Remobilization of barium in continental margin sediments

Geochimica et Cosmochimica Acta, Vol. 58, No. 22, pp.4899-4907, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in the USA.All rights reserved 00 I6-7037/94$6.00+ .OO

Pergamon

~~6-7037(94~263~

Remobili~tion of barium in continental margin sediments JAMESMCMANUS, I,* WILLIAMM. BERELSON,I GARY P. KLINKHAMMER,’TAMMY E. KILGORE,’ and DOUGLASE. HAMMOND’ ‘University of Southern California, Department of Geological Sciences, Los Angeles, CA 90089-0740, USA *Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331-5503, USA (Received August 16, 1993; accepted in revisedform May 19, 1994)

Abstract-The rate of Ba release from California continental margin sediments has been measured, using an in situ benthic flux chamber, and the range of values (25-50 nmol cmm2 y-‘) is larger than any previously published benthic flux estimate for this element. The magnitude of the Ba flux suggests that a si~~~nt fraction of the Ba raining from the euphotic zone is recycled at the seafloor. BaSi regeneration ratios from these margin sediments increase with depth, demonstrating that Ba is decoupled from Si during the earliest stages of diagenesis. On the other hand, Ba regeneration rates and CaCOs dissolution rates covary; the coupling between these two constituents is supported by the observation that the Ba: CaC03 dissolution flux ratio ( 1.7 + 0.4 X 10 -3) is independent of bottomwater depth-even in sediments underlying the oxygen minimum zone along the continental margin. Furthermore, this flux ratio is consistent with both the water column Ba:alkalinity ratio for the world’s ocean, as well as the Ba:CaC03 ratio in ~diment-up solid phases from the Equatorial Pacific f 1.1-2.2 X IO-‘). However, the constancy of the Ba:alkalinity ratio over geologic time remains in question, because the mechanism that controls this relationship remains a mystery. Our flux measurements suggest that diagenesis does not significantly influence the Ba:Ca ratio in the upper 0.5 mm of Pacific sediments, thereby supporting the idea of using the Ba concentration in surface-dwelling benthic forams as a proxy for deepwater chemical conditions (LEA and BOYLE, 1989, 1990). On the other hand, we predict that if a foraminifer lives 0.5 mm or more below this interface, then diagenetic effects could influence the Ba:Ca ratio that foram species would record. The carrier phase of the particulate Ba reactive during early diagenesis does not appear to be organic matter, oxyhydroxides, or calcium carbonate, but rather a mineral phase related to marine barite or perhaps celestite. though seawater appears to be undersaturated with respect to pure bar&e. Although the exact mechanism for Ba precipitation is unknown, BISHOP( 1988) has suggested that barite precipitation may occur within the remains of siliceous organisms, suggesting a possible link between Si and Ba biogeochemistry. In addition, it has recently been demonstrated that dissolution of acantharian-derived celestite ( SrS04), in which the Ba:Sr ratio is approximately ten times greater than the surface seawater ratio, may provide a microenvironment suitable for barite formation (BERNSTEINet al., 1992). As these skeletons dissolve, Ba and SO4 concentrations become elevated in the dissolution microenvironment, relative to seawater concentrations, producing an environment that is supersaturated with respect to bar&e. In the Pacific, particulate Ba rain rates to the deep-sea range between 1 and 75 nmol cm-’ y-’ (COLLIER and EDMOND, 1984; DYMONDet al., 1992). Since the river flux of Ba to surface waters is 4-6 nmol cmm2 y-l (HANOR and CHAN, 1977), remobilization of Ba, either within the water column or within the sediments, is required to balance the oceanic Ba budget. Data presented here suggest that a significant fraction of the pa~cu~ate Ba that falls through the water column is remobilized within the sediments.

INTRODUCTION

DESPITETHE WEALTHof data on the distribution of Ba in the oceanic water column, obtained prima& during the GEOSECS (Geochemical Ocean Sections) expeditions, surprisingly little is known about the marine geochemistry of this element. Historically, interest in the distribution of Ba in seawater stems from its use as a possible stable analogue to Ra (CHOW and GOLDBERG, 1960; WOLGEMUTH and BROECKER,1970; BACONand EDMOND, 1972; CHAN et al., 1977). More recently, it has been suggested that Ba in sedimentary solid phases may serve as a proxy for both modern (BISHOP, 1988; DEHAIRSet al., 1991, 1992; DYMONDet al., 1992) and paleoceanographic conditions ( SCHMITZ, 1987; LEA and BOYLE, 1989, 1990; ELDERFIELD,1990; DYMOND et al., 1992). Thus, while Ba has the potential to serve as a powerful geochemical tool, a better understanding of its geochemistry, particularly during early diagenesis, is required, before we can interpret these distributions with confidence. Barium appears to precipitate in the form of barite ( BaSO.,) within microenvironments of decaying biological debris (DEHAIRSet al., 1980; COLLIERand EDMOND,1984; BISHOP, 1988 ), This process would explain why barite precipitation occurs as particulate material falls through the upper water column (DEHAIRS et al., 1980, 1990, 1992; COLLIER and EDMOND, 1984; BISHOP, 1988; DYMONDet al., 1992), even

EXPERIMENTAL Benthic fluxes were measured using a free-vehiclehenthic lander and HAMMOND, 1986;BEREL~ON et al., 1990). Thelanders used in this study are th~~harn~r~ devices. Each chamber (BERELSON

* Present address: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 9733 l-5503, USA.

has a volume of approximately 7 L, covers a sediment s&ace area of 730 cm*, and is stirred with a paddle. The landers were deployed

4899

J. McManus et al.

4900

Table 1. Station

Station locations and bottom depths Latitude Longitude Depth (meters) North West

[02]bw (PM)

I

102

35” 35’

121” 15’

7

653

35” 12’

121” 18’

IX

3

lOGil

35” 28’

121’36’

25

4

2145

36” 08’

122” 26’

80

38 -

138

3360 36” 04 122” 36 [Ozfb, is bottom water oxygen at each station.

37

Santa Crw

-

113

across the highly productive margin off the coast of central California 1992 as part of a program (TEFLON) designed to constrain the flux of several trace elements from continental margin sediments (JOHNSON et al., 1992). Locations and water depths for lander deployments are presented in Table I, and a map of the study area is presented in Fig. 1. Two of these stations were positioned between water depths of 600 and 1000 m, where the oxygen minimum impinges on the continental slope (Table I ). Incubation chamber samples were drawn into polyethylene reservoirs at preset times during the experiment. Once the lander was recovered, these samples were filtered (0.45~pm filter) into HCIleached bottles and acidified with redistilled 6 N HCI. Separate splits of this water were also filtered and analyzed for nutrients, pH, and alkalinity. Analysisof dissolved Ba was carried out by isotope dilutioninductively coupled plasma quadrupole mass spectrometry ( KLINKHAMMER and CHAN, 1990) using a Fisons VG PlasmaQuad Plus at Oregon State University. Measurements have been corrected (4%) for dilution of chamber water with bottomwater, during sample

.

36 -

i

(MARTIN et al., 1987) during June

withdrawal as described by BERELSON et al. ( 1990). RESULTS Benthic oxygen consumption rates and silica remobilization rates in our study area vary from 1 to 13 mmol m-* day-’ (W. M. Berelson, unpubl. data) and 1 to 7 mmol mm2 day-’ (Table 2), respectively. Data from our benthic chamber measurements show that there is a flux of Ba out of these sediments (Fig. 2, Table 2). The Ba flux estimates labeled “chamber” in Table 2 are derived from a time series of two to six measurements from a single chamber (Table 2). The “average” flux value (Table 2) is a simple average of this result and two other fluxes from the same lander that are calculated from a two-point flux calculation. The tlux for these latter two chambers is estimated by the change in concentration over time between the bottomwater sample and one chamber water sample. The uncertainties in our Ba flux

1

102

49+4

653

23f: 12

3

1000

25+4

4

2145

24 rf:8

26.7 f 4.5 40.0 f 16.2

.

.

.

.

123

122

121

Longitude (degrees West)

FIG. I. Map of the study area showing approximate station locations.

estimates are based on the measurement precision, which is typically <5%, the uncertainty of the chamber height (< 15%), and the uncertainty in the fit of the data to a line representing the change in Ba over time. We can be fairly certain that the benthic flux chamber did not contaminate our samples with Ba or that our samples do not suffer from any severe sampling artifacts, based on several lines of evidence. First, the agreement between our bottomwater Ba estimates and the water column Ba values taken from the nearest GEOSECS station suggest that there was no contamination in our sample tubes ( Fig. 3 ) . Second, the absence of a sample tube blank is further supported by the observation that the Ba data, when plotted as a function of time, forms a line that goes through the bottomwater intercept (Fig. 2). Third, as discussed later, our benthic flux measurements are entirely consistent with what we know about Ba geochemistry and the magnitude of the Ba rain rate in this region. Finally, the Ba:Si flux ratio for each station can be calculated from a regression of Ba vs. silicic acid for chamber samples (Fig. 4; ABa/ASi in Table 2) and an estimate of the station-averaged silica dissolution flux, which is derived from two to three incubation chambers (up to eighteen total samples). The uncertainties for the ABa/

Table 2. Lander data for dissolved constiauente from Central Caiifd Margin ABalASi S111caflux Ba flux Station Deth Stn. Ave. ratio (x 103) (n)l (nmol cm-2 y-l) (mZers) (mol cm-* y-l) average3 chamber 2

‘1;; Monterey

Ba flux normalized* (nmol CM2 y-l)

0.18If:O.O2

4

265It 15

48+8

0.41 f 0.07

8

59+7

24f6

0.59

3

43 f 3

25 + 2

0.77 + 0.13

8

51f2

39f8 43f 13

44f2 0.98 f 0.26 5 5 3360 36f23 ai’dBfl ‘callted 1. (n) is the total number of samples used for the determination of the ABaJASi ratio. 2 The B”, flE’:e cziated from the Ba:Si ratio and the average silicic acid flux at each station. The uncertainties in the:%% from the uncertainties in the silica flux and the Ba:Si ratio. 3. The average is the simple average of the fluxes observed in the chamber with multiple draws and those observed in chambers based on a two point calculation.

4901

Ba in continental margin sediments

10

5

IS

0

10

Time (hr)

10

20

30

40

se

Time (br)

30

40

so Time (hr)

Time (hr)

160

20

b 0

I 20

40

I 60

I 80

1 100

Time (br)

FIG. 2. Barium as a function of time for all lander deployments, by station (Table 2). The different symbols at stations 2 and 4 represent different chambers from the same lander. Flux estimates from the three chambers at stations 2 and 4 were weighted equally to obtain station averages in Table 2. Lines are linear regressions to data from each chamber.

ASi ratio (Table 2) represent uncertainties in the slope of this relationship. Because the uncertainties in this ratio are smaller than the uncertainties in the measured Ba flux, we use this approach to ascertain whether there are any potential problems inherent in the Ba vs. time relationship. The Ba flux calculated in this fashion yields a “normalized” Ba flux that is consistent with the measured Ba flux (Table 2); therefore, the measured Ba fluxes do not appear to suffer from any severe sampling artifacts. It is likely that each predicted flux represents the average Ba regeneration rate for each site. The dissolution flux of CaC03 ( .ha~o,) was estimated from an alkalinity budget from the chamber where we have multiple Ba samples (cf. BEREL~ONet al., 1987). This was accomplished by measuring fluxes of titration alkalinity, ammonia, and nitrate ( Jalk, JNH, , and JNoJ, respectively), making an estimate of net sulfate reduction ( Jso,), and assuming that other reactions involving Fe*+ and Mn2+ are negligible to the alkalinity budget ( BERELSONet al., 1987; JOHNSON et al., 1992), where

Jcaco,= ‘h(Jane+

JNO)

-

JNH,)

-

Jso,.

(1)

J taco) I JNO> > JNH,, and JSO, are expressed in units mmol me2 day-‘, and Jalk is expressed in units meq me2 day-’ (Table

3). The nitrate, alkalinity, and ammonia fluxes were estimated in the same manner as described for Ba. An example of the changes in these constituents over time is presented in Fig. 5, and values for each parameter are presented in Table 3. Jso, was estimated from sedimentation rate (estimated from REIMERSet al., 1992) and the average fraction of sedimentary S (total S minus the S from seasalt) in surfrcial sediments (W. M. Berelson et al., unpubl. data). For this estimate, the end product of sulfate reduction is assumed to be S- that reacts with Fe” (BERELSONet al., 1987). Note that the nitrate flux has a negative value, indicating nitrate uptake in these sediments. The dissolution flux ratio incorporates the uncertainties for all terms in Eqn. 1 (Table 3). DISCUSSION

Benthic Barium Fluxes Previous work by others had predicted a flux of Ba from marine sediments. Concentrations of dissolved Ba in marine porewaters are generally at or above saturation with respect to barite, while seawater is highly undersaturated (CHURCH and WOLGEMUTH, 1972; LI et al., 1973; BRUMSACKand GIESKES, 1983; KLINKHAMMERand LAMBERT, 1989; J.

J. McManus et al [Ba]

(nmol/kg)

% I

(b)

FIG. 3. Water column (a) Ba and (b) silicic acid as a function of depth for the GEOSECS test station in the northeast Pacific (28”29’N, 12I o 38’W) and TEFLON (this study) stations along a transect near Pt. Piedras Btancas (35”40’N, 12 I ot7’W f. There were no published silicic acid data from the GEOSECS test station having Ba data; therefore, the data presented here are from the nearest GEOSECS station where silicic acid data were available. The TEFLON data represent bottomwater Ba and silicic acid concentrations at the depths of our lander deployments. For the three shallowest stations ( l-3), bottom-water was obtained from a Niskin sampler mounted on the side of the lander. For the two deeper stations, Niskin data were not available; therefore, we chose the time-zero intercept determined from the Ba vs. time relationship for the chamber samples. The agreement between our data and the historical data suggests that our samples were not subject to significant contamination during collection and processing.

McManus and G. P. ~inkhammer, unpubl. data). This situation maintains a concentration gradient across the se.diment-seawater interface that should drive a flux of Ba from

marine sediments to the overlying water. Published estimates suggest that the diffusive fiux of Ba from deep-sea sediments is between 3 and 16 nmol cm-’ y-’ (CHAN et al., 1977; FISCHER et al., 1986; FALKNER et al., 1993 ). These estimates

are based on diffusive fluxes calculated from porewater profiles (reviewed by FALKNER et al., I993 ), water column budgets for dissolved Ba (CHAN et al.. 1977). and particulate Ba budgets ( FISCH~X et al.. 1986). There are potential problems with these indirect estimates. For example. Ba in marine porewaters may be complexed with dissolved or colloidal organic material, thus retarding molecular diffusron, or the down-core sample resolution may not be adequate to resolve significant near-surface porewater gradients. Furthermore. water column budgets may reflect in situ processes not related to the sediments. Our work removes these ambiguities by providing a direct measure of the benthic Ba fiux (Fig. 2. Table 2). These data suggest that the benthic Ba flux for this region of the world’s ocean is higher than any previously published flux estimate. 25-50 nmol cm--’ y ’ (Table 2). It is not yet clear whether differences in our flux estimates. when compared to the previous flux estimates, are due to geographic variability in the benthic flux or, in the case of the porewater estimates. are indicative of low flux estimates in the literature because ofproblems in establishing the Ba vs. depth gradient. The higher Ba fluxes observed in this study are consistent with measured rain rates of biogenic Ba under the California Current (60-75 nmol cm ’ y I< DYMOND et al.. 1992). If we assume that the reported range in Ba rain rates represents upper and lower limits and we use our range of remobilization fluxes (Table 2). then we calculate that between 30 and 80% of the barium being deposited at the sediment-water interface is regenerated. This result is not completely unexpected as a recycling term of this m~nitude is required to maintain steady-state conditions in the oceans, given a riverine input of 4-6 nmol cm-’ y ’ . a hydrothermal flux that is only IO20% of the river flux, and particulate Ba fluxes up to 75 nmol cm-’ y ’ ( HANOR and CHAN, 1977; COLLIER and EUMONI?, 1984: VON DAMM et al., 1985; DVMONU et al., t9Y? 1. Moreover, our results are consistent with the work of DY%loNJI et al. ( 1992), who predict that, based on comparisons between rain rates of particulate Ba and the burial rate, - 30% of the non-detrital particulate Ba raining through the water column is preserved in sediments. It is not yet clear what diagenetic reactions produce elevated Ba coneentmtions in marine porewaters. If barite is the pri-

250 --T----r-

,

! -----7

sm.5 200

Stn. 4

4 * A/

Stn. 3 150

100

Stn. 2

J

~~

so++OFI

0

I

so

I

100 150 [Silicic acid] pM

200

250

RG. 4. Barium vs. siiicic acid in each of the benthic chambers. Lines are regressions through each dataset.

4903

Ba in continental margin sediments Table 3. Fhuea wed for cakalation of CaCO3 budget Station Jalk JNHa JNo3

Jso.,

Jcscog

Ba:CaCOs flux ratio (x ld)

2 3

2.61f 0.77 1.60f 0.40 0.65+ 0.30

-0.25f 0.05 -0.73f 0.07 -0.45f 0.04

0.73f 0.15 0.03f 0.02 0.00f 0.01

0.15f0.15 0.02f 0.02 0.02f 0.02

0.67f 0.42;24 f 15 0.40f 0.19; 15 f 7 0.08f 0.30; 3 f 11

4

1.09f 0.28

-0.13 f 0.02

0.01 f 0.01

0.02 f 0.02

0.46 f 0.28; 17 f 10

1.4f0.9

5

1.12f0.23

-0.11 fO.O1

0.01 f 0.00

0.02 f 0.02

0.48 f 0.23; 18 f

2.0f 1.6

1

8

2.0f 1.3 1.5f 1.0 (8 f 29)

1.7f0.4

Average

All fluxunits are mm01 m-2 day-t except for J& which are in meq m-2 day-t and those inbold which have been converted into pmol cm-2 y-1. Note that positive values represent fluxes out of the sediment. Fluxes of alkalinity, nitrate, and ammoniaare froma single chamber (chamber Y) of the lander deployment at each station. The sulfate flux is an estimate based on sediment accumulation rate and solid sulfur data.

mat-y solid phase of particulate Ba, then sulfate reduction and consequent barite dissolution within sediments underlying the oxygen minimum zone (CHURCH and WOLGE-

2^ 2

21.5

2 3 E

21.0 20.5 20.0 0

5

10 Time (hours)

15

20

MUTH, 1972) is one possibility. While this process is extremely important for Ba cycling in some environments (DE LANGE et al., 1990; VON BREYMANNet al., 1990) recent data indicate that net sulfate reduction in this region of the ocean is small (REIMERS et al., 1992; W. M. Berelson, unpubl. data). Although net sulfate reduction is low and neither the flux chamber nor our porewater data show a flux of sulfide from the sediments, we cannot exclude the possibility of Ba release from anoxic microenvironments. However, this possibility seems remote, since we observe the smallest Ba remobilization fluxes in the region around the oxygen minimum, where we might expect total sulfate reduction rates to be highest (Tables 1 and 2). Based on this evidence, we conclude that it is unlikely that changes in barite saturation, driven by sulfate reduction alone, are controlling the Ba fluxes measured in this study.

Relationship Between Barium and Biogenic Constituents

2270

2260

2240 0

5

0

5

10

I5

20

10 Time (hours)

15

20

Time (hours)

6.U

FIG. 5. Alkalinity, nitrate, and ammonium as a function of time from a benthic incubation chamber at station 1 (Tables I and 3).

Data from GEOSECS test station 1 in the northeast Pacific ( WOLGEMUTH, 1970) illustrate the salient features of the Ba distribution in seawater (Fig. 3a). In general, Ba profiles resemble those of silicic acid (Fig. 3b) and alkalinity, both of which result from the dissolution of primary biogenic hardparts produced in the surface ocean. These similar distributions suggest similar geochemistries; however, as CHAN et al. ( 1977) point out, Ba distributions in the water column do not always mimic these other constituents. In some environments, such as in the Southern Ocean, there is clear evidence for a decoupling of Ba from Si and, to a lesser extent, from alkalinity ( CHAN et al., 1977). Our results indicate that there is a decoupling between Ba and Si during the earliest stages of diagenesis on the seafloor. Moreover, the benthic incubation chamber data show that the Ba:Si regeneration ratio increases with water depth (Table 2; Fig. 6). It has been suggested that Ba precipitates in the form of barite in decaying siliceous debris (BISHOP, 1988); thus, it might be expected to observe a constant regeneration ratio for these constituents if both these constituents have similar locations of regeneration. Spatial variability in the Ba:Si regeneration ratio could result either from compositional differences in the material arriving at the seafloor or from differences in dissolution rates of these constituents. Compositional differences would be expected if, for example, Ba were being continuously precipitated as siliceous debris falls through the water column (BISHOP, 1988; DEHAIRS et al.,

4904

J. McManus et al

FIG. 6. Ba:Si regeneration ratio as a function of depth. The figure demonstrates the increase in this ratio with increasing depth.

199 1, 1992; DYMONDet al., 1992)) leading to an increase in the Ba:Si ratio in the particulate rain with water depth. The BaSi ratio found in material falling through the water column (700-3500 m) in the Equatorial Pacific ranges from 1.2 to 2.5 X 10T3 (data from DYMOND and COLLIER, 1988 and DYMONDet al., 1992)) whereas, the Ba:Si regeneration ratio measured in this transect across the continental margin varies from 0.2-1.0 X lo-’ (Table 2). Although it is difficult to compare these datasets, because they are from different marine provinces, the observation that the Ba:Si regeneration ratio is smaller than the ratio in the material reaching the sediments may indicate that the primary phase of particulate Ba is buried preferentially with respect to biogenic silica. Alternatively, it may mean that biogenic silica is not the primary barium bearing phase. Although we see clear evidence for a decoupling between Ba and Si, either because of processes occurring within the water column or during early d&genesis, there is no concomitant change in the Ba:CaC03 regeneration ratio (Table 3). The Ba:CaC03 regeneration ratio ranges between 1.4 X 10 -3 and 2.0 X 10e3, if we ignore the value exceeded by its uncertainty. This value from station 3 results from a low total alkalinity flux estimate, one that is roughly half the station average alkalinity flux. If an overall station average (combination of three chambers) is considered for station 3, the predicted CaCOJ dissolution flux increases to 0.39 mm01 m-* day-’ and the resulting Ba:CaC03 dissolution ratio becomes 1.9 X 10e3. Thus, for all stations in this transect, the average Ba:CaC03 dissolution ratio is 1.7 f 0.4 X 10W3.This ratio is consistent with both the ratio found in sediment trap material ( - 1.1 - 2.2 X 10m3, from DYMOND and COLLIER, 1988 and DYMOND et al., 1992), as well as the ratio that would be expected from the water column ABa:Aalkalinity ratio ( -0.7 X 10e3; LEA, 1993), assuming that most of the water column alkalinity is generated by carbonate species (yielding a Ba:CaC03 ratio of - 1.4 X 10m3). The similarity between the Ba:CaC03 regeneration ratio and the ABa:Aalkalinity water column ratio is meaningful if the sediments are a major source for both Ba and alkalinity to the ocean. It has been suggested that carbonate dissolution on the seafloor may provide the major source of alkalinity to the deep ocean ( BERELSONet al., 1994). In addition, this work shows that the magnitude of the benthic Ba flux in this

area is 5-10 times larger than the river flux per unit area of seafloor, and is comparable in magnitude to the nondetrital particulate Ba flux ( DYM~ND et al., 1992). thus, implying that the seafloor is also the major site of Ba regeneration. While we observe a Ba:CaCO, regeneration ratio that is consistent with the water column ratio, this does not necessarily imply that Ba is a suitable proxy for alkalinity in past oceans. Without knowing the mechanism that controls the Ba:CaC03 relationship, it is possible that the relationship is fortuitous. If the regeneration rates for these constituents are controlled by completely independent mechanisms, then this ratio may have been different in the past. Essentially, because the sediments are a major source for both Ba and alkalinity, the water column ratio should adjust to the flux ratio. The coupling between Ba regeneration and CaC03 dissolution on the seafloor has possible ramifications for the use of Ba:Ca ratios in benthic forams as a proxy for alkalinity in past oceans (LEA and BOYLE, 1989, 1990). Assuming that Ba is a proxy for alkalinity, it has been proposed that measurements of the Ba:Ca ratio in foraminifera can be used to predict the concentration of deep water dissolved Ba (LEA and BOYLE, 1989). The essence of this model is contained in the expression presented by LEA and BOYLE ( 1989 ): D( Ba/Ca ),, watel= ( WCah,m,nllern.

(2)

where D is the effective distribution coefficient for Ba:Ca between forams and seawater (0.37 f 0.06 for the species studied by LEA and BOYLE, 1989). Inherent in this method, is the assumption that the chemical reactions occurring during early diagenesis do not significantly alter the Ba:Ca ratio in waters inhabited by these benthic forams. The observed Ba flux can be used to predict the Ba:Ca porewater ratio relative to the

= 75 “Mi

= 50

D (1

t 0

/ 10

I

20

30

40

CT)

1

_--I

50

60

nnf :

70

Ba Flux (nmol cm.* yi’) FIG. 7. The Ba:Ca ratio that a benthic foram would record living at 1 mm depth as a function of the Ba flux at different bottom water Ba concentrations (Ba,,). The sensitivity limit represents the point at which the porewater concentration of Ba will significantly affect the Ba:Ca ratio recorded by a foram ifthe foram lives at 1 mm depth. To the right of the sensitivity line, porewater Ba concentrations become significantly higher than the overlying water concentration, whereas to the left of the line, there is no significant difference between bottomwater and porewater Ba concentrations. Note that for some fluxes measured in this study, a benthic foram would need to be living in the upper 0.5 mm of sediment to fall to the left of the sensitivity line.

4905

Ba in continental margin sediments bottomwater value, an important consideration if a benthic foraminifer derives any of its Ba from porewater. To constrain the potential impact of porewater Ba enrichments on the recorded Ba:Ca ratio, we use the fractionation expression of LEA and BOYLE( 1989) to calculate the Ba:Ca ratio that a foram living at 1 mm depth in the sediment would record, as a function of the benthic Ba flux at different bottomwater Ba concentrations (Fig. 7). To constrain the porewater Ba and Ca concentrations we have assumed that the observed Ba and Ca flux (from CaC& dissolution) is produced by molecular diffusion from porewater and that the profile for each of these constituents in the top 1 mm can be approximated by a linear con~ntration vs. depth gradient. We also use the average Ba:CaC03 regeneration ratio given in Table 3 and the diffusion coefficients for Ba2+ and Ca*” given in LI and GREGORY ( 1974). The Ba:Ca foram ratio at zero Ba flux (Fig. 7) represents bottomwater Ba:Ca ratios, and the sensitivity limit is an upper limit for D, based on the uncertainty ( 1 u) for this parameter (LEA and BOYLE,1989). Thus, the predicted Ba:Ca ratio in forams is calculated, using D = 0.43 in Eq. 2 to generate the dashed line in Fig. 7. Combinations of conditions above and to the left of the dashed line will not significantly influence the Ba:Ca ratio in shells, while those below and to the right of the dashed line will have a signifi~nt impact. At be&tic Ba fluxes of < 10 nmol cm-’ y-l, we predict that diagenetic enrichments of porewater Ba will not significantly affect the Ba:Ca ratio of a benthic foram that lives within the top millimeter of sediment. However, for Ba fluxes in excess of 30 nmol cm-’ y -’ , which are consistent with the margin sediments from this study, there may indeed be a ~gnifi~nt diagenetic effect on the Ba:Ca ratio recorded by forams living at depths of 1 millimeter or greater within the sediments. In the case of margin sediments with fluxes near 50 nmol cm-’ y -’ , forams must live in the upper 0.5 mm of sediment, otherwise, they may be exposed to fluids that are significantly different from bottomwater. In summary, although forams living immediately at the sediment surface may not be strongly influenced by the concentration of Ba in porewater, it is possible that if a benthic foram lives just below the sediment surface for even part of its life, then near-surface enrichments in dissolved Ba could bias the relationship between bottomwater Ba and the foram Ba:Ca ratio (LEA and BOYLE, 1989). This diagenetic effect will likely be important only in regions where the Ba flux is large. Based on measurements of the Ba rain rate ( DYMOND et al., 1992 ) , high-benthic Ba fluxes should be largely confined to continental margin regions, The Phase of Particulate Barium Althou~ the relatively constant ~lationship between Ba remobilization and CaCOs dissolution might suggest that Ba is in some way incorporated within the structure of CaC03, the Ba:Ca ratio in foram shells is about 10e6 (LEA and BOYLE, 1,989; LEA and SPERO, 1992 ) , far too low to account for a Ba:CaCOs dissolution flux of -10e3 (Table 3). However, some reiation~ip might exist because LEA and SPERO( 1992) find that there are small barite crystals associated with the surfaces of planktonic foraminifera shells. Also, based on sequential leach experiments, DYMOND et al. (1992) report that between 22 and 43% of the total Ba raining through the

water column in the eastern tropical Pacific is released into weak acid and must be adsorbed onto some biogenic phase or be calcite bound. Barium may also be associated with iron-manganese oxyhydroxides (DE LANGE et al., 1990; DYMONDet al., 1992; FALKNERet al., i 993 ) . DE LANGEet al. ( 1992) propose that Ba cycles with Mn at the oxic/anoxic interface in the Bannock Basin; however, at three other anoxic marine sites, FALKNER et al. ( 1993) suggest that while these two metals may cycle together in some environments, there is simply not enough Mn in these systems to dominate the Ba cycle. In addition, based on their sequential leach experiments, DYMONDet al. ( 1992) find a significant fraction of Ba associated with oxyhydroxides in only one of their six samples. Thus, although some of the Ba flux may be due to dissolution of oxyhydroxides, it is more likely that Ba is associated with some other particulate phase. Barite is thought to be the primary phase of particulate Ba settling through the water column to the sediments-independent of its precipitation mechanism ( DEHAIRSet al., 1980, 1990, 1992; BISHOP, 1988 ). There is, however, some debate over whether or not the barite precipitated within the water column is pure or impure ( FALKNERet al., 1993) and consequently there are questions about barium solubility in ~rewate~-~~icularly in the upper centimeter of sediment. Finally, recent data also suggest that celestite could be an important carrier phase for Ba in shallow waters (< 1500 m) (BERNSTEINet al., 1992). BERNSTEINet al. (1987) report a maximum celestite flux of 0.6 lcmol cma2 y-’ ; this estimate is based on sediment trap measurements, where their maximum flux was measured at a water depth of 400 m, and this estimate is consistent with their advection~ffusion models for the water column Sr budget (assuming Sr removal is controlled by acantharians). Assuming an acantharian Ba:Sr ratio of 4 X 10e3 (BERNSTEINet al., 1992) this maximum Sr flux corresponds to a Ba flux of 2 nmol cm-’ y -’ , which is a factor of ten or more lower than our benthic flux estimates. However, because acantharians are highly soluble in seawater (BERNSTEINand BETZER, 199 1) it is likely that most acantharians will dissolve before they can be sampled. Thus, a possible scenario for the control of Ba rain through the water column may be that precipitation of Ba (as barite) occurs within acantharian shells as they dissolve, this barite is then transported out of the upper water column along with other forms of biogenic debris that are less labile than the celestite shells (BERNSTEINet al., 1992). Although we cannot elucidate the primary phase of particulate Ba from our data, given the relatively small amounts of Ba that are directly incorporated into living plankton and biogenic hard parts (COLLIERand EDMOND, 1984; LEA and BOYLE, 1989; LEA and SPERO, 1992; DYMONDet al., 1992), the large Ba fluxes reported here suggest that this flux is supported by a phase of particulate barium that is probably associated with, but not incorporated into, biological debris, and marine barite is a possible candidate. CONCLUSION The benthic Ba fluxes measured along the California continental margin are 25-50 nmol cm-* y-l, and exceed pre-

J. McManus et al.

4906

viously reported estimates from other regions of the world’s ocean by as much as a factor of ten. Although the exact mechanism for the release of Ba into porewaters remains unclear, in the North Pacific continental margin area, Ba is decoupled from biogenic Si. either because of processes occurring within the water column, or during the earliest stages of diagenesis, or both. There are, however. no significant variations in the Ba:CaC03 dissolution flux ratio in this region. This latter finding, along with our calculations that demonstrate that, at low-Ba regeneration rates, the Ba:Ca ratio seen by benthic forams living in the upper 1 mm of sediment is not significantly different from that of bottomwater, are observations consistent with the use of this ratio as a proxy for paleochemical conditions. However, our calculations also indicate that if a benthic foram lives more than I mm below the surface, or if the magnitude of the Ba flux is much greater than 25 nmol cm-’ y - ’ , then this foram may no longer be recording bottomwater Ba concentrations. In addition, without knowing the geochemical mechanism regulating the observed Ba:CaC03 dissolution flux ratio, it may be premature to assume that this regeneration ratio would have been the same in the past. Thus, caution should be exercised when interpreting the Ba:Ca ratio measured in benthic forams. Additional studies will be required, before we understand how Ba is remobilized and what role, if any, biogenic constituents play in the dissolution and diffusive flux of Ba back into the deep sea. The results from this study indicate that flux data obtained from in situ benthic incubation chambers may provide important insights into the geochemistry of trace elements during early diagenesis. ,.i~krro~l~~gmmts-We thank S. Tanner, K. Johnson, and K. Coale for their cont~butions to this study. We are also grateful to the captain and crew of the R/V 1vrsv~~~r~~~j~. Andy Ungerer helped USwith the ICP-MS. Appreciation is also extended to J. Dymond, K. IS. Falkner, D. McCorkle, T. Pedersen, M. Torres, and an anonymous reviewer for their thoughtful comments. This work was supported in part by NSF grant OCE-8923024 to W. Berelson, NSF grant OCE8923057 to K. Johnson and K. Coale, and NSF grant OCE-9 I! 5357 to G. Klinkhammer.

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