Mud volcanoes, gas hydrates and methane-derived authigenic carbonates from the South Portuguese Margin and the Gulf of Cadiz

http://la.cesam.ua.pt/highlights/2007/EMM_v1_LuisMenezesPinheiro.htm

Luis Menezes Pinheiro & Vitor Magalhães

CESAM (Centre for Environmental and Marine Studies) & Department of Geosciences

University of Aveiro, Portugal

The South Portuguese Margin and the Gulf of Cadiz are characterized by extensive evidence of hydrocarbon-rich gas seepage, manifested by the occurrence of mud volcanoes (MVs), pockmarks, diapiric ridges and methane-derived authigenic carbonates (Figs. 1, 2). Mud volcanoes were discovered in this area in 1999 (Gardner, 2000), with Portuguese participation (IGM Marine Geology Dep. and Univ. Aveiro). Since then, this area has been extensively investigated in the scope of national and international projects: INGMAR (IGM/FCT), MVSEIS (ESF/FCT) and, more recently, HERMES (EU). 38 of the 41 mud volcanoes (MVs) were discovered by Principal Investigators of the Euromargins MVSEIS project (2003-2008), approved by the European Science Foundation (http://www2.geo.ua.pt/mvseis/).

Figure 1. Location of the mains mud volcanoes, mud diapirs and areas of methane-related authigenic carbonates (MDAC) in the Gulf of Cadiz.

Combined interpretation of seismic profiles, high-resolution bathymety, sampling and side-scan sonar data acquired during the MVSEIS and related cruises, allowed a much better constrained understanding of the area and a clearer picture of the complex faulting that controls the migration of the hydrocarbon-rich fluids originated in deep areas of the basin (Pinheiro et al., 2003; Hensen et al., 2007) (Fig. 2). Most MVs in this tectonically active region are located along major fault systems or at fault intersections and are confined to the interpreted Gulf of Cadiz accretionary wedge (Gutscher, 2000).

Figure 3. Left: gas hydrates recovered from the Bonjardim Mud Volcano, in 2006 (sample stored in the University of Aveiro). Right: Image of the burning of a gas hydrate.

Large areas covered with methane-derived carbonate crusts and chimneys, precipitated from methane-rich fluids mediated by a consortia of sulphate-reducing bacteria and archaea were also discovered in this area (Figs. 1, 4), mainly along major NE-SW elongated diapiric ridges, associated with major strike-slip faults (Fig. 4).

Figure 4. Left: Carbonate chimneys recovered from the Gulf of Cadiz; Centre: underwater image of a carbonate chimney; Right: Multibeam bathymetry showing the NE-SW fault-controlled ridges where the methane-derived carbonate chimneys and crusts are found.

Of great importance and a topic of current investigation is the estimation the extent of the areas of gas hydrates in the Gulf of Cadiz and unravelling the likely effect of the periodic and past variability of the Mediterranean Outflow Water (MOW) and past sea-level changes on the dissociation of gas hydrates, with consequent release of methane to the hydrosphere and atmosphere and contribution to the greenhouse gas effect. Pore water analyses and methane oxidation measurements in sediment cores show that, at present, thermogenic methane and associated higher hydrocarbons are being consumed in subsurface sediments. Anaerobic oxidation of methane (AOM) and sulphate reduction rates (SRR) show maxima in distinct subsurface sediment horizons at the methane-sulphate transition zone (SMT), at depths from 20 to 200 cm below sea floor. Lipid biomarker and 16S rDNA clone library analysis showed that AOM is being mediated by a mixed community of anaerobic methanotrophic archaea and associated sulphate reducing bacteria (SRB). In periods of MV inactivity such as at present, the contribution of methane from MVs to the atmosphere is therefore insignificant; however this contribution is potentially important during eruptions.

Selected publications (see also http://www2.geo.ua.pt/mvseis/):

1. Hensen, C, Nuzzo, M, Hornibrook, E, Pinheiro, L M, Bock, B, Magalhães, V, W Bruckmann (2007). Sources of mud volcano fluids in the Gulf of Cadiz: indications for hydrothermal imprint. Geochim. Cosmochimica Acta, 71, 1232-1248.

2. Niemann, H, Duarte, J, Hensen, C, Omoregie, E, Magalhães, V, Elvert, M, Pinheiro, L M, Kopf, ., Boetius, A (2006). Microbial methane turnover at mud volcanoes of the Gulf of Cadiz. Geochim. Cosmochimica Acta, 70, 5336-5355.

4. Pinheiro, L M, Ivanov, M, Sautkin, A, Akhmanov, G, Magalhães, V, Volkonskaya, A, Monteiro, J, Somoza, L, Gardner, J, Hamouni, N, Cunha, M; TTR10 Cruise party (2003). Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise. Marine Geology, 195, 131-151.

5. Somoza, L, Diaz-del-Rio, V, Léon, R, Ivanov, M, Fernandez-Puga, M C, Gardner, J, Hernandez-Molina, J, Pinheiro, L M, Rodero, J, Lobato, A, Maestro, A, Vasquez, J T, Medialdea, T, Férnandez-Salas, L M (2003). Seabed morphology and hydrocarbon seepage in the Gulf of Cadiz mud volcano area: acoustic imagery, multibeam and ultra-high resolution seismic data. Marine Geology,195, 153-176.

6. Mazurenko, L, Soloviev, V., Belenkaya, I, Ivanov, M and Pinheiro, L M (2002). Mud volcano gas hydrates in the Gulf of Cadiz. Terra Nova, V.14, No.5, 321-329.

Biodata

Luis Menezes Pinheiro started his academic career in applied geophysics in 1981. He graduated in Mining Engineering, University of Porto, and did his PhD in Marine Geophysics in Imperial College, London. His research interests in Marine Geology and Geophysics include: geophysical data processing and interpretation, structure and evolution of continental passive margins, rifting mechanisms and lithospheric structure at the Ocean-Continent Transition, neotectonics, mud volcanism, gas hydrates and hydrocarbon-rich fluid seepage, and Seismic Oceanography. He is an Associate Professor in the Geosciences Department of the University of Aveiro, and a member of CESAM.

Vitor Magalhães graduated in Geology from the University of Porto. He started his career in the Marine Geology Department of the Portuguese Geological and Mining Institute (now INETI). He did his PhD at the University of Aveiro, in collaboration with Moscow State University, on the origin of the methane-related carbonates from the Gulf of Cadiz. He is presently doing post-doc research in the University of Chicago and the University of Aveiro on the stability of gas hydrates and its possible impact on the formation of authigenic carbonates and climate change.