JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 103, NO. B5, PAGES 9931-9944, MAY 10, 1998
Hydrothermal fluid circulation through the sediment of Crater Lake,
Oregon: Pore water and heat flow constraints
C. GeoffreyWheat
•
Instituteof Marine Sciences,Universityof Alaska, Fairbanks
JamesMcManus, JackDymond, and RobertCollier
Collegeof OceanicandAtmospheric
Sciences,OregonStateUniversity,Corvallis
Michael
Whiticar
Centrefor Earth and OceanResearch,Universityof Victoria, Victoria, BritishColumbia,Canada
Abstract. We presentevidencefor porewaterflow throughthe sedimentof CraterLake,
Oregonbasedon systematicvariationsin porewaterchemicalcompositions
andthermal
gradients.Porewaterwasextractedfrom sedimentby centrifugation
anddiffusiveexchange
usinga gravitycorerdeployedfroma surfacevesselanda boxcorerandpeepersdeployed
fromthe submersible
DeepRoverin a knowngeologiccontext.Depthprofilesof sediment
temperaturewere measuredusingtwo probesdeployedfromthe submersible.One probewas
connected
to the submersible
whereasthe otherwasself-contained
anddeployedfor 7 days.
On the basisof measuredandcalculateddepthprofilesof porewaterNa, Ca, Mg, K, Li, and
temperature,we showthatporewaterupwellsin zonesof focusedupflowat speedsof meters
to hundredsof metersper year. Thesezonesof focusedflow arepatchyandusuallycover
severalsquaremetersto hundredsof squaremetersandaremarkedby iron-manganese-rich
crusts,bacterialmats,andsalinepools. In contrast,mostof the lake floor consistsof
sedimentderivedfrom the calderawalls and hasa low heatflow with porewater velocitiesof
millimetersper year. The chemicalcomposition
of theporewaterthatupwellsthroughthe
sampledsectionof the sedimentcolumndiffersfrom coreto core. This differenceresults
from mixing a hydrothermalfluid in igneousbasementbelowthe lake with lake water before
the final ascentthroughthe sedimentcolumn. Elementalratiosof thisthermallyand
chemicallyalteredfluid in basementmatchthosecalculatedbasedon massbalance
considerations.
Calculationof massbalanceandgeothermometry
constrainthe temperature
in basementandultimatelythe poweroutput,whichis about30 MW. This poweroutputis in
agreementwith two otherestimatesthat were calculatedusingtemperaturedatafrom the
water column and measurements of sediment heat flow.
1. Introduction
Nelson, 1990]. Recentevidencesupports
the hypothesis
that
chemicallyandthermallyalteredwaterrelativeto lake wateris
CraterLake, Oregonis locatedin the southernsectionof the presentlyissuingintothe lake. Evidenceincludesanomalously
Cascademountainrange(Figure 1) in the completelyenclosed high concentrations
of C1and sulfatein CraterLake relativeto
caldera of Mount Mazama, an andesiticvolcano. Crater Lake
formedshortlyaftera cataclysmiceruptionroughly6850 years
B.P. [Bacon, 1983] and volcanicactivity continuedfor several
concentrations
in neighboring
lakes[Van Denburgh,1968],
chemicalandphysicaldatafromthewatercolumn[Simpson,
thousandyears with the last eruptionabout 4000 yearsB.?.
[Baconand Lanphere, 1990]. TheseeruptionsformedWizard
Island, Wizard Platform,and Merriam Cone (Figure 1). This
volcanism was accompaniedby hydrothermal activity as
indicatedby the alterationof andesiticstockwork[Barberand
Collier et al., 1990a,b;McManuset al., 1992, 1993, 1996],
measurements
of heatflow [Williamsand VonHerzen,1983],
andcalculations
of massbalances[Nathenson,
1990].
All of theaboveworkprovideindirectevidencefor inputof
•NowatWestCoastandPolarRegionsUndersea
Research
Center,Moss
Landing,California.
activeventingof hydrothermal
fluids,suchas waterissuing
fromchimney-likefeatures,mudvolcanos,shimmering
water,
ornonlinearprofilesof sediment
temperature.
In thispaperwe
presentdirect evidencefor hydrothermalpore water flow
through the sedimentof Crater Lake basedon systematic
variationsin pore water chemicalcompositions
and thermal
Copyright1998by theAmericanGeophysical
Union.
Papernumber97JB03391.
0148-0227/98/97JB03391 $09.00
1970a,b; Dymond et al., 1989; Dymondand Collier, 1990;
chemicallyand thermally alteredwater into Crater Lake. In
1989, submersibleoperationswere employedto find sitesof
9931
9932
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
Steel Bay
CALl
F ,'
_ 57o30'N
/
Basin ß
2GC
East Basin
3GC
Wizard\
ß
Platform9GCß
'OG•
Basin
8GC
O0
1000
m
Eagle Point
I
122oo5'w
Figure 1. Locationof gravitycores(GC) in CraterLake,Oregon.Bathymetric
contoursare 100 m. The box
definesthe arearepresented
in Figure2.
gradients.Theseporewaterdataprovidea measureof the speed
of pore water flow throughthe sediment,constrainthe pattern
of fluid circulation through the sedimentand underlying
basement,
andallow usto estimatethe compositionof the fluid
in basementand the flux of dissolvedchemicalspecies,heat,
andwater from basementto the deeplake.
acrossand are mostly composedof iron-oxidizing,sheathforming bacteriaof the genera Gallionella and Leptothrix
[Dymondet al., 1989] and salinepools,which havesalinities
and temperaturesthat are greaterthan that measuredin the
surroundinglake water. A more detailedphysical,chemical,
and thermal descriptionof the mats and pools is given by
Collier et al. [ 1991].
2. Benthic Setting
3. Methods
CraterLake,Oregon,is enclosed
by calderawalls,whichrise
hundredsof metersabovethe calderafloor andarecomposedof
Sedimentandporewaterwas sampledusinga gravitycorer,
andtwo temperature
probesto characterbasalticandesite[Bacon and Lanphere, 1990]. Thesewalls a box corer,"peepers,"
andthermalgradientof
definethe drainagebasin,obstructsurfaceoutflow,andrestrict izeprofilesof thechemicalcomposition
sedimentaryinputs. Sediment in Crater Lake is mostly pore water and to quantify sedimentphysicalproperties.
aluminosilicate debris. Net accumulations of aluminosilicate
Gravitycores(GC) upto 170 cm in lengthwerecollectedfrom
debris, biogenic debris from the euphoticzone, and iron- a surfacevessel. Thesecoreswere usedto targetgeneralareas
manganese-rich
precipitatesfrom inputof hydrothermalwater of interestwhere longersedimentsectionswere desired. Box
at the lakebottom[Dymondand Collier, 1990] differ from tens cores (BC), "peepers"(lettersA throughH, Figure2), and
of centimeterson the slopesof the calderawalls to several temperatureprobeswere usedto target specificsedimentary
meterson Wizard Platformto about100 m in basementdepres- features
in a knowngeologicsettingusingthesubmersible
Deep
Rover. Niskin bottlesalsoweredeployedfrom the submersible
sions[Barberand Nelson, 1990].
Most of the lake floor is covered by a smoothlayer of to collectwater from mats and pools on the lake floor and are
aluminosilicate
debristhat is undisturbed
by benthicorganisms. reportedas lake floor springsamplesherein.
Sedimentsamplesfor porewater extractionweretakenfrom
In the SouthBasin, other sedimentaryand biologicalfeatures
were observed. For example, we located and sampled 14 gravitycores(Figure1) and8 boxcores,whichwere0.06to
iron-manganese-rich
cruststhat containabout35 weight per 0.2 m in length(Figure2). Gravitycoreswerepackedin snow
to a van, held at 3ø -5øCwheretheyweresplit,
cent(wt %) Fe andlessthan1 wt % A1. Thesecrustsformmulti andtransported
andsampledfor sedimentphysicalproperties
andfor
coloredpavements(ochreto dark brown) that covertens to described,
hundreds
of squaremeters,fieldsof pebblesthatalsocovertens pore water extractionby centrifugationwithin 12 hours of
to hundreds
of squaremeters,andmultipleochre-colored
layers retrieval. Box cores were retrieved using a miniature
that are visible in areasof sedimentslumping[Dymondand Soutar-typebox corerthatwas developedfor deployment
by a
Collier, 1990]. Other features that were sampled include submersible
[Soutaret al., 1981]. Thesecoreswerepackedin
bacterialmats,which are 2 to 20 m in lengthand severalmeters snow and sampledimmediatelyby whole-coresqueezing
WHEAT ET AL ß HYDROTHERMAL INPUTS TO CRATER LAKE, OREGON
9933
1800
1600
1400
.................
;:;';;;¾;¾;_':t::;;:'
......
12OO
...........................
::.• .........
...................
............................
!ii•.'!i!iiiii!iiii
.....
'................
'..........
'.........
.......
41.0,
iii:ii:i:"
........
..........
400
600
800
1000
1200
1400
! 600
1800
Meters E of Eagle Point
Figure 2. Locationof box cores(BC), peepers(LettersA throughF), andTTT deployments
with respectto
bathymetry
(20 m contours).The locationof EaglePointis shownin Figure1. Two of theeightbox coreswere
collected out of this area.
[Benderet aL, 1987]. In addition,discretesedimentsamples storedin vacutainers.One aliquotwas spikedwith 10 gL of
redistilled1Nhydrochloricacidandanalyzedfor severalmetals.
Si was measuredon a differentaliquot.
3-15 hoursuntil they couldbe centrifuged.
Dissolvedsilicawasmeasuredusinga standardcolorimetric
Supernatantfrom centrifugedsedimentand pore water
extractedby whole-coresqueezingwere filteredthrough0.45 technique[Parsonset al., 1984]with a precisionof about2%
of CI weredetermined
by ion chromatog!amfiltersand subsampled
into severalaliquots. Aliquotsfor (1o). Concentrations
CI and sulfatewere sealedin glassampules.Aliquotsfor Na, raphy with a precision of <2% (lo). Atomic absorption
techniques
wereusedto measureconcentrations
of
Ca, Mg, K, andLi (4-6 mL) were spikedwith 10 !xLof redis- spectrometry
tilled 1 N hydrochloricacid and storedin acid-washed,
high- Na, Ca, Mg, K, and Li with precisionsof lessthan 5% and
densitypolyethylenebottles. An additionalaliquotwas stored typicallylessthan 2% (1 o). Somesamplingand/orstorage
of Na increased
in polystyrene
testtubesfor analysisof nutrients,
pH, andtotal artifactsexist. For example,the concentration
CO2, all of which were measuredwithin 1 to 2 daysof core by 0.3 mM in all of the peepersamples,which were storedin
retrieval. Someof thesedataare omittedin thispaperbecause glassvacutainers. This resultis basedon a systematicoffset
versusbox
they do not provide additionalargumentsfor the principal observedin plotsof Na versusLi for peeperSamples
hypothesis
of hydrothermal
circulationin CraterLake, Oregon. core,lakefloor spring,andgravitysamples,whichwere stored
of K are0.02 mMhigherin
Pore water was collectedwith "peepers"to obtainsamples in plastic.Similarly,concentrations
by centrifugationrelativeto thosefrom
that were not chemicallyalteredby samplingprocedures. pore water processed
Peepersweremadefrom 1.27 cm thick polycarbonate
sheetsin peepersand lake floor springsasindicatedby a positiveoffset
whichcavities1 cm in lengthweremilled every2 cm downthe of 0.02 mM K in the upper-mostsamplesfrom gravitycores.
reactionsthat
lengthof the sampler. Thesecavitieshold about4.5 mL of This artifactprobablyresultsfrom ion-exchange
andpressure
during
water, and two cavitieswere milled at each sampledepth. occurasa resultof changesin temperature
Cavities were filled with deoxygenateddistilled water and samplehandling[de Langeet at'., 1992].
Two temperatureprobeswereused,oneof whichis 60 cm in
coveredby a 0.45 !amdialysismembrane.Peeperswere stored
mountedat 20, 40, and60 cm alongthe
in deoxygenated
distilledwateruntiltheywereinsertedintothe lengthwiththermistors
sediment.A plasticT handlewasattachedto the peeperto aid probe. The resolutionof each thermistoris 0.01øC with an
in deployment,
andthe bottomedgeof the peeperwasbeveled accuracy
of + 0.2øC,andthethermistors
werecalibratedagainst
to causeminimal sedimentdisturbance.Four peeperswith 20 knownresistance
valuesperiodicallythroughoutthe sampling
to an OmegaModel 5830
cavitiesand one peeperwith 36 cavitieswere deployedfor program.This probewas connected
about1 week(Figure2), duringwhichtimewaterin thecavities ThermistorThermometerin the submersible. Temperature
were recordedon the videorecordof the operaequilibrated
with adjacent
porewatervia diffusion.Aliquotsof measurements
pore water from the peeperswere taken upon retrievaland tion. This recordwas then usedto determinethe depthof
were taken near the base of these cores and stored in snow for
9934
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
penetration.
Forty-eight
temperature
measurements
weremade; 4. Models for Pore Water Data
however, 28 of thesemeasurements
were made with only one
Porewaterdepthprofilesof Na fromrepresentative
coresare
thermistor
insertedintoa particularfeature.Impenetrable
crusts
shownin Figure3. Profileshapesof Ca, C1,Mg, Li, K, andSi
or underlying
rocksarethelikelycauses
of incomplete
penetraconcentrations
in porewaterfrombox coresandpeepersparallel
tion of the probe.
the
shape
defined
by Na data. Likewise, chemicalprofile
We alsouseda Sea-BirdElectronicsSeaCatSBE 19 Tempershapes
of
pore
water
datafrom gravitycoresgenerallyparallel
atureProfiler(TTT) with threethermistors
epoxy-potted
inside
Na profiles.The clearexceptionis Si (Figure3). For example,
a 61 cm long,0.635 cm diameterstainless
steeltube. When
fully insertedintothesediment,
thermistor
depthsare35 andare the profile of Si from GC 1 is curved,but the Na profile is
linear, suggesting
that differentprocesses
controlthe shapeof
5 cm below the sediment-water interface and 5 cm above the
sediment-water
interface.The SeaCatprofilerrecordedtemper- theseprofiles. Chemicalandthermalprofilesof porewaterare
diffusive,advective,andreactive
ature
profiles
2 timesa second
witharesolution
of0.001øCand influencedby time-dependent,
processes. For the one-dimensionalcase a mathematical
an accuracyof +0.01øCover a temperature
rangeof-5øC to
representation
of theseprocesses
is
35øC. All threethermistorsregisteredidenticaltemperatures
a(ooc)/at= a(ooD.,.(ac/az))/oza(½•vc)/az
+ •,ZR,
(1)
during descent. Only after the initial penetrationdid the
temperature
readingsdivergein response
to thermalgradients
time = diffusion + advection + reaction
within the sediment.All threethermistorsregisteredidentical
temperatures
duringtransitto thesecond
siteandduringascent.
The TTT recordeddatacontinuouslyat one site(TTT-1) for 3 where $ is porosity, C is concentration,t is time, D.,.is the
sedimentdiffusioncoefficient,z is depth(positivedowncore),
daysandat a secondsite(TTT-2) for 4 days(Figure2).
Sodium (mM)
0
1
2
3
Sodium (mM)
4
5
0
1
2
-5
•--H1_130
cm/yr
r,
-
4
4
i
i
,
,
ii,
,
-
_ BC6
5
GC '13
13 crn/yr
GC 8
9.5 ½m/yr
I,
15
3
•. BC8
GC1
-
210cm/yr b.110crn/yr
GC 11
3.1 cm/yr
I
20
!
I.,
0-05
Icm/yr
I
I,
I
Silica (mM)
0
0.5
Silica (mM)
1
-5
,,
I
1.5
0.5
1
1.5
,
20
-•-40
GC 1
BC 6
•
80
D
15
100
GC 11
BC8
20
I
I
120
Figure3. Profiles
of porewaterNa andSi fromrepresentative
cores.Curvesshownin plotswiththeNa data
werecalculated
fromequation
(5), andspeeds
of porewaterupwelling
areidentified.Linesconnect
the Si data.
Note the differencein the depthscalbs.
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
9935
v isvelocity(upwellingis negative),and•R includesall terms about24%, stemsfrom a decreasein porosityfrom about0.85
relatedto biogeochemical
interactions
influencinga particul•ar near the sediment-water interface to about 0.65 at 1.7 m below
solute
ortemperature
[Berner,1980].Thismodelissimplified the lake floor. Electrical effects,which causethe more mobile
by evaluating
therelativeimportance
of eachtermandeliminat- ions to diffuse slower and the less mobile ions to diffuse faster
inginsignificant
terms.In thefollowingdiscussion
we present [Berner, 1980], are producedby gradientsof chargedions
argumentsthat supportan advection-diffusion
model for generating an electrical potential while maintaining
chemical
species
thatare"conservative"
in thesediment
column electroneutrality.This effect was not includedin the calculathelackof a calculated
chargebalanceresulting
andfortemperature
andanadvection-diffusion-reaction
model tions,because
for Si, which is non conservativein the sedimentsection fromincompleteanalysesanderrorsin usingpH andtotal CO2
collectedwith gravity cores.
datato estimatealkalinit),in somesamplesmakesthesecorrections equivocal. Becauseof the uncertaintiesin estimating
4.1. Chemical
Profiles
changesin temperatureand porosityand calculatingcharge
The diffusive term is a function of the sediment diffusion
balance,we useda uniform sedimentdiffusion coefficient with
depthbasedonthe coefficients
listedby Li and Gregory[ 1974]
adjusted
to
the
temperature
of
deep lake water (3.7øC) and
electrical
effects[Berner,1980].Forcalculating
thediffusive
term, we used the temperatureof bottom lake water, even accountingfor Archie'sLaw with a uniformporosityof 0.85.
thoughthe maximumtemperature
recordedin hydrothermal Usinga uniformdiffusioncoefficientdoesnot alterthe general
featuresis about 4ø to 15øC above ambient(Table 1). Our results or conclusions based on the model calculations.
We simplified(1) furtherby examiningthe needfor a timerational for t•singlake water temperatureis that no measureterm. For example,if we assumethatthe concentramentsof temperature
weremadewheregravitycoresweretaken dependent
andthe maximumtemperature
wherebox coreswereretrieved tion of lakewaterchangedfrom 4 mMNa to itspresentvalueof
from specificgeologicfeatureswas less than 10øC. This about0.5 mM for a rangeof timesbeforepresent,then calcutermscan
differencein temperatureresultsin an increasein the sediment latedprofilesbasedon diffusiveandtime-dependant
diffusioncoefficientby at most40% [Li and Gregory,1974]. approximate
measured
profiles
(Figure
4). However,
a rangeof
with a well-mixed lake with
Because
gravitycoreswerenotcollected
fromgeologicfeatures, timescalesis required,inconsistent
thetemperature
wouldbe lessandthushavelessof an effecton a stable chemical compositionfor the past 50-100 years
the diffusioncoefficient. In contrastto the temperatureeffect [Nathenson,1990]. We alsohave calculatedprofilesin which
the downcorechangein tortuosity,whichis a functionof the the concentrationof Na in the basementfluid has changed.
formationfactorand porosityand is numericallyestimatedby These calculatedprofiles do not match measuredprofiles
Archie'slaw [McDuff and Ellis, 1979], resultsin a decreasein (Figure 4b). Thus time-dependantprocesseshave been excoefficient, which varies with temperature,tortuosity, and
the sediment diffusion coefficient. This decrease,which is
cluded from the models.
Table 1. TemperatureProfilesObtainedFrom SpecificGeologicFeaturesUsingthe Temperature
Probe
Dive
Sample
X,
Y,
T1,
Z1,
T2,
Z2,
T3,
Z3,
Velocity,
Type
m
m
*C
cm
*C
cm
*C
cm
cmyr'l
bottom water
CD
3.74
216
sediment
609
1431
CD 217
sediment
978
1664
3.78
20
4.46
40
4.54
60
3.96
40
4.08
60
CD 218
sediment
1040
1253
4.16
10
5.86
50
CD 222A
sediment
924
1337
5.55
30
6.35
50
6.92
70
CD 222B
sediment
949
1438
4.21
20
4.72
40
5.33
60
CD 222C
sediment
945
1361
5.00
25
5.85
45
6.65
65
CD 223
sediment
539
1207
3.80
15
4.05
35
4.24
55
-3000
-1000
-400
CD 226
sediment
1062
1363
5.30
20
7.90
40
10.20
60
CD 228A
sediment
2884
6062
4.60
2.0
10.20
22
10.80
42
-6000
CD 228B
sediment
2917
5954
3.68
-5.0
6.10
15
8.85
35
-500
CD 228C
sediment
3.68
-10
5.80
10
9.77
30
-200
CD 228D
sediment
3.69
-5.0
6.37
15
9.66
35
-300
CD 207
mat
843
1528
7.56
30
CD 215
mat
951
1150
CD 226
mat
999
1327
7.50
CD 228
mat
2884
6062
5.60
CD 230
mat
612
1421
4.84
CD 211
CD 228
CD 230
pool
pool
pool
634
2907
618
1452
5984
1420
4.60
4.30
7.76
50
8.10
70
-3000
9.00
10
9.60
30
-14000
12
11.08
32
12.90
52
-2200
15
7.15
35
8.11
55
-1600
10
5.75
30
6.00
50
-4000
20
10
4.35
5.00
5.00
20
40
30
4.45
5.60
5.21
40
60
50
-6000
-500
-3000
Fluidvelocitiesareestimated
by fittingthedatato manipulations
of equation(9). Upwellingis negative.Negativedepthsare
distancesabovethe sediment-water
interface.X is the distancenorthof EaglePoint,and Y is the distanceeastof Eaglepoint.
No entry denoteseitherno navigationor inconclusiveresults,which couldin partbe a resultof a porewatervelocitythat is
lessthan severalhundredcentimetersper year.
9936
WHEAT ET AL.' HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
Sodium (mM)
0
1
2
3
4
I
I
Sodium (mM)
5
0
1
2
3
4
5
0
20
100
•
GC8
40
GC 8
v
•200
I
I
• 80
GC 11
'
300
I
I
lOO
GC 1
--
-1 yr
4OO
' ' ' 50 yrs
a
.....
500 yrs
120
500
Figure4. Representative
profiles
ofNa plotted
withprofiles
calculated
witha timedependent-diffusion
model.
Calculatedprofilesat severaltimeincrements
areshownthatrepresent
(a) a changein a uniformporewater
profileof 4 mMNa to thepresent-day
concentration
of 0.5mMNa in bottomlakewaterand(b) a change
in a
uniformporewaterprofileof 0.5mMNato a concentration
of 4 mMNa at a depthof 5 m. Although
measured
datafit calculated
profilesin Figure4a, no changes
in theNa concentration
of the lakewatershavebeen
observed
in recentyears[Nathenson,
1990].Changes
inthecomposition
of thebasement
fluidgenerate
concave
up profilesin contrast
to themanyobserved
profileswithconcave
downshapes.Therefore
time-dependant
processes
havebeeneliminated
frommodelsof porewaterin CraterLake,Oregon.
Is there a need for a reaction term? Terms for reaction are
In thetext that follows,we will usethe porewaterchemical
typicallyincluded
forprofilesof dissolved
chemical
species
that datato calculateupwellingspeeds.We will showthatthe cores
are influencedby sediment-water
reactions. For example, listed in Table 2 have upwellingporewater speedsthat range
peryear. As suggested
dissolutionof amorphoussilica in sedimentincreasesthe fromseveralto hundredsof centimeters
concentrationof Si abovevalues in deep lake water. Once by Table 2, the advectiveflux is ordersof magnitudegreater
steadystateis reached,
theconcentration
of Si remainsuniform than a flux that could be generatedfrom reaction. Thus we
thattheporewaterwith the fastestupxvelling
speedis
with depth[e.g.,Boudreau,1990; Wheatand McDuff, 1994]. conclude
of the fluid
Similarprofilesof Si aswell asotherdissolved
ions(e.g.,Na, the leastalteredrelativeto the originalcomposition
C1,Mg, Ca,Li, andK) wereobserved
in CraterLake(Figure3). that enteredthe sedimentsectionand that this originalpore
If we assume
thattheseprofilesaregovernedby onlydiffusive water at the base of the sediment section exhibits "conservative"
and reactiveprocesses
with a reactiontermthat is first order ]nixingwith lakewater. Therefore,by eliminatingthereaction
with respectto the degreeof undersaturation
(k(C-Css),
where termandavoidingelementsthatarehighlyreactive(e.g.,Si), we
k is the first-order rate constantand Cs.¾
is the steady state canusethe porewaterdatato calculateporewaterupwelling
of thefluidbelowthe
concentration),
thencalculated
ratesof reactionforNa, C1,Mg, speedsandto constrainthe composition
Ca,Li, andK rangefromabout10'•4s'• in GC 1,to 10'9s4 in
GC l 1,to 10-8s'• in GC 8 andGC l 3, to 10-6s'• in boxcoreand
peepersamples.Ratesof reactionvarywith temperature
and
mineralassemblages.
Yet, thiscalculated
rangeis notconsistent Table 2. Pore Water Chemical Fluxes From the Sediment to the
with a temperature
changeof at most15øCanda singlesedi- OverlyingWaterColumnBasedon a Reaction-Diffusion
Model
mentarysource(primarilyweathered
andesitc
fromthecaldera andComparedwith Burial Rates
walls). Furthermore,calculatedchemicalfluxes from the
sediment
usingthismodelareordersof magnitudegreaterthan
Chemical
BC 8
PD
BC 6
GC 13
Burial
measured
valuesof burialfluxes(Table2). Thusmodelsof ion
Species
Ratea
profilesmustincludean advectiveterm.
Na +
200
83
25
18
1.4
Althoughsediment-water
reactionmayexistin somecores
Ca2+
50
19
5.7
2.7
0.87
for someions (i.e., Si in gravitycores),no reactionterm is
Mg2+
8.2
29
5.3
2.5
0.82
includedin the followinganalysis.Thisdoesnot precludelowK+
19
8.2
4.9
2.8
0.38
temperature
water-rockreactions
belowthesampledsectionor
Li +
2.3
9.1
0.46
0.29
0.0058
reactions
in the sedimentsection.The analysispresented
above
Si
32
17
8.5
4.6
1.4
confirmsthattheresultingflux generated
by anysediment-water
reactions,which must be relatively uniform over the area
sampled,is negligiblefor someelements(e.g.,CI, Na, andLi)
Values
aregivenin gMcm-2yr'•.
relative to advective fluxes in some cores.
aDymond
andCollier [ 1990].
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
9937
Table 3. Vertical Fluid VelocitiesEstimatedFrom the Pore Water Data and Manipulationsof Equation(5)
ChemicalSpecies
Best
Core
Na+
Ca2+
Mg2+
Cl'
K+
Li+
Estimate
GC 1
+0.05
-0.6
+0.05
+0.05
+0.05
+0.05
GC 8
-9.5
-5.2
-4.0
-11
-16
-10
+0.05
-9.0
GC 11
-3.1
-1.5
-1.4
-3.6
-3.6
-1.5
-3.0
GC 13
-13
-9.6
-8.0
-12
-7.0
-11
PD
-130
-100
-80
-200
-110
-120
BC 6
-210
-220
-220
-240
-220
BC 8
-110
-150
-120
-80
-120
Upwelling
is negative.
No entryfor CI- denotesinsufficientdata.Becauseof samplingartifacts,no velocitieswerecalculated
2+
2+
forCa andMg fromboxcores.Velocities
aregivenincmyr4.
sedimentsection.Eliminatingthe term for reactionreduces(1)
to
Ds(O2C/az2)v(aC/az)
= 0.
(2)
numberof measurements
fromthecurvedportionof theprofile,
thepresence
of sediment-water
reactions
in thesampled
section,
and artifactsinducedby samplingtechniques.For example,
dissolution
of carbonates in GC
1 causes the calculated
The solutionto (2) given the followingboundaryconditions:
C:
CO
z :0,
(3)
12
2 = 2bot,
(4)
C = CO- •z/1+ •z/1exp (vz/D,O,
(5)
'•• •,1o •= o•
8
C = Cbo
t
where
GC11
10
'I/ '/'/' I"
!:: _''/ I '/ !5')', ''• ,"
''
• 5 /
20
'
A I -' {Cbo
t - Go}/{ exp (VJbot/Ds)
- 1},
Cbo
t is the concentration
at the baseof the coreand2bo
t is the
lengthof the sampledsection.
Velocities of pore water flow through the sediment are
calculated by minimizing the deviation between measured
profilesandthosecalculatedusing(5) and a rangeof velocities
(Figure 3 and Table 3). A measureof the uncertaintyin these
.
• x4
0
the sumof the deviationssquareddividedby the variance. The
sedimentdiffusion coefficients.Theseexamplesillustratethe
uncertainties
in modelingchemicalprofilesfromareaswith pore
water upwelling speedsof the order of centimetersper year
comparedwith larger uncertaintieswhen estimatingvelocities
of hundredsof centimetersper year. Note that at the slower
velocitiesthe estimatedvelocity is well constrained.For faster
upwelling speedsthe larger uncertaintyresultsfrom having
fewer data in the curved portion of the profile. In addition,
uncertaintiesare greaterfor higherupwellingspeedsbecause
diffusion
after
core retrieval
and disturbance
of the sedi-
ment-waterinterfacewill alterthe upperseveralmillimetersof
the profile [Wheat and McDuff, 1995]. Diffusion and disturbancetendto decrease
the in situgradientresultingin calculated
velocitiesthat are minima. In areaswhereporewaterupwells
at speedsgreaterthan severalhundredcentimetersper year,
better estimatesof velocity are obtainedusing profiles of
temperature.
4
6
8
10
Upwelling Speed (cm/yr)
velocitiesis estimatedby thevalueof thereducedX2,whichis
bestfit to the dataresultswhenthe reducedX2 is lessthanthe
5% value,which meansthatthe parameters
usedin calculating
a profile have a high probabilityof being the correctset of
parametersthat describesthe observedprofile. Two examples
are shown in Figure 5 for a range of upwelling speedsand
2
'E
12J-t! 7'
ooZE8
8
B/.4
BC6 "
V' ' "/*
" ';
.. . 1.2 •
2
:
'•• 4
0
'
•
100
200
300
400
UpwellingSpeed (cm/yr)
Figure 5. Contourplotsof reducedX2 as a functionof the
sediment
diffusioncoefficientandupwellingspeedfor Na data
from two representativecores:GC 11 and BC 6. The 5%
reducedX2valueis 1.8usinga varianceof 0.0021mM2thatis
calculated
fromconcentrations
of porewaterin uniformsections
of profiles.Valuesgreaterthan1.8represent
a low probability
for the givenparameters
to be the correctparameters
for the
model.Theseplotsillustrate
thelargeruncertainty
in estimating
Table 3 presentspore water velocitiesthat were calculated upwellingspeedat speedsgreaterthanseveralhundredcentimeusingdatafrom six differentchemicalspecies.Differencesin tersperyear,primarilybecauseof continueddiffusionaftercore
calculated velocities result from uncertainties in the calculated
retrieval[Wheatand McDuff, 1995] andthe lack of datain the
diffusioncoefficient,the qualityof analyticalmeasurements,
the curvedportionof the profile.
9938
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
upwelling speedbased on the Ca profile to be an order of
magnitudegreaterthanthe speedestimatedfromthe otherions.
Similarly, whole-core squeezingcausesan increasein the
concentration
of Ca andMg relativeto Na becauseof dissolution or adsorption-exchange
reactionsthat occurasporewater
is forcedthroughthe overlyingsediment[Benderet al., 1987].
Further,the analyticaluncertaintyis smallerfor Na than for C1
or Li, andwe havegreaterdown-corecoveragefor Na thanfor
theseotherelements.Becauseof thevariedqualityandquantity
of datafromwhichthesevelocitiesarecalculated,
an averageof
the calculatedvelocitiesis not appropriate,
andthebestestimate
that is reportedin Table 3 is heavily weightedto the Na result.
To differs by at most 0.3øC in the deep lake, based on
conductivity-temperature-pressure
measurementsfrom the
submersible[McManuset al., 1993] andby recordings
of the
TTT thermistorprobethat rests5 cm abovethe sediment-water
interface(Figure7). Calculatedvelocitiesrangefromlessthan
severalhundredcentimeters
per yearto greaterthan 10,000cm
yr-•. Uncertainties
inthedepthandangleof penetration
andthe
possibilityof havinglakewaterseepinto the sedimentduring
insertionof the probeaffectthe precisionof calculatedvelocities. For example,ifthe depthof penetrationof CD 222 or CD
230 (Table 1) is +3 cm or the angleof penetrationis 30ø,then
the calculatedvelocityis + 25% of the value listedin Table 1.
Errorsin the depthandangleof penetrationare lessthanthose
listed above.
4.2. Temperature Profiles
Twenty-eightotherattemptsto obtaintemperatureprofiles
resultedin havingonly onethermistorpenetratethe sediment.
Eighteenmeasurements
were madein bacterialmatswherethe
highesttemperaturerecordedwas 18.9øC, and six of the 18
measurements
were greaterthan 10øCat a depthof lessthan20
(6)
D•(O2
T/Oz2)
- v(aT/az)= o,
cm. Nine measurements
were made in iron-manganese-rich
sediment
wheretemperatures
rangefrom 10.5øto 11.2øC.The
where T is temperature(in degreesCelsius)and D r is the
wasmadein a poolat 6.45øC.No pore
thermaldiffusivity(2 x 10-3cm2s-] [Williamsand VonHerzen, remainingmeasurement
water velocitieswere calculatedfrom these data, but they
1983]). Given the followingboundaryconditions:
probablyare fasterthanspeedsreportedin Table 1.
The TTT temperature
probewasdeployedfor 1 week(Figure
T = To
z = 0,
(7)
7). Measuredtemperatures
wereuniformfor the first2 daysof
deployment,but on the third day, higher temperatures
were
r = rbot
J = Jbot,
(8)
recorded by the two thermistorsnear the sediment-water
interface,reflectingeithera changein the upwellingspeedor a
the solutionto (6) is
changein the depth of penetrationproducedby the probe
settlinginto the sediment. After 3 daysthe TTT probewas
[r-ro]/[rbot - To]= [1 - exp(vz/Dr)]/[1- exp(vZbot/Dr)
]. (9)
moved 30 m away to an area where highertemperatures
were
Data from 28 temperatureprofilesare presentedin Table 1.
Thesedataarecomparedto profilescalculatedfrom the following advection-diffusionequationin one dimension:
recorded in the water column and in the sediment where it
Pore water velocities are estimated by minimizing the
remainedfor 4 days. Duringthese4 daysthe threethermistors
squareddeviationof the datato calculations
of (9) thatemploy
recordedtemperatures
thatwereabout0.8øCwarmerthanthose
a varietyofvelocitiesanda Toof 3.74øC(Figure6 andTable 1).
recordedduringthe first3 daysof deployment.Thesewarmer
measurements
for theuppermost
probe,whichis normally5 cm
abovethe sediment-water
interface,are greaterthanthe measuredrangeof deeplake watertemperature
(3.5ø to 3.9øC).
Temperature (øC)
4
6
8
10
12
lo
20
30
40
5. Pattern
CD 222A
1000cm/y
-•
CD 228A
Circulation
--
CD 228
_
1600 cm/yr
,
Mat
I
Sediment
5.1.
Pore Water Flow Through the Sediment-Water
Interface
-
6000 cm/yr %
-
7o
of Fluid
-
5o
6o
This is not an instrumentoffset, becauseall of the thermistors
measure3.56øC when they were exposedto deeplake water
bothbeforeand afterdeployment.Instead,this differenceis a
productof the probepenetratingdeeperinto the sedimentthan
was planned. The depthof penetrationwas adjustedbasedon
visual observations
and a bottomwatertemperature
of 3.9øC
[McManuset aL, 1993]in calculations
using(9) forminimizing
the leastsquareddeviationto the data(Table 4).
o
_
_--
-
8o
Figure 6. Representative
profilesof temperatureplottedwith
calculatedprofilesusingequation(9). Upwelling porewater
speedsare listed.
The spatialdistribution
of porewaterflow fromthe sediment
to the deepwaterof CraterLake is characterized
by two distinct
settings. One settingcoversmuch of the lake bottomwhere
measurements
of heatflow arerelativelylow (about0.14 W m-2
[Williams and VonHerzen, 1983]) and sedimentporewater
velocitiesare tenthsof a centimeterper year. In contrast,the
othersettingconsistsof sitesseveralsquaremetersto several
hundreds
of squaremeterswith iron-manganese-rich
crustsand
bacterialmats,highheatflowsof tensof wattspersquare
meter,
andporewatervelocities
thatrangefromtensof centimeters
per
year to greaterthantensof thousands
of centimeters
peryear.
Further,porewaterflow fromthesesitesdiffersmarkedlyover
WHEAT ET AL.' HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
9939
1
AREA #1
AREA
Dive CD226
i
Dive CD222
DEEP THERMISTOR
( 35 cm into the sediment)
6
O
[
MIDDLETHERMISTOR
5 cm into lhe sediment)
5
I
TOP
THERMISTOR
above sedimentin water )
4
1100
i
i
0000
0000
0000
24 Aug 25 Aug 27 Aug
0000
0000
28 Aug 30 Aug
31 Aug
1710
31 Aug
Deployment Time Scale
- 4 Hour Increments
( some data omitted for clarity )
Figure7. An abbreviated
7 daytimeseriesrecordof theTTT temperature
probe.For clarity,we omitteddata
fromAugust25-26andfromAugust28-29, 1989. Eachof thethermistors
in theprobemeasured
identicalvalues
in thewatercolumnduringtransit.Offsetin thetemperature
seriesbetweenthe two sitesresultsfrom different
depthsthat the probepenetrated.
distancesof only 50 m. For example,GC 13 and BC 6 are
locatedwithin 30 m of TTT 1, but the calculatedvelocitiesfrom
the three measurements
differ by three ordersof magnitude
(Tables 3 and 4). Anotherexampleis illustratedby BC 8, D,
two gravity cores(GC 10 and 14 with porewater upwelling
profiles(CD 216 sediment,
CD 230 mat, andCD 230 pool)
(Tables 1, 3, and4). Theseexamplesare consistent
with the
patchynatureof observedhydrothermal
features(i.e., mats,
pools,and crusts).
Mostof thecoresprovideevidence
for porewaterflow from
speeds
of 3 and8 cmyr '•, respectively),
andthreetemperature the sedimentto the deeplake,but only onecore(GC 6) has
chemicalprofilesthat are concaveup. Theseprofilesare
consistent
with lakewaterthatdownwellsthroughthe sediment
Table 4. Average TTT Measurements
for EachDay of Deploymentand section[e.g.,Mottl, 1989]. A downwardspeedof about1 cm
Calculated Velocities
yr -• in (5) providesthebestfit to the datawith an advectiondiffusionmodel. Redmond[1990] suggests
thatthe average
i
ProbeTemperature,
*C
Date
Aug. 24, 1989
Aug. 25, 1989
Aug. 26, 1989
Date
Aug. 27,
Aug. 28,
Aug. 29,
Aug. 30,
Aug. 31,
1989
1989
1989
1989
1989
Area -5 cm
1
1
1
3.80
3.95
4.03
Velocity,
speedof porewaterdownwellingacrossthe surfaceareaof the
5 cm
35cm
cmyr4
4.35
4.43
4.43
5.62
5.60
5.6
-4000
-3700
-2900
lakeflooris about130cmyr '•, basedonmassbalancecalculations. BecauseGC 6 is locatednearothercoreswhereupfiow
exists,we suggest
thatthisdownward
movingporewateris part
of a localcirculationcell anddoesnotrepresent
a pathwayfor
mass seepage. Local circulationcells may developfrom
heterogeneities
in lateral pressuregradientsresultingfrom
differencesin heatandporewaterflow [Wheatand McDuff
1994]. An alternativeand lesslikely hypothesisis that the
ProbeTemperature,
øC
Area 11cma 21cma
51cma
2
2
2
2
2
4.68
4.68
4.75
4.63
4.63
5.22
5.22
5.27
5.19
5.19
6.44
6.44
6.45
6.44
6.44
Velocity,
cmyr-]
-1100
-1100
-1400
-1000
-1000
Negativedepthsare the distanceabovethe sediment-water
interface.
Upwellingis negative.The adjusteddepthaccounts
for the depththatthe
probeover penetrated.
aAdjusteddepth.
profiles from GC 6 are a result of a recent increasein the
concentration
of dissolvedchemicalspecies
nearthe baseof the
core (e.g., Figure 4), presumablya productof openingand
closingpathwaysfor fluid flow.
Pore water velocities were calculated from data collected
during a 7-day deploymentof the TTT probeto accessthe
degreeof temporalvariabilityin hydrothermalflow. A similar
timeserieswasrecorded
by Leinenet al. [ 1987] in the Mariana
Mounds, a ridge-flankhydrothermalsystem. Both data sets
9940
WHEAT ET AL.: HYDROTHERMAL INPUTS TO CRATER LAKE, OREGON
suggestthat flow throughthe sedimentis uniformfor a period
of at leastseveraldays. Similarly,Schultzet al. [1992] measuredvelocitiesof diffuse flow from a mid-oceanridge-axis
hydrothermalsystemin whichthe averagevelocitywasuniform
for periodsof severaldays,buthourlyvaluesfluctuatedwith the
tidal cycle. Land-basedhydrothermalsystemsalso may be
affected by tidal and barometricchanges[Rinehart, 1972].
None of theselow-frequency
effectsareevidentin the data,but
some temperaturefluctuationswere recordedby the deepest
thermistor. Pore water flow in Crater Lake must be fairly
uniformfor periodsof at leastseveralyears,basedon thriving
bacterialmatassemblages
thatwereobserved
duringsuccessive
years. In addition,pore water chemicaland thermalprofiles
suggest
that focusedflow may be maintainedfor tensof years.
At leastseveraltensof yearsare requiredto generatea steady
statechemicalprofilewhereporewaterupwellsat a speedof 10
cmyr'• through
a theoretical
5 m thicksediment
column.More
time is requiredif upwellingspeedsare sloweror if the sediment column
is thicker.
Because sediment thickness is not
northeast
sectionof the lakeor by fracturepermeabilityassociated with the ring fracturezone in the caldera[Baconand
Lanphere, 1990]. In the basementreservoirthe extent of
chemicalalterationis directlyrelatedto temperature.The
compositionof this fluid in basementmust be uniform for a
periodof at leasttensto hundreds
of years,andit is saltierand
warmer than lake water, basedon pore water data and the
modelspresented
above.Whenthisfluid upwells,it mixeswith
lake waterbelowthe sampledsedimentsection.Mixing may
occur at different depthsand with differentratiosof the two
sources,
basement
andlakewaters.Thusdifferentmixingratios
resultin differentconcentrations
at the baseof the cores(Table
5), yet elementalratiosof waterin zonesof focusedupflowwill
be maintained because lake water is much more dilute than the
fluidin basement,
andadvective
fluxesareordersof magnitude
greaterthan reactivefluxes. This reservoirmodelsupportsa
singletrendin plotsof onechemicalspeciesversusanotherin
zonesof focusedupflow. Deviationsfrom this singletrend
result from non conservativebehaviorin a portion of the
hydrothermalsystemin which fluid velocities must be much
temporalvariability of flow throughthe sedimentis uncertain, slower;thusfluxesfromreaction
maybe a significant
portionof
but it must persistfor decades(centuries)in areaswith pore the total chemical flux for these cores. Whereas the reservoir
water velocities on the order of tenths of centimeters to several
model supportsa single trend, a multiple reservoirand/or
reaction model supporta family of trends in plots of one
centimetersper year.
chemicalspeciesversusanother.The rangein thesetrendsis
5.2. Fluid Flow Within the Upper Basement
definedby thatfromthedifferentreservoirs
asindicated
by data
known in the exact locations where cores were taken, the
We define the upperbasementas the regionwhere fluids
developtheirchemicalsignature
beforeupwellinganddiffusing
throughthe sediment. This definition allowsthe upperbasementto be eithera permeablesedimentary
layer(severalmeters)
or igneousbasement(tensto hundredsof meters)below the
sampledsection.Knowledgeof the compositionof the fluid in
the upper basementprovidesconstraintsfor the sourceor
sourcesof thermallyandchemicallyalteredfluidsthat enterthe
lake. The composition
of thesefluidsis estimated
by extrapolating chemicalprofilesthroughout
the sedimentcolumnby fitting
the datato calculations
of (5) whereZbo
t is sedimentthickness
basedon the seismicdataof Barber and Nelson [ 1990] and v is
the bestestimatelistedin Table 3. Cbo
t is thenvariedfor the
best fit to the data [e.g., Wheat and Mottl, 1994]. Given
velocitiesgreaterthan severalcentimetersper year, Cbo
t is
simplythe asymptotic
concentration.
Calculations
of Cbo
t differ
by a factorof 4 for severalcores(Table 5). This differenceis
consistentwith a reservoirmodel for the southernportion of
from zonesof focusedupflow(> 100 cm yr-•) andfromareas
with slowflow (<0.1 cmyr-•).
A plotof CI versusNa illustrates
a singletrendforporewater
fromzonesof focusedupflow(e.g.,samplescollectedwith the
box corer,solidtrianglesin Figure8) andthus supports
the
reservoirmodel. AssumingCI is conservative,
deviationsfrom
this trend resultfrom low-temperature
reactionsat depththat
increase the concentration of Na as the fluid ascends to the
sampledsection. SimilarC1/element
linearrelationships
exist
in data from coreswith porewaterupwellingspeedsthat are
greaterthantensof centimeters
peryear. Becauseof thepaucity
of datafor C1,we haveplottedLi andK versusNa to illustrate
these singularmixing trendsin coresfrom zonesof focused
upflow(Figure8). Notethatfor coreswith porewatervelocities
less than tens of centimetersper year, severalelement/Na
relationshipsmay exist (e.g., Li/Na from GC 1, 2, and 3 in
Figure 8). GC 1, 2, and 3 presentexamplesof the effectsof
low-temperaturewater-sedimentreactionsin CraterLake, and
anyporewaterflow associated
withthesecoresis a likely result
CraterLake in which lake water flows downwardinto basement,
of sedimentcompaction.ThesereactionsaddNa to the pore
reactswith basement,
upwells,andmixeswith morelakewater
water, but concentrations
of Li remainunchanged.With the
beforeenteringthe sedimentsection.
exceptionof C1, Li is the most conservativeelementthat we
The downwardseepageof lakewater is probablyregulated
measuredin pore water from CraterLake.
by the bulk permeabilityassociated
with freshlava flows in the
The reservoirmodel doesnot precludeother "reservoirs"
beingpresentunderthe lake. All of the datausedto illustrate
thepresence
of a singlereservoirarefromcoreswith porewater
Table 5. Projected Concentrationsfor Pore Fluids at the Baseof
that upwellsat speedsgreaterthanabout10 cm yr -t in the
the Sediment Column
southernbasin. Thus otherreservoirsmay feed hydrothermal
Chemical
Bottom
systems
in the EastandNorth Basins. In fact,onesamplewith
Species
BC 6
BC 8
PD
GC 8
GC 13
Water
anomalouslyhigh concentrations
of Na (Figure8) wasrecoveredby the submersiblein the North Basin,but the hydrotherNa+
0.85
2.4
2.1
4.4
3.8
0.455
mal systemthat generatedthis samplemust have a minimal
CI0.57
1.2
2.1
2.0
0.274
impact on heat and salt fluxes to the lake [McManus et al.,
Ca2+
-•0.2
-0.8
0.68
0.91
0.92
0.17
Mg2+
-•0.2
-1.0
0.90
0.98
0.94
0.11
K+
Li +
0.11
0.014
0.19
0.035
0.14
0.025
0.43
0.078
0.42
0.075
0.044
0.0066
Si
0.59
0.88
0.79
1.1
1.2
0.32
No entry denoteslack of data. Values are given in millimolars.
19931.
6. Hydrothermal Fluid and Chemical Fluxes
Hydrothermalfluid and chemicalfluxes for Crater Lake are
calculatedusing a massbalanceequationof the hydrologic
budgetwhich is
WHEAT ET AL.: HYDROTHERMAL INPUTS TO CRATER LAKE, OREGON
9941
Table 6. Fluid Flux and Chemical Parameters for Mass Balance
2.5
Calculations
:• 2 []Gravity
g:
x Submersible
x•
[]
[]
v 1.5ßBox
• ß
't-'
X
Water
Budget
FluidFlux
107m3yr'l
Li+,
gM
Na+,
mM
K+,
mM
Ci',
mM
Springrunoff
2a
1
0.1
0.025
0.015b
Precipitation
11a
1c
0.013
b
0.006
b
0.014
b
Evaporation
6.3d
0.1e
NetSeepage
6.7d
6.7
0.455
0.044
0.274
0.14
290
19
1.1
12
0.0013
e 0.0006
e 0.0014
_
0.5
_
Hydrothermal
_
0 -, , , , I , , , , I , , , , I ....
0
1
2
I • • • • I • • • •-
3
4
5
6
aAbout
85%ofthefluidinputisfromdirect
precipitation
[Nathenson,
b1990]
foratotal
input
of13.1
m3yr'•[Redmond,
1990].
0.1 , , , , i , , , , i , , , , i , , , , i , • , , i • • •',
_
Simpson[ 1970a].
_
CAssumes
nowater-rock
interaction
instream
runoff,
thus
equal
to
•• 0.08• Gravity
- x Submersible
•
o Peepers
th• valueinstream
runoff.
_
• 0.06
•
ß Box
•
-
•
0.04
•
0.02
•
•Redmond[ 1990].
eEstimated
tobeonetenth
ofthatinprecipitation.
•
Q
•
0
-
GC 1, 2, 3
,,,1•,,,1•,,,I,,,,I
0
1
2
....
3
4
x
-
fluxesin the lake aredefinedby combining(10) and(11) and
accountingfor the chemicalcompositionof eachfluid flux'
-
Vhydro
[ Chydro
' Clake]
= Clake
Vnet
seepage
+
I ....
-
5
6
_
_
_
_
- • Gravity
• 0.4- xSubmersible
•
o Peepers
•
-
,ox
'• 0.2
Cevap
Vevap-Cprecip
Vprecip-Cruno
ff rmnoff;(12)
whereC is concentration
and V is volumeflux. For Na, C1,Li,
and K we assumethat(1) no termsarerequiredfor diagenetic
reactionas the hydrothermalfluid upwellsinto the lake, (2)
thereareno dissolution,
precipitation,
adsorption,
or desorption
reactionsin the water column,and (3) thereare no dominant
biogenicsources
or sinksof theseelements.Giventheconcentrationsandwaterfluxesin Tables6 and7, we reduce(12) to a
seriesof equationsfor eachelement.For example,
-
Vhydr
o [ Nahydr
o- Nalake
] = 2.7x 107molNayr'•. (13)
The dominatetermin (12) is the termfor net seepage,
whichis
equalto thevolumetricnetflux of waterto basement
multiplied
0
1
2
3
4
5
6
by the compositionof lake water. The compositionof lake
wateris knownto withina few percent;however,thevolumetric
Sodium (mM)
net seepage is probably only known to within 10-20%
Figure 8. Concentrations
of C1,Li, andK versusNa. Lines [Redmond,1990]. C1,Li, Na, andK fluxesfrom hydrothermal
represent
mixingrelationships
definedby massbalancecalcula- sourcesare 8.8, 2.5, 7.9, and 1.6 times, respectively,greater
tions. Symbolsrepresentthe differentsamplingtechniques thanthe combinedfluxesfrom precipitationandrunoff. In the
(opensquares,gravi• cores; solidtriangles,box cores; open calculations
abovewe assumed
thatdiageneticandburialfluxes
diamonds,
peepers;andcrosssigns,submersible
samples).All
0 -, , , , I , , , , I , , , , I , , , , I , , , , I , , , ,-
of the data are shown.
Table 7. Parameters for Mass Balance Calculations
precipitation+ runoff = evaporation+ net seepage,
(10)
whereeachtermrepresents
a measuredfluid flux. Net seepage
of water from the lake is definedby
net seepage+ mixing =
total seepage-hydrothermal+ mixing.
Parameter
Value
Volume of lake•
17.3 km3
Areaofdrainage
basin
b
67.8km2
Areaof lakeb
53.2km2
Areaoflakedeeper
than350m
(11)
Power
output
below350m,MW
22.3km2
15to31c
23+ 8d 30+ 5e
aRedmond
[ 1990].
The mixing flux accountsfor waterthat seepsinto the upper
basement,
mixeswith hydrothermal
fluids,andupwellsthrough
the sedimentcolumninto the lake. Althoughthe mixing terms
cancelin (11), they are includedbecausethey are importantin
conceptualizingfluid flow throughthe basement. Chemical
bphillips
[1968].
CThe
conductive
power
output
thatisnotassociated
withthereservoir
fluid is at most3 MW [Williamsand VonHerzen,1983].
dMcManus
etal.[1993].
eThiswork.
9942
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
were not significantfor theseelements. For example,the
upwellinghydrothermalfluids. This areais about33% of the
basedon observadiagenetic
fluxofK tothelakebased
onthediffusional
gradient areabelow350 m andis largerthanexpected
of pore water concentrations
from GC 1, 2, and 3 and the
surfacearea of the lake is about9% of the hydrothermalflux,
tions. In contrast,porewaterthat flowsthroughbacterialmats
and manganesecrustshas measuredupwellingspeedsin the
whereasthe depositionalflux of K to the sediment-water range
of1000to14,000
cmyr-•.Onthebasis
ofthisrange
of
interfaceis 11% of the hydrothermalflux. Similarly, .the velocitiesthe areathroughwhichporewatermustflow is about
diagenetic
anddepositional
fluxesforNa areabout4% and3%, 1.5km2(6.7%)to 0.10 km2(0.45%). A smallerareais required
respectively,
of thehydrothermal
flux. Thesefluxesfor Li and if flow is more focused. Consideringthe arealextentof mats
C1are lessthan 2% of calculatedhydrothermalfluxes.
andmanganese
crustsin the southemportionof the SouthBasin
We canthencombineanytwo of theseequations
(e.g.,(13)) andthemanyhightemperature
measurements
atlessthan20 cm
to determinemixing relationships
betweenthe hydrothermal depththat reflect fasterspeedsof upwellingthan the results
herein,we canaccountfor all of theporewaterflow
fluid in basement
andlakewaters(Figure8). Providedthereis presented
primarilyone sourceof hydrothermal
fluidsto the lake,the fromthesediment,buttheportionof thisflow thatoriginatesas
composition
of the end-member
hydrothermal
fluid mustfall pristinehydrothermalfluid from deepwithin the basementis
alongthisline. Samples
thatbestfit theselinearrelationships equivocal.
are thosefrom box coresandpeepers,which haveporewater
upwelling
speeds
thatareatleastanorderof magnitude
greater 7. Constraints on the Composition
thanthat calculatedfrom gravitycoredata. Thesefasterflows
allow lesstime for water-rock/sedimentreactionsto occur, and
fluxesfromth:sereactionsaresmallrelativeto transport
fluxes;
of the Hydrothermal Fluid
The compositionof the fluid in basementis constrainedby
thuschangesin concentration
are lesslikely to be observed thecomposition
of themostalteredporewater,elementalratios
[Wheat and McDuff, 1994]. The fit betweenthesedata and
mixing lines supportearlier conclusionsthat most of the
from sitesthat have the fastestporewatervelocities,and mass
balanceconsiderations.In this sectionwe usethesecomposi-
hydrothermalinput to the lake is in the southernbasin tionsandratiosin conjunction
withempirical
geothermometers
,
[McManuset al., 1993]andsuggestthatwe locatedthe sitesof which are based on measuredtemperaturesand chemical
active venting that accountfor the observedchangesin the analysesof fluids from a rangeof land-based
hydrothermal
water column.Ion-ion relationships
shownin Figure 8 further
illustratethe effects of low-temperaturewater-rock/sediment
reactionon the upwellingporewater. If we assumethat C1is
conservative
duringupwellingfrombasement,
thentheselowtemperaturereactionsadd Na, Li, and K to porewater. We
expectthesereactions
to occurbelowthe sampledsectionin an
settings[e.g.,Fournier and Rowe, 1966;Fouillacand Michard,
1981;Fournier, 1981, 1990; Giggenbach,1988; Kharaka and,
Mariner, 1989], to place limits on the maximum extent of
chemicalalterationof the hydrothermalfluid andto estimatethe
temperature
at depthin basement.Determiningthermodynamic
boundariesfor mineral phasesis not in the purview of this
environment which is warmer than lake water, because the paper,which focusessolely on the subsurfaceflow regime as
products
of thesereactions
aremuchdifferentthanthosedefined constrainedby pore water chemicalandthermaldata.
Two geothermometers
useconcentrations
of Li andNa
by GC 1, 2 and3 fromareaswith low heatandporewaterflow
andwhereflow is probablyrestricted
to thatfromsediment to predictfluid temperature[Fouillac and Michard, 1981]:
compaction.
Log (NaJLi)=-0.38 + 1000/ [ T (øC) + 273.15],
(14)
Equation(12) alsoprovidesa meansto calculatethe residence time of ions in the lake. The calculated residence time for
Na, Li, K, and C1 is about250 years. Note that thesecalculaLog (Li)= 1.44- 2258 / [ T (øC) + 273.!51,
(15)
tionsareindependent
of theparticularvolumeflux or concentration of the hydrothermalfluid; however,theseparametersdo where concentrations
are in molar units. Anothergeothermoprovidesomeconstraints
on the residence
time of waterin the meteris basedon NaJK. We usedTruesdelrs[1976] equation:
lake. For example,if thehydrothermal
fluid hasa concentration
Log (NaJK)=- 0.8573 + 855.6 / [ T(øC) + 273.15], (16)
of 4.6 mMNa, thehighestconcentration
measured
in porewater
samples,thenthehydrothermal
fluid flux is at least5% of that
are in unitsof milligramsper kilogram,
fromprecipitationandrunoff,andtheresidence
time for water where concentrations
in the lake is about130 years. If the concentration
of Na were becauseit providesthe best fit to data below 250øC, yet it
are below 150øC. We
higher,thenthehydrothermal
waterflux wouldbe lower,but an providesa poormeasureif temperatures
additionalflux of lakewateris requiredto dilutethis hydrother- chosethesethreegeothermometers
because(1) our samplesare
mal fluid to concentrations
observedin theporewaterdata. The from surfaceexpressionsof venting that are susceptibleto
overalleffectof thisdilutionwill producethe samenet resultas chemicalalterationwhile upwellingintothe lake, in contrastto
described
before. Much of the hydrothermalfluid that upwells samplesfrom boreholesthat provide a meansfor collecting
intothe lakeis probablydilutedto a concentration
of 2.3 mM pristine fluids from basement and are the basis for
Na, which is consistentwith results from BC 8 and D, or geothermometers
and (2) they includethe least reactive elepossiblyas low as0.85 mM, asobservedin BC 6. Thisvaria- mentsin pore watersfrom box coresand peeperscollectedin
tion resultsin a calculatedrangeof waterresidencetimesfrom the CraterLake systemfromwhicha geotemperature
canbe
of Li substitutedinto (15)
90 to 130 yearsandwaterfluxes(hydrothermal
plusmixing) calculated.Measuredconcentrations
thatrangefrom 65ø to 135øC.
from6.85x 107to 0.65 x 107m3yr -l. Thisnetwaterflux from resultin calculatedtemperatures
the sediments(hydrothermalplus mixing) is consistentwith These temperaturesare minimums becausedilution of the
and
submersible
observations
of the extentof porewaterflow in the hydrothermalfluid with lakewaterlowersthe concentration
southernbasin. If mostof the porewater flow from the sedi- thus the calculatedtemperature. By substitutingLi/Na and
ment has a concentrationof 2.3 mM Na and is upwelling at a K/Na ratios defined by mass balance calculations(equation
are 166øCand
speedof 200 cmyr -l, ascalculated
fromprofilesof boxcores (13)) into(14) and(16), calculatedtemperatures
and peepers,then an area of 7 km2 is requiredto vent the 197øC, respectively. Low-temperaturewater-rock/sediment
WHEAT ET AL.: HYDROTHERMAL
INPUTS TO CRATER LAKE, OREGON
reactions
in CraterLakeresultin lowerLi/Na andhigherK/Na;
thus 160øCis likely to be a lower bound,and 200øC is likely to
be an upperbound. If we usethe averagetemperature
of 180øC
in (14) through(16), then the end-memberhydrothermalfluid
hasa compositionof 19 mMNa. Given a 19 mMNa hydrothermal fluid and the massbalanceequation(13), the composition
ofthe hydrothermal
fluid is 0.24 mMLi, 1.2 mMK, and 12 mM
C1. Mixing lines betweenthis fluid and lake waterfit the pore
water data from box core and peepersamples(Figure 8). As
noted above, these samplesrepresentthe most pristinefluids
9943
Oregon. Pore water issuesfrom the lake floor at speedsof
tenthsof a centimeterper year to tensof thousands
of centimetersperyear and are derivedfrom mixing deeplake waterwith
thermally and chemicallyaltered fluids from basement. We
have estimateda hydrothermalend-membertemperatureand
composition based on geothermometersand mass balance
calculations.Whenthesecompositions
aresubstituted
into flux
equations (e.g., (13)), a volumetric hydrothermalflux is
calculated.
This studyillustratesthe powerof porewaterchemicaldata
andthermalprofilesin sedimentto constrain
thepatternof fluid
setting.Similarstudieshavebeen
Other geothermometerssuch as Si [Fournier and Rowe, circulationin a hydrothermal
conductedin hydrothermalsystemson the flanks of the mid1966, 1977],Na-K-Ca-Mg [FournierandPotter, 1979], K-Mg
[Giggenbach,1988], andNa-K-Mg [Fournier,1990] havebeen ocean ridge axes [e.g., Wheat and Mottl, 1994; Wheat and
developedbut are inappropriatefor use with samplesfrom McDuff, 1995], and the sametechniquesemployedhereinare
applicableto groundwaterflow into freshand estuarinewater.
Crater Lake, becausethe rate of reactionof Si, Ca, and Mg
thatis elucidated
by these
within sedimentand basementis fast enoughandtoo complex Oneof themostimportantparameters
to reducethe data to a single trend that can then be used in techniquesis an estimatefor the chemicalcompositionof the
fluid in basement.Thiscomposition
thenconstrains
thepathfor
calculationsfor the temperatureat depth.
Given a temperatureof 180øC in the reservoir and the fluid flow, the extent of reaction in basement,and mass,
geothermometers
listedin (14)-(16), we calculateda basement thermal, and chemical fluxes from fluid circulation.
reservoirconcentrationfor Li, Na, and K. By substitutingthis
Acknowledgments.Many individualscontributed
theirtime
concentration
into appropriateequations
thataresimilarto (13),
we calculatethe volumetric hydrothermalfluid influx to the andeffortduringthe field and laboratoryportionsof this work.
of the generous
help
lake. This fluid flux coupled with a temperatureof 180øC The authorsareparticularlyappreciative
resultsin a total power input to the lake of 30 + 5 MW. This providedby M. Buktenica,J. Milestone,andthe restof the lake
calculationis basedsolely on geothermometers
and general crewat CraterLakeNationalPark. G. WhippleandC. Meredith
hydrologicmassbalancedataandis consistent
with (1) elemen- provided analytical assistance. C. G. W. is indebtedto R.
tal ratiosof porewater concentrations
from zonesof focused McDuff for financial supportduring the field portionof this
thethoughtfulreviewsfrom D.
upflow;(2) Williamsand l/on Herzen's[1983] estimateof 15 to program.We greatlyappreciate
31 MW, basedon 62 measurementsof sedimentheat flow and Issler,I. Hutcheon,andan anonymousreviewer. This research
temperatureprofilesin the water column;and(3) McManuset wasfundedby theU.S. NationalParkServiceundercooperative
al.'s [1993] estimateof 23+8 MW, basedon the watercolumn agreement CA 9000-3-0003, CPSU, College of Forestry,
thermalinventory.The agreementbetweenthesethreeestimates OregonStateUniversity. IMS, University of Alaska Fairbanks
is significant,becauseall threeestimatesare basedon different contribution 2488.
data setsand techniques. Further, this agreementprovides
credibility to our estimateof the temperatureand composition
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