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

<p>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 (CH4) 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 (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 conditions 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 CH4.</p>