Jan 15, 1995 - construct a freshwater budget for the Mackenzie shelf, the nearshere within which ... Table 1. the Shelf. Inflow of the Mackenzie River and Area of ..... 0. 10. 20. 30. 40. Salinity (psu). Figure 4. Salinity versus 5180 for the water column data collected in ... Kugmallit Bay, 181-182 cm (n - 24) at Sachs Harbour,.
JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. C1, PAGES 895-919, JANUARY
15, 1995
The freshwater budget and under-ice spreading of Mackenzie
River
water
in the
based on salinity and and
Canadian
Beaufort
Sea
measurementsin water
ice
Robie W. Macdonald, David W. Paten, and Eddy C. Carmack Institute of Ocean Sciences, Sidney, British Columbia, Canada
Anders
Omstedt
Swedish Meteorological and Hydrological Institute, Norrk;Sping
Abstract. Observations of salinityand oxygenisotopecomposition (5180) were made for the Beaufort shelf-Mackenzie estuary waters in September 1990, just prior to ice formation, and for both the water column and ice in April-May 1991, at the end of winter. These measurementsare used to determine the apportioning of fresh water in the estuary between its two main souxces,runoff and sea ice xttex[. •nanges in dispositionof water between seasonsand amounts frozen into the growing ice sheet are also derived. Two domains are consideredin order to constructa freshwaterbudget for the Mackenzie shelf, the nearsherewithin which
landfast ice growsin winter and the outer shelf. Most of the winter inflow from the Mackenzie River appears to remain impounded as liquid under the ice within
the landfast zone at the end of winter, and about 15% of it is incorporatedinto
the landfastice. Oxygenisotopes (Slso) in ice corescollected from acrossthe shelf record the progressbeneath the ice of new Mackenzie inflow as it invades the
nearsherethroughoutwinter. Rates of spreadingare about 0.2 cm s-• away from the coastand 1.3 cm s-1 alongthe coast. As this inflow spreadsacrossthe shelf, it progressivelyshuts off convectiondriven by brine production at locations within
the landfastice. Salinityand 5•80 in the offshorewatercolumnsuggest that about 3 m of sea ice was formed in the outer shelf domain. Since both brine and newly formed sea ice can be advectedoff the shelf, a completebudget for brine or sea ice production cannot be establishedwithout first measuring the advection of one of these two components.
1. Introduction
posedlarge-scaleoil developmentboth in the Canadian
Recent oceanographicstudies of the Arctic tend to be motivated by issuesrelating to climate changeand contaminants. Within the Arctic Ocean the relatively dynamic margins, which make up about a third of the surfacearea, are arguably the most important regions, both for their physicaland chemicalprocessesand their biology. It is at thesemarginsthat changesin ice thicknessand duration of the ice seasonare expected to be most pronounced,should global warming occur. The shelvesare also the logxcalentry point for many contaminants becauserivers drain into them, becausethey provide large open areas with which to exchangewith the atmosphere,and becausethey are the sites of pro-
and Russian sectors of the Arctic. Processes that control the under-ice
flow of river wa-
ter in major Arctic estuariesstronglyinfluencethe wintertime oceanographyand transport pathwaysover the
shelves[Macdonaldand Carmack,1991].Fewmeasurements of the behavior of river water plumes under ice have been made owinglargely to instrument limitations
at the slowspreadingvelocitiesinvolvedlingyam,198!; Ingram and Zarouche,1987]. In the presentpaper we use observationsof oxygen isotope ratios and salinity to determine the evolution and spreading rates of the under-ice plume arising from the Mackenzie River inflow to the Beaufort shelf from October 1990 to May 1991.
Arctic shelves are important sources of both posiCopyright1995by the AmericanGeophysical Union.
tive (f•e•h w•t•)and negative(brine)buoyancyfluxes [Macdonaldand Carmack,1991].On the shelves,input
Papernumber94JC02700.
of fresh water by river inflow, ice melt, and net precipitation increasesthe averall stability of Arctic sur-
0148-0227/95/94JC-02700505.00 895
MACDONALDET AL.: FRESHWATERBUDGET OF MACKENZIERIVER
896
Table 1. the Shelf
Paramete• Total Mackenzie
only the fourth largest river entering the Arctic Ocean
Value
River flow 1990
MackenzieRiverinflow(October-May) River waterfrozen(winter) Net inflow(winter) Total
Arctic shelves[Macdonaldet al., 1987; Thomaset ai., 1986;Aa#aardand Carmack,1989]. The Mackenzieis
Inflow of the Mackenzie River and Area of
Area of shelf
Area of landfast ice Area of outer sheff
(Table 1) (seealsoAa#aardand Carmack[1989]),but
sincetheadjacent shelf.•srelatively small(64,000 km2,
300 km $ 73 km s 11 km s 62 km s
Figure 1) the annual freshwateryield for the shelfis about 5-6 m [Brunskill,1986;Macdonaldet al., 1987]. Becausethe Mackenzie ILlvet draws on large headwater
64,000km2 16,000km2 48,000km2
lakes,the winterdischarge remainshigh (3500m3 s-x) and delivers amounts of water comparable to the Ob
(4000m3 s-x) andYenisey(5500m3 s-x) (V.K. Parlor and S.L. Pfirmen, Hydrographicstructure and variability of the Kara Sea: Implications for pollutant distri-
bution, submittedto Deep Sea Research,1994). De-
facewaters,therebypromotingwinterice growthand suppressing deepconvection, both in the centralArctic and, possibly,by advectionout of the system,in the NorthAtlantic[Aagaard and Carmack,1989].At the sametime,cold,brine-enriched watersproduced by freezingseawater serveto coolanddeepenthe Arctic halocline,therebyfavoringfurther ice growthby sustaininga thick,cold,insulating layerto shieldthe surface,whereiceis formed,fromthe deeperwaterwhich contains heat[Aagaard et al., 1981;MellingandLewis, 1982].
spite this strong, year-round supply of buoyancy the Mackenzie shelf is known in certain winters to produce sufficient brine-enhanced water from sea ice formation
to ventilate the offshorehalocline[Melling and Lewis,
1982;MellingandMoore,•995]. Two circumstances appear to be critical for this to occuras follows:first, the upperlayersmust be preconditionedby removalof freshwatercomponentsprior to freeze-up[Macdonald et al., 1989; Melling and Moore, 1995; Omstedtet al., 1994];and second,the dilutinginfluenceof new winter inflow of river water must be blocked from areas fa-
Just as the Arctic Ocean, itself, acts as an estuary vorableto brine enrichment[Macdonaldand Carmack, for the world ocean, so do the shelvesact as estuarThe shelves,which behave as a short-term buffer, ies for the Arctic Ocean, and the Mackenzieshelf of the Canadian Beaufort Sea is the most estuarine of all modify fresh water before it is exported to the Arc-
71øN
ß 30 ß 29
200m
ß 50
ß28
ß 52 70 ø
ß:24 ß 23 ß 22 ß21
49
38 ß36 35
69 ø
140øW
135ø
130 ø
Figure1. Bathymetry andstation locations for(a) thefallof1990showing thelocation ofthe packiceat thetimeof sampling (patterned area)and(b) thespring of 1991showing thelarge flawleadthat startedto openin late Februaryof 1991(cross-hatched area).
MACDONALD
ET AL.:
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
897
50 m
(b) 71øN 200 m
CD-4' CD-3'
C-6 ß
CB-7 CB-40
CD-2 ' L-3
50 rn
70 ø
'5 ' CM-4 ß
PI-•
L-1 ß
PI-2 ß PI-!
ß GI-2
AP-3. AP-2 ' AP-1
' CM-3
CM-2 ' CM-I'
ß GI-1
....... ktoyaktuk
140øW 130 ø
135 ø
Figure 1. (continue. d)
tic interior water
ocean.
on Arctic
Therefore
the residence
shelves and within
time of fresh
the Arctic
halocline
progresses,the shelf clears of ice so that by September
the edgeof the permanentpack is well offshore(Fig-
[(•stlund, 1982;M•cdon•ld et •l., 1989;Schlosser et •l., ures la and 2a). During the open-waterseasonthe 1994] and the effect of freshwaterrunoff on the heat Mackenzie plume dominates the nearshore. Autumn budget of the Arctic Ocean and global thermohaline (Figure 2b) is characterized by decliningriver inflow circulation have been• and will continue to be, impor- and intensewinds[Fisseland Melling, 1990]whichtotant topicsof investigation(see,for example,the new gether produce well-mixed, relatively saline nearshore program proposedby the Surface Heat Budget of the water [Carmacket al., 1989]. New ice usuallybegins
ArcticOcean(SHEBA) ScienceWorkingGroup[1994]). to form in October(Figure 2c) with relativelysmooth Models
of Arctic shelves tend to treat them as a homolandfast ice occupyingthe region inshore of the 20-m geneouscompartment into which runoff is added, ice isobath(Figure lb). The mostimportantattribute of formed or melted, and for which there is some charac- the landfast ice in the context of this paper is that it reteristic residencetime or flushingtime [cf. Macdonald mains in place throughout winter, although ridges may et al., 1989]. Recently,Omstedtet al. [1994]modeled form within it. At the beginningof the ice-growingseafreshwater cycling on the Mackenzie shelf by parame- son, ice forming in shallow regionsnear the mouths of terizing the fluxes of river inflow, ice advection, wind- the Mackenzie River incorporates fresh water from the driven transport, and vertical mixing. Because this plume which is spreadingbeneath it, while farther offmodel uses only a single, vertically resolvedreservoir shore, sea ice formation is accompaniedby brine rejeccoupled to the open ocean, it cannot describe the lat- tion (Figure2c). Duringwintera flawlead opensspoeral transport within the estuary. To construct a model radically at the outer edge of the landfast ice, usually that allows a shelf simultaneouslyto accept fresh water at about the 30-m isobath, in responseto wind-driven and producebrine requiresunderstandingthe distribu- movementof the offshorepackice (Figures2c and 2d)
tion of freshwaterwithin the system(ice,ice melt, and [Giovandoand Herlinveau•, 1981]. The flaw lead is a river runoff) and the spatialand temporaldisposition region of enhancedice and brine production in winter. of important storage compartmentson the shelf. The seasonalcycle for the Mackenzie shelf is shown conceptually in Figure 2. In summer the Mackenzie River dischargesabout 75% of its annual inflow be-
As winter advances,the seaward edge of the landfast ice becomesprogressivelymore heavily ridged, and ex-
tensiverubblefieldsare produced(Figure2d). These rubblefields(stamukhi)act as an inverteddam to trap
tweenJune and September(Figure 3). At the start lessdensefresh water in the nearshore. The landfast ice, of freshet the shelf is mostly ice covered. As summer
which consistsof large, fiat fields separated by relatively
898
MACDONALD
ET AL.:
FRESHWATER
BUDGET
(b}
OF MACKENZIE
RIVER
W•ND
(c)
is$ light
is$ heavy
........
Figure 2. A schematicdiagramof the runoff and ice cyclein the Mackenzieestuaryshowing
(a) summerfreshet,characterized by stronginflowfromthe Mackenzie River,plumesandfronts on the shelf,and the permanentice pack,locatedwell offshore;(b) autumn, characterized by declininginflow, and autumn stormswhich mix and redistributefresh water on a largely icefree shelf;(c) early winter, characterized by freezingtemperatures with the Mackenzieinflow beginningto spreadunderthe newlyforminglandfastice, leavingits imprint asisotopicallylight ice(hatchedarea). Brine(circleB) produced in theouterregions wherethewinterinflowfromthe MackenzieRiverhasnot penetrated,helpsto alestabilize and convectthe watercolumn;and (d) the end of winter just beforebreakup where the winter inflow is containedbehind the stamukhi
zone(SZ) that hasprogzessively grownthroughout winter,at the endof the landfastice. Beyond the stamukhi,enhancedbrineproductioncontinues in the flaw lead (FL). The growingsheetof icein Figures2c and 2d hasincorporatedvaryingquantitiesof river wateras evidencedby a light
oxygenisotopecomposition (•sO).
low-relief ridges, has the potential to record horizontal shelf more than 97% of the meteoric water comes from patterns from the underlying surface water, provided the Mackenzie River; annual local precipitation yields there is a conservativetracer of the pattern. The pat- only 0.10-0.13 m of fresh water [Bur•s, 1974]. Since tern of interest here is the spreadingof new inflow water ice forms with minor fractionation at the isotopic comas a thin plume directly beneath the ice; an appropriate position of the water from which it grows, landfast ice records the horizontal spreading with time of the isotraceris the oxygenisotopicratio expressed as •80. Oxygen isotope measurementstogether with salinity topically light plume over the isotopicallyheavier seaare used in polar oceansto distinguishbetween mete- water as illustrated schematicallyin Figures 2c and 2d. oric water, which includes river runoff and precipitaHere we useseasonaloxygenisotopeand salinity meation, and sea ice melt, the other major sourceof fresh surements in both ice and water to address two main water [Redfielda;zdFriedrna;,,1969; Ta;, a;zdStrai;,, themes. First, we distinguish between the processes
1980;Vetshteyn et al., 1974;(Sstlund andHut,1984]. that Meteoric water is isotopically light compared to seawater
and the ice formed
from
it.
For the Mackenzie
affect
the freshwater
balance
on the Mackenzie
shelf-river inflow and the formation and melting of sea ice. Second, we use the isotopic record in the ice at
MACDONALD
ET AL.:
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
899 -15
30,000
Period of Core Growth
o,
20,000
.
"'-' " ' Winter Values (minimum flow) I :1 .,' \Summer Values -20
10,000
0
• 0
I 100
•
I 200
I
I 300
I
I 400
I
I 500
-25
Day(starting January 1, 1990) Figure 3. The hydrograph(solidline) for the MackenzieRiver from January1, 1990 to May
11, 1991(datasupplied by Environment Canada).The dashed lineshows the predicted 5180 compositionof the river below the confluencewith the Liard River as a function of time using KrouseandMackay's[1971]measurements, togetherwith flow data for the two riversabovethe confluence.
the end of winter to establish the pattern and rate of within the lead (station ISZ) and anotherwas taken river plume spreadingthat has occurredunder the ice from the ice on the northernedgeof the lead (station throughout winter. C-6). Ice coring, water samplingfor chemicaldeter-
minations,and conductivity-temperature-depth (CTD) profiling were carried out along sections that encom2.
Methods
Field Sampling During September 15-16, 1990, water sampleswere collectedby hydrocastsfrom the Canadian Coast Guard
Ship Henry Larsenalongtwo transectsacrossthe Mack-
enzieshelf (Figure la). At the time of samplingthe shelf was completely clear of ice as shown in Figure la. Sampling details and su•pporting oceanographic data
are givenby Macdonaldet al. [1991]. FromApril 26 to May 11, 1991,watersamplesand/or ice coreswere collectedat the locationsshownin Figure lb. In late February 1991 a lead 7-8 km wide opened intermittently about 60 km to the north of the Tuktoyaktuk Peninsula. By mid-April a continuouslead was open to the north of the peninsula up to the southwest side of Banks Island. At the time of samplingthe width
of thisleadexceeded 60 km in someplaces(Figurelb). We collectedmost of our samplesfrom the ice inshore of the lead, but one station was taken from a small boat
passedthe landfastice to beyondthe stamukhi(Figure lb). The general plan was to sample along designated transcots, but the actual site for each ice core was selected carefully from the air; we chose broad, flat regions of first year ice that were well away from ridges. The depth of snow was measured in the vicinity of the site, snow was cleared from the site, and a Sipre corer was used to obtain the ice in segmentsof about 70 cm in length. Water was collected from just under the ice through the hole made by the ice corer. Discontinuities in the ice were noted. The total length of the core was carefully reconciled with the depth of the hole, a procedure found to be particularly important for river ice which was brittle and tended to fracture easily during the coring process.The ice coreswere immediately cut with a saw into 10-era segmentswhich were placed in sealed plastic containers. The containers were returned to the laboratory in Tuktoyaktuk, where the contents were thawed, homogenized,and subsampledfor salinity and oxygen isotope determinations. Sampling details
900
MACDONALD
ET AL.:
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
ters(ViennaStandardMeanOceanWater (V-SMOW), Universityof Washington,Seattle (-11.55% o), and ter column sampleswere obtained independently of the Antarcticstandards(-33.40%0)). Two in-housestanand ancillary measurementsare reported by Macdon-
ald et al. [1•1, 1•2] and Paton et al. [1•4]. Wa-
dards that were calibrated by the University of Wash-
ice-coringoperation by augeringa 25-cm hole and using either a pump or hydrowire/5-L Niskin bottle to collect water at various depths. Water was subsampled into glasssalinity bottles with care to prevent freezing during sampling or shipment back to the laboratory.
ingtonagainstV-SMOW (LTW3,-16.62% o and Tuk Snow, -27.17% o) were run daily. Equilibrationwas
carried out by stirring at 20øC for 15 hoursin batches of 12 sampleswith four standards. Moisture was frozen out of the gas usinga Pelltier coolerand the dried gas Analytical Methods fed to the inlet systemof a Nuchde ratio massspectromSalinity was analyzed at the Institute of Ocean Sci- eter for isotopic ratio determination. Errors were meaences(September1990 samples)and at the Depart- sured through replicate analysesand repeat measurement of Fisheries and Oceans laboratory in Tukyoy- ments of in-house standards. Results are expressedin standard 5180 notation 'with the V-SMOW
aktuk (May 1992) usinga GuildlincAutosal (model 8400A)instrument;data are reportedin practicalsalinity units [œewis aridPerkin, 1978].SampleswerestandardizedagainstStandardSeaWater (StandardSeawater Service,Institute of Oceanography, Wormley,U.K.).
the referencevalue. Agreement between replicate sam-
plesrun on the sameday was usuallywithin 4-0.1%o. Over the period of months required to analyze the sam-
ple suite, precisionwascloserto 4-0.2%o.
Repeat determinations •ndicate a precision of -t-0.003. Overall uncertainty based on bottle duplicateswas better than
standard as
3. Observations
4-0.02.
Oxygen isotopic compositionwas measuredby a me-
Water
Column
Data
thod similar to one developedby Whaite[1982].A de-
Salinity-5180dataforautumn(September 1990)and
tailed discussionof the procedure and error is presented
by Pator• et al. [1994]. Briefly, the analysiswas per- winter (April-May 1991)are summarized in Figure4. formed by equilibrating 5 mL of the water samplewith At salinities above 32 both the summer and winter data Matheson(instrumentgrade) CO2 of isotopiccompo- for salinityand/P80 covary.The samerelationshiphas sition -27.63% 0 establishedwith three referencewa- beenobservedin previousyears(September1986,Mac-
Sea Ice
•Oo
Late Summer 1990
o8 o
-5
[]
O
o
+
cO
+
Late Winter 1991
o RiverIce -15
RiverWater(-:•8.3) -2O
0
10
20
30
40
Salinity(psu) Figure 4. Salinityversus5180 for the watercolumndata collected in September 1990(solid squares),for the waterdata collectedin April-May 1991(crosses), andfor the ice cores(average of coresamplesis shown)collectedin April-May 1991(opencircles).
MACDONALD ET AL.' FRESHWATER BUDGET OF MACKENZIE RIVER
901
donaldet al. [1989];April, 1987andAugust1987,Mac- averagedabout 8 cm (as snow)which,again,is normal donaldand Carmack[1991]).At salinitieslessthan 32 for the regionat thistime of year lice Centre,1992].We the data pointsfall into two groups;the uppergroup found no apparent relationship between snowdepth and (Figure4, solidsquares) comprises the September 1990 amount of ice formed at a site. The 5x80 valuesshowcoherentpatternsboth within data,andthelowergroup(crosses) comprises the AprilMay 1991 data. The late September data have been eachicecoreandbetweenicecores(Figure6). Nearthe affected by the addition of sea ice melt. River water
top of a given core, which containsthe ice formed early
(bottomleft) hasalsoinfluenced manyof thesesamples in the season, 5180 tendsto be high(-5 to -1% 0). to a greater or lesser degree. The winter data extend The 5180 valuesusuallyincreaseslightlywith depth toward the bottom left cornershowinga stronginflu- near the top of the core, but for many coresthis trend enceof river water at somestations,but they are also is reversedabruptly at somedepth and 5180 decreases displacedto the right relative to the summerdata. This sharplywith depththereafter(for example,seecorePIdisplacementindicates the addition of salt to the water 2). The 5180 valuedropstowardbut not pasta limit column during sea ice production. of-15.7% 0 which is the value for ice growingfrom Ice
Core
Data
The ice cores collected from the zone of landfast
ice
pure Mackenzie River water. This sharp transition correspondedto depths where during sample collectionwe often noted the texture ½,fthe ice to changefrom plastic
were,with a few exceptions, of uniformlength (solid (seaice) to brittle and transparent(riverice). The ice coresrecord the sequenceof isotopicallylight portionsof the histogramsin Figure 5; rangeof 163 cm to 202 cm; average length 1.87-•- 0.11 m, number of water invading many but not all of the siteso For exsamplesn = 23). Theseice depthsare normalfor the ample,ice coresalongthe PI section(Figure 6) shift time of year; the Ice Centre[1992]givesaverageice to lighter5180 valuesat 20-30 cm in PI-1, 70-110cm depthsforApril 26 to May 11as203-210cm(n -- 4) at in PI-2, 120-150cm in PI-3, and 170-200 cm in PI-4;
KugmallitBay,181-182cm (n - 24) at SachsHarbour, on the seaward side of the stamukhi zone at PI-5 the and 180 (n - 27) at Cape Parry. Four of the coresin shiftto lighter5180doesnotoccurat all. Considering Figure 5 are much shorter than the others. Two of these that ice growsdownwar(!as winter progresses and that short coreswere collectedfrom beyondthe landfast ice MackenzieRiver wateris isotopicallylight comparedto zone(stationsCD-5 and AP-5), wherethe particular seawater, theicecoresections (PI, GI, CM, andAP) can patch of ice we sampledmay have startedgrowingin be seenas new winter inflow spreadingoutwardunder leads that opened late in the season. The other two the ice towardthe stamukhizone. Similarpatternsare shortcoreswerecollected fromthe river (April 25, HW- evidentin sectionsrunningparallel to the Tuktoyaktuk 13, 80 cm, and HW-14, 150 cm), wherethe conditions Peninsula (CM-1, L-2, AP-1, L-3, andCD-2). affectingicegrowth(e.g.,temperature,snowcover,and Salinitydata for our ice cores(not shown)tend to heatflow)clearlydifferfromthosein the estuaryandon have a high variance due to the nonconservativebethe shelf. For comparison,the norm for the Mackenzie haviorof salt duringand after freezing[Ma•lkut,1985]. Riverat Inuvikin April is 120q-27 cm (n - 29). Snow This behaviortends to make salinity a poor tracer for
cover(shaded portionsof the histograms in Figure5)
river ice that has formed beneath sea ice because brine
25O
200
•
•
"',
',
8
100
0
Location
Figure 5. Histograms showing the lengthsof the icecorescollected duringthe study(solid portion) andthedepthofsnow cover at eachsitewhenthecorewascollected (stippled portion).
902
MACDONALD
ET AL.'
FRESHWATER
BUDGET
OF MACKENZIE
ß ß
ß
ß ß
ß
8
ß ß ß ß ß ß ß
: ß
ß
RIVER
MACDONALD
ET AL.: FRESHWATER BUDGET OF MACKENZIE
draining downward from the sea ice will contaminate ice grown later. Under these circumstanceswe find that salinity can be used only as a very crude guide to dif-
chosen for the reasons
RIVER
outlined
below.
903 Sea ice melt
(SIM) hasa specialattributein that it can be positive (seaice is melted)or negative(seaice is formed).
œerentiate river ice from •ea ice [cf. Jeffrieset aL, 1988, Fractionation of' 5180 When Water Freezes 1989; Jeffries and Krouse, 1988]. T•ends within the cores,likethoseshownsoclearlyin the 5i80 data (FigUnder equilibrium conditions,ice forms with a small ure 6), are not observedin the salinitydata exceptin isotopicfractionationot' about 3% 0 heavier than the extreme caseswhere seawater has been completely re- liquid from which it was formed [O'Neil, 1968; Beck placed by fresh water at some time during ice growth andMuennich,1988;Le,•rnannand Siegenthaler, 1991]. (e.g., at PI-1 and PI-2). However,if the salinitiesand Our measurements of •lSO at the bottom of the ice 5180 valuesare averaged for the 15-20sections analyzed and in the water just under the ice showa fractionation within eachcore,then an increasing trend of 5180 with offsetof 2.66q-0.28%0 as the interceptof a very good salinityemerges(Figure4, opencircles). linearregression (Figure7). It shouldbe notedthat this observedfractionation applies to ice growing slowly at
4. Processes Affecting the Composition
the end of the seasonwhen it is 1.6-2 m thick. The slope
of Water
of the regression (0.999q-0.020) showsthat this offset doesnot dependon the 51so value nor, by inference,
and
Ice
on the salinity. Takingthe averagedifference for our
Water Types and Composition
paired data, we find ice to be heavier than the water
At any site in the Mackenzie estuary and adjacent
it growsfrom by 2.57 q- 0.10%0. This fractionationis
slightly higher than that determined by Melling and the followingthree primary types of water: a saline end Moore[19951(2.09%0) thr ice growingfrom water of a mem• (•o• m• •y• (P•L)), • •o• o• me- restrictedsalinity range of 29-31 on the Beaufort shelf. shelf the surface
waters
can be viewed
as a mixture
of
teoricfreshwater (MW) (essentiallyMackenzieRiver Polar mixed layer (PML), saline water. We runoffaugmentedby minor amountsof precipitation), have selected a seawater end member appropriate to anda source/sink of brackishwater(seaicemelt (SIM)
/formation)[Macdonald et al., 1989;Macdonald and the study period by referring to Figure 4. Below the
Carmack,1991; Melling and Moore, 1995]. To deter- point definedby 32.2, -2.5% 0, the 1990and 1991data
branch into two groups; above this the summer mine the compositionof a given sample with reference sets data narrow onto a well-defined curve which has been to these primary water types requires the measurement observedin previous years. We use this branch point of at leasttwo conservative properties(e.g.,salinityand 5180) and an appropriate assignment of properties to to define the underlying saline water mass commonto 1990 and April !991. This end-member repeachoftheprimary watertypes.6sflsnd andHut[1984] September resentssalinity of water at the depth to which winter outlined the principles of the calculationsas they apply mixing by brine-driven convectionand winter stirring to the Arctic Ocean am! gave massbalance equations has occurred. The properties of this saline end-member
to find the recipefor any samplewith salinity(S) and (PML) will differ from :yearto year dependingon the 5180(5)values water properties prior to freeze-up and the amount of
(1) ice grown over winter. (2) Mackenzie River meteoric water (MW). The (3) Mackenzie River has a well-established summer 5180 value of about -20.3% 0 [Krouseand Mackay, 1971; suggest where the subscriptsrefer to the three primary water Macdonaldet al., 1989],but our measurements types,F is the fractionof eachprimarywater type, and that a slightlyhigher51Sovalue (-18.3% o) prevails S and 5 refer to the salinity and 5180 compositions, in winter. Cyclic variationin the river's51so can be respectively, of the subscriptedwater types. The end- accountedfor by seasonalvariation in the flow of the FPMLq- FMWq-FSIM -- 1 FpMLSpML 4- FMWSMW4- FsIMSSIM= $ FpMLSPML q- FMWSMW q-FSIMSSIM= •
member compositions are listed in Table 2 and were _
isotopically lighter (21.3ø/•
Liard River. a major trib-
Table 2. Water Propertiesand ConversionFactorsfor Mass BalanceCalculations
WaterType
Salinity
MW (summer) MW (winter)
0.15 0.15
PML SIM River ice Fr actionation
32.2 6.2 0 -
0.07 0.07 0.1 0.4 0.1 -
--20.3 --18.3
-2.5 0.07 -15.7 2.57
0.66 0.09 0.1 0.1 0.06 0.12
Densityof ice is 900 kg m-a. Abbreviationsare s•, the standarderror of the mean; MW, meteoric water; PML, polar mixed water; and SIM, sea ice melt.
904
MACDONALD
ET AL.'
FRESHWATER
BUDGET
OF MACKENZIE
ß
ß
ß
RIVER
/
ß ß
//
/
ß ß // / / / / / / /
-5
/ / / / /
o
/ / / /
ß
-15
/ /
-20
' -20
i
I
-15
•
I
i
-10
I
i
-5
0
Water (•l•O
Figure ?. The •180 at the bottomof the ice coreplottedagainstthe •180 in the waterjust beneath the ice at the time of collection. The isotopic fractionation that occursduring freezing is evident as the offset of data from the diagonal which would be observedif there were no fractionation.
utary of the Mackenzie[cf. KrouseandMacka•t,197!]. (2.57%0). Similarly,for river ice the salinity is taken Above the confluence of the Liard the Mackenzie has to be 0 and 5180 to be -15.7% 0.
a 5180of-17.4%0. UsingKrouseandMackay's 5•80 valuesfor the two rivers abovetheir confluence,together
River ice melt (RIM).
The three massbalance
with flow data (Water Surveyof Canada),we predict equations allow us to solve only for the three primary
the 5180 belowthe confluenceto rise from -20.2%0 typesof water (PML, MW, and SIM). However,the efat peak flow in summerto a constant-18.2 -4-0.2%0 fect of a fourth water type, river ice me]t, must be considered. In previousstudiesthat have used5180 meathroughoutwinter(seedashedlineandright-handscale
in Figure3). Ourmeasurements onriverwaterandon surementsto distinguish between the Arctic Ocean's the two ice corestaken at HW-13 and HW-14 (Figure 6, 5180 - -15.7 4-0.06% 0, n- 23) confirmthat the river had a constant5180 value of about -18.3%o throughoutthe period of ice growth, providedwe allow for the 2.57%0 fractionation. The summerinflow of the Mackenzie River to the Beaufort Sea accounts for
freshwatersources;fractionationduring the freezingof meteoric water has been ignored. While this probably constitutes insignificant error for the Arctic Ocean interior, for the shelveswherethere is a substantialquantity of runoff, the effect of altering the compositionof freshwater by freezingsomeof it and then later melting
75% and the winter inflow, 25% of the total flow (Ta- it must be evaluated. For lack of a third conservative ble 1). For our massbalancecalculations wewill assume property we are left only with the option of determining that the summer5180 valueis appropriatefor Septem- what error the fractionationof MW into ice might produce. Using the end-memberassignmentsgiven in Taber 1990 and for stations outside the landfast zone in April-May 1991, where the winter inflow appears not ble 2 and applying the conservationequations,we find
to havepenetrated. The winter5180valuefortheriver that 1 kg of riverice (salinityof 0, 5180of-15.7% o)is
willbeusedwithinthelandfast zonefortheApril-May solvedalgebraicallyas an apparent mixture of 0.86 kg
of MW plus 0.17 kõ of SIM minus 0.03 kõ of PML. Therefore the conservationof properties requires that Sea ice melt ($IM). As seenin Figure4 (open the water out of which ] kg of river ice grew must, accircles),ice from the Mackenzieestuarydoesnot have cording to the algebra, contain an apparent or virtual uniform 5180 OI salinity.Here we treat ice as a two- extra 0.14 kg of MW and -0.17 kg of SIM. The algecomponentmixture of seaice and river ice. The salinity braic solutionbalancessalt by removinga smallamount of first-year sea ice is estimated as 6.2 from the six ice (0.03 kg) of PML. In terms of properties,the water corestaken from the seawardsideof the stamukhizone. would be slightly lighter isotopicallybut would witness The5180ofseaice,+0.07%0,isto bethe5180ofthe no changein salt content. If this ice is subsequently salineend-member (PML) plusthe fractionationfactor melted and mixed back into the water, then the net 1991 stations.
MACDONALD
ET AL.'
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
905
position returns to 1 kg of MW. Provided we can independently estimate how much meteoric ice has been formed in the estuary, we can apply an appropriate correction to the algebraicsolutions.
estimated from the water column data should, in principle, provide independentaccountingof salt and fresh
5. The
and
Distribution
water
for the outer shelf.
Distributions Sea Ice
of Freshwater
Components in Ice and Water Ice
and
Water
Before discussingthe water column budgets, we must consider
River
of Meteoric Melt
Sea Ice
the
error
introduced
into
the
mass
balance
1000kg m-a). From Table 3, whichsummarizes the
equations by isotopic fractionation during the freezing of river water. We have estimated that an average of 0.72 m of MW was captured into the growing landfast ice; therefore consideringthe effect of fractionation during freezingas disscussedabove, this will produce an apparent 0.1 m of MW and an apparent -0.12 m of SIM in the water beneath the ice. That is• the equivalent heights of MW and SIM calculated from the mass balance equations will, on average, overestimate MW and underestimate SIM by these amounts, or, sincenegative values for SIM imply the formation of sea ice, the mass balance calculations will overestimate sea ice production by this amount. T},•.eseerrors are small relative to
equivalentheightof seaiceis 1.00m (Table4).
in Table 4).
An estimateof the equivalentheight (liquid) of river water
R and
seawater
H-
R that
has frozen
into
a
sample of ice from any location can be calculated from
61sO,the average61sOvaluewithin the ice core;
(61sO- 2.57- 61SOs•M)
(4) R-- •i-•-M•• •i•O-•I•IM) Hpice/Pwater where H is the measured depth of ice and Pice and
Pwater are,respectively, the densityof ice (900kg m-a) and water (taken here to be the freshwatervalue of
integrated componentsfor each station, we find the av- the estimatedequivalentheightsand their ranges(Taerage equivalent height of river ice for all ice corescol- ble 3). We haveusedthe estimatederror only to correct lected from the landfast ice is 0.72 m and the average the averagevaluesfor landfastice zonein winter (given The equivalent height calculations for river ice and
September 1990. 'The late summer water properties, shownschematicallyin Figures 2b and 4, are shown the relatively flat and stable landfast zone. However, in section in Figures 8a-Sd as isopleths of fractional beyond the landfast zone, ice can advect out of the re- composition. Autumn storms have smoothed out plume gion during winter as evidencedby the large polynya structure, and inflow from the Mackenzie has declined. sea ice at each station
are reasonable
estimates
within
which openedin late February 1991 (Figure lb) [see Sections of MW at this time show the river water to be Melling and Riedel, 1993]. Conversely,advectioncan distributed relatively uniformly through the water colbring offshoreice onto the shelves,and, particularly for umn among the stations. Only near the surface at the
sites that open later in the season,the observedthickness of new ice will not be a reliable
estimate
inner
stations
of the eastern
section
do we see a struc-
of the to-
ture identifiableas the MackenzieRiver plume (Figure tal amountof ice actuallygrownat that site (e.go,AP-5 8a, stations 21-22; also see Figure 4 where these two and CM-5). The ice on the outershelftendsto be more pointsare displacedtoward the bottom left). There is
heavily ridged which means that measurement of ice at flat locationsgood for landing aircraft will considerably underestimate the net ice production for the shelf even if the fraction of ice coveris correctly estimated. Using height and frequencydata for ice ridges, Prinsenberg
minor variation in the equivalent height of MW within each section, shown as bars at the top of Figure 8, but the westernmostsectioncontainssignificantlylarger
tributed an additional 25 cm of ice in Hudson Bay and as much as 58 cm of ice in Foxe Basin where extremely roughice is found; thesefigurestranslate into a net con-
In contrast to MW, SIM is distributed predominantly in the upper 10-15 m and it tends to be more concen-
quantitiesof MW (2.93 m versus2.37 m; probability P < 0.05). The averageequivalentheight of MW for [1988]estimatedthat ice accumulatedinto ridgescon- all of the Septemberstationsis 2.57 4-0.12 m (r, - 20).
tribution
of about 30% over what would be estimated
trated towardthe outer shelf(Figures8c and 8d). At
from ice thickness measurements alone. Melling and
this are negative valuesindicating a lens of water that Riedel[1993]suggestthat 30% is probablyan underes- contains remnant brine from sea ice production during
timate since the under-ice keels are much larger than the above ice sails used in the calculations.
The advan-
tage therefore of using the mass balance equations to integrate the water-type content for the water column is that we can determine the image of ice production
left behind as salt in the water (negativeSIM), that is, water will capture the imprint of sea ice produced whether
or not the ice is later
advected
off the shelf or
piled into ridges. Comparing the measurementsgiven in Table 3 for sea ice thickness and sea ice production
the previous winter. •his deeper• negative SIM tends to o•set •he positive S•M in the surface layer with the result that the equiw]ent heights c•culated for the top 40 m are less than whnt is contained just in the surface 10 m. •he average equivalent height for SIM in
Septemberis 0.82• 0.11m (•-
20).
April-May 1991. The distribution of MW under the ice at the end of the followingwinter is shownfor the
stationsin Figure lb as four sections(Figures9a-9d).
906
MACDONALD
ET AL.'
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
Table 3. Equivalent Heights in Meters of Freshwater Components Water
Station
Latitude, øN
Longitude, øW
Column
Ice
MW, top 40 m
SIM, top 40 m
Rice
021
69 51.11
133 19.17
2.61
0.14
-
022
69 55.28
133 23•39
2•55
0.04
-
023 024
70 01.63 70 08.30
133 24.43 133 26.03
1.71 1.76
0.18 0.89
-
025 026
70 13.12
133 33.73
70 18.20
133 40.20
2.11 2.00
0.21 0.78
-
Sice
Tice
-
027
70 23.37
133 46.14
2.23
0.31
-
028
70 29.10
133 53.14
2.62
0.51
-
029 030
70 33.81 70 39.21
133 57.78 133 59.47
2.37 2.56
0.91 0.77
-
032 033
70 49.29 70 55.59
134 17.15 134 23.68
2.59 2.95
0.28 1.32
-
034 035
70 59.09 69 08.00
134 29.78 137 41.62
2.74 1.97
1.30 0.83
-
036
69 16.90
137 51.02
3.16
1.47
-
038
69 24.65
137 58.60
2.54
1.11
-
042 049
69 30.85 69 37.95
138 04.47 138 14.09
2.66 2.99
1.41 0.97
-
050
70 24.89
139 03.39
3.58
0.72
052
70 09.74
138 48.34
3.63
1.94
-
HI-1 HI-2 KP-2 GI-1 GI-2 PI-1 PI-2 PI-3 PI-4 PI-5 CM-1 CM-2 CM-3 CM-4 CM-5 ISZ C-6 AP-1 AP-2 AP-3 AP-4 AP-5 L-1
69 69 69 69 69 69 69 69 69 70 69 69 69 70 70 70 70 69 70 70 70 70 69
28.00 36.82 23.94 30.24 38.05 43.84 48.75 54.54 57.63 02.28 45.01 52.05 56.15 00.99 08.06 11.60 28.29 59.74 04.21 08.52 15.44 18.05 53.00
138 138 138 136 136 134 134 134 134 135 133 133 133 133 133 133 133 131 131 131 131 131 133
50.58 47.84 07.47 09.49 21.95 31.26 38.23 48.54 52.94 01.50
1.96 6.11 5.38 4.21 1.91
L-2 L-3 CD-2 CD-3 CD-4 CD-5
69 70 70 70 70 70
54.03 16.06 25.01 31.46 39.95 48.32
132 130 129 129 129 129
18.00 21.32 21.57 24.05 25.23 39.73 51.47 27.97 25.20 25.16 29.24 21.28 59.98 13.88 31.69 39.98 39.47 40.83 40.35
4.68 5.49 2.64 1.70 1.42 2.12 1.81 5.51 5.05 2.16 0.83 1.02 1.31 4.52 2.83 -
-0.05 -0.40 -1.36 -1.92 -2.15 -0.53 -1.26 -2.11 -2.09 -1.85 -2.52 -1.88 -0.60 -1.35 -2.17 -1.25 -1.43 -1.86
0.36 0.24 0.36 1.71 1.07 1.58 1.04 0.71 0.39 0.18 1.14 0.97 0.27 0.18 0.74 0.40 0.25 0.13 0.14 -
-0.94 -0.51 -
0.74 0.38 0.18 0.11 0.13 0.06
1.33
1.69
1.46
1.70
1.33
1.69
0.01
1.72
0.59
1.65
0.18
1.76
0.66
1.70
1.09
1.80
1.41
1.80
1.58
1.76
0.53
1.67
O.7O
1.67
1.22
1.49
1.56
1.74
-
1.71
-
0.99
0.82
1.56
1.11
1.51
1.5
1.75
1.47
1.6
1.24
1.38
-
1.80
1.08
1.82
1.30
1.68
1.47
1.65
1.36
1.47
1.64
1.77
0.82
0.88
Water column is the top 40 m. Abbreviationsare MW, meteoric water; SIM, sea ice melt; P•ice,river ice; Sice,seaice; Tice, total ice.
MW is contained predominantly withina 4- to 5-mdeep spreadingout to the stamukhi will be further discussed layer of fresh water under the ice at nearshore stations.
below. Despite the apparent containment of new winter
For each section there is a front across which stratifi-
inflow inside this stamukhi
cationall but vanishes(see,e.g., PI-4 to PI-5 or AP-3
tains a substantial quantity of MW at the end of winter
zone the offshore water con-
to AP-4). Thisboundaryof winterinflowspreading in (Table 3, averageequivalentheightof MW is 1.54m). late winter generally follows the stamukhi zone at the
Mixing by autumn storms,followedby brine production
edgeof the landfastice; the rate and extentof plume
in winter
at these outer stations
redistributes
the MW
MACDONALD
ET AL.:
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
907
Table 4. Average Equivalent Heights of Meteoric Water, Sea Ice Melt, Sea Ice, and Meteoric
Ice for the Landfast
Zone and the Offshore
Landfast Region
m
Offshore Region
Period
WatexType
s•n)
m
s•n)
Sept. 1990
meteoricwater seaice melt
2.$1 0.73
0.14(10) 0.16(10)
2.82 0.90
May 1991
meteoricwater meteoricice seaice melt seaice
4.16 0.72 --0.94 1.00
0.39(13) 0.11(17) 0.19(13) 0.11(13)
1.54 0.15 --1.88 1.47
0.15(10) 0.16(10) 0.18(7) 0.02(8) 0.16(7) 0.05(7)
Abbreviations are m, mean; s•, standard error of the mean; n, the number of data points.
uniformly throughoutthe top 40 m where it contributes
On the cross-shelf sections(Figures9e-g) the mini3-5% of the water (Figures9a-9d). Furthermore, wa- mum (most negative)valuesof SIM tend to be found ter displaced from the landfast zone by winter inflow nearthe outsideedgeof the landfastzone(PI-4, CM-3, or exchangedbelow the depth of the ice keels has con- and AP-3) rather than in the polynyabeyondit where tributed
additional
MW
to the offshore zone.
Within
enhanced ice production occurs. The location of this
the landfast ice zone the equivalentheightsof MW are maximum is probably due to the capture of the rejected muchhigherthan they are offshore(Table 3 and solid brine in the buoyantspreadingplume itself, which then barsat the top of Figures9a-d; the averageis 4.16m). prevents the water column from conveeringto the botThe equivalent height of MW at some of the stations tom and thereby mixing the brine through a larger volnearest to shore is simply limited by depth, which ex- ume. This "snowplow"effect of the spreadingplume is plains why a maximum is reachedat the central stations more than of academic interest as it would undoubtedly (5-6 m at PI-2, PI-3, and CM-2). be an important processin the transport of contamiThe
total
amount
of winter
inflow
that
has frozen
nants released beneath
t.he landfast
ice. The nearshore
into the ice at eachsite (calculatedaccordingto (4)) stations, which obviously produced brine early in the decreases as oneproceedsawayfrom the river (Table 3 season,have apparently 'beenflushedby the winter river and cross-hatchedportion of the bars depicting equiva- inflow, and it is probably this brine that tends to aug-
lentwaterheightoficein Figures9a-d). At the stations ment the middle stations. The histogramsat the tops of beyond the stamukhi, MW makes only a small contri- Figures9e-h showthat the amountof seaice (hatched bution to the ice formed(0.06-0.19m). The MW in- part of the icehistograms) increases with distanceaway corporated into the offshoreice comesmost likely from runoff left over from the previous summer rather than
from the river mouth and that the equivalentheight of negative SIM contained in the water beneath the ice
from new river inflow in winter.
(solidbars)similarlytendsto increase.Within thelandIn contrastto the situationin autumn(Figures8cand fasticezonethe averageseaiceproduction(1.0 m) is al8d), all valuesfor the fractionof SIM in the water col- most identical to the equivalentheight of negative SIM umn are negativein late winter (Figures9e-h), clearly (-1.06 m) contained in the waterbelow.However,the
indicating that salt has been added. The shift from summer to late winter conditionscan be seen clearly in
similarity of these two numbers does not imply that the salt rejected by the sea ice exactly balances the brine the salinity-51soplot (Figure4); in summerall points added to the water column during the current winter; lie to the left of the MW-PML mixing line, indicating becausethere was an average+0.73-m equivalentheight the addition of SIM. In late winter all points are found of SIM in September, there must have been either a loss
salt which is seenin/he water columnas negativevalues or an inflow of negative.SIM from outer parts of the
for SIM. Near the branchon Figure 4 (32.2, -2.5%0) thereare a numberof points(crosses) displacedfar to the right. These points, arhichindicate that large quantities of brine have been injected, come from stations at
shelf(or a combinationof both processes) to balance the salt budget. Winter
Inflow
From
the
Mackenzie
River
the entranceto LiverpoolBay (Figure 9h), a location We calculated the total winter inflow of the Mackenshown to be particularly effective at enhancing brine in the water due to its shallow depth and semienclosed zie River from freeze-up(October16, 1990)until the
shape(E.G. Carmacket al., Water and ice relatedphe- day we collectedour last ice core (May 10, 1991) to
of Canada are nomenain the coastalregion of the Beaufort Sea: Some be73km3 (datafromtheWaterSurvey parallelsbetweennative experienceand westernscience, plottedin Figure3). To determinethe net amountof river water delivered to the estuary, we must subtract submittedto Arctic,1994).
908
MACDONALD
ET AL.' FRESHWATER
BUDGET
OF MACKENZIE
RIVER
33 34
II 2o
ß3o
1.
•o 10-
30. 0.050 4O
-4
-3
ß-• 'r' G) •
2!
0
.-
22
2:3
-
"
24 ß ß
21627
25 -,
ß
ß
2__829 ß 3o . ß
ß
ß
:32 i
33 34 I I ß
.2 1
ß
0
ß
lO
10
•
ß
.s:: 20
.. ß
30
..
4O
.•
2,
'r'
1-
• •o"
:
' --•.•-0.025
---
36
• K
10 \
:
ß ß L0.025 •••........••
I /
!
38
,
,
,
42
I
I
0.10•
50
49 I :
52
ß
0075•0.10_ '•
•oo•,,.o,o•
I :
•
40
0
50
1O0
150
Distance (kin)
Figure 8. Data for September 1990stations(Figurela) showingcontours of fractionalcompositionfor meteoricwater(MW) at (a) stations21-34and(b) stations35-52andfor seaicemelt (SIM) at (c) stations21-34and(d) stations35-52. Solidhistograms at the topsof the diagrams give the equivalentheight in metersof the freshwatertypes.
MACDONALD
ET AL.' FRESHWATER
7'
ß •'
Pl-2
PI-3
PI-4
5
Pl-i
PI-.S
ISZ
1 o
909
6
•' 5
:•
RIVER
7
6'
._o•4 3• 3
BUDGET OF MACKENZIE
4 3 1
o
o
o
0.1'
lO
ß
'
0.0S
10
. ß
2Q
40
10
0
20
30
40
50
CM-2 CM-I
CM-3 CM-4
ISZ
CM-S
C-6
0.1
2O
•. 2o-
ß
40
E•, 6
AP-I
50
55
40
80
90
AP-I
AP-2
CM-1
L.-2 L-3
AP-3
• •
AP-4
21
CD-2
AP-5
LB-I
CB-4 CBTb
0
10.
lO
10-
lO
20.
20
20
20
30-
-30
1•0
2'0
3•
30
.30
,40
40
0
50
100
150
200
2•0
Distance (km)
Figure 9. April-May 1991stations(Figurelb) showing contoursof fractionalMW composition at (a) •ectionPI, (b) 8eelionG•, (½)•ectionAP, and(d) section alongtheTuktoyaktuk ?eninsula andSIM at (e) sectionPI, (f) sectionCM, (g) sectionAP, and(It) sectionalongthe Tuktoyaktuk Peninsula. I-Iistog•amsor the equivalentliquid height or the variousr•,hwa•e• component•
dxown;•oliclba• •ere• •o integ•ateclwate• •olumn values,o•en/hatdteclba• •ere• to ice. t•atd•eaportionor the ice histogram•ere, to the •ame r•,hwate• componentgivenin the •olia •a• ro• eachaiag•am(i.•., MW ro• 0a-0a aria SI• ro• 0e-0h). eo• SI•, neeativewate•column e•uivalent height• a•e sitownas black ba•s below the line. Dashea hi•tog•am• a•e given
locations wi•ereonlythecle•thoricewa•rnea,•ea. Openwater(0aand0•; 0eariaOr)i• •hown
a•a •i•lea •u•race neaxly levelwititthetopor•the ice(iceeee•oa•a w. usually about•0
i 40
910
MACDONALD
ET AL.' FRESHWATER
BUDGET
OF MACKENZIE
RIVER
_3'1 PI-1PI-2.A-.B•/z,.,.••L' PI-3 PI-4 PI-S ISZ 13 ':•..:•':.•....,::........::::•:...•.'-,----..•.•,,'"'• I \ 'ß o 40' "'•:.! :'(":'"'":' ":"•l ':":'/" 0
10
20
30
, 50
40
40
,3O
ß40 90
0
10
L-2
CM-1
•
20
AP-1
:30
L-3
CD-2
40
LB-1
CB-4 CB7b
:
-0 075
--0.125
'
.:.:.•3• .......iiiiiiiii........... •:.....•...:•• •oo '7111
10
••'øø
2O
4O
•
5o
1oo
150
200
Distance (km)
Figure 9. (Continued)
2,•o
MACDONALD
ET AL.:
FRESHWATER
BUDGET
OF MACKENZIE
RIVER
91!
that the ice corescollectedat HW-13 and HW-14 (in the MackenzieRiver) and the very shortcoretaken at
the portion of this inflow that has been captured into the growinglandfast ice. The calculated averageequiv-
alentheightof riverice (0.72m) spreadoverthe region CM-5 will be treated separately. For all coreswe assign
withinthelandfast ice(16,000 km2) accounts forabout 12 km3 or about 15% of the winter inflow. The amount of Mackenzie River left to spread under the ice in the
the collection
date to the bottom
of the core.
Ice growth models have been reviewed by Maykut
water column from October 16, 1990 to May 10, 1991 is therefore about 61 km3.
[1985,1986],Orastedtand Wettlaufer[1992],and Leppdfanta [1993]. For simplicitywe will adopt an empirical modeldevisedby Anderson[1961]. For youngice,
6. Plume Spreading' The Record
Anderson found the relationship between the thickness of the ice H, and the cumulative freezing-degreedays
in the
(0 - fj(T! - T•) dt where T! isthefreezing pointof
Ice
water,Ta is the air temperature,andt is time) to be
Three stepsare requi,ed to use 5•80 profilesin ice cores to infer
the
estuarine
circulation
under
+
the ice.
The first and most important step is to convert depth in the ice core at eachsite into "time" by estimating the growth rate of the ice. The secondstep is to convert the
Accordingto Maykut [1986], good estimatesare obtained with Anderson's[1961]model without specific knowledge of winds, cloudiness,snowfall, or oceanic
5•sO valuein the coreto the corresponding 5•sO value heat flux. The cumulative freezing-degreeday record in the water from which it originated;this is simply the from Tuktoyaktukand Paulatuk(on the southerncoast fractionation correction shown in Figure 7. The third of AmundsenGulf about 300 km to the east) are alstep is to infer the water's salinity from the calculated mostidenticalfor the reriod in question(Figure 10). water 51sO.In this way,the 5•sO ice coredata can be Freezing-degreedays started to accumulateat the end usedto produce a seriesof maps showingthe evolution of September 1990, but presumably during the first 2 of surfacesalinity under the ice throughout the winter. weeksof October the seasurfacewas being cooledto the freezing point. Using the freezing-degreeday record at Assigning a Time Scale to the Cores
Tuktoyaktukstartingon October16, we find from (5) Weeklyice charts(EnvironmentCanada)from Oc- that by aboutthe endof December(Figure11) the first meter of ice had grown. During this time period only about 10 cm of snow had accumulated at Tuktoyaktuk.
tober 2, 1990, to November 27, 1990, show that new landfast
ice started
to form after October
9 when most
of the nearshorewater was open and before October 16 when most of the region was coveredby new ice. Comparison of the October 9 and October 16 charts shows that the landfast ice from which we gathered most of our cores became coveredby new ice almost synchronouslyø
The Anderson[1961]modelappliedfor the entirewinter predicts the ice thicknessto grow to a maximum of
1.70m (Figure 11, bottomline) whichis lessthan the
average 1.87 m we observedfor our ice cores. The inclusion in the model of a snow cover, which insulates We thereforeassignOctober16, 1990(4-1 week),asthe the ice, would tend to decreasethe predicted ice depth date at the top of all of the ice cores. Note, however, at the end of winter and thus cannot bring the pre5,000
4,000
3,000
Tuktoyaktuk 2,000
aulatuk 1,000
•
0 0
•'•
I 50
I
I 100
I
I
I
150
I 200
i
I 250
Day (startingOctober16, 1990)
Figure 10. Cumulatiw•.freezing-degree days starting October 16, 1990, and finishingMay 10,
1991(the last daya corewascollected),at Tuktoyaktuk(dashedline), and raulatuk (solidline).
912
MACDONALD
ET AL.: FRESHWATER
BUDGET
OF MACKENZIE
RIVER
Kugmallit Bay
2!f-
•,
!
Sachs Harbour
o ø•,•'"•
/
.. •Anderson
o
o
o•
0.5• • • 0
50
100
150
200
Day(startingOctober16, 1990) Figure 11. Ice thicknessas a function of time. The bottom line is producedfrom Anderson's [1961]formula usingthe freezing-degree day recordat Tuktoyaktukstarting October 16, 1990
(data suppliedby EnvironmentCanada).The opencirclesshowthe averagethickness of iceasa functionof time for KugmalhtBay duringwinter[/ce Centre,1992].The middle(SachsHarbour)
andtop (KugmaHitBay)linesshowthefit to "average" icecorescollected onday200(May 2) at the respectivesites using the method describedin the text. The first meter of the averagecore is assumedto conform t• the Anderson model with the 1990-1991 keezing-degreeday record at Tuktoyaktuk.
dieted
value closer to the observations.
Therefore
for
the deepersectionof the ice we have proceededby sim-
at Inuvik lice Centre,1992]and thereforemustbe considered
normal
for their
location.
From
the Ice Cen-
ply assumingthat after the first meterof growth,the tre data we could estimate depth as a function of time ice thicknessincreaseda• the squareroot of time. Each
for each river core simply by assumingthe minimum corewastreated separatelyaccordingto its total length growth scenario fo• the short co•e and the maximum ...
and th e' date it was collected by fitting an equation of
growthscenario for the longcore.Sincethe •zso signal
the form H - a + bt•/2 where a and b are fitted con- does not vary significantly within either of these two stants and t is time.
We have evaluated
the uncer-
cores(Figure6), thereis little benefitto be gainedby
tainty in this method of dating the coresby applying assigninga time frame except to note that the long core
it to the Ice Centre[1.c•92] averageice growthstatis- andprobablythe shortcorespanthe winterstartingin tics for KugmalhtBay (4 yearsof observations, Figure mid-October and therefore fairly representthe Macken11, opencircles)and SachsHarbour(28 yearsof ob- zie River •So composition throughoutthe periodof ice
servations, Figure11,closedcircles).We havefit these cover. For the anomMouslyshort core at CM-5 the Anaveragedata as if they werean actual corecollectedby derson model taken, together with the freezing-degree usonApr• 26;weused(5) with the 1990-1991freezing- day record(workingbackwardkom the time of collecdegreeday recordand assumedthat the corestarted to tion), suggests that thisicestartedto formaboutFebrugrow on October 16. The fit is shownas the middle ary 15, 1991. The •zso signalin the coreis consistent line for Sachs Harbour and the top line for Kugmalht
Bay in Figure 11. Kugmalht Bay givesthe worstfit in
with this picture; nevertheless, we do not assume any rigoroustime scalefor this core.
the regionwherethe ice thickness jumpsbetweenday 1!0 and day 120. Here our modellags the average Estimating Water Salinity from •180 CC
•,
data by about 2 weeks. For SachsHarbour, agreement
is usuallywithinabout5 days.Therefore weestimate Water salinity at the time of freezing is calculated
the uncertainty involvedin the datingprocess for our from the salinity-•lso correlationdiagram. However, ice coresto be -4-1weekat the beginning,perhaps-4-2 becausethe summer and winter correlationsdiffer, some adjustmentbetweenthe seasons is required. One month before freeze-up the water on the middle and outer The two river cores(HW-13 and HW-14) bracketthe shelveshad the •lSO-sa.linitypropertiesshownby the rangeof ice depthsreportedfor the Mackenzie River solidsquaresin Figure 4. Only two of the sampleswere weekstoward'the middle of the core, and within a few days toward the bottom of the core.
MACDONALD
ET AL.:
FRESHWATER
BUDGET
s•tonglyinfluencedby •he MackenzieRivet (surfacea• s•a•ions021 and 022). The MackenzieRivet•ssummer composition (-20.3) is wellknownfrom previousmeasurements in the riverandestuary[KrouseandMackarl,
OF MACKENZIE
RIVER
913
of changeof salinity, as evidencedby the slope of the salinity as it shifts from the top trend line toward the
bottom trend line, is surprisinglyconsistentamong the
cores(0.35-0.45d-•).
1971; Macdonald et al., 1989; Macdonald and Carmack,
The timing of river water invasion varies with dis1991]. Thereforewe are confidentthat the landfastice tance from the source; Figures 13a and 13b show the started to grow from surface water that was a mixture
time distribution
of the shelf surface water: and Mackenzie
trol buoyancyat a particularsite (i.e., salinitypasses the maximum)and the time at whichthe river plume
River water as
approximated by the to.p line in Figure 4. At the end of winter there is a closerelationshipbetween5•80 and
when river water
first starts
to con-
"front" as definedby the 15 isohalinearrives at a partic-
salinity (bottom curveon Figure 4), but it is different ular location. Once river water starts to control buoyfrom
the one established
in late
summer.
To convert
ancy at a site, it essentially reversesthe density en-
the 51sOwater valueto salinity,we haveusedthe two hancementtrend driven by brine rejectedfrom growing relationshipsshown in Figure 4, starting with the top
sea ice.
oneandmovinglaterallyto thebottomoneusinga t 1/2
Figures 13a and 13b show that the plume spreads faster along the coast of the Tuktyoyaktuk Peninsula
scale to mimic the rate of ice formation.
We estimate
the error in salinity determined this way to be about (1.3 cms-•) than it doesperpendicular to the coast +2.5 for the saltiestsurt;ace water early in the growing (0.2 cms-1). Coresat HI-1 and KP-2 yield a r•te of season but less than this for freshet water and for the spreadingalong the west coast of MackenzieBay that late winter
values.
is intermediate
to the •bove
two values.
Toward
the
end of winter, the under-ice river plume, as defined by
Spreading Rate of the Mackenzie Plume
the 15 isohaline,coversan area of about 16,000km2. Surface water salinities
Combining the growth rate of the ice with the cal-
measured
at the time the cores
werecollectedshowa gradientin salinity(at a salinity
culatedsalinityyieldsthe water'ssurfacesalinityas a
functionof time at eachice corelocation(Figure12). of 15)of about0.4km-• alongthecoastand1.9km-•
(perpendicularto the coast). Multiplying the spread-
The patterns are remarkably clear. Most coresshow a
salinitymaximumin time. They start to growin water ing rate by the respectivesalinity gradientwith distance with a surfacesalinity of 20-30. The steady increase suggeststhat the rate of changeof salinity at the various of surface salinity due to brine release and convection sitesshouldbe about 0.3-0.45 d-•; thesevaluesmatch of the water can be se,min the top cluster of trend the rates of change of salinity at the various sites in lines;for the outerlocations(PI-5, AP-5, AP-4, CD-3, Figure 12 calculated solely from the recordsin the ice CM-4, and HI-2) surfacesalinity eventuallyincreases cores. The thick dashedline in Figures 13a and to 30-32.5. At the inst•ore stations the surface salinshowsthe boundary of influence of winter inflow from ities depart from the top trend line and drop toward the Mackenzie River at the end of winter; the salinity zero as river water invadesa particular site. The rate at stations beyond this boundary was either constant
AP-4 AP-S CO-3
3O
CM-4 HI-2
AP-3
Pl-4
I
0
I
50
I
I
100
•
I
150
•
I
200
Day(startingOctober16, 1990)
Figure 12. Surfacewater salinity shownas a functionof time throughoutwinter at the ice core sites.Valuesweredeter]ninedusingthe 5•sO recordin the ice coresas describedin the text.
914
MACDONALD
ET AL.: FRESHWATER
BUDGET
OF MACKENZIE
RIVER
i
(.) 71øN
200 m
50 m
ß
70 ø
6g ø
140øW
130 ø
135 ø
(b) 71øN
200 m
70 ø
140øW 135 ø
130 ø
Figure 13. (a) Isochv•nsfor the day at whichsalinityreachedits maximumand beganto decrease within the landfastice and (b) isoehrons for the day at whichthe salinitydroppedto 15 within the landfast ice. The thick dashed line showsthe boundary beyond which salinities continued to increase up to the date the core was collected.
or continuingto increasewith time at the date of sam- made it to the outer siteIf through displacement, expling. The thick dashedline coincidesroughly with the changeof deeper water below the level of the ice keels, nearshore
side of the stamukhi
zone found
on the south
and leakagethroughthe ice boundary[Ornstedtet al.,
side of the polynya shown in Figure lb. The heavily 1994]. The advancingfront of the freshenedwater and its efridged ice along the southern boundary of the polynya is apparently an effective backstop for the winter inflow fect on surfacesalinity with time throughoutthe winter seasoncan also be visualizedby contouringsalinity on less• some of the water in the nearshore has probably a diagramof distanceversustime (Figures14a-14d).
MACDONALD
ET AL.: FRESHWATER
BUDGET
OF MACKENZIE
RIVER
915
Thesediagramsshowa ridge of high salinity (dashed line) which signifiesthe divide wherebysalt enhancementfromseaiceformation(belowthe dashedline) be-
2OO
comesoutstripped by freshwater supply from the river
1•0
inflow(abovethe dashecl line). If we visualizepassage of time along a sectionas the z axis movingupward at a constant rate in the various panels of Figure 14, we see the winter inflow progressivelyshutting off convection as the line crossesthe ridge at each of the locations. The advancingfreshwaterplume essentiallycaptures any brine rejected from growing ice, thus cutting off convectionto the bottom. As shown earlier in Figures 9e-9h, this advancing front acts as a "plow" to carry with it the brine or other substancesintroduced
100 •0
200
CM-1
CM-2
CM-3
I
•
I
iso (b) /
CM-4
at the inner
/ •
stations.
7. Budgets and Fluxes for MW, and
100
SIM,
Ice
The data presentedin Figures 8 and 9 and the average equivalent heights categorized by area in Table 4 may be used to summarizeregional budgetsfor MW
50
and SIM
for the shelf and to infer the fluxes that
must
have occurred between the two seasons. Considering
{o
the distributionof winterinflow(Figures13aand 13b), we divide the shelfinto two regions,the nearshorezone
AP-1
AP-2
AP-3
AP-4
(16,000kin2),whichincorporates theareawhereland-
AP-5
fast ice growsand into which the Mackenzie River continues to flow and spread during winter, and the outer
2oo] 1•0 t ' ' / ' /[•
• •oo t t o
shelfdeeper thanabout20m (48,000 kin2),whichdoes not receive much winter
inflow.
The choice of this divi-
sionis supportedby eartier work [Macdonaldand Cavmacl•,1991],and it is clearthat the processes occurring under
the landfast
ice out to its stamukhi
•.one bound-
ary are different than those on the outer shelf in winter. Meteoric
{o
Water
Figure 15a showsthe dispositionof MW betweenMay and September. The total volumesfor the sampling L-3
CD-2
'
'
CD-3
periodare shownin boldnumbers(in cubickilometers), while fluxes betweencompartmentsare given in italics within
the arrows.
The fluxes refer to volumes of the
watertype (cubickilometers)that haveenteredor left the compartmentsbetweenthe samplingperiods(i.e., fluxesin the late summer panel refer to the period from
late winterto late summ,•r).For example,weknowthat iv .LL •,,,
AS
.L.L L A.L
a
ß
During winter, 73 km of river water wasadded, 12 km
of river ice wasformed,and 67 km3 remainedin the water o
40
80
120
160
column
at the
end oœ winter.
To balance
this
budgetrequires thelossof31km3 fromtheinnerregion
during winter. Figure 15a shows that the nearshore tends to hold greater quantities of MW in winter than in summer, and the reverseis true for the outer shelf. Figure 14. Contours of salinity with distance versus of MW goinginto ice production accountfor timefor sections (a) PI, (b) CM, (c) AP, and(d) thesec- Amounts 15% of the inflow. In the nearshore the winter inflow tion running along the 3•ktoyaktuk coast. The dashed exactlybalanced by the estimated line showsthe top of the salinity ridge indicating the di- (73km3) is almost vide between salinity increasingwith time and salinity volume of MW in the water plus that which has gone into ice production. Nevertheless,this still requires a decreasingwith time at the variousstations. Distance(km)
916
MACDONALD
(a) MW
ET AL' FRESHWATER BUDGET OF MACKENZIE RIVER
Late Summer
6
•
23•> •37
3
' ,'
Late Winter
4
-•-
10
135
!
0.4
0.1
2.3
8
,
•
11
8
,
2.8
36
[
o.,
[
o.,•
l
1.0
I
3.0
I
•44
43
!
0.5
'•0.7
0.8
' ,
•0.9
'
•>•-0.9 ! •_ -1.9
Figure 15. Budgets(in cubickilometers) and fluxes(cubickilometers transferred between
seasons) of (a) MW and(b) SlM fortheshelf.Thenearshore zonecoincides with20 m depth, theapproximate positionof thelandfastice. Budgetsareestimated fromdatagivenin Table• for thetwosampling periods(September 1990andApril-May1991).Fluxeshavebeencalculated by
addingtheknownriverftowfortheperiod(ArcticY•edmyerstation,Wate•Surveyof Canada) andbalancingthe startingbudgetwith the endingbudget.The smallarrowsat the top of each panelfor late summerreferto ice;it hasbeenassumed herethat half the iceformedin eachzone
is melted within the gonein summer. Seetext for descriptionsof numbers.
substantial leakage of MW (31km•) to theoutershelf
Sea Ice
Melt
throughoutthe period. The volumesgivenin the top panelof Figure15ahavebeenconvertedinto equivalent SinceSIM (Figure15b) can haveboth positiveand heights(bottompanel)by usingthe respective surface negativevalues,fluxesc,fSIM in one directioncan alareasfor the regions(Table 1). For the fluxesbetween ternativelybe representedas fluxesof negativeSIM in the inner and outer boxes,however,we must give two the oppositedirectiono: by a combinationof both. In equivalentheights;one for the box being left and one Figure 15b we have adoptedthe conventionof giving for the box beingentered.The magnitudeof the fluxes the fluxes as positive SIM. As was the case for MW, comparedto the contentsof the boxessuggeststhat the nearshore SIM is utmost balanced in late winter, betempting to equate the 16km3 ofsea the flushingtime of MW on the shelfis longerin winter andit would
SIM in thewa(about6 months) thanin summer (about4 months) and iceformedwiththe 14km3 of negative that it is very short in the nearshorezone in summer ter. However,in Septemberthere remaineda significant
(1 month).
quantity ofSIM(11km3)sothatsome ofthismusthave
MACDONALD
ET AL.: FRESHWATER
BUDGET
OF MACKENZIE
RIVER
917
leakedto the outer shelfor negativeSIM must havebeen curs in winter but not in sufficientquantities to turn off imported as brine from 'the outer shelf. A combination convectionthere. Three processescombine to suppress oftheseprocesses is probably responsible for the9 km• the transport of fresh water to the outer shelfin winter: flux requiredto balancethe system,but we have no way Mackenzie River inflow is reduced, some of the inflow is of separating the two. frozen into ice, and the stamukhi zone acts as a backThe amount of sea ice produced on the outer shelf stop to impede progressof the plume in late winter. cannot be estimated with certainty from measurements Conversely,in summerthe impoundedand frozen fresh of ice cover and thickness alone. Even if one accounts for water is suddenlyreleasedand, togetherwith intensified the ice that has goneinto forming ridgesby using some inflow from the Mackenzie River in freshet, enters the appropriatefactor [e.g.,Pri•se•berg• 1988;Melli•g a•d ocean in a short time. Ice therefore amplifies the seaRiedel, 1993] and makes reasonableestimates of the sonality of the freshwater cycle on the Mackenzie shelf. amount of open water using remote sensing,a problem We can presently produce a reasonable accounting remainsin determiningexactly how much of the ice that of meteoricfreshwater (runoff) as it transitsthe shelf formed on the shelf within that year was subsequently because(1) we can distinguishthis water from seaice advected off the shelf. The difference in the outer shelf melt and estimate its quantity using isotopic composiwater column content of SIM between September and tion and (2) we havereliablemeasurements of MackenMay amounts to about143km3 allowing for the net zie inflow throughout the year. The other processthat influxof9 kms fromthenearshore (Figure15b).This affectsfreshwater balanceis the formation and melting
quantityof negativeSIM is equivalentto the growthof of ice (SIM). As wasthe casefor meteoricwater,wecan an averageof 3.0 m of seaice (equivalent liquidheight) distinguish SIM andestimateits quantityusingisotopic over the region or an ice thicknessof about 3.3 m. In
composition. However,sincewe presentlydo not have
Figure15bwe havebalancedthe SIM budgetsimplyby reliablemeasurements of the source(new ice producproducingsufficientice. While an averageof 3.3 m of tion) or sink (lossof brine from the shelf), we cannot new ice production over the outer shelf appears to be a reasonableestimate, it must be noted that any brine that has been formed and subsequentlyescapedfrom
as yet produce a complete budget. Acknowledgments.
We are extremelygratefulfor the
the region(bottomdashedarrow,Figure15b) would logistic support provided to us by officersand crew of the not leave a record in the water
and therefore
would be
missedin this budget. Melling and Lewis [1982]suggest that in regionsof .,;mallbottom slope, densewater may linger for many months and that drainage of salinewater from the shelf is of seasonalor longer duration.
Under
these circumstances
the water
column
CCGS Henry Larsen and the staff of the Polar Continental
StieffProjectw.ithout whomwe wouldnot havebeenableto collect the samples. In particular, we thank Captains Dave Johns, Steve Gomes, and Ivan C5t4 of the Canadian Coast Guard, Barry Hough and Claude Brunet of the Polar Continental Sheff Project, and Ron Sprang and Pieterre Paros., the aircraft pilots for our spring work. Adrian Abehennah and William Grieve assisted with the 5•sO determinations
record of ice production is probably a better estimate than one based on the ice content plus ice advection. at IOS. We are indebtedto the Frozensea ResearchGroup Unlessone carefully measuresthe total brine produced (IOS) for supportand loan of equipmentand to Gerry La-
andOceans, Winnipeg)forgenerously sharing (watercolumnbudgetchangeplusexport)or the total cho(Fisheries laboratory spaceat Tuktoyaktnk. Paul Squires(Water Surnew ice produced(budgetof new ice plus new ice ex- vey) providedflowdata for the Macken•.ieRiver system.The ported), one cannotproducea completebudgetas has work was funded by Indian and Northern Affairs Canada as
beenproposedfor Arctic climatestudies[World Climate part of the Northern Oil and Gas Action Program; we'thank David Stone and Russ Shearer for their continued interest in ResearchProgram,1994]. and support for our work. We acknowledgeRosalie Rutka
8. Conclusions
forthemanycarefulhoursspentonproducing diagrams and organizingthe final versionof the text. Finally, we greatly
appreciate colleagueswho took time to review e•lier drafts Measurements of salinityand $180 in the water col- and suggest numerous improvements; they are Knut Aa-
umn at the beginningand end of winter and in the ice gaard,KellyFalkner, GSte•stlund,Stephanie Pfirman,and grownthat winter showthat the Mackenzieshelfdivides Peter Scklosser. into two domainsduring winter. in the nearshore,invasion by Mackenzie River winter inflow progressively suppressesconvectionand producesa slowly advancing References
front (0.2-1 cms-1). In the offshore, seaice formation enhances the salt content of the water, thereby promoting convection. The border area between these two
domainscanbe narrow(lessthan a kilometer)andgenerally follows the stamukhi zone in late winter. The within-shelf partition in winter cannot be ignored if we
are to producemodelsthat correctlysimulatehowwater and associatedproperties transit the shelf to the interior Arctic Ocean. It •'isprobable that leakage of some fresh water from the nearshore to the offshore oc-
Aagaard, K., and E. C. Carmack, The role of sea ice and other fresh water in the Arctic circulation, J. Geophys. Res., 9.•, 14,485-14,498: 1989.
Aagaard, K., L. K. Coach:man,and E. C. Carmack, On the halocline of the Arctic Ocean, Deep Sea Res., Part A, •8• 529-545,
1981.
Anderson, D. L., Growth rate of seaice, J. Glacio!., 3• 11701172, 1961.
Beck, N., and Ko O. Muexmich, Freezing of water: Isotopic
fractionation(abstract) Chem. Geol., 70• 168, 1988.
918
MACDONALD
ET AL'
FRESHWATER
Brunskill, G. J., The Mackenzie River basin, in The Ecology oj• River Systems, edited by B. R. Davies and K. F. Walker, pp. 435-471, Dr. W. Junk, Dordrecht, The Netherlands, 1986. Burns, B. M., The climate of the Mackenzie Valley, vol. 1,
Publ. EN 57 7/24,227 I•P., Environ. Can., Toronto, 1974. Carmack, E. C., R. W. Macdonald, and J. E. Papadakis,
BUDGET
OF MACKENZIE
RIVER
Macdonald, R. W., R. Pearson,D. Sieberg,F. A. McLaughlin, M. C. O'Brien, D. W. Paton, E. C. Carmack, J. R. Forbes, and J. Barwell-Clarke, NOGAP B.6 Physical and chemical
data collected in the Beaufort
Sea and Mackenzie
River delta, April-May 1991, Can. Data Rep. Hydrogr. Ocean Sci. 10•, 154 pp., Inst. of Ocean Sci., Sidney, B.C., 1992.
Maykut, G. A., The ice e.avironment,in Marine BiologyPolar Regions, edited by R. Horner, pp. 21-82, CRC shelf estuary, J. Geoph•s. Res., 94, 18,043-18,055, 1989. Press, Boca Raton, Fla., 1985. Fissel, D. B., and H. Melling, Interannual variability of oceanographic conditions in the southeastern Beaufort Maykut, G. A., The surface heat and mass balance, in The Geophysicsof Sea Ice, edited by N. Untersteiner, pp. 395Sea, Can. Contract Rep. Hydrogr. Ocean Sci. 35, 102 463, Plenum, New York, 1986. pp., Inst. of Ocean Sci., Sidney, B.C., 1990. Giovando, L. F., and R. H. Herlinveaux, A discussionof Melling, H., and E. L. Lewis, Shelf drainage flows in the factorsinfluencingdispersionof pollutantsin the Beaufort Beaufort Sea and their effect on the Arctic Ocean pycnoSea, Pac. Mar. Sci. Rep. 81-4, 198 pp., Inst. of Ocean cline, Deep Sea Res., Part A, 29, 967-985, 1982. Sci., Sidney, B.C., 198!. Melling, H., and D. A. Riedel, Draft and movement of pack Ice Centre, Ice Thickness Climatology, 1961-1990 Normals, ice in the Beaufort Sea, April 1990-March 1991, Can. Publ. EN 57-•8•1961-•.990, 277 pp., Minist. of Supply Tech. Rep. Hydrogr. Ocean Sci. 151, 79 pp., Inst. of and Serv., Ottawa, Ont., 1992. Ocean Sci., Sidney, B.C., 1993. Ingram, R. G., Characteristics of the Great Whale River Melling, H., and R. M. Moore, Modification of halo cline plume, J. Geophys. Res., 86, 2017-2023, 1981. sonrce waters during freezing on the Beaufort Sea Shelf: Ingram, R. G., and P. Larouche, Variability of an under-ice Evidence from oxygen isotopes and dissolvednutrients, river plume in Hudson Bay, J. Geophys. Res., 92, 9541Cont. Shelj• Res., in press, 1995. 9547, 1987. Omstedt, A., and S. Wettlaufer, Ice growth and oceanicheat Jeffries, M. O., and H. R. ]•rouse, Salinity and isotope analflux: Models and measurements,J. Geophys. Res., 97, 9383-9390, 1992. ysis of some multi-year landfast sea-ice cores, northern Ellesmere Island, Canada, Ann. Glaciol., 10, 63-68, 1988. Omstedt, A., E. C. Carmack, and R. W. Macdonald, Mode•llingthe seasonalcycleof salinityin the Mackenzieshelf/ Jeffries, M. O., W. M. Sackinger, H. R. Krouse, and H. estuary, J. Geophys. Res., 99, 10,011-10,021, 1994. V. Setson, Water circulation and ice accretion beneath Ward Hunticeshelf(northernEllesmere Island,Canada), O'Neil, J. R., Hydrogen and oxygen fractionation between ice and water, J. Phys. Chem., 72, 3683-3684, 1968. deduced from salinity •nd isotope analysis of ice cores, Ann. Glaciol., 10, 68-72, 1988. •)stlund,H. G., The residence timeof the freshwater component in the Arctic Ocean, J. Geophys. Res., 87, 2035Jeffries,M. O., H. R. Krouse, W. M. Sackinger,and H. V. Water
mass structnre
a.nd boundaries
in the Mackenzie
Setson,Stable-isotope (LsO/•60)tracingof fresh,brackish, and sea ice in muir's-yearland-fast sea ice, Ellesmere Island, Canada, J. Glaciol., 35, 9-16, 1989.
Krouse,H. R., andJ. R. Mackay,ApplicationofH•sO/H•60 abundancesto the problem of lateral mixing in the LiardMackenzie River system, Can. J. Earth Sci., 8, 11071109, 1971.
3684, 1982.
•)stlund,H. G., and G. Hut, Arcticoceanwatermassbalance from isotope data, J. Geophys.Res., 89, 6373-6381, 1984.
Paton, D. W., A. Abehennah, W. Grieve, and R. W. Macdonald, NOGAP B.6 Oxygen isotopedata from water and ice cores from the Beaufort Sea, September 1990, May 1991 and September 1991, Can. Data. Rep. Hydrogr. OceanSci. 13•, 61 pp., Inst. of Ocean Sci., Sidney,B.C.,
Lehmann, M., and U. Siegenthaler,Equilibrium oxygenand hydrogen isotope fractionation between ice and water, J. Glaciol., 37, 23-26, 1991. 1994. Lepp&ranta, M., A review of analytical models of sea-ice Prinsenberg, S. J., Ice-cover and ice-ridge contributions to growth, Atmos. Ocean, 31, 123-138, 1993. the freshwater contents of Hudson Bay and Foxe Basin, Lewis, E. L., and R. G. P :rkin, The practical salinity scale Arctic, •1, 6-11, 1988. 1978: Conversionof existing data, Deep Sea Res., Part A, Redfield, A. C., and I. Friedman, The effect of meteoric 28, 307-328, 1978.
Macdonald, R. W., and E•. C. Carmack, The role of largescale underice topography in producing property flux di-
vergencein an Arctic shelf/estuary,Atmos. Ocean,29, 37-53, 1991.
Arctic
Macdonald, R. W., C. S. Wong, and P. Erickson, The distribution
of nutrients
in the southeastern
Beaufort
Sea: Im-
plications for water circulation and primary production, J. Geophys. Res., 9•, 2939-2952, 1987. Macdonald, R. W., E. C. Carmack, F. A. McLaughlin, K. Iseki, D. M. Macdonald, and M. C. O'Brien, Composition and modification
water, melt water and brine on the composition of Polar Sea water and of the deep waters of the ocean, Deep Sea Res., 16, suppl., 197-214, 1969. Scltlosser,P., D. Grabits, R. Fairbanks, and G. BSnisch,
of water
masses in the Mackenzie
shelf
estuary, J. Geophys. Res., 9•, 18,057-18,070, 1989. Macdonald, R. W., E. C. Carmack, F. A. McLaughlin, D. Sieberg, M. C. O'Brien, D. Paton, R. Pearson, Y. Liangfeng, and C. Gobeil, Oceanographicdata collected
from the Henry Larsen in the Beaufort Sea, AugustSeptember, 1990, Can. Data Rep. Hydrogr. Ocean Sci.
97, 142 pp., Inst. of OceanSci., Sidney,B.C., 1991.
river-runoff:
Mean
residence
time
on the shelves
and in the halocline, Deep Sea Res., 41, 1053-1068, 1994. SHEBA ScienceWorking Group, New program to research issuesof global climate in the Arctic, Eos Trans. A 75, 249-253, 1994.
Tan, F., and P.M. Strain, The distribution of sea ice meltwater in the eastern Canadian Arctic, J. Geophys. Res., 85, 1925-1932, 1980.
Thomas, D. J., R. W. Macdonald, and A. B. Cornford, Geochemical
mass-balance
calculations
for the coastal
Beau-
fort Sea, N.W.T., Canada. Rapp. P.-V. R•un. Cons. Explor. Met., 186, 165-184, 1986. Vetshteyn, V. Ye, G. A. Malyuk, and V. P. Rusanov, Oxygen -18 distribution in the central Arctic Basin, Oceanology, lg, 514-519, 1974.
MACDONALD
ET AL.:
FRESHWATER
Whaite, P., An automated sample line for the preparation
BUDGET
OF MACKENZIE
RIVER
Institute of Ocean Sciences,P.O. Box 6000, Sidney, B.C.,
of lSO/160 isotopeanalyses fromwatersamples, M.Sc. CanadaViL 4B2. (e-mail:robie•ios.bc.ca) thesis, 165 pp., Univ. of B.C., Vancouver, 1982. World Climate ResearchProgram, Arctic Climate System
A. Omstedt, SwedishMeteorologicaland HydrologicalInstitute, S-601 76 NorrkSping, Sweden.
Study (ACSYS), Initial implementationplan, WCRP-85, Geneva, 1994.
E. C. Carmack, R. W. Macdonald, and D. W. Paton,
(ReceivedApril 20, 1994;•evisedAugust 16, 1994; acceptedSeptember16, 1994.)
