GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 10, NO. 3, PAGES 527-541, SEPTEMBER 1996
Calcite dissolutionin sedimentsof the Ontong-Java Plateau:
In situ measurementsof pore water Oz and pH
Burke Hales
Lamont-Doherty
EarthObservatory
of ColumbiaUniversity,Palisades,
New York
Steve Emerson
Schoolof Oceanography,
Universityof Washington,
Seattle
Abstract. We presentin situelectrode
measurements
of sediment
resistivity,
porewateroxygen,
andporewaterpH fromthreestations
between2300and3000m depthontheOntong-Java
Plateauin thewesternequatorial
Pacific.Oneof thesestations
is alsothesiteof a concurrent
benthicchamberincubation
experiment
[Jahnkeet al., 1994]. Theporewateroxygendataanda
steadystatediffusionandreactionmodelconstrain
thedepth-dependent
rateof oxicrespiration
in
thesediments
andimplya diffusive
fluxofoxygen
tothesediments
of 10-21gmolcm-2yr-1.
Giventheserespiration
rates,theporewaterpH datacannotbeexplained
withoutcalcite
dissolution
drivenby metabolically
produced
CO2. Thedissolution
necessary
to explainthe
observations,
quantified
bya statistical
approach,
is3.5-6gmolcm-:yr-1,whichcorresponds
toat
least20-40% of the calciterain to thesesediments.Over 65% of the total dissolutionis drivenby
metabolic
CO:. Oxygenfluxesandnetcalcitedissolution
constrained
by theelectrode
dataare
compatible
withthebenthicchamber
measurements
of Jahnkeetal. [ 1994]. Thedissolution
flux,
whilea significant
partof theearlydiagenesis
of calcitein thesesediments,
is lessthanwouldbe
predicted
by earliermodelsof dissolution,
andJahnkeetal. [1994]probably
couldnotdistinguish
it from zerowith thebenthicchambertechnique.The dissolution
ratesfoundin thisstudyare
lowerthanpreviousestimates
because
therespiration
reactionis concentrated
nearthesedimentwaterinterface,andthe calcitedissolution
rateconstants
arevery small. The statisticalevaluation
of theporewaterpH dataandmodelconstrain
thecalcitedissolution
rateconstant
to 0.005-0.16%
d'1,followingthegeneraltrendof lowervaluesdetermined
byin situtechniques
ratherthanby
laboratorymethods.
Introduction
We describea studyof the early diagenesisof organiccarbon
and calcium carbonate in pelagic sedimentsof the western
equatorialPacific. Our goals were (1) to determinethe total
respirationand dissolutionfluxes within the sedimentsand
comparethem to the ratesof supplyto, and burial within, the
sediments;(2) to separatethe effects on calcium carbonate
dissolution due to bottom water undersaturation from those due to
metabolicaddition of CO2 within the sediments;and (3) to
determinethe empiricalformsof the kineticsof thesereactions
within the sediments.
Understanding
the mechanismof calcitedissolutionis critical
becausethe balancebetweenthe influx of alkalinity from rivers
andthelossby oceanicburialof calciumcarbonatecandetermine
theCO2partialpressure
(Pco2)
of thesurface
wa•ers
andhenceof
metabolically producedCO2 can drive atmospheric CO2
differencesbetweenglacialandinterglacialperiodsif the ratioof
the supplyratesof organiccarbonand calciteto the sediments
changedover thosetimescales.It is clear that calciumcarbonate
accumulationin the sedimentsis linked to glacial cycles [e.g.,
Farrell and Prell, 1989]; however, separatingthe effects of
dissolution(eitherdue to bottomwatersaturationstatechangesor
increased supply of organic matter to the sediments)from
changesin productivityis difficult without quantificationof the
kinetics of dissolution.
These are not new questions,but there is a fair amountof
disagreementbetween historical attempts to answer them.
Emersonand Bender[ 1981] first quantifiedthe amountof calcite
that could dissolvein sedimentsin responseto metabolically
producedCO2. Subsequent
in situpH electrodemeasurements
in
pore watersof sediments[Archer et al., 1989;Hales et al., 1994;
theatmosphere
on glacialtimescales
[ArcherandMaier-Reimer, Cai et al., 1995] both above and below the saturation horizon
1994; Emersonand Archer, 1992; Broecker, 1989;Boyle, 1988;
could not be explained without dissolution in responseto
Broeckerand Peng, 1987]. Archer and Maier-Reimer [1994] metabolicallyproducedCO2. Studiesof pore water chemistry,
argued that dissolution in pelagic sediments driven by
measuredin pore waterscollectedwith in situ samplers,also
implicateddissolutiondriven by metabolicCO2 [Sayles,1980,
1985; Saylesand Curry, 1988;Martin and Sayles,1996]. Bulk
Copyfight1996by the AmericanGeophysical
Union.
sediment calcite content and sediment dissolution indices based
on foraminiferal assemblages,however, often do not show
significantchangesabove the depth of the saturationhorizon
PaperNumber96GB01522.
0886-6236/96/96GB-01522512.00
527
528
HALES AND EMERSON: CALCITE DISSOLUTION IN SEDIMENTS
Pacific
Ocean
5øN
......
Ontong-JavaPlateau
In
Situ
Electrode
Measurements
(this
study)
Benthic Flux Measurements (Jahnke et al. 1994)
•k ElectrodeandBenthicFluxMeasurements
•
•500
40
00-2øS
• • I
155øE
160 ø
'
165 ø
Figure 1. Map of theOntong-Java
Plateaustudyarea.Depthcontours
in meters.
[Berger et al., 1982]. Jahnke et al. [1994], basedon direct
measurementsof calcium and alkalinity fluxes at the seafloor,
questioned
earlierestimatesof metabolicdissolution.They found
significant alkalinity and calcium fluxes out of sedimentsat
depthsbelow the saturationhorizonbut nonefrom depthsnearor
abovethe saturationhorizon. Fluxes predictedwith respiration
and dissolution rate models used previously [Emerson and
Bender, 1981; Archer et al., 1989; Archer and Maier-Reimer,
1994] were significantly greater than the benthic chamber
measurements,
while modelsincludingdissolutiondrivenonly by
bottom water undersaturationwere in agreement with these
observations.
The rateof calciumcarbonate
dissolution
in seawater
Recis given
by the empiricalexpression:
Rd,
c=kd,
c[CaC03(s)
](1-•c)nc
whereKsp.c
is thethermodynamic
solubility
of calcite[UNESCO,
1987]. The rate constantlq.c and the reactionordernc are
determinedempirically. Field studiesof calcitedissolutionhave
generallyassumedthat the exponent,nc, is 4.5 [Archeret al.,
1989; Hales et al., 1994; Jahnke et al., 1994;Berelsonet al.,
1994;Caietal., 1995;MartinandSayles,
1996],followingKeir
[1980],although
otherlaboratory
studies
haveshown
thatnc may
be as low as 2 [Walter and Morse, 1985]. There is less
agreementabout the value of kd.c. Keir [1980]performed
laboratory measurementsof the rate of dissolutionof several
differentkindsof naturalandsynthetic
calcite,findingthatthe
best value for bulk sedimentwas at least 1000% d'l, which
appears
tobein accord
withthe•4Cages
of themixed
layers
of
deeppelagiccarbonatesediments[Keir and Michel,'1993].
(1)
Determinations
of kd.
cbased
onfielddatagenerally
haveyielded
much lower values. Measurementsof in situ benthic fluxes
[Keir, 1980; Morse, 1978; Walter and Morse, 1985]. The
saturationstate of the surroundingwater with respectto calcite
[Jahnkeet al., 1994;Berelsonet al., 1994],in situmicroelectrode
porewaterpH profiles[Archeret al., 1989;Cai et al., 1995;
FIc is definedby
Haleset al., 1994],or carbonate
chemistry
of porewaters
collectedwith in situsamplers[Martinand Sayles,1996],
interpreted
with modelsof the porewaterreactions,
give
estimates
of k•cfrom0.2to 100% d'•.
ac--[Ca2+][CO32-]
(2)
Ksp,
C
HALES AND EMERSON: CAI•ITE
DISSOLUTION IN SEDIMENTS
529
Table 1. Ontong-JavaPlateauStationDescriptions
BottomWater Properties
Station
Depth,
m
Oxygen,
gmol
kg-1
ALK,
geqkg-1
2A
2322
128
2404
2B
3
2335
2966
128
140
2404
2411
DIC,
•
2345
2345
2358
91_+ 13
gmol
kg-1
C,bw'
%
*
91_+ 13
75_+ 11
*Theuncertainty
in •C,bw
isduetotheuncertainty
incalculating
carbonate
ionconcentration
from the alkalinity(ALK) and dissolvedinorganiccarbon(DIC) measurements
and the
uncertainty
in the 1-atmcalcitesolubilityproduct.
Setting
Three stationswere occupiedon the Ontong-JavaPlateau,a
topographic
highin thewesternequatorial
Pacific(Figure1), on a
cruiseby the R/V Moana Wave in Juneof 1991. Two of the
stationsat --2300 m are spatiallyseparatedby less than 1 km,
verticallyby lessthan15 m, andareconsidered
duplicates.
The plateauhashighcalcite(>70%by dry mass)from 1500m
to nearly5000 m, andis in a uniformlylow productivityportion
of the world's oceans[Berger et al., 1987]. This location is
ideally suited to addressthe stated goals because(1) the
sedimentsare well characterizedhere, including estimatesof
dissolution
from sedimentary
indicators[e.g.Bergeret al., 1982]
and rates of sedimentaccumulationand benthicmixing [Berger
and Killingley, 1982], and(2) Jahnkeet al. [1994] basedtheir
conclusionsregardingmetabolicdissolutionon benthicflux
chamber results from two sites here. One of these, the station
referredto by Jahnkeet al. as"NS",is thesameasourstation3.
This studyrepresents
the first successful
attemptto quantify
the rate of calcite dissolution with both benthic chamber and in
situ electrode measurementsat the same site. The pertinent
bottom water characteristics of the stations are summarized in
The vertical spacingbetween measurementsis known very
well; the locationof the interfacerelativeto eachoxygenandpH
electrodewas determinedfrom their respectiveprofiles' Each
profile includeda seriesof measurements
in the overlyingwater
that showed little change as the depth increased. The first
measurement
that deviatedsignificantlyfrom thesewas assumed
to be below the sediment-water interface, and the location of the
interfacewas determinedby interpolation(estimatedaccuracyis
about 20% of the vertical interval or 0.5-1.0 mm). The interface
for the resistivityelectrodewas determinedby interpolationas
the depthwherethe resistivitywas 10% higherthanit wasin the
bottom
water[Archer,1990].Theinterface
determined
by
measuringthe lengthdifferencebetweenthe resistivityelectrod
e
and each of the others and assuminga fiat seafloor was not
systematically
differentfrom the abovemethod.
Electrode
driftwasassessed
bymakingtwomeasurements
of
each electrode in the overlying water after the profile was
completed. If thesevaluesdiffered from thosein bottom water
prior to entering the sediment by more than 10% (15% for
resistivity) of the total changerecordedby the electrode,that
profile was discarded. Two of the four oxygen electrodes
performed acceptably at each station. Two pH electrodes
Table 1. The rangein estimatedbottomwater saturationstate
with respectto calciteis due to uncertainties
in boththe bottom
water carbonate ion and the estimated solubility product for
performed
acceptably
at station
2A,butonlyoneperforme
d
calcite.
relationbetweenthe signalin bottomwater(concentratio
n
Methods
acceptablyat stations2B and3.
Oxygenelectrodeswere calibratedin situby assuminga linear
determinedby Winklet titration)and the opencircuitreading
(zero oxygenconcentration).We have shownthat the assumption
of
an open circuit reading at zero oxygen is valid with
In Situ Electrode Measurements
measurements
in oxygendepletedsedimentsin PugetSound(B.
The microelectrode
measurements
were madein situby an Hales, unpublished data, 1990). The pH electrodes were
calibratedon the ship prior to deploymentwith pH 7 andp H 8
updatedversion of the profiler used by Hales et al. [1994],
deployed on a free-vehicle lander. This a substantial buffers,madeup to the ionic strengthof seawaterwith KC1. The
improvement
overthebox-coredeployment
methodfollowedby differencebetweenthesesolutionsand seawatermay meanthere
Hales et al. [1994] andArcher et al. [1989], becauseof decreased are differentliquidjunctionpotentialsfor the electrodes
between
wire time and the decreasedpotentialfor introducingartifacts the buffersand seawater.This is not a significanterror,however,
associated
with takinga core. On eachdeployment,
the profiler since we are interestedin the pH differencesbetweenbottom
water and pore water (ApH), not the absolute value of the
was configuredwith one resistivity,four oxygen,and fourpH
electrodes.All electrodes
usedwereof thesametypeasusedby seawaterpH. The slope thus determinedwas correctedfor the
Hales et al. [1994]. Electrodemeasurements
were made at 0.25- temperaturedifferencebetweenthe laboratoryand the bottom
0.5-cm vertical intervals from a few centimeters above the
water using the Nernst equation. Error bars on each electrode
sediment-water interface to as much as 9 cm below. Each
measurementbelow the interfaceincludethe uncertaintydue to
measurement
is theaverageof thelastfive readingsoutof a suite both short-termvariability (i.e., the standarddeviationof the five
readingsof eachelectrodeat eachdepth)andthe uncertaintydue
of 15 readingsmadeonceevery10 s.
530
HALES AND EMERSON: CALCITE DISSOLUTION IN SEDIMENTS
to the differences between the measurements in bottom water
all cases were bottom water concentrations
at the interface and
beforeand after profiling(i.e., "drift"). Most of the uncertainty zero-derivativesat the deepestpart of the model. Both models
derives from the latter.
were solvedby discretizingthe differentialequationsdescribing
the systemandinvertingtheresultingmatricesusinga tridiagonal
Bulk Sediment Measurements on Retrieved Cores
approach[Presset al., 1989]. The oxygenmodel was linear and
independentof the pH model, so it could be solveddirectly.
Sedimentsampleswere taken from subcoresof Soutarbox
coresat two locations. One set of sampleswastakenat 2-3-mm Numerical solutionswere verified by comparisonto analytical
resolutionoverthe upper10 cm of the sediment,andanotherwas solutions. The pH modelrequiredsimultaneoussolutionof the
takenat =1 cm resolutionover the upper30 cm of the sediment.
Bothsetsof sampleswereanalyzedfor particulate
organiccarbon
andtotalcarbonby a Carlo-ErbaCHN analyzer.Organiccarbon
wasmeasuredon samplesthathadbeentreatedwith HC1vapor
followedby directadditionsof ultrapureHC1to eachsampleto
removeall solid carbonates[Hedgesand Stem, 1984]. Calcium
carbonate content was calculated assuming the difference
betweenacid-treatedand untreatedsampleswas entirelydue to
calciumcarbonate.Porositywascalculatedfrom measurement
of
thedry massof thefine-resolution
samplesandthesaltcontentof
the samples.
Bottom Water Properties
Temperature and salinity were taken from the nearest
Geochemical Ocean Sections Study (GEOSECS) station, at
corresponding
depths. Bottomwater oxygenwasdeterminedon
the ship by Winkler titration on bottle samples. Bottomwater
alkalinity (ALK) and dissolvedinorganiccarbon(DIC) were
determinedby Gran titrationwith ultrapureHC1in a closedcell
monitoredby a RosspH electrode. The titrationwascalibrated
with ALK andDIC standards
(A. Dickson,ScrippsInstitutionof
Oceanography,personalcommunication,1991). Precisionof
replicatealkalinity and DIC measurements
was lessthan0.25%.
There was an artifact associated with the volume of the titration
cell, which appearedto be systematicallyincreasingin volume
overthe courseof thecruise. As a result,samplestitratedlaterin
the cruiseyieldedestimates
of ALK andDIC thatwereup to 1%
higher than samplesfrom the same locations titrated earlier.
However, the most importantparameterfor this study was the
distributions
of ALK, DIC, anddissolved
calcium,
Ca2*. Species
included
in ALK and/orDIC wereCO2'(CO2(aq)
q-H2CO•),
HCO•',CO•2',andB(OH)4'.Thespecies
HCO•'andB(OH)4'
were
eliminatedalgebraicallyfrom the equations,usingthe acid-base
equilibriumrelationships
betweenthemandthe otherspeciesand
the dissociationconstantsfor carbonicand boric acidssuggested
by UNESCO [1987]. This left a combinationof threecoupled,
nonlinear
differential
equations
withCO2',CO32',
andCa2*asthe
dependentvariables. Becauseof the nonlinearity,the tridiagonal
inversionof the matricesdefinedby thesediscretizedequations
had to be executediteratively,usingNewton's method[Presset
al., 1989]. The pore water pH data are presentedas ApH, the
differencebetweenpore waterpH and bottom water pH. For
comparisonto the data,modeledpore waterApH wascalculated
fromtheresulting
modelsolution
for CO•2'andCO2'fromthe
relationship
where the subscriptsBW and PW distinguishconcentrations
in
bottom water and pore water, respectively. This result is
relatively insensitiveto the choice of equilibrium constantsfor
carbonicand boric acids,as long as a consistentbuffer systemis
employed. Diffusivity in the pore waters is approximatedby
Di.•,w
= Di/F [Bemer,1980;McDuffandEllis, 1979]whereD•is
the diffusioncoefficientfor speciesi in "clear"waterandF is the
sedimentformationfactor,determinedby measurements
with the
resistivityelectrode. Diffusion coefficientswere takenfrom Li
carbonate
ionconcentration
([CO32']).Theaccuracy
withwhich andGregory[1974](forCa2+,HCO•'andCO32');
Broecker
and
[CO32']
couldbedetermined
wasrelatively
insensitive
tothiscell Peng[1974](forCO2');andWiseandHoughton
[1966](for02).
volume error, since it affectedALK and DIC similarly. There
Borateion (B(OH)4') was assumedto have the samediffusion
coefficientas HCO3'. The modelsincludeda diffusivesublayer
titrationsperformedearlierandlater in the cruise. Calculationof
that was 0.5 mm thick; diffusivitiesin this layer were assumedto
[CO•2']isverysensitive
toerrors
in ALKandDIC;theanlaytical equal the clear water diffusivities;respirationand dissolution
precisionof 0.25% in both measurementsyields uncertaintyin
rateswere zero in the diffusivesublayer.
wasno significant
difference
between
[CO•2']determined
from
the calculated
[CO•2-] of about+6%. Uncertainties
in the
dissociation constants of carbonic and boric acids [UNESCO,
1987]havelesseffect,sincecalculation
of [CO•2']fromALKand
DIC dependson the ratios of the constantsand not the absolute
values.Evengiventheuncertainty
in calculation
of [CO32'],
these
stationsare more undersaturatedwith respectto calcite than
similardepthsat the nearestGEOSECSstation,whichis nearly
2000 km east of the Ontong-JavaPlateau. This follows the
generaltrend of a westward-shoaling
of the calcite saturation
horizonevidentin theGEOSECSequatorialdata.
Pore Water
Models
The pore water 02 andp H data were interpretedusingonedimensional, steady state mathematical models simulating
diffusionand reactionsin the sediment.Boundaryconditionsin
Results
Bulk sedimentpropertiesfor the top 10 cm of the sediments
are presentedin Figure 2. Calcite contentsare fairy constant
with depthat 85-92%. Thereis slightlyhighercalciteat station2
than
at station
3, but the difference
is similar
to the
reproducibility of each measurement(based on duplicate
measurements of the same sample). Organic carbon is
consistently
low (0.1-0.4%) at bothstations,with station3 being
slightly enrichedover station2. The organiccarboncontentat
bothstationsdecreases
weaklyoverthetop 10 cm,butthechange
is small relative to the measurementerror, and the decreaseis not
by any meanscontinuous.For thisreasonthe organiccarbondata
were not usedto constrainthe ratesof respirationwithin, or the
flux of metabolizable carbon to, the sediment.
HALES AND EMERSON: CALCITE DISSOLUTION IN SEDIMENTS
g CaC03/100g
g OrganicCarbon/100g
0.1
I
80
0.5
0.3
l•
ß
I
531
85
90
95
B)
O
I
$I
•
ll•
I
/
W
I
I
ß
4
Depth
(cm)
Sta. 2
5
ll
Sta. 3
$
ll
I
/
!
II
/
',
I
....
I
....
I
....
I
,
Figure 2. Solidcarboncontentof Ontong-Java
Plateausediment,as percentages
of the total solids: (a) organic
carbonand (b) calcium carbonate.
thepH profilesandthereturntoward,andevenbeyond(at station
2A), their bottom water values at depths >2 cm below the
interface. This could not happenwithout dissolutionof some
carbonatemineralat depthsof a few centimeters
in the sediment.
These stations are strongly undersaturatedwith respect to
aragonitein thebottomwaters,andit is unlikelythata significant
1
amountof thismineralcouldpersistto thesedepths[e.g.Hales et
• = F •'
(4)
al., 1994]. Indeed,Berger et al., [1982] observedno aragonitein
[Andrewsand Bennett,1981];porositymeasuredon subsamples the sedimentsthey collectedfrom the Ontong-JavaPlateau at
thesedepths. The only conclusionthat can be drawn is that
from box coresdecreasedto constantvaluesof about70% by 3-4
cm below the interface (data not shown), implying that the calciteis activelydissolvingwithinthe sediments.
exponentb is about2. Oxygen (Figure 3b) decreases
smoothly
throughoutthe measurementregion, showingcurvatureeven at
Discussion
the deepestpenetrationof the electrodes. The pH data is
We interpretedthe pore water databy simulatingthe vertical
presentedas ApH, the differencebetweenpore water pH and
distributions of oxygen and pH with steady state, onebottomwaterpH (Figure3c). Negativevaluesof ApH meanthat
the pore water is more acidic than the bottom water; positive dimensional diffusion and reaction models to evaluate the
pertinent reaction rates in the sediment. The approachis
valuesindicatethe opposite.All electrodesat all stationsshowa
essentiallythe same as used by Hales et al. [1994], and the
shift towardlower pH below the interfaceto a minimum, and a
governing equationsand numerical solutionsare describedin
gradualreturn towards(in one caseeven beyond)bottomwater
values. There are severalinterestingqualitativefeaturesof these detailby Hales [ 1995].
data. The first is the significantdifferencebetweenthe two sets
of oxygenprofilesat Stas.2A and2B, implyingdifferencesin the The Pore Water O2Model
The microelectrode
dataare shownin Figure3. The formation
factor (Figure 3a), defined as the ratio of resistivity in the
sedimentsto that in the overlyingwater,reachesa fairly constant
value of about 2 by 3-4 cm below the interface at all stations.
Porosityq• is relatedto formationfactorF by therelationship
oxygenflux into the sedimentsbetweenthesetwo stations.
The pH profilesare alsodifferentbetweenthe two stations,with
thepH minimumat station2B beingroughlytwice as low as at
station 2A.
Pore wateroxygensimulationsare sensitiveto the depthdependent
rateof oxygenconsumption
Ro2(Z
) assumed
to follow
the empiricalform:
Since the saturation state of calcite must be similar at
the two sites,the moreacidicporewatersat stationB mustbe due
to the greaterrespirationratesin the sediments,
asimpliedby the
oxygengradients. Also remarkableis the existenceof minima in
Ro2(Z
): (1-q•)(rl
e-r2z
+r3e-r4z
);
(5)
whereq•is the porosity(calculatedfrom the formationfactordata
532
HALF3 AND EMERSON:CAI_E:ITEDISSOLUTIONIN SEDIMENTS
Station 2A-- 2322m
Formation Factor
Oxygen(gmol/kg)
1
1.5
2
30 50 70 90 110130
-1 b.O,
. ß [ ....
• , ._ P .........
'.........
'.........
'.........
'.......t•"-1
ApH
-0.04
-0.02
0
"'''1
.........
I.......
• .......
c)
I
I
•'
ß
I
=
I
d•
3
t
Depth
(cm) 4
-
I
oi
ß
I
I
I
I
I
i
I
;.I .........
I .........
I .........
I .........
I ........ .I ..... ,•
Station 2B-- 2335m
Formation Factor
1
1.5
2
-1
'•' ' ' ' ....
[''
Oxygen(gmol/kg)
30
50
70
90
ApH
110130
-0.04
.... [ ........
-0.02
0
[ ..........
• .... :
,
ß
I
I
I
2
-
I
I
I
I
I
Depth
3:,
(cm)4:'
I
I
I
I
.
-[
I
I
.'[
I
I
5
I
'[
6
I
I
I
I
I
I
I
I
-
I
I
I
I
I
7
I
-
-
-
I
[
,
I
I
I
'
I
...I
.........
I .........
I ........
'
Figure 3. In situelectrodedatafromthe Ontong-Java
Plateau:(a) formationfactor,definedastheratioof the
resistivitymeasured
in the sediment
to thatmeasured
in theoverlyingwater,(b) porewater
oxygen,and(c)
porewater
pH, expressed
asApH,thedifference
between
porewater
pH andbottomwaterpH. In figures3a-3c,
depthis expressed
positivedownward,with the sediment-water
interfaceat zero (solidhorizontalline). The
vertical
dashed
linesrepresent
thebottom
watervalueof eachproperty.
Different
shaped
symbols
in thesameplot
represent
measurements
madeby separate
electrodes.Profilesrepresented
by the diamonds
will be referredto as
profile1,thoserepresented
bythecircles
willbereferred
toasprofile2. In allplots,thelarger,opensymbols
1 cm
.abovethe interfaceare measurements
madeaftercompleting
the sediment
measurements,
represented
by the
smallersolidsymbolof thesameshape.In somecases,
thesepointsarepartiallyobscured
by themeasurements
prior to penetrationof the sediment.All measurements
belowthe sedimentwaterinterfacehaveerrorbars;those
notvisiblearesmaller
thanthesymbols
themselves.
Sincethemeasurements
abovetheinterface
areall replicate
measurements
of bottomwater,theirscatter
isindicative
of theirreproducibility
anderrorbarsarenotgiven.
HALF3 AND EMERSON: CALCrrE DISSOLlrrION
IN SEDIMF_.aN•
533
Station 3-- 2966m
1
Formation
1.5
Factor
2
-1•'•
'''....'''
0 oA)
ApH
Oxygen(gmol/kg)
-0.04
30 50 70 90 1101)0
u.........
I.........
I.........
I.........
I........
'1"•
-0.02
0
"'"1
I........
• ......
:
C).........
ß
_
1._•
.!
ß
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ß
.!
D•
,.
ß
ß
ptl•'-'
;.
g
,
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.
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6•
ß
i
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.
7"-!
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.
8"
,!
I
I
i
I
L
i
I
I
I
I
I.
I
I
I
I
I
i
9"
;I ......... I......... I ......... I......... I ......... I.... h•
: ....
I .........
I .........
I ........
.
.
.
i
Figure 3. (continued)
and (4)) and rl, r2, r3, and r4 are adjustable
parameters.This
expression
of Ro2is similarto thatusedby Hammond
et al.,
[ 1996]andis slightlymoredetailedthanthatusedby Haleset al.,
explicitly include the dependenceon the solid fraction of the
model
sediments,
of organic
(1- {). carbon
Although
degradation
conceptually
[Berner,
similar1980],
to a "multi-G"
our goal
[ 1994],whousedat mostthreeadjustable
parameters,
anddid not
was only to matchthe oxygenprofile as accuratelyas possible.
Determinationof the optimum combinationof four adjustable
parameters manually is difficult, so the oxygen model was
executedby an optimizationroutine(Powell'smethod,Presset
al., 1989] that adjustedrl, r2, r3, andr4 to minimizethedeviation
betweenthe data andthe modeloutput.
Figure4 illustratesthe needfor inclusionof all four adjustable
rate parameters(two exponentialterms)in orderfor the modelto
fit the oxygendatathroughoutthe profile;resultsfor othercases
Oxygen (gmol/kg)
60
80
100
120
140
are summarized in Table 2, where there are some trends worth
noting. In all simulations, most (>70%) of the oxygen
consumption
is dueto the first exponentialtermin (5), whichhas
a scaledepth(1! r2) of 1.7-3.5 mm. This meansthat mostof the
CO2 wasgeneratedvery nearthe interface.As expected,oxygen
fluxes calculatedfrom the model outputat station2B are greater
than those at station 2A. This difference is greater than the
differencebetweenindividualprofilesat eachstation,but is only
Depth
(cm)
Figure 4. Exampleoxygenmodeloutputfor station3, compared
to oxygenprofile 2. Model outputwith respirationratesthat are
constantwith depth (heavydashedline); a single,exponentially
decayingfunctionof depth(heavysolidline); or a combination
of
a constantand a singleexponential(light dashedline) do not
provideadequateagreementwith the data over the entiredepth
range,while that with a two-exponentialformulationdoes(light
solid line). The inflection near the interfacein the profile with
respirationdescribedby a singleexponential(heavysolidline) is
due to the fact that the gradientin formationfactoris significant
relativeto thereactiontermin thatdepthinterval,Notethateach
exponentialterm containstwo empirical parameters(see, e.g.
(5)).
534
HALES AND EMERSON: CAI_L'TrE DISSOLUTION IN SEDIMF_2q'I•
Table 2. OxygenModel Results
OxygenConsumption
Rate Parameters*
r •,
r2,
rs,
r4,
Integrated(Net)
Consumption,
gmol02
gmol02
(cmssed.solids)
4
(cmssed.solids)
4
gmol02
Station
Profile
yr4
cm4
yr-•
cm4
cm-2yr4
2A
1
2
111
192
2.85
4.25
8.50
6.57
0.352
0.277
9.7
9.6
2B
3
1
262
2.87
6.86
0.318
18.7
2
305
3.47
2.69
0.132
16.8
1
2
281
854
4.18
6.17
3.09
3.45
0.200
0.228
14.0
21.1
*The rate parametersr•, r 2, r s, and r4from (5) are determinedby an optimizationroutineas the combinationthat
minimizesthe deviationbetweenthe model simulationand the porewateroxygendata [seeHales, 1995]. The integrated
consumption
is calculatedfromthisbest-fitsimulationandis oppositeandequalto the flux at the sediment-water
interface.
slightlygreaterthanthe differencebetweenthe two estimates
at
station3, implyingthatthe variabilityin benthicoxygenflux is as
largeover spatialscalesof a few centimeters
as it is overa few
thousand meters. The two estimates of oxygen flux to the
carbon Rco2. Uncertainties in the magnitude and depth
dependences of these reactions must be addressed before
attemptingto ,quantifythe calcitedissolutionimplied by thepH
sediment
at station3, 14 and21 gmolcm'2yf•, arein good
agreementwith the flux predictedby the benthic chamber
Assumingthat the order of the dissolutionreactionnc is
known to equal 4.5, the importantparameterscontrollingthe
measurements,
18+8gmolcm'2yr4.
calcitedissolution
ratearethe dissolution
rateconstant
ka,c and
Consideringthe detailedsensitivityanalysisof thepH model
that follows, some discussionof the uncertaintiesassociatedwith
the ri parametersof (5) is justified. There are, of course,
uncertainties
in theseparameters
for a givenoxygenprofilethat
are not shownin Table 2. Becauseof the shallowscaledepthof
the first exponentialterm in (5), (even when the increasein the
term (l-q>)is considered,the scaledepthof this term is of the
orderof 2.5-5 mm), the parametersr• and r2 are the leasttightly
data.
the saturationstateof the bottomwater with respectto calcite
flc,bw.The rate constanthas a wide rangeof reportedvalues.
Keir [1980] determined
that1•,c wasover 1000%d'• with
laboratory measurementsof the dissolution rate of natural
sediments,while Jahnkeet al. [1994] reporteda rangeof 0.05-
0.5% d'•, basedon in situmeasurements
of seafloorcalciumand
alkalinity
fluxes.Thisis a relative
difference
of over104,leaving
essentially no predeterminedlimits on this parameter. We
constrained, and we will limit the rest of this discussionto those
examinedrate constants
in the range10'3-104%
d'•, which
terms.
includesall previousestimates.The bottomwatersaturationstate
Constraint
of these
values
is difficult
to address
unequivocally,becausetheyare sodependent
on oneanother.If,
for example,r• is held constant,adjustments
of only a few percent
in r2 generatemodel profiles that do not approximatethe data
acceptably;the sameis true if r2 is held constantand r• varied.
depends
onboththethermodynamic
solubility
product
Ksp,c
and
the bottom water carbonate and calcium ion concentrations.
uncertainties
The
in the measured bottom water DIC and ALK lead to
However, if both are allowed to vary in such a way that the
implied 02 flux to the sedimentremainswithin a few percentof
the optimum,r• andr2 valuesas muchas 20% differentfrom the
optima in Table 2 yield acceptable approximationsof the
uncertaintiesin the calculatedcarbonateion concentration
of up
to 6%. The UNESCO [1987] solubility product is about 8%
higherthan the lower limit of estimatesbasedon Mucci's [1983]
1-atmsolubilityandsimilarlylower thanthe upperlimit of those
basedon Plath et al.'s [1980] 1-atm solubility measurements.
electrode data. These uncertainties,however, are smaller than the
This meansthatourestimateof flc,bw
hasuncertainties
of about
differences implied by inter-electrode variability. In the
followingsection,uncertainties
in Ro2will be limitedto only the
different setsof ris implied by the different electrodes.In that
way, Ro2is treatedas a singleinput parameterto thepH model,
ratherthanfour, with limits that are definedby the optimumfits
to thetwo observed02 profilesat eachstation.
+14%, which we used to define the practical limits. This
assumesthat Millero's [1983] pressuredependence
is correct;at
thesedepths,the pressuredependenceof Ingle [1975] predicts
only abouta percenthighersolubility.
MetabolicCO2 productionis linearly proportionalto the rate
of oxygenconsumption:
The Pore Water pH Model
Model simulationsof pore water pH are sensitiveto two
reactions,
thedissolution
rateof calcite(Ra,
c, asexpressed
by (1))
and the rate of CO• productionduring metabolismof organic
Rco2= flRo2,
where B is a stoichiometric
constant.
(6)
At each station there are
two estimatesof Ro2,determinedby the two oxygenprofilesat
each station(seeTable 2). As statedpreviously,the differences
HALES AND EMERSON: CALCITE DISSOLUTION IN SEDIMENTS
ApH
-0.1
-0.05
ApH
0
-0.1
-0.05
535
ApH
0
-0.1
-0.05
2
Depth
(cm)
3
Figure 5. Model pH curvesat (a) station2A, (b) station2B, and (c) station3, for threescenarios:no dissolution
(heavysolidlines),nometabolic
CO2production
(heavydashed
lines),andthebestfit to thedata(lightsolidline).
betweenthesetwo estimates
of Ro2aregreaterthanuncertainties
in eachestimate,and the practicallimits on this parameterare
definedby the two estimates.The constant13can be between
0.55 (alkaneequivalentorganiccarbon;seeHaleset al. [1994]or
0.77 (Redfield organiccarbon). A recentwater columnstudy
[Andersonand Sarmiento,1994] estimatedthis parameterto be
0.69, anda benthicflux chamberexperiment[Hammondet al.,
1996] gave an estimateof 0.67, but the reportederrorbarswere
large enough that these estimatesdid little to discriminate
betweenthe upper and lower limits statedabove. We usedthe
aboverange(0.55 to 0.77) asthe practicallimitson 13.
contrast to all the observations. Furthermore, the no dissolution
simulations greatly overpredict the acidification of the pore
waters. It is clear that neither case excluding the effects of
metabolic dissolutioncan reproducethe observations,and that
dissolutiondriven by metabolic CO2 productionmust play a
significantrole in determiningthe porewater chemistryat these
stations.
To addressthe apparentdisagreement
betweenthe pH data
and the no metabolic dissolution scenarios, we determined the
constraints
posedby thepH dataon the dissolutionflux predicted
by the model,giventhe practicallimits on the four modelinputs
The primary goal of the pH modelingexercisewas to assess discussedabove. Figure 6 demonstratesthe sensitivityof the
the importance of calcite dissolution driven by metabolic modelsimulationsto theseparameters.Sensitivityto 13(Figure
processes
relativeto thatdrivenby bottomwaterundersaturation. 6a) over its entire practical range is similar to a factor of 2
As a first step, we addressedtwo simplecasesthat includedno
adjustment
in the rateconstant(Figure6b). The mostsensitivity,
metabolic dissolution of calcite.
These are (1) the "no
dissolution" case, where the respiration reaction produces
carbonicacid,but no calciumcarbonatedissolves(effectivelythe
however, is to the bottom water saturation state. A similar shift
in modelpH resultsif this valueis alteredonly abouta percent
fromits optimumvalue. This sensitivity
to f•c.bw
meansthat,in
sameas assuming
thatkd.c is zero),and(2) the "no CO2"case, somecases,thepH datathemselvesprovidea betterconstrainton
of alkalinity
where the respirationreactiondoesnot producecarbonicacid this parameterthanthe bottomwatermeasurements
while consumingoxygen(effectivelythe sameasassumingthat13 and DIC.
The first step in constrainingthe model simulationswas
is zero), and calcitedissolvesonly in responseto bottomwater
undersaturation.The rangein pH for thesetwo casesis presented choosinga statisticthat quantifiedthe deviationbetweenmodel
in Figure5, alongwitha scenario
including
metabolic
dissolution simulations and the data. We chose the "average relative
that providesthe best fit to the pH data in the range of input absolutedeviation"(ARAD), definedby:
parameters investigated (described below). The two no
(7)
dissolutioncurvesrepresentthe rangeof possiblesolutionsgiven
N i=l O'i
the two different estimatesof the oxygen consumptionrate at
eachstationand the practicalrangeof 13;the two no CO2curves
representthe rangeof solutionsfor the saturationstateof the whereN is thenumberof datapointsin a givenprofile,mi is the
modeloutputat depthi, di is the valueof the dataat thatpoint,
bottomwatersgiven in Table 1 and dissolutionrate constantsof
0.05%and0.5%d4 (therangesuggested
byJahnke
etal. [1994]). and oi is the analytical standarddeviation of the data at that
It is worthnotingthat no combinationof dissolutionparameters point, as describedin the methodssection. The ARAD is a linear
in the no CO2 casecan generatenegativevaluesof ApH, in version
of thefamiliarZ2whichisthenaveraged
overthenumber
ARAD__I•Imi-dil
536
I-•ES ANDEMERSON:
CALCITEDISSOLUTION
IN SEDIMENTS
ApH
-0.04
'"'1
ApH
-0.02
.........
-0.04
I .........
'''l
mmmmmmmmm
ApH
-0.02
.........
0
-0.04
ß
ß
B = 0.77
(cm)
•
I
0
m.
1
Depth
-0.02
I .........
.
k =0.02
I
ß
!
i
•
i
$
i
$
i
•
i
i
I
$
i
%
!*,
,.
.,,
t
'.
ß
I
'k = 0.01
' B-0.S:
.
!' ',
mm'
•'
I
I
m
m-
'
'
-
•",
m
m-
_
'
e"-
m
m
ß
•
m
m
.
,.
'...I ........
m
m
le !
I
I
!
i
I-
I
I .
I
I
s
ß
ß
ß
I ,•..l...ml.'
..., .......
l.,
i
...I .....
m
.
.
'
m I I m i m, m m
i
m mI , m
Figure6. Demonstration
of thesensitivity
of thepH modeloutputat station3 to (a) thedepth-dependent
rateof
CO2generation,
(b) dissolution
rateconstant,
and(c) bottomwatersaturation.
In all cases,
thelightsolidlineis the
model
output
withkd,
½= 0.014%
d'x,•½,bw
= 76%,B= 0.64,andtheoxygen
consumption
ratedetermined
byoxygen
profile1. The othercurvesin figures6a-6carethe sameasthiscurve,with oneof theseparameters
altered.In
Figure6a,theheavysolidlineis theresultif Bis 0.77'theheavydashed
linehasB= 0.55. Theheavydottedlineis
output
if theoxygen
consumption
ratedetermined
byoxygen
profile2 isused.Figure6B shows
theresultif kd,½
=
0.01%
d'•(heavy
solid
line)or0.02%
d'•(heavy
dashed
line).Figure
6Cshows
theoutput
for•½,bw
= 75%(heavy
dashed
line)and•½,•w= 77%(heavysolidline).
of datapointsin the profile. It is a more'robust'statisticthan
ApH
-0.02
-0.01
0
I
I
stepwasto searchtherangeof inputparameters
untilwe founda
I
minimumvalueof theARAD. Because
everydataprofilehas
different analyticalerror bars and different degreesof
"noisiness",
therewasno universally
acceptable
valueof this
statisticfor all profiles,and we visuallyevaluatedthe model
outputto assess
the "goodness"
of fit. Thisis preciselywhat
Presset al. [1989]referto as"chi-byeye",butthepresence
of
variability
in theobservations
thatwecannot
expectthemodelto
I
I
fit
%%
,
I
I
%
'acce'
I
3
Depth
(crn)
4
higher-order
statistics
liketheZ2,because
thedependence
onIm
idllis lesssensitive
to outlying
pointsthanhigher-order
termslike
(mi_di)2
[Presset al., 1989]. (Thisstatistic
is theoneminimized
in theoxygenratedetermination
described
above.)The second
I
I
reproduce
withintheanalyticaluncertainties
of themeasurements
I
(e.g.,thevariability
in thepH dataat station
2A below4 cm)left
I
us with no other alternative.
I
I
I
The minimum ARAD, or "best fit" case was evaluated for
eachpH profiletodecide
whether
ornotthefit wasgoodenough
I
I
I
I
I
Figure 7. Examplep H model profilesdefiningthe "region of
agreement"with the data. The solid line is the model solution
with the lowest ARAD,
or best fit.
The two dashed lines are
modelsimulationsthat fit the dataacceptably;simulationsthat fit
worsethan thesedashedlines have greaterARAD statisticsand
are deemedunacceptablereproductions
of the data. Data shown
here are p H profile 2 from station 2A; this procedurewas
repeatedfor pH profile 1 at this stationand the profile at station
3.
15M
3M
?5
•C,bw
(%)
?5
•C,bw
80
Of•C,bw
pre_determined
lower
85
3M
85
from
oxygen
profile
1
••Ro2
from
oxygen
profile
2
90
15M
90
log(ka,cl(%d-•)
)
-1
-2
-3
-1
log(ka,cl(%d-•)
)
Limits on
B)
7o
?5
?5
:ainedupper
80
of
(%)
80
85
90
Agreement
with
pit :2
Agreement
with
pH 1
Region
ofagreement;
B=0.55
ofagreement;
B
.•Re gion
B=
=0.64
0.T
-3
90
-1
-2
0
-3
-2
-1
log(k,•,cl(%d43)
this
1øg(ka'c/(%d4))
the
dissolution
rate
parameters,
demonstratecl
for
station
Figure
8.
The
procedure
for
constraining
Contour
plot
of
the
bRAD
statistic
for
profile
2,
with
oxygen
of
the
log
of
the
dissolution
rate
constant
and
the
bottom
water
ß
es
inFigure
7.Solutions
was
also
followed
for
station
3.(a)
of
the
bRAD
and
corresponds
to
procedure
the
maximum value
hedashed
lin
urs
represent
consumption
rate
1,[5
0.64,
as
afunction
allowable
•dding
ines;
soluU
3Mrepresen .
saturation
state.
The
contour
labeled
....
.
ß solutions
ma,,--
k and•c,• ....
thedashed
1 .....
'OhS
outsloe
m •-
ß
t tooactb•,•-
.
will
mours
labeled _ _•adW,F•res 8b-Sd
commnauuns
o[_.
a,c
data
•etter_than
•_ RAD't•at•s,c•,.,
.... :••im•re
•. potct• w
contour
fitthe
ev•u<ofA
eanne•ß....
changes
within
the
..... ximum
allowabl
...... <}•'
tha•
the
dash ofthe
region
ofacceptable
fitto
mult'p _ __u
aata
bya
M).(b)•e sensitivity
•1es
cla•ty
only
the
contours
with
B=0.64
will
be
shown
in
the
ot
me
•,,• factor
ot
mre•
,,,•to
pH
•rofile
2for
oxygen
consumption
rates
determined
by
deviate
from
me
p,•....
. ra
.teisO
more
appropriate
for
show
only
the
region
of
acceptable
fit
(ARAD
•
s
iration
reaction
B.
•or
ß
,hich
respiraUon
c
bm
with
region
instoichiomet•
•fthe
re
p o•
.....
•table
(c)
Regions
a•e--__
:•fit
nowa•ofknowing
• • qame
asFigure
• , •,•,n nrofile
l;
Figures
8cand
_=0.64.•*--•.
ince
there
,•" must
l u,
.....
• 81%
• ......
atible
rt'
•tins
......ot
e
. • 8d.d2 B
•m
cases
•-,,sidetea.
_....r than
•sC•,,,v
..... rwin,
day))
tot
en rofilcs
1an _'• •,reemem
for..... _.... ,,,• of•c• gr•a•. -•,•-(k.J(percem
v• •- .• value
oxyg P
ionu,•
.
d •o •m•
'.
value
....
.
rofile
2,}•ereg•
. 1suenmpos•
ß_•a them•mum
-• ot•rt•,this
stauon,
the •immu,,,
• o-•ves
thefinal
P•P
t wire
pHurof•
eavetheP•ame
vmue
ot-•c.•,,
.... o•• c ='t•vo
• eS lrauu
ß -=•n
•arametet
'••'
a eemen
r ll•t
sinceboth..P • o • qven the Pre
..... nceaurex•
•
a ga) is -u.o. •
._. ß n The sameuTM
•t• = •'•;'
..... , •er day)) •s -• ....
log(k•d[
pe•c•,,•
parameters-
emits'on
the
dissolution
rate
538
HALESANDEMERSON:CALCITEDISSOLLrrION
IN SEDIMENTS
to merit discriminationof othermodelsimulationsagainstit. The
modelcurvesin Figure5 showthat, while not perfect,the bestfit
solutionsfor the two profiles at station 2A and the profile at
station3 did a goodjob of describingmostof the variationin the
data,while the bestfit at station2B did not do nearlyaswell. We
will limit the rest of the analysisto Stas.2A and 3. Given this
"optimum"combinationof parametersfor eachprofile, we then
adjustedthe input parametersaway from that optimumuntil the
simulationno longer agreedwith the given profile (Figure 7).
The value of ARAD correspondingto these simulationswas
of acceptablefit, andloosenthe constrainta givenpH profilehas
on thedissolutionparameters.
Addinga secondpH profiledoesprovidefurtherconstrainton
the field of acceptablefit in k-fl space(Figure 8d), as the two
profiles must have the same value of bottom water saturation.
While it is not clear that they must necessarilyhave the same
deemed
uncertainties
in Rco2,the dissolutionrate constantat this station
the
maximum
allowable
deviation
from
the
data.
Simulationswith ARAD lessthanor equalthis valuefit the data
better than the dashedlines in Figure 7 and were acceptedas
reasonableapproximations
of the data;thosewith greaterARAD
values fell outside the dashedlines in Figure 7 and were not
acceptedasreasonablesimulations.
The ARAD was tabulatedas a functionof the model input
parametersand plottedin parameterspacefor eachpH profile.
Figure 8a demonstrateshow the ARAD varies as a function of
ka.c and•c,b•, The closedcontour,labeled"M," represents
the
boundaryof the "field of acceptablefit" to pH profile2 at station
2A, drivenby the respirationrate determinedby oxygenprofile 2
(at that station), with a stoichiometryof 0.64, as a functionof
value of ka,c (e.g., becauseof spatialheterogeneity
in the size
distribution, surface area, or mass fraction of the calcite), the
requirement
of equalflc,•wprovidedsomefurtherconstraint
on
the value of ka,c (Figure 8d). Taking into accountthe
mustbebetween0.025%and0.16%d" (Table3), andthebottom
watersaturationratio with respectto calcitemustbe betweenthe
practicallower limit of 78% (determinedpreviously,seeTable
1), anda maximumconstrained
by the dataof 81%.
Includingtheuncertainty
in flc,•wcaused
by uncertainty
in the
carbonateion concentration,this upper limit implies that the
solubilityproductat station2A mustbe at least5% greaterthan
the UNESCO [1987] value, which is about equal to the stated
uncertainties. It is, however, incompatible with the lower
solubility suggestedby Mucci [1983], which implies that the
bottom water saturation state can be no lower than 85%.
The
upperlimit of •C,b•at station2A alsoprovides
a morerestrictive
log(kd.
c/(percentperday))andflc,bw
(percent).Any combination upperlimit of •C.b• at station3, because
the calcitesolubility
of kd,c andflC,•w
withinthiscontourgivesan acceptable
fit to the product
(Ksp,c)
at station
3 mustbe 11%higherthanat station
2A,
electrodedata (see Figure 7); combinationsoutsidethe contour
generatemodel simulationsthat deviate unacceptablyfrom the
observations.The fact that the field of acceptablefit has finite
becauseof the difference in depth between the two stations.
Following the above procedure,this additional constraintat
station
3 restricts
thevalueofka,
cto0.005-0.05%
d"there(Table
dimensions
in k•,c and•C,bw
impliesthatthepH datais capable
of
3). The inclusiverangefor bothstations,
0.005-0.16%d" is at
constrainingboth of these parametersmore tightly than the
predetermined
limits discussed
above.
Figures 8b and 8c illustrate how the field of acceptablefit
movesin this 'k-fl space'in responseto changesin Rco2. Figure
8b demonstrates
the effectof variable13.(For the sakeof clarity,
this responsewill not be shown in Figures 8c and 8d.) The
dependenceon Ro2 is shown in Figure 8c. Note that both
estimatesof Ro2 and the entire practical range of 13generate
modelsimulationsthat agreeacceptablywith the datawithin the
least 4 ordersof magnitudelower than the valuesKeir [1980]
determinedfrom laboratoryrate measurements.It is alsolower
than any other value previouslydeterminedin the field but
followsthe generaltrendof lower rate constantsobservedin field
experiments
thanin the laboratory[Archeret al., 1989;Cai et al.,
practicallimitsof bothkd,
c andflC,b•,Thismeansthat,unlikethe
dissolutionrate parameters,constraintson the respirationrate
parameters13and Ro2 are not improvedover the a priori limits
statedabove. An additionalproblem is that we do not know
which rate of respiration was more appropriatefor this pH
profile; therefore we had to use the union (rather than the
intersection)of the two fieldscorresponding
to the two estimates
of Ro2as the regionof acceptablefit. A consequence
of this is
that additionalestimatesof Ro2canpotentiallyexpandthe region
1995; Hales et al., 1994; Berelson et al., 1994; Jahnke et al.,
1994;Martin and Sayles,1996].
Using the constraints
of thepH profileson thesecontrolling
parameters,we quantifiedthe calcitedissolutionfluxes. The total
dissolutionwas plottedin parameterspaceand presentedas
contours,
muchastheARAD. Simplysuperimposing
thefieldof
acceptablefit as determinedaboveover thesecontoursgavethe
rangeof dissolution
flux constrained
by thedata. Thisprocedure
is illustratedin Figure 9 for station2A, 13= 0.55 and 0.77, and
oxygenconsumption
rate 1 from Table 2.
Note that the field of acceptable
fit is essentiallyparallelto
the flux contoursin Figure9 for a given valueof 13.This means
thatthe uncertaintyin dissolution
flux is relativelyinsensitive
to
Table 3. The pH Model Results
Station
2A
3
kdc,
%•'•
0.025 - 0.16
0.005 - 0.06
flc,bw,
%
78 - 81
68 - 80
TotalDissolution,
gmolcm'2yr4
3.5 - 6
3.5 - 6
The rangesin all of thesequantitiesincludeuncertainties
relatedto B andRo2. The
lowerlimitof •c bwat station2A is thepredetermined
limitbasedontheprecision
of the
measurement
of i'fiealkalinityanddissolved
inorganic
carbonandtheuncertainty
in the
solubilityproduct.The pH observations
constrainthe upperlimit at stations2A and3,
and the lower limit at station 3.
HALES AND EMERSON: CAIX21TE DISSOLUTION IN SEDIMENTS
A)
539
Table 3. Calcitedissolutionfluxesat both stationsare 3.5-6 gmol
13= 0.77
cm'2yr'l. Because
a 40%increase
in 13(from0.55to0.77)givesa
10
70
70% increasein dissolutionflux, thereappearsto be a superlinear
dependenceon 13,which is probably due to the high-order
dissolution
kinetics.
The
fact that the dissolution
fluxes
at
stations2A and 3 are similar despite the differencesin total
oxygen flux and bottom water saturation state is due to the
shallowerscale depthsof the respirationreactionsat station3,
75
andthelowerrangeof ka.c constrained
by thepH datathere.
(%)
The predictedrate of calcite dissolutioncorrespondsto 2040% of the calcite rain to these sediments,given the sediment
calcite content and independentestimatesof the sedimentation
rate [Berger and Killingley, 1982]. This is probablya lower
boundof the fractionof the rain thatis dissolved.For example,if
a 1:1 ratio is assumedbetween the organic carbon and calcite
Lower
limit
offlC,bw
80 _ _
_
pperlimitof D•,bw
85
fluxes to the sediment, then our estimates of dissolution
2
correspondto 37-64% of the rain. In either case, dissolution
accountsfor a significantfraction of the rain to the sediments.
Model runswith the samerangeof bottom-watersaturationstates
90
-2
-3
-1
log(kd,½/(%d-•))
and dissolution rate constants as in Table 3, but without
metabolicCO2 production(i.e., 13= 0) predict only about 1.2
gmol cm'2yr'l of calcitedissolution,
implyingthatmetabolic
dissolutionis responsible
for 65-80% of thetotal.
B)
13=0.55
Conclusions
70
(%)
80
Lowerlimitof •C,bw
Upperlimitof
85
We find that there is significant calcite dissolution in the
sedimentsof the Ontong-JavaPlateauand that the dissolutionis
driven. primarily by metabolic CO2. Jahnke et al. [1994]
concludedfrom benthic flux, chamber experimentsat station 3
thatdissolutiondrivenby metabolicCO2wasnot asimportantas
previous models had predicted. Fluxes predicted by both
methods are summarized in Table 4. As noted previously,
electrode-based
oxygenfluxesare within the methoderrorsof the
fluxes determined by the benthic chamber measurement.
Somewhatsurprisingis the fact that the range of calcium and
alkalinity fluxes predictedfrom the modelsof pore waterpH in
this study overlapswith thosereportedby the benthicchamber
technique. The reason for the apparent difference in the
conclusions about the importance of metabolic dissolution
between
9O
this work and that of Jahnke et al. is due to different
representations
of the respirationkineticsin the two studies.
Jahnkeet al. [1994] simulatedconcentrationchangesin the
-3
-2
-1
0
chamberas a functionof time usinga sediment/porewater model
log(kd,c/(%d-•))
that approximatedrespirationas a reactionthat was first-order
Figure 9. Exampleof dissolutionflux estimationat station2A
with respectto organiccarbon,following the work of Archer et
for theoxygenconsumption
ratedetermined
by oxygenprofile1, al. [1989] andArcher and Maier-Reirner[1994]. This gave a
with B equal(a) 0.77, and(b) 0.55. The heavy,labeledcontours respirationrate that varied with depthin the sedimentby means
arecalcitedissolution
fluxes,withunitsof gmolcm'2yr4. The of a single exponential term. Given the stated bioturbation
regionslabeled"pill" and"pH2" arethe limitsof acceptable
fit
coefficient and respirationrate constant,the scaledepth for the
to the pH data determinedpreviously. The allowablerangein
respirationreactionin Jahnkeet al.'s [ 1994] model was about 1.5
dissolutionflux is constrained
by thesetwo regionsandthe limits
placedon Flc.bwby the proceduredescribedin the Figure8 cm. In contrast,the porewater oxygenprofilesmeasuredby the
electrodesrequiredtwo exponentialtermsto fit the data. Most
caption.
(>70%) of the oxygen consumptionwas accountedfor by the
term with scaledepthsof 0.2-0.4 cm. Thus moreof the metabolic
theaboveuncertainties
in k•candFlc,•w,
in thisfieldof k-Flspace. CO2 productionwas shiftedtowardthe sediment-waterinterface
where it had a greaterchanceof diffusing out of the sediments
As a result,mostof the uncertaintyin the implieddissolutionflux
comesfrom the uncertaintyin 13. Repeatingthis exercisefor before it could promote dissolution. We ran several model
both stationsand takinginto accountthe rangesin the estimates simulationscomparingthe dissolutionflux driven by the twoof 13andRo2,yieldedtheestimates
of dissolution
summarized
in exponentialmodel of respirationto that driven by a first-order
540
HALESAND EMERSON:C3J.LTI'E DISSOLUTIONIN SEDIMENTS
Table 4. Comparison
to BenthicChamberResultsat Station3
Flux
Method
Benthic chamber
[Jahnke et al., 1994]
Porewaterprofiles
Oxygen,
Calcium,
Alkalinity,
gmol
cm
'2yr'•
gmol
cm
'2yr4
gmol
cm
'2yr4
- 18+_8
0.35_+4.2
1.4_+6.3
- 14- -21
3.5- 6
5.5- 10
(this study)
Negative
signs
ontheoxygen
fluxesindicate
thatthefluxistothesediment,
whilethefluxesof
calciumandalkalinityareoutof thesediment.Calciumfluxesbasedon theporewater
electrode
measurements
areequaltothetotaldissolution
(Table3). Alkalinityfluxesarecalculated
from
FA = 2Fca+0.15fiFo
2
where
F^,Fca,andFo2arethealkalinity,
calcium,
andoxygen
fluxes,
respectively,
and0.15Bisthe
stoichiometric
relationship
bet•veen
oxygenconsumption
andnitrateproduction,
assuming
Redfield
C:N stoichiometry.
Pacificimplyoxygen
fluxestothesediments
of 10-21
model. For all otherparametersequal,the first-orderrespiration Equatorial
reactionwith a scaledepthof 1.5 cm droveabouttwice as much gmol
cm'2yf•andcalcite
dissolution
rates
of3.5-6gmolcm'2yr
'•.
dissolution(for the observedrangeof ka,c and •2c,bw)
as a
formulation consistentwith the pore water oxygen data. In
addition,Jahnkeet al. [1994] assumedthat the organiccarbon
undergoingdegradationhad Redfield stoichiometry(B = 0.77).
In the previous section,B was found to be one of the most
important factors influencing the magnitudeof the estimated
dissolutionin our sensitivityanalysis,with a 70% increasein the
predicteddissolutionfor B = 0.77 overthatpredictedfor B = 0.55.
These two differences in the description of the respiration
Thisdissolution
ratecorresponds
to at least20-40%of thecalcite
rain to thesesediments.Dissolutiondrivenby CO2producedby
metabolism
of organicmatterin thesediments
accounts
for 6580% of the total; the observed
pH datawereincompatible
with
any scenarioexcludingthe effectsof metabolicallydriven
dissolution.
Thepredicted
oxygenfluxeswerein goodagreement
with fluxes determined at one of these stationsby a benthic
Andersonand Sarmiento[ 1994], our estimatesof the calciumand
alkalinity fluxes would fall entirely within the error bars of the
chamberincubationexperiment[Jahnkeet al., 1994]; calcium
andalkalinityfluxescorresponding
to thecalcitedissolution
rate
predicted
by theporewatermodelwerealsocompatible
withthe
chamber-determined
fluxes. Our pore water oxygenprofiles
demonstrated
that the majorityof the metabolicCO2production
was very near the interface,which allows a greaterfraction to
diffuse out of the pore water before it reactswith calcite. The
benthic chamber measurements.
pore waterpH data constrainthe calcite dissolutionconstantto
The sensitivity of the estimated dissolution flux to the
respiration reaction illustrates the necessity of accurate
quantification of its depth dependencewhen attempting to
constrain dissolution rate parameters. Without accurate,
quantitative information about the depth-dependence
of the
respirationreaction, kinetic parametersand the proportionof
0.005%-0.16%d'l, the lowestvalueof thisparameter
ever
reported. This combinationof kinetic parametersdrives less
dissolution than previous models: our representation of
respirationdrivesabouthalf the metabolicdissolutionpredicted
by simpleexponentialmodelsof respirationwhichreleaseCO2at
greater depths in the sediment, for the observedranges in
metabolic dissolution cannot be assessed from flux data alone. A
dissolution rate constant and bottom water saturation state.
reaction could lead to as much as a factor of 3 difference in the
predictedmetabolicdissolution.Had we restrictedthe valueof B
in our modeling exercise to 0.69, following the results of
factor-of-two difference in dissolutionflux implies ordersof
magnitudedifferencesin the predicteddissolutionrate constant
Acknowledgements. NSF Grant OCE 9217233 fundedthis research.
(for theserangesin ka,c and•2c,bw,
seeFigure9). Thissensitivity We greatlybenefitedfrom the insightfulreviewsof R. A. Jahnkeandtwo
anonymnous
reviewers.We would like to thankJ. Kaczynskifor making
to the respirationrate also has implicationsfor the atmospheric the solid-phaseorganiccarbonandcalcitemeasurements.
CO2 reductionscenariopresentedby Archer and Maier-Reirner
[1994], who used a simple exponentialmodel of respiration. If
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BurkeHales,Lamont-DohertyEarthObservatory
of Columbia
University,Palisades,
NY 10964. (e-mail:hales@ldgo.columbia.edu)
SteveEmerson,Schoolof Oceanography,
Universityof Washington,
box 357940, Seattle,WA 98195-7940. (email:
semerson
@ocean.washington.edu)
(ReceivedJanuary17, 1996;revisedMay 13, 1996;
acceptedMay 14, 1996.)