The solubility of calcite and aragonite in seawater of 35%. salinity at 25°C and atmospheric pressure

Georhimicn er Cosmwhimica Am Vol. 44. pp. 85 to 94 0 Pergamon Press Ltd. 1980. Printed in Great Britain

The solubility of calcite and aragonite in seawater of 35x0 salinity at 25°C and atmospheric pressure JOHN W. MORSE,ALFONSOMuccr

and FRANK J. MILLERO

Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, U.S.A. (Received 25 April 1979; accepted

in reoisedJorm

10 September 1979)

Abstract-The solubilities of synthetic, natural and biogenic aragonite and calcite, in natural seawater of

357;,,,salinity at 25°C and 1 atm pressure, were measured using a closed system technique. Equilibration times ranged up to several months. The apparent solubility constant determined for c&cite of 4.39( k5.20) x lo-’ moI* kgmZ is in good agreement with other recent solubility measurements and is constant after 5 days equilibration. When we measured aragonite solubility we observed that it decreased with increasing time of equilibration. The value of 6.65( kO.12) x lo-’ mol* kg-‘, determined for equilibration times in excess of 2 months, is significantly less than that found in other recent measurements, which employed equilibration times of only a few hours to days. No statistically significant difference was found among the synthetic, natural and biogenic material. Solid to solution ratio, contamination of aragonite with up to 10 wt% calcite and recycling of the aragonite made no statisti~lly significant difference in solubility when long equilibration times were used. Measured apparent solubility constants of aragonite and calcite are respectively 22( +3)“/, and 20( f 2)% less than apparent solubility constants calculated from thermodynamic equilibrium constants and seawater total activity coefficients. These large differences in measured and calculated apparent solubility constants may be the result of the formation of surface layers of lower solubility than the bulk solid.

Consequently, the problem of determining the solubility of pure calcite or aragonite in seawater is basically one of knowing the activity of calcium and carbonate in seawater under a specified set of conditions. To avoid the problems associated with the precise determination of activity coefficients, marine chemists (e.g. F’YTKOWICZ, 1969) have frequently used apparent constants based on total ion concentrations. Ideally, for pure calcite and aragonite:

INTRODUCI’ION

THE STUDY of the chemical behavior of calcium carbonate in seawater has long been of major interest to investigators attempting to understand the oceanic carbon dioxide system, and the accumulation and diagenesis of carbonate minerals in marine sediments. Although the existence of many complex relationships among physical, chemical and biological factors has been clearly demonstrated, a major difficulty has persisted in precisely determining a firm reference point to which these factors can be clearly related. This reference point is the solubility of the two most important calcium carbonate phases in the marine environment; calcite and aragonite. Until the solubility problem is resolved, the importance and influence of other factors will remain ambiguous. Ideally a classical the~~ynamic calculation should prove sufficient, where the solubility product is simply equal to the thermodynamic equilibrium constant. K, = ~a~+-i=iiKa = =%z+=%~-

K: = ii&+ii&-

(3)

K: = $ax+=%+-

(4)

Where K’ is the apparent solubility constant and m is the total (free plus complexed) ion concentration in moi per kg of seawater in equilibrium with the solid phase. The apparent and thermodynamic constants should be related in the following ways: Kf =

(1)

K; =

(2)

K is the thermodynamic equilibrium constant for calcite or aragonite as denoted by the subscript c or a, la and H indicate the equilibrium activities of calcium or carbonate for calcite and aragonite, respectively, and the activity of the solid phase is defined to be unity. Histori~lly, the major problem has been relating ion concentrations in seawater to their thermodynamic activities, particularly when the effects of variable temperature, pressure and salinity are considered.

KC YCd+Ycoi K, YCaJ+Yco: -

(3

(6)

Where y is the total ion activity coefficient which includes the effects of complexing. The major difficulty with using apparent constants for the solubility of solid phases in complex electrolyte solutions such as seawater is the necessary assumption of ideal reversibility. This assumption is simply that the stoichiometry of the dissolution and precipitation reactions which occur during equilibration are the same as the bulk solid composition, 85

X6

J. W. MORSE.A. Mucci and F. J. MUERO

There is abundant evidence (e.g. BERNER, 1975) that this assumption is false for the precipitation of calcite and aragonite from seawater. Consequently, the resulting apparent solubilities may not be representitative of the pure bulk solids, but rather of impure surface phases. We have undertaken this investigation to determine the effects of experimental variables on the apparent solubilities of calcite and aragonite in seawater, and in an effort to resolve the differences in other recently determined values for the apparent solubility constants of calcite and aragonite in seawater (MAcINTYKE, 1965: INC~LErt ul., 1973; BERNER, 1976; PLATH, 1979). This investigation differs from previous investigations in the variety of materials studied, the use of long equilibration times and its examination of such factors as solid to solution ratios, calcite contamination of aragonite and recycling of solids. It has been restricted to determining calcite and aragonite solubilities at 25°C and 1 atm total pressure in filtered natural seawater of 35”;,,, salinity.

of the seawater had a significant effect on the carbonic acid dissociation constants and solid apparent solubility constants. MACINTYRE’S (1965) experimental data has been used here to recalculate the apparent soiubility constants of calcite and aragonite in seawater. This was done by using the K; value of MEHRBACH et cd. (1973) for seawater of lx”,,,, chlorinity at 25”C, modified by the method of BEN-YAAKOV and GQLDHABER(1973) for the change in composition due to increased calcium and alkalinity. The same modiiication technique was used on the resulting K;. and K; values. The values of k’:. and h’:, are 4.3X(&0.26) x 10~7mo12kg~’ and 7.12(&0.31) x IO-’ mo12 kg- ‘, respectively. where the value in parentheses is the standard deviation. INC;LEet uf.(1973) determined the solubility of caicite in synthetic seawater using the saturometry method (WEYL, 1961). The average value of K: is 4.60( f0.10) x lo-’ ’ mol’ kgu2. This value is 5.3”,, larger than the value of Ki calculated from the data of MACINTVRE (1965). BERNER (1976) determined the solubility of aragonite in seawater using an open reaction flask mainCOMP,~RtSON OF RESULTS OF tained at a constant P,ol. The average of Ki is DIFFERENT INVESTIGATORS 8.21(+0.25) x lo-’ mol’ kg--*, which 15.3:,;; is greater than the value calculated from the data of There have been numerous measurements of the MAC.INTYRE(1965). apparent solubility constants of calcite and aragonite PLATH (1979) determined calcite and aragonite in seawater. The most notable measurements in recent solubilities using a s~turometer technique similar to years of calcite or aragonite solubility in seawater are those of MACINTYRE (1965), INGLE et ui.(1973), INC~LE that of INGLE et cd. (1973). Only one run of 8-12 hr duration using reagent grade calcite was made from (1975) BERNER(1976) and PLATH (1979). Earlier work and undersaturation at 25°C in seahas been reviewed by MAC~NTYRE (1965), EDMOND supersaturation water of 34.57”,,,, salinity the result was (1970) and INCLE et ai.(1973). Relatively pure (greater Ki = 4.70(+0.10) x IO-’ mol” kgw2. The value at than 99?; CaCO,) natural and synthetic, calcites and calculated using PLATH’S(1979) temperature saliaragonites were used in these measurements. Solubili35”<,*, nity equation is 4.62 x lo- ’ mol’ kg-*, which is 5.5”~~ ties were reported in terms of the total ion molat or greater than the value determined by MACINTYRE molar product of calcium and carbonate ions. All of (1965) and 0.4”; greater than the value determined by the measurements, with the exception of INGLE (1975) INC~LEer (11.(1973). The solubilities of Bermuda oolites were restricted to 1 atm pressure. The following comand synthetic aragonite contaminated with an undeparison of results is restricted to those obtained at termined amount of calcite at 25°C in seawater of 25°C and 1 atm pressure in seawater of approximately 35“,,,, salinity. In order to compare the results, it is 32.6?; salinity were 8.69( +0.49) x lo-’ mol’ kg--’ and 5.00( 10.32) x IO--’ mol’ kg- ‘, respectively. necessary to compute all the solubilities using the PLATH (1979) recommended rejection of the synthetic same carbonic acid system apparent constants and aragonite data due to its contamination with calcite. concentration units. The constants used in all calculaAlthough not specified, the equilibration time is pretions here are those of MEHRRACHet (II. (1973) as refit sumed to be the same as that for calcite (X- 12 hr). by MILLERO (1979). PL.ATH (1979) found that the oolite aragonite soluMACINTYRE (1965) measured caicite and aragonite bility was 2.05 times greater than calcite at the same solubilities in seawater using both reaction flask and temperature and salinity. IJsing this ratio at 25 C and agitated closed system methods. A major problem in 35”,,,, salinity results in a calculated aragonite soluinterpreting his data has resulted from his use of a bility of 9.46( 50.49) x IO ’ mo12 kg -‘, which is 33”,, partial pressure of carbon dioxide (Pro,) equal to greater than the value determined by MA~INTYRE I atm. This produced equilibrated seawater solutions (1965) and 15.2’?;, greater than the value determined having signi~~ntly greater alkalinity and calcium by BERNER(1976). concentrations than normal seawater. The effect was equivalent to a change in salinity of approximately Calculation of K.k md K:fiorn K, ctnd K, 2”,,,,.To correct for this, MACINTYRE (1965) diluted his The thermodynamic equilibrium constants for calstarting seawater solutions so that the final salinity cite and aragonite of BERNER (1976), and a value of would be approximateiy 35”<,,,. BEN-YAAKOV and 0.21( &O.OCtSestimated) for the total activity coeffiG~LDHABER(1973) noted that the altered composition

Solubility

of calcite

and aragonite

cient of calcium (MUERO, 1974) and 0.030(+0.002 estimated) for the total activity coefficient of carbonate (PYTKOWICZ, 1973, in seawater of 35% at 25°C and 1 atm pressure, were used to calculate the theoretical values of K: and K: according to equations 5 and 6. The resulting values are K; = 5.65(+0.48) x lo-’ mol’ kg-* and K: = 8.40( kO.86) x lo-’ mol’ kg-‘.

MATERIALS

AND

METHODS

Mallinckrodt brand reagent grade calcium carbonate was used for all experiments in which synthetic calcite was studied. The 125420 urn size oortion of foraminifera tests from a deep sea core’collected from 2247 m depth on the Ontong Java Plateau by Berger, was also used for calcite solubility measurements. Aragonite was synthesized by the method of WRAY and DANIELS (1957) as modified by KATZ er ul. (1972) at a temperature of 70°C. X-ray diffraction spectra and SEM photomicrographs did not indicate the presence of any vaterite and less than I wt% calcite. A sample of the ground stalagtitic aragonite used in previous- aragonite solubility experiments (BERNER, 1976) was furnished bv Berner. Pterbpod tests in the greater than 125 pm size range from a core collected at 1935 m depth at Joides site 411 in the Atlantic Ocean were also used in the aragonite solubility measurements. Natural seawater was used in all of the solubility determinations. Gulf Stream near-surface seawater of approximately 36.5% salinity was collected 5-7 km off the coast of Miami, filtered through a glass fiber filter and a 0.4 pm Nuclepore filter. The filtered seawater was diluted to 35”‘{,,,and stored for at least 3 months in a glass carboy. After this period of time and prior to its use, the water was refiltered through a 0.4pm Nuclepore filter. It contained undetectable amounts of reactive phosphate (co.1 pg PO4 - P/l). The method of STRICKLAND and PARFQNS (1972) was used to determine the reactive phosphate concentrations. This procedure was used to minimize biological activity during the long term equilibration experiments. Solubility measurements were carried out in a closed system. Sealed bottles containing CaCO, suspensions were stirred on a rotating table mounted in a constant temperature bath. Equilibration was followed from both undersaturated and supersaturated solutions. Natural seawater diluted to 35&,,, was used without further alteration for the supersaturated (approximately 5 times with respect to calcite) solution, and was acidified with 1 N hydrochloric acid to reduce the saturation state to approximately 30% with respect to calcite for the undersaturated solution. Air from outside the laboratory, saturated with water, was equilibrated with the solutions prior to adding the solids. The Pco2 of the solutions was monitored by measuring pH and total alkalinity. The solutions were equilibrated with air until the calculated Pco, was 360 + IOppm. Brown polypropylene bottles capable of holding 70ml of solution were used as reaction vessels. Known quantities of calcite or aragonite were added to these bottles first. They were then filled with either the undersaturated or supersaturated seawater solution, which was allowed to overflow slightly before being sealed with polypropylene caps. The bottles were transferred to a PVC rotating table, capable of holding 42 bottles, which was immersed in a constant temperature bath maintained at 25 k O.l”C by a temperature controlled circulating system. The table was rotated at a rate of approximately 2 rpm, to constantly stir the solutions. After various equilibration periods, the bottles were taken from the rotating table and quickly placed in a water bath also maintained at 25°C. The calcium carbonate was

in seawater

87

allowed to settle before the pH of the solution was measured. The bottle was uncapped, and a combination electrode, fitted with a piece of ‘Tygon’ tubing and ‘Parafilm’ to form an air tight seal, was immediately inserted in the neck of the bottle. After measurement of pH the solution was drawn from the bottle using a 5Occ syringe and filtered through a 0.4nm Nuclepore syringe filter. Total alkalinity and calcium were then determined on the filtered solution. The pH of the initial and equilibrated solutions were measured using a Leeds and Northrup combination glass electrode No. 117184. The electrode was calibrated by using a set of three buffers (6.862, 7.410, 9.18) calibrated on the NBS scale at 25°C. The buffers and equilibrated solutions were kept in the constant temperature bath at 25°C prior to and during the measurements. The electrode was recalibrated after every four pH measurements. Electrode drift was generally less than 0.003 pH units. The total alkalinity was determined on 10 ml of solution using the gran titration technique described by GIESKES (1974). The precision was +0.3x. The boric acid contribution to total alkalinity was subtracted from total alkalinity to obtain the carbonate alkalinity. Calcium was determined by EGTA titration using the method described by GIESKES (1974) which is based on a technique devised by TSUNOGAI et al. (1968). The titrant was standardized using standard seawater. The calcium concentration could also be estimated by the change in, carbonate alkalinity between the initial and equilibrated solution, assuming a two to one relationship, ACa = i_AAc. The measured and calculated values of Ca’+ in solution agreed to within 0.5% or better. The effects of opening up the closed system to insert the electrode and syringe filter the solution, on pH, alkalinity and calcium concentration were not detectable within our analytical precision. RESULTS The

analytic

results

and

calculated

apparent

solu-

calcite and a mixed assemblage of pelagic foraminifera tests in natural seawater are presented in Table 1. The average value of K: for synthetic calcite exposed to seawater, for periods in excess of 4 weeks, is 4.36( kO.20) x lo-’ mol’ kg- ‘. This is in good agreement with the average values of K: obtained for synthetic calcite exposed to seawater for 5-14 days of 4.39(f0.27) x lo-’ mol’kg-* and the mixed assemblage of pelagic foraminifera tests of 4.39( kO.20) x lo-’ mol* kg-*. Analytic results and calculated apparent solubility products for synthetic aragonite, ground stalagtitic aragonite and a mixed assemblage of pelagic pteropod tests, in natural seawater, are presented in Table 2. The apparent solubility product determined for synthetic aragonite during exposures of 1. 2 and 4 weeks is 6.97(+0.20) x lo-’ mol’ kg-*. There are no statistically significant differences in the values of K: for these different periods of exposure to seawater. However, a lower apparent solubility product, of 6.65(&0.12) x lo-’ mol* kg-‘, is found for exposure periods in excess of 2 months. The average K: values obtained for the ground stalagtitic aragonite and mixed assemblage of pelagic pteropod tests, during exposures in excess of 50 days, of 6.59 x lo-‘mol* kg-* and 6.73 x lo-’ mol* kg-*, are in excellent agreement with the long term exposure K: of synthetic aragonite. bility

products

of synthetic

R

R

R

R

S"P

und

SUP

S'-'Q

und

R

SUP

R

R

und

sup

R

und

R

R

SUP

R

R

SUP

SUP

R

sup

und

R

und

R

R

und

R

R

und

und

R

Approach

und

Type oE Calcite

14

14

14

11

11

11

11

11

11

7

7

7

7

5

5

5

5

5

5

10.00

10.33

10.33

10.24

10.26

10.15

10.43

10.45

10.47

10.01

10.00

10.29

10.29

10.31

10.34

10.28

10.45

10.47

10.38

c

inseawater

1.745

0.757

0.740

1.769

1.856

1.745

1.188

1.154

1.134

1.766

1.760

0.659

0.665

1.717

1.726

1.715

1.027

1.077

1.045

-1 (equiv.kg x103)

A

* mCa Equilibration (mole kgv1x103) (days)

apparent

constants

1. Calcite

solubility

Table

7.545

7.870

7.895

7.508

7.497

7.550

7.695

7.711

7.718

7.565

7.565

7.935

7.935

7.558

7.539

7.560

7.726

7.706

7.699

PH

4 51

4 04

4 15

4 32

4 43

4 63

4 43

4.46

4.46

4.77

4.75

4.00

4.04

4.71

4.55

4.71

4.10

4.13

3.91

(mole2 kge2x107)

K' c

6

5 E

L

‘1?

6 e

z

E

C

28 28 28

R

R

R

und

sue

aup

62

108 108 123 123

50 50 50 50 50

R

R

R

R

R

R

F

F

F

F

F

F

auP

und

und

aup

und

sup

und

und

und

auP

aup

aup

9.98

kg-lx

10.17

10.12

10.23

10.32

10.35

10.47

10.20

10.64

10.36

10.50

10.71

10.01

10.04

10.44

10.35

9.96

10.02

10.40

10.41

(mole

“Ca* 103)

kg-’

*c

2.107

2.009

2.232

0.888

0.958

1.204

1.704

1.529

1.832

1.431

1.545

1.774

1.831

0.981

0.779

1.674

1.794

0.900

0.920

1.719

(equiv.

(Continued)

x 103)

7.462

7.501

7.399

7.819

7.778

7.660

7.547

7.553

7.520

7.594

7.575

7.520

7.512

7.769

7.862

7.582

7.530

7.792

7.788

7.555

PH

(mole2

4.62

4.77

4.20

4.26

4.23

4.18

4.51

4.28

4.64

4.32

4.57

4.35

4.42

4.28

4.09

4.67

4.50

4.11

4.17

4.53

brand

kge2x 107>

R’c

Und = approach to equilibrium for undersaturation; sup = approach to equilibrium from supersaturation; R = Mallinckdodt reagent grade calcite; F = pelagic foraminifera tests in the 125 to 420 pm size range.

50

108

62

R

auP

62 62

R

R

und

und

28

R

14

R

aup

Equilibration (days)

und

Approach

Type of Calcite

TABLE 1.

10.34 10.34 10.01 9.98 10.37

7 I

14 14 14 14 14 14 14 14 28 28 28 28 62

S

s

s

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

"nd

und

S"P

S"P

und

und

S"P

S"P

und

und

SUP

S"P

und

und

S"P

SUP

und

und

SUP

SUP

und

7

7

7

7

7

7.738 7.740

1.764 1.705

6.88 6.74

7.684 7.722 8.086

1.950 1.809 0.816

10.04 10.38

10.11

7.03 6.88

7.01

6.70

6.92

6.85

7.05

7.06

6.95

7.24

6.89

8.121

8.122

8.165

0.715

0.792

8.170 0.730

7.654

7.721

1.868 2.087

7.905

7.883

1.265

7.01

7.05

7.04

7.17

6.71

6.52

7.45

7.10

K'a -2 (mole' kg x 107)

0.796

10.37

10.33

9.84

10.31

1.261

7.745

1.761

10.01 10.30

7.748

8.192

8.209

7.691

7.675

7.893

7.910

1.760

0.700

0.691

1.914

1.916

1.331

1.224

PH

10.01

10.33

10.33

9.90

9.95

10.35

10.33

s

Approach 7

Type of Aragonite

+ITime of % *c Equilibration -1 -1 3 (mole kg x 10 ) (equiv.kg x 103) (days)

Table2.Aragonite apparent solubility constants inseawater

8

S

und

51 51 51

P

Bl

SW

und

51 62

B2

SUP

62 62 62

BL

Bl

B2

B2

und

sup

und

SUP

Ac

9.81

19.07

10.39

10.13

10.46

la.11

10.64

10.06

IO.55

10.22

10.61

10.43

10.71

1.878

0.851

1.997

0.993

1.964

1.360

2.314

1.803

2.217

1.748

2.690

2.026

2,062

1.578

2.010

1.490

1.815

1.822

lCl3,

7.719

7.999

7.645

7.952

7.730

7.769

7.620

7,648

7.609

7.712

7.500

7.629

7.638

7.751

7.685

7.810

7.708

7.698

8.077

PW

a

7.12

5.93

6.50

6.34

7.65

6.05

7.08

6.15

6.73

6.88

6.52

6.73

6.40

6.60

6.78

6.74

6.70

6.58

6.67

(lnole2kg-*x 107)

K’

Und = appraach to equilibrium from undersaturation; sup = approach to equilibrium from supersaturation; 5 = synthetic aragonite; BI and B2 =i 2 different samples of ground stalagtitic aragonite provided by Berner; P = pelagic pteropod tests greater than 125 pm in size.

62

62

P

P

und

sue

51

BI

B2

SUP

und

51.

102

S

P

SUP

und

10.38

87 102

9.91 9.64

87

S

S

S

sup

und

10.04

62

S

sup

SW

LO,04

62

S

und

Approach

Type of Aragonite

of %3++ Equilibration -1 3 hole kg x10 ) (equiv.kg% (days) -_ 62 0.821 10.38

Time

TABLE 2. (Continued)

J.W. MORSE,A. MUCCIand F. J. MILLERO

92

Table 3. Examination of ex~rimental

factors which may influence apparent solubility constants

Solid Solid Solution

Solid Calcite

Aragonite

to Solution

Ratio

K' sP -2 (mole' kg x 107)

to Ratio

Number of Determinations

1:70

4.36(+

0.17)

8

1:140

4.39(*

0.24)

6

1:70

6.63(i 0.17)

.l:LOO

6.61(lt0.29)

1:140

6.67(r 0.07)

1:230

6.84(+ 0.13)

1:300

6.57(+ 0.62)

1:350

6.73(+ 0.67)

Calcite Contamination of Aragonite Number of Determinations

Weight % Calcite 2

6.66(+ 0.18)

10

10

6.65(4 0.23)

10

Recycling Aragonite K: -2 (mole* kg x 10')

Number of Determinations 10

6.65(+ 0.55) The results of solubjlity measurements to determine the effects of solid to solution ratio, contamination of aragonite with calcite, and previous equilibration of aragonite on apparent solubility are summarized in

Table 3. None of these factors has a statistically significant influence on solubility over the ranges studied. DlSCUSSION

The results of this study and all other recent measurements of the apparent solubility of calcite in

seawater at 25°C. 1 atmosphere total pressure and 35:;,, salinity agree to within 5% or better (see Table 4). Considering the different calcite sources, experimental techniques, equilibration times, and solid to solution ratios used by the various investigators, this good agreement is indicative that the apparent soiubility of calcite in seawater is well established. The apparent solubiiity of calcite in seawater is 22(*3)% less than that calculated for pure calcite in seawater. Direct observations, using depth profiling scanning Auger spectroscopy, by MORSEer al. (1979) indicate that the apparent solubility of calcite in sea-

Table 4. Summary of calcite and aragonite apparent solubility constants at 25°C and 35’:,, salinity

Kc’ 2 -2 (mole kg X 10’) MacIntyre Ingle

(1965)

et al.

(1973)

4.38(%

0.26)

Ka’ -2 (mole' kg x 107) 7.12(t

0.31)

Ka’

K' c 1.63(+

0.17)

4.60(% 0.10) 8.21(+ 0.25)

Berner (1976) Plath (1979)

4.62(+ 0.10)

9.46(? 0.49)

2.05(+ 0.15)

This study

4.36(? 0.20)

6.65(? 0.12)

1.53(+ 0.10)

Calculated

5.65(t 0.48)

8.4O(Lt0.86)

1.49(+ 0.28)

Solubility

of calcite

and aragonite

93

in seawater

IO

F;______,_ --

N-

9L I

‘2

I

N

E hx -Y

-t

f

B

I

8-1

I

I

I

I - \,<

7-

___I -- -

Y& I

61 0

1

---__

1

20

$-

--w-w--

1

I

1

40

I

I

60 Time

I 80

4

I 100

I

120

(days)

Fig. I. Apparent solubility of aragonite in seawater versus equilibration time. Vertical bars represent calculated uncertainty in Kg. P = PLATH (1979); B = BERNER(1976); M = MACINTYRE (1965) as calculated in this paper; X = Results of this study; T = Theoretical value calculated from the thermodynamic equilibrium constant for aragonite and total activity coefficients in seawater as described in text.

water may be established by the formation of a Mgcalcite surface phase containing between 2 and 6 mol% Mg. Determinations of the apparent volubility of aragonite in seawater made by different investigators are in poor agreement. At present it is not possible to establish the reasons for the differing results. One major variable is the time of equilibration. A consistent trend in the results is a decrease in measured apparent solubility with increasing time of equilibration (see Fig. 1). The results of this study and MACINTYRE (1965) are statistically indistinguishable for the same period of equilibration. The results of BERNER (1976) and PLATH (1979), which are based on relatively short equilibration times, agree within the uncertainty of 10% with the calculated apparent solubility of aragonite in seawater. The results of this study for equilibration times in excess of 2 months are 20( + 2)% less than the calculated apparent solubility of aragonite in seawater. These observations indicate that the solubility behavior of aragonite in seawater is complex. The slow inversion of the aragonite surface to a Mg-calcite or formation of an aragonitic phase of greater stability than pure aragonite may be responsible for this behavior. It is, however, important to note that once the aragonite surface is conditioned by extended exposure to seawater, that dissolution cannot start until seawater becomes undersaturated with respect to the surface phase. CONCLUSIONS The apparent solubility constant of calcite in seawater determined in this study is in good agreement with other recent measurements (MACINTYRE, 1965; INGLE et al., 1973; PLATH, 1979). Time of equilibration is important for the determination of aragonite solu-

bility in seawater. Values for the apparent solubility constant of aragonite determined for equilibration times in excess of 2 months, in this study, are significantly less than those determined on other recent investigations (MACINTYRE, 1965; BERNER, 1975; PLATH, 1979), where equilibration times of only a few days were used. Synthetic, natural and biogenic calcium carbonates of greater than 99% purity have the same apparent solubility constants within a statistical uncertainty of approximately 5%. Solid to solution ratio, contamination of aragonite with up to lOwt% calcite and recycling of aragonite have no statistically significant influence on solubility when long equilibration times are used. The measured apparent solubility constants of calcite and aragonite are respectively 22( *3)% and 20( f 2)% less than apparent solubility constants calculated from thermodynamic equilibrium constants and total ion activity coefficients in seawater. A possible explanation is that the apparent solubilities of both calcite and aragonite in seawater are determined by the formation of a surface layer of different composition and lower solubility than the bulk solid. If true, this has serious implications for the use of apparent solubility constants in understanding the relationship between water chemistry and calcium carbonate behavior in the marine environment. Acknowledgements-We thank Dr R. A. BERNER for stimulating discussions, Dr R. M. PYTKOWICZ for his comments, SARA ~OTOLONGO for analytical assistance and CATHERINE TUTTLE for aid in preparing this paper. Support provided by NSF Marine Chemistry Program grant OCE78-18072.

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