Seepage of hydrocarbon bearing fluids at the Carlos Ribeiro and Darwin mud volcanoes (Gulf of Cadiz)

Abstract

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Mud volcanoes (MVs) occur in abundance on all oceanic plate margins, so fluxes of methane (CH4) and other chemical constituents from mud volcanism are likely to be significant for the oceanic budgets of some elements. Here, I present a detailed study of the spatial and temporal variation in fluid and chemical fluxes and mud flow activity at the Carlos Ribeiro and the Darwin MVs in the Gulf of Cadiz. Analyses of the chemical composition of pore fluids, sediments and authigenic carbonates are combined with a 1-D transport-reaction pore fluid model. Pore fluids from both MVs contain high concentrations of hydrocarbons (up to 16 mM), while pore fluids from Carlos Ribeiro MV (CRMV) are also enriched in lithium (Li+) and boron (B) but depleted in chloride relative to seawater. Oxygen, hydrogen and strontium isotope data suggest that the pore fluids are derived from depth and are affected by the transformation of smectite to illite. This process also produces pore fluids that are depleted in chloride and potassium, while B and Li+ appear to be leached from the sediments during this transformation process. The CRMV is the most active of the two MVs: fluid flow velocities are as high as 4 cm yr?1 at the eye of the MV but rapidly decrease to 0.4 cm yr?1 at the periphery. The associated fluxes of B, Li+ and CH4 are 7-301, 0.5-6 and 0-806 mmol m?2 yr?1, respectively. Fluid flow velocities at the Darwin MV are lower, ?0.09 cm yr?1, and show little spatial variation: seepage activity appears to be controlled by the distribution of slabs of authigenic carbonate that are found on the seafloor at the summit of the MV. Results of radiocarbon dating of the hemipelagic sediments covering the CRMV suggest that there has been recent mudflow activity at the eye of the MV, and frequent mud expulsions over the past ?1000 Cal yrs BP. The distribution of barite fronts at the margin of the MV and within the mudflow to the SE of the summit suggests that fluid advection has ceased over the past 340 Cal yrs, but degassing of these mudflows is ongoing and is potentially an important source of CH4. Geochemical and petrographic analyses of carbonates from the Darwin MV suggest that the MV formed in stages, with periods of intense fluid flow alternating with phases of mud extrusion and tectonic uplift. The results of this study demonstrate that fluid (and chemical) fluxes from MVs vary over relatively small time and space scales and that mud volcanism may contribute significantly to the oceanic inventories of Li+ and B. Moreover, anaerobic oxidation of methane appears to be an important control on methane emissions into the overlying water column, and a better understanding of this process is urgently required to properly quantify the impact of mud volcanism on the global oceanic methane budget.

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Identifiers

ORCID for Rachael James: orcid.org/0000-0001-7402-2315

ORCID for Rachel Mills: orcid.org/0000-0002-9811-246X

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