charactactivventforearcbsinsundaindonesia wiedicke etal,2002

7/30/2019 CharactActivVentForeArcBsinSundaIndonesia Wiedicke Etal,2002

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Characteristics of an active vent in the fore-arc basin ofthe Sunda Arc, Indonesia

M. Wiedicke a;, H. Sahling b, G. Delisle a, E. Faber a, S. Neben a,H. Beiersdorfa, V. Marchig a, W. Weiss a, N. von Mirbach b, A. Aat c

a Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germanyb GEOMAR, Kiel, Germany

c Trisakti University, Jakarta 11440, Indonesia

Received 11 April 2001; accepted 24 September 2001

Abstract

RV Sonne cruise SO139 discovered active fluid venting at an anticline structure in the fore-arc basin of the Sunda

Arc south of Java. Fluid venting is indicated by methane anomalies in the water column, elevated heat flow, vent-

typical macrofauna, authigenic carbonate precipitation at the sea floor and methane-rich pore fluids of the sediments.

The vent site Snail Hill is located at an up to 90 m elevated topographic high which formed immediately atop the

folded basin sequence. Gas escaping from the site is the source for methane anomalies in the water column with values

up to 5000 nl/l; depth positions of maximum concentrations correspond to the peak elevation of the topographichigh. Heat-flow values close to the site are elevated three to five times relative to regional background values. The

main venting area at 2910^2920 m water depth is restricted to an elongated area near the summit. It is characterised

by clusters of giant white bivalves (probably Vesicomyidae), black sulphidic sediment patches and authigenic

carbonate slabs. In addition, we observed low seep activity over a relatively large area as suggested by the widespread

distribution of burrowing bivalves (Acharax sp.) and of pogonophoran tube worms (Lamellisabella sp. and probably

Oligobrachia sp.). Pore water of reduced salinity at the vent site suggests destabilisation of gas hydrates. Bottom-

simulating reflectors (BSRs) rising steeply towards the vent location and potential pathways for rising fluids observed

on seismic records support this interpretation; the lack of a (visible) BSR below the vent suggests a perforation of the

hydrate-stability zone in the crestal part of the fold. The position of the vent site on top of a tectonic structure which

is linked to oblique subduction suggests that the Snail Hill site is not a singular vent phenomenon. We speculate that

venting is a common process along compressional/transpressional zones (Ujung Kulon fault zone, Mentawai fault

zone) in the northwestern fore arc of the Sunda convergent margin. 2002 Elsevier Science B.V. All rights reserved.

Keywords: seep; heat ow; methane; gas hydrate; fore-arc basins; transpression; Sunda Arc

1. Introduction

Within the past decades uid venting has be-

come a widely recognised process at active mar-

gins. Tectonically controlled seepage of uids has

been discovered in most accretionary complexes

0025-3227 / 02 / $ ^ see front matter 2002 Elsevier Science B.V. All rights reserved.

PII : S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 7 8 - X

* Corresponding author. Tel.: +49-511-643-2793;

Fax: +49-511-643-3663.

E-mail address: [email protected] (M. Wiedicke).

Marine Geology 184 (2002) 121^141

www.elsevier.com/locate/margeo


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(e.g. Cascadia: Kulm et al., 1986; Nankai: Le-

Pichon et al., 1992; Barbados: Martin et al.,

1996; Aleutians: Wallmann et al., 1997; Suess et

al., 1998; Peru: Dia et al., 1993; Makran: von

Rad et al., 2000). Critical zones of uid escapeare considered to be the front of deformation,

including the youngest thrust sheets of the accre-

tionary prism. Expulsion of uids near the front

of accretion is favoured by considerable reduction

of the pore space of sediment sequences due to the

onset of lateral tectonic compression (Minshull

and White, 1989; Bekins and Dreiss, 1992) and

by the rise of uids from below the accretionary

complex along the decollement (Kastner et al.,

1993, 1997; Henry et al., 1996). Elevated heat-

ow values measured at or near the accretionaryfront (Fisher and Hounslow, 1990; Delisle et al.,

1998; Kaul et al., 2000) support this conceptual

understanding of the dewatering process during

the subduction process.

Less attention has been paid to other structural

units that may signicantly contribute to uid

venting at active margins, in particular to fore-

arc basins with quickly accumulating sediment se-

quences that may experience compression and

faulting during their complex evolution. The ob-

jective of this paper is to present an integratedmultidisciplinary investigation of an active vent

location in the fore-arc basin of the Sunda Arc

behind the accretionary prism proper. Although

far from the deformation front, this vent site

owes its origin to the compressional processes

inherent to the evolution of this convergent mar-

gin.

The results are based on investigations carried

out during the Sonne cruises SO137 (Reichert et

al., 1999) and SO139 (Beiersdorf et al., 1999) in

1998/1999 within the project GINCO (Geoscien-

tic investigations at the active convergence zone

between the Eastern Eurasian and Indo-Austral-

ian plates o Indonesia).

2. Geological setting

The Sunda Arc displays a typical morphologi-

cal succession of trench, accretionary prism, outer

arc ridge, fore-arc basin and active volcanic chain

at the islands (Karig et al., 1980; Hamilton,

1979). Along its trench the Indo-Australian Plate

is being subducted beneath the Eurasian Plate

(Fitch, 1972) at a rate of 60^70 mm/yr (DeMets

et al., 1990; Fig. 1). Recent global positioning

system measurements dene a convergence rate

of 65 mm/yr o Java (Tregoning et al., 1994).The subduction of the Australian Plate is directed

towards the north (Jarrard, 1986; McCarey,

1991). Thus, with increasing curvature of the con-

vergent zone towards the northwest, the mode of

subduction changes from normal to oblique (Fig.

1; Fitch, 1972; Huchon and LePichon, 1984). Ob-

liquity of subduction favours the partitioning of

movements at the leading edge of the overriding

plate into a normal subduction-oriented and a

trench-parallel shear component (McCarey,

1991; Diament et al., 1992; Baroux et al., 1998;McCarey et al., 2000). In geological terms, a

major tectonic response to oblique subduction

then would be the generation of right-lateral

strike-slip faults. The dextral great Sumatra fault

at the island of Sumatra has long been known to

function in this way (Katili, 1970; Karig et al.,

1980 ; Sieh and Natawidjaja, 2000). In the fore-

arc region the Mentawai fault zone (Fig. 1) has

been discovered and is interpreted in much the

same way (Diament et al., 1992). The area be-

tween the two fault systems has been named the

Mentawai sliver plate, which moves towards the

Fig. 1. Structural map of the Sunda Arc from the westernmost part of Java to southern Sumatra. Slip partitioning due to obli-

que subduction generates microplates and causes trench-parallel slices of the fore arc o Sumatra to move towards the northwest.

The vicinity of the Sunda Strait acts as a relay zone and is characterised by (i) E^W-oriented extension and (ii) incipient evolu-

tion of NW^SE striking zones of transpression (Ujung Kulon fault zone) (modied after Diament et al., 1992; Malod and Ke-

mal, 1996 and Huchon and LePichon, 1984). The enlarged section shows the bathymetry of the working area in the southern

part of the fore-arc basin. Venting occurs about 10 km north of the slope to the outer arc high at a low ridge (Snail Hill). A sec-

ond vent site is postulated c. 5 km further east. The positions of the reection seismic lines (Fig. 2), of the Parasound lines A^D

(Fig. 3) and of the detailed map of Fig. 4 are also presented.

M. Wiedicke et al./ Marine Geology 184 (2002) 121^141122


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M. Wiedicke et al./ Marine Geology 184 (2002) 121^141 123


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northwest relative to Sumatra and accommodates

part of the trench-parallel shear stress (Malod and

Kemal, 1996).

The Mentawai platelet and the great Sumatra

fault are thought to end south of Sumatra; theneighbouring Sunda Strait and the western part of

Java act as a transition or hinge zone to the area

of frontal subduction further east (Huchon and

LePichon, 1984).

The transition zone is characterised by the evo-

lution of graben structures cutting the fore arc

(e.g. Semangko graben) and crustal stretching

on the order of 50^70 km (Huchon and LePichon,

1984; Harjono et al., 1991; Diament et al., 1992).

An arc-parallel zone of deformation and faulting

(Ujung Kulon fault zone; Malod and Kemal,1996), analogous to the Mentawai fault zone, ex-

tends in the western part of the Java fore-arc area

but is poorly constrained and may be a response

to the beginning of oblique subduction at the

western Java trench. Our survey area is located

in the prolongation of the Ujung Kulon fault

zone (Fig. 1) in the fore-arc basin.

This basin is exceptionally well developed with

a width of up to 100 km and a sedimentary ll of

more than 5 km thickness (Hamilton, 1979;

Reichert et al., 1999). A widespread bottom-sim-

ulating reection (BSR) indicates the occurrenceof gas hydrates in both the Java and the Menta-

wai fore-arc basins (Neben et al., 1999). From the

active accretionary prism the basin is separated by

the outer arc high and therefore traps the entire

sediment input from the island arc. Recent reec-

tion-seismic investigations south of Java (Reichert

et al., 1999) recorded deformation of the basin ll

parallel to the arc, which illustrates compression

along this structural feature.

3. Methods and data acquisition

Navigation of RV Sonne utilises a dierential

global positioning system. The average naviga-

tional accuracy is estimated to be better than5 m.

A special data processing sequence was applied

to the multichannel-seismic (MCS) reection lines

of cruise SO137 with emphasis on the velocity

analyses. Optimum stacking velocity was deter-

mined interactively, using velocity spectra based

on the semblance method. The multiple suppres-

sion was performed by normal move-out (NMO)

overcorrection, prestack fk-ltering, NMO over-

correction removal, an inside mute and NMO

stretch mute. Deconvolution processes were ap-plied to the data before and after stacking. Fi-

nally, an g-x-migration was used to perform thetime migration of the seismic data.

Bathymetric mapping is based on the use of the

swath-mapping multibeam sonar system HYDRO-

SWEEP (Grant and Schreiber, 1990) and the sys-

tems own software for generation of processed

bathymetric maps. The rened and detailed map

of the active vent area incorporates continuous

depth information of several photo sledge runs

(with built-in CTD) and of TV-guided grab sta-

tions (using cable lengths).Shallow high-resolution sediment echosounding

data were acquired with the hull-mounted para-

metric PARASOUND system on board RV

Sonne (Grant and Schreiber, 1990). We used a

frequency of 4 kHz and recorded at analogue pa-

per records using the DESO plotter.

Water sampling was achieved using a rosette

water-sampler (with gas-tight Niskin bottles) com-

bined with a CTD. To extract dissolved gases

Fig. 2. (A) Migrated seismogram section of line SO137-01 running along the eastern end of Snail Hill, about 500 m aside of the

vent location (triangle; for location of line see Figs. 1 and 4). Note light-coloured zones and seismic wipe-out features stretching

from the core of the anticline upwards towards the ridge and to the sea oor further to the NE (arrows). These zones are

thought to indicate faulting and the rise of charged uids. Between shotpoints (SP) 450 and 420 a crosscutting negative reector

0.7^0.9 s TWT below the sea oor is interpreted as a BSR, commonly thought to represent the base of gas-hydrate-bearing sedi-

ments. Small arrows indicate sites where uid venting is possible. (B) Line SO137-04 crosses line SO137-01 slightly SW of the

vent location (marked). The abrupt topographic termination of the ridge near the crossing point is combined with a narrow fold-

ing of reectors and suggests a fault along which a diapiric rise of uids/sediment was initiated. A second potential vent location

is marked near SP 1300 (small arrow). A BSR is observed in the east (SP1160^1220) rising from 5.05 s to 4.8 s TWT. Note the

dierent vertical exaggeration (see horizontal scales) in A and B.



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from the sea water samples on board the ship, the

combined vacuum/ultrasonic technique, modied

after Schmitt et al. (1991), was applied. The quan-

tity and chemical composition of gases dissolved

in sea water were immediately determined by gaschromatography (Shimadzu Mini 3). The isotopic

composition of methane was analysed using mass

spectrometry (Finnigan Mat 252) at BGR, Hann-

over, Germany.

The heat-ow probe, built by BGR according

to the so-called violin-bow concept, consists of a

4.2 m long glass bre rod with a parallel mounted

(10 cm distance) thin steel tube housing seven

thermistors. A 1.4-t weight atop the glass bre

rod ensured penetration into the sea oor.

The towed photo sledge for bottom observa-tions is equipped with both black-and-white and

colour video cameras and a CTD; the images and

data are transmitted to the ship and displayed in

real time. They are used to continuously adapt the

distance of the instrument to sea oor (ca. 2 m).

In addition, the sledge is equipped with ashlights

and two still cameras capable of shooting up to

800 colour photographs. Photos were taken at

regular time intervals of 8 or 15 s.

For sediment sampling a heavy (3-t) TV-con-

trolled hydraulic grab was deployed. It sampledan area of 1.8 m2 with a penetration depth of 40^

50 cm. This instrument also allows real-time sea-

oor observations during the slow drift of the

ship. After arrival on deck we collected macro-

faunal organisms, carbonate precipitates, and ex-

tracted 40-cm-long push cores used for pore-water

analysis. In the vicinity of the vent area a gravity

corer (diameter 12.5 cm) was used to recover sedi-

ment cores.

Pore water was extracted from wet sediments

using a low-pressure nitrogen and/or argon

squeezer and ltering samples with 0.4-Wm cel-lulose acetate lters in a cold room at 4C. Hy-

drogen sulphide and ammonia were analysed us-

ing standard spectrophotometric procedures

(Grassho et al., 1983). Chloride and sulphate

were determined by ion chromatography, the rel-

ative standard deviation of replicate measure-

ments is 6 1% for chloride and 6 5% for sul-

phate.

Stable isotope analyses were conducted from

cemented bulk carbonate samples and soft sedi-

ment samples by mass spectrometry following

standard procedures in the laboratories at the

University of Erlangen and GEOMAR, Kiel. All

values are reported in per mil relative to PeedeeBelemnite (PDB).

A more detailed description of methods is pre-

sented in Beiersdorf et al. (1999) and Reichert et

al. (1999).

4. Results

4.1. Location

During our survey of the accretionary marginsouth of Java and Sumatra we discovered an ac-

tive vent, indicated by abundant vent-typical mac-

rofauna, carbonate slabs and high methane con-

centrations in pore-water samples and in the

water column. The vent site is located atop an

anticline structure in the thickly sedimented

fore-arc basin about 60 km south of Java in

3000 m water depth (Fig. 1). The local high

with the vents was named Snail Hill, due to

the noticeably high abundance of buccinid gastro-

pods.

4.2. Multichannel seismics

Two previously acquired MCS lines (Reichert

et al., 1999) near the seep site provide insight

into the subsurface structure of the sedimentary

basin ll in the vicinity of the seep (Fig. 2).

Line SO137-01 (Fig. 2A) shows a huge dome or

anticline structure aecting the entire basin ll

about 10 km north of the basins southern bound-

ary (see Fig. 1). Between 4.0 and 4.5 s reection

time (TWT; V400 mbsf, vp =1600 m s31) abovethe dome-like structure the upbent sedimentary

reections show a chaotic reection pattern and

the reection strength of individual reectors is

considerably reduced (blankening). The upbent

and discontinuous reectors are interrupted by

several steep faults. The area aected by faulting

and upbending is V2 km wide. A BSR occurs

0.6^0.9 s TWT below sea oor (540^810 mbsf;

vp =1600 m s31 ; Fig. 2A). However, the reector



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can be detected only in the central part of the

record where it crosses the inclined strata of the

fold-like structure; in part of the record the close

succession of more or less horizontal reections

probably masks the BSR.

Line SO137-04 (Fig. 2B) runs parallel to the

basin axis, approximately along the crest of the

subsurface structure observed on line SO137-01.

At the crosspoint to line SO137-01 (below the

vent site) reectors are faulted or bent upwards.

A second steeply rising and faulted zone is dis-

played in the centre of Fig. 2B (SP1280), about

4.8 km further to the east. A BSR is observed in

the easternmost part of the record; it rises from

Fig. 3. Line drawings of four shallow high-resolution sediment echosounding records (Parasound) close to the Snail Hill vent

site. The lines cross the anticline structure (indicated by dark grey shading) and show that compressional deformation is weaker

towards the SE (line D). Note a small, particularly steep feature of low reectivity at the centre of line C, which may be caused

by the rough, hard (cemented) surface of a potential second vent location about 5 km away from the Snail Hill site. For location

of lines see Fig. 1.



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1.0 s to 0.8 s TWT below sea oor when ap-

proaching the faulted zone further west.

4.3. Sediment echosounding (Parasound)

Four high-resolution echosounder proles (Pa-

rasound, PS) show details of the uppermost 30^40

m of the subsurface structure (Fig. 3). These lines,

labelled A^D, run perpendicular to the basin axis

and cover a 20-km-wide area in an E^W direction

in the vicinity of the vent site (Fig. 1). A ridge

traces the crest of the deformed subsurface struc-

ture at the sea oor. This ridge is partially buried,

but its E^W extent can be followed over (at least)

the 20 km covered by our lines (E^W axis). It

terminates at or close to our easternmost Line

D, where reectors are near parallel and horizon-

tal. Towards the west, the ridge is successively

Fig. 4. Topography and sampling sites in the close vicinity of the vent site. Venting occurs at the southeastern end of the ridge

which we called Snail Hill. Heat-ow measurements indicate a ve-fold increase towards the vent site (grey shading south of the

ridge crests presents approximate position of heat-ow zonation based on ve probing stations). E^W-oriented grey pattern

marks the zone of ridge uplift as dened in PS lines of Fig. 3, above the crestal zone of the anticline (Fig. 2). Positions of Figs.

5 and 6 are marked in the centre of the map.



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more uplifted (Fig. 3). As the upper sediment se-

quences in the basins at both anks of the ridge

thin towards the ridge (Figs. 2A and 3), recent

uplift along the axis of the ridge is evident. Ero-

sional truncation (toplap) is also observed in theeastern part of the axial MCS line (right side of

Fig. 2B).

Diuse acoustic reections at the centre of Line

C, rising steeply 20^30 m above the general sea-

oor level, indicate either a rough surface (hard

rock outcrop) or interference with the acoustic

signal by e.g. gas bubbles. This is the position

where the MCS line SO137-04 displays faulting

(Fig. 2B, centre).

4.4. Bathymetry

Topographically, the seep is situated at the c.

1-km-wide crest of a 50^60-m-high ridge (Fig. 4).

Sea-oor observation and sampling revealed thatmost active venting is restricted to a small area on

Snail Hill, as indicated by abundant vent-typical

macrofauna and authigenic carbonates. Snail Hill

forms the eastern end of the ridge where it termi-

nates abruptly, exposing a steep and slightly in-

dented frontal slope (Fig. 6). It displays a bifur-

cated ridge crest, which is also shallower than the

ridge crest further west (see also peak at Fig. 2B).

The northern spur of the crest has a summit

Fig. 5. Methane concentrations of sea water samples (in nl/l, blank curves; background concentration indicated by grey shading)

and carbon isotope data of extracted methane (in x PDB, dotted curves): the prole composed of four water stations coversthe vent site and runs along the axis of the anticline. A remarkable methane anomaly was observed at Station 80MS in water

depths corresponding to the depth of the Snail Hill site (2900 m) and up to 2800 m. The N13C values of the methane-anomaly

peaks range between 359 and 363x PDB. Despite large concentration changes the isotope data show no signicant gradient.

Compared to the likely methane source at the vent site (44GA), the methane in the water column is already slightly oxidised, as

can be deduced from the relative enrichment of 13C of the methane in sea-water samples. The isotope data indicate a bacterial

origin of the methane. For location of prole see Fig. 4.



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height of 2900 m water depth rising about 90 m

from the basin oor in the east (Figs. 4 and 6).

4.5. Methane and uids

Four vertical water-sample proles down to the

sea oor, all located within 1 km distance of the

active vent, revealed methane enrichments up to

150 times above the local background of 30 nl/l(Fig. 5). The methane distribution in the water

column shows two or more local methane anoma-

lies over background concentrations (30 nl/l): an

increase towards the sea oor at all stations, a

rst maximum around 2900^2860 m water depths

and a second maximum slightly higher in the

water column which is well isolated from the rst

anomaly. The maximum in methane concentra-

tion (s 5000 nl/l) coincides with the depth of

the main venting eld. Elevated methane concen-

trations (9 400 nl/l) were found at water depths

between 2790 and 2930 m, indicating either sig-

nicant dispersion of the methane from the local

source or (less likely) more than one source. The

increase in methane concentrations near the bot-

tom indicates signicant methane release at the

sea oor.

The carbon isotope signatures (N13C) of the ex-tracted methane from the depths of high CH4content range from 359 to 363x PDB (Fig.

5). This is about 8^12x enriched in 13C relative

to the value dened for the likely source area

(44GA). Sediments of vent grabs 43 and 44GA

contained high methane concentrations of 19 500

ppb and 17 600 ppb, respectively (in wet sedi-

ment), in 20^30 cm subbottom depths. The ex-

tracted hydrocarbons did not contain signicant

Fig. 6. Distribution of vent biota and carbonates at the active vent location at Snail Hill. Outcropping authigenic carbonates and

conspicuously dark (reduced) sediment patches occur at a 30^50-m-wide stripe where clusters of living bivalves were observed.

This band is surrounded by a 50^150-m-wide zone characterised by tube worm colonies. Processed swath bathymetry has been

rened by depth measurements resulting from various photo-sledge proles and video-guided grab deployments (tracks are indi-cated). The depression running parallel to the relatively steep slope with active venting suggests that the vent site is fault-con-

trolled.



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amounts of higher hydrocarbons (40 and 140 ppb,

respectively). The carbon isotopic signature of the

methane was N13C =371x PDB (44GA). Thepore water extracted from the sediments of

44GA was signicantly inuenced by uid vent-ing, as indicated by decreasing concentrations of

SO234 and Cl3 (Fig. 7). Hydrogen sulphide con-

centrations in the pore water had maximum con-

centrations exceeding 4 mM at 12 cm depth, sug-

gesting local production (Fig. 7). While sulphate

decreases readily with increasing sediment depth,

ammonia concentrations increase only slightly.

The molar ratio SO234 :N with values of 53:6 0.2

(44GA) indicate sulphate reduction coupled to

methane oxidation rather than sulphate reduction

coupled to organic matter oxidation with typical

molar ratios of 53:s 10 (Fig. 8).

4.6. Heat-ow measurements

The heat-ow probe was deployed at ve loca-

tions in the vicinity of the vent site (Fig. 4): four

sites are located south or east of Snail Hill along

the ank of the ridge and at its eastern slope; one

was placed close to the vent. Heat-ow density q

increases considerably towards the eastern end of

the ridge (Fig. 4). The highest q values were mea-

sured near the vent (86HF:174 mW m32, c. 150 m

Fig. 7. H2S, SO234 , and Cl

3 concentrations in the pore water of the vent grab 44GA. Bottom water concentration is indicated

with a lled circle (at 0 cm depth).



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distance) and at the foot of the eastern slope of

the ridge (27HF: 72 mW m32, c. 600 m distance;

Fig. 6). Two sites (35HF and 36HF) show q val-

ues of 55 and 58 mW m32, respectively. The site

located at the edge of the ridge slope (85HF)yielded a low value of 34 mW m32. The rise of

the BSR under the hill site is evidence for a deep-

seated source of the ascending uids.

4.7. Vent fauna

Sea-oor observations revealed that vent-typi-

cal macrofaunal species on the eastern end of

the ridge vary considerably in distribution and

density, probably reecting dierences in uid-

venting activity. The main uid-venting site (Fig.6) was characterised by clusters of giant living

bivalves, probably belonging to the family Vesi-

comyidae, associated with carbonate slabs and

black patches of sediments (Plate I). Sampling in

the central uid-venting area at stations 43/44GA

recovered abundant pogonophoran tube worms,

authigenic carbonates and sulphide- and meth-

ane-rich pore uids. In a wider area surrounding

the main venting area, pogonophoran tube worms

(Lamellisabella sp. and probably Oligobrachia sp.,

E. Southward, personal communication) were ob-served in densely populated patches (Plate IC),

but mostly in less dense populations surrounded

by empty bivalve shells. Only a few of the recov-

ered tube worm specimens were alive, and many

tubes were empty. White circular patches of a few

centimetres in size found in this area around holes

and in small depressions are considered to be l-

amentous bacteria. In addition to the main uid-

venting zone and its surrounding area character-

ised by abundant seep-typical macrofauna, sea-

oor samples recovered living specimens of the

burrowing bivalve Acharax sp. (Solemyidae,33GA) and shells of thyasirid bivalves in a

much larger region.

4.8. Carbonate in sediment

The surface sediments in the fore-arc basin con-

sist of olive-coloured mud with planktonic fora-

minifera and siliceous tests (mainly diatoms). Oc-

casionally we observed thin silty^sandy turbiditePlateI.Ventfaunaandauthigeni

ccarbonates.

(A)Centralpartofactiveventar

eaatSnailHill(89FS,shot36).Clustersofgiantlivingbivalves(cf.Vesicom

yidae)surroundedbycoloniesofpogo

nophorantube

worms(Lamellisabellasp.andprobablyOligobrachiasp.).Notepresence

oflargebuccinidgastropodshells,anddarkgreydecimetre-sizedcarbonateslabs.

(B)Centralpartofactiveventarea(89FS,shot45).Scattered10^50-cm

-longpatchesofdarksediment.Pogo

nophoraatupperright.Abundantme

gafauna(holo-

thurians,galatheidcrab,sh,buccinidgastropods).

(C)Marginalzoneofventarea(89FS,shot100).Areadenselypopulated

bypogonophorawithscatteredemptybivalveshells.Atcentrelargespecimen

ofholothuria.

(D)Authigeniccarbonateslabsw

ith2^3-cm-wideopenchannelsprobab

lycausedbymacrofauna.Notelight-c

olouredthincementlininginholeatr

ight.Theslabs

consistofsedimentcementedbymicriticMg-calcite;carbonisotopesign

atureisN13C=341x

PDB.

LengthofscalebarinA^Cisapproximately0.5m



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layers composed of volcanic debris. Commonly,

carbonate content is lower than 10%. However,

the soft sediments of the two vent grabs (43/

44GA) contained 20^50% carbonate. At thesesites we also recovered irregularly shaped large

carbonate slabs (650 cm diameter) that cover

much of the sediment surface (Plate IA,D).

Some of these blocks have 2^3-cm-wide, tube-

like, open holes. The slabs consist of sediment

cemented by micritic Mg-rich calcite. Their car-

bon isotope signatures (N13C) range from 330 to341x PDB, that of the disseminated carbonate

in soft sediment ranges from N13C =337 to339x PDB.

5. Discussion

5.1. Active venting at Snail Hill

Our data collected at Snail Hill demonstrate

active venting and enable us to present a plausible

explanation of origin, pathways and reactions of

uids as they migrate from the subsurface to the

water column.

Sea-oor samples from the central venting area

showed advection of uids with high methane

concentration (17 600^19 500 ppb). The isotopic

signature of the methane (N13C =371x PDB)

and the lack of signicant amounts of higher hy-drocarbons indicate a biogenic origin. The source

of methane is probably the dissolution of gas hy-

drates, which is corroborated by the decreased

chloride concentrations in the pore water (Martin

et al., 1996; Suess et al., 1999). However, the

freshening of pore water can also be caused by

dehydration and transformation of clay minerals

(Martin et al., 1996). Sulfate concentration de-

creases readily in the pore water of the uppermost

sediment in the vent grab 44GA, suggesting that

sulphate reduction is dominantly coupled tomethane oxidation (Hoehler et al., 1994), as is

also found at many other cold seep sites (e.g.

Wallmann et al., 1997). Sulphate reduction by or-

ganic matter degradation can be excluded, as we

did not observe an associated increase in dissolved

ammonia of the pore uids (Fig. 8). Anaerobic

oxidation of advected methane accounts for pre-

cipitation of authigenic carbonates and produc-

tion of hydrogen sulphide (Hoehler et al., 1994).

The origin of the authigenic carbonates from

oxidised methane is supported by their highly neg-ative carbon isotope signature (N13C =330 to341x PDB), as is known from other actively

venting localities world-wide (Kulm et al., 1986;

Roberts and Aharon, 1994; von Rad et al., 1996;

Bohrmann et al., 1998). The observed enrichment

of carbonate in soft surface sediments results from

nely disseminated authigenic carbonate precipi-

tation fostered by anaerobic oxidation of meth-

ane; the negative N13C =337 to 339x PDBclearly support this interpretation (Suess and

Whiticar, 1989). At vent sites at the Aleutian con-

vergent margin, similar disseminated precipitateswere observed in the upper 35 cm of the sediments

(Wallmann et al., 1997). Disseminated precipita-

tion might reect the initial phase of the process

responsible for the precipitation of massive carbo-

nates at the sea oor. The carbonate slabs (Plate

I) showed frequent inclusion of planktonic fora-

minifera and sediment particles supporting the

origin by near-surface cementation of existing

soft sediment. The oxygen isotope composition

Fig. 8. SO234 versus NH4 concentrations in the pore-water

samples of the vent grab 44GA. Methane oxidation coupled

to sulphate reduction accounts for the steep decrease in sul-

phate concentration; if organic matter remineralisation was

of signicance, ammonia concentrations should show a stron-

ger increase.



15/21

of about N18O = + 5x PDB is in agreement withformation in equilibrium with cold bottom-water

temperatures at 3 km water depth.

Further evidence of uid venting comes from

biological indicators at the sea oor which were

used to map the extent of the venting area. Most

species of pogonophoran tube worms and all spe-

cies of the bivalve families Vesicomyidae, Sole-

myidae and Thyasiridae investigated so far de-

pend on the symbiosis with chemoautotrophic

sulphur-oxidising bacteria (Southward et al.,

1981; Fisher, 1990). The prerequisite for the spe-

cialised symbiotic biota to thrive (supply with suf-

cient hydrogen sulphide) is achieved by the an-

Fig. 9. 3D perception of the vent site and its position atop a major fold structure in the sediment ll of the fore-arc basin. Seep-

age at the vent location appears fault-controlled. Fluids rise along conduits from the core of the anticline (possibly also along

permeable inclined sediment layers). Depth positions of BSRs rise towards the vent site. We have added hypothetical authigenic

carbonate layers below the vent to represent a potential history of the site, considering the time-scale involved for the evolution

of the anticline. Regional compression is indicated by the two bold arrows. (Not to scale.)



16/21

aerobic oxidation of advecting methane discussed

above. Vesicomyid clams, which occur at many

cold seeps around the world, indicate particularly

high sulphide uxes (Sahling et al., 2002). There-

fore, clusters of giant living clams, as well as slabsof authigenic carbonates, and dark (sulphidic)

sediment patches dene the most active (meth-

ane-venting) area with an extent of at least

300U50 m near the hill top (Fig. 6). The wide-

spread occurrence of pogonophoran tube worms,

white bacterial patches and empty bivalve shells at

the hill top marks a zone which may temporarily

or locally experience venting with uctuating in-

tensity. Besides activity at the main venting area,

we have evidence of widespread, low-intensity

seepage. Living specimens of the Acharax sp.were recovered more than 1 km away from the

main area (33GA). Solemyidae have been shown

to occur in environments with considerable but

low sulphide concentrations (Sahling et al., 2002).

Active venting of methane is further conrmed

by signicant methane anomalies in the water col-

umn; the positions of the methane plumes in

near-bottom waters correspond to the water

depth of the hill top (2900 m) where our vent

site is located. The methane-rich uids at cold

vents are readily mixed with the ambient bottomwater and tend to generate horizontal plumes, as

observed at other active margins (Suess et al.,

1998). The N13C values of extracted methanefrom samples at the peaks of the methane anoma-

lies in the water column fall in a narrow range of

359 to 363x PDB. Since the isotope signature

of methane plumes in the water column is about

10x heavier than at the likely source sampled at

station 44GA (N13C =371x PDB, Fig. 5), thedata suggest bacterial oxidation. In highly con-

centrated methane plumes, ongoing bacterial oxi-

dation changes both the concentration and the

isotope signature quickly (often within weeks;

DeAngelis et al., 1993). Therefore, a correlation

between concentration and isotope signatureshould be expected. However, despite variable

concentrations, the stations do not dier signi-

cantly in their isotope signatures. Thus our data

argue for a process of fairly rapid dispersion of

the methane in the lowermost 150 m of the ocean

water. Apart from the well pronounced methane

plumes in the water column, the repeatedly ob-

served increase in concentration towards the sea

oor may indicate the release of methane from the

sediments in a wide area. Such a large-scale (mod-

erate) seepage would explain the observed pres-ence of vent-typical biota outside the central vent-

ing area on Snail Hill.

Active venting is also corroborated by the heat-

ow data. Regional heat ow in the relatively

cold fore arc o Java is considered to be 30^50

mW m32 (Delisle et al., 2001), which is consistent

with data from analogous structural units in other

convergent settings. The heat ow at marginal

sites around the vent location was found to be

in a similar range; even so, the values vary con-

siderably within a short distance (85HF: 34,35HF: 55, 36HF: 58 mW m32). The 72^174

mW m32 determined in the vicinity of the venting

area is anomalously high ; the three- to ve-fold

increase can be best explained by expulsion of

warm uids from the sediment ll of the basin.

The BSR depth under Snail Hill (0.6^0.9 s TWT)

indicates a background q value of approximately

35 mW m32 (0.9 s TWT), which increases under

the hill site to 55 mW m32. The two very high q

values (27HF and 86HF) near the vent site are

Table 1

Dependence of heat ow anomaly as a function of velocity of the rising uid and depth extent of convective heat transport ( d);

thermal conductivity of rising mud = 1.2 W m31 K31

Fluid velocity

m s31 1.0U10312 1.0U10311 1.0U10310 1.0U1039 3.0U1039 5.0U1039

m yr31 3.1U1035 3.1U1034 3.1U1033 3.1U1032 9.5U1032 0.15

q in mW m32 34.3 34.7 38.6 91.4 249.7 414.4

d= 700 m

q in mW m32 34.9 36.0 48.3 242.2 723.5 1201.7

d= 2000 m



17/21

made up of this background value plus a convec-

tive heat transport component, which expresses

itself in a steeper temperature gradient, measur-

able in the sediments right below the sea oor.

The rise of the BSR under the hill site is rmevidence for a source of the ascending uids

which is seated considerably deep.

5.2. Subsurface structure and tectonic framework

We think that the architecture of the underlying

sequence favours the ascent of charged uids

from several hundred metres depth. The seismic

records across this structure display several poten-

tial pathways for ascending uids. Crossing MCS

lines (Fig. 2A,B) suggest that the abrupt termina-tion of the ridge is caused by a major, approxi-

mately N^S-trending fault (oset/bending of re-

ectors), which provides a plausible explanation

for the alignment of the chaotic reection pat-

terns. The convex-shaped reectors below the

abrupt ridge end (Fig. 2B, left) pose a question

as to the geological nature of this acoustic phe-

nomenon: based on accompanying faults and the

compressional setting we suggest diapiric mobili-

sation of sediment and uids at this axial setting.

The observed authigenic carbonate precipitationat the present sea oor also allows us to consider

a dierent origin: (i) buried hydrocarbon-derived

carbonate build-ups, as described at the Louisiana

slope (Roberts and Aharon, 1994) or cold-water

reefs (Huvenne and Henriet, 2000) and/or (ii) a

zone of high sound velocity caused by carbonates

(Anselmetti and Eberli, 1997) below the vent lo-

cation, resulting in the pull-up of seismic reec-

tors (Fig. 2B). The latter scenario does not require

(buried) constructive carbonate build-ups but sim-

ply assumes sucient carbonate precipitation near

the sea oor with a long-standing history to alterthe acoustic properties at the root zone of the

vent location. O Vancouver Island Spence et

al. (2000) have reported an analogous case: on

seismic records they found a low-amplitude blank

zone below a vent surrounded by a high-ampli-

tude rim. Given the uniform geometric congura-

tion with inclined reectors stretching several

hundred metres aside of the vent location, we fa-

vour the interpretation of initial diapirism.

The huge anticline aecting the entire sedimen-

tary basin ll (Reichert et al., 1999) indicates sig-

nicant compression. Compression and uplift of

the central part of the structure are continuing, as

inferred from the thinning of the upper sedimentsequence towards the ridge despite a nearly hori-

zontal basin oor (Fig. 2A). Thus, we consider

the process of tectonic compression combined

with crosscutting faulting (potentially related to

the E^W extension in the western fore-arc basin;

Huchon and LePichon, 1984; Malod and Kemal,

1996) to be the driving force of the rise of uids.

This interpretation is supported by the position of

the BSR; over a lateral distance of 1 km the BSR

rises 270 m (vp = 1600 m s31) close to the anks of

the ridge with the vent location (Fig. 2A). Directlybelow the ridge no BSR is observed. Similarly, the

BSR about 8 km east of the vent site rises when

approaching the site from the east (Fig. 2B, right;

Fig. 9). The signicant changes in the BSR depth

indicate the rise of material and uids over several

kilometres into the crestal part of the anticline,

accompanied by faulting.

The magnitude of the measured geothermal

anomaly on the hill allows us to estimate the min-

imum amount of uids rising annually to the sea

oor. Following the approach of Bredehoeft andPapadopoulos (1965), we can calculate the in-

crease in heat ow as a function of uid velocity

in a vertical direction and the length of ascending

path. Table 1 presents the relationship whereby

an average background heat-ow density of 35

mW m32 (typical for a back-arc basin with high

sedimentation rates) was chosen. Table 1 shows

that the required uid ow for heat-ow values

of 120^170 mW m32 is on the order of 0.8^

2.2U1039m s31 (0.025^0.07 m yr31). This rate

refers to a situation where uids rise uniformly

through the sediment column. Multiplied by aconservative estimate of the vent site extent (500

mU500 m, based on TV-camera surveys), the an-

nual ux into bottom waters would then be on the

order of 6250^17 500 m yr31. This amount ap-

pears to be insucient to support the measured

methane anomaly of about 5000 nl/l at station

80MS (Fig. 5). We suspect that larger quantities

rise along conduits (faults) which we were unable

to precisely locate. In this scenario our heat-ow



18/21

measurements would have been placed in the ther-

mal halos surrounding these preferred ow con-

duits.

This survey cannot present hard proof for dia-

piric extrusion at the sea-oor surface, such asmud volcanoes, chaotic texture of sediments or

mixed stratigraphic ages. But in a similar struc-

tural position o Sumatra (Mentawai fault zone)

mud diapirism is known to occur (Samuel and

Harbury, 1996). Pathways are thought to exist,

as indicated by the MCS records. A permeable

gas-hydrate layer could explain the lack of a

BSR in the axial zone of the anticline. Free gas,

often accumulating below the gas-hydrate stabil-

ity zone and providing the necessary acoustic con-

trast to generate a BSR (e.g. Holbrook et al.,1996; Sain et al., 2000), might rise to the sea oor.

Further support comes from the reduced salinity

in pore-water samples from the vent location

(44GA) and the nearby coring site 88SL (Fig.

6), suggesting that destabilisation of gas hydrates

(Paull and Matsumoto, 2000) at least contributes

to the vented methane. From a number of venting

areas it is reported that destabilised gas hydrates

may play a major role in the process of seepage

(Martin et al., 1996; Suess et al., 1999; Spence et

al., 2000).The position of the vent site is fault-controlled,

not only due to the major crosscutting fault but

also because a second minor fault runs parallel to

the ridge (Fig. 9). The latter interpretation is sup-

ported by the elongate active venting area and the

dissected crestal topography (Fig. 6). Geographi-

cally, the investigated fold structure relates to the

Ujung Kulon fault zone (Malod and Kemal,

1996). This fault zone is thought to be transpres-

sional in character, owing its origin to the evolv-

ing oblique subduction south of western Java.

Therefore, we postulate that this vent site is nota singular feature; vents are probably common

along the fault zone, especially as compression

and strike-slip motion should increase towards

the northwest according to the concept of oblique

subduction (McCarey, 1991; Malod and Kemal,

1996). In fact, there are indications that addition-

al seeps exist where we have recorded diuse

acoustic reections at the crestal zone of the wan-

ing anticline (Fig. 3C) 5 km west of Snail Hill.

The scattering of acoustic reections at the sea

oor (exposed hard rock or escaping gas) and,

additionally, a faulted zone beneath (potential

conduit for uids; Fig. 2B, centre) support this

idea.

6. Summary and conclusion

Active venting is presently occurring in the

thickly sedimented fore-arc basin south of Java.

MCS records give evidence of existing pathways

rising steeply from the core of an underlying deep-

reaching anticline which developed in the sedi-

mentary basin ll. Methane anomalies of up to

5000 nl/l in the near-bottom water above the anti-cline and three to ve times elevated heat-ow

values support this nding. At the sea oor three

dierent zones of the seep area can be distin-

guished:

(i) The major seep zone is located at a ridge

crest and extends at least 50U300 m, as indicated

by clusters of Vesicomyid clams, large authigenic

carbonate slabs and black sulphidic sediment at

the sea-oor surface (Plate IA,C). The most fo-

cused discharge zone of methane-rich uids oc-

curs in this area at a depth of 2910 m, whichcorresponds to the largest methane anomaly en-

countered in sea water samples. Methane oxida-

tion coupled to sulphate reduction is the domi-

nant process at shallow sediment depths of a

few tens of centimetres, leading to sulphide pro-

duction and carbonate precipitation. The location

appears fault-controlled as suggested by its posi-

tion at the bifurcated eastern end of the ridge.

(ii) The main seep zone is surrounded by a

wider area populated by pogonophoran tube

worms, living specimens of the burrowing bivalve

Acharax sp., and white patches of lamentousbacteria. Heat-ow values measured in this zone

are extremely high (86HF). The extent of this area

was used to calculate a minimum uid ux.

(iii) Apart from the main uid-venting zone and

its surrounding area with typical macrofauna at

the sediment surface, a much larger area is af-

fected by moderate methane seepage. The follow-

ing arguments support this : methane concentra-

tions rise widespread when approaching the sea



19/21

oor above the anticline (80MS, 81MS, 82MS,

87MS); heat-ow values are elevated but variable

(27HF, 35HF, 36HF); living specimens of sole-

myid bivalves (33GA) and shells of tyasirid bi-

valves were present.Recent uplift of the anticline indicates ongoing

tectonic movement fostering the rise and expul-

sion of uids. The comparatively steep rise of a

BSR towards the axis of the anticline and the

reduced chlorinity encountered at the vent suggest

that destabilisation of gas hydrates may contrib-

ute to venting of methane. On the basis of this

structural set-up indications of additional seeps in

the vicinity of this site suggest to us that Snail Hill

is not an isolated vent phenomenon. Given the

tectonic setting with compression and strike-slipfaulting due to (the beginning of) oblique subduc-

tion, it appears most likely that venting is a com-

mon process along compressional/transpressional

zones (Ujung Kulon fault zone, Mentawai fault

zone) in the fore arc of the Sunda convergent

margin.

Acknowledgements

We wish to thank captain H. Papenhagen andhis crew on board RV Sonne for their support. U.

von Rad and A. Lu ckge provided helpful com-

ments on the manuscript. We also acknowledge

M. Joachimski, University of Erlangen, for the

stable isotope determinations. We are indebted to

M. Torres and a second anonymous reviewer for

their constructive comments. RV Sonne cruise

SO139 was funded by the German Bundesminis-

terium fu r Bildung und Forschung (BMBF),

Grant 03G 0139A.

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