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 concretions and chimneys which were recovered dur-

A program of dives 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,

” Corresponding author. E-mail: mtorres@oce.orst.edu

ing 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 chim- neys, 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-

0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved P/i SOOl2-821X(96)00163-X

470 ME. Torres et al. /Earth and Planetary Science Letters 144 (1996) 469-481

ning electron microscopy (SEMI of the barites show

a dendritic arrangement of clean rosette-type struc- tures 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 remo-

bilized by sulfate depletion, coupled with a low-tem-

perature hydrodynamic regime of fluid flow through

the sediments and venting of these fluids at the seafloor. If the barium-rich fluids encounter sulfate-

bearing 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 evi-

dence: (1) the existence of a non-detrital barium

source in the Peru margin sediments and remobiliza-

tion of this barite in the zone of sulfate depletion;

and (2) data on flux rates which document the

contemporaneous release of barium at cold tempera-

tures 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

6’ -

PERU

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 associ-

ated 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 sea- ward of the upper slope scarp. Multibeam, seismic

surveys and submersible observations have been used

to infer that this escarpment is the result of a catas- trophic 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 chim-

neys 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 sample from the Chiclayo Canyon (Station

NP2-3 1); (C) thin barite crust from the Paita middle slope scarp (Station SO78/ 180- 1).

M.E. Torres et ul./ Earth and Planetary Sciewe Letters 144 (1996) 469-481

A

C

472 M.E. Torres et al./Earth and Planetary Science Letters 144 (1996) 469-4NI

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 Obser-

vatory 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 occu-

pied stations in the Paita, Chimbote and Chiclayo areas to conduct hydrocasts, deployments of a ben-

thic 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 CTD- rosette 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 se- quence using a benthic chamber deployed with a lander (VESP). Even though venting was also ob- served 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 temper- ature 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 teleme-

try 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 acid-

leached 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 homoge- neous and consist mainly of terrigenous mud with

small amounts of diatoms and foraminifera. The

barite deposits at the Paita middle slope and Chi- clayo Canyon sites occur in areas characterized by

very steep morphological gradients and by outcrop-

ping 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-con- trolled (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 poly- ethylene vials.

Samples from the water column, vent sites and pore fluids were analyzed for dissolved barium by isotope-dilution inductively coupled plasma (ICP)

Fig.

bari

3. SE

te San

M.E. Tnrrrs et al./Eartll and Planemy Scienc~e Letters 143 (1996) 469%481 473

-100 pm -

-4Opm - )graph? illustrating (A) the concentric layers (Station SO78/ 180- I) and (B) dendritic structures (Station NP-? -3l)of

474 M.E. Torres et al. /Earth and Planetary Science Letters 144 (1996) 469-481

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-di- luted 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 dis-

solved sulfate using a GAT/WESCAN single-col-

umn ion chromatograph at the Institute fur

Meereskunde in Kiel, Germany. The procedure, de- scribed in [ 181, resulted in a precision better than 1%

for measurements of IAPSO seawater standard. Se-

lected sediment samples (200-250 g) were frozen

and analyzed on-board for their total methane con-

centration. The method used was that described in [ 191. Samples were treated with concentrated H,PO,

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 tem-

perature data, which show no indication of a hy- drothermal 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) dur- ing 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 Location Water depth Comments

(m)

Chic,layo Canyon

NP2/34-8 6”50.4O’S 8 l”23.05’W

NP2/34-10 6”49.2O’S 81”23.OO’W

SO78/ 157-5 6”49.9O’S 81”14.95’W

SO78/157-6 6”50.8O’S 81”26.56’W

Poito Middle Slope

SO78/ I 77-I 5Y6.3 I’S 81”38,44’W

SO78/ 177-Z 5”36.03’S 8 l”38.58’W

SO78/ 180-4 Y36.03’S 81”38.83’W

SO78/ 180-Z 5”35.51’S 81”38.92’W

Chirnhote Upper/Middle Slope

ODPI 12/684 8”59.49S 79”54.35’W

SO78/ 167-2 _ 9?.61’S 79”47,06’W

SO78/165-3 Y35.99’S 79”54.11’W

SO78/168-2 9’%.26’S 80’=07,7O’W

SO78/ 168-3 9%.28’S 80°07.76’W

Chimhote Lmvr- Slope

ODPI I2,‘685 9”06.78’S 80”35.Ol’W

Rrf&wc~a St&m

SO78/152-1 7-04.42’S 88”27.52’W

3785 Vent site: barite concretion sampled by DSRV Nmtile

3779 Vent site: barite concretion sampled by DSRV Noutile

3255 Hydrocast

4695 Hydrocast

3423

3350

3309

437

1293

1763

3692

373 1 Dredge: recovered no barites

508 I

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

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

High concentration of dissolved barium in pore fluids [33]

4146 Hydrocast

M.E. Torres et al. /Earth and Planetary Science Letters 144 (19961 469-4633 415

180-4

160

S S

m” 150

I-t”

0 10 20 30

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 contempora-

neous 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 de- posits were observed at the Chimbote vent sites. No

barium anomaly was observed in any of the hydro- cast 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 mea- sured 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

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 as-

sociated 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 sedi-

ments (up to 2580 ppb, Fig. 5). The barium distribu- tions at this site are consistent with a structurally

confined migration of fluids highly enriched in dis-

solved 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 sedi- ments, which contain sulfate, before the fluids reach

the sediment surface. Hence. we postulate that bar- ium release to the seafloor, and consequently, barite

formation, are restricted to areas where deep se- quences are exposed so that the barium-rich migrat-

ing fluids do not encounter sulfate-bearing pore flu- ids 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, barium-

rich fluids which were discharged following the sedi- ment 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 trans- ported to the venting site, where it precipitates upon mixing with ambient bottom water.

Deposition of barite from submarine hydrothermal

476 M.E. Torres et al. /Earth and Planetary Science Letters 144 (1996) 469-481

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 polymetal-

lit sulfides. Temperature measurements do not show

any evidence for contemporaneous hydrothermal ac-

tivity 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 ex-

tremely 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),

0.0

Ba (PM) 0.2 0.4 0.6

100 -

z 200 - s

r, $ 0 300 -

400 -

0

S076/165-3 500 I . I .

0 10 20 30

SO4 0-W

0 100 200 300

CH4 twb)

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 bar- ium-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 mi-

croenvironments of decaying biological debris [27- 291. The term ‘biogenic barites’ has been used to

designate these deposits as a way of stressing their

Ba (PM) 0 10 20 30

100

200

300

400

500

0 10 20 :

SO4 (mM)

0 1500 3000

CH4 @iW

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) 469-481 477

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

remobilization of barium within the sediments trig- gered by sulfate depletion, and subsequent reprecipi- tation in authigenic fronts, was proposed by Gold- berg 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).

478 M.E. Torres et al. / Earth and Planetary Science Letters 144 (1996) 469-481

data collected during deep sea drilling at continental margins has been used to further document the remo-

bilization 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 bar-

ium concentration in the pore fluids. Core

SO78/167-2, from the Chimbote slope, revealed the

presence of an anomalously high barium concentra- tion 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 hydro-

geological regime characterized by active fluid mi-

gration [21]. The areas of fluid seepage at this mar- gin represent the seafloor manifestation of the migra-

tion 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 indica- tive 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 spread-

ing 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 remobiliza-

tion 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 previ-

ously 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

M.E. Torres et al. /Earth and Planera? Science Letters 144 (1996) 469-481 419

sites has *‘Sr/ 86 Sr 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 stron-

tium 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 signa- ture [22]. The decoupling of these two parameters

(barium and strontium) indicates that the high dis-

solved 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 mecha- nism 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 obser-

vations 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 con-

vection around the vent, but, rather, the observed

cold seeps are indeed the surface manifestation of a large-scale fluid flow regime through the accre-

tionary margin.

6. Summary and conclusions

The Peru margin is characterized by intense bio-

logical 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 flu- ids 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-

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. Simi- larly, 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 recov-

ered 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 sed-

iments. 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 Univer- sity) for access to the ICP-MS and computer facili- ties. M. Torres is specially thankful to K.K. Falkner for her generous advice and support; and to CA.

480 M.E. Tomes et al. /Earth and Planetar?, Science Letters 144 (1996) 469-481

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

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