Project using measurements of 7Be
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. C2, PAGES 3369-3378, FEBRUARY
15, 2000
Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic Ocean (SHEBA)
Projectusingmeasurements of 7Be David
Kadko
Marine and AtmosphericChemistry,RosenstielSchoolof Marine and AtmosphericSciences,Universityof Miami, Miami, Florida
Abstract. During the SurfaceHeat Budgetof the Arctic Ocean (SHEBA) projectin October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used
to reconstructthe evolutionof the mixed layer over the previousseasonand confirmed that the reservoirof heat beneaththe fall mixed layer was emplacedin the summer,rather than input cumulativelyover severalseasonsor advectedin from distantsources.As suggested by McPheeet al. [1998], severaltimes as much heat was emplacedat SHEBA than at Arctic Ice DynamicsJoint Experiment 20 yearsearlier. A likely mechanismfor this would be substantiallygreaterlead openingcoupledwith the positivefeedbackloop betweenincreasedopen water, increasedheat absorption,and further ice melting.In
addition, a rateof netprimaryproduction >_13gCm-2 for thepreceding spring-summer period was derived usingthis mixed layer historywith an oxygenprofile from the fall. SHEBA mixed layer was the result of cumulativeinput over severalsummersor was advectedin from elsewhere.Basically, at SHEBA duringthe fall of 1997offeredonly One of the important considerationsin the summersurface the observations were made energybalance of the Arctic Ocean is the fate of the energy a "snapshot"of the systembecauseno observations input (mostlyshortwaveradiation) in the heterogeneousice- duringthe precedingsummer.It is thereforedifficultto reconoceansystemduring the melt season.Downwellingsolar radi- structthe mixedlayer evolutionand heatinghistoryof this site ation throughleadsis the primarysourcefor the directheating prior to the initial SHEBA occupation. To addressthis question,the heat exchangebetween the of the ocean,but the fate of the solarenergyabsorbedthrough leads and underlyingmixed layer of the SHEBA site is evaluthe leadsis not well understood[Rind et al., 1995]. It is not ated here usinga naturallyoccurring radioactive tracer,7Be. knownfor example,the extentto whichthisheat laterallymelts the ice in contactwith the leads,is conveyedto depth and melts Beryllium7 is a cosmicray-producedspeciesthat is delivered the ice from below, or is transportedaway from the ice. The to the Earth's surface and, as such, is a form of proxy for fate of the heat has direct consequencetherefore upon the incomingsolar radiation. Becauseof its 53.3 day half life, it is 1.
Introduction
persistenceof the ice cover and the associatedsummertime albedo.
The SurfaceHeat Budget of the Arctic Ocean (SHEBA) programwas undertakenin October 1997 to studythe feedbacksassociatedwith the ice-wateralbedosystem,in whichthe dispositionof the summerheat input is of primary importance [e.g.,Ingramet al., 1989].The initial observations made in the Fall of 1997 indicatedthat the upper oceanwas lesssalineand warmer than expected[McPheeet al., 1998].The temperature abovefreezing(tST) of the mixedlayerduringSHEBA (1997) had increasedby a factor of 2.5 over that which occurred during an earlier program,Arctic Ice DynamicsJoint Experiment (AIDJEX) (1975). Additionally,the mixedlayer salinity in 1997was ---27.6ppt, comparedto 29.7 ppt in 1975 [Maykut and McPhee;1995;McPheeet al., 1998].From this data it was concludedthat the oceanicheat flux was considerablygreater duringthe 1997 summer.Becausethe albedoof openwater is considerablylessthan that of seaice, it wassuggested that the percentageof open water between1975 and 1997 had tripled to allow a greater amount of heatingto occur [McPheeet al., 1998]. It is possible,however,that the heat stored in the
an ideal tracer of water that has been in contact with the sea
surfaceover the pastseason[Kadkoand Olson,1996].Profiles
of 7Bemeasured in thefallprovideanintegration of themixed layer behaviorover the previoussummerand can offer insight into the mixed layer evolutionand heatinghistory.
2.
Background
Beryllium 7 is a cosmicray-producedradioactivenuclide with a radioactive meanlife of 76.9days(the meanlife is defined as the 1/e decaylevel or half life/0.693) and, as such,is well suited for studyingseasonalphenomenon.Beryllium 7 is deposited upon the Earth's surface by precipitation and is homogenizedwithin the surfacemixedlayer of the oceanrapidly with respectto its decayrate [e.g.,Silker,1972;Youngand
Silker,1980;KadkoandOlson,1996].In snowthe 7Beactivity (equivalentto concentration,here expressed as disintegrations
perminute(dpm)perm3) is ---2ordersof magnitude greater thanthat in the oceanmixedlayerbecause a given7Befluxis
Paper number 1999JC900311.
diluted in mixed layerstypicallytens of meters deep but depositedin snowlayers(over a 77 day period) that are at least an order of magnitude shallower,thereby concentratingthe nucliderelativeto the surfaceocean[e.g.,Cooperet al., 1991]. In the oceanthe mixedlayer depth is a criticalparameterthat
0148-0227/00/1999JC900311509.00
largelydetermines the7Besurfaceactivitybecause for a given
Copyright2000 by the American GeophysicalUnion.
3369
3370
KADKO: MODELING
THE ARCTIC
MIXED
LAYER USING 7BE
Al 7o
Barrow
Dead
Horse
Tuktoyaktuk 65
18,0
12O
•
1so
Figure 1. Locationmap of the SurfaceHeat Budgetof the Arctic Ocean(SHEBA) projectsite.Point A is the initial deploymentlocationin October1997.Point B is the locationin July 1998.
7Beinput,the activityis concentrated in shallow mixedlayers placedin a marenellibeaker,whichin turn wasplacedover a and dilutedin deepermixedlayers.As will be describedbelow, low background germanium gammadetector.The 7Behasa waterin contactwiththe oceansurfaceis "tagged" with7Be, readilyidentifiablepeak at 478 keV. The detectoris calibrated and profilesof this isotopeprovide a record of the seasonal for this geometryby addinga commerciallypreparedmixed changesin the mixed layer depth. solution of known gamma activities to an ashed fiber and countingit in the marenelligeometry.The countingefficiency for the 478 keV gamma in this configurationwas 0.03. The 3.
Methods
limit of detection used in this work defined at the 99.7% con-
fidence level, is SL = --> 3frO, where O'b is the standard Verticalprofilesof 7Bewerecollected in the BeaufortSea deviationof the blank measurements[Rubinson,1987]. The from October8 to 12, 1997(Figure 1). To collectthe samples, detection limitof a sample increases withtimebecause of 7Be a hydroholewas melted through -2 m of ice throughwhich
samples for7Bewerecollected bypumping. In thisprocess, 700 L of seawaterwere passedthrough iron-impregnatedacrylic fiberspackedin cylindricalcartridges[e.g.,Krishnaswami et al., 1972;Lal et al., 1988;Lee et al., 1991;Kadkoand Olson,1996]. The water wastaken at variousdepthsthrougha 1.5"hoseat the endof whichwasa conductivity-temperature-depth (CTD) system.The pumpingwas achievedwith a centrifugalpump, poweredby a gasoline-fueled generatoron the ice,at a rate of
-14 L min.-• Thuseachsampleddepthrequired-50 min.In mostcases,doubleor triple sampleswere collectedat any one depth and later combined.On one cast (October8), where only singlecartridgeswere usedat eachdepth,three samples from different depthsin the mixed layer were combinedas representativeof the mixed layer activity.The fiber efficiency, testedwith stableBe in the lab or by countingtwo cartridges placedin serieswas 0.90 _+.02. This is an improvementover earlier fibers [Kadkoand Olson, 1996] and reflectsa higher retention of iron in the fibersproducedin the laboratoryfor this study. On land the fibers are dried
and then ashed. The ash was
decayand,for oursystem, is(12.7/V) x[exp(XD)] dpmm-3,
whereD is the numberof daysafter samplecollection,V is the
volume(m3),andX is thedecayconstant (0.013d-•). To estimate theatmospheric deposition of 7Be,samples of snow(m2 in area)weretaken,andalso,plasticbuckets were deployedto collectfallout over knowntime periods.The snow was melted, and 0.5 mL of concentrated HC1 and stable Be
yieldtracerwereadded[e.g.,Dibb,1990].The 7Bewasremoved by coprecipitationwith iron hydroxide, dried, and countedby gammaspectrometry. The stableBe in the precipitate wasthen measuredby atomicabsorptionto calculatethe
7Berecovery duringprecipitation. Thebuckets weredeployed for periodsrangingbetween 1 and 3 weeksin October 1997 and July-August1998.After collectionthe bucketswere rinsed with diluteHC1.Subsequently, the HC1rinsewastreatedasthe snowsamplesdescribedaboveand counted[e.g.,Baskaranet al., 1993].The countingsystemis calibratedfor all samplesby preparinga commercialstandardin geometriesidenticalto the samples. The error for each measurementis the statisticalcounting
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3371
error•r andthe uncertainty in the blank,X/o-2 + •r2t,,multi- Table 1. Beryllium 7 From SHEBA, October 1997 plied by
[(XCT)/[1 - exp (-XCT)]][exp (XD)/(CE. FE ßPE)], where CT is the countingtime, CE is the countingefficiencyof the 478 keV gamma, FE is the fiber (or precipitation)effi-
Date
Depth,m
7Be,dpmm-3
Oct. 8
10, 20, 25
100.8 _+16.6
Oct. 9
Oct. 11
ciency, andPE isthephotonemission fraction(0.104)for 7Be. 4.
Results
and Discussion
Oct. 12
24
103.5 _+ 15.0
35 25 33 39.6 45.5 25 34 45
24.8 _+ 17.7 133.45 + 14.0 30.2 _+ 9.2 19.1 _+ 6.4 5.95 + 6.00 104.7 _+ 5.9 17.8 _+ 3.75 12.5 +_ 5.2
Comments three samplescombined D.L. a=
30.6
D.L. a=
12.4
The typicaltemperatureand salinityprofile from SHEBA in October 1997 is shownin Figure 2. The mixed layer extended aD. L. is the detection limit. to •30 m depth, below which was found a layer •0.5øC warmer than the mixed layer. McPheeet al. [1998] note that this layer is appreciablywarmer than a similarlayer measured during AIDJEX in 1975. The salinity of the mixed layer in allow phytoplanktonactivity.Second,while the initial oxygen SHEBA is alsoconsiderablyfreshet than that of AIDJEX. The contentis not known and an absolute02 utilization rate cannot ?Bemeasurements in October1997are presented in Table 1 be determined,the persistenceof the oxygenpeak well into that the bacterialrespirationrate is low. As and displayedwith the •T (temperatureabovefreezing)profile October suggests in Figure 3a. In two casesthe valuesare closeto or below the discussedbelow, the data and model presented here allow detection limit defined earlier but are retained because of someestimateof primary productionfor the summerof 1997
consistency withtheothervalues(Table1). The remnant?Be
to be determined.
belowthe mixedlayer marksthe warm water ashavingbeen in contactwith the sea surfacewithin the previous77 days.Thus the heated layer beneath the October mixed layer at SHEBA was remnant of an earlier, deeper mixed layer and was the resultof very activeheat input duringthe previouslate spring-
The flux determinationsare presentedin Table 2. For 1997, if it is assumedthat by October13 snowhad beenaccumulating for 3 weeksat the SHEBA site,then the flux determinedby the snowinventoryagreeswith two out of the three bucketdeployments at that time. The average for these measurementsis
summer.The ?Beshowsthat the higherheatlayercouldnot
0.0020+_0.0009dpmcm-2 d-•. In the summerof 1998the
have been advectedfrom afar or emplacedcumulativelyover
average flux from nine bucket deploymentswas 0.0122 _+
several
0.0070dpmcm-2 d-•.
seasons.
Corresponding to theheatand7Bebeneaththemixedlayer is a peak in oxygen(Figure 3b; profile collectedby E. Sherr).
4.1.
The 7Be givessomeinsight,at leastqualitatively, into the
Thedistribution of ?Becanprovidequantitative information on mixedlayerevolutionbecause featuresof the 7Bedistribu-
history of this as well. First, it suggeststhat there had to be substantialprimary productionearly in the melt seasonwhen the mixedlayerwasdeep(of the order of 50 m). The presence of substantialopenwater, a suggested by McPheeet al. [1998], would have contributed to sufficientlight being available to
Modeling the Mixed Layer Evolution
tion demandcertain aspectsof mixed layer history.For exam-
ple,thepresence of 7Beat a depthof 50 m requiresthemixed layer to havebeen that deep early in the melt season.Furthermore, the mixedlayer had to persistlong enoughat that depth to accumulate sufficient 7Be to be detected months later in
October.In the followingthe 7Be is usedto derivea mixed layer historythat then can be appliedto other tracers,notably
Potential Temp ('C) -1
-2
0
0
1
•
•
2
heat.
Themajorfeaturesof oceanic 7Beprofilescanbegenerated by accountingfor the seasonaldeepeningand shoalingof the mixedlayer on the basisof empiricalobservations of the mixed
10
layerhistory. For a giveninputof 7Bethemixedlayerdepthis a criticalparameterthat largelydeterminesthe 7Be surface activity; shallow mixedlayersconcentrate 7Bewhilein deeper
20
mixedlayersthe concentrationis diluted.The model usedhere is not dynamicallydriven, in that there are no surfaceforcing
terms,butsimplyillustrates how7Be(andothersurfacetracers) will respondto a prescribedmixedlayer history,an input function, and relevant tracer decay terms. With this under50
-
60
-
70 26
standing an observed 7Beprofilealongwithknowledge of 7Be input can yield insightinto the mixed layer history. Details of the model are presentedelsewhere[Kadko and Olson, 1996]. It is basedon the finite differenceform of the •
I
•
27
28
29
30
SALINITY
Figure 2. Temperatureand salinityprofilesfrom October 11, 1997.This profile was typicalof thosetaken betweenOctober 8 and 12.
one-dimensional analyticalexpression governing the 7Bedistribution:0c/0t= gzO2C/OZ 2 - •c, wherec is the concentrationof 7BeandKz is the verticaleddydiffusivity. The 7Beis assumedto be mixed rapidly with respect to its radioactive decay in the mixed layer and is mixed with a finite K z in the pycnocline.As an illustration,the model representationof a
3372
KADKO: MODELING THE ARCTIC MIXED LAYER USING 7BE Be-7 (dprn/rn"5) o
20
o
40
60
80
ml/I 100
120
140
•
0 0
0.2
o
(^)
oxygen above safurafion 0.4
0.6
!
(B)
10
lO
2O
20
3O
5o
[] ß v ß
40
10/8/97 10/9/97 10/11/97 10/12/97
40
50
50
60
I
0.0
0.5
•
I
60
1.0
I
[
I
1.5
DelfoT (øC)
Figure 3. (a)The7Be measurements inOctober 1997 aredisplayed withthe•T profile. Theprofile issimilar to thatexpected fromthemodel (Figure 4c).(b)Thedifference (02 concentration minus 02 saturation) plotted versus depth forOctober 24,1997(datacourtesy ofE. Sherr, Oregon StateUniversity). generic seasonal cycleof mixedlayerdepthisshownin Figure shoalas a resultof heatandfreshmeltwaterinputuntil 4a,whereat sometime(Tmax) themixedlayerisat itsdeepest coolingrecommences anddeepening occurs. (Lmax). In thisexample, Tmax is50 m, andKz for thepycno- As the mixedlayeronceagaindeepens (Figure4c), the clineis assigneda value of 10-6 cm2 s-•. mixedlayer?Beconcentration decreases because of dilution, Monthlyprofiles of ?Beactivity aregenerated asthemixed andtheupperthermoclineactivityistruncated.Thisisbecause layershoals,leavingwaterbeneaththe mixedlayerthat is the?Bedeposited muchearlierin theyearhasnowdecayed isolatedfrom the atmospheric inputI, whichis chosento be away.Thisiswhatwasexpected at SHEBAin October1997,at constant inthisexample. The?Beactivity intheshoaling mixed whichtime the leadsand melt pondswerefrozenandmixed layerincreases, whilethe?Beisolated at depthdecays radio- layerdeepeninghad begun. actively, whichproduces theobserved profiles (Figure4b).In In thisworkthebasicmodelof KadkoandOlson[1996]is the Arctic,in late SpringandSummer, the mixedlayerwill modifiedfor the SHEBA site.The parameters usedfor the Table
2.
Flux Measurements Collection
Sample Bucket
1 Bucket 2 Bucket 3 Snow 1 Snow 2
Collection
Oct. Oct. Oct. Oct. Oct.
Dates
2-10, 1997 3-15, 1997 4-15, 1997 13, 1997 13, 1997
Time,
Flux,
days
dpmcm-2 d-i
7.9 11.8 10.9 21" 21"
Average Oct. 1997 Bucket
4
Bucket
6
Bucket Bucket Bucket Bucket Bucket Bucket Bucket
7 8 9 10 11 12 13
July 5-20, 1998 July 5-20, 1998 July 6-20, 1998 July 12-29, 1998 July 20 to Aug. 8, 1998 July 20 to Aug. 8, 1998 July 20 to Aug. 8, 1998 July 21 to Aug. 10, 1998 July 30 to Aug. 10, 1998
Average JulyAugust 1998 aAssumed snow accumulation time.
15.2 15 14 17 17 17 17 20.1 10.75
0.0020 0.0005 0.0021 0.0031 0.0022 0.0020 _+ 0.0009 0.0092
0.0028 0.0058 0.0233 0.0192 0.0177 0.0127 0.0143 0.0052 0.0122 _+ 0.0070
KADKO:MODELINGTHE ARCTICMIXED LAYERUSING?BE
I
F
10
_1
40
x
50
I
i
I
i
120
180
240
3373
I
_
Tmo• i
6o
0
60
i
.300
360
DAYS o
(Bq IIII I I+1120 I I II III
I•
-
I I I•
20
..••=-"•-•+ 180 t ......__J -80 days .//J/ +60 _. _ t -50days
40
1/i'-ox
lO
ZO days
5o
max I
6o o
-2 -1 -2d-1 Xo.006 dpm cm d I II--0•,006 dpm cm I
100 200
I
300
I
400
I
500
I
600
L
0
I 100
I 200
I 300
i
400
i
500
i
600
Be-7
Be-7 (d pm/m"5)
Figure 4. (a)Themodel representation ofanidealized seasonal cycle ofmixed layer depth where atsome
time(Tmax) themixed layer isatitsdeepest (Lr.ax)' Inthis example theinput of7Be, I, isconstant over the entire seasonal cycle, and Kzinthepycnocline ischosen tobe10-6m2s-•. (b)The7Be activity forashoaling mixed layer. Profiles of7Bearegenerated forvarious times (+ days after Tmax). Asshoaling progresses, the
7Beactivity ofthemixed layer increases, while the?Be isolated atdepth decays radioactively, which produces theobserved tailing. (c)Asthemixed layer once again deepens, themixed layer 7Beconcentration decreases because ofdilution andtheupper thermocline activity istruncated. Thisisbecause the7Bedeposited much earlier intheyear hasnow decayed away. Profiles thatmight beexpected atSHEBA inthefallduring mixed layerdeepening wouldbesimilar to thesemodelcurves.
1997summer-fall in SHEBAaredisplayed in Figure5, andthe
19toSeptember 20,thereisonlyatmospheric input.In stage 3,
of icecover.Themodelisrun model output isshown inFigure 6.Monthly profiles of7Beare thereiszero7Beinputbecause
derived asa function of mixedlayerevolution and7Beinput. until October 11, at which time the SHEBA site was comi.e., isolated fromatmoInitially, it isassumed thatthereisno7Beintheocean follow- pletelyfrozenandsnowcovered, spheric or melt input. Likely, it had been in this stateof the ingthewinterwhentheice-covered upperoceanhadbeen isolatedfrom atmosphericinput.
orderof 3 weeks.Therefore,from September20 to October
Theinputfunction hereissomewhat different thanthatof 11,thereiszero*Beinput(i.e.,thereisonly*Bedecay). fluxis determined asfollows: onOctober KadkoandOlson[1996]in thatinputof 7Beisnotconstant. Theatmospheric 11 the depth-integrated standing stock of 7Be was 0.357dpm Hereit iscomprised notonlyof atmospheric inputbutaddithisto September 20,thestanding stockof tionally, duringlatespring, of rapidinputfrommelting snow cm-2. Correcting
wouldbe0.463dpmcm-2, corresponding thathasaccumulated ?Beoverthewintermonths. The ?Be 7Bebeforeisolation to flux of 0.463X or 0.006dpmcm-2 d-•, whichis theinput inputfunction consisted ofthreestages (Figure 5).In stage 1, used between July 19 andSeptember 20 (stage2). It is then fromJune21 (summer solstice) untilJuly19,?Bewasinput that the atmospheric fluxaccumulating in the snow through atmospheric fluxandsnow melt.In stage 2,fromJuly assumed
3374
KADKO: MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
0.020
I
-
Stage
1
I
-
I
_
-•
ß
_
50
_
60
x
:•
I
,
_
I
,
'---------C--0.012
',
I
I
Stage 2
_
•Stage5 i
I
i
i
i
i
180
Jun
0.008
-
I
- • i
0.010
_
I
i
i
,
200
,
i
i
i
i
•
220
21
i
!
!
!
240
i
i
z
.-• c
o.006
I
I i
_
I
I
70
•
0.014
-
40
I
0,016
I
ß
m
0,018
!
260
i
!
i
.
i
0.004
3
0.002
•
o.ooo
3
28(J
DAYS
Oct.• •
Figure5. Parameters usedin theSHEBA?Bemodelasa functionof time.Thethreestages of ?Beinputare shown.
during the winter monthswas the same as during the stage2
Inputtotal= InpUtmelt+ Inputatto
flux,andthusthe standing stockof 7Bein the snowwouldbe 0.463dpmcm-2. Assuming snowmeltoccurred predominately overa month'stime,this7Beis inputinto thewatergradually
= {(0.463/28)[1 - exp(-(X28)]/(X28)}
+ 0.006dpmcm-2 d-•.
overthe 28 dayperiodof stage1. The formulationfor stage1 is
20 /2s • •0 ,m., tm 40
Max ML = 50 m
50
60
0
20
40
60
80
100
120
140
160
i
180
200
i
116
-•
'1 •z,
9/15 _
•o/•/• 10/9/97 10/11/97 10/12/97
60 0
20
40
60
80
100
120
• 140
• 160
180
200
Be-7 (d pm/m"3) Figure6. The modeloutputshowing the evolutionof the 7Beprofileat SHEBAwithtime.Data fromthe period October8-12 are superimposed upon the model profiles.The modelprofile for October 11 is shown as a dashed line.
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3375
10
20
30
---0 m2/s 10-6 rn2/s - -- 10-5m2/s
40
!
50
Max
ML-
50
m 2
Input - .006 dpm/cm 60
0
i 20
i 40
60
80
100
'
120
/d 140
Be-7 (d pm/m"S) Figure7. The modeloutputfor the 7Beprofileat October11 for threedifferentvaluesof Kz in the pycnocline: 0, 10-6, and10-s m2 s-].
Fromthemaximum depthof the7Beit is surmised thatat the 4.2. Modeling the Evolution of Mixed Layer Temperature onset of melting the maximumdepth of the mixed layer was 50 m. This assumes that mechanisms suchasdownwellingwere not a factor, which is reasonable as the SHEBA site was not in
the centerof an anticyclonicgyre. StartingJune 21 (summer solstice)this depth is maintainedfor 12 daysfollowingwhich the mixed layer decreasesto a minimum of 10 m on July 19.
With the 7Beinputfluxuseda late springmixedlayermaintained at this depthfor this time period is requiredto achieve
the 7Beprofileobserved in October.The maximummixed layer depth modeled here is similar to that reported for the CanadianBasinat ice islandT-3 from 1970 to 1973 [Morison and Smith,1981]and for the AIDJEX projectin 1975[Maykut and McPhee,1995]. The effectof differentpycnoclineKz on the modelresultsis shown in Figure 7. These results suggestthat in the highly stratifiedwater belowthe mixedlayer (see salinitygradientin
The evolution of the 7Be distribution in the model described
aboveis similarto the evolutionof bT describedbyMaykutand McPhee[1995].In late winter,there is a deep,well-mixedlayer. After 2 months,spanningthe summersolstice,there is warming throughoutthe mixed layer and fresheningdue to melt water introduced at the surface.By late summer the mixed layer has shoaled,leavingdeeperwater behindthat had been heatedearlier in the season.The maximumin bT is interpreted to be the temperature to which the mixed layer was heated
aroundthe time of the solsticeand,like the deep7Beis a remnantfeature of the deepermixedlayer of late spring/early summer. In a sensethis is a form of subduction in which water,
initially in contactwith the surface,becomesisolatedfrom the atmosphere.The heat is trappedin this layer until subsequent wintertime
convection.
In the modelingshownbelow the temperaturemaximumis Figure2), verticalmixingis not muchgreaterthan10-6 cm2 similarly formed, i.e., as a consequenceof meltwater capping s-• andthat the 7Beprofileis mainlya response to the fall and represents a residual mixed layer. However, it is shown deepeningof the mixedlayer.Higher valuesof Kz would draw that the bT maximumlikely corresponds to the temperatureto more7Befromthe mixedlayerintothepycnocline water. The fluxusedin themodel(0.006dpmcm-2 d-•) liesbe- whichthe mixedlayerwasheatedby mid-July(as opposedto tween the flux determinationsof October 1997 (0.0020 dpm the solstice)andmarksthe depthof the mixedlayerat that time. The parametersused in the temperatureevolutionmodel cm-2 d-•) andJuly-August 1998(0.0122dpmcm-2 d-•). As such, the model value is not unreasonable, but there is no are shownin Figure 8. The model is the sameas that usedfor historical 7Be flux data from this area of the world with which
to compareit. Taken at facevalue, the ratio of the model flux (basedon the summer1997water inventory)to the measured summer1998flux (--•0.5)couldsuggestthat in 1997the summertimelead coveragewasof the order of 50%. This appears
7Beexceptthatthereis no radioactive decaytermandit uses the mixedlayerevolutionderivedfrom the 7Bedistribution
(compareFigures5 and 8). The heat input functionwas derived from the pattern of summerheat content of the water columnduringAIDJEX in 1975 (Figure 9 [from Maykutand large and is certainlybiasedby 7Be input from meltwater McPhee,1995,Figure9]). At AIDJEX (Snowbirdsite),starting runoff. However, the large heat inventorymeasuredin Octo- approximatelyJune 21, the heat contentincreasedat a rate of ber 1997 suggeststhat indeed, substantialopen water must 9.64W m-2 for a periodof 28 days(Figure8, linea). Forthe next39 daysthenetheatincrease wasat a rateof 0.48W m-2 havebeen present.This will be discussed in section4.2.
3376
KADKO: MODELING
•,
THE ARCTIC MIXED
LAYER USING 7BE
0 40 -
I
_
I
-
I
-
I •
I
I
_
I
C ß
40
/
o
•
60
•.
20
z
10.•
-->. 5o -_, ß'o
.•o
......Inpu ear
I
_
-10
-
-20
i
-
i
...........
X
180
200
220
240
260
)
2801
• 70,i,•....•....I....I....I....,, Jur,
2•
DAYS
oot. l•
Figure 8. The parametersusedin the temperatureevolutionmodelplottedasa functionof time. The model
usesthe mixedlayerevolutionderivedfromthe 7Bedistribution.
(Figure 8, line b). Beyondthat date, there was a net heat loss 4.3. Modeling the Evolution of Mixed Layer Oxygen
of -4.4 W m-2 (Figure8, linec).To achieve thetemperature and Implications for Primary Production profile observedon October 11, 1997, with this model, the same relative heat flux for each period was used, but the
absolute valueshadto bequadrupled (39,2, and-18 W m-2, respectively). This is consistent with McPheeet al. [1998],who suggestthat the lead openingsin 1997 relative to AIDJEX (1975) tripled, and with observations made at SHEBA in October that indicatedextensivesummertimemelting. These includedthinnerice than anticipated(--•1.2 m asopposedto 3-4 meters),extensivemelt pond (by then frozen) coverage,and melt pondsthat had melted through to the underlyingocean (D. Perovich,personalcommunication, 1998).This is alsocon-
sistentwith the high7Beinventory(relativeto flux) of the
By analogywith the temperatureprofile the peak in oxygen measuredin October 1997 is remnant of spring-summerproduction,while the water aboveit has subsequentlydegassed duringmixedlayer deepening.Neglectingfor the momentloss effectsthrough respiration,the upper 50 or so meters must
havebeenat least0.5 mL L-• or 35 g m-2 abovesaturation. Thisis of the orderof 13 gC m-2 net production but likely representsa minimum because(1) if this productioncommenced in June, then the water column started out below
saturationasa resultof respirationduringthe dark season,(2) 02 is lost throughgasexchange,and (3) respirationoccurred subsequentto the summer.The 1998 spring-summerobserva-
water columnmeasuredin October 1997,whichrequiressub- tions at SHEBA are consistentwith this estimate. Although stantialleadopening for the7Beto entertheupperocean. direct comparisonmay not be possiblebecausethe SHEBA The model output is shownin Figure 10 comparingthe case sitehad drifted far westfrom the October1997location(Figof AIDJEX and SHEBA. It is seen that the &T maximum on ure 2), the June-Julynet productionof oxygensuggested a net October11 corresponds to the temperatureto whichthe mixed community primaryproduction of --•17gC m-2 (B. Sherr, layerwasheatedby mid-Julyandmarksthe depthof the mixed personalcommunication,1998). layer at that time. These resultsare consistentwith the conclusiondrawnby McPheeet al. [1998] that the mixedlayerwas 5. Conclusions of the order of 50 m deepearlyin the summer,wasextensively heatedbefore shoaling,and by Octoberhad givenup its availThispaperreportsthefirstmeasurements of 7Beevermade ableheat.Byusing7Be,similarconclusions areindependentlywithin the Arctic Ocean. The profile from October 1997was drawn. usedto reconstructthe evolutionof the mixed layer over the
i
,
'
'- .....
,
!
40 .....
!
i
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-1 ......
'b
i
r
.......
'
i
E 20
o
140
1,,Fe!5 180
200
220
240
260
280
300
Day of 1975 Figure 9. The pattern of summer heat content of the water column during Arctic Ice DynamicsJoint Experiment(AIDJEX) in 1975 [from Maykutand McPhee,1995].The heat input for SHEBA (1997) was derivedby quadruplingthe slopeof the three linespicturedhere.
KADKO: MODELING
(a)
THE ARCTIC MIXED LAYER USING 7BE
delta T (oC) 0 0
0 1
0
I
10
delta T (oC)
0.2
ß
3377
0 0
0.1
0.2
I
I
I
75
973
8 6
2o •
3O
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DJEX 1975
40
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I
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10 75 •
6/ 8
/
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I
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19
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/SHEBA
50
0.2
I
7
40 0.0
I
I
/•
0.4
0.6
0.8
1.0
delta T (oC)
0.0
0.2
•
•
0.4
0.6
0.8
1.0
delta T (oC)
Figure 10. The modeltemperatureoutputfor (a) AIDJEX and (b) SHEBA. Note the changein temperaturescale.It is seenthat the &T maximumon October11 corresponds to the temperatureto whichthe mixed layerwasheatedbymid-July(asopposed to the solstice) andmarksthe depthof themixedlayerat thattime.
previousseasonand confirmedthat the reservoirof heat beneaththe fall mixedlayerwasemplacedin the summer,rather than input cumulatively over severalseasons or advectedin from distantsources.In addition,a reasonablerate of primary
Acknowledgments.This work was supportedby NSF Polar Program (grant OPP-9701067)and by the NSF ChemicalOceanography Program(grant OCE-9809168).The authorwould like to thank J. Morison, M. McPhee, and B. Welch for many valuable discussions in
regardsto thispaperand two anonymous reviewersfor their helpful productionfor the spring-summer periodwas derivedusing comments.Thanks also to E. Sherr and B. Sherr for permissionto presenttheir oxygendataandto M. Stephens for field andlaboratory thismixedlayerhistorywith an oxygenprofilefrom the fall. assistance. Finally, the author acknowledges the crew of the CCGC As suggested byMcPheeet al. [1998],severaltimesasmuch DES Groseilliersand the SHEBA logisticsgroup for their support heatwasemplacedat SHEBA than at AIDJEX 20 yearsear- duringthe field operationof SHEBA.
lier. A likely mechanismfor this is substantially greaterlead openingcoupledwith the positivefeedbackloopbetweenincreasedopenwater,increasedheat absorption,andfurtherice References melting.Note thatwhilethe heatobserved in Octoberwasnot Baskatari,M., C. H. Coleman,and P. H. Santchi,Atmosphericdepositionalfluxesof ?Beand•mPbat Galveston andCollegeStation, cumulative(i.e., the heat is reseteachwinter),a portionof the salinitychangecan be cumulative.Consistentwith observa- Texas,J. Geophys.Res.,98, 20, 555-20, 571, 1993. tionsof accelerating diminishment of Arcticseaice [Johannes- Cooper,L. W., C. R. Olsen,D. K. Solomon,I. L. Larsen,R. B. Cook and J. M. Grebmeier,Stableisotopesof oxygenand natural and senet al., 1995], the changein salinitymight be an ongoing fallout radionuclidesusedfor tracingrunoff duringsnowmeltin an processthat has been occurringover many summersof inArctic watershed, Water Resour. Res., 27, 2171-2179, 1991. Dibb, J. E., Beryllium-7and lead-210in the atmosphereand surface creasedoceanicheat absorption.
3378
KADKO: MODELING
THE ARCTIC MIXED
snow over the Greenland ice sheet in the summer of 1989, J. Geo-
phys.Res.,95, 22, 407-22, 415, 1990. Ingram, W. J., C. A. Wilson, and J. F. B. Mitchell, Modeling climate change:An assessment of sea ice and surfacealbedofeedbacks,J. Geophys.Res.,94, 8609-8622, 1989. Johannessen, O. M., M. Martin, and E. Bjorgo,The Arctic's shrinking sea ice, Nature, 376, 126-127, 1995. Kadko, D., and D. Olson, Be-7 as a tracer of surfacewater subduction and mixed layer history,Deep SeaRes.,Part II, 43, 89-116, 1996. Krishnaswami,S., D. Lal, B. L. K. Somayajulu,F. S. Dixon, S. A. Stonecipher,and H. Craig, Silicon,radium, thorium and lead in seawater;In-situ extractionby syntheticfibre,Earth PlanetSci.Lett., 16, 84-90, 1972.
Lal, D., Y. Chung, T. Platt and T. Lee, Twin cosmogenicradiotracer studiesof phosphorusrecyclingand chemicalfluxesin the upper ocean,Limnol. Oceanogr.,33, 1559-1567, 1988. Lee, T., E. Barg and D. Lal, Studiesof verticalmixingin the Southern
LAYER USING 7BE
Fresheningof the upper oceanin the centralArctic:Is perennialsea ice melting?,Geophys.Res.Lett., 25, 1729-1732, 1998. Morison,J. H., andJ. D. Smith,Seasonalvariationsin the upperArctic as observedat T-3, Geophys.Res.Lett., 8, 753-756, 1981. Rind, D., R. Healy, C. Parkinson, and D. Martinson, The role of sea-icein 2 x CO2 climatemodel sensitivity,Part 1, The total influence of sea ice thicknessand extent., J. Clim. 8, 449-463, 1995. Rubinson, K. A., ChemicalAnalysis, Little, Brown, Boston, Mass., 1987.
Silker, W. B., Beryllium-7 and fissionproductsin the GEOSECS II water columnand applicationsof their oceanicdistributions, Earth Planet. Sci. Lett., 16, 131-137, 1972.
Young, J. A., and W. B. Silker, Aerosol depositionvelocitieson the
Pacificand Atlantic Oceanscalculatedform 7Be measurements, Earth Planet. Sci. Lett., 50, 92-104, 1980.
D. Kadko, RSMAS/MAC, Universityof Miami, 4600 Rickenbacker
California Bightwithcosmogenic radionuclides 32pand7Be,Limnol. Causeway, Miami, FL 33149.([email protected]) Oceangr.,36, 1044-1053, 1991. Maykut, G. A., and M. G. McPhee,Solarheatingof the Arctic mixed layer,J. Geophys.Res.,100, 24, 691-24, 703, 1995. McPhee, M. G., T. P. Stanton, J. H. Morison and D. G. Martinson,
(ReceivedJanuary23, 1999;revisedSeptember10, 1999; acceptedOctober6, 1999.)
RESEARCH, VOL. 105, NO. C2, PAGES 3369-3378, FEBRUARY
15, 2000
Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic Ocean (SHEBA)
Projectusingmeasurements of 7Be David
Kadko
Marine and AtmosphericChemistry,RosenstielSchoolof Marine and AtmosphericSciences,Universityof Miami, Miami, Florida
Abstract. During the SurfaceHeat Budgetof the Arctic Ocean (SHEBA) projectin October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used
to reconstructthe evolutionof the mixed layer over the previousseasonand confirmed that the reservoirof heat beneaththe fall mixed layer was emplacedin the summer,rather than input cumulativelyover severalseasonsor advectedin from distantsources.As suggested by McPheeet al. [1998], severaltimes as much heat was emplacedat SHEBA than at Arctic Ice DynamicsJoint Experiment 20 yearsearlier. A likely mechanismfor this would be substantiallygreaterlead openingcoupledwith the positivefeedbackloop betweenincreasedopen water, increasedheat absorption,and further ice melting.In
addition, a rateof netprimaryproduction >_13gCm-2 for thepreceding spring-summer period was derived usingthis mixed layer historywith an oxygenprofile from the fall. SHEBA mixed layer was the result of cumulativeinput over severalsummersor was advectedin from elsewhere.Basically, at SHEBA duringthe fall of 1997offeredonly One of the important considerationsin the summersurface the observations were made energybalance of the Arctic Ocean is the fate of the energy a "snapshot"of the systembecauseno observations input (mostlyshortwaveradiation) in the heterogeneousice- duringthe precedingsummer.It is thereforedifficultto reconoceansystemduring the melt season.Downwellingsolar radi- structthe mixedlayer evolutionand heatinghistoryof this site ation throughleadsis the primarysourcefor the directheating prior to the initial SHEBA occupation. To addressthis question,the heat exchangebetween the of the ocean,but the fate of the solarenergyabsorbedthrough leads and underlyingmixed layer of the SHEBA site is evaluthe leadsis not well understood[Rind et al., 1995]. It is not ated here usinga naturallyoccurring radioactive tracer,7Be. knownfor example,the extentto whichthisheat laterallymelts the ice in contactwith the leads,is conveyedto depth and melts Beryllium7 is a cosmicray-producedspeciesthat is delivered the ice from below, or is transportedaway from the ice. The to the Earth's surface and, as such, is a form of proxy for fate of the heat has direct consequencetherefore upon the incomingsolar radiation. Becauseof its 53.3 day half life, it is 1.
Introduction
persistenceof the ice cover and the associatedsummertime albedo.
The SurfaceHeat Budget of the Arctic Ocean (SHEBA) programwas undertakenin October 1997 to studythe feedbacksassociatedwith the ice-wateralbedosystem,in whichthe dispositionof the summerheat input is of primary importance [e.g.,Ingramet al., 1989].The initial observations made in the Fall of 1997 indicatedthat the upper oceanwas lesssalineand warmer than expected[McPheeet al., 1998].The temperature abovefreezing(tST) of the mixedlayerduringSHEBA (1997) had increasedby a factor of 2.5 over that which occurred during an earlier program,Arctic Ice DynamicsJoint Experiment (AIDJEX) (1975). Additionally,the mixedlayer salinity in 1997was ---27.6ppt, comparedto 29.7 ppt in 1975 [Maykut and McPhee;1995;McPheeet al., 1998].From this data it was concludedthat the oceanicheat flux was considerablygreater duringthe 1997 summer.Becausethe albedoof openwater is considerablylessthan that of seaice, it wassuggested that the percentageof open water between1975 and 1997 had tripled to allow a greater amount of heatingto occur [McPheeet al., 1998]. It is possible,however,that the heat stored in the
an ideal tracer of water that has been in contact with the sea
surfaceover the pastseason[Kadkoand Olson,1996].Profiles
of 7Bemeasured in thefallprovideanintegration of themixed layer behaviorover the previoussummerand can offer insight into the mixed layer evolutionand heatinghistory.
2.
Background
Beryllium 7 is a cosmicray-producedradioactivenuclide with a radioactive meanlife of 76.9days(the meanlife is defined as the 1/e decaylevel or half life/0.693) and, as such,is well suited for studyingseasonalphenomenon.Beryllium 7 is deposited upon the Earth's surface by precipitation and is homogenizedwithin the surfacemixedlayer of the oceanrapidly with respectto its decayrate [e.g.,Silker,1972;Youngand
Silker,1980;KadkoandOlson,1996].In snowthe 7Beactivity (equivalentto concentration,here expressed as disintegrations
perminute(dpm)perm3) is ---2ordersof magnitude greater thanthat in the oceanmixedlayerbecause a given7Befluxis
Paper number 1999JC900311.
diluted in mixed layerstypicallytens of meters deep but depositedin snowlayers(over a 77 day period) that are at least an order of magnitude shallower,thereby concentratingthe nucliderelativeto the surfaceocean[e.g.,Cooperet al., 1991]. In the oceanthe mixedlayer depth is a criticalparameterthat
0148-0227/00/1999JC900311509.00
largelydetermines the7Besurfaceactivitybecause for a given
Copyright2000 by the American GeophysicalUnion.
3369
3370
KADKO: MODELING
THE ARCTIC
MIXED
LAYER USING 7BE
Al 7o
Barrow
Dead
Horse
Tuktoyaktuk 65
18,0
12O
•
1so
Figure 1. Locationmap of the SurfaceHeat Budgetof the Arctic Ocean(SHEBA) projectsite.Point A is the initial deploymentlocationin October1997.Point B is the locationin July 1998.
7Beinput,the activityis concentrated in shallow mixedlayers placedin a marenellibeaker,whichin turn wasplacedover a and dilutedin deepermixedlayers.As will be describedbelow, low background germanium gammadetector.The 7Behasa waterin contactwiththe oceansurfaceis "tagged" with7Be, readilyidentifiablepeak at 478 keV. The detectoris calibrated and profilesof this isotopeprovide a record of the seasonal for this geometryby addinga commerciallypreparedmixed changesin the mixed layer depth. solution of known gamma activities to an ashed fiber and countingit in the marenelligeometry.The countingefficiency for the 478 keV gamma in this configurationwas 0.03. The 3.
Methods
limit of detection used in this work defined at the 99.7% con-
fidence level, is SL = --> 3frO, where O'b is the standard Verticalprofilesof 7Bewerecollected in the BeaufortSea deviationof the blank measurements[Rubinson,1987]. The from October8 to 12, 1997(Figure 1). To collectthe samples, detection limitof a sample increases withtimebecause of 7Be a hydroholewas melted through -2 m of ice throughwhich
samples for7Bewerecollected bypumping. In thisprocess, 700 L of seawaterwere passedthrough iron-impregnatedacrylic fiberspackedin cylindricalcartridges[e.g.,Krishnaswami et al., 1972;Lal et al., 1988;Lee et al., 1991;Kadkoand Olson,1996]. The water wastaken at variousdepthsthrougha 1.5"hoseat the endof whichwasa conductivity-temperature-depth (CTD) system.The pumpingwas achievedwith a centrifugalpump, poweredby a gasoline-fueled generatoron the ice,at a rate of
-14 L min.-• Thuseachsampleddepthrequired-50 min.In mostcases,doubleor triple sampleswere collectedat any one depth and later combined.On one cast (October8), where only singlecartridgeswere usedat eachdepth,three samples from different depthsin the mixed layer were combinedas representativeof the mixed layer activity.The fiber efficiency, testedwith stableBe in the lab or by countingtwo cartridges placedin serieswas 0.90 _+.02. This is an improvementover earlier fibers [Kadkoand Olson, 1996] and reflectsa higher retention of iron in the fibersproducedin the laboratoryfor this study. On land the fibers are dried
and then ashed. The ash was
decayand,for oursystem, is(12.7/V) x[exp(XD)] dpmm-3,
whereD is the numberof daysafter samplecollection,V is the
volume(m3),andX is thedecayconstant (0.013d-•). To estimate theatmospheric deposition of 7Be,samples of snow(m2 in area)weretaken,andalso,plasticbuckets were deployedto collectfallout over knowntime periods.The snow was melted, and 0.5 mL of concentrated HC1 and stable Be
yieldtracerwereadded[e.g.,Dibb,1990].The 7Bewasremoved by coprecipitationwith iron hydroxide, dried, and countedby gammaspectrometry. The stableBe in the precipitate wasthen measuredby atomicabsorptionto calculatethe
7Berecovery duringprecipitation. Thebuckets weredeployed for periodsrangingbetween 1 and 3 weeksin October 1997 and July-August1998.After collectionthe bucketswere rinsed with diluteHC1.Subsequently, the HC1rinsewastreatedasthe snowsamplesdescribedaboveand counted[e.g.,Baskaranet al., 1993].The countingsystemis calibratedfor all samplesby preparinga commercialstandardin geometriesidenticalto the samples. The error for each measurementis the statisticalcounting
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3371
error•r andthe uncertainty in the blank,X/o-2 + •r2t,,multi- Table 1. Beryllium 7 From SHEBA, October 1997 plied by
[(XCT)/[1 - exp (-XCT)]][exp (XD)/(CE. FE ßPE)], where CT is the countingtime, CE is the countingefficiencyof the 478 keV gamma, FE is the fiber (or precipitation)effi-
Date
Depth,m
7Be,dpmm-3
Oct. 8
10, 20, 25
100.8 _+16.6
Oct. 9
Oct. 11
ciency, andPE isthephotonemission fraction(0.104)for 7Be. 4.
Results
and Discussion
Oct. 12
24
103.5 _+ 15.0
35 25 33 39.6 45.5 25 34 45
24.8 _+ 17.7 133.45 + 14.0 30.2 _+ 9.2 19.1 _+ 6.4 5.95 + 6.00 104.7 _+ 5.9 17.8 _+ 3.75 12.5 +_ 5.2
Comments three samplescombined D.L. a=
30.6
D.L. a=
12.4
The typicaltemperatureand salinityprofile from SHEBA in October 1997 is shownin Figure 2. The mixed layer extended aD. L. is the detection limit. to •30 m depth, below which was found a layer •0.5øC warmer than the mixed layer. McPheeet al. [1998] note that this layer is appreciablywarmer than a similarlayer measured during AIDJEX in 1975. The salinity of the mixed layer in allow phytoplanktonactivity.Second,while the initial oxygen SHEBA is alsoconsiderablyfreshet than that of AIDJEX. The contentis not known and an absolute02 utilization rate cannot ?Bemeasurements in October1997are presented in Table 1 be determined,the persistenceof the oxygenpeak well into that the bacterialrespirationrate is low. As and displayedwith the •T (temperatureabovefreezing)profile October suggests in Figure 3a. In two casesthe valuesare closeto or below the discussedbelow, the data and model presented here allow detection limit defined earlier but are retained because of someestimateof primary productionfor the summerof 1997
consistency withtheothervalues(Table1). The remnant?Be
to be determined.
belowthe mixedlayer marksthe warm water ashavingbeen in contactwith the sea surfacewithin the previous77 days.Thus the heated layer beneath the October mixed layer at SHEBA was remnant of an earlier, deeper mixed layer and was the resultof very activeheat input duringthe previouslate spring-
The flux determinationsare presentedin Table 2. For 1997, if it is assumedthat by October13 snowhad beenaccumulating for 3 weeksat the SHEBA site,then the flux determinedby the snowinventoryagreeswith two out of the three bucketdeployments at that time. The average for these measurementsis
summer.The ?Beshowsthat the higherheatlayercouldnot
0.0020+_0.0009dpmcm-2 d-•. In the summerof 1998the
have been advectedfrom afar or emplacedcumulativelyover
average flux from nine bucket deploymentswas 0.0122 _+
several
0.0070dpmcm-2 d-•.
seasons.
Corresponding to theheatand7Bebeneaththemixedlayer is a peak in oxygen(Figure 3b; profile collectedby E. Sherr).
4.1.
The 7Be givessomeinsight,at leastqualitatively, into the
Thedistribution of ?Becanprovidequantitative information on mixedlayerevolutionbecause featuresof the 7Bedistribu-
history of this as well. First, it suggeststhat there had to be substantialprimary productionearly in the melt seasonwhen the mixedlayerwasdeep(of the order of 50 m). The presence of substantialopenwater, a suggested by McPheeet al. [1998], would have contributed to sufficientlight being available to
Modeling the Mixed Layer Evolution
tion demandcertain aspectsof mixed layer history.For exam-
ple,thepresence of 7Beat a depthof 50 m requiresthemixed layer to havebeen that deep early in the melt season.Furthermore, the mixedlayer had to persistlong enoughat that depth to accumulate sufficient 7Be to be detected months later in
October.In the followingthe 7Be is usedto derivea mixed layer historythat then can be appliedto other tracers,notably
Potential Temp ('C) -1
-2
0
0
1
•
•
2
heat.
Themajorfeaturesof oceanic 7Beprofilescanbegenerated by accountingfor the seasonaldeepeningand shoalingof the mixedlayer on the basisof empiricalobservations of the mixed
10
layerhistory. For a giveninputof 7Bethemixedlayerdepthis a criticalparameterthat largelydeterminesthe 7Be surface activity; shallow mixedlayersconcentrate 7Bewhilein deeper
20
mixedlayersthe concentrationis diluted.The model usedhere is not dynamicallydriven, in that there are no surfaceforcing
terms,butsimplyillustrates how7Be(andothersurfacetracers) will respondto a prescribedmixedlayer history,an input function, and relevant tracer decay terms. With this under50
-
60
-
70 26
standing an observed 7Beprofilealongwithknowledge of 7Be input can yield insightinto the mixed layer history. Details of the model are presentedelsewhere[Kadko and Olson, 1996]. It is basedon the finite differenceform of the •
I
•
27
28
29
30
SALINITY
Figure 2. Temperatureand salinityprofilesfrom October 11, 1997.This profile was typicalof thosetaken betweenOctober 8 and 12.
one-dimensional analyticalexpression governing the 7Bedistribution:0c/0t= gzO2C/OZ 2 - •c, wherec is the concentrationof 7BeandKz is the verticaleddydiffusivity. The 7Beis assumedto be mixed rapidly with respect to its radioactive decay in the mixed layer and is mixed with a finite K z in the pycnocline.As an illustration,the model representationof a
3372
KADKO: MODELING THE ARCTIC MIXED LAYER USING 7BE Be-7 (dprn/rn"5) o
20
o
40
60
80
ml/I 100
120
140
•
0 0
0.2
o
(^)
oxygen above safurafion 0.4
0.6
!
(B)
10
lO
2O
20
3O
5o
[] ß v ß
40
10/8/97 10/9/97 10/11/97 10/12/97
40
50
50
60
I
0.0
0.5
•
I
60
1.0
I
[
I
1.5
DelfoT (øC)
Figure 3. (a)The7Be measurements inOctober 1997 aredisplayed withthe•T profile. Theprofile issimilar to thatexpected fromthemodel (Figure 4c).(b)Thedifference (02 concentration minus 02 saturation) plotted versus depth forOctober 24,1997(datacourtesy ofE. Sherr, Oregon StateUniversity). generic seasonal cycleof mixedlayerdepthisshownin Figure shoalas a resultof heatandfreshmeltwaterinputuntil 4a,whereat sometime(Tmax) themixedlayerisat itsdeepest coolingrecommences anddeepening occurs. (Lmax). In thisexample, Tmax is50 m, andKz for thepycno- As the mixedlayeronceagaindeepens (Figure4c), the clineis assigneda value of 10-6 cm2 s-•. mixedlayer?Beconcentration decreases because of dilution, Monthlyprofiles of ?Beactivity aregenerated asthemixed andtheupperthermoclineactivityistruncated.Thisisbecause layershoals,leavingwaterbeneaththe mixedlayerthat is the?Bedeposited muchearlierin theyearhasnowdecayed isolatedfrom the atmospheric inputI, whichis chosento be away.Thisiswhatwasexpected at SHEBAin October1997,at constant inthisexample. The?Beactivity intheshoaling mixed whichtime the leadsand melt pondswerefrozenandmixed layerincreases, whilethe?Beisolated at depthdecays radio- layerdeepeninghad begun. actively, whichproduces theobserved profiles (Figure4b).In In thisworkthebasicmodelof KadkoandOlson[1996]is the Arctic,in late SpringandSummer, the mixedlayerwill modifiedfor the SHEBA site.The parameters usedfor the Table
2.
Flux Measurements Collection
Sample Bucket
1 Bucket 2 Bucket 3 Snow 1 Snow 2
Collection
Oct. Oct. Oct. Oct. Oct.
Dates
2-10, 1997 3-15, 1997 4-15, 1997 13, 1997 13, 1997
Time,
Flux,
days
dpmcm-2 d-i
7.9 11.8 10.9 21" 21"
Average Oct. 1997 Bucket
4
Bucket
6
Bucket Bucket Bucket Bucket Bucket Bucket Bucket
7 8 9 10 11 12 13
July 5-20, 1998 July 5-20, 1998 July 6-20, 1998 July 12-29, 1998 July 20 to Aug. 8, 1998 July 20 to Aug. 8, 1998 July 20 to Aug. 8, 1998 July 21 to Aug. 10, 1998 July 30 to Aug. 10, 1998
Average JulyAugust 1998 aAssumed snow accumulation time.
15.2 15 14 17 17 17 17 20.1 10.75
0.0020 0.0005 0.0021 0.0031 0.0022 0.0020 _+ 0.0009 0.0092
0.0028 0.0058 0.0233 0.0192 0.0177 0.0127 0.0143 0.0052 0.0122 _+ 0.0070
KADKO:MODELINGTHE ARCTICMIXED LAYERUSING?BE
I
F
10
_1
40
x
50
I
i
I
i
120
180
240
3373
I
_
Tmo• i
6o
0
60
i
.300
360
DAYS o
(Bq IIII I I+1120 I I II III
I•
-
I I I•
20
..••=-"•-•+ 180 t ......__J -80 days .//J/ +60 _. _ t -50days
40
1/i'-ox
lO
ZO days
5o
max I
6o o
-2 -1 -2d-1 Xo.006 dpm cm d I II--0•,006 dpm cm I
100 200
I
300
I
400
I
500
I
600
L
0
I 100
I 200
I 300
i
400
i
500
i
600
Be-7
Be-7 (d pm/m"5)
Figure 4. (a)Themodel representation ofanidealized seasonal cycle ofmixed layer depth where atsome
time(Tmax) themixed layer isatitsdeepest (Lr.ax)' Inthis example theinput of7Be, I, isconstant over the entire seasonal cycle, and Kzinthepycnocline ischosen tobe10-6m2s-•. (b)The7Be activity forashoaling mixed layer. Profiles of7Bearegenerated forvarious times (+ days after Tmax). Asshoaling progresses, the
7Beactivity ofthemixed layer increases, while the?Be isolated atdepth decays radioactively, which produces theobserved tailing. (c)Asthemixed layer once again deepens, themixed layer 7Beconcentration decreases because ofdilution andtheupper thermocline activity istruncated. Thisisbecause the7Bedeposited much earlier intheyear hasnow decayed away. Profiles thatmight beexpected atSHEBA inthefallduring mixed layerdeepening wouldbesimilar to thesemodelcurves.
1997summer-fall in SHEBAaredisplayed in Figure5, andthe
19toSeptember 20,thereisonlyatmospheric input.In stage 3,
of icecover.Themodelisrun model output isshown inFigure 6.Monthly profiles of7Beare thereiszero7Beinputbecause
derived asa function of mixedlayerevolution and7Beinput. until October 11, at which time the SHEBA site was comi.e., isolated fromatmoInitially, it isassumed thatthereisno7Beintheocean follow- pletelyfrozenandsnowcovered, spheric or melt input. Likely, it had been in this stateof the ingthewinterwhentheice-covered upperoceanhadbeen isolatedfrom atmosphericinput.
orderof 3 weeks.Therefore,from September20 to October
Theinputfunction hereissomewhat different thanthatof 11,thereiszero*Beinput(i.e.,thereisonly*Bedecay). fluxis determined asfollows: onOctober KadkoandOlson[1996]in thatinputof 7Beisnotconstant. Theatmospheric 11 the depth-integrated standing stock of 7Be was 0.357dpm Hereit iscomprised notonlyof atmospheric inputbutaddithisto September 20,thestanding stockof tionally, duringlatespring, of rapidinputfrommelting snow cm-2. Correcting
wouldbe0.463dpmcm-2, corresponding thathasaccumulated ?Beoverthewintermonths. The ?Be 7Bebeforeisolation to flux of 0.463X or 0.006dpmcm-2 d-•, whichis theinput inputfunction consisted ofthreestages (Figure 5).In stage 1, used between July 19 andSeptember 20 (stage2). It is then fromJune21 (summer solstice) untilJuly19,?Bewasinput that the atmospheric fluxaccumulating in the snow through atmospheric fluxandsnow melt.In stage 2,fromJuly assumed
3374
KADKO: MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
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Figure5. Parameters usedin theSHEBA?Bemodelasa functionof time.Thethreestages of ?Beinputare shown.
during the winter monthswas the same as during the stage2
Inputtotal= InpUtmelt+ Inputatto
flux,andthusthe standing stockof 7Bein the snowwouldbe 0.463dpmcm-2. Assuming snowmeltoccurred predominately overa month'stime,this7Beis inputinto thewatergradually
= {(0.463/28)[1 - exp(-(X28)]/(X28)}
+ 0.006dpmcm-2 d-•.
overthe 28 dayperiodof stage1. The formulationfor stage1 is
20 /2s • •0 ,m., tm 40
Max ML = 50 m
50
60
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20
40
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80
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140
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Be-7 (d pm/m"3) Figure6. The modeloutputshowing the evolutionof the 7Beprofileat SHEBAwithtime.Data fromthe period October8-12 are superimposed upon the model profiles.The modelprofile for October 11 is shown as a dashed line.
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3375
10
20
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Be-7 (d pm/m"S) Figure7. The modeloutputfor the 7Beprofileat October11 for threedifferentvaluesof Kz in the pycnocline: 0, 10-6, and10-s m2 s-].
Fromthemaximum depthof the7Beit is surmised thatat the 4.2. Modeling the Evolution of Mixed Layer Temperature onset of melting the maximumdepth of the mixed layer was 50 m. This assumes that mechanisms suchasdownwellingwere not a factor, which is reasonable as the SHEBA site was not in
the centerof an anticyclonicgyre. StartingJune 21 (summer solstice)this depth is maintainedfor 12 daysfollowingwhich the mixed layer decreasesto a minimum of 10 m on July 19.
With the 7Beinputfluxuseda late springmixedlayermaintained at this depthfor this time period is requiredto achieve
the 7Beprofileobserved in October.The maximummixed layer depth modeled here is similar to that reported for the CanadianBasinat ice islandT-3 from 1970 to 1973 [Morison and Smith,1981]and for the AIDJEX projectin 1975[Maykut and McPhee,1995]. The effectof differentpycnoclineKz on the modelresultsis shown in Figure 7. These results suggestthat in the highly stratifiedwater belowthe mixedlayer (see salinitygradientin
The evolution of the 7Be distribution in the model described
aboveis similarto the evolutionof bT describedbyMaykutand McPhee[1995].In late winter,there is a deep,well-mixedlayer. After 2 months,spanningthe summersolstice,there is warming throughoutthe mixed layer and fresheningdue to melt water introduced at the surface.By late summer the mixed layer has shoaled,leavingdeeperwater behindthat had been heatedearlier in the season.The maximumin bT is interpreted to be the temperature to which the mixed layer was heated
aroundthe time of the solsticeand,like the deep7Beis a remnantfeature of the deepermixedlayer of late spring/early summer. In a sensethis is a form of subduction in which water,
initially in contactwith the surface,becomesisolatedfrom the atmosphere.The heat is trappedin this layer until subsequent wintertime
convection.
In the modelingshownbelow the temperaturemaximumis Figure2), verticalmixingis not muchgreaterthan10-6 cm2 similarly formed, i.e., as a consequenceof meltwater capping s-• andthat the 7Beprofileis mainlya response to the fall and represents a residual mixed layer. However, it is shown deepeningof the mixedlayer.Higher valuesof Kz would draw that the bT maximumlikely corresponds to the temperatureto more7Befromthe mixedlayerintothepycnocline water. The fluxusedin themodel(0.006dpmcm-2 d-•) liesbe- whichthe mixedlayerwasheatedby mid-July(as opposedto tween the flux determinationsof October 1997 (0.0020 dpm the solstice)andmarksthe depthof the mixedlayerat that time. The parametersused in the temperatureevolutionmodel cm-2 d-•) andJuly-August 1998(0.0122dpmcm-2 d-•). As such, the model value is not unreasonable, but there is no are shownin Figure 8. The model is the sameas that usedfor historical 7Be flux data from this area of the world with which
to compareit. Taken at facevalue, the ratio of the model flux (basedon the summer1997water inventory)to the measured summer1998flux (--•0.5)couldsuggestthat in 1997the summertimelead coveragewasof the order of 50%. This appears
7Beexceptthatthereis no radioactive decaytermandit uses the mixedlayerevolutionderivedfrom the 7Bedistribution
(compareFigures5 and 8). The heat input functionwas derived from the pattern of summerheat content of the water columnduringAIDJEX in 1975 (Figure 9 [from Maykutand large and is certainlybiasedby 7Be input from meltwater McPhee,1995,Figure9]). At AIDJEX (Snowbirdsite),starting runoff. However, the large heat inventorymeasuredin Octo- approximatelyJune 21, the heat contentincreasedat a rate of ber 1997 suggeststhat indeed, substantialopen water must 9.64W m-2 for a periodof 28 days(Figure8, linea). Forthe next39 daysthenetheatincrease wasat a rateof 0.48W m-2 havebeen present.This will be discussed in section4.2.
3376
KADKO: MODELING
•,
THE ARCTIC MIXED
LAYER USING 7BE
0 40 -
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Figure 8. The parametersusedin the temperatureevolutionmodelplottedasa functionof time. The model
usesthe mixedlayerevolutionderivedfromthe 7Bedistribution.
(Figure 8, line b). Beyondthat date, there was a net heat loss 4.3. Modeling the Evolution of Mixed Layer Oxygen
of -4.4 W m-2 (Figure8, linec).To achieve thetemperature and Implications for Primary Production profile observedon October 11, 1997, with this model, the same relative heat flux for each period was used, but the
absolute valueshadto bequadrupled (39,2, and-18 W m-2, respectively). This is consistent with McPheeet al. [1998],who suggestthat the lead openingsin 1997 relative to AIDJEX (1975) tripled, and with observations made at SHEBA in October that indicatedextensivesummertimemelting. These includedthinnerice than anticipated(--•1.2 m asopposedto 3-4 meters),extensivemelt pond (by then frozen) coverage,and melt pondsthat had melted through to the underlyingocean (D. Perovich,personalcommunication, 1998).This is alsocon-
sistentwith the high7Beinventory(relativeto flux) of the
By analogywith the temperatureprofile the peak in oxygen measuredin October 1997 is remnant of spring-summerproduction,while the water aboveit has subsequentlydegassed duringmixedlayer deepening.Neglectingfor the momentloss effectsthrough respiration,the upper 50 or so meters must
havebeenat least0.5 mL L-• or 35 g m-2 abovesaturation. Thisis of the orderof 13 gC m-2 net production but likely representsa minimum because(1) if this productioncommenced in June, then the water column started out below
saturationasa resultof respirationduringthe dark season,(2) 02 is lost throughgasexchange,and (3) respirationoccurred subsequentto the summer.The 1998 spring-summerobserva-
water columnmeasuredin October 1997,whichrequiressub- tions at SHEBA are consistentwith this estimate. Although stantialleadopening for the7Beto entertheupperocean. direct comparisonmay not be possiblebecausethe SHEBA The model output is shownin Figure 10 comparingthe case sitehad drifted far westfrom the October1997location(Figof AIDJEX and SHEBA. It is seen that the &T maximum on ure 2), the June-Julynet productionof oxygensuggested a net October11 corresponds to the temperatureto whichthe mixed community primaryproduction of --•17gC m-2 (B. Sherr, layerwasheatedby mid-Julyandmarksthe depthof the mixed personalcommunication,1998). layer at that time. These resultsare consistentwith the conclusiondrawnby McPheeet al. [1998] that the mixedlayerwas 5. Conclusions of the order of 50 m deepearlyin the summer,wasextensively heatedbefore shoaling,and by Octoberhad givenup its availThispaperreportsthefirstmeasurements of 7Beevermade ableheat.Byusing7Be,similarconclusions areindependentlywithin the Arctic Ocean. The profile from October 1997was drawn. usedto reconstructthe evolutionof the mixed layer over the
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Day of 1975 Figure 9. The pattern of summer heat content of the water column during Arctic Ice DynamicsJoint Experiment(AIDJEX) in 1975 [from Maykutand McPhee,1995].The heat input for SHEBA (1997) was derivedby quadruplingthe slopeof the three linespicturedhere.
KADKO: MODELING
(a)
THE ARCTIC MIXED LAYER USING 7BE
delta T (oC) 0 0
0 1
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3377
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Figure 10. The modeltemperatureoutputfor (a) AIDJEX and (b) SHEBA. Note the changein temperaturescale.It is seenthat the &T maximumon October11 corresponds to the temperatureto whichthe mixed layerwasheatedbymid-July(asopposed to the solstice) andmarksthe depthof themixedlayerat thattime.
previousseasonand confirmedthat the reservoirof heat beneaththe fall mixedlayerwasemplacedin the summer,rather than input cumulatively over severalseasons or advectedin from distantsources.In addition,a reasonablerate of primary
Acknowledgments.This work was supportedby NSF Polar Program (grant OPP-9701067)and by the NSF ChemicalOceanography Program(grant OCE-9809168).The authorwould like to thank J. Morison, M. McPhee, and B. Welch for many valuable discussions in
regardsto thispaperand two anonymous reviewersfor their helpful productionfor the spring-summer periodwas derivedusing comments.Thanks also to E. Sherr and B. Sherr for permissionto presenttheir oxygendataandto M. Stephens for field andlaboratory thismixedlayerhistorywith an oxygenprofilefrom the fall. assistance. Finally, the author acknowledges the crew of the CCGC As suggested byMcPheeet al. [1998],severaltimesasmuch DES Groseilliersand the SHEBA logisticsgroup for their support heatwasemplacedat SHEBA than at AIDJEX 20 yearsear- duringthe field operationof SHEBA.
lier. A likely mechanismfor this is substantially greaterlead openingcoupledwith the positivefeedbackloopbetweenincreasedopenwater,increasedheat absorption,andfurtherice References melting.Note thatwhilethe heatobserved in Octoberwasnot Baskatari,M., C. H. Coleman,and P. H. Santchi,Atmosphericdepositionalfluxesof ?Beand•mPbat Galveston andCollegeStation, cumulative(i.e., the heat is reseteachwinter),a portionof the salinitychangecan be cumulative.Consistentwith observa- Texas,J. Geophys.Res.,98, 20, 555-20, 571, 1993. tionsof accelerating diminishment of Arcticseaice [Johannes- Cooper,L. W., C. R. Olsen,D. K. Solomon,I. L. Larsen,R. B. Cook and J. M. Grebmeier,Stableisotopesof oxygenand natural and senet al., 1995], the changein salinitymight be an ongoing fallout radionuclidesusedfor tracingrunoff duringsnowmeltin an processthat has been occurringover many summersof inArctic watershed, Water Resour. Res., 27, 2171-2179, 1991. Dibb, J. E., Beryllium-7and lead-210in the atmosphereand surface creasedoceanicheat absorption.
3378
KADKO: MODELING
THE ARCTIC MIXED
snow over the Greenland ice sheet in the summer of 1989, J. Geo-
phys.Res.,95, 22, 407-22, 415, 1990. Ingram, W. J., C. A. Wilson, and J. F. B. Mitchell, Modeling climate change:An assessment of sea ice and surfacealbedofeedbacks,J. Geophys.Res.,94, 8609-8622, 1989. Johannessen, O. M., M. Martin, and E. Bjorgo,The Arctic's shrinking sea ice, Nature, 376, 126-127, 1995. Kadko, D., and D. Olson, Be-7 as a tracer of surfacewater subduction and mixed layer history,Deep SeaRes.,Part II, 43, 89-116, 1996. Krishnaswami,S., D. Lal, B. L. K. Somayajulu,F. S. Dixon, S. A. Stonecipher,and H. Craig, Silicon,radium, thorium and lead in seawater;In-situ extractionby syntheticfibre,Earth PlanetSci.Lett., 16, 84-90, 1972.
Lal, D., Y. Chung, T. Platt and T. Lee, Twin cosmogenicradiotracer studiesof phosphorusrecyclingand chemicalfluxesin the upper ocean,Limnol. Oceanogr.,33, 1559-1567, 1988. Lee, T., E. Barg and D. Lal, Studiesof verticalmixingin the Southern
LAYER USING 7BE
Fresheningof the upper oceanin the centralArctic:Is perennialsea ice melting?,Geophys.Res.Lett., 25, 1729-1732, 1998. Morison,J. H., andJ. D. Smith,Seasonalvariationsin the upperArctic as observedat T-3, Geophys.Res.Lett., 8, 753-756, 1981. Rind, D., R. Healy, C. Parkinson, and D. Martinson, The role of sea-icein 2 x CO2 climatemodel sensitivity,Part 1, The total influence of sea ice thicknessand extent., J. Clim. 8, 449-463, 1995. Rubinson, K. A., ChemicalAnalysis, Little, Brown, Boston, Mass., 1987.
Silker, W. B., Beryllium-7 and fissionproductsin the GEOSECS II water columnand applicationsof their oceanicdistributions, Earth Planet. Sci. Lett., 16, 131-137, 1972.
Young, J. A., and W. B. Silker, Aerosol depositionvelocitieson the
Pacificand Atlantic Oceanscalculatedform 7Be measurements, Earth Planet. Sci. Lett., 50, 92-104, 1980.
D. Kadko, RSMAS/MAC, Universityof Miami, 4600 Rickenbacker
California Bightwithcosmogenic radionuclides 32pand7Be,Limnol. Causeway, Miami, FL 33149.([email protected]) Oceangr.,36, 1044-1053, 1991. Maykut, G. A., and M. G. McPhee,Solarheatingof the Arctic mixed layer,J. Geophys.Res.,100, 24, 691-24, 703, 1995. McPhee, M. G., T. P. Stanton, J. H. Morison and D. G. Martinson,
(ReceivedJanuary23, 1999;revisedSeptember10, 1999; acceptedOctober6, 1999.)
