Vitor Magalhaes

Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise

A new deep water mud volcano field (between 2000 and 3500 m water depth) was discovered in the deep South Portuguese margin, as well as several new mud volcanoes in the South Spanish and Western Moroccan margins of the Gulf of Cadiz, during the TTR-10 (Training Through Research, UNESCO/IOC) cruise, in July/August 2000. This work followed the discovery of a large mud volcano field in the Gulf of Cadiz, first investigated during the TTR-9 cruise [Gardner (2001) Geophys. Res. Lett. 28, 339–342; Kenyon et al. (2000) IOC, Technical series no. 56]. The discoveries were made based on a SEAMAP side-scan sonar mosaic and multibeam bathymetry (SEABEAM) collected in the area by the Naval Research Laboratory (NRL), Washington, DC, USA, in 1992, kindly released for this purpose. Single-channel seismics, long-range side-scan sonar (OKEAN), TV-controlled grab, hull-mounted 3.5-kHz profiler and coring were used to investigate several seafloor features observed on the side-scan sonar imagery, in the South Iberia, Spanish and Moroccan margins of the Gulf of Cadiz, which were confirmed to be mud volcanoes. The typical structures related to fluid venting in the Gulf of Cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m. Some of these structures appear to be aligned along major conjugate NE–SW and NW–SE trending faults that can be identified on the side-scan sonar imagery. The new field discovered in the South Portuguese margin is the deepest in the Gulf of Cadiz area and includes three new mud volcanoes – Bonjardim, Olenin and Carlos Ribeiro – which seem to be quite active, with near-surface gas hydrate occurrence and a high saturation in H2S and hydrocarbon gases (mainly methane) in the mud breccia and overlying pelagic sediments. Gas hydrates were recovered from the Bonjardim mud volcano. An intensely gassified mud breccia, with one fragment of semi-consolidated claystone with a thin bituminous veneer at the surface, was recovered from the Carlos Ribeiro mud volcano. The fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of Foraminifera. Three new mud volcanoes were also discovered in the NW Moroccan margin: Rabat, Student and Jesus Baraza. These show a richer fauna that includes several species of molluscs, polychaetes, pogonophoran worms, crustaceans, echinoderms and some fragments of dead coral (Madrepora and Lophelia). Carbonate crusts were recovered from the Student mud volcano. The Ginsburg mud volcano, discovered during the previous TTR-9 cruise, was revisited and gas hydrates recovered once again. A new mud volcano was also discovered in the Spanish margin, Tasyo, where evidence was found of coral build-ups on the hard substratum of the mud volcanic edifice.

Strike-slip faults mediate the rise of crustal-derived fluids and mud volcanism in the deep sea

We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

Shallow Microbial Recycling of Deep-Sourced Carbon in Gulf of Cadiz Mud Volcanoes

Based upon the molecular and isotopic composition of hydrocarbons it has been proposed that the source of CH 4 in Gulf of Cadiz mud volcanoes (MV) is a mixture of deep sourced thermogenic CH 4 and shallow biogenic CH 4 . We directly investigated this possibility by comparing porewater CH 4 concentrations and their δ 13 C values with the potential for Archaeal methanogenesis in Gulf of Cadiz mud volcano (MV) sediments (Captain Arutyunov, Bonjardim, Ginsburg and Porto) using 14 C-rate measurements. The CH 4 has a deep sourced thermogenic origin ( δ 13 C ∼ −49) but becomes 13 C-depleted in and beneath the zone of anaerobic oxidation of methane (AOM) where the rates of hydrogenotrophic methanogenesis increase. Thus we infer that a portion of AOM-produced CO 2 is being recycled to CH 4 by methanogens yielding further 13 C-depleted CH 4 , which might be misinterpreted as indicative of a fully shallow biogenic origin for this gas. Production of H 2 is related to compositional changes in sedimentary organic matter, or to upward flux of substrate-enriched fluids. In contrast to other MVs in the Gulf of Cadiz, Ginsburg MV fluids are enriched in SO 4 2− and contain very high concentrations of acetate (2478 μ M below 150 cmbsf); however, the high levels of acetate did not stimulate methanogenesis but instead were oxidized to CO 2 coupled to sulphate reduction. Both anaerobic oxidation of thermogenic CH 4 linked to shallow methanogenesis and fluid geochemistry control the recycling of deep-sourced carbon at Gulf of Cadiz MVs, impacting near-surface δ 13 C-CH 4 values.

Rare earth elements in mud volcano sediments from the Gulf of Cadiz, South Iberian Peninsula

Eight gravity cores (GC) were retrieved from the deep mud volcanoes Sagres, Bonjardim, Soloviev and Porto in the Gulf of Cadiz. Cores with 137 to 317 cm long were sliced in intervals of 15 to 20 cm thickness, and 46 samples were analyzed for grain size distribution, loss on ignition, Al, Fe, Ca, Mg, Mn, Sr, Ba and the rare earth elements (REE) La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. REE profile normalized to Post-Archean Australian Average Shale (PAAS) was dominated by a mid-REE bulge with a pronounced Eu enhancement. Sediment reducing conditions resulting from the anaerobic oxidation of methane probably contributed to the positive-Eu anomaly (1.18-2.19, PAAS normalization). Most likely, reactions near the sulfate-methane transition zone such as the precipitation of barium sulfate found at layers around 50-cm depth explain the enhancement of Eu/Eu* ratios. The Ce anomalies (0.93-1.09) were almost absent.