Joan Gardner

Seabed morphology and hydrocarbon seepage in the Gulf of Cádiz mud volcano area: Acoustic imagery, multibeam and ultra-high resolution seismic data

Abstract Extensive mud volcanism, mud diapirism and carbonate chimneys related to hydrocarbon-rich fluid venting are observed throughout the Spanish–Portuguese margin of the Gulf of Cadiz. All the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome/accretionary complex units which were emplaced in the Late Miocene in response to NW-directed convergence between the African and Eurasian plates. The study area was investigated by multibeam echo-sounder, high and ultra-high resolution seismic profiling, dredging and coring. The structures observed on multibeam bathymetry, at water depths between 500 and 1300 m, are dominated by elongate mud ridges, mud cones, mud volcanoes and crater-like collapse structures ranging in relief from 50 to 300 m and size from 0.8 to 2 km in diameter. The main morphotectonic features, named the Guadalquivir Diapiric Ridge (GDR) and the Cadiz Diapiric Ridge (CDR), are longitudinally shaped diapirs which trend NE–SW and consist of lower–middle Miocene plastic marly clays. The GDR field and the TASYO field, which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development, are described in this paper. The GDR field is characterised by numerous, single, sub-circular mud volcanoes and mud cones. The single mud volcanoes are cone-shaped features with relatively gentle slopes of 3°–6°, consisting of several generations of mud breccia deposition with indications of gas-saturation, degassing structures and the presence of H2S. The mud cones have asymmetric profiles with steep slopes of up to 25° and contain large surficial deposits of hydrocarbon-derived carbonate chimneys and slabs. The TASYO field is characterised by an extensive concentration of small, sub-circular depressions, oval and multi-cone mud volcanoes and large sediment slides. Mud volcanoes in this area are characterised by moderate slopes (8°–12°), have bathymetric relief ranging from 100 to 190 m and consist of sulphide-rich mud breccia, calcite chimneys, carbonate crusts and chemosynthetic fauna (Pogonophora tube worms). We propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic/accretionary wedge. This stress results in the uplifting and squeezing plastic marly clay deposits. Additionally, the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon-enriched fluids to migrate.

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.

Looking for clues to paleoceanographic imprints: A diagnosis of the Gulf of Cadiz contourite depositional systems

A new morphosedimentary map of the Gulf of Cadiz is presented, showing the contourite depositional system on the gulfs middle slope. This map is constructed from a broad da- tabase provided by the Spanish Research Council and the U.S. Naval Research Laboratory. Our map shows that this contourite depositional system comprises five morphosedimentary sectors: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) contourite deposition; and (5) submarine canyons. The Gulf of Cadiz con- tourite depositional system stems directly from the interaction between Mediterranean Out- flow Water and the seafloor; its morphosedimentary sectors are clearly related to the sys- tematic deceleration of the Mediterranean Outflow Waters westward branches, bathymetric stress on the margin, and the Coriolis force. The slopes depositional system can be consid- ered as a mixed contourite and turbidite system, i.e., a detached combined drift and fan.

Mud volcanoes revealed and sampled on the Western Moroccan continental margin

We surveyed and sampled two active and 3 inactive mud volcanoes on the Atlantic continental margin of Morocco. The active mud volcanoes (named Yuma and Ginsburg) are each about 4 km in diameter, rise between 150–250 meters above the seafloor and are the first active methane-related mud volcanoes to be identified in this region. The inactive volcanoes range in diameter from 1–3 km, with relief between 50 and 80 meters. Gravity cores from the crest of the active mud volcanoes yielded methane hydrates and mud breccia deposits. Chemosynthetic communities of Pogonophora worms and the bivalve Solemya were found on the surface of the active mud volcanoes. Sediment cores from the inactive volcanoes contained hemipelagic sediments, rich in foraminifera and ahermatypic coral debris, overlying mud breccia

Gas hydrates in the Ginsburg and Yuma mud volcano sediments (Moroccan Margin): results of chemical and isotopic studies of pore water

Abstract Two active mud volcanoes on the Atlantic continental margin of Morocco were sampled during a 1999 expedition on the R/V Professor Logachev. The active mud volcanoes (named Yuma and Ginsburg) are each about 4 km in diameter, rise 150–250 m above the seafloor and are the first active methane-related mud volcanoes identified in this region. Gravity cores from the crest of the active mud volcanoes recovered methane hydrates (clathrates). Thirty-five pore water samples were extracted and analyzed from seven gravity cores taken in and around the active mud volcanoes. Estimated gas hydrate content in mud volcano sediments is 2–11% by volume and 4–19% by pore space. Chloride ion concentrations of the original mud volcano water are similar to those of surrounding seawater, but are isotopically different. The water is very high in oxygen (to +35‰) and light in hydrogen (to −70‰). Pore water with these features has not been previously described in the published literature.

Numerous methane gas‐related sea floor structures identified in Gulf of Cadiz

Until recently, the importance of intense sea floor emissions of hydrocarbon-enriched fluids related to the ongoing movement and development of the olistostrome-accretionary wedge complex located in the Gulf of Cadiz (Figure la), remained uncertain. The Gulf of Cadiz is located in the transitional zone between the Gloria transform fault zone, which is the African-Eurasian plate boundary in the Atlantic, and the western-most part of the Alpine-Mediterranean orogenic belt.

Coincident multiscale estimates of Arctic sea ice thickness

Recent dramatic changes in the characteristics of the Arctic sea ice cover have sparked interest and concern from a wide range of disciplines including socioeconomics, maritime safety and security, and resource management, as well as basic research science. Though driven by different priorities, common to all is the demand for an improved ability to monitor and forecast changes in the sea ice cover. Key to meeting this demand is further improvement in the quality of observations collected from remote platforms. Satellites provide an important platform for instruments designed to monitor basin-wide changes in the volume of the ice cover, a function of ice extent and thickness. Remote techniques to monitor sea ice extent in all seasons are well developed—these observations reveal a dramatic decline in summer sea ice extent since 1979, when satellite records became available. Further, they indicate that the decline has been facilitated by a dramatic decrease in the extent of perennial (i.e., multiyear) ice. Combined estimates of ice thickness derived from submarine records between 1958 and 2000, and Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry from 2003 to 2008, provide the longest-term record of sea ice thickness observations. These data suggest a decrease in the mean overall thickness of the sea ice over a region covering about 38% of the Arctic Ocean.