CALCIUM
CARBONATE
SATURATION
IN SEAWATER:
EFFECTS OF DISSOLVED
ORGANIC MATTER’
Keith E. Chase and Erwin Suess2
Hawaii Institute of Geophysics and Department of Oceanography,
University of Hawaii, Honolulu 96822
ABSTBACI
Surface seawater is generally supersaturated with CaCOs; but carbonate will not prcdpitate from natural seawater in convenient experimental times. If the supersaturation is
increased by the addition of Ca2’ or C03’- ions, precipitation can occur in minutes or hours.
Seawater, with 0.1 m Na&Os solution added to give a pH of 9.5, will begin rapid CaCOs
(aragonite) precipitation in as little as 15 min. The length of time between the addition
of the N&CO3 and rapid precipitation increases with an increase in the dissolved organic
content of the water. If CaCL, which produces no pI1 increase, is added to seawater,
CaSOc.21140 (gypsum) is the only precipitate that will form within 12 hr.
from filtered
seawatcrs.
CaC03
INTRODUClTON
natural
and artificial
The supersaturation of CaC03 in surface
seawater was first noted in the central AtEXPERIMENTS
lantic by Wattenberg
and Timmerman
Calcium carbonate will not precipitate
( 1936). Others have since observed this from naturally
occurring
supersaturated
phenomenon
in many widely
scattered
surface seawater in reasonable experimenareas (e.g., Cloud 1962; McIntyre
and tal times, Thus, to study rates of nucleaPlatford 1964; Pytkowicz and Fowler 1967; tion, it is necessary to increase the degree
Lyakin 1968). It appears that most surface of supersaturation artificially.
This is easwaters of the world’s oceans are supersatuily acco’mplished by the addition of Ca”+
rated. This disequilibrium
state of the or COs2- ions. If these are added in the
CaC03 system in surface seawater has form. of CaCl2 or Na&On solutions,, the
generally been attributed to some nonspe- gross chemistry of the seawater changes
cific, inorganic reaction kinetic control. On little since Na-’ and Cl- are the dominant
the basis of laboratory experiments, Pyt- ions of seawater.
kowicz (1965) showed the effect of magnesium in solution in seawater in the
regulation of carbonate nucleation kinetics.
Natural seawaters from a variety of enSimkiss (1964) has shown that dissolved
vironments around Hawaii were passed
phosphorus compounds inhibit CaC03 pre- through 0.8-p Millipore filters and continuously stirred. Portions ( 160 ml) of the
cipitation.
Chave ( 1965) and Chave and Suess filtrates were brought to p’H 9.5 by addition of 0.1 m Na2C03 solutions.
The
( 1967) suggested that naturally occurring
solution required to
organic compounds inhibited reactions be- amount of Na&03
tween carbonate minerals and seawater. reach this pH ranged, with one exception,
from 5.7-6.7 ml; about 0.6 mmoles of C032This report presents the results of experiper sample was added.”
ments to determine the effect of dissolved
organic compounds on the nucleation of
3 To approximate a consistent degree of super1 This work was supported by Office of Naval
Research Contract NR 083-194. Contribution 374
from the Hawaii Institute of Geophysics.
2 Present address: Geologisch-Palaontologischcs
Institut der Universitgt, 23 Kiel, Olshausenstrassc
40/60, West Germany.
saturation by addition of Na2COs to various seawaters, the choice of adding a constant amount of
NaEOs, or of adding NaE03 to a constant pH,
is arbitrary. Because of the disequilibrium in either
system, it is impossible to describe the absolute
supersaturation. Addition of NazCOa to a constant
pH was chosen for these experiments.
633
634
KEI’I’I-I
9.5
E.
CHAVE
AND
ERWlN
TAIHX
-‘--A:+,,
‘Y=c
‘.
9.4
\;-‘--*---.+.
\
?\ .
Effect
pH 9.5
-.
3.0 mGC/L
’
0
I
IO
,
20
I
30
TIME,
0.1 m
Na,C08
added/
Dissolved
org-C
rnpid
pII de-
7.64
5.69
0.5
16
8.09
6.65
1.1
22
8.14
6.21
1.2
261
8.12
5.71
1.2
3x2
7.92,
6.10
1.9
GO
8.01
6.10
2.0
48
8.00
5.90
2.0
55
7.94
6.10
2.9
120
-
-
8.83
3.00
\
Scawnter
sample
9.0
of dissolved organic carbon. on
from seawater raised to
CaCOs precipitation
.
9.3 1
1.
SUES3
I
40
1
50
.
I
60
I
70
MIN
PIG. 1. Change in VI-I with time after addition
of NanCOs solutions. Rapid precipitation of CaCO:,
is indicated by a rapid decrease in pII. Dissolved
organic carbon (mg C/liter ) of each sample is
indicated.
Plots of pII change with time after the
addition of Na&08,
Eor three typical expcrimcnts, arc shown in Fig. i.- In the
Lxpcrimcnts, the $1 dccrca&s slowly at
first and then rapidly as a thick white
precipitate of CaCOs (aragonite) is formed.
Total dissolved o,rganic carbon was dctermined on each of-the filtered seawater
samples bcforc the prccipita tion expcriments, using the method of Menzel and
Vaccaro ( 1964). The length of time bctween the addition of Na2CO:I and the
rapid precipitation
of carbonate, as indicated by a rapid pH dccrcasc, appears to
be related to the dissolved organic content
of the water (Fig. 1, Table 1).
Two samples of scawatcr from laboratory
aquaria, with unusually high dissollved organic contents, did not precipitate CaCOs
in 3 to 4 hr after additions of Na&03.
The
longer
experiments were not continued
bccausc WC:felt that after this period other
factors such as microbiolo,gical
activity,
contaminaevaporation, or water-surface
tion, might influence the results.
To clucidatc the relationship
between
dissolved organic compounds aid CaCOR
prccipitationr wc collcctcd samples of precipitatcd material periodically
during another set of experiments.
The Na2COzI
solution was added to 8-lo-liter
samples
of Filtered scawatcr until the pII was 9.5.
ArtificiaI
Kancohe Bay
( surface )
Kancohc Bay
( surface )
Coconut
Jsland
(surface)
Lab circulating
( well)
Lab circulating
( well)
Kaneohc Bay
( surface )
Aquarium”
( natural )
Aquarium*
( natural )
Aquarium *
(natural)
Time
to
Initial
j?II
* Various aquaria around the laboratory
ganisms in nntural seawater.
t Iyo precipitation
obscrvcd.
3.3, >240<
4.0
>240t
containing
--
or-
Immediately after addition of the Na&O:I,
1 liter of the experimental solution was
withdrawn and filtered through a precombusted Gclman type A glass-Fiber filter.
The filter was then washed with 5 ml of
distilled water. Withdrawal
and filtering
of 0.5-1.0-l&r
aliquots continued at 5-l@
min intervals until the initial seawater samplc was exhausted, or until the CaCO:l
prccipitatc was very heavy.
The glass-fiber filters were dried over
silica gel, and analyzed for CaC03 carbon
and organic carbon. WC measured CaCO:$carbon (in a Beckman IR-215 CO2 analyzcr) in the form of CO, rclcascd by
treatment with 15% IIaPOL~. Organic carbon was measured as CO2 rcleascd by
oxidation with l&$208 after acid treatment.
Both reactions took place in the same vesscl. The method is described by Klim
(1969).
The results of two of these cxpcriments
are shown in Fig. 2. Organic carbon is
precipitated rapidly from the NazCOR-cn-
chCo3
SATURATION
IN
635
SEAWATER
2. Rates of CaCOs nuchtion from Udifida1 seuwater raised to pH 9.5 (clissolved mg-C,
0.5 mg C/liter)
TABLE
1.0-
---
NORMAL SEAWATER
ORQANIC-RICH
SEAWATER
0.1 171.
0.8 -
I
II
0’
II
0I
< 0.6 43
5
z$
%
0 0.4 -
0
Mg concn
(M)
P
I
ARAGONITE
L
i
60
30
TIME,
\::
Initial
PI-1
7.891
7.73
7.64
<lo*
2.41 x lo-’
5.23 x 1O-2
I
I
I
Na,CO,
added/ 100 ml
(ml)
Time to
rapid pH
decrease
(mj.4
2..81
3.12
5.68
1
8
18
compounds, comprising about 10% of the
total organic carbon, associate with CaCOs
nuclei, This association may render the
nuclei inactive as sites for further CaCOa
Consequently, rapid CaCOn
precipitation.
precipitation
does not occur until these
are removed from
organic compounds
solution.
90
MIN
FE. 2. Composition of precipitate from NaLIOsenriched seawater as a function of time. Organic
carbon is removed from solution before rapid
CaC,Os precipitation occurs.
richcd seawaters at the beginning of the
expcrimcnts, before the major part of the
Organic compounds
CaCOs precipitates.
may be prccipitatcd because the increased
pH causes a decrease in their solubility,
or they may be removed from solution by
association with the few Ca.C03 nuclei that
folrm early in the experiment (Fig. 2.). In
either case, organic compounds are rcmoved from solution before rapid CaCOs
precipitation
occurs.
The maximum. amounts of dissolved organic carbon precipitated
during the cxperiments arc 0.10 mg C/liter for normal
seawater and 0.25 mg C/liter for organicrich ( aquarium)
water. This represents
about 10% of the initial dissolved organic
carbon in these two types of samples. Suess
(1970) reported that between 10 and 14%
of the dissolved organic carbon in seawater
associates with calcite surfaces when the
mineral is added to seawater as a fine
powder. It is likely that in the expcriments reported here, dissolved organic
Effect
of magnesium
Pytkowicz (1965) demonstrated that magncsium in solution influences the rate of
In his experiments,
C&O3
nucleation.
magnesium-free
artificial
seawater nucleated much faster than natural seawater or
magnesium-enriched natural seawater. Pytkowicz did not report the dissolved organic
content of theso waters.
In an attempt to evaluate the relative
importance of magnesium and organics in
nucleation inhibition, we did an experiment
in which the concentration
of dissolved
organics was kept constant and the magnesium concentration varied. Magnesium
concentrations lower than those in seawater can only be obtained in artificial
seawater.
Artificial
seawaters containing no magnesium, about half the magnesium of seawater, and the magnesium concentrations
of seawater (NaCl substituted for MgCl:!
to keep the ionic strength constant) were
brought to pH 9.5 by addition of 0.1 m
Na&O:I solution. The pH change-indicative of prccipita tion-was
then monitored.
The results are shown in Table 2.
The effect of magnesium on CaC03 nucleation is striking. Magnesium at seawater
concentration inhibits nucleation from these
high plH waters for 18 min. On the other
636
KEITH
3.
E.
CHAVl3
Precipitates formed within 12 hr by
addition of CaCL to seawater
AND
ERWIN
SUESS
critical mechanism of inhibition,
and the
way in which it operates to allow the maintenance of supersaturation in surface seaDissolved
CaCl, added (g/100 ml)
Mg
water, is not obvious.
org-C
concn
1.25
1.0
1.50
1.75 (mg C/liter)
bd
Magnesium forms a stable MgCO$ complex
in seawater (Garrels, Thompson, and
Artificial seawater
Siever 1961)) and likewise, many organic
<lo”
N*
GYP GYP GYP
0.5
compounds reported in seawater may com2.41 x 1O-2 N
GYP GYP GYP
0.5
N
GYP GYP
0.5
5.23 x 1O-2 N
plcx Ca2+ or COs2- ions. In addition, organic molecules are adsorbed from solution
Natural seawater
onto CaC03 surfaces in such a way as to
ND
N
N
N
GYP
1.2
block nucleation sites (Chave and Suess
ND
N
N
GYP GYP
1.1
ND
N
N
N
GYP
1.2
1967). Each or all of these mechanisms
may inhibit the precipitation
of CaCOZI
*N = none; GYP = gypsum; ND = not determined.
from seawater.
Supersaturation of surface seawater canhand, Table 1 indicates that under the not be explained solely on the basis of
same conditions, dissolved organic matter
complexing of Ca2+ or COs2- ions. The
in concentrations found in natural seawater
degree of supersaturation is lowered by this
inhibits nucleation for as long as an hour.
process (Garrels and Thompson 1962)) but
complexing does not lower the activity of
CaCld experiments
these ions in seawater below the ion acSolutions of CaC12 can be added to sea- tivity product for calcite. Natural surface
water with little change in its pH. Thus,
seawater is actually
supersaturated,
as
increasing the CaC03 supersaturation
by has been demonstrated by Weyl ( 1961))
addition of CaClz results in quite a differSchmalx and Chave ( 1963), Chave and
ent chemical system from Na&Os-enriched
Suess ( 1967)) and others.
water.
Complcxing dots not appear to slow the
Samples ( 100 ml) of natural and artifikinetics of ionic interaction to the degree
cial seawaters were passed through 0.8-p suggested by Pytkowicz ( KM%), who felt
Millipore
filters and varying amounts of that it would take 70,000 yr before precipiCaClz solutions were added. Dry CaClz in tation could occur, nor do the experiments
amounts ranging from 1.0 to 1.75 g were reported here. Chave and Schmalz (19661)
weighed out and dissolved in a minimum of and Chave and Suess (1967) showed that
distilled water for addition to the seawater. when sufficient CaC03 nuclei were added
The mixtures were stirred continuously fo,r to natural seawater (containing
normal
magnesium concentrations ) to reduce the
12 hr and watched for precipitate formation. The only solid precipitated in any of effective dissolved organic content through
between
surface adsorption, equilibration
the experiments was gypsum: CaS04*2Hz0
the solid and water occurred within an
(Table 3).
Calcium carbonate does not precipitate
hour.
from the calcium-enriched seawaters within
The best explanation of the supersatura12 hr. The data perhaps suggest that both
tion of surface seawater, based on our data,
magnesium and dissolved organic carbon is that CaC03 nuclei are probably forming
continuously in natural or Ca2+- o,r COs2-inhibit gypsum nucleation.
enriched seawater. The rate of nucleus
DISCUSSION
formation may be influenced by either disPrecipitation of CaCOs from seawater is solved magnesium or phosphorus (Simkiss
inhibited
by magnesium in solution, as 1964) or both. The nuclei are inactivated
shown by Pytkowicz ( 1965)) and by dis- immediately by adsorption of dissolved orsolved organics, as indicated here. The ganics from solution onto the carbonate
TABLE
C&03
SATURATION
surfaces. This suggestion is in agreement
with the observation by Chave and Suess
(1967) that the adsorption of dissolved
organic matter from seawater onto carbonate surfaces is faster than the precipitation
of CaC& onto the same surfaces.
Furthermore,
in our experiments with
enriched seawater, rapid CaC03 precipitation took place only at high pH values,
after organics had -been removed from
solution (Fig. 2).
Finally, inactivated nuclei may account
for the CaC03 associated with organic aggregates observed in suspension in shallow
and deep ocean water (Wangersky and
Gordon 1965 ) .
On the basis of this model of carbonate
supersaturation in surface seawater, the sea
can probably maintain a high degree of
supersaturation
if pH remains near 8.0,
and equilibration
will occur only if pH is
raised considerably,
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