that

© 2014 John Wiley & Sons Ltd Loki’s Castle barite chimney 317 sulfate reducers in the barite chimney also consume seawa- signature of framboidal pyrite in the barite chimney and in ter sulfate, but this process is not evidently expressed in the sediments of the target area highlight the existence of a the sulfur isotopic composition of the framboidal pyrite in prominent sulfate reduction zone in this hydrothermal the barite chimney. mound (Figs 3 and 4). Microbial sulfate reduction during Most of the barite in hydrothermal systems (hydrother- degradation and oxidation of organic matter is a common mal barite) is formed by mixing between a hydrothermal and widely distributed process in marine sediments ﬂuid and seawater (Fig. 3). However, additional evidence (D’Hondt et al., 2002; Bottcher€ et al., 2004), which also indicates that barite formation in other hydrothermal sys- fosters the reductive dissolution of barite. In contrast, as tems other than Loki’s Castle is linked to microbial sulfate organic matter in ocean crustal rocks and hydrothermal sys- reduction. d34S values of barite that are higher than those tems is scarce, it is assumed that microbial communities in of seawater sulfate are reported for the Guaymas Basin these settings are mainly sustained by reduced compounds

(Lonsdale & Becker, 1985), the northern Baja California such as H2 and CH4 that are formed by water–rock interac- (Paytan et al., 2002), the San Clemente Basin (Lonsdale, tions (e.g., Kelley et al., 2002; Bach & Edwards, 2003). 1979) and the Henry Seamount (Klugel€ et al., 2011). The The subsurface mixing of cold seawater and high-tempera- 87 86 Sr/ Sr ratios of the barite from the hydrothermal ture hydrothermal ﬂuids that are high in CH4 (up to

Guaymas Basin are less radiogenic than seawater and are 15.5 mM) and H2 (up to 5.5 mM), and the relatively con- therefore comparable to the Loki’s Castle barite chimney stant supply of these potential electron donors in concert (Lonsdale & Becker, 1985). More evidence for a global sig- with elevated temperatures are the most likely mechanism nificance is needed, but the conditions for such barite fueling the microbial sulfate reduction in this hydrothermal deposits in hydrothermal systems, a supply of barium by system. Indeed, several H2-utilizing sulfate-reducing leaching of the oceanic crust and sulfate by seawater circula- organisms have been isolated from hydrothermal environ- tion, are characteristic for hydrothermal systems (Von ments (e.g., Alazard et al., 2003; Alain et al., 2010; Steins- Damm, 1990; Schultz & Elderfield, 1997). Furthermore, bu et al., 2010), and anaerobic methanotrophic archaea low-temperature venting areas represent a larger area in (ANME) have also been identified in such habitats (Holler hydrothermal systems, but our knowledge of the influence et al., 2011; Biddle et al., 2012; Merkel et al., 2013). Con- of the subsurface biosphere and knowledge about the sortia of ANME and sulfate-reducing micro-organisms are microbial community and about chemical fluxes in those well known for anaerobic oxidation of methane with sulfate areas are still in its infancy (Perner et al., 2011, 2013; Wan- (Knittel & Boetius, 2009). The question arises whether kel et al., 2011). We thus consider that such barite deposits microbial sulfate reduction could also be a major subsub- will enhance our understanding of the interaction of abio- surface process within the underlying ocean crust, where genic and biogenic processes in hydrothermal systems and circulating seawater and hydrothermal fluids are likely to further offer possibilities for interdisciplinary research. interact. Indeed, several studies report sulfur isotope values of sulfide minerals in oceanic basalts suggesting that these minerals are, at least partly, the result of microbial sulfate Barite chimneys as a recorder of subsurface microbial reduction (Rouxel et al., 2008; Ono et al., 2012). processes The d34S/d18O ratios of barite deposits can potentially The d18O, d34S, and Δ33S values of the barite chimney as be used to identify the seepage rate (Feng & Roberts, well as the existence and the multiple sulfur isotope 2011). d34S/d18O ratios that are higher than those of microbial sulfate reduction (>4) are interpreted as indicator 0.2 for low seepage rates, whereas values <4 are believed to be 0.15 indicative of high seepage rates (Aharon & Fu, 2000; Feng d34 d18 0.1 & Roberts, 2011). The relatively low S/ O ratios for

) the Loki’s Castle barite chimney in concert with the obser-

0.05 SR + SDP ‰ ( 34S (‰) vation of clear, shimmering ﬂuids above chimneys indicate

0 S

3 indeed fast seepage, which is in accordance with the seep- 3 20 40 60 80 –0.05 SR age model suggested by Aloisi et al. (2004). In contrast to –0.1 the fast seepage and/or shallow ﬂuid mixing in the hydro-

–0.15 thermal mound that is recorded by the relative easily acces- sible barite chimneys at the seafloor, hidden areas with –0.2 lower flow rates and/or fluid mixing in deeper parts of the –0.25 surrounding basaltic basement are likely to occur. Such

Fig. 5 Plot of d34Svs.D33S illustrating steady state conditions for a global deep mixing processes could sustain a signiﬁcant deep bio- sulfate model (for details see Farquhar et al., 2008). The ﬁelds indicate sul- sphere community. Thus, barite chimneys and their isoto- fate reduction (SR) and reduction plus sulfur disproportionation (SDP). pic composition can therefore be used to identify and

© 2014 John Wiley & Sons Ltd 318 B. EICKMANN et al. reconstruct subseafloor processes in the low-temperature proposed in Fig. 6. Our results indicate that the barite areas surrounding hydrothermal systems. chimneys and the framboidal pyrite in both the chimney and the sediment of the hydrothermal mound are clearly related to microbial sulfate reduction. This finding identi- Formation model for the Loki’s Castle barite chimney fies the existence of a prominent sulfate reduction zone in Based on our observations and results, a model for the for- the mound in which ascending hydrothermal fluids that mation of barite chimneys at the Loki’s Castle vent field is are rich in CH4,H2,NH4, and Ba mix with seawater. Here, sulfate reducers consume the seawater sulfate and produce a fluid that contains sulfate enriched in 18O and 34S, but at low concentrations. The migration of this modified fluid through the hydrothermal sediments could remobilize additional Ba from fallout and chimney debris in the mound, because barite dissolves under sulfate- depleted conditions, leading to fluids even higher in dissolved Ba. Based on the close proximity to the high-temperature vents and the 87Sr/86Sr ratios of the barite chimney, we conclude that Ba in the mound is largely of

hydrothermal origin (SrHF) and was originally discharged into the seawater by the high-temperature vent fluids, where it combined with seawater sulfate. The sediment- influenced high-temperature vent fluids (Pedersen et al., 2010a) have likely shifted 87Sr/86Sr ratios toward more radiogenic values (Fig. 3B). However, incorporation of

seawater Sr (SrSW) in the barite cannot be excluded. In the barite field, the modified fluids discharge into the oxic seawater and barite precipitates immediately on the seafloor. Continued and focused discharge of these low-temperature fluids subsequently led to the formation of barite chimneys. The first barite precipitated and solidified on the seafloor would have acted as a barrier separating the ambient seawater from the discharging low-temperature fluids that bear a microbial sulfate reduction signature. During inward chimney growth, the seawater influence diminished and Fig. 6 Conceptual formation model for the barite chimneys at Loki’s Castle. the microbial sulfate signature (high d34S and high d18O, ~ ° 33 (i) The high-temperature vent fluid ( 320 C) is characterized by high CH4, low D S values) becomes more prominent. Consequently, + H2 and NH4 concentrations, which likely reflect an influence of sediments the inner part of the barite chimney indicates a higher con- buried below the volcanic ridge hosting the vent field (Pedersen et al., 2010a). The high-temperature, Ba-rich vent fluids discharge into the oxic tribution from sulfate that was affected by microbial sulfate seawater, and the hydrothermal Ba combines with seawater sulfate to form reduction and yields a non-radiogenic Sr isotope signature, barite. SrHF and SrSW indicate incorporation of non-radiogenic and radio- both of which record a decreasing seawater influence. genic Sr into barite. This barite accumulates in the hydrothermal mound as Thus, barite chimneys in hydrothermal systems can be fallout of the hydrothermal plume and as chimney debris over time. (ii) A regarded as windows to unravel subseafloor geochemical fault line intersecting the hydrothermal mound facilitates migration and mixing of ascending high-temperature vent fluid with percolating seawater and microbial processes. at the eastern flank of the mound and generates a habitat favorable for microbial sulfate reduction. Microbial sulfate reduction in the mound modi- CONCLUSIONS fies the seawater and generates hydrogen sulfide (H2S) that reacts with iron to framboidal pyrite of biogenic origin. The fluid that is now low in sulfate In this study, evidence is provided which constrain the bio- remobilizes hydrothermal barite in the sulfide mound, leading to even higher Ba concentrations in the ascending low-temperature (~20 °C) fluid. geochemical conditions during the formation of barite The subseafloor microbial sulfate reduction and mixing between percolating chimneys in a low-temperature area of the Loki’s Castle seawater and a high-temperature hydrothermal fluid are recorded in the vent field at the AMOR. This low-temperature area and isotopic composition of the barite chimneys precipitating at the seafloor. the formation of barite chimneys result from the interplay (iii) Microbial sulfate reduction could possible also be a major subsurface between high-temperature, H2- and CH4-rich, hydrother- process within the underlying basaltic basement, where circulating seawater mal fluids and percolating seawater that is modified by and hydrothermal fluids rich in H2 and CH4 are likely to interact. Note that the model is not in scale. The distance between the high-temperature vents microbial sulfate reduction in the subseafloor. Sr isotope and the barite field is approximately 50 m. ratios of the barite chimney similar to those of the Loki’s

Castle black smoker fluids identify these fluids as the pri- from the East Pacific Rise at 21°N. Earth and Planetary Science mary Ba source. In addition, microbial sulfate reduction in Letters 55, 229–236. the hydrothermal sediments of the barite field could play a Alford SE, Alt JC, Shanks WC (2011) Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of key role in remobilization of accumulated barite, which uplifted lower oceanic crust: insights from ODP Hole 735B. subsequently reprecipitates to form chimneys. The sulfur Chemical Geology 286, 185–195. isotopic composition and occurrence of framboidal pyrite Aloisi G, Wallmann K, Bollwerk SM, Derkachev A, Bohrmann G, in the hydrothermal sediment and within the chimney flow Suess E (2004) The effect of dissolved barium on channel underline the existence of an extensive sulfate biogeochemical processes at cold seeps. Geochimica et Cosmochimica Acta 68, 1735–1748. reduction zone in the subsurface. The CH4 and H2 con- Alt JC, Shanks WC (2011) Microbial sulfate reduction and the tents of the high-temperature fluids are most likely fueling sulfur budget for a complete section of altered oceanic basalts, the microbial sulfate reduction. Thus, this type of barite IODP Hole 1256D (eastern Pacific). Earth and Planetary chimneys may also occur in other active vent fields at mid- Science Letters 310,73–83. ocean ridges and may represent a formation mechanism Bach W, Edwards K (2003) Iron and sulfide oxidation within the basaltic ocean crust: implications for chemolithoautotrophic that might be more common than previously recognized. microbial biomass production. Geochimica et Cosmochimica Acta Our study demonstrates the great advantage of using mul- 67, 3871–3887. tiple sulfur isotopes to unravel subseafloor processes and Baumberger T (2011) Volatiles in marine hydrothermal systems. the need for in situ measurements of diffuse vent fluids for PhD thesis, ETH Zurich,€ Switzerland, http://dx.doi.org/10. an in-depth understanding of the processes in low-temper- 3929/ethz-a-007230100. Biddle JF, Cardman Z, Mendlovitz H, Albert DB, Lloyd KG, ature venting areas. Boetius A, Teske A (2012) Anaerobic oxidation of methane at different temperature regimes in Guaymas Basin hydrothermal sediments. The ISME Journal 6, 1018–1031. ACKNOWLEDGMENTS Bottcher€ ME, Khim B-K, Suzuki A, Gehre M, Wortmann UG, We thank the crew of R/V G.O. Sars for making this study Brumsack H-J (2004) Microbial sulfate reduction in deep sediments of the Southwest Pacific (ODP Leg 181, Sites 1119- possible. This work was supported by the Research Council 1125): evidence from stable sulfur isotope fractionation and of Norway and by the EuroMARC programme of the pore water modeling. Marine Geology 205, 249–260. European Science Foundation (ESF) through the Brand WA, Coplen TB, Aerts-Bijma AT, Bohlke€ JK, Gehre M, Geilmann H, Groning€ M, Jansen HG, Meijer HAJ, ‘H2DEEP—Ultraslow-spreading- and hydrogen-based deep biosphere’ project. We thank A. Fugmann, A. Lutter, H. Mroczkowski SJ, Soergel K, Stuart-Williams H, Weise SM, Werner RA (2009) Comprehensive inter-laboratory calibration Chmiel, H. Baier, E. Gjerløw, and O. Tumyr for laboratory of reference materials for d18O versus VSMOW using various assistance, and E. Bjørseth for assistance with graphic illus- on-line high-temperature conversion techniques. Rapid tration. 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