Authigenic barite records of methane seepage at the Carlos Ribeiro mud volcano (Gulf of Cadiz). Heleen Vanneste ⁎, Rachael H. James, Boris A. Kelly-Gerreyn 1 , Rachel A. Mills Ocean and Earth Science, National Oceanography Centre, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK abstract article info Article history: Received 20 December 2012 Received in revised form 5 June 2013 Accepted 11 June 2013 Available online 20 June 2013 Editor: U. Brand Keywords: Authigenic barite Mud volcano Gulf of Cadiz Pore fluid modelling X-ray fluorescence (XRF) data Submarine mud volcanoes (MVs) are dynamic features that episodically expel gas-charged fluids and mud onto the seafloor, transferring various chemical constituents into the overlying water column. The temporal variability in MV activity is, however, poorly understood, so their importance as a source of methane (CH 4 ) and higher hydrocarbons for the oceanic carbon budget, although thought to be significant, cannot be properly constrained. In this study, the history of fluid and gas seepage at the Carlos Ribeiro MV (Gulf of Cadiz) is assessed via geochemical analyses and transport-reaction modelling 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 data reveal that Ba fronts are absent at the eye, while a single front (with up to 1740 ppm Ba) is present at the margin of the summit. Three Ba fronts occur at 45, 85 and 130 cm depth within a mudflow to the southeast of the crater. Spectrometric analyses indicate that barite is the Ba-rich mineral in these layers. Upward advecting pore fluids are enriched in barium but depleted in calcium (Ca 2+ ) relative to seawater. Modelling of the Ba 2+ and Ca 2+ 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 yr before present, but degassing of the mudflow is ongoing and is potentially an important source of CH 4 . © 2013 Elsevier B.V. All rights reserved. 1. Introduction Submarine mud volcanoes (MVs) have been extensively described and studied across the globe (e.g. MacDonald et al., 1994; Gardner, 1999; Graue, 2000; Hensen et al., 2004; Omoregie et al., 2009; Chao et al., 2011) as they are extremely efficient in transporting hydrocar- bons (especially methane) from deeply buried sediments, to shallow sediments, 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 because methane 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-day oceanic methane budget (e.g. Mau et al., 2006; Sauter et al., 2006; 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 methane venting at MVs on the global carbon cycle. Authigenic barite (BaSO 4 ) is considered to be a useful proxy for assessing past fluxes of methane gas on continental margins (Dickens, 2001). This is because gas-charged fluids generally lack sulphate (SO 4 2- ) but contain elevated concentrations of dissolved barium (Ba 2+ )(Torres et al., 1996b). During the ascent of the fluids through the sediment column, barite precipitates on contact with 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): Ba 2þ aq ð Þþ SO 4 2- aq ð Þ↔BaSO 4 s ðÞ: ð1Þ The pore water sulphate gradient at MVs is typically regulated by the methane flux from depth via the anaerobic oxidation of methane (AOM; CH 4 þ SO 4 2- →HCO 3 - þ HS - þ H 2 O) at the sulphate–methane transition (SMT; e.g. Aloisi et al., 2002; Bohrmann et al., 2003; Borowski et 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 depth of sulphate depletion, its presence records the depth of the SMT (Von Breymann et al., 1992; Dickens, 2001; Kasten et al., 2003; Snyder et al., Chemical Geology 354 (2013) 42–54 ⁎ Corresponding author at: EcoLab (Laboratoire Ecologie Fonctionnelle et Environnement); Campus INPT- ENSAT, Avenue de l'Agrobiopole — BP 32607, 31326 Castanet Tolosan Cedex, France. Tel.: +33 5 34 32 37 56. E-mail addresses: heleen.vanneste@ensat.fr (H. Vanneste), r.h.james@noc.ac.uk (R.H. James), B.Kelly-Gerreyn@bom.gov.au (B.A. Kelly-Gerreyn), ram1@noc.soton.ac.uk (R.A. Mills). 1 Present address: Observations and Engineering Branch, Bureau of Meteorology, GPO Box 1289 Melbourne VIC 3001, Level 8, 700 Collins Street, Docklands, VIC 3008, Australia. 0009-2541/$ – see front matter © 2013 Elsevier B.V. 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