Authigenic barites and fluxes of barium associated with fluid seeps in the Peru subduction zone

EPSL Earth and Planetary

Science Letters 144 (1996) 469-481

Authigenic barites and fluxes of barium associated with fluid seeps in the Peru subduction zone Marta E. Torres a.*, Gerhard Bohrmann b, Erwin Suess b ’ Oregon Stare University. College of Oceanic and Atmospheric Sciences, Oceanography Administration Building 104. Cowallis, OR 9733 l-5503, USA h GEOMAR, Research Center for Marine Geosciences, l-3 Wischhofstrasse. D-24148 Kiel. German) Received 28 November

1995: accepted 3 August 1996

Abstract Large deposits of batite were discovered in association with biological communities, indicative of active fluid seepage on the middle slope of Paita and in the Chiclayo Canyon, in the Peru margin. We postulate that the barium source for the deposits is associated with the high concentration of non-detrital barite buried in sediments from this high productivity region. Barite is remobilized within the sediment column due to sulfate depletion. Subsequent flushing of the barium-rich fluids from the sediment to the bottom water, leads to the formation of barite deposits at the cold vent sites. High barium concentrations measured in pore fluids of sediments are consistent with remobilization of barium sulfate below the zone of sulfate depletion. Fluid samples -collected in a time sequence using a benthic chamber in the Paita middle slope vent sites - document a contemporaneous release of barium to the bottom water at a rate of 23 pmol cm-’ yr- ‘. Fluid seepage in the Peru margin is not restricted to the middle slope of Paita and the Chiclayo Canyon where barite deposits occur, but is also evident in the upper and lower slopes of Paita and in the Chimbote upper slope. Deployment of a benthic chamber on the Chimbote upper slope site show no measurable release of barium; even though the dissolved barium concentration in the pore fluids is high. These observations indicate that the barite deposits associated with fluid seepage in the Peru margin are restricted to areas where slope failure has exposed sequences deep enough such that the barium-rich fluids do not encounter sulfate-bearing pore fluids before emanating at the seafloor. Keywords: Peru-Chile

Trench; barites; seepage; vents

1. Introduction with the deep submersible Nautile during the spring of 1991 (NAUTIPERC) revealed the presence of communities of clams and serpulae as well as venting fluids on the Peru margin slope (Fig. 1) [ 1,2]. Of particular interest was the discovery of deposits of barite in the form of crusts, A program

” Corresponding

of dives

author. E-mail: [email protected]

0012-821X/96/$12.00 Copyright P/i SOOl2-821X(96)00163-X

concretions and chimneys which were recovered during these Nautile dives, and subsequently sampled during R/V Some cruise 78 in 1992. The barite deposits were found along a scarp failure on the Paita middle slope as well as on the walls of the Chiclayo Canyon (Fig. 1). The barite occurs in the form of light yellow to brown concretions or chimneys, up to 15 cm in height, as well as white crusts only a couple of millimeters thick (Fig. 21, and are composed of very pure barium sulfate crystals. Scan-

0 1996 Elsevier Science B.V. All rights reserved

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and Planetary Science Letters 144 (1996) 469-481

ning electron microscopy (SEMI of the barites show a dendritic arrangement of clean rosette-type structures for chimney samples, and a concentric-layer structure for barite concretions and crusts (Fig. 3). Several episodes of barite deposition are indicated by concentric generations of the barite crystals. The porous nature of these deposits results in a low bulk density, with values ranging from 2.8 to 3.4 g/cm’, whereas the specific gravity of barite is 4.5 g/cm3. The 87Sr/ *‘Sr ratio in these deposits range from 0.710152 to 0.71 1186 [2]. In this paper we propose a mechanism of barite formation at these cold vent sites. It calls for a source of non-detrital barium sulfate which is remobilized by sulfate depletion, coupled with a low-temperature hydrodynamic regime of fluid flow through the sediments and venting of these fluids at the seafloor. If the barium-rich fluids encounter sulfatebearing pore fluids before venting, the barite deposits will form within the sediment column. Thus, this set of conditions can also explain the barite deposits previously found in sediments recovered by drilling in this margin at Ocean Drilling Program (ODP) Site 684 [3,4]. We present arguments using two lines of evidence: (1) the existence of a non-detrital barium source in the Peru margin sediments and remobilization of this barite in the zone of sulfate depletion; and (2) data on flux rates which document the contemporaneous release of barium at cold temperatures from vents in the Paita middle-slope scarp.

2. Area of study Extensive geophysical and geological surveys document the development of the continental margin off Peru (e.g., [5-91). From east to west, this margin is characterized by three distinct morphological and structural domains; namely the upper, middle and lower slopes. A scarp which parallels the strike of the slope separates the upper from the middle slope areas. This upper slope scarp is approximately 1000 m high off Chimbote and ranges from 400 to 700 m

820

800

78’W

PERU 6’

-

loos Fig. 1. Map showing the areas surveyed during the NAUTIPERC and Sonne expeditions (shaded areas), location of the stations and relevant sites drilled during ODP Leg 112. H = hydrocasts; V and VESP = deployments; D = dredges; 0 = OFOS surveys: N = barite recovered during Nautile dives; and C = kasten cores.

high in the Paita area. In both cases the scarp is thought to correspond to a major detachment fault [ 1,101. Venting of fluids was observed in association with the upper slope scarp in both the Chimbote and Paita areas, but there are no barite deposits associated with either of these sites. The middle slope off Chimbote is characterized by a relatively flat terrace; in contrast, in the Paita area, a 1000-l 200 m high second scarp can be observed at about 10 km seaward of the upper slope scarp. Multibeam, seismic surveys and submersible observations have been used to infer that this escarpment is the result of a catastrophic debris avalanche which occurred 13.8 f 2.7 kyr ago [9]. This gravity failure has exposed upper Miocene to Pleistocene sequences in the middle slope of Paita [9,10]. Large colonies of clams and serpulid worms as well as extensive barite crusts and chimneys have been observed along this middle slope scarp. The deformation front of the subduction zone marks the base of the slope. Venting of fluids has

Fig. 2. Plate showing: (A) barite deposits at a vent site in the Paita middle slope; (B) chimney NP2-3 1); (C) thin barite crust from the Paita middle slope scarp (Station SO78/ 180- 1).

sample from the Chiclayo

Canyon (Station

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been observed on a 300-500 m scarp on the Paita lower slope, as was the case with the upper slope sites, there are no barite deposits associated with fluid venting in the lower slope. Venting of fluids was also observed along the walls of the Chiclayo Canyon. This canyon is an east-northeast feature 100 km south of the Paita area. Surveys in the canyon during the NAUTIPERC expedition yielded samples of large continental metamorphic basement outcrops as well as barite chimneys from the vent sites [ 111.

3. Methods A survey of the barite distribution on the seafloor was conducted using the French submersible Nautile in 1991 as well as with OFOS (Ocean Floor Observatory System) by the R/V Some in 1992 @O/78). During the SO78 cruise, the slope and shelf areas offshore Paita and Chimbote were also surveyed with a narrow-beam subbottom profiler (PARASOUND). This surveying technique allowed for the mapping of fine layers of sediment with very high vertical and lateral resolution. Such resolution is not achievable with analog records of conventional 3.5 kHz subbottom profiling systems [ 121. Analysis of PARASOUND data has been used successfully to determine the presence of gas in marine sediments [l3]. During the R/V Sonne expedition we also occupied stations in the Paita, Chimbote and Chiclayo areas to conduct hydrocasts, deployments of a benthic chamber and sediment coring. The locations of the stations are given in Table 1. Seawater samples were taken with 5 I bottles on a Hydrobios CTDrosette system from three hydrographic stations (SO78/ 157-5, SO78/ 157-6 and SO78/ 180-2) near the sites of fluid venting and from one station (SO78/ 152-l) in the Peru Basin, which we used as a reference site. Unfiltered water samples were drawn directly into acid-leached, 1 1 polyethylene bottles and acidified with 1.0 ml concentrated HNO,. Fluid samples from three stations (SO78/168-2, SO78/177-2 and SO78/180-4) in the Paita and Chimbote vent sites were collected in a time sequence using a benthic chamber deployed with a lander (VESP). Even though venting was also observed on the Chiclayo Canyon, the steep walls of

the canyon did not allow deployments of VESP at these locations. The sampling instrument consists of a TV-controlled device for the deployment of water samplers and a CTD-probe mounted inside a barrel. The CTD-probe allows for monitoring of the temperature inside the chamber during the deployment. The bottom of the barrel is open and can be pushed into the sediments to assure a seal over the vent sites. The barrel encloses 0.238 m2 of the sediment surface and has an internal displacement volume of 284 1. Five Niskin water bottles (1.7 1) are mounted vertically around a cylindrical frame, and they are tripped sequentially by a motor located in the center of the frame. The sampling cycle is activated by the telemetry unit on board the ship. A complete description of this instrument and its operation can be found in [14]. Water samples collected in this manner were drawn from the Niskin bottles into 125 ml acidleached polyethylene bottles and acidified with 100 pl concentrated HNO,. We recovered sediment cores from two sites in the upper slope off Chimbote (stations SO78/165-3 and SO78/167-2). The sediments are very homogeneous and consist mainly of terrigenous mud with small amounts of diatoms and foraminifera. The barite deposits at the Paita middle slope and Chiclayo Canyon sites occur in areas characterized by very steep morphological gradients and by outcropping massive mudstones. Hence, no sediment cores could be obtained from the vent sites. However, we were able to retrieve a surface sediment sample during one of the deployments of the VESP at a vent site in the Paita middle slope (station SO78/180-4). Pore fluids from all sediment samples were squeezed immediately after retrieval in a temperature-controlled (4°C) titanium squeezer similar to the steel instrument described in [15]. Sediments were squeezed in a hydraulic press at pressures up to 50 kN. Interstitial water was collected from the squeezer directly into 50 ml all-plastic syringes from which various aliquots for analysis were filtered through an on-line 0.2 p,m polysulfone filter. Subsamples ( 10 ml) for barium analysis were acidified with 10 pl of concentrated HNO, and stored in acid-washed polyethylene vials. Samples from the water column, vent sites and pore fluids were analyzed for dissolved barium by isotope-dilution inductively coupled plasma (ICP)

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and Planemy

bari te San

)graph? illustrating

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pm -

-4Opm Fig. 3. SE

Scienc~e Letters

(A) the concentric

layers (Station

SO78/

180-I)

and (B) dendritic

structures (Station

NP-?-3l)of

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quadrupole mass spectrometry (MS) using a Fisions VG Plasma Quad 2+ instrument at Oregon State University. For the seawater and vent-fluid samples, a 250 ~1 aliquot of 13’Ba-enriched spike (“8Ba/ “‘Ba ratio of 0.03812) was added to an equal volume of sample and diluted twenty-fold in 0.2 N HCl. Pore fluid samples were pre-diluted in 0.2 N HCI (lo- to 50-fold); equal amounts of pre-diluted sample and ‘35Ba-enriched spike were then also diluted 20-fold in 0.2 N HCl. The procedure used for the ICP-MS measurements is that described in [16,17]. Relative standard deviations (2~) were better than 3%. Pore fluid samples were also analyzed for dissolved sulfate using a GAT/WESCAN single-column ion chromatograph at the Institute fur Meereskunde in Kiel, Germany. The procedure, described in [ 181, resulted in a precision better than 1% for measurements of IAPSO seawater standard. Selected sediment samples (200-250 g) were frozen and analyzed on-board for their total methane concentration. The method used was that described in [ 191. Samples were treated with concentrated H,PO,

Science Letters 144 (1996) 469-481

under vacuum and constant heating and stirring. The CO, in the extracted gas was removed by a KOH trap, and the CH, measured by gas chromatography. For this work we used a Shimadzu 14A FID-GC with a Porapak column and nitrogen as a carrier gas. The reproducibility of measurements achieved with this set-up was better than 3%.

4. Results VESP deployments allow for collection of temperature data, which show no indication of a hydrothermal source for the fluids. Temperatures ranged from 1.68” to 1.71”C, and in no case was there evidence for a temperature increase ( * O.Ol”C) during the duration of the deployment. The dissolved barium data for the sequential sampling of vent fluids using VESP are shown in Fig. 4. The data show changes in the barium content of the enclosed bottom water with time during two deployments in the Paita middle slope sites (stations SO78/177-2 and SO78/180-4) which are associated with barite

Table I Location of stations Station

Water depth

Location

Comments

(m) Chic,layo Canyon NP2/34-8 NP2/34-10 SO78/ 157-5 SO78/157-6 Poito Middle Slope SO78/ I 77-I SO78/ 177-Z SO78/ 180-4

6”50.4O’S 6”49.2O’S 6”49.9O’S 6”50.8O’S

8 l”23.05’W 81”23.OO’W 81”14.95’W 81”26.56’W

5Y6.3 I’S 81”38,44’W 5”36.03’S 8 l”38.58’W Y36.03’S 81”38.83’W

5”35.51’S 81”38.92’W SO78/ 180-Z Chirnhote Upper/Middle Slope ODPI 12/684 8”59.49S 79”54.35’W 9?.61’S 79”47,06’W SO78/ 167-2 _ Y35.99’S 79”54.11’W SO78/165-3 9’%.26’S 80’=07,7O’W SO78/168-2 9%.28’S 80°07.76’W SO78/ 168-3 Chimhote Lmvr- Slope 9”06.78’S 80”35.Ol’W ODPI I2,‘685 Rrf&wc~a St&m 7-04.42’S 88”27.52’W SO78/152-1

sampled by DSRV Nmtile sampled by DSRV Noutile

3785 3779 3255 4695

Vent site: barite concretion Vent site: barite concretion Hydrocast Hydrocast

3423 3350 3309

Barite crusts recovered by dredge Vent site: fluid samples and barite concretions sampled by VESP Vent site: fluid samples, surface sediment and barite concretions recovered by VESP Hydrocast

437 1293 1763 3692

Barite layers within sediments recovered by drilling [Z] Kasten core Kasten core Vent site: fluid sampling by VESP; fluid flow measured by thermistor flowmeter [13J Dredge: recovered no barites

373

1

508 I

High concentration

4146

Hydrocast

of dissolved barium in pore fluids [33]

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180-4

160 S S m”

150

I-t”

0

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Time (min) Fig. 4. Dissolved barium concentration in samples collected in a time sequence over vent sites using a benthic barrel (VESP). Two deployments in the Paita middle slope sites (Stations SO78/177-2 and SO78/180-4) show an increase in the barium content of the enclosed bottom water with time. There were no significant changes in the barium content with time during the deployment in the Chimbote upper slope scarp (Station SO78/168-2). a site devoid of barite deposits at the seafloor. The arrow demarks the barium content of bottom seawater. The location of the stations is given in Table I.

deposits. These data clearly document a contemporaneous release of barium to the bottom water at the cold-vent sites. In contrast, no change in barium concentration was observed during one deployment over a vent site at the Chimbote upper scarp site (station SO78/ 168-2). Significantly, no barite deposits were observed at the Chimbote vent sites. No barium anomaly was observed in any of the hydrocast samples along the margin (Table 1). These results are not totally unexpected because the flow rates at convergent margins are relatively low [141 and barite formation removes at least some - if not most - of the barium discharged at the vent site. Nevertheless, the water column profiles provide a background measurement of barium concentration of the bottom water, which is consistent with that measured in the VESP samples. The barium concentration measured in pore fluids of a surface sediment recovered from station SO78/180-4 is 820 nM. The dissolved sulfate and barium concentration of the pore fluids from the Chimbote sediment cores (stations SO78/ 165-3 and SO78/167-2) is illustrated in Fig. 5. The barium

415

distribution in the pore waters of sediments at site SO78/165-3 shows a small increase with depth, which probably reflects diagenetic remobilization and diffusive transport of this element. This pattern is typical for steady-state early diagenetic reactions associated with sulfate reduction [20]. In contrast, site SO78/167-2 shows a large increase in the dissolved barium content in the deeper samples. Pore fluids from this site record low sulfate concentrations and anomalously high levels of the methane in the sediments (up to 2580 ppb, Fig. 5). The barium distributions at this site are consistent with a structurally confined migration of fluids highly enriched in dissolved barium, which has been documented by drilling at this margin during ODP Leg 112 [4,21,22]. In spite of the barium enrichment in the pore fluids, no barium is currently being released at the seafloor (Station SO78/ 168-2, Fig. 4), nor were any barites recovered off Chimbote. We believe that, at this location, the dissolved barium is stripped from the pore fluids in the near-surface hemipelagic sediments, which contain sulfate, before the fluids reach the sediment surface. Hence. we postulate that barium release to the seafloor, and consequently, barite formation, are restricted to areas where deep sequences are exposed so that the barium-rich migrating fluids do not encounter sulfate-bearing pore fluids before emanating at the seafloor. as it is the case for the cold seeps in the Paita escarpment zone and in the Chiclayo Canyon.

5. Discussion It is important to determine the source of the barium in these deposits as this knowledge might help understand the mechanism of barite formation. Two scenarios are possible: (1) the barites might have been deposited in association with hot, bariumrich fluids which were discharged following the sediment slide by tapping into a hydrothermal reservoir; or (2) the barium associated with biogenic deposition in this high productivity region is remobilized in the zone of sulfate depletion, and subsequently transported to the venting site, where it precipitates upon mixing with ambient bottom water. Deposition of barite from submarine hydrothermal

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sources along the East Pacific Rise [23], the Gorda Ridge [24] and the Guaymas Basin [25,26] has been well documented. In these cases, barite is found in association with high-temperature fluids and other deposits characteristic of hydrothermal activity; namely, manganese and iron oxides, and polymetallit sulfides. Temperature measurements do not show any evidence for contemporaneous hydrothermal activity nor were there any hydrothermal deposits found in the Peru slope. Furthermore, recent precipitation is suggested by the fragility of the deposits and by the absence of any significant sediment coating on the chimneys or infillings of the void space of the crystallites in an environment characterized by extremely high sediment accumulation rates. If the barites were formed by a hydrothermal plume at the time of the sediment slide in the Paita slope (14 ka),

the high sediment accumulation rates of the Peru margin, would have rapidly buried the crystallites and in the process coated them with sediments. These observations lead us to postulate that the barium source for the barite deposits at the cold-seep sites is associated with remobilization of barite from the sediments, and subsequent transport of the barium-rich fluids to the cold-vent sites. Surface sediment samples from the slope and shelf of Peru have been analyzed to document a large increase in the concentration of barium over the detrital level [22]. This barium excess (up to 3000 ppm) is thought to result from the precipitation of barium sulfate within the water column in microenvironments of decaying biological debris [27291. The term ‘biogenic barites’ has been used to designate these deposits as a way of stressing their

Ba(PM)

Ba(PM) 0.0

100

-

200

-

0.2

0.4

0.6

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0

10

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:

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200 z s r, $ 0

300 300

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500 0 S076/165-3 I

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SO40-W

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200 twb)

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Fig. 5. Distribution of dissolved barium and sulfate in pore fluids from Stations SO78/165-3 and 167-2 located in the Chimbote upper slope. The solid rectangles represent the concentration of methane measured in sediment samples at these sites. Note the difference in barium and methane concentration scales between these two stations.

M.E. Tomes et al./ Earth and Planetan, Science titters 144 (1996) 469-481

relationship with primary productivity; however, since there is as yet no clear biogenic mechanism to explain their formation, we refer to the excess barite as ‘non-detrital’. The formation of authigenic barite deposits by

477

remobilization of barium within the sediments triggered by sulfate depletion, and subsequent reprecipitation in authigenic fronts, was proposed by Goldberg and Arrehenius [30] and restated in numerous publications (e.g., [31-331). Since this time, new

Fig. 6. PARASOUND profile from the middle slope of Chimbote, illustrating the location of core SO78/167-2. The letters A and B show the lateral distribution of the weak sediment reflections in sediments at a depth 6-8 m below the sea floor. The weak reflectors extend for approximately 1 km and are thought to represent the presence of gas-charged sediments [ 121. These results are in agreement with the high levels of methane (2580 ppb) measured at 5.01 m depth in core SO78/167-2 (Fig. 5).

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data collected during deep sea drilling at continental margins has been used to further document the remobilization of non-detrital barite at several localities in the Pacific Ocean, including the Peru margin [4]. This margin is characterized by intense biological productivity and a large barite flux to the seafloor. In these sediments, which are undergoing strong anoxic diagenesis, barite is partially dissolved in intervals depleted of interstitial sulfate, resulting in high barium concentration in the pore fluids. Core SO78/167-2, from the Chimbote slope, revealed the presence of an anomalously high barium concentration in pore fluids at the bottom of the core (Fig. 5). These results are consistent with a pattern of barium remobilization, which was documented by drilling at this margin during ODP Leg 112 [22]. Results from Leg 112 also document a tectonically driven hydrogeological regime characterized by active fluid migration [21]. The areas of fluid seepage at this margin represent the seafloor manifestation of the migration of fluids. PARASOUND surveys of the area around station SO78/ 167-2 imaged an acoustically transparent section 6-8 m below the sea floor (Fig. 6). This horizon was mapped for approximately 1 km and is thought to reflect the presence of gas-charged sediments. These data, placed in the context of the hydrogeology of this margin, suggest that the high barium and methane concentrations measured in the deepest samples from core SO78/ 167-2 are indicative of fluids which have migrated from deeper sediment sequences. The flow of fluids through the sediments provides a mechanism by which a high concentration of dissolved barium in the pore fluids is transported and subsequently discharged at the seafloor, where barite precipitates because of the high sulfate concentration in seawater. Barite deposits in the marine environment have been observed in association with oceanic fracture zones in the California Borderland [34], in the Sea of Okhotsk [35], and in the Gulf of Mexico [36]. Large masses of baritic sinter along the San Clemente fault in the California Rorderland were recovered from an area which did not show any evidence of active hydrothermal discharge 1341. Furthermore, the barite deposits do not have coatings of manganese oxides nor are they associated with massive sulfide deposits. Inference of hydrothermal discharge as the origin of these bar&es was based solely on their association

with vestimeniferan tube worms and other fauna typical of hydrothermal plumes at mid-ocean spreading centers [34]. We suggest that these deposits, as well as those from the Peru margin, are all the result of discharge of barium-rich fluids at cold seeps. Using the data from the sequential sampling by two VESP-deployments (Fig. 4) we can estimate the barium flux at the Paita escarpment site to average 23 pm01 cm-’ yr _ ’ . A diffusive barium flux from non-venting marine sediments is a thousand times smaller, based on indirect estimates [20] as well as on direct measurements using a benthic lander [37]. These fluxes, which are associated with remobilization of barium during early diagenesis are in the order of 3-50 nmol cm-’ yr-‘. The barium flux associated with fluid seeps in the Paita escarpment by far exceeds the benthic flux of non-venting pelagic sites. Using the barium flux data we attempted to estimate whether the barium venting at the cold seeps can account for the barite deposits on the seafloor. For a first-order estimate we assume that the barites cover 40% of the vent area and that the chimneys or crusts are on average 5 cm high. These assumptions are based on visual observations using the DSRV Nuutile as well as repeated video and photographic records of the sea floor at vent sites obtained with OFOS. Such a coverage requires 6 X IO” g of BaSO, per square meter. This requirement is consistent with regeneration of barite from a 500 cm deep sediment column containing about 0.5% non-detrital barite. If we further assume that the barium flux measured by the short-term benthic chamber deployments represents an average rate of discharge which has been active since the start of the vent regime off Paita (marked by debris flow avalanche 14 kyr ago) [9], then the seeping fluids would have expelled a total of 75 X 10” g of BaSO, per square meter. These rough estimates indicate that there is sufficient barium within the sediments and more than enough barium seeping through the seafloor at the cold vent sites to account for the massive barite deposits accumulating in the Paita middle slope. Apart from providing information on a not previously discussed mechanism for the formation of barite deposits, the cold-seep barites also contain long-term records of the nature and source of the advecting fluids. The barite recovered at the vent

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sites has *‘Sr/ 86Sr values which range from 0.710152 to 0.711186 [2]. This value indicates that the fluid responsible for the transport of barium to the vent site may have a continental component, consistent with the strontium isotopic composition of the pore fluids recovered by drilling at Site 685 [38]. It is interesting to note that fluids recovered from Site 688 (located farther south at 9%) show strontium isotopic values less radiogenic than seawater, indicating that this site is part of a separate fluid regime which appears to have interacted with oceanic basement [38]. Nevertheless, large dissolved barium concentrations were observed in areas with both a continental and a basaltic strontium isotopic signature [22]. The decoupling of these two parameters (barium and strontium) indicates that the high dissolved barium concentration in the pore fluids is not necessarily related to the source from which the fluids originate. Instead, the accumulation of barium in the pore fluids, a common feature throughout the Peru continental slope, is consistent with a mechanism of diagenetic remobilization of this element in sulfate depleted sediments. It is likely, then, that dissolved barium has accumulated in the sediment as a result of sulfate depletion and it is subsequently transported by advecting fluids, which have, at some point, interacted with continental crust. These observations are consistent with a large circulation cell for the fluids venting at this margin. Thus, the recharge fluids cannot come from a small-scale satellite convection around the vent, but, rather, the observed cold seeps are indeed the surface manifestation of a large-scale fluid flow regime through the accretionary margin.

6. Summary and conclusions

The Peru margin is characterized by intense biological productivity and a large flux of barite to the sediments. Remobilization of this barite in the zone of sulfate depletion leads to large concentrations of dissolved barium in the pore fluids [22]. This margin is also characterized by channelled migration of fluids within the accreted sediments [21]. Venting of these barium-rich fluids at the Paita escarpment zone and in the Chiclayo Canyon result in formation of barite deposits at the vent sites. Barium concentra-

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tion in the pore fluids of sediments drilled and cored in the slope off Chimbote are greatly elevated (Fig. 5). If the barium-rich fluids encounter sulfate-bearing pore fluids before venting, the barite deposits will form within the sediments, as has been documented at ODP site 684 [3,4]. This is a likely scenario for the Chimbote vent sites, where there are no barite deposits, nor is there any evidence of barium release from the venting fluids at this location (Fig. 4, site SO78/168-21. At the Paita middle slope, a major detachment fault exposes fluids which are flowing well below the hemipelagic sediment cover [9], along what we think are sulfate-depleted sequences. Similarly, barite deposits recovered from the Chiclayo Canyon are also associated with venting sites where deep strata are exposed. Thus, we conclude that the authigenic/massive barite deposits on the seafloor, which are not associated with hydrothermal activity, are only found in areas where tectonism has exposed deep sequences, so that fluids do not pass through the sulfate-bearing sediments before venting at the seafloor. Our hypothesis suggests the same mechanism of barite formation for the origin for the barites recovered in association with faults in the San Clemente and Sea of Okhotsk, although they were assumed to be of hydrothermal origin [34,35]. Both areas are characterized by high biogenic barium accumulation in the sediments. It is possible that fracture zones associated with the venting sites provide an escape route for over-pressured fluids. These fluids carry high concentrations of dissolved barium that has been remobilized from sulfate-free, organic-rich sediments. This is particularly so for the San Clemente deposits; where there is no temperature or chemical data to support a hydrothermal origin for the barites recovered at the vent sites.

Acknowledgements The authors wish to thank the crew members of the R/V Nadir and R/V Sonne for their helpful assistance during the expeditions to the Peru margin. We are grateful to the COAS (Oregon State University) for access to the ICP-MS and computer facilities. M. Torres is specially thankful to K.K. Falkner for her generous advice and support; and to CA.

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Ungerer and S. Moore for valuable analytical assistance. Comments by M. Kastner, A. Paytan, G. Shimmield and two anonymous reviewers greatly improved the manuscript. Financial support was provided by the Bundesministerium fur Forschung und Technologie (Contr. 03R418-9; Bonn), and the Forschungzentrum fur Marine Geowissenschaften GEOMAR (Kiel, Germany). CMILI

[ 121

[13]

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