ALBERT

Description: The Gulf of Cadiz, off SW Iberia and the NW Moroccan margin, straddles the cryptic plate boundary between Africa and Eurasia, a region where the orogenic Alpine compressive deformation in the continental collision zone passes laterally to the west to strike-slip deformation. A set of new multibeam bathymetry, multi-channel and single-channel seismic data presented here image the main morphological features of tectonic origin of a significant part of the Gulf of Cadiz from the continental shelf to the abyssal plain. These morphotectonic features are shown to result from the reactivation of deeply rooted faults that changed their kinematics from the early Mesozoic rifting, through the Late Cretaceous–Paleogene collision, to the Pliocene–Quaternary thrusting and wrenching. The old faults control deep incised, more than 100 km long canyons and valleys. Several effects of neotectonics on deep water seabed are shown. These include: i) the complex morphology caused by wrenching on the 230 km long WNW–ESE faults that produced en echelon folds on the sediments; ii) the formation of up to 5 km wide crescent shaped scours at roughly 4 km water depth by reactivation of thrusts; iii) 10 km long creep folds on the continental slope; and iv) the formation of landslides on active fault escarpments. The present day deformation is partitioned on NE–SW thrusts and WNW–ESE to W–E strike-slip faults and is propagating northwards on N–S trending thrusts along the West Iberia Margin from 35.5°N to 38°N, which should be considered for seismic hazard.

Description: Mud volcanism in the Gulf of Cadiz occurs over a large area extending from the shelf to more than 3500 m water depth and is triggered by compressional stress along the European–African plate boundary, affecting a deeply faulted sedimentary sequence of locally more than 5 km thickness. The investigation of six active sites shows that mud volcano (MV) fluids, on average, are highly enriched in CH4, Li, B, and Sr and depleted in Mg, K, and Br. The purity of the fluids is largely controlled by the intensity of upward directed flow. Flow rates could be constrained by numerical modelling and vary between 〈0.05 and 15 cm yr−1. Application of δD–δ18O systematics identifies clay mineral dehydration, most likely within Mesozoic and Tertiary shales and marls, as the major source of fluids. Hence, Cl and Na in the pore fluids are mostly depleted below seawater values, following a general trend of dilution. However, deviations from this trend occur and are likely caused by the dissolution of halite in evaporitic deposits. Other secondary processes overprinting the original fluid composition may occur along the flow path, such as dissolution of anhydrite or gypsum and/or the formation of calcite and dolomite. Different sources of fluids are also indicated by variations in 87Sr/86Sr, which range from 0.7086 to 0.7099 at the different sites. Dehydration may be induced primarily by overburden and tectonic compression; however, very high concentrations of Li and B, specifically at Captain Arutyunov MV (CAMV) indicate additional leaching at temperatures above 150 °C, which could be explained by the injection of hot fluids along deep penetrating, major E–W strike–slip fault systems. This hypothesis is supported by the occurrence of generally thermogenic, but significantly CH4-enriched, light volatile hydrocarbon gases at CAMV which cannot be explained by shallow microbial methanogenesis. Li and Li/B ratios from different types of hot and cold vents are used to infer that high temperature signals seem to be preserved at various cold vent locations and indicate a closer coupling of both systems in continental margin environments than outlined in previous studies.

Description: The Gulf of Cadiz is a tectonically active area of the European continental margin and characterised by a high abundance of mud volcanoes, diapirs, pockmarks and carbonate chimneys. During the R/V SONNE expedition ‘‘GAP–Gibraltar Arc Processes (SO-175)’’ in December 2003, several mud volcanoes were surveyed for gas seepage and associated microbial methane turnover. Pore water analyses and methane oxidation measurements on sediment cores recovered from the centres of the mud volcanoes Captain Arutyunov, Bonjardim, Ginsburg, Gemini and a newly discovered, mud volcano-like structure called ‘‘No Name’’ show that thermogenic methane and associated higher hydrocarbons rising from deeper sediment strata are completely consumed within the seabed. The presence of a distinct sulphate–methane transition zone (SMT) overlapping with high sulphide concentrations suggests that methane oxidation is mediated under anaerobic conditions with sulphate as the electron acceptor. Anaerobic oxidation of methane (AOM) and sulphate reduction (SR) rates show maxima at the SMT, which was found between 20 and 200 cm below seafloor at the different mud volcanoes. In comparison to other methane seeps, AOM activity (〈383 mmol m�2 year�1) and diffusive methane fluxes (〈321 mmol m�2 year�1) in mud volcano sediments of the Gulf of Cadiz are low to mid range. Corresponding lipid biomarker and 16S rDNA clone library analysis give evidence that AOM is mediated by a mixed community of anaerobic methanotrophic archaea and associated sulphate reducing bacteria (SRB) in the studied mud volcanoes. Little is known about the variability of methane fluxes in this environment. Carbonate crusts littering the seafloor of mud volcanoes in the northern part of the Gulf of Cadiz had strongly 13C-depleted lipid signatures indicative of higher seepage activities in the past. However, actual seafloor video observations showed only scarce traces of methane seepage and associated biological processes at the seafloor. No active fluid or free gas escape to the hydrosphere was observed visually at any of the surveyed mud volcanoes, and biogeochemical measurements indicate a complete methane consumption in the seafloor. Our observations suggest that the emission of methane to the hydrosphere from the mud volcano structures studied here may be insignificant at present.