Chemical Geology 354 (2013) 42–54

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Chemical Geology

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Authigenic barite records of methane seepage at the Carlos Ribeiro mudvolcano (Gulf of Cadiz).

Heleen Vanneste ⁎, Rachael H. James, Boris A. Kelly-Gerreyn 1, Rachel A. MillsOcean and Earth Science, National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK

⁎ Corresponding author at: EcoLab (Laboratoire Ecologie FCampus INPT- ENSAT, Avenue de l'Agrobiopole — BP 32607France. Tel.: +33 5 34 32 37 56.

E-mail addresses: heleen.vanneste@ensat.fr (H. Vann(R.H. James), B.Kelly-Gerreyn@bom.gov.au (B.A. Kelly-G(R.A. Mills).

1 Present address: Observations and Engineering BrGPO Box 1289 Melbourne VIC 3001, Level 8, 700 CollinAustralia.

0009-2541/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.chemgeo.2013.06.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 December 2012Received in revised form 5 June 2013Accepted 11 June 2013Available online 20 June 2013

Editor: U. Brand

Keywords:Authigenic bariteMud volcanoGulf of CadizPore fluid modellingX-ray fluorescence (XRF) data

Submarine mud volcanoes (MVs) are dynamic features that episodically expel gas-charged fluids and mudonto the seafloor, transferring various chemical constituents into the overlying water column. The temporalvariability in MV activity is, however, poorly understood, so their importance as a source of methane (CH4)and higher hydrocarbons for the oceanic carbon budget, although thought to be significant, cannot be properlyconstrained. In this study, the history of fluid and gas seepage at the Carlos RibeiroMV (Gulf of Cadiz) is assessedvia geochemical analyses and transport-reactionmodelling of pore fluids and barium (Ba) rich layers (Ba fronts)in sediment cores, recovered along a transect from the eye to the periphery of the MV. X-ray fluorescence datareveal that Ba fronts are absent at the eye, while a single front (with up to 1740 ppmBa) is present at themarginof the summit. Three Ba fronts occur at 45, 85 and 130 cm depthwithin amudflow to the southeast of the crater.Spectrometric analyses indicate that barite is the Ba-rich mineral in these layers. Upward advecting pore fluidsare enriched in barium but depleted in calcium (Ca2+) relative to seawater. Modelling of the Ba2+ and Ca2+

pore fluid profiles indicates that the positions of the Ba fronts reflect both the present-day hydrodynamic condi-tions as well as higher fluxes of methane in the past. Fluid advection appears to have decreased since 340 cal yrbefore present, but degassing of the mudflow is ongoing and is potentially an important source of CH4.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Submarine mud volcanoes (MVs) have been extensively describedand studied across the globe (e.g. MacDonald et al., 1994; Gardner,1999; Graue, 2000; Hensen et al., 2004; Omoregie et al., 2009; Chaoet al., 2011) as they are extremely efficient in transporting hydrocar-bons (especially methane) from deeply buried sediments, to shallowsediments, to the overlying water column (Dimitrov, 2002), and po-tentially to the atmosphere (e.g. Dimitrov, 2003; Sauter et al., 2006).Quantifying methane fluxes from MVs has proven to be a real chal-lenge, not only because MVs are extremely dynamic, but also becausemethane fluxes are modified by a number of biogeochemical processes,such as anaerobic oxidation within the sediments (Boetius et al., 2000)and aerobic oxidation in the water column (Higgins and Quayle,1970). Most MV studies focus on their contribution to the present-dayoceanic methane budget (e.g. Mau et al., 2006; Sauter et al., 2006;

onctionnelle et Environnement);, 31326 Castanet Tolosan Cedex,

este), r.h.james@noc.ac.ukerreyn), ram1@noc.soton.ac.uk

anch, Bureau of Meteorology,s Street, Docklands, VIC 3008,

rights reserved.

Wallmann et al., 2006), yet mud volcanism is known to be episodic(Kopf, 2002; Lykousis et al., 2009; MacDonald and Peccini, 2009).Hence, there is a need for more information on how hydrocarbon emis-sions have varied in the past, to understand the true impact of methaneventing at MVs on the global carbon cycle.

Authigenic barite (BaSO4) is considered to be a useful proxy forassessing past fluxes of methane gas on continental margins (Dickens,2001). This is because gas-charged fluids generally lack sulphate (SO4

2−)but contain elevated concentrations of dissolved barium (Ba2+) (Torreset al., 1996b). During the ascent of the fluids through the sedimentcolumn, barite precipitates on contactwith downward-diffusing seawa-ter sulphate close to or even at the seafloor (Eq. (1); Aquilina et al.,1997; Fu et al., 1994; Gingele and Dahmke, 1994; Kasten et al., 2003;Torres et al., 1996a):

Ba2þ aqð Þ þ SO42− aqð Þ↔BaSO4 sð Þ: ð1Þ

The pore water sulphate gradient at MVs is typically regulated bythe methane flux from depth via the anaerobic oxidation of methane(AOM; CH4 þ SO4

2−→HCO3− þHS− þH2O) at the sulphate–methane

transition (SMT; e.g. Aloisi et al., 2002; Bohrmann et al., 2003; Borowskiet al., 1999; de Beer et al., 2006; Haese et al., 2003; Niemann et al., 2006;Werne et al., 2004). As authigenic barite builds up just above the depthof sulphate depletion, its presence records the depth of the SMT (VonBreymann et al., 1992; Dickens, 2001; Kasten et al., 2003; Snyder et al.,

43H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

2007a) and thus, can be used to document the history of gas seepage oncontinental margins.

Depending on the availability of sulphate and barium, barite precipi-tates at cold seep sites vary frommicrocrystalline phases that formwith-in the sediment column (e.g. Peru Margin; Torres et al., 1996a) to blocksand columns up to 10 m high scattered over the seafloor (e.g. Sea ofOkhotsk; Greinert et al., 2002). At MVs, barite tends to be present as aminor authigenic phase coexisting with themore widespread and abun-dant calcium carbonate concretions, slabs and chimneys e.g. in the Gulfof Cadiz (Díaz-del-Río et al., 2003; Vanneste et al., 2012) and in theNile deep-sea fan (Gontharet et al., 2007). Although the Ba cycle is welldocumented at localities characterised by diffuse and/or advectivemeth-aneflow (e.g. Torres et al., 1996a; Aquilina et al., 1997; Naehr et al., 2000;Dickens, 2001; Greinert et al., 2002; Torres et al., 2002; Riedinger et al.,2006), studies of authigenic barite layers and their linkage to methaneventing in mud volcano settings, are limited to two MVs in the Gulf ofMexico (Castellini et al., 2006).

In this study, we investigate the history of venting activity at theCarlos Ribeiro MV, which is located on the Portuguese margin of the

Fig. 1. A. Bathymetric map of the Gulf of Cadiz (modified from Zitellini et al., 2009) showingtriangles: mud volcanoes; black dotted line: external boundary of the accretionary wedge;reverse faults; yellow lines with rhombuses: axis of inactive anticline (Medialdea et al., 20location of the core sites. C. Fluid flow regime at the present-day at the MV (u0 = upwardrepresent the best fit between the model simulated chloride (Cl−) pore fluid profiles and m

Gulf of Cadiz. The majority of the mud volcanoes in the Gulf ofCadiz are considered to be in a dormant stage, however mud volca-nism has been estimated to be active since 2.4–2.6 Ma on for instancethe Moroccan margin (Van Rensbergen et al., 2005; Perez-Garciaet al., 2011). To date mainly geophysical tools have been applied toinvestigate the episodic nature of MVs in this area. Here we use anITRAX XRF core scanner to locate authigenic mineral fronts in sedi-ment cores recovered from four sites along a transect from the centreto the periphery of the MV. Chemical analyses of the solid phase andpore fluids are used together with a numerical 1-D transport-reactionmodel to estimate past variations in methane fluxes. Finally, radiocar-bon dating of hemipelagic sediments on top of the MV is used to pro-vide an absolute chronology of the mud extrusion history of the mudvolcano.

2. Geological setting

The Gulf of Cadiz is located in the North Atlantic Ocean on theEuropean continental margin, west of the Gibraltar Strait (Fig. 1A).

the position of the Carlos Ribeiro mud volcano and the main geological structures. Blackblack lines: deep rooted faults that affect the seafloor; red lines with triangles: active09; Zitellini et al., 2009). B. Shaded relief map of the Carlos Ribeiro MV, showing thefluid flow velocity). Measured data are shown by the black dots while the solid lineseasured data (after Vanneste et al., 2011).

Table 1Location of core sites. PC = piston core; GC = gravity core; MC = multi core.

Corecode

Location Latitude(°N)

Longitude(°W)

Core length(cm)

Water depth(m)

PC-43 2.9 km SW of the MV(background site)

35°46.04′ 8°26.551′ 546 2344MC-44 35°46.042′ 8°26.554′ 18 2344GC-52 Eye, on the summit 35°47.255′ 8°25.327′ 201 2173PC-53 35°47.259′ 8°25.320′ 548 2174MC-57 35°47.250′ 8°25.324′ 33 2175GC-49 Off-centre, on the

summit35°47.224′ 8°25.290′ 124 2177

MC-50 35°47.221′ 8°25.292′ 20 2176PC-58 Margin, on the

summit35°47.196′ 8°25.269′ 231 2177

MC-63 35°47.197′ 8°25.269′ 36 2177GC-60 Mudflow, SE of the

summit35°47.076′ 8°25.229′ 187 2189

44 H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

The tectonic interaction between the Eurasian and African plates hascontrolled the geological history of this region since the Triassic(200 Ma ago; e.g. Gracia et al., 2003; Malod and Mauffret, 1990;Srivastava et al., 1990). Its current tectonic setting was achievedafter the westward movement of the Gibraltar Arc domain (Deweyet al., 1989; Maldonado et al., 1999). The collision of this domain withthe South Iberian and North African palaeomargins, caused the em-placement of a large wedge shaped sedimentary body in the Gulf ofCadiz, in the Late Tortonian (Torelli et al., 1996; Maldonado et al.,1999; Medialdea et al., 2004). It is the thickest sedimentary unit (atleast 10 km thick in the eastern sector thinning towards the west)in the Gulf (Thiebot and Gutscher, 2006; Iribarren et al., 2007), andwas reactivated in the Late Miocene as a result of the oblique collisionof Iberia and Africa along a NW–SE boundary, which is continuingtoday (Medialdea et al., 2009; Zitellini et al., 2009).

The tectonic activity in this area is reflected in numerousfluid escapestructures on the seabed, including MVs (e.g. Baraza and Ercilla, 1996;Gardner, 1999; Kenyon et al., 2003; Pinheiro et al., 2003; Somozaet al., 2003; Fernández-Puga et al., 2007; León et al., 2007; Medialdeaet al., 2009; León et al., 2012). The MVs are concentrated on the IberianandMoroccanmargins and range in size from 800 to 4000 m in diame-ter and up to 200 m in height (e.g. Pinheiro et al., 2003). The MVs con-sist mainly of grey Miocene plastic marls with rock fragments datingfrom the Late Cretaceous to the Pliocene (Ovsyannikov et al., 2003).Although marly clay and salt diapirism (e.g. Medialdea et al., 2009;Perez-Garcia et al., 2011) are considered to play an important role inthe formation of the MVs, seismic profile analysis (Gardner, 2001; VanRensbergen et al., 2005; Medialdea et al., 2009) as well as geochemicaldata (Nuzzo et al., 2009) indicate that mud volcanism in the Gulf ofCadiz is fault-controlled. The presence of hydrate-bearing sediments(e.g. Mazurenko et al., 2002; Pinheiro et al., 2003), authigenic carbon-ates (e.g. Magalhães et al., 2012; Vanneste et al., 2012) and seep relatedbiota (e.g. Vanreusel et al., 2009) at some MVs suggests that methaneseepagewas high in the past and/or is today. However, spontaneous es-cape ofmethane from the seabed has only been observed at the CaptainArutyunov MV and the Mercator MV (Haeckel et al., 2007; Sommeret al., 2009); themajority of theMVs do not show recentmud extrusionactivity (Pinheiro et al., 2003; Van Rensbergen et al., 2005).

The Carlos Ribeiro MV is located on the Portuguese margin at2173 m below sea level, on the lower slope of the sedimentary wedge(Fig. 1B). The MV is circular with a flat-topped summit, extending to~1500 m in diameter and ~80 m in height (Pinheiro et al., 2003). Theeye (i.e. the focus of activity) of the MV lies slightly to the north of itscentre, and it is surrounded by a series of concentric ridges whichbuild up the MV. Mud extrusions have formed a mudflow to the south-east of the summit. At the present-day, methane emissions into theoverlying water column are relatively low (0–806 mmol m−2 yr−1),which has been attributed to low fluid flow velocities (0.4–4 cm yr−1)coupled with efficient microbial methanotrophy (Vanneste et al., 2011).Fluidflowactivity is focused at the eye of theMV, and gradually decreasestowards its periphery (Vanneste et al., 2011).

3. Methods

3.1. Sample collection

A suite of sediment cores was recovered from the Carlos RibeiroMV during RRS James Cook Cruise 10 (May–June 2007). These includepiston, gravity and multi cores from four sites along a transect thatconnects the eye of the MV to mudflow pathways to the southeastof the summit (Fig. 1B and Table 1). For comparison, a backgroundsite located 2.9 km to the southwest of the MV was also cored. On re-covery, the cores were taken into a cold room (~6 °C). The piston andgravity cores were sampled every 20 cm, and multi cores were slicedinto 1–3 cm thick sections. Sub-samples were taken for hydrocarbongas analyses, pore water chemistry and porosity analyses immediately

after opening the cores. Pore waters were extracted in a glove bagunder a N2 atmosphere by pressure-filtration through 0.2 μm celluloseacetate membrane filters.

All of the piston and gravity cores, from the MV, consist exclusivelyof mud breccia, which is composed of greenish-grey clay with variousamounts of clay- and sandy siltstones. Cores PC-58 and GC-60 also con-tain a thin layer of hemipelagic sediment at the top of the core (see coredescription, Supplementary Data). The absence of oxidation fronts sug-gests that a single mudflow layer was sampled. Multi cores taken at theeye, off-centre and margin sites provided intact samples of the upper-most ~40 cm of the sediment column, consisting of bottom seawater,a thin layer of hemipelagic sediment, and then mud breccia. A singlecarbonate concretion was found in GC-60 (mudflow site) at 130 cmdepth.

3.2. Onshore analyses

3.2.1. Pore fluid analysesPore water barium ([Ba2+]) and calcium ([Ca2+]) concentrations

were determined by inductively coupled plasma optical emission spec-troscopy (ICP-OES; Perkin Elmer Optima 4300DV). The external repro-ducibility of these analyses, determined by repeat analyses (n = 3) ofIAPSO seawater and four pore water samples, is less than 4%. Repeatanalyses of an in-house seawater standard gave Ba2+ and Ca2+ concen-trations of, respectively, 0.36 μM and 10.0 mM compared to consensusvalues of, respectively, 0.36–0.37 μM and 9.9–10.0 mM. Sulphate con-centrations were measured by ion chromatography (Dionex ICS2500).Repeat analyses of IAPSO seawater as well as single anion standards in-dicates that the external reproducibility is better than 1%.

3.2.2. Sediment analysesTo optimize the sediment sampling strategy, piston (PC-43 and 58)

and gravity (GC-52, 49 and 60) cores were first analysed using theITRAX core scanner housed in BOSCORF at the National OceanographyCentre in Southampton (UK). The ITRAX provides semi-quantitativeX-ray fluorescence (XRF) measurements of the elemental compositionof the sediments (Croudace et al., 2006), thus giving an indication ofdown-core geochemical changes. The voltage and current of the X-raysource, a 3 kWMo tube,was set to 30 kV and30 mA respectively. Amea-surement step-size of 1000 μm and an exposure time of 40 s were used.The data were processed using Q-Spec software (Croudace et al., 2006).

Based on the XRF results, 39 sediment samples were selected forquantitative chemical analysis. The samples were oven dried, groundto powder and dissolved using Aqua Regia, HF and HClO4. Major andminor element concentrations were determined by ICP-OES (PerkinElmer Optima 4300 V) at the NOCS (U.K.). The reproducibility of Ba,Ca and titanium (Ti) data, which are presented here, is b2% basedon replicate analyses (n = 3) of five sediment samples. The accuracyof the measurements was assessed by analysis of two internationalcertified reference materials (MAG-1 and SCo-1). Measured values forall elements were within ±6% of the certified values.

45H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

The total inorganic carbon (TIC) content of 24 sediment sampleswas determined using a UIC CM 5012CO2 coulometer. The inorganiccarbon was converted to CO2 by treating the sample with 10% phos-phoric acid, and the carbonate (CO3) content is calculated: CO3

(wt%) = TIC ∗ 60.01 / 12.01. Repeat analyses (n = 3) of two sedimentsamples consistently gave concentrations within 1%.

In order to confirm the presence of barite in the Ba-rich layers, threesediment samples from the mudflow station (one from each of theBa-enriched sediment layers) were subjected to a sequential leachingprocedure to separate the barite grains. The samples were weighed,and then leachedwith acetic acid, sodium hypochlorite, hydroxylaminehydrochloride and anHNO3:HFmixture. The residuewas treatedwith a1:1mixture of saturated AlCl3 and 1 MHNO3 and subsequently ashed inamuffle furnace at 700 °C for 1 h (Paytan et al., 1993; Eagle et al., 2003).After extraction the residual material was mounted onto a metal stub,coated with gold and examined using a Leo 1450VP Scanning ElectronMicroscope (SEM) equipped with an Energy Dispersive Spectrometer(EDS) at the NOCS (U.K.).

3.2.3. Radiocarbon analysesThree cores from the eye (MC-57), the margin (MC-63) and the

mudflow (GC-60) sites on the mud volcano were subsampled forradiocarbon analyses. The aim of these analyses was to determinethe absolute age of the mud extrusion episodes that build up theMV. Approximately 10 cm3 of hemipelagic sediment was taken fromjust above its boundarywith the underlyingmud breccia (sample depthsare given in Table 2). The sediments were thenwashed through 250 and150 μmsieveswithMilli-Qwater and dried under a hot lamp. 10–15 mgof planktonic foraminifera was picked from each sample, coated ingraphite at the NERC Radiocarbon Facility (Environment) in Scotlandand analysed for 14C at the SUERC AMS Laboratory. The age of thesamples is reported in conventional radiocarbon years BP (i.e. rela-tive to AD 1950), and converted to the calendar year timescale by ap-plying theMarine04 dataset (Hughen et al., 2004). This was achievedusing the calibration programme CALIB 5.0 which incorporates atime-dependent global ocean reservoir correction of about 400 yrs(Stuiver and Reimer, 1993). The range in the calibrated radiocarbonage represents the 95% confidence interval (i.e. 2σ) of the analyses(Table 2).

3.3. Geochemical modelling

A 1-D transport model, described in detail by Vanneste et al. (2011),was used to generate the pore water concentration-depth profiles ofCa2+ and Ba2+. Themodel represents the depth distribution of dissolvedspecies in terms of molecular diffusion, fluid advection and bioirrigation(Boudreau, 1996), as follows:

∂φCi

∂t ¼ Di∂∂x

φϑ2

∂Ci

∂x

� �−∂φuCi

∂x −φα xð Þ⋅ Ci xð Þ−Ci 0ð Þð Þ

where Ci is the concentration of the dissolved pore water species i, Di isthe diffusion coefficient of species i corrected for bottom water salinity(34.2), temperature (4.3 °C) and pressure (220 bar), ϑ2 (= 1 − ln(φ2))is the tortuosity correction for diffusion (Boudreau, 1997),φ is themea-sured sediment porosity, u is the velocity of the upward fluid flow (atany depth), α(x) is the irrigation exchange coefficient (Ci(x) − Ci(0))

Table 2Conventional and calibrated radiocarbon ages for the hemipelagic veneer at the Carlos Ribe

Core code Sample depthcm

Sample ID Conventional ryr BP

MC-57 0–1 SUERC-26762 –

MC-63 2–4 SUERC-26763 636GC-60 8–12 SUERC-26764 1512

is the difference between the concentration of species i at depthand in bottom water, t is time and x is sediment depth. Fluid flow

velocities (u0 with u ¼ u0⋅φ0

φ xð Þ ) and irrigation parameters (xmix and

α′ with α(x) = α′ ⋅ exp[−(xb − xmin)] where xb is the depth belowthe irrigation zone) were obtained by fitting the model results to themeasured pore water concentration profiles of Cl, Na and B (which areconsidered to behave conservatively, at least for the depth of sampling)using a least squares technique (Fig. 1C, Vanneste et al., 2011). Valuesfor all of the above parameters are given in Appendix A (Table A.1).

This model does not account for any reactions between the porefluids and the sediments, so any differences between the measuredand modelled pore fluid profiles of Ca2+ and Ba2+, point to their in-volvement in diagenetic reactions such as precipitation of calciumcarbonate and barite at the time of sampling.

In addition, a 1-D transport-reaction model (Vanneste et al., 2011)was applied to pore fluid concentration-depth profiles of sulphate toassess how changes in the depth of the SMT are linked to past changesin a) pore fluid flow velocity, and b) dissolvedmethane concentrations.In this model, the depth distribution of sulphate and methane is de-scribed by a partial differential equation (Boudreau, 1996):

∂φCi

∂t ¼ Di∂∂x

φϑ2

∂Ci

∂x

� �−∂φuCi

∂x −φα xð Þ⋅ Ci xð Þ−Ci 0ð Þð Þ þ φ∑Ri

where Ri is a reaction term for pore water species i which accounts forsulphate reduction, methanogenesis and anaerobic oxidation. Ri isdiscussed in detail in Appendix A and values for all of themodel param-eters are given in Table A.2.

4. Results

4.1. Pore fluid profiles

Figs. 2 and 3 show pore fluid concentration-depth profiles forBa2+ and SO4

2− from the four sites on the CRMV and the backgroundsite, respectively. A detailed description of the pore fluid SO4

2− data isgiven in Vanneste et al. (2011). Pore fluid Ca2+ data are shown inFigs. 3 and 4. For all sites on the MV, the pore fluid Ba2+ concentra-tions are low (b1 μM; Supplementary Table 1) and do not change inthe uppermost 50 cm of the sediment column. Below 50 cm depth,dissolved Ba2+ concentrations rise. At the mudflow and margin sites,the increase in [Ba2+] is more gradual than it is at the eye andoff-centre sites. The depth of the increase in [Ba2+] differs at each siteand coincides with the depth of the SMT (Fig. 2). The highest Ba2+ con-centration (78 μM) was measured in pore fluids from the margin site.The dissolved Ba2+ concentration at the background site is b0.5 μMthroughout the core (Fig. 3). Concentrations of Ca2+ in the pore fluidsgradually decreasewith depth at all sites, from seawater concentrations(~10 mM) near the seafloor down to 2–6 mM at the base of the cores(Figs. 3 and 4).

4.2. Sediment chemistry

Ba/Ti profiles determined by the ITRAX XRF as well as the quantita-tive measurements of Ba content and Ba/Ti are shown in Fig. 2; profilesof sedimentary Ca and inorganic carbon for all sites are given in Fig. 4.

iro mud volcano.

adiocarbon age Uncertaintyyr (1σ)

Calibrated radiocarbon age range

2σ cal yr BP

– –

37 255–30435 997–1132

Eye Mudflow

MC-57GC-52PC-53

MC-50GC-49

SO42-

Ba2+

Ba Ba/Ti (XRF)

Ba/Ti

Ba2+ model

MC-63, PC-58

SMT

GC-60

MarginOff-centre

Ba (ppm) SO42- (mM)

Ba/Ti

Ba2+ (µM)Ba/Ti (XRF)

Dep

th (

cmb

sf)

0 0 10 20 301,000 2,000 0 0 10 20 301,000 2,000 0 0 10 20 301,000 2,000 0 0 10 20 301,000 2,000

0

25

50

75

100

125

150

175

200

225

Ba (ppm) SO42- (mM) Ba (ppm) SO4

2- (mM) Ba (ppm) SO42- (mM)

0 0 25 50 75 1000.1 0.2

0 0.1 0.2 0.3 0.4

Ba/Ti

Ba2+ (µM)Ba/Ti (XRF)0 0 25 50 75 1000.1 0.2

0 0.1 0.2 0.3 0.4

Ba/Ti

Ba2+ (µM)Ba/Ti (XRF)0 0 25 50 75 1000.1 0.2

0 0.1 0.2 0.3 0.4

Ba/Ti

Ba2+ (µM)Ba/Ti (XRF)0 0 25 50 75 1000.1 0.2

0 0.1 0.2 0.3 0.4

Fig. 2. Profiles of solid phase barium (Ba), Ba/Ti and Ba/Ti (XRF) (shown on the left), and pore fluid Ba2+and sulphate (SO42−) (shown on the right), for the four core sites on the

Carlos Ribeiro mud volcano. Results of numerical modelling of dissolved Ba2+ profiles are shown by the dark blue solid line. The yellow and grey shaded boxes represent thehemipelagic veneer and the sulphate–methane transition (SMT) zone, respectively. The dark grey line indicates the depth of the SMT, i.e. the shallowest depth of sulphate depletion.

46 H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

The shapes of the XRF profiles correspond well with the quantitativeelement/Ti profiles for all elements at all sites. Thus, the X-ray fluores-cence data appear to reliably reflect the elemental composition of thesediments at the Carlos Ribeiro MV.

Bulk sediment Ba contents range from 129 to 1740 ppm (AppendixB Table B.1). The eye and background site show little variation in Bacontent throughout the core, with an average [Ba] value of, respectively,266 and 403 ppm. Higher [Ba] values are recorded at the other sites,although these are confined to narrow (13–27 cm) depth intervals.The off-centre site shows one [Ba] peak of 617 ppm at ~25 cm depth.Note that because of the offset between the XRF data and the quantita-tive Ba data at this site, this peak is not considered in the discussionbelow. A maximum Ba content of 1740 ppm was measured at 18 cmdepth at the margin site, while three [Ba] peaks occur at the mudflow

Ba/Ti

Ba/Ti (XRF) Ba2+ (µM) Ca

CaBa (ppm) SO42- (mM)

Dep

th (

cmb

sf)

Back

Fig. 3. Profiles of solid phase barium (Ba), calcium (Ca) and carbonate (CO3), Ba/Ti, Ba/Ti (ground site located 2.9 km southwest of the Carlos Ribeiro mud volcano.

site, at 45 (407 ppm), 85 (785 ppm) and 130 cm (748 ppm) depths.The lowest [Ba] in sediments from the Carlos Ribeiro MV are measuredjust below the enriched horizons. The highest [Ba] was measured in aconcretion that was found embedded in the sediment at 130 cm depthat the mudflow site (4940 ppm). From its chemical composition it isclear that this concretion is mainly composed of CaCO3 (Table B.1), andit seems that the high Ba content can be attributed to the occurrence oftransparent-white crystals of barite, on the surface of the concretion.

In general, the sediments have relatively homogenous Ca contentwith exception of the sediment cores from the margin and mudflowsite where the sediments at the top of the core, in the hemipelagiclayer, are enriched in Ca (7.3–13.8 wt%) relative to the rest of the core(~3 wt% on average; Appendix B Table B.1). In addition, the Ca sedimentprofile at the margin site shows a slight enrichment (by ~2 wt%) at

PC-43MC-44

Ca/Ti

/Ti (XRF) Ca2+ (mM)

CO3 (wt%) (wt%)

ground

XRF), Ca/Ti, Ca/Ti (XRF), and pore fluid Ba2+, Ca2+ and sulphate (SO42−) for the back-

Mudflow

SMT

Ba front

Ba front

Ba front

GC-60

Off-centre

SMT

MC-50GC-49

Eye

SMT

MC-57GC-52PC-53

Margin

SMT

Ba front

PC-58MC-63

Ca2+ model

Dep

th (

cmb

sf)

Dep

th (

cmb

sf)

Dep

th (

cmb

sf)

Dep

th (

cmb

sf)

Ca (wt%) CO3 (wt%)

Ca/Ti (XRF) Ca2+ (mM)

Ca/Ti

Fig. 4. Profiles of solid phase calcium (Ca) and carbonate (CO3), Ca/Ti, Ca/Ti (XRF), andpore fluid Ca2+, together with the results of numerical modelling of dissolved Ca2+

profiles (dark blue solid line) for the four core sites on the Carlos Ribeiro mud volcano.The yellow, light grey and dark grey shadedboxes represent, respectively, thehemipelagicveneer, the sulphate–methane transition (SMT) zone and the location of the Ba fronts ateach site.

47H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

40 cm depth, just above the SMT zone (Fig. 4). This corresponds to anincrease in the CO3 content of the sediments.

Downcore variations in the chemical composition of sedimentsnot only reflect the precipitation or dissolution of mineral phases, theycan also reflect changes in the relative proportion of different primaryminerals such as aluminosilicates (i.e. clays). To negate the effect of var-iable clay content on the chemical composition of the sediments, ele-ment concentrations are normalised to Ti, which is both refractory anda major component of clays. Although normalisation to Al is more com-monly used for marine sediments (Van der Weijden, 2002), the ITRAXcore scanner is rather insensitive to Al (Croudace et al., 2006). In anycase, Fig. 2 demonstrates that downcore profiles of Ba/Ti and Ca/Ti arevery similar in shape to the profiles of Ba and Ca, respectively, with ex-ception of the surface sample at themudflow site, which is locatedwith-in the hemipelagic sediment layer. This suggests that the Ba- and Ca-richhorizons within the mud breccia are not associated with changes in theproportions of clay minerals.

In the following discussion, depth intervals with high [Ba] (> 258 ±11 ppm) are referred to as ‘Ba fronts’. The value of 258 ppm representsthe average Ba content of the mud breccia in the uppermost part of thecores, i.e. the ‘background’ level. The barite in the mud breccia in theshallow subsurface sediments is not thought to have been altered sincethe mud was extruded.

4.3. Radiocarbon dates

Results from the radiocarbon analyses are given in Table 2. Thehemipelagic layer at the eye site contains a component of carbon ofmodern origin, which means that the mud breccia was extruded veryrecently. Themudflow at themargin of theMV is significantly older, be-tween 255 and 304 cal yr BP. The mudflow pathways to the southeastof the summit date from 997 to 1132 cal yr BP.

4.4. Pore fluid modelling

Comparison of themeasured andmodelled pore fluid data (Fig. 2) in-dicates that, at the time of sampling, Ba2+ is being taken up into the solidphase (i.e. the measured pore fluid Ba2+ concentrations are lower thanthe modelled values) at all sites with the exception of the off-centresite. On the other hand, Ba2+ is being released from the sediments intothe pore fluids, between 117 and 180 cm depth, at the margin site(pore fluid Ba2+ concentrations are higher than modelled values). Themeasured Ca2+ concentrations are generally lower than those predictedby themodel, indicating that Ca2+ is being removed from the pore fluids.

5. Discussion

5.1. Origin of Ba fronts at the Carlos Ribeiro MV

All observed Ba fronts occurwithin themud breccia that builds up theMV (see core description, Supplementary Data). This mud breccia hasgenerally lower concentrations of Ca (~2.8 wt.%) and Ba (~258 ppm) rel-ative to the background site (Ca: 12.8 wt.% and Ba: ~404 ppm; AppendixB Table B.1), which strongly suggests that the Ba fronts are authigenic inorigin, i.e. they were formed after the eruption of themud breccia withinwhich they occur. Indeed, EDS investigations confirm the presence ofmi-crocrystalline barite within the Ba fronts at themudflow site (Fig. 5). Theabsence of a Ca front at themargin site indicates that the Ba front is due tothe presence of barite rather than a Ba-rich carbonate phase.

The main source of Ba for the formation of authigenic barite inmarine sediments is generally considered to be the dissolution of bio-genic barite in sulphate-depleted pore waters (Brumsack, 1989; VonBreymann et al., 1992; Torres et al., 1996a; McManus et al., 1998). ‘Bio-genic’ barite forms in the water column within microenvironments ofdecaying biological debris (Dehairs et al., 1980; Bishop, 1988; Dymondet al., 1992; Paytan et al., 1993; Gingele and Dahmke, 1994), so barite

48 H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

fluxes to the seafloor are highest in areas of high productivity, such asalong continental margins (Von Breymann et al., 1992). An additionalsource of Ba2+ is sedimentary material: detrital aluminosilicates cancontain up to 1000 ppm of Ba (Dymond et al., 1992). Hence dissolvedBa2+ in the pore fluids from the Carlos Ribeiro MV might originatefrom pore fluid–sediment interactions at moderate temperatures atdepth, as is the case for Li+ and B (Vanneste et al., 2011).

The Ba enrichments that we record at the CRMV are relatively small(407 to 1740 ppm) compared to sedimentary Ba fronts observed atMVsin the Gulf of Mexico, which have Ba contents of 14,600 to 23,000 ppm(Castellini et al., 2006). However, the Ba2+ concentration in the porefluids at these MVs are also much higher (up to 1.2 mM; Castelliniet al., 2006) than we observe at the CRMV, because of dissolution of de-trital potassium-feldspar in arkosic sandstone horizons (Macpherson,1989). The barium in pore fluids from all other seep sites studied todate, such as San Clemente Fault (Torres et al., 2002), Monterey Bay(Naehr et al., 2000) and Peru Margin (Torres et al., 1996a), comesfrom the remobilisation of biogenic barite in intervals depleted indissolved sulphate. Concentrations of pore fluid Ba2+ at these sites(8 to 146 μM; Naehr et al., 2000; Torres et al., 1996a; Torres et al.,2002) are similar to those that we have measured at the Carlos RibeiroMV (up to ~68 μM), suggesting that biogenic barite is the main sourceof Ba2+.

5.2. Present-day barium cycle at the Carlos Ribeiro MV

Methane rich pore fluids are actively seeping at the Carlos RibeiroMV today, with the consumption of sulphate in the shallow subsurfaceas a result of AOM(Vanreusel et al., 2009; Vanneste et al., 2011). At seepsites, barite fronts have been shown to form in sediments at or justabove the SMT, as ascending Ba-rich fluids interact with dissolved sul-phate present in pore fluids in shallow sediment layers (Greinert etal., 2002; Aloisi et al., 2004; Castellini et al., 2006).Whether these frontsare actively forming today, or were formed in the past (hence markingthe position of a ‘relict’ SMT), can be assessed from pore fluid profiles.Profiles of Ba2+ in the shallow sediments of the CRMV are similar tothose observed at othermethane-charged systems, that can be attribut-ed to the precipitation of barite at the SMT (Torres et al., 1996a;Dickens,2001; Aloisi et al., 2004; Castellini et al., 2006; Snyder et al., 2007a;Snyder et al., 2007b). The application of the 1-D transport model tothe Ba2+ pore fluid data confirms the formation of barite as the modelresults show that Ba2+ is being removed from the pore fluid and takenup into the sediments at the eye, margin and mudflow sites. Themodel generated pore fluid profiles only coincide with the measured[Ba2+] data at the off-centre site, indicating that no significant sedi-ment–pore fluid reactions are occurring at this site at the time ofsampling.

0 2 4 6 8 100

500

1000

1500

2000

SBa

O

Cou

nts

Energy [keV]

Ba

Fig. 5. Energy Dispersive Spectrometer (EDS) spectra of residue remaining after sequen-tial extraction of a sample recovered from sediment core GC-60 at the mudflow site at130 cm depth on the Carlos Ribeiro mud volcano.

At the eye site, measured pore water barium concentrations appearto be lower than the modelled values between ~25 and 85 cm depth(Fig. 2). As this interval has low sulphate, barite precipitation is inhibited,so Ba2+ must be taken up into an alternative mineral phase such as car-bonate. The Ca2+porewater profile shows a concave-upwards inflectionabove the SMT, consistent with the precipitation of a carbonate phasewithin the SMT zone (as the model also implies; Fig. 4). Accordingly, itseems likely that Ba2+ is taken up into a carbonate phase at the eyesite, rather than barite. Nonetheless, neither a Ba nor Ca front is observedin the sediment record. There could be a number of reasons for this. Onepossibility is that present-day hydrodynamic conditions (e.g. fluidfluxes) were established only recently. As pore water chemistry adjustsmuch more quickly to changes in hydrodynamic conditions thanthe solid phase, it is possible that significant levels of Ba and Cahave not yet had time to build up in the sediments at the depth ofthe present-day SMT. This is consistent with the position of the eyesite on the MV, which is the centre of activity today (Vanneste et al.,2011). In addition, radiocarbon analyses show that the date of thelast mud extrusion event is indistinguishable from the present-day(Table 2). This means that any Ba and Ca fronts formed in the past,would have been broken up andmixed inwith the newmud as it forceditself up to the seafloor.

By contrast, the pore fluid data at the mudflow site support theprecipitation of barium just above the SMT, and this is indeed ob-served in the sedimentary record (Fig. 2). An inflection in the Ca2+

profile occurs at the same depth (Fig. 4), which implies that bothcalcium carbonate and barite are actively precipitating at this site at130 cm depth. This is in agreement with the presence of a carbonateconcretion with barite crystals at 130 cm. As the authigenic baritefront occurs immediately above the depth of SMT, the uppermosttwo Ba fronts would appear to delineate the position of relict SMTsand thus a decrease in methane flux since the emplacement of themudflow.

The margin site is the only site where the measured dissolvedBa2+ concentrations are higher than those predicted by the model(Fig. 2). The release of Ba2+ from the sediment is restricted to a rela-tively narrow depth interval of ~58 cm, which is located underneaththe SMT (at 117 cm depth) and underlain by a zone of barite precip-itation. As the pore fluids contain no sulphate at this depth, Ba2+ ismost likely released through dissolution of barite. The position ofthe largest Ba front at the CRMV is in the shallow subsurface, wellabove the present-day SMT, and the absence of any fronts immediatelyabove the SMT suggests that the flux of methane from depth has fallenover time, resulting in a downward shift of the SMT zone. The sedimen-tary and pore water Ca data support this interpretation: Ca2+ uptakeinto the sediment coincides with a Ca front which is located withinthe present-day SMT zone but below the Ba front (Fig. 4).

5.3. Venting history of the Carlos Ribeiro MV

The solid phase and porefluid data suggest that the single Ba front atthe margin site, and the uppermost two sedimentary Ba peaks at themudflow site, mark the location of relict SMTs and can thus potentiallyprovide information about past methane fluxes (ref. Dickens, 2001).

In the case of the mudflow site, the barite fronts are likely to haveformed as the mudflow degassed over time since its emplacement.Freshly-erupted mud is generally saturated in methane and will degaswhen emplaced onto the seafloor because of the drop in pressure(methane solubility decreases with decreasing depth; Duan et al.,1992). Because of this pressure drop, and also because of the strong con-trast between the concentration of methane in the pore waters in themudflow and the concentration of methane in the overlying watercolumn (which promotes diffusion), methane will rapidly escape intothe water column as the mud extrudes. This might also explain whythe uppermost Ba front is significantly smaller than the lowermosttwo. As the methane pressure within the pore fluid of the mudflow

49H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

will decrease rapidly during this process, the SMT will deepen just asfast and there is less time to accumulate barite at the SMT. To verifythis interpretation, an attempt was made to reconstruct the sulphatepore water profiles at the time that each of the Ba fronts formed,using the 1-D transport-reaction model. This is based on the observa-tion that Ba fronts are formed just above the pore fluid sulphate mini-mum (Von Breymann et al., 1992; Torres et al., 1996a).

The dissolved sulphate profile within the sediments can largely bedescribed in the model by two parameters (1) the advective fluidflow velocity (u0), and (2) the dissolved methane concentration at thebase of the modelled sediment column (CH4L) (Vanneste et al., 2011).Assuming that the Ba fronts at the mudflow site are generated by diffu-sion from the degassingmud, i.e. u0 is zero, then CH4L is varied until thedepth of the SMT coincides with the base of the barite front (Vannesteet al., 2011). Modelling predicts that the value of CH4L must be>150 mM for the uppermost Ba front, 30 mM for the middle front,and 8 mM for the lowermost front (Fig. 6). These results demonstratethat the pore fluids in the extruded mud were initially oversaturatedin methane as pore water methane concentrations exceeded the mini-mum methane concentration required for gas hydrate formation(68 mM at the CRMV, calculated using equations given in Tishchenkoet al., 2005). As the model presented here only simulates the dissolvedphase, CH4L values greater than 68 mM imply that methane must alsohave been present as a free gas. This is consistentwith observations thatdocument the coexistence of free gas and gas hydrates at a number ofcold seeps (e.g. Sauter et al., 2006; Sahling et al., 2009). The modelresults therefore support the contention that the uppermost front issmall because of the rapid escape of methane gas following the deposi-tion of the mudflow. The estimated CH4L values for the two lowermostBa fronts are significantly lower than saturation, which suggests thatthe lowermost Ba fronts formed when degassing had ceased and meth-anewas only transported towards the seafloor by diffusion, i.e. dissolvedin the pore water. As diffusion is relatively slow (Clennell et al., 2000), itfacilitates the formation of bigger Ba fronts at depth in the sedimentcolumn as the SMT is stationary for a longer period of time. Also the pres-ence of a carbonate concretion at 130 cm depth indicates that the avail-ability of bicarbonate was higher than in the upper two Ba fronts. Themain source of bicarbonate at cold seep sites is AOM. Hence for porefluids to reach saturation with respect to calcium carbonate, pore fluidmethane concentrations need to be high (~mM), but not saturated, as

0 500 1000 1500 2000

0

25

50

75

100

125

150

175

200

225

0 5 10 15 20 25 30

SO42- (mM)

Dep

th (

cmb

sf)

Ba (ppm)

8

30150

CH4L (mM)

Fig. 6. Solid phase barium (Ba; black symbols) profile and results of numerical model-ling of dissolved sulphate (SO4

2−; blue lines) for the mudflow site. CH4L is the dissolvedmethane concentration at the base of the core. The dashed light grey line indicates thevalue for background Ba concentrations at this site.

only dissolved methane is accessible to methanotrophic archaea (Aloisiet al., 2004). Thus, the model results agree with the sediment geochem-istry, in that both support degassing of the mudflow as the underlyingcause of Ba front formation at the mudflow site. These data also under-line the importance of mudflows as a possible source of methane tothe ocean which, to date, has been neglected.

In case of the margin site, which is located on the summit of theMV, a drop in the methane flux and thus in the depth of SMT, couldhave been caused either by a reduction in the fluid flow velocityand/or by lower [CH4]. It is likely that both of these parametershave changed in the past. Unfortunately, it is not possible to deter-mine the relative influence of each of the variables on the sulphatepore water profile at the time that the Ba front formed, using our1-D transport-reaction model. Nevertheless, we can assess the effectof varying these parameters, one at a time. This approach makes it pos-sible to estimate the upper limit of fluid flow velocities and dissolvedmethane concentrations that enable the depths of the SMT and thebase of the Ba front to coincide.

Past fluid flow velocities can be derived in two independent waysusing the 1-D transport-reaction model. Firstly, they can be estimatedby matching model-calculated dissolved Ba2+ fluxes to Ba2+ fluxvalues based on the amount of ‘excess Ba’. ‘Excess Ba’ is the differencebetween the Ba content measured in the Ba front and the Ba contentin ‘background’ sediment (258 ppm; see Section 4.2). Following theapproach used byDickens (2001 and references therein), the integratedBa peak area at the margin site (grey zone on Fig. 7) is estimated to be12,300 ppm cm. This value is then converted to volumetric unitsbased on the measured porosity (i.e. 0.58, Vanneste et al., 2011) and agrain density of 2.65 g cm−3. Accordingly, the authigenic Ba front atthe margin site contains ~1 × 10−4 mol of excess Ba per cm2 of sedi-ment. Given that the Ba front at the margin station had 312–361 yrs(relative to 2007, i.e. the year of sampling) to form, a dissolved Ba2+

flux of 2.8–3.2 mmol m−2 yr−1 is required to supply the excess Ba inthe sediments at the margin site. This Ba2+ flux can be obtained fromthe model by varying the fluid flow velocity (u0). Assuming that[Ba2+] has remained constant over time, i.e. ~70 μM (the Ba2+ concen-tration at the base of the core; Appendix A Table A.1), a fluid flow veloc-ity of ~6 cm yr−1 is required to form this Ba front. The alternativeapproach, is similar to the method used for the mudflow site, exceptthat in this case only u0 is varied in order to fit the depth of the SMTto the base of the barite front. The obtained best-fit value for the fluidflow velocity is ~4 cm yr−1 (Fig. 7A). Thus, both approaches generatesimilar (same order of magnitude) fluid flow velocities.

Alternatively, the downward shift in the SMT could have beencaused by only a reduction in themethane concentration of the advectingfluids. If this is the case, then [CH4]would need to be>200 mM to form aBa front at 18 cm depth (Fig. 7B). This is rather unlikely as the porewaters currently being expelled at the centre of the CRMV have a [CH4]value of b120 mM (Vanneste et al., 2011).

Thus, our data suggest that the velocity of fluid flow at the marginsite has changed within the last 255–304 cal yr BP. However, themagnitude of this change is difficult to quantify, because the changein [CH4] over this time interval cannot be specified, but it seems likelythat u0 was never greater than 6 cm yr−1. These findings are in agree-ment with model results of Aloisi et al. (2004) which indicate that mi-crocrystalline phases and concretions of barite and calcium carbonate,similar to those found at this MV, are formed within the sediment col-umn at lowfluid flowvelocities (0.14–20 cm yr−1)whilemore vigorousfluid flow (>20 cm yr−1) would give rise to continuous barite–carbon-ate crusts near the seafloor. Nevertheless, the estimated values of fluidflow, ~4 and 6 cm yr−1, are an order of magnitude greater than thoseestimated for this site at the present-day (0.4 cm yr−1; Fig. 1C),underlining the episodic nature of fluid flow at the Carlos Ribeiro MV.

As the fluid flux always seems to have been relatively low at theCRMV (b20 cm yr−1), the varying occurrence of relict calcium car-bonate fronts (whether or not absent or coinciding with barite fronts)

B0 500 1000 1500 2000

0

25

50

75

100

125

150

175

200

225 A0 500 1000 1500 2000

0 5 10 15 20 25 30 0 5 10 15 20 25 30

SO42- (mM) SO4

2- (mM)

Ba (ppm) Ba (ppm)

Dep

th (

cmb

sf)

u0 = 3.5 cm yr-1

CH4L = 75 mM

u0 = 0.4 cm yr-1

CH4L = 200 mM

Fig. 7. Solid phase barium (Ba; black symbols) and results of numerical modelling ofdissolved sulphate (SO4

2−; blue line) for themargin site at the Carlos Ribeiromud volcano.A.Model assumes that the concentration of dissolvedmethane (CH4L) is constant, and thedepth of the sulphate–methane transition (SMT) varies as a function of the upward fluidflow velocity (u0) B. Model assumes that u0 is constant and the depth of the SMT varies asa function of the dissolved methane concentration. The values of the underlined parame-ters represent present day conditions at the margin site (Vanneste et al., 2011). Thedashed light grey line indicates the value for background Ba concentrations at this site.

50 H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

is due to changes in pore fluid chemistry (Aloisi et al., 2004). Calciumcarbonates are a by-product of AOM (Ritger et al., 1987), so their pre-cipitation rate is proportional to the rate of AOM. The higher thedissolved CH4 concentration, the higher the rate of AOM (Treudeet al., 2003), so carbonate precipitation is enhanced when pore fluidCH4/Ba2+ ratios are high (Aloisi et al., 2004). As calcium carbonatefronts were only observed at the depth of the present day SMT atthe CRMV (at the margin and mudflow sites), this suggests thatpore fluid CH4/Ba2+ ratios have increased over time. Moreover, com-petition between methane and barium seems to play a bigger role indetermining the mineralogy of authigenic deposits at MVs than itdoes at cold seeps in general. Fluids expelled at MVs in the Gulf ofMexico have extremely high [Ba2+] concentrations, yet calcium carbon-ate is the principal authigenic phase (Castellini et al., 2006). On theother hand, massive barite deposits covering the seafloor in MontereyBay (Torres et al., 2002), the Peru Margin (Torres et al., 1996a) andthe San Clemente Fault (Naehr et al., 2000) are fed by pore fluids withmodest [Ba2+] concentrations and consist of almost pure barite. Atthese localities, the barite deposits appear to have formed during asingle major event of fluid venting, enabling the majority of the meth-ane to escape the microbial AOM filter. As a result, production of bicar-bonate is suppressed and carbonate does not precipitate. This underliesthe importance of seepage rate (as well as pore fluid CH4/Ba2+) on themineralogy of authigenic precipitates (Aloisi et al., 2004).

5.4. Utility of Ba fronts as a record of fluid and gas seepage at MVs

Although there have been numerous investigations into the timingand duration of mud eruptions from submarine mud volcanoes (e.g.Neurauter and Roberts, 1994; Lykousis et al., 2009; MacDonald andPeccini, 2009; Perez-Garcia et al., 2011), it is now clear that significantfluid flow and seepage of methane can occur long after the eruptionhas ceased (e.g. at the Håkon Mosby MV, de Beer et al., 2006; Fesekeret al., 2008). Although benthic chamber studies allow assessment ofthe variation in fluid flow over timescales of several hours (e.g. Linkeet al., 2005), information as to the variability in fluid and gas seepage

over long timescales is, to our knowledge, absent from the literatureto date. Filling this gap is important not just for assessing the impor-tance of mud volcano activity for the oceanic carbon budget, but alsofor the development and conservation of chemosynthetic communitiesthat depend on the flux of reduced inorganic substrates (methane,sulphide; Levin, 2005 and references therein) and take tens of years toreach maturity (Nix et al., 1995; Smith et al., 2000).

This study demonstrates that the position of barite fronts presentin the sedimentary record at MVs, combined with pore fluid model-ling, can provide this information. Nevertheless, some limitationsapply. If present-day fluid flow is high, then the SMT will be close tothe seafloor, and any barite fronts that formed when the rate offluid flow was lower are likely to have dissolved. Moreover, in orderto date the emplacement of the last mudflow, a complete record ofthe hemipelagic sediment layer covering the MV is required. Thus,this approach is most useful for MVs that have low rates of fluidflow, andwhose activity has decreased over time. These include thema-jority of MVs in the Gulf of Cadiz (e.g. Van Rensbergen et al., 2005), andin theMediterranean Sea (e.g. Lykousis et al., 2009; Gennari et al., 2013;Panieri et al., 2013).

6. Conclusions

ITRAX XRF data, together with quantitative geochemical analyses,indicate that seepage of barium and methane rich fluids at the CarlosRibeiro MV results in the precipitation of microcrystalline barite withinnarrow zones in the sediment column. At themudflow site, the depth ofthe Ba front coincides with the depth of the sulphate–methane transi-tion. Possible sources of Ba2+ include the dissolution of biogenic baritein deeper, pore water sulphate-depleted sediments during the burialprocess and/or sedimentary material altered at low to moderate tem-peratures at depth. In either case, the Ba2+ released is subsequentlytransported upwards through the sediment column by advecting fluids.

At the eye of the MV, no Ba (or Ca) fronts can be identified withinthe sediment column, which suggests that fluxes of material fromdepth have been highly variable, both today and in the past. Towardsthe margin of the MV, Ba fronts are observed, suggesting that at thesesites, fluxes of material from below are more stable. However, the Bafront at themargin site is not forming at the present-day; rather, it musthave formed at a timewhenmethanefluxeswere higher. By contrast, Bafronts at the mudflow site appear to have formed by gradual degassingof the mudflow since its extrusion. This study shows that the sedimentrecord at MVs, combined with chemical data for pore fluids and geo-chemical modelling, provides valuable information onmethane seepageactivity both now and in the past.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2013.06.010.

Acknowledgments

This project is part of a Ph.D. study, funded by the Graduate Schoolof the National Oceanography Centre Southampton (University ofSouthampton, U.K.) and the HERMES project of the EuropeanCommission's Sixth Framework Programme (EC contract no. GOCE-CT-2005-511234). Radiocarbon analyseswere carried out at the Natural En-vironment Research Council Radiocarbon Facility (Environment) andfunded by Radiocarbon Analysis Allocation 1388.0409. The authorsthank Dr. K. Heeschen for organizing the cruise and the shipboard scien-tific party (JC-10 cruise) as well as the officers and crew of the RRS JamesCook for their invaluable support at sea. Dr. D. Connelly, D. Green and B.Alker are thanked for their assistance with shipboard and shore-basedchemical pore fluid analyses. All of the core material described inthis paper is archived in BOSCORF at the National Oceanography Centrein Southampton.We are extremely grateful to G.R. Dickens and an anon-ymous reviewer for their constructive comments and suggestions thathave helped to improve this manuscript.

51H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

Appendix A

Table A.1Parameters used in the numerical transport model.

Parameter Symbol Unit Eye Off-centre Margin Mudflow

Molecular diffusion coefficient of calciuma DCa cm2 yr−1 132.29Molecular diffusion coefficient of bariuma DBa cm2 yr−1 145.83Length of model columnb L cm 500.5 110 217 179.5Porosity at sediment surfacec,d φ0 0.75 0.76 0.75 0.69Porosity at lower boundaryc,d φ∞ 0.58 0.57 0.53 0.51Attenuation coefficient for porosity decreasec,d β cm−1 0.0605 0.08 0.05 0.04Upward fluid flow velocityd u0 cm yr−1 4 3.3 0.4 0Irrigation depthd xmix cm 32 32 40 50Irrigation mixing coefficientd α′ yr−1 3.1 2.5 0.6 0.6Ca2+ concentration at sediment surfacee Ca0 mM 11 10.6 10.5 10.1Ca2+ concentration at depthe CaL mM 1.9 2.1 3.2 5.5Ba2+ concentration at sediment surfacee Ba0 μM 0.7 0.2 0.3 0.5Ba2+ concentration at depthe BaL μM 43.3 49 68.3 28

aCalculated from equations given in (Boudreau, 1997), using measured values for the in situ temperature (4.3 °C), salinity (34.2) and pressure (220 bar).bLength of the sediment core (Table 1).cObtained by fitting an exponential equation φ(x) = (φ0 − φ∞)e− βx + φ∞ to the measured porosity depth profile (Vanneste et al., 2011).du0 is the upward fluid flow velocity at the water–sediment interface while u is the upward fluid flow velocity at any depth where u ¼ u0⋅φ0

φ xð Þ (Vanneste et al., 2011).

The irrigation coefficient, α(x), is defined by xmix and α′ as follows: α(x) = α′ ⋅ exp[−(xb − xmin)] where xb is the depth below which irrigation ceases (Vanneste et al., 2011).eMeasured in pore waters extracted from the respective sediment cores.

(continued)

Parameter Symbol Unit Margin Mudflow

Porosity at sediment surfacec,d φ0 0.75 0.69Porosity at lower boundaryc,d φ∞ 0.53 0.51Attenuation coefficient forporosity decreasec,d

β cm−1 0.05 0.04

Irrigation depthd xmix cm 40 50Irrigation mixing coefficientd α′ yr−1 0.6 0.6Upward fluid flow velocityd u0 cm yr−1 0.4 0Rate constant of AOMd kAOM (mM yr)−1 10.5 10.1CH4 concentration at sedimentsurfacee

CH40 mM 0.001 0.002

aCalculated from equations given in Hayduk and Laudie (1974), using measured valuesfor the in situ temperature (4.3 °C), salinity (34.2) and pressure (220 bar).bLength of the sediment core (Table 1).cObtained by fitting an exponential equation φ(x) = (φ0 − φ∞)e− βx + φ∞ to themeasured porosity depth profile.dVanneste et al. (2011)eMeasured in pore waters extracted from the respective sediment cores.

Table B.1Elemental composition of sediments and a carbonate concretion from the CarlosRibeiro MV.

Sample Depth intervalcm

Cawt%

Tiwt%

CO3

wt%Bappm

Background sitePC-43 21–25 16.1 0.3 26 405PC-43 224–227 10.4 0.4 17 473PC-43 344–348 11.3 0.4 380PC-43 525–527 13.5 0.3 23 356

Carlos Ribeiro MVEye

Table A.2 (continued)

A.1. Description of the reaction term used in thetransport-reaction model.

The reaction term for sulphate andmethane in the transport-reactionmodel (Vanneste et al., 2011) is described as follows:

φ∑RSO4¼ φ⋅ −kG⋅

Corg

2 ⋅SO4

2−h i

KSO42− þ SO4

2−� �−RAOM

0@

1A

φ∑RCH4¼ φ⋅ kG⋅

Corg

2 ⋅KiSO4

2−

KiSO42− þ SO4

2−� �−RAOM

!

where kG is the kinetic constant for organic matter degradation(1 × 10−6 yr−1), Corg is the organic matter concentration (625 mM),KSO4

2− is the half-saturation constant for sulphate reduction (1 mM),KiSO4

2− is the inhibition constant for initiation of methanogenesis(1 mM) and RAOM is the reaction rate for the anaerobic oxidation ofmethane. The latter is defined as (Treude et al., 2003):

RAOM ¼ kAOM⋅ CH4½ �⋅SO4

2−h i

KS;AOM þ SO42−� �

where [CH4] and [SO42−] are the concentrations of dissolved CH4 and

SO42− in the pore water, kAOM is the kinetic constant for AOM and KS,

AOM is a Monod constant defining the inhibition of AOM at low [SO42−]

(KS,AOM = 1 mM). kAOM was determined by Vanneste et al. (2011) byfitting the model results to the pore water SO4

2− data and the obtainedvalues are given in the table below.

Table A.2Parameters used in the numerical transport-reaction model.

Parameter Symbol Unit Margin Mudflow

Molecular diffusion coefficientof methanea

DCH4 cm2 yr−1 289.38

Length of model columnb L cm 217 179.5

Appendix B

GC-52 18–12 2.5 0.5 264GC-52 69.5–70.5 2.7 0.5 273GC-52 171–173 2.2 0.6 4 277PC-53 370–375 2.8 0.5 256PC-53 505–510 2.5 0.5 5 259

Off-centreGC-49 11–12 1.8 0.5 3 245GC-49 17–18 2.6 0.5 4 363

(continued on next page)

(continued)

Sample Depth intervalcm

Cawt%

Tiwt%

CO3

wt%Bappm

Off-centreGC-49 24–25 2.8 0.5 5 617GC-49 52-53 2.7 0.5 186GC-49 56.5–57.5 2.7 0.5 4 311GC-49 62–63 2.7 0.5 333GC-49 100–101 3.0 0.5 5 248

MarginPC-58 0–5 7.3 0.4 12 253PC-58 12–15 2.7 0.5 4 435PC-58 17–18.5 3.0 0.5 5 1740PC-58 30–31 3.1 0.5 5 209PC-58 35.5–36 3.5 0.5 230PC-58 40–41 5.6 0.5 9 186PC-58 44–45 4.2 0.5 132PC-58 50–51 3.9 0.5 7 129PC-58 70–75 3.0 0.5 133PC-58 124.5–125.5 2.5 0.5 4 174PC-58 215–219 2.7 0.5 224

MudflowGC-60 0–2 13.8 0.3 268GC-60 35.5–36.5 2.6 0.5 253GC-60 44.5–45.5 2.8 0.5 4 407GC-60 51.5–52.5 2.9 0.5 5 197GC-60 79.5–80.5 2.8 0.5 5 467GC-60 84.5–85.5 2.6 0.5 5 785GC-60 87–88 2.9 0.5 5 765GC-60 90.5–91.5 2.8 0.5 5 188GC-60 122.5–123.5 2.5 0.5 4 299GC-60 129.5–130.5 2.3 0.5 4 748Carbonate concretion 130 30.5 0.1 50 4940GC-60 135–136 3.0 0.5 5 136GC-60 172–173 2.5 0.4 6 208

Table B.1 (continued)

52 H. Vanneste et al. / Chemical Geology 354 (2013) 42–54

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