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Marine barite and celestite saturation in seawater
Marine Chemistry 69 Ž2000. 19–31 www.elsevier.nlrlocatermarchem
Marine barite and celestite saturation in seawater Ahmed I. Rushdi a
a,b
, James McManus
c,)
, Robert W. Collier
a
College of Oceanic and Atmospheric Science, Oregon State UniÕersity, 104 Ocean Admin. Building, CorÕallis, OR 97331-5503, USA b Department of Oceanography, Faculty of Science, Sana’a UniÕersity, Sana’a, Yemen c Large Lakes ObserÕatory, UniÕersity of Minnesota, 10 UniÕersity Dr., Duluth, MN 55812, USA Received 23 July 1998; accepted 18 August 1999
Abstract U The stoichiometric solubility product, K sp,T , of barite and celestite in seawater has been calculated using thermodynamic constants, K s0 , and the activity coefficients for barium, strontium, and sulfate in seawater. An equation of the form:
U ln K sp,T s A q BlnT q
C T
q DS n
has been used. The constants A, B, C, D and n are derived from the calculated stoichiometric Žor total. solubility product of barite and celestite in seawater as a function of temperature and salinity. T is the absolute temperature ŽK. and S is the U is also calculated. Comparing the solubility products determined from this equation salinity. The effect of pressure on K sp,T and the pressure effect equation to the distribution of Ba, Sr and SO4 in seawater, we conclude that the upper surface water of the Southern Ocean is likely supersaturated with respect to pure barite, in agreement with Jeandel et al. wJeandel, C., Dupre, B., Lebaron, G., Monnin, C., Minster, J.F., 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 43, 1–31.x and Monnin et al. wMonnin, C., Jeandel, C., Cattaldo, T., Dehairs, F., 1999. The marine barite saturation state of the world oceans. Mar. Chem. 65, 253–261.x and that the oceanic water column is typically - 30% saturated with respect to celestite. The model, which includes the thermodynamic solid–solution behavior of barite in seawater at 258C and 1 atm, suggests that this mineral may contain up to 13 mol% SrSO4 at equilibrium. Accordingly, we have determined the stoichiometric solubility products of strontian barite as a function of salinity and temperature: X lnBa K sp s 247.88 y 38.333lnT y
15421 T
q 1.27S 0 .3
Using our model results for the total solubility product of the Sr-barite phase and seawater Ba and SO4 concentration data, we conclude that the maximum saturation level of the oceans with respect to marine barite is 63% in the North Atlantic, 88% in the Indian Ocean, and 111% in the North Pacific. The depth of this maximum saturation level is shallower in the Atlantic
)
Corresponding author. Tel.: q1-218-726-7384; E-mail: [email protected]
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 9 9 . 0 0 0 8 9 - 4
20
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Ocean Žabout 1000 m. than in the Pacific and Indian oceans Žabout 2000 m.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: barite; celestite; seawater
1. Introduction The processes that control the formation of barite in the marine environment are poorly understood ŽChow and Goldberg, 1960; Church, 1970, 1979; Church and Wolgemuth, 1972; Dehairs et al., 1980; Bishop, 1988; Bernstein et al., 1992.. However, the mechanism most frequently advocated is that barite is formed in organic-rich microenvironments during the decay of biogenic debris ŽDehairs et al., 1980; Bishop, 1988; Gingele and Dahmke, 1994.. It is also known that the injection of hydrothermal fluids into the deep ocean provides the necessary conditions for barite precipitation ŽDymond, 1981.. The association of barium with biogenic processes results in a dissolved Ba distribution where barium is generally depleted in the surface ocean and enriched in the deep ocean ŽWolgemuth and Broecker, 1970; Li et al., 1973.. This association, coupled with the observation that barium-rich sediments are often found underlying biologically productive regions, were behind the initial suggestion that barium may be a proxy for paleoproductivity ŽArrhenius, 1963; Turekian and Tausch, 1964; Goldberg and Arrhenius, 1969; Church, 1970.. Further evidence for the utility of barium as a paleoproductivity proxy has been derived from sediment trap correlations between Ba and organic carbon ŽDymond et al., 1992; Francois et al., 1995. and surface sediment barite correlations with upper water column productivity ŽPaytan et al., 1996.. The low solubility of barite in pelagic sediments ŽDymond et al., 1992. and the fact that marine sediment pore waters quickly saturate with respect to barite are the primary reasons why barite is thought to be well-preserved in most oceanic sediments. Both barite and celestite coexist in the marine environment with significant fractions of Sr and Ba in solid solution. In the marine environment celestite, SrSO4Žs. , is primarily found as the skeletal component of the marine planktonic organism acantharian ŽBottazzi, 1978; Rieder et al., 1982; Bernstein et al., 1992.. This phase contains up to 5800 ppm Ba
ŽBernstein et al., 1992.. Celestite tends to dissolve within the water column due to its high solubility relative to seawater saturation. Barite crystals found in the oceanic water column ŽDehairs et al., 1980. or inorganically precipitated in the laboratory ŽHanor, 1968, 1969; Church, 1970, 1979. contain variable amounts of Sr. Data from the upper water column of the oceans demonstrates that the mole fraction of SrSO4Žs. in barite may exceed 20% ŽDehairs et al., 1980.. Thus, the marine barite and celestite solubility systems may be linked and the influence of Sr on the solubility of marine barite is of particular interest. A quantitative understanding of barite and celestite solubility is therefore essential for understanding the dissolved distribution of Ba in oceanic environments and the preservation of marine barite as a paleoproductivity proxy ŽChurch and Wolgemuth, 1972.. Accordingly, we derive here a generalized empirical equation for estimating the stoichiometric equilibrium solubility product of barite and celestite in seawater as a function of salinity and temperature based on available thermodynamic data and activity coefficients for barium, strontium and sulfate. The effect of pressure on the equilibrium solubility product of barite and celestite is also considered and estimated. One of the goals of this paper is to present a set of equations that are based on the best available data and information that will allow us to directly use the ionic concentration products of Ba, Sr, and SO4 in seawater, thus avoiding the effort of calculating the mean activity coefficients of aqueous BaSO4 and SrSO4 in seawater. We use these equations to estimate the saturation state of seawater with respect to pure barite at selected stations in the Atlantic sector of the Southern Ocean. These results compare favorably with previously published estimates that used slightly different calculation approaches — supporting the veracity of a number of different approaches to estimate barite solubility. We then explore the idea that ‘‘marine barite’’ suspended in seawater is better represented as a mixed Ba–Sr–SO4 phase. The effect of strontium on the solubility of marine barite is then included in an empirical equa-
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
tion for calculating the solubility products of barite in seawater. This equation is then used to estimate the saturation state with respect to the strontian barite for seawater at selected stations in the Atlantic, Indian, and Pacific Oceans. We propose that this modified equation offers a more accurate view of barite solubility in the marine environment.
21
and: Sr
Ks0 s
Ž g Sr 2q . T Ž g SO 42y . TSr K spU ,T
Ž 4b .
aSrSO 4Ž s .
where the stoichiometric solubility product equilibrium constants of barite and celestite are: Ba
U K sp ,T s Ž m Ba 2q . T ,e Ž m SO 42y . T ,e
Ž 5a .
and: 2. Pure barite and celestite phase solubility at STP The solubilities of pure barite and celestite in an aquatic system are expressed by the following reactions: BaSO4 Žs . and SrSO4 Žs .
m Ba
m Sr
2q
q SO42y
2q
q SO42y
Ž 1a . Ž 1b .
and the thermodynamic solubility product constants of barite, Ba K s 0 , and celestite, Sr K s 0 , are: a Ba 2q aSO 42y Ba Ks0 s Ž 2a . aBaSO 4Ž s . and Sr
Ks0 s
aSr 2q aSO 42y aSrSO 4Ž s .
Ž 2b .
where, aBa 2q, aSr 2q and aSO 42y are the activities of Ba2q, Sr 2q and SO42y, respectively, and aBaSO 4Ž s . and aSrSO 4Ž s . are the activities of barite and celestite solids. The activity of an ion i, a i , as a function of ionic medium ŽJohnson and Pytkowicz, 1978; Pytkowicz, 1983; Rushdi et al., 1998. is: a i s g i ,T m i ,T s g i ,F m i ,F
Ž 3.
where g i is the activity coefficient and m i is the concentration of the ion i in that ionic medium and the subscripts T and F refer to total and free, respectively. Therefore, the relationship between the thermodynamic solubility product constants, K s0 , and the stoichiometric solubility product equilibrium U constants, K sp,T of barite and celestite are: Ba
Ks0 s
Ž g Ba 2q . T Ž g SO 42y . TBa K spU ,T a BaSO 4Ž s .
Ž 4a .
Sr
U K sp ,T s Ž m Sr 2q . T ,e Ž m SO 42y . T ,e
Ž 5b .
The subscript e refers to equilibrium. Eqs. Ž4a. and Ž4b. can be rewritten: as Ba
U K s 0 s Ž g Ba2q . T Ž g SO 42y . TBa K sp ,T
Ž 4c .
and Sr
U K s 0 s Ž g Sr 2q . T Ž g SO 42y . TSr K sp ,T
Ž 4d .
and they are valid for pure barite and pure celestite in ionic media, where the activity of the pure solid is assumed to have a value of unity. 3. Solubility effects of temperature, salinity and pressure U U The values of Ba K sp,T , and Sr K sp,T , as a function of temperature and salinity, can be estimated by knowing the values of thermodynamic solubility constants, Ba K s 0 , and Sr K s 0 at different temperatures, and the values of the total activity coefficients of Ba2q, Sr 2q and SO42y at different salinities and temperatures. For the present case Ba K s 0 , and Ba K s 0 , as a function of temperature, are calculated from the following empirical equation ŽRaju and Atkinson, 1988, 1989.: T T C Ih Ig ln K s 0 s Aln q B q y y Ž 6. 2 R 2R RT R RT where the constants A, B, C, I h and Ig are listed in Table 1, R is the gas constant, and T is the absolute
Table 1 The values of the constants used in Eq. Ž6. to compute the thermodynamic solubility constants of barite and celestite as a function of temperature Žfrom Raju and Atkinson, 1988, 1989. A
B
C
Ih
Ig
Barite 594.534 y1.91171 40.0731eq6 200,488 3740.12 Celestite 641.541 y1.90146 y4.28eq7 y251,748 4102.24
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
22
Table 2 The values of the coefficients for the equation logg i s a0 q a1 q . . . qa n q S n to calculate the activity of barium and sulfate in seawater at 258C Ion 2q
Ba Sr 2q SO42y
a0
a1
a2
a3
a4
a5
R
4.9375e y 1 5.1774e y 1 4.4850e y 3
y3.8288e y 2 y3.9703e y 2 y4.5890e y 4
2.4683e y 3 2.7407e y 3 2.9708e y 5
y8.8411e y 5 y1.04840e y 4 y1.0840e y 4
1.6193e y 6 2.0016e y 6 2.028e y 6
y1.1795e y 8 y1.518e y 8 y1.5077e y 8
0.99998 0.99998 1.00000
temperature ŽK.. This equation shows good agreement with barite experimental Ba K s 0 values ŽTempleton, 1960; Blount, 1977; Monnin and Galineir, 1988.. The estimated error between the predicted value from Eq. Ž6. and that derived experimentally is - 0.05 p K unit. The equation also predicts p K for celestite ŽCulberson et al., 1978; Roger, 1981; Reardon and Armstrong, 1987; Felmy et al., 1990. within 0.03 p K unit of experimentally derived values. Millero and Schrieber Ž1982. have estimated and tabulated the activity coefficients of free and total ions in seawater at various salinities at 258C by using the mean salt method, Pitzer Formalism and ion pairing model. From the total activity coefficients for Ba2q, Sr 2q and SO42y the following relationship is derived: logg i s a0 q a1 q . . . qa n S
n
Ž 7.
where the constants a 0 , a 2 , . . . , a n of the equation are listed in Table 2. The temperature effects on the total activity coefficients are estimated from the extended Debye–Huckel equation using the Debye– ¨ Huckel parameters obtained from Hamer Ž1968.. ¨ The stoichiometric solubility products of barite and celestite are then calculated as a function of salinity and temperature using Eqs. Ž4c. and Ž4d. U ŽTable 3; Fig. 1.. These values of K sp,T are then U used to derive an equation to compute the K sp,T values in seawater as a function of salinity and temperature following the method introduced by Ives and Moseley Ž1976. and adapted by Ramette Ž1977.. The obtained empirical equation has the following form: U ln K sp ,T s A q BlnT q
C T
q DS n
Ž 8.
where A, B, C, D and n are constants ŽTable 3.; T and S are the absolute temperature ŽK. and the
salinity of the sample, respectively. The uncertainty U in pK sp,T using Eq. Ž8. is - 0.015 p K unit relative to the fitted data in Table 3 ŽFig. 2.. The effect of pressure on the equilibrium solubility product of a mineral can be estimated from Table 3 The stoichiometric solubility product constants Žin p K unit. of barite and celestite in seawater at different salinities and temperatures obtained from Eqs. Ž4c. and Ž4d.; and the constants of the U s Aq BlnT qCr T q DS s equation ln K sp,T Barite Salinity Temperature 20 Ž8C.
25
30
35
40
5 10 15 20 25 30 35 40
8.776 8.64 9 8.53 3 8.42 7 8.33 0 8.239 8.157 8.08 2 B y38.3326
8.70 0 8.572 8.43 4 8.346 8.24 7 8.155 8.070 7.99 3 C y15421.2
8.64 3 8.51 3 8.394 8.28 5 8.18 5 8.09 0 8.00 4 7.92 4 D 1.2645
8.58 7 8.456 8.335 8.22 4 8.12 2 8.02 6 7.93 7 7.856 n 0.3
Temperature 20 Ž8C.
25
30
35
40
5 10 15 20 25 30 35 40
5.13 4 5.10 1 5.075 5.059 5.04 3 5.035 5.03 2 5.03 4 B y40.6673
5.06 3 5.02 8 5.00 0 4.98 0 4.96 5 4.956 4.951 4.951 C y12691.4
5.011 4.975 4.94 7 4.92 5 4.90 8 4.89 7 4.89 1 4.89 0 D 1.1832
4.959 4.92 2 4.89 2 4.86 8 4.850 4.838 4.83 0 4.82 6 n 0.3
8.86 6 8.74 1 8.62 6 8.52 2 8.42 7 8.338 8.258 8.18 5 A 247.616 Celestite Salinity
5.21 9 5.18 8 5.16 4 5.146 5.135 5.12 9 5.12 8 5.13 2 A 259.542
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
23
pressibility, k, for Ba2q, Sr 2q SO42y and BaSO4Žs. and SrSO4Žs. as a function of temperature in seawater are obtained from Millero Ž1982; 1983.. The values of VŽBaSO 4Ž s . . and VŽSrSO 4Žs . . in seawater are 52.10 cm3 moly1 and 46.25 cm3 moly1 , respectively ŽMillero, 1983. and k ŽBaSO 4Ž s . . and k ŽSrSO 4Žs . . are assumed to be zero ŽMillero, 1982, 1983.. DV U and D kU are calculated at various temperatures from Eqs. Ž10. and Ž11. and the following empirical equation is obtained: ln
K spp
ž / K sp0
s
Ž Õ 0 q Õ1 t . P RT
q
Ž k 0 q k1 t . P 2 2 RT
Ž 12 .
where Õ 0 , Õ 1 , k 0 and k 1 are constants and their values are shown in Table 4, t is the temperature Ž8C., T is the absolute temperature Ž8K., P is the pressure in atmospheres and R equals 83.1465 cm3 bar moly1 Ky1 . We use the values of solubility product constants for barite as a function of temperature, salinity, and
U Fig. 1. p K sp,T of barite and celestite vs. temperature at different salinities obtained from the thermodynamic solubility product and activity coefficients for barium, strontium, and sulfate at different salinities and temperature.
ŽOwen and Brinkley, 1941; Lown et al., 1968; Millero, 1976, 1982.: ln
Up K sp ,T
ž / U0 K sp ,T
yDV U P sy
D kU P 2 q
RT
2 RT
Ž 9.
where DV U and D kU are the partial molal volume change and the partial molal compressibility change at atmospheric pressure P, respectively. They are expressed as: DV U s V MS O 4Ž s . y V M 2qy VSO 42y
Ž 10 .
and D kU s k MS O 4Ž s. y k M 2qy k SO 42y
Ž 11 .
where M 2q is the cation Ba2q or Sr 2q. The values of partial molal volume, V, and partial molar com-
U ŽTable 3. Fig. 2. The difference, D, between the estimated p K sp,T U and p K sp,T calculated from Eq. Ž8. for barite and celestite.
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
24 Table 4 The values of the constants for Eq. Ž12.
Barite Celestite
Õ0
Õ1
k0
k1
45.61714 44.75463
y0.25097 y0.21603
y14.445ey3 y14.74ey3
1.251ey4 1.25Xey4
pressure reported by others Že.g., Church and Wolgemuth, 1972; Falkner Kenisson et al., 1993; Monnin, 1999. to compare with those estimated from our model ŽTable 5.. Here, the thermodynamic solubility products, K s0 , of barite which were reported by Falkner Kenisson et al. Ž1993. and Monnin Ž1999. U were converted to K sp,T by using the aqueous mean activity coefficients of BaSO4 Ž"g BaSO 4 . Žfrom Monnin, 1999.. As shown in Table 5, our model for U of barite in seawater is in good estimating K sp,T agreement with Monnin Ž1999. at temperature ranges between 58C and 258C as a function of salinity and U pressure. Our p K sp,T values generally deviate by - 0.05 p K unit. They are higher than the values obtained by Falkner Kenisson et al. Ž1993. and deviate by ) 0.10 p K unit. The deviation is much higher when compared to the values of Church and Wolgemuth Ž1972. Ž) 0.36 p K unit.. By combining Eqs. Ž8. and Ž12. we can calculate the saturation state of a water parcel with respect to a
pure mineral phase at a given S, T, and P. In Section 4, we apply these equations to several stations in the Atlantic sector of the Southern ocean. We choose these stations to compare our model with a previous approach applied to the same data set ŽJeandel et al., 1996; Monnin et al., 1999.. In Section 6, we examine the potential impact of impurities on the barite solubility product and then reexamine the saturation state of the oceans with respect to this impure marine barite.
4. Comparison of our model with previous work The degree of water column saturation, % V , with respect to pure barite or pure celestite can be defined by: %V s
100% Ž IP . in-situ
Ž 13 .
Ž K sp . in-situ
where ŽIP. in-situ , is the in-situ ionic product for barium and sulfate or strontium and sulfate ions. A value of % V s 100 represents saturation, and the values above or below 100 represent super and undersaturation, respectively. The in-situ stoichiometric solubility product, Ž K sp . in-situ , of barite or celestite, is
Table 5 The values of p K sp of barite and celestite in seawater computed by Eq. Ž8. compared with other values from literature P Žbar.
Temperature Ž8C.
Salinity Ž‰.
U ŽTW. ylog K sp
U ŽF. ylog K sp
U ŽM. ylog K sp
1 10 20 1 10 20 1 100 200 1 10 20 1 1 1 500
5 5 5 7 7 7 9 9 9 20 20 20 25 25 1 1
22 22 22 22 22 22 22 22 22 22 22 22 18 35 35 35
8.86 6 8.859 8.850 8.81 3 8.80 6 8.79 8 8.76 2 8.68 3 8.60 3 8.50 9 8.50 2 8.495 8.46 9 8.181 6 8.78 0 8.34 9
8.751 8.739 8.72 6 8.70 3 8.68 8 8.677 8.653 8.550 8.457 8.40 6 8.396 8.38 7 8.594
8.82 1 8.81 5 8.80 5 8.778 8.771 8.76 1 8.73 3 8.56 7 8.51 2 8.50 6 8.49 8 8.51 4 8.21 6 8.696 8.275
U ŽCW. ylog K sp
8.13 0 8.296 7.98 6
ŽTW. s This Work; ŽF. s Falkner Kenisson et al. Ž1993.; ŽM. s Monnin Ž1999.; ŽCW. s Church and Wolgemuth Ž1972..
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
25
Fig. 3. % V for pure barite from the same stations used by Jeandel et al. Ž1996. ŽStations 07, 36, 79. and Monnin et al. Ž1999. ŽStations 78, 89, 82..
corrected for the effect of pressure and temperature at a given salinity. The in-situ ion product is estimated using GEOSECS dissolved Ba profiles Ž m Ba,T ., Sr profiles from Brass and Turekian Ž1974., and salinity-normalized SO4 concentrations. Fig. 3 shows the saturation profiles for the three stations discussed by Jeandel et al. Ž1996. Žstations 07, 36 and 79. and Monnin et al. Ž1999. Žstations 78, 89, and 82.. These authors used ‘‘Saturation Index, ŽSI.’’, which is equivalent to our % V . Eq. Ž13. reproduces similar % V profiles to the same profiles shown by Jeandel et al. Ž1996. and Monnin et al. Ž1999. for pure barite, thus showing the equivalence of both modeling approaches. It is clear that the upper Southern Ocean water column is slightly supersaturated with respect to pure barite and undersaturated at depth below 2000 m in agreement with Jeandel et al. Ž1996. and Monnin et al. Ž1999..
Fig. 4. % V with respect to celestite for Stations 79 and 217.
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
26
The % V of oceanic water with respect to pure celestite is illustrated in Fig. 4. Both stations are clearly undersaturated with respect to pure celestite. Station 37 reaches a maximum at approximately 1000 m depth, decreasing slightly at depth and Station 79 reaches a much shallower maximum - 500 m, followed by a larger decrease to ; 3500 m.
5. Solubility effects of strontium substitution Marine barite is found to contain minor, but significant, concentrations of strontium ŽHanor, 1968; Church, 1979; Dehairs et al., 1980., which results because of the high concentrations of strontium in seawater. The incorporation of this Sr into the barite crystal lattice will influence the solubility product of barite in seawater. The K sp value for pure barite and celestite computed using Eq. Ž8., are listed in Table 6, along with the experimental values in seawater obtained from the literature ŽKrauskopf, 1956; Burton et al., 1968; Culberson et al., 1978.. There is excellent agreement between the experimental value U determined in seawater at available for celestite K sp 258C. ŽCulberson et al., 1978. and our estimate using Eq. Ž8.. On the other hand, the estimated stoichiometric solubility product of barite is much smaller than the experimental value in seawater at 258C ŽBurton et al., 1968.. We suggest that this discrepancy may be caused by Sr x Ba 1yx SO4Žs. solid solution equilibrium. Because the presence of x SrSO 4Žs . in the barite solid may increase the activity of barite, it may not have a value of unity. We assume the
activity of the solid can be calculated from the general relationship: a MS O 4Ž s . s
aSr 2q
ž / aBa 2q
fKpP soln
U p K sp
8.282 8.185
8.185"0.23 Krauskopf Ž1956. 8.050"0.03 Whitfield Ž1975. Žobtained from Burton, personal communication. 4.909"0.03 Culberson et al. Ž1978.
T Reference Ž8C. Žthis work. ŽLiterature. Barite
20 25
Celestite 25
4.908
ž
x SrSO 4 Ž s . x BaSO 4Ž s .
/
Ž 15 . Solid
where x SrSO 4Žs . and x B aSO 4Ž s. are the mole factions of SrSO4Žs. and BaSO4Žs. in the solid, the subscript soln represents solution, and K p is the partition coefficient, which can be defined as: Sr
U p K sp,T
Ž 14 .
U K sp ,T
where a MS O 4Ž s . is the activity of the solid, either U BaSO4Žs. or SrSO4Žs. , K sp,Žs. , is the experimental stoichiometric solubility product for the impure solid and is the stoichiometric solubility product for pure barite or celestite calculated from Eq. Ž8.. The estimated barite activity at equilibrium with seawater at 208C and 258C is found to be 1.25 and 1.36, respectively Žfrom Table 6., and for celestite, it is 1.001. In the case of barite, the involvement of other cations, such as Ca and K, in the crystal structure could affect the activity of barite but it has been reported that Sr is likely to be the primary contaminant ŽChurch, 1979; Dehairs et al., 1980.. For the purpose of demonstrating the effect of Sr x Ba 1yx SO4Žs. solid solution behavior on the stoichiometric solubility product, the distribution of Ba and Sr between aqueous solution and solid–solution can be approximated Ža more rigorous analysis approach is currently underway, Rushdi, unpublished. by the relation:
Kpf Table 6 The values of ylog K sp of barite and celestite in seawater computed by Eq. Ž8., compared with experimental values from literature
U K spŽs.
K s0
ž / Br
Ž 16 .
Ks0
Following Eqs. Ž15. and Ž16. we obtain:
ž
m Sr 2q m Ba 2q
/ ž f
soln
x SrSO 4 Ž s .
/ ž /ž /
x BaSO 4Ž s . Sr
P
Ba
Ks0
Ks0
Solid–soln
P
g Sr 2q
g Ba 2q
In the case of seawater at 258C ŽRaju and Atkinson, 1988., 10 y1 0
Ž 17 . soln Ba Sr
K s 0 s 1.10 = K s 0 s 2.42 =
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Fig. 5. % V with respect to strontian barite at selected stations in the Atlantic Ža., Pacific Žc., Indian Žb., and Southern Žd. Oceans.
27
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
28
10y7 ŽCulberson et al., 1978., g Ba 2qs 0.197, g Sr 2qs 0.231 and g SO 42y s 0.085 ŽMillero and Schrieber, 1982.. From the experimental study on barite solubility in natural seawater ŽBurton et al., 1968., the concentration of barium at equilibrium is about 3.24 = 10y7 mol kgy1 at 258C and assuming that the strontium concentration is f 8.9 = 10y5 mole kgy1 , then from Eq. Ž17., the ratio of Ž m Sr rm Ba . soln s 2.75 = 10q2 , which is equivalent to about 13 mol% SrSO4Žs. . For estimating the mixed solid solubility in seawater, the following congruent reaction is assumed: Sr x Ba 1yx SO4Žs.
m xSr
2q
q Ž 1 y x . Ba2qq SO42y
Ž 18 .
6. Saturation state of the oceans with respect to strontian barite As a first step towards assessing the degree of seawater saturation with respect to strontian barite, one could assume that the solubility of barite in natural seawater is higher by a factor of 1.3 than that calculated using Eq. Ž8. for pure barite. However, considerably more experimental measurements are needed to verify this assumption. The following modified equation is derived and we used it to estimate the %V of seawater with respect to the impure barite phase: X ln K sp s 247.88 y 38.333lnT y
1521.20
and the stoichiometric solubility product of X , is expressed by: Sr x Ba 1yx SO4Žs. , K sp X K sp s
K s0 Ž aBaSO 4 Ž s . .
Ž g Ba .
1yx
1yx
Ž aSrSO .
x
4Žs .
x
Ž g Sr . g SO 42y
Ž 19 .
X The value of K sp is estimated to be 8.51 = 10y9 , for Sr0.13 Ba 0.87 SO4Žs. which is closer to the value reported by Whitfield Ž1975. Ž8.91 = 10y9 ..
T
q 1.265S 0.3
Ž 20 . The % V of seawater with respect to marine barite Že.g., strontian barite. for selected stations from the Atlantic, Pacific, Indian and Southern oceans are shown in Fig. 5. The surface waters in the Southern Ocean still appear to be slightly saturated with respect to a mixed barite phase but are much closer to equilibrium. In all the other ocean basins examined,
Table 7 The depth of % V and maximum % saturation for selected stations from GEOSECS data and Jeandel et al. Ž1996. Station no.
Position
Depth of Max. % V Žm.
Atlantic a 29 37 54
35858 N–47800 W X X 12801 N–50859 W X X 15803 S–29831 W
Pacific a 217 227 241
44836 N–176850 W X X 42859 N–170805 E X X 04833 N–179800 E
X
X
Maximum %V
1042 950 1001
40 48 63
X
1912 2092 1997
111 101 101
Indianb 36
06809 S–50855 E
Southern 82 a 79 b
56815 S–24855 W X X 64810 S–84802 E
a b
X
X
X
1889
88
X
X
82 97
120 123
Data obtained from GEOSECS. Data obtained from Jeandel et al. Ž1996..
Depth range of ) 100% saturation – – –
1042–2990 1698–2092 1997–2297
–
50–1400 48–1186
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
surface water is undersaturated. In the North Atlantic the profile tends to be undersaturated with respect to barite, with a maximum % V of about 63% below 1000 m. The saturation profile in the North Pacific exhibits a maximum of 111% at depths between 1000–2000 m, while the bottom water is about 66% of saturation. The 100% saturation horizon obtained for the selected stations is shown in Table 7, which demonstrates that oceanic water is undersaturated in the Atlantic Ocean and increases in the degree of saturation towards the Pacific, the maximum degree of the saturation becomes deeper and broader towards the Pacific as illustrated in Table 7 and Fig. 6, reflecting the nutrient-like behavior of dissolved Ba. We should mention that while the oceanic water column is generally slightly above or below the 100% saturation level, this contention is not true for marine pore waters ŽMcManus et al., 1998.. There are at least two possible interpretations of this latter observation: Ž1. that marine pore waters are everywhere supersaturated with respect to marine barite,
Fig. 6. The maximum degree of saturation of oceanic waters with respect to strontian barite and their depths for selected stations from the Atlantic, Pacific, Indian and Southern Oceans.
29
or Ž2. that the solubility of the sedimentary phase of marine barite is considerably higher than that discussed here for the oceanic water column. As discussed elsewhere ŽMcManus et al., 1998., quantifying the degree of pore water saturation is important for understanding the controls on Ba burial — which has direct bearing on the utility of Ba as a paleoproductivity proxy ŽMcManus et al., 1999..
7. Conclusion Marine barite and celestite activities at equilibrium with seawater are estimated to be ; 1.36 and 1.00 at 258C, respectively. Marine celestite can be treated as solid phase with activity of one in seawater ionic medium, while this is not the case for marine barite. The consideration of the Sr effect in our calculation of barite solubility from thermodynamic relationships shows an agreement between our calculations and the experimental constant values ŽBurton et al., 1968.. Therefore, to quantify the saturation state of seawater with respect to barite, the effect of impurities including Sr needs to be considered. While we have estimated these effects here, this issue can be approached by determining the solubility of barite in seawater as a function of salinity, temperature and ŽSr 2q . in the laboratory. Assuming that the dissolution of marine barite is congruent and the solubility is higher by a factor of 1.3 than that of pure barite in seawater, Eq. Ž20. along with Eq. Ž12. can be used to estimate the degree of saturation of seawater. In general the upper water column in the world’s oceans are undersaturated with respect to marine barite. The maximum % V increases from the Atlantic to the Pacific — consistent with the regeneration pattern as water masses age for bio-active elements. The Atlantic is generally undersaturated at its maximum % V whereas the Pacific is slightly supersaturated. Also the depth of the % V maximum increases from the Atlantic to the Pacific. The Southern ocean is somewhat variant to this pattern in that the first 50 m of surface waters of the Southern Ocean are found to be undersaturated, then become supersaturated and decrease with depth.
30
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Acknowledgements A.I.R. would like to thank Prof. B.R.T. Simoneit for providing the space and the facilities in his laboratory. The authors also express their appreciation to Professor T. Church and two other anonymous reviewers for their constructive comments and suggestions. Discussions with Prof. J. Dymond and Dr. M. Torres at various stages of this work were fruitful. This work was supported by NSF grant OCE-9530056 and OCE-9617929 to J.M.
References Arrhenius, G., 1963. The Sea, Vol. 3. In: Hill ŽEd.., Wiley-Interscience, New York. Bernstein, R.E., Byrne, R.H., Betzer, P.R., Gerco, A.M., 1992. Morphologies and transformations of celestite in seawater: the role of acantharians in strontium and barium geochemistry. Geochim. Cosmochim. Acta 65, 3273–3279. Bishop, J.K.B., 1988. The barite–opal–organic association in oceanic particulate matter. Nature 331, 341–343. Blount, C., 1977. Barite solubilities and thermodynamic quantities up to 3008C and 1400 bars. Am. Miner. 62, 942–957. Bottazzi, E.M., 1978. Boll. Zoo. 45, 133. Brass, G., Turekian, K.K., 1974. Strontium distribution in GEOSECS oceanic profiles. Earth Planet. Sci. Lett. 28, 141– 148. Burton, J.D., Marshall, N.J., Phillips, A.J., 1968. Solubility of barium sulfate in seawater. Nature 217, 834–835. Chow, T.J., Goldberg, E.D., 1960. On the marine geochemistry of barium. Geochim. Cosmochim. Acta 20, 192. Church, T.M., Wolgemuth, K., 1972. Marine barite saturation. Earth Planet. Sci. Lett. 15, 35–44. Church, T.M., 1970. Marine barite. PhD Thesis, University of California. Church, T.M., 1979. Marine Minerals. In: Burns, R.G. ŽEd.., Mineralogical Society of America, Vol. 6. Washington, DC, pp. 175–209. Culberson, C.H., Lathan, G., Bates, R.G., 1978. Solubility and activity of calcium and strontium sulfates in synthetic seawater at 0.5 and 258C. J. Phys. Chem. 82 Ž25., 2693–2699. Dehairs, R., Chesselet, R., Jedwab, J., 1980. Discrete suspended particles of barite and the barium cycle in the open ocean. Earth Planet. Sci. Lett. 49, 529–550. Dymond, J., 1981. Geochemistry of Nazca Plate surface sediments: an evaluation of hydrothermal, biogenic, detrital and hydrogenous sources. Geol. Soc. Am. Mem. 154, 133–174. Dymond, J., Suess, E., Lyle, R., 1992. Barium in deep sea sediment: a geochemical proxy for paleoproductivity. Paleoceanography 7, 163. Falkner Kenisson, K., Klinkhammer, G.P., Bowers, T.S., Todd, J.F., Lewis, B.L., Landing, W.M., Edmond, J.M., 1993. The
behavior of barium in anoxic marine waters. Geochim. Cosmochim. Acta 57, 537–554. Felmy, A.R., Rai, D., Amonette, J.E., 1990. The solubility of barite and celestite in sodium sulfate: evaluation of thermodynamic data. J. Solution Chem. 19 Ž2., 175–185. Francois, R., Honjo, S., Manganini, S.J., Ravizza, G.-E., 1995. Biogenic barium fluxes to the deep sea: implication for paleoproductivity reconstruction. Gingele, F., Dahmke, A., 1994. Discrete barite particles and barium as tracers of paleoproductivity in south Atlantic sediments. Paleoceanography 9, 151–168. Goldberg, E., Arrhenius, G., 1969. Chemistry of pelagic sediment. Geochim. Cosmochim. Acta 33, 894. Hamer, W.J., 1968. Theoretical mean activity coefficients of strong electrolytes in aqueous solutions from 0 to 1008C. National Standard Reference Data Series, National Bureau of Standards, Publ. No. 24, Washington. Hanor, J.S., 1968. Frequency distribution of compositions in the barite–celestite series. Am. Miner. 53, 1215–1222. Hanor, J.S., 1969. Barite saturation in seawater. Geochim. Cosmochim Acta 33, 894–898. Ives, D.J.G., Moseley, P.G.N., 1976. Deviation of thermodynamic functions of ionization from acidic dissociation constants. J. Chem. Soc., Faraday Trans. I 72 Ž1., 1132–1143. Jeandel, C., Dupre, B., Lebaron, G., Monnin, C., Minster, J.F., 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 43, 1–31. Johnson, K.S., Pytkowicz, R.M., 1978. Ion association of Cly, with Hq, Kq, Ca2q, and Mg 2q aqueous solution at 258C. Am. J. Sci. 278, 1428–1447. Krauskopf, K.B., 1956. Factors controlling the concentration of thirteen rare metals in seawater. Geochim. Cosmochim Acta 9, 1. Li, Y.H., Ku, T.L., Mathieu, G.G., Wolgemuth, K., 1973. Barium in the Antarctic Ocean and implications regarding the marine geochemistry of Ba and 226Ra. Earth Planet. Sci. Lett. 19, 352. Lown, D.A., Thirsk, H.R., Wynne-Jonnes, L., 1968. Effect of pressure on ionization equilibria in water at 258C. Trans. Faraday Soc. 64, 2073–2080. McManus, J. et al., 1998. Geochemistry of barium in marine sediments: implications for its use as a paleoproxy. Geochim. Cosmochim. Acta 62, 3453–3473. McManus, J., Berelson, W.M., Hammond, D.E., Klinkhammer, G.P., 1999. Barium cycling in the North Pacific: implications for the utility of Ba as a paleoproductivity and paleoalkalinity proxy. Paleoceanography 14, 53–61. Millero, F.J., 1976. The effect of pressure on the solubility of calcite in seawater at 258C. Geochim. Cosmochim. Acta 40, 983–985. Millero, F.J., 1982. The effect of pressure on the solubility of minerals in water and seawater. Geochim. Cosmochim. Acta 46, 11–22. Millero, F.J., 1983. Influence of pressure on chemical processes in the sea. In: Riley, J.P., Chester, R. ŽEds.., Chemical Oceanography, Vol. 8, 1–88.
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31 Millero, F., Schrieber, D.R., 1982. Use of the ion pairing model to estimate activity coefficients of the ionic components of natural waters. Am. J. Sci. 282, 1508–1540. Monnin, C., 1999. A thermodynamic model for the solubility of barite and celestite in electrolytic solutions and seawater to 2008C and to 1 kbar. Chemical Geology 153, 187–209. Monnin, C., Galineir, C., 1988. The solubility of celestite and barite in electrolyte solutions and natural waters at 258C: a thermodynamic study. Chemical Geology 71, 283–296. Monnin, C., Jeandel, C., Cattaldo, T., Dehairs, F., 1999. The marine barite saturation state of the world oceans. Mar. Chem., in press. Owen, B.B., Brinkley, S.R., 1941. Calculation of the effect of pressure upon ionic equilibrium in pure water and salted solutions. Chem. Rev. 29, 461–474. Paytan, A., Kastner, M., Chavez, F.P., 1996. Glacial to interglacial fluctuations in productivity in the Equatorial Pacific as indicated by Marine Barite. Science 274, 1355–1357. Pytkowicz, R.M., 1983. Equilibria, non-equilibria and natural waters, Vol. II. Wiley Interscience, New York, 353 pp. Raju, K., Atkinson, G., 1988. Thermodynamics of ‘‘scale’’ mineral solubilities: 1. BaSO4Žs . in H 2 O and aqueous NaCl. J. Chem. Eng. Data 33, 490–495. Raju, K., Atkinson, G., 1989. Thermodynamics of ‘‘scale’’ mineral solubilities: 2. SrSO4Žs . in aqueous NaCl. J. Chem. Eng. Data 34, 361–364.
31
Ramette, R.W., 1977. On deducing the pK-Temperature equation. J. Chem. Educ. 54, 280–283. Reardon, E.J., Armstrong, D.K., 1987. Celestite ŽSrSO4Žs . . solubility in water, seawater and NaCl solution. Geochim. Cosmochim. Acta 51, 63–72. Rieder, N., Ott, H.A., Pfundstein, P., Schoch, R., 1982. J. Protozool. 29, 15–18. Roger, P.S.Z., 1981. Thermodynamics of geothermal fluids. PhD dissertation. University of California, Berkeley, 242 pp. Rushdi, A.I., Chen, C.T., Suess, E., 1998. The solubility of calcite in seawater solution of different magnesium concentrations at 258C and 1 atm total pressure: a laboratory re-examination. La mer. 36, 9–22. Templeton, C.C.J., 1960. Solubility of barium sulfate in sodium chloride solution from 258C to 958C. J. Chem. Eng. Data 5, 514–516. Turekian, K.K., Tausch, E.H., 1964. Barium in deep-sea sediment of the Atlantic Ocean. Nature 201, 696. Whitfield, M., 1975. The extension of chemical models for seawater to include trace components at 258C and 1 atm. pressure. Geochim. Cosmochim. Acta 39, 1545–1557. Wolgemuth, K., Broecker, W.S., 1970. Barium in seawater. Earth Planet. Sci. Lett. 8, 372.
Marine barite and celestite saturation in seawater Ahmed I. Rushdi a
a,b
, James McManus
c,)
, Robert W. Collier
a
College of Oceanic and Atmospheric Science, Oregon State UniÕersity, 104 Ocean Admin. Building, CorÕallis, OR 97331-5503, USA b Department of Oceanography, Faculty of Science, Sana’a UniÕersity, Sana’a, Yemen c Large Lakes ObserÕatory, UniÕersity of Minnesota, 10 UniÕersity Dr., Duluth, MN 55812, USA Received 23 July 1998; accepted 18 August 1999
Abstract U The stoichiometric solubility product, K sp,T , of barite and celestite in seawater has been calculated using thermodynamic constants, K s0 , and the activity coefficients for barium, strontium, and sulfate in seawater. An equation of the form:
U ln K sp,T s A q BlnT q
C T
q DS n
has been used. The constants A, B, C, D and n are derived from the calculated stoichiometric Žor total. solubility product of barite and celestite in seawater as a function of temperature and salinity. T is the absolute temperature ŽK. and S is the U is also calculated. Comparing the solubility products determined from this equation salinity. The effect of pressure on K sp,T and the pressure effect equation to the distribution of Ba, Sr and SO4 in seawater, we conclude that the upper surface water of the Southern Ocean is likely supersaturated with respect to pure barite, in agreement with Jeandel et al. wJeandel, C., Dupre, B., Lebaron, G., Monnin, C., Minster, J.F., 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 43, 1–31.x and Monnin et al. wMonnin, C., Jeandel, C., Cattaldo, T., Dehairs, F., 1999. The marine barite saturation state of the world oceans. Mar. Chem. 65, 253–261.x and that the oceanic water column is typically - 30% saturated with respect to celestite. The model, which includes the thermodynamic solid–solution behavior of barite in seawater at 258C and 1 atm, suggests that this mineral may contain up to 13 mol% SrSO4 at equilibrium. Accordingly, we have determined the stoichiometric solubility products of strontian barite as a function of salinity and temperature: X lnBa K sp s 247.88 y 38.333lnT y
15421 T
q 1.27S 0 .3
Using our model results for the total solubility product of the Sr-barite phase and seawater Ba and SO4 concentration data, we conclude that the maximum saturation level of the oceans with respect to marine barite is 63% in the North Atlantic, 88% in the Indian Ocean, and 111% in the North Pacific. The depth of this maximum saturation level is shallower in the Atlantic
)
Corresponding author. Tel.: q1-218-726-7384; E-mail: [email protected]
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 9 9 . 0 0 0 8 9 - 4
20
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Ocean Žabout 1000 m. than in the Pacific and Indian oceans Žabout 2000 m.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: barite; celestite; seawater
1. Introduction The processes that control the formation of barite in the marine environment are poorly understood ŽChow and Goldberg, 1960; Church, 1970, 1979; Church and Wolgemuth, 1972; Dehairs et al., 1980; Bishop, 1988; Bernstein et al., 1992.. However, the mechanism most frequently advocated is that barite is formed in organic-rich microenvironments during the decay of biogenic debris ŽDehairs et al., 1980; Bishop, 1988; Gingele and Dahmke, 1994.. It is also known that the injection of hydrothermal fluids into the deep ocean provides the necessary conditions for barite precipitation ŽDymond, 1981.. The association of barium with biogenic processes results in a dissolved Ba distribution where barium is generally depleted in the surface ocean and enriched in the deep ocean ŽWolgemuth and Broecker, 1970; Li et al., 1973.. This association, coupled with the observation that barium-rich sediments are often found underlying biologically productive regions, were behind the initial suggestion that barium may be a proxy for paleoproductivity ŽArrhenius, 1963; Turekian and Tausch, 1964; Goldberg and Arrhenius, 1969; Church, 1970.. Further evidence for the utility of barium as a paleoproductivity proxy has been derived from sediment trap correlations between Ba and organic carbon ŽDymond et al., 1992; Francois et al., 1995. and surface sediment barite correlations with upper water column productivity ŽPaytan et al., 1996.. The low solubility of barite in pelagic sediments ŽDymond et al., 1992. and the fact that marine sediment pore waters quickly saturate with respect to barite are the primary reasons why barite is thought to be well-preserved in most oceanic sediments. Both barite and celestite coexist in the marine environment with significant fractions of Sr and Ba in solid solution. In the marine environment celestite, SrSO4Žs. , is primarily found as the skeletal component of the marine planktonic organism acantharian ŽBottazzi, 1978; Rieder et al., 1982; Bernstein et al., 1992.. This phase contains up to 5800 ppm Ba
ŽBernstein et al., 1992.. Celestite tends to dissolve within the water column due to its high solubility relative to seawater saturation. Barite crystals found in the oceanic water column ŽDehairs et al., 1980. or inorganically precipitated in the laboratory ŽHanor, 1968, 1969; Church, 1970, 1979. contain variable amounts of Sr. Data from the upper water column of the oceans demonstrates that the mole fraction of SrSO4Žs. in barite may exceed 20% ŽDehairs et al., 1980.. Thus, the marine barite and celestite solubility systems may be linked and the influence of Sr on the solubility of marine barite is of particular interest. A quantitative understanding of barite and celestite solubility is therefore essential for understanding the dissolved distribution of Ba in oceanic environments and the preservation of marine barite as a paleoproductivity proxy ŽChurch and Wolgemuth, 1972.. Accordingly, we derive here a generalized empirical equation for estimating the stoichiometric equilibrium solubility product of barite and celestite in seawater as a function of salinity and temperature based on available thermodynamic data and activity coefficients for barium, strontium and sulfate. The effect of pressure on the equilibrium solubility product of barite and celestite is also considered and estimated. One of the goals of this paper is to present a set of equations that are based on the best available data and information that will allow us to directly use the ionic concentration products of Ba, Sr, and SO4 in seawater, thus avoiding the effort of calculating the mean activity coefficients of aqueous BaSO4 and SrSO4 in seawater. We use these equations to estimate the saturation state of seawater with respect to pure barite at selected stations in the Atlantic sector of the Southern Ocean. These results compare favorably with previously published estimates that used slightly different calculation approaches — supporting the veracity of a number of different approaches to estimate barite solubility. We then explore the idea that ‘‘marine barite’’ suspended in seawater is better represented as a mixed Ba–Sr–SO4 phase. The effect of strontium on the solubility of marine barite is then included in an empirical equa-
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
tion for calculating the solubility products of barite in seawater. This equation is then used to estimate the saturation state with respect to the strontian barite for seawater at selected stations in the Atlantic, Indian, and Pacific Oceans. We propose that this modified equation offers a more accurate view of barite solubility in the marine environment.
21
and: Sr
Ks0 s
Ž g Sr 2q . T Ž g SO 42y . TSr K spU ,T
Ž 4b .
aSrSO 4Ž s .
where the stoichiometric solubility product equilibrium constants of barite and celestite are: Ba
U K sp ,T s Ž m Ba 2q . T ,e Ž m SO 42y . T ,e
Ž 5a .
and: 2. Pure barite and celestite phase solubility at STP The solubilities of pure barite and celestite in an aquatic system are expressed by the following reactions: BaSO4 Žs . and SrSO4 Žs .
m Ba
m Sr
2q
q SO42y
2q
q SO42y
Ž 1a . Ž 1b .
and the thermodynamic solubility product constants of barite, Ba K s 0 , and celestite, Sr K s 0 , are: a Ba 2q aSO 42y Ba Ks0 s Ž 2a . aBaSO 4Ž s . and Sr
Ks0 s
aSr 2q aSO 42y aSrSO 4Ž s .
Ž 2b .
where, aBa 2q, aSr 2q and aSO 42y are the activities of Ba2q, Sr 2q and SO42y, respectively, and aBaSO 4Ž s . and aSrSO 4Ž s . are the activities of barite and celestite solids. The activity of an ion i, a i , as a function of ionic medium ŽJohnson and Pytkowicz, 1978; Pytkowicz, 1983; Rushdi et al., 1998. is: a i s g i ,T m i ,T s g i ,F m i ,F
Ž 3.
where g i is the activity coefficient and m i is the concentration of the ion i in that ionic medium and the subscripts T and F refer to total and free, respectively. Therefore, the relationship between the thermodynamic solubility product constants, K s0 , and the stoichiometric solubility product equilibrium U constants, K sp,T of barite and celestite are: Ba
Ks0 s
Ž g Ba 2q . T Ž g SO 42y . TBa K spU ,T a BaSO 4Ž s .
Ž 4a .
Sr
U K sp ,T s Ž m Sr 2q . T ,e Ž m SO 42y . T ,e
Ž 5b .
The subscript e refers to equilibrium. Eqs. Ž4a. and Ž4b. can be rewritten: as Ba
U K s 0 s Ž g Ba2q . T Ž g SO 42y . TBa K sp ,T
Ž 4c .
and Sr
U K s 0 s Ž g Sr 2q . T Ž g SO 42y . TSr K sp ,T
Ž 4d .
and they are valid for pure barite and pure celestite in ionic media, where the activity of the pure solid is assumed to have a value of unity. 3. Solubility effects of temperature, salinity and pressure U U The values of Ba K sp,T , and Sr K sp,T , as a function of temperature and salinity, can be estimated by knowing the values of thermodynamic solubility constants, Ba K s 0 , and Sr K s 0 at different temperatures, and the values of the total activity coefficients of Ba2q, Sr 2q and SO42y at different salinities and temperatures. For the present case Ba K s 0 , and Ba K s 0 , as a function of temperature, are calculated from the following empirical equation ŽRaju and Atkinson, 1988, 1989.: T T C Ih Ig ln K s 0 s Aln q B q y y Ž 6. 2 R 2R RT R RT where the constants A, B, C, I h and Ig are listed in Table 1, R is the gas constant, and T is the absolute
Table 1 The values of the constants used in Eq. Ž6. to compute the thermodynamic solubility constants of barite and celestite as a function of temperature Žfrom Raju and Atkinson, 1988, 1989. A
B
C
Ih
Ig
Barite 594.534 y1.91171 40.0731eq6 200,488 3740.12 Celestite 641.541 y1.90146 y4.28eq7 y251,748 4102.24
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
22
Table 2 The values of the coefficients for the equation logg i s a0 q a1 q . . . qa n q S n to calculate the activity of barium and sulfate in seawater at 258C Ion 2q
Ba Sr 2q SO42y
a0
a1
a2
a3
a4
a5
R
4.9375e y 1 5.1774e y 1 4.4850e y 3
y3.8288e y 2 y3.9703e y 2 y4.5890e y 4
2.4683e y 3 2.7407e y 3 2.9708e y 5
y8.8411e y 5 y1.04840e y 4 y1.0840e y 4
1.6193e y 6 2.0016e y 6 2.028e y 6
y1.1795e y 8 y1.518e y 8 y1.5077e y 8
0.99998 0.99998 1.00000
temperature ŽK.. This equation shows good agreement with barite experimental Ba K s 0 values ŽTempleton, 1960; Blount, 1977; Monnin and Galineir, 1988.. The estimated error between the predicted value from Eq. Ž6. and that derived experimentally is - 0.05 p K unit. The equation also predicts p K for celestite ŽCulberson et al., 1978; Roger, 1981; Reardon and Armstrong, 1987; Felmy et al., 1990. within 0.03 p K unit of experimentally derived values. Millero and Schrieber Ž1982. have estimated and tabulated the activity coefficients of free and total ions in seawater at various salinities at 258C by using the mean salt method, Pitzer Formalism and ion pairing model. From the total activity coefficients for Ba2q, Sr 2q and SO42y the following relationship is derived: logg i s a0 q a1 q . . . qa n S
n
Ž 7.
where the constants a 0 , a 2 , . . . , a n of the equation are listed in Table 2. The temperature effects on the total activity coefficients are estimated from the extended Debye–Huckel equation using the Debye– ¨ Huckel parameters obtained from Hamer Ž1968.. ¨ The stoichiometric solubility products of barite and celestite are then calculated as a function of salinity and temperature using Eqs. Ž4c. and Ž4d. U ŽTable 3; Fig. 1.. These values of K sp,T are then U used to derive an equation to compute the K sp,T values in seawater as a function of salinity and temperature following the method introduced by Ives and Moseley Ž1976. and adapted by Ramette Ž1977.. The obtained empirical equation has the following form: U ln K sp ,T s A q BlnT q
C T
q DS n
Ž 8.
where A, B, C, D and n are constants ŽTable 3.; T and S are the absolute temperature ŽK. and the
salinity of the sample, respectively. The uncertainty U in pK sp,T using Eq. Ž8. is - 0.015 p K unit relative to the fitted data in Table 3 ŽFig. 2.. The effect of pressure on the equilibrium solubility product of a mineral can be estimated from Table 3 The stoichiometric solubility product constants Žin p K unit. of barite and celestite in seawater at different salinities and temperatures obtained from Eqs. Ž4c. and Ž4d.; and the constants of the U s Aq BlnT qCr T q DS s equation ln K sp,T Barite Salinity Temperature 20 Ž8C.
25
30
35
40
5 10 15 20 25 30 35 40
8.776 8.64 9 8.53 3 8.42 7 8.33 0 8.239 8.157 8.08 2 B y38.3326
8.70 0 8.572 8.43 4 8.346 8.24 7 8.155 8.070 7.99 3 C y15421.2
8.64 3 8.51 3 8.394 8.28 5 8.18 5 8.09 0 8.00 4 7.92 4 D 1.2645
8.58 7 8.456 8.335 8.22 4 8.12 2 8.02 6 7.93 7 7.856 n 0.3
Temperature 20 Ž8C.
25
30
35
40
5 10 15 20 25 30 35 40
5.13 4 5.10 1 5.075 5.059 5.04 3 5.035 5.03 2 5.03 4 B y40.6673
5.06 3 5.02 8 5.00 0 4.98 0 4.96 5 4.956 4.951 4.951 C y12691.4
5.011 4.975 4.94 7 4.92 5 4.90 8 4.89 7 4.89 1 4.89 0 D 1.1832
4.959 4.92 2 4.89 2 4.86 8 4.850 4.838 4.83 0 4.82 6 n 0.3
8.86 6 8.74 1 8.62 6 8.52 2 8.42 7 8.338 8.258 8.18 5 A 247.616 Celestite Salinity
5.21 9 5.18 8 5.16 4 5.146 5.135 5.12 9 5.12 8 5.13 2 A 259.542
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
23
pressibility, k, for Ba2q, Sr 2q SO42y and BaSO4Žs. and SrSO4Žs. as a function of temperature in seawater are obtained from Millero Ž1982; 1983.. The values of VŽBaSO 4Ž s . . and VŽSrSO 4Žs . . in seawater are 52.10 cm3 moly1 and 46.25 cm3 moly1 , respectively ŽMillero, 1983. and k ŽBaSO 4Ž s . . and k ŽSrSO 4Žs . . are assumed to be zero ŽMillero, 1982, 1983.. DV U and D kU are calculated at various temperatures from Eqs. Ž10. and Ž11. and the following empirical equation is obtained: ln
K spp
ž / K sp0
s
Ž Õ 0 q Õ1 t . P RT
q
Ž k 0 q k1 t . P 2 2 RT
Ž 12 .
where Õ 0 , Õ 1 , k 0 and k 1 are constants and their values are shown in Table 4, t is the temperature Ž8C., T is the absolute temperature Ž8K., P is the pressure in atmospheres and R equals 83.1465 cm3 bar moly1 Ky1 . We use the values of solubility product constants for barite as a function of temperature, salinity, and
U Fig. 1. p K sp,T of barite and celestite vs. temperature at different salinities obtained from the thermodynamic solubility product and activity coefficients for barium, strontium, and sulfate at different salinities and temperature.
ŽOwen and Brinkley, 1941; Lown et al., 1968; Millero, 1976, 1982.: ln
Up K sp ,T
ž / U0 K sp ,T
yDV U P sy
D kU P 2 q
RT
2 RT
Ž 9.
where DV U and D kU are the partial molal volume change and the partial molal compressibility change at atmospheric pressure P, respectively. They are expressed as: DV U s V MS O 4Ž s . y V M 2qy VSO 42y
Ž 10 .
and D kU s k MS O 4Ž s. y k M 2qy k SO 42y
Ž 11 .
where M 2q is the cation Ba2q or Sr 2q. The values of partial molal volume, V, and partial molar com-
U ŽTable 3. Fig. 2. The difference, D, between the estimated p K sp,T U and p K sp,T calculated from Eq. Ž8. for barite and celestite.
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
24 Table 4 The values of the constants for Eq. Ž12.
Barite Celestite
Õ0
Õ1
k0
k1
45.61714 44.75463
y0.25097 y0.21603
y14.445ey3 y14.74ey3
1.251ey4 1.25Xey4
pressure reported by others Že.g., Church and Wolgemuth, 1972; Falkner Kenisson et al., 1993; Monnin, 1999. to compare with those estimated from our model ŽTable 5.. Here, the thermodynamic solubility products, K s0 , of barite which were reported by Falkner Kenisson et al. Ž1993. and Monnin Ž1999. U were converted to K sp,T by using the aqueous mean activity coefficients of BaSO4 Ž"g BaSO 4 . Žfrom Monnin, 1999.. As shown in Table 5, our model for U of barite in seawater is in good estimating K sp,T agreement with Monnin Ž1999. at temperature ranges between 58C and 258C as a function of salinity and U pressure. Our p K sp,T values generally deviate by - 0.05 p K unit. They are higher than the values obtained by Falkner Kenisson et al. Ž1993. and deviate by ) 0.10 p K unit. The deviation is much higher when compared to the values of Church and Wolgemuth Ž1972. Ž) 0.36 p K unit.. By combining Eqs. Ž8. and Ž12. we can calculate the saturation state of a water parcel with respect to a
pure mineral phase at a given S, T, and P. In Section 4, we apply these equations to several stations in the Atlantic sector of the Southern ocean. We choose these stations to compare our model with a previous approach applied to the same data set ŽJeandel et al., 1996; Monnin et al., 1999.. In Section 6, we examine the potential impact of impurities on the barite solubility product and then reexamine the saturation state of the oceans with respect to this impure marine barite.
4. Comparison of our model with previous work The degree of water column saturation, % V , with respect to pure barite or pure celestite can be defined by: %V s
100% Ž IP . in-situ
Ž 13 .
Ž K sp . in-situ
where ŽIP. in-situ , is the in-situ ionic product for barium and sulfate or strontium and sulfate ions. A value of % V s 100 represents saturation, and the values above or below 100 represent super and undersaturation, respectively. The in-situ stoichiometric solubility product, Ž K sp . in-situ , of barite or celestite, is
Table 5 The values of p K sp of barite and celestite in seawater computed by Eq. Ž8. compared with other values from literature P Žbar.
Temperature Ž8C.
Salinity Ž‰.
U ŽTW. ylog K sp
U ŽF. ylog K sp
U ŽM. ylog K sp
1 10 20 1 10 20 1 100 200 1 10 20 1 1 1 500
5 5 5 7 7 7 9 9 9 20 20 20 25 25 1 1
22 22 22 22 22 22 22 22 22 22 22 22 18 35 35 35
8.86 6 8.859 8.850 8.81 3 8.80 6 8.79 8 8.76 2 8.68 3 8.60 3 8.50 9 8.50 2 8.495 8.46 9 8.181 6 8.78 0 8.34 9
8.751 8.739 8.72 6 8.70 3 8.68 8 8.677 8.653 8.550 8.457 8.40 6 8.396 8.38 7 8.594
8.82 1 8.81 5 8.80 5 8.778 8.771 8.76 1 8.73 3 8.56 7 8.51 2 8.50 6 8.49 8 8.51 4 8.21 6 8.696 8.275
U ŽCW. ylog K sp
8.13 0 8.296 7.98 6
ŽTW. s This Work; ŽF. s Falkner Kenisson et al. Ž1993.; ŽM. s Monnin Ž1999.; ŽCW. s Church and Wolgemuth Ž1972..
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
25
Fig. 3. % V for pure barite from the same stations used by Jeandel et al. Ž1996. ŽStations 07, 36, 79. and Monnin et al. Ž1999. ŽStations 78, 89, 82..
corrected for the effect of pressure and temperature at a given salinity. The in-situ ion product is estimated using GEOSECS dissolved Ba profiles Ž m Ba,T ., Sr profiles from Brass and Turekian Ž1974., and salinity-normalized SO4 concentrations. Fig. 3 shows the saturation profiles for the three stations discussed by Jeandel et al. Ž1996. Žstations 07, 36 and 79. and Monnin et al. Ž1999. Žstations 78, 89, and 82.. These authors used ‘‘Saturation Index, ŽSI.’’, which is equivalent to our % V . Eq. Ž13. reproduces similar % V profiles to the same profiles shown by Jeandel et al. Ž1996. and Monnin et al. Ž1999. for pure barite, thus showing the equivalence of both modeling approaches. It is clear that the upper Southern Ocean water column is slightly supersaturated with respect to pure barite and undersaturated at depth below 2000 m in agreement with Jeandel et al. Ž1996. and Monnin et al. Ž1999..
Fig. 4. % V with respect to celestite for Stations 79 and 217.
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
26
The % V of oceanic water with respect to pure celestite is illustrated in Fig. 4. Both stations are clearly undersaturated with respect to pure celestite. Station 37 reaches a maximum at approximately 1000 m depth, decreasing slightly at depth and Station 79 reaches a much shallower maximum - 500 m, followed by a larger decrease to ; 3500 m.
5. Solubility effects of strontium substitution Marine barite is found to contain minor, but significant, concentrations of strontium ŽHanor, 1968; Church, 1979; Dehairs et al., 1980., which results because of the high concentrations of strontium in seawater. The incorporation of this Sr into the barite crystal lattice will influence the solubility product of barite in seawater. The K sp value for pure barite and celestite computed using Eq. Ž8., are listed in Table 6, along with the experimental values in seawater obtained from the literature ŽKrauskopf, 1956; Burton et al., 1968; Culberson et al., 1978.. There is excellent agreement between the experimental value U determined in seawater at available for celestite K sp 258C. ŽCulberson et al., 1978. and our estimate using Eq. Ž8.. On the other hand, the estimated stoichiometric solubility product of barite is much smaller than the experimental value in seawater at 258C ŽBurton et al., 1968.. We suggest that this discrepancy may be caused by Sr x Ba 1yx SO4Žs. solid solution equilibrium. Because the presence of x SrSO 4Žs . in the barite solid may increase the activity of barite, it may not have a value of unity. We assume the
activity of the solid can be calculated from the general relationship: a MS O 4Ž s . s
aSr 2q
ž / aBa 2q
fKpP soln
U p K sp
8.282 8.185
8.185"0.23 Krauskopf Ž1956. 8.050"0.03 Whitfield Ž1975. Žobtained from Burton, personal communication. 4.909"0.03 Culberson et al. Ž1978.
T Reference Ž8C. Žthis work. ŽLiterature. Barite
20 25
Celestite 25
4.908
ž
x SrSO 4 Ž s . x BaSO 4Ž s .
/
Ž 15 . Solid
where x SrSO 4Žs . and x B aSO 4Ž s. are the mole factions of SrSO4Žs. and BaSO4Žs. in the solid, the subscript soln represents solution, and K p is the partition coefficient, which can be defined as: Sr
U p K sp,T
Ž 14 .
U K sp ,T
where a MS O 4Ž s . is the activity of the solid, either U BaSO4Žs. or SrSO4Žs. , K sp,Žs. , is the experimental stoichiometric solubility product for the impure solid and is the stoichiometric solubility product for pure barite or celestite calculated from Eq. Ž8.. The estimated barite activity at equilibrium with seawater at 208C and 258C is found to be 1.25 and 1.36, respectively Žfrom Table 6., and for celestite, it is 1.001. In the case of barite, the involvement of other cations, such as Ca and K, in the crystal structure could affect the activity of barite but it has been reported that Sr is likely to be the primary contaminant ŽChurch, 1979; Dehairs et al., 1980.. For the purpose of demonstrating the effect of Sr x Ba 1yx SO4Žs. solid solution behavior on the stoichiometric solubility product, the distribution of Ba and Sr between aqueous solution and solid–solution can be approximated Ža more rigorous analysis approach is currently underway, Rushdi, unpublished. by the relation:
Kpf Table 6 The values of ylog K sp of barite and celestite in seawater computed by Eq. Ž8., compared with experimental values from literature
U K spŽs.
K s0
ž / Br
Ž 16 .
Ks0
Following Eqs. Ž15. and Ž16. we obtain:
ž
m Sr 2q m Ba 2q
/ ž f
soln
x SrSO 4 Ž s .
/ ž /ž /
x BaSO 4Ž s . Sr
P
Ba
Ks0
Ks0
Solid–soln
P
g Sr 2q
g Ba 2q
In the case of seawater at 258C ŽRaju and Atkinson, 1988., 10 y1 0
Ž 17 . soln Ba Sr
K s 0 s 1.10 = K s 0 s 2.42 =
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Fig. 5. % V with respect to strontian barite at selected stations in the Atlantic Ža., Pacific Žc., Indian Žb., and Southern Žd. Oceans.
27
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
28
10y7 ŽCulberson et al., 1978., g Ba 2qs 0.197, g Sr 2qs 0.231 and g SO 42y s 0.085 ŽMillero and Schrieber, 1982.. From the experimental study on barite solubility in natural seawater ŽBurton et al., 1968., the concentration of barium at equilibrium is about 3.24 = 10y7 mol kgy1 at 258C and assuming that the strontium concentration is f 8.9 = 10y5 mole kgy1 , then from Eq. Ž17., the ratio of Ž m Sr rm Ba . soln s 2.75 = 10q2 , which is equivalent to about 13 mol% SrSO4Žs. . For estimating the mixed solid solubility in seawater, the following congruent reaction is assumed: Sr x Ba 1yx SO4Žs.
m xSr
2q
q Ž 1 y x . Ba2qq SO42y
Ž 18 .
6. Saturation state of the oceans with respect to strontian barite As a first step towards assessing the degree of seawater saturation with respect to strontian barite, one could assume that the solubility of barite in natural seawater is higher by a factor of 1.3 than that calculated using Eq. Ž8. for pure barite. However, considerably more experimental measurements are needed to verify this assumption. The following modified equation is derived and we used it to estimate the %V of seawater with respect to the impure barite phase: X ln K sp s 247.88 y 38.333lnT y
1521.20
and the stoichiometric solubility product of X , is expressed by: Sr x Ba 1yx SO4Žs. , K sp X K sp s
K s0 Ž aBaSO 4 Ž s . .
Ž g Ba .
1yx
1yx
Ž aSrSO .
x
4Žs .
x
Ž g Sr . g SO 42y
Ž 19 .
X The value of K sp is estimated to be 8.51 = 10y9 , for Sr0.13 Ba 0.87 SO4Žs. which is closer to the value reported by Whitfield Ž1975. Ž8.91 = 10y9 ..
T
q 1.265S 0.3
Ž 20 . The % V of seawater with respect to marine barite Že.g., strontian barite. for selected stations from the Atlantic, Pacific, Indian and Southern oceans are shown in Fig. 5. The surface waters in the Southern Ocean still appear to be slightly saturated with respect to a mixed barite phase but are much closer to equilibrium. In all the other ocean basins examined,
Table 7 The depth of % V and maximum % saturation for selected stations from GEOSECS data and Jeandel et al. Ž1996. Station no.
Position
Depth of Max. % V Žm.
Atlantic a 29 37 54
35858 N–47800 W X X 12801 N–50859 W X X 15803 S–29831 W
Pacific a 217 227 241
44836 N–176850 W X X 42859 N–170805 E X X 04833 N–179800 E
X
X
Maximum %V
1042 950 1001
40 48 63
X
1912 2092 1997
111 101 101
Indianb 36
06809 S–50855 E
Southern 82 a 79 b
56815 S–24855 W X X 64810 S–84802 E
a b
X
X
X
1889
88
X
X
82 97
120 123
Data obtained from GEOSECS. Data obtained from Jeandel et al. Ž1996..
Depth range of ) 100% saturation – – –
1042–2990 1698–2092 1997–2297
–
50–1400 48–1186
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
surface water is undersaturated. In the North Atlantic the profile tends to be undersaturated with respect to barite, with a maximum % V of about 63% below 1000 m. The saturation profile in the North Pacific exhibits a maximum of 111% at depths between 1000–2000 m, while the bottom water is about 66% of saturation. The 100% saturation horizon obtained for the selected stations is shown in Table 7, which demonstrates that oceanic water is undersaturated in the Atlantic Ocean and increases in the degree of saturation towards the Pacific, the maximum degree of the saturation becomes deeper and broader towards the Pacific as illustrated in Table 7 and Fig. 6, reflecting the nutrient-like behavior of dissolved Ba. We should mention that while the oceanic water column is generally slightly above or below the 100% saturation level, this contention is not true for marine pore waters ŽMcManus et al., 1998.. There are at least two possible interpretations of this latter observation: Ž1. that marine pore waters are everywhere supersaturated with respect to marine barite,
Fig. 6. The maximum degree of saturation of oceanic waters with respect to strontian barite and their depths for selected stations from the Atlantic, Pacific, Indian and Southern Oceans.
29
or Ž2. that the solubility of the sedimentary phase of marine barite is considerably higher than that discussed here for the oceanic water column. As discussed elsewhere ŽMcManus et al., 1998., quantifying the degree of pore water saturation is important for understanding the controls on Ba burial — which has direct bearing on the utility of Ba as a paleoproductivity proxy ŽMcManus et al., 1999..
7. Conclusion Marine barite and celestite activities at equilibrium with seawater are estimated to be ; 1.36 and 1.00 at 258C, respectively. Marine celestite can be treated as solid phase with activity of one in seawater ionic medium, while this is not the case for marine barite. The consideration of the Sr effect in our calculation of barite solubility from thermodynamic relationships shows an agreement between our calculations and the experimental constant values ŽBurton et al., 1968.. Therefore, to quantify the saturation state of seawater with respect to barite, the effect of impurities including Sr needs to be considered. While we have estimated these effects here, this issue can be approached by determining the solubility of barite in seawater as a function of salinity, temperature and ŽSr 2q . in the laboratory. Assuming that the dissolution of marine barite is congruent and the solubility is higher by a factor of 1.3 than that of pure barite in seawater, Eq. Ž20. along with Eq. Ž12. can be used to estimate the degree of saturation of seawater. In general the upper water column in the world’s oceans are undersaturated with respect to marine barite. The maximum % V increases from the Atlantic to the Pacific — consistent with the regeneration pattern as water masses age for bio-active elements. The Atlantic is generally undersaturated at its maximum % V whereas the Pacific is slightly supersaturated. Also the depth of the % V maximum increases from the Atlantic to the Pacific. The Southern ocean is somewhat variant to this pattern in that the first 50 m of surface waters of the Southern Ocean are found to be undersaturated, then become supersaturated and decrease with depth.
30
A.I. Rushdi et al.r Marine Chemistry 69 (2000) 19–31
Acknowledgements A.I.R. would like to thank Prof. B.R.T. Simoneit for providing the space and the facilities in his laboratory. The authors also express their appreciation to Professor T. Church and two other anonymous reviewers for their constructive comments and suggestions. Discussions with Prof. J. Dymond and Dr. M. Torres at various stages of this work were fruitful. This work was supported by NSF grant OCE-9530056 and OCE-9617929 to J.M.
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