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Quaternary International, Vol. 10-12, pp. 123-142, 1991. 1040-45182/91 $0.00 + .50 Printed in Great Britain. All rights reserved. © 1992 INQUA/Pergamon Press Ltd

THE LAST INTERGLACIAL AS RECORDED IN THE GREENLAND ICE SHEET AND CANADIAN ARCTIC ICE CAPS

Niels Reeh Danish Polar Center, c/o The Geological Survey of Greenland, Oster Voldgade 10, DK-1350 Copenhagen K,

Denmark

The Greenland ice-sheet and Canadian Arctic ice-cap record of the last interglacial (understood broadly as marine Isotopic Stage 5) is critically reviewed. Lack of age control on the deep ice-core records leaves open two largely different interpretations of the climate in marine Isotopic Stage 5: either most of Stage 5 (with the exception of Substage 5e) experienced a cold climate, and the main change from interglacial to glacial conditions took place at the boundary between Substages 5e and 5d; or, during much of Stage 5 the climate was of interglacial type, i.e. as warm as, or warmer than at present, with Substage 5d as the only period with a glacial type climate. It is concluded that most of the 'hard' evidence is in favour of the latter scenario.

Ice-sheet dynamic model studies, using the 'warm' marine Isotopic Stage 5 scenario, indicate that the Greenland ice sheet in the warmest periods of Stage 5 had split up into a main ice sheet covering central and northern Greenland, and a much smaller ice cap over the southeastern highlands.

The Greenland record of marine Isotopic Stage 5 is then discussed in relation to other paleoclimatic and paleoenvironmental records. The possibility of major inter-hemispheric differences of the climate in mid and late marine Isotopic Stage 5 is discussed. It is concluded that it is important to study the entire marine Isotopic Stage 5 (not only Substage 5e) in order to understand possible differences in the global climatic responses to nearly identical peaks in orbitally driven summer insolation changes in the northern hemisphere.

INTRODUCTION

In this contribution, information about the last interglacial (understood broadly as marine Isotopic Stage 5) as recorded in the Greenland ice sheet and Canadian Arctic ice caps will be discussed. The discussion will primarily be based on the deep ice-core records from the central areas of the ice masses, and on surface-ice records from the ice margins. However, also ice-sheet-dynamic model studies provide information about past extent, thickness, and flow pattern of ice masses, and thereby contribute to a better understand- ing of past climatic and environmental conditions. Consequently, ice-dynamic model studies of the Green- land ice sheet during the last glacial-interglacial cycle will also be discussed in the following.

On the other hand, glacial geological records from Greenland and the Canadian Arctic, and marine sediment records from the adjacent seas will only be peripherally discussed since these records are dealt with in other contributions to this volume (Funder, 1991; de Vernal, 1991; Sejrup, 1991).

ICE SHEET RECORDS

The ice sheets in Greenland and Antarctica, and the ice caps in the Canadian high Arctic, are rich sources of information about past climatic and environmental conditions because all types of fallout, airborne terrestrial dust and biological material, volcanic debris, sea

Contribution to the NATO Advanced Research Workshop on the "Climate and Environment of the Last Interglacial in the Arctic and Subarctic'. Hanstholm, Denmark, October 19-22, 1990.

salts, cosmic particles, isotopes produced by cosmic radiation, and naturally and artificially produced che- micals are incorporated in the snow. Layers of snow containing these contaminants are gradually compressed and transformed into solid ice which also includes small cavities containing samples of the atmosphere. The layers are buried by subsequent snow falls, and sink into the ice sheet under continuous vertical thinning, initially as a result of densification, by which the snow is transformed into ice, but then mainly due to flow-induced vertical compressive strain. In this pro- cess, the layers are stretched horizontally until they are advected by the ice motion into the ablation zone, where the ice either melts away or is removed by calving of icebergs. The flow pattern is illustrated in Fig. 1 which shows particle paths in a cross section of an ice sheet. In principle, a complete sequence of all the deposited layers can be obtained either by deep drilling in the accumulation zone (the region of positive mass input) or by surface sampling in the ablation zone from the equilibrium line (the line separating the accumulation and ablation zones) to the ice margin. By analysis of deep ice cores from the central ice-sheet regions, or surface-ice samples collected near the ice margin, records of climatic and environmental parameters can be established (e.g. Robin, 1983; Langway et al., 1985; Oeschger and Langway, 1989; Reeh et al., 1987; Reeh et al., 1991).

Once the climatic and environmental information has been extracted by physical or chemical analysis, the next step is to establish a historical record (a time series). This step, which, among other things involves dating and correction for advective transport due to

123

124 N. Reeh

~ 0 100 200 300 400 500

4 ICE

ABLA ION ACCUMULATION

(FLOW LINE)

6 16o 2oo a6o 4oo soo D I S T A N C E ( k m )

FIG. 1. Flow in a cross section of an ice sheet. Particle paths (flow lines) are indicated, connecting points with same 6(180) values in the accumulation and ablation zone,

motion within the ice sheet, constitutes one of the major problems for the interpretation of climatic and environmental records from ice sheets (see e.g. discussion by Reeh, 1989a,b, 1990). At present, there are only a few well-established methods for absolute, experimental dating of ice in ice sheets, and none of these methods allow dating more than a few tens of thousands of years back in time (Hammer, 1989; Stauffer, 1989). Consequently, dating of ice older than this age depends on theoretical flow-model calculations (Lorius et a l . , 1985), which becomes an increasingly difficult and uncertain task to perform the further back in time the dating is extended (see discussion by Reeh, 1989a), or on correlation with other dated records (Dansgaard et al. , 1971, 1982; Reeh et al. , 1991). These dating methods suggest that probably no more than one glacial cycle is found in the Canadian Arctic ice caps, that at least one glacial cycle is found in the Greenland ice sheet, and that several glacial cycles can be found in the Antarctic ice sheet.

Whereas the full marine isotopic Stage 5 has been identified in the Vostok ice core from central East Antarctica (even the equivalents of Substages 5a,b,c,d,e can be easily identified (Lorius et al. , 1985; Jouzel et al. , 1987; see also Fig. 3)), the situation for the existing Greenland and Canadian Arctic ice cores is more complicated. In these cores, ice interpreted as originating from marine Isotopic Stage 5 has been retrieved from the deepest few metres (or tens of metres) i.e. immediately above the base of the ice sheets (Dansgaard et al. , 1982; Paterson et al. , 1977; Koerner et al. , 1987; Fisher, 1987, see also Fig. 3). In none of the cores has ice been identified, which unambiguously can be referred to marine Isotopic Stage 6 (the previous glacial). These facts, combined with the poor absolute age control, complicate the

interpretation of the records from marine Isotopic Stage 5 in the existing Greenland and Canadian Arctic ice cores. However, ice sheet records also have clear advantages in comparison with other paleo-records. For instance, they contain a continuous sequence of layers, except possibly for the near-bottom section where discontinuities may occur (Fisher, 1979), and, in general, they have a high resolution.

The missing ice from marine Isotopic Stage 6 led Koerner (1989) to suggest that extensive melting of the Greenland ice sheet and the Canadian Arctic ice caps occurred in the Eemian (marine Isotopic Stage 5e). Moreover, Koerner (1989) mentions that massive (and possibly complete) retreat of the Greenland ice sheet would account for a large part (or all) of the 6 metre sea-level rise in the Eemian. However, complete melting away of the Greenland ice sheet, is contra- dicted by the 6(~SO) record from P~kitsoq on the West Greenland ice-sheet margin (Fig. 3) which seems to cover the full marine Isotopic Stage 5, and even a part of marine Isotopic Stage 6 (Reeh et al. , 1991). If this interpretation is correct, a large part of the Greenland ice sheet must have survived the warm periods of marine Isotopic Stage 5. Consequently, the ongoing deep drilling efforts in the Summit area of the Green- land ice sheet (for location see Fig. 2) should provide a unique, high resolution record through marine Isotopic Stage 5 with an expected average annual layer thickness on the order of one centimetre. Moreover, the core section covering marine Isotopic Stage 5 is likely to be found so high above the base of the ice sheet (Dahl- Jensen, 1989) that one would not expect the record to be too much influenced by disturbances from the bottom.

Even if the problem of dating the deep ice core records could be solved, this would not mean that the

The Last Interglacial 125

• A G A S S I Z . ICE CAP

D E V O N

ICE CAP • CAMP CENTURY

A F F I N .

B A Y

• BARNES ICE CAP

G R E E N L A N D

• S U M M I T

| I /

4**

' P A A K I T S O Q

DYE 3

o

ODP SITE 6 4 6 0 500 km

FIG. 2. Map showing the locations of Greenland ice sheet and Canadian Arctic ice cap b(~80) records discussed in the text. The flow line from Summit to the ice margin at P~kitsoq is drawn to illustrate that Pleistocene ice now exposed at the ice-sheet surface at Phkitsoq was originally deposited in the Summit area of Central Greenland. The ocean drilling site O D P SITE 646 is

also shown in the figure.

problem of interpreting the records would then also have been solved. One reason is that several environmental parameters measured on ice cores show large spatial variations, when measured in the surface layers of the ice sheets. Therefore , the variation of a quantity recorded in an ice core, will be affected by the original spatial variation of that parameter, since the ice in the core did not all originate at the same location, unless the drill site was always located at a dome or horizontal crest. Consequently, in order to extract a series of temporal variations from an ice core record, corrections must be made for the advective transport from the region upstream of the core location. This must be done by means of ice sheet dynamic model calculations. The precision of such corrections depends on how well

the upstream distribution of the relevant glaciological parameters is known. Not only the present accumulation rate, ice thickness, ice temperature, and ice-flow- law parameters upstream of the core location must be known. Even past changes of these parameters must be known or estimated over the time span covered by the ice-core record. Obviously, the corrections become more and more uncertain with increasing age of the record.

Although the emphasis in this contribution is on marine Isotopic Stage 5, it is necessary to deal with the entire pre-Holocene units of the ice sheet records, because of the dating problem mentioned previously. The locations from where the records have been obtained are indicated on the map in Fig. 2. As far as

126 N. Reeh

6 5e 5d 5c 5 b S a 4 3 2 1

" v o , , o k . . . . . . . . . . . . . . . .

~ -55 '

D73

D72

-35

FIG. 3. Pleistocene records from the Greenland ice sheet (Camp Century, Pfikitsoq, and Dye3), Antarctica (Vostok), and the Canadian Arctic (Agassiz ice cap, A84, A79, and A77 and Devon ice cap, D72 and D73). Units assumed to be of marine Isotopic Stage 5 age are indicated by thick lines. Dashed thick lines indicate units

subject to different interpretations.

the Greenland ice sheet is concerned, the records to be discussed are the deep ice core records from Camp Century on the Northwest Greenland ice sheet, Dye3 on the South Greenland ice sheet, and the ice margin record from Pfikitsoq in central West Greenland. These records are all believed to cover marine Isotopic Stage 5, partly or fully. The records from the Canadian Arctic comprises the deep ice core records from Barnes ice cap on Baffin Island, Devon ice cap on Devon Island, and Agassiz ice cap on Ellesmere Island. Of these records, only those from Devon ice cap and Agassiz ice cap contain ice that, with some confidence, can be referred to marine Isotopic Stage 5.

6(lSo) RECORDS

Since the emphasis in this contribution will be on 6(lSO) records, a discussion of ice-sheet 6(lSO) records seems appropriate.

As shown by Dansgaard et al. (1973) and Johnsen et al. (1989), a close relationship exists between the ratio of heavy H2180 to light H2160 (usually expressed as the relative difference from a standard value SMOW (Standard Mean Ocean Water) in parts per thousand, and designated 6(lsO)) in the surface snow from the Greenland ice sheet, and the mean annual temperature at the snow deposition site. This has led to the suggestion that 6( lSo) provides an estimate of the atmospheric condensation temperature at the sampling site. For sites on the Greenland ice sheet with elevations above 1500 m, linear fits give 6-temperature gradients in the range from 0.62 %0 6 per K to 0.67 %0 6 per K (Dansgaard et al., 1973; Johnsen et al., 1989; Reeh et al., 1990). Similar gradients were found by Koerner and Russel (1979) and Koerner (1979) on the slopes of Canadian Arctic ice caps facing Baffin Bay. Elsewhere on the Canadian Arctic ice caps, there was no significant 6-elevation (6-temperature) gradient. However, a significant correlation was found between 6(lSO) and the distance from Baffin Bay, which was assumed to be a significant moisture source for the precipitation over the region.

Below an elevation of 1000 m, data from the Northwest Greenland ice sheet (Reeh et al., 1990) and from central West Greenland (Reeh, unpublished data) indicate a reverse a-elevation relationship (increasing 6 with elevation). Similar inverse relation- ships were found by Koerner (1979) for the same elevation range on the Canadian Arctic ice caps.

The 6(lSO) variations found in Greenland and the Canadian Arctic can to some extent be explained by means of global and local Oxygen Isotope models (Fisher and Alt, 1985; Johnsen et al., 1989; Fisher, 1990). Although there still seems to be some disagreement between the various authors as regards details, it is generally agreed that, for the present-day situation, the subtropical part of the North Atlantic is the primary moisture source for the precipitation on the high elevated areas of the Greenland ice sheet. At lower elevations, local moisture sources contribute significantly to the precipitation.

Since climatic temperature variations are minor in the subtropical North Atlantic, the ~-temperature relationship derived for the high elevated areas of Greenland should be expected to hold as long as major changes in the global circulation pattern do not occur. Based on a combined study of 6(1SO) and 6(D) (deuterium) on ice from the Dye3 deep core, Johnsen et al. (1989) suggested that, even under glacial conditions, the moisture source was unchanged. This would mean that the present-day a- temperature relation should be applicable as a reasonable approximation also for the glacial 6(1SO) records from interior Greenland ice sheet locations. This conclusion is


supported by the good agreement between the temperature shift at the Pleistocene-Holocene transition on the South Greenland ice sheet (10.5 K), derived from the 6(180) shift in the Dye3 deep ice core record (Dansgaard and Oeschger, 1989) and the corresponding temperature shift (12 + 2 K) derived from the observed temperature profile in the Dye3 deep bore hole by means of ice-dynamic modelling (Dahl-Jensen and Johnsen, 1986).

However, even for the high elevated Greenland ice sheet locations, the existence of a simple 6(lSo) - temperature relationship does not mean that the interpretation of 6(tSo) records in terms of climatic temperature variations is straight-forward. The reason is that 6(t80) is also influenced by processes such as for example the ratio of summer to winter precipitation (Fisher et al. , 1983), deposition noise (Fisher et al. , 1985), advective transport within the ice sheet (see Fig. 1), and changes in ice-sheet surface elevation. In particular, past changes in flow pattern and surface elevations may blur the climatic 6(~80) signal in ice- core records (Reeh, 1990).

As mentioned above, the situation on the Canadian Arctic ice caps may be even more complicated, since a simple 6(~So)-temperature relationship may not exist over the main part of these ice caps.

In Fig. 3 the Pleistocene Greenland and Canadian Arctic 6(180) records are displayed. Moreover, as a reference, the 6(180) record from the deep ice core drilled at Vostok, East Antarctica is shown on the top of the figure. As mentioned previously, equivalents of the marine isotopic stages can be unambiguously identified in the Vostok record, and these stages are indicated in the figure. For the Greenland and Cana- dian Arctic records, the sections interpreted as originating from marine Isotopic Stage 5 are shown. For several of the records two interpretations have been given in the past, in which case both interpretations are indicated in Fig. 3.

AGE-DEPTH RELATIONSHIP (TIME SCALE)

The records in Fig. 3 are plotted against depth or distance above the bottom (ice-core records), or against distance from the margin (the ice-margin record). Notice the large difference in length scales for the Greenland, the Canadian Arctic, and the Vostok records: In the Greenland records the Pleistocene unit (marine Isotopic Stages 5-2) is 250--600 m long, whereas Pleistocene ice only constitutes the near- bottom 5-12 m of the Canadian Arctic cores. In the Vostok core, the section spanning marine Isotopic Stages 5-2 is about 1600 m long. These large differences are primarily due to the large variations of ice thickness and accumulation rate between the core sites, see Table 1. Not only the length of the Stage 5-2 unit varies from core to core. Also the location of this unit relative to the total record length varies as indicated in the last column of Table 1. Inspection of the numbers in this column clearly shows a decreasing trend from the Vostok record, via the Greenland records (disregarding the P~kitsoq ice-margin record), to the Canadian Arctic records. In other words, the record of marine Isotopic Stages 2-5 is, relatively speaking, located closer to the bottom in the Canadian Arctic cores than in the Greenland cores, and closer to the bottom in the Greenland cores than in the Vostok core. Since the time scale (the age-depth relationship) generally becomes more and more compressed with relative depth, an increasing non-linearity of the timescales with depth is therefore to be expected in the succession: Vostok, Greenland, the Canadian Arctic. However, this general pattern is modified for example by temporal changes of accumulation rate and ice thickness. In particular, the time scale may 'expand' considerably in periods of large ice-thickness increases, as for example the period of initial ice-sheet build-up. This may be an important effect to consider for explaining the time scale of the near-bottom sections of the Canadian

TABLE 1. Greenland ice sheet and Canadian Arctic ice cap records

Core

Surface Length of Relative height accumulation Pleistocene above base of

rate Ice thickness record boundaries of (m/year) (m) (m) Pleistocene ice

Agassiz ice cap A77 0.175 338 7.7 0-0.023 A79 0.115 139 10.8 0-0.078 A84 0.100 127 12.3 0-0.097

Devon ice cap D72, D73 0.22 299 4.5-5 0-0.016

Greenland ice sheet Camp Central 0.38 1388 233 0-0.168 Phkitsoq 22000* 585 0-0.03¢ Dye3 0.55 2037 251 0-0.123

Antarctica Vos tok 0.024 3700 1600:~ 0.49-0.92

*Appropriate distance from ice margin to equilibrium line. tLength of record relative to the distance from ice margin to SMarine Isotopic Stages 2-5.

equilibrium line.

128 N. Reeh

Arctic records, but may also explain the apparent expansion of the time scale in the near-bottom part of the Camp Century and Dye3 records (Reeh, 1990).

CAMP CENTURY, NORTHWEST GREENLAND

The deepest 300 m of the 6(180) record from Camp Century is shown in Fig. 8b. The Pleistocene-Holocene transition, as indicated by an abrupt increase of 6(180), is found about 235 m above the bottom. This transition has been dated to 10,750 + 150 BP (Hammer et al . , 1986). Two largely different interpretations of the pre-Holocene part of the record have been given in the past: One interpretation (Dansgaard et al . , 1971) places the sharp drop in 6(180) around 50 m above the bottom at an age of 60-70 ka BP, i.e. at the boundary between marine Isotopic Stages 5 and 4, see Fig. 4c. The other interpretation (Dansgaard et al . , 1982) places the same event at the Stage 5e-5d boundary (ca. 120 ka BP), see Fig. 4b. The first mentioned dating was derived from Fourier spectral analyses of a preliminary theoretical relationship of 6 versus time, assuming a persistent 2500-year climatic periodicity (Dansgaard et

al . , 1971). The second dating was derived by comparing the Camp Century deep 6(tSo) record with a deep-sea foraminifera 6(180) record (Dansgaard et al . , 1982), see Fig. 4a.

The lack of absolute age control makes it impossible to settle which of the interpretations is the better. However, as discussed by Reeh (1990) and Reeh et al. (1991), recently published t°Be concentration measurements on the Camp Century deep ice core (Beer et al . ,

1988) support the original interpretation of Dansgaard et al. (1971), at least as far as the dating of the drop in 6(tSo) around 50 m above the bottom is concerned. Two distinct peaks in the Camp Century 1°Be concentration profile can be correlated with two similar peaks in the a°Be profile from the well-dated deep ice core drilled at Vostok, East Antarctica (Raisbeck et al. ,

1987). Raisbeck et al. (1987) concluded that two distinct peaks at ca. 35 and 60 ka in the Vostok L°Be concentration record are most likely due to production- rate changes in the atmosphere caused by extraterres- trial disturbances. Therefore, the 35 and 60 ka events should be detectable also in Greenland 1°Be ice-sheet records. As shown in Fig. 8c, there are, in fact, two distinct 1°Be concentration peaks also in the Camp Century record at distances of ca. 75 and 155 m above the bottom. If ages of 60 and 35 ka are assigned to these levels, the core section with high 6-values shown in Fig. 3b from 13-38 m obviously must be of mid to late marine-Isotopic Stage 5 age.

In Fig. 5, the 6(180) and 1°Be records from Camp Century, and the 6(180) and 1°Be records from

Indian Ocean Camp Century

40 :._._ I -

) l,ooq ! _ k " " / "

~ ka B.P. -35 -30

3 2 5('80) (%*) ~(1'o) (~)

'c) I ~ ' - ~ = ' / 20

40

6O

lOO

120 " ~ .L m ka

-40 -30 B.P. ~("O) (~)

FIG. 4. Different time scales for the Camp Century Pleistocene 6(180) record. (a) Shows deep-sea foraminifera 8(180) record (Hays et al., 1976). (b) Shows the chronology derived by Dansgaard et al. (1982), obtained by comparing the record with the deep-sea foraminifera 6(t80) record. (c) Shows the chronology derived from fourier spectral analysis, assuming a persistent

2500-year periodicity (Dansgaard et al., 1971).


o -45

a) -40"

~ -35'

-30' ~

b)

I U

co ~ 2

i i

--0' 0 . . . . 5~0

50 1 O0 150 • i , • , , l I I

Camp Century

al. (1971)

Beer et al. (1988)

; I i I I

200 i = i |

A

6 ~ E l U

4 ~

, , i I , i = | , , , ,

100 150

Raisbeck et al. ( 1 9 ~ -55

v°s'°:.A f / o

i i r i i I I I r i i I i

0 200

FIG. 5. (a) 6(~O) and (b) "~Be records from Camp Century, Northwest Greenland, and (c) 6(!sO) and (d) ~Be records from Vostok, East Antarctica plotted on a time scale. The correlation by means of two "~Be peaks dated at 35 and 60 ka BP in the Vostok record (Raisbeck et a l . , 1987) is indicated. (a) and (b) are replotted from Robin (1983), and Beer et al. (1988),

respectively. (c) and (d) are from Raisbeck et al . (1987).

Vostok are plotted on a time scale. For the Vostok record, the time scale suggested by Lorius et al. (1985) is used. For the Camp Century record, ages of 10.75, 35, and 60 ka have been assigned to the levels 233, 155, and 75 m above the bottom, in accordance with the discussion above. Beyond ca. 75 ka BP the dating of the Camp Century record is uncertain.

Further evidence in support of the Camp Century time scale shown in Fig. 5 is provided by comparing accumulation-rate estimates derived by different methods. By means of ice-dynamic model calculations Reeh (1990) showed that a consequence of the timescale shown in Figure 4b would be that accumulation rates on the Northwest Greenland ice sheet upstream of Camp Century must have been 4-5 times lower than at present during the period 60-10 ka BP. If, on the other hand, the time scale shown in Fig. 5 is used, the accumulation rate in the 60-10 ka BP interval would have to be reduced by only a factor between 2-3 in respect to the present value. A similar reduction of the accumulation rate is estimated from the generally 2-3

times higher-than-present L~JBe concentrations during this period, disregarding the 35 and 60 ka peak values discussed above. Also the estimate of accumulation rate obtained by means of the 5(180) record, assuming an 8% change in accumulation rate for 1%0 change in 6 as derived from Central Greenland shallow ice core studies (Clausen et al., 1988), suggests a 2-3 times reduction of the accumulation rate in the period 60-10 ka BP.

The dashed lines shown in the Camp Century ~(lSO) diagrams in Figs 4b and 8b represent the steady-climate 6(tSo) trend lines, i.e. the expected 6(~80) variations in the case that past temperatures had always been like those of the present. The steady climate references have been determined by ice- dynamic model calculations using the flow-line model described by Reeh (1988). Past changes in ice-flow velocities due to changing accumulation rates are considered, but ice-thickness changes are not taken into account. The decreasing trend back in time is due to the fact that the older the ice, the farther inland (i.e.

130 N. Reeh

the higher and, consequently, the colder) the ice was originally deposited (see Fig. 1). Deviations of the 6(t~O) records from the steady-climate reference curves will have to be intepreted in terms of either climatic change or changes in surface elevation.

Ice-sheet dynamic model calculations (see section on Ice - shee t m o d e l l i n g ) suggest that surface elevations in the interior region of the Greenland ice sheet were only moderately changed, compared to the present elevations in periods with a climate colder than the present. Also in periods with a climate slightly warmer than at present, changes in surface elevation seem to have been small. However, as far as Northwest Greenland and South Greenland are concerned, the model studies indicate that surface elevations were likely to be significantly lower than at present during periods with a climate 3-5 K warmer than now (Letr6guilly et al . , 1991a,b). These model studies support the suggestion by Koerner (1989), who, based on 6 0 8 0 ) , total gas content, CO2 and visible dust content analyses, concluded that the deep ice in the Camp Century and Dye3 cores, most likely was deposited in a warm interglacial climate at a much lower elevation than presently found at the core locations.

The different time scales in Figs 4b and 5a lead to quite different interpretations of the climate in North- west Greenland during marine Isotopic Stage 5. If the time scale of Fig. 4b is accepted, then most of Stage 5 (with the exception of Substage 5e) had a cold climate, and the main climatic change from interglacial to glacial conditions took place at the boundary between Sub- stages 5e and 5d. Even Substages 5a and 5c were 5-6 K colder than now, if 6(~80) is translated into temperature by means of the conversion factor 0.62 %0 6 per K (Dansgaard et a l . , 1973). For Substage 5e (the Eemian) a similar procedure leads to the conclusion that the temperature at the surface of the Northwest Green- land ice sheet was 8-10 K warmer than now. It is unlikely, that this represents a pure climatic temperature change, since an unknown part of the warming most likely was due to a significant lowering of the surface due to a general retreat of the Northwest Greenland ice sheet during the warm period, cf. the discussion above.

If, on the other hand, the time scale of Fig. 5a is accepted, the main shift in climate would have taken place at the boundary between Stage 5 and Stage 4. Moreover, in this case, the temperature at the surface of the Northwest Greenland ice sheet would have been 8-10 K warmer than now during the Interstadials 5a and 5c. Again, an unknown part of this warming would have to be ascribed to lower surface elevations.

As previously mentioned, the question of which time scale is correct remains open, due to the lack of absolute age control. However, most evidence seems to be in favour of the latter scenario, suggesting that the climate in Northwest Greenland was warmer than at present during more than one period of marine Isotopic Stage 5, i.e. in Substages 5e and 5c, and probably also in Substage 5a.

DYE3, SOUTH GREENLAND

A time scale for the Dye3 deep ice core record was originally established by correlation with the 6(180) record from Camp Century (Dansgaard et al . ,

1982), see Fig. 6. However, Reeh et al. (1991) argued that, if their re-interpretation of the Camp Century record is accepted, then the time scale for the Pleis- tocene record from the Dye3 core would also have to be changed, assuming the correlation indicated in Fig. 6 to be correct. Recently, a study by Ram and Gayley (1991) indicated that a visible dust layer found 87 m above the bottom of the Dye3 ice core, which was previously claimed to be of bedrock origin (Hammer et al . , 1985), is in fact a volcanic ash layer. Ram and Gayley (1991) suggest that this volcanic signal could correspond to the Z2 ash layer (dated to ca. 65 ka BP) observed by Ruddiman and Glover (1972) in sediment cores from the North Atlantic. The age of 65 ka BP is close to the age of 73 ka BP suggested by Dansgaard et al. (1982) for the same depth in the Dye3 core. Therefore, if the visible layer 87 m above the bottom in the Dye3 core is the Z2 ash layer, and if the identification of the 60 ka t°Be peak in the Camp Century record is correct, then the correlation shown in Fig. 6 cannot be right. A detailed X°Be concentration analysis of the Pleistocene ice from the Dye3 core, and identification by chemical analysis of the Z2 eruption in the Camp Century record would help to settle the important question of correlation between the Pleistocene records

Depth

]

ast iclat|on

.,'6 :d~ ":~'6 ~ '

1100

1200

1300

m

FIG. 6.6('~O) profiles along the deepest 300 m of the ice cores from Dye3 and Camp Century, as interpreted by Dansgaard et al. (1982). The numbered arrows indicate layers of assumed simultaneous

deposition. Modified from Dansgaard et al. (1982).


from the Camp Century and Dye3 deep ice cores, and would also provide a much needed improvement of the age control on these records.

At present, we are left with two possible climatic scenarios for the South Greenland ice sheet (Dye3) in marine Isotopic Stage 5, viz. the same two scenarios that have already been discussed for Northwest Green- land (Camp Century): If the time scale of Dansgaard et al. (1982) is accepted, then most of Stage 5 (with the exception of Substage 5e) had a cool climate, and the main climatic change from interglacial to glacial conditions took place at the boundary between Substages 5e and 5d. If, on the other hand, the re-interpretation of the time scale by Reeh et al. (1991) is accepted, the main shift in climate would have taken place at the boundary between Stage 5 and Stage 4, and the Dye3 record goes back only to the Substage 5c/5b transition. However, the best time markers yet available (the possible Z2 ash (65 ka BP) in the Dye3 record, and the possible 60 ka X°Be peak in the Camp Century record) suggest that the correlation between the two records indicated in Fig. 6 does not hold. This leaves open the interesting possibility that there was a major difference of the climates in Northwest and South Greenland during marine Isotopic Stage 5, with alternating cold and warm periods occurring in Northwest Greenland during mid and late marine Isotopic Stage 5, while the climate stayed cool in South Greenland in the entire period.

PAKITSOQ, CENTRAL GREENLAND

The 6(t80) records from the ice-sheet margin at

Pglkitsoq in central West Greenland (for location see Fig. 2) have been discussed in detail by Reeh et al.

(1987, 1991). As appears from Fig. 7, the two records sampled along parallel lines transverse to the ice margin about 50 m apart, are nearly identical consider- ing the rather low resolution of the 1985 record. This indicates that the surface 6(t80) records are reprodu- cible in spite of the irregular, sometimes rather chaotic appearance of the ice surface. As explained in the section on Ice sheet records a complete sequence of all the layers deposited in the ice sheet can be obtained by surface sampling in the marginal ablation zone from the equilibrium line to the ice edge. The layers become progressively older with decreasing distance from the ice edge. Therefore, Pleistocene ice is expected to be found nearest to the ice edge. An interpretation of the Pleistocene record in terms of marine isotopic stages is indicated in Fig. 7. The marked 6(180) shift at 580 m in the 1988 record marks the Holocene/Pleistocene transition, and the AllerOd/Younger Dryas oscillation is also clearly indicated in the record.

The interpretation of the PgLkitsoq record in terms of marine isotopic stages is to some extent based on correlation with the re-interpreted Camp Century record, see Fig. 8. However, it is rather obvious to interpret the unit from 150-240 m in the P~kitsoq record as representing marine Isotopic Stage 5. Other- wise, one would have to postulate severe disturbances of the continuity of the layer sequence, e.g. caused by large-scale folding of the ice at the margin. Although discontinuities of the layer sequence cannot be ruled out, there is no indication of folding of the marginal ice, which, on the contrary, displays a large-scale linear

-45

Distance from margin (m) o 1 oo 200 soo 400 soo coo 7oo 8oo goo 1 ooo

l i l , i i . I , , , i , , . i , , , i , , , i . . , l , , , i , , , i i , , i i i , i ,

A

V -30

C~/as

";'o 1 9 8 8

o ~ - 3 s

- 3 0

5 4 1 2 5

s 'O' ,~ ' ~ so' ~o~.~so" 300' ~ o , ; o ' , ' ~so' ,oo' ,so' ~oo' ~ o 800 isfance f rom marg in (m)

FIG. 7. The two 6(180) records from the ice margin at P~kitsoq, Central West Greenland. The sampling profiles run approximately parallel to each other at a distance of 50-100 m. The interpretation of the 1988 record in terms of Emiliani stages

is shown in the figure.

132 N. Reeh

Cb) (o/oo) - 4 0 - 3 5 - 3 0 - 2 5 o

I I I I .

o .)Pakitsoq I ' ~ b) ~ c) j :~

Zoo. 2

~ __34 ~ ~ ~ ! f o o

- 6 Camp Century

° - ~ s - 4 0 - 3 s - ~ o ' ' ' ~ ' ~ ' 0

6(180) (o/oo) l°Be (10 4 atoms/g)

FIG. 8. Comparison of the 6(~80) records from the Central Greenland ice-margin (a) and the Camp Century deep ice core (b). The arrows connect events assumed to be simultaneous. The dashed lines indicate steady-climate reference lines as determined by ice- dynamic model calculations. They show the 6(~80) variations to be expected in the case that past climate had always been like the present. (c) Shows the "'Be concentration record measured along the Camp Century ice core. The ages of 60 and 35 ka of the distinct peaks 75 and 155 m above the bottom are obtained by correlation with similar peaks in the ~°Be-concentration profile measured along the well-dated deep ice core from Vostok, East Antarctica (Raisbeck et al., 1987). The interpretation in terms of Emiliani isotopic stages are shown at the left and right of the figure. The Camp Century b(180) profile is adapted from Robin (1983). The l°Be-profile is

redrawn from Beer et al. (1988).

foliation parallel to the ice edge (Reeh et aL, 1991). The dashed lines in Fig. 8 represent the steady-

climate 6(tSo)-trend lines. As explained in the section on the Camp Century record, deviations of the 6(ISo) record from the steady-climate reference curve will have to be interpreted in terms of either climatic changes or changes in ice-sheet surface elevation. Calculations with a time-dependent ice-dynamic model (see section on Ice sheet modelling) indicate that the changes of the position and elevation of the Summit of the central Greenland ice sheet were moderate during the last glacial cycle: This is even true for climatically warm periods when the marginal areas of the ice sheet suffered considerable thinning and retreat. Since the Pleistocene ice in the Pfikitsoq record was originally deposited in the Summit area (see Fig. 2), past surface- elevation changes are likely to have made only minor contributions to the Pleistocene P~kitsoq 6(~80) record. The deviations of the 6(lSo) record from the steady-climate reference curve can therefore be translated into temperature changes with respect to the present temperature by means of the conversion factor 0.62 %0 6(180) per K established for the Central Greenland ice sheet (Dansgaard et al., 1973), see discussion in the section on 6(180) records. The derived temperature record is shown in Fig. 9. Origi- nally, the time scale derived by Lorius et al. (1985) for the deep ice core from Vostok, East Antarctica was applied to the Pg~kitsoq record (Reeh et al., 1991). However, the Vostok time scale has been re- interpreted (Lorius et al., 1990), accounting for the recent interpretation of the Vostok dust record which suggests that the marine and Vostok records are roughly in phase also before 110 ka BP (Petit et al., 1990). In Fig. 9 the temperature record is plotted versus the changed time scale, which better fits the chronology established for marine sediment records. Fig. 9 indi-

P a k i f s o q , C e n t r a l G r e e n l a n d f e r n p e r a f u r e

0 5

• 0

0

0 ° ~

@

E ~. -10 E 0

5 0 1 O 0 1 5 0 i i i i J i i • • i , , , , i . , , ,

i t J

~ ~. , 1 _

" 1 1 ' ,, l

2 0 0

5

-5

-10

- 1 5 . . . . , , ~ , . . . . z . . . . - 1 5 0 5 0 1 O 0 1 5 0 2 0 0

Age (ka)

FIG. 9. Derived isotopic temperature deviations with respect to present temperatures for Central Greenland. Modified from Reeh et aL (1991).


cates large temperature variations in Central Green- land during marine Isotopic Stage 5, from 3-5 K warmer than now in Substages 5e and 5c to 2-8 K cooler than at present in Substage 5d. Substage 5a also had a warm climate (up to 3 K warmer than now), whereas Substage 5b was up to 2 K cooler than at present.

DEVON ICE CAP

The two Pleistocene 6(180) records from Devon ice cap shown in Fig. 3 are from two boreholes only 27 m apart, about 8 km from the highest point of the ice cap (Paterson et al., 1977). Whereas the 6(t80) records from the two boreholes closely resemble each other over most of the 299 m long cores, this is certainly not the case for the Pleistocene records near the base of the ice, see Fig. 3. A composite record was constructed (Fig. 10), assuming that no part of the record was missing from both cores, which may not be true as noted by Paterson et al. (1977)..A chronology was established for the composite record by comparison with the Camp Century record, using the time scale suggested by Dansgaard et al. (1971). Support for the alignment of the Devon ice cap and Camp Century

a("o) (%4 -35 -31 -27 -23 0 t I c I L I I I

5- a) 10- 72

, 72

72 i ~ ~ ~

-41 -37 -33 -29

Camp Ce

I I I 1 [ 1 1 1 1

-27 l 0

20

,~o n" ei

60 ~ "

gO

120

FIG. 10. (a) 6(lsO) profile for Devon ice cap. The Pleistocene section was obtained by combining the records from the lowest few metres of the two cores (see Fig. 3). While more nearly complete than either record, the combined one probably still has gaps. (b) 6(~O) profile for Camp Century, Northwest Greenland plotted on the time scale suggested by Dansgaard et al. (1971). From Paterson et

al. (1977).

6(180) records came from comparing a reconstructed microparticle record for Devon ice cap and a silicon concentration record from Camp Century (Fisher, 1979). Furthermore, the comparison of the insoluble impurity profiles in the Devon ice cap and Camp Century records with silicon concentration records from dated Labrador Sea marine cores led Koerner and Fisher (1986) to reject the revision of the Camp Century time scale suggested by Dansgaard et al. (1982). Moreover, in a discussion of pollen analysis and time scales in Canadian ice cores, Koerner et al. (1988) maintain the point of view that the marked drop in 6(lSo) 2.6 m above the bottom in the Devon ice cap cores represents the marine Isotopic Stage 5/4 transition. Although the Stage 5/4 transition is put at nearly the correct age in Fig. 10, the time scale in general is not reliable. For example, beyond 70 ka BP, the time scaleshould most likely be expanded, reducing the age of the bottom ice. The reason for the time-scale expansion near the bottom is probably that the corresponding ice was deposited at the surface of the initial, small and thin ice sheet that began to grow towards the end of a warm episode in marine Isotopic Stage 5. As discussed in the section on the Age-depth relationship (time scale), the time scale corresponding to such a period will be greatly expanded. Koerner et al. (1988) and Koerner (1989) provide evidence that the basal ice in the Devon ice cap core (and in other Canadian Arctic ice cores) was originally deposited at the surface of small and thin ice caps. They suggested that the present ice caps probably started to form towards the end of the last interglacial. In view of the warm climatic episodes suggested for mid and late marine isotopic Stage 5 by the Phkitsoq (and Camp Century?) 6(1~O) records (see Fig. 5a and Fig. 9), the build-up of the present Devon ice cap (and the other ice caps in the Canadian Arctic) could be referred to a late period of marine isotopic Stage 5. This would explain why the pronounced cold Substage 5d peak seen in the P~kitsoq (and Camp Century?) records, is apparently missing in the Canadian Arctic ice cap records (see Fig. 3).

Due to the obvious gaps and discontinuities of the Pleistocene 6(]SO) records from Devon ice cap, and also due to lack of age control, it is hardly possible to derive direct climatic information about marine isotopic Stage 5 from the 6(180) records. Paterson et al. (1977) discuss the lowest 2.6 m of the Devon ice cap records, which, except for short intervals, have 6(JSO) values 0.5-3 %0 less negative than at the peak of the Holocene climatic optimum. An obvious interpretation is that, during the period when the near-bottom ice was deposited, temperatures were higher and the ice thinner than at present, which would fit well with other indications (Koerner et al., 1988; Koerner, 1989) that the near-bottom ice was originally deposited on a thin ice cap in a warm climate. However, Paterson et al. (1977) also discuss an alternative explanation of the relatively high 6(180) values near the bottom, viz. that during marine Isotopic Stage 5, the major source area

134 N. Reeh

of precipitation on the ice cap could have been different from the present one. This could explain why 6(180) was higher than at present in a period, when other evidence suggests that world sea level was below the present level, and, therefore, that glaciation in some parts of the world must have been more extensive than it is now. This topic will be further discussed in a later section.

AGASSIZ ICE CAP

The three published Pleistocene 6(180) records from Agassiz ice cap (Koerner et al . , 1987; Fisher, 1987) are all from the same flow line. A84 (drilled in 1984) is at the top of a local dome, and A79 and A77 are located 1 km and 2 km respectively along a flow line originating at this dome. Koerner et al. (1987) have interpreted the A77 and A79 records in terms of marine isotopic stages. They suggest that the marked change in 5(180) at 3.5 and 5.5 m above the bottom in cores A77 and A79, respectively, represents the Stage 5/4 transition (Fig. 3). The chronology of the A79 record was further discussed by Koerner et al. (1988), based on pollen analysis of the Pleistocene record (Fig. 11). The high pollen concentrations in section C and D (in fact, the highest concentrations found in the whole core, see Fig. 11) suggest an interglacial-period origin for the ice, and therefore could be taken as evidence for an interglacial type climate also in late marine Isotopic Stage 5 (samples 3 and 4 in Fig. 11). However, the 6(180) values of section C show a decreasing trend from Holocene-like values at the bottom of the section to colder-than-Holocene values at the top. Referring to lower sea levels than at present in mid and late marine Isotopic Stage 5, and to the glacial-geological record from Baffin Island and the Labrador-Ungava penin- sula, Koerner et al. (1988) conclude that high pollen

concentrations need not on their own provide an unequivocal indication of warm climate conditions. On the other hand, Koerner et al. (1988) also state that high pollen concentrations in the deepest sample (section D), coupled with the least negative 6(t80) values in the whole core, strongly suggest that the deepest ice in the core has formed from surface ice which was deposited in a warmer climate than today.

A chronology for the Pleistocene A84 record has not been published. The tentative placing of the marine Isotopic Stage 5/4 transition suggested in Fig. 3 is based on correlation with the A79 record.

ICE SHEET MODELLING

A recurrent topic in the discussion of the marine Isotopic Stage 5 record from the Greenland ice sheet and the Canadian Arctic ice caps has been the extent of these ice masses during periods with a climate warmer than the present. To elucidate this topic, the dynamics of the Greenland ice sheet has been investigated by means of a three-dimensional, time dependent , fully coupled thermomechanical ice-sheet model (Huyb- rechts et al . , 1991). Boundary conditions are the basal topography of the ice sheet as determined by radio- echo soundings, and parameterizations of surface mass balance and surface snow/ice temperatures in terms of snow accumulation and air temperature (Reeh, 1991b). The model solves the full set of coupled thermomechanical equations on a fine mesh in three dimensions. It is t ime-dependent and also includes the response of the underlying bedrock to changing ice load. The model was checked by performing a forward run until an equilibrium state was reached, using the present distri- butions of snow accumulation and air temperatures. The calculated equilibrium state, which agrees well with observations (Fig. 12), was also used as a

a) 6 ('"o) 7** b) 6 ('sO) */** C) GRAINS/LITRE - 4 0 -35 - 3 0 -25 - 4 0 - 3 5 - 3 0 -25 0 2 4 6 8 10 12 14 16

1,q, R"c°ntt . . . . . . ; . . . . . . . . . .

20 11/2 ,10 \ \ 10 // " 9 ' i

.'~e '40 /114 8

• ;6o I ,, I I 6 t = = ,

i '

1001 I \ \ \ x c_...~..__...___~- 0 / 1201

1 2 0 ~

FIG. 11. 6(tsO) and pollen concentration profiles from the 1979 Agassiz ice cap (Ellesmere Island, Canadian Arctic) core (A79). (a) Plotted on an ice-equivalent depth scale. (b) Plotted as metres of ice above the bed. (c) Pollen concentrations plotted by sample number. The uppermost sample in (c) is from four 5 m cores collected in 1983. In (c) values above the mean are

shaded. From Koerner et al. (1988).


a)

90 70 50 30 10 0 , / ,/ I I I I ~ \ \ \

(

I I

b) w 90 7,0 50 30 10 0

/ / / / | I ~ \ \ \ / '

9o

! o / 7 5 ~75 k~o 75

~70 70

~65

~. ~ ÷

5'o 4b ~3o

FIG. 12. Surface topography of the Greenland ice sheet. (a) Measured. (b) Steady-state model simulation. Surface contour lines are drawn for each 200 m. Triangles indicate deep drilling sites in Greenland: Camp Century in the northwest, Summit in the

Centre and Dye3 in the south. From Letr6guilly et al. (1991b).

reference against which to compare the calculated ice sheet evolutions for changing climates.

In another series of model runs, the response of the ice sheet to different degrees of climatic warming was studied (Letr6guilly et al. , 1991a), see Fig. 13. Starting with the topography of the present-day reference ice sheet as initial condition, the model was run to steady state for a range of temperature rises between 1 K and 8 K. A likely increase in accumulation rate with increasing temperature was accounted for by keeping the present distribution pattern of the accumulation- rate, but letting the amount of accumulation increase by 5.3% for a climatic temperature increase of 1 K. This relationship is suggested by correlating a series of annual accumulation rates measured in shallow ice cores from central Greenland (covering approximately the last 100 years of accumulation) with the corresponding annual 8(180) values (Clausen et al. , 1988), and converting 8(180) to temperature by means of the factor 0.62 %o 8(180) per K valid for present day central Greenland conditions (Dansgaard et al. , 1973).

Even a small increase in temperature will cause a decrease of the ice-sheet volume (Huybrechts et al. , 1991). However, the effect, in terms of changed surface topography, is hardly detectable for temperature changes up to 2 K on the map scale of Fig. 13, For a temperature increase of 3 K, most of the change in ice- sheet topography occurs at the ice-sheet margin, particularly in the southwestern part, which retreats by up to 60 kin. It takes a temperature increase of 4 K to really change the configuration of the ice sheet: the ice sheet then splits up into two parts, one large part covering central and northern Greenland, and a much

DT=3 DT=4

a b

,I

' DT=6

c d

FIG. 13. Modelled steady-state ice sheets for temperature increases (DT) of 3-6 K as compared to the present. Surface contour lines are drawn for each 200 m. The thick closed contour line in central Greenland in (a), (b), and (c) is the 3000 m elevation contour. From

Letr6guilly et al. (1991a).

136 N. Reeh

smaller ice cap over the southern mountains. It is interesting to note that the Dye3 deep-drilling site becomes ice free. With a 5 K temperature rise, the main ice sheet is shown to shrink even more, and northern Greenland, including the Camp Century deep-drilling site, then also becomes ice free. For a temperature increase of 6 K, the main ice sheet disappears completely, leaving behind only four small ice caps on the southern and eastern mountains.

The rate of change of the ice-sheet volume was also studied. For the warming experiments illustrated in Fig. 13, most of the changes occurred in the first 10 ka, and equilibrium was reached after approximately 30 ka (Letrrguilly et al. , 1991a). In this connection, it is interesting to note that the warm episodes in marine Isotopic Stage 5, as derived from the Pglkitsoq 6(lSO) record (Fig. 9), have a duration of about 10 ka. Moreover, the temperature increase in respect to the present temperature is between 3 K and 5 K. Thus, these climatic events should have the potential of causing significant volume losses from the Greenland ice sheet.

As mentioned above, changes in accumulation rate is parameterized only in terms of climatic temperature changes. A geographical variation of the temperature change (for example a likely larger change in the north than in the south) is not considered. Neither are changes of the accumulation rate distribution due to changing orographic conditions, or changes of the atmospheric general circulation pattern accounted for. As long as the ice sheet topography remains essentially like the present, such changes may not be important. However, when the ice sheet undergoes large changes, the atmospheric large scale circulation as well as local orographic precipitation patterns are likely to be changed significantly. As mentioned above, such changes were not considered. However, the sensitivity of the model results to accumulation rate variations was studied in model runs, in which the temperature- accumulation rate feedback was neglected. These calculations indicated only a moderate influence of the changed accumulation rates.

The evolution of the Greenland ice sheet during the past 150 ka was also studied (Letrrquilly et al. , 1991b), see Fig. 14. The Pfikitsoq isotopic temperature record (Fig. 10) was used as climatic forcing. This calculation confirmed that marine Isotopic Substage 5e as well as Substage 5c were sufficiently warm and long to cause the ice margins to retreat, and the ice sheet to split up into a main ice sheet covering central and northern Greenland, and a small ice cap over the southern highlands. The Dye3 deep-drilling site is located in the ablation area very close to the ice margin. Either an increase of the temperature rise by 1 K during the warm episodes or a slightly longer duration (both possibilities are within the range of uncertainty of the climatic record) would make the ice disappear from the Dye3 location, and thus explain why ice from marine Isotopic Stage 6 is not found in the Dye3 deep ice core.

Independent evidence that large areas in South

Greenland were free of ice during marine Isotopic Substage 5e is provided by the palynological record from ODP site 646 about 200 km off the Southwest Greenland coast (Hillaire-Marcel and de Vernal, 1989). Very high pollen and spore concentrations, and the absence of correlation with the East Canadian palynostratigraphy led Hillaire-Marcel and de Vernal (1989) to suggest that the Stage 5e pollen and spore assemblages of ODP site 646 correspond to input from different sources: an atmospheric input from a southwest origin (a forest region) and a dominant input from a shrub tundra type vegetation. The latter was attri- buted to fluvial input from a deglaciated South Green- land, which accordingly at that time was occupied by a dense vegetation dominated by shrubs and pterido- phytes. Moreover, the species assemblage suggested the existence of subarctic or even temperate climates in the source area (southern Greenland) which, in fact, agree with our climatic r~construction for Greenland for marine Isotopic Stage 5 with temperatures up to 5 K warmer than at present.

In Northwest Greenland, the ice margin also re- treated substantially during marine Isotopic Substages 5 e and 5c (Fig. 14), and the ablation zone was not far from Camp Century. An increase of the amplitude of the climatic temperature change for North Greenland in agreement with predictions by GCM model calculations, and actually indicated by the larger amplitude of the Camp Century 6(~80) record in respect to the amplitude of the Central and South Greenland records (see Fig. 3 and Fig. 8), would result in larger ice sheet retreat in North Greenland (see Fig. 13) than that indicated in Fig. 14. This would make the ice margin retreat to a position inland of Camp Century, which could then explain the missing marine Isotopic Stage 6 ice in the Camp Century record. Independent evidence for a warm climate in Northwest Greenland in marine Isotopic Stages 5e and 5c-a is provided by Quaternary geological studies in the Thule area (Funder, 1990), see Discuss ion .

Another interesting result of the model studies is that a large ice sheet with essentially unchanged position and elevation of the dome (Summit) remains over northern and central Greenland even in the warm periods of marine Isotopic Stage 5. Furthermore, the results of the model studies shown in Fig. 13 indicate that complete disappearance of the ice sheet in marine Isotopic Stage 5 is highly unlikely. For the ice sheet to melt away in total, a temperature increase of 6 K sustained over 20 ka, or a temperature increase of 8 K sustained over 5 ka is required (Letrrguilly et al. , 1991a). None of these climate scenarios are likely to have occurred during marine Isotopic Stage 5. The survival of a central Greenland ice sheet also agrees with the fact that ice from marine Isotopic Stage 6 is apparently present in the Pfikitsoq ice-margin record (Fig. 7), the Pleistocene part of which was originally deposited in central Greenland.

The essentially unchanged elevation of the central Greenland ice sheet indicated by the calculations, also


15000 years BP 5000 years BP PRESENT

FIG. 14. Extent and surface topography of the Greenland ice sheet at selected times during the 150 ka reconstructed evolution. Surface contour lines are drawn for each 200 m. The thick closed contour line in central Greenland is the 3000 m elevation contour. Marine isotopic stages are indicated in the bottom right corners. The time scale is the one suggested by Lorius et al, (1985) for the Vostok ice-core record, putting the peak of marine Isotopic Stage 5e at 130 ka BP. Triangles indicate deep drilling sites in Greenland: Camp Century in the northwest, Summit in the centre, and Dye3 in the south. From Letr6guilly et al.

(1991b).

justifies the exclusion of the influence of surface elevation change on the P~kitsoq 6(180) record cf. the section on Pg~kitsoq, Central Greenland.

The calculated volume change of the Greenland ice sheet during the last 150 ka is displayed in Fig. 15. A scale showing the equivalent contribution to global sea- level change is added to the right. In terms of sea-level change, the Greenland contribution ranges between a

rise of about 2 m in marine Isotopic Stages 5e and 5c to a lowering of less than 1 m during the late glacial maximum. Since a complete disappearance of the Greenland ice sheet in marine Isotopic Stage 5 is highly unlikely, for the reasons mentioned above, one will have to look for another source that could have contributed about 4 m to the total sea-level rise of 6 m inferred for marine Isotopic Substage 5e from coral-

138 N. Reeh

4 . 0 e + 1 5 ' ' " . . . . . . . . . . . .

, g

I l l 3 . 5 4 ÷ 1 5 ~ ~l~

:E 3:

. . I ..,.I

W 3.~+15 .1 ~

-2 2 . 5 e + 1 8 . . . . , . . . . , . . . .

-~ ~oooo .looooo -soooo

TIME (years BP)

FIG. 15. Volume changes of the Greenland ice sheet during the 150 ka reconstructed evolution. The time scale is the one suggested by Lorius et al. (1985) for the Vostok ice core record, putting the peak of marine Isotopic Stage 5e at 130 ka BP. The vertical scale on the right indicates the corresponding contribution to sea-level change. From

Letr6guilly et al. (1991b).

reef studies (Chappell and Shackleton, 1986). The West Antarctic ice sheet, which has been suggested to be potentially unstable for a climatic warming, is a likely candidate (Mercer, 1968).

DISCUSSION

In connection with the presentation of the Greenland ice sheet and Canadian Arctic ice cap records, various problems concerning chronology and correlation of the records have already been discussed. In this section, the Greenland record for marine Isotopic Stage 5 will be discussed in relation to other paleoclimatic and paleoenvironmental records. In Fig. 16 the isotopic temperature record from Pfikitsoq and two different interpretations of the Camp Century 6(lSo) record (those of Dansgaard et al. , 1982, and Reeh et al. , 1991) are displayed together with the orbitally driven variations of mean July insolation at 65°N (Berger and Pestiaux, 1984), the temperature record derived from the Grand Pile pollen record (Guiot et al., 1989), the Huon sea level record (Chappell and Shackleton, 1986), mean seawater 6(i80) (Labereyrie et al., 1987), the Specmap oxygen isotope record (Imbrie et al., 1984), and the Vostok isotopic temperature record (Lorius et al., 1990).

It appears that there are major differences between the P~kitsoq and Vostok isotopic temperature records in marine Isotopic Stage 5. Whereas the Pfikitsoq record shows an interglacial-type climate in much of marine Isotopic Stage 5 with a climate at least as warm as at present in Substages 5a,c, and e, the Vostok record indicates that a rather cold climate prevailed during most of EIS 5, with Substage 5e as the only period displaying an interglacial-type climate, some- what warmer than now. On the other hand, the derived central Greenland temperatures are in agreement with the climate variations inferred from recent glacial- geological studies in North-west Greenland (Funder,

1990, 1991). These studies conclude that during the Qamat interstade (TL-dated to between 114 and 69 ka BP, i.e. marine Isotopic Stages 5a,b,c) the West Greenland current was stronger and had a larger warm- water component than is known from the Holocene, while at the same time summers on land were significantly warmer than during the Holocene (Funder and Houmark-Nielsen, 1990). For the Saunders O interstade (TL-dated to between 136 and 120 ka, i.e. marine Isotopic Stage 5e), it is concluded that marine conditions were similar to those of the present or warmer (Feyling-Hanssen and Funder, 1990). The Qamat and Saunders O warm episodes were separated by a glacial advance (the Narssarsuk stade) TL-dated to 114 ___ 10 ka BP. This also agrees with the P~tkitsoq isotopic temperature record, showing a cold climate during marine Isotopic Stage 5d.

As shown by Funder and Houmark-Nielsen (1990), the glacial and marine events in the Thule area can be correlated with similar events further south in West Greenland and on Baffin Island, suggesting that during marine Isotopic Stage 5, marine and glacial events in the Baffin Bay region were in phase. Moreover, a 140 ka climate reconstruction based on European pollen records indicates warm interglacial-type climates not only in marine Isotopic Stage 5e, but also in marine Isotopic-Stage 5a and 5c when temperatures were similar to present temperatures (Guiot et al., 1989; Fig. 16d), in agreement with the Pfikitsoq isotopic temperature record. Thus, there is evidence that the relatively warm climate in marine Isotopic Stages 5a and 5c was geographically extended to other parts of the northern hemisphere than just Northwest and Central Green- land, suggesting that the climate in significant parts of the northern hemisphere behaved differently from that of the southern hemisphere (Antarctica) during mid and late marine Isotopic Stage 5.

An interglacial type climate in the northern hemisphere in marine Isotopic Stage 5e,c, and a would cause the continental ice sheets in North America and Eurasia to disappear or at least retreat significantly, and thus sea level should be close to the present level in these stages. The Huon coral-reef sea-level studies indicate 6 m higher, 12 m lower, and 18 m lower sea level stands in marine Isotopic Stage 5e, marine Isotopic Stage 5c, and marine Isotopic Stage 5a, respectively (Chappell and Shackleton, 1986), see Fig. 16e. Revisions of the continental ice-volume record (Shackelton, 1987; Labeyrie et al. , 1987, Fig. 16f) confirm the high sea level in marine Isotopic Stage 5e, and support the moderate sea-level lowering in marine Isotopic Stage 5c and 5a, compared with earlier estimates of a sea-level lowering of about 30 m in the last mentioned stages, see the Specmap record, Fig. 16g. A 12-18 m lower sea level stand in Substages 5a and 5c, however, indicates that an amount of ice equivalent to 2-3 times the volume of the present Greenland ice sheet was, at those times, stored on the continents in excess of the present ice sheets and glaciers. Even this rather moderate amount of extra ice


0 GO 100 1GO

d~..,.. G,~.a P.. AQ. (ko)

17o

~ -o.51 ~" -1.o /

i . . . . . . . . . . . ! f - - o-"4/.1 co p C.. -ry /

i -0: , .~i) Voslok . - - / I

~ - I I 1

E

- t ~ . . . . , . . . . , . . . . ,

50 ~00 150

Age (ko) FIG. 16. Paleoclimatic and paleoenvironmental records for the last glacial cycle. (a) Variation of mean July insolation at 65°N (Berger and Pestiaux, 1984). (b) The Camp Century, Northwest Greenland 6('80) record plotted on the time scale suggested in this work. (c) The Pglkitsoq, Central Greenland isotopic temperature record (Reeh et al., 1991) plotted on a revised time scale. (d) European reconstructed temperatures derived from La Grand Pile Pollen record (Guiot et al., 1989). (e) Huon coral-reef sea-level record (Chappell and Shackleton, 1986). (f) Mean sea water 6('"O), representing continental ice volume (Labeyrie et al., 1987). (g) The Specmap stacked marine isotopic record (Imbrie et aL, 1984). (h) The Camp Century, Northwest Greenland 6('80) record plotted on the time scale suggested by Dansgaard et al. (1982). (i) The Vostok isotopic temperature record

plotted on the revised timescale (Lorius et al., 1990).

140 N. Reeh

seems to be high for a period with an interglacial type climate. So, there seems to be a disagreement in middle and late marine Isotopic Stage 5 between continental climate records from the northern hemisphere and the sea level records derived from coral reef studies and marine foraminifera isotopic records.

One should note though, that the interpretation of the Camp Century record by Dansgaard et al. (1982) suggests a climate during marine Isotopic Stage 5 in general agreement with the climate indicated by the Vostok isotopic temperature record and the marine isotopic records, compare Figs 16f-i. On the other hand, it is not unreasonable to think that the waxing and waning of northern hemisphere ice sheets in response to orbitally driven variations in northern hemisphere summer insolation, could have a large influence on the climate in the northern hemisphere, and cause larger fluctuations than those occurring in the distant, isolated Antarctica. The peaks in the mean July insolation for 65°N at ca. 105 and 85 ka BP (Fig. 16a), although not as high as the peak at ca. 125 ka BP, are comparable to the peak associated with the Holo- cene climatic optimum. The Pfikitsoq isotopic temperature record suggests an interglacial-type climate in Greenland associated with all three insolation maxima in marine Isotopic Stage 5. If, as indicated by other paleoenvironmental records, this was not the case, one will have to look for explanations why the marine Isotopic Stage 5e peak and the Stage 1 peak (the Holocene climatic optimum) were associated with interglacial climates, whereas the peaks in Substages 5a and 5c were not. This emphasizes the importance of studying the entire marine Isotopic Stage 5 (not only Substage 5e), in order to document whether the northern hemisphere climate responded differently to nearly identical summer insolation maxima, and if so, to look for the causes.

In this context a matter of crucial importance is the establishment of reliable chronologies for the ice sheet records, with the purpose of synchronizing records from the northern and southern hemispheres, and also synchronizing terrestrial ice-sheet records and marine sediment records. Until absolute dating methods for the deep ice-core records become available, the synchronizing must depend on identification of markers, representing synchronous events. For inter- hemispheric correlations, peaks in l°Be concentration profiles seem to be promising as time markers (Fig. 5), whereas volcanic signals have a potential for providing land-sea correlations, see section on Dye3, South Greenland.

Recently it has been suggested that the atmospheric dust load can be used as a stratigraphic marker for north-south as well as land-sea synchronizations of sediment records through the last glacial cycle (Petit et al., 1990). Petit et al. (1990) reported on high correlations over the last climatic cycle between the dust record from the Vostok ice core, magnetic susceptibil- ity measured on the Xifeng loess sequence, and the magnetic remanence parameter (HIRM) in a North

Atlantic deep sea sediment core. Apparently, the micro-particle record from Dye3, South Greenland (Hammer et al,, 1985), shows a similar pattern as the above mentioned records, suggesting that the use of atmospheric dust as a stratigraphic marker can be extended also to Greenland ice-sheet records (Reeh, unpublished data).

Except for the possible Z2 ash layer in the Dye3 deep ice core, volcanic signals in ice cores from Greenland and the Canadian Arctic have so far been detected only as layers of increased acidity (conductivity) (Hammer, 1989; Fisher et al., 1983). This technique can at present only be applied to the Holocene part of these records, because the acid volcanic signals are suppressed in ice deposited during most of the last glacial period. This ice is alkaline as a result of the calcareous high aerosol load during the glacial, that neutralized the atmospheric acids (Hammer et al., 1985). Volcanic eruptions during the last glacial-interglacial cycle are recorded by tephra layers in sediments from the surrounding seas (e.g. Ruddiman and Glover, 1972; Sejrup et al., 1989). In general, the identification of these events in the ice cores requires microparticle analysis of the ice. This is a much more time-consuming task than making conductivity (ECM) measurements, and such studies have hence not been performed systematically on glacial ice from Greenland cores.

For the existing Greenland and Canadian Arctic cores, the situation is even worse as far as the 'last interglacial' is concerned. As previously mentioned, ice of 'last interglacial' age is, in these cores, found so close to the base that large concentrations of silt, presumably of bedrock origin, are present in most of the ice cores. This probably precludes the detection of volcanic signals. However, two new deep ice cores are currently being drilled near 'Summit' on the Central Greenland ice sheet (GRIP and GISP2). In these cores, ice from marine Isotopic Stage 5 is likely to be found sufficiently high above the ice-sheet base that there should be no risk of debris inclusions from the base. Therefore, in these cores, the large volcanic signals recorded in deep- sea sediment records of marine Isotopic Substage 5e age from the Greenland sea (Sejrup et al., 1989) should be detectable, perhaps even by means of conductivity measurements.

Also, because of the expected high resolution of the Summit ice core records from marine Isotopic Stage 5 with an expected average annual layer thickness on the order of one centimetre, the new deep ice cores should provide unique climatic and other environmental records of the 'last interglacial', elucidating not only the temperature variations, but probably also changes of precipitation, composition of the atmosphere, the general atmospheric circulation, volcanic activity etc.

REFERENCES

Beer, J., Siegenthaler, U., Bonani, G., Finkel, R.C., Oeschger, H., Suiter, M. and Wolfli, W. (1988). Information on past solar activity and geomagnetism from "JBe in the Camp Century ice core. Nature, 331, 675-679.


Berger, A. and Pestiaux, P. (1984). Accuracy and stability of the Quaternary terrestrial insolation. In: Berger, A.L. et al. (eds), Milankovitch and Climate, Part 1, pp. 83--111. Reidel.

Chappell, J. and Shackleton, N.J. (1986). Oxygen isotopes and sea level. Nature, 324, 137-140.

Clausen, H.B., Gundestrup, N.S., Johnsen, S.J., Bindschadler, R. and Zwally, J. (1988). Glaciological investigations in the Cr6te area, Central Greenland: a search for a new deep-drilling site. Annals of Glaciology, 10, 10-15.

Dahi-Jensen, D. (1989). Two-dimensional thermo-mechanical modelling of flow and depth-age profiles near the ice divide in central Greenland. Annals of Glaciology, 12, 31-36.

Dahl-Jensen, D. and Johnsen, S.J. (1986). Palaeotemperatures still exist in the Greenland ice sheet. Nature, 320, 250-252.

Dansgaard, W. and Oeschger, H. (1989). Past environmental long- term records from the Arctic. In: Oeschger, H. and Langway, C.C. Jr (eds), The Environmental Record in Glaciers and Ice Sheets, pp. 287-318. Wiley.

Dansgaard, W., Johnsen, S.J., Clausen, H.B. and Langway, C.C. Jr (1971). Climatic record revealed by the Camp Century ice core. In: Turekian, K.K. (ed.), The Late Cenozoic GlacialAges, pp. 37-56. Yale University Press, New Haven.

Dansgaard, W., Johnsen, S.J., Clausen, H.B. and Gundestrup, N. (1973). Stable isotope glaciology. Meddelelser om GrCnland, 197, No. 2, 1-53.

Dansgaard, W., Clafisen, H.B., Gundestrup, N., Hammer, C.U., Johnsen, S.J., Kristinsdottir, P.M. and Reeh, N. (1982). A new deep Greenland ice core. Science, 218, 1273-1277.

de Vernal, A., Miller, G.H. and Hillaire-Marcel, C. (1991). Paleoenvironments of the last interglacial in northwest North Atlantic region and adjacent mainland Canada. Quaternary International, 10-12, 95-106.

Fisher, D.A. (1979). Comparison of 10 "~ years of oxygen isotope and insoluble impurity profiles from the Devon Island and Camp Century ice cores. Quaternary Research, 11, 29%305.

Fisher, D.A. (1987). Enhanced flow of Wisconsin ice related to solid conductivity through strain history and recrystallization. In: Waddington, E.D. (ed.), The physical basis of ice sheet modelling. (Proceedings of the Vancouver Symposium, August 1987.) IAHS Publication, 170, 45-51.

Fisher, D.A. (1990). A zonally-averaged stable-isotope model coupled to a regional variable-elevation stable-isotope model. Annals of Glaciology, 14, 65-71.

Fisher, D.A. and Alt, B.T. (1985). A global oxygen model - - semi- empirical, zonally averaged. Annals of Glaciology, 7, 117-124.

Fisher, D.A., Koerner, R.M., Paterson, W.S.B., Dansgaard, W., Gundestrup, N. and Reeh, N. (1983). Effect of wind scouring on climatic records from ice-core oxygen-isotope profiles. Nature, 302, 205-209.

Fisher, D.A., Reeh, N. and Clausen, H.B. (1985). Stratigraphic noise in time series derived from ice cores. Annals of Glaciology, 7, 76-83.

Feyling-Hanssen, R.W. and Funder, S. (1990). Fauna and Flora. In: Funder, S. (ed.), Late Quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland. Meddelelser om GrCnland, Geoscience, 22, 1%32.

Funder, S. (1990). Late Quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland. Meddelelser om Grcnland, Geoscience, 22, 63 pp.

Funder, S. and Houmark-Nielsen, M. (1990). Local events and regional correlation. In: Funder, S. (ed.), Late quaternary stratigraphy and glaciology in the Thule aea, Northwest Greenland. Meddelelser om GrOnland, Geoscience, 22, 40-45.

Funder, S., Hjort, C. and Kelly, M. (1991). Isotope Stage 5 (130-74 ka) in Greenland a review. Quaternary International, 10-12, 107- 122.

Guiot, J., Pons, A., de Beaulieu, J.L. and Reille, M. (1989). A 140,000-year continental climate reconstruction from two Euro- pean pollen records. Nature, 338, 309-313.

Hammer, C.U. (1989). Dating by physical and chemical seasonal variations and reference horizons. In: Oeschger, H. and Langway, C.C. Jr (eds), The Environmental Record in Glaciers and Ice Sheets, pp. 9%121. Wiley, New York.

Hammer, C.U., Clausen, H.B., Dansgaard, W., Neftel, A., Kristins- dottir, P. and Johnsen, E. (1985). Continuous impurity analysis along the Dye3 deep core. In: Langway, C.C. Jr, Oeschger, H. and Dansgaard, W. (eds), Greenland Ice Core: Geophysics, Geoche- mistry, and the Environment, pp. 90-94. American Geophysical Union, Geophysical Monograph, 33, Washington, D.C.

Hammer, C.U., Clausen, H.B. and Tauber, H. (1986). Ice core dating of the Pleistocene/Holocene boundary applied to a calibra- tion of the 14C time scale. Radiocarbon, 28, 2A, 284-291.

Hays, J.D., Imbrie, J. and Shackleton, N.J. (1976). Science, 194, 1121.

Hillaire-Marcel, C. and de Vernal, A. (1989). Isotopic and palynological records of the late Pleistocene in eastern Canada and adjacent ocean basins. Gdographie physique et Quaternaire, 43(3), 263-290.

Huybrechts, P., Letrfguilly, A. and Reeh, N. (1991). The Greenland ice sheet and greenhouse warming. Global and Planetary Change, 89, 399-412.

Imbrie, J., Hays, J.D., Martinson, D.G., Mclntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., PreU, W.L. and Shackleton, N.J. (1984). The orbital theory of Pleistocene climate: support from a revised chronology of the marine t5(180) record. In: Berger, A.L. et al. (eds), Milankovitch and Climate, Part 1, pp. 26%305. Reidel.

Johnsen, S.J., Dansgaard, W. and White, J.W.C. (1989). The origin of arctic precipitation under present and glacial conditions. Tellus, 41B, 452-468.

Jouzel, J., Lorius, C., Petit, J.R., Genthon, C., Barkov, N.I., Kotlyakov, V.M. and Petrov, V.M. (1987). Vostok ice core: a continuous isotope temperature record over the last climatic cycle (160,000 years). Nature, 329 , 403-408.

Koerner, R.M. (1979). Accumulation, ablation, and oxygen isotope variations on the Queen Elizabeth Islands ice caps, Canada. Journal of Glaciology, 22(86), 25-41.

Koerner, R.M. (1989). Ice core evidence for extensive melting of the Greenland ice sheet in the last interglacial. Science, 244, 964- 968.

Koerner, R.M. and Fisher, D.A. (1986). The Devon Island ice core and the glacial record. In: Andrews, J.T. (ed.), Quaternary Environments. Eastern Canadian Arctic, Baffin Bay and Western Greenland, pp. 309-327. Allen and Unwin, Boston.

Koerner, R.M. and Russell, R.D. (1979). 61~O variations in snow on the Devon Island ice cap, Northwest Territories, Canada. Canadian Journal of Earth Science, 16(7), 1419-1427.

Koerner, R.M., Fisher, D.A. and Paterson, W.S.B. (1987). Wiscon- sinan and pre'-Wisconsinan ice thicknesses on Ellesmere Island, Canada: inferences from ice cores. Canadian Journal of Earth Sciences, 24, 296--301.

Koerner, R.M., Bourgois, J.C. and Fisher, D.A. (1988). Pollen analysis and discussion of time-scales in Canadian ice cores. Annals of Glaciology, 10, 85-91.

Labeyrie, L.D., Duplessy, J.C. and Blanc, P.L. (1987). Variations in mode formation and temperature of oceanic deep waters over the past 125,000 years. Nature, 327, 477--482.

Langway, C.C. Jr, Oeschger, H. and Dansgaard, W. (1985). Greenland Ice Core: Geophysics, Geochemistry, and the Environ- ment. American Geophysical Union, Geophysical Monograph, 33, Washington, D.C.

Letr6guilly, A., Huybrechts, P. and Reeh, N. (1991a). Steady state characteristics of the Greenland ice sheet under different climates. Journal of Glaciology, 37(125), 14%157.

Letr6guilly, A., Reeh, N. and Huybrechts, P. (1991b). The Green- land ice sheet through the last glacial-interglacial cycle. Global and Planetary Change, 90, 385-394.

Lorius, C., Jouzel, J., Ritz, C., Merlivat, L., Barkov, N.I., Korotkevich, Y.S. and Kotlyakov, V.M. (1985). A 150,000-year climatic record from Antarctic ice. Nature, 316, 591-596.

Lorius, C., Jouzel, J., Raynaud, D., Hansen, J. and Le Treut, H. (1990) . The ice-core record: climate sensitivity and future greenhouse warming. Nature, 347, 139-145.

Mercer, J,H. (1968). Antarctic ice and Sangamon sea level. IAHS Commission of Snow and Ice. General Assembly of Bern, Oct. 1967. IAHS Publication, 79, 217-225.

Oeschger, H. and Langway, C.C. Jr (1989). The Environmental Record in Glaciers and Ice Sheets. Wiley, New York.

Paterson, W.S.B., Koerner, R.M., Fisher, D., Johnsen, S.J., Clausen, H.B., Dansgaard, W., Bucher, P. and Oeschger, H. (1977). An oxygen-isotope climatic record from the Devon Island ice cap, arctic Canada. Nature, 266, 508-511.

Petit, J.R., Mounier, L., Jouzel, J., Korotkevich, Y.S., Kotlyakov, V.I. and Lorius, C. (1990). Palaeoclimatological and chronological implications of the Vostok core dust record. Nature, 343, 56-58.

Raisbeck, G.M., Yiou, F., Bourles, D., Lorius, C., Jouzel, J. and Barkov, N.I. (1987). Evidence for two intervals of enhanced ~°Be deposition in Antarctic ice during the last glacial period. Nature, 326, 273-277.

142 N. Reeh

Ram, M. and Gayley, R.I. (1991). Long-range transport of volcanic ash to the Greenland ice sheet. Nature, 349, 401-404.

Reeh, N. (1988). A flow-line model for calculating the surface profile and the velocity, strain rate, and stress fields in an ice sheet. Journal of Glaciology, 34(116), 46-54.

Reeh, N. (1989a). Dating by ice flow modelling: a useful tool or and exercise in applied mathematics. In: Oeschger, H. and Langway, O.O. Jr (eds), The Environmental Record in Glaciers and Ice Sheets, pp. 141-159. Wiley, New York.

Reeh, N. (1989b). Dynamic and climatic history of the Greenland ice sheet. In: Fulton, R.J. (ed.), Quaternary Geology of Canada and Greenland, pp. 795-822. Geological Survey of Canada, Geology of Canada, No. 1 (also: Geological Society of America, The Geology of North America, K-l).

Reeh, N. (1990). Past changes in precipitation rate and ice thickness as derived from age-depth profiles in ice sheets; application to Greenland and Canadian Arctic ice core records. In: Bleil, U. and Thiede, J. (eds), Geological History of the Polar Oceans: Arctic Versus Antarctic, pp. 255-271. Kluwer, Dordrecht.

Reeh, N. (1991). Parameterization of melt rate and surface temperature on the Greenland ice sheet. Polarforschung, 59/3, 113-128.

Reeh, N., Thomsen, H.H. and Clausen, H.B. (1987). The Greenland ice sheet margin - - a mine of ice for paleo-environmental studies. Palaeogeography, Palaeoclimatology, Palaeoecology, 58, 229-234.

Reeh, N., Thomsen, H.H., Frich, P. and Clausen, H,B. (1990). Stable isotope studies on ice margins in the Thule area. In: Funder, S. (ed.), Late quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland. Meddelelser om Gremland, Geo- science, 22, 47-56.

Reeh, N., Oerter, H., Letr6guilly, A., Miller, H. and Hubberten, H.-W. (1991). A new, detailed ice-age oxygen-18 record from the ice-sheet margin in central West Greenland. Global and Planetary Change, 90, 373-383.

Robin, G. de Q. (1983). The Climatic Record in Polar Ice Sheets. Cambridge University Press, Cambridge.

Ruddiman, W.F. and Glover, L.K. (1972). Vertical mixing of ice- rafted volcanic ash in North Atlantic sediments. Geological Society of America Bulletin, 83, 2817-2836.

Sejrup, H.P. and Larsen, E. (1991). Eemian-Early Weichselian N-S temperature gradients: North Atlantic-NW Europe: Quaternary International, 10-12, 161-166.

Sejrup, H.P., Sj~holrri, J., Furnes, H., Beyer, I., Eide, L., Jansen, E. and Mangerud, J. (1989). Quaternary tephrachronology on the Iceland Plateau, north of Iceland. Journal of Quaternary Science, 4, 109-114.

Stauffer, B.R. (1989). Dating of ice by radioactive isotopes. In: Oeschger, H. and Langway, C.C. Jr (eds), The Environmental Record in Glaciers and Ice Sheets, pp. 99-121. Wiley, New York.