organic-walled microfossils and geochemical tracers: sedimentary indicators of productivity changes...

Deep-Sea Research II 49 (2002) 5277–5295

Organic-walled microfossils and geochemical tracers:sedimentary indicators of productivity changes in the North

Water and northern Baffin Bay during the last centuries

Dominique Hamela,*, Anne de Vernalb, Michel Gosselina, Claude Hillaire-Marcelb

a Institut des sciences de la mer de Rimouski (ISMER), Universit!e du Qu!ebec "a Rimouski, 310 all!ee des Ursulines, Rimouski, Qu!e.,

Canada G5L 3A1bCentre de recherche en g!eochimie isotopique et en g!eochronologie (GEOTOP), Universit!e du Qu!ebec "a Montr!eal, C.P. 8888,

Succursale Centre-ville, Montr!eal, Qu!e., Canada H3C 3P8

Received 24 November 2000; received in revised form 20 July 2001; accepted 8 November 2001

Abstract

Analyses performed on 26 surface sediment samples collected with a box corer at 17 stations throughout the North

Water and northern Baffin Bay (75–791N; 68–801W) revealed abundant organic-walled microfossils, mostly

dinoflagellate cysts (103–104 cysts g�1) and organic linings of benthic foraminifers (102–103 OL g�1), as well as high

organic carbon concentrations (0.87–2.81% dry weight). These data indicate high productivity in both the pelagic and

benthic environments of the North Water and slightly lower productivity in northern Baffin Bay. The data also showed

calcium carbonate and biogenic silica dissolution throughout the study area. The dinocyst assemblages were relatively

uniform in the North Water and dominated by heterotrophic taxa (Algidasphaeridium? minutum and Brigantedinium

spp.), whereas northern Baffin Bay assemblages were dominated by autotrophic taxa, notably Operculodinium

centrocarpum and Spiniferites elongatus. The difference between these two assemblages may be related to higher

diatomaceous primary production in the North Water than in northern Baffin Bay, since diatoms constitute the

principal food source of heterotrophic dinoflagellates. The biogeographical boundary between the North Water and

northern Baffin Bay has been maintained for at least the last few centuries as shown by analyses of microfossils and

geochemical tracers in two sediment cores, one taken in the southeastern part of the North Water (761170N, 721020W)

and the other in northeastern Baffin Bay (751350N, 701480W). The analyses of the North Water core revealed relatively

uniform microfossil assemblages and organic carbon fluxes ranging from 1.1 to 1.5mg Corg cm�2 yr�1 for the last few

centuries, which corresponded to 4–6% of the present annual primary production in the euphotic zone. These data

suggest high productivity and relatively stable conditions in the polynya on a decadal time scale. In the northeastern

Baffin Bay core, the analyses indicated generally lower organic carbon fluxes, ranging from 0.3 to 0.6mg Corg cm�2 yr�1,

and (from the microfossil data) significant variations in sea-surface conditions at this lower latitude over the last centuries.

r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Arctic marine ecosystems are known to reactquickly to a changing environment, especially

*Corresponding author. Fax: +1-418-724-1842.

E-mail address: dominique [email protected] (D. Hamel).

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 9 0 - X

when the ice-cover conditions vary (Matishov,1999). Since the last glaciation, the biologicalproductivity of arctic marine environments hasbeen subjected to important fluctuations related topartial ice-cover melt (N�rgaard-Pedersen et al.,1998). Several studies using micropaleontologicaltracers also have documented changes in biologicalproductivity related to sea-ice variations on timescales covering centuries or millennia (Mudie andShort, 1985; Baumann and Matthiessen, 1992;Mudie, 1992; Kunz-Pirrung, 1998; de Vernal andHillaire-Marcel, 2000; Levac et al., 2001). Little isknown, however, about variations in biologicalproductivity of the High Canadian Arctic over thelast centuries, which have been marked byimportant climate changes including the LittleIce Age (e.g., Overpeck et al., 1997; Dahl-Jensenet al., 1998).

In polar regions, polynyas (areas of open watersurrounded by sea ice) constitute marine ecosys-tems of various sizes that are partially or totallyfree of sea ice, even when the water temperaturereaches the freezing point (Smith et al., 1990).Polynyas that occur at approximately the sametime and place each year (Smith et al., 1990) arecharacterized by high biological productivity(Stirling, 1980; Smith and Rigby, 1981). TheNorth Water, located in northern Baffin Bay(75–791N, 68–801W), is the largest recurringpolynya in the Canadian Arctic. It was firstreported by William Baffin in 1616 (Stirling,1980; Dunbar, 1981). The northern edge of theNorth Water is bordered by an ice bridge formedin Smith Sound and linking Ellesmere Island(Canada) to Greenland (Denmark). This bridgeis located at approximately the same place everyyear. The southern margin is more difficult todefine since it varies seasonally and depends uponclimate conditions (Stirling, 1980; Smith et al.,1990). In summer, the North Water constitutes anarea of approximately 80,000 km2 (Stirling, 1980;Steffen and Ohmura, 1985; Fig. 1).

The present study, undertaken within the frame-work of the International North Water PolynyaStudy (NOW), aimed at characterizing biologicalproduction from sedimentary records in order toreconstruct past changes in productivity. Its mainobjective was to assess the stability level of the

North Water ecosystem through recent time.Micropaleontological and geochemical analyseswere performed on surface sediment samplescollected throughout the North Water to documentthe spatial distribution of modern biogenic fluxes tothe seafloor. Two cored sequences, one from theNorth Water and the other from its southeasternmargin (northern Baffin Bay), were analyzed toevaluate burial rates of carbon and to reconstructchanges in biogenic fluxes over the last centuries.

For oceanographic context, the bottom topo-graphy of the North Water is variable with a meandepth of 400–500 m; the center of the basin has anorth–south channel that reaches a depth of about700 m (B#acle, 2000). Throughout the North Water,the upper 200–300 m of the water column areoccupied by the Arctic Layer (Muench, 1971;B#acle et al., 2002). This layer can be divided intotwo sub-strata. The top 75–100 m constitutes thesurface water, which is modified seasonally bysolar heating and meltwater admixture. Thesecond sub-stratum is composed of Arctic waterunmodified by surface boundary processes andderived from two distinct sources (Fig. 1). Thecentral and western parts of the basin are occupiedby Arctic Basin Water that enters the North Watervia Nares Strait and flows southward along theCanadian coast (B#acle, 2000). In the eastern partof the North Water, the West Greenland Currentof Atlantic origin flows northward along theGreenland coast below a depth of 200 m (B#acle,2000) and may influence the physical properties ofthe Arctic Layer through turbulent exchange(Melling et al., 2001). Most of the North Wateris partially free of sea ice, with ice covering lessthan 50% of the total area from April, when thepolynya begins to extend southward, until Octoberwhen sea ice starts to form again (Dunbar, 1969;Smith and Rigby, 1981; Steffen and Ohmura, 1985).

In the eastern part of the North Water, the totalannual production of particulate organic matterby phytoplankton in the euphotic zone, calculatedfrom the beginning of April to early October(April–July 1998, August–October 1999), reachedabout 250 g C m�2 yr�1, while it was three timeslower in the western part (75 g C m�2 yr�1, Kleinet al., 2002). Flux measurements, however, madewith multi-cup sediment traps moored at about

D. Hamel et al. / Deep-Sea Research II 49 (2002) 5277–52955278

250 m below the surface from September 1997 toJuly 1999, indicated that the annual fluxes ofparticulate organic carbon in the western part ofthe North Water were higher (7–12 g C m�2 yr�1)than elsewhere in the basin (0.7–8 g C m�2 yr�1),with the lowest fluxes recorded in northern BaffinBay at the southern margin of the polynya(Hargrave et al., 2002).

2. Materials and methods

2.1. Study area and sampling

Sediment samples were collected with a 0.25-m2

USNEL box core at three stations (the northern-most) in Kane Basin, at 11 stations in the NorthWater, and at three stations (the southernmost) in

Fig. 1. Location of the North Water (75–791N, 68–801W) in northern Baffin Bay and sea-surface circulation patterns (Arctic Layer):

AB, Arctic Basin Water; WGC, West Greenland Current (adapted from Blake, 1998).

D. Hamel et al. / Deep-Sea Research II 49 (2002) 5277–5295 5279

Baffin Bay in April and July 1998 and in Augustthrough October 1999 (Fig. 2, Table 1). Somestations in the North Water were sampled on twoto four occasions (Table 1). At each station, theupper cm of the sediment was collected over asurface of approximately 20 cm2 and stored in aplastic bag at 41C. These samples were used todetermine the horizontal distribution of coarsesand, palynomorphs (mostly dinoflagellate cystsand organic linings of foraminifers), calcareousshells of foraminifers, and geochemical tracers(organic carbon, total nitrogen, calcium carbo-nate, biogenic silica). At stations 54D and 76F(Fig. 2), two PVC push-cores (26-cm length, 12-cminternal diameter) were collected from the box corefor the determination of vertical profiles ofdinoflagellate cysts, organic linings of foramini-fers, organic carbon, calcium carbonate, and C/Nratios. The sediments were extruded from thetubes on a piston table designed for this purpose:the core was kept in a vertical position above thepiston that was used to extrude the sediments outof the tube. No compaction was observed whenextruding the sediments. The sediment of the twocores was cut at 1-cm intervals and each sub-sample kept at 41C in a plastic bag.

2.2. Geochemical and isotopic analyses

Accumulation rates and sediment mixing depthsin cores collected at stations 54D and 76F weredetermined from 210Pb analyses. Lead-210 con-centrations were obtained from alpha spectro-metry measurements of its daughter isotope 210Po(T1=2 ¼ 138:4 d; a ¼ 5:30 MeV). The analyses wereperformed several months after sampling to ensuresecular equilibrium between 210Pb and 210Po. Thesediment samples were spiked with 209Po andchemically treated for Po extraction and purifica-tion. The polonium samples were finally depositedon silver disks (see Flynn, 1968). The spectro-meters used for the measurement of 209–210Poactivities were equipped with silicon surface-barrier detectors (EGG&ORTEC type 576A).Counting statistics allow the determination of210Po (and thus 210Pb) activities with a standarddeviation of 73%.

Organic carbon (Corg), nitrogen (N), andinorganic carbon contents were measured using aCarlo-ErbaTM elemental analyzer. One sedimentsample aliquot was dried, ground, and analyzedfor the measurement of its total carbon and totalnitrogen contents. A second aliquot was acidifiedtwice with HCl (1N) to dissolve carbonates, thenwashed and analyzed for its N and residual Ccontent, which is considered to represent only theCorg content. Inorganic carbon was then calculatedby balancing the difference between the twomeasurements. Corg and N contents were expressedin dry weight percent of total sediment andrepresent the mean of two analyses (Table 2).Uncertainties (71s), as determined from replicatemeasurements of standard materials, averaged75% (relative) for C and N contents. This methodmay result in biases for inorganic carbon contentswhen hydrolyzable minerals other than carbonatesare present in the sediment (e.g., Leventhal andTaylor, 1990; Rainswell et al., 1994), thereforecaution must be taken when interpreting inorganiccarbon concentrations.

The biogenic silica content in sediment wasdetermined according to the protocol of Mortlockand Froelich (1989), which consists of Na2CO3

wet-alkaline extraction coupled with a molybdate-blue spectrometry determination. This methodensures a reproducibility level of about 76%(1s). The results are presented as the mean ofduplicate samples in weight percent of SiOPAL

(SiOPAL=2.139�%SiO2, SiOPAL=2.4�%OPAL;Mortlock and Froelich, 1989).

2.3. Micropaleontological analyses

The organic-walled microfossils (palynomorphs)are biological remains composed of refractoryorganic matter that is not affected by degradationduring sinking or after deposition on the seafloor.They constitute tracers of biogenic production andorganic fluxes (Jansonius and McGregor, 1996).The most frequent palynomorphs recovered inmarine sediments include dinoflagellate cysts (ordinocysts), organic linings of foraminifers, pollengrains, and pteridophyte or bryophyte spores.Dinocysts are hypnozygotes that result from thesexual reproduction of some dinoflagellate taxa in


Fig. 2. Map showing location of the sediment sampling stations in the North Water and northern Baffin Bay, including locations

(asterisks) of the two cores used for the time-series analyses and typical northern and southern boundaries (dashed lines) of the ice

cover in April (Dunbar, 1969). Isobaths are in meters.


the water column (only 10–15% of the dinofla-gellate taxa are known to form fossilizableorganic-walled cysts; Wall and Dale, 1968; Head,1996). They provide a fragmentary picture ofautotrophic and heterotrophic production in theupper water column. The organic linings offoraminifers (OL) consist of the inner-coatingscomposed of refractory organic matter thatcharacterize several species of benthic foramini-fers; they provide an indication of the benthicproduction. In palynological slides, foraminiferlinings are recognized by their series of semi-

transparent to brownish chambers linked by fora-mens (de Vernal et al., 1992).

The surface sediment samples and the sedimentsof core 54D that were analyzed for the identifica-tion and count of palynomorphs were processedaccording to the standard method described by deVernal et al. (1999). The palynological preparationincluded sonication (15–30 s) and wet sievingthrough 10- and 120-mm meshes to eliminate finesilts and clay particles in addition to coarse sands.The fraction greater than 120 mm, which containsalmost exclusively mineral material, was dried andweighed to evaluate the percentage of coarse sandvs. the total dry weight of the samples as reportedin Table 2. The 10- to 120-mm fraction wassubmitted to repeated manual washing in hydro-chloric (10% HCl) and hydrofluoric (49% HF)acids, coupled with centrifugations to eliminatecarbonate and silica particles. For the sedimentsamples of core 76F, the chemical treatments weredone automatically using a Microdigest 3.6tsystem according to the protocol of Loucheur(1999). Reproducibility tests have demonstratedthat the use of either method does not induceanalytical bias (Loucheur, 1999). After chemicaltreatment, the samples were stored in distilledwater with a few drops of 20% phenol and keptcool until they were fixed on slides with Kaiser

glycerine–gelatine. The dinocysts, organic liningsof foraminifers, pollen grains, spores, tintinnidlorica, and thecamoebians were counted system-atically using a light microscope (400� to1000� ). The dinocysts were identified to thespecies or variety level following the nomenclaturein Rochon et al. (1999). For the North Water andnortherly Kane Basin samples, which showed alow dinocyst diversity and a dominance of onlytwo taxa, a minimum of 150 dinocysts per slidewere counted. For samples from northern BaffinBay, where species diversity was higher, a mini-mum of 300 dinocysts per slide were counted tocalculate percentages of occurrence in the assem-blages. The palynomorph concentrations wereestimated using the marker-grain method (deVernal et al., 1987), which requires the additionof a known amount of exotic grains (here,Lycopodium spore tablets) to each sample at thebeginning of the palynological preparation. The

Table 1

Sampling date, location, and water depth of study sites in the

North Water, including Kane Basin, and in northern Baffin Bay

Site Sampling

date

Latitude

(N)

Longitude

(W)

Depth

(m)

2Aa 10 Apr 1998 781210 741420 579

7Aa 12 Apr 1998 781590 731200 257

16A 16 Apr 1998 771510 741470 680

22A 18 Apr 1998 771210 761290 420

35A 19 Apr 1998 761590 751030 553

40A 03 May 1998 761570 721290 461

44A 21 Apr 1998 761230 771250 370

49A 30 Apr 1998 761160 741400 454

3Da 11 July 1998 781210 741290 560

35D 17 July 1998 771000 751000 561

40D 16 July 1998 771000 721380 400

44D 07 July 1998 761220 771170 345

50D 03 July 1998 761170 741150 445

54Db 19 July 1998 761170 721020 570

58D 05 July 1998 761190 701140 259

68Dc 01 July 1998 751130 741560 499

35E 01 Sept 1999 761480 741560 490

40E 05 Sept 1999 761580 721240 611

44E 30 Aug 1999 761240 771280 338

50E 29 Aug 1999 761170 741090 466

54E 07 Sept 1999 761190 711520 609

14F 25 Sept 1999 771470 751120 670

32F 22 Sept 1999 771050 761390 259

40F 20 Sept 1999 771000 721200 544

66Fc 30 Sept 1999 751160 761050 405

76Fb,c 01 Oct 1999 751350 701480 560

aStations located in Kane Basin.bUsed for time-series comparison.cStations located in northern Baffin Bay.


Table 2

Geochemical and micropaleontological characteristics of surface sediments in North Water and northern Baffin Bay areas

Region Site Coarse sand Geochemical tracers Micropaleontological tracers

>120

mm (%)

Corg

(%)

N

(%)

C/N

(at./at.)

CaCO3

(%)

SiOPAL

(%)

Dinocysts

(g�1)

Dinocyst

assemblagea (h/a)

OLb

(g�1)

CSb

(g�1)

CaCO3 dissolution

index (log [OL/CS])

Kane Basin 2A 26 0.87 0.10 10.15 22.09 NA 4651 185.0 425 59 0.9

3D 10 1.29 0.15 10.03 14.39 1.48 4634 53.3 2900 18 2.2

7A 79 0.29 0.04 8.68 25.94 NA NA NA NA 80 NA

Northern North Water 14F 5 1.03 0.13 9.24 13.22 2.29 6820 173.0 2783 20 2.1

16A 8 1.08 0.13 9.69 15.05 2.31 17274 Abs. 2440 5 2.7

22A 29 1.18 0.18 7.65 12.20 NA 6106 158.0 499 5 2.0

32A 33 1.59 0.14 13.25 3.72 2.81 3849 152.0 805 0 c.d.

35A 17 2.10 0.21 11.67 5.47 1.99 10244 76.0 11441 3 3.6

35D 17 1.28 0.21 7.11 11.13 2.98 20866 Abs. 1508 0 c.d.

35E 43 1.18 0.15 9.18 6.66 2.84 4933 48.7 596 5 2.1

40A 5 2.08 0.23 10.55 2.53 3.76 11679 54.7 3684 5 2.9

40D 37 1.81 0.19 11.11 2.61 2.43 7100 30.0 916 0 c.d.

40E 14 1.65 0.21 9.17 2.04 NA 9097 12.6 1172 11 2.0

40F 27 1.05 0.16 7.66 2.66 3.45 6612 18.3 1073 2 2.7

Southern North Water 44A 2 2.65 0.28 11.04 2.26 4.65 13850 80.5 2444 1 3.4

44D 10 2.22 0.24 10.79 3.56 NA 8041 79.0 1156 9 2.1

44E 60 0.99 0.10 11.55 5.71 1.77 2173 31.6 347 0 c.d.

49A 17 2.21 0.24 10.74 4.60 3.15 12097 29.4 1990 8 2.4

50D 41 1.37 0.15 10.66 4.59 2.04 9172 54.3 2763 1 3.4

50E 44 1.52 0.16 11.08 3.79 2.44 6943 18.3 1217 0 c.d.

54D 9 2.43 0.28 10.13 6.49 4.09 11240 24.8 1015 1 3.0

54E 6 2.81 0.29 11.30 2.36 3.83 9402 20.4 815 2 2.6

58D 67 0.10 0.03 4.67 1.39 0.34 NA NA NA NA NA

Northern Baffin Bay 66F 23 2.60 0.27 11.23 2.30 2.64 4535 2.8 756 1 2.9

68D 19 1.60 0.18 10.37 2.85 3.63 3597 1.9 1870 17 2.0

76F 6 1.13 0.15 8.79 4.20 3.11 3219 0.3 1574 1 3.2

ah/a indicates the ratio of heterotrophic to autotrophic taxa.bOL and CS indicate organic linings and calcareous shells of benthic foraminifers, respectively.

Abs. indicates absence of autotrophic taxa.

c.d. indicates complete dissolution of biogenic CaCO3.

NA indicates no data available

D.

Ha

mel

eta

l./

Deep

-Sea

Resea

rchII

49

(2

00

2)

52

77

–5

29

55283

reproducibility of this method is 711% (2s; deVernal et al., 1987). The concentration results arepresented as the number of palynomorphs per unitdry weight of sediment (cysts g�1 and OL g�1). Thefluxes (cysts cm�2 yr�1 and OL cm�2 yr�1) werecalculated from concentrations expressed by unitvolume and the sedimentation rates as determinedfrom 210Pb measurements. We present here onlythe concentrations and fluxes of dinocysts andorganic linings of foraminifers; concentrations ofother palynomorphs determined in this area arereported in Hamel (2001).

Quantitative assessment of carbonate dissolu-tion in marine sediments was determined accord-ing to the method proposed by de Vernal et al.(1992). This method is based on the comparison ofthe number of calcareous foraminifer shells in the>120-mm fraction of sediment (obtained afterthe wet-sieving process described above) with theconcentration of organic linings obtained fromstandard palynological counts (see above). Thedissolution level is presented as the logarithm ofthe ratio of OL to calcareous shell (CS) concen-trations (log [OL/CS]). Dissolution in sediments ismarked by positive index values. A high level ofCaCO3 preservation is indicated by negative indexvalues (de Vernal et al., 1992).

3. Results

3.1. Characteristics of the surface sediments

3.1.1. Grain size and geochemical characteristics

The coarse sand component of the sediment wasexpressed as the proportion of particles exceeding120 mm in percent dry weight. It ranged from 2%to 44%, except at the most northern (7A), eastern(58D), and western (44E) stations, where thecoarse fraction reached 79%, 67% and 60%,respectively (Table 2). Such high values indicateconsiderable ice-rafting deposition. Throughoutthe study area, surface sediment samples werecharacterized by relatively high Corg content,ranging from 0.87% to 2.81% with an average of1.66%, except at stations 58D (0.10%) and 7A(0.29%), where the Corg values were particularlylow (Table 2; see also Grant et al., 2002). Elevated

concentrations of Corg were observed at stationslocated in the southern part of the North Waterand in northern Baffin Bay. The C/N ratio(organic carbon/total nitrogen) was relativelyhomogeneous throughout the study area, withvalues ranging from 7.1 to 11.7 (atom/atom),except at stations 58D, where it was much lower(4.7), and 32F, where it was particularly high(13.3; see also Grant et al., 2002).

The CaCO3 content in surface sedimentsallowed us to divide the study area into two zones.The first zone, which includes the northernstations (2–22), was characterized by high CaCO3

content (12–26%), whereas most southern stations(except 35D) were characterized by CaCO3 con-tents below 7%. The CaCO3 content was sig-nificantly higher (p o 0.05, Kruskal–Wallis test;Zar, 1999) in Kane Basin than in the other regionsof the study area. The visual microscopic sedimentexamination revealed little or no biogenic carbo-nates such as calcareous foraminifer shells orcoccoliths. Finally, Siopal contents ranged from0.34% to 4.6% (average of 2.8%) and did notpresent any particular pattern throughout thestudy area.

3.1.2. Micropaleontological characteristics:

palynomorph concentrations

In the surface sediments of the study area, theconcentration of dinoflagellate cysts ranged from2000 to 21,000 cysts g�1 (Table 2). The concentra-tion was significantly higher (po0:05; Kruskal–Wallis test) in the North Water region (2173–20,866 cysts g�1) than in Kane Basin (4634–4651cysts g�1) or northern Baffin Bay (3219–4535cysts g�1). The high concentrations of organiclinings (from 350 to nearly 11,500 OL g�1) con-trasted with the low number of calcareousforaminifer shells (o80 CS g�1) in the sediments.As for CaCO3, the CS content in surface sedimentswas significantly higher (po0:05; Kruskal–Wallistest) in Kane Basin (18–80 CS g�1) than in theother regions (0–20 CS g�1). The CaCO3 dissolu-tion index ranged from 1 to 3.6, indicating a highdissolution level of calcareous material within thesediments. The index could not be calculated forseveral samples due to the absence of CS,indicating a complete dissolution of CaCO3.


3.1.3. Micropaleontological characteristics: the

dinoflagellate cyst assemblages

Dinoflagellates are present in most aquaticenvironments; their cysts constitute good indica-tors of biological productivity as well as pastsea-surface conditions. Two independent classifi-cations are used for organic-walled cysts (dino-cysts) and thecal stages (motile stages). We usedthe paleontological nomenclature of the cyst stage(cf. Rochon et al., 1999; for cyst and thecataxonomic equivalence, see Head, 1996). Theassemblage at station 76F in northern Baffin Baywas composed of nine taxa, whereas all otherstations throughout the study area had lowerdiversities (two to five taxa). The dinocyst taxa canbe divided into two groups according to thetrophic level of their motile stage: heterotrophic/mixotrophic (Peridiniales and Gymnodiniales) orautotrophic (Gonyaulacales). The ratio of hetero-trophic to autotrophic dinocyst taxa was signifi-cantly higher (po0:05; Kruskal–Wallis test) inKane Basin and the North Water than in northernBaffin Bay (Table 2). In the three northern regions(>761N), the heterotrophic/autotrophic ratiosranged from B13 to 185 compared to o3 in theBaffin Bay samples. The dinocyst assemblages ofKane Basin and the North Water were dominatedby heterotrophic taxa (93–100%), mainly Algidas-

phaeridium? minutum (47–85%) and Brigantedi-

nium spp. (12–52%). In contrast, autotrophic taxa(27–77%), principally Operculodinium centrocar-

pum (20–67%) and Spiniferites elongatus sensu lato(s.l.) (6–7%), dominated in northern Baffin Bay.

Throughout the study area, the concentrationsof Corg; N, Siopal, dinocysts, and OL correlatednegatively (po0:05; Spearman rank correlations(rs); Zar, 1999) to the proportion of coarseparticles (>120 mm) in the surface sediments.The coarse sand reflects terrigenous deposition,likely through ice rafting, whereas all other tracersmentioned above are related to biogenic inputs.

3.2. Geochemical and micropaleontological tracers

in time series

3.2.1. Geochemical tracers210Pb measurements in sediments from the

southern North Water area and northern Baffin

Bay suggested a mixed layer of about 2–3 cm andaccumulation rates of B0.1 cm yr�1 at station 54D(Fig. 3) and B0.05 cm yr�1 at station 76F (Fig. 4).Therefore, core 54D represented a sedimentarysequence of about 275 years and core 76F, a longerinterval of about 530 years. Taking into accountthe thickness of the mixed layer and the sedimen-tation rates, smoothing due to biological mixingwould account for about 30–40 years of sedimen-tation in both cores.

Despite the smoothing effect of bioturbation,the Corg and, to a lesser extent, the CaCO3

concentrations showed variations with depthat station 54D (Fig. 3). The Corg fluxes wereestimated to vary between B1.1 and1.5 mg cm�2 yr�1. The terrigenous CaCO3 fluxesranged between 3.39 and 5.35 mg cm�2 yr�1. Asignificant decrease in Corg was recorded between 8and 12 cm that seems to match the slightly higherinput of ‘‘terrigenous’’ CaCO3. The C/N ratio(organic carbon/total nitrogen) at this stationseemed to increase slightly with depth, showinglower C/N values towards the surface. Therewas an exception at 2 cm below the surface, wherethe C/N ratio was equivalent to the ratio observedat the bottom of the core. The proportion ofcoarse particles (>120 mm) in the sediments ofcore 54D was relatively uniform and neverexceeded 2.6% except for the first centimeter,where this proportion reached 4.7% (data notshown).

At station 76F, variations in Corg below 10 cmcorrelated inversely with CaCO3 concentration(rs ¼ 20:86; po0:001) and positively with C/Nratios (rs ¼ 0:81; po0:001) (Fig. 4). At the surfaceof the core, however, the Corg concentrations wereparticularly low, even though the CaCO3 con-centrations were minimal. The Corg fluxeswere estimated to vary between B0.3 andB0.6 mg cm�2 yr�1 and the CaCO3 fluxes, betweenB0.8 and B2.3 mg cm�2 yr�1. The C/N ratioprofile showed some small variations throughoutthe core, with values ranging from 8.6 to 10.7. Theproportion of coarse particles (>120 mm) in core76F reached 6% and 7% at the top and thebottom of the core, respectively, whereas it neverexceeded 3% for the rest of the core (data notshown).


3.2.2. Micropaleontological tracers

The temporal (depth-related) profiles of dino-cyst abundance, percentages of dinocyst taxa, and

abundance of organic linings of foraminifersrevealed differences between stations 54D (Fig. 5)and 76F (Fig. 6). In core 54D, relatively uniform

26

24

22

20

18

16

14

12

10

8

6

4

2

2 2.2 2.4 2.6 2.8

Depth (cm)

9 9.5 10 10.5 11

Ln Excess 210Pb (dpm/g)

�

�

�

�

�

�

�

�

7.5

6.5

5.5

4.5

3.5

2.5

1.5

0.5

0 0.5 1 1.5 2 2.5

Depth (cm)

Mixed layer

(a) (b)

y = 3.277-0.328x

Corg (%) CaCO3 (%)

6.5 7.5 8.5 9.5

C/N (at./at.)

Fig. 3. (a) Excess 210Pb vs. depth in a box core from station 54D, where bioturbation is assumed to have little influence on the gradient

below the mixed layer (B3 cm); and (b) vertical profiles of the concentrations of organic carbon (Corg) and calcium carbonate (CaCO3)

and of the ratio of Corg to total nitrogen (C/N) in the sediment.

26

24

22

20

18

16

14

12

10

8

6

4

2

1 1.2 1.4 1.6 1.8

Depth (cm)

8 9 10 113 4 5 6 7

�

�

�

�

�

4.5

3.5

2.5

1.5

0.5

0 0.5 1 1.5 2 2.5

Depth (cm)

Mixed layer

(a) (b)

y = 2.7857-0.6389x

Ln Excess 210Pb (dpm/g) Corg (%) CaCO3 (%) C/N (at./at.)

Fig. 4. (a) Excess 210Pb vs. depth in a box core from station 76F, where bioturbation is assumed to have little influence on the gradient

below the mixed layer (B2 cm); and (b) vertical profiles of the concentrations of organic carbon (Corg) and calcium carbonate (CaCO3)

and of the ratio of Corg to total nitrogen (C/N) in the sediment.


dinocyst concentrations were observed in theupper half of the profile, with 104 cysts g�1 onaverage. Taking into account the accumulationrate, these concentrations represent fluxes of about102–103 cysts cm�2 yr�1. The lower part of the core(below 15 cm; i.e. older than 160 years BP) wascharacterized by larger variations with minimalconcentrations between 17 and 20 cm. Within thedinocyst assemblages, Algidasphaeridium? minutum

(66–90%) and Brigantedinium spp. (7–34%), spe-cies that are associated with heterotrophic produc-tion, dominated and varied in opposition to eachother. The only taxon belonging to an autotrophicspecies was Operculodinium centrocarpum (0–3.5%), which recorded a slight increase towardsthe surface; i.e. in sediments representing the lastcentury. The concentrations of organic linings offoraminifers were fairly high throughout the coreand varied from 102 to 103 OL g�1, suggesting

benthic productivity of 10–100 foraminifers cm�2

per year.At station 76F (Fig. 6), a trend of decreasing

dinocyst concentrations towards the top of thecore was observed. The concentrations variedfrom 3� 103 to 1� 104 cysts g–1, with a peakconcentration (B1.7� 104 cysts g�1) at 22 cm. Theassemblages were dominated throughout the coreby autotrophic taxa, mainly Operculodinium cen-

trocarpum (65–84%). This taxon was accompaniedby Spiniferites elongatus s.l. and Pentapharsodi-

nium dalei. A few cysts belonging to the hete-rotrophic taxa Brigantedinium spp andAlgidasphaeridium? minutum were present. Occa-sional occurrences of Impagidinium pallidum,Spiniferites ramosus, and Nematosphaeropsis labyr-

inthus also were recorded. The organic linings offoraminifers showed some variations in concentra-tions in this core, between 102 and 103 OL g�1,

(c)0 20 40 60 80 100 0 20 40 0 2 4

26

24

22

20

18

16

14

12

10

8

6

4

2

103 104 105 10 310 2 10 4(a) (b)

Depth (cm)

Dinocysts g

−1

Algidasphaeridium?

m

inutum (%)

Briganted

inium spp. (%

)

Operculodinium

c

entrocarpum (%

)

Organic linings g

−1

Fig. 5. Summary diagram of dinocyst assemblages in the box core from station 54D: (a) dinocyst concentration; (b) percentage of the

three dominant dinocyst taxa; and (c) organic lining concentration.


which would represent high benthic production onthe order of 102 OL cm�2 yr�1.

4. Discussion

4.1. Characteristics of the surface sediments

The Corg content of North Water sediment,which averaged 1.66%, is significantly higher(pp0:001; sign tests; Zar, 1999) than that of otherpolar environments, such as the St. LawrenceIsland polynya (Bering Sea, average of 0.62%,Grebmeier and Cooper, 1995), the NortheastWater polynya (East Greenland Shelf, average of0.77%, Ambrose and Renaud, 1995), the ChukchiSea (average of 0.94%, Mayer, 1994), the Ross Sea(average of 0.63%, DeMaster et al., 1996), and theArctic Ocean (average of 1.01%, Clough et al.,1997). The Corg contents of sediments measuredthroughout the North Water are more comparableto those of temperate marine environments, such

as the Gulf of Maine (average of 1.75%, Mayer,1994) and the Gulf of St. Lawrence (1.3–2.6%,Silverberg et al., 2000; 1.6–2.4%, Mucci et al.,2000). Only a few sites, such as the most northernstation 7A and most near-shore (eastern) station58D, had particularly low Corg content. Thesestations are covered by sea ice during a longerperiod than the other stations (Dunbar, 1969),which could explain this large difference. The icecover may indeed delay the productive season bypreventing light penetration into the water columnand may consequently result in reduced phyto-plankton photosynthetic activity (Klein et al.,2002; see also Kashino et al., 2002). Limitedbiological production in turn limits the potentialexport of organic carbon to the seafloor. Stations7A and 58D are also the two sites where theproportion of coarse sand >120 mm related to ice-rafting deposition was the highest (79% and 67%,respectively), which results in a dilution of thebiogenic inputs to the sediments. In general, thestations in the study area that had higher Corg

0 5 10 0 10 20 0 5 10 150 5 100 20 40 60 80 100

26

24

22

20

18

16

14

12

10

8

6

4

2

103 104 105 102 103 104

Depth (cm)

Dinocysts g

-1

Algidasphaeridium?

m

inutum (%)

Briganted

inium spp. (%

)

Operculodinium

c

entro

carpum (%

)

Pentapharso

dinium

d

alei (%

)

Spiniferit

es

el

ongatus (%)

Organic lin

ings g-1

(a) (b) (c)

Fig. 6. Summary diagram of dinocyst assemblages in the box core from station 76F: (a) dinocyst concentration; (b) percentage of the

five dominant dinocyst taxa; and (c) organic lining concentration.


contents also had a lower proportion of coarsesediments.

The relatively low C/N ratios at most stationsindicate that organic matter inputs were relatedprimarily to marine production (Pocklington,1973). However, the atomic C/N ratio in the studyarea (average of 9.9) was in the upper range ofvalues observed in the St. Lawrence Islandpolynya (5.8–10.9, Grebmeier and Cooper, 1995),the Northeast Water polynya (6.3–10, Ambroseand Renaud, 1995), and the Ross Sea (6.4–9.3,DeMaster et al., 1996). This difference in theelemental composition of the surface sedimentsmay be explained by a higher regeneration of thesinking particles (Kawakita and Kajihara, 1990) inour study area than in other polynyas (Huston andDeming, 2002; Sampei et al., 2002) or by a higherterrigenous and/or redeposited organic matterinput (Stein, 1991), especially at stations wherethe C/N ratio exceeded 10. Higher organic matterinput would explain the slightly higher Corg

content observed in the North Water, as comparedto other polynyas and Arctic areas.

No biogenic carbonates such as calcareousforaminifer shells or coccoliths were observed inthe sediments. Thus, the CaCO3 content insediments of the study area likely had a detritalterrigenous origin. This terrigenous material couldbe related to the erosion of sedimentary carbonaterock formations in the Canadian Arctic and to thesubsequent deposition through ice rafting. Thehigh CaCO3 content in the northern part of thestudy area (12.2–25.9%) would therefore reflecthigh terrigenous input from the north or fromEllesmere Island. Since most rocks that are asource of reworked Cinorg also contain Corg, a partof the organic carbon content in sediments fromthe study area also may have a terrigenous origin.

The Siopal concentrations in the North Watersediments (average of 2.8%) are slightly lowerthan those recorded in the North Atlantic (averageof about 4%, DeMaster, 1981) and similar to orslightly higher than those of Arctic Ocean sedi-ments (about 2%, DeMaster, 1981). However,compared to the Siopal concentrations found inAntarctic sediments (Ross Sea, average of about29%, DeMaster et al., 1996; Weddell Sea, 4–15%,Schl .uter and Sauter, 2000), the Siopal accumulation

in the North Water appears low. Michel et al.(2002) measured rapid settling of diatom frustulesand a low remineralization rate of biogenic silicacompared to the remineralization rate of particu-late organic nitrogen and carbon in the upper100 m of the North Water water column. The lowSiopal concentrations in the sediments togetherwith sparse and fragmented diatoms recovered insediments of the study area (Hamel, unpublisheddata) is explained by a partial dissolution ofbiogenic silica in the deep water layer or in thesediments. This dissolution is possibly related tothe degree of diatom silicification (Schl .uter andSauter, 2000). According to Hutchins and Bruland(1998), the degree of diatom frustule silicificationdepends on iron availability, with Fe-limitationleading to more heavily silicified individuals. In theNorth Water, as in the North Atlantic, theterrigenous inputs are sufficient to provide suitableconcentrations of dissolved iron in the watercolumn, thus less heavily silicified diatoms areexpected. This mechanism, coupled with the highertemperature in the deep waters of the study area ascompared to surface waters, is likely to enhancethe dissolution rate of biogenic silica in the bottomlayer of the water column and in surface sediments(Schl .uter and Sauter, 2000; Michel et al., 2002;Tremblay et al., 2002).

4.2. Micropaleontological characteristics:

palynomorph concentrations

Dinocyst concentrations reaching 2� 104

cysts g�1 (Table 2) represent high fluxes to theseafloor, especially when taking into account thehigh sedimentation rates on the order of 1.0 and0.5 mm yr�1 as calculated for stations 54D and76F, respectively (Figs. 3 and 4). At station 54D,for example, the dinocyst flux was evaluated at 335cysts cm�2 yr�1. This value contrasts with datafrom central Baffin Bay (about 701N, 651W),where dinocyst fluxes were estimated to be lowerthan 1 cyst cm�2 yr�1 (Rochon and de Vernal,1994). Planktonic productivity in the study areatherefore appears significantly higher, by twoorders of magnitude, than in central Baffin Bay.

The dinocyst distribution in the study area isheterogeneous. The low dinocyst concentrations


(o5� 103 cysts g�1) found in Kane Basin andnorthern Baffin Bay relative to the North Waterregion might be related to the lower annualprimary production observed in these regions byKlein et al. (2002). These regions remained ice-covered until the beginning of June (Mundy andBarber, 2001), preventing light penetration intothe water column and thereby limiting phyto-planktonic production. From then on, the phyto-plankton production and biomass remained low(Klein et al., 2002; but see also Odate et al., 2002).Annual fluxes of particulate organic carbon in thenorthern Baffin Bay region also have beenidentified as the lowest of the study area byHargrave et al. (2002). However, the measurementof sedimentation rates at each coring site would benecessary to further assess the distribution ofdinocyst fluxes and organic matter burial rates.

The high concentration of organic linings offoraminifers in sediments throughout the studyarea (347–11,441 OL g�1) revealed high benthicforaminifer production. These results indicate thatthe transfer of organic matter from the surface tothe seafloor is high enough to sustain an abundantbenthic population. Hence, the absence of calcar-eous foraminifer shells in sediments has to berelated to CaCO3 dissolution occurring in themixed sediment layer (Figs. 3 and 4). Someadditional dissolution, however, also may occurafter sampling, depending upon storage conditions(Leduc, 2001). In the case of cores 54D and 76F,which represent time series, we believe that post-sampling dissolution had only a minor impact onshell preservation, since the samples were pro-cessed rapidly after sediment extrusion from thetube. Poor preservation of biogenic CaCO3 is acommon feature of surface sediments of Baffin Bayand Davis Strait (Aksu, 1983; de Vernal et al.,1992). The high biogenic CaCO3 dissolution in thestudy area could be related to high pCO2 (e.g.Aksu, 1983; Mucci et al., 2000) and poor oxygenventilation in stratified bottom waters (Aksu, 1983).

4.3. Micropaleontological characteristics: the

dinoflagellate cyst assemblages

The almost exclusive co-occurrence of Algidas-

phaeridium? minutum and Brigantedinium spp., as

observed in the North Water samples, is com-monly associated with cold or sub-arctic waters(Mudie and Short, 1985; de Vernal et al., 1997;Rochon et al., 1999). The maximum abundance ofA.? minutum is observed in waters generallycovered by sea ice from 8 to 12 months of theyear. The distribution of Brigantedinium spp.,which is one of the most ubiquitous dinocyst taxa,is more equivocal: it shows no clear relationshipwith temperature, salinity, or nutrients (Devillersand de Vernal, 2000). The abundance of Brigante-

dinium spp., however, might be related to theavailability of diatom cells, which constitute animportant food source for heterotrophic dinofla-gellates (Dale, 1996; Saetre et al., 1997). In thenorthern Baffin Bay stations, Operculodinium

centrocarpum and Spiniferites elongatus s.l. weremuch more abundant than in the North Waterstations. Like Brigantedinium spp., O. centrocar-

pum is an ubiquitous species, occurring in rela-tively large numbers in all types of environmentsfrom middle to high latitudes (de Vernal et al.,1997; Rochon et al., 1999). The taxon S. elongatus

s.l. includes two species, S. elongatus and S.

frigidus, which are difficult to tell apart due tointergradations in morphology. S. frigidus isassociated with low temperatures ranging from–11C to 41C and a salinity of about 29, whereas S.

elongatus is associated with cool temperate condi-tions. These assemblages, indicative of autotrophicproduction, show the same characteristics as thosein central Baffin Bay and reveal a West GreenlandCurrent signature at the southern margin of theNorth Water region (Mudie and Short, 1985;Rochon et al., 1999).

The dominance of taxa associated with hetero-trophic production in the North Water can beexplained by a high abundance of suitablepelagic prey, mostly diatoms (Booth et al., 2002),which are the principal food source for hetero-trophic dinoflagellates (Dale, 1996; Saetre et al.,1997). This hypothesis is further supported by thework of Mostajir et al. (2001), who measured ahigher abundance of nanophytoplankton (mainlydiatoms) in the North Water than in northernBaffin Bay (stations 66F and 76F) during thefall season. Our results indicate that the physi-cal and chemical conditions (see Paerl, 1988;


Anderson, 1997) in northern Baffin Bay are morefavorable for the growth of autotrophic dinofla-gellates than in the North Water.

4.4. Geochemical and micropaleontological tracers

in time series

4.4.1. Geochemical tracers

The accumulation rates measured at stations54D (B0.1 cm yr�1) and 76F (B0.05 cm yr�1) arerelatively high compared to those in other polarregions such as Baffin Bay (average of 10 cm kyr�1,Aksu and Piper, 1987), the deep Arctic Basin (0.2–0.3 cm kyr�1, Ku and Broecker, 1965) or theAntarctic shelf (3.5 cm kyr�1, DeMaster, 1981).High accumulation rates in the range of0.05–0.1 cm yr�1 are generally observed in coastalor estuarine environments of temperate regions(Peru-Chile coast, 0.05–0.16 cm yr�1, DeMaster,1981; Gulf of St. Lawrence, 0.08 cm yr�1, Silver-berg et al., 2000). These elevated accumulationrates cannot be generalized to the entire NorthWater/Baffin Bay region, since they constituteonly punctual information. However, theypermit interesting comparisons. Indeed, the Corg

burial fluxes in the southeastern part of theNorth Water region (station 54D; B1.1–1.5 mg cm�2 yr�1) is about three times higherthan in northeastern Baffin Bay (station 76F)and about ten times higher than in centralBaffin Bay (o0.1 mg cm�2 yr�1, Stein, 1991).These fluxes correspond to 4–6% of the annualprimary production in the euphotic zone inthe Greenlandic side of the North Water (Kleinet al., 2002) and are comparable to thosemeasured in the Ross Sea in Antarctica(0.1–0.8 mg cm�2 yr�1, DeMaster et al., 1991).The slight variations in the Corg and CaCO3

content of both cores (Figs. 3 and 4) are difficultto interpret in terms of fluxes, since they can berelated to changes in the amount of ice-raftedmaterial deposited at these stations that may alsohave modified sediment accumulation rates. Forexample, in the core collected at station 76F, thedecrease of the Corg and CaCO3 content in theupper centimeter corresponded to higher amountsof coarse particles.

4.4.2. Micropaleontological tracers

Dinocyst fluxes calculated from core 54D weremore than two orders of magnitude higher thanthose estimated for deep central Baffin Bay (about701N, 651W, Rochon and de Vernal, 1994) and didnot vary significantly with time. The two dominantheterotrophic taxa, Algidasphaeridium? minutum

and Brigantedinium spp., varied in opposition toeach other all along the profile, but a slightincrease toward the surface of the autotrophictaxa Operculodinium centrocarpum was also ob-served. This increase might be related to anenhanced contribution of the northward compo-nent of the West Greenland Current, whichinfluences the physical properties of the ArcticLayer through turbulent exchange in the NorthWater (Melling et al., 2001). The variations of theorganic lining concentrations are difficult tointerpret, since they may represent changes inforaminifer assemblages (i.e. percentage of taxawith or without organic linings) or productivity.Nevertheless, these OL concentrations suggestdecadal variations in the benthic environment.

At station 76F, the dinocyst concentrationsdecreased towards the top of the core. If weassume sedimentation rates to have been relativelyconstant, the concentrations would indicate adecrease in dinocyst fluxes over the last fewcenturies. The assemblages dominated by auto-trophic species clearly differed from those ofstation 54D. Slight variations in the relativeabundance of the five main taxa, especiallyAlgidasphaeridium? minutum and Pentapharsodi-

nium dalei, suggest changing sea-surface condi-tions, probably in relation to the northward inflowof the sub-polar West Greenland Current.

5. Conclusions

The geochemical and micropaleontological ana-lyses of surface sediment collected in the NorthWater area showed high productivity in the watercolumn as evidenced by the very high organiccarbon content and dinocyst concentrations. Theanalyses also indicated high benthic production asshown by the abundance of organic linings offoraminifers. One particularity of the dinocyst


assemblages of the North Water was the almostexclusive dominance of dinocysts belonging toheterotrophic taxa, which depend on prey produc-tion, notably diatoms (the often dominant primaryproducer in the North Water), to satisfy their foodrequirement. On the whole, the sediments from theNorth Water stations present geochemical andmicropaleontological characteristics that contrastwith those of northern Baffin Bay, where muchlower organic carbon and microfossil contentswere recorded and where dinocyst assemblageswere dominated by dinocysts belonging to auto-trophic taxa.

The detailed analyses of two 26-cm longsedimentary cores indicated that the differentbiogenic fluxes that presently characterize theNorth Water and northern Baffin Bay region haveexisted for at least the last few centuries. At theNorth Water site (station 54D), the data revealedrelatively uniform biogenic fluxes and sea-surfaceconditions through time. The northern Baffin Bayrecords (site 76F) indicated less stable conditions,notably with respect to the dinocyst assemblages.This relative instability suggests changes in sea-surface conditions on a secular time scale,probably in relation to the northward inflow ofthe sub-polar West Greenland Current. The NorthWater constitutes an environment more stablethan Baffin Bay, and hydrographically distinctfrom it, and thus would represent a good site forlong-term study to assess the response of aproductive arctic ecosystem to eventual climatechanges.

Acknowledgements

This study was made possible through financialsupport from the Natural Sciences and Engineer-ing Research Council of Canada and the Fonds

FCAR of Qu!ebec as well as through extensivelogistical support from the Polar Continental ShelfProject (Energy, Mines and Resources Canada)and the Canadian Coast Guard. D. Hamelreceived post-graduate scholarships from theFonds FCAR and the Institut des sciences de la

mer de Rimouski (ISMER) and financial supportfrom the Department of Indian and Northern

Affairs Canada for fieldwork. The authors extendtheir thanks to captain R. Dubois and the crew ofthe Canadian icebreaker NGCC Pierre-Radisson

for their outstanding help during the expedition;C. Lalande and M. Sampei for field assistance; L.Cournoyer, M. Henry, and V. Loucheur (GEO-TOP) for laboratory assistance; B. Ghaleb (GEO-TOP) for performing the 210Pb analyses; and J.Grant and D. Adler (Dalhousie University) andA.T. Fisk (Carleton University) for courteouslyproviding sediment samples from the 1998 NOWexpedition. Special thanks are also due to G.Desrosiers (ISMER) and the Bedford Institute ofOceanography for graciously providing respec-tively the box core and the winch in 1999. We alsothank L. Devine, J.-F. Ouellet and G. Arturi fortheir help during the writing of this paper.Comments by A. Rochon and P. Hill (GeologicalSurvey of Canada), J. Grant, J. Deming (Uni-versity of Washington) and an anonymous re-viewer were most helpful. This is a contribution tothe International North Water Polynya Study(NOW).

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