Paleoenvironments of the last interglacial in northwest north atlantic region and adjacent mainland Canada

Quaternary International, Vol. 10--12,pp. 95-106, 1991.

104(~6182/91 $0.00 +.50 © 1992 INQUA/PergamonPress Ltd

Printed in Great Britain. All rights reserved.

PALEOENVIRONMENTS OF THE LAST INTERGLACIAL IN NORTHWEST ATLANTIC REGION AND ADJACENT MAINLAND CANADA

NORTH

A n n e de Vernal,* Gifford H. Millert and Claude Hillaire-Marcel* *GEOTOP, Universitd du Qudbec ~ Montrdal, CP 8888, Suec. A, Montreal, Qudbec H3C 3P8, Canada t l N S T A A R , University of Colorado, Boulder, CO 80309-0450, U.S.A.

The paleoenvironmental conditions that prevailed in the northwest North Atlantic regions and adjacent mainland Canada during the last interglacial are documented through lithostratigraphical and paleontological data in terrestrial sections and marine cores, with special reference to the deep-sea records that allow global correlations based on their isotopic stratigraphy. The onshore paleovegetational data and the offshore planktonic faunal and algal records demonstrate much warmer than present terrestrial and marine environments during the climate optimum of the last interglacial sensu stricto (Isotopic Substage 5e). Optimal conditions persisted in surface water of the Labrador Sea during the early glacial inception (Isotopic Substage 5e/5d transition) when extensive ice masses developed over Arctic Canada. A deterioration of sea-surface temperature in Labrador Sea followed the maximum ~80 peak of Isotopic Substage 5d. During the later part of Stage 5, climatic conditions similar to the present prevailed over southeastern Canada and a recurrent warming is recorded in south central Labrador Sea; fluctuating ice volumes in the Canadian Arctic led to episodic dilution of surface water masses in Baffin Bay and the eastern Labrador Sea, and subarctic conditions apparently prevailed in nearshore environments of Baffin Island despite an ice marginal context.

episode by referring to the terrestrial and marine stratigraphies.

INTRODUCTION

The last interglacial is commonly considered as an interval of relatively warm climate throughout the high THE TERRESTRIAL RECORD latitudes of the northern hemisphere. However, the last interglacial sensu lato (Isotopic Stage 5) that spanned Southeastern Canada In contrast to western Europe where continuous Late about 55,000 years (130-75 ka) was marked by large amplitude changes in summer radiation over circumpo- Pleistocene sequences have been recovered (e.g. Woillar regions (e.g. Berger, 1978) that were accompanied lard and Mook, 1982; de Beaulieu and Reille, 1984), by large scale fluctuations in terrestrial ice volume, as the sedimentary record of the last interglacial in illustrated through isotopic stratigraphy and sea level northeast North America is discontinuous in time and variations (up to 70 m; Shackleton, 1977, 1987). In view space. Relatively well preserved Late Pleistocene of the location of the main centers of ice growth during records are nevertheless found in areas which experithe Quaternary (eastern Canada and Greenland), the enced ice marginal conditions during the last ice age, paleoenvironmental changes in the northwest North such as the Great Lake region (Fig. 1, region C; Atlantic regions deserve special attention. However, Karrow, 1990), southern Quebec (Fig. 1, site B; knowledge of the Late Pleistocene climatostratigraphy Anderson et al., 1990) and the Atlantic provinces of of these regions is fragmentary, notably because of the Canada (Fig. 1, region C; Mott and Grant, 1985; de vertical and lateral discontinuity of the corresponding Vernal and Hillaire-Marcel, 1987; Mott, 1990). In sedimentary records, partly due to glacial erosional many of the records which yielded non-finite 14C ages, processes. Although paleoclimatic schemes have been the microflora indicates climate conditions as warm or proposed for the Atlantic provinces of Canada (e.g. warmer than the present. However, the chronological Mott and Grant, 1985; de Vernal et al., 1986), the control is often too vague to firmly assign a precise age Hudson Bay region (Shilts, 1984), the eastern Cana- to most interglacial deposits which can belong to any dian Arctic (e.g. Andrews and Miller, 1984; Miller, part of the last interglacial (from 130 to 75 ka), and 1985) and Greenland (e.g. Funder, 1989), the difficulty eventually may be older. Nevertheless, the paleoecoloof establishing an 'absolute' chronological framework gical study of the numerous non-glacial sequences often prevents unequivocal correlation with the refer- exposed in Atlantic Canada (Mott et al., 1982; Mott ence oceanic ~80 climatostratigraphy. The marine and Grant, 1985; de Vernal and Mott, 1986), coupled record of transitional basins adjacent to eastern Canada with Th/U measurements on associated wood (Causse and Greenland is therefore of primary interest since it and Hillaire-Marcel, 1986), provided the basis for a may allow establishment of direct links between the reasonably well constrained regional Late Pleistocene terrestrial records and the reference oceanic stratigra- stratigraphy (de Vernal et al., 1986, de Vernal and phy. Herein, we intend to document the paleoenviron- Hillaire-Marcel, 1987a; Fig. 2). This paleoclimatologic mental changes that took place over the northwest scheme of the last interglacial was established on the North Atlantic regions during the last interglacial basis of palynological successions observed through 95

96

A. de Vernal et al.

FIG. 1. Map of the northwestern North Atlantic, showing the location of marine sites and terrestrial regions discussed in the text. The numbers refer to ODP Sites (cf. Srivastava et al., 1987). The arrows illustrate schematically the modern circulation in surface water masses. The letters refer to the terrestrial sites or regions with a stratigraphy representative of the last interglacial to which we refer in the text. Region A refers to the Atlantic provinces of Canada, where tens of Late Pleistocene sedimentary sequences have been studied (for regional synthesis see de Vernal and Hillaire-Marcel, 1987a; Matt, 1990). Site B refers to sequences studied by Anderson et al. (1990). Region C corresponds to the area around Toronto where reference Late Pleistocene sections are exposed (see Karrow, 1990). Region D includes several sites along Hudson Bay attributed to the last interglacial (for regional synthesis see Dredge et al., 1990; Matt and DiLabio, 1990). The Late Pleistocene stratigraphy of region E (CR = Clyde River site; Q = Quvitu site), along the Baffin Island coast is reported in detail by Miller (1985).

J

~,c.(~

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Minimum r h / U ages

Proposed correlation with oceanic 180 cLJmotostratigrophy

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MiddLe II I-2 b o

62000 +__ 5000

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e

=:11 !

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,26 oo+,5ooo

Substage 5a

Substage 5e (--5c ?)

.•

rO*/o
FIG. 2. Summarized Late Pleistocene palynostratigraphy, established from a composite sequence of the Atlantic provinces o~ Canada (region A on Fig. 1; cf. de Vernal et al., 1986).

Paleoenvironments of the Last Interglacial composite sequences, and consists of two distinct palynostratigraphical units. (1) Unit I has been identified in several sections and dated to about 120 ka (de Vernal et al., 1986). This unit is characterized by pollen assemblages dominated by Pinus and Ostrya which reflect the existence of hydroclimatic conditions drier than present and annual temperatures at least 4°C warmer than at present over Atlantic Canada (Mott, 1990). At some locations, the pollen assemblages of Unit I replace tundra-type assemblages reflecting cold conditions which have been attributed to the penultimate glacial episode (Illinoian; Mott and Grant, 1985). Unit I no doubt corresponds to the early part of the last interglacial and can be correlated with the Isotopic Substage 5e of the oceanic stratigraphy. (2) The palynostratigraphical unit II, dated to about 85 ka (Causse and Hillaire-Marcel, 1986) has been identified in several sections, including one (Addington Forks section; cf. Mott and Grant, 1985) where it appears to overlie Unit I. Unit II is notably characterized by high percentages of A b i e s and the occurrence of Tsuga and Fagus, indicating cool temperate and humid climate conditions similar to the present (Mott et al., 1982; de Vernal and Mott, 1986). In most sequences, Unit II is followed by a third unit with dominant Picea pollen and decreased percentages of thermophilous tree taxa. Such a transition marks a significant regional cooling that may be attributed to the climatic transition at the end of the last interglacial (Isotopic Stage 5/4 transition; de Vernal et al., 1986). Lacking from the stratigraphy of the Atlantic provinces of Canada is evidence for major climate reversals during the last interglacial. If the regional climatostratigraphical scheme summarized above is correct, the last interglacial sensu lato would have been marked by a gradual cooling trend rather than by cyclical climate changes as observed elsewhere, such as in western Europe (e.g. Guiot et al., 1989). This would point to discrepancies in climate variations over eastern and western North Atlantic regions, as during the younger Dryas for example (e.g. Broeker et al., 1985).

H u d s o n Bay L o w l a n d s

Non-glacial deposits that represent interglacial and interstadial ice-free intervals are exposed in sections along streams draining the Hudson Bay Lowland (Fig. 1, region D), the geographic center of the former Laurentide Ice Sheet. Paleocurrent directions associated with these deposits indicate unimpeded drainage into Hudson Bay, thereby requiring that the bay, and its connection to the open ocean through Hudson Strait, were free of glacier ice. Sub-till exposures of marine sediments and forest beds in the Lowland were first reported by Bell (1887), but systematic analysis of the ecological indicators was delayed until the studies of Terasmae and Hughes (1960). The results of research carried out over the last decade on the stratigraphy in the Lowland is summarized by Mott and

97

DiLabio (1990), Dredge et al. (1990) and Thorleifson et al. (1992). Locally, inter-till sediments include a complete deglacial sedimentation cycle, similar to that of the last deglaciation, beginning with a marine facies deposited during isostatic readjustment from an earlier interval of glacial loading, followed by peat and/or forest litter indicative of a terrestrial environment similar to present. Base level for the fluvial systems draining the Lowland was lower than present, suggesting a more complete glacio--isostatic recovery than has yet occurred in the present interglacial. At some sites, the uppermost unit consists of glacio--lacustrine silts interpreted to reflect ponding of regional rivers by a newly formed ice sheet. The complete succession was formally named the Missinaibi Formation by Skinner (1973), with the basal marine sediments, the Bell Sea member, deposited during a glacio-isostatic high sea level episode. Non-glacial waterlain sediments are found between till sheets that stratigraphically overlie the Missinaibi Formation and correlate throughout the Hudson Bay Lowlands (e.g. Skinner, 1973; Dredge and Nielsen, 1985; Thorleifson et al., 1992), although at no site is the Missinaibi Formation directly overlain by inter-till organic-bearing deposits. The stratigraphic position of many of the isolated occurrences of inter-till organic beds is uncertain. The pollen assemblages in these beds are generally dominated by Picea (40-60%), with modest percentages of Pinus (20-30%) and Betula (ca. 10%), indicating vegetation and climate rather similar to the present day. Neither the taxonomic composition nor the vegetation succession are sufficiently different between sites to evaluate whether the organic accumulations are representative of a single interglaciation, or whether they represent two or more warm, ice-free intervals. The Missinaibi Formation has been correlated with the last interglacial (e.g. Prest, 1970), based on its geographic location, sea level considerations, and the presence of terrestrial vegetation similar to present. Stuiver et al. (1978) obtained a radiocarbon age of >72,500 BP on wood from the forest beds using isotopic enrichment techniques. Mean amino acid D/L (alle/Ile) ratios in Hiatella arctica from the Bell Sea member range from 0.22 to 0.24 (Andrews et al., 1983; Wyatt, 1989; Thorleifson et al., 1992), compatible with, but no younger than a last interglacial age for the event. The correlation and absolute age of other inter-till nonglacial sediments across the Lowland is uncertain, and has been hampered by the lack of suitable material for absolute dating. Amino acid D/L ratios have been measured in molluscan fossils from associated marine beds and as erratics in till (Andrews et al., 1983; Wyatt, 1989), and from wood in the forest beds (Nielsen et al., 1986). The D/L ratios have been used to correlate disjunct deposits (Dredge et al., 1990; Thorleifson et al., 1992), and to define a preliminary absolute chronostratigraphy assuming the Missinaibi formation is of last interglacial age (Andrews et al., 1983). Thermoluminescence dating has also been used to date waterlain

98

A. de Vernal et al.

inter-till sediments at 32--46 ka (Berger and Nielsen, 1990), and 73 __+ 10 ka (Forman et al., 1987). Amino acid D/L ratios from the latter site on in situ molluscs of the Prest Sea (Thorleifson et al., 1992) support correlation of the Missinaibi Formation to the last interglaciation. In summary, the Hudson Bay Lowland contains a complex packet of tills and interbedded non-glacial sediment. The Missinaibi Formation at the type section in the Moose River Basin is correlated to the last interglacial (sensu stricto) on a variety of lines of evidence. It includes a cool-warm-cool cyclic sequence of paleoecological indicators, with the interval of maximum warmth similar to present conditions. Elsewhere, organic-bearing beds of similar character are correlated to the last interglacial (sensu lato; e.g. Dredge et aLl 1990), with paleoecological indications (particularly pollen and beetle) of conditions both similar to, and cooler than present. The units with indicators of maximum warmth cannot be conclusively ascribed to the last interglacial (sensu stricto), but at no site do the most optimal conditions exceed those of the Holocene optimum (ca. 5-6 ka BP). Conditions during the late interglacial (sensu lato) cool interval are estimated to be 3--4°C lower than present in July (Dredge et al., 1990), although this may have been well after ice-sheet growth began elsewhere in northern Canada, as the first indications of the Wisconsin Glaciation are of an ice margin advancing into the area from the NE. We conclude that the last interglacial sensu stricto was broadly similar to the present; maximum last interglacial warmth was similar to the Mid-Holocene optimum, although climatic interpretations may be limited by the relative insensitivity of many of the sites within the boreal forest biome.

Eastern Canadian Arctic Coastal exposures along northeastern Baffin Island (Fig. 1, region E) contain interbedded glacial, glacialmarine, marine and occasional eolian sediment, with evidence of intermittent subaerial exposure. The buried surfaces are marked by soils or organic accumulations, most of which contain pollen. Based on stratigraphic position, relative sea level inferences, and limiting age control supplied by radiocarbon dates and amino acid D/L ratios on in situ molluscs from underand over-lying marine beds, some of the paleosurfaces have been ascribed to the last interglaciation (sensu stricto). The floral evidence and paleoecological interpretations are given by Miller et al. (1977), Mode (1985), and Andrews et al. (1986). The working definition for the last interglacial is the deposit closest to the present surface that requires relative sea level near present, has a non-finite radiocarbon age, and faunal or floral evidence requiring terrestrial summer temperatures higher than present (Miller et al., 1992); inshore marine surface waters were warmer than present more than once during Isotope Stage 5 and cannot be used to identify the last interglacial sensu

stricto. Although we recognize the inherent limitations of this definition, the lack of secure chronological control precludes a more rigorous, climate-independent definition. The Kogalu aminozone (Miller, 1985) is considered to represent the last interglacial sensu lato; it includes terrestrial sediments of last interglacial sensu stricto age, as well as glacial, glacio-marine, and marine sediments deposited later during Isotope Stage 5. Pollen samples from levels deposited within the Kogalu aminozone on Qivitu Peninsula contain 6-12% Betula, well above the current levels (2%; cf. Mode, 1985). Betula percentages exceed the 5% at present only in areas within the range of shrub birch; consequently, it is inferred that birch was growing on the peninsula at that time, north of its present limit. At Clyde the present influx of Betula pollen is less than 1%, yet several pollen samples from buried organic-rich horizons with non-finite radiocai'bon dates contain 20 to more than 50% Betula (Mode, 1985); they are interpreted to indicate that shrub birch was growing on the foreland, more than 300 km north of its present limit. The precise chronostratigraphic position of these samples is not always established, but buried soils found at three localities directly beneath Kogalu aminozone marine sediments contain more than 50% Betula (Mode, 1985). Two of the buried soils were developed on Cape Christian marine sands, subsequently glacially overridden, and are overlain by sediment of the Kogalu aminozone. They satisfy the working definition to be of last interglacial age, and contain pollen assemblage dominated by Betula, and significant percentages of A l n u s (Miller et al., 1977). Both the absolute abundances and pollen assemblages at all three sites indicate terrestrial summer temperatures well above the Holocene optimum. Miller et al. (1977) defined the Cape Christian interglacial based on the pollen assemblages in two of these soils, and correlated it with the last interglacial sensu stricto. Recently, the absolute ages of the aminozones have been revised (Miller, 1985), but the soils on which the Cape Christian Interglaciation was defined are in the correct stratigraphic position to be of last interglacial age, although an older age cannot be excluded. The Cape Christian Interglacial at Clyde was correlated with the organic beds at Flitaway Lake and the Isortoq beds near the Barnes Ice Cap (Terasmae et al., 1966; Miller et al., 1977). However, the deposits in the vicinity of the Barnes Ice Cap are now recognized to be of pre-Quaternary age, hence the correlation is invalidated. Despite uncertainties with precise chronologies, pollen analysis of three paleosurfaces directly underlying the Kogalu aminozone and of several other organic horizons from within sediments of the Kogalu aminozone, document Betula at levels indicating shrub birch was growing 100--300 km north of its Holocene limit. Based on the pollen assemblages in the paleosurfaces at Clyde, summer temperatures must have been 3°C higher than present, with ecological conditions similar

99

Paleoenvironments of the Last Interglacial

to those of northernmost Labrador or southern Greenland.

THE MARINE RECORD

Proposed isotopic

sto~es ~m 0 | (°/oo) / 4

TWC

,

Q.

/

In the northwest North Atlantic, the Labrador Sea and Baffin Bay constitute subpolar and polar deep-sea basins adjacent to eastern Canada• These basins contain good records of regional climate changes in relation to the glacial fluctuations of the Quaternary: (1) in view of their location proximal to the main centers of ice growth, they constituted transitional basins between the ice sheets and the open ocean; (2) surface water mass circulation in these subpolar basins plays a major role in the northward heat transport that controls the hydroclimatic regime of northeastern Canada; and (3) high rates of sediment accumulation (> 5 cm/ka; e.g. de Vernal, 1986) make possible high time resolution studies. Since the late seventies, several studies have been undertaken to establish the regional climatostratigraphy in the Labrador Sea and Baffin Bay (Aksu, 1981; Fillon and Duplessy, 1980; de Vernal, 1986). However, because of the high sedimentation rates that characterize these basins, most piston cores did not penetrate deposits of the last interglacial optimum. It was only during Leg 105 of the Ocean Drilling Program (ODP) in 1985 that hydraulic coring allowed recovery of longer sequences spanning the Late Pleistocene (cf. Srivastava et al., 1987). In the Labrador Sea, two main sequences include the last interglacial: one is from south central Labrador Sea (ODP Site 647 and cores 84-030-001/-002/-003; Fig. 3) and the other was collected on the southwest Greenland rise (ODP Site 646; Fig. 4). In central Baffin Bay, the most probable interglacial sequence is from the ODP Site 645 (Fig. 5). These three sequences constitute a south to north transect from subpolar regions adjacent to the North Atlantic• South Central Labrador Sea

In south central Labrador Sea (Site 647), surface water masses represent a westward branch of the warm North Atlantic Drift, these mix with cold water from the Labrador Current to form a mid-ocean gyre. At Site 647, sedimentation rates were about 5 crn/1000 years throughout the Late Pleistocene. In addition to ODP cores, the collection of site survey piston cores (84-030-001/-002/-033) made possible high resolution studies (sampling interval of 5 cm, i.e. ca. 1000 years time resolution) for isotopic and micropaleontological analyses. The isotopic stratigraphy of south central Labrador Sea conforms to that of the open ocean, allowing global correlation. However, the amplitude of 8180 fluctuations is up to 3.1%o, notably at the Isotopic Stage 6/5 boundary (Fig. 3A) which is attributed to significant dilution in surface water due to the direct meltwater discharge from the Laurentide Ice sheet at the time of ice decay (cf. de Vernal and Hillaire-Marcel, 1987b,c).

3

2

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.

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.

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I

--.. __ "~'_.F

2 P o "~"

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6

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s

. ~ .

I0

FIG. 3. Summarized stratigraphy in cores 84-030-001 and 84-030-003 (ODP Site 647 on Fig. 1). A, Oxygen Isotope stratigraphy after de Vernal and Hillaire-Marcel (1987b) and Scott et al. (1989).

The glacial/last interglacial transition in the isotopic stratigraphy (Stages 6/5) was rapidly followed by the development of subarctic microfauna (Fig. 3B) and microflora (Fig. 3C) in surface water masses• The dinoflagellate cyst assemblages indicate the establishment of conditions much warmer than present, by about 4°C in summer, during the climate optimum of the Isotopic Substage 5e. Both planktonic foraminifers (Scott et al., 1989) and dinoflagellate cyst assemblages (de Vernal, 1986) indicate that subarctic conditions prevailed in surface water masses during the subsequent ice-sheet inception of Isotopic Substage 5d. The persistence of optimal climate conditions during the trend of increasing 8180 values at the Isotopic Substage 5e/5d transition is also shown by planktonic foraminifer assemblages in some sequences of the open North Atlantic domain (CLIMAP, 1984). An impoverishment of planktonic assemblages is recorded at the maximum i51so peak of Isotopic Substage 5d and through most of Substage 5c. It is attributed to a strong cooling in surface waters. Late in the last interglacial (Substages 5b and 5a) a recurrence of subarctic conditions, as warm or warmer than present, marked

A. de Vernal et el.

100

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i

TWC

PLanktonic Foraminifer Concentrations

~. to o ~ oo

i

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( X 10 3 . c m - 3

Sub-arctic species (%)

)

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FIG. 3. Summarized stratigraphy in cores 84-030-001 and 84-030-003 (ODP Site 647 on Fig. 1). B, Planktonic foraminifer stratigraphy (Scott et al., 1989): the subarctic species include all planktonic foraminifers except Neogloboquadrina pachyderma leftcoiling. C, Palynostratigraphy (de Vernal and Hillaire-Marcel, 1987a). The dinoflagellate cyst assemblages are expressed in term of concentrations: Impagidinium spp., Nematosphaeropsis labyrinthus and Bitectatodinium tepikiense constitute temperate to subarctic indicators. In the summary, pollen assemblage NAP stands for non-arboreal pollen and the trees include mainly Pinus and Picea.

Paleoenvironments of the Last Interglacial

101 ~180 VS P D B

..~

Plonktonic forominifer

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N. p o c h y d e r m o

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(%1

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FIG. 4. Summarized Late Pleistocene stratigraphy at ODP Site 646 (cf. Aksu et al., 1989). A, Sand, planktonic foraminifer and b~sO stratigraphy. N. pachyderma dextrogyre, G. bulloides and G. quinqueloba are the three main subarctic species of the assemblage. B, Dinoflagellate cyst stratigraphy• Note that Spiniferites mirabilis is a cool temperate taxa indicating sea surface temperature at least 14°C during summer (Turon, 1984). which is 6°C warmer than present• surface water masses. The stratigraphy of the south central Labrador Sea appears, therefore, to be characterized by a discrepancy between ice fluctuations as recorded in the 180 stratigraphy, and changes in surface water masses, as shown by micropaleontological analyses, demonstrating that the early ice inception of Isotopic Substage 5d preceded the cooling trend in regional surface waters. In the southern Labrador Sea record, the pollen content of sediments provides additional information on paleoclimates as they relate to the terrestrial

vegetation of the surrounding land masses and to subsequent transport through atmospheric circulation (de Vernal and Hillaire-Marcel, 1987b). L o w pollen concentrations and the dominance of P i n u s throughout most of the sequence reflect long distance atmospheric input controlled by S W - N E trending air masses (in summer), as at the present time. H o w e v e r , the interval spanning the late Substage 5c through 5a is marked by increased pollen influx with moderately high Picea percentages. Together, these indicate a relatively close vegetational source dominated by boreal forest• In-

102

A. de Vernal et al.

o

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.=_

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13.

~o ~

PoLLen concentration

~[80

&

vs PDB oo (%°)

rn

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IO

I0

60

I0 I(

3O

2O

400

5

I

x IO00.cm-

fO

4

3

2

zl x I00, cm-

FIG. 4. Summarized Late Pleistocene stratigraphy at ODP Site 646 (cf. Aksu et al., 1989). C, Pollen and spore stratigraphy•

Oinofloget LaLe cysts AssembLage

a.

a.

_c~ -¢:: o~.~. " ~.

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ol

~x =~

~ &..9

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03

-~ ~180 vs / PDB (%*) |

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t)

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400

300

J

500

im.

r

I 600

I 12oo

I I 5 4 3 z

I I

5e

I0 cm-3

FIG. 5. Summarized Late Pleistocene palynostratigraphy and 6t80 stratigraphy at ODP Site 645 (cf. Hillaire-Marcel et al., 1989).

Paleoenvironments of the Last Interglacial

203

controlled by SW-NE atmospheric trends from the spruce forest of eastern Canada (de Vernal and Hillaire-Marcel, 1987c). Such is not the case for pollen and spore assemblages of the lower part of the last interglacial. The absence of correlation with the palynostratigraphy of eastern Canada (cf. supra) indicates northeastern North America did not constitute the principal vegetational source. Moreover, the very high Eastern Labrador Sea In the eastern Labrador Sea, off southwest Green- concentrations, an order of magnitude higher than land, surface water circulation is controlled by the West present, indicate a much more proximal vegetational Greenland Current, which is formed by a westward source that was probably located on Greenland. In fact, branch of the North Atlantic Drift mixing with the the overall pollen and spore assemblages of the Isotopic polar East Greenland Current. Hydraulic piston coring Substage 5e suggests input from different sources: long at ODP Site 646 permitted the recovery of a complete distance atmospheric inputs from the southwest (see Late Pleistocene sequence. Sediment accumulation the Pinus curve) and a dominant input from a proximal rates are variable through the sequence, averaging 9 shrub-tundra type vegetation. The latter may be cm/1000 years (Hillaire-Marcel et al., 1989; Aksu et al., attributed to fluvial, inputs from Greenland, which 1989). The higher sampling resolution allowed by ODP strongly suggests that large areas of southern Green(20 cm) led to isotopic and micropaleontological studies land were free of ice (see also Reeh, 1991) and occupied by a dense vegetation dominated by shrubs with an average time resolution of 2200 years. Despite large amplitude fluctuations in the 6180 and Pteridophytes. The high percentages of Osmunda record (up to 3%o), the isotopic stratigraphy is not as cf. cinnamomea, which has a modern distribution south clear as in southern Labrador Sea. Isotopic Substage 5e of 51°N in eastern Canada, indicates the existence of is easily distinguished but other subdivisions within subarctic or even cool temperate regional climates Stage 5 would be arbitrary. Full subarctic conditions (Hillaire-Marcel and de Vernal, 1989). Such a climate were established in surface water masses rapidly after is favorable for forest development, which was probthe glacial/last interglacial transition as shown by the ably limited on Greenland because its insular context planktonic foraminifer (Fig. 4A) and dinoflagellate cyst was unfavorable for rapid tree migration. Following the (Fig. 4B) assemblages. Both microfossil assemblages Isotopic Substage 5e there is a decrease in pollen indicate much warmer conditions than present, with concentration concomitant with increased Picea persummer temperature of about 14°C. As in the southern centages. This trend suggests decreasing inputs from Labrador Sea, such subarctic conditions prevailed the proximal southern Greenland source and the during the terrestrial ice inception of the Isotopic restoration of input from eastern Canada, controlled by Substage 5e/5d transition, also suggesting ice growth SW-NE to WSW-ENE air mass trajectories. Thus, the started before cooling of surface water masses. The assemblages reflect the existence of spruce forest to increase in 6180 values related to terrestrial ice growth tundra in eastern Canada during the upper part of the in the Substage 5e/5d transition is accompanied by last interglacial, while a decline in the vegetational increased accumulation of coarse debris due to ice cover on Greenland may have been due to significant rafting and, therefore, to glacial activity. An impover- ice growth. ishment of the planktonic foraminifer and dinoflagellate cyst assemblages coincides with a first maximum Central Baffin Bay 6aSo peak, which probably belongs to the Isotopic Baffin Bay constitutes an epicontinental basin Substage 5d. The upper part of Isotopic Stage 5 is bounded to the north by the Canadian Arctic Archipecharacterized by polar planktonic foraminifer assem- lago and isolated from the Labrador Sea by the Davis blages and by relatively sparse dinoflagellate cyst Strait, which has a sill at about 800 m of depth. Surface assemblages. Despite a subarctic component related to water masses are characterized by the penetration of a tenuous but persistent penetration of North Atlantic northward West Greenland Current branch to the east water into Labrador Sea, the overall dinoflagellate cyst and Arctic water outflow through the Archipelago assemblages reveal low productivity in surface water channels forming the cold Baffin Land Current that masses, probably due to dense seasonal sea ice cover. flows southward to the west. Both currents mix to form Nevertheless, abundance peaks with dominant Brigan- a gyre in central Baffin Bay. Baffin Bay is characterized tedinium spp. are associated with episodes of higher by a shallow lysocline responsible for calcium carbonproductivity under low salinity conditions (< 30%0; ate dissolution in deep sea sediments. Because of Mudie and Short, 1985) related to meltwater discharges dissolution and low biogenic productivity, planktonic along the Greenland margins. foraminifers are not abundant in Baffin Bay cores (e.g. Sediments from Site 646 are characterized by rela- Aksu, 1981, 1983), preventing the establishment of a tively abundant pollen grains and spores, especially in continuous isotopic stratigraphy. In addition, because the Isotopic Substage 5e interval (Fig. 4C). From high of the epicontinental character of the basin, the isotopic resolution studies of Holocene sediments, it has been stratigraphy has a predominantly regional signature demonstrated that postglacial pollen inputs are mainly with large amplitude fluctuations: any melting event

fluxes from eastern Canada are related to trajectories during summer of WSW-ENE to W-E air masses. Such influxes also suggest the existence of spruce forest and subarctic climate, similar to the present, along the eastern Canadian coasts during the later part of the last interglacial.

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would contribute to strong dilution in water masses and make correlations with the world ocean isotopic stratigraphy difficult. The peculiar character of isotopic records from Baffin Bay and Davis Strait has led to controversial chronostratigraphic interpretations (e.g. Aksu, 1981; Mudie and Aksu, 1984; de Vernal et al., 1987). The establishment of a magnetostratigraphy at ODP Site 645, in addition to AMS-14C dates on planktonic foraminifers allowed to establish a reasonably well constrained isotopic stratigraphy spanning approximately 125 ka in a ca. 22 m composite sequence (Fig. 5; Hillaire-Marcel et al., 1989). The base of this sequence is marked by low 6180 values and subarctic dinoflagellate cyst assemblages that are indicative of much warmer conditions than present, with summer temperature up to about 10°C. This interval is attributed to Isotopic Substage 5e although independent confirmation is lacking. The pollen assemblage of this particular interval is characterized by relatively high concentrations (up to 800 grains/cm3) as compared to the rest of the sequence. The moderately high percentages of Picea reflect a northward position of the boreal forest limit as compared to the present and/or strong northward atmospheric trends; the significant occurrence of Betula, A l n u s crispa and herb pollen suggests the existence of shrub tundra on the surrounding land masses. Such an assemblage may allow correlations with the buried paleosols of Baffin Island, which contain abundant Betula pollen grains and have been attributed to the last interglaciation (Terrasmae et al., 1966; Miller et al., 1977; Mode, 1985). Above the interval associated with the Isotopic Substage 5e, increases in 5XSo values and impoverishment of dinoflagellate cyst and pollen assemblages are related to the early glacial inception and cooling in regional surface waters. Throughout the Isotopic Stage 5, fluctuations in 6180 values and peaks of Brigantedinium spp. and Algidasphaeridium? minutum, together indicate episodic phases of meltwater discharge from the ice caps on surrounding lands. The most salient feature in the Baffin Bay record is undoubtedly the abundance of reworked preQuaternary palynomorphs that characterize Isotopic Substages 5d-5a. These palynomorphs indicate intense erosion of the Canadian Arctic Archipelago where the original sedimentary formations occur. On this basis and from field evidence (e.g. Miller et al., 1977, 1992; Andrews et al., 1984; Klassen, 1985), it has been argued that maximum glacial activity in eastern Arctic Canada occurred early during the Late Pleistocene, probably during Isotopic Substages 5d-5a. Deep-sea evidence for meltwater discharge, and the occurrence of subarctic nearshore faunal assemblages along Baffin Island and Greenland coasts (Miller et al., 1977, 1992), indicate phases of subpolar-type climate during the later part of the last interglacial. SUMMARY

The overview of paleoecological trends throughout

the last interglacial sensu lato in the northwest North Atlantic regions allow some considerations about environmental changes in relation to climate and glacial fluctuations. (1) Much warmer conditions than the present prevailed from the south to north along the eastern Canadian coast, in southern Greenland, and in surface waters of adjacent marine basins during the climatic optimum of Isotopic Substage 5e, and no doubt much longer at some locations, at least in the Labrador Sea. (2) The global ice volume increase of the Isotopic Substage 5e/5d transition corresponds to initial ice sheet growth, which no doubt affected Arctic Canada and perhaps extended southward over Hudson Bay. However, warm optimal conditions persisted, notably in Labrador Sea, until the ice volume reached its early maximum extent (peak of Substage 5d). Relatively warm surface waters during the glacial inception probably contributed to the northward transport of warm and humid air masses favorable for precipitation and ice accumulation over circum-Arctic regions. (3) Paleoenvironmental data of the upper part of the last interglacial indicate relatively warm conditions, similar to the present, over southeastern Canada and the southern Labrador Sea, which was characterized by the advection of temperate North Atlantic waters. At that time, the eastern Canadian Arctic was marked by intense glacial activity with ice margin fluctuations that led to episodic flows of meltwater discharge offshore. The development of a subarctic fauna in nearshore environments during phases of ice retreat suggest warming of surface water masses late in Isotope Stage 5. Throughout the last interglacial, the regions adjacent to the northwest North Atlantic were marked by large amplitude environmental changes, especially a t the highest latitudes where terrestrial areas experienced ice growth. From the marine and terrestrial data it appears clear that the glacial inception (Substages 5e/5d transition) occurred in circum-Arctic regions while optimal climate conditions, warmer than present, prevailed elsewhere. It is also clear that climatic conditions similar to the present existed during the later part of the last interglacial despite extensive glacial activity in the eastern Canadian Arctic. Although glacial fluctuations no doubt occurred in northeastern Canada, the available non-glacial proxy-climatic data from both terrestrial and marine environments of the northwest North Atlantic regions lack evidence for a cyclicity in phase with the global oceanic 6180 stratigraphy and insolation variations during the last interglacial interval.

ACKNOWLEDGEMENTS We sincerelythank all the participantsto the NATO workshopon the last interglacialfor their stimulatingdiscussions.We are grateful to Michelle Laithier (UQAM) for the drawing of figures. G.H. Milleracknowledgeslong term support fromthe Divisionof Earth Sciencesat the US NationalSciencefoundation.A. de Vernal and C. Hillaire-Marcel acknowledge support from The F.C.A.R. Funds of Quebec, and NSERC-Canada.

Paleoenvironments of the Last Interglacial

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