Earth and Planetary Science Letters 203 (2002) 181^194
www.elsevier.com/locate/epsl
Fluid seepage along the San Clemente Fault scarp:
basin-wide impact on barium cycling
Marta E. Torres a; , James McManus b;1 , Chih-An Huh c
a
b
College of Oceanic and Atmospheric Sciences, 104 Ocean Administration Building, Oregon State University,
Corvallis, OR 97331-5503, USA
Large Lakes Observatory, University of Minnesota, 10 University Drive, 109 RLB, Duluth, MN 55812, USA
c
Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei 11529, Taiwan
Received 30 October 2001; received in revised form 7 May 2002; accepted 28 June 2002
Abstract
Material fluxes associated with fluid expulsion at cold seeps and their contribution to oceanographic budgets have
not been accurately constrained. Here we present evidence that the barium released at cold seeps along the San
Clemente Fault zone may significantly impact the geochemical budget of barium within the basin. Barium fluxes at
seep localities on the fault scarp, measured with benthic chambers, reach values as high as 5 mmol m32 day31 . This is
the largest dissolved barium flux measured to date at a cold seep. The discharge of barium-rich fluids results in
formation of massive barite deposits along the escarpment wall. The deposits are young (approximately 8 yr) and
appear to grow at a minimum rate of 0.2 cm yr31 . This rapid growth rate requires a barium efflux rate that is about
two orders of magnitude higher than the measured dissolved flux. We believe that the discrepancy reflects a highly
localized seepage system and that chambers positioned as close as possible to the growing chimneys did not sample
the foci of fluid discharge. Transport of fine barite particles from the seeps may be responsible for excess rates of
barium accumulation throughout the San Clemente Basin, relative to other basins in the California Margin. Based on
a preliminary budget, we estimate that cold-seep barite is accumulating at the basin floor in San Clemente at a rate of
2 Wmol m32 day31 , a value that is comparable to the total barium accumulation rates driven by detrital and biogenic
components in neighboring basins. Remobilization of cold-seep barite on the basin floor adds to that driven by the
biogenic barium flux and results in benthic barium recycling rates (effluxes) within the San Clemente Basin that are as
much as seven times higher than the effluxes from surrounding borderland basins. Our estimates imply that processes
associated with fluid seepage along the San Clemente Fault significantly contribute to the basin’s barium cycle. The
strontium isotopic composition of the seep barite is significantly different from marine ‘biogenic’ barite, which is
known to accurately record seawater composition. In addition, the seep deposits are depleted in 226 Ra relative to their
modern biogenic counterparts, and are likely to be a source of radium-depleted particulate barium to the basin. Thus
the impact of barite transport from seeps on the San Clemente escarpment to the basin floor might also have
implications for the geochemistry of elements other than barium.
* Corresponding author. Fax +1-541-737-2064.
E-mail addresses: mtorres@coas.oregonstate.edu (M.E. Torres), jmcmanus@coas.oregonstate.edu (J. McManus),
huh@earth.sinica.edu.tw (C.-A. Huh).
1
Present address: College of Oceanic and Atmospheric Sciences, 104 Ocean Administration Building, Oregon State University,
Corvallis, OR 97331-5503, USA.
0012-821X / 02 / $ ^ see front matter = 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 8 0 0 - 2
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M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
= 2002 Elsevier Science B.V. All rights reserved.
Keywords: barium; cold seeps; geochemical cycle; basins; Northeast Paci¢c
1. Introduction
Fluid £ow through oceanic crust and continental margin sediments transports material that discharges onto the sea£oor in what are usually designated as cold seeps. Seeps are a general feature
of the hydrogeology of active and passive margins. These £uid systems support biological communities, alter the rocks they £ow through, re£ect
processes at depth and a¡ect the chemistry of the
ocean and atmosphere. Submarine £uid venting at
continental margins plays a signi¢cant role in
mass transfer and may have as much impact on
the global geochemical budgets as does hydrothermal £uid circulation at mid-ocean ridges (e.g. [1^
4]). The total area covered by cold seeps is essentially unknown, but the ubiquity of seepage on
continental margins is becoming apparent as additional sites of £uid expulsion are discovered.
Methane is commonly used as a geochemical
tracer of cold seepage (e.g. [5]) and its concentration has been used to evaluate £uid £ow rates [6].
Other elements, however, are also mobilized during £uid migration through the sediments. A signi¢cant release of barium from cold seeps was
¢rst documented along the Peru Margin, where
Torres et al. [7] measured a barium £ux of 0.63
mmol m32 day31 . Similarly, barium £uxes ranging from 0. 55 to 1.9 mmol m32 day31 were observed in the Cascadia Margin [8]. This barium
e¥ux often results in the formation of authigenic
barite deposits at the sea£oor, where barite supersaturation is locally achieved. Such deposits have
been recovered from the Peru Margin [7], the
Monterey Canyon [9], the Sea of Okhotsk [10],
and the Gulf of Mexico [11]. The deposits are
often quite large. More than 50 kg of barite
dredged from the Sea of Okhotsk included several
single barite blocks larger than 40 cm in diameter
[12]. Barite chimneys in the San Clemente Fault
can reach 10 m in height [13] and sites of barite
deposition were observed from DSV Alvin for
over 3 km along the fault [14]. These observations
indicate that barium discharge at cold-seep sites is
common and that continental margins could represent a signi¢cant oceanic barium source.
2. Study site
The San Clemente Basin, located approximately
100 km southwest of San Diego, is one of a series
of submarine basins distributed on a broad continental margin o¡ the coast of Southern and Baja
California (Fig. 1) [15]. The basin is divided into
distinct northern and southern sections, which are
bounded to the east and west by a series of banks
that reach depths of less than 200 m below sea
level. The banks are separated from the coastal
shelf by the 1200 m deep San Diego Trough.
Descanso Plain sediments comprise the major
body of sediments in the San Clemente Basin
[16]. Deep-sea fan deposits that formed as a result
of sediment over£ow through the Navy Channel
overlay the Descanso Plain sequences. The Navy
Fan comprises a section 100^200 m in thickness
and is characterized by turbidite sands and mud
interspersed with foraminifera and radiolariabearing hemipelagic mud [17]. Sediments in the
San Clemente Basin are deformed by the San
Clemente Fault, which cuts through the basin
along a northeast trend [18].
During our ¢eld program we acquired multibeam coverage over several active segments of
the San Clemente Fault [14]. These data were
combined with swath bathymetry from previous
cruises to compile a nearly complete coverage of a
225 km length of the fault zone [19]. Alvin dives
examined the steep-sloped scarp along the San
Clemente Fault previously described by Lonsdale
[13] and found vertical fault scarps that ranged in
height from several centimeters to a few meters.
Di¡erent ages of faulting were apparent, based
upon the degree of bioturbation and degradation
of the scarp surface [19]. The presence of chemosynthetic organisms along the fault traces and re-
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183
Table 1
Station summary
Station
1
2
6
7
10
12
13
17
19
21
22
Dive on scarp (AD 3486)a
Multicore deployed 5 km from scarp
Multicore: over the scarp
Dive on scarp (AD 3534)
benthic barrel
PC10b : at seep site
Dive on scarp (AD 3535)
benthic barrel
PC4: at seep site
PC18: 1 m from seep
PC14: backgroundc
Multicore: 10 km from seep, not on turbidite fan
Dive on scarp (AD 3536)
benthic barrel
PC3: seeps at the foot of the scarp, clam ¢eld
PC14B: backgroundc
Multicore: North San Clemente, 6 2 km from fault
Dive on scarp (AD 3537)
PC2: seep site
PC16: 1 m from seep
Multicore: North San Clemente, 6 2 km from fault
San Clemente Basin (AD 3538)
PC3: basin £oor
benthic lander
Latitude North
Longitude West
Depth
(m)
32‡13.197P
32‡13.455P
32‡13.4P
117‡43.029P
117‡46.607P
117‡42.5P
1800
1766
1766
32‡13.190P
32‡13.190P
117‡43.023P
117‡43.023P
1805
1805
32‡13.190P
32‡13.190P
32‡13.190P
32‡13.192P
32‡10.725P
117‡43.023P
117‡43.023P
117‡43.023P
117‡43.023P
117‡48.734P
1805
1805
1805
1815
1897
32‡12.748P
32‡12.748P
32‡12.748P
32‡04.177P
117‡42.334P
117‡42.334P
117‡42.338P
117‡31.048P
1848
1848
1850
1540
32‡12.847P
32‡12.847P
32‡36.0P
117‡42.375P
117‡42.375P
118‡06.0P
1817
1817
2048
32‡25.830P
32‡25.830P
118‡08.539P
118‡08.539P
1906
1906
a
AD denotes Alvin dive number.
PC designates Alvin push core.
c
Background push cores were collected with Alvin at the foot of the escarpment, s 100 m away from any sign of active seepage.
b
cent seismicity in the area [20] further established
current fault activity.
3. Methods
Approximately 3 km of the escarpment wall
was surveyed during Alvin dives in 1999 and
2000. The dive locations are shown in Fig. 1C,
and the stations are listed in Table 1. Barite deposits were recovered at active vent sites along the
3 km transect. These samples were analyzed for
their elemental composition by electron microprobe at Oregon State University. A 74 g sample
of barite (sample 7-B11) was dried, homogenized
and analyzed for a suite of natural decay series
isotopes at the Academia Sinica, Taipei. 226 Ra
and 228 Ra (via 228 Ac) were measured by gamma
spectrometry, whereas 210 Pb (via 210 Po) and iso-
topes of U and Th were measured by alpha spectrometry using 209 Po, 236 U and 229 Th, respectively,
as the yield determinants. The results showed that
the material is virtually free from uranium and
long-lived thorium isotopes (230 Th and 232 Th),
and thus 226 Ra can be readily analyzed without
interference by gamma spectrometry, based on its
photo peak at 185.99 keV. Another two samples
(7-B1 and 7-B11) were selected for analyses of
their strontium isotopic composition, following
strontium separation using cation exchange columns. Strontium isotope ratios were measured
at the University of Bochum, Germany.
To obtain barium e¥ux rates we deployed
benthic chambers using Alvin at sites of active
seepage, as well as at locations within the San
Clemente Basin where we believe the £uxes out
of the sea£oor are dominated by di¡usion (Fig.
2). These instruments are similar to those de-
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M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
Fig. 1. (A) Map of the study site showing location of the borderland basins and Descanso Plain (D. P.). (B) Perspective shadedrelief bathymetric-image of the Southern California Borderland region o¡shore San Diego, showing the San Clemente Fault.
Circled numbers indicate location of multicore stations (Table 1). Box demarcates location of the area detailed in panel C. Image
provided by C. Gold¢nger, COAS. (C) Bathymetry of the scarp wall surveyed by Alvin in 1999 and 2000. Dark shading in core
depictions indicates the presence of sand layers, which are common within Navy Fan sequences [17]. These layers are 3^5 cm in
thickness and may represent discrete pathways for £uid migration. Locations of stations listed in Table 1 are demarcated by
circled numbers.
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M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
185
Fig. 2. (A) Benthic barrel at active seep site on the sea£oor. This instrument isolates a volume of water of 186 l over an area of
0.26 m2 and allows for collection of six 1.7 l samples at pre-programmed times [21]. (B) Benthic lander at the sea£oor on the
San Clemente Basin. This instrument isolates a volume of V10 l and allows for collection of eight 10 ml samples during incubation. The exact chamber volume for each deployment is calculated by injecting a known concentration of CsCl during incubation
and subsequent ICP-MS analyses of the Cs concentration in the samples after recovery. (C) Barite deposits at the sea£oor along
the San Clemente escarpment, showing tubeworm colonies usually associated with the barite. (D) S.E.M. image of barite crystals
recovered from the San Clemente seeps suggestive of spiral crystal growth sequences, usually associated with precipitation from
low to moderately supersaturated solutions [27].
scribed elsewhere [21,22]. They permit collection
of sequentially timed water samples within a
chamber placed directly over the sediments. The
deployment periods range from 2 to 24 h. Venting
£uids were collected via 2 h benthic barrel deployments at active seeps on the escarpment wall (Stations 7 and 10). During these deployments the
instrument was positioned as close as possible to
the barite deposits and surrounding tubeworm
thickets (Fig. 2A). The ¢nal location of the instrument was constrained by the slope gradient and
the need to obtain a good seal between the bot-
tom of the chamber and the sediments. Di¡usive
barium £uxes from the basin £oor, approximately
20 km away from the fault (Station 22), were
obtained by two deployments of the benthic lander (Fig. 2B), which complement data previously
collected in this basin [23].
Once the chambers were recovered, a split of
the sample was used for dissolved gas analyses
and a second split was ¢ltered into HCl-leached
bottles and acidi¢ed with quartz-distilled HNO3 .
Methane was analyzed onboard by extracting dissolved methane into headspace and subsequent
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M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
injection into a gas chromatograph (GC) with
£ame ionization detection. For these analyses,
100 ml of water was drawn into 140 ml syringes
to which 40 ml of UHP helium was added. The
samples were then shaken on a wrist-action shaker for 5 min and the gaseous headspace was injected into the 10 cm3 loop of the GC. Barium
analyses in chamber £uids were conducted on ¢ltered and acidi¢ed samples using isotope dilution
inductively coupled mass spectroscopy (ICP-MS)
at the W.M. Keck Collaboratory, Oregon State
University, following the procedure of Klinkhammer and Chan [24].
Sediment samples at the escarpment wall were
collected with Alvin push cores at various distances from the seeps. Background samples were collected with Alvin by sampling at the foot of the
escarpment, s 100 m away from any seepage site,
as well as by multicore deployments within the
central basin (Table 1). All the cores were processed immediately ( 6 2 h) after recovery. Pore
waters were extracted by sectioning and centrifuging sediment slices (7^10U103 rpm) at in situ temperatures (4‡C), under a nitrogen atmosphere.
Pore water samples were ¢ltered (0.45 Wm ¢lter)
under a nitrogen atmosphere and the £uids were
subsampled through a three-way stopcock into
containers appropriate for the various analytes.
Dissolved sul¢des were measured spectrophotometrically at sea [25]. Total dissolved CO2 was
also analyzed onboard; sul¢des in the sample
were trapped with AgNO3 and the CO2 was measured in a coulometric cell. Pore water samples
for dissolved barium and sulfate analyses were
collected in HCl-leached bottles. Barium in the
pore £uids was measured by ICP-MS and sulfate
was determined by ion chromatography.
4. Results and discussion
4.1. Cold-seep barites
Prominent barium sulfate deposits characterize
the seeps along the San Clemente escarpment.
These deposits occur as large blocks on the sea£oor and serve, in some instances, as an anchor
for tubeworm thickets (Fig. 2C). The barites are
s 92% BaSO4 , with Sr concentrations ranging
from 0.5 to 3% [26]. The porous nature of these
deposits results in a low bulk density that averages 3.6 U 0.2 g cm33 , whereas the speci¢c gravity
of crystalline barite is 4.5 g cm33 . The crystal
morphology (Fig. 2D) suggests spiral growth se-
Fig. 3. Sr/Ba, 87 Sr/86 Sr and 226 Ra/Ba ratios measured in the San Clemente barite. Published data from seawater, sediment trap,
core tops from pelagic deep-sea sediments and other seep barite samples are included for comparison. All seep barite samples
show signi¢cant deviations in the 87 Sr/86 Sr ratios from the coeval seawater [28,32] and core top [28] samples. These deviations
are thought to re£ect the type of rock through which water £ows before venting at the sea£oor [7,9,12]. The Ra/Ba ratio measured in the San Clemente barite is also signi¢cantly lower than the ratios measured on core top samples [30] and in sediment
trap particles [31]. Transport of radium-depleted cold-seep barite particles from continental margin deposits might explain the
anomalies in the Ra/Ba ratios of particles collected in the Eastern Paci¢c at depths s 1200 m [31].
EPSL 6329 25-9-02 Cyaan Magenta Geel Zwart
M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
quences, usually associated with barite precipitation from low to moderately supersaturated solutions [27]. In contrast, barites collected from sediment trap and surface sediments underlying high
productivity zones show an ovoid and irregular
crystal structure [28], which Bertram and Cowen
[29] believe re£ects biogenic crystal formation.
Radioisotope measurements yielded activity ratios of 1.28 U 0.10 for the 228 Th/228 Ra pair, and of
0.211 U 0.004 for the 210 Pb/226 Ra pair. Assuming
that the 228 Th and 210 Pb measured in the San
Clemente barite are produced from the decay of
Ra isotopes after the formation of the barite deposit, the 228 Th/228 Ra pair yields an age of 8 U 1 yr
for the sample and the 210 Pb/226 Ra pair, 8 U 2 yr.
If we assume that the chimneys have been growing at a constant rate during the past 8 yr, and
that the outermost layer of the sample is less than
1 yr old, we can estimate a minimum growth rate
for the barite chimney of 0.2 cm yr31 . For comparison, ages obtained from a cold-seep barite
sample recovered from the Monterey Canyon suggest a growth rate for these deposits of 1 cm yr31
[9].
Despite the young age estimated for the San
Clemente sample, its 226 Ra activity (6.6 U 0.6 Bq/
g) is about one third of that measured in barite
from modern deep-sea sediments [30] and sediment trap [31] samples (Fig. 3). This discrepancy
implies that the barite at cold seeps formed in
equilibrium with a 226 Ra-depleted source. Since
226
Ra is a highly mobile species, it di¡uses easily
from sediments into bottom water, resulting in
226
Ra depletion (relative to 230 Th) in sur¢cial sediments. Thus, remobilization of radium from this
source should generate low 226 Ra activity in pore
£uids and lead to the formation of radium-depleted barite crystals.
The 87 Sr/86 Sr composition of the two samples
analyzed yielded ratios of 0.708482 U 0.000006
and 0.708358 U 0.000006, which are signi¢cantly
lighter than the modern seawater value of
0.7092 (Fig. 3) [28,32]. Similar deviations from
coeval seawater have been reported in barite samples from other cold-seep sites (e.g. [9,12]) and are
thought to re£ect modi¢cation of the strontium
composition of the migrating £uids by interaction
with less radiogenic sources such as dacitic tu¡s
187
[12] or detrital plagioclase [9]. Fluids transporting
barium to the seeps on the Peru Margin have
been modi¢ed by interactions with continental
sources, thus the seep barite there has 87 Sr/86 Sr
ratios that are more radiogenic than seawater
[7,33].
Fig. 4. Pore water data from Alvin push cores. (A) Station 7,
PC-10; (B) Station 10, PC-4; (C) Station 19, PC-16. Remobilization of barium sulfate during transport of £uids depleted
in SO4 results in high dissolved barium concentration in the
pore £uids.
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M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
4.2. Formation mechanisms
Movement of £uids through sediment sequences depleted in sulfate result in remobilization of
barium, which in turn leads to very high dissolved
barium concentrations in the pore £uids [34,35].
The dissolved barium distribution within the escarpment wall cores is high compared to typical
marine pore waters from this region ( s 500 nM,
Fig. 4). In all cores, the highest concentration
corresponds to samples at the bottom of the
core, where further penetration was prevented
by the presence of sand layers. Higher dissolved
barium concentrations are likely to occur deeper
in the sediment sequences, as observed in pore
£uids recovered by ocean drilling [35].
Expulsion of barium-enriched £uids leads to the
formation of barite deposits at the cold vent sites.
This mechanism is similar to that described for
barite deposits along the Peru Margin [7] and
Monterey Bay seeps [9]. Deposition of barite
and dense colonies of tubeworms along the San
Clemente Fault were ¢rst reported by Lonsdale
[13]. He postulated that these deposits formed as
a result of hydrothermal activity. Temperature records collected during the benthic barrel deployments show no temperature anomalies in the discharging £uids, clearly establishing that these are
indeed sites of cold-£uid seepage.
The barrel deployments also allowed us to
document currently active barium and methane
discharge at these sites, consistent with the very
young age (approximately 8 yr) of the barite deposits. By positioning the benthic barrel in close
proximity to the barite chimneys, we measured a
dissolved barium £ux of 5 mmol m32 day31 at
Station 10 (Fig. 5A). The actual barium £ux out
of these seeps, however, may be signi¢cantly higher than this value, since it is not possible to deploy
the barrel directly over the barite chimneys. The
dissolved barium £ux measured during Alvin dive
3534 (Station 7; Fig. 5B) is not accompanied by
a methane increase. These data suggest that,
although the instrument was deployed in proximity to the chimneys, in this instance it was clearly
not over a focus of active discharge. The heterogeneity of seepage along the wall is consistent
with the presence of discrete and, in some instan-
Fig. 5. (A) Change in barium and methane concentration
with time during the deployment of a benthic barrel at Station 10 on the escarpment wall. The barium £ux at this location is 5 mmol m32 day31 , which is the largest £ux measured
to date at a cold seep. (B) The small barium discharge recorded at Station 7 (0.26 mmol m32 day31 ) is not accompanied by a methane increase, suggesting that the barrel was
not deployed directly above a seep discharge site.
ces, very thin ( 6 3 cm) sand layers [17], which we
believe act as discrete horizons for £uid transport.
An alternative approach for estimating the barium £ux from the seeps is to evaluate the amount
of barium needed to form the barite chimneys. As
a ¢rst-order approximation we can use our minimum estimate for the growth rate of the San
Clemente deposits of 0.2 cm yr31 and that of
the Monterey chimneys, which are growing at
1 cm yr31 [9]. To support these fast rates of
growth, barium must be emanating from the seeps
at rates ranging from 150 to 720 mmol m32 day31
(Table 2). The discrepancy between these values
and the measured dissolved barium £ux is likely
due to the fact that the benthic barrel needs to
form a seal with the sediments and thus cannot be
placed directly over barite deposits or tubeworm
colonies. In addition, barium sulfate precipitation
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189
Table 2
Barium £uxes at cold seeps compared with e¥uxes measured in California Margin basins and in deep-sea sediments from the
Equatorial Paci¢c
Site
Flux rate measured with
benthic chambers
mmol m32 day31
Seep sites
Peru (Paita)
Cascadia (¢rst ridge)
San Clemente
Monterey Canyon
Hemipelagic and deep-sea sediments
Equatorial Paci¢c
California Margin
San Clemente Basin (Station 9, 1994)
San Clemente Basin (Station 9, 1995)
San Clemente Basin (Station 22, 2000)
0.63 U 0.15
0.55^1.9
0.26^5.0
Flux rate estimated from
barite growth rates
mmol m32 day31
s 150
V720
0.00080 U 0.00059
0.00091 U 0.0003
0.0027 U 0.0007
0.0041 U 0.0005
0.0070 U 0.0035
Source
[7]
[8]
This study
[9]
[36]
[36]
[36]
[36]
This study
is characterized by very short induction times and
a rapid growth [27], thus some of the dissolved
barium in the migrating £uids will precipitate at
the sediment^water interface, and thus it is not
sampled by the benthic chambers.
4.3. Transport of cold-seep barite to the basin
A comparison of published barium recycling
rates (e¥uxes) in the San Clemente Basin with
those from surrounding areas (Fig. 6 and Table
2) shows that the e¥ux rates in San Clemente are
as much as a factor of 4 higher than the benthic
£uxes from other borderland basins [23,36]. During the Alvin cruise in 2000, we measured a barium £ux at Station 22 of 7.0 U 3.5 Wmol m32
day31 . This value is seven times higher than the
typical £uxes from the California Margin basins,
which average 0.91 U 0.3 Wmol m32 day31 (Table
2). The high £uxes in San Clemente are not related to direct £uid venting at the basin £oor, as
there is no indication of advective £uid £ow in
this region of the basin. Nevertheless, the sum
of the barium benthic e¥ux and burial rate measured in San Clemente are two to seven times
higher than those supported by typical hemipelagic barium rain rates (e.g. [37]). In other words,
some process other than the rain of material from
the upper water column must be contributing barium to the basin inventory.
Cold-seep barite is extremely porous and frag-
Fig. 6. Barium accumulation rates and benthic £ux data obtained in 1994 (white bars) and 1995 (dark bars) at Station 9
in the San Clemente Basin (SCL, shaded area); compared to
data from San Pedro (SP), Santa Monica (SM), Catalina
(CAT), and Tanner (TB) basins and Patton escarpment (PE)
sites [23,36]. Outer basins (CAT, TB and PE) have higher accumulation rates of biogenic barium relative to the inner
borderland basins (SP, SM). In the San Clemente Basin, biogenic barium is accumulating at approximately 2 Wmol
cm32 day31 in excess of the neighboring borderland outer basins.
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ile ; indeed, we observed during our Alvin dives on
the escarpment wall that barite microcrystals
easily separate from the sea£oor deposits and
are transported as particles within the water column. We postulate that the transport of ¢ne (submicrometer) barite particles across the basin may
be responsible for the high accumulation rates of
barium throughout the San Clemente Basin. If so,
as cold-seep barite deposited within the basin
undergoes diagenetic remobilization, it may result
in an enhancement of benthic barium £uxes. Consequently, both the accumulation rate and barium
e¥ux would be signi¢cantly larger than the values
measured in other California Borderland basins,
which is what we observe (Fig. 6).
In Table 3 we summarize the data obtained
from surface sediment samples collected within
the San Clemente Basin, which can be used to
test the feasibility of this hypothesis. The barium
concentration at a site above the escarpment is
0.16%, a value that is comparable to barium concentrations measured in the Santa Catalina and
Tanner basins [36]. In contrast, cores collected
at non-venting sites at the foot of the escarpment
wall have barium concentrations of up to 0.73%.
Table 3
Sediment composition and barium acumulation rates within
San Clemente Basin
Location
Station
Ba
(%)
Al
(%)
Above escarpment
Escarpment wall
6
7-PC10
10-PC4
10-PC18
13-PC3
19-PC2
19-PC16
10-PC14
13-PC14
9a
17
21
22-PC3
12
2
CATa
TBa
0.158
41.2
28.6
19.6
0.151
8.80
1.88
0.490
0.730
0.268
0.250
0.221
0.216
0.218
0.217
0.150
0.178
0.37
0.05
0.20
0.26
0.44
0.36
0.38
0.33
0.40
6.42 0.38
0.36
0.35
0.35
0.35
0.39
6.47
3.33
Escarpment foot
Basin £oor
Santa Catalina Basin
Tanner Basin
Ti
(%)
Data for the Santa Catalina and Tanner basins are included
for comparison.
a
Stations 9, CAT and TB are from [36].
These cores have a visible abundance of white
barite particles, which were likely transported
downslope from the seeps. Within the basin, cores
collected approximately 2 km away from the fault
have 0.26% barium, whereas in the four cores
collected at distances ranging from 5 to 20 km
from the fault, the barium concentration was
0.22%. These data are consistent with a transport
of barite particles from the seeps that signi¢cantly
impacts barium accumulation within the entire
basin.
A ¢rst-order estimate for barium accumulation
rate in the basin can be obtained if we use an
average value for sediment accumulation rate in
the San Clemente Basin of 15 mg cm32 yr31 [38].
We further assumed that the detrital barium in
this basin is 0.05%, based on estimates made by
McManus et al. [36] for the outer borderland basins. With these assumptions, we estimate that
non-detrital barium in cores collected at distances
ranging from 5 to 20 km away from the seeps is
accumulating at an approximate rate of V5 Wmol
m32 day31 (Fig. 7). Similarly, the accumulation
rate at a site above the escarpment wall is estimated to be V3 Wmol m32 day31 . This value is
consistent with biogenic barite accumulation rates
of 2.8 and 3.7 Wmol m32 day31 reported for the
Santa Catalina and Tanner basins [36], where
there is no cold-seep contribution. If the excess
barium in the San Clemente Basin (V2 Wmol
m32 day31 , relative to the biogenic estimate) originates from cold-seep barite transport, the magnitude of its accumulation is approximately 40% of
the total barium £ux. These preliminary estimates
indicate that barite from cold seeps might be a
signi¢cant fraction of the total barium in surface
sediments.
To evaluate whether this estimate is reasonable,
we need an estimate of the total barite volume
along the fault. Based on our Alvin observations,
barites cover on average 5% of each square meter
of active seepage and average 1 m in height. The
barite density is 3.6 g cm33 , thus at each seep site
there is approximately 1.8U105 g BaSO4 m32 .
Approximately 2% of the fault surveyed (Fig.
1C) is covered with active seeps and thus we estimate the seepage area surveyed to be 3000 m2 . If
our survey identi¢ed only a third of the seepage
EPSL 6329 25-9-02 Cyaan Magenta Geel Zwart
M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
191
support a £ux to the basin of 6.5 Wmol m32
day31 . This £ux would be enough to maintain
barite chimney buildup and transport of particulate barium to the basin £oor. We recognize that
these estimates are highly uncertain; however, as
a ¢rst approximation they show that submarine
£uid venting can indeed support an accumulation
rate of barium in the San Clemente Basin that is
comparable to the magnitude of that driven by
the sum of the detrital (V1.4 Wmol m32 ) and
biogenic (V3 Wmol m32 ) inputs.
Pore water data from multicores deployed within the San Clemente Basin, although highly variFig. 7. Bathymetric pro¢le across the San Clemente Basin,
showing the location of the cold seeps and barite deposits on
the escarpment fault. Navy Fan sediments comprise a 100^
200 m thick section, characterized by the presence of turbidite sands, which may act as high permeability horizons for
£uid £ow. Subsurface £uid £ow through Navy Fan sequences
transports dissolved Ba, which, upon release, forms large
barite deposits at the sea£oor. First-order estimates indicate
that transport of particulate Ba from the cold-seep source to
the basin constitutes approximately 40% of the total Ba £ux.
These ¢rst-order approximations include rough estimates of
detrital Ba contribution and sedimentation rates from neighboring basins, and measured Ba content on the limited samples available. The accumulation rate estimates shown in
boxes (e.g. Babio V3) are in units of Wmol m32 day31 . We
further assumed that in cores collected above the escarpment
there is no cold-seep contribution, thus the excess Ba in the
basin represents the cold-seep £ux.
sites along the 120 km of fault that crosses the
San Clemente Basin, the total amount of barium
trapped in barite deposits would be 6.75U106
mol. If 10% of this barium is transported o¡ the
fault every year, and deposited on the ocean £oor
deeper than 1800 m, these estimates yield an accumulation rate of cold-seep barite of 3.1 Wmol
m32 day31 . This value would be su⁄cient to supply the estimated excess barium rain in the San
Clemente Basin (Fig. 7).
Can the barium £ux at the seeps support this
large £ux to the basin? If we use an average
growth rate for the barite chimneys of 0.6 cm
yr31 , the barium £ux from the seeps must be approximately 140 mmol m32 day31 . Using our previous estimates of area of active seepage (9U103
m2 ) and that of the basin deeper than 1800 m
(6U108 m2 ), the barium £uxes at the seeps can
Fig. 8. Upper panels show solid phase Ba/Al data for Station
9 in the San Clemente Basin compared with data from two
neighboring borderland basins. Middle and bottom panels
compare dissolved pore water data in the San Clemente Basin with those measured in pore £uids from the Catalina and
San Pedro basins.
EPSL 6329 25-9-02 Cyaan Magenta Geel Zwart
192
M.E. Torres et al. / Earth and Planetary Science Letters 203 (2002) 181^194
able, show in some cases values that are signi¢cantly higher than those typical for the other borderland basins (Fig. 8). This enhanced dissolved
barium concentration in pore £uids is consistent
with the impact of particulate barium transport
from cold seeps to the basin £oor, and subsequent
remobilization during diagenesis. Such enhanced
pore water concentrations are consistent with the
large barium e¥ux rates measured at this basin
(Fig. 6).
In summary, the high barium concentration in
the sediments and the large e¥ux rates in the San
Clemente Basin re£ect the input of components
associated with £uid seepage at the escarpment
zone and suggest a potential basin-wide impact
on barium geochemical processes.
Apart from the measured changes in barium
geochemistry within the San Clemente Basin, we
believe that seep barite transported from continental margins might also play a role in the behavior of other geochemically important elements.
For example, particulate transport of barium
from continental margin seeps might explain the
discrepancies in the strontium isotopic record of
barite relative to concurrent carbonate phases
documented in deep-sea sediment cores [39].
Cold-seep barite could ‘contaminate’ the isotopic
record in areas of low biogenic barite accumulation since the strontium composition of cold-seep
barites is signi¢cantly di¡erent from the modern
biogenic counterparts (Fig. 3). Thus, the contribution of these cold-seep barites could prove to be
important in sediments underlying low productivity areas, where the non-biogenic barite source
may hinder attempts to reconstruct seawater paleoceanographic conditions from barite preserved
in hemipelagic sediments.
In addition, cold-seep barite transport might
also in£uence the radium oceanic budget. The radium distribution is driven by a balance of input
from the sediments with vertical mixing, radioactive decay and removal in the particulate £ux. As
Ra and Ba have similar chemistries, an understanding of the 226 Ra/Ba distributions was once
thought to o¡er tremendous promise in unraveling deep-ocean mixing. However, after investigations during the GEOSECS project (e.g. [40,41]),
outstanding issues on the 226 Ra cycle remained
and interest in the use of this isotope as an oceanographic tracer has waned. Moore and Dymond
[31] documented the presence of a Ba source to
deep particles, which has a lower Ra activity than
that measured in shallow ( 6 1200 m) sediment
trap samples. They conclude that the transport
of ‘old’ barite from continental margins, which
have lost some Ra by radioactive decay, can explain the enrichment of Ba relative to Ra in deep
particles. The barites recovered at San Clemente
have an activity of 6.6 Bq/g, which is approximately one third of the activity measured in modern ‘biogenic’ barite [30] and one tenth of that
measured in sediment trap particulates (Fig. 3).
Thus the anomalous Ra/Ba ratios in cold-seep
barites might prove to be pertinent to the 226 Ra/
Ba relationships in deep water.
Acknowledgements
We appreciate the support at sea by the master
and crew of R/V Atlantis and the dedication and
expertise of the Alvin teams during the 1999 and
2000 cruises. Marie de Angelis (HSU) provided
the methane data. G. Bohrmann (GEOMAR)
provided the strontium isotope data. C. Gold¢nger (OSU) provided the high-resolution bathymetry and structural analyses of the San Clemente
Fault zone. We sincerely thank them for their
contribution to this manuscript. We also want
to thank B. Haley, C. Meredith, D. Hubbard,
S. Kohlbry, C. Willie, E. Heinen, B. Cumberland,
J. Watson and S. Newell for their excellent assistance at sea and with post-cruise analyses. Barium
measurements were done at the W.M. Keck Collaboratory for Plasma Spectrometry and at the
LLO plasma facility. The research was supported
by WCNURP Grant PF808254.[BOYLE]
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