Mud volcanoes, gas hydrates and
methane-derived authigenic carbonates from the South Portuguese Margin and the
Luis Menezes Pinheiro & Vitor Magalhães
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 (
Figure 1. Location of the mains mud volcanoes,
mud diapirs and areas of methane-related authigenic carbonates (MDAC) in the
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
Figure 2. (a) Simplified mechanism for
the formation of mud volcanoes and mud diapirs by migration to the surface of
overpressured gas-charged shales; also shown area of stability of gas hydrates;
(b) High resolution bathymetry image of the Anastasya mud volcano and
corresponding high resolution seismic section showing the typical
“christmas-tree” structure characteristic of multi-pulse activity.
Gas hydrates (Fig. 3), a likely
important future energy resource (1 cm3 of hydrate releases by
dissociation ca. 160 cm3 of gas) were retrieved from 4 MVs and
likely occur in other MV’s and elsewhere in the study area. High percentages of
C2+ homologues indicate a thermogenic origin of the gas, suggesting the
existence of petroleum reservoirs at depth. A Bottom-simulating reflector at
the base of the gas hydrate stability zone and active gas bubbling were
observed on the Mercator MV, one of the shallowest ever reports of hydrate
accumulations.
Figure 3. Left: gas hydrates recovered from the Bonjardim Mud Volcano, in
2006 (sample stored in the
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
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.
Luis Menezes Pinheiro started his
academic career in applied geophysics in 1981. He graduated in Mining
Engineering,
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