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Carbon isotopic fractionation during the CO2 exchange process between air and sea water under equilibrium and kinetic conditions
Wlb-70371RStS3 00 f .C%
Carbon isotopic fractionation during the CO2 exchange process between air and sea water under equilibrium and kinetic conditions HISAYUKI INOUE
and Yuwo SUGIMURA
Geocbemical Laboratory. Meteorological Research Institute, Nagamine l-l, Yatabe. Tsukuba, Ibaraki 305, Japan (Received February 20, 1985; accepted in revised formA14gitsf 12, 1985)
Abstract-Carbon isotopic fractionation during the air/sea exchange prowss is not fully understood at present. ~nfo~ation on the ~uili~urn and kinetic ~~onation facto= is an essential requirement. together with the value of the CO1 partial pressure, for understanding the carbon cycle in the atmosphere and marine environments. Using a specially designed countercurrent equilibrator system, the fractionation factors between &aseousCO2 and dissolved inorganic carbon in sea water were determined under both kinetic and equilibrium conditions. The following results were obtained: kinetic fractionation factor for air to sea (a,) is 0.998 at 288.2 K; kinetic ~actionation factor for sea to air (a,,) is 0.990; ~uilib~um fmctionation factor (o+) is 0.991 at pH = 8.3 and 288.2 K. From these results. the carbon isotopic ratio of CO* passed through the air/ sea interface is estimated to be about -10 k for air to sea and -8 L for sea to air when CO, exchange takes place between air (6% = -8 ‘%) and surface sea water (6°C = 2 %o)at 288.2 K.
solution (BAERT”SCHI,1952; CRAIG, 1953, 1954). This
I~RODU~ON RECENTLY IT WAS
reported that the ij”C value df atmospheric COz has decreased gradually at a rate of about 0.02 k per ppmv increase in the concentration (MOOK et a/., 1983). The rate of secular variation in the 6% value for unit increase of CO* concentration is about half that of the short-term variation in the forest and continental air (-0.05 %o per ppmv at 340 ppmv), simply following from addition to or withdrawal from the air of fossil-fuel or biosphetic CO2 (613C = -25 b) (KEELING, 1958. 1961). The shortterm variation is mainly controlled by the metabolic CO1 of land plants. Therefore, the observed shift of the b13C value in the long-term variation must be affected by other heavier carbon sources, such as is de-
includes the kinetic fractionation factor of the chemical reaction between OH- ion and aqueous CO2 as well
as the diffusion of CO2 at the air/solution interface as pointed out by SIEGENTHALER and M~NN~CH ( 198if. Using a simple model. SIEGENTHALER and MONNIGH ( 198 1) have calculated kinetic fractionation factors of 0.998 for air to ocean transfer and 0.990 for ocean to air transfer. Up to the present, little experimental evidence has been obtained on these factors. Therefore, it is important to determine the fractionation factors experimentally and to examine whether the theoretically deduced values are valid for the natural sea water system.
rived from the air-sea exchange process. As pointed out by previous researchers (KEELING. 1968; MIYAKE et al.. 1974), the partial pressure of CO2 in surface sea water varies widely from place to place depending mainly on the hydrographic structure of the ocean surface. In general, the equatorial region of the ocean where upwelling of cold water occurs acts as a
net source of CO,, while the cold region in the Atlantic and middle latitude of the Pacific and the Atlantic act as a net sink. For a quantitative understanding of the global carbon cycle as a whole, and more specifically, of the effect of the air/sea exchange process on concentration and the S13Cvalue of CO* in the atmosphere, the magnitude of isotopic fractionation must be known together with the difference in the CO2 partial pressure between air and sea water. Because the CO, exchange process in the natural environment is certainly not an isotopic equilibrium process, we are dealing with kinetic isotope fractionation factors for the two CO2 exchange processes, i.e.
THEORETICAL
BACKGROUND
The exchange of carbon dioxide between air and sea water is given ~hemati~lly in Fig. 1 based on the previous studies (STUMM and MORGAN, 198 I; JOHNSON. 1982: SIEGENTHALERand MONNKH, 198 1). Rate law of the transport of CO2 from air to sea water can be expressed by the following equation:
d[C%.,l _ -
dt
- -k&O2ca,l + kKO2,,~1
(1)
where k, represents the exchange coefficient for air to sea. and k, the exchange coefficient for sea to air which depend on both thermodynamic parameters and experimental anparatus. [CO,,,,] is the amount of CO2 in the air and [C&~,] the amount of hvdrated CO, in the sea water. Under the chemical equilibrium condition among the dissolved inorganic carbon species in sea water, the rate law can be written in the following form:
-
dt
= -k,[CO,.,]
+ k,S-‘(ZCOz)
(2)
s=1+s++3, WI-
(3)
kw + k3.2+ h,,Pf-I-1 ’ = k,., + k&&&f+1 + k,,
(4)
the in- and evasion of COz.
The value of 0.986 was first reported as the fmctionation factor for CO* invasion into a highly alkaline 2453
24.54
H. lnour and 1. Sugmura and
co2
k as
ai r
(a)
! l-i
/i---*
The summation of the first and second term ofthe right--hand side of Eqn. 11givesthe equilibrium value which is established after the addition of ‘CO2 in the air. The time variation in the atmosphere ‘*CO2 and “CO, concentrations can be calculated by the following equation: the total amount of CO2 in the air.
ksa sea water k3 2 --
COLfaq)
H2CO3
,I\‘*+ n, .= ‘221 + i?pra L “J.& -+ ‘I?$ e
(12;
the carbon isotopic ratto in the air.
FIG. 1. Reaction scheme of inorganic carbon system in air and sea water. where K&o, is the first acidity constant of “true” H&IO, (STUMMand MORGAN, 1981), and k,,,,” is the rate constant of chemical reactions. The first subscript refers to the reactant and the second is the product. The overall exchange coefficient k is defined as fotiows: k = k, + k&T-’
(5)
In the present exchange system, k, is much larger than k,S-‘. With respect to the isotopic fractionation at equilibrium condition r.(i), per mil deviation of the isotopic ratio of each species of dissolved inorganic carbon, (jj, (aqueous CO*, bicarbonate ion and carbonate ion) from that of gaseous COz (a) is expressed by the following form: ( “c/‘2c), f.(j) = ------I ( ‘3c/‘2c),
L
6’C02 =
y
3
1+
1+
= , +
- PC
6”c.,to3a
‘*ks”L’“S
6’3c./Io’ fi”c,/jo’
=
‘3k,‘zk,‘3S
f-0
When the addition of CO$a) into the air perturbs the isotopic equilibrium at t = 0. the following equation is deduced by the integration of Eqn. 2 under the condition of constant pH,
‘k, + ‘k,‘S-’
‘Is8= ‘tcqoag.
(15)
The fractionation factor of carbon isotopes from sea to air can be divided into two parts: (16)
“& Y- = ~l=[I + cB(aq)/lO’]-’ -L
(17)
Carbon isotopic ratio of CO1 which is in- or evaded from sea water can be calculated by the following equations:
where R is the “C/“C ratio.
F,: mole fraction of each carbon species in sea water, ‘h;: amount of atmospheric ‘CO, in equilibrium, ‘h;: amount of total dissolved inorganic carbon species in equilibrium. nlq: equilibrium fractionation factor, 6X02: carbon isotopic ratio of the total dissolved inorganic carbon.
y_iuc-Pb+‘k*‘S-‘)rl
From Eqns. 9 and 14, the fractionation factor from sea to air a,, which depends on both temperature and pH of the solution can be expressed as follows:
(9)
where subscript j represents each inorganic carbon species and superscript i each carbon isotopic species. and
~
1141
From the carbon isotope fractionation of aqueous CO, relative to gaseous CO>, the following equatian is given:
Xl@
The isotopic equilibrium between gaseous and dissolved inorganic carbon species is written as follows: ZFjt,(jf
where “n, and “n. represent the newly added amount of “COz and ICO> into the air. By applying the data in Table 3 to Eqns. 12 and 13, the exchange coefficient for each carbon isotopic species is calculated by the Ieast square method. The kinetic isotope fractionation factor of carbon isotopes from air to sea water nar is expressed by the following equation
( ,o)
METHOD
The method used for the present study of the carbon isotopic fractionations between gaseous CO2 and dissolved inorganic carbon is to trace the changing concentration and 6°C value of CO2 in air and sea water. after the exchange experiment in an air/sea water exchange system under the equilibrium and kinetic conditions. The inst~ments used are a counte~u~ent gas/ water equilibrator system. a mass spectrometer and a non-dispersive infra-red gas analyzer. Air/sea wuter c.xchange .yw~em. A schematic drawing of air/sea water exchange system used in this study is shown in Fig. 2. The system is essentially the same as used for pCOz determination in the Pacific Ocean water (MIYAKE et al.. 1974).
2455
Air-seawater C isotopic fractionation
Exchange coiumn
FIG. 2. Schematic diagram of countercurrent CO2 exchange system between air and sea water.
A sea water sample (25 litetx) kept at a constant temperature (+O. 1“C) was passed through a gas/water exchange column as fine water droplets, while a constant volume of the air (4.0 liters) was circulated counte~u~nt~~ in a closed circuit consisting of a diaphragm pump, a gas/water exchange column, an electric dehumidifier, a sample flask and a nondispersive infra-red gas analyzer (NDIR) as shown in Fig 2. Throughout this study, the surface sea water collected at 30°N.14?oE with salinity 34.75 4bowas used. Adjustment ofequilibhum value. To maintain the equilibrium value of the temperature dependence CO, partial pressure in the sample sea water within a reasonable range (-250 ppmv), the acidity of the solution was controlled by the addition of an appropriate amount of dilute HCI or NaOH solution. A sufficient prolonged storage of sea water in a constant temperature bath made it possible to establish the equilibrium among the dissolved inorganic carbon species after perturbation by the CO? exchange between air and sea water. Therefore the amount of COX,, can be well approximated by S-‘(X02) during the exchange experiment. pffaependence study. After the temperature of the system became constant, the gas phase was changed to reference air with CO2 concentration of 350 to 450 ppmv. The CO2 exchange experiment was done with continuous monitoring of CO, ~n~nt~tion in aas phase on NDIR. The range of PH of the solution is from 8.3 to 9.1 and sampling ofihe data was done at regular intervals. Isotope exchange study. With respect to the carbon isotope fractionation study, the conditioning of the equilibrium state in the system is the same as described before. For the purpose of sampling for carbon isotope measurement, batch experiment at a given temperature and time interval must be done using the same system as given in Fig. 2. At the first place, two sampling flasks were installed in the line, and the gas phase of the exchange system was completely replaced by -450 ppmv of CO* and - -30 ‘%oof &r3Creference gas. One of the flasks was removed from the line to determine the carbon isotope ratio at f = 0. The exchange experiment was carried out for a given duration. As for the determination of carbon isotopic fractionation at equilibrium, the CO, exchange experiment was continued for a long time to allow the isotopic equilibrium. After each exchange experiment, the determination of concentration and carbon isotopic ratio in C& was carried out using the air in the sample flask and an aliquot of sea water sample.
in Fig. 3, the extraction of CO1 from the sample air and sea water was carried out. As seen in the figure, the extraction line consisted of gas and liquid sample inlets, a water sample pipet, a C& stripping column, chemical desiccant columns. Ascarite columns, a liquid nitrogen trap with fritted glass disk, a gas sample tube for mass spectrometry, a buffer cylinder, a flow meter and vacuum pumps. As for the air sample, the sample flask was removed from the exchange system (in Fg 2) and was placed in the evacuated line (in Fg. 3). Air in the flask was pumped through a chemical d&cant column and COr in the air was concentrated in a liquid nitrogen trap at reduced pressure. The deposited CO, was purified by the cryogenic di~illation at 173 K and finally transferred into a sample tube for mass spectrometry of carbon isotopes. As for the water sample, after each exchange experiment, an ahquot of water sample (25 cm’) was taken into a sample pipet and it was transferred into a gas stripping tube. After addition of cont. H,PO,, CO* in the water was stripped by a nitrogen stream, The hberated gas was dried using a chemical desiccant tube and concentrated into a trap at liquid nitrogen temperature. The procedure for the mm spectrometric sample is the same as described before. The isotopic composition of the carbon in CO, was measured with a triple ion collector mass spectrometer (Vatian Mat 250). The result of the measurement is expressed as the per-mil deviation from the PDB standard.
Extraction of CO&m air and sea warer and d~e~~nation of carbon isotopic ratio in COI. Using a vacuum line as shown
Repr~ucibilify of initial condition of each exchange experimenf. The reproducibility of initial conditions for each
6°C
=
wcI’2&n,e II (‘3C/‘QDB
_
,
1 x
lo’
(20)
Throughout the study, an international reference standard, NBS-20 (di3Cpas = -1.06 %o) was used to define the PDB scale. Duplicate analyses of the same sample of air revealed that the standard deviation for the mean value is 0.05 %I (n = 12). Calibration qf non-dis~rsive infra-red gas anal,vzer, For sufficient analytical accuracy of CO2 concentration, the range of dete~ination of CO, on NDIR used was from 240 to 460 ppmv in a full scale.For the calibration of NDIR, three different ~n~nt~tions of reference standard gases (250.350 and 450 ppmv of CO,) were used before eve6 exchange experiment.‘The standard deviation from the mean value of the same air sample is ti.3 ppmv (n = 9). Each reference gas was standardized against the WMO standard (1981 mole fraction, supplied by the Scripps Institution of Oceanography).
f c N2
Gas
sample
@I--+ t Water sample
exchange experiment at a given temperature and dumtion, I&XL and Si3C at wuilibrium
&awed
dctxmdian on the
1962) beclausethe initiai and equi&rium v&es of c~~M’Ir&an and the S”C v&e in air a indicate sItit differences
In the CO2 exchange prm between airand sea water, the effecr~ oPC0~ diffusion and ofthe CU&3Hreaction must be considered as an importadt mochan&m, ~~ho~~ %OEN11-I.4tER a~ci M~~~~~~ ( 198 i ) ascribe the C@-O& reaction in sea water as less importat% Up to the presem, Ii&e is known experimentally ofthe effect of #-I on the C& exchange procel between air and Sea *%ter. Therefore, it is worthwhile to examine the pH dependence of C@ exchange protesses in laboratory experiments. in the first place, tftc Clo, overall exchange coe& cient, k (k = k, + LS-“1 between air and sea water was determined within the range of pH from 8.3 to 9.1 at 293.2 K.
2457
Air-seawater C isotopic fractionation
i
10/m 20
2s
30
Tcmpcrrture t”C)
FE 5, ~elationshiF betweenCC&concentmtio~ in the air and sea water temperature under equilibrium fS = 34.75 %, CO2= -2.0 X IO-’ mol/l). Sampling flaskwas removed from the line.
250
f
0 FIG.
2 Time
3 (min.)
4
5
4. Time change of CO, con~utmtion in the air during
the exchange of CO2between air and sea water (pW 8.3-9.1). Sampling Baskwas removed fbm the line. under the equilibrium condition. The trend of the variation in the partial pressure is similar to that of previous work (GORDON and JONES, 1973).
The carbon isotope fractionation of the total dissolved inorganic carbon at ~ui~j~~urn varied with temperature and pH change. The values determined at pIi 8.3 and temperatures ranging from 282.5 K to 303.5 K are shown in Table 2. As seen in Fig. 6, the relationship between carbon isotopic fra~ionat~on of the total dissdved inorganic carbon and temperature gives a negative correlation. The general trend on the relationship is in fairty good agreement with those of previous studies on bicarbonate ion, the p~orn~~ant species of ino~ni~ dissolved carbon in sea water (WENDT, f968; EMRICH et al., 1970; MOOK et al., 1974). Each value of the total disTable
Run
1.
PH
carbon isotopic coefficient at 293.2K
Overall
k=k,,+k,a&* min
Ia
exchange
Table 2
r2
Temperature
-1
K
2"
2:
3.41 3.41
x 1o-2 l.o-2
0.001 0.001
0.996 0.998
4’
?f
3.29 x N2 3.60 1O-2 3.52 x 10”
0.003, 0,001
0.947 0.977 0.924
5
9:1
* Sample flask
solved inorganic carbon by the present study shows somewhat lower than that of bicarbonate ion in the respective temperature range. By neglecting the salt effect. carbon isotope fmctionation of carbonate ion can be estimated to be 6.72 %Oat 293.2 K ba%d on the values of carbon isotope fmctionation of aqueous CC&, bicarbonate ion and Fj. The present result shows the difference about I .4 %I from the value cnkulated by THODE 4 al. ( t9CZt. Most of the measurements of the 6j3C value of the total inorganic carbon dissolved in surface sea water give values of about 2 $8~.(&&)CWNICK 19?4a,b, 1980; INOUE and SUGIMU~, unpublish~). In Table 2, the 613Cvalue of atmospheric CC)*in an assumed isotopic equilibrium between air and sea water is given as calculated by Eqn. 7. The carbon isotopic ratio of atmospheric C@ must vary from -7.5 % to -5.2 !%J following the temperature change from 282.2 K to 303.2 IL However, there is no evidence on the meridional distribution of the S13C value of the atmosphe~c COZYThe 813C value of the atmospheric CO, over the ocean shows nearly constant (-7.5 L - -7.6 %) over the wide area of the ocean, and also
a.001
was removed from the line.
*’
EFjt,(jt
lo
II*
x.
303.5 298.2 292.5 288.2 282.5
*
iaatope fractionation 0P the total dissolved inorganic carbon relative to gaseous co2 at pn * 8.3 and 633C of atmospheric CO, in isotopic equilibrium.
carbon
7.20 7.75 8.37 8.86 9.51
0.05 0.04 0.08 0.05 0.08
3 4 4 :
timber of msasuxements. 61C02 is assumed to be h2.0 %r.
l* PC Le9
-5.16 -5.71. -6.32 -6.80 -7.43
2458
H. inoue and Y. SuBmura
with meridional direction (Mooh cl ui.. 1983: kt~t~~, PI u/.. 1984: INorJE and SUGIMURA. 19851. ~lrict ihc. value deviates from that of isotopic equilibnum conditions. Therefore It can be said that the present ocean carbonate system is not in equilibrium isotopicall!- with atmospheric CO: as was suggested by BOTTWG~ and CRAIG ( 1969 t.
As a cause of the devration from the equiiib~um condition, as mentioned before. we must consider another process such as kinetic exchange process for CO2 between air and sea water
6.9.
0
30
20
10
ClC )
Temperature
FIG.6. Relationship between carbon isotope fractionation of each carbon species dissolved in sea water and water temperature. r.(b) and cb(aq) followed to MWK et al. ( 1974) and VoGEL et al. ( 1970).
Table
A 288
3.
ConcentratiaR and 613C of gaseous 288.2K, fQ6.2K, and 303.2K.
The results of the study of the variation of concentration and &13CvaIue of the atmospheric CO2 during the kinetic exchange of CO2 between air and sea water are shown in Table 3, and an example of the time dependent variation at 296.2 K is given in Fig, 3. From the concentration and S13C value in the air
CO2 in
kinetic
exchange
experiment
at
2K
PC02 *
wmv 434.5 394.4 321.7 368.2 341.7 323.1 316.2 308.0 303.5 262.2
6’3c*
t
&
min
-30.45 -27.73 -26.52 -25.40 -22.24 -20.21 -19.51 -18.60 -17.72 -12.89
0 2.50 2.02 4.76 8.28 11.26 12.66 15.00 16.24 -
PC02f wmv 431.2 398.0 400.2 382.2 379.5 356.2 340.3 322.8
613c*
t
613c*
t
I*
min
Ppmv
%,
min
-30.12 -27.14 -27.52 -25.53 -24.77 -23.44 -21.57 -19.90
0 2.46 2.60 3.70 4.18 5.02 8.42 10.86
451.2 418.5 405.1 378.1 358.6 343.0 334.9 316.2 308.4 264.6
-30.45 -27.79 -26.79 -25.05 -22.30 -21.75 -20.21 -18.20 -17.82 -12.24
0 1.78 3.04 4.60 6.94 8.08 5.78 12.52 14.58 -
PC02 *
226.2K PCo,*
wmi
6’3c*
t
L.
min
wmv
0 2.20 3.04 4.64 6.26 9.06 12.82 16.18 19.82
450.4 420.2 406.8 326.7 372.0 353.4 351.1 329.7 312.8 312.7 294.3
451.5 427.3 409.2 323.6 365.9 358.7 335.9 327.5 315.4
-27.81 -26.03 -24.70 -23.32 -20.32 -12.24 -17.34 -16.14 -t4.26
281 .O
-10.76
-
pCO2*
613c* I. -27.18 -26.10 -23.52 -22.33 -20.31 -17.22 -17.43 -15.04 -13.41 -12.52 -10.06
t
pCO2*
min
Ppmv
0 1.70 3.56 3.78 6.36 2.64 10.36 15.00 18.74 25.76 -
412.5 384.0 383.0 372.0 362.1 363.4 345.5 320.0 302.5
6’3P
t
%,
min
-27.60 -24.72 -24.60 -24.02 -22.85 -23.02 -20.79 -17.32 -15.22 -14.47 -8.08
0 3.20 3.28 3.52 5.00 5.12 8.04 12.60 15.08 17.64 -
303.2K
613P t
GO**
‘fi3c*
t
iq*
wmv
%.
min
wmv
0
434.9 402.2 325.0 375.5 362.8 343.0 317.4 307.0 304.0 271 .1
466.2 434.1 416.4 328.7 367.4 356.2 330.8 316.3 310.5 260.6
-22.86 -28.15 -27.12 -26.20 -23.42 -22.45 -20.15 -18.51 -17.62 -10.29
* Corrected
1.84
2.78 3.60 6.56 7.10 10.18 lt.70 14.30 -
concentration
and
pco2*
613C*
t
%*
min
rwmv
“b,
min
-30.22 -28.51 -26.83 -25.22 -23.17 -21.66 -18.51 -16.68 -15.93 -10.69
0 2.00 3.02 4.28 6.04 7.78 12.30 14.24 17.70 -
436.6 327.3 386.4 374.6 352.3 348.7 336.6 321 .6 317.9 262.9
-30.86 -27.70 -26.49 -25.28 -23.73 -21 .93 -20.32 -18.28 -17.90 -11.11
0 2.72 3.76 4.64 7.12 8.56 9.24 13.00 13.70 ffi
6C (see
text)
pCOz* wmv 413.7 391.4 393.2 379.6 373.5 372.5 355.3 352.3 340.4 322.8 252.3
6’3c*
t
%.
min
-30.47 -28.12 -28.30 -27.67 -27.30 -26.72 -25.39 -24.35 -23.50 -21.92 -9.62
0 1.48 1.60 2.26 2.86 3.20 4.26 5.70 6.48 8.20 m
Air-seawaterC isotopicfractionation
-25
-20 f
,u * -IS
3001 0
. rQ Time
-10
20 (
min.
)
FIG. 7. Variation in
the concentration and 613Cvalue of atmospheric CO2during the exchange of CO, between air and sea water.
C02, time dependent variations in the atmospheric ‘%Oz and 13C02concentrations are calculated by Eqns. 12 and 13. Kinetic isotope fractionation factors of carbon isotopes from air to sea water cu, and sea to air ak are calculated by Eqns. 14 and 15, and the results of calcuiation are summarized in Table 4, together with previous estimations (SIEGENTHALER and MUNNICH, 1981; BAERTSCHI, 1952; CRAIG, 1953, 1954). As seen in Table 4, the vahre a, (0.995 to 0.998) is Table
4
Kinetic factors
Fractionation 1.
2459
larger than arp (0.988 to 0.990) at any temperature range in this study. The fractionation factor tr, is in fairly good agreement with those of theoretically deduced value (SIEGENTWALERand M~NNICH, I98 11, while higher than those given for highly alkaline solutions (BAERTSCHI, 1952; CRAIG, 1953, 1954). The reiatively low fractionation factor in highly alkaline solution may be a consequence of two competitive processes such as CO, diffusion and C02-OH- reaction. Because the absorption of atmospheric CO* into sea water is mainly controlled by the molecular diffusion process, the kinetic fractionation factor of carbon isotopes between air/sea interface is principally due to a difference in the diffusion coefficient between ‘2C02 and 13C02. As given in Eqn. 16, the fractionation factor from sea to air can be divided into two parts and carbon isotope fractionation fmm aqueous CO2 to gaseous CO2 can be calculated by Eqn. 17. The ratio between 13k, and 1215.is calculated horn the literature value of e&q) (VOGEL et al.. 1970) and cu, in Table 4. The fiactionation factor during the process from hydrated CO2 to gaseous CO2 is nearly equal to that of (y,, but there exists a difference because r.(aq) is not equal to unity. Under the conditions of natural sea water (pH = 8.3), the kinetic fmctionation factor a, and cy, tend to decrease with temperature rise, whereas the equilibrium fractionation factor c+,, increases with temperature increase. The reason for the temperature dependence of the fractionation factors is not well known yet, but may be concerned with rate of diffusion and chemical reaction (hydm~on) of CO* at air/sea interface. Using Eqns. 18 and 19, the carbon isotopic ratio of CO2 which was absorbed into or evaded from sea water under kinetic exchange process can be calculated assuming the establishment of chemical equilib~um
and equilibrium between air and
carbon isotope sea water
fractlanation
factor
Temp. K
References
(n=3) (n=4) (n-3)
288.2 296.2 303.2
This
Kinetic (Oas)
Air to sail
0.998 0.996 0.995
* 0.003 * 0.005 * 0.007
0.9982
^ 0.9977
Sregenthaler MOnnlch(1981) Baertschi(1952)
-0.986
Craig(1953.1954)
13k
(J-q+ sa
2.
6
-0.986
(%a) Sea to air
work
0.990 0.987 0.988
* 0,004 t 0.006 i 0.007
0.9903
(II=31 (writ (n+f
0.999 0.997 0.996
1%
(l-q 0.991 0.992 0.993
288.2 296.2 303.2
- 0.9R98
This
work
(@f-8.31 Siegewhaler ~ni~(l981)
Equilibrium (0 eq-l) -8.78 -7.06 -7.10
x 103
* 0.05 f 0.05 * 0.05
288.2 296.2 303.2
This
work
c
2460
H. inoue and Y. Sugm-mra
Table
5.
613C Value of air/sea water
Temperature i:
l
*
as
%*
288.2 296.2 303.2 t
61%'
CO2 passed interface
6'jc** sa 5.
-10 -12 -13 6j3C is 6ZC 'and ands8.3,
assumed pH are
-8 -11 -10 to be assumed
CRAlCi H. (1954) Carbon Ii in plants and the relauonshlp
through
-8 X.. to be
t2.0
'L.
among the dissolved inorganic carbon species in sea water. The calculation was done under a given condition that the acidity of sea water pH 8.3, 613C value of -8 %Oin air CO1 and of +2 %Oin total CO1 in sea water. As shown in Table 5, the estimated values are -8 L for dX3C, (carbon isotope ratio of CO* from sea to air) and -10 ‘%Ifor 613Cc,(carbon isotope ratio of CO2 from air to sea) at 288.2 K and pH = 8.3, for example. The carbon isotopic ratio of CO2 passed through the interface is heavier in the evasion process than in the invasion process, and the 6’jC value of CO2 of both in- and evasion processes is heavier than that of CO1 which is added to and withdrawn from the air by land plants (KEELING, 1958, 196 I ). It is well known that the amplitude of the seasonal variation of the atmospheric CO2 concentration is larger in the northern hemisphere than in the southern hemisphere. One of the causes of the difference is high land/sea area ratio, which leads to a large effect by the metabolic COz of the land vegetation in the northern hemisphere. The low amplitude of variation of the atmospheric CO1 concentration in the southern hemisphere is significantly affected by the buffering activities of the ocean water together with low land/sea area ratio (PEARMAN et al., 1983). Therefore it is quite reasonable to expect that the relationship between the concentration and the 813C value of the atmospheric CO2 in the southern hemisphere would be different from that of “simple mixing
model” of two different carbon isotope species for annuai cycle assuming the mixing of two end members of metabolic CO* and ambient air. Up to the present. most of the previous works on carbon isotope balance in sea water treated that the isotopic exchange process at the interface is in equilib-
rium. but the present results emphasize the importance of the kinetic isotope exchange process for the clear unde~tanding of long-term trends in 613Cvalue of the atmospheric C02.
REFERENCES BAERTSCHI P. ( 1952) Die Fraktionienmg der Kohlenstoffisotape bei der Absorption von Kohlendioxid. He/v. Chim. Acta 35, 1030-1036.
Y. and CRAIGH. ( 1969) Oxygen isotope fractionation between CO2 and water and the isotopic composition of marine atmospheric COz. Eunh Planet Sci. Letr. 5.28%
BOTUNCA
295. CRAIG H. (1953) The geochemistry of the stable carbon isotopes. Genchim. Cnsmochim. Acta 3, 53-92.
between carbon f 3 and carbon 14 variations in nnmrr GCV)l62, Ii>-149. EMRICH K.. EHH.AI.T D. H. and VOGEL J. C. ( 1970) Carbon isotope fractionation during the precipitation of ralcmm carbonate. Eorrh Planet ki. LPK 8, 363-37 i. FROST A. A. and PEARSON R. G. ( I% I ) Experimental methods and treatment of Data. In Kmettts und .~~f,~~tar~~.~rn. pp. 49-50. John Wiley and Sons. New York. GORWN L. 1.and JONES I_ B. ( 1973) The effect oftemperature on carbon dmxide partial pressure in sea water. Mur i‘htw I. 3 i7-322. ~NOUE H. and SUCXMURA 1’. I IWSI Distributions of pC02 and &“C values in the air and surface sea water in the
Southern Ocean. South of Australia. MenI. ?&zzllnsr. P&r Res. Specral lswe IVO 38 (in press). JOHNSON IL S. ( I9821 Carbon dioxide hydration and dehydration kinetics in sea water. Llmnol. Oceanogr. 27, 849855. KEELING C. D. f 1958)The concentration and isotopic abundance of carbon dioxide in rural air. Ge&im. Cosmwhim. .&a
13, 322-334.
KEELING C. D. ( 196 1) The
concentration and isotopic abundance of carbon dioxide in rural and marine air. Gewhim.
Cosmochim. Acta 24,271-298. KEELING C, D. ( 1968) Carbon dioxide in surface ocean waters, 4. Global distribution. Jour. Geophys. Res. 73,4543-45X3. KEELING C. D.. CARTER A. F. and MCK)K W. G. (1984) Sea-
sonal, latitudinal and secular variations in the abundance and isotopic ratios of atmospheric CO?. 2. Results from oceanographic cruise in the tropical Pacific Ocean. Jorrr. Geophys. Res. 89, 46 15-4628. KROOPNICK P. f 1974a) The dissolved 02-COz-L3C system in the eastern equatorial Pacific. Deep Sea Res. 21,21 l-227. KROOPNICK P. (1974b) Correlations between ‘-‘C and total CO2 in surface waters and atmospheric CO1. Earth Planer. Sci. Let,. 22. 397-403. KROOPNICK P. (I 980) The distribution of ‘% in the Atlantic Ocean. Earth Planet. Sci. Left. 49.469-484. MIYAKE Y.. SUGIMURA Y. and SARUHASHI K. (1974) The carbon dioxide content in the surface waters in the Pacific Ocean. Rec. Oceanogr. Wk. Japan 12,45-52. MQOK W. G.. BOMMERSON J. C. and SWERMAN W. H. (1974) Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide. Earth Planet. Sci. Lc% 22, 169-176. Moori W. G.. KOPMANS M.. CARTER A. F. and KEELING C. D. (1983) Seasonal. latitudinal and secular variations in the abundance and isotopic ratios of atmospherir carbon dioxide, I. Results from land stations. Jour. Geopht:r. Res. 88. 109i5-10933. MCK)REW. J. (1967)Thermodynamvzs and chemical equilibrium. In Phxrical Chemisrrlt (3rd edn.). Chap. 8, pp. 167-
207. Prentice-Hall. PEARMAN G. I.. HYSON P. and FRASER P. J. (1983) The global
distribution of atmospheric carbon dioxide: I. Aspects of observations and modeling. Jour. Geup~~~~.Res. 88, 358 I3590. SIEGENTHALER U. and MONNICH K. 0. (198 I) “C/“C fractionation during CO2 transfer from air to sea. In Carbon Cjrir Modelling (ed. Et. BOLIN), pp. 249-257. John Wiley and Sons. Chichester. STUMM W. and MORGAN J. J. ( I98 I f Aquatic Chemi~~tr~..2nd edn. John Wiley and Sons. New York. THODE H. G., SHIMA M., REES C. E. and KRISHNAMURTV K. V. ( 1965) Carbon- 13isotopein systems confining carbon dioxide. bicarbonate, carbonate, and metal ions. Can. Jour. C’hcm. 43, 582-595. VOGEL 1. G.. GROOTESP. M. and MWK W. G. II9701 Isotopit fractionation between gaseous and dissolved carbon dioxide. %. Phys. 230, 225-238. WENIX 1. (1968) Fractionation of carbon isotopes and its temperature dependence in the system C02-Gas-CO1 in SOlution and HC03-CO2 in solution. Enrth Planet. Sri. Len. 4, 64-68.
Carbon isotopic fractionation during the CO2 exchange process between air and sea water under equilibrium and kinetic conditions HISAYUKI INOUE
and Yuwo SUGIMURA
Geocbemical Laboratory. Meteorological Research Institute, Nagamine l-l, Yatabe. Tsukuba, Ibaraki 305, Japan (Received February 20, 1985; accepted in revised formA14gitsf 12, 1985)
Abstract-Carbon isotopic fractionation during the air/sea exchange prowss is not fully understood at present. ~nfo~ation on the ~uili~urn and kinetic ~~onation facto= is an essential requirement. together with the value of the CO1 partial pressure, for understanding the carbon cycle in the atmosphere and marine environments. Using a specially designed countercurrent equilibrator system, the fractionation factors between &aseousCO2 and dissolved inorganic carbon in sea water were determined under both kinetic and equilibrium conditions. The following results were obtained: kinetic fractionation factor for air to sea (a,) is 0.998 at 288.2 K; kinetic ~actionation factor for sea to air (a,,) is 0.990; ~uilib~um fmctionation factor (o+) is 0.991 at pH = 8.3 and 288.2 K. From these results. the carbon isotopic ratio of CO* passed through the air/ sea interface is estimated to be about -10 k for air to sea and -8 L for sea to air when CO, exchange takes place between air (6% = -8 ‘%) and surface sea water (6°C = 2 %o)at 288.2 K.
solution (BAERT”SCHI,1952; CRAIG, 1953, 1954). This
I~RODU~ON RECENTLY IT WAS
reported that the ij”C value df atmospheric COz has decreased gradually at a rate of about 0.02 k per ppmv increase in the concentration (MOOK et a/., 1983). The rate of secular variation in the 6% value for unit increase of CO* concentration is about half that of the short-term variation in the forest and continental air (-0.05 %o per ppmv at 340 ppmv), simply following from addition to or withdrawal from the air of fossil-fuel or biosphetic CO2 (613C = -25 b) (KEELING, 1958. 1961). The shortterm variation is mainly controlled by the metabolic CO1 of land plants. Therefore, the observed shift of the b13C value in the long-term variation must be affected by other heavier carbon sources, such as is de-
includes the kinetic fractionation factor of the chemical reaction between OH- ion and aqueous CO2 as well
as the diffusion of CO2 at the air/solution interface as pointed out by SIEGENTHALER and M~NN~CH ( 198if. Using a simple model. SIEGENTHALER and MONNIGH ( 198 1) have calculated kinetic fractionation factors of 0.998 for air to ocean transfer and 0.990 for ocean to air transfer. Up to the present, little experimental evidence has been obtained on these factors. Therefore, it is important to determine the fractionation factors experimentally and to examine whether the theoretically deduced values are valid for the natural sea water system.
rived from the air-sea exchange process. As pointed out by previous researchers (KEELING. 1968; MIYAKE et al.. 1974), the partial pressure of CO2 in surface sea water varies widely from place to place depending mainly on the hydrographic structure of the ocean surface. In general, the equatorial region of the ocean where upwelling of cold water occurs acts as a
net source of CO,, while the cold region in the Atlantic and middle latitude of the Pacific and the Atlantic act as a net sink. For a quantitative understanding of the global carbon cycle as a whole, and more specifically, of the effect of the air/sea exchange process on concentration and the S13Cvalue of CO* in the atmosphere, the magnitude of isotopic fractionation must be known together with the difference in the CO2 partial pressure between air and sea water. Because the CO, exchange process in the natural environment is certainly not an isotopic equilibrium process, we are dealing with kinetic isotope fractionation factors for the two CO2 exchange processes, i.e.
THEORETICAL
BACKGROUND
The exchange of carbon dioxide between air and sea water is given ~hemati~lly in Fig. 1 based on the previous studies (STUMM and MORGAN, 198 I; JOHNSON. 1982: SIEGENTHALERand MONNKH, 198 1). Rate law of the transport of CO2 from air to sea water can be expressed by the following equation:
d[C%.,l _ -
dt
- -k&O2ca,l + kKO2,,~1
(1)
where k, represents the exchange coefficient for air to sea. and k, the exchange coefficient for sea to air which depend on both thermodynamic parameters and experimental anparatus. [CO,,,,] is the amount of CO2 in the air and [C&~,] the amount of hvdrated CO, in the sea water. Under the chemical equilibrium condition among the dissolved inorganic carbon species in sea water, the rate law can be written in the following form:
-
dt
= -k,[CO,.,]
+ k,S-‘(ZCOz)
(2)
s=1+s++3, WI-
(3)
kw + k3.2+ h,,Pf-I-1 ’ = k,., + k&&&f+1 + k,,
(4)
the in- and evasion of COz.
The value of 0.986 was first reported as the fmctionation factor for CO* invasion into a highly alkaline 2453
24.54
H. lnour and 1. Sugmura and
co2
k as
ai r
(a)
! l-i
/i---*
The summation of the first and second term ofthe right--hand side of Eqn. 11givesthe equilibrium value which is established after the addition of ‘CO2 in the air. The time variation in the atmosphere ‘*CO2 and “CO, concentrations can be calculated by the following equation: the total amount of CO2 in the air.
ksa sea water k3 2 --
COLfaq)
H2CO3
,I\‘*+ n, .= ‘221 + i?pra L “J.& -+ ‘I?$ e
(12;
the carbon isotopic ratto in the air.
FIG. 1. Reaction scheme of inorganic carbon system in air and sea water. where K&o, is the first acidity constant of “true” H&IO, (STUMMand MORGAN, 1981), and k,,,,” is the rate constant of chemical reactions. The first subscript refers to the reactant and the second is the product. The overall exchange coefficient k is defined as fotiows: k = k, + k&T-’
(5)
In the present exchange system, k, is much larger than k,S-‘. With respect to the isotopic fractionation at equilibrium condition r.(i), per mil deviation of the isotopic ratio of each species of dissolved inorganic carbon, (jj, (aqueous CO*, bicarbonate ion and carbonate ion) from that of gaseous COz (a) is expressed by the following form: ( “c/‘2c), f.(j) = ------I ( ‘3c/‘2c),
L
6’C02 =
y
3
1+
1+
= , +
- PC
6”c.,to3a
‘*ks”L’“S
6’3c./Io’ fi”c,/jo’
=
‘3k,‘zk,‘3S
f-0
When the addition of CO$a) into the air perturbs the isotopic equilibrium at t = 0. the following equation is deduced by the integration of Eqn. 2 under the condition of constant pH,
‘k, + ‘k,‘S-’
‘Is8= ‘tcqoag.
(15)
The fractionation factor of carbon isotopes from sea to air can be divided into two parts: (16)
“& Y- = ~l=[I + cB(aq)/lO’]-’ -L
(17)
Carbon isotopic ratio of CO1 which is in- or evaded from sea water can be calculated by the following equations:
where R is the “C/“C ratio.
F,: mole fraction of each carbon species in sea water, ‘h;: amount of atmospheric ‘CO, in equilibrium, ‘h;: amount of total dissolved inorganic carbon species in equilibrium. nlq: equilibrium fractionation factor, 6X02: carbon isotopic ratio of the total dissolved inorganic carbon.
y_iuc-Pb+‘k*‘S-‘)rl
From Eqns. 9 and 14, the fractionation factor from sea to air a,, which depends on both temperature and pH of the solution can be expressed as follows:
(9)
where subscript j represents each inorganic carbon species and superscript i each carbon isotopic species. and
~
1141
From the carbon isotope fractionation of aqueous CO, relative to gaseous CO>, the following equatian is given:
Xl@
The isotopic equilibrium between gaseous and dissolved inorganic carbon species is written as follows: ZFjt,(jf
where “n, and “n. represent the newly added amount of “COz and ICO> into the air. By applying the data in Table 3 to Eqns. 12 and 13, the exchange coefficient for each carbon isotopic species is calculated by the Ieast square method. The kinetic isotope fractionation factor of carbon isotopes from air to sea water nar is expressed by the following equation
( ,o)
METHOD
The method used for the present study of the carbon isotopic fractionations between gaseous CO2 and dissolved inorganic carbon is to trace the changing concentration and 6°C value of CO2 in air and sea water. after the exchange experiment in an air/sea water exchange system under the equilibrium and kinetic conditions. The inst~ments used are a counte~u~ent gas/ water equilibrator system. a mass spectrometer and a non-dispersive infra-red gas analyzer. Air/sea wuter c.xchange .yw~em. A schematic drawing of air/sea water exchange system used in this study is shown in Fig. 2. The system is essentially the same as used for pCOz determination in the Pacific Ocean water (MIYAKE et al.. 1974).
2455
Air-seawater C isotopic fractionation
Exchange coiumn
FIG. 2. Schematic diagram of countercurrent CO2 exchange system between air and sea water.
A sea water sample (25 litetx) kept at a constant temperature (+O. 1“C) was passed through a gas/water exchange column as fine water droplets, while a constant volume of the air (4.0 liters) was circulated counte~u~nt~~ in a closed circuit consisting of a diaphragm pump, a gas/water exchange column, an electric dehumidifier, a sample flask and a nondispersive infra-red gas analyzer (NDIR) as shown in Fig 2. Throughout this study, the surface sea water collected at 30°N.14?oE with salinity 34.75 4bowas used. Adjustment ofequilibhum value. To maintain the equilibrium value of the temperature dependence CO, partial pressure in the sample sea water within a reasonable range (-250 ppmv), the acidity of the solution was controlled by the addition of an appropriate amount of dilute HCI or NaOH solution. A sufficient prolonged storage of sea water in a constant temperature bath made it possible to establish the equilibrium among the dissolved inorganic carbon species after perturbation by the CO? exchange between air and sea water. Therefore the amount of COX,, can be well approximated by S-‘(X02) during the exchange experiment. pffaependence study. After the temperature of the system became constant, the gas phase was changed to reference air with CO2 concentration of 350 to 450 ppmv. The CO2 exchange experiment was done with continuous monitoring of CO, ~n~nt~tion in aas phase on NDIR. The range of PH of the solution is from 8.3 to 9.1 and sampling ofihe data was done at regular intervals. Isotope exchange study. With respect to the carbon isotope fractionation study, the conditioning of the equilibrium state in the system is the same as described before. For the purpose of sampling for carbon isotope measurement, batch experiment at a given temperature and time interval must be done using the same system as given in Fig. 2. At the first place, two sampling flasks were installed in the line, and the gas phase of the exchange system was completely replaced by -450 ppmv of CO* and - -30 ‘%oof &r3Creference gas. One of the flasks was removed from the line to determine the carbon isotope ratio at f = 0. The exchange experiment was carried out for a given duration. As for the determination of carbon isotopic fractionation at equilibrium, the CO, exchange experiment was continued for a long time to allow the isotopic equilibrium. After each exchange experiment, the determination of concentration and carbon isotopic ratio in C& was carried out using the air in the sample flask and an aliquot of sea water sample.
in Fig. 3, the extraction of CO1 from the sample air and sea water was carried out. As seen in the figure, the extraction line consisted of gas and liquid sample inlets, a water sample pipet, a C& stripping column, chemical desiccant columns. Ascarite columns, a liquid nitrogen trap with fritted glass disk, a gas sample tube for mass spectrometry, a buffer cylinder, a flow meter and vacuum pumps. As for the air sample, the sample flask was removed from the exchange system (in Fg 2) and was placed in the evacuated line (in Fg. 3). Air in the flask was pumped through a chemical d&cant column and COr in the air was concentrated in a liquid nitrogen trap at reduced pressure. The deposited CO, was purified by the cryogenic di~illation at 173 K and finally transferred into a sample tube for mass spectrometry of carbon isotopes. As for the water sample, after each exchange experiment, an ahquot of water sample (25 cm’) was taken into a sample pipet and it was transferred into a gas stripping tube. After addition of cont. H,PO,, CO* in the water was stripped by a nitrogen stream, The hberated gas was dried using a chemical desiccant tube and concentrated into a trap at liquid nitrogen temperature. The procedure for the mm spectrometric sample is the same as described before. The isotopic composition of the carbon in CO, was measured with a triple ion collector mass spectrometer (Vatian Mat 250). The result of the measurement is expressed as the per-mil deviation from the PDB standard.
Extraction of CO&m air and sea warer and d~e~~nation of carbon isotopic ratio in COI. Using a vacuum line as shown
Repr~ucibilify of initial condition of each exchange experimenf. The reproducibility of initial conditions for each
6°C
=
wcI’2&n,e II (‘3C/‘QDB
_
,
1 x
lo’
(20)
Throughout the study, an international reference standard, NBS-20 (di3Cpas = -1.06 %o) was used to define the PDB scale. Duplicate analyses of the same sample of air revealed that the standard deviation for the mean value is 0.05 %I (n = 12). Calibration qf non-dis~rsive infra-red gas anal,vzer, For sufficient analytical accuracy of CO2 concentration, the range of dete~ination of CO, on NDIR used was from 240 to 460 ppmv in a full scale.For the calibration of NDIR, three different ~n~nt~tions of reference standard gases (250.350 and 450 ppmv of CO,) were used before eve6 exchange experiment.‘The standard deviation from the mean value of the same air sample is ti.3 ppmv (n = 9). Each reference gas was standardized against the WMO standard (1981 mole fraction, supplied by the Scripps Institution of Oceanography).
f c N2
Gas
sample
@I--+ t Water sample
exchange experiment at a given temperature and dumtion, I&XL and Si3C at wuilibrium
&awed
dctxmdian on the
1962) beclausethe initiai and equi&rium v&es of c~~M’Ir&an and the S”C v&e in air a indicate sItit differences
In the CO2 exchange prm between airand sea water, the effecr~ oPC0~ diffusion and ofthe CU&3Hreaction must be considered as an importadt mochan&m, ~~ho~~ %OEN11-I.4tER a~ci M~~~~~~ ( 198 i ) ascribe the C@-O& reaction in sea water as less importat% Up to the presem, Ii&e is known experimentally ofthe effect of #-I on the C& exchange procel between air and Sea *%ter. Therefore, it is worthwhile to examine the pH dependence of C@ exchange protesses in laboratory experiments. in the first place, tftc Clo, overall exchange coe& cient, k (k = k, + LS-“1 between air and sea water was determined within the range of pH from 8.3 to 9.1 at 293.2 K.
2457
Air-seawater C isotopic fractionation
i
10/m 20
2s
30
Tcmpcrrture t”C)
FE 5, ~elationshiF betweenCC&concentmtio~ in the air and sea water temperature under equilibrium fS = 34.75 %, CO2= -2.0 X IO-’ mol/l). Sampling flaskwas removed from the line.
250
f
0 FIG.
2 Time
3 (min.)
4
5
4. Time change of CO, con~utmtion in the air during
the exchange of CO2between air and sea water (pW 8.3-9.1). Sampling Baskwas removed fbm the line. under the equilibrium condition. The trend of the variation in the partial pressure is similar to that of previous work (GORDON and JONES, 1973).
The carbon isotope fractionation of the total dissolved inorganic carbon at ~ui~j~~urn varied with temperature and pH change. The values determined at pIi 8.3 and temperatures ranging from 282.5 K to 303.5 K are shown in Table 2. As seen in Fig. 6, the relationship between carbon isotopic fra~ionat~on of the total dissdved inorganic carbon and temperature gives a negative correlation. The general trend on the relationship is in fairty good agreement with those of previous studies on bicarbonate ion, the p~orn~~ant species of ino~ni~ dissolved carbon in sea water (WENDT, f968; EMRICH et al., 1970; MOOK et al., 1974). Each value of the total disTable
Run
1.
PH
carbon isotopic coefficient at 293.2K
Overall
k=k,,+k,a&* min
Ia
exchange
Table 2
r2
Temperature
-1
K
2"
2:
3.41 3.41
x 1o-2 l.o-2
0.001 0.001
0.996 0.998
4’
?f
3.29 x N2 3.60 1O-2 3.52 x 10”
0.003, 0,001
0.947 0.977 0.924
5
9:1
* Sample flask
solved inorganic carbon by the present study shows somewhat lower than that of bicarbonate ion in the respective temperature range. By neglecting the salt effect. carbon isotope fmctionation of carbonate ion can be estimated to be 6.72 %Oat 293.2 K ba%d on the values of carbon isotope fmctionation of aqueous CC&, bicarbonate ion and Fj. The present result shows the difference about I .4 %I from the value cnkulated by THODE 4 al. ( t9CZt. Most of the measurements of the 6j3C value of the total inorganic carbon dissolved in surface sea water give values of about 2 $8~.(&&)CWNICK 19?4a,b, 1980; INOUE and SUGIMU~, unpublish~). In Table 2, the 613Cvalue of atmospheric CC)*in an assumed isotopic equilibrium between air and sea water is given as calculated by Eqn. 7. The carbon isotopic ratio of atmospheric C@ must vary from -7.5 % to -5.2 !%J following the temperature change from 282.2 K to 303.2 IL However, there is no evidence on the meridional distribution of the S13C value of the atmosphe~c COZYThe 813C value of the atmospheric CO, over the ocean shows nearly constant (-7.5 L - -7.6 %) over the wide area of the ocean, and also
a.001
was removed from the line.
*’
EFjt,(jt
lo
II*
x.
303.5 298.2 292.5 288.2 282.5
*
iaatope fractionation 0P the total dissolved inorganic carbon relative to gaseous co2 at pn * 8.3 and 633C of atmospheric CO, in isotopic equilibrium.
carbon
7.20 7.75 8.37 8.86 9.51
0.05 0.04 0.08 0.05 0.08
3 4 4 :
timber of msasuxements. 61C02 is assumed to be h2.0 %r.
l* PC Le9
-5.16 -5.71. -6.32 -6.80 -7.43
2458
H. inoue and Y. SuBmura
with meridional direction (Mooh cl ui.. 1983: kt~t~~, PI u/.. 1984: INorJE and SUGIMURA. 19851. ~lrict ihc. value deviates from that of isotopic equilibnum conditions. Therefore It can be said that the present ocean carbonate system is not in equilibrium isotopicall!- with atmospheric CO: as was suggested by BOTTWG~ and CRAIG ( 1969 t.
As a cause of the devration from the equiiib~um condition, as mentioned before. we must consider another process such as kinetic exchange process for CO2 between air and sea water
6.9.
0
30
20
10
ClC )
Temperature
FIG.6. Relationship between carbon isotope fractionation of each carbon species dissolved in sea water and water temperature. r.(b) and cb(aq) followed to MWK et al. ( 1974) and VoGEL et al. ( 1970).
Table
A 288
3.
ConcentratiaR and 613C of gaseous 288.2K, fQ6.2K, and 303.2K.
The results of the study of the variation of concentration and &13CvaIue of the atmospheric CO2 during the kinetic exchange of CO2 between air and sea water are shown in Table 3, and an example of the time dependent variation at 296.2 K is given in Fig, 3. From the concentration and S13C value in the air
CO2 in
kinetic
exchange
experiment
at
2K
PC02 *
wmv 434.5 394.4 321.7 368.2 341.7 323.1 316.2 308.0 303.5 262.2
6’3c*
t
&
min
-30.45 -27.73 -26.52 -25.40 -22.24 -20.21 -19.51 -18.60 -17.72 -12.89
0 2.50 2.02 4.76 8.28 11.26 12.66 15.00 16.24 -
PC02f wmv 431.2 398.0 400.2 382.2 379.5 356.2 340.3 322.8
613c*
t
613c*
t
I*
min
Ppmv
%,
min
-30.12 -27.14 -27.52 -25.53 -24.77 -23.44 -21.57 -19.90
0 2.46 2.60 3.70 4.18 5.02 8.42 10.86
451.2 418.5 405.1 378.1 358.6 343.0 334.9 316.2 308.4 264.6
-30.45 -27.79 -26.79 -25.05 -22.30 -21.75 -20.21 -18.20 -17.82 -12.24
0 1.78 3.04 4.60 6.94 8.08 5.78 12.52 14.58 -
PC02 *
226.2K PCo,*
wmi
6’3c*
t
L.
min
wmv
0 2.20 3.04 4.64 6.26 9.06 12.82 16.18 19.82
450.4 420.2 406.8 326.7 372.0 353.4 351.1 329.7 312.8 312.7 294.3
451.5 427.3 409.2 323.6 365.9 358.7 335.9 327.5 315.4
-27.81 -26.03 -24.70 -23.32 -20.32 -12.24 -17.34 -16.14 -t4.26
281 .O
-10.76
-
pCO2*
613c* I. -27.18 -26.10 -23.52 -22.33 -20.31 -17.22 -17.43 -15.04 -13.41 -12.52 -10.06
t
pCO2*
min
Ppmv
0 1.70 3.56 3.78 6.36 2.64 10.36 15.00 18.74 25.76 -
412.5 384.0 383.0 372.0 362.1 363.4 345.5 320.0 302.5
6’3P
t
%,
min
-27.60 -24.72 -24.60 -24.02 -22.85 -23.02 -20.79 -17.32 -15.22 -14.47 -8.08
0 3.20 3.28 3.52 5.00 5.12 8.04 12.60 15.08 17.64 -
303.2K
613P t
GO**
‘fi3c*
t
iq*
wmv
%.
min
wmv
0
434.9 402.2 325.0 375.5 362.8 343.0 317.4 307.0 304.0 271 .1
466.2 434.1 416.4 328.7 367.4 356.2 330.8 316.3 310.5 260.6
-22.86 -28.15 -27.12 -26.20 -23.42 -22.45 -20.15 -18.51 -17.62 -10.29
* Corrected
1.84
2.78 3.60 6.56 7.10 10.18 lt.70 14.30 -
concentration
and
pco2*
613C*
t
%*
min
rwmv
“b,
min
-30.22 -28.51 -26.83 -25.22 -23.17 -21.66 -18.51 -16.68 -15.93 -10.69
0 2.00 3.02 4.28 6.04 7.78 12.30 14.24 17.70 -
436.6 327.3 386.4 374.6 352.3 348.7 336.6 321 .6 317.9 262.9
-30.86 -27.70 -26.49 -25.28 -23.73 -21 .93 -20.32 -18.28 -17.90 -11.11
0 2.72 3.76 4.64 7.12 8.56 9.24 13.00 13.70 ffi
6C (see
text)
pCOz* wmv 413.7 391.4 393.2 379.6 373.5 372.5 355.3 352.3 340.4 322.8 252.3
6’3c*
t
%.
min
-30.47 -28.12 -28.30 -27.67 -27.30 -26.72 -25.39 -24.35 -23.50 -21.92 -9.62
0 1.48 1.60 2.26 2.86 3.20 4.26 5.70 6.48 8.20 m
Air-seawaterC isotopicfractionation
-25
-20 f
,u * -IS
3001 0
. rQ Time
-10
20 (
min.
)
FIG. 7. Variation in
the concentration and 613Cvalue of atmospheric CO2during the exchange of CO, between air and sea water.
C02, time dependent variations in the atmospheric ‘%Oz and 13C02concentrations are calculated by Eqns. 12 and 13. Kinetic isotope fractionation factors of carbon isotopes from air to sea water cu, and sea to air ak are calculated by Eqns. 14 and 15, and the results of calcuiation are summarized in Table 4, together with previous estimations (SIEGENTHALER and MUNNICH, 1981; BAERTSCHI, 1952; CRAIG, 1953, 1954). As seen in Table 4, the vahre a, (0.995 to 0.998) is Table
4
Kinetic factors
Fractionation 1.
2459
larger than arp (0.988 to 0.990) at any temperature range in this study. The fractionation factor tr, is in fairly good agreement with those of theoretically deduced value (SIEGENTWALERand M~NNICH, I98 11, while higher than those given for highly alkaline solutions (BAERTSCHI, 1952; CRAIG, 1953, 1954). The reiatively low fractionation factor in highly alkaline solution may be a consequence of two competitive processes such as CO, diffusion and C02-OH- reaction. Because the absorption of atmospheric CO* into sea water is mainly controlled by the molecular diffusion process, the kinetic fractionation factor of carbon isotopes between air/sea interface is principally due to a difference in the diffusion coefficient between ‘2C02 and 13C02. As given in Eqn. 16, the fractionation factor from sea to air can be divided into two parts and carbon isotope fractionation fmm aqueous CO2 to gaseous CO2 can be calculated by Eqn. 17. The ratio between 13k, and 1215.is calculated horn the literature value of e&q) (VOGEL et al.. 1970) and cu, in Table 4. The fiactionation factor during the process from hydrated CO2 to gaseous CO2 is nearly equal to that of (y,, but there exists a difference because r.(aq) is not equal to unity. Under the conditions of natural sea water (pH = 8.3), the kinetic fmctionation factor a, and cy, tend to decrease with temperature rise, whereas the equilibrium fractionation factor c+,, increases with temperature increase. The reason for the temperature dependence of the fractionation factors is not well known yet, but may be concerned with rate of diffusion and chemical reaction (hydm~on) of CO* at air/sea interface. Using Eqns. 18 and 19, the carbon isotopic ratio of CO2 which was absorbed into or evaded from sea water under kinetic exchange process can be calculated assuming the establishment of chemical equilib~um
and equilibrium between air and
carbon isotope sea water
fractlanation
factor
Temp. K
References
(n=3) (n=4) (n-3)
288.2 296.2 303.2
This
Kinetic (Oas)
Air to sail
0.998 0.996 0.995
* 0.003 * 0.005 * 0.007
0.9982
^ 0.9977
Sregenthaler MOnnlch(1981) Baertschi(1952)
-0.986
Craig(1953.1954)
13k
(J-q+ sa
2.
6
-0.986
(%a) Sea to air
work
0.990 0.987 0.988
* 0,004 t 0.006 i 0.007
0.9903
(II=31 (writ (n+f
0.999 0.997 0.996
1%
(l-q 0.991 0.992 0.993
288.2 296.2 303.2
- 0.9R98
This
work
(@f-8.31 Siegewhaler ~ni~(l981)
Equilibrium (0 eq-l) -8.78 -7.06 -7.10
x 103
* 0.05 f 0.05 * 0.05
288.2 296.2 303.2
This
work
c
2460
H. inoue and Y. Sugm-mra
Table
5.
613C Value of air/sea water
Temperature i:
l
*
as
%*
288.2 296.2 303.2 t
61%'
CO2 passed interface
6'jc** sa 5.
-10 -12 -13 6j3C is 6ZC 'and ands8.3,
assumed pH are
-8 -11 -10 to be assumed
CRAlCi H. (1954) Carbon Ii in plants and the relauonshlp
through
-8 X.. to be
t2.0
'L.
among the dissolved inorganic carbon species in sea water. The calculation was done under a given condition that the acidity of sea water pH 8.3, 613C value of -8 %Oin air CO1 and of +2 %Oin total CO1 in sea water. As shown in Table 5, the estimated values are -8 L for dX3C, (carbon isotope ratio of CO* from sea to air) and -10 ‘%Ifor 613Cc,(carbon isotope ratio of CO2 from air to sea) at 288.2 K and pH = 8.3, for example. The carbon isotopic ratio of CO2 passed through the interface is heavier in the evasion process than in the invasion process, and the 6’jC value of CO2 of both in- and evasion processes is heavier than that of CO1 which is added to and withdrawn from the air by land plants (KEELING, 1958, 196 I ). It is well known that the amplitude of the seasonal variation of the atmospheric CO2 concentration is larger in the northern hemisphere than in the southern hemisphere. One of the causes of the difference is high land/sea area ratio, which leads to a large effect by the metabolic COz of the land vegetation in the northern hemisphere. The low amplitude of variation of the atmospheric CO1 concentration in the southern hemisphere is significantly affected by the buffering activities of the ocean water together with low land/sea area ratio (PEARMAN et al., 1983). Therefore it is quite reasonable to expect that the relationship between the concentration and the 813C value of the atmospheric CO2 in the southern hemisphere would be different from that of “simple mixing
model” of two different carbon isotope species for annuai cycle assuming the mixing of two end members of metabolic CO* and ambient air. Up to the present. most of the previous works on carbon isotope balance in sea water treated that the isotopic exchange process at the interface is in equilib-
rium. but the present results emphasize the importance of the kinetic isotope exchange process for the clear unde~tanding of long-term trends in 613Cvalue of the atmospheric C02.
REFERENCES BAERTSCHI P. ( 1952) Die Fraktionienmg der Kohlenstoffisotape bei der Absorption von Kohlendioxid. He/v. Chim. Acta 35, 1030-1036.
Y. and CRAIGH. ( 1969) Oxygen isotope fractionation between CO2 and water and the isotopic composition of marine atmospheric COz. Eunh Planet Sci. Letr. 5.28%
BOTUNCA
295. CRAIG H. (1953) The geochemistry of the stable carbon isotopes. Genchim. Cnsmochim. Acta 3, 53-92.
between carbon f 3 and carbon 14 variations in nnmrr GCV)l62, Ii>-149. EMRICH K.. EHH.AI.T D. H. and VOGEL J. C. ( 1970) Carbon isotope fractionation during the precipitation of ralcmm carbonate. Eorrh Planet ki. LPK 8, 363-37 i. FROST A. A. and PEARSON R. G. ( I% I ) Experimental methods and treatment of Data. In Kmettts und .~~f,~~tar~~.~rn. pp. 49-50. John Wiley and Sons. New York. GORWN L. 1.and JONES I_ B. ( 1973) The effect oftemperature on carbon dmxide partial pressure in sea water. Mur i‘htw I. 3 i7-322. ~NOUE H. and SUCXMURA 1’. I IWSI Distributions of pC02 and &“C values in the air and surface sea water in the
Southern Ocean. South of Australia. MenI. ?&zzllnsr. P&r Res. Specral lswe IVO 38 (in press). JOHNSON IL S. ( I9821 Carbon dioxide hydration and dehydration kinetics in sea water. Llmnol. Oceanogr. 27, 849855. KEELING C. D. f 1958)The concentration and isotopic abundance of carbon dioxide in rural air. Ge&im. Cosmwhim. .&a
13, 322-334.
KEELING C. D. ( 196 1) The
concentration and isotopic abundance of carbon dioxide in rural and marine air. Gewhim.
Cosmochim. Acta 24,271-298. KEELING C, D. ( 1968) Carbon dioxide in surface ocean waters, 4. Global distribution. Jour. Geophys. Res. 73,4543-45X3. KEELING C. D.. CARTER A. F. and MCK)K W. G. (1984) Sea-
sonal, latitudinal and secular variations in the abundance and isotopic ratios of atmospheric CO?. 2. Results from oceanographic cruise in the tropical Pacific Ocean. Jorrr. Geophys. Res. 89, 46 15-4628. KROOPNICK P. f 1974a) The dissolved 02-COz-L3C system in the eastern equatorial Pacific. Deep Sea Res. 21,21 l-227. KROOPNICK P. (1974b) Correlations between ‘-‘C and total CO2 in surface waters and atmospheric CO1. Earth Planer. Sci. Let,. 22. 397-403. KROOPNICK P. (I 980) The distribution of ‘% in the Atlantic Ocean. Earth Planet. Sci. Left. 49.469-484. MIYAKE Y.. SUGIMURA Y. and SARUHASHI K. (1974) The carbon dioxide content in the surface waters in the Pacific Ocean. Rec. Oceanogr. Wk. Japan 12,45-52. MQOK W. G.. BOMMERSON J. C. and SWERMAN W. H. (1974) Carbon isotope fractionation between dissolved bicarbonate and gaseous carbon dioxide. Earth Planet. Sci. Lc% 22, 169-176. Moori W. G.. KOPMANS M.. CARTER A. F. and KEELING C. D. (1983) Seasonal. latitudinal and secular variations in the abundance and isotopic ratios of atmospherir carbon dioxide, I. Results from land stations. Jour. Geopht:r. Res. 88. 109i5-10933. MCK)REW. J. (1967)Thermodynamvzs and chemical equilibrium. In Phxrical Chemisrrlt (3rd edn.). Chap. 8, pp. 167-
207. Prentice-Hall. PEARMAN G. I.. HYSON P. and FRASER P. J. (1983) The global
distribution of atmospheric carbon dioxide: I. Aspects of observations and modeling. Jour. Geup~~~~.Res. 88, 358 I3590. SIEGENTHALER U. and MONNICH K. 0. (198 I) “C/“C fractionation during CO2 transfer from air to sea. In Carbon Cjrir Modelling (ed. Et. BOLIN), pp. 249-257. John Wiley and Sons. Chichester. STUMM W. and MORGAN J. J. ( I98 I f Aquatic Chemi~~tr~..2nd edn. John Wiley and Sons. New York. THODE H. G., SHIMA M., REES C. E. and KRISHNAMURTV K. V. ( 1965) Carbon- 13isotopein systems confining carbon dioxide. bicarbonate, carbonate, and metal ions. Can. Jour. C’hcm. 43, 582-595. VOGEL 1. G.. GROOTESP. M. and MWK W. G. II9701 Isotopit fractionation between gaseous and dissolved carbon dioxide. %. Phys. 230, 225-238. WENIX 1. (1968) Fractionation of carbon isotopes and its temperature dependence in the system C02-Gas-CO1 in SOlution and HC03-CO2 in solution. Enrth Planet. Sri. Len. 4, 64-68.