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Deep-Sea Research I 58 (2011) 922–931

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Deep-Sea Research I

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Dissolved and particulate organic carbon in hydrothermal plumes from the East Pacific Rise, 91500N

Sarah A. Bennett a,n, Peter J. Statham a, Darryl R.H. Green b, Nadine Le Bris c, Jill M. McDermott d,1, Florencia Prado d, Olivier J. Rouxel e,f, Karen Von Damm d,2, Christopher R. German e a School of and Earth Science, National Centre, Southampton SO14 3ZH, UK b National Environment Research Council, National Oceanography Centre, Southampton SO14 3ZH, UK c Universite´ Pierre et Marie Curie—Paris 6, CNRS UPMC FRE3350 LECOB, 66650 Banyuls-sur-mer, France d University of New Hampshire, Durham, NH 03824, USA e Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA f Universite´ Europe´enne de Bretagne, European Institute for Marine Studies IUEM, Technopoleˆ Brest-Iroise, 29280 Plouzane´, France article info abstract

Article history: Chemoautotrophic production in seafloor hydrothermal systems has the potential to provide an Received 19 November 2010 important source of organic carbon that is exported to the surrounding deep-ocean. While hydro- Received in revised form thermal plumes may export carbon, entrained from chimney walls and biologically rich diffuse flow 23 June 2011 areas, away from sites of venting they also have the potential to provide an environment for in-situ Accepted 27 June 2011 carbon fixation. In this study, we have followed the fate of dissolved and particulate organic carbon Available online 3 July 2011 (DOC and POC) as it is dispersed through and settles beneath a hydrothermal plume system at 91500N Keywords: on the East Pacific Rise. of both DOC and POC are elevated in buoyant plume samples Hydrothermal that were collected directly above sites of active venting using both DSV Alvin and a CTD-rosette. Dissolved organic carbon Similar levels of POC enrichment are also observed in the dispersing non-buoyant plume, 500 m Particulate organic carbon downstream from the vent-site. Further, sediment-trap samples collected beneath the same dispersing East Pacific Rise plume system, show evidence for a close coupling between organic carbon and Fe oxyhydroxide fluxes. We propose, therefore, a process that concentrates POC into hydrothermal plumes as they disperse through the deep-ocean. This is most probably the result of some combination of preferential adsorption of organic carbon onto Fe-oxyhydroxides and/or microbial activity that preferentially concentrates organic carbon in association with Fe-oxyhydroxides (e.g. through the microbial oxidation of Fe(II) and Fe sulfides). This potential for biological production and consumption within hydrothermal plumes highlights the importance of a multidisciplinary approach to understanding the role of the in deep-sea hydrothermal systems as well as the role that hydrothermal systems may play in regulating global deep-ocean carbon budgets. & 2011 Elsevier Ltd. All rights reserved.

1. Introduction are enriched in Fe(II) but depleted in organic carbon (Von Damm, 1995a; Lang et al., 2006). Therefore, any elevated concentrations is an important source and sink of of organic carbon present within a hydrothermal plume must be elements to the ocean (Elderfield and Schultz, 1996; German and entrained from adjacent areas of diffuse flow and chimney walls Von Damm, 2004). In particular, hydrothermally sourced iron (Cowen et al., 1986; Winn et al., 1986; Lang et al., 2006) and/or (Fe), stabilised by organic complexes within dispersing plumes produced in-situ within the plumes themselves (Roth and may be important to global ocean budgets (Bennett et al., 2008; Dymond, 1989; McCollom, 2000; Lam et al., 2004). Toner et al., 2009; Tagliabue et al., 2010; Wu et al., 2011). Hydrothermal plumes are dynamic, 3-dimensional biologically In a typical basalt hosted hydrothermal field, end-member fluids active zones in the deep-sea and biological communities within the hydrothermal system rely on chemoautotrophic fueled by hydrothermal fluids (McCollom, 2000). As hot, chemically n Corresponding author. Now at NASA Jet Propulsion Laboratory, Caltech, 4800 reduced fluids mix with oxygenated , complex redox Oak Grove Drive, Pasadena, CA 91109, USA. Tel.: þ1 6266765372. disequilibria are established that provide chemical energy for E-mail addresses: [email protected]. microbial . Areas of diffuse flow and chimney walls are [email protected] (S.A. Bennett). 1 Now at Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA. well known for their rich and symbiotic bacteria, with 2 Deceased. elevated concentrations of dissolved and particulate organic carbon

0967-0637/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2011.06.010 S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 923 in the surrounding waters (Rau and Hedges, 1979; Karl et al., 1980; particulate organic carbon (POC and DOC) analyses are lacking for Comita et al., 1984; Lang et al., 2006). Free living microbes within this setting. Initial vent end-member carbon measurements made hydrothermal plumes are also an important, but often overlooked, by Lang et al. (2006), suggested that the flux of carbon from primary producer (Cowen et al., 1999; Lam et al., 2008; Dick and hydrothermal vents to the open ocean is minor compared to most Bradley, 2010; German et al., 2010; Sylvan et al., in review). Both oceanic source and sink terms. However, the dynamic plume symbiotic and non-symbiotic microbes play an important role in environment has been suggested to be a source of ‘new’ organic chemical cycling in these environments. carbon to the deep-ocean (Karl et al., 1984). Early plume studies detected elevated microbial biomass at This study focuses on bulk organic carbon variations, both plume depths both near- and far-field, entrained from bottom waters particulate and dissolved, directly above a hydrothermal source, near the vents (Cowen et al., 1986; Winn et al., 1986; Straube et al., its evolution through a dispersing plume and its presence within 1990). This was followed by the demonstration of chemoautotrophic sinking particles settling beneath the buoyant plume. carbon fixation and biological removal of organic matter in the hydrothermal plume (Roth and Dymond, 1989). Bacteria and viruses have been detected within hydrothermal plumes (Juniper et al., 1998; Ortmann and Suttle, 2005; German et al., 2010), along with 2. Sample collection, processing and analysis directly above the plume, apparently grazing off the hydrothermal constituents (Burd and Thomson, 1994; Vereshchaka All samples for this study were collected during 2006 from the and Vinogradov, 1999; Cowen et al., 2001). well-characterized basalt hosted vent system at 91500N, East The microbial community within the hydrothermal plume has Pacific Rise (EPR), which is one of the best characterized vent been inferred to include methane (Cowen et al., 2002), ammonia systems worldwide. Samples were collected from four separate (Lam et al., 2004; Lam et al., 2008), hydrogen and sulfide oxidizers research cruises of the area (Table 1) during the months imme- (Jannasch and Mottl, 1985; Sunamura et al., 2004), as well as diately following a volcanic eruption event that occurred in microbes that utilize Mn and Fe (Cowen et al., 1998; Dick et al., winter 2005–2006 (Tolstoy et al., 2006; Cowen et al., 2007). 2009). Sulfide and hydrogen oxidation is considered to be impor- Within the axial summit collapse trough (ASCT), five sites of tant in the buoyant or early non-buoyant plume, whereas metha- high- hydrothermal activity remained actively vent- notrophy and ammonia oxidation are more likely to be important ing after the eruption: Biovent, Bio9, P vent, Ty and Io (Fig. 1). in non-buoyant plumes (Lam et al., 2008). The plume environ- There were also microbial mats covering basaltic substrates and ment is biologically and chemically rich and is able to host both areas of diffuse flow, occupied by patches of Tevnia and Alvinella. auto- and hetero-trophic communities, yet bulk dissolved and The majority of the Riftia and vent mussels present prior to the eruption had been destroyed. It is unlikely that the volcanic eruption affected our samples to a major extent, even though increased microbial activity and generation of biomass within Table 1 diffuse flow areas have been reported in previous eruptions at Details of cruise number and dates during which the samples for this study were collected. mid-ocean ridges (Haymon et al., 1993; Juniper et al., 1995; Juniper et al., 1998; Huber et al., 2003). The samples collected Cruise Dates Sample collection in this study were obtained at least 6 months following the number eruption event, and even though microbial mats were still largely

AT15-6 June 2006–July 2006 Sediment trap deployment visible in the surrounding diffuse vents, chemosynthetic mega- AT15-12 October 2006–November 2006 Sediment trap recovery fauna already appeared to dominate the biomass of vent biologi- AT15-13 November 2006–December Vent fluid collection cal assemblages, with Tevnia dominating. This stage in the 2006 colonization of vent systems was reported by Shank et al. AT15-14 December 2006–January 2007 Plume samples (1998), 10 months post the 1991 eruption at 91500N EPR.

Fig. 1. Locations of CTD stations 83 and 91 relative to the positions of the Ty, Io, Bio90, P vent and Biovent vent sites at 91500N EPR. 924 S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931

2.1. Vent fluid samples et al. (2009) for both pH and sulfide. The tips of the electrodes were combined with a thermistance sensor in order to define End-member vent fluid samples were collected with titanium temperature at measurement point (NKE, France). The combined ‘majors’ bottles using DSV Alvin as reported in Von Damm (2000). probes were placed on the Alvin basket, about 1 m from the Niskin High-temperature vent fluid samples were collected from four bottles. Electrode signals were calibrated in the laboratory before smoker orifices; Bio90, P vent, Biovent and Ty, with titanium major the dive, and later converted into pH and values sampler pairs using the DSV Alvin. Bottles were triggered when using the sulfide acidity constant given in Rickard and Luther the Alvin high-temperature probe or inductively coupled link (2007). The calculated temperature, pH and free sulfide content temperature probes, both calibrated to a NIST traceable standard, were averaged over 15 s around the time the maximum tempera- stabilised at a maximum temperature. ture was reached. Shipboard sample treatments were as reported in Von Damm A further suite of samples was collected by CTD et al. (1995b, 2000). On the day of sampling, high-temperature rosette, from both buoyant and non-buoyant plumes overlying 0 vent fluid samples were processed shipboard for H2S (5% preci- the EPR 9150 N vents, using 10 L Ocean Test Equipment (OTE) sion, standardised to Dilut-its standard ) by iodimetric bottles internally sprung with silicone tubing. These bottles were starch titration. The remaining vent fluid samples were acidified mounted on a 24-position rosette equipped with a SBE with 0.5–1.0 ml of concentrated 3x quartz distilled HCl, and 9/11þCTD system and a SeaPoint STM in-situ sensor. stored in high-density polyethylene (HDPE) bottles. Any remain- Initially, the rosette was deployed directly above the 91500N vent ing precipitates were rinsed out of the bottles during the sample sites in order to collect buoyant plume samples, as detected by draw and saved in HDPE bottles. the presence of , density and temperature anomalies (CTD At the University of New Hampshire (UNH), USA, high-tem- 83). This was followed by a second CTD cast occupied 0.5 km perature vent fluid samples were filtered through 0.45 mm nucle- southwest of the vent sites, selected to be down-plume along the pore filters in a laminar flow bench. The acidified, filtered fluid prevailing direction, which was detected with a lowered- fraction samples were stored at room temperature in HDPE acoustic Doppler current profiler (L-ADCP) attached to the CTD bottles, while the filters and filtered particles were saved in rosette. Here, a non-buoyant plume was intersected 150 m separate HDPE bottles. Fluid fractions were analyzed for Fe and above the seafloor and detected by the presence of particle Mn (both 1% precision, standardised to Ricca Chemical Company anomalies (CTD 91, Fig. 1)(Baker, 1998). AA standards) by flame atomic absorption spectroscopy (FAAS). On recovery of the CTD rosette or DSV Alvin aboard ship, water Fe and Mn end-members were calculated for a high-temperature samples were collected from the OTE/Niskin bottle, after shaking, vent by performing a least-squares regression of an individual directly into Teflon bottles (with 3-fold rinsing). Water collected chemical species versus Mg and assuming a linear relationship directly above the vent orifice was sub-sampled into acid-cleaned passing through the ambient bottom seawater composition, and LDPE bottles and acidified to pH 1.6 using HNO3 (Fisher Scientific, extrapolated to 0 mmol/kg Mg. Reported Fe and Mn end-mem- OPTIMA grade). In a dedicated clean area of the shipboard bers were calculated using only data derived from filtered laboratory, 1 L of sample was filtered through pre-combusted samples (dissolved fraction). As such, this does not take into (400 1C, 44 h) GF/F filters (0.7 mm, 25 mm diameter, Whatman) account the precipitated or suspended fractions (but does include into 40 ml I-CHEM, certified low-level total organic carbon (TOC) colloidal material), and so will under-estimate the true end- vials. These were acidified with 40 ml trace metal grade HCl member values. (Fisher Scientific, stored in glass) to pH 2 and stored at 4 1C for Detailed studies on the vent fluid evolution and chemical analysis in our shore laboratories. All GF/F filters were folded in evolution following the winter 2005–2006 eruptions are beyond combusted foil and frozen, with volumes of seawater filtered the scope of this paper and will be reported in detail elsewhere recorded for subsequent calculations of particulate material con- (Von Damm et al., unpublished data). In this study, we will use centrations per litre of ocean/plume water. end-member vent-fluid Fe/Mn ratios to help calculate the relative Analyses of total dissolvable Fe and Mn (TdFe and TdMn) in the dilution factors that can be ascribed to buoyant plume samples near-field plume samples collected directly above the vent orifices collected directly above each vent-site and H2S concentrations to were carried out at the National Oceanography Center, Southampton determine the potential for microbial sulfide oxidation. (NOCS), UK. A 4% of each sample in 0.4 MdHNO3 was analyzed using a Perkin Elmer Optima 4300DV ICP-OES that was 2.2. Plume samples calibrated against matrix-matched standard solutions that bracketed the concentration ranges of the sample solutions. Stan- Near-field buoyant plume samples were collected from above dard solutions were prepared using single element 1000 mgL1 four high-temperature vent sites; Bio90, P vent, Biovent and Ty, standards (Specpure, Spex) of Fe and Mn. A 4% IAPSO seawater using 1 L externally sprung Niskin bottles placed in the basket standard was also run, even though it contained very low Fe and Mn attached to DSV Alvin. A total of eight samples was collected, with concentrations, to check for contamination. The limit of detection of two samples from each site obtained sequentially as DSV Alvin the technique was 6.5 nM for Fe and 1.3 nM for Mn, determined by flew a few meters above the active chimneys. The distance of DSV repeated analysis of the seawater matrix blank (3s, n¼10). While Alvin above the seafloor was recorded with a 200 kHz these detection limit values are high compared to open-ocean trace (ex-Datasonics) Model PSA-900D altimeter. The samples were metal concentrations, they are much lower than the concentrations collected based on increases in water temperature (up to 15 1C) reported for the near-vent hydrothermal plume samples investi- recorded by the temperature probe on Alvin and on direct gated here (mM concentrations). observations of black particle-laden plumes through the submer- Determination of DOC was carried out on a coupled high- sible view-ports. Duplicate Niskin samples were collected at each temperature combustion total organic carbon-nitrogen chemilu- sampling location along with in-situ measurements of pH, free minescence detection (HTC TOC–NCD) system at NOCS. The sulfide and temperature. The comprised pH and free sulfide measurements were performed using a Shimadzu TOC 5000A concentrations within the plume were estimated from an auton- total carbon analyzer coupled with a Sievers NCD 255 nitrogen omous electrochemical probe. The probe was made of pH and chemiluminescence detector (Pan et al., 2005). A procedural blank sulfide electrodes connected to autonomous potentiometric data of MQ water, gave an analytical precision of 70.8 mM(1s, n¼3) loggers, as used in Le Bris et al. (2003, 2006) for pH and Laurent and the accuracy of the technique was determined from the S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 925 analysis of a certified reference material. The standard deviation were sieved to retain coarse fragments and the remaining o1mm of each sample was obtained from 3 to 5 injections of the same fraction passed through a 10-port rotating wet sediment splitter. sample into the instrument. Total dry mass for mass flux calculations was determined from three Determination of POC was carried out at the Plymouth Marine of the sample splits. Dried subsamples (5–10 mg) were analyzed for Laboratory, UK. The GF/F filters were unfolded, removed from particulate inorganic carbon and POC using a CHN analyzer, and Al, the foil and placed in small polystyrene containers. The filters were Fe, Mn and V, after acid digestion, using ICP–OES and ICP–MS. A then treated with sulfurous acid under vacuum for 24 h to remove number of geo-reference standards (BHVO-1 and IFG) were ana- the inorganic carbonates (Verardo et al., 1990). The filters were lyzed along with the samples to confirm analytical accuracy, which dried at 60 1C for 24 h, quartered and packaged in pre-combusted was better than 5% for all elements reported. aluminum foil (Hilton et al., 1986). The samples were then analyzed on a Thermo Finnigan Flash EA1112 Elemental analyzer using acetanilide as a calibration standard. Filter blanks were used 3. Results for blank correcting all samples. The analytical precision, deter- mined from analysis of the filter blanks, was 0.55 mg(1s, n¼6). 3.1. Vent fluid samples

2.3. Sediment trap samples The end-member temperature, Fe, Mn and H2S concentrations for each high-temperature vent are listed in Table 2. The end- Sediment trap samples analyzed for this study were collected member concentrations for vent-fluids were sampled over several using a McLane 21-position time-series trap deployed at the Bio9 days. End-member regressions were calculated from major ion vent-site at 9150.290N, 140117.490W and at a depth of 2505 m, analyses for 2–6 discrete samples. 3 m above the seafloor. This ‘R1’ trap was deployed during AT15-6 from the RV Atlantis and repositioned during Alvin 3.2. Plume samples dive 4206 on 30th June, 2006. The sediment trap was positioned 25 m laterally away from the Bio9 vent-site with the intention The POC and DOC concentrations in the near-field vent samples that the trap would then be sufficiently close to underlie (hence, together with their respective TdFe and TdMn concentrations, collect particles settling from) both the buoyant and non-buoyant Fe/Mn ratios, maximum temperature and calculated percent end- portions of the hydrothermal plume dispersing away from this member fluid in these samples (see later) are listed in Table 2.The vent-site. The R1 trap then collected samples continuously, with POC concentrations ranged from 0.87 to 3.81 mMandtheDOC each sample representing a 14.8-day cycle (to coincide with concentrations ranged from 37.9 to 47.1 mM. The averaged tem- spring/neap tidal cycles in the overlying water column) until its perature, pH and free sulfide recorded by the in-situ probes for these recovery during Alvin dive 4262 (3rd November, 2006, RV samples and height of DSV Alvin above the seafloor during each Atlantis AT15-12). Further information on this deployment will sampling event are shown in Table 3. The CTD depth profiles of POC be reported in German et al. (manuscript in preparation, 2011). concentrations, DOC concentrations, potential temperature anoma- Prior to deployment, each 250 mL polyethylene sample cup lies and particle anomalies are shown in Fig. 2 and the depth vs was filled with dimethyl sulfoxide (DMSO) and buffered to pH density plots for these two CTD casts are shown in Fig. 3.Potential 9.070.5. This preservative prevents any biological activity con- temperature and particle anomalies were determined as described tinuing within the sample-cups, post-collection, while retaining in Baker (1998). The POC concentrations in the plume ranged from sample integrity for mineralogical, bulk geochemical, and mole- 0.07 to 0.46 mM compared to an average ‘background’ value of cular microbiological investigations (Comtet et al., 2000). Upon 0.1670.05 mM measured above and below the plume. DOC con- trap recovery, each sample cup was capped and refrigerated at centrations were approximately one order of magnitude greater 4 1C for return to the laboratory and shore-based analysis. than POC concentrations and ranged from 35.3 to 43.2 mMinthese All subsequent sample processing was conducted at Woods Hole samples compared to ‘background’ values of 38.270.9 mMabove Oceanographic Institution using standard methods established for and below plume-height. The upper and lower background limits deep-ocean sediment trap analyses (Honjo et al., 1995). Samples are shown on Fig. 2.

Table 2 Chemical characteristics of vent fluids and near-field buoyant plume samples collected at 91500N EPR.

Vent fluids Near-field buoyant plume

Temperature [Fe] [Mn] [H2S] Max temp [Fe]Td [Mn]Td Fe/Mn % end-member [POC] (lM) [DOC] (1C) (lM) (lM) (mM) (1C) (lM) (lM) fluid (lM)

Biovent 321 854 523 12.3 6 3.50 2.00 1.8 0.38 0.87 40.0 2.83 1.70 1.7 0.33 37.9 Bio90 382 3870 149 11.4 13 7.10 0.30 23.8 0.20 1.48 46.1 335 4080 156 8.6 6.18 0.27 22.6 0.18 43.9 P vent 392 1080 92.2 12.8 11 4.74 0.53 8.9 0.56 3.74 41.5 386 1300 97.1 10.6 6.80 0.77 8.8 0.81 47.1 Ty 359 3780 582 28.7 7 1.75a 0.60a 2.9a 0.10a 3.81a 44.0a 2.87 0.59 4.8 0.10 39.2 Background – –––2 –––– 0.1670.05 38.270.9

[Fe] and [Mn] are the concentrations of Fe and Mn with subscript ‘Td’ indicating ‘total dissolvable’. Reported vent fluid concentrations have been corrected to allow for seawater entrainment (Section 2.1), [POC] is the concentration of particulate organic carbon and [DOC] is the concentration of dissolved organic carbon. The Fe/Mn ratio was calculated using the concentrations of total dissolvable Fe and Mn measured in the near-field buoyant plume samples. The % end-member fluid represents the dilution of the vent fluids with the surrounding seawater and was calculated using Mn as a conservative tracer (see Section 4.1 for further details). a One of the Niskin bottles fired at Ty, was not shaken prior to sample collection. 926 S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931

Table 3 Plume characteristics as measured by the in-situ probe on board DSV Alvin.

Site Dive number Time Height above Average pH Free H2S(lM) % end-member seafloor (m) temp (1C) fluid

BioVent 4295 20:52:25–20:52:40 7.5 5.0 6.9 22 0.9 Bio90 4292 20:04:16–20:04:34 7.0 7.7 7.0 26 1.5 P Vent 4292 20:47:34–20:47:49 4.1 7.2 7.0 26 1.3 Ty 4294 21:07:51–21:08:11 4.2 4.7 7.6 9 0.8

Data were averaged over 15 s at the maximum temperature anomaly. The height of DSV Alvin above the seafloor was recorded with an altimeter attached to the bow of the submersible. The % end-member fluid was calculated using the average temperature measured in the plume and the maximum temperature measured in each end-member (Table 2), subtracting the background temperature from each value.

POC (μM) POC (μM) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2200 CTD 83 CTD 91 2250

2300

2350 Depth (m) 2400

2450 Temperature Turbidity POC 2500 0.00 0.02 0.04 0.06 0.00 0.02 0.04 0.06 Potential temperature anomalies, Δθ (°C) Potential temperature anomalies, Δθ (°C) -0.2 -0.1 0.0 0.1 -0.2 -0.1 0.0 0.1 Particle anomalies, Δc (NTU) Particle anomalies, Δc (NTU) DOC (μM) DOC (μM) 30 40 50 60 70 30 40 50 60 70 2200 CTD 83 CTD 91 2250

2300

2350 Depth (m) 2400

2450 Temperature Turbidity DOC 2500 0.00 0.02 0.04 0.06 0.00 0.02 0.04 0.06 Potential temperature anomalies, Δθ (°C) Potential temperature anomalies, Δθ (°C) -0.2 -0.1 0.0 0.1 -0.2 -0.1 0.0 0.1 Particle anomalies, Δc (NTU) Particle anomalies, Δc (NTU)

Fig. 2. Depth profiles of POC, DOC, potential temperature anomalies and turbidity anomalies for CTD stations that intercepted a buoyant and non-buoyant plumeat91500N EPR. The gray line indicates temperature, the solid line indicates turbidity, the solid circles indicate POC and the open circles indicate DOC. The vertical dashed lines on the plots indicate the upper and lower limits of the background concentration.

3.3. Sediment trap samples we have calculated the relative abundance of hydrothermally

sourced Fe-oxyhydroxide material, [Fe-oxy]hyd, in each of the The sediment trap flux data for elements pertinent to this sediment trap samples, using a protocol previously described for study are shown in Table 4 and Fig. 4. A more complete discussion hydrothermal sediments, sediment trap samples and hydrother- of post-eruption EPR sediment trap fluxes will be presented in mal plume particles (German et al., 1997; German et al., 2002; German et al. (manuscript in preparation, 2011). For this study, Bennett et al., 2009). This method exploits the scavenging S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 927 properties of Fe oxyhydroxides for oxyanions (e.g. vanadium (V)) In order to correct for these continental eolian inputs, we have within the water column, resulting in V:Fe ratios which vary used sediment data reported in Kyte et al. (1993) to provide us systematically with the composition of the ambient seawater with detrital vanadium:aluminum ratios and assumed that total (Feely et al., 1998). By knowing the concentration of vanadium Al content within the sediment traps is a result of such inputs that is derived from hydrothermal Fe-oxyhydroxide scavenging, (German et al., 1997). From this, we can calculate the predicted we can calculate the concentration of [Fe-oxy]hyd. The chemical detrital concentration of vanadium in each sample using constituents within the trap will be sourced from both eolian inputs sinking through the water column and hydrothermal ½Vdet ¼½Altrap ð½V=½AlÞeolian ð1Þ inputs dispersed laterally within the hydrothermal plume. There- fore, we must first determine how much of the vanadium in the where [V]det is the predicted detrital concentration of vanadium trap samples was sourced from the hydrothermal system as in a sediment trap sample, [Al]trap is the measured aluminum opposed to eolian inputs. concentration in a sample and [V]/[Al]eolian is the average V:Al ratio in the upper 0–1 m of sediment as reported by Kyte et al. (1993) and shown in Table 4. The excess of vanadium in each sample is assumed to be hydrothermal plume fall out, [V] , 2200 hyd CTD 83 CTD 91 100 10 3.0 2250 Mass flux Organic carbon 80 FeOOH 2.5 8 2300 60

/day) 2.0 /day) 2 40 2

6 /day) 2 2350 20 1.5 Depth (m) 4 (mg/m 0 org 1.0 2400 C FeOOH (mg/m

Mass flux (mg/m -20 2 0.5 -40 2450 -60 0 0.0 0 20 40 60 80 100 120 140 2500 Day 27.71 27.72 27.72 27.72 27.73 27.73 Fig. 4. Time profile of the mass flux, organic carbon (Corg) and hydrothermally Potential density (Kg/m3) sourced Fe oxyhydroxides (FeOOH) for sediment trap (R1) deployed 25 m from the Bio9 vent site. The error for organic carbon analysis is too small to show on the Fig. 3. Depth vs density plot demonstrating the of the plume at CTD 83. plot (RSD¼0.92%).

Table 4 Sediment trap data.

Days Mass Fe Corg Al V [Fe-oxy]hyd RSD elapsed mg/m2/day mg/m2/day mg/m2/day mg/m2/day lg/m2/day mg/m2/day %

6 81.0 14.4 1.8 1.09 5.07 1.27 12.3 12 79.6 16.2 1.6 1.44 6.29 1.49 13.1 18 16.9 2.6 2.3 0.30 1.28 0.30 13.2 24 20.0 3.0 1.0 0.29 1.44 0.36 12.1 30 26.3 3.8 2.1 0.50 2.02 0.46 13.7 36 12.5 2.5 0.5 0.09 0.67 0.19 10.5 42 22.0 5.6 0.8 0.17 1.10 0.30 11.0 48 25.8 5.0 0.9 0.14 0.80 0.21 11.3 54 46.1 9.1 1.7 0.81 3.84 0.94 12.5 60 26.8 3.7 3.8 0.40 1.96 0.49 12.1 66 26.9 1.8 3.6 0.42 1.54 0.34 14.3 72 18.5 2.0 1.1 0.22 1.03 0.26 12.2 78 33.8 5.8 1.7 0.39 2.04 0.53 11.7 84 17.0 1.7 1.1 0.12 0.76 0.21 10.8 90 18.2 3.0 1.5 0.21 1.22 0.33 11.1 96 19.0 1.7 1.3 0.19 1.05 0.28 11.4 102 23.7 3.2 2.2 0.17 1.25 0.36 10.3 108 52.3 7.9 4.7 0.57 3.79 1.08 10.5 114 60.0 9.1 4.4 0.68 4.95 1.43 10.3 120 57.2 9.2 2.8 0.43 3.64 1.08 10.0 N. Pac. clay 98.5 mg/g 152 mg/g Average 01 m from LL44-GPC3 (Kyte et al., 1993)

Fe, organic carbon (Corg), Al and V flux data are shown for bulk compositions within the trap samples. These data were used

to calculate the Fe-oxyhydroxide hydrothermal component ([Fe-oxy]hyd), as described in the text (Section 3.3), along with a propagated relative standard deviation (RSD). 928 S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 such that also reveals an instability in the water column coincident with the positive temperature anomalies from the seafloor up to 2460 m, ½V ¼½V 2½V ð2Þ hyd trap det confirming that we had intercepted a buoyant (rising) hydro- where [V]trap is the measured vanadium concentration in the thermal plume at these depths (Lupton, 1995). During this same sediment trap sample. The uptake of V by Fe oxyhydroxides along cast, a lowered-acoustic Doppler current profiler (L-ADCP) the EPR has a V/Fe ratio of 0.0028 (Feely et al., 1998), therefore, attached to the CTD rosette, measured a current velocity at 1 the hydrothermal Fe oxyhydroxide component ([Fe–ox]hyd) of the 2400 m depth of 8.6 cm s along heading 1991. Because the sediment trap samples can be calculated using depth of the ASCT is only 20 m, this rising plume would not have been constrained by the local topography but, rather, should ½Fe-oxhyd ¼ 1=0:0028 ½Vhyd ð3Þ have been dispersed away from the ridge axis along the prevailing The results of these calculations are shown in Table 4. current direction. At CTD 91, occupied 500 m southwest of the 91500N vents (i.e. in the projected down-plume direction), particle anomalies 4. Discussion were observed at a typical non-buoyant plume height of between 2350 and 2275 m, 150–225 m above the seafloor. At these depths, 4.1. Dilution of near-field plume samples no density anomalies were observed indicating that the plume was no longer buoyant (Fig. 3). Further, the L-ADCP measured a Within the short time frame of buoyant plume rise (1 h), current velocity at plume-height of 5.8 cm s1 on a heading of from emission at the vent orifice to emplacement at non-buoyant 2331, consistent with the 91500N high-temperature vents being plume height (Baker et al., 1995), dissolved Mn can be treated as a the source for this plume. conservative tracer. By contrast, Fe does not show conservative behavior over this time frame, due to the formation of polyme- 4.3. DOC and POC in the near-field plume tallic sulfide precipitates within the first few seconds of venting and the rapid oxidation of Fe(II) to form Fe oxyhydroxide POC concentrations in our near-field plume samples exhibit precipitates. Both of these particulate Fe species can fall out of a elevated concentrations over background, ranging from 0.87 to plume during buoyant plume rise. By contrast, Mn oxidation is 3.81 mM with highest concentrations at the Ty and P vents much slower and dissolved reduced Mn species can be considered (Table 2). The DOC concentrations in these same near-field plume to be quantitatively transported to and emplaced within non- samples ranged from 37.9 to 47.1 mM, a range that spans the local buoyant hydrothermal plumes. value for background seawater (38.270.9 mM), but also reflects Using the vent fluid data collected in November 2006 (Table 2), enrichments of up to 20% at three of the sites investigated. The we can investigate the evolution of near-field plumes and analysis of POC and DOC is operationally defined as the separation calculate the percent dilution for each near-field buoyant plume of particles through a combusted GF/F glass fiber filter (0.7 mm sample using nominal pore-size). A direct consequence is that the majority of any POC fraction measured is often dominated by microorgan- % endmember fluid ¼½MnBP=½MnVF 100 ð4Þ isms, including microbial biomass and larvae (Mullineaux et al., where [Mn]BP is the Mn concentration measured in the near-field 2010), whereas DOC sources commonly include extracellular buoyant plume samples and [Mn]VF is the corresponding vent release from bacteria, grazer mediated release and excretion, fluid end-member Mn concentration. Dilution factors calculated release via cell lysis (from viruses and bacteria), solubilization by this method fall in the range of 125- to 1000-fold dilution with of particles and bacterial transformation and release (Carlson, ambient seawater (i.e. the samples contain just 0.1–0.8% end- 2002). DOC sinks include biotic consumption (e.g. by hetero- member vent fluid, Table 2), demonstrating the dilution that is trophic bacteria) and abiotic sorption onto particles. associated with turbulent mixing within the first few meters of Winn et al. (1986), estimated living carbon concentrations buoyant plume rise. Except for Ty, duplicate near-field plume within a hydrothermal plume by using ATP measurements as an samples taken successively at each site exhibit very similar Fe/Mn indicator of microbial biomass, and reported particulate living ratios, demonstrating that each plume was rather homogeneous. carbon concentrations of 1.2 mM at 20 m above a vent-chimney Dilution factors calculated from in-situ temperature measure- and 0.3 mM at 50 m off-bottom at hydrothermal sites along the ments ranged from 0.8% to 1.5%. These values were calculated as Juan de Fuca Ridge. These concentrations are similar to the total for Mn but using the average temperature measured within the POC measured in our near field plume samples collected from plume and the maximum temperature measured within the vent DSV Alvin as well as in the buoyant and non-buoyant plume 1 fluid at each site (and subtracting background temperature of 2 C samples collected during CTD casts 83 and 91 (POCr0.46 mM; from each value) (Table 3). These lower dilution ratios reflect Fig. 2). minimum dilution conditions experienced in the core of the We do not believe that EPR 91500N vent fluids can be the plume, as illustrated by steep peaks simultaneously recorded for source for the POC and DOC enrichments in our near-field plume all three parameters (temperature, pH and sulfide) while DSV samples, because the required end-member DOC and POC con- Alvin flew a few meters above the high-temperature chimneys. In centrations would be unreasonably high. Typically, thermal comparison, Niskin sample contents have integrated several degradation of organic matter is to be expected at the high- mixing fractions from the turbulent plume, with larger seawater associated with typical ‘black smoker’ venting. For dilution. example, at the Main Endeavor field, and at Axial Volcano, on the Juan de Fuca Ridge, average DOC concentrations in end-member 4.2. Buoyant plume rise and dispersal fluids are 15 and 17 mM, respectively (Lang et al., 2006). By contrast, if our Alvin-collected Niskin samples (average DOC As buoyant plumes rise up into the water column, further concentration of 42.5 mM) have experienced a 125- to 1000-fold dilution continues. At CTD 83, directly above the EPR 91500N dilution with seawater (Table 2), the required end-member vent- vents, strong particle anomalies coupled with positive tempera- fluid DOC concentration would fall in the range 5.3–42.5 mM, ture anomalies were detected from just above the seafloor up to some three orders of magnitude higher than previously measured 2460 m. Importantly, the density profile for this station (Fig. 3) at the Juan de Fuca Ridge vents. S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 929

A more plausible source for the DOC and POC in our near field 23 mg of C per kg of fluid (when 2.2 g biomass contains 1 g of C plume samples is from entrainment of diffuse flow and chemosyn- (Battley, 1998)) or 1.9 mM C. In a diluted buoyant plume, this thetic life on the chimney walls into the base of the buoyant plume would only result in a 1.9 mM increase in POC concentrations over (Cowen et al., 1986; Karl et al., 1988; Bailly-Bechet et al. 2008). At background which, for EPR 91500N (background¼0.1670.05 mM), the time of sampling, the high-temperature chimney walls were would imply maximum predicted POC values within our Alvin- covered with Alvinella colonies and it is expected that hydrodynamic collected near-field samples of 2.1 mM. Intriguingly, that value constraints will favor the entrainment of organic material from the compares extremely well with the POC concentrations that we chimney walls or alvinellid colonies (Bailly-Bechet et al., 2008). In did measure in our Niskin samples, processed aboard ship, which addition, diffuse systems typically host the most abundant chemo- fall in the range 0.87–3.81 mM(Table 2). synthetic communities associated with seafloor - ing and during our dive program, areas of high biomass were 4.4. POC and DOC in the dispersing plume observed in the areas surrounding the vents at 91500NEPR.Within these environmental niches, DOC concentrations have been As hydrothermal plumes rise and become progressively more observed to reach values (39–69 mM) that are nearly double back- dilute, they can be tracked readily using in situ optical sensors ground deep-ocean values (Lang et al., 2006)andareclosertothose that respond to the high concentrations of suspended particulate measured in the most productive waters associated with the matter within those plumes (Baker et al., 1995). In this study, equatorial Pacific region, where DOC concentrations reach such a phenomenon is most readily apparent in cast CTD 91 67 mM(Carlson and Ducklow, 1995). where a particle-rich lens of water at 2275–2350 m depth Elevated POC concentrations (of up to 18.3 mM) have also been provides clear evidence for a dispersing non-buoyant hydrother- observed in warm (20 1C) diffuse-flow fluids at 211N, EPR (Comita mal plume (Fig. 2). What is particularly notable at this station is et al., 1984). In that study, DOC concentrations were 53–71 mM, that the profile of measured POC concentrations directly mimics similar to the range reported by Lang et al. (2006), who concluded this turbidity profile, indicating that highest values of POC occur that the high DOC concentrations present in diffuse flow vent- toward the core of the dispersing non-buoyant plume (Fig. 2). fluids were a result of high microbial activity sustained by energy Further, the maximum POC concentrations observed at CTD 91 produced from the oxidation of sulfur species and H2 (McCollom are directly comparable to the highest buoyant plume concentra- and Shock, 1997; Sarradin et al., 1999). Thus, while entrainment tions observed at CTD 83, situated immediately above the of pre-formed DOC and POC into our plume samples could explain seafloor. Given that we should expect plume samples at CTD 91 the data reported here, we cannot preclude the possibility that to have undergone at least one order of magnitude further on-going microbial within the buoyant and non- dilution, relative to samples from the deep water column at CTD buoyant plume might also play an important role in producing 83 (Lupton, 1995), this indicates that there must have been some biomass7organic ligands7exopolymers. further process resulting in the addition of POC to the dispersing Certainly, in-situ production of microbial biomass has the poten- non-buoyant plume. The close correlation observed between tial, energetically, to occur within plumes themselves, due to the particle anomalies and POC data, suggests that an additional redox disequilibria set up between the chemically reduced vent- source of POC to the plume may result from passive scavenging fluids and oxygenated seawater (McCollom, 2000; Lam et al., 2008; of dissolved organic carbon onto plume particles and/or from a Dick et al., 2009). Of course, any in-situ production within the microbial community active within the dispersing plume. buoyant plume samples obtained using Alvin, prior to their collec- Adsorption of organic carbon onto plume particles should result tion, is likely to be minimal because the samples were taken from inashiftinthesizefractionationofthetotalorganiccarbonpresent, turbulent rising plumes just a few meters above each vent orifice. increasing the POC fraction and decreasing the DOC fraction. Like- In-situ production could have occurred subsequently, however, wise, a hetero- or mixotrophic microbial community would con- within the closed Niskin bottles, during the ascent of Alvin but prior sume DOC (decreasing that concentration) while increasing its to being filtered aboard the ship. This would be expected to result in biomass (leading to increased POC concentrations). In either case, the production of biomass (increasing POC) as well as the produc- one would expect any increase in POC to coincide with a comple- tion and consumption of DOC. Within the early stages of buoyant mentary decrease in DOC concentrations. In our plume samples— plume rise, microbial activity is most likely the result of hydrogen and within the precision of our measurements—none of the DOC and sulfide oxidation (Lam et al., 2008), with the oxidation of concentrations measured at plume height differ significantly from elemental sulfur and sulfide precipitates providing the biggest background. Nevertheless, acrossthedepth-rangeforthenon- biomass potential (McCollom, 2000). Hydrogen sulfide in the vent buoyant plume at CTD 91, as defined from in-situ turbidity signals, fluids ranged from 8.6 to 28.7 mM and the free sulfide concentra- there does appear to be a concave curvature in the profile of DOC tions measured in-situ as DSV Alvin flew through the plume were that mirrors the concave curvatures observed in both the POC high compared to background seawater (Table 3). It is also impor- profile and the in-situ turbidity sensor profile. This could be tant to remember that free sulfide represents only part of the total consistent with scavenging and/or biological consumption of DOC sulfide transported by hydrothermal plumes. Since aqueous and from the dispersing non-buoyant plume (Fig. 2).Atthecoreofthe - polymeric iron sulfides in equilibrium with H2SandHS may plume, however, maximum POC concentrations are 0.4 mMrepre- contribute more than 60% of the total ‘dissolved’ sulfide pool senting an increase of just 0.2 mM over background. By contrast, (Luther et al., 2001; Le Bris et al., 2003) the total amount of reduced DOC concentrations appear to decrease from 39 to 37 mMover sulfur that was trapped within each Niskin bottle, and hence the same depth range—a deficit of 2 mM. If this much larger available for microbial oxidation following each sampling event, decrease were due to uptake of DOC onto/into particulate matter at was most probably still larger than the values reported in Table 3. plume-height, the apparent net deficit would need to be explained Previously, McCollom (2000) carried out a theoretical study to by some additional removal process—e.g. in the form of settling quantify the component of biomass production within hydro- plume . thermal plumes that can be directly attributed to inorganic Evidence that such a mechanism is plausible can be gained from a chemical reactions. He determined that approximately 50 mg of consideration of fluxes to sediment traps underlying the EPR 91500N biomass could potentially be produced per kg of vent fluid (from plume (Fig. 4). In those sediment traps, particulate organic carbon EPR 211N OBS vent) as the fluid dilutes 1000-fold with the fluxes during the period immediately prior to our Alvin and surrounding seawater in the rising plume. This is equivalent to CTD-rosette based sampling appear to relate more closely to the 930 S.A. Bennett et al. / Deep-Sea Research I 58 (2011) 922–931 calculated Fe oxyhydroxide (FeOOH) fluxes than to total mass-flux or Baker, B.J., German, C.R., Elderfield, H., 1995. Hydrothermal plumes over spread- total Fe flux (Fig. 4, Table 4). Since this Fe oxyhydroxide fraction ing-center axes: Global distributions and geographical Interferences. In: Humphris, S.E., Zierenberg, R.A., Mullineaux, L.S., Thomson, R.E. (Eds.), Seafloor represents just 1–2% of the total mass flux, this would appear to be Hydrothermal Systems: Physical, Chemical, Biological, and Geological Inter- consistent with a more specific Corg-transporting process than just actions. 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