Carbon isotope fractionation between gaseous carbon dioxide and aqueous bicarbonat

Degens, E. T.

Deuser, W. G.

English

NASA Technical Reports Server

C .  . . 
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WOODS 
CARBON ISOTOPE FRACTIONATION IN THE 
SYSTEM co2 (gas) -CO* (aqueous) -HCO~- (aqueous) 
GPO PRICE $,- 
I ff 653 July 65 
https://ntrs.nasa.gov/search.jsp?R=19670018374 2020-03-12T11:36:23+00:00Z
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Abs t rac t .  
and aqueous bicarbonate  decreases from 9.2 t o  6.8%0 over  t h e  temperature 
range 0 t o  3OoC. 
i n  t h e  passage o f  atmospheric C02 through t h e  air-water i n t e r f a c e .  
Carbon i s o t o p e  f r a c t i o n a t i o n  between gaseous carbon dioxide 
This f r a c t i o n a t i o n  occurs i n  t h e  hydra t ion  s t a g e ,  no t  
. .  
I t  has  long been recognized 
i n  C13 r e l a t i v e  t o  t h e  ino rgan jc  
- 3- 
t h a t  atmospheric carbon d ioxide  i s  deple ted  
carbon pool  of t h e  oceans. Reid and Urey 
(1) first measured t h e  equi l ibr ium f r a c t i o n a t i o n  i n  t h e  exchange C1302 + 
HC O3 
C. 
r e l a t i v e  t o  oceanic  b icarbonate  and Abelson and Hoering (3) redetermined a 
value of  1.0083 f o r  t h e  f r a c t i o n a t i o n  i n  t h e  above equi l ibr ium a t  25OC. 
l a t t e r  va lue  i s  apparent ly  t h e  only one der ived  from l abora to ry  experiments 
us ing  improved a n a l y t i c a l  techniques t h a t  have become a v a i l a b l e  s i n c e  t h e  
e a r l y  work of Reid apd Urey (1 ) .  
on t h e  change o f  t h i s  f r a c t i o n a t i o n  over t h e  temperature  range 0 t o  30'C 
which is  t h e  most important  one wi th  regard t o  n a t u r a l  condi t ions  involv ing  
t h e  atmosphere and t h e  oceans. 
f r a c t i o n a t i o n  between oceanic  bicarbonate  and molecular  carbon dioxide d i s -  
so lved  i n  t h e  water. Both f r a c t i o n a t i o n  e f f e c t s  a r e  of  geochemical as  we l l  
as b i o l o g i c a l  i n t e r e s t .  Knowledge of t h e i r  magnitude may a i d ,  for example, 
i n  determining whether c e r t a i n  marine p l an t s  u t i l i z e  d i s so lved  carbon dio-  
x ide  o r  b icarbonate  o r  poss ib ly  both spec ies  during photosynthes is .  
t h e r e f o r e  decided t o  p e r f o m  s e v e r a l  l abora tory  experiments t o  determine 
t h e  s ize  of t h e s e  f r a c t i o n a t i o n s  under equi l ibr ium condi t ions .  
12  - 4 13 - + 
T- C1'02 + HC O3 and a r r i v e d  a t  a value of  1.0120 - 0.0025 a t  25' 
Craig  (2) found a C13 dep le t ion  o f  some 7%0in atmospheric carbon d ioxide  
This  
No experimental  d a t a  a t  a l l  seem t o  e x i s t  
Data are a l s o  lack ing  on carbon i s o t o p e  
We 
Round-bottom, three-neck d i s t i l l i n g  f l a s k s  of 300 m l  c apac i ty  were used 
as r e a c t i o n  v e s s e l s .  Attached were a gas sample tube ,  a l iquid-sampling p i -  
p e t t e  c o n s i s t i n g  of two s topcocks i n  s e r i e s  wi th  a 3 - m l  volume between them, 
a modif ied stopcock whose bulb  contained H3P04 and a main stopcock f o r  i s o -  
l a t i n g  t h e  whole system. A l l  stopcocks and j o i n t s  were l u b r i c a t e d  with a 
minimum of s i l i c o n e  grease i n  brder  t o  avoid con tac t  between s o l u t i o n  and 
grease.  
w a s  f i l l e d  with t h e  d e s i r e d  amount of C02 and t h e  p i p e t t e  and t h e  stopcock 
P repa ra t ion  of a run proceeded as follows: The gas sample tube 
-4- 
c 
con ta in ing  H PO were evacuated and closed a f t e r  t h e  a c i d  was thoroughly de- . 3  4 
gassed; 
f l a s k  and frozen by applying a mixture o f  dry i ce  and acetone around t h e  
f l a s k .  
a p r e s s u r e  o f  about 
t h e  same cyc le  was repeated s o  as t o  remove most o f  t h e  gases d i s so lved  i n  
t h e  s o l u t i o n .  After t h e  second thawing t h e  C02 i n  t h e  gas sample tube w a s  
r e l e a s e d  i n t o  t h e  f l a s k  and t h e  e n t i r e  r eac t ion  vessel with attachments 
was immersed i n  a water b a t h  whose temperature was c o n t r o l l e d  t o  - 0.5OC. 
A 50 m l  .a l iquot  o f  t h e  sample s o l u t i o n  was then p i p e t t e d  i n t o  t h e  
When t h e  s o l u t i o n  was completely frozen t h e  f l a s k  was evacuated t o  
t o r r  and closed. After thawing o f  t h e  s o l u t i o n ,  
+ 
A minimum o f  18 hours was allowed for. e q u i l i b r a t i o n  during which time t h e  
vessel was shaken p e r i o d i c a l l y .  
A t  t h e  end o f  a run, a p o r t i o n  o f  the s o l u t i o n  was drawn i n t o  t h e  p i -  
p e t t e  f o r  l a t e r  pH measurement and t h e  gaseous C02 was condensed back i n t o  
t h e  gas sample tube.  
carried o u t  a t  t h e  p a r t i c u l a r  temperature o f  t h e  experiment. 
Both t h e s e  s t e p s  as well as t h e  pH measurement were 
Following 
t h e  i s o l a t i o n  o f  t h e  gas phase, t h e  s o l u t i o n  was a c i d i f i e d  by opening and 
d r a i n i n g  t h e  stopcock containing t h e  phosphoric ac id .  
e x t r a c t e d  and p u r i f i e d  by two passes through a t r a p  cooled with dry ice  and 
acetone. 
t h e  same p u r i f i c a t i o n  before  t r a n s f e r  t o  t h e  mass spectrometer .  
meter used f o r  t h e  i so tope  r a t i o  measurements i s  a conventional 6-inch, 60' 
instrument  with dua l  i n l e t  system and double c o l l e c t o r ,  manufactured by Nuc- 
l i d e  Corporation. 
The evolved C02 was 
The CO c o n s t i t u t i n g  t h e  equi l ibr ium gas phase was sub jec t ed  t o  2 
The spec t ro -  
In  t h e  first s e t  of experiments mole o f  C02 with a 6C13 o f  -27 .0% 
-5-  
(4) was e q u i l i b r a t e d  with a s o l u t i o n  of mole o f  NaHCO with a 6C13 o f  3 
- 6 . 0 % 0 .  
carbon w a s  p re sen t  i n  bicarbonate  (5).  The 6C13 measured on t h e  C02 de r iv -  
The pH of t h e  s o l u t i o n  w a s  8.4, ensuring t h a t  roughly 98%. of t h e  
ed by a c i d i f y i n g  t h e  s o l u t i o n  a f t e r  the run, t h e r e f o r e ,  c l o s e l y  r ep resen ted  
t h e  va lue  f o r  t h e  bicarbonate  i n  equi l ibr ium with t h e  C02 atmosphere 
above t h e  s o l u t i o n .  
t i o n  temperatures 
Fig., 1 shows t h e  d a t a  obtained at 0 and 3OoC e q u i l i b r a -  
The r e p r o d u c i b i l i t y  i n  a l l  experiments descr ibed h e r e  
i s  o f  t h e  o rde r  o f  0 .2  t o  0 . 4 b f o r  A ,  i . e .  t h e  d i f f e rence  between t h e  6- 
va lues  o f  t h e  e q u i l i b r a t e d  phases.  
ate sample p repa ra t ions  and analyses  i n  a d d i t i o n  t o  t h e  e r r o r s  i nhe ren t  i n  
As t h e  e r r o r  i n  A depends on two sepa r -  
t h e  e q u i l i b r a t i o n  and s e p a r a t i o n  s t ages ,  t h i s  level of p r e c i s i o n  segms q u i t e  
accep tab le .  
s imilar t o  t h e  one above, where t h e  i n i t i a l  carbon dioxide was i s o t o p i c a l l y  
Fig.  1 a l s o  inc ludes  t h e  d a t a  obtained i n  a s e p a r a t e  experiment, 
h e a v i e r  (6C13 = + 3.6% than t h e  bicarbonate  ( 6 C 1 3  = -6.0%. This experiment 
w a s  performed i n  o r d e r  t o  approach equi l ibr ium from the  opposi te  d i r e c t i o n .  
As is ev iden t  from Fig. 1, a switchover was achieved and a f t e r  t h e  run t h e  
C02 above t h e  s o l u t i o n  w a s  9.6 % l i g h t e r  t han  t h e  bicarbonate .  Within t h e  
l i m i t a t i o n s  o f  t h e s e  experiments, t h i s  value can be  considered i d e n t i c a l  t o  
t h e  o t h e r s  obtained at  t h e  saqe temperature, v i z .  9.3, 8.8, 9.1% The C13/ 
C1* d i f f e r e n c e  between atmospheric carbon dioxide and aqueous b i ca rbona te  a t  
O°C may thus  be taken t o  be 9.2 - 0.4% 
determined d i f f e rences  and t h e i r  mean values f o r  0' 
+ ---  Table 1 l ists  t h e  experimental ly  
IO,. 
x Z O O ,  and 3OoC. The 
- last  column o f  t h e  Table gives t h e  f r a c t i o n a t i o n  f a c t o r s  a, def ined  as 
- 
d =  
13 1 2  
(C / c  lCO, 
-5-  
for each temperature .  
a l l  t h e  d a t a  i n  a p l o t  of 1000 In= vs.  lo6 T-2 where T i s  t h e  abso lu te  tem- 
These values  were c a l c u l a t e d  from t h e  b e s t  fit t o  
p e r a t u r e .  
Another s e r i e s  of experiments was aimed a t  deteqnining t h e  i s o t o p i c  
composition of  d i sso lved  C02 i n  equi l ibr ium with aqueous b icarbonate  and 
atmospheric  CO 
6.20 t o  6 . 5 3  were allowed t o  e q u i l i b r a t e  wi th  carbon dioxide gas. 
- 
To t h i s  end buf fered  s o l u t i o n s  o f  pH values  ranging from 2 ’  
The 
b u f f e r s  used were mixtures of  KH2P04 and Na HPO d isso lved  i n  d i s t i l l e d  
water .  
2 4  
E q u i l i b r a t i o n  temperature  was O°C i n  o rde r  t o  maximize f r a c t i o n a -  
t i o n  e f f e c t s .  
above except  t h a t  i n  t h e s e  runs a l l  t h e  carbon was i n i t i a l l y  i n  t h e  form 
Prepara t ion  procedure was i d e n t i c a l  t o  t h e  one descr ibed  
o f  C02 gas .  
C02 e n t e r e d  t h e  s o l u t i o n  and p a r t l y  hydrated and d i s s o c i a t e d  t o  H 
HC03-. 
t e d  H2C03) i n  t h e  s o l u t i o n  t o  the  bicarboqate  is  dependent upon the  pH and 
t h e  temperature  and can be obta ined  from t a b l e s  of  values  c a l c u l a t e d  by Saru- 
hash i  ( 5 ) .  
w a s  then  only necessary t o  measure t h e  pH of t h e  s o l u t i o n  a t  t h e  end of  t h e  
experiment and t h e  6 C  
i n  t h e  s o l u t i o n  (6 
ous CO 
Upon b r ing ing  the  gas and s o l u t i o n  i n t o  contac t  some of t h e  
+ 
and 
The molar r a t i o  of f r e e  C02 ( t h i s  inc ludes  roughly 0.1% undissocia-  
To determine t h e  i s o t o p i c  composition of t h e  d i s so lved  C 0 2 ,  it 
13 values  of t h e  gaseous C02 and o f  t h e  t o t a l  carbon 
) .  The 6C13 of t h e  b icarbonate  and of t h e  aque- W a q )  
were then  c a l c u l a t e d  from t h e  following equat ions 2 
+ A 
- 7- 
where A is  t h e  p rev ious ly  determined d i f f e rence  between t h e  6 ' s  o f  C02 and 
b i ca rbona te  i n  equ i l ib r ium ( 9 . 2 % 0  a t  O'C) and Q is  t h e  molar r a t i o  of HC03 
t o  C02 i n  t h e  s o l u t i o n ,  a value obtained from Saruhash i ' s  t a b l e s  f o r  t h e  
pH and temperature of  t h e  experiment. 
- 
\ 
\ 
Table 2 shows t h e  d a t a  obtained i n  t h r e e  runs i n  which Q v a r i e d  from 
i s  i d e n t i c a l  t o  0.41 t o  0.89. I n  a l l  t h r e e  cases t h e  ca l cu la t ed  6 
t o  w i th in  - 0 . 5 t o r  b e t t e r ,  an e x c e l l e n t  agreement t h e  measured6 
cons ide r ing  t h e  fact t h a t  t h e  e r r o r s  a s soc ia t ed  with A ,  Q and two independ- 
co2 (aq) 
+ 
co2 (gas) * 
e n t  sample p repa ra t ions  and analyses en te red  i n t o  t h e  c a l c u l a t i o n .  
To o b t a i n  a f u r t h e r  check on t h e  da t a  p re sen ted  i n  Table 2 ,  one add i t -  
i o n a l  experiment was performed i n  which t h e  pH o f  t h e  s o l u t i o n  was lowered 
t o  5.2.  A t  t h i s  low a pH, 96% of t h e  carbon i n  s o l u t i o n  is i n  t h e  form of 
C02 and t h e  b i ca rbona te  f r a c t i o n  can be neg lec t ed  f o r  t h e  purposes o f  t h e  
p r e s e n t  experiment. I n  terms of  equation (2)  above, Q was 0.04 which made 
, As t h e  s o l u b i l i t y  of  cC(a;i) ' 'CoZ(aq) t h e  second term n e g l i g i b l e  and 6 
C02 a t  a pH o f  5 .2  i s  appreciably lower than at  t h e  pH values o f  t h e  o t h e r  
experiments t h e  C02 p r e s s u r e  above t h e  s o l u t i o n  was r a i s e d  by allowing 20 
cc STP of C02 t o  e q u i l i b r a t e  with t h e  s o l u t i o n  i n s t e a d  o f  t h e  usual  2 .25  cc. 
Upon completion of  t h e  run g r e a t  ca re  was taken n o t  t o  allow any o f  t h e  d i s -  
so lved  C02  t o  escape t h e  s o l u t i o n .  The stopcock of t h e  gas sample tube i n t o  
which t h e  gaseous CO, was being frozen was qu ick ly  closed as soon as t h e  
first bubbles appeared i n  t h e  s o l u t i o n  due t o  t h e  sudden p r e s s u r e  drop above 
t h e  s o l u t i o n .  I t  i s  conservat ively est imated t h a t  a t  l eas t  99% o f  t h e  gase- 
\ 
\ 
ous C02 were recovered and t h a t  no more than  1% of t h e  d i s so lved  C02 escaped 
. .  
I 
- 8- 
t h e  s o l u t i o n  before  c los ing  of t h e  stopcock. 
i ed ’  t o .  l i b e r a t e  t h e  d isso lved  CO2. 
t i o n  o f  t h e  gas samples showed t h a t  about 16% o f  t h e  gas had gone i n t o  
The s o l u t i o n  w a s  then a c i d i f -  
Comparison of t h e  volumes a f t e r  p u r i f i c a -  
s o l u t i o n .  
involved,  had a 6 C 1 3  o f  -27% 
t h e  gaseous and d isso lved  C02 were -26.5 and -27.1%0 re spec t ive ly ,  again i d e n t i -  
tal values  wi th in  t h e  e s t a b l i s h e d  p rec i s ion .  
The i n i t i a l  C02 of t h e  experiment, comprising t h e  t o t a l  carbon 
After e q u i l i b r a t i o n  the  measured values  f o r  
\ 
The experiments have shown t h a t  (a) t h e  f r a c t i o n a t i o n  of  carbon i so topes  
between atmospheric carbon dioxide and aqueow bicarbonate  decreases  from 
9.2  - 0 . 4 % 0 a t  O°C t o  6.8 - 0.4%,at 3OoC and (b) t h e r e  i s  no measurable f r a c t i o n -  + + 
a t i o n  between atmospheric carbon dioxide and carbon dioxide d i s so lved  i n  water .  
In t h e  absence of complete stagnancy equ i l ib r ium appears t o  be e s t a b l i s h e d  
r a t h e r  qu ick ly  s o  t h a t  t h e  d i f f e rence  i n  t h e  k i n e t i c s  of  passage through t h e  
13  a i r - w a t e r  i n t e r f a c e  between t h e  two i s o t o p i c  s p e c i e s  C1’02 and C 0 can- 2 - 
n o t  s i g n i f i c a n t l y  con t r ibu te  t o  t h e  C02-HC03 
e d  t h a t  t h e  f r a c t i o n a t i o n  occurs i n  t h e  hydra t ion  s t e p  where carbon bonds a r e  
f r a c t i o n a t i o n .  I t  may be assum- 
rear ranged  and not  i n  t h e  d i s s o c i a t i o n  though t h i s  cannot be a s c e r t a i n e d  ex- 
pe r imen ta l ly  because of  t h e  low r e l a t i v e  abundance of t h e  undissoc ia ted  
phase.  
gu ish  t h e  ino rgan ic  carbon source  of a q u a t i c  p l a n t s .  
C02 only,  t h e i r  carbon should be i s o t o p i c a l l y  i d e n t i c a l  t o  t h a t  o f  a i r  b rea th -  
One b i o l o g i c a l  imp l i ca t ion  of t h i s  i s  t h a t  i t  is  p o s s i b l e  t o  d i s t i n -  
If t h e  p l a n t s  u t i l i z e  
i n g  p l a n t s  as long as t h e  u t i l i z a t i o n  proceeds along t h e  same pa ths  and t h e  
2 CO can e q u i l i b r a t e  with t h e  atmosphere, I f  a p l a n t  can u t i l i z e  e i t h e r  CO 2 
or bicarbonate  depending on t h e i r  r e l a t i v e  a v a i l a b i l i t y ,  t h e  p ropor t ion  of 
t h e  two w i l l  be r e f l e c t e d  i n  t h e  p l a n t ’ s  carbon i s o t o p e  conten t .  
extreme case of  exc lus ive  bicarbonate  uptake,  t h e  p l a n t  will, depending upon 
In  t h e  
-9- 
mean growth temperature ,  be 7 t o  9 'bheavier  than i n  t h e  case of  pure C02 
uptake.  
of t h e  k i n e t i c  e f f e c t  i n  the  passage of  CO2 molecules through t h e  a i r -wa te r  
i n t e r f a c e  a l s o  c a s t s  doubt on t h e  r e a l i t y  of  t h e  p o s t u l a t e d  l a r g e  k i n e t i c  
The apparent ease  o f  e q u i l i b r a t i o n  and consequent i n s i g n i f i c a n c e  
' e f f e c t  (6) i n  the  passage of C02 from t h e  atmosphere i n t o  t h e  p l a n t  cyto- 
plasm during photosynthes is .  
In t h e  l i g h t  of  t h e s e  experiments,  carbon i so tope  d i f f e rences  between 
most f r e s h  waters  ( l i g h t )  and marine water  (heavy) can no longer  be i n t e r -  
p r e t e d  t o  be exc ius ive ly  a r e s u l t  of t h e  l a r g e  b iogenic  C02 con t r ibu t ions  
t o  rivers and lakes .  
a c i d  subs tances  i n  s o l u t i o n  appears t o  be a p r i n c i p a l  f a c t o r  i n  determin- 
i n g  t h e  u l t ima te  6C13 i n  a given water body. 
The pH and consequently t h e  r a t i o  of t h e  carbonic  
W.G. Deuser 
Department of Chemistry 
Woods Hole Oceanographic I n s t i t u t i o n  
Woods Hole, Massachusetts 
E.T.  Degens 
. .  
- 10- 
1. 
2. H. Craig, Geochim. Cosmochim. Acta 3, 53  (1953) and J. Geol. 62 
A.F. Reid and H.C. Urey, J. Chem. Phys. - 11, 403 (1943). 
- -’ 
115 (1954). 
I 3. P.H. Abelson and T.C. Hoering, Proc. Natl. Acad. S c i .  47, 623 (1961). -
A l l  values r epor t ed  are r e l a t i v e  t o  t h e  Chicago PDB s t anda rd .  
5 .  K. Saruhashi ,  Pap. Met. Geophys. Tokyo 6 ,  38 (1955). 
6. R. P a r k  and S. Eps t e in ,  Geochim. Cosmochim. Acta 21, 110 (1960). 
7. 
- 
-
This  work was supported by NSF Grant GP 4083. 
(D 
cd 
It 
a 
b" 
0 r 
F 
r; 
II 
a 
0" 
0 
I r r )  
0 
0 
I 
CD 
cd 
II 
a 
0" 
0 
t m  
0 
0 
I 
-11- 
' C  . .  
I .  
-12- 
Table 1. Equil ibr ium f r a c t i o n a t i o n  o f  carbon i so topes  between atmospheric 
carbon dioxide and aqueous b icarbonate .  
- - 
a 
A 
Temp. C02 - HC03 A 
+ 
(add - 0.0003) 
0 
0 
0 
0 
10 
10 
20 
20 
30 
30 
9.3 
8.8 
9.1 
9:6 
9.2 
8.1 
8.1 
8.1 
7.6 
7.0 
7.3 
6.6 
7.1 
6.8 
1.0093 
1.0084 
1.0076 
1.0069 
c - 13- 
13 
Table 2 .  Comparison o f  6C values (@ of C02(gas) and C02(aq) i n  e q u i l i -  
brium a t  O°C. 
PH Q 'IC Cd9) (measured) 
%02(aq) ' 
( c a1 cu 1 a t  e d) D i f f .  
6.53 0.89 5.7 1.6 1.2 +0.4 
6.27 0.48 
6.20 0.41 
5.5 
5.7 
2.8 
2.7 
2; 4 
3.2 
+O,. 4 
-0.5 


Carbon isotope fractionation in the system CO2/gas/-CO2/aqueous/-HCO3-/aqueous/

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