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The carbonate system in the western Mediterranean sea
~ S e a Research,Vol. 26A,pp. 1395to 1404 © PergamonPressLtd 1979.Printed in Great Britain
0011-7471/79/1201-1395 $02.00/0
NOTE The carbonate system in the western Mediterranean Sea FRANK J. MILLERO*, JOHN MORSE* and CHEN-TUNG CHENt (Received 20 February 1979; in revisedform 21 August 1979; accepted 23 August 1979) Abstract--Measurements of the pH and total alkalinity have been made on waters collected in the western Mediterranean Sea. These results have been used to examine the elements of the carbonate system, HCO~, CO~-, CO2, F~CO2,Pco2, and specificalkalinity. The saturation of Mediterranean waters with respect to calcite and aragonite has also been determined. The entire water column near Gibraltar is supersaturated with respect to calcium carbonate. Our results for the saturation state (f~) for Mediterranean waters are in good agreement with the results of ALEKIN(Geochemistry, 206, 239 242, 1972)and those calculated fromthe GEOSECS (GeochemicalOcean Sections)test station in the eastern Mediterranean. The saturation state of calcite and aragonite in deep Mediterranean waters is higher than that of deep North Atlantic waters. IN RECENT years the carbonate system in seawater has received attention because of the interest in the fate of CO 2 from fossil fuels. In September 1976, a cruise was made in the western Mediterranean Sea to study the carbonate system. This paper summarizes the measurements made on the carbonate system during the cruise. A complete compilation of the physical and chemical observations is available elsewhere (MILLERO, MEANS and MILLER, 1978a; MILLERO, MORSE and CHEN, 1978b). OBSERVATIONS The samples were collected in 3-1. Niskin bottles. The locations of the 20 stations are shown in Fig. 1. Measurements of salinity (S), pH, and total alkalinity (AT) were made on board. The temperatures and depths were obtained from reversing thermometers. The salinities were determined with a Guildline salinometer. The pH determinations were made on the water as soon as the samples were taken from the Niskin water bottles. The combination glass electrode (Corning) and pH meter (Leeds & Northrup) were calibrated before and after each series of measurements at a given station with NBS buffers (pH = 4.0, 7.0, and 9.0). The precision of the pH measurements was 0.003 pH units and the accuracy is thought to be 0.02 pH units. The values of pH at 25°C were determined from equations (MILLERO et al., 1978b) generated from the tables of HARVEY (1963). These tables are based on the work of Bucn and NYNXS (1939). The values o f p H E s pHt calculated from these equations agree with those derived by BEN-YAAKOV(1970) [based on LYMAN'S(1956) constants] to 0.007 over the temperature, salinity, and pH range of our measurements. They also give values o f p H 2 s - p H t that agree to 0.008 (MILLERO, 1979) with values generated from the constants of MEHRBACH, CULBERSON, HAWLEYand PYTKOWICZ (1973). * Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A. + School of Oceanography, Oregon State University, Corvallis, OR 97331, U.S.A. 1395
1396
FRANK J. MILLERO,JOHN MORSEand CHEN-TUNGCHEN
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58"N
57"N
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Fig. 1. Locations of the hydrographic stations in the Mediterranean Sea.
The alkalinity determinations were by the methods developed by EDMOND (1970). The titrations of seawater with 0.1 M HC1 (in 0.6 M NaCl) were at 25-t-0.02°C. The e.m.f. measurements were made with an Orion or Leeds & Northrup pH meter and a combination glass electrode. The end-points of the titration were determined by using the Gran method and a computer program developed by GIESKES (personal communication). The volume alkalinities were converted to weight values using the densities determined on the same samples (MILLERO et al., 1978a). The precision of the total alkalinities (AT) was 0.003 meq k g - ~ and the accuracies are thought to be 0.01 meq k g A summary of the pH25 and A T measurements is shown in Fig. 2 along with the salinity and temperatures. The average values of pH25 and A T for the surface and deep waters are given in Table 1. The surface waters have a pH25 = 8.19___0.02 and A x = 2 . 3 9 + 0 . 0 3 m e q k g -1, and the deep waters have a pH25=8.05___0.01 and A T = 2.63 + 0.03 meq kg- 1. The specific alkalinity S.A. = AT/C1 (where C1 = S/1.80655, UNESCO, 1966) has also been calculated for the samples. A summary of these results is shown in Fig. 2. The scatter in the data is due to the error in Aa- (an error of +0.02 m e q k g -1 in A T yields an error of _0.001 in S.A.). The surface waters have a S.A. = 0.122+0.001 while the deep waters have a S.A. = 0.124+0.002. The apparent increase in S.A. is within the scatter of the data. The summary of the data represented in Fig. 2 shows quite a lot of scatter in the various properties. Part is due to experimental errors; however, much of it is due to variations in the surface waters. This can be demonstrated by examining the temperature, salinity, pH25, A T, and S.A. for one station. Station 11 was selected because we have tritium data for this station (MILLERO et al., 1978b). The values of the different variables are plotted as a function of depth in Fig. 3.
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The carbonate system in the western Mediterranean Sea
1399
Table 1. Summary of results for the Mediterranean Sea* Parameter
S(°/~) T(°C) 0(°C) PH25oc PHi, situ AT(meq kg 1) AT/CI(%o) [ n c o 3 ] , mmol kg i [CO3], mmol kg 1 [COz], mmol kg I ZCO2, mmol kg -1 Pco~, 106 atm f~eal~le ~'~aragonite
Surface waters (0 to 100 m)
Deep waters (300 m to bottom)
36.8+0.2 (43) 23.5+0.5 (20) -8.19+0.02 (41) 8.20 + 0.02 (41) 2.39+0.03 (78) 0.1218 + 0.001 (99) 1.91+0.03 (51) 0.23+0.01 (52) 0.0125___0.001 (73) 2.18+0.05 (81) 393+31 (80) 5.2 + 0.2 (41) 3.1 + 0.1 (41)
38.44+0.02 (67) 13.05+0.05 (81) 12.83 4-0.07 (71) 8.05+0.01,, (109) 8.17 + 0.014 (109) 2.63+0.03 (163) 0.1243 ___0.0016 (123) 2.18+0.03 (110) 0.179-t-0.006 (139) 0.016,,+0.001 (145) 2.36+0.03 (136) 425+23 (100) 2.44 + 0.06 (8)t 1.43 + 0.03 (8)'t
* The number of data points used is given in parenthesis. * From the deepest stations (~2500 m). RESULTS AND CALCULATIONS
A full description of the calculations of the various parameters for the carbonate system is given in the data report by MILLEROet al. (1978b). We will briefly outline the methods used in this section. The measured values of pH were corrected to the in situ temperature using equations (MILLEROet al., 1978b) generated from the tables of HARVEY(1963). As the determinations of pH were made within I°C of the in situ temperature, this correction was quite small. The effect of pressure on the pH was determined from equations (MILLERO et al., 1978b) generated from the pressure coefficients published by CULBERSON and PYTKOWICZ(1968). As the temperatures of the surface waters are close to 25°C, the in situ pH is only 0.01 unit higher than pH25. The average value of the in situ pH in the deep waters is 8.17+0.01, only 0.03 pH units lower than the surface waters. The various parameters of the carbonate system were determined using the standard e q u a t i o n s (SKIRROW, 1965) A c = A T - K'a[B]T/(K'a + an)
[HCO3] = A J(1 + 2K~/aH) [CO 2- ] = AcK~/(an + 2K~) [CO2]
=
(AcaH/K'I)/(1 + 2K~/aH)
2CO2 = [ H C O ; ] + [CO3z-] + [CO2] Pco2 = [CO2]/ct I.P. = [ C a 2 + ] [ C O ] - ] n = I.P./K'~p
(1) (2) (3) (4) (5) (6) (7)
(8)
where the definition of the symbols have their usual meaning (SKIRROW, 1965). The values of the apparent constants (K~, K~, K~) were taken from LYMAN(1958) and MEHRBACHet al. (1973). The pressure coefficients were taken from CULBERSONand PYTKOWICZ(1968). The values of total boron [B]T and calcium [Ca 2 +] at a given salinity were calculated from the
1400
FRANKJ. MILLERO,JOHNMO~E and CHEN-TUNGCHEN
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The carbonate system in the western Mediterranean Sea
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1402
FRANK J. MILLERO, JOHN MORSE and CHEN-TUNG CHEN
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Fig. 6. Profilesof the saturation state (f~)of calcite and aragonite in the western (Sta. ll this study), central (ALEKIN,1972),and eastern (GEOSECS test station) Mediterranean Sea.
equations of MILLERO (in press). The values of ~ (the solubility coefficient for C O 2 ) w e r e taken from the equations of WEISS (1974). The values of K~ for calcite were calculated from the equation of INGLE (1975), while the values for aragonite were taken from the work of BERNER (1976). Errors of 4-0.01 meq kg -~ in A T and _+0.01 in pH give errors of +_0.02 mmoi kg- ~ in H C O 3 , _+0.004 mmol kg- ~ in CO 2-, _ 0.002 mmol k g - ~ in CO2, _+30 × 10- 6 atm in Pco2 and _+0.02 mmoi k g - 1 in y CO 2. The various parameters for the carbonate system for the Mediterranean stations are given elsewhere (MILLERO et al., 1978b). Profiles summarizing all the data are shown as a function of depth in Figs 4 and 5. A summary of the average values of [ H C O £ ] , [CO 2-], [CO2], ZCO2 and Pco2 for the surface and deep waters is given in Table 1. The ~CO 2 increases by 0.18_+ 0.08 mmol kg-1 from the surface to deep waters, while Pco2 shows little or no change within the scatter of the data ( + 30 × 10 -6 atm). The increase in ~CO2 (as well as the other carbonate parameters) with depth is largely due to the increase in the salinity of the water. The small changes in Pco2 indicate that little biological oxidation occurs in the Mediterranean waters (in contrast to deep Atlantic and Pacific waters). The fast turnover time of the Mediterranean waters (SVERDRUP, JOHNSONand FLEMING, 1942) causes the deep waters to have nearly equilibrium values of Pco2. Our results for the various components of the carbonate system are in good agreement with the results of CHERNYAKOVA(1976) in the Straits of Sicily, ALEKIN (1972) in the northwestern Mediterranean, and the GEOSECS test station (404) in the eastern Mediterranean. A summary of the values o f f l for calcite and aragonite for the Mediterranean samples is shown in Fig. 5. The surface waters have f2caI = 5.0 and flarag = 2.8, while the deep waters have values as low as Q~z = 2.5 and Qatar = 1.5. All the Mediterranean waters are supersaturated with respect to calcite and aragonite (in agreement with the work of
1403
The carbonate system in the western Mediterranean Sea
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Comparisons of profiles of the saturation state (t~) in the western Mediterranean Sea (Sta. 11) and the North Atlantic (GEOSECS Sta. 15).
ALEKIN, 1972, and the G E O S E C S test station in the eastern Mediterranean, Fig. 6). The deep waters are ~ 50~ less saturated than the surface waters. This decrease is largely due to the effect of pressure on the solubility of CaCO3. The deep waters of the Mediterranean are more highly saturated with respect to calcite and aragonite than are Atlantic waters (GEOSECS Sta. 115) at the same depth (Fig. 7). This is due to the lower values of pH in deep Atlantic waters (due to more biological oxidation). CHERNYAKOVA, 1976;
A cknowledgements--The authors wish to acknowledge the support of the Office of Naval Research (N00014-75-C0173) and the Oceanography Section of the National Science Foundation (OCE73-0035 l-A01) for this study. We also wish to thank Dr GOTE OSTLUND, head of the Miami tritium laboratory, for making the tritium measurements.
REFERENCES ALEkXNO. A. (1972) Saturation of Mediterranean Sea water with calcium carbonate. Geochemistry, 206, 239-242. BEN-YAAKOVS. (1970) A method for calculating the in situ pH of seawater. Limnology and Oceanography, 15, 326-328. BERNER R. A. (1976) The solubility of calcite and aragonite in seawater at atmospheric pressure and 34.5%,% salinity. American Journal of Science, 276, 713-730. Bucn K. and O. NYN~,S(1939) Studien iibenere pH-methodik mit besonderer Beriicksichtigung des Meerwassers. Academa Mathematics Physics, 12, 41. CHERNYAKOVA A. M. (1976) Elements of the carbonate system in the Straits of Sicily (Tunis Strait) area. Oceanology, 16, 36-39. CULBERSON C. and R. M. PYTKOWXCZ(1968) Effect of pressure on carbonic acid, boric acid, and the pH in seawater. Limnology and Oceanography, 13, 403-417. EDMONDJ. M. (1970) High precision determination of titration alkalinity and total carbon dioxide content of sea water by potentiometric titration. Deep-Sea Research, 17, 737-750. HARVEY H. W. (1963) The chemistry and fertility ofseawaters, Cambridge University Press, 240. pp. INGLE S. E. (1975) Solubility of calcite in the ocean. Marine Chemistry, 3, 301-319.
1404
FRANK J. MILLERO, JOHN MORSE and CHEN-TUNG CHEN
LYMAN J. (1956) Buffer mechanism of seawater. Ph.D. Thesis, University of California, Los Angeles, 196 pp. MEnRBACH C., C. H. CULnERSON J. E. HAWLEY and R. M. PYTgOWICZ (1973) Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography, 18, 897-907. MILLERO F. J. (1979) The thermodynamics of the carbonate system in seawater. Geochemica et Cosmochimica Acta, 43, 1651-1661. MILLERO F. J. (in press) The thermodynamics of seawater, In: Oceans handbook, R. A. HORrCE,editor, Chapter 4, Marcel Dekker. MILLERO F. J., D. MEANSand C. MILLER (1978a) The densities of Mediterranean Sea waters. Deep-Sea Research, 25, 563-569. MILLERO F. J., J. W. MORSE and C.-T. CHEN (1978b) Chemical oceanographic data from the western Mediterranean. Technical Report, National Science Foundation, 62 pp. SKIRROW G. (1965) The dissolved gases--carbon dioxide. In: Chemical oceanography~ J. P. RILEy and G. SK1RROW, editors, Academic Press, pp. 227-322. SVERDRUP H. U., M. W. JOHNSON and R. H. FLEMING (1942) The oceans, Prentice-Hall, 1087 pp. UNESCO (1966) Second report of the joint panel on Oceanographic Tables and Standards. UNESCO Technical Papers in Marine Science, No. 4 (mimeographed), 7 pp. WEISS R. F. (1974) Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Marine Chemistry, 2, 203-215.
0011-7471/79/1201-1395 $02.00/0
NOTE The carbonate system in the western Mediterranean Sea FRANK J. MILLERO*, JOHN MORSE* and CHEN-TUNG CHENt (Received 20 February 1979; in revisedform 21 August 1979; accepted 23 August 1979) Abstract--Measurements of the pH and total alkalinity have been made on waters collected in the western Mediterranean Sea. These results have been used to examine the elements of the carbonate system, HCO~, CO~-, CO2, F~CO2,Pco2, and specificalkalinity. The saturation of Mediterranean waters with respect to calcite and aragonite has also been determined. The entire water column near Gibraltar is supersaturated with respect to calcium carbonate. Our results for the saturation state (f~) for Mediterranean waters are in good agreement with the results of ALEKIN(Geochemistry, 206, 239 242, 1972)and those calculated fromthe GEOSECS (GeochemicalOcean Sections)test station in the eastern Mediterranean. The saturation state of calcite and aragonite in deep Mediterranean waters is higher than that of deep North Atlantic waters. IN RECENT years the carbonate system in seawater has received attention because of the interest in the fate of CO 2 from fossil fuels. In September 1976, a cruise was made in the western Mediterranean Sea to study the carbonate system. This paper summarizes the measurements made on the carbonate system during the cruise. A complete compilation of the physical and chemical observations is available elsewhere (MILLERO, MEANS and MILLER, 1978a; MILLERO, MORSE and CHEN, 1978b). OBSERVATIONS The samples were collected in 3-1. Niskin bottles. The locations of the 20 stations are shown in Fig. 1. Measurements of salinity (S), pH, and total alkalinity (AT) were made on board. The temperatures and depths were obtained from reversing thermometers. The salinities were determined with a Guildline salinometer. The pH determinations were made on the water as soon as the samples were taken from the Niskin water bottles. The combination glass electrode (Corning) and pH meter (Leeds & Northrup) were calibrated before and after each series of measurements at a given station with NBS buffers (pH = 4.0, 7.0, and 9.0). The precision of the pH measurements was 0.003 pH units and the accuracy is thought to be 0.02 pH units. The values of pH at 25°C were determined from equations (MILLERO et al., 1978b) generated from the tables of HARVEY (1963). These tables are based on the work of Bucn and NYNXS (1939). The values o f p H E s pHt calculated from these equations agree with those derived by BEN-YAAKOV(1970) [based on LYMAN'S(1956) constants] to 0.007 over the temperature, salinity, and pH range of our measurements. They also give values o f p H 2 s - p H t that agree to 0.008 (MILLERO, 1979) with values generated from the constants of MEHRBACH, CULBERSON, HAWLEYand PYTKOWICZ (1973). * Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, U.S.A. + School of Oceanography, Oregon State University, Corvallis, OR 97331, U.S.A. 1395
1396
FRANK J. MILLERO,JOHN MORSEand CHEN-TUNGCHEN
6*W
4*W
2*W
0
2*E
4*E
I
I
I
I
I
I
I
l
i
!
i
i
41*N 40*N
39"N
58"N
57"N
36"N 35"N
Fig. 1. Locations of the hydrographic stations in the Mediterranean Sea.
The alkalinity determinations were by the methods developed by EDMOND (1970). The titrations of seawater with 0.1 M HC1 (in 0.6 M NaCl) were at 25-t-0.02°C. The e.m.f. measurements were made with an Orion or Leeds & Northrup pH meter and a combination glass electrode. The end-points of the titration were determined by using the Gran method and a computer program developed by GIESKES (personal communication). The volume alkalinities were converted to weight values using the densities determined on the same samples (MILLERO et al., 1978a). The precision of the total alkalinities (AT) was 0.003 meq k g - ~ and the accuracies are thought to be 0.01 meq k g A summary of the pH25 and A T measurements is shown in Fig. 2 along with the salinity and temperatures. The average values of pH25 and A T for the surface and deep waters are given in Table 1. The surface waters have a pH25 = 8.19___0.02 and A x = 2 . 3 9 + 0 . 0 3 m e q k g -1, and the deep waters have a pH25=8.05___0.01 and A T = 2.63 + 0.03 meq kg- 1. The specific alkalinity S.A. = AT/C1 (where C1 = S/1.80655, UNESCO, 1966) has also been calculated for the samples. A summary of these results is shown in Fig. 2. The scatter in the data is due to the error in Aa- (an error of +0.02 m e q k g -1 in A T yields an error of _0.001 in S.A.). The surface waters have a S.A. = 0.122+0.001 while the deep waters have a S.A. = 0.124+0.002. The apparent increase in S.A. is within the scatter of the data. The summary of the data represented in Fig. 2 shows quite a lot of scatter in the various properties. Part is due to experimental errors; however, much of it is due to variations in the surface waters. This can be demonstrated by examining the temperature, salinity, pH25, A T, and S.A. for one station. Station 11 was selected because we have tritium data for this station (MILLERO et al., 1978b). The values of the different variables are plotted as a function of depth in Fig. 3.
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2400
2000
1600
DEPTH, m
1200
800
400
i
.~•
•
I
i
i I
1200
8OO
.
39.00
2.40 0
400
Ii
DEPTH, m
i
!
|
I
!
f
Ir
e
2400
L
•
I i
i
e e ~ • mum~
2400
I
•
i
2000
I t •
I
.~" I'#.°
•
Be
•
i
38.00
2000
•
• ooeo
;~',
$
•
i
S(%.)
1600
i
•
.....',;.
•
3ZOO
1600
DEPTH, m
1200
8OO
400
•
A T / C I (%*), meq kg "1 0.120 0.121 0.122 0123 0.124 0125 o
~ , ~.~,;~
36.00
,
AT,
•%
,
2.50
,
,
kg "1 i
• •
.
j°,"
i
i i
~
..;:-.
::°e#
~
".-#t~
~, .,..
• ~,e
",~o • • I Io:~
....
260 i
,
•
•
•
270 i
2400
2000
• .., are,•. . o •
meq
m
1600
DEPTH,
1200
800
400
0
'•e
8.00
•
i
".'.a
• ••0
•0
• lll~o •
pH
•
•
,
•
•
i
~'~.-"-
t..~:"~
..
8.10
i
8.20
M e d i t e r r a n e a n Sea.
Fig. 2. Profiles of t e m p e r a t u r e (T, °C), salinity (S, %o), pH25, titration alkalinity (AT, m e q kg 1), a n d specific alkalinity (AT/CI% o) for the western
2400
2000
1600
DEPTH, m
1200
800
400
o
12.0
T. °C 14.0 1 6 . 0 180 20.0 22.0 24,0
i
8.30
a
0 I:
P~
I
I
II
i
i
T, U
6.0
Tritium S t a t i o n II
40
y 2.Ù
8.0
I
Stahon II
Fig. 3. Profiles of salinity (S, °/oo, temperature (T, -C), tritium (T.U. tritium units), pH25, total alkalinity (AT, meq kg t l and specific alkalinity (AT/CI% o, meq k g - 1 ) for Sta. 11 in the Mediterranean Sea.
i
2400
2000
2000
2000
Station II
1600
1600
1600
2400
DEPTH, m
DEPTH, rn
DEPTH, m
|
1200
2400
0
SPECIFIC A L K A L I N I T Y , meq kg -~ 0.125 0.121 0.122 0.1231 0124
1200
270
1200
i
2.60
2400
1800
DEPTH, m
1200
800
'
250
A T , meq kg -I
0
600
800
240 0
I
II
I
8OO
8.30
I
Station
,
400
Station
8.20
PH25
I
,
240
400
8.10
I
,~I~2~
T E M P E R A T U R E , °C 160 200
40O
o
8.00
2400
2000
2000
I
1600
1600
I
DEPTH, m
DEPTH, m
2400
1200
1200
I
120 0
800
'
390
800
Station II
380
400
'
570
S A L I N I T Y , %o
400
0
360
=
Z
z -4
~=
d3
I-L
©
Z
=
o
F
z
"11
MD Oo
The carbonate system in the western Mediterranean Sea
1399
Table 1. Summary of results for the Mediterranean Sea* Parameter
S(°/~) T(°C) 0(°C) PH25oc PHi, situ AT(meq kg 1) AT/CI(%o) [ n c o 3 ] , mmol kg i [CO3], mmol kg 1 [COz], mmol kg I ZCO2, mmol kg -1 Pco~, 106 atm f~eal~le ~'~aragonite
Surface waters (0 to 100 m)
Deep waters (300 m to bottom)
36.8+0.2 (43) 23.5+0.5 (20) -8.19+0.02 (41) 8.20 + 0.02 (41) 2.39+0.03 (78) 0.1218 + 0.001 (99) 1.91+0.03 (51) 0.23+0.01 (52) 0.0125___0.001 (73) 2.18+0.05 (81) 393+31 (80) 5.2 + 0.2 (41) 3.1 + 0.1 (41)
38.44+0.02 (67) 13.05+0.05 (81) 12.83 4-0.07 (71) 8.05+0.01,, (109) 8.17 + 0.014 (109) 2.63+0.03 (163) 0.1243 ___0.0016 (123) 2.18+0.03 (110) 0.179-t-0.006 (139) 0.016,,+0.001 (145) 2.36+0.03 (136) 425+23 (100) 2.44 + 0.06 (8)t 1.43 + 0.03 (8)'t
* The number of data points used is given in parenthesis. * From the deepest stations (~2500 m). RESULTS AND CALCULATIONS
A full description of the calculations of the various parameters for the carbonate system is given in the data report by MILLEROet al. (1978b). We will briefly outline the methods used in this section. The measured values of pH were corrected to the in situ temperature using equations (MILLEROet al., 1978b) generated from the tables of HARVEY(1963). As the determinations of pH were made within I°C of the in situ temperature, this correction was quite small. The effect of pressure on the pH was determined from equations (MILLERO et al., 1978b) generated from the pressure coefficients published by CULBERSON and PYTKOWICZ(1968). As the temperatures of the surface waters are close to 25°C, the in situ pH is only 0.01 unit higher than pH25. The average value of the in situ pH in the deep waters is 8.17+0.01, only 0.03 pH units lower than the surface waters. The various parameters of the carbonate system were determined using the standard e q u a t i o n s (SKIRROW, 1965) A c = A T - K'a[B]T/(K'a + an)
[HCO3] = A J(1 + 2K~/aH) [CO 2- ] = AcK~/(an + 2K~) [CO2]
=
(AcaH/K'I)/(1 + 2K~/aH)
2CO2 = [ H C O ; ] + [CO3z-] + [CO2] Pco2 = [CO2]/ct I.P. = [ C a 2 + ] [ C O ] - ] n = I.P./K'~p
(1) (2) (3) (4) (5) (6) (7)
(8)
where the definition of the symbols have their usual meaning (SKIRROW, 1965). The values of the apparent constants (K~, K~, K~) were taken from LYMAN(1958) and MEHRBACHet al. (1973). The pressure coefficients were taken from CULBERSONand PYTKOWICZ(1968). The values of total boron [B]T and calcium [Ca 2 +] at a given salinity were calculated from the
1400
FRANKJ. MILLERO,JOHNMO~E and CHEN-TUNGCHEN
1.80 0
[HCO3-], mmol kg -I 1.90 2 . O 0 2 . i O 2.20 /
C032-], 0.I0 0
2.30
0.15
mmol
0.20
kg -I
0.25
I
• "-%".ItS#,i&" • . t . . , . • • "&%'.o
.; ~'...
...~
400
•
•
.e~l#
800
400
.e
e
800
.~o
-.
1200
:.
DEPTH, m
.e
•
#
1200 DEPTH, m
•
1600
o
1600 o
2000
2000
2400
•
,
,
,
2400
el •
I,"
0
I
0
i
[C02+ H2C03], mmol kg "1 O.O1 0.02 0.03 0.04 0.05 i___
0
,
t
I
,N
400
800
%
12OO DEPTH, m
,,h
1600
2000
2400
• i
i •
I.
i
I
I
Fig. 4. Profiles of the total concentration of HCO~ (mmolkg-l), CO~- (mmol kg 1) and CO2 + H2CO3 (mmol kg-l)in the western Mediterranean Sea.
0.30
The carbonate system in the western Mediterranean Sea
Pco2 X I03, otto
COz, mmol kg"i 2.00
.
0 •
_
2.20
2.60
2.40
i
1401
0.35
0.30
0
i
~';" ~,~r,.¢'~
0.40
0.45
•
0 .a5 0
-". • oeo ~
18o
400
400
•
•
odd
•
800
800
•
so
°oeo~e
e~
•
•
•
.
DEPTH, m
DEPTH, m
•
.'"
1200
1200
e
ee
•
•
e
1600
1600
"ee
0
2000
2000
..
•
2400 2400
I
I
I
ee
i
'I
1.00
2.00 3.00
1200
6.00
J.
400
800
4.00 5 . 0 0
•k #
Aragonite
•
•
Qe •
DEPTH, m
Calcite
1600
2000
2400
Fig. 5. Profiles of the total carbon ~CO2, mmol kg- ~), the partial pressure of carbon dioxide (Pc02 x 103, atm) and the saturation state (fl) of calcite and aragonit¢ in the western Mediterranean Sea.
1402
FRANK J. MILLERO, JOHN MORSE and CHEN-TUNG CHEN
ARAGONITE
CALCITE 0
6.0
4.0
2.0
0
MED
6.0
4.0 I
WATERS
• Western
400
210 I
I
• •
400
•
0
o Ao
a
•0 0
• Eastern
800
• oo
o o
oCentral
Ao 800
Ao o
0 h
1200
1200 m
DEPTH, m
1600
1600
DEPTH,
MED
2000
2000
WATERS
Weslern
•
Cenlral
o
Eastern
•
2400
2400 I
I
Fig. 6. Profilesof the saturation state (f~)of calcite and aragonite in the western (Sta. ll this study), central (ALEKIN,1972),and eastern (GEOSECS test station) Mediterranean Sea.
equations of MILLERO (in press). The values of ~ (the solubility coefficient for C O 2 ) w e r e taken from the equations of WEISS (1974). The values of K~ for calcite were calculated from the equation of INGLE (1975), while the values for aragonite were taken from the work of BERNER (1976). Errors of 4-0.01 meq kg -~ in A T and _+0.01 in pH give errors of +_0.02 mmoi kg- ~ in H C O 3 , _+0.004 mmol kg- ~ in CO 2-, _ 0.002 mmol k g - ~ in CO2, _+30 × 10- 6 atm in Pco2 and _+0.02 mmoi k g - 1 in y CO 2. The various parameters for the carbonate system for the Mediterranean stations are given elsewhere (MILLERO et al., 1978b). Profiles summarizing all the data are shown as a function of depth in Figs 4 and 5. A summary of the average values of [ H C O £ ] , [CO 2-], [CO2], ZCO2 and Pco2 for the surface and deep waters is given in Table 1. The ~CO 2 increases by 0.18_+ 0.08 mmol kg-1 from the surface to deep waters, while Pco2 shows little or no change within the scatter of the data ( + 30 × 10 -6 atm). The increase in ~CO2 (as well as the other carbonate parameters) with depth is largely due to the increase in the salinity of the water. The small changes in Pco2 indicate that little biological oxidation occurs in the Mediterranean waters (in contrast to deep Atlantic and Pacific waters). The fast turnover time of the Mediterranean waters (SVERDRUP, JOHNSONand FLEMING, 1942) causes the deep waters to have nearly equilibrium values of Pco2. Our results for the various components of the carbonate system are in good agreement with the results of CHERNYAKOVA(1976) in the Straits of Sicily, ALEKIN (1972) in the northwestern Mediterranean, and the GEOSECS test station (404) in the eastern Mediterranean. A summary of the values o f f l for calcite and aragonite for the Mediterranean samples is shown in Fig. 5. The surface waters have f2caI = 5.0 and flarag = 2.8, while the deep waters have values as low as Q~z = 2.5 and Qatar = 1.5. All the Mediterranean waters are supersaturated with respect to calcite and aragonite (in agreement with the work of
1403
The carbonate system in the western Mediterranean Sea
0
CALCITE .Q 1.0 2.0 3.0 4.0
'
0
'
o0
~°".
~1/
0
,
I000
4.0
5.0
I
|
4
2000
2000
DEPTH, m
DEPTH, m
o
3000
3000
4000
4000 MED
•
ATL
5000
5000'
"
-" I
I
Fig. 7.
,~..~
oo I
•
oS" o0 .0
IO00
ARAGONITE 1.0 2.0 3.0
0
5.0
I
I
I
I
MED
•
ATL
• I
I
I
Comparisons of profiles of the saturation state (t~) in the western Mediterranean Sea (Sta. 11) and the North Atlantic (GEOSECS Sta. 15).
ALEKIN, 1972, and the G E O S E C S test station in the eastern Mediterranean, Fig. 6). The deep waters are ~ 50~ less saturated than the surface waters. This decrease is largely due to the effect of pressure on the solubility of CaCO3. The deep waters of the Mediterranean are more highly saturated with respect to calcite and aragonite than are Atlantic waters (GEOSECS Sta. 115) at the same depth (Fig. 7). This is due to the lower values of pH in deep Atlantic waters (due to more biological oxidation). CHERNYAKOVA, 1976;
A cknowledgements--The authors wish to acknowledge the support of the Office of Naval Research (N00014-75-C0173) and the Oceanography Section of the National Science Foundation (OCE73-0035 l-A01) for this study. We also wish to thank Dr GOTE OSTLUND, head of the Miami tritium laboratory, for making the tritium measurements.
REFERENCES ALEkXNO. A. (1972) Saturation of Mediterranean Sea water with calcium carbonate. Geochemistry, 206, 239-242. BEN-YAAKOVS. (1970) A method for calculating the in situ pH of seawater. Limnology and Oceanography, 15, 326-328. BERNER R. A. (1976) The solubility of calcite and aragonite in seawater at atmospheric pressure and 34.5%,% salinity. American Journal of Science, 276, 713-730. Bucn K. and O. NYN~,S(1939) Studien iibenere pH-methodik mit besonderer Beriicksichtigung des Meerwassers. Academa Mathematics Physics, 12, 41. CHERNYAKOVA A. M. (1976) Elements of the carbonate system in the Straits of Sicily (Tunis Strait) area. Oceanology, 16, 36-39. CULBERSON C. and R. M. PYTKOWXCZ(1968) Effect of pressure on carbonic acid, boric acid, and the pH in seawater. Limnology and Oceanography, 13, 403-417. EDMONDJ. M. (1970) High precision determination of titration alkalinity and total carbon dioxide content of sea water by potentiometric titration. Deep-Sea Research, 17, 737-750. HARVEY H. W. (1963) The chemistry and fertility ofseawaters, Cambridge University Press, 240. pp. INGLE S. E. (1975) Solubility of calcite in the ocean. Marine Chemistry, 3, 301-319.
1404
FRANK J. MILLERO, JOHN MORSE and CHEN-TUNG CHEN
LYMAN J. (1956) Buffer mechanism of seawater. Ph.D. Thesis, University of California, Los Angeles, 196 pp. MEnRBACH C., C. H. CULnERSON J. E. HAWLEY and R. M. PYTgOWICZ (1973) Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography, 18, 897-907. MILLERO F. J. (1979) The thermodynamics of the carbonate system in seawater. Geochemica et Cosmochimica Acta, 43, 1651-1661. MILLERO F. J. (in press) The thermodynamics of seawater, In: Oceans handbook, R. A. HORrCE,editor, Chapter 4, Marcel Dekker. MILLERO F. J., D. MEANSand C. MILLER (1978a) The densities of Mediterranean Sea waters. Deep-Sea Research, 25, 563-569. MILLERO F. J., J. W. MORSE and C.-T. CHEN (1978b) Chemical oceanographic data from the western Mediterranean. Technical Report, National Science Foundation, 62 pp. SKIRROW G. (1965) The dissolved gases--carbon dioxide. In: Chemical oceanography~ J. P. RILEy and G. SK1RROW, editors, Academic Press, pp. 227-322. SVERDRUP H. U., M. W. JOHNSON and R. H. FLEMING (1942) The oceans, Prentice-Hall, 1087 pp. UNESCO (1966) Second report of the joint panel on Oceanographic Tables and Standards. UNESCO Technical Papers in Marine Science, No. 4 (mimeographed), 7 pp. WEISS R. F. (1974) Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Marine Chemistry, 2, 203-215.