Widespread fluid expulsion along the seafloor of the Costa Ricaconvergent margin

Gerhard Bohrmann,1* Katja Heeschen,1 Carmen Jung,1 Willi Weinrebe,1 Boris Baranov,2

Beatrice Cailleau,1 Richard Heath,1 Veit Huhnerbach,3 Matthias Hort,1 Douglas Masson3

and Irmgard Trummer11GEOMAR Research Center, Wischhofstr.1–3, 24148 Kiel, Germany; 2Institute of Oceanology, Russian Academy of Sciences, Nakhimovsky

Pz. 38, Moscow 117851, Russia; 3Southampton Oceanography Centre, Empress Dock, Dock Gate 4, Southampton SO14 3ZH, UK

Introduction

It has been established that fluid flowand dewatering of sediments thataccrete in or subduct beneath marginwedges are fundamental processesalong the world’s convergent margins(Moore and Vrolijk, 1992). Such pro-cesses may not only affect the geo-chemical budget of the ocean (Kastneret al., 1991), but fluids and volatilesalso play an essential part in thegeneration of natural disasters suchas tsunamis in response to submarinelandslides and earthquakes (vonHuene et al., in press). The seafloormanifestation of fluid expulsion ischaracterized by the occurrence ofspecialized vent ecosystems (Suesset al., 1985; Sibuet and Olu, 1998),geochemical and geothermal anomal-ies in near-seafloor sediments (Kastneret al., 1991; Henry et al., 1992;Wallmann et al., 1997), as well aslithological indicators such as authi-genic carbonates (Kulm et al., 1986)and barites (Torres et al., 1996). Inaddition, in-situ flow rate measure-ments from seafloors reveal ventingrates that vary between 86 and1751 L m–2 d–1 (Carson et al., 1990;Linke et al., 1994; Suess et al., 1998).

This type of seafloor venting has beenstudied in a number of subductionareas, e.g. the Cascadian Margin(Kulm et al., 1986; Suess et al.,2001), offshore Peru (Dia et al.,1993), along the Mediterranean Ridge(Cita et al., 1995), in the Nankaitrough (Lallemand et al., 1992), atthe Barbados prism (Le Pichon et al.,1990), at Makran accretionary margin(von Rad et al. 2000) and in theeastern Aleutian subduction zone(Suess et al., 1998).Surface investigations of fluid flow

were conducted along the MiddleAmerican Trench, during a suite of20 ALVIN dives forming a pre-sitesurvey for ODP Leg 170, only in asmall area south of Nicoya Peninsula,off Costa Rica (Kahn et al., 1996). Incontrast to these first sparse investi-gations on surficial fluid venting, theCosta Rica Margin area has beenexceptionally well imaged with both2D and 3D seismic reflection data(Shipley et al., 1992; Hinz et al.,1996), with wide-angle seismic refrac-tion data (Ye et al., 1996), detailedheat-flow data (Langseth and Silver,1996) and swath bathymetric data(von Huene et al. 2000). Geophysicalanalyses in the recent past have led toa change in the tectonic interpretationof the margin. Although it was ini-tially interpreted to be built by accre-tion of sediments (e.g. Shipley et al.,1992), it is now proposed that thelong-term tectonic process along the

Middle American convergent marginis tectonic erosion of the upper plate(Ranero and von Huene, 2000).Limited by the focus of several

research initiatives planned for thefuture (e.g. ODP Leg 203), a searchwas carried out for active zones offluid escape along a broad area of thecontinental slope during one of thePAGANINI cruises using RV Sonne(SO144-2). The seafloor was surveyedfrom south of Nicoya Peninsula toapproximately 200 km to the south-east (Bialas et al., 1999). Based onprevious geophysical information, itwas possible to concentrate the searchon specific target areas such as theseamount subduction scars and rela-ted tectonic features, landslide sites,scarp headwalls, mud volcanos andareas of high reflectivity in TOBIsidescan sonar backscatter (Bialaset al., 1999).

Geological setting

South-west of Costa Rica, the CocosPlate subducts beneath the CaribbeanPlate at a rate of nearly 90 mm yr–1

(Kimura et al., 1997). The Cocos Plateformed at the East Pacific Rise and atthe Cocos–Nazca spreading centre(Fig. 1). In the collision zone, theincoming oceanic plate is separatedinto two types of crust of differentages. North-west of Fisher Seamount,the crust is 22 Myr old and has arelatively smooth surface; to the

ABSTRACT

Active fluid and gas transport were measured and observedalong more than 200 km of the convergent margin of Costa Ricaduring cruise SO144-2 aboard RV Sonne. Ten profiles were runwith the TV-sled OFOS, eight of which detected the denseoccurrence of cold vent sites. This discovery shows that seafloorfluid expulsion is widely spread along the Pacific margin ofCosta Rica. Surficial evidence of fluid expulsion is indicated bythe appearance of chemosynthetic vent organisms such asbacterial mats, vesicomyid, solemyid and mytilid bivalves and

tubeworms. Numerous active vents were indicated by elevatedmethane concentrations (£ 200 nmol L–1) in the bottom water.Although fluid-venting activity was known previously from asmall area south of Nicoya Peninsula, the present studydocuments active seepage at landslides, headwall scarpsrelated to seamount subduction, morphological intersectionsof faults and mid-slope mud volcanoes.

Terra Nova, 14, 69–79, 2002

*Correspondence: Gerhard Bohrmann,

GEOMAR Research Center, Wischhofstr.

1–3, 24148 Kiel, Germany. Tel.: +49 4 31

600 2319; fax: +49 4 31 600 2930; e-mail:

gbohrmann@geomar.de

� 2002 Blackwell Science Ltd 69

south-west, the oceanic crust isapprox. 18–19 Myr old (Barckhausenet al., 2001) and exhibits a roughtopography with several seamounts(Fig. 1). Recent drilling has confirmedthe marginal wedge to be of contin-ental origin, reflecting a nonaccretion-ary scenario (Kimura et al., 1997;Meschede et al., 1999). A 400–500-m-thick pelagic sequence of calcareousand siliceous biogenic sedimentsabove the basaltic basement entersthe subduction zone (Kimura et al.,1997). At Costa Rica, the marginwedge is composed of igneous rock(Ranero and von Huene, 2000), cov-ered by 1–2-km-thick slope sediments.On the lower slope, a small zoneof deformed sediments, with distinctmorphologies shown in seismic sec-tions, is termed the frontal prism (vonHuene et al., 2000). The thickness ofthe margin wedge decreases to the

south-east and is thinnest in the areaof subducted seamounts. Ranero andvon Huene (2000) have shown twomechanisms of basal erosion along theCosta Rica convergent margin fromseismic records: tectonic erosion byseamount tunnelling and rock re-moval of mega lenses of a distendingupper plate.Fluid venting has been documented

on the Costa Rica slope duringALVIN dives (McAdoo et al., 1996).Active venting was most abundantmore than 5 km behind the deforma-tion front whereas on the lowest partof the slope, the frontal prism revealedno evidence for fluid flow. At waterdepths between 3800 and 3480 m,biological evidence for fluid ventingwas observed along thrust faultscarps, indicating fracture permeabil-ity as the fluid conduit (Kahn et al.,1996). Fluid expulsion was also

evident at mid-slope mud diapirsbetween 3140 and 2980 m waterdepths. Higher upslope, active ventsites were observed at depths ofapproximately 1610–1590 m, wherelarge canyon walls cut stratigraphiclayers which may be fluid pathways(Kahn et al., 1996). The chemistry offluids obtained from squeezed pushcores taken by ALVIN revealed signi-ficant differences between vent-relateddeposits and nonvent-related back-ground sediments. Pore waters fromthe mud volcano sediments indicatedpossible admixing of fluid from great-er depth (Zuleger et al., 1996). Inter-stitial waters from the sedimentarycover of the slope drilled by ODPshow variations in sulphate and alka-linity that are typical for sedimentsrich in organic matter (Kimura et al.,1995). The occurrence of gas hydratesin the slope sediments was proven by

Fig. 1 Study site covered during RV Sonne SO144-2. The lower continental slope along the Pacific margin of Costa Rica wassurveyed by 10 TV-sled observation tracks (OFOS; Table 2) and nine sites, from which methane distribution in the water columnwas measured in eight profiles (CTD stations; Table 1). Index map: CB, Caribbean Plate; SAP, South American Plate; EPR, EastPacific Rise; CNSC, Cocos–Nazca spreading centre.

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their recovery and by resistivity andvelocity logs. The presence of gashydrates in the sediments of threeODP leg 170 sites is also evident inlow chlorinity values (Kimura et al.,1995). Gas hydrate layers are notapparent in sediments below thedecollement, as shown by the ODPporewater profiles.

Methods

Hydrocasts equipped with a conven-tional CTD (Seabird 911 plus) and arosette, and the Ocean Floor Obser-vation System (OFOS) were usedduring R/V Sonne cruise SO144–2(Tables 1 and 2). Water samples forCH4 analyses were obtained frompotential vent sites at eight stations(Table 1; CTD 2–9). For CH4 analy-sis, a modified vacuum degassingmethod was used (Rehder et al.,1999): 400 mL seawater samples weretaken using a glass syringe from theNiskin bottles and injected into a pre-evacuated 600 mL glass bottles; theoverhead gas and water phases wereequilibrated by shaking for at least30 min; subsequently the gas phase

was recompressed to atmosphericpressure and the CH4 mole fractiondetermined by gas chromatographyusing a flame ionization detector(FID); the dissolved CH4 concentra-tion was calculated as the product ofmole fraction in the extracted gasphase and the total gas content(STP) in the sample. The latter wascalculated by measuring the dissolvedoxygen concentration (Winkler Titra-tion; Grasshoff et al., 1982) and assu-ming that nitrogen and argon were100% saturated relative to theiratmospheric partial pressures. Forcalibration, bottle mixtures of10.00 p.p.m. ± 5% and 904 ± 2%methane in synthetic air were used.The TV-sled observations were con-

ducted using the Ocean Floor Obser-vation System (OFOS). OFOS wasequipped with a SIMRAD SSBLonline navigation system, allowingexact positioning of the TV-sled.Two video cameras, a colour and ablack and white camera, as well as astill camera are installed on the sled.The OFOS is equipped with a sensorfor recording pitch and roll, an alti-meter and a CTD (Seabird). The sled

is manually adjusted by the winchoperator and is towed at 0.8 knotsship speed 1.5 m to 2 m above theseafloor (Bialas et al., 1999).

Results and discussion

During the search for vents at CostaRica, both TV surveys and methanemonitoring of the lower water columnwere used. Based on detailed bathy-metric and seismic background infor-mation, near-bottom hydrocasts atnine sites (Table 1; Fig. 1) and 10OFOS transects (Table 2; Fig. 1) wereconducted. Active fluid venting wasindicated by bacterial mats, solemyid,vesicomyid, mytilid bivalves andpogonophoran tubeworms along eightof the 10 OFOS transects (Fig. 2).Most species rely on chemoautotro-phy with reduced sulphur compundsas an energy source (Fisher, 1990);however, some species of the familyMytilidae have been shown to bemethanotrophic (Cary et al., 1988).The vent sites are characterized by

elevated methane concentrations inthe water column, found in six of theeight profiles. Station CTD 1, in thedeepest part of the trench, wasdesigned to measure the sound velocityof the water column for calibratingthe HYDROSWEEP multibeam sys-tem. No water samples were taken atthis station. The highest methane con-centrations are mostly limited to thebottom »50–150 m of the water col-umn. Three methane profiles areshown in Fig. 3. Stations CTD 8 and4 document two distinct near-bottomplumes, where background values of2–4 nmol L–1 and lower are enrichedto 20 and 35 nmol L–1. At both

Table 1 CTD/hydrocast stations conducted during R/V Sonne SO144-2 cruise (see

Fig. 1 for locations)

Station Latitude (N�) Longitude (W�) Water depth (m)

CTD 1 08� 50.97 84� 41.02 3575

CTD 2 09� 06.90 84� 50.86 1640

CTD 3 08� 55.22 84� 40.10 2921

CTD 4 08� 57.51 84� 39.03 1626

CTD 5 08� 51.03 84� 12.98 408

CTD 6 09� 23.86 85� 06.96 1461

CTD 7 09� 19.17 85� 28.79 2000

CTD 8 09� 20.01 85� 17.20 592

CTD 9 09� 39.38 85� 52.94 1919

Table 2 List of start and end points from TV-sled tracks (OFOS) deployed during R/V Sonne SO144-2 cruise (see Fig. 1 for

locations)

Begin/on seafloor End/off seafloor

Station Latitude (N�) Longitude (W�) Water depth (m) Latitude (N�) Longitude (W�) Water depth (m)

OFOS 1 09� 11.32 84� 47.88 763 09� 10.82 84� 48.20 744

OFOS 2 09� 10.75 84� 48.43 755 09� 04.65 84� 52.28 2440

OFOS 3 09� 00.15 84� 36.21 1366 08� 55.41 84� 36.30 2675

OFOS 4 08� 51.55 84� 12.72 222 08� 47.52 84� 16,16 817

OFOS 5 09� 28.47 85� 05.32 345 09� 22.62 85� 07.81 1707

OFOS 6 09� 00.77 84� 44.21 1745 08� 56.77 84� 43.01 2481

OFOS 7 09� 19.61 85� 24.01 1088 09� 15.14 85� 25.37 2371

OFOS 8 09� 19.34 85� 16.22 455 09� 22.41 85� 15.85 826

OFOS 9 09� 21.85 85� 34.99 2340 09� 14.45 85� 37.03 2877

OFOS 10 09� 41.30 85� 53.02 1836 09� 39.48 85� 53.04 1931

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stations, seafloor vents were observedby TV. CTD station 6 reveals nomethane injection to the water column(Fig. 3). Subsurface maxima inmethane contents up to 17 nmol L–1

(Fig. 3) at 50–170 m water depth werefound in all hydrocasts and are theresult of biogeochemical cycling(Tilbrook and Karl, 1995). A distinctthermocline at about 30 m decreasesthe gas exchange between the watercolumn and the atmosphere.

Landslide areas

Previous detailed morphologicalinvestigations have detected regionsof landsliding in the Costa Rica mar-gin, among which the 50-km-wideNicoya Slump (Fig. 1) had a tsunam-ogenic potential (von Huene et al.2000). During OFOS 9, large areaswere covered in the northwestern partof the Nicoya Slump without detec-tion of any clear indications for fluidventing (Fig. 1). OFOS survey 7 star-ted its observation track north of theNicoya Slump and crossed the steepflanks of the headwall. Numerouscarbonates were detected along thetrack indicating inactive, former vent-ing. Along the OFOS track, the head-wall of the landslide was reached ataround 1780 m water depth. ScatteredCalyptogena clams and shell debriswere observed from 1800 m to 2300 mwater depths. Most of these were deadclams. Seepage is thought to occur inareas of extensional fractures associ-ated with the landsliding process.Landsliding may be less active atpresent, which could explain the lim-ited amount of seafloor venting. Sim-ilar results were encountered duringtrack OFOS 5 that covers the CaboBlanco Landslide from the headwalldown to the foot (Fig. 1). Althoughno active venting has been detectedalong OFOS track 5, this process isnot excluded because the observationsof the TV-survey did not cover theentire landslide area.The situation is quite different in the

area of Quepos Landslide, wherenumerous vent indicators were found.OFOS 4 was performed from thenorth-east at 214 m water depth tothe south-west down to 854 m waterdepth (Fig. 1; Table 2). The trackstarted above 270 m water depth, ona rapidly steepening slope (Fig. 4),where outcrops of consolidated sedi-ments were covered by a very thinlayer of soft sediments. The rocksshowed visible stratification withslope-parallel dipping layers. Thelandslide headwall is a steep-dippingsurface between 270 m and 410 mdepth. The rocks became massiveand jointed downslope and stratifi-cations were not clearly visible. Farmore visibly, talus covered the scarpbase at 365 m. The Quepos landslide,starting at 400 m, shows remarkablemorphologies: its surface consists of

several small ridges of up to 20 mheight, which step upslope to present asaw-like sea-bed. It was observed thatthe downslope faces of the highs arevery steep scarps where sedimentaryrocks outcrop. They are synthetic tothe main slide plane and can beinterpreted as being derived by tiltedblocks owing to rotational sliding.Spectacular bacterial mats were

observed and coincided with the slope-ward projection of methane maximaon CTD 5 (Fig. 4). Methane values of40 nmol L–1 were measured between367 and 377 m water depth and can becorrelated with an uppermost occur-rence of bacterial mat patches (Fig. 4).The most widespread bacterial matswere observed around 400–410 m,which coincides with the methaneconcentration of 75 nmol L–1 around402 m water depth. The white, blue/grey and orange bacterial mats com-pletely cover the seafloor and extendhorizontaly along the OFOS profilefor about 220 m (Fig. 2E). At the rimof the bacterial patches tiny unknownorganisms were observed, which havewhite shells (Fig. 2F). Careful exam-ination of all the images suggestedthat, in most cases, they have twoshells. This would indicate a bivalvelike the small clam, Calyptogena.Many smaller-sized bacterial matsare dispersed downslope around theslide tail and, deeper than 550 m,numerous larger-sized vesicomyidclams and, less often, Acharax-shellswere observed. Other benthic lifeincludes dispersed snails, crabs andcrinoids. Authigenic carbonates areexposed on the crest of a small ridgenear the track termination. Carbonatesurfaces are rough, irregular and showthe texture of chemoherm carbonates(Greinert et al., 2001).The spectacular occurrences of

extensive bacterial mats around400 m water depth are typically asso-ciated with a distinct intersection ofthe morphology, which we interpret asthe upper outcrop of the slide plane.As in other landslide areas, the basalshear surface should follow downparallel to the slope by decreasing tiltangle (Fig. 4). This slide plane couldact as a pathway for ascending fluidsand gas. If estimates are correct, theslide plane could range down to 800 mwater depth. Figure 5(B) shows thecalculated stability curve for gashydrates using the Colorado School

Fig. 3 Examples of CH4 distribution inthe oceanic water column over the CostaRica margin. Near-bottom injection ofmethane is indicated north of Rio BongoScarp (CTD station 8) and at ParritaScar (CTD station 4). Bottom depths are592 m at CTD 8 and 1026 m at CTD 4.Within hydrocast station 6 (CaboBlanco Landslide) no methane plumewas detected close to the bottom.

Fig. 2 Seafloor images from active ventsites off Costa Rica indicated by theabundance and prolific colonies ofchemosynthetic organisms. (A) Pogono-phora colony; (B) bivalve cluster: vesi-comyid clams (white shells), solemyidbivalves (black periostracum) andmytilid mussels (brown shells; leftforeground); (C) scattered bivalves, bac-terial mat, and carbonate fragments; (D)pogonophorans attached to authigeniccarbonates; (E) large bacterial mat(thick individual strands recognizable)surrounding vent orifice; (F) scatteredsmall bivales and small bacterial colonies.

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Fig. 4 Seafloor observation and geological interpretation along OFOS 4 track covering approximately the length of Queposlandslide. Methane distribution in the water column close to the headwall of the landslide indicates injection of methane between350 and 400 m water depth.

Fig. 5 (A) Oxygen values at CTD station 4 revealed a water depth of 200–600 m for the oxygen-minimum layer in the watercolumn. The sensor measurements were calibrated using water samples on which the oxygen values were measured by Winklertitration (Grasshoff et al., 1982). (B) Temperature profiles measured at seven CTD stations between 50 and 1200 m water depths.Theoretical gas hydrate stability curves (Sloan, 1997) for 100% methane (left curve) and for a gas mixture of 97.5% methane: 2.5%H2S (right curve) in water of 3.5% salinity.

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of Mines Program (Sloan, 1997).Based on the approximate intersectionof water temperature profiles takenfrom CTD stations 1–7, the gashydrate stability zone is reachedaround 580–600 m water depth atCosta Rica margin. Below that depth,gas hydrates can be stable and thepresence of buried gas hydrates isdocumented by the widespread Bot-tom Simulating Reflectors (BSR)mapped in seismic profiles by Pecheret al. (1998). BSRs were observed nearthe region of Quepos landslide, inseismic lines 5 and 6. Therefore, it isobvious that gas hydrates shouldoccur in the sediments of the lowerQuepos landslide structure, at least inparts deeper than 600 m water depth.Although the causal relationship

between gas hydrates and slides arenot clear, in this case warmer fluidsfrom below could migrate upwardsalong the slide plane. During theirascent the fluid may absorb methanefrom decomposing gas hydrates. Thehigher temperature of the fluids maytrigger the decomposition of the gashydrates. In any case, fluids that exitat the slide plane outcrop, containhigher methane amounts that maynurture the bacterial mats and injectmethane into the water column, asindicated by the methane plume atSite CTD 5 (Fig. 4). Because thisoutflow of the slide plane is withinthe oxygen minimum zone of thewater column (Fig. 5A), large-sizedchemosynthetic animals that needoxygen for their metabolism are miss-ing. Below the oxygen minimum zone(> 550 m water depth) Calyptogenaclams may find enough oxygen topopulate fluid expulsion sites. Frac-tures may act as fluid pathways.

Scars of subducting seamounts

The morphology along the continen-tal slope of the Costa Rica marginsouth-east of a line terminated by thesoutheastern boundary of NicoyaPeninsula on land and the FisherSeamount on the ocean floor, ischaracterized by the subduction ofseamounts (Ranero and von Huene2000; von Huene et al., 2000). Theseamounts are transported on therapidly subducting Cocos Plate andhave led to a characteristic patternof upper plate deformation. Eachseamount creates a broad furrow

from the deformation front to itscurrent position, which is clearlyindicated by a circular uplift (vonHuene et al., 2000). The uplifteddomes are associated seaward withsedimentary landslides and steepscarps are impressive features in themorphology of the entire margin(Fig. 1). Seamount subduction maybe important for many geologicalprocesses like earthquakes (Prottiet al., 1994) or subduction erosionprocesses (Ranero and von Huene2000). The specific deformation pro-cess associated with seamount sub-duction may modify the fluidcirculation significantly within themargin sediments. In order to studythe relation of active seepage to cur-rent subducting seamounts, we inves-tigated two uplifted areas with theirseaward scarps (Jaco Scarp andParrita Scarp; Fig. 1).At Jaco Scar, OFOS tracks 1 and 2

(Table 1; Fig. 6A) ran along multi-channel seismic line 13 (von Hueneet al., 2000), which crosses from NEto SW. This includes the head of theseamount, the uplifted area over thescar and the slide structure locatedwithin the middle slope (Fig. 6A,B).OFOS 1 was cancelled after one hourowing to a technical defect in theelectric power cable and OFOS 2continued the track (Fig. 6A). Thefirst part of the track (OFOS 1)showed a seafloor covered by softsediments with no indications of seep-age. Near the beginning of the OFOS2 observation, the smooth bottomsurface was disturbed by 8–10 scarpswith outcrops of carbonate. Thescarps face upslope and are up to40 m high. These features are connec-ted with a radial fracturing that likelyresulted from the uplift of the upperplate above the seamount (Dominguezet al., 1998; Ranero and von Huene2000). Because no chemosyntheticorganisms were observed near thefractures, it can be concluded that nofluid activity presently exists. Thecarbonates, however, are clear indica-tions of former seepage activity andfuture sampling will determine whe-ther chemosynthetic organisms areincorporated into these precipitates.Based on the visible observation, thecarbonate outcrops look rough andirregular like chemoherm carbonates,which are known from other seepareas to represent locations of long-

time seepage, dominantely controlledby tectonic structures (Trehu et al.,1999).The headwall of the Jaco Scar

consists of two scarps related to faultsdeveloped during slope failure. Thedip of the lower scarp is shallower,suggesting rotation. Both scarps are280 m high. Many outcrops of grey-green sedimentary rocks, sometimesaccompanied with several bacterialmat colonies, are observed. Compar-ison with multichannel seismic profile13 (von Huene et al., 2000) shows thatthese outcrops correspond to stratifiedreflectors, extending over a 250-mthick layer. A particular strong im-pedance contrast has been measuredwithin this layer and is termed theBottom Simulating Reflector (BSR).The BSR is probably formed betweenhydrate-bearing sediment above andgas-filled pore space below (Pecheret al., 1998; Trehu et al., 1999), whichtallies with the presence and locationof the observed bacterial colonies(Fig. 6B). Below 1050 m depth, asecond type of layer displays chaoticreflectors but has an undeterminedcomposition owing to the lack ofoutcrops Smaller bacterial mats, livingclams and shells of dead clams weredispersed at the headwall and at thesite of slide deposition. The lower partof the track is dominated by talusdeposition, covered by a thin blanketof sediments (Fig. 6B). Three exten-sive clam fields were observed; theuppermost at 1400 m depth associatedwith bacterial mats and Pogonophora,the middle at 1800 m and the thirdand most extensive clam field at1900 m.Hydrocast station 2 was taken close

to the lowermost clam field andrevealed the highest methane concen-trations, higher than 200 nmol L–1,between 1900 and 1854 m waterdepths with a maximum of237 nmol L–1 at 1900 m water depth(Fig. 6B). A second high methaneconcentration of 195 nmol L–1 wasmeasured at 1815 m water depth andcorresponds closely with the middleclam field. The uppermost clam fieldaround 1400 m has no correspon-dence with the methane profile ofstation 2. Because the sample locationhas a horizontal distance of more than2 km to the clam field, a plumegenerated by the clam field may notreach the sample site, or the injection

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of gas may be less vigorous at theventing depth of 1400 m.On the lower part of the debris

deposit, OFOS 2 observed boulders,

probably carbonates, with a blacksurface. These are the results of a longexposure on the seafloor and mangan-ese oxidation, and indicate an older

age of the debris at this location,compared to the debris in upper levels.Dispersed sea anemones, crinoids,corals, brittle stars, crabs and rattailfish were identified on the seafloorthroughout the area. Sea urchins,sponges and cucumbers only appearat the upper part of the track where noclams were observed. The corals typ-ically attach themselves to hard sub-strates – usually exposed boulders orcarbonates. The corals and crinoidsare also orientated in the dominantsea-bottom current direction.OFOS 3 was conducted across Par-

rita Scar south-east of Jaco Scarp,from the top of the seamount at1000 m water depth down to 2600 m(Fig. 1). Between 1100 m and 1300 mwater depths, on the upper slope,several clam shells, clams and clamfields were present within a large fieldof talus. Scattered clams and clam-shell debris occurred at 1600 m waterdepth and correspond very well withhighermethane values (£ 40 nmol L–1)at the hydrocast station 4 (Fig. 3). Theupper occurrences of seafloor ventactivities showed very small or noexpression in methane concentrations.At the end of the OFOS 3 profile at2500 m, dispersed bacterial mats werediscovered surrounded by scatteredclams.

Mud volcanoes

Mud diapirs or mud volcanoes at theCosta Rica margin were first detectedbeween 2400 and 3100 m water depthsin the area of the 3D survey and ODPLeg 170 south of Nicoya Penisula(Shipley et al., 1992; Kimura et al.,1997). Many other mud volcanoes,dispersed in a zone of mid-slopedepths, were observed during detailedswath bathymetry mapping off Ni-coya Peninsula to Nicaragua (Raneroet al., 2000). Based on the Hydro-sweep data, a mud volcano was selec-ted at around 2000 m depth and theaim was to cover the morphologicalelevation four times (Fig. 7A,B) dur-ing OFOS track 10 (Table 2). How-ever, owing to strong currents in thedeeper water column the mound wascrossed only N–S and S–N. The mudvolcano with a diameter of approxi-mately 1000 m stands above thesurrounding seafloor by as muchas 100 m as shown by the cross-section profiles (Fig. 7B). The central

(A)

(B)

Fig. 6 (A) Hydrosweep bathymetry of Jaco Scar. OFOS tracks 1 and 2 were combinedand covered a profile from the actual area of uplift above the subducting seamountover the erosional scar down to the bottom of Jaco Scar. (B) OFOS observations alongthe track including interpretation from RV Sonne seismic line 13 (von Huene et al.,2000) and methane distribution of the water column measured at CTD station 2.

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depression was approximately 30 mdeeper than the rim and hosted spec-tacular massive accumulations ofchemoautotrophic biological commu-nities (Fig 2A–D). Furthermore, coldvent-like carbonates are also present(Fig. 2D). They occur as massive,yellowish-grey, irregular edifices thatoften cover the crest of the mudvolcano. Future sampling and carbonisotopic investigations will determinewhether or not the dominant carbonsource is methane. Methane measure-ments (CTD 9) in the bottom waterdirectly above the mud volcano showdistinct near-bottom plumes reachingconcentrations of 94 nmol L–1. Thisclearly indicates methane injectionsfrom the active vent sites of the mudvolcano.The fluid expulsion sites in the

centre of the mud volcano were inha-bited by bacterial mats, vesicomyidand solemyid clams (Fig. 2B,C), andto a lesser extent by Bathymodiolus-like mussels (Fig. 2B), and by hugebunches of Pogonophora (Fig. 2A,D).In the Gulf of Mexico, mytilid musselswere generally observed at high-flowseep sites while vesicomyid clams andvestimentiferans prefer to settle inareas of low seepage (Aharon, 1994).The mytilid species may even usemethane as their sole source of carbonand energy as they harbour methano-trophic endosymbionts (Cary et al.,1988). Vesicomyid and solemyidclams, in contrast, get most of theircarbon from sulphur-oxidizing bac-teria (Fiala-Medioni et al., 1993) thatuse the hydrogen sulphide expelledfrom the mud volcano sediments.Hydrogen sulphide has not been veri-fied by chemical measurements,because the sediments have not yetbeen sampled. However, photographstaken using a high-resolution stillcamera revealed dark olive grey toblack-coloured patches on the sea-floor, interpreted as sulphide-richsediments (Fig. 2C,D). During OFOStrack 10, very dense bunches of

Fig. 7 (A) Hydrosweep bathymetry andtrack line of TV-sled observation alongOFOS profile 10. (B) Simplified inter-pretation of the OFOS observationsalong N–S tracks crossing the activemud volcano south-west of Nicoyapeninsula.

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Pogonophora were observed (Fig. 2A).In all cases, these occurred close tosteep walls that often revealed car-bonate outcrops. Other nonvent-rela-ted organisms were predatorybuccinid snails and crabs. Exception-ally dense accumulations of deadshells of Calyptogena sp. and Acharaxsp. were observed on the top of amud flow in the centre of the mudvolcano.The dense population as well as the

diversity of chemosynthetic fauna pro-vide evidence of concentrated and/orepisodic fluid and mud release fromthe centre of the volcano, and advec-ting through the slope sediments. Thesource depths of the muds and fluidsare unclear. Studies by Shipley et al.(1990) assumed that the margin wedgewas accretionary in nature and thefluids came from accreted sedimentsof the wedge or deeper from under-plated sediments. However, the struc-tural interpretation of the convergentarea has changed from an accretion-ary to an erosive margin (Ranero andvon Huene, 2000). Moreover, chem-ical investigations from Zuleger et al.(1996) have argued for shallowersource of the fluids. Up to now, noseismic records have crossed this act-ive mud volcano. Seismic line 7arecorded during SONNE cruise SO81(Ranero, pers. comm.) covered a mudvolcano 3 km south-west of the inves-tigated structure. In general, seismicstudies of the area have revealed anapproximately 1-km-thick layer ofslope sediments underlain by 4–5 kmthick continental basement (Raneroand von Huene, 2000). Close to thearea within the 3D seismic survey,mud volcanoes were clearly visible inseismic lines. Fluid conduits could beseen as well as some shallow high-amplitude anomalies which may rep-resent trapped gas (Shipley et al.,1992). As the formation of gashydrates reduces the permeability,hydrates may play an important rolein the material ascent in mud volca-noes; hopefully, detailed seismicstudies in future will help to clarifythis context.

Concluding remarks

Investigations by TV-sled andmethane monitoring in the lower watercolumn along the Costa Rica conver-gent margin encountered remarkably

widespread seafloor venting associatedwith diverse geological structures:• The highest density of chemosyn-thetic vent organisms was detectedon a mid-slope mud volcano, indi-cating that diapirism and mudvolcanisms may be an effectivetransport mechanism for fluids inthe overpressured slope sediments.

• The diverse pattern of fault systemsrelated to seamount subductiontraces cause variations in the per-meability. This, in turn, focusesascending fluids that feed thechemosynthetic vent communitieson the seafloor.

• Next to the headwall of landslides,manifestations of seafloor venting,such as bacterial mats and Calypto-gena clam fields, could be documen-ted in numerous cases.

Acknowledgments

We greatly appreciate the support of cap-tain H. Papenhagen and the crew of RVSonne. M. Muller, H. Florianova, T. Kath,C. Rockmann, D. Matthew, I Rouse andR. Wallace provided excellent technicalsupport. H. Sahling helped in evaluatingthe chemosynthetic organisms. The studywas supported by grant 03G144A of theFederal Ministry of Education, Research,Science, and Technology (BMBF, Bonn) aswell as by the Ocean Technology Divisionof Southampton Oceanography Centre(SOC). This publication is contributionno. 5 of the Sonderforschungsbereich 574‘Volatiles and Fluids in Subduction Zones’at Kiel University.

References

Barckhausen, U., Ranero, C., von Huene,R., Cande, S. and Roeser, H., 2001.Revised tectonic boundaries in theCocos Plate off Costa Rica: Implicationsfor the segmentation of the convergentmargin and for plate tectonic models.J. Geophys. Res., 106, 19,207–19,220.

Bialas, J., Fluh, E. and Bohrmann, G.,1999. FS SONNE, cruise report SO144/1& 2 PAGANINI; San Diego – Caldera(September 7 – November 7, 1999).GEOMAR Rep., 94, 1–437.

Carson, B., Suess, E. and Strasser, J.C.,1990. Fluid Flow and Mass Flux Deter-minations at Vent Sites on the CascadiaMargin Accretionary Prism. J. Geophys.Res., 95(B6), 8891–8897.

Cary, C., Fisher, C.R. and Felbeck, H.,1988. Mussel growth supported bymethane as sole carbon and energy.Science, 240, 78–80.

Cita, M.B., Woodside, J.M., Ivanov, M.K.,et al., 1995. Fluid venting from a mudvolcano in the Mediterranean Ridge dia-piric belt. Terra Nova, 7, 453–458.

Dia, A.N., Aquilina, L., Boulegue, J.,et al., 1993. Origin of fluids and relatedbarite deposits at vent sites along thePeru convergent margin. Geology, 21,1099–1102.

Dominguez, S., Lallemand, S.E., Malavi-eille, J. and von Huene, R., 1998. Upperplate deformation associated with se-amount subduction. Tectonophysics, 293,207–224.

Fiala-Medioni, A., Boulegue, J., Ohata, S.,Felbeck, H. and Mariotti, A., 1993.Source of energy sustaining the Caly-ptogena populations from deep trenchesin subduction zones off Japan. Deep-SeaRes. I, 40, 1241–1258.

Fisher, C.R., 1990. Chemoautotrophic andmethanotrophic symbioses in marineinvertebrates. Rev. Aquat. Sci., 2,399–436.

Grasshoff, M., Ehrhardt, K. and Kremling,K., 1982. Methods of Seawater Analyses.Verlag Chemie, Weinheim, 419pp.

Greinert, J., Bohrmann, G. and Suess, E.,2001. Gas hydrate-associated carbonatesand methane venting at Hydrate Ridge:Classification, distribution and origin ofauthigenic lithologies. In: Natural GasHydrates: Occurrence, Distribution, andDetection, Washington (C. Paull andW. Dillon, eds). Am. Geophys. Un.,Monogr. Ser., 124, 99–113.

Henry, P., Foucher, J.-P., Le Pichon, X.,et al., 1992. Interpretation of tempera-ture measurements from the Kaiko-Nankai cruise: Modeling of fluid flow inclam colonies. Earth Planet. Sci. Lett.,109, 355–371.

Hinz, K., von Huene, R., Ranero, C. andPACOMAR working group, 1996. Tec-tonic structure of the convergent Pacificmargin. Tectonics, 15, 54–66.

Kahn, L.M., Silver, E.A., Orange, D.,Kochevar, R. and McAdoo, B., 1996.Surficial evidence of fluid expulsion fromthe Costa Rica accretionary prism.Geophys Res. Lett., 23, 887–890.

Kastner, M., Elderfield, H. and Martin,J.B., 1991. Fluids in convergent margins:what do we know about their composi-tion, origin, role in diagenesis andimportance for oceanic chemical fluxes.Phil. Trans. R. Soc. A, 335, 243–259.

Kimura, G., Silver, E., Blum, P. et al.,1997. Proc. Init. Rep. Ocean drill. Proj.,170, 458 pp.

Kulm, L.D., Suess, E., Moore, J.C., et al.,1986. Oregon subduction zone: venting,fauna, and carbonates. Science, 231,561–566.

Lallemand, S.E., Glacon, G., Lauriat-Rage, A., et al., 1992. Seafloormanifestation of fluid seepage at the topof a 2000-metre-deep ridge in the eastern

Fluid expulsion along the Costa Rican convergent margin • G. Bohrmann et al. Terra Nova, Vol 14, No. 2, 69–79.............................................................................................................................................................

78 � 2002 Blackwell Science Ltd

Nankai accretionary wedge: Long-livedventing and testonic implications. EarthPlanet. Sci. Lett., 109, 333–346.

Langseth, M.G. and Silver, E.A., 1996. TheNicoya convergent margin – A region ofexceptionally low heat flow. GeophysRes. Letts, 23, 891–894.

Le Pichon, X.P., Henry, P. and Lallement,S., 1990. Water flow in the BarbadosAccretionary Complex. J. Geophys. Res.,95, 8945–8967.

Linke, P., Suess, P., Torres, M., et al.,1994. In situ measurement of fluid flowfrom cold seeps at active continentalmargins. Deep-Sea Res., 41, 721–739.

McAdoo, B.G., Orange, D.L., Silver, E.A.,et al., 1996. Seafloor structural observa-tions, Costa Rica accretinary prism.Geophys. Res. Lett., 23, 883–886.

Meschede, M., Zweigel, P. and Kiefer, E.,1999. Subsidence and extension at aconvergent plate margin: evidence forsubduction erosion of Costa Rica.Terra Nova, 11, 112–117.

Moore, J.C. and Vrolijk, P., 1992. Fluids inaccretionary prisms. Rev. Geophys., 30,113–135.

Pecher, I.A., Ranero, C.R., von Huene, R.,Minshull, T.A. and Singh, S.C., 1998.The nature and distribution of bottomsimulating reflectors at the Costa Ricanconvergent margin. Geophys. J. Int., 133,219–229.

Ranero, C.R. and von Huene, R., 2000.Subduction erosion along the MiddleAmerica convergent margin. Nature,404, 748–752.

Ranero, C.R., von Huene, R., Weinrebe,W., McIntosh, K. and Reichert, C.,2000. Mass Transfer and Fluid FlowPaths Related to Subduction Erosion atthe Middle America Convergent Margin(Fall Meeting Supplement). Eos, Trans.Am. Geophys. Un., 81, 48.

Rehder, G., Keir, R., Suess, E. and Rein,M., 1999. Methane in the northern

Atlantic controlled by microbial oxida-tion and atmospheric history. Geophys.Res. Lett., 26, 587–590.

Sibuet, M. and Olu, K., 1998. Biogeogra-phy, biodiversity and fluid dependence ofdeep-sea cold-seep communities at activeand passive margins. Deep-Sea Res., 45,517–567.

Sloan, E.D., 1997. Clathrate Hydrates ofNatural Gases. Marcel Dekker, NewYork.

Shipley, T.H., Stoffa, P.L. and Dean, D.F.,1990. Underthrust sediments, fluidmigration paths, and mud volcanosassociated with the accretionary wedgeoff Cosa Rica: Middle America trench.J. Geophys. Res., 95, 8743–8752.

Shipley, T.H., McIntosh, K.D., Silver,E.A. and Stoffa, P.L., 1992. Three-dimensional imaging of the Costa Ricaaccretionary prism: structural diversityin a small Volume of the lower slope.J. Geophys. Res., 97, 4439–2259.

Suess, E., Carson, B., Ritger, S.D., et al.,1985. Biological communities at ventsites along the subduction zone offOregon. Biol. Soc. Wash. Bull., 6, 475–484.

Suess, E., Bohrmann, G., von Huene, R.,et al., 1998. Fluid venting in the easternAleutian subduction zone. J. Geophys.Res., 103(B2), 2597–2614.

Suess, E., Torres, M., Bohrmann, G. et al.,2001. Sea floor methane hydrates atHydrate Ridge, Cascadia Margin. In:Natural Gas Hydrates: Occurrence, Dis-tribution, and Detection, Washington (C.Paull and W. Dillon, eds). Am. Geophys.Un., Monogr. Ser., 124, 87–98.

Tilbrook, B.D. and Karl, D.M., 1995.Methane sources, distributions and sinksfrom California coastal waters to theoligotrophic North Pacific Gyre. Mar.Chem. 49, 51–64.

Torres, M., Bohrmann, G. and Suess, E.,1996. Authigenic barites and fluxes of

barium associated with fluid seeps in thePeru Subduction Zone. Earth Planet.Sci. Lett, 144, 469–481.

Trehu, A., Torres, M., Moore, G., Suess,E. and Bohrmann, G., 1999. Temporaland spatial evolution of a gas hydrate-bearing accretionary ridge on the Ore-gon continental margin. Geology, 27,939–942.

von Huene, R., Ranero, C. and Weinrebe,W., 2000. Quaternary convergent margintectonics of Costa Rica, segmentation ofthe Cocos Plate, and Central Americanvolcanism. Tectonics, 19, 314–334.

von Huene, R., Ranero, C. and Watts, P.,in press. Tsunamigenic slope failurealong the Middle America Trench intwo tectonic settings. In: Prediction ofUnderwater Landslide Hazards(P. Watts, C.E. Synolakis andJ.-P. Bardet, eds). A.A. Balkema,Rotterdam.

von Rad, U., Berner, U., Delisle, G. et al.,2000. Gas and fluid venting at theMakran Accretionary Wedge offPakistan. Geo-Mar. Lett., 20, 10–19.

Wallmann, K., Linke, P., Suess, E., et al.,1997. Quantifying fluid flow, solutemixing, and biogeochemical turnover atcold vents of the eastern Aleutian sub-duction zone. Geochim. Cosmochim.Acta, 61, 5209–5219.

Ye, S., Bialas, J., Flueh, E., et al., 1996.Crustal structure of the Middle americanTrench off Costa Rica from wide-angleseismic data. Tectonics, 15, 1006–1021.

Zuleger, E., Gieskes, J. and You, C., 1996.Interstitial water chemistry of sedimentsof the Costa Rica Accretionary Complexoff the Nicoya Peninsula. Geophys. Res.Lett., 23, 455–466.

Received 19 June 2001; revised versionaccepted 24 October 2001

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