14SJ. Rcch. OciGnolraphique, 1991 ,!foL 23",°4: 14S-lSJ

DIATOMS AS INDICA TORS OF COASTAL PROCESSES AND SEDIMENTARYENVIRONMENTS IN THE CANADIAN ARCTIC

Stephane CAMPEAU1, Arnaud HEQUEITE2 and Reinhard PIENITZ1

I Centre d'EtIJdes nordiquea, Universite Laval, Sainte-Foy, Que~ Canada, OIK 7P4

Email: sIepbane.campeaU@oowrier.ggr.ulavalca2 Departement de oeographie, Universite du littoral, 2 Chaussee des Danes, F-S9140~.

Keywords: Diatoms, coastal environments, paleoenvironmental reconstruction, Beaufort Sea

Abstract: Surface sediment diatom assemblages from 74 stations along the southeastern Beaufort Sea coasts were analysedto provide modern analogues for future stratigraphic studies. The modern environments sampled are believed to represent thcfull range of sedimentary facies that may have been preserved in Holocene stratigraphic records of the Beaufort Shelf.Samples were collected from coastal freshwater environments, backbarrier environments, the shoreface, the inner shelf, andthe Mackenzie Delta front area. A total of 225 diatom taxa representing 60 genera were identified. The results of this studysuggest that diatoms represent excellent indicators of environmental parameters such as the proximity of ponds and otherfreshwater bodies to the sea, the degree of communication of lagoonal environments with the open sea, the magnitude of tidalcurrents, the importance of storm-induced overwash processes; the salinity fluctuations induced by the Mackenzie Riverplume and other freshwater inputs from the land, the proximity of the polar pack ice and duration of the landfast ice, and thewater depth.

COTIERSLES DIATOMF.ESEN TANT QU'INDICATRICES DE PROCESSUSD'ENVIRONNEMENTS SF-DIMENTAIRES DANS L' ARCllQUE CANADIEN

ET

Mots cles: Diatomees. environnenlents cotiers. reconstitution paleoenvironnementale. mer de Beaufort.

Resume: Les assemblages diatamiflres des sediments de surface des cotes de la mer de Beaufort furent analyses afin deproduire un cadre de reference pour l'identification des environnements sedimentaires preserves dans les sequencesstratigraphiques de la plate-forme continentale. Les 74 echanti/lons preleves proviennent d'un large eventail d'environ-nements, leis que des etangs cotiers, des lagunes, l'avant-cote, la plate-forme continentale et Ie front deltarque du jleuveMackenzie. Pres de 225 taxa de diatomees furent identifies, repartis en 60 genres. Les resultats de celie etude suggerent queles diatOlneeS constituent d'exce/lents indicateurs pour la reconstitution de parametres paleoenvironnementaux tels que ladistance separant les etangs cotiers de la mer, Ie degre de communication entre les lagunes et la mer, la force des courantsde moree, l'intensite des deborde111ents de tel11pete, les fluctuations de salinite provoquees par Ie panache de turbidite dujleuve Mackenzie et par d'autres apports d'eau continentale, la proxil11ite de la banquise polaire et la duree des gelssaisonniers en milieu colier, et la profondeur de l'eau.

INTRODUCnONDiatoms are unicellular algae composed of siliceousvalves which are usually well preserved in thestratigraphic record since the Cretaceous. They are idcaJpaleoenvironmental proxies since they are singJe-celledorganisms that respond rapidly to environmental changesand are very abundant in most marine, coastal andlacustrine environments. Diatoms can be used asindicators of environmental parameters such as waterchemistry, paleosalinity, paleodepth, paleo-temperature,paleocurrents and paleonutrient concentrations. Mostdiatom taxa found in Quaternary sediments are extant andthus sound paleoecological interpretations are based uponsimilarities between populations found in modern habitatsand recovered microfossil assemblages. Paleoenviron-mental interpretations of downcore diatom data that havebeen based on previous studies of the surface sedimentassemblages of modern environments have proven to beparticularly valuable for example in the Bering Sea(Starratt, 1993), Baltic Sea (Witkowski, 1994), Thamesestuary (Juggins, 1992), Pacific Northwest (Shennan eta/., 1996), northwest England (Shennan et a/., 1995;Zong, 1997) and in numerous paleolimnological studies

(e.g., re\iewed in Charles & Smol, 1994; and Moser etal., 1996). In coastal areas, the characterization of moderndiatom assemblages may yield reliable criteria to identifyancient analogues of sedimentary environments andprovide a valid basis for the elucidation of paleo-oceanographic conditions and changes in sea level. TheBeaufort Sea coasts represent an area of particular interestto studies of coastal paleoenvironments and sea-levelvariations as they lie within an area especially sensitive toboth short- and long-term environmental changes. In theshort-term. rates of coastal retreat in excess of 2 m yr") arecommon (Hequette & Ruz, 1991), while over a longertimc-scale the area has undergone a persistent rise in sealevel since the Late Pleistocene (Hill et al., 1985). Thepurpose of this project is to provide a firmer basis for theuse of diatoms as indicators of coastal paleoenvironmentsin the Beaufort Sea. To this end the diatom assemblagespreserved in the surface sediments of a variety of coastalsub-environments, from salt marshes to the inner shelf,were analysed to define the present biofacies of thesesedimentary basins and provide modern analogues forfuture stratigraphic studies.I

146

SETflNG

The Beaufort Sea is the southernmost part of the ArcticOcean. The southeastern Beaufort Sea is bordered by theTuktoyaktuk Coast1ands which are part of the ArcticCoastal Plain between the Mackenzie Delta andAmundsen Gulf (Rampton, 1988), and comprise theTuktoyaktuk Peninsula and Richards Island (figure 1).The southwestern part of the Tuktoyaktuk Peninsula isprimarily fonned of ice-contact deposits, moraines ormorainal veneer overlain by lacustrine sediments ofHolocene age, while the northeastern part consists ofglacial outwash sands which are in places covered byeolian or lacustrine sediments (Rampton, 1988). TheTuktoyaktuk Peninsula is believed to have beencontinuously ice-free for at least the past 13 ka (Ritchie,1984; Vincent, 1989). Between 13 to 8 ka BP, climate wassignificantly wanner than at present, possibly as a resultof an early Holocene Milankovitch insolation maximum(Ritchie et 01., 1983; Ritchie, 1984). Thennokarstoccurred widely on the Tuktoyaktuk Coastlands(Rampton, 1988; Mackay, 1992), as the active layerdeepened regionally and numerous retrogressive thawslumps developed (Rampton, 1974). Rampton (1988)suggested that thennokarst basins fonned where thedeepening active layer intercepted massive icy beds.Ponds fonned at such locations and developed intothennokarst lakes as the basins expanded, mainly byretrogressive thaw slumping. The paleoecological rccordfrom lake sites indicated that a gradual climatic coolingbegan ca 8 ka BP. The limit of the forests shifted south-wards, reaching its present position at about 4.5 ka BP(Spear, 1983; Ritchie, 1984). The present-day climate ofthe southern Beaufort Sea coastlands is characterized byshort cool summers and long harsh winters. The meanannual temperature at Tuktoyaktuk is -10 °C. Precipita-tion is low, with an annual mean of 142 rom, roughly halfof which is accounted for by winter snowfall (Environ-ment Canada, 1993). Cold climatic conditions during thePleistocene have led to the formation of pennafrostthroughout the region. Permafrost is widespread beneathland areas, ranging from 200 to 500 m in thickness on theTuktoyaktuk Peninsula, and over 700 m on RichardsIsland (Judge et 01.,1987). Much of the topography in thearea can be attributed to the presence of subsurfaceground-ice. Thermokarst lakes cover about 35 % ofRichards Island and the Tuktoyaktuk Peninsula, andnearly 70 % of the northeastern part of the peninsula.Forbes (1980) suggested that the relative sea level (RSL)of the Beaufort Sea has been rising over the last 15 ka.Hill et 01. (1985) reported a number of additional ages andproposed a Late Quaternary sea-level curve for theCanadian Beaufort Sea. According to this curve, the RSLrose from -140 mat 27 ka BP to a relative highstand of-40 m at approximately 15 ka BP and was th~n lowered toa Late Wisconsinan lowstand of -70 m. During the Holo-cene, the RSL rose from -70 m to its present position. Therate of RSL rise during the Holocene was furtherinvestigated by Hill et 01. (1993). During the EarlyHolocene, this rate was on the order of 4 to 5 mm yr", andthen increased to 7 to 14 mm yr" during the mid-Holocene. Over the last 3 ka, the rate of RSL rise slowedmarkedly. Recent radiocarbon dates on peats from modem

coastal marshes on the Tuktoyaktuk Peninsula alsosuggest a slow rise in sea level during the last 1 ka (Hill eta/., 1990). Although not statistically significant, tide-gauge data from Tuktoyaktuk suggest that relative meansea level is still rising at a rate of about 1 mm yr-t (Forbes.1980).The coastal areas of the Tuktoyaktuk Peninsula aregenerally low lying with local relief less than 30 m- Thecoast consists mainly of bluffs developed in ice-bondedQuaternary sediments, and of barrier islands and spitsenclosing, partially or completely, lagoons andembayments formed by the brooching of thermokarstlakes. Spits and barrier islands form approximately 30 0;0of the length of the coastline east of the Mackenzie Delta(Harper, 1990). Hequette & Ruz (1991) calculated thatbarrier islands migrate onshore at a mean rate of3.1 m yr-t while spits are retreating at an average rate of1.7 m yr-t. Causes of the widespread coastal retreat alongthe southeastern Beaufort Sea include: 1) wave-inducederosion; 2) thermal erosion (Harper, 1990)~ and 3) theongoing relative sea-level rise (Hill et 01., 1993). TheCanadian Beaufort Shelf extends offshore to 60-100 mwater depth and is characterized by a very gentle 8r3:dient.The area is divided into three distinct physiographicregions: the narrow western shelf adjacent to the U.S.border, the Mackenzie Trough, and the broad easternshelf. The fine surficial sediments (silts and clays) of theinner shelf are mainly derived from deposition ofsuspended sediments from the Mackenzie River.The coastal ice regime is marked by four "seasons": openwater, freeze-up, winter and break-up. Coastal ice formsand becomes intermittently stationary during the freeze-upseason, usually from October to mid-Dccember. Thewinter season, usually from mid-January through May, ischaracterized by stable coastal ice (fast ice). The break-upseason from June to mid-July is associated withdeterioration of the fast ice. This period is followed by theopen water season, from mid-July to early October.During the open-water season, winds originate mainlyfrom the east, southeast and northwest quadrants. Stormwinds (>40 km hOt) are usually from the northwest. Thcpresence of sea ice during eight to nine months limitswave activity during most of the year and, even during theopen-water season. wave generation is limited by thefetch-restricting pack ice. As a result, the Beaufort Sea isa moderate wave-energy environment and nearly 80 % ofdeep-water waves are less than 1 m in height (Harper &Penland, 1982). Tidal range is small, with typical rangesof 0.3 m for neap tides and 0.5 m for spring tides. Stormsurges, however, are known to be significant. Surveys oflog debris lines stranded on the tundra during these stormevents indicate surge elevations up to 2.4 m above meansea level in the Tuktoyaktuk area (Harper et 01., 1988).The Tuktoyaktuk Coastlands are covered by low Arctictundra vegetation. Upland sites are dominated by dwarfshrubs and lichens, whereas poorly drained peaty areasshow ice-wedge polygon and frost hummock developmentwith dominance by Eriophorum vaginatum and Carexstands (Corns, 1974). Sedge tussock Oats occur aroundmany lakes, on the bottom of drained lakes and alongcoastal lowlands (Mackay, 1963).

147

Figure I:The southeastern Beaufort Sea.

Figure J : fA sud-est de 10 mer de Beouforl

METHODS

SamplingSurface sediment samples were co11ected at 74 stationsalong the southeastern Beaufort Sea (figures 2 and 3). Thestations were selected to encompass an environmentalspectrum as broad as possible. The modem environmentssampled in this study are thought to represcnt the fu11range of sedimentary facies preserved in the Holocenestratigraphic records of the Beaufort Shelf. Samples wereco11ected from coastal freshwater environments, back-barrier environments, the shoreface, the inner shelf, andthe Mackenzie Delta front area. Sampling was assisted byusing a 200 kHz depth recorder (Raytheon Fathometer~.Surface grabs or top core sediments were collected. ALivingston coring system was utilized in the cases wheresediments were unbonded and when they were anticipatedto be very soft and highly organic (e.g. in breached lakebasins). A portable vibracoring system was used inlocations where sediments were unbonded and firmer thancould be penetrated by the Livingston system. Ice bondedmaterial was co11ected using a CRREL auger system witha Stihl power head mounted on a Winke Unipress dri11stand. The upper 2 to 5 cm of sediment in cores or surfacegrabs was used. These surface sediments likely contain atime-averaged assemblage composed of diatoms depositedduring a number of discrete events. The distributionalpatterns derived from these samples do not represent theecological preferences of each species but may reflect allthe processes that have led to the formation of theassemblages, including the ecological requirement of eachspecies of the biocenosis. and the processes that have ledto the final composition of the thanatocenosis. Seasonalvariability in the composition of the biocenosis anddiscrete. events responsible for the reworking of theassemblage may therefore be averaged out, but the timeperiod represented by the sample wi11 be unknown.

BEAUFORT SEA

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&\. Breached Ilk.. S.mplinc silOS

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Figure 2: The A1kinson Point area and location of sampling sites.Figure 2 : Localisation des sites d'echantil/onnage dans Ie secteur de fapointe Atkinson.

148downweighting of rare ta.'<3. CCA detects patterns ofvariation in species data that can be explained by theobserved environmental variables (Jongman et 01., 1995).Diatom ta.'<3 that occurred in at least three of the 74sampling sites Mth a relative abundance of ?1% wereincluded in the CCA. Of the 225 taxa identified in thesurface sediments, 105 met the above criterion. Inaddition to the measured water depth at each station, theavailable environmental data consist of somesedimentological data, such as the percentage of sand(2:63 JUD), mud «63 JUD) and organic matter, and thedistance of the sampling stations from the MackenzieDelta mouth. These variables had skewed distnoutionsand were log-transformed (in(x+I» prior to statisticalanalyses. In addition, the nine main sedimentaryenvironments occurring in our set of sampling sites,including the freshwater environments (ponds, salt pondsand coastal peats), salt marshes, semi-enclosed breachedlakes, tidal channels, breached lakes and lagoons,shoreface, inner shelf, delta front and delta front lagoons,were included as dummy (value 0 or 1) passive variables.Sununary statistics for the variables used in the CCA aregiven in Table I. The environmental variables in the CCAbiplot are represented by arrows, whereas species andsampling sites are marked by numbers. Variables Mthhigh positive correlations have small angles between theirbiplot arrows, while high negative correlations are usuallydepicted by arrows pointing in opposite directions(Jongman et al., 1995). Variables Mth long arrows havehigh variance and their prox.imity to the a.xcs surnm:lrizcsthe relative weight of each variable in determining eacha.xis (ter Braak. 1987). The direction of each arrowindicates ascending values for each environmentalvariable. Each taxon's position in the ordination space isan approximation of its weighted-average optimum inrelation to other ta.'<3 and each explanatory variable(Jongman et al., 1995).

Figure 3: 11IC Nor1b Had (A) uMI Tuktoyaktuk. (8) -- uMI ~ 01sampling .ilea.Fig.". J : Localilalion du Iitel d'khanlillOllllage donI lei lectellrs deNOt11I Head (A) ., . TIIkIoyakIIlk (B)..

Oeanin2 and countingIn the Paleoecology laboratory at Laval University. ahomogenized sediment subsample (ca. 1 cm') from eachof the surface samples was digested in hot acid (30 %H2OV. The siliceous material was then repeatcdly washedand decanted in order to remove acids. Coarse sand wasremoved by decanting. For samples with high proportionsof clay-sized material. heavy-liquid separation withsodium polytungstate was used to concentrate diatomvalves. An aliquot of the resulting slurry was evaporatedonto coverslips. which were subsequently mounted ontoglass slides with Naphrax.. Diatoms were identified andenumerated along transects using a Leica DMRBmicroscope under phase contrast illumination at amagnification of lOOOx. When possible. 200-300 valveswere counted per sample. Several slides had sometimes tobe analysed to reach as high a number of diatom valves aspossible. particularly in samples from shorefaceenvironments. Broken valves consisting of more than halfof the valve were counted as one valve. Diatomidentifications were made to the lowest taxonomic levelpoSSlole. The Shannon-Wiener diversity index (H') of theassemblages was calculated from the relative abundancedata (If = - ~ Pi log. Pi. where P, is the proportion ofspecies i in the assemblage). The ecology (salinitypreferences and life-forms), distribution and taxonomy ofthe Beaufort Sea coastal diatoms is presented in Campeaueta/. (1998).

Table I: Sun.IIary llatisti~ f« 1hc v.nabl. used in 1hc canonicalCUTespondcncc analysis.Table 1 : Stoti"tiqU." IOIIImairu d." variobk" ulili".." don" fonoly".C'GlIOIIiqu. du CorTe.rpondaIlCU.

Ordination techniquesCanonical correspondence analysis (CCA) was performedon surface samples data using the computer programCANOCO version 3.12 (ter Braak. 1987), with

I Mud: <63 JIm2 As detemIiDed by loa 00 iFitiooS Distance to !he ~ Delta .-.Ih (1OU1henmK)It part of Kittigazuit

Bay (69" 2O'OO"N; l~ OO'OO"W). Coastal peals 8nd ponds

149

RESULTS

A total of 225 diatom taxa representing 60 genera wereidentified in the 74 sediment samples. With respect to thesalinity requirements, brackish-water forms were mostabundant The majority of the taxa had freshwater-brackish (42 %) or brackish water (34 %) affinities. Fewhad marine or marine-brackish (14 %) or freshwateraffinities (9 %). In general, the relative abundance offreshwater-brackish species tended to increase towards theMackenzie Delta front, while brackish and marine formstended to increase towards the northeastern coasts of theTuktoyaktuk Peninsula. Freshwater diatoms also reachedgreater abundances in some restricted areas along thepeninsula, especially in lagoons influenced by freshwaterinputs. The dominance of euryhaline taxa indicatesimportant fluctuations in osmotic pressure due to thedispersion of the Mackenzie River plume alongsoutheastern Beaufort Sea coasts. Benthic diatoms werepredominant, with only 10.5 % of planktonic ortychoplanktonic forms. The dominance of benthic speciesreflects the preponderance of shallow water habitats in oursample set Of the 225 taxa idcntified, 47 % were epipelic,19 % epipelic or epiphytic, 8.5 % euplanktonic, 8.5 %epipsammic, 7 % epontic (sea ice diatoms), 7 % aero-philic and 2 % tychoplanktonic.The CCA biplot (figure 4) presents a clear separationbetween shallow and deeper environments on the firstaxis, and between brackish-marine and freshwalcr-brackish environmcnts on the second a.xis. Fivc mainclusters can be easily distinguished (figure 4B) whichregroup the sites located in deep muddy environments(upper left quadrant), the delta front (uppcrmost part ofthe ordination), shallow or subaerial environmcnts withhigh organic matter contents (upper right quadrant),sandy backbarrier environmcnts (lower right quadrant)and the shoreface (1owcr lcft quadrant). Axis 1 of theCCA ordination appcars to represent a water depthgradient whereas axis 2 seems to represent a salinity andgrain-size gradient In general, as one moves from the topto the bottom of the biplots, the percentage of sand, thedistance from the Mackenzie Delta and the salinityincrease, whereas water depth increases from the right tothe left. Diatom ta.xa that plot in the upper right quadrant(figure 4A) are mostly aerophilic forms that live insubaerial environments with high organic matter contents(salt marshes and ponds). Species plotting in the lowerright quadrant are primarily epipsammic diatoms thatprefer sandy shallow brackish environments such aslagoons and breached lakes. Species plotting on the lowerleft quadrant are epipelic, epipsammic and planktonictaxa present on the shoreface. Deeper sites are positionedin the upper left quadrant Diatom taxa that plot in closeproximity to these sites are mostly planktonic forms. Sitesfrom the Mackenzie Delta front are plotted near the originof the first axis as they have intermediate water depths.They contain species that are present both in shallowfreshwater environments and in the inner shelf, but havefew species in common with brackish backbarrierenvironments. Sites 60 and 65 (figure 4B), located in theMackenzie Delta front, have diatom assemblages closelyrelated to those of the southeastern inner shelf. Stations 66to 68 are located in shoreface environments from theMackenzie Delta front and contain diatom assemblagesclo~ly related to shoreface assemblages of the

Tuktoyaktuk Peninsula (lower left quadrant). Samples 33,37, 45 and 52 have been collected from the lowershoreface. Their diatom assemblages share similaritieswith stations botll from the shoreface and the inner shelf.From the CCA biplot. it appears that surface sedimentdiatoms exhibit distinct distributional patterns which maybe related to sedimentary environments. In coastal ponds,diatom assemblages were mostly dominated by epipelicand epiphytic freshwater diatoms, such as Eunotiapraerupta and Caloneis tenuis. Coastal peat assemblageswere dominated primarily by freshwater-brackishaerophilic and epiphytic diatoms, such as Diatoma cr.vulgaris, whereas salt marsh assemblages consistedmainly of brackish aerophilic and epipelic taxa, such asDiploneis interrupta. As a consequence of the highlyvariable environmental conditions and the predominanceof sandy material, breached lake and lagoon assemblageswere dominated by euryhaline epipsammic diatoms withbroad ecological tolerances. The epipsammon consists ofsmall appressed species or species with very short stalks,which occupy depressions in the surface of sand grains.Such epipsammic taxa occur generally in intertidalenvironments with pronounced currents and sedimentdisplacement (de Jonge, 1985). Along the TuktoyaktukPeninsula, Achnanthes delicatula ssp. hauckiana was thedominant epipsammic diatom of backbarrierenvironments. The other epipsammic taxa of considerableabundance were Achnanthes lemmermannii, Opephoraolsen ii, O. cf parva, O. marina, Fragilaria cassubica, F.schulzii, and Navicula perminuta. Despite the dominanceof epipsammic taxa, diatom assemblages varied accordingto an exposure gradient to coastal processes, from semi-enclosed breached lakes to tidal channels. Althoughdominated by epipsammic species, semi-enclosedbreached lake assemblages contained more epipelic andepiphytic taxa than lagoons. Lagoonal assemblages werecharacterized by an increase in epipsammic diatoms and adecrease in species diversity. These tendencies wereamplified in tidal channels which were almost entirelydominated by epipsammic taxa and had the lowest speciesdiversity index (2.5) of all the environments investigated.Both backbarrier and shoreface environments consistedmainly of sandy sediments. However, backbarriersediments, although subjected to strong tidal currents, arelittle reworked by waves, whereas shoreface sediments arestrongly reworked by breaking and shoaling waves. Thisresults in a shift in diatom assemblage composition.Backbarrier environments were dominated by epipsammicspecies, while upper shoreface assemblages weredominated by motile epipelic taxa which are able toreposition themselves in the top layer of sediments onunstable wave-exposed sandy beaches. Lower shorefaceassemblages contained mostly planktonic species that hadsettled out of the water column during fairweatherconditions, as well as some epipelic and epipsammicdiatoms. Inner shelf assemblages were primarilydominated by euplanktonic and tychoplanktonic formsthat were able to settle owing to the low energyenvironment of the shelf.Lower shoreface and inner shelf assemblages alsocontained a number of planktonic and tychoplanktonicforms with freshwater-brackish affinities. The occurrenceof these species in inner shelf sediments reflects thefreshwater influence of the Mackenzie River plume alongthe Tuktoyaktuk Peninsula. For examole. AulacoseiraI

ISOcontained less epip~c species, more freshwater-brackish taxa, and more epipelic species adapted tomuddy surfaces than lagoonal assemblages of theTuktoyaktuk Peninsula. The most common diatomspresent were Dip/oneis smithii, Nitzschia sigma, andTryb/ione//a acuminata. Finally, although neverabundant. sea ice (epontic) diatoms were a commoncomponent of the Beaufort Sea coastal assemblages.

;s/andica, the most common species of inner shelfassemblages, was also the dominant species of MackenzieDelta front sediments.Delta front assemblages contained strongly silicifiedplanktonic species, such as Au/acose;ra is/andica and theresting spores of Chaetoceros spp., and epipelic species.Due to the proximity of the Mackenzie River, delta frontlagoons differ from the lagoons of the TuktoyaktukPeninsula through their lower surface water salinities andfiner sediments. Consequently, diatom assemblages

Figure 4: Canonical co rTespondenI:e analysis (CCA) for the lOS most cocmnoll diatom taxa (A) and the 74 sampling sites (B). A list of the full diatom names withcorresponding nurnbets used in the CCA is given in Table ll. The sampling sites are i1lusllated in figures 2 and 3. Abbreviations as follows: ACTIVEVARIABLES; DEPTH: Water depth (m); SAND: Sand (%);MUD: Mud (8/0); OM: Organic matter (%)'. DIST: Distance to the Mackenzie Delta (km);SEDIMENTARY ENVIRONMENTS (dwmny ~ve variables); FRESH: Freshwater environments; MARSH: Sah manhes; SBL: Semi-enclosed breached lakes;BI..: Breached lakc and lagoons; TC: Tidal clIamels; SHORE: Shoreface; SHELF: Inner shelf; DELTA: Delta fi'oo1; DLAG: Delta Iion1tagoons.Figur. 4: Analyse cononiqu. des correspondances (ACC) pour res 105 taxa de diatom4es res plus abondants (A) .t res 74 sites d'khantillonnage (B). La listecompUte des diatomees pnsenles dans I~CC est donnee dans Ie Tableau II. La localisation des sites d'echanti1lonnage esl presentee sur les figures 2 e13.

151Table u: Maximum relative ~ (Max.) of the lOS most abundant diatom taxa. their nwnber of ~ (Num. Occ.) and the num~ used in the CCA(figure 4).Table 1/ : Abondances relati~s maximalu (Max;) des 105 taxa de diatomees les plus abondant.r, lewrs occvnnces (Hum. OccJ et lu numiros utilises dansl~CC Qigure 4).

N° Taxon MaLJ!!l~4.1

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I NavicI,/a cinct~~!p)_~~sin Pritchardl..!J!!vicula CTucic"loides Brol:lan:umI Navicula 50. 2I Navicula cry"tonella Lan2e-BertalotI ~'a..icula dieitoradillla (Gre~o~) RaJfs b1 PritchardI Navicula dUma v.r.~~~.I Navicuill~~ka (Oeve & Gnmow) CleveJ NQVicu~ ~~alica GnmowI Navicu/apraparia DonJdn, Naviculaiamalinensis Cleve In Cleve & Grunow

~

,59 , Na..icula cr. microdieiloradiata L8i1ie-Bertalot

~

12

lS2Table lI(~~tinued) N-

~-E--

244541011

,-;-.-L

19~~~

14-;-'"~-l.!..

4~~

15

MaL..00

4.97..6Sol!.4

..!:!.-8.1U

~~

8.916.217..71.6

TaxonN°

, l!!!!-!!E!:I'" r;-ir. .ubadnata 1I_~~dt)170I Na~",bil!nata Gnmow in 9~eI NIJY~lIMitalU Oeve~ Ne~ amoliatum (Ehrenbere) K~er\ N/ttr~ bilobiiiSmithI Nlt:.fc/ria ~ii HantzschI N"1t'""chio dis.f!!!~(KUtDn2) GnmowINlttSchia s~ma ~.!!!!!n.) Smith

78 t798081 KI'aISb82 ~. on & M8JIJI

1518311.29.1

~5.8

~4..5,2

10.8ill4.J15.2U3..

I Petroneir marinll_~aIfs) Mann

5351322828313219191410V"

WIllIams &. Round

~eld ~ 17,8-\_~ssios;,a h""erborea var. s~~ionalis (Gronow) Hasle 1989(99 8.6

9.3!!:!-U15

~

I 1 00- (~las.r;o:;;;Qhv"erboreo (Gru~) J!asle-

~ 103_ILi;;Jif!Qneila apicuJo Gn/,oryi

CONCLUSIONAcknowledgements: This project was funded by theNatural Sciences and Engineering Research Council ofCanada through a postgraduate scholarship to the seniorauthor and operating grants to A. Hequette and R Pienitz.and by the Fonds pour la Formation de Chercheurs etI~ide a la Recherche (Quebec). Field work was supportedby the Geological Survey of Canada. Further logisticsupport ~'as provided by the Polar Continental ShelfProject (PCSP) in Tuktoyaktuk. Ron Felix:, MichelCloutier, Marc Desrosiers, Mathieu Avery et Marie-PierreOuellon helped with sample collection. We are grateful tothe staff of Centre d'etudes nordiques at Laval Universityfor their technical support.

The analysis of the relationships betwcen modem diatomassemblages and sedimentary environments has enabledthe definition of "analogues" of modern facies that mayoccur in sedimentary sequences of the Beaufort Sea.Comparison of microfossil assemblages with modempopulations could yield valuable information on thepaleoenvironment. For example, high abundances ofaerophilic freshwater species and a variable amount ofbrackish water taxa could reflect the close proximity ofponds and other freshwater bodies to the sea. The relativeabundance of epipsammic taxa could serve as anindication of the degree of communication of lagoonalenvironments with the open sea and of the magnitude oftidal currents. High occurrences of shoreface diatoms inbackbarrier environments could point to intense storm-induced overwash processes. High frequencies offreshwater planktonic and ichtyoplanktonic species on theinner shelf could reflect the extent of the Mackenzie Riverplume, whereas high frequencies of such species inbackbarrier environments could be indicative of localizedfreshwater inputs from the land. Unusual high abundancesof sea ice diatoms in the coastal zone could point to acloser proximity of the polar pack ice or to changes in theduration of the landfast ice. Finally, the ratio betweenbenthic and planktonic species along the shoreface profilecould be used to infer the water depth.

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Accepted for publication: 20-07-1998

I