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Quaternary Science Reviews 23 (2004) 245–260

*Correspondi

3635.

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doi:10.1016/j.qu

Size-dependent isotopic composition of planktic foraminifers fromChukchi Sea vs. NW Atlantic sediments—implications for the

Holocene paleoceanography of the western Arctic

C. Hillaire-Marcela,*, A. de Vernala, L. Polyakb, D. Darbyc

a GEOTOP, Universit!e du Qu!ebec "a Montr!eal, C.P.8888, Succursale Centre Ville, Montr!eal, PQ Canada H3C 3P8b Department of Geological Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, OH, USA

c Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA, USA

Accepted 15 August 2003

Abstract

In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadrina pachyderma (left coiled)—

Npl—occur along the pycnocline, and their d13C and d18O-values are size and weight dependent. However, whereas the Npl 18O data

from the NW Atlantic indicate near-equilibrium conditions with ambient waters and a positive relationship between shell weight and18O-content, assemblages from box-cored sediments of the Chukchi Sea (western Arctic) are depleted by B2% with respect to

equilibrium values with modern conditions, and depict a negative relationship between shell weight and its d18O-value

(�0.1570.03%/mg on VPDB scale). A similar feature is also depicted by the dextral form of N. pachyderma (Npd). We associate

the reverse shell-size or weight vs. d18O relationship to the reverse temperature gradient observed along the thermocline between

the surface cold and dilute water layer, and the underlying near 3�C-warmer saline North Atlantic water mass. The analysis of two

late to post-glacial sedimentary sequences from the Chukchi Sea indicates that such a water mass stratification with a reverse

thermocline persisted throughout the Holocene, thus reflecting an early onset of the modern-like linkage between the Arctic Ocean

and the North Atlantic. Moreover, lower d18O-values in both Npl and Npd together with larger d18O-gradients between the different

shell sizes at ca 9–7 ka BP suggest B3�C higher temperatures in the upper North Atlantic water mass, in comparison with the

present (approximately +1�C, at the study site), thus likely a higher inflow rate of this water mass during the early Holocene.

r 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Deep dwelling planktic foraminifers, such as Neoglo-

boquadrina pachyderma (left coiled)—henceforth Npl—have been shown to form their calcite at variable waterdepths, ranging from the mixed surface layer down to afew hundred meters, along the underlying pycnocline,depending upon local hydrographic conditions and thedegree of encrustation of their shells (e.g., Ravelo andFairbanks, 1992; Kohfeld et al., 1996; Ortiz et al., 1996;Bauch et al., 1997). In most settings, carbon isotoperatios of Npl shells depict an isotopic offset X�1%from the equilibrium with ambient dissolved inorganiccarbon (DIC) (e.g., Kohfeld et al., 1996; Bauch et al.,

ng author. Tel.: +1-514-987-4080; fax: +1-514-987-

ss: [email protected] (C. Hillaire-Marcel).

front matter r 2003 Elsevier Ltd. All rights reserved.

ascirev.2003.08.006

1997), i.e., a ‘‘vital effect’’ that is believed to be linked tometabolic factors and food supply rates (Ortiz et al.,1996).

In temperate to subarctic settings, stable oxygenisotope ratios of Npl shells, encrusted or not, indicatecalcite precipitation near isotopic equilibrium conditionswith ambient waters and also suggest growth andencrustation of tests when sinking along the pycnocline(e.g., Wu and Hillaire-Marcel, 1994a; Kohfeld et al.,1996; Ortiz et al., 1996). This property allowed severalresearchers to use d18O-values in Npl assemblages fromdeep-sea sediment cores as a probe into past tempera-ture, salinity, and thus density conditions along thepycnocline (e.g., Andreason and Ravelo, 1997; Faulet al., 2000; Duplessy et al., 2001; Hillaire-Marcel et al.,2001a, b; de Vernal et al., 2002), which provideimportant paleoceanographic information notably forsites where deep water formation may occur. In contrast

ARTICLE IN PRESSC. Hillaire-Marcel et al. / Quaternary Science Reviews 23 (2004) 245–260246

to the NW Atlantic, the Arctic specimens of Npl havebeen shown to be systematically depleted in 18O, withoffsets from equilibrium values varying between �1 and�2%, depending on the study area (e.g., Van Donk andMathieu, 1969; Kohfeld et al., 1996; Bauch et al., 1997;Aksu and Vilks, 1988; Simstich, 1999; Duplessy et al.,2001).

This paper investigates the relationship of d18O-valuesin Arctic Npl to the hydrographic properties of theupper water column. To accomplish this, we examinedthe size- and weight-dependent isotopic gradientsdepicted by Npl assemblages from box cored sediments.Besides the 13C-enrichment generally observed betweenthe small and the large, more or less encrustedspecimens, Npl shells depict significant size- andweight-dependent d18O gradients, which appear relatedto the structure of the pycnocline between the surfaceand subsurface water masses during the growth season(Kohfeld et al., 1996; Candon, 2000; Hillaire-Marcelet al., 2001a; de Vernal et al., 2002). In the present study,we use this property as a means to constrain changes intemperature and/or salinity gradients in the upper watercolumn of western Arctic during the Holocene, usingsamples from two cores raised from the lower con-

Fig. 1. Location map of the study cores to which we refer in the text (see Tab

in the Arctic Ocean at depths ranging 200–1700 m (cf. Rudels et al., 1994; J

tinental slope of the Chukchi Sea (B15 and P49; Fig 1),at the very margin of the present Arctic pack ice insummer. More detailed discussions of the Holocenepaleoceanography of the Chukchi Sea region are to befound elsewhere (Darby et al., 2001). Here, we focus onthe information that can be extracted from size-dependent isotopic data on planktic foraminifer assem-blages, including Npl and Neogloboquadrina pachyder-

ma (dextral)—henceforth Npd—and on theirrelationship with the isotopic and thermohaline proper-ties of the water masses. Thus, we address the historyof the relatively warm North Atlantic water that ispresent today below the relatively dilute cold surfacewater layer of the Arctic Ocean (e.g., Bauch et al., 1995;Jones, 2001).

2. Material and methods

2.1. Coring sites

This study is based on cores collected along a transectfrom the edge of the continental shelf of the ChukchiSea, downslope to the adjacent Northwind Basin. These

le 1). The arrows indicate the circulation pattern of the Atlantic waters

ones, 2001).

ARTICLE IN PRESSC. Hillaire-Marcel et al. / Quaternary Science Reviews 23 (2004) 245–260 247

cores were initially sampled in order to study the flux ofsediment and nutrients from the shelf into the basin aspart of the Shelf–Basin-Interaction Project (Darby et al.,2001). Piston core AR-92-P1 (P1) and associated boxcore AR-92-B3 (B3) are from the shelf edge in 201 mwater depth and contain a 220 cm Holocene sequenceand at least 2 m of marine isotope stage 2 sediment.Cores AR-92-P49 (P49) and box core AR-92-B15 (B15),which were collected much deeper in 2103 and 2135 mwater depth, respectively, contain only about 10 cm ofHolocene, but much higher amounts of preservedcalcareous foraminifers than the upper slope and shelfcores.

In the present study, we will use material from coresB15 and P49 (Table 1; Fig. 1), which consist insequences characterized by very low sedimentationrates, of the order of 1 cm per thousand years, as shownby a few 14C measurements in core B15. In theAmerasian Basin (western Arctic), sedimentary recordswith higher accumulation rates than those of cores B15and P49 are hardly attainable (Darby et al., 1997; Pooreet al., 1999) except from areas of turbidite deposition(Grantz et al., 1996). Holocene sequences characterizedby higher sedimentation rates were recovered fromshallower sites on the Chukchi shelf and slope (Darbyet al., 2001). However, they could not be used for thepresent study because of carbonate dissolution and/orpaucity of planktonic foraminifers in the sediment, incontrary to cores B15 and P49 which contain wellpreserved Npl and Npd shells throughout the Holocenesection.

The particularly low sedimentation rates recorded inthe deep Arctic basins are in part due to the permanentpack ice, which results in extremely low biogenicproduction and fluxes as well as in limited sedimentaryinputs. As a consequence of reduced carbon fluxes to thesea floor, there is a sparse biological activity in benthicenvironments which accounts for limited, although nottotally negligible, bioturbations in sediment. In coreB15, 210Pb measurements at 0.5 cm intervals (unpub-lished data) indicate mixing in the upper first centimeter,and a more discrete penetration of benthic organismsbetween 2 and 3 cm (cf. the ‘‘non-local mixing distribu-

Table 1

Core location

Core number Latitude N Longitude W Depth (m)

Western Arctic (Chukchi Sea)

AR-92-B15 (B-15) 075�44.030 160�51.630 2135

AR-92-P49 (P-49) 075�44.650 160�55.730 2103

NW North Atlantic (Labrador and Irminger seas)

90-013-017 (B-17) 58�12.500 48�21.600 3379

91-045-051 (B-51) 59�29.560 39�18.430 2949

91-045-060 (B-60) 59�50.960 33�34.930 2255

91-045-093 (B-93) 50�12.280 45�41.150 3448

tion’’ of Boudreau, 1986). Thus, using 1-cm thicksamples, one may achieve a millennial time-resolutionin this sequence despite some smoothing. This is likely tohave dampened millennial frequency oscillations, but weshould still be in a position to document large amplitudepaleoceanographic changes at the Holocene time scale.

In order to compare the isotopic data in foraminiferafrom the western Arctic with those from the northernNorth Atlantic, we will use data from four box-coresthat were raised from the Labrador and Irminger seasduring cruises 90-013 and 91-045 of the CSS-Hudson(Hillaire-Marcel et al., 1990, 1992; Table 1, Fig. 1). Theyrepresent late Holocene sequences with relatively highsedimentation rates of B20 cm/ka (Table 1). Detailedisotopic measurements of several test sizes and variousplanktic species or subspecies from these cores havealready been reported in Candon (2000) and Hillaire-Marcel et al. (2001a).

2.2. Stable isotope measurements in foraminifers

The foraminiferal samples used for isotopic investiga-tions in cores B15 and P49 consist of sieved then hand-picked sub-assemblages of Npl and Npd and a benthicspecies Cibicides wuellerstorfi. We distinguish thefollowing shell size classes for Npl and Npd: 100–150,150–250 and >250 mm. All sub-samples were weightedand the shells were counted in order to calculate a meanweight for each size-class. Routine procedures were usedfor isotopic measurements (e.g., Shackleton, 1974). Thebenthic shells were roasted at 200�C for one hour underhelium flux in order to destroy the organic lining whenpresent, then all samples were reacted with 100%orthophosphoric acid, either using an Autocarbtpreparation device on line with a Prismt instrument,or using a Multicarbt preparation device online with adual inlet IsoPrimet instrument. The standards usedinclude the Carrara marble, NBS 19 (e.g., Coplen, 1996)and our home UQ6 carbonate standard (cf. Hillaire-Marcel et al., 1994). The overall analytical uncertaintydetermined from replicate measurements of UQ6,during each run, is routinely better than 0.05% withthe first technique, and better than 0.03%, with thesecond one. All results are reported in d-values againstthe VPDB standard (Coplen, 1996).

2.3. Hydrographic data and isotopic composition in the

water column

Hydrographic data (salinity and temperature) andisotopic composition of water masses (Figs. 2–3) wereextracted from the Goddard Institute sea-water oxygen18 database (cf. Schmidt et al., 1999). Because of therarity of hydrographic measurements in the ArcticOcean, especially those collected along vertical profiles,a very large area had to be considered to get a

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0

100

200

300

400

500n=913

Water depth (m)

30 32 3634 -2 0 2 -4 -2 0δ18O-Water � vs. VSMOWTemperature °CSalinity

Western Arctic data set(120-180 °W; 70-90 °N)

(a) (b) (c)

Fig. 2. Salinity, temperature and isotopic composition of water in August and/or September in the western Arctic sector (data from the Goddard

Institute Database). Note the reverse thermocline between the dilute and cold surface water layer and the underlying warm, saline water mass

originating from the North Atlantic.

C. Hillaire-Marcel et al. / Quaternary Science Reviews 23 (2004) 245–260248

reasonable number of data points. The western Arcticdata set we used includes all data comprised between120� and 180� west and between 70� and 90� north.These data were collected mostly in August and/orSeptember, when ice free conditions prevailed, thuspermitting scientific expeditions.

From temperature (t in �C) and water d18O (dw) data,we calculated equilibrium calcite d18O vs. PDB (dc)using Eq. (1) below after Shackleton (1974), with theoffset value of �0.27% (see Eq. (2)) standing forconversion into the VPDB scale after Coplen (1988).

t ¼ 16:9 � 4:38ðdc � AÞ þ 0:10ðdc � AÞ2; ð1Þ

A ¼ dw � 0:27: ð2Þ

Beyond the rarity of data from the western ArcticOcean, a critical parameter in establishing relationshipsbetween hydrographic and isotopic data concerns the

incidence of sea ice formation and melting on isotopiccompositions of ambient waters (e.g., B!edard et al.,1981; Moore et al., 1983; Ostlund and Hut, 1984;Melling and Moore, 1995; MacDonald et al., 1995). Asillustrated in Fig. 4, oxygen isotope data from theBeaufort and Chukchi seas suggest that the simple two-end members linear mixing between local fresh-watersand ‘‘normal’’ sea water (thus a linear salinity–d18Owater

relationship), is strongly altered by the production andsinking of isotopically light brines, during sea ice growthintervals, and by the release of isotopically heavy, lowsalinity waters, when sea ice melts. In opposition to thispattern, waters from the NW Atlantic illustrate theclassical linear relationship between salinity and waterd18O-content (Fig. 4). Adding to these sea ice effects onthe salinity vs. water d18O-content relationship, therelatively wide range of d-values of fluvial supplies intothe Arctic (e.g., L!etolle et al., 1993; Bauch et al., 1995;

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Temperature °C δ18O-CaCO3-eq.

Chukchi & Beaufort Seas

E. Siberian & Laptev Seas

Temperature °C δ18O-CaCO3-eq.Potential density

Potential density

0

100

200

300

400

500

0

100

200

300

400

500

-2 0 2 3.5 4.526 2827

-2 0 2 3.5 4.526 2827

Depth (m)

(a) (b) (c)

(d) (e) (f)

4

Fig. 3. Temperature and oxygen isotope compositions (d18O-CaCO3-eq. vs. VPDB) for a calcite precipitated in isotopic equilibrium with ambient

water in the eastern vs. western sectors of the Arctic Ocean (the East Siberian and Laptev seas and the Chukchi and Beaufort seas, respectively; data

are from the Goddard Institute Database). The reverse thermocline results in a reverse trend of d18O-CaCO3 values vs. depth, between 100 and 250 m,

that is more pronounced in the eastern basins than in the western Arctic seas due to higher temperatures on top of the North Atlantic water mass.

C. Hillaire-Marcel et al. / Quaternary Science Reviews 23 (2004) 245–260 249

MacDonald et al., 1995; Azetsu-Scott and Tan, 1997)may also play a role in the scatter of values observed(Fig. 4). Such processes are likely to have had a variableimpact in the past, and may also result in differential

isotopic responses depending upon the size and depthhabitat of planktonic foraminifera. The small light Npland Npd specimens would be more likely influenced bysea-ice meltwater, whereas the large dense specimens

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y0 = -20.3±0.4�

n=753

Salinity

20

30

40

-8 -6 -4 -2 0 2

NW Atlantic data set (40-65°N; 35-60°W)

δ18O � vs. VSMOW

Western Arctic data set (120-180°W; 70-90°N)

δ18O � vs. VSMOW

-8 -6 -4 -2 0 2-10

Salinity

0

5

10

15

20

25

30

35

40

n=913

ISOTOPICALLYLIGHT BRINES

DILUTIONBY SEA-ICEMELTWATER

SEA ICE

Fig. 4. Comparison of the water-d18O and salinity relationship in the western Arctic (right) and NW Atlantic (left) sectors (data from the Goddard

Institute Database). Y and X scales are different for the two plots. Note the scatter in the western Arctic sector due to dilution by isotopically heavy

sea-ice meltwater vs. addition of isotopically light brines resulting from sea ice growth.


would rather be under direct influence of the addition ofbrines resulting from sea ice formation, on top of thepycnocline on which they form their shell or seesecondary calcite overgrowth to occur.

2.4. Relationships between the isotopic composition in

foraminifers and hydrographic data

The correspondence between the modern data fromthe water column and data from box-core tops cannotbe perfectly accurate because the hydrographical dataillustrate conditions recorded punctually during a fewcruises, which are thus representative of a few yearsduring the last few decades, whereas the core-topsamples represent the last centuries to a thousand years.Moreover, the few instrumental measurements availablefrom the Arctic do not provide statistically strong basisto describe the modern situation and that of the lastcentury, especially when taking into account the climatefluctuations which characterized the northern hemi-sphere on decadal to centennial time scales (e.g., Mannet al., 1999). In the western Arctic, in particular, variousproxies suggest that the last millennium has beenmarked by a significant trend of cooling in surfacewaters (Bauch and Polyakova, 2000; Darby et al., 2001).

Nevertheless, despite these uncertainties, the hydro-graphic data provide some grounds for understandingthe general features of the foraminiferal isotopiccompositions. In the NW Atlantic, higher sedimentationrates result in lesser smoothing of the record than in thewestern Arctic but, even there, box core top assemblagesstill represent a few centuries, often a couple ofthousand years due to bioturbations (see Wu andHillaire-Marcel, 1994b, for example), whereas thehydrographic database does not exceed a few decades.The natural variability in hydrographic conditions overpolar latitudes and the statistical weakness of moderninstrumental data constitute a limitation for calibratingquantitative relationship with isotopic proxies fromsediment records.

3. Results

Temperature, salinity and oxygen isotope data inmodern waters (Fig. 2) were used to calculate d18O-values for calcite precipitated in isotopic equilibriumduring the summer months in the Chukchi andBeaufort seas and compared with values from the EastSiberian and Laptev seas (Fig 3). Under these modern


conditions, one would expect d18O-values as high as +4to +4.25% for the large Npl specimens developingusually along the deeper end of the pycnocline (e.g.,Kohfeld et al., 1996; Bauch et al., 1997). However, Nplshells in all analyzed size classes from the late Holoceneassemblages on the Chukchi borderland have system-atically lower d18O-values (Tables 2–3; Fig. 5), indicat-ing a –2 to –3% offset from the isotopic equilibriuminferred from modern oceanographic data. A similaroffset characterizes d18O-values in Npd. Most Npdshells are even more depleted in 18O than those of Npl(Table 2 and Fig. 5). This difference could be related toNpd development during exceptionally warm summers(see also Darby et al., 2001) or to an increased offsetfrom isotopic equilibrium with ambient water due to anenhanced metabolic response of Npd to the harshenvironmental conditions of the western Arctic. Theoffset we observe in the Chukchi Sea between the 18O

Table 2

Isotopic composition of foraminiferal assemblages in core B-15

Sample

depth

(cm)

Cal.14C

age (a)

Interp.

age (a)

Cibicides

wuellerstorfi

Neogloboquadrina pachyderma l.

d13C d18O 100–150 mm 150–250 mm >

mg d13C d18O mg d13C d18O m

0–1 3321 3321 1.52 3.70 3.0 0.83 1.84 8.4 1.25 1.69

2–3 4562 1.51 3.60 2.8 0.76 1.89 9.2 1.24 1.56

3–4 5210 1.52 3.69 3.0 0.79 1.94 4.9 1.14 1.65

4–5 5877 1.52 3.71 2.6 0.71 2.13 5.1 1.09 1.64 1

5–6 6565 6565 1.42 3.75 2.8 0.64 1.93 6.0 0.97 1.33

6–7 7278 1.42 3.81 3.2 0.54 1.78 6.4 0.90 1.23

7–8 8020 1.19 3.95 2.5 0.37 1.98 7.7 0.84 0.51

8–9 8790 1.42 3.86 4.6 0.55 1.59 7.0 0.95 0.98

9–10 9590 1.09 4.16 2.7 0.31 1.75 6.0 0.61 0.96

10–11 10419 1.01 4.04 2.4 0.12 1.92 4.8 0.63 1.11

11–12 11277 11277 0.99 4.03 2.0 �0.01 1.96 4.8 0.34 1.70

12–13 12148 1.38 3.60 1.3 �0.04 2.43 2.9

Table 3

Isotopic composition of foraminiferal assemblages in core P-49

Sample

depth (cm)

Cibicides wuellerstorfi Neogloboquadrina pachyderma l.

d13C d18O 100–150mm 150–250mm

d13C d18O d13C d18O

0–1 1.50 3.69 0.76 1.78 1.21 1.56

1–2 1.48 3.70 0.78 1.83 1.14 1.45

2–3 1.46 3.74 0.69 1.94 1.14 1.48

3–4 1.57 3.78 0.79 1.81 1.14 1.69

4–5 1.41 3.74 0.73 1.73 0.99 1.40

5–6 1.38 3.76 0.46 1.58 0.99 0.24

6–7 1.35 3.85 0.33 1.53 0.89 0.17

7–8 1.01 3.92 0.11 1.18 0.47 0.78

8–9 1.04 4.04 0.05 0.98 0.64 1.38

9–10 1.01 3.96 �0.00 1.98 0.24 1.19

10–11 0.99 4.11 0.04 2.07 0.07 2.17

content in foraminifer shells of both Npl and Npd andthe isotopic equilibrium with ambient water is notperfectly understood, but seems to be a consistentfeature of Arctic environments (e.g., Van Donk andMathieu, 1969; Kohfeld et al., 1996; Bauch et al., 1997;Aksu and Vilks, 1988; Simstich, 1999; Duplessy et al.,2001). Recent findings by Darling et al. (2000) andKucera and Darling (2002) on ‘‘cryptic genetic’’ types ofNeogloboquadrina pachyderma suggest that the Arcticspecimens could well represent such types that woulddrastically differ, with respect to calcite precipitationmechanisms, from the sub-Arctic types.

The most striking feature depicted by Npl assem-blages from the western Arctic is the reverse linearrelationship between shell weight or size and its 18Ocontent (Fig. 5), in opposition to the North Atlanticassemblages that depict a positive linear relationship(Fig. 6) (see also Hillaire-Marcel and Bilodeau, 2000 or

Neogloboquqdrina pachyderma d.

250 mm 100–150 mm 150–250 mm >250 mm

g d13C d18O mg d13C d18O mg d13C d18O mg d13C d18O

9.9 1.36 1.17 3.1 0.84 1.59 9.2 1.11 1.34 12.1 1.25 1.31

9.9 1.37 1.32 2.5 0.88 1.99 8.0 1.13 1.49 12.8 1.39 1.58

8.7 1.19 0.89 2.8 0.74 1.75 7.3 1.01 0.99 9.5 1.31 1.13

0.1 1.31 1.13 3.0 0.75 1.73 7.0 1.01 1.36 10.4 1.15 0.47

9.9 1.19 1.38 3.3 0.69 1.78 7.3 1.03 1.26 11.0 1.25 0.54

8.3 1.12 0.31 4.2 0.28 1.27 6.3 0.89 1.14 13.8 1.10 0.63

8.7 1.03 �0.29 3.4 0.13 1.12 7.1 0.56 0.38 10.6 0.98 �0.32

6.7 1.01 0.90 3.7 0.45 1.45 6.6 0.93 0.79 10.7 1.10 1.04

7.3 1.14 0.82 2.7 �0.04 0.70 6.1 0.67 0.46

7.8 0.87 0.44 5.2 0.53 0.95

5.8 0.23 1.52

Neogloboquadrina pachyderma d.

>250mm 100–150mm 150–250mm >250mm

d13C d18O d13C d18O d13C d18O d13C d18O

1.25 1.34 0.81 1.81 1.07 1.33 1.10 1.06

1.32 1.27 0.81 1.66 1.07 1.49 1.28 1.74

1.23 1.29 0.69 1.54 1.04 1.30 1.16 1.50

1.30 1.36 0.68 1.80 1.05 1.20 1.15 1.18

1.25 1.37 0.65 1.81 0.94 1.14 1.14 1.37

1.11 0.47 0.42 1.58 0.83 0.44 1.00 0.58

1.09 �0.05 0.30 1.39 0.74 0.39 0.91 �0.06

0.75 0.15 0.54 0.63

0.52 0.78 0.32 1.00

0.52 1.62 0.33 1.36

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NplNpd

2.5

2

1.5

1

Mean δ13C�1.5

1

0.5

Mean δ18O� 2

1.5

1

100-150µm 150-250µm >250µm

1.5

1

0.5100-150µm 150-250µm >250µm

Mean δ13C�

Mean δ18O�

B-15 core top P-49 core top

Fig. 5. The d18O and d13C vs. size relationships in Npl and Npd from the Chukchi Sea core top sediments (B-15: 0–4 cm; P-49: 0–5 cm). The symbols

and bars represent mean values and the standard deviations in each size range sieved. Npd and Npl do not differ significantly from each other within

standard deviation. The larger specimens show a larger scatter (see text) than the small ones for both Npl and Npd. However, this scatter is somewhat

larger for Npd than Npl assemblages.


Hillaire-Marcel et al., 2001b). Adding to a generallystrong negative trend for the d18O vs. shell-weightrelationship, some scatter is noticeable for the larger,18O-depleted specimens of the western Arctic (Fig. 5). Incontrast to their distinct features with respect to oxygenisotope compositions, both the North Atlantic andwestern Arctic assemblages show similar behaviors withrespect to d13C-values, which are positively correlatedwith shell-size or weight (Figs. 5 and 6). Isotopiccompositions of Npd are similar to those of Npl,showing positive d13C-size and negative d18O-sizegradients (Fig. 5), but Npd has a slightly larger standarddeviation around mean values in each size class,particularly in the >250 mm fraction.

4. Discussion

4.1. Oxygen isotope composition vs. shell density in the

NW Atlantic assemblages

The box core-top assemblages from the NW Atlantic(Fig. 6) illustrate a typical relationship between Npl shellsize or weight and its isotopic composition. The densestspecimens that calcify deeper down the seasonalpycnocline (from May until September; e.g., B!e andTolderlund, 1971; Tolderlund and B!e, 1971), thus in

more saline and colder waters, show the highest d18O-values. In contrast, the lightest specimens that calcifytowards the upper end of the pycnocline, in warmer andless saline waters, have lighter isotopic compositions(e.g., Hillaire-Marcel et al., 2001b). Depending on thesite, the seasonal pycnocline is either primarily tempera-ture-controlled (as in the Iceland basin) or primarilysalinity-controlled (as in the Labrador Sea). This patternis related to the fact that the Labrador Sea constitutes amajor conduit for the cold, dilute sub-polar currents,whereas more saline North Atlantic Drift waters exert atighter control on the salinity of the Iceland Basin.Nevertheless, be it salinity- or temperature-driven, thedevelopment of the seasonal pycnocline results in a largespreading of size-dependent d18O-values in Npl shells,which can be used to reconstruct density gradients in theupper water column (e.g., Hillaire-Marcel et al.,2001a, b; de Vernal et al., 2002). In general, Npl fromthe NW Atlantic sediments yield d18O-values compatiblewith precipitation in equilibrium with ambient watersalong the pycnocline (Hillaire-Marcel et al., 2001a).However, large encrusted specimens may show higherd18O-values than those calculated for a calcite precipi-tated in isotopic equilibrium with water in the upper fewhundred meters of the water column. This is possiblebecause secondary calcite precipitation may occurdeeper in the water column or even, in the sediment,

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18O = (2.26±0.17) - (0.15±0.03) X ; R2 = 0.5

13C = (0.14±0.09) + (0.12±0.01) X ; R2 = 0.7

18O = (2.20±0.06) + (0.04±0.01) X ; R2 = 0.35

13C = (-0.07±0.06) + (0.09±0.01) X ; R2 = 0.62

NW ATLANTIC (B93, B17, B60, B72)

WESTERN ARCTIC (BX-15)

3

2

1

0

-1

3

2

1

0

-1

0 2 4 6 8 10 12 14Mean weight (µg)

δ

δ

δ

δ

δ -

valu

esδ

- va

lues

Fig. 6. Isotopic composition of Npl shells as a function of the mean

shell weight in the fraction used for the measurement (see text for

details). Top: Assemblages in box cores from the NW Atlantic

(Irminger, Iceland and Greenland Seas; data from Candon, 2000);

bottom: Assemblages from core B-15 from the Chukchi Sea. Note the

overall low (below equilibrium) and d18O-values in the Arctic

specimens and the reverse d18O-weight relationship in comparison

with those from the NW Atlantic sites. Carbon isotope values are

comparable in both sectors, and show an almost similar positive linear

relationship with the shell weight.


in relation to organic carbon mineralization processes(e.g., Gehlen et al., 1999; Silverberg et al., 2000).

4.2. Oxygen isotope composition vs. shell density in the

western Arctic assemblages

Arctic Npl assemblages show a variable, but generallylarge negative offset of more than �1% between theird18O-values and those of a calcite precipitated inequilibrium with ambient waters (e.g., Kohfeld et al.,1996; Bauch et al., 1997; Volkmann and Mensch, 2001).Although not fully understood, it is generally admittedthat this offset is linked to metabolic fractionationeffects (e.g., Bauch et al., 1997; Aksu and Vilks, 1988),eventually in relation with the occurrence of distinct‘‘cryptic genetic’’ types (Darling et al., 2000; Kucera andDarling, 2002). These effects are possibly controlled byshell growth rates, which tightly depend upon the

duration of the growth stage (e.g., Volkmann andMensch, 2001). Positive d18O-gradients between smalljuvenile and large mature specimens have often beenattributed to similar metabolic effects (e.g., Aksu andVilks, 1988; Volkmann and Mensch, 2001). It has alsobeen shown that secondary calcite overgrowth occurringalong the pycnocline may significantly increase both thedensity and the 18O content of the shells (Kohfeld et al.,1996).

Our data from the western Arctic are partly inagreement with the above-mentioned observationsabout Arctic Npl assemblages, notably with regard tothe large departure of d18O-values from isotopicequilibrium with ambient waters that ranges up to�2.5% as estimated from modern hydrographic data(Figs. 3 and 4). However, our data do not support theassumption that the d18O gradient is controlled bydifferential physiological effects leading to heavier d18Ovalues in mature specimens as described by Aksu andVilks (1988) or Volkmann and Mensch (2001) in othersettings. In contrary, the samples from the Chukchi Seashow a clear reverse relationship between specimenweight or size and d18O-values, not only in Npl, but alsoin Npd (Fig. 5). Such a negative relationship cannot beexplained by differential fractionation mechanisms. Itmay instead reflect the particular structure of the upperwater masses in the western Arctic, which is character-ized by an increasing temperature gradient downward,from the surface mixed layer to the top of theintermediate Atlantic waters, at about 150–200 m. Anear 2.5�C temperature increase is indeed observed from100 m down to 250 m approximately, i.e., along thepycnocline between the cold surface layer and therelatively warm underlying water mass originating fromthe North Atlantic. This temperature gradient couldaccount for the lighter d18O values of the large speci-mens, which calcify deeper, but in warmer water.Because of extremely scattered hydrographic data fromthe western Arctic, the isotopic composition (d18O-CaCO3-eq.) calculated for a calcite precipitated inisotopic equilibrium does not show a clear trend alongthe pycnocline (Fig. 3). A much stronger temperaturegradient and a larger reverse isotopic shift are seen in theEast Siberian and Laptev seas, where a decrease of 0.6%in d18O-CaCO3-eq. values can be calculated along thepycnocline, i.e., between 100 and 250 m (Fig. 3). Thisvalue is nearly that observed between specimens in the100–150 and >250 mm size fractions of the most recentassemblages in cores B-15 and P-49 (Fig. 5). Therefore,we suggest that despite an offset from equilibrium,the d18O-values in shells vary as a function oftemperature as has been observed, for example, fromexperimental determination of stable isotope variabilityin Globigerina bulloides (Gb) by Spero and Lea (1996).These authors conclude that ‘‘shell d18O varies aspredicted by the paleotemperature equation, but is


offset from equilibrium’’. We thus make the assumptionthat the reverse size vs. d18O relationship that char-acterizes the recent planktic foraminifera from thewestern Arctic can be interpreted in terms of tempera-ture gradients along the pycnocline, as it is the case inother oceanic environments. On these grounds, the lateHolocene planktic foraminifera from the western Arcticseem to yield a stable-isotopic signature, which wouldreveal conditions similar to those prevailing today in theEast Siberian and Laptev seas. This would imply slightlyhigher temperature at the top of the North Atlanticwater mass than at present in the western Arctictogether with a possibly higher inflow rate of the NorthAtlantic water mass in the Arctic Ocean.

4.3. The dextral vs. sinistral N: pachyderma in the

western Arctic

Npd is present in all Holocene samples of cores B15and P49. In core B15 which was submitted to micro-faunal counts in the greater than 63 mm fraction, theyounger than 10 kyr sediment is characterized by 4–7%Npd, which represents concentrations in the order of102 shells/g (Febo, pers. comm.; cf. Febo and Polyak,2001). Such a low but significant occurrence of Npdindicates that conditions suitable for its development orat least its survival prevailed on a regular basis in thewestern Arctic waters during the Holocene. Ecologicalrequirements for Npd are not yet well understood. Thissub-species has generally been associated with warmerwaters than Npl, but recent studies suggest that severalgenotypes might belong to this taxon, one of them beinggenetically identical to Npl (Bauch et al., 2003).

The behavior of Npd with respect to its isotopiccomposition is variable at the sites we examined in theNW Atlantic. It often shows d18O-records similar tothose of Gb, but it may also follow those of Npl. In theLabrador Sea, for example, Gb shows isotopic composi-tions indicating calcite precipitation from August untilearly September, between 40 and 80 m, above thepycnocline, whereas 150–250 mm Npl show isotopiccompositions suggesting calcite precipitation during alonger growth season, from late spring until latesummer, much deeper along the pycnocline (Hillaire-Marcel and Bilodeau, 2000). Npd may follow one or theother pattern, and may even show both patterns in thesame cores, alternating in phase with glacial/interglacialor interstadial episodes. This heterogeneous behavior ofNpd in the Labrador Sea with respect to its isotopiccomposition supports the hypothesis of the existence oftwo distinct cryptic species of Npd close to the Arcticfront as recently proposed by Bauch et al. (2003).

In the western Arctic, Npd follows closely the Nplpattern with respect to both its d13C and d18O-values.Npd and Npl show a similar distribution of stableisotope values between small and large specimens,

including the same negative trend for d18O-values withincreasing shell size, although there is a slightly largerscatter of Npd values (Fig. 5). Variations in the stableisotope record of Npd during the Holocene are similarto those of Npl, notably during the early Holocene(Figs. 7 and 8). This similarity strengthens the use of Npland Npd isotopic signatures for constraining thepaleoceanographical history of the area. Moreover, thecoherency of isotopic data from Npl and Npd supportsthe hypothesis that both taxa actually belong to thesame genotype as proposed by Bauch et al. (2003) forpolar Neogloboquadrina pachyderma, occurring north of70�N.

4.4. Carbon isotopes in Npl and Npd

Both Npl and Npd are reported to show 13C-depletedd13C-values with reference to equilibrium conditionswith atmospheric CO2. This behavior seems to resultfrom 13C-depleted ambient DIC and/or to vital respira-tion effects (e.g., Ortiz et al., 1996; Spero and Lea, 1996;see a review in Volkmann and Mensch, 2001). Ingeneral, the major temperature control seems indirect,i.e., through its incidence on metabolic rates. Reversetemperature gradients along the pycnocline, as observedin the western Arctic, should result in negative d13C-gradients between small and large specimens if tempera-ture-controlled isotopic fractionation was the onlyprocess involved. This is not the case for the studiedassemblages (Figs. 5 and 6).

In the open ocean, surface d13C-DIC values generallydecrease from near equilibrium values with atmosphericCO2, to 13C-depleted values deeper, in relation withoxidation rates of organic matter, which is maximal atthe depth of the oxygen minimum (e.g., Duplessy, 1972).In such settings, this should result in a similar trend ford13C-values in Npl shells. The small ones, living near thesurface, should be 13C-enriched, compared with thelarge ones that have added calcite to their shell at depthswhere DIC should be 13C-depleted. Such a pattern isillustrated somewhat by data from plankton tows in theNortheast Water (NEW) polynya (Kohfeld et al., 1996).However, in the western Arctic sediment samples, aswell as in the NW Atlantic ones, the large Npl shells aresystematically 13C-enriched in comparison with thesmall specimens. Higher calcite precipitation and meta-bolic (respiration) rates in juvenile specimens couldaccount for this behavior. Such an effect discards thepossibility examined above that the large specimenscould be 18O-depleted due to higher temperaturedependent precipitation rates. Before concluding thisdiscussion on d13C-gradients in the western Arcticassemblages, it is worth mentioning that Npd followsexactly the Npl d13C vs. shell size pattern, and that bothNpl and Npd depict a stratigraphic trend from thedeglacial minimum to the late Holocene 13C-maximum,

ARTICLE IN PRESS

Core B-15

4.5 4 3.50

5000

10000

15000

CalibratedAge yr BP

0.5 1 1.5 2 2 1 0 2 1 0 10-0.5 0.5 1.5 10-0.5 0.5 1.5

100-150 µm

150-250 µm

>250 µm

>250 µm

150-250 µm

100-150 µm

14C-datedlayers

δ18O � CW δ13C � CW δ18O � Npl δ18O � Npd δ13C � Npl δ13C � Npd

YD ?

100-150 µm

150-250 µm

>250 µm

100-150 µm

150-250 µm

>250 µm

Fig. 7. Isotopic records of benthics and of Npl and Npd shells as a function of their size, in the Chukchi Sea core B-15. The shaded stripe highlights

conditions during the early Holocene climate optimum. Data on the large Npl and Npd specimens suggest that the North Atlantic water mass, below

the surface layer, may have been warmer (by up to 4�C) and/or more dilute, than at present, during this interval. Note the lesser variability suggested

for the surface water layer small Npl and Npd data sets. The chronology is established after 14C ages obtained from foraminiferal populations, which

were normalized for a d13C of �25%, corrected by �750 years to take into account the regional air-sea CO2 reservoir difference (Mangerud and

Gulliksen, 1975), and calibrated using the Calib software (Stuiver et al., 1998). The age-model is based on a linear interpolation between the three

dates and suggests near constant sedimentation rates.


that is practically identical to that observed in mostNorth Atlantic records (Fig. 6; see also Duplessy et al.,1988; Hillaire-Marcel et al., 1994).

Because primary productivity is highly variable, in theArctic, and because hydrographic conditions also varyfrom site to site, and from one sampling year to the next(resulting notably in highly variable growth rates),studies of water column Npl specimens yielded a largearray of d13C vs. size relationships. They range fromstrongly positive to strongly negative relationships,without unequivocal explanations for the observedpattern (e.g., Kohfeld et al., 1996). Obviously thecarbon isotope metabolism of Npl is still unclear. Oneconclusion arises from the detailed investigation ofplankton tow collected Npl in the NEW polynya byKohfeld and others (1996): the deepest collected speci-mens, both encrusted and non-encrusted, showed d13C-values (B0.5% vs. VPDB) with an almost constant�1% offset vs. equilibrium conditions with ambientDIC. Allowing for some offset value to be determined,this suggests that the large specimens from the westernArctic should reflect the d18C-DIC values on top of theNorth Atlantic water mass, where they developed. If this

Arctic isotopic composition were identical to thatobserved in the NEW polynya (B1.5% at a depth of200 m), then, the offset should be almost negligible, inview of the isotopic composition of the large Npl andNpd specimens (B1.2–1.3% vs. VPDB).

4.5. Limitation on the paleoceanographic interpretation

of isotopic data in the western Arctic

Several problems are encountered when trying tointerpret past Npl and Npd d18O and d13C-records(Figs. 7 and 8). As can be seen, all size classes show arelatively large scatter, especially downcore. The scatteris particularly large for the >250 mm specimens. Part ofthis scatter can be simply due to variable fluxes of eachclass of size of Npl or Npd, due to ecologicalconstraints. Mixing would then redistribute the shells,with possibly size-dependent mixing equations (e.g.,Bard, 2001), and also varying as a function of mixingintensity, thus of sedimentation rates and benthiccarbon fluxes (e.g., Wu and Hillaire-Marcel, 1994b). Inlow sedimentation rate records as for B-15 and P-49,such processes could play an important role and alter

ARTICLE IN PRESS

δ18O � CW δ13C � CW δ18O � Npl δ18O � Npd δ13C � Npl δ13C � Npd

4.5 4 3.5 0.5 1 1.5 2 2 1 0 2 1 0 10-0.5 0.5 1.5 10-0.5 0.5 1.50

2

4

6

8

10

12

YD ?

100-150 µm

150-250 µm

>250 µm

100-150 µm

150-250 µm

>250 µm

100-150 µm

150-250 µm

>250 µm

>250 µm

150-250 µm

100-150 µm

Depth (cm)

Core P-49

Fig. 8. Isotopic records of benthics and of Npl and Npd shells as a function of their size, in the Chukchi Sea core P-49. Compared with site B-15

(Fig. 4), the records suggest enhanced smoothing due to bioturbation. Nevertheless, as highlighted by the shaded area values, notably for large

specimens, higher temperatures and/or lower salinity conditions can be inferred for the North Atlantic water mass during the early Holocene climate

optimum.


the isotopic signatures. Another uncertainty here lies inthe fact that one ignores the impact, if any, of the lateraltransportation of shells, on top of the North Atlanticwater mass, along major current pathways (Fig. 1). Onealso ignores the duration of the Npl life cycle (a fewweeks?), thus the maximum potential distance of lateraltransportation. Nonetheless, such a process is likely to‘‘fractionate’’ low density and/or small shells, vs. highdensity and/or large shells. Therefore, part of theisotopic gradient observed between classes of size couldwell reflect differential calcification conditions ‘‘up-stream’’ in major current pathways, with the largest,longer lived specimens representing overall conditionsfrom more distant growth places.

Another element of concern is the incidence of sea iceformation and melting on isotopic compositions ofambient waters, as illustrated by many studies of ArcticOcean waters (e.g., B!edard et al., 1981; Moore et al.,1983; Ostlund and Hut, 1984; MacDonald et al., 1995;Melling and Moore, 1995; ). As shown in Fig. 4, oxygenisotope data from the Beaufort and Chukchi seassuggest that a simple two-end member, linear mixingbetween local fresh-waters and ‘‘normal’’ sea water (thusa linear salinity–d18Owater relationship), is stronglyaltered by the production and sinking of isotopicallylight brines during sea ice growth intervals, and by the

release of isotopically heavy, low salinity waters, when itmelts. In opposition to this pattern, waters from the NWAtlantic illustrate the classic linear relationship betweensalinity and water 18O-content (Fig. 4). Adding to thesesea ice effects on the salinity vs. water 18O-contentrelationship are the relatively large spread of riverined-values supplied to the Arctic (e.g., L!etolle et al., 1993;Bauch et al., 1995; MacDonald et al., 1995; Azetsu-Scottand Tan, 1997). This may also play a role in the scatterof values observed (Fig. 4). Nevertheless, such processes,occurring near the modern pack-ice limit could have hada variable incidence on isotopic composition of plank-tonic foraminifera in the past due to any shift in theposition of the packice summer margin. Anotherlimitation on a direct interpretation of oxygen isotopegradients between classes of sizes of Npl or Npd in termsof properties of the water masses involved arises fromour ignorance of the actual relationship between the sizegroups. As indicated earlier, we cannot presume that thelarge specimens represent mature stages of the smallones. If this were the case, then a weighted masscalculation would permit a calculation of the isotopiccomposition of calcite added during each growth stageand thus constrain hydrographic parameters at a givengrowth depth. Such a calculation has been made for theNW Atlantic assemblages (Fig. 9). Large uncertainties

ARTICLE IN PRESS

Fig. 9. Mean isotopic composition of Npl assemblages in the size

fractions o125, 125–150, 150–212, >212mm from NW Atlantic box-

core tops (data from Candon, 2000). The dashed lines and empty

symbols represent values calculated for overgrowth from one size to

the next, assuming that the large specimens represent calcite over-

growth on smaller ones (see text). The thick lines and filled symbols are

actual measurements.


in the isotopic composition of calcite increments areseen in the largest specimens. Fig. 9 also illustrates thefact that a critical isotopic threshold is found at precisely150 mm. A threshold usually retained for the setting ofNpl isotopic records. Also interesting is the fact that inmost investigated populations, the abundance of speci-mens in the 125–150 mm range is such that using the125–212 mm range of sizes for Npl, instead of the150–250 mm, shifts the isotopic records by approxi-mately 0.4% towards lower values (e.g., Hillaire-Marceland Bilodeau, 2000). Nevertheless, the fact that wecannot ascertain the significance of the calculations(Fig. 9), leads us to conclude that the isotopiccomposition measured on the larger specimens (150–250 and >250 mm) provides only a lower limit for theisotopic composition of the calcite added during thefinal growth stages and/or of the secondary calciteovergrowth. Thus the paleohydrographic parametersreconstructed, whenever possible, represent minimumvalues for paleosalinity, for example, when the pycno-cline is essentially controlled by salinity gradients (as inthe case of the Labrador Sea), or for temperaturegradients, as in the case of the western Arctic situation.

4.6. Constraints on the Holocene paleoceanography of

the western Arctic

Our paleoceanographic interpretation of isotopicrecords in planktonic foraminifers is limited, in view of

the above caveats. Nevertheless, the data presented hereprovide important information on paleoceanographicconditions in the western Arctic. We have hypothesizedthat the reverse d18O vs. planktic size gradients observedin the Chukchi Sea assemblages were due to the presenceof the warm North Atlantic water mass, below the coldsurface water layer. An examination of the Holocenerecords of Figs. 7 and 8 leads us to conclude that thissituation prevailed very early during the Holocene. Npland Npd, in both B-15 and P-49 records, show a strongdeparture of the largest specimens towards lowerisotopic compositions during the early Holocene, witha maximum between ca 9 and 7 ka BP, contrasting withrelatively stable isotopic values for the small specimens(Figs. 7 and 8). This shift exceeds 1%, thus suggesting aminimum temperature shift of 3�C in the correspondingwater mass, although a decrease in its salinity that couldaccount for part of the isotopic shift cannot be totallyruled out. Reconstructions based on dinocyst assem-blages also indicate warmer conditions during the earlyHolocene (cf. Darby et al., 2001). Moreover, Mg/Caratio in ostracode shells from cores B-3 and P-1collected on the Chukchi shelf indicate that the bottomwater (201 m) mass was warmer than at present by atleast 1�C during most of the Holocene with rapid andlarge amplitude warming pulses of 1–2�C during theearly mid Holocene (cf. Darby et al., 2001). Comparedwith the modern situation, a +3�C shift in thetemperature of the North Atlantic water mass wouldraise its temperature to near 4�C, at the study site. Thiswould imply a significant enhancement of NorthAtlantic water inflow rates in the Arctic Ocean, or theinflow of much warmer waters than today. In thehypothesis that part of the 18O gradient between smalland large shells would be inherited from distinct lateraltransportation processes, an enhanced penetration ofNorth Atlantic water should also result in larger isotopicgradients, due to the more distant origin of the largestspecimens.

An inflow of North Atlantic waters significantlywarmer than at present during the early Holocene isconsistent with the sea-surface temperature estimatesbased on various proxies in the northeast North Atlanticand Nordic seas, which reveal marked cooling trendfrom the early to the late Holocene (e.g., Ko@ et al.,1993; Klitgaard-Kristensen et al., 2001; Marchal et al.,2002). Moreover, in accordance with the hypothesis of ahigher inflow rate of North Atlantic water during theearly Holocene, we may put forth recent findings byDuplessy et al. (2001) indicating a significant increase inthe inflow of North Atlantic waters in the Barents Seabetween ca 7.8 and 6.8 ka BP (see also Lubinski et al.,2001). Due to the smoothing of our records, we cannotachieve a similar precision in the dating of the event inthe western Arctic, but it broadly matches that of theBarents Sea. Other findings that would concur with an


early Holocene high inflow, are those linked to theinception of the modern-like thermohaline circulation(THC) in the NW Atlantic. Evidence for a high outflowof North-East Atlantic Deep Waters (NEADW) im-mediately after the Younger Dryas is found in theenhanced supply of Reykjanes Ridge indicators (smec-tites/illite ratios, eNd values; see Fagel et al., 1997 andInnocent et al., 1997) in the Labrador Sea. At ca 7 kaBP, the production of Labrador Sea Water started(Hillaire-Marcel et al., 2001a, b), suggesting a majorreorganization of the THC at the same time. It seemsthus to be related to a reduced inflow of North Atlanticwaters to the Arctic, based notably on Duplessy et al.(2001) and the present study, with a concomitantreduction of NEADW overflow.

In the studied cores, a significant increase indinoflagellate cyst concentrations is associated withproductivity increase during this critical early Holoceneinterval (cf. de Vernal, unpubl. data). The small negatived13C-oscillation that is observed in all size classes thatmatches the d18O shift in large Npl and Npd shellsbetween ca 9 and 7 ka (Figs. 7 and 8), could thus be aresponse to the higher precipitation temperature of theinterval and thus, to a reduced DIC-calcite isotopicfractionation.

5. Conclusions

Despite limitations due to the low sedimentationrates, the poor resolution of time-series and themonospecific character of planktonic foraminifer popu-lations, there are several elements from this study thatdeserve attention. One is the fact that a comparison ofisotopic composition in several sizes of plankticforaminifers representing epi- to mesopelagic environ-ments may provide indication on the structure of theupper water column. Furthermore, when using Npl inthe 150–250 mm size fraction, it is clear that one does notget an indication on the isotopic composition of thesurface water layer, but of a sub-surface water horizon,along the underlying pycnocline, as also shown byinvestigations on live specimens from the Arctic Ocean(e.g., Kohfeld et al., 1996). Another conclusion is thefact that Npd seems to behave in a fashion quite similarto that of Npl in the western Arctic, indicating that theyshare a same ecological niche and supporting thehypothesis that they represent actually two polarmorphotypes of a same genotype (cf. Bauch et al.,2003). From a paleoceanographical viewpoint, the factthat the small Npl specimens of the 9–7 ka BP intervalare practically isotopically unchanged, whereas the largespecimens developing deeper are depleted in 18O, is veryinteresting inasmuch as it does provide further insightsinto the gradient in the water column and on drivinghydrographic changes. In the particular context of the

western Arctic, this indicates enhanced inflow of NorthAtlantic waters during the early Holocene. A finalconclusion we would like to draw concerns the fact thatwestern Arctic cores from the deep basins near thesummer ice-pack margin may provide valuable insightsinto the paleoceanography of the area and on oceanscale. In particular, the reconstruction of large ampli-tude hydrographic changes, such as that of the earlyHolocene, prior to 7 ka BP, is consistent with the onsetstage of the modern THC, when the eastern routethrough the Iceland basin was probably the most activeone, until the western components started operating inthe Labrador Sea and, most likely, the Greenland Sea.

Acknowledgements

This study was funded by NSF-OPP-9817051 as partof the NSF western Arctic Shelf–Basin-Interactioninitiative and a contribution to the Climate System,History and Dynamics project, supported by theNational Science and Engineering Research Council ofCanada, and to the international IMAGES program.Complementary support from the Fonds pour laFormation de Chercheurs et l’Aide "a la Recherche ofthe Quebec Province and by the Canadian Foundationfor Climate and Atmospheric Sciences (project no. GR-240) is acknowledged. We are grateful to John Andrewsand an anonymous reviewer of the Journal for theirhelpful comments on the manuscript and to GuyBilodeau and Julie Leduc for their analytical supportat GEOTOP.

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