-
12. INTERSTITIAL WATER STUDIES, LEG 15 - STUDY OF CO2 RELEASED
FROM STOREDDEEP SEA SEDIMENTS1
Jaw-Long Tsou, Douglas Hammond, and Ross Horowitz,
Lamont-Doherty GeologicalObservatory, Columbia University,
Palisades, New York
ABSTRACT
Thirty-five samples of deep sea sediments were collected onDSDP
Leg 15 and stored in sealed kettles for over a year. ThePCO2 of
these kettles was monitored as a function of time andtemperature.
When CO2 is removed for analysis, the decreaseobserved in kettle
PCO2 is consistent with the assumption thatequilibration of water
and calcite controls the alkalinity and pHof the interstitial
water. An empirical relation between ΣCC>2 andPCO2 is selected
which allows correction for CO2 removal andcomparison of PCO2 to
other carbonate parameters (i.e., pH)measured on Leg 15. The
resolution of the data is limited, but pHand PCO2 appear to be
consistent within 0.1 pH units.
Calculation of the degree of calcite saturation is used
tosuggest that pressure may significantly influence ionic
equilibriabetween clay and pore waters, causing samples from Site
149 toappear understaurated with calcite.
Sediment samples from regions with strong [SC 4=] gradientsshow
increases in PCO2 with time, indicating active sulfatereduction
continuing in the kettles. Comparison of the CO2generation rate in
the laboratory with that in situ at Sites 147 and148 suggests that
the process may be limited by the availability ofsuitable organic
substrates, which is a function of temperature.
INTRODUCTION
The carbonate chemistry of the interstitial fluids of deepsea
sediments presents a challenging problem to geo-chemists. The
system, defined here as including alkalinity,total CO2, pH, the
partial pressure of CO2 (PCO2), Ca
+ +
(titrated Ca), total HCO3~, and total CO 3= , may be
completely defined by a knowledge of any two parameters,the
equilibrium constants of the reactions which relatethem, and the
degree of calcite saturation.
A number of processes rpay affect the system:1) Bacterial
oxidation of organic matter to produce
CO 2.2) Solution and precipitation of carbonate minerals.3)
Cation and hydroxyl exchange between clay minerals
and interstitial water which may be very important inbuffering
pH.
4) Solution of conjugate bases such as NH3, PO 4= , FeO,
and reduction of weak bases to stronger bases (such asSO 4
= to S=).In an attempt to study these processes, and also to
evaluate the pH measurements made on shipboard, 35samples of
sediment were collected in sealed kettles onDSDP Leg 15 and
returned to Lamont-Doherty GeologicalObservatory where the PCO2 was
monitored as a functionof time and temperature. Thirteen of these
samples are
Lamont-Doherty Geological Observatory Contribution No. 1911.
from the Cariaco Trench (Site 147), twelve from the AvesRise
(Site 148), and ten from the Venezuelan Basin (Site149). One
core-storage kettle was broken in transportation.
These three sites are interesting to geochemists, becausethey
cover a wide range of sedimentary and chemicalenvironments. The
Cariaco Trench is a structural depressionon the continental shelf
surrounded by water less than 200meters deep. Because it is
separated from the deep water ofthe Caribbean Sea and receives a
large amount of organicmaterial, its bottom water has become anoxic
and has anunusually high temperature (17°C). Site 147 (882 m
belowsea level) is a rather uniform calcareous clay with
authigenicdolomite, calcite, and pyrite. A high sedimentation rate
(50cm/103y) prevails, the material is rich in organic material(dry
weight carbon 2%), and is consequently ananaerobic environment. The
Venezuelan Basin is a deep(Site 149 is 3472 m below sea level),
open basin with loworganic content (~0.1%), and low sedimentation
rate (Av.0.8 cm/103y). Sulfate changes very little with depth in
thecore, indicating limited biological activity. The
sedimentchanges from a calcareous clay above 250 meters to adiatom
ooze below. The Aves Rise site (1223 m deep) isalso a calcareous
clay with an average sedimentation rate of3 cm/103y. Sulfate is
reduced in the upper 100 meters ofthe sediment.
COLLECTION AND ANALYTICAL PROCEDURE
The shipboard handling of samples is described inHorowitz et al.
(this volume). Briefly, a 10 to 15 cm sectionwas cut from the core
and moved to a dry box flushed with
851
-
J-L. TSOU, D. E. HAMMOND, R. HOROWITZ
argon. In an effort to avoid oxidation, the sediment surfacewas
scraped away and the remainder sealed in kettles forstorage. The
kettles were approximately half filled withsediment. CO2 loss from
the sediment during this processand during coring is difficult to
estimate, but should besmall except for samples from the Cariaco
Trench wherethe gas content was so high it caused difficulties in
drilling.The gas pressure was sometimes sufficient to blow
thesediment out from the pipe on the ship.
The kettles were refrigerated (T = ~ 4°C) from the timeof
collection until the first set of measurements was carriedout,
about 5 months later. While still refrigerated, analiquot of gas
(about 1/6 of the total) was taken from eachkettle for infrared
(IR) analysis and the kettle refilled withnitrogen to about 1
atmosphere total pressure. To deter-mine the CO2 content, each gas
sample was diluted 10 to50 times with N2 and compared with
laboratory standards.Results are listed in Table 1.
TABLE 1Partial Pressure of CO2 at 4°C and 22°C
KettleNumber
Depth(m) DSDP No.
15-147 Cariaco Trench (10°42.65
123456789
10111213
4.225.2
14204.5
566484
104128148177
15-148 Aves Rise
141516171819202122232425
610263563709090
107107161210
147B-1-3147A-1-2147B-1-4147B-2-2147B-2-6147B-1-4147B-6-2147B-7-4147B-9-4147B-11-3147C-2-2147C44147C-7-4
SampleInterval
(cm)
4'
pco2a
(mb)
'C
NPCO/I
(mmol/1)
PCO/L
(mb)
'N, 65° 10.46'W) Depth below sea surface:
93-11060-9695-11038-5338-532-15
96-14077-97
111-12669-84
0 4 10-34
119-129
9.92(0.284)6.454.47
11.17.959.33
12.731.116.312.312.27.65
1.3_
0.870.451.20.771.41.62.82.51.61.70.92
(13°24'N, 63°45'W) Depth below sea surface:
148-14148-2-1148-3-31484-3148-7-3148-8-3148-10-3148-10-3148-124148-124148-18-2148-234
15-149 Venezuelan Basin (15° 06
262728293031323334
73101136157
323
230315369
149-9-5149-12-5149-164149-18-3149-2-2149-4-3149-26-2149-35414941-5
135-150135-15070-85
120-135135-15045-60
75-9090-10590-105
120-135
.25'N, 69C
135-150105-120
0-15105-120135-150135-150105-120105-120105-120
2.041.932.782.474.376.272.85_
2.882.742.002.16
'21.95'W)
1.060.4781.952.001.271.185.651.132.28
0.230.360.370.290.280.500.19_
0.210.190.140.12
20.4—
13.27.64
20.716.413.025.950.722.621.016.015.3
1223 m.
5.844.881.116.81
10.910.78.099.056.666.755.185.80
NPCO2d
z(mmol/1)
882 m.
2.4—
1.61.62.21.43.14.57.36.45.24.35.7
0.671.11.31.01.11.41.90.821.11.00.600.50
Depth below sea surface: 3472 m.
0.0730.0310.150.160.120.110.490.0870.20
3.331.522.451.511.802.918.325.681.59
0.320.130.280.560.190.311.010.470.17
22° C
Pco2b
(mb)
18.7—
13.5(4.30)19.316.512.916.646.220.418.911.912.5
5.874.597.975.98
(15.0)11.0
8.649.226.387.004.536.36
2.941.382.140.8581.372.497.255.090.731
PCH4b
(mb)
n.d.-
n.d.n.d.
8.46n.d.
0.9241.706.701.200.3
-0.10.838
n.d.-
n.d.n.d.
-n.d.n.d.n.d.
1.170.593.364.99
n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.
Pco2a
(mb)
19.7-
13.7——
17.7—--——
11.8-
5.805.157.787.13
10.1--
9.476.70---
3.311.522.260.943—---—
aGas chromatograph data.bInfrared data.cTotal CO2 lost from 1
liter of pore water to gas phase in bottles before the first
measurement at 4°C. (Calculated from
estimated volume of the sample sediments and measured water
content of nearby sediments. DSDP data).
"Total CO2 lost from 1 liter of pore water before the first
measurement at 22°C.n - d Not detectable.
852
-
STUDY OF CO2 RELEASED FROM STORED DEEP-SEA SEDIMENTS
When this set of measurements was completed, thekettles were
removed from cold storage and allowed to sitat room temperature (T
= 23°±1.5°C). The extremesensitivity of PCO2 to temperature (see
Table 1 wherePCO2 at 22°C is a factor of 2 greater than PCO2 at
4°C)indicated that a better control of temperature was needed.The
kettles were moved to a thermostated laboratory andplaced in
styrofoam boxes where the temperature wasnearly constant
(22°±0.5°C) for 100 days. A set of PCO2measurements was then made
(Table 1). One hundred dayslater, a second set of PCO2 measurements
was made at22°C. Some of this second set of measurements were
madeusing IR analysis and all using a gas chromatograph (GC)which
required only 0.5 cc of gas (about 1% of the gas ineach kettle). At
this time, the partial pressure of methane(PCH4) was also
determined in each kettle. All these resultsare listed in Table
1.
The radius of the core sediments is about 4 cm. If wetake 2 ×
10~5 cm2/sec as the diffusion constant for CO2, amean
transportation distance for CO2 in the sediments in100 days is
calculated to be 14 cm. Tortuosity and porosityare not taken into
account. However, cracks on thesediments are sufficient to cover
their effects. Therefore thePCO2 we measured is considered to be
the PCO2 of thecarbonate system of pore waters. To establish the
reliabilityof PCO2, another comparison of pH measurements ismade.
Several kettles were opened and punch-in pH (PIpH)was determined
with an Orion pH meter (model 801 digitalpH/mV meter) and two
electrodes (a Beckman No. 40471glass electrode and an Orion 90-02
double junction refer-ence electrode). They are listed in Table 2
along with twosets of pH calculated from the most recent PCO2
measure-ments: one set was derived from PCO2 and Gieskes'
(thisvolume) titration alkalinity, and one from PCO2 andcalcium
(see Gieskes; Presley et al.; Sayles et al.; Hammond,this volume),
assuming calcite saturation.
A third of the discrepancy between measured andcalculated pH can
be attributed to pH measurement error(±0.04 pH unit). The remainder
is probably due to changesin Ca++ and alkalinity which have
occurred during storage(a discrepancy of 0.08 pH units could be
explained by a20% change during storage in ALK, [Ca++], or a 20%
errorin the calcite saturation assumption). The reasonableagreement
between measured pH and that calculated fromPCO2 and alkalinity
indicates that the observed PCO2 isclose to being in equilibrium
with the interstitial water after100 days. The even closer
agreement between measured pH(±0.06 pH unit, Table 2) and that
calculated from PCO2and [Ca++] is strong evidence that the water is
inequilibrium with calcite and that [Ca++] is less susceptibleto
change during storage than is alkalinity.
Derivation of the equilibrium constants used for thiscalculation
is discussed in the following section.
EQUILIBRIUM CONSTANTS
The chemical equilibria of carbonate system in sea watercan be
shown as follows (Li, 1967):
PC02)gas
[C0,=]3 J aq
aq
TABLE 2Comparison of Laboratory pH and PÇQ Measurements
KettleNo.
919202122232830
Pco2a
(mb)
46.211.08.649.476.707.002.141.37
ALKb
(meq/1)
29.148.086.906.905.625.622.322.44
Cac
(mmol/1)
5.615.125.025.025.595.59
21.9910.03
p H d
7.357.387.427.387.457.437.527.73
P He
6.997.317.367.347.397.387.357.62
P Hf
7.117.257.307.397.507.417.357.53
aMeasured at 22°C.Gieskes, this volume.
cSee Hammond, this volume."Calculated from alkalinity +
PCO2eCalculated from calcium + Pço
-
J-L. TSOU, D. E. HAMMOND, R. HOROWITZ
K* f H C O 3 - L
K s p .*
CafCO
Kf C a + + ff Ca CO
sp
(3)
(4)
where
: fraction of free ions in total concentration.
* : denote pore water quantities (others are seawater
quantities).
L : Lyman's apparent constants for sea water(Lyman, 1957).
K : apparent solubility product of calcite (Maclntyre,1965).
a : solubility of CO2 in sea water (Li and Tsui,1971).
Thus, using the major element data determined on squeezedwater,
equilibrium constants can be computed for eachsample.
EFFECT OF TIME AND CO2REMOVALON PCO2
After 100 days, the gas and water should be inequilibrium. When
the system is perturbed by removingCO2 from the gas phase for
analysis, a new equilibriumPCO2 will be attained. This PCO2 offers
a clue about theinteractions occurring between the water and the
solidphases over this 100-day time span. Several mechanismsmay
operate when CO2 is removed for analysis:
1) Solid phases and water do not exchange:Σ C O 2 = initial —
gas removedALK = initial
2) Solid carbonate (calcite) equilibrates with the ECO2dissolved
in the water:
Σ C O 2 = initial — gas removed — calcite ppt.ALK = initial — 2
× calcite ppt.[Ca++1 = initial - calcite ppt.[Cθ3=]=Ks*p/[Ca++]
3) Clay minerals exchange ions with the solution keep-ing the pH
constant:
Σ C O 2 = initial — gas removedpH = initial
4) Clay minerals fix pH and water equilibrates withcalcite:
Σ C O 2 = initial — gas removed + calcite dissolved[Ca++] =
initial + calcite dissolved[CO3
=]=K*p/[Ca++]pH = initial
In Figure 1, the percentage of PCO2 change from thefirst set of
22°C measurements to the second set is plottedagainst the
percentage of CO2 removed (NPCO2/ALK X100, assuming titration
alkalinity Σ C O 2 ) . Also plottedare curves representing cases 1,
2, 3, and 4. Cases 1 to 4 arequite similar for samples with low
alkalinity and high
calcium, or those with high alkalinity and low calcium. Ifthere
were no CO2 released from sediments to the gas phasein the kettle,
after an aliquot of gas was removed for IR,analysis PCO2 would be
reduced by 15 to 22% (see A, B inFigure 1). However, almost all the
points distribute along aline traversing cases 1, 2, 3, and 4. The
deviation of all thepoints are caused by the following factors:
1) Molecular diffusion of CO 2.2) Reactions between sediments
and pore water.3) Bacterial activities.If diffusion controls the
PCO2, all the points should lie
on the line representing case 1. However, almost all of
themtraverse it toward reactions lines (cases 2, 3, 4) and
evencross case 4 to the region where PCO2 increases.
The group of samples (solid symbols) showing anincrease in PCO2
may be attributed to CO2 production bysulfate-reducing bacteria and
will be discussed later. Manyof the remainder (open symbols) fall
between cases 1 and 2,indicating that some exchange takes place
between solidand aqueous phases. Although it is possible that the
silicatephases may contribute to this exchange, the change inPCO2
can be explained by assuming that calcite precipita-tion is the
mechanism controlling pH and alkalinity in thesystem over this time
span.
Magnesium-rich coatings (Weyl, 1967) and organic coat-ings
(Chave, 1965; Suess, 1970) have been shown to inhibitequilibration
between calcite and sea water, and Cooke(1971) has shown that the
composition of the fluid maycontrol the chemistry of the surface
rather than vice versa.Apparently some of these processes prevent
the attainmentof complete equilibration.
Although suggesting short-term pH and alkalinity con-trol by
carbonate minerals, these observations do noteliminate the
possiblity that clay minerals exert stronginfluences, i.e., ion
exchange and pH buffering, over thelonger time span available for
diagenesis.
CONSISTENCY OF LABORATORY PCO2AND SHIPBOARD pH MEASUREMENTS
ON INTERSTITIAL WATER
One purpose of this study was to test the consistency ofpH and
PCO2 measurements. To do this, a relation must befound between PCO2
and the CO2 removed for analysis.Since stochiometric reactions
(such as calcite equilibration)cannot be written, the best approach
is to extrapolate thedashed line on Figure 1 to the amount of CO2
removedbefore the first analysis. This should be comparable to
theconditions under which warm squeezed (WS) samples werecollected.
The slope of the line is —5, i.e., for every percentof initial Σ C
O 2 removed, there is 5 percent decrease inPCO2. The same
correction can be made for PCO2measurements at 4°C, enabling them
to be compared withcold squeezed (CS) samples. These extrapolations
areplotted in Figure 5.
Figure 2 schematically illustrates the calculation modelsused to
compute pH. In one series (model A), theextrapolated PCO2, the
titration alkalinity, and the calciumdata were used to compute Σ C
O 2 and the degree of calcitesaturation at the storage temperature
(T§) (saturationresults are discussed in the following section).
This Σ C O 2
854
-
STUDY OF C O 2 RELEASED FROM STORED DEEP-SEA SEDIMENTS
- 6 - 4
NPCO2/ALK 1%)
2 0 2 8
15
10
0
CMO
oα.CM
Ou0-
-10
-15
-20
-25
- 3 0
extrapolation\ \ choice
SulfateReducingRegion
o Cariaco Trench
A Aves Risea Venezuela Basin
i 2
29
C 0 2 ADDED
TO WATER
COfe REMOVED-*
FROM WATER
Figure 1. PCO2 change between two successive measurements at
22°C (APCO2/PCO2) vs. the ratio of CO2 removed foranalysis to total
alkalinity (NPCO2/ALK). (Solid symbols are in or near regions of
strong sulfate gradients; dashed lineis chosen to relate PCO 2 and
ΣCO2; s e e text for explanations of model curves.)
855
-
J-L. TSOU, D. E. HAMMOND, R. HOROWITZ
PCO2 (Ts)
measuredNPCO2/ALK
(Ts)
COz~ removal correction I
TITRATION
ALK (Ts)
Extrapoluted
Pco (Ts)
C a + +
(Ts)
USE CA**(Ts)
CALCULATEDEGREE ofCALCITE
SATURATIONtTs
Assume calpite soturotion
CALCULATE
ΣCOZ Aik(Ts)
CONSERVE ICOjALK
MODEL A'PH
AT Tm
Lj MODEL A" '
Pco,AT 2 2 * |
EQUILIBRATE WITH CALCITE
, Ij MODEL A** J
I P P A I
MODEL A'J>H
AT Tm
ICON;
{ MODEL B'7], PCO2 ,
, AT 22" |T — -
EQUILIBRATE
XI MODEL B** J
CONSERVE TCθ2 ALK
MODEL B'pH
AT Tm
1WITH CALCITE
I
AT 22*
MODEL B *pH
AT Tm
Figure 2. Flowchart of calculation for comparing extrapo-lated
PCO2 data with other pore water data (Ts = 4° or22°C: temperature
of squeeze and storage; Tm =temperature of which pore water pH was
measured;dashed boxes are calculations done only on 4°C PCO2).
and the alkalinity were then used to calculate pH at the
pHmeasurement temperature (T M , model A'). To estimate theeffect
on pH which could be caused by calcite precipitationbetween water
collection and pH measurement, model A*was used, which assumes that
calcite precipitates until itreaches saturation. Models A" and A**
were used tocalculate PCO2 at 22°C on the basis of the 4°C
PCO2measurements. They have the same constraints as A' andA* and
the results are compared to the 22° extrapolatedPCO2 in Figures
5a-c.
The second approach (model B) was to take [Ca++] andPCO2 and,
assuming calcite saturation, calculate Σ C O 2 , andalkalinity.
These two quantities were used, as in model A,to obtain model B'
(pH calculated from Σ C O 2 and ALK atT M ) and model B* (pH of
calcite-equilibrated solution).Models B" and B** were used for
calculation of PCO2 at22°C from the 4°C measurements (Figures
5a-c).
The results of models A', A*, B', B*, and the pHmeasurements are
plotted in Figures 3a-c for 4°C samplesand in Figures 4a-c for 22°C
samples. In Site 148 and theupper 50 meters of Site 147, model pH
is consistently 0.2pH units lower than the observed values. This
may beattributed to CO2 production in SO 4
= reducing regions andwill be discussed later. The remaining
regions show generalagreement (about 0.1 pH units) between the
cluster of fourmodel pH values and the observed value.
Unfortunately,there is insufficient resolution to choose one of the
fourmodels as superior, and all that can be established is arather
crude agreement of about 0.1 pH units betweencalculated pH and
measured pH. The models are interestingin illustrating the effect
calcite precipitation betweencollection and pH measurement can have
on pH, usually adecrease of about 0.1 pH unit.
The extrapolated PCO2 (at 22°C and 4°C) is plotted inFigure 5a-c
for comparison to that predicted from the 4°CPCO2 with models A",
A**, B", B**. Again, the sulfate-
reducing regions show higher observed PCO2 than themodels
predict, suggesting the continuance of CO2 produc-tion. The other
regions show crude agreement (about 25%)between observed and model
PCO2, but again the data lacksufficient resolution to choose a
superior model.
It has been suggested (Hammond, this volume) that anincrease in
temperature, which causes uptake of divalentcations and release of
univalent cations (see Table 3), mayalso cause release of H+ ion
(or uptake of OH~) andsolution of carbonate. This effect would make
the observedPCO2 only about 10% greater than the model PCO2,
andcannot be resolved.
DEGREE OF CALCITE SATURATION
The problem of calcite saturation has been discussedextensively
in the literature and, as pointed out earlier,calcite and water are
rarely in equilibrium. Keeping this inmind, the degree of
saturation (DS) of calcite is defined as:
DS = [CO,
DS was calculated using the extrapolated PCO2, [Ca+ +],
and alkalinity (from Gieskes; Presley et al., Sayles et al.,and
Hammond, this volume). Results are listed in Table 4and show
considerable variation with temperature and fromhole to hole. The
high values ( 1.8) in the Cariaco Trenchare probably due to CO2
loss which has not been correctedfor during sediment collection,
and consequently theextrapolated PCO2 is too low. The 4°C results
on samplesfrom the Aves Rise are reasonable but the 22°C
resultsappear undersaturated. As mentioned earlier, many of
thesesamples appeared to have active sulfate reducers which hadbeen
at work longer and at a higher CO2 production rate(as will be shown
later) before the 22° C measurements thanbefore the 4°C
measurements. Since the PCO2 wasobserved to increase with time
despite CO2 removal foranalysis, the extrapolated PCO2 was probably
much toohigh.
Sediments from the Venezuelan Basin appear to beconsiderably
undersaturated by nearly the same degreecalculated by Hammond (this
volume) with pH andalkalinity measurements. The cause of this
discrepancy isnot obvious, but the answer probably lies in the
effects ofpressure on ionic equilibria between water and
silicatephases. The DS calculated by Hammond for Sites 147, 148,and
149 shows smaller values for progressively deeper sites.If a
decrease in pressure favors a rapid ion exchangereaction between
clays and water, which might be writtenas
N a + ) clayC a H2° N a + + H C a + + clay
and perhaps be driven by a net increase in volume for thesystem,
the alkalinity of the water would appear smallerand the pH more
acid than in situ conditions. If the kineticsof calcite solution
are not sufficiently rapid for equilibriumto be re-attained before
squeezing takes place, the waterwill appear undersaturated. It is
interesting to note that onesample (149-16-4-WS) was squeezed
overnight and had analkalinity 25 percent greater than the
corresponding CS
856
-
6.7pH(C.S.)
7.1 7.3 7.5 7.7
50
100
150 -
200
Dep\h
o×
_ A Δ × O
A Δ<
A ΔX
6.8
X O
lα) Cαriαco TrenchI i I . I
100
^ 200
3 0 0
4 0 0
pH(C.S.)
7.0 7.2 7.4 7.6 7.8 6.6 6.8
A QΔ ×A
-
00
00
6.7pH (W.S.)
7.1 7.3
50
E- 100SZ
α.
s
150
2 0 0
-6.20
O X
(α) Cαriαco Trench
I I I I I I I
6.8 7.0pH(W.S.)
7.2 7.4 7,6 7.8 6.6
100 -
ε 200 -
300 -
4 0 0
I 1o
0
90m |
o
AΔ0 A Δ
JD Δác
OA Δ»
OA C*
A Δ
X PH
Δ B'
Δ A" J
• A*
(b)Aves
1 • '
• Ix 1
X
i\
>
>
measured
Models
Rise
• I
\
(W.S.
1
1
(7.92)
X
)
100
E- 2 0 0
a.Q
3 0 0
4 0 0
pH IW.S.)7.0 7.2
> A
7.4 7.6
A Δ O( c ) Venezuela Basin
Figures 4a-c. Comparison of the measured warm squeeze pH with
those calculated with different models (see Figure 2).
-
2 0
PC02 (mb)40 60 80 100
5 0
100
150
2 0 0
PC0 2 (mb)8 12 16 2 0
+ * x
(α) Cαriαco Trench
100
T; 2 0 0
3 0 0
4 0 0
+ Extrαpoloted PCOa ( 4 β C )x Extrapolated PCO2 (22°C)
o Model A"
• Model A * *Model B"
* Model B * *
(b) Aves RiseI
P C 0 2 (mb)
8 12 16 2 0
100
E-£200
300
400
Qß A
(ci Venezuela Basin
Figures 5a-c. Comparing the extrapolated PCO2 at 22°C with those
calculated from different models using the extrapolated PCO2 at
4°C. (See dashed boxes inFigure 2.)
-
J-L. TSOU, D. E. HAMMOND, R. HOROWITZ
TABLE 3Effects of Squeezing Temperature on Some Major
Elements
Location
1. Cariaco Trench e
2. Aves Risee
3. Venezuelan Basine
4. San Pedro Basina
Cold
466.1462.2464.3
Warm
470.4467.8470.4
Δ(W-C)
+4.3+5.6+6.1
(W-C)/C
+0.93%+1.2 %+1.3 %
Cold
7.987.576.73
Warm
9.419.237.78
(W-C)
+1.43+1.66+1.05
(W-C)/C
+17.9%+21.9%+15.6%+13.3%
Cac Mgc
5.607.95
21.2
5.587.56
19.3
-0.02-0.39-1.9
-0.4%-4.9%-9.1%-4.9%
39.046.144.3
36.742.942.8
-2.3-3.2-1.5
Cl< ALKC
-5.9%-6.9%-3.4%-2.5%
543.1549.4553.9
_
543.1555.1555.0
_
±0+1+1_
.7
.1
±0+0.3%+0.2%+1.4%
20.144.372.57 d
_
20.444.502.5 7 d
_
+0.30+0.13±0.00_
+1.5%+3.0%±0.0%
_
aBischoff et al., 1970; T c = 5°C; T w = 22.5°C.
°Sayles et al., this volume.cGieskes, this volume.d I n upper 16
samples (0 - 230 m), average warm squeeze datum (2.54) is higher
than cold datum (2.49) by
1.9%. In lower 8 samples (259 - 379 m), average warm squeeze
datum (2.64) is lower than cold datum (2.74)by 3.2%. For the whole
core, the average change is zero.
Concentration in mmol/1. T c = 4°C; T w = 22°C.
sample, which was squeezed for only 30 minutes. Almostall other
samples had CS and WS alkalinities within a fewpercent of each
other. The longer time available for calciteequilibration in this
WS sample after a rapid clay reactioncould be the source of this
anomaly. Thus, the DScalculations from PCO2 portray a picture
similar to thosefrom pH. Their divergence from the equilibrium
value of1.0 is probably due to the unmeasured amounts of CO2
lostduring sampling and changes in pore water alkalinity
duringstorage.
2C(org.) + SO 4= + 2H2O 2HCO3~ + H2S
H2S + FeO FeS + H2O
Ca+ + + 2HCO3 ~ CaCO3 + H 2O + CO2
so one unit of CO2 (i.e., NPCO2) is produced per unit ofSO4
= reduced. From Figure 6, the average production rateis 3 X lO~1
mmol/1 per 100 days or 3 × 10~6 mol/1 day.This can be compared with
the in situ rate by fitting asteady state model to the observed
[SO4
=] profile.Choosing
CO2GENERATION DURING STORAGE
In Figure 1, one group of samples showed PCO2increases with
time. Virtually all of these came from, or justbelow, regions which
showed strong gradients in [SO4
=],implying that viable communities of SO4
= reducers (i.e.,CO2 producers) are present in these sediments.
Thedistance of each point from the dashed line in Figure 1 onthe
NPCO2/ALK axis should approximate the amount ofCO2 generated over
the 100 day period. Plotting thisagainst [SO4~] (Figure 6) shows
little correlation, althoughan unexplained correlation exists
between NPCO2/ALKand [SO4
=] (not shown). Apparently, in these sediments,the rate at which
CO2 is produced is independent of[SO4
=] and depth and may, instead, be limited by the rateat which
suitable organic substrates are produced. Althoughthe stoichiometry
of the reaction is uncertain, it might bepostulated as
= R
Cz = [SO4=] at depth z
CQ = [SO4=] at depth 0
z=0
860
-
STUDY OF CO2 RELEASED FROM STORED DEEP-SEA SEDIMENTS
Table 4DEGREE OF SATURATION (D.S.) CALCULATED FROM Ca+\ ALK, AND
EXTRAPOLATED P c o 2
KettleNo.
Depth(m)
15-147 Cariaco Trench16345789
10111213
4.24.55.2
1420566484
104128148177
15-148 Aves Rise141516171819202122232425
610263563709090
107107161210
Ca+ +
C.S.
3.61.51.759.14.23.55.236.155.456.156.406.30
9.227.387.377.585.795.574.724.726.026.028.97
10.1
15-149 Venezuelan Basin303126272829323334
32373
101136157230315369
15.417.122.822.510.511.9528.530.630.95
(mmol/l)a
W.S.
3.561.981.729.254.093.585.235.615.856.005.816.15
8.097.157.107.335.385.125.025.025.595.597.129.72
10.0311.6414.7616.3321.9921.7227.9230.6630.37
ALK (meq/l)a
C.S.
17.717.615.2
8.1813.820.024.627.830.032.931.423.7
4.003.505.235.027.617.666.72_
5.275.275.723.05
2.292.441.791.291.812.202.902.542.27
W.S.
17.7217.6015.17
7.9014.2210.0624.6429.1434.2332.8930.8023.74
Median
4.163.335.374.767.578.086.906.905.625.623.993.03
Median
2.442.722.061.292.322.292.972.422.22
Median
4°C
1.530.851.031.700.930.423.451.883.807.156.786.91
1.8
0.880.490.840.951.050.631.10—
0.830.900.920.640.90
0.540.770.590.840.410.620.362.330.81
0.59
D.S.
22° C
1.531.230.931.551.110.613.362.207.828.54
10.15.46
1.8
0.580.270.540.530.770.810.620.800.680.700.650.440.63
0.960.790.460.541.271.420.490.692.68
0.79
Gieskes; Presley et al.; Sayles et al.; Hammond, this
volume.
R = sulfate utilization rate
D = diffusion coefficient of sulfate
Using D = 9.8 XIO~6 cm2/sec (Y.-H. Li, personal com-munication)
and fitting this equation to the [SO4~]profiles in 147 and 148
(Figure 7) yields R l 4 7 = 1 X 1CT
7
mol/1 day and R 1 4 8 = 2 X 10-10 m o l / l day. Although
thecorrelation may be coincidental, the increase in R
withtemperature (in situ T147 17°C; T148 6-7°C) isstriking and may
be a crucial factor in controlling sulfatereduction rate.
In Figure 7, PCH4 in the kettles is also plotted. Methaneis
absent from regions with significant sulfate, as expectedon
microbiological grounds (Desulfovibrio inhibits Methan-obacterium,
B. Mechalas, personal communication). PCH4and [Sθ4=] show an
inverse correlation in regions of low
sulfate, suggesting that the sulfate is probably a contami-nant
in these samples and has destroyed some methaneduring storage.
CONCLUSIONS
Monitoring PCO2 in kettles of stored deep-sea sedimentsuggests
several ideas:
1) pH measurements on interstitial water are probablyaccurate to
0.1 pH units. Insufficient resolution exists inthis PCO2 study to
be any more precise, or to accuratelydetermine the degree of
saturation of calcite.
2) The pU and alkalinity of interstitial water areprobably
controlled by the interaction between bacterialmetabolism products
such as CO2 and CH4 and carbonateminerals (at a constant
temperature and pressure) over atime span of months to a few
years.
861
-
J-L. TSOU, D. E. HAMMOND, R. HOROWITZ
0.7 -
(Vj(VI
S0.5I-
•SO.3 |
Sm
sOo
0.1
1
X6
(X6)
×25
1
XI9v20
CVI
lO
(VI
(VI
(VI
X
X
1 1
Time interval =110 ± 4days
average rate
^ X17 —* M
1 i
-
XI5
10
[so4=]T15
mmol/l
20
Figure 6. CO 2 generated at 22° C in HOdays vs. interstitial
water [SO 4' ]. (Presley et al; Sayleset al.)
3) Pressure reduction from in situ to atmospheric maycause clay
minerals to exchange cations and hydroxyl ions,significantly
decreasing alkalinity and pH, and creatingundersaturation for
calcite, which cannot dissolve rapidlyenough to compensate for this
before interstitial watersamples are collected. This problem must
be investigatedfurther before alkalinity and pH measurements can
berelied on to discuss carbonate saturation.
4) The rate of CO2 production from sulfate reductionappears to
increase with temperature. No correlation of ratewith depth of
[SO4H was observed, suggesting that at aconstant temperature the
process may be limited by theavailability of suitable organic
substrates.
ACKNOWLEDGMENTS
The authors thank Dr. W. S. Broecker for suggesting thisproject,
assisting in sample collection, and reviewing themanuscript. Dr. J.
Simpson also reviewed the manuscriptand, with Dr. T. Takahashi,
made many helpful suggestions.Drs. Sayles, Gieskes, and Presley
kindly made their dataavailable prior to publication.
We also wish to express our thanks to the staff ofGlomar
Challenger and the following persons: Tsung-HungPeng for guidance
in using his IR and pH-meter, R. Pichulofor comments on the paper,
G. Mathieu for designing thedilution system, and Marylou Zickl and
Diann Warner fordoing the typing and drafting.
Financial support from National Science Foundationgrant NSF
C-482 to collect the data and Atomic Energy
Commission grant AT( 11-1)2185 to interpret the results
isgratefully acknowledged.
REFERENCES
Bischoff, J. L., Greer, R. E. and Luistro, A. O.,
1970.Composition of interstitial water of marine
sediments.Temperature of squeezing effect. Science. 167, 1245.
Chave, K. E., 1965. Carbonates: Association with organicmatter
in surface sea water. Science. 148, 1723.
Cooke, R. C, 1971. The lysocline and calcium
carbonatecompensation depth in the sea. Ph.D. Thesis,
DalhousieUniversity.
Garrels, R. M. and Thompson, M. E., 1962. A chemicalmodel for
sea water at 25°C and one atmosphere totalpressure. Am. J. Sci.
260, 57.
Li, Y.-H., 1967. The degree of saturation of CaCC«3 in
theoceans. Ph.D. Thesis. Columbia University, New York.176 p.
Li, Y.-H. and Tsui, T.-F., 1971. The solubility of CO2 inwater
and sea water. J. Geophys. Res. 76, 4203.
Lyman, J., 1957. Buffer mechanism of sea water. Ph.D.Thesis.
Univ. California, Los Angeles.
Maclntyre, W. G., 1965. The temperature variation of
thesolubility product of calcium carbonate in sea water.Fisheries
Res. Board Canada Manuscript Rept. Ser. No.200.153 p.
Suess, E. 1970. Interaction of organic compounds withcalcium
carbonate — I. Association phenomena andgeochemical implications.
Geochim. Cosmochim. Acta.34,157.
Weyl, P. K., 1967. The solution behavior of carbonatematerials
in sea water. Stud. Trop. Oceanogr. Miami. 5,178.
862
-
STUDY OF C O 2 RELEASED FROM STORED DEEP-SEA SEDIMENTS
5 0
100
150
200 i-
SO (mmol / I)
15 25
Δ—
A
- Δ
— Δ.ith
c
-
-
-
Δ
A*7 \' \
) A
A
A
i i
v\
-"—'
-__A
/
A
\\
&
C, = C0 - 7
---
_ * /
^ A
——
\
\•
\
\
\
\
i f
—
\
j 1-
+ 5×IO~'Z2
Cz s C
—
»
i i i
s
° •
Δ
A
O
•
1 —
/**
-
sea_ water
-O
_
0.33Z+IXI0"3Z2.
[so4•]
[ S 0 4 s ]PCH4
PCH4
1 1
•
147
148
147 "
148 -
-
-
-
-
-
-
4 6 8KETTLE PCH4 (mb)
Figure 7. [SO4 ] (Presley et al; Sayles et al.) and
partialpressure of CH4 (in storage kettles} vs. depth. (Solidlines
are the equations fitted to the sulfate profile.)
863