Jun 15, 1997 - Lee W. Cooper, 1,2 Terry E. Whitledge, 3 Jacqueline M. Grebmeier,4, 5 and Tom .... COOPER ET AL. ..... Clarence on the Seward Peninsula.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. C6, PAGES 12,563-12,573,JUNE 15, 1997
The nutrient, salinity, and stable oxygen isotope composition of Bering and Chukchi Seas waters in and near the Bering Strait LeeW. Cooper, 1,2TerryE. Whitledge, 3 Jacqueline M. Grebmeier,4, 5 andTomWeingartner • Abstract. Seawaternutrient,salinity,andoxygen18 datacollectedfrom 1990 to 1993 in the BeringandChukchiSeaswereusedto identifypotentialsources of nutrientsandwatermasses thatresultin formationof theArcticOceanupperhaloclineandits associated nutrient
maximum. Watermatching the•5180 values oftheArcticOcean upper halocline andcontaining sufficient,or a nearlysufficient,nutrientandsalinityconcentration wascollectedin subsurface watersin thesummerin portions of theBeringSea,particularly theGulf of Anadyr.However, nutrientconcentrations significantly declinedin thisnorthflowingwateroverthe shallow continental shelfbeforeit reached theBeringStrait,asa consequence of biologicalutilization, anddilutionwithnutrient-poor andoxygen18-depleted freshwater.Therefore it doesnotappear likelythattheflow of unaltered waterthroughtheBeringStraitin thesummer playsa critical role in theformationof theArcticOceanupperhalocline.The role of othermechanisms for contributingPacific-derivedwatersto the Arctic Oceannutrientmaximumis considered.
which in turn promotes a hydrological regime that is
Introduction
conducive
Nutrient-rich and low-salinity waters flowing northward
to sea ice formation
identification
Arctic
associated with Pacific
nutrient
maximum.
This
nutrient
maximum
climate
[Aagaard and Carmack, 1989; Rudels, 1989]. However, the
through BeringStraitare an important component of the• Ocean
and its associated
and direct measurement
Ocean-derived
of conservative
tracers
waters in the Arctic
coincideswith the upper halocline, between 100 and 160-m depth, characterizedby a salinity of 33.1 practical salinity units (psu), silica concentrationsof approximately45 gM and nitrateconcentrations of 20 gM [Aagaardet al., 1981;Aagaard
Ocean have not been extensiveand have often dependedon indirect inferences. For example, BjOrk's [1990] onedimensional circulation model that explains vertical nutrient
and Carmack, 1989; Jones and Anderson, 1986; Macdonald et
value of one samplefrom the Bering Strait region. Since the salinity associatedwith the upper halocline is higherthan that of any water passingthroughBering Strait in
al., 1989; BjOrk, 1990; Salmon and McRoy, 1994; Melling and Moore, 1995]. The Pacific Ocean-derived waters of the
upper halocline enhancethe strong, permanentstratification between a relatively fresh surface layer (0-100 m) and
underlying salinewater(salinity>34 psu;5180 ~ 0.2 %0)of Atlantic origin in the Arctic Ocean's Canada Basin. The freshwater inflow through Bering Strait, when normalized
against a salinityof 34.8psu,is estimated to be ~1670km3 y1, which is greater than any single fiver discharginginto the
Arctic,andhalf of thetotal3300km3 yr-1 fiverrunofffor the entireArctic basin [Aagaardand Carmack, 1989]. Bering Strait inflows are thereforeimportantin controlling Arctic Ocean nutrient budgets[Codispotiand Richards, 1962] and in maintaining the stratification of the Arctic Ocean, 1Environmental Sciences Division,OakRidgeNationalLaboratory, Oak Ridge, Tennessee.
2AlsoatDepartment ofEcology andEvolutionary Biology, Universityof Tennessee, Knoxville. 3Marne ScienceInstitute,Universityof Texasat Austin.
4Department ofEcology andEvolutionary Biology, University of Tennessee,Knoxville.
5Alsoat Environmental Sciences Division,Oak RidgeNational Laboratory,Oak Ridge,Tennessee. 6Instituteof Marine Science,Universityof AlaskaFairbanks.
Copyright 1997bytheAmerican Geophysical Union. Papernumber97JC00015. 0148-0227/97/97JC-00015509.00
distributions in the ArcticOceanusesthe salinityand•5180
summer [Walsh et al., 1989], increases in Chukchi Sea salinity during sea ice formation in the winter have been assumedto be importantin the modificationof Pacific Ocean sourcewaters north of Bering Strait [Coachman and Barnes, 1961]. These brine-enhancedwaters sink and come in contact with the seabedbefore flowing off the continentalshelf. As a result, Jones and Anderson [1986] and Moore et al. [1983]
suggestedthat nutrients released from Bering and Chukchi shelf sedimentscould play a role in developmentof the Arctic Ocean
nutrient
maximum.
These
sediments
are a source of
nutrients, particularly inorganic nitrogen produced by sediment microfauna [Lornstein et al., 1989; Henriksen et al., 1993] and macrofauna [Grebrneier et al., 1988; Grebrneier and
Barry, 1991; Highsmith and Coyle, 1990]. Consequently,to determine how the Arctic Ocean nutrient maximum is createdrequiresunderstanding how Pacific Ocean waters are modified by salinity (density) changesfollowing sea ice formation and nutrient exchangebetweenbottom sediments and bottom waters flowing off the continental shelf. We expectedthat stableisotopemeasurements of Bering and Chukchi seawaters, in conjunction with measurementsof salinity and nutrients, might overcome these difficulties and provide insights into the fate of waters and entrainednutrients flowing through Bering Strait. This expectationwas based
uponresultsof previoussurveysof 5180 valuesof bottom 12,563
12,564
COOPERET AL.: BERING STRAIT TRACER DISTRIBUTIONS
Table 1. Origin of Water Samplesand Methods for Nutrient Analyses Ship,CruiseNumber
Locationof Sampling
Date
Nutrient Analyses
RV Alpha Helix, 139
St. LawrenceIsland
June 1990
RV Khromov
Chukchi Sea
Surveyor RV Alpha Helix, 165
ChukchiSea AlaskaChukchicoast
September1990 September1990 August-September 1992
RV Alpha Helix, 166
ChukchiSea
September-October1992
RV Alpha Helix, 171
Bering Sea/Strait
June 1993
onboardautoanalyzer onboardautoanalyzer onboardautoanalyzer shipboardand lab analysesfrom frozen samples lab autoanalyzedfrom frozen samples lab autoanalyzedfrom frozen samples
watersin the Bering Sea and limited samplingin the Chukchi Sea [Grebmeieret al., 1990; Grebmeierand Cooper,1995]. The presentdata are derived from samplescollectedon sev-
was derivedfrom conductivity-temperature-depth (CTD) offsets applied following each cruise, basedupon comparisonswith bottle
salinities,
eral cruisesandprovidea description of the 5180 valuesof waters of the Bering and Chukchi Seas. We have combined these analyseswith salinity and nutrient concentrationsof silica, phosphate, ammonia, and nitrate/nitrite. Waters were sampledin the Bering and Chukchi Seas, as well as a small portion of the Beaufort Sea in the vicinity of Barrow Canyon. Our main goal has beento evaluatethe apparentchangesin the salinity, nutrient, and oxygen 18 contentof Bering Sea water as it passesinto the ChukchiSea. Two other questionswe soughtto answerin analyzingthese data were the following: 1. Is ammoniumin bottom water over Bering and Chukchi Sea continental shelf sediments a significant source of regeneratedinorganic nitrogen for maintenanceof the Arctic Ocean nutrient maximum?
2. Can the use of freshwater end-member regressionsof
salinity-/5180separatethe contributions of sea ice melt, freshwater entrained in Bering Strait inflows, and direct river contributionsto the Arctic Ocean surfacelayer?
Results Summer nutrient concentrations on the Chukchi shelf and in
the Bering Strait are less than thoseof the nutrient maximum of the Arctic Ocean.As an example,the 1990 Khromovcruise data for nitrate and silica, (Figures1 and 2) showthat concen-
trationsof silica greaterthan 30 gM were observedat only threeChukchiSea stations,which rangedin depthfrom 74 to 105 m (plus symbolson Figures1 and 2). Thesehigh concentration waters were geographicallydisjunctfrom the nutrientrich plumeof water flowing northwardthroughBering Strait, observablefor both silica (Figure 1) and nitrate (Figure 2). Due to sea ice conditions,only three stationsdeeperthan 105 m were sampled on the Khromov cruise (boxed circles on Figures 1 and 2, deepest station, 168 m). Silica and nitrate concentrations decreasedbelow 105 m, indicatingthat the 74to 105-m depth range defines the depth of the nutrientmaximum at the time of sampling. Salinity below 105 m also in-
creased above34.0psu(Figure2), and•5180valuesincreased above-0.3%0(Figure 1). Theseincreasesare associatedwith a decline in silica (Figures 1 and 2) and thereforeindicate the Water samples were collected and nutrient analyses presenceof water at these depthsof Atlantic origin. Samples performed between June 1990 and June 1993 as detailed in from portionsof Barrow Canyon(20 BM nitrate concentrasalinitymeasurements was+ 0.001 psu,while accuracy of the tion observedin the upper haloclinewatersthat were sampled measurements wasbetterthan0.01 psu. The accuracyestimate (Figure 5). Methods
COOPERET AL.: BERING STRAIT TRACER DISTRIBUTIONS
12,565
>30 +••..1•20
Bottom water CHUKCHI SEA
•,--•---'""O • (•),' ' 10 - •'••"'• ß,
.
ß ß•
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>10 ',.•:.•':'•'•:.;':
5-10
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ß'
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ALASKA
69øN
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=:•=. :•.:::=•::: ::=:,•;•::•.•]':...
67øN
.='•:i:•ering• rai ß 175øE
180ø
175øW
[] 170øW
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ß
.
I
I
ß
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-zo••
[]•
165øW
u./•
Atlantic influenced
w.,er ß -10
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ß
•
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'•-.....,......• 71øN
ß
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160øW
w +0.2 to -0.3
-05
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c.ukc.• S•A -•.o. ...... cast •lDerlan
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Bottom water
.H=;'•:'.'
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. :.!::. ;'-:;;:.:.' .. .. j.....?:•i: •.•;:'• ::'•'"'
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ß• '"•2ß ß.. :::.:..:.;.:.
':" A•K•A 69øN
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67øN
,
'
Figure1. (top)Silicaconcentrations and(bottom) 5180 valuesfor bottomseawaters collected on RV Khrornov,September 1990.Deeperwatersamplesare 74-105m (plusses) and 130-168m (boxedcircles).All otherdepthsare lessthan74 m.
Therelationship between salinityand•5180valueswaswell
Discussion
correlatedfor many individual categoriesof samplescollected (Figure 6; Table 2). However, the categoriesof samplescluster along mixing lines with different Y intercepts, indicating mixing from several end-membersourcewaters. These sources include seawater influenced by sea ice melting, brine injection, and meteoric contributions.
This study seeks to define the potential mechanismsby which Arctic upper halocline water acquires its specific nutrient, salinity, and stable isotope content.Typical nutrient concentrations in Bering Sea summerwater that is transported north throughBering Strait [Walshet al., 1989;Hansell and Plots of subsurface silica and nitrate concentrations relative Goering, 1990; Whitledgeet al., 1992; Hansell et al., 1993] to salinityand•5180valuesalsoshowthemixingof Chukchi are lower than nutrient concentrationsin the 33.1%o salinity water associatedwith the Arctic Oceanupperhalocline:20 pM shelf water of Pacific origin and deeperArctic Ocean water of Atlantic origin; the latter has comparativelyhigher salinity, nitrateand 45 pM silica [Macdonaldet al., 1989 and references
higher•5180values, andlowernutrient concentrations (Figures
cited therein].
7, 8, 9, and 10).
Chukchi
Bottom water nutrient concentrations on the
shelf are consistent with concentrations
observed in
12,566
COOPERET AL.' BERING STRAIT TRACERDISTRIBISTIONS 12
Initrate (pM) I
15
bottom water CHUKCHI
7
3
,
SEA
71øN
33 0 '
(•)ß
ß
ßß
''
ß
.,•_3•'20 ' "•. '!'.':"::![?.".:½',:!6.:',.• _ '/
•'•'•....•
/ ß
32..0
' ß'
"•
..• 71 øN
....'?; .........:%..ß
/ •,::•.'•i":-"•
' '. •
32.0 to33.0 ! /
ß
•
' ß
ß
31.0.__,•' /32.0 to33.0 .
3({•
160øW
•'••.,•. , 34.0 ß', 32.9
33.0 \ 32.0
165øW
67øN
.-•::!:'!" ALASKA -
69øN
..
'
'' 67øN
CHUKCHI PENINSULA ?iiii..r•i!i?:[!." '";:::;!:....."• •I•'•/./•..']:;,!::i?:•.: :::::::::::::::::::::: ' "!'!:i:;
.
,• ::.:::::.:'
j
Figure 2. (top) Nitrateconcentrations and(bottom)salinityfor bottomseawaters collectedon RV Khromov, September 1990.Deeperwatersamples are74-105m (plusses) and130-168m (boxedcircles).All otherdepths are less than 74 m.
summerwater transportedthroughBering Strait (Figures1 and 2)[Walsh et al., 1989; Hansell and Goering, 1990; Whiffedge et al., 1992; Hansell et al., 1993]. This past work, as well as our current study, has not produced any evidence for a continuouspath for nutrients acrossthe Chukchi Sea shelf during the summer that would permit formation of the peak nutrient concentrationsobservedin the upper halocline. Depth of Origin of Pacific-Derived and Transport Timescales
Nutrients
High nutrientwatersupwelled onto the Bering Sea shelf are depletedin nutrientsduringthe summertransitthroughBering Strait [Walsh et al., 1989; Hansell et al., 1993]. Although the
nutrient concentrations of water transported through Bering Strait in winter are poorly known, it can be reasonably assumedthat nutrientdepletiondue to biological activity is minimal [Hansell et al., 1993]. Moreover, salinities are higher due to the decrease in freshwater
runoff
contributions
and in-
creasingbrine injection from growing sea ice [Schumacheret al., 1983; Muench et al., 1988]. The initial nitrate and silica concentrationsof waters upwelled onto the Bering shelf in the
summer,before any biological depletion, might approximate the winter nutrient concentrationsin Bering Strait once freshwater dilution is taken into account[Hansell et al., 1993]. By plotting our silica and nitrate data from Bering Sea continental
shelfbottomwatersagainst •5180values(Figures 8 and10)(see also Grebmeier and Cooper [1995]), it is clear that water with
COOPERET AL.: BERING STRAIT TRACER DISTRIBUTIONS
12,567
o
o lOO
1 oo
200
200
300
300
400
400
27
29
31
33
35
•
o
o •
o
100
•'.1 •- •
•
200
300
300
N•trate
(•M)
0
--'
60
ß
400
10
,
i
ß
ArcticOcean
N
,
I
0 HX
165-017
20
-2100 -1.50-1.00 -0.50 0.0
HX165-028 HX165-032'(404 m) •m I ...J-IX
165-034
(315 m)
(3•o m)
71o30,N •I I
40
100
200
400
20
I
*HX 165-042 (145 m)
/
t
/•,•nt Barrow
(30m, /,.r.,_ • L•
•j•.j.,,.2N
.
1 60øW
1 58øW
1 56øW
1 54øW
Figure3. Salinity,silica,;5180values,andnitrate(clockwise fromtopleft) profilesfor watersamples collectedin BarrowCanyon,September 1992,from RV Alpha Helix, Cruise165. The bottommap shows stationlocations,with symbolsfor corresponding data.
the nitrate concentrations(20 gM), silica concentrations (45
I•M), and6180 values(-1.1%o) corresponding to theArctic
33.1%o and the appropriate silica and nitrate concentrations can be identified (Figures 7 and 9) relative to water samples
Oceanupperhaloclinecorewater[Macdonald et al., 1989]is
withtheappropriate ;5180valuesandnutrient content (Figures
found in summerin the Bering Sea. These samplesare col-
8 and 10). In winter, higher salinitiesare observedas a result
lectedprimarilyin the Gulf of Anadyrat depthsof lessthan of decreasedfreshwater inflows and brine rejection during sea 100 m, wherenutrient-richwatersare first advectedup ontothe northernshelf. Salinitiesof 33.1 psu (equivalentto the upper
ice formation.As a generalcase,northernBering Sea bottom salinities in winter are elevated as a result of sea ice formation
[Schurnacheret al., 1983; Muench et al., 1988]. Roach et al. [1995] reported wintertime salinities in Bering Strait specificallyto be as high as 34.8 psu. We suggestthat some of this higher-salinity winter water passingthrough Bering Oceannutrienttracerbecausenitrate and phosphatehave other Strait will reach the Chukchi shelf break prior to the onset of significant sources in the ArcticOcean,e.g.,Atlantic-derived melting and signficantprimary production. We estimatethat deeperwatersandnitrifledammoniareleased fromcontinental the minimum transit time required for winter water to traverse
haloclinesalinity)are alsofoundin the Gulf of Anadyr,but at depthsgreaterthan100 m [Grebrneier et al., 1990],andwith higher nutrientconcentrations (nitrate --30 pM, silica--70 BM) [Whitledge et al., 1992]. Silicais themostusefulPacific
shelf sediments. Nevertheless, there is a modest enhancement
of silica from river sourcesin the Chukchi Sea (Figures7 and
the Chukchi
This
shelf is 3-4 months.
estimate
is
based
on
several
sources.
First,
Weingartneret al. [1997] estimatethe advectivetime between Kotzebue Sound. Fewer indications of freshwater contribuBering Strait and Barrow Canyon to be 2-3 months. tionsof silica can be seenin the Bering Sea (Figures7 and 8, Preliminary inspection of recently obtained current meter top andmiddlepanels),exceptin a few surfacewatersample records from Herald Valley, between Bering Strait and the Chukchi shelf break (and earlier records from K. Aagaard, collections made near the Yukon River delta and in Port Clarence on the Seward Peninsula. personal communication,1990-1991) suggesta minimum For waterstransitingnorth throughBering Strait, salinity transit time of 2 months to Herald Valley. Assuming a mean
8, bottom panel), correspondingto samplescollected in
flowof7 cms'1 along theseavalley axisimplies a is moresubject to modification than;5180values because sea northward fluid parcel transporttime of 1.5 monthsto reach the shelf oxygenisotopecomposition.This is apparentin the data; break from Herald Valley (-300 km). On the basis of a fewer Bering Sea water sampleswith a salinityapproaching temperature time series, [Weingartner et al., 1997], we ice formation has a much larger impact on salinity than on
12,568
COOPERET AL.' BERINGSTRAITTRACERDISTRIBUTIONS Sediment
[]
o
o
ß" J..•
ß [] []1ß
ß ß 26.0
ß
ß ß
24.0
,
2
4
6
o
ßo
[3 ß []
o. o iDi
I
o o
[]
[3
0
o
ß surface,BeringSea o bottomBeringSea, 150 rn []
-4.0
o
[3
0 •' -3.0
trient concentrations. At least in the case of ammonium, our
data (Figure 4) support this suggestion. The source of this higher bottom water ammonium is likely to be benthic sediment metabolism [Grebmeier et al., 1988, 1989; Lornstein et al., 1989; Highsmith and Coyle, 1990; Henriksen et al., 1993]. However, even after including these contributionsfrom regenerated organic nitrogen, waters transiting the Chukchi Sea shelf in the summer do not contain enough inorganic nitrogen to fully accountfor the >20 BM nitrate concentration observed in the upper halocline waters that were sampled (Figure 5). For all Bering and Chukchi Sea bottom water samples
n
ß..•,•-.
28.0
Nutrients
Strait into bottom waterscan contributeto upper haloclinenu-
[]l
ß
of
Moore et al. [1983] and Jonesand Anderson[1986] suggestedthat nutrientreleasesfrom sedimentsnorth of Bering
34.0
30.0
Contributions
(/•M)
Figure 4. Concentrations of ammoniumfor varioustypesof
watersamples relativeto (top) salinityand(bottom)8180 values.
mean
concentration
of ammonium
in bottom
shelf
waters, regenerated nitrate derived from benthic sediment metabolismin the Bering and Chukchi Seas would contribute abouthalf of the remainingnitrate requiredin order to increase the nitrate concentrationsobservedat a depth of 64 m in the Gulf of Anadyr (-15 BM) to that observed in the upper halocline of the Arctic Ocean (-20 BM). Alternately, if the nitratein the upperhaloclineis derivedfrom Bering Sea waters only 10 m deeper,at a depthof 74 m in the Gulf of Anadyr,no significant contributions of nutrients from sediments are required. Although Atlantic-derived waters are another potential nitrate source, these waters have lower core nitrate (13.8 gM) and silica (8 gM) concentrations,as well as higher
8180 values[Macdonaldet al., 1989]. Atlanticwatersare apparent as secondary,independentmixing lines for deeper Arctic Ocean water samples (inverted darkened triangles) on bottompanelsof Figures9 and 10.
estimate that the advective timescale between Bering Strait and the central Chukchi shelf (-71ø N, 170ø W) is -2 months.
Recently recoveredcurrent meter recordsfrom the central Chukchi shelf north of this point suggestthat a water parcel wouldrequirean additional2 monthsto reachtheshelfbreak. These estimates are based upon limited current measurements anddo not adequatelydescribethe magnitudeof interannualvariabilityin currentcirculation.Nevertheless, all water parcel transit estimatessuggestthat a portion of northernBering shelf winter water crossesthe Chukchishelf prior to the onsetof ice melt. Moreover,duringthe spring months,low-salinity meltwateris surfacestratified,so nearbottom waters will retain their winter temperatureand salinity propertieseven as melting commences.
TOTAL INORGANIC
NITROGEN (gM)
Jeg 1-KROMOV, ] 1990
CHUKCHI SEA
71 ø
....'1 69 ø
For bottom waters collected on the continental shelf of the
' :'•!i==;:,:.... ß
Bering Sea during summer,simpleregressions betweenthe -• sampledepth and silica and nitrate concentrations indicate that a depthof 65 to 75 m approximates the level at which45 gM silicaand20 gM nitratecan be expectedin the northern BeringSea,particularlyin thoseareasinfluencedby the high nutrientAnadyr Current.For silica, depth(meters)= 28.98 +
•
ß•:•'ii!.:Y=:::•.. =-:•;.r
'::%,..:.•.. ..'
..'.
'•'•...'•i•2i!•;!i;;..,.:,:'....../..... ) ....
ß.':•,.:..:•..,q, •_'$ ... ß ß ß ß ........
.g...."..
.•[ ':%.=...•'•l =!::,!;,•., .'C•.... •'•--?:..i.;'! .' •CHU ß : '' •...,=;,J,ji: ."'=J';"'"i']'hh??= ..•. :-r. KCHI PENINSULA '::!ii'•i•[ ""!••.,,• ?"'"-" ....
0.81' [silica](r 2 = .34;n = 101;p < 0.0001;thestandard er-
175ø
180ø
175ø
170ø
165ø
67 ø
160ø
0.0001; the standarderrror associatedwith the Y interceptis
Figure 5. Total inorganic nitrogen (nitrate+nitrite+ammonium), in bottom waters, collected on RV Khromov, September1990. Deeperwater samplesare 74-105 m (plusses) and 130-168 m (boxed circles). All other depthsare less than
+2.4 m).
74 m.
ror associatedwith the Y intercept is +3.4 m). For nitrate,
depth = 32.52+ 2.09* [nitrate] (r2 = 0.48,n = 102,p
150 m) Bering Sea shelf bottom (, 30 Bering Sea (Table 2) suggeststhat Bering Sea continental shelf bottom waters do not contain any significantcomponent • z8 of melted sea ice. The freshwatercomponentalso appearssim-
ilar in oxygenisotopecomposition to the weighted5180
z8 Chukchi Sea waters (except surface)
value for inland precipitation in the greater Arctic Ocean.
Although thestandard errorassociated withtheY intercept for thesedata is +2.3%o,this weight-averaged •5180valueof -21.1%o is close to the freshwater
contribution
of the Yukon
River,thelargest riverentering theBeringSea(/5180valueof ~-22%o [Cooper et al., 1991]). It is also within the standard error of the estimate Kipphut [1990] obtained for Gulf of
z4
0
,
10
20
30
i
40
50
nitrate (gM)
Figure 9. Salinity and nitrate relationships for several categories of (top) subsurfaceBering Sea samples, (middle) surface(0-2 m) Bering and Chukchi Sea samples,and (bottom) nonsurfaceChukchi Sea samples.
COOPER ET AL.:BERINGSTRAITTRACERDISTRIBLrIIONS
12,571
northward.Thus salt-enriched shelfwatersare exportedto the
Chukchishelfrelativelyquickly.In contrast,the prevailing
o
northerly winter winds push newly formed ice southward,
x(•O O •
0
where it melts on the southeastBering shelf. Sluggish northerlydrift [Kinder and Schumacher,1981] eventually
-2
carries some or most of this meltwater back onto the northern
-3
Bering shelf. Therefore contemporaneousmodification of shelf waters by both sea ice melt and formation occurs in winteron the BeringSeashelf.However,the productsof these processes of seaice formationandmelt transitBeringStraitin
Bering Sea waters (except surface) ,
,
lO
20
,
,
....
ß surface,BeringSea o bottomBeringSea, 150 m -1
[] bottom, ChukchiSea, 150 m
tions were made in June 1990 and 1993 south of St. Lawrence
x Unimak Pass
O•
-3
ß surface Chukchi Sea
Island, shortly after dissolution of ice cover, so these esti-
•, Arctic Ocean, 100-150 m
matesof seaice melt contributions in Bering Sea surfacewa-
O BeringStrait
ters are probably upper limits. The shortresidencetime of sur-
+ AnadyrStrait
facewaterson the northernBeringshelfsuggests thatthe sea ice melt component diminishesduringthe restof the summer.
Bering and Chukchi Sea surface waters 10
20
30
--Atlantic
40
The /5•80 (Y) intercept for the freshwater end-member in-
mixingline
creasesin ChukchiSea continentalshelf waters(150 m, also indicates water sampleswere collected in coastalAlaska waters between
thatthesewatersarenota majorcomponent of watersflowing theSewardPeninsula andBarrownearmeltingseaice. over the shelf and throughBering Strait. At the depthof the Arctic Oceanhalocline(100-150 m), The/S180-salinity relation forBeringSeacontinental shelf ChukchiSeawatershavean apparentfreshwater/5•80 value surfacewatersindicatesinfluenceby seaice melt becausethe interceptof-21.8%,,. This is a reversalof the geographical freshwaterend-memberisotopic compositionincreasesto (southto north)trendtowardmorepositive /S180-salinity in-19.1%,,. Thereis no masonto expectthatriver inflowsin the terceptsobservedin Bering to Chukchi Sea continentalshelf northBeringSeaarelessisotopically depleted in •80 (i.e.,ex- waters.Thistrendtowardmorepositive/5•80freshwater endhibitlessnegative/5•80values)thanriver contributions fur- membersis drivenby increasingice melt influencein a south
thersouth. Therefore thedecline in the/5•80valueof theap- to north direction,as well as greaterseaice melt influencein parentfreshwaterend-memberin Bering Sea surfacewaters shelf surface waters relative to the subsurface. One mechanism mustbe largelyattributable to seaice melting. If we assume for reversing, anddecreasing, thefreshwater/5•80 intercept at thatmeteoricfreshwater contributions to the northBeringSea the upperhaloclinedepthwould be mixing with watersof
thatarefreeof seaicemelthavea weight-averaged/5•80 value Atlantic origin found below the halocline. Another of --21%o and use estimated/5180 valuesof -2 to -3%ofor mechanism wouldbe the additionof BeringSea shelfwater melted sea ice, then the fresh water componentof surface Bering Sea sampleswe have collectedhas itself a sea ice melt
thathasa similar freshwater /5180_salinity intercept (--21)
component of between11 to 16%. The remainingfreshwater
andhascrossed theChukchishelfwithoutmixingwithseaice meltwater.The latter mechanism is likely to occurin winter
is contributedby river runoff.
andwouldbeconsistent withthescenario described previously
(Notethatthismeltwatercontribution to BeringSeasurface for transportingnutrientsat the appropriateconcentrations wateris consistent with Pease's[1980]description of Bering andsalinitiesinto the Arctic Oceanupperhalocline. Sea ice processes. Ice productionis vigorouson the northern Craig and Gordon [1965] provided a freshwater/5•80 shelf, particularlyin the Gulf of Anadyr and Chrikov Basin interceptestimateof-20.6%,, for high-latitudeNorth Atlantic [Cavalieri and Martin, 1994], where the flow is swift and surface waters(