Barite fronts in continental margin sediments: a new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts

INCL”D,NC

ISOTOPE GEOSCIENCE Chemical Geology

127 (1996)

125-139

Barite fronts in continental margin sediments: A new look at barium remobilization in the zone of sulfate reduction and formation of heavy barites in diagenetic fronts M.E. Ton-es ap*, H.J. Brumsack b, G. Bohrmann ‘, K.C. Emeis d a College of Oceanic and Atmospheric Sciences, Oregon State Universiv, Corvallis, OR 97331 , USA h Institutfiir Chemie and Biologie des Meeres, Carl Ossietzky Universitiit Oldenburg, D-26111 Oldenburg, Germany ’ GEOMAR, 1-3 Wishhofstrasse, D-24148 Kiel, Germany d Geologisch-Paliiontologisches Institut, Universitiit Kiel, Olshausenstrasse 40/60, D-241 18 Kiel , Germany

Received 20 September 1994; accepted 19 July 1995 after revision

Abstract Micro-crystalline barites recovered by deep-sea drilling from Site 684 on the Peru margin and Site 799 in the Japan Sea are highly enriched in the heavy sulfur isotope relative to seawater ( 84S up to + 84%0). This isotopic composition is consistent with remobilization of biogenic barite triggered by sulfate reduction, and subsequent reprecipitation as a diagenetic barite front. The high levels of barium sulfate in these deposits (lo-50%) cannot be explained by a diffusive transport model in sediments experiencing a constant rate of sedimentation. When sedimentation rates change radically, the barite front will remain at a given depth interval leading to large accumulations of barium sulfate. Such conditions may have generated the barite deposits at Site 799. At Site 684, on the other hand, there is evidence that the barite deposits are a result of the tectonically-driven advection of sulfate-bearing fluids through the sediment column.

1. Introduction

Barite deposits occur in a wide range of deep-sea locations. Their distribution, composition and correlation with sediment type has been discussed by a number of authors. Particularly good reviews are those of Cronan ( 1974) and Dean and Schreiber (1978). A variety of mechanisms, including hydrothermal, biogenie and diagenetic processes, result in accumulation of barium sulfate in the marine sedimentary environment. Barite can be formed by direct precipitation when a barium-enriched hydrothermal fluid reacts with seawater sulfate. These deposits are restricted to the vicin* Corresponding author. ISRl Elsevier Science B.V. SSDlOOO9-254 1(95)00090-9

ity of hydrothermal activity, as observed, for example, along the East Pacific Rise (Church, 1979)) the Gorda Ridge, northeastern Pacific (Koski et al., 1988) and the Guaymas Basin, Gulf of California (Koski et al., 1985; Peter and Scott, 1988). High concentrations of micro-crystalline barite (up to 30 X 50 pm in size) in sediments from the Eastern Equatorial Pacific, the Indian Ocean (Goldberg and Arrhenius, 1958) and the Antarctic convergence zone (Brahms et al., 1992) are thought to result from the precipitation of barium sulfate within microenvironments of decaying biological debris in the water column (Dehairs et al., 1980; Collier and Edmond, 1984; Bishop, 1988, Dehairs et al., 1990). Such a mechanism is supported by a relationship between upper ocean

126

M.E. Torres et al. /Chemical

Geology

127 (1996) 125-139

(4 cm

30 -

Fig. 1, a. Core photograph of an irregular vein filled with finely crystalline barite in section 112.684B-4H-6 b. SEM photographs of sample 112.684B-48-6 (at 31 mbsf), indicating the presence of barite crystals.

biological processes and barium flux to the seafloor obtained by sediment-trap studies (Dymond et al., 1992). These “bio-barites” are highly pure and average 1% in weight in the carbonate-free fraction of the sediments; maximum concentrations of 7-9% barite (carbonate free) have been reported in sediments from the Eastern Equatorial Pacific ( Arrhenius and Bonatti, 1965).

(at 31 mbsf).

There is, however, no consensus as to the origin of non-hydrothermal barite deposits which occur in layers or concretions in marine sediments. Such deposits have been reported in the Gulf of California (Goldberg et al., 1969) and in the banks of the Japan Sea (Sakai, 197 1) This type of sediment-hosted barite with no associated massive sulfides also occurs in a variety of geologic settings around the world. Examples are found

M.E. Torres et al. /Chemical

Fig. 2. SEM photomicrographs

of sample 112-684B-6H-4

in the Middle Devonian Rocks in the Appalachian Basin Valley and Ridge province of North America (Nuelle and Shelton, 1986) ; Devonian Slaven Chert in Nevada, U.S.A. (Jewel1 and Stallard, 1991), Ordovician-Carboniferous deposits of Stevens County, Washington, U.S.A. (Mills, 197 1)) Devonian carbonate rocks of northeastern British Columbia, Canada (Morrow et al., 1978) and Aptian-Albian black shales of the Vocontian basin of France (Breheret and Delamette, 1989). Several diagenetic models have been proposed for the origin of such non-volcanogenic barites. For example, Goldberg et al. (1969) suggested that barite nodules from the coast of California originated in a coastal lagoon or a shallow hydrothermal environment. In contrast, Dean and Schreiber (1978) concluded that the barite at DSDP Sites 369 and 370

Geology I27 (1996) 125-139

127

(at 49 mbsf), indicating the presence of barite crystals.

deposits formed diagenetically in sediments exposed at the sediment-water interface during long hiatuses. Still another scenario is presented by Jewel1 and Stallard ( 1991) ; they suggest that the bedded barite deposits of central Nevada formed beneath an anoxic basin in a paleo-upwelling zone. We discuss a mechanism for the formation of diagenetic barite deposits by remobilization of diagenetic barium (triggered by sulfate reduction) and subsequent reprecipitation in authigenic fronts. The development of such barite fronts - a phenomenon similar to the well-known metalliferous redox horizons in pelagic sediments - was proposed by Goldberg and Arrhenius (1958) and was restated by Brumsack (1986) as a mechanism leading to the formation of barite deposits at chemical boundaries and in lag deposits. Sulfur iso-

128

M.E. Torres et al. /Chemical

Geology 127 (1996)

125-139

Fig. 3. SEM photomicrographs of barite crystals from two layers recovered at Site 799. The upper panel corresponds 2 (569 mbsf) and the bottom photographs are from section 12%799A-36X-5 (at 323.2 mbsf).

to section 128-799B-14R-

M.E. Tomes et al. /Chemical

Geology 127(1996)

12.5-139

129

Fig. 4. Location of areas along the Circum-Pacific where diagenetic barite deposits have been observed or inferred based on pore water and sediment analysis (Table 1). Dissolved barium and sulfate profiles at these localities are shown in Fig. 5.

topic data from marine barite deposits from the Peru margin and the Japan Sea are used to support a diagenetic model for the origin of sedimentary barite deposits enriched in s4S.

2. Diagenetic barites Micro-crystalline diagenetic barites have been recovered by deep sea drilling from Site 684 on the Peru margin and Site 799 in the Japan Sea. Both deposits have similar characteristics with respect to the host lithology, sedimentation rates and complete sulfate consumption by organic matter diagenesis. A good description of the sedimentology and geochemistry of these sites can be found in Suess et al. (1988) and in Ingle et al. ( 1990). Site 684 is located in a small sediment pond on upper slope of the Peru margin beneath the seaward trail of frequent upwelling plumes that cross this shelf. Authigenie barite was observed at two depths in the sediment column of this site (Suess et al., 1988) : (1) at - 31 metres below seafloor (mbsf) , finely crystalline barite occur as filling of a small vein (sample 684B-4H-6, 354 1 cm; Fig. 1A) ; and (2) at 49 mbsf coarse barite crystals were observed in a phosphatic-glauconitic

mud (sample 684B-6H-4, 30 cm). The concentration of barium sulfate at 31 mbsf ranges from 10 to 35%. X-ray diffraction (XRD) analysis of samples from this vein confirmed the occurrence of barite, as well as revealed the presence of clay minerals, gypsum, quartz, feldspar and calcite. Scanning electron microscope (SEM) photographs of the vein samples show a dense filling of long barite needles 2-5 pm in diameter and 10-50 pm in length (Fig. 1B). Deeper in the core, at 49 mbsf, the concentration of barium sulfate ranges from 30% to 50%. Under the SEM, a sample from this barite occurrence revealed large crystals (30-60 pm), that often contain various impurities in small quantities, particularly biogenic opal and clay minerals (Fig. 2), Only the smaller barite crystals show euhedral shapes; the rounded nature of the larger crystals suggests that these crystals are very fragile, since the rounding might have resulted from shaking of the sample prior to the preparation of the SEM slides. Site 799 is located in the Kita Yamato Trough in the south-central Japan Sea. Barite at this site occurs as a 2.5-cm layer in section 799A-36X-5 at 323.2 mbsf and as a 0.8-cm layer in section 799B-14R-2 at 569 mbsf. XRD analysis of samples from these layers show that biogenic opal and barite are the only two minerals present in significant quantities. SEM studies revealed the

130

M.E. Torres et ul. /Chemical

Geology 127 (1996) 125-139

Peru Margin Barium (PM)

Barium (PM)

688 2oou

0

10

20

Sulfate

(mM)

0

30

Sulfate

(mM)

10

Sulfate

20

(mM)

Gulf of California Barium (FM)

Sulfate

(mM)

Japan Sea Barium (PM)

Barium (PM)

0

10

Sulfate

20

(mM)

30

0

10

Sulfate

Barium (PM)

20

(mM)

Fig. 5. For caption, see p.131.

0

10

20

Sulfate

(mM)

30

ME. Tomes et al. /Chemical Geology I27 (1996) 125-139

Astoria fan

and smooth appearance. The concentration of barium sulfate in this layer is N 13%. Based on the composition of the pore fluids, we have also inferred the possibility of diagenetic barite deposits in the Gulf of California and the northeast Pacific margin (Fig. 4). All of these settings coincide with zones of high productivity of biogenic opal. This observation is not unexpected since the development of barite fronts requires a labile barium source which is not likely to be elastic detritus but rather the large “bio-barite” flux observed in areas of high biogenic opal productivity (Dehairs et al., 1980, 1990; Collier and Edmond, 1984; Bishop, 1988; Dymond et al., 1992).

Barium (NM)

0

10 Sulfate

20

131

30

3. Diagenetic redistribution

(mM)

Aleutian Trench f3arlum (vM)

Sulfate

(mM)

Barlum (PM)

Sulfate

(mM)

Fig. 5. Dissolved barium and sulfate distribution profiles showing barium remobilization in the zone of sulfate reduction. The armws indicate areas where barite deposits have been identified or inferred from chemical analysis of the sediments. These occurrences are listed in Table 1, The shaded areas highlight sections where sulfate concentrations increase below the sulfate reduction zone. Data for the Peru margin sites are from von Breymann et al. ( 1990); the Gulf of California, from Bmmsack and Gieskes ( 1983); the Japan Sea, from Brumsack et al. (1992) and von Breymann et al. (1992); and the Astoria fan and Aleutian trench, from Waterman et al. ( 1973).

presence of small barite crystals 3-9 pm in length in association with opal-A (Fig. 3). Energy-dispersivexray analysis (EDAX), in conjunction with the SEM, was used to confirm barium and sulfur as the main constituents of these crystals. The layer at 323.2 mbsf occurs within a fairly homogeneous diatomaceous ooze of Pliocene age. It has a light-green color and a fine

Deposition of ‘‘bio-barite” microcrystals on the seafloor provides a source of labile barium, which can be subsequently remobilized within the sediment column, in the zone of sulfate depletion. As a consequence, continental margin settings characterized by high accumulation rates of opaline silica and by organic matter decomposition via sulfate reduction, usually result in high concentration of dissolved barium in the pore fluids below the zone of sulfate depletion. Fig. 5 summarizes data available for dissolved barium and sulfate concentrations from several localities in the Pacific Ocean (given in Table 1) , results that clearly illustrate the remobilization of labile barite in these environments. 3.1. Isotopic evidence During sulfate reduction, bacteria preferentially metabolize sulfate containing the lighter 32S isotope (Mizutani and Rafter, 1973; McCready and Krouse, 1980), and thus the residual sulfate becomes progressively enriched in the heavy 34S isotope (Goldhaber and Kaplan, 1980). Hence, the diagenetic barites formed within the sediment column, and constituting the barite front, should be isotopically heavy relative to seawater. In contrast, the barites formed by precipitation of hydrothermal barium-rich fluids at the seafloor, show 634S-values not significantly different from seawater (Koski et al., 1988). In order to use this information as supportive evidence for the diagenetic origin of the barite, we have

132

M.E. Tomes et al. /Chemical

Table 1 Barite occurrences fate depletion IDa

Site

in areas of barium mobilization

Location

Criteria

triggered by sul-

Barite

Source

(%) a

685

Peru margin

excess barium”

0.10

b

688

Peru margin

excess barium

0.15

C

684

Peru margin

XRD

d

684

Peru margin

XRD, SEM

e

E17

Gulf of California

excess barium

> 0.05

f

798

Japan Sea

excess barium

0.03

g

799

Japan Sea

excess barium

0.3

i

799 796

Japan Sea Japan Sea

XRD, SEM excess barium

j

k

1

174

178

Astoria fan

Aleutian trench

XRD

XRD

10-35

13 0.25

2.1

2-22

von Breymann et al. (1990) von Breymann et al. ( 1990) Suess et al. (1988) Suess et al. (1988), this study Brumsack (1986) von Breymann et al. (1992) von Breymann et al. (1992), this study this study von Breymann et al. (1992) Kulm et al.

Geology 127 (1996) 125-139

filtration and drying reacted with V20, in sealed glass ampoules at 1OW’C. In all cases, the SO2 was analyzed on a Finnigan@’ MAT 251 mass spectrometer at the Geochemisches Institut at the University of Gottingen. The reproducibility was better than f 0.2%. All values are given as the deviation (in parts per thousand) relative to the Canyon Diablo Troilite (CDT) standard. No pore fluid samples from the Peru margin were available for these analysis. The sulfur isotopic composition for dissolved sulfate for Sites 798 and 799 in the Japan Sea is given in Table 2 and plotted vs. depth in Fig. 6. This figure illustrates the enrichment of the sulfate reservoir in the heavy 34S isotope. The sulfur isotopic distribution of the Japan Sea fluid samples and its relationship to sediment accumulation and sulfate reduction rates has been discussed by Brumsack et al. ( 1992). Here it suffices to state that precipitation of authigenic barites from diagenetically remobilized barium with sulfate from this residual pool should be isotopically heavy. Indeed, the Japan Sea and the Peru margin barite deposits are highly enriched in the heavy sulfur isotope relative to seawater ( S34S up to + 84%0 rel. CDT). The data, presented in Table 3, also include sulfate isotope values available in the literature for other non-volcanogenic marine bar&es. 3.2. Diagenetic model Using the concentration of dissolved barium in pore fluids from the Peru margin and the Japan Sea, we can

(1973). Dean and Scbreiber (1978)

Table 2 Sulfur isotopic composition

Kulm et al.

Site

Depth (mbsf)

8?S (%, vs. CDT)

SO, (M)

798

1.25 1.85 2.77 4.00 5.75 12.85 23.75 36.96 2.65 16.50 16.55 27.75 35.85 43.90

+ 29.6 +33.1 +35.0 +43.6 +52.8 n.a. na. n.a. +41.4 +71.7 n.a. n.a. n.a. n.a.

25.1 24.4 21.5 17.7 12.0 0.3 0.1 0.0 19.6 2.9 2.9 0.1 0.0 0.0

(1973). Dean and Schreiber (1978) aThe location of the barite within the sediment column is identified by letters a through 1 in Fig. 4. ‘When barite was not identified by XRF or SEM, the presence of a barite front was inferred based on an excess of barium over the background detrital level.

799

analyzed pore fluids and authigenic barites from the Peru and Japan sites for their sulfur isotopic composition. For this analysis, the sulfate was trapped as barium sulfate and converted to HPS by a tin(II)-phosphoric acid reaction (Kiba and Kishi, 1957). The liberated H,S was trapped as CdS, converted to Ag,S and after

of pore fluids at Sites 798 and 799

mbsf = meters below sea floor; n.a. = not analyzed.

M.E. Torres et al. /Chemical

0

20

40

60

80

30

0 Site 799 40

1 0

10

20

30

0

SO4 VW

,

I

.

I 20

10

40 30

SO4 WV

Fig. 6. Downcore profiles of dissolved sulfate. and its sulfur isotopic composition in the pore fluids from Sites 798 and 799. The arrows denote seawater values. Table 3 Sulfur isotopic composition

of marine diagenetic

barites

Location

S?S (%o vs. CDT)

Source

Peru margin (684-6H-4, 80 cm) Japan Sea (799-36X-5, 23-26 cm) Japan Banks Kaipoara Harbor, NZ

+ 83.8

this study

+58.4

this study

California

borderlands

+54 sakai (1971) +45.7 to + 57.9 Rafter and Mizutani (1967) + 74.5 to + 78.5 Goldberg et al. (1969)

describe the diagenetic redistribution of barium in terms of the dissolution of a primary mineral, its migration and reprecipitation at a different horizon in the sediment column. If the rate of dissolution and precipitation are controlled by the transport of dissolved barium via molecular diffusion, then me processes can be modeled by using Fick’s second law of diffusion (Nielsen, 1961; Berner, 1980). On the other hand, if the rates of crystal growth and dissolution are limited by surface reactions, then it is necessary to determine and quantify the rate-controlling step at the mineral’s surface. In this case, a simple calculation of the time of

Geology 127(1996)

125439

133

formation of concretions and layers is possible only when the rate laws and rate constants for the various processes are known. There is indeed evidence to suggest that the dissolution of barite occurs by surface controlled processes (Berner, 1978)) as well as by the mediation of bacterial activity (Bolze et al., 1974). Nevertheless, the use of a diffusion controlled mechanism does make possible the calculation of an upper limit for rates of diagenetic redistribution, and thus we consider this approach useful for a semiquantitative evaluation of the effects of various parameters in the possible formation of authigenie barite layers. The model used is based on that developed by Bemer ( 1980) for the remobilization of iron and manganese (Fig. 7). Dissolved barium is almost totally removed by barium sulfate formation above the zone of sulfate depletion (z= 0). If the dissolved barium in the pore fluids is derived from the dissolution of scattered fine particles of barite in the sulfate-depleted zone, and if both the dissolution and reprecipitation are diffusion controlled, then a depletion zone of soluble barium is formed below the zone of sulfate depletion and the concentration profile would be linear across this zone. The barium flux Jo at z =0 is given by equation (Berner, 1980) : J=_D”(C 0

-C)

L

-

(1)

O

Sediment-water

Dissolved barium

Fig. 7. Schematic representation of formation of a monolayer deposit in a “barite front” as the result of diffusion-controlled port of diagenetically remobilized barium.

barite trans-

134

M.E. Torres et al. /Chemical Geology 127 (1996) 125-139

Table 4 Parameters Site

used in estimating Sedimentation (m Myr-‘)

685’ 798” 799”

100 120 127

the formation rate

of barium sulfate layer in a diagenetic front driven by diffusion controlled transport

Geothermal (“C km-‘)

gradient

Porositya

30 111 98

Diffusion coefficien? ( IO~crr?s-r)

Asymptotic

(%) 75 12 15

4.3 6.2 6.0

500 800 1,300

“Average values for the upper 300 mbsf. “The diffusion coefficient was estimated at 200 m, using the site specific geothermal McDuff and Gieskes ( 1976). ‘Data for Site 685 is from Stress et al. ( 1988). dData for Sites 798 and 799 is from Ingle et al. ( 1990).

where L is the thickness of the zone of sulfate depletion; C, is the asymptotic concentration of dissolved barium at z > L, which might or might not be the equilibrium value; 4 is the porosity; D, is the bulk sediment diffusion coefficient; and C,, is the concentration at z = 0. We assume that the linear profile between z = 0 and z = L represents a steady state, that there is no sulfate present below z = 0, and that the porosity remains constant in this zone. Furthermore, the adsorption of barium and the flow of water due to compaction or externally impressed flow are assumed to be negligible. Under these conditions, the thickness of the barite layer (A) and the zone of depletion (L) will vary with time in accordance to the relationships (Berner, 1980) :

$=%(C&- C”)

(2)

$=$+,-Co)

(3)

d

where u represents the molar volume of barite (52 cm3 mol - ’) ; and F, and Fd are the volume fractions of the precipitating and dissolving barite in bulk sediments, respectively. Combining Eqs. l-3 we may estimate the time necessary for growth of the barite layer (t) , by: t=

gradient and the relationships

concentration

(pm)

of Khnkenberg

( 1951) and

and Gieskes ( 1976). In order to look at a realistic situation, we have selected the parameters for porosity and C, observed in holes drilled in the Japan Sea (Site 799) and the Peru margin (Site 685). These values, together with the estimates of the diffusion coefficient are given in Table 4. The amount of barium available for remobilization has been chosen to vary between 100 and 5000 ppm; values that correspond to the range of concentration of “biogenic” barium in productive areas of the Pacific Ocean (Goldberg and Arrhenius, 1958; Gurvich et al., 1978; M.E. Torres, unpublished data, 1994). We then obtain a first-order estimate of the time necessary to produce a layer of barite ranging from 0.1 to 2 cm in length; as illustrated in Fig. 8. It is apparent, that even for the best-case scenario of Site 799 it takes a rather long time to grow a layer 1 cm thick. During this time it is likely that the horizon of barite precipitation would have moved downwards in the sediment column due to sediment deposition. The sedimentation rate (w) at Site 799 is on the order of 100 m Myr- ‘. The mechanism of diagenetic redistribution will result in an enrichment of barium at z=O, which may be estimated by: BaSO, enrichment

(%) = lOOh/ [h + ( wt) ]

h2F$ 2$‘F,uD,( ccc - c,)

(4)

In order to solve Eq. 4 we must know the value for the bulk sediment diffusion coefficient, D,. This parameter has been estimated at each site using the value for the diffusion coefficient for barium in seawater and correcting it for tortuosity and temperatureeffects using the relationships of Klinkenberg ( 195 1) and McDuff

We have estimated this enrichment in the Japan Sea and the Peru margin sites by assuming values of biogenie barium in the solid phase of 1000,400 and 600 ppm for Sites 799, 798 and 685, respectively. We are not able to model the diagenetic distribution at Site 684 since there are no pore water samples available for barium analysis. Fig. 9 illustrates the barium enrichment as a function of time, and shows that at approxi-

h4.E. Tomes et al. /Chemical

Site

Geology 127 (1996) 125-139

13.5

constant sedimentation rate and no advection of fluids, diagenetic redistribution of barium cannot account for the formation of barite deposits such as those found at Sites 799 and 684.

665

3.3. Mechanisms for barite accumulation

20

10

Layer thickness

(cm)

500

ea = 100

Site

799

Ba = 400

0

2

1 Layer thickness

3

(cm)

Fig. 8. Changes in the size of a barite layer with time for conditions at Sites 685 and 799. The parameters used are given in Table 4, the amount of barium available for remobilization has been chosen to vary between 100 and 5000 ppm.

mately steady state, it will correspond to 450, 230 and 280 ppm for Sites 799,798 and 685, respectively. Even though no barite horizon was identified by microscopy at Sites 798 and 685, chemical analysis of tne sediments by neutron activation (von Breymann et al., 1990; von Breymann et al., 1992) revealed an enrichment of barium over the detrital values at depths corresponding to the bottom of the zone of sulfate depletion (Fig. 10). This enrichment is in the order of 300 ppm at Site 798 which is in agreement with the estimated value from the diagenetic redistribution model. At Site 685, the observed enrichment is - 1000 ppm; a value that is higher than predicted from the calculation. More significantly, the barium concentration at Site 799 is -7.7%, and in the sand layer at Site 684 it reaches values as high as 30%. The above calculations show that under steady-state conditions, with

Several mechanism can enhance the accumulation of barite in diagenetic fronts. During periods of reduced sediment accumulation the “barite front” would remain at a fixed depth relative to the sediment-water interface, allowing for the formation of larger deposits or concretions. A genetic link between pauses of sedimentation and barite crystallization was evoked by Dean and Schreiber ( 1978) to explain the barite occurrence in Upper Albian black shales at DSDP Site 369 (off Cape Bojador). Similarly, Breheret and Delamette ( 1989) suggest an early diagenetic trapping of barium sulfate during pauses in the rate of sedimentation as the mechanism responsible for the formation of Mid-Cretaceous barite nodules in the Vocontian basin (SE France). Another mechanism that can promote large accumulations of barium sulfate, is the advection of sulfatebearing fluids from deep within the sediment column. Large-scale fluxes of fluids within the sediment sequences are common occurrences in convergent margin settings (Gieskes et al., 1990; Kastner et al., 1990;

Time

Fig. 9. Barium enrichment text for discussion.

(Kyr)

vs. time for Sites 799, 798 and 685. See

136

M.E. Torres et ul. /Chemical

Non-detrital barium (ppm)

Geology 127 (1996) 125-139

Non-detrital barium (ppm)

0

50

f g r, % n

100

150

I0 200

Fig. IO. Non-detrital

Site 665

Iq

barium profiles for Sites 798 and 685. Data and details on the calculation

of non-detrital component

are from van Breymann

et al. ( 1990, 1992).

Kulm and Suess, 1990). Movement of fluids with extreme chemical composition through the sediment package profoundly influences the diagenetic processes in these environments (Paul1 and Newmann, 1987; Ritger et al., 1987). Sulfate-bearing fluids in deep sequences - below the sulfate reduction zone - have been observed in various convergent margins. Examples of such anomalies in the Astoria fan (Site 174), the Aleutian trench (Site 178) and in the Peru shelf (Sites 680 and 681) are shown in Fig. 5 and Fig. 11. Estimates of sedimentation rates at Site 799 are based upon magnetic reversal stratigraphy and selected biostratigraphic datum levels (Ingle et al., 1990). Unfortunately, diagenetic alteration of the microfossils severely limited biostratigraphic age assignments within lower Upper Miocene through Lower Miocene sediments at this site. Only three microfossil datum levels and nine magnetic reversals constrain estimated rates of sedimentation in Quaternary through Upper Miocene sediments at Site 799 (O-440 mbsf) Within this interval, sedimentation rates vary between 15 and 175 m Myr- ’ (Ingle et al., 1990). The barite layer at this site was recovered at 323.2 mbsf, within a sediment sequence of particularly poor biostratigraphic control. Fig. 8 shows that for the conditions of this site, and assuming a biogenic barium input of 1000 ppm, a 2-

SO4VW

Fig. 11. Sulfate distribution in pore fluids from Peru shelf Sites 680, 681 and 684. Data from Suess et al. ( 1988). Sulfate concentrations are expected to also increase at Site 684 below the recovered section, based on a decrease in the methane concentrations at depth. The increase in sulfate at depth reflects the presence of a southward flowing subsurface brine documented by Kastner et al. (1990) and Martin et al. ( 1993). A supply of dissolved sulfate from the migrating fluids is though to drive the precipitation of barium sulfate along the fluid path.

M.E. Tomes et al. /Chemical Geology 127 (1996) 125-139

cm barite layer could have developed in - 84 kyr. Detecting such a short hiatus is not possible with the available data. However, since there is no evidence of fluid advection, soft-sediment deformation or anoxic events in the sediments from this site we postulate that a small pause in sedimentation acted in concert with high biogenic input of barium and its remobilization in a sulfate depleted zone to generate the diagenetic barite front observed at 323 mbsf. In contrast, Site 684 is characterized by the presence of a subsurface brine, which results in large systematic downhole increases in salinity, chloride, calcium and magnesium (Suess et al., 1988). Sulfate reduction proceeds rapidly at this site, the zone of sulfate depletion extends from 36 mbsf to the bottom of the hole. However, sulfate concentrations are expected to increase at depth due to the influence of the brine. Such an effect is suggested by the decrease in methane concentrations towards the bottom of the hole (Suess et al., 1988). Analysis of the distribution of major ions in the pore fluids recovered from other sites drilled on the Peruvian shelf further document the presence of the subsurface brine (Kastner et al., 1990; Martin et al., 1993). Based on the pore fluid data, Kastner et al. ( 1990) proposed a southward migration of hypersaline fluids which have been flowing at a rate of - 3-4 cm yr- ’since the Early Miocene. They have also shown that Site 684 is the closest of the drilled sites to the brine source. In an environment characterized by high accumulation rates of opaline silica and biogenic barium (Suess et al., 1987; von Breymann et al., 1993)) the hydrogeochemical regime described by Kastner et al. ( 1990) provides an ideal setting for barite formation. In this scenario, biogenic barium is remobilized in the zone of sulfate depletion; a supply of dissolved sulfate from the subsurface brine will then result in reprecipitation of barium sulfate along the fluid path. It is thus not surprising that the barite deposits at this site were found as vein fillings and in a sandy layer: both are high-permeability structures within the sediment package. The presence of halite and gypsum observed in association with the barite at this site (Fig. 1) is consistent with the idea of barite precipitation driven by the brine flow. This scenario may well be responsible for the genesis of veintype barite occurrences in Nevada reported by Papke ( 1984) and Berg ( 1988). Fig. 5 shows an increase in the concentration of sulfate at depth in Sites 796 (Japan Sea) and 174 (Astoria

137

fan). Even though no barite layers have been documented at these sites, these localities may represent other environments where barite may be forming due to the advection of sulfate-rich fluids. Site 178 (Aleutian margin) also shows dissolved sulfate anomalies. The barite deposits (up to 22% BaSO,) reported in the drilled sequences at this site (Fig. 5; Kulm et al., 1973; Dean and Schreiber, 1978) may be associated with the presence of an Upper Miocene hiatus between 3 18 and 358 mbsf (Ingle, 1973; Waterman et al., 1973). However, Linke et al. ( 1995) have recently reported barite crusts in sediments from the Aleutian subduction zone associated with sites of fluid venting at this margin. These deposits are consistent with barite precipitation driven by fluid flow in this convergent margin.

4. Summary and conclusions We have documented a diagenetic mechanism in which remobilization of biogenic barium in sulfate depleted zones, and subsequent reprecipitation, results in accumulation of authigenic barite in several continental margin settings. Authigenic barites are likely to occur in areas where intensive high productivity results in a large flux of biogenic barium to the ocean floor (von Breymann et al., 1993). This labile bio-barite is partially dissolved at depth intervals depleted of interstitial sulfate in sediments undergoing strong anoxic diagenesis. In this scenario, the precipitation of barite involves sulfate ions which come, at least partially, from a reservoir that has been enriched in the heavy 34S isotope by selective consumption of the lighter isotope during bacterial oxidation of organic matter. Thus, the barites will be isotopically heavy relative to seawater; such heavy barites (up to + 84%0 rel. CDT) were recovered by deep-sea drilling in the Japan Sea and the Peru margin. In the absence of advective flow, upward barium diffusion to the sulfate reducing zone leads to the in situ formation of crystalline barite in these sediments. Barite precipitation is thought to occur in that section of the sediments where there is sufficient sulfate, generally supplied by downward diffusion, to cause supersaturation with respect to this mineral. A simple diagenetic model has shown that large accumulations of this mineral cannot form under steady-state conditions of constant sedimentation rate. When sedimen-

138

M.E. Tomes et ul. /Chemical

tation rates change radically, an authigenic barite front will remain at a given depth interval; a condition that can lead to larger barite deposits or concretions. Such a mechanism is likely to have generated the barite deposits at Site 799. In contrast, the presence of a subsurface brine coupled with evidence of fluid flow within the sediments of the Peru margin shelf, provides a source of sulfate and a mechanism for precipitation of barite along the paths of fluid flow at Site 684. The diagenetic barites described here can be considered as modern analogs to ancient rocks with similar lithological and environmental affinities such as the deposits described by Papke ( 1984)) Berg ( 1988)) and Breheret and Delamette ( 1989).

Acknowledgements

Financial support for this work was provided by a grant (No. Br 1334/l-l) from the Deutsche Forschungsgemeinschaft (DFG) and the Forschungzentrum fur Marine Geowissenschaften GEOMAR. We also wish to acknowledge support from the College of Oceanic and Atmospheric Sciences (Oregon State University). Discussions with Robin Keir were extremely valuable for the diagenetic model calculations. We are grateful for the comments and suggestions provided by Paul Jewel1 and by an anonymous reviewer which significantly improved the manuscript.

References Arrhenius, G. and Bonatti, E., 1965. Neptunism and volcanism in the ocean. In: M. Sears (Editor), Progress in Oceanography, Vol. 3. Pergamon, London, pp. 7-22. Berg, R.B., 1988. Barite in Nevada. Mont. Bur. Mines Geol., Mem. 61. 100 pp. Berner. R.A., 1978. Rate control of mineral dissolution under earth surface conditions. Am. J. Sci., 278: 1235-1252. Berner. R.A., 1980. Early Diagenesis - A Theoretical Approach. Princeton University Press, Princeton, N.J., 241 pp. Bishop, J.K.B., 1988. The barite+pal+rganic carbon association in oceanic particulate matter. Nature (London), 331: 341-343. Bolze, C.E., Malone, P.G. and Smith, M.J., 1974. Microbial mobilization of barite. Chem. Geol., 13: 141-143. Brahms, CC., Bohrmann, G., Schliiter, M. and Rutgers v.d. Loeff, M.. 1992. Biochemical cycle of barium in the South Atlantic. Forschungszent. Mar. Geowiss., Kiel, GEOMAR Rep., 15: 76. Breheret, J.G. and Delamette, M., 1989. The mid-Cretaceous barite nodules of the Vocontian domain (SE France) as markers of

Geology 127 (1996) 125-139

basinal discontinuities within marly deposits, C.R. Acad. Sci. Paris, Ser. II, 308: 1369-1374 Brumsack, H.J., 1986. The inorganic geochemistry of Cretaceous black shales ( DSDP Leg 41) in comparison to modem upwelling sediments from the Gulf of California. In: C.P. Summerhayes and N.J. Shackleton (Editors), North Atlantic Paleoceanography. Blackwell, London, pp. 447-62. Brumsack, H.J. and Gieskes, J.M., 1983. Interstitial water tracemetal chemistry of laminated sediments from the Gulf of California, Mexica Mar. Chem., 14: 89-106. Brumsack, H.J., Zuleger, E., Gohn, E. and Murray, R.W., 1992. Stable and radiogenic isotopes in pore waters from Leg 127, Japan Sea. In: K.A. Pisciotto, J.C. Ingle, Jr., M.T. von Breymann, J., Barron, et al. (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 127/ 128 (Part 1). ODP (Ocean Drill. Prog.), College Station, Texas, pp. 635-650. Church, T.M., 1979. Marine barite. In: R.G. Burns (Editor), Marine Minerals. Mineral Sot. Am., Rev. Mineral., 6: 170-210. Collier, R.W. and Edmond, J.M., 1984. The trace element geochemistry of marine biogenic particulate matter. Prog. Oceanogr., 13: 113-199. Cronan, D.S., 1974. Authigenic minerals in deep sea sediments. In: E.D. Goldberg (Editor), The Sea, Vol. 5. Wiley, New York, N.Y., pp.491-498. Dean, W.E. and Schreiber, B.C., 1978. Authigenic bar&e, Leg 41 Deep Sea Drilling Project. In: Y. Lancelot, E. Seibold, et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Vol. 41. U.S. Gov. Print. Off., Washington, D.C., pp. 915-931. Dehairs, F., Chesselet, R. and Jedwab, J., 1980. Discrete suspended particles of barite and the barium cycle in the ocean. Earth Planet. Sci. Lett., 49: 528-50. Dehairs, F., Goeyens, L., Stroobants, N., Bernard, P., Goyet, C.. Poisson, A. and Chesselet, R., 1990. On suspended barite and the oxygen minimum in the Southern Ocean. Global Biogeochem. Cycles, 4: 85-102. Dymond, J., Suess, E. and Lyle, M., 1992. Barium in deep sea sediment: a geochemical indicator of paleoproductivity. Paleoceanography, 7: 163-181. Gieskes, J.M., Vrolijk, P. and Blanc, G., 1990. Hydrogeochemistty of the northern Barbados accretionary complex transect: Ocean Drilling Project Leg 110. J. Geophys. Res., 95: 880998819. Goldberg, E.D. and Arrhenius, G.O.S., 1958. Chemistry of Pacific pelagic sediments. Geochim. Cosmochim. Acta, 13: 153-212. Goldberg, E.D., Somayajulu, L.K., Galloway, J., Kaplan, I.R. and Faure, G., 1969. Differences between barites of marine and continental origins, Geochim. Cosmochim. Acta, 33: 287-289. Goldhaber, M.B. and Kaplan, I.R., 1980. Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the Gulf of California. Mar. Chem., 9: 95-143. Gurvich, Y.G., Bogdanov, Y.A. and Lisitzin, A.P., 1978. Behavior of barium in Recent sedimentation in the Pacific. Geokhim., 3: 359-374 (in Russian). Ingle, J.C., 1973. Neogene Foraminifera from the Northeastern Pacific Ocean, Leg 18, Deep Sea Drilling Project. In: L.D. Kulm. R. von Huene, et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Vol. 18. U.S. Gov. Print. Off.. Washington, D.C.. pp. 517-567.

M.E. Torres et al. /Chemical Geology I27 (1996) 125-139

lngle, Jr.. J.C., Suyehiro, K., von Breymann, M.T., et al. (Editors), 1990. Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 128. ODP (Ocean Drill. Prog.), College Station, Texas. Jewell, P.W. and Stallard, R.F., 1991. Geochemistry and paleoceanographic setting of central Nevada bedded barites. J. Geol., 99: 151-170. Kastner, M., Elderfield, H., Martin, J.B., Suess, E., Kvenvolden, K.A. and Garrison, R.E., 1990. Diagenesis and interstitial water chemistry at the Peruvian continental margin - major constituents and strontium isotopes. In: E. Suess, R. von Huene, et al. (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 112. ODP (Ocean Drill. Prog.), College Station, Texas, pp. 4 13440. Kiba, T. and Kishi, I., 1957. Microcolorimetric determination of sulfate by reduction to hydrogen sulfide with tin(II)-strong phosphoric acid. Chem. Sot. Jpn. Bull., 30: 44-48. Klinkenberg, L.J., 195 1. Analogy between diffusion and electrical conductivity in porous rocks. Geol. Sot. Am. Bull., 62: 559-562. Koski. R.A., Lonsdale, P.F., Shanks, W.C., Bemdt, M.E. and Howe, S.S., 1985. Mineralogy and geochemistry of a sediment hosted hydrothermal sulfide deposit from the southern trough of the Guaymas basin, Gulf of California. J. Geophys. Res., 90: 66956707. Koski, R.A., Shanks, W.C., Bohrson, W.A. and Oscarson, R.L., 1988. The composition of massive sulfide deposits from the sediment covered floor of Escanaba Trough, Gorda Ridge: Implications for depositional processes. Can. Mineral., 26: 655-673. Kulm, L.D. and Suess, E., 1990. Relationship between carbonate deposits and fluid venting: Oregon accretionary prism. J. Geophys. Res.. 95: 8899-8916. Kulm, L.D., von Huene, R., et al., 1973. Initial Reports of the Deep Sea Drilling Project, Vol. 18. U.S. Gov. Print. Off., Washington, DC, 1077 pp. Linke, P., Bohrmann, G. and Sues& E., 1995. Manifestations of venting in the Aleutian subduction zone. Annu. Meet. Geol. Assoc. Can., Victoria, B.C. (abstract). Martin, J.B., Gieskes, J.M.. Torres, M. and Kastner, M., 1993. Bromine and iodine in Peru margin sediments and pore fluids: Implications for fluid origins. Geochim. Cosmochim Acta, 57: 43774389. McCready, R.D.L. and Krause, H.R., 1980. Sulfur isotope fractionation by De.su&~~ibn’o oulgaris during metabolism of BaSO,. Geomicrobiol. J., 2: 55-61. McDuff R.E. and Gieskes, J.M., 1976. Calcium and magnesium profiles in DSDP interstitial waters: Diffusion or reaction? Earth Planet. Sci. L&t., 33: l-10. Mills, J.W., 1971. Bedded barite deposits of Stevens county, Washington. Econ. Geol., 66: 1157-l 163. Mizutani, Y. and Rafter, T.A., 1973. Isotopic behavior of sulfate oxygen in the bacterial reduction of sulfate. Geochem. J., 6: 183191. Morrow, D.W., Krause, H.R., Ghent, E.P., Taylor, G.C. and Dawson, K.R., 1978. A hypothesis concerning the origin of barite in Devonian carbonate rocks of Northeastern British Columbia. Can. J. Earth Sci., 15: 1391-1406.

139

Nielsen, A.E., 196 1. Diffusion controlled growth of a moving sphere: The kinetics of crystal growth in potassium perchlorate precipitation. J. Phys. Chem., 65: 46-49. Nuelle, L.M. and Shelton, K.L. 1986. Geologic and geochemical evidence of possible bedded barite deposits in Devonian rocks of the Valley and Ridge province, Appalachian mountains. Econ. Geol., 81: 1408-1430. Papke, K.G., 1984. Barite of Nevada. Nev. Bur. Mines Geol., Bull. 98, 125 pp. Paull, C.K. and Newmann, A.C., 1987. Continental margin brine seeps: Their geological consequences. Geology, 15: 545-548. Peter, J.M. and Scott, S.D., 1988. Mineralogy, composition and fluid inclusion microthermometry of seafloor hydrothermal deposits in the Southern trough of Guaymas basin, Gulf of California. Can. Mineral., 26: 567-587. Rafter, T.A. and Mizutani, Y., 1967. Oxygen isotopic composition of sulfates and the relationship to their environment and to their 8% values. N.Z. J. Sci., 10: 816-840. Ritger, S., Carson, B. and Suess, E., 1987. Methane-derived authigenie carbonates formed by subduction-induced pore water expulsion along the Oregon/Washington margin. Geol. Sot. Am Bull., 98: 147-156. Sakai, H., 1971. Sulfur and oxygen isotopic study of barite concretions from the banks in the Japan Sea off the Northeast Honshu, Japan. Geochem. J., 5: 79-93. Suess, E., Kulm, L.D. and Killingley, J.S., 1987. Coastal upwelling and the history of organic-rich mudstone deposition off Peru. In: J. Brooks and A.J. Fleet (Editors), Marine Petroleum Source Rocks. Geol. Sot. London, Spec. Publ., 26: 181-197. Suess, E., von Huene, R., et al. (Editors), 1988. Proceedings of the Ocean Drilling Program, Inititial Reports, Vol. 112.ODP (Ocean Drill. Prog.), College Station, Texas. von Breymann, M.T., Emeis, K.C. andcamerlenghi, A., 1990. Geochemistry of sediments from the Peru upwelling area: results from Sites 680, 685 and 688. In: E. Suess, R. von Huene, et al. (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 112. ODP (Ocean Drill. Prog.), College Station, Texas, pp. 491-504. von Breymann, M.T., Emeis, K.C. and Suess, E., 1993. Water-depth and diagenetic constraints in the use of barium as a paleoproductivity indicator. In: C.P. Summerhayes, W. Prell and K.C. Emeis (Editors), Evolution of Upwelling Systems since the Early Miocene. Geol. Sot London, Spec. Publ., 64: 273-284. von Breymann, M.T., Brumsack, H. and Emeis, K., 1992. Depositional and diagenetic behavior of barium in the Japan Sea. In: K. Pisciotto, J.C. Ingle, M.T. von Breymann and J. Barron (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 128. ODP (Ocean Drill. Prog.), College Station, Texas pp. 65 l-665. Waterman, L.S., Sayles, F.L. and Manheim, F.T., 1973. Interstitial water studies on small core samples, Legs 16, 17 and 18. In: L.D. Kulm, R. von Huene, et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Vol. 18. U.S. Gov. Print. Off., Washington, D.C., pp. 1001-1012.