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

L.M. Pinheiro a;b;�, M.K. Ivanov c, A. Sautkin c, G. Akhmanov c,V.H. Magalha‹es b, A. Volkonskaya c, J.H. Monteiro b, L. Somoza d,

J. Gardner e, N. Hamouni f , M.R. Cunha g

a Departamento de Geocie“ncias, Universidade de Aveiro, 3800 Aveiro, Portugalb Departamento de Geologia Marinha, Instituto Geolo¤gico e Mineiro, Alfragide, Portugal

c UNESCO Centre on Marine Geology and Geophysics, Moscow State University, Geology Faculty, Moscow, Russiad Division de Geologia Marina, Instituto Geologico y Minero, Madrid, Spain

e Naval Research O⁄ce, Washington, DC, USAf Mohamed 5 University, Rabat, Morocco

g Departamento de Biologia, Universidade de Aveiro, 3800 Aveiro, Portugal

Received 25 March 2002; accepted 5 November 2002

Abstract

A new deep water mud volcano field (between 2000 and 3500 m water depth) was discovered in the deep SouthPortuguese margin, as well as several new mud volcanoes in the South Spanish and Western Moroccan margins of theGulf of Cadiz, during the TTR-10 (Training Through Research, UNESCO/IOC) cruise, in July/August 2000. Thiswork followed the discovery of a large mud volcano field in the Gulf of Cadiz, first investigated during the TTR-9cruise [Gardner (2001) Geophys. Res. Lett. 28, 339^342; Kenyon et al. (2000) IOC, Technical series no. 56]. Thediscoveries 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 forthis 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, inthe 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 mudvolcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m. Some of thesestructures appear to be aligned along major conjugate NE^SW and NW^SE trending faults that can be identified onthe side-scan sonar imagery. The new field discovered in the South Portuguese margin is the deepest in the Gulf ofCadiz area and includes three new mud volcanoes ^ Bonjardim, Olenin and Carlos Ribeiro ^ which seem to be quiteactive, with near-surface gas hydrate occurrence and a high saturation in H2S and hydrocarbon gases (mainlymethane) in the mud breccia and overlying pelagic sediments. Gas hydrates were recovered from the Bonjardim mudvolcano. An intensely gassified mud breccia, with one fragment of semi-consolidated claystone with a thin bituminousveneer at the surface, was recovered from the Carlos Ribeiro mud volcano. The fauna recovered consists mainly ofpogonophoran worms belonging to several species and undetermined species of Foraminifera. Three new mudvolcanoes were also discovered in the NW Moroccan margin: Rabat, Student and Jesus Baraza. These show a richer

0025-3227 / 02 / $ ^ see front matter G 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0025-3227(02)00685-0

* Corresponding author. Fax: +351-234370605. E-mail address: lmp@geo.ua.pt (L.M. Pinheiro).

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fauna that includes several species of molluscs, polychaetes, pogonophoran worms, crustaceans, echinoderms andsome fragments of dead coral (Madrepora and Lophelia). Carbonate crusts were recovered from the Student mudvolcano. The Ginsburg mud volcano, discovered during the previous TTR-9 cruise, was revisited and gas hydratesrecovered once again. A new mud volcano was also discovered in the Spanish margin, Tasyo, where evidence wasfound of coral build-ups on the hard substratum of the mud volcanic edifice.G 2002 Elsevier Science B.V. All rights reserved.

Keywords: mud volcanoes; gas hydrates; hydrocarbon seepage; cold seeps; chemosynthetic communities; Gulf of Cadiz;South Portuguese margin; South Spanish margin; Northwest Moroccan margin

1. Introduction

A considerable quantity of work on the topicsof mud volcanism, submarine gas hydrates, coldseeps and hydrocarbon-rich £uid venting has beendone over more than two decades, both by indus-try and academia (see, for example, references inGinsburg and Soloviev, 1998, and in Milkov,2000). A particular e¡ort has been put into under-standing the role of tectonics in the occurrence ofmud volcanism, particularly in compressional re-gimes (e.g. Westbrook and Smith, 1983; Barber etal., 1986; Limonov et al., 1996; Galindo-Zaldivaret al., 1996; Woodside et al., 1997; COifcOi et al.,1997; Suess et al., 1999). Also, in the last fewyears, the international scienti¢c community hasdevoted much attention to the widespread occur-rence of gas hydrates (mainly methane, which is agreenhouse gas) in the ocean sub-bottom sedi-ments and their importance to global carbon£uxes, climate change, slope instabilities and asa potential energy resource for the future (e.g.Hovland and Judd, 1988; Mienert et al., 1996;Dickens et al., 1997; Judd et al., 1997; Nisbetand Piper, 1998; Ginsburg and Soloviev, 1998;Henriet and Mienert, 1998; Baraza et al., 1999;Milkov, 2000; Paull and Dillon, 2001).The occurrence of mud volcanism, cold seep-

age, hydrocarbon venting and gas hydrates inthe Gulf of Cadiz has been intensively investi-gated since 1996 (Baraza and Ercilla, 1996; Ba-raza et al., 1999; Gardner, 2001; Kenyon et al.,2000; Ivanov et al., 2000, 2001; Somoza et al.,2000; Mazurenko et al., 2001). Within the frame-work of the UNESCO/IOC Training ThroughResearch Program, two research cruises have in-vestigated the Gulf of Cadiz for this speci¢c pur-pose: TTR-9, in 1999, and TTR-10, in 2000. Dur-

ing the TTR-9 cruise (Gardner, 2001; Kenyon etal., 2000), a large mud volcano ¢eld was discov-ered in the Moroccan and Spanish margins of theGulf of Cadiz. This discovery was based on theinterpretation of a side-scan sonar mosaic andmultibeam bathymetry (SEAMAP) acquired inthat area by the Marine Physics Branch of theNaval Research Laboratory (NRL), Washington,DC, USA, in cooperation with the Hawaii Map-ping Group and the Naval Oceanographic O⁄ce,in 1992. This was followed by three other cruises:(1) two cruises in the Spanish and Portuguesesectors of the Gulf of Cadiz, within the frame-work of the Spanish Tasyo project, where, besidesnew mud volcanoes, carbonate chimney structuresrelated to £uid venting were recovered (Somoza etal., 2000, 2003); (2) the TTR-10 cruise (Leg 2).In this paper we present the results of the TTR-

10, Leg 2. The main objective of this cruise was toinvestigate several sea£oor structures observed onthe NRL side-scan mosaic, that occur in theSouth Portuguese, the Western Moroccan andthe Spanish margins of the Gulf of Cadiz. Theseoften exhibit a signi¢cant morphological expres-sion on the multibeam bathymetry and were in-terpreted as mud volcanoes. They were investi-gated with seismic pro¢ling, long-range side-scansonar (OKEAN), hull-mounted 3.5-kHz pro¢ler,TV grab and were con¢rmed by coring to be mudvolcanoes (Fig. 1). This work complements theprevious work done in the Moroccan and Spanishmargins during the TTR-9 cruise.

2. Geological setting

The Gulf of Cadiz has had a very complex geo-logical history and has undergone several episodes

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of rifting, compression and strike-slip motion,since the Triassic (Wilson et al., 1989; Dewey etal., 1989; Maldonado et al., 1999). During theMesozoic and the Early Cenozoic, the area wasundergoing rifting, with the formation of half-gra-ben structures and carbonate platforms (Maldo-nado et al., 1999). The westward migration of theGibraltar arc during the Late Tortonian causedthe Gulf of Cadiz to form as a forearc basinand induced the emplacement of an olistostrome,in an accretionary wedge-type depositional envi-ronment, strongly deforming the sediments de-rived from the South Iberian and North Africanmargins of Thetys (Maldonado and Comas, 1992;Maldonado et al., 1999) (Fig. 2). The end of theemplacement of the olistostrome took place in theLate Miocene and coincided with an acceleratedtectonic subsidence and the development of thick

progradational and aggradational depositional se-quences.Throughout this area, extensive mud volcanism

(Fig. 2), mud diapirism, carbonate mounds andchimney structures related to hydrocarbon-rich£uid venting are observed (Somoza et al., 2003).The observed mud volcanism has been interpretedas indicating the existence at depth of gas-richoverpressured sediments (Somoza et al., 2000).The migration of these £uids through the sedi-ments to the sea£oor appears, in some places, tohave been controlled by faulting, since some ofthe mud volcanoes are located along majorNW^SE and NE^SW trending faults identi¢ableon the NRL side-scan imagery (Gardner, 2001).The migration of the £uids along these faults ispossibly related both to along-slope gravitationalsliding and to the tectonic compression of the

N

Fig. 1. Map of South Iberia and Gulf of Cadiz, showing the location of the mud volcanoes discovered in the area during theTTR-9 (circles) and the TTR-10 cruises (stars). Also shown the area covered by the NRL side-scan mosaic (shaded gray) and themain core locations, the single-channel seismic lines and the OKEAN side-scan sonar coverage obtained during the TTR-10cruise.

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olistostrome and accretionary complex of theGulf of Cadiz, due to the Africa^Eurasia conver-gence (Somoza et al., 2000). It has also been sug-gested that some of the widespread shallow £uidventing on the sea£oor, particularly in the North-ern Gulf of Cadiz, could be explained by the de-stabilization of gas hydrate-rich sediments in con-tact with the Mediterranean Out£ow Water(MOW) (Somoza et al., 2000).

3. Geophysical data acquisition

During the TTR-10 cruise, additional geophys-ical surveys with airgun single-channel seismic,OKEAN side-scan sonar and 3.5-kHz pro¢ler

were carried out in the areas chosen for bottomsampling (Fig. 1).

3.1. Seismic re£ection pro¢les

Single-channel seismic re£ection pro¢les wereacquired in the Gulf of Cadiz area during theTTR-10 cruise. The source consisted of one 3.5-lairgun, at a pressure of 120 bar (12 MPa), towedat a depth of approximately 2^2.5 m and ¢redevery 10 s (i.e. approximately every 30 m). Thestreamer consisted of one active section, 7.5 mlong, with 16 hydrophones, towed at a depth ofapproximately 2.5^3 m. The o¡set between theseismic source and the middle of the hydrophonearray was 180 m. The signals were analogue-¢l-

N

Fig. 2. Geological setting of the study area. (Modi¢ed from Maldonado et al., 1999; Somoza et al., 1999). A simpli¢ed bathyme-try and the location of the mud volcanoes discovered during the TTR-9 and TTR-10 cruises are also shown.

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tered to 50^250 Hz during the acquisition stage.The sample interval was 1 ms and the recordlength 3 s. The data were acquired digitally usingsoftware developed by Moscow State University(MSU). Preliminary processing was done onboard with RadExPro software, provided to theUnesco MSU Centre for Marine Geosciences byGSD Productions, Moscow, Russia. The basicprocessing sequence, after the de¢nition of thesource-receiver geometry, consisted of the appli-cation of static shift corrections, amplitude recov-ery by spherical divergence correction, and But-terworth bandpass ¢ltering (20^230 Hz).

3.2. OKEAN side-scan coverage

Due to time constraints and also to the factthat the NRL mosaic provided excellent detailin the western part of the survey, where the geol-ogy appears to be simpler, only the eastern part ofthe study area (Lines PSAT 152^156) was sur-veyed with the long-range 10-kHz OKEAN side-scan sonar available on the Professor Logachev(Fig. 1). In this area, the geology is very complex

with a strong mix of tectonic and sedimentaryfeatures. This survey was necessary to provide amore detailed image of the sites selected for cor-ing. It covers the Tasyo mud volcano and part ofthe Tasyo ¢eld in the north.

4. Bottom sampling results

A total of 21 sites in the South Portuguese, theWestern Moroccan and the Spanish margins ofthe Gulf of Cadiz were chosen for sampling (seelocation in Table 1). The precise sampling loca-tion was based on the interpretation of the NRLmosaic, complemented by additional seismic, 3.5-kHz pro¢ler and OKEAN side-scan imagery.Twenty gravity cores were taken, with a total re-covery of 36.87 m. One site was investigated witha TV-controlled grab system (Student mud volca-no). The location of the 13 most important coresis depicted in Fig. 1. Macrofauna was recoveredfrom the top 30 cm of each gravity core and fromsieved sediments of the grab sample. The listing ofthe taxa recovered is shown in Table 2.

Table 1Location of the cores (G) and the TV-grab station (GR), obtained during the TTR-10 cruise

Core N. Latitude Longitude Depth Site(m)

AT-226G 35‡27.603PN 09‡00.023PW 3059 Bonjardim mud volcanoAT-227G 35‡27.851PN 09‡00.028PW 3060 Bonjardim mud volcanoAT-228G 35‡28.234PN 09‡00.030PW 3150 Bonjardim mud volcanoAT-229G 35‡29.129PN 08‡15.442PW 1962 diapiric structureAT-230G 35‡35.539PN 07‡11.693PW 1159 Jesus Baraza mud volcanoAT-231G 35‡35.452PN 07‡12.048PW 1091 Jesus Baraza mud volcanoAT-232G 35‡30.849PN 07‡08.744PW 940 Student mud volcanoAT-233G 35‡31.068PN 07‡08.964PW 955 Student mud volcanoAT-234G 35‡25.724PN 07‡10.556PW 1140 diapiric structureAT-235G 35‡18.858PN 07‡07.997PW 1060 Rabat mud volcanoAT-236G 35‡22.409PN 07‡05.328PW 910 Ginsburg mud volcanoAT-237G 35‡22.410PN 07‡05.343PW 910 Ginsburg mud volcanoAT-238G 35‡22.409PN 07‡05.330PW 910 Ginsburg mud volcanoAT-239GR 35‡30.853PN 07‡08.816PW 955 Student mud volcanoAT-240G 36‡12.624PN 07‡14.540PW 845 submarine channelAT-241G 35‡45.967PN 07‡06.720PW 1100 Tasyo mud volcanoAT-242G 35‡45.962PN 07‡06.684PW 1098 Tasyo mud volcanoAT-243G 35‡47.217PN 08‡25.313PW 2200 Carlos Ribeiro mud volcanoAT-244G 35‡47.221PN 08‡25.333PW 2200 Carlos Ribeiro mud volcanoAT-245G 35‡35.007PN 08‡37.901PW 2614 Olenin mud volcanoAT-246G 35‡27.610PN 09‡00.030PW 3060 Bonjardim mud volcano

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4.1. The South Portuguese ¢eld

Several structures observed on the NRL SEA-MAP mosaic of the deep South Portuguese mar-gin of the Gulf of Cadiz (water depths between

ca. 2000 and 3200 m) were interpreted as eithermud volcanoes or gas seepage structures. Foursites, expressed by high re£ectivity sub-circularspots, were chosen for bottom sampling. Threeof them were con¢rmed by coring to be newmud volcanic structures and were named: Bonjar-dim, Carlos Ribeiro and Olenin.

4.1.1. The Bonjardim mud volcanoThis was the ¢rst structure, presumed to be a

mud volcano, investigated during the TTR-10cruise. It is located at 35‡27.6PN and 09‡00.0PW.On the NRL side-scan mosaic, it appears as anapproximately circular, isolated feature, with adiameter of ca. 1 km and a strong backscatter(Fig. 3a). The single-channel seismic pro¢lesPSAT-146 and PSAT-147 reveal that it has aheight of approximately 100 m (Fig. 3b) with asea£oor depression at the rim of the cone. Fourcores were taken on this structure, all of whichrecovered mud breccia and con¢rmed it to be amud volcano.The ¢rst core, AT-226G, was taken from the

top of the Bonjardim mud volcano, at a depthof 3059 m (Fig. 3b). The recovery consisted of151 cm of mud breccia matrix, supporting angularand subangular clasts of variable size (from milli-metric to 3 cm) and lithology, mainly representedby brownish gray mudstone. There was no pelagicveneer, and only the uppermost few cm of themud volcanic deposits were oxidized, suggestingthat this mud volcano was recently active. Di¡er-ent layers of mud breccia were observed along thecore, alternating lighter and water saturated withdarker and more consolidated mud breccia. Thismay indicate di¡erent £ows and stages of activityof the volcano. A strong H2S smell was detectedand Pogonophora (two undetermined species)were recovered from this core.The second core, AT-227G (234 cm), was taken

from a terrace just below the top of the mudvolcano, at a depth of 3066 m (Figs. 1 and 3);the mud breccia recovered was very similar to theone previously sampled (although it was morehomogenous) but the top 42 cm comprised amarly sediment, soupy in the ¢rst 6 cm and thenbioturbated and rich in Foraminifera. A strongH2S smell was present and a Pogonophora-rich

Table 2Taxonomical list of the macrofauna collected during TTR-10in the mud volcanoes of the Gulf of Cadiz

Taxa Sampling stations

Poriferaundetermined species S (239-GR)CnidariaHydrozoa S (232-G, 239-GR);

R (235-G)Anthozoa S (233-G, 239-GR)Sipunculaundetermined species T (242-G); S (239-GR)AnnelidaPolychaetaNothria hispanica (241-G)Maldanidae B (246-G)undetermined species S (233-G; 239-GR)

PogonophoraOligobrachia sp. B (246-G); CR (244-G)Polybrachia sp. B (227-G)Siboglinum sp. JB (231-G); S (232-G,

239-GR); R (235-G);G (236-G, 237-G, 238-G)

undetermined species B (226-G; 227-G);CR (243-G)

MolluscaGastropodaundetermined species S (239-GR)BivalviaCuspidariidae S (239-GR)Limopsis sp. S (232-G, 239-GR)Thyasiridae CR (243-G)

ArthropodaPycnogonidaHedgepethia atlantica S (239-GR)CrustaceaCirripedia S (239-GR)Procampylaspis sp. S (239-GR)Liropus cf. elongatus S (239-GR)Tanaidacea sp. 1 B (228-G)Tanaidacea sp. 2 S (239-GR)Anthuridae S (239-GR)Munidopsis sp. S (232-G)

EchinodermataEchinidae S (239-GR)Ophiuridae S (239-GR)

Portuguese sector: Bonjardim (B) and Carlos Ribeiro (CR);Spanish sector: Tasyo (T); Moroccan sector: Jesus Baraza(JB), Student (S), Rabat (R) and Ginsburg (G).

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Fig. 3. Bonjardim mud volcano. (a) Side-scan image (NRL mosaic); (b) single-channel seismic pro¢le across the structure (southern end of Line PSAT 146). SeeFig. 1 for location. Also shown are the locations of the cores taken during the TTR-10 cruise (AT-226G, AT-227G, AT-228G and AT-246G).

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(mainly Polybrachia sp.) layer was observed at thevery top of the core, indicating active £uid vent-ing. Foraminifera and empty polychaete tubeswere also recovered from this core.A third core, AT-228G (283 cm), was taken

from the £ank of the mud volcano, at a depthof 3150 m, in order to de¢ne the extension ofthe mud £ows (Fig. 3). The recovery consistedof a thick intercalation of olive brown pelagicmarl and brown marly turbidites enriched in Fo-raminifera, easily distinguishable due to the sharpboundaries and the coarser texture. Bioturbationactivity was very strong throughout the wholecore. No mud breccia was recovered. One crusta-cean (Tanaidacea) and empty Pogonophora tubeswere recovered from this core.Finally, one last core, AT-246G (306 cm), was

taken, at a depth of 3060 m, on the top of theBonjardim mud volcano, in an attempt to recovergas hydrates. The recovery was comparable tothat of the station AT-226G, with a 12-cm layerof soupy and oxidized structureless mud brecciaat the top, with angular and subangular clasts ofdi¡erent size and lithology. The contact betweenthe oxidized and the unoxidized parts was verysharp. Between 135 and 156 cm, an interval ofdarker mud breccia was observed. Gas hydrates,in the form of small crystals, were ¢nally sampledand found below 156 cm. A strong H2S smell wasevident and some small pyrite crystals were ob-served. Foraminifera, Pogonophora (Oligobrachiasp.) and polychaete fragments (Maldanidae) wererecovered from this core.

4.1.2. The Carlos Ribeiro mud volcanoThe next mud volcano in the South Portuguese

margin that was investigated during the TTR-10cruise was named Carlos Ribeiro, after a famousPortuguese geologist (1813^1882). It is locatedat 35‡47.217PN and 08‡25.313PW, at a depth of2200 m. It is expressed as a high re£ectivity sub-circular spot on the NRL SEAMAP mosaic (Fig.4a). The 3.5-kHz pro¢ler con¢rms its conicalshape, with a height of approximately 80 m anda diameter of ca. 1.5 km (Fig. 4b).Two cores, separated by approximately 30 m,

were taken from the top of this structure: AT-243G and AT-244G (Fig. 4; Table 1). The cores

are similar, showing almost no pelagic sedimentson top, and a succession of gray structureless verypoorly sorted mud breccia, slightly oxidized andbioturbated at the very top (only a few cm thickon core AT-244G); this indicates that this is arecently active structure. The mud breccia recov-ered is highly gasi¢ed, with an intense H2S smell.The clasts are subangular or angular rock frag-ments of di¡erent lithologies and shapes, withvariable sizes sometimes reaching 15 cm. Amongthem was found one fragment of semi-consoli-dated claystone with a thin bitumenous pelliculeat the surface. The strong H2S smell throughoutboth cores and the lack of a pelagic veneer at thetop of both successions con¢rms an intense recentactivity of this mud volcano. Some empty shellsand one live bivalve (Thyasiridae), Foraminifera,empty Thecosomata shells and Pogonophora (Oli-gobrachia sp. and another undetermined species)were recovered from this site.

4.1.3. The Olenin mud volcanoThe next structure to be investigated was the

Olenin mud volcano (Figs. 1 and 5), named aftera famous Russian scientist (1923^1982). On theSEAMAP mosaic, it appears as a dome-like struc-ture with high re£ectivity, located at approxi-mately 35‡45PN and 08‡26PW, on a slope. It wassurveyed with a 3.5-kHz pro¢ler. Two crossingpro¢les were collected, and one core, AT-245G,was taken near the summit of this structure (Fig.5). The 287 cm of core recovered is mainly repre-sented by pelagic yellowish brown and olive graymarl with olive brown and gray clay. The sedi-ments were bioturbated and contained a silty andforaminiferal admixture. Several oxidized layerswere observed throughout the core and traces ofa reduction front were described for the lower-most part of the succession, suggesting possiblepast £uid venting in the site. A minor part ofthe core consisted of two thin layers of mud vol-canic deposits: one at the 60^65-cm and the otherat the 210^210-cm intervals. They both were char-acterized by sharp irregular contacts and con-tained millimetric to centimetric subangular clastsof di¡erent lithologies in a silty clayey matrix. Thefauna recovered from this site included Forami-nifera, empty Thecosomata shells and a few frag-

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(a) (b)Fig. 4. Carlos Ribeiro mud volcano. (a) Side-scan image (NRL mosaic); (b) 5.1-kHz pro¢le across this structure. See Fig. 1 for location. Also shown are the loca-tions of the two cores (AT-243G and AT-244G) taken during the TTR-10 cruise.

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(b)(a)

Fig. 5. Olenin mud volcano. (a) Side-scan image (NRL mosaic); (b) 5.1-kHz pro¢le across this structure. See Fig. 1 for location. Also shown is the location of theAT-245G core taken during the TTR-10 cruise.

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ments of empty pogonophoran tubes. It has beeninterpreted as a small, non-active vent/volcano.

4.1.4. Dome-shaped structure. Mud volcano?One dome-shaped structure was also observed

on the OKEAN side-scan sonar records and onthe seismic pro¢le PSAT-148, at approximately35‡29PN, 08‡15PW (Fig. 1). One core, AT-229G,was taken in this structure. A hemipelagic succes-sion of alternating light olive brown and stronglybioturbated marl rich in Foraminifera and moregrayish, also intensively bioturbated marl was re-covered. Although no mud breccia was recoveredfrom this site, a mud volcanic origin of the dome-like structure cannot be excluded. Empty tubes ofPogonophora and Polychaeta were also retrievedfrom this site.

4.2. The Spanish ¢eld

In the Spanish ¢eld, several occurrences of gasseepage and mud volcanism were discovered dur-ing the TTR-9 (Kenyon et al., 2000) and TTR-10cruises. In the northern part of this sector, a sig-ni¢cant amount of very high quality data, includ-ing multibeam bathymetry, high resolution seis-mics, coring and dredging, was collected in theTasyo ¢eld (Somoza et al., 1999, 2003). Duringthe TTR-9 cruise, a very interesting mud volcanowas discovered in this area ^ the St. Petersburgmud volcano (Gardner, 2001; Kenyon et al.,2000). During the TTR-10 cruise, another dome-like structure, discovered during the Spanish Geo-logical Institute expedition in 1999 (L. Somoza,personal communication) was investigated: theTasyo mud volcano.

4.2.1. The Tasyo mud volcanoThe Tasyo mud volcano is located approxi-

mately at 35‡45PN and 07‡05PW. During theTTR-10 cruise, both OKEAN and seismics wereused to investigate this structure prior to coring.The seismic Line PSAT-152 crossed the TASYOmud volcano (Figs. 1 and 6).Two cores were taken on the top of this mud

volcano (Table 1). The ¢rst core, AT-241G (Fig.6), at a depth of 1100 m, penetrated carbonate-rich super¢cial sediments (marls with dead corals

^ Lophelia), overlying a mud breccia with gasvesicles. Empty shells of Thecosomata and othergastropods, Sipuncula (undetermined species) andPolychaeta (Nothria hispanica), were recoveredfrom this site. The second core, AT-242G, at awater depth of 1100 m, penetrated marls andmud breccia like AT-241G, with Foraminifera,although with less dead corals (Lophelia). Thetwo cores recovered showed a marly unit on thetop, of di¡ering thickness. In both cases the yel-lowish brown marl was found to be rich in Fora-minifera and contained a coral-supported layer atabout 15 cm below the top of the core. In the coreAT-241G, coral fragments showed up again in thelower part of the marly unit between 50 and 69cm, where the sediment appeared to be moregrayish and enriched in Foraminifera. The secondhalf of these cores was a homogenous and struc-tureless mud breccia with a large number of clastsof di¡erent roundness and size (in some cases upto 9 cm in size). Both cores showed evidence of apossible development of coral build-ups on hardsubstrate, namely carbonate crusts at the surfaceof the mud volcanic edi¢ce.

4.3. The Western Moroccan ¢eld

In the Western Moroccan ¢eld, where theYuma, Ginsburg, Kidd, Adamastor and TTRmud volcanoes were discovered during the TTR-9 cruise (Gardner, 2001; Kenyon et al., 2000), ¢vesites were chosen for bottom sampling (see loca-tion in Fig. 1). Three new mud volcanoes werediscovered: Jesus Baraza, Student and Rabat.Three cores were also taken from the previouslyknown Ginsburg mud volcano, and more gas hy-drates were recovered from them.

4.3.1. The Jesus Baraza mud volcanoThis was the second structure to be cored dur-

ing the TTR-10 cruise. It also proved to be a mudvolcano and was named Jesus Baraza, after a re-cently deceased Spanish geologist who worked in-tensively on methane seeps. On the side-scan mo-saic it shows up as a circular feature with a strongbackscatter (Fig. 7a). On the seismic pro¢les (east-ern end on Line PSAT-151; Fig. 1) it shows as aprominent high with a maximum height above

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(b)(a)Fig. 6. Tasyo mud volcano. (a) Side-scan image (NRL mosaic); (b) single-channel seismic pro¢le across the structure (Line PSAT 152). See Fig. 1 for location.Also shown is the location of the AT-242G core taken during the TTR-10 cruise.

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(b)(a)Fig. 7. Jesus Baraza mud volcano. (a) Side-scan image (NRL mosaic); (b) single-channel seismic pro¢le across the structure (Line PSAT 151). See Fig. 1 for loca-tion. Also shown are the locations of the two cores AT-230G and AT-231G taken during the TTR-10 cruise.

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sea£oor of about 150 m (Fig. 7b). Two cores werecollected in this mud volcano.The ¢rst core, AT-230G, was taken from the

£ank of the mud volcano, at the base of the slope,at a water depth of 1159 m. A hemipelagic marlysuccession was recovered with bioturbation andForaminifera distributed throughout the wholecore. The usual few centimeters of soupy andwater-saturated brownish marl were found atthe top, with the sediment becoming more consis-tent downcore. Slump structures were visible be-low 120 cm, where darker layers and burrowswere tilted and bent in an oblique orientation.No mud breccia was recovered. This indicatesthat hemipelagic and slump deposition was prev-alent on this slope for a relatively long period.Only a few specimens of Foraminifera and emptyThecosomata shells were recovered from this core.The second core, AT-231G, at a water depth of

1091 m, sampled the top of the mud volcano. The¢rst 4 cm consisted of light brown soupy andwater-saturated marl very rich in Foraminifera,with a few Pogonophora (Siboglinum sp.). Therest of the core could be subdivided in two mainunits. The ¢rst unit (4^27 cm) showed structure-less water-saturated brownish mud breccia withclayey matrix and numerous clasts of di¡erentcolor, roundness and lithology. The oxidation ofthis interval and the bioturbation with burrows,¢lled with sediment from the upper marl layer,might indicate that the mud volcano has notbeen active recently. The second unit (27^1254cm) presented the same mud breccia found abovebut more consolidated and without oxidation orbioturbation. A strong H2S smell was detected.The mud breccia unit was relatively well strati¢ed,with several intervals representing di¡erent mud£ows. Mud breccia £ows therefore occurred onthis mud volcano but they did not reach the siteof the ¢rst of these two cores.

4.3.2. Student. Another mud volcano?Two cores, AT-232G and AT-233G, and one

TV-controlled grab station (AT-239GR) were tak-en on this high re£ectivity, sub-circular featureobserved on both the NRL mosaic and theOKEAN sonar mosaic obtained during theTTR-9 cruise (Fig. 8). They con¢rmed that this

structure is related to cold seepage, and it wasnamed Student. A carbonate crust and dead cor-als (Madrepora and Lophelia) were retrieved fromthe top part of the cores.The ¢rst core, AT-232G, showed a poor recov-

ery, with only some material in the core catcher.It was represented by gray mud enriched in Fo-raminifera, with numerous dead coral branches ofLophelia cf. pertusa and Madrepora cf. oculataand abundant fragments of mollusc shells andechinoderm spikes. Many Pogonophora (Sibogli-num sp.), some hydroids, one decapod (Munidop-sis sp.) and one bivalve (Limopsis sp.), were alsorecovered. Three carbonate crusts were collected.The second core, AT-233G, recovered 36 cm of

yellowish brown and grayish brown marl, veryenriched in Foraminifera with fragments of bi-valve shells and dead coral branches (Madreporaand Lophelia). One anthozoan, a polychaete frag-ment and a juvenile pogonophoran (probably Si-boglinum sp.), were collected from this sample. Acarbonate crust (5 cm in size) was also found inthe middle of the recovered sequence.Finally, one grab station (AT-239GR), posi-

tioned with a TV camera, recovered carbonatecrusts and marls. A video pro¢le across this struc-ture con¢rmed the assumption about recent seep-age in the area, and showed the presence of gas-related carbonate build-ups, dense clam beds(probably Solemya mediterranea), living coral,and coral debris. A number of yellowish andgrayish brown porous carbonate crusts were col-lected, reaching 50 cm in size. The fauna recov-ered from this sample was very abundant anddiversi¢ed: 50 specimens belonging to 17 di¡erenttaxa were collected. These included Porifera, Cni-daria (Hydrozoa and Anthozoa), Sipuncula, Poly-chaeta, Pogonophora (Siboglinum sp.), Mollusca(undetermined gastropod, Cuspidariidae-like bi-valves, Limopsis sp.), Pycnogonida (Hedgepethiaatlantica), Crustacea (Cirripedia, the cumaceanProcampylaspis sp., the amphipod Liropus cf.elongatus, a tanaid and an anthurid isopod) andEchinodermata (undetermined species of Echini-dae and Ophiuridae).

4.3.3. The Rabat mud volcanoThis mud volcano (see Fig. 1 for location) ap-

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(b)(a)Fig. 8. Student mud volcano. (a) Side-scan image (NRL mosaic); (b) seismic pro¢le across the structure (Line PSAT 146). See Fig. 1 for location.

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pears on the NRL side-scan mosaic as anothercircular feature. One core, AT-235G, was takenfrom the top of this structure. 288.5 cm of sedi-ment was recovered. The ¢rst 19 cm consisted ofyellowish brown marl intensively bioturbated andwater-saturated for the ¢rst 6 cm. Foraminifera,Pogonophora (Siboglinum sp.), shell^coral debrisand dead coral (Lophelia) were found throughoutthis interval. From 19 to 129 cm the core showedthe same marl but with a grayish color becomingdarker towards the bottom. Coral debris werepresent throughout this interval. Bioturbationwas extremely intense, with large burrows ¢lledwith soupy, sandy silty clay, presumably froman underlying interval. The rest of the core wasmud breccia, with a color varying from gray todark gray; the intervals separated by sharpboundaries were interpreted as di¡erent mud£ows. Surprisingly the mud breccia showed in-tense bioturbation; this, together with the exis-tence of a marly sediment upper cover, appearsto indicate that the mud volcano has been inactivefor a relatively long period of time.

4.3.4. The Ginsburg mud volcanoThree cores were collected and gas hydrates re-

trieved from the top of the Ginsburg mud volcano(see location in Fig. 1), discovered during theTTR-9 cruise (Gardner, 2001; Kenyon et al.,2000): AT-236G, AT-237G and AT-238G. Therecovery varied from 145 to 182 cm and they allshowed the same lithology: a structureless, gray,very gassi¢ed mud breccia with a thin oxidizedinterval at the very top. The target, to recovergas hydrate crystals, was successfully accom-plished in core AT-238G, from which ice-likegas hydrates up to 5 cm in size (Fig. 9) werecollected from the interval below 150 cm. Thesewere stored in a cold laboratory for later geo-chemical analyses. According to the data obtainedduring the TTR-9 and TTR-10 cruises, the Gins-burg mud volcano appears to be presently themost active mud volcano in the Gulf of Cadiz.Foraminifera and Pogonophora (Siboglinum sp.)were recovered from this site.

4.3.5. Diapir-like structure. Mud volcano?One core, AT-234G, was also taken from the

top of dome-shaped structure shown as a highre£ectivity spot on both the NRL mosaic andthe OKEAN side-scan data from the TTR-9cruise. 241 cm of a marly succession was recov-

Fig. 9. Gas hydrates recovered from the Ginsburg mud volcano during the TTR-10 cruise.

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ered, showing an intensive bioturbation through-out the core and the presence of Foraminifera,shells and dead coral fragments at some intervals.The lower half of the succession was representedby an unusual intercalation of oxidized and re-duced lamina and was characterized by an intenseH2S smell. All this evidence seems to indicate theexistence of £uid venting through the sea£oor.Although no more cores were taken from thisstructure, its gas escape origin was con¢rmed.Thus, this site clearly deserves further investiga-tion.

5. Micropaleontological investigation of the matrixfrom the mud volcanic deposits

Micropaleontological studies were carried outon board, to date the matrix of the mud volcanicdeposits using calcareous nannofossils. Samplesfrom the matrix of the mud breccia were takenfrom the Bonjardim, Carlos Ribeiro, Jesus Bara-za, Rabat, Ginsburg and Tasyo mud volcanoesfor preliminary determination of the time range.Smear slides were prepared from raw samples andexamined under the microscope. The method ofcalcareous nannofossils identi¢cation of Perch-Nielsen (Bolli et al., 1985) was applied to cocco-liths.Calcareous nannofossils from di¡erent ages

have been found in the matrix of the mud brec-cias. Preservation of the calcareous nannofossils ismoderate to poor. They seem to be partially dis-solved, which makes it di⁄cult to identify smallforms. Relatively large coccoliths show good pres-ervation. In general, discoasters are found to bedamaged and at times their arms broken, makingtheir identi¢cation di⁄cult. Some representativesof the discoasters show overcalci¢cation patternsthat obscure important features and cause com-plications during the process of identi¢cation andclassi¢cation of species.Some Upper Cretaceous species are common in

the samples from the mud volcanoes. In eachsample, four to six species of Upper Cretaceouscoccoliths were identi¢ed in the mud volcanoes ofthe Moroccan and the Spanish margins. The mostabundant species are Micula decussata and Watz-

naueria barnesae. These species could have beenderived from a hemipelagic source or from slopesediment by the process of re-sedimentation. Theyare also present in the pelagic sediments observedon the super¢cial sediments of some mud volca-noes. Thus, the presence of these species is notconclusive evidence of an Upper Cretaceous age.Only the mud volcanoes discovered in the SouthPortuguese margin show an assemblage of UpperCretaceous coccoliths. It is possible that UpperCretaceous deposits are located near the roots ofthe mud volcanoes on the Portuguese margin.Paleocene species occur only sporadically in

only two sediment samples. The most probableexplanation for this is the absence of signi¢cantPaleocene deposits near the roots of the studiedmud volcanoes. Calcareous nannofossils, typicalof Eocene deposits, are found in abundance.Among them Reticulofenestra umbilica, R. hillae,Discoaster barbendiensis, D. saipanensis, Ericsoniaformosa are frequently observed in smear slides.Based on these nannofossils it is con¢rmed thatthere are Eocene deposits in the roots of the mudvolcanoes. There are also some transitional spe-cies through the Oligocene (D. de£andrei, Cycli-cargolithus £oridanus, C. abisectus, Dictycoccitesbisectus). However, the presence of the Oligocenesediments cannot be con¢rmed because of lack ofevidence of marker species. The most abundantspecies in the matrix are Miocene^Pliocene cocco-liths and discoasters. There are numerous speciesin all the samples of the matrix that were studied.The following species were observed most fre-quently in the smear slides: Discoaster variabilis,D. druggii, D. pansus, Reticulofenestra pseudoum-bilica, R. gelida, Reticulofenestra spp., Dictycoc-cites spp., Sphenolithus heteromorphus, Calcidiscusmacintyrei ; several other species were alsopresent. The presence of Miocene^Pliocene speciessuggests that sediments of these ages have beenactively involved in the process of mud volcanismin the region.Quantitative analysis of the coccoliths was car-

ried out in the samples of matrix from mud brec-cia in core AT-227G. Amongst the 145 specimensof calcareous nannofossils, 45 are from the UpperCretaceous (mainly Micula decussata 26, Watz-naueria barnesae 15, Cribrospherella ehrinbergii

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5, etc.), 25 specimens are from the Eocene (Eric-sonia formosa 7, Reticulofenestra umbilica 5,R. hillae 3), 15 specimens are transitional fromthe Upper Eocene through the Oligocene to thelower Miocene, and 37 specimens are from theMiocene^Pliocene (mainly di¡erent types of Dis-coasters, small Reticulofenestra, R. pseudoumbili-ca, R. gelida and Dictycoccithes sp.). The otherspecies are recent coccoliths namely Emilianiahuxley, Gephyrocapsa sp., Coccolithus pelagicus,Calcidiscus leptoporus, Rhabdosphaera clavigerand Syracosphaera pulchra.

6. Discussion and conclusions

From the combined results of the TTR-9, TTR-10 and Tasyo cruises, there is now vast evidenceof extensive cold seepage, carbonate mound for-mation including cold-water coral communities(Lophelia skeletons), hydrocarbon-rich gas vent-ing and mud diapirism in the South Portuguese,Spanish and Western Moroccan margins of theGulf of Cadiz (Baraza and Ercilla, 1996; Barazaet al., 1999; Kenyon et al., 2000, 2001; Ivanov etal., 2000; Somoza et al., 2000, 2003; Gardner,2001; Mazurenko et al., 2001). This has been in-terpreted as indicating the existence of gas-richoverpressured sediments at depth and the migra-tion of these £uids through the sediments and/oralong major faults to the sea£oor (Somoza et al.,2000).As regards the tectonic control of the location

of the mud volcanoes, the NRL SEAMAP mosaicshows that some of these structures are alignedalong major fault structures, probably strike-slip,with two main orientations: NW^SE and a con-jugate NE^SW set (Gardner, 2001). Therefore,although some of the widespread shallow £uidventing on the sea£oor in the northern part ofthe Gulf of Cadiz may be explained by the de-stabilization of gas hydrate-rich sediments in con-tact with the warmer MOW (Somoza et al., 2000),a signi¢cant number of the mud volcanoes in thedeeper areas of the basin appear to be controlledby major faults that acted as conduits for the£uids. The overpressure in the sediments at depthis possibly related to fast sediment supply, along-

slope gravitational sliding and to the tectoniccompression of the olistostrome and accretionarycomplex caused by the Africa^Eurasia conver-gence (Somoza et al., 2000).There is also strong evidence that the mud vol-

canoes located in the western and deeper parts ofthe Gulf of Cadiz are the most active, with theexception of the Ginsburg mud volcano, on theMoroccan sector. Gas hydrates were recoveredfrom both the Bonjardim and the Ginsburg mudvolcanoes. The signi¢cance of this is not yet fullyunderstood but is possibly related to the distribu-tion of the tectonic stress in this area.The typical structures related to £uid venting in

this area are conical mud volcanoes with diame-ters ranging from several tens of meters to ca.4 km and heights that can reach 200 m (Ivanovet al., 2000). In some of the cores recovered fromthese structures (e.g. Bonjardim mud volcano), astrong H2S smell could be felt. This, together withthe lack of a pelagic veneer and abundant Pogo-nophora at the very top of the core, appears toindicate active venting. Evidence of multiple £owswas also observed on several mud volcanoes.As regards the age of the gas-rich layer, the

oldest clasts found in the cores are of Upper Cre-taceous age (Bonjardim, Carlos Ribeiro and Ta-syo), particularly in the Portuguese sector,although the majority of the clasts are of Eo-cene/Miocene age. Since the study area is an ac-cretionary complex with an olistostrome emplace-ment, this does not necessarily imply that theoverpressured gas-rich mud layer is older, becausethe Upper Cretaceous clasts may belong to theoverlying olistostrome and come from the easternnappes. Further research is necessary to clarifythis problem.Carbonate crusts and cold-water coral com-

munities were found only in the eastern sectorsof the Gulf of Cadiz, i.e. the Spanish and Moroc-can sector. Seepage contributes to the enrichmentof dissolved organic carbon in the water columnwhich seemingly favors the development of suchcommunities (Hovland et al., 1998). However, thepresence of a hard substrate in some of the mudvolcanoes, namely the existence of carbonatecrusts or mud breccias, may be crucial to the ini-tial settlement of the coral larvae. This question

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also deserves further investigation. The fauna col-lected in the Moroccan sector exhibited a highdiversity that can be explained mainly by thehigh habitat heterogeneity due to the availabilityof di¡erent substrates (soft muddy sediments, car-bonated crust, dead coral beds).The macrofauna collected in the Gulf of Cadiz

included several species of pogonophoran wormsbelonging to the genera Siboglinum, Polybrachiaand Oligobrachia. Variations in the species distri-bution are mainly depth-related with a higher di-versity occurring at the deeper Portuguese mudvolcanoes. Thyasirid bivalves (one living specimenand several empty shells), whose complex burrowsystems oxygenate the sediments to a depth ofmore than 8 cm, were also recovered in the Por-tuguese sector. Video footages from the Moroc-can sector, collected during this cruise, showedevidence of dense clam beds (probably Solemyamediterranea). These pogonophoran and bivalvespecies are known to contain bacterial symbiontsand form chemosynthetic communities that aresustained by £uid expulsion through mud volca-noes (Southward, 1985, 1986; Southward et al.,1981; Sibuet and Olu, 1998).Chemoautotrophic organisms may interact with

the sediment properties: high levels of bioturba-tion are associated with the dense beds of thebivalves that can cause signi¢cant changes in thechemical characteristics of the sediment due to theactivity of their symbionts (Dando and South-ward, 1986). On the other hand these organismsrespond to the environment characteristics andmay also be used as indicators of £uid emission:variations in the density of Pogonophora (numberof tubes per m2) can be related to the intensity orchemistry and regularity or persistence of £uidemissions in seepage areas (Sibuet and Olu,1998). No Pogonophora were recovered fromthe Spanish ¢eld, while the highest densitieswere found at the Student, Ginsburg (Moroccansector) and Bonjardim (Portuguese sector) mudvolcanoes. This biological information agreeswith the geological evidence of recent activity es-pecially in the two latter volcanoes.Based on all the available data, the main con-

clusions that can be drawn from the bottom sam-

pling results, integrated with the geophysicaldata and the preliminary work carried out, canbe summarized as follows:^ A large variety of sediment types present in

the Gulf of Cadiz area were recovered and de-scribed, implying a superposition of di¡erent de-positional environments in this region of verycomplex geology and £uid £uxes.^ The mud volcanism is a widespread phenom-

enon in the Gulf of Cadiz and six new mud vol-canoes were discovered in the area during theTTR-10 cruise. Most of them were proven to berecently active, in particular Bonjardim, CarlosRibeiro and Ginsburg. Gas hydrates were recov-ered from Bonjardim and Ginsburg.^ The mud volcanism of the Gulf of Cadiz is

characterized by a zonal pattern in terms of themud breccia lithology typical of di¡erent mudvolcanoes. In general, relatively thin mud breccia£ows, intensively bioturbated at the top, andsemi-consolidated clasts within matrix with highcarbonate content are characteristic of the mudvolcanoes sampled on the Spanish and Moroccanmargins of the Gulf of Cadiz. Non-strati¢ed thicksti¡ mud breccia, only occasionally bioturbated,and numerous well lithi¢ed clasts within matrixlacking signi¢cant carbonate admixture werefound to be typical of the mud volcanoes fromthe Portuguese margin of the Gulf of Cadiz.^ The co-existence of coral-rich carbonate

build-ups and mud volcanic activity was con-¢rmed to be widely present in the Gulf of Cadiz.However, this co-existence appears to be re-stricted to the shallower Spanish and Moroccansectors (900^1200 m) and apparently absent onthe deeper Portuguese sector (2200^3200 m).^ Faunal communities probably sustained by

hydrocarbon-rich £uid emission through mud vol-canoes were also widely present in the Gulf ofCadiz.^ Di¡erences in composition and structure of

the chemosynthetic-based communities in themud volcanoes may be related to the intensityor chemistry, or to the regularity or persistenceof the hydrocarbon-rich £uid emission. These dif-ferences may be used as indicators of temporalvariations in the activity of the mud volcanoes.

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Acknowledgements

The authors thank the NRL, Washington, DC,USA, for kindly having released their SEAMAPmosaic and multibeam bathymetry for the TTR-10 cruise. We also wish to thank Eve Southwardand Roger Bamber, for the identi¢cation of thePogonophora and Pycnogonida, respectively, andAlan Judd, Martin Hovland and Jean-PaulFoucher, for their careful revision of this paper,which helped us to greatly improve the overallquality of the ¢nal version. Finally, we thankthe Captain and crew of the R/V Professor Loga-chev for a splendid time on board and for thequality of data acquired during the whole cruise.Contributions by J. Gardner to this project weresupported by the O⁄ce of Naval Research PE#0601153N. This work and the shiptime ofTTR-10 were funded jointly by the UNESCO/IOC, the University of Moscow and the Portu-guese Geological and Mining Institute (throughthe INGMAR Project ^ PLE/4/98, ¢nanced bythe Portuguese Foundation for Science and Tech-nology ^ FCT).

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