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A new, detailed ice-age oxygen-18 record from the ice-sheet margin in central West Greenland
Palaeogeography, Palaeoclimatology, Palaeoecology(Globaland Planetary Change Section), 90 (1991) 373-383
373
Elsevier Science Publishers B.V., Amsterdam
A new, detailed ice-age oxygen-18 record from the ice-sheet margin in central West Greenland Niels Reeh, H a n s Oerter, A n n e Letr6guilly, H e i n z Miller a n d H a n s - W o l f g a n g H u b b e r t e n Alfred-Wegener-lnstitutfiir Polar- und Meeresforschung, Postfach 120161, D-2850 Bremerhaven, Germany (Revised version accepted February 28, 1991)
ABSTRACT 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. Palaeogeogr., Palaeoclimatol., Palaeoecol.(Global Planet. Change Sect.), 90: 373-383. A new detailed oxygen-18 record measured on surface-ice samples from a West Greenland ice-margin location reveals the hitherto longest climatic record from the Greenland ice sheet, spanning the last c. 150,000 years. The new record implies that the Greenland deep ice-core records from Dye3 and Camp Century need to be re-interpreted. A comparison with the deuterium record from the Vostok deep ice core, Antarctica indicates that climate behaved differently in the northern and southern hemispheres during the last glacial/interglacial cycle, with major differences occurring in Emiliani isotopic stage 5.
Introduction
The large ice sheets in Greenland and Antarctica are rich sources of information about climate and environmental changes during the past c. 150,000 years (150 ka) and probably much more, as demonstrated by the results of deep ice-core drilling programmes (e.g. Dansgaard et al., 1982; Lorius et al., 1985; Jouzel et al., 1987). However, the old ice found at depth in the central regions of the ice sheets can also be retrieved from the surface of the ice-sheet margins, where the ice is found in a sequence with the oldest ice nearest to the ice edge (Lorius and Merlivat, 1977; Reeh et al., 1987) Here, we report on a detailed oxygen-18 record measured on surface-ice samples collected from the ice margin at Pakitsoq, Central West Greenland ( 6 9 ° 2 6 ' N , 50°16'W). The record spans a period of at least 150 ka covering the early part of the present interglacial (the Holocene), the last glacial, the last interglacial, and also a part of the previous glacial. The Pre-Holocene part of the record was originally deposited in the Summit 0921-8181/91/$03.50
region of Central Greenland (Fig. 1), and thus represents the climate in this region where deep ice-core drilling projects are yet being undertaken. The record has been translated into a Greenland temperature record covering the past 150 ka on the time scale derived for the deep ice core from Vostok, East Antarctica (Lorius et al., 1985). The derived isotopic temperatures indicate large temperature variations in Emiliani's isotopic stage 5 (EIS 5, approximate age 130-75 ka B.P.), with a climate warmer than at present not only in substage 5e, but also in substage 5c and during a short period of substage 5a. This is in contrast to the temperature record derived from the Vostok deuterium record, which shows colder-than-present temperatures during most of EIS 5 with substage 5e as the only exception. Our study, therefore, suggests that the climate was different in the northern and southern hemispheres during a large period of the last glacial-interglacial cycle. A correlation of the ice-margin oxygen-18 record with the corresponding records from the Greenland deep ice cores from Camp Century (CC) and Dye3 indicates that neither of these
© 1991 - Elsevier Science Publishers B.V. All rights reserved
374
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records reach back to the previous glacial, p r o b ably due to substantial thinning a n d retreat of the ice sheet in N o r t h and South G r e e n l a n d in the w a r m p e r i o d s d u r i n g EIS 5 ( K o e r n e r , 1989; Reeh, 1990; Letr~guilly et al., 1991).
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The b a c k g r o u n d for the ice-margin isotopic study is the following: An ice sheet can be considered a s e d i m e n t a r y d e p o s i t consisting of sequences of layers d e p o s i t e d a n n u a l l y in the a c c u m u l a t i o n zone (i.e. the region of positive mass i n p u t ) as snow accumulation. The snow layers sink into the ice sheet subject to c o n t i n u o u s thinning, initially as a result of densification, by which snow is t r a n s f o r m e d into ice, but then mainly due to flow-induced vertical c o m p r e s s i v e strain. In this process the layers are stretched h o r i z o n t a l l y until they are advected by the ice m o t i o n into the a b l a t i o n zone, where the ice is r e m o v e d b y melting or calving of ice bergs. In areas where a b l a t i o n by melting is p r e d o m i n a n t , the layers move u p w a r d s relative to the surface where they eventually melt away. The ice flow pattern, illustrated in Fig. 2, shows particle p a t h s ill a cross section of an ice sheet. It a p p e a r s from the figure that the higher the ice is originally d e p o s i t e d in the a c c u m u l a t i o n area, the d e e p e r into the ice sheet it will sink, a n d the nearer to the ice-sheet m a r g i n it will r e a p p e a r
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Fig. 1. The Greenland ice sheet. Sampling location on the ice margin at Pakitsoq, and deep drilling locations at Camp Century, and Dye3, and Summit are indicated. The flow line from Summit to Pakitsoq illustrates that Pre-Holocene ice now found at the ice-sheet surface at Pakitsoq was originally deposited in the Summit area of Central Greenland.
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at the surface in the ablation zone. Consequently the oldest ice is found near the base and along the margin of the ice sheet. In principle, a complete sequence of all the deposited layers can be obtained either by deep core drilling from the surface to the base in the accumulation zone or by surface sampling from the equilibrium line (the line separating the accumulation and ablation zones) to the ice margin. The layers become progressively older with depth, respectively with distance from the equilibrium line.
Surface ice sampling and reproducibility Nearly 1500 surface-ice samples were collected in a profile on the ice margin at Pakitsoq, c. 40 km N E of Jakobshavn, West Greenland (for location see map in Fig. 3). The profile extends 750 m in from the margin, and runs approximately parallel
375
WEST GREENLAND
to the main ice-flow direction. The sampling site is located at 380 m elevation, and is a region of strongly divergent flow (stagnation zone). The ice thickness decreases from about 200 m one kilometre inland to probably less than 10 m at the margin (Thorning and Hansen, 1987). Over the same distance, the ice-flow velocity at the surface decreases from 12 m / y r to nearly zero. The annual ablation rate is 2-2.5 m / y r of ice. At the time of the sampling (August, 1988) which was late in the ablation season, the surface generally consisted of bubbly glacier ice. Cryoconite holes of varying diameter (from a few millimetres to about one metre) and depth (0.1-0.5 m), and surface irregularities with wavelengths and waveheights of typically 2 - 5 m and 0.5 m, respectively are of common occurrence. Blue bands, 1 - 5 0 cm wide, running roughly parallel to the ice margin, often over distances of several hundred metres, are
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also c o m m o n . They consist of clear coarse-grained ice with 6~SO values several %e higher than those of the s u r r o u n d i n g ice. A n e x p l a n a t i o n of this 6~SO a n o m a l y is not yet known, but, to j u d g e from d e t a i l e d s a m p l i n g across the bands, they d o not seem to break the c o n t i n u i t y of the 6~SO record. A p a r t from the blue b a n d s which cause the large scale linear foliation p a t t e r n seen on the p h o t o graph in Fig. 4, strong foliation is found only in a b a n d between 25 and 137 m from the margin. However, as a p p e a r s from Fig. 4 there is no indication of folding at the ice surface. F r o m an oxygen-I 8 profile s a m p l e d in 1985 at the same location, it was k n o w n that ice of Pleistocene origin constitutes a c. 600 m wide b a n d parallel to the ice margin (Reeh et al., 1987). However, the previous s a m p l i n g at spacings of 2 - 5 m was not detailed enough to convincingly resolve the details of the Pleistocene 6~SO record. In the new profile, s a m p l i n g distances are 0 . 5 1 m, a n d a 170 m section through the Pleistocene
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Holocene transition has even been s a m p l e d continuously as 20-cm samples. The two 81SO-records are shown in Fig. 5. Due to a small difference in the o r i e n t a t i o n of the profile lines, a n d the fact that the foliation (as indicated by the blue b a n d s ) forms an angle of a p p r o x i m a t e l y 60 ° with the profile lines, there is a small difference of the length scales along the profiles. In o r d e r to facilitate the c o m p a r i s o n , the length of the 1985 profile has a c c o r d i n g l y been reduced by 9% relative to the length of the 1988 profile. W i t h this a d j u s t m e n t , the two records are nearly identical, considering the low resolution and the degree of r a n d o m n e s s i n t r o d u c e d in the 1985 profile due to spot s a m p l i n g at intervals of 2 - 5 m. It thus a p p e a r s that the surface 8~SO-record is r e p r o d u c i b l e (at least within d i s t a n c e s of the o r d e r of 50 m) in spite of the irregular, sometimes rather chaotic, a p p e a r a n c e of the ice surface. Part of the e x p l a n a t i o n is that the nearer to the ice margin a layer is found, the steeper is its dip, so
Fig. 4. Photograph of the sampling location at the ice-margin. The photograph is taken from the southeast. The lake on the righthand side of the photograph is the lake labeled 326 on the map in Fig. 3. The length of the ice margin contact with the mountain slope is approximately 1.5 km. The photograph illustrates the large-scale linear foliation (blue bands) parallel to the margin. The irregular pattern on the ice surface is caused by numerous melt streams. A rather large stream is seen in the foreground to the left.
ICE-AGE OXYGEN-18
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FROM ICE-SHEET MARGIN
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WEST GREENLAND
were filled in plastic bottles which were closed with tight covers. The ice samples eventually melted to fill more than half the bottle, thus limiting isotopic fractionation due to evaporation to insignificant amounts. The samples were transported as water samples from Greenland to the Alfred Wegener Institute, Bremerhaven, West Germany, where they were stored at - 30 ° C until analysis in a Finnigan mass spectrometer with a precision of 0.2%~ 8180.
that the original near-horizontal stratification found in the interior accumulation areas ends up as a near-vertical stratification (foliation) at the ice margin (Hook and Hudleston, 1978). This justifies using the term "horizontal core" for the surface-ice records, since the layers are tilted to nearly 90 o and are sampled progressively, as are the horizontal layers cut by vertical coring in the interior regions of ice sheets. As regards the reproducibility of ice margin records over longer distances, a 3]80 record from an ice margin location c. 5 km away reveals broadly similar variations as the records shown in Fig. 5. The sampling procedure was as follows: The upper 10-20 cm thick weathered surface layer was cut away by means of an ice axe. Then, ice was sampled by means of a chisel along a 20-cm section of the freshly exposed ice. The samples
The oxygen-18 record As can be seen from Fig. 5, the ice-margin oxygen-18 record shows large variations. However, generally the variations display a degree of continuity, the transitions from valleys to peaks and vice versa being supported by several values. There
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Fig. 5. The two 8]80 records from the ice margin at Pakitsoq, 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 isotopic stages is shown in the figure.
378
are exceptions however, in the form of single-point spikes, all showing higher isotopic values than those of the adjacent ice. These spikes can all be explained as containing blue-band ice, which generally has a 81SO value which is several %o (from 1 to 16%o) higher than that of normal glacier ice, see discussion above. In very broad terms, the 1988 ice-margin record is characterized by relatively high 6180 values (between -31%o and -33%o) from 750 m to c. 580 m, and relatively low 81SO values (between -38%o and - 41%~,) from 580 m to the ice margin. Since the 1985 record shows that the high 61~O values continue for at least 250 m farther inland, it was concluded that the marked 8180 shift in the 1988 record at 580 m marks the HolocenePleistocene transition (Reeh et al., 1987). Along several sections of the Pleistocene record, 81SO reaches values similar to those of the Holocene ice. This is the case at 560, 210 and 160 m. High 6~SO values, not quite reaching Holocene levels, however, are found at 520, 350, 270, and 235 m. A tentative interpretation of the Pleistocene ~ S O variations in terms of Emiliani isotopic stages is shown in Fig. 5. The Allerod/Younger Dryas oscillation at the Pleistocene-Holocene transition is also indicated in the figure. According to this interpretation, the record reaches back at least into EIS 6, i.e. into the previous glacial. However, as far as the outermost 137-m section of the record is concerned, one should be cautious about interpreting the details of the record. Along this section, the ice surface displays a strongly foliated structure entirely different from the bumpy irregular ice surface along the rest of the profile. The reason for the change in surface structure and the possible influence on the 8180 profile is not yet understood.
Comparison with ice-core records The deep sections of the oxygen-18 records from Camp Century (CC) and Dye3 show a high degree of correlation (Dansgaard et al., 1982), see Fig. 6. The record from CC seems to reach furthest back in time. It is displayed in Fig. 7 together with the 1988 ice-margin record from Pakitsoq. The " b l u e - b a n d " spikes in the ice-
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Fig. 6. 8180 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 simultaneous deposition. Modified from Dansgaard et al. (1982).
margin record have been left out, and the 20-cm samples from the continuously sampled section have been averaged over 1-m increments. Correlation of undated time series is certainly a delicate matter, and it must be agreed that there is more than one way of correlating the two 6lSO-records shown in Fig. 7. However, the most plausible correlation of the main trends of the two records is indicated in the figure by arrows pointing at similar features. Since the ice-margin record was not sampled continuously, little importance has been attached to correlate the high frequency oscillations. A consequence of the correlation is that the unit from 13 to 38 m above the bottom in the CC record displaying relatively high 8a80 values must
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be referred to EIS 5(a,b,c), as originally suggested by Dansgaard et al. (1971) and not to EIS 5e as later suggested by Dansgaard et al. (1982). If at all present in the CC record, EIS 5e (the Eemian) seems to be represented only by the high 8180 value of -23%0 at the bottom of the core. This interpretation of the CC isotopic record, (which is more or less a return to the original suggestion by Dansgaard et al. (1971)) is supported by correlat-
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WEST GREENLAND
ing two distinct peaks in the 1°Be-concentration profile measured along the CC core (Beer et al., 1988) (see Fig. 7) with two similar peaks in the t°Be profile from the well-dated deep ice core drilled at Vostok, East Antarctica (Raisbeck et al., 1987). Raisbeck et al. (1987) concluded that the two distinct peaks at c. 35 ka and 60 k a in the Vostok t°Be concentration record are most likely due to production-rate changes in the atmosphere
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Fig. 7. Comparison of the 8180 records from the Central Greenland ice-margin (a) and the C a m p 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 8180 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 ka and 35 ka of the distinct peaks 75 and 155 m above the bottom are obtained by correlation with similar peaks in the l°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 top and bottom of the figure. The C a m p Century 8180 profile is adapted from Robin (1983). The ]°Be-profile is redrawn from Beer et al. (1988).
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caused by extraterrestrial disturbances. Therefore, the 35 ka and 60 ka events should be detectable also in Greenland l°Be records. As shown in Fig. 7, there are, in fact, two distinct l°Be-concentralion peaks also in the CC record at distances of c. 75 m and 155 m above the bottom. If ages of 60 ka and 35 ka are assigned to these levels, the 13-38 m unit in the CC record with high 8lSO-values obviously must be of mid-late EIS-5 age. One might argue that the less distinct peak shown in Fig. 7 at c. 187 m above the bottom could be the 35 ka l°Be-peak. However, if this peak and the peak at 155 were taken to represent the 35 and 60 ka events, then the accumulation rate in the C a m p Century area must have been 4-+5 times lower than at present in the period 60-10 ka B.P. as shown by ice-dynamic model calculations (Reeh, 1990). If, on the other hand, the peaks at 155 m and 75 m are used, model calculations indicate that the accumulation rate in the last glacial was reduced by only a factor between 2 and 3 in respect to the present value (Reeh, 1990). This agrees better with the generally 2--3 times higherthan-present I°Be-concentration level during the last glacial, see Fig, 7, and with most other estimates of the accumulation rate in Greenland during this period, see discussion by Reeh (1990). We take this as support for the chronology in terms of isotopic stages shown in Fig. 7.
The ice-age climate The dashed lines in Fig. 7 represent the steadyclimate 81SO-trend lines, i. e. the expected alSO variation in the case that past temperatures had always been like those of the present. The steadyclimate reference lines have been determined by ice-dynamic model calculations, considering changes in past ice-flow velocities caused by changing accumulation rates, but neglecting the effect of ice-thickness changes. The decreasing trends back in time are due to the fact that the older the ice, the farther inland (i.e the higher) was the ice originally deposited (see Fig. 2). The decreasing trends simply reflect the present low 6J80 values at these higher deposition sites. Deviations of the oxygen-18 records from the steady-climate reference curves will have to be
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Fig. 8. Comparison of derived isotopic temperature deviations with respect to present temperatures for Central Greenland and Vostok, Antarctica. The Vostok temperature curve is from Jouzel et al. (1987).
explained in terms of either climatic changes or changes in ice-sheet surface elevations. Calculations based on a time-dependent ice-dynamic model using the climate history derived from the ice-margin oxygen-18 record (Fig. 8) as climate forcing, indicate that the changes in the position and elevation of the '+summit" of the Central Greenland ice sheet were moderate during the last glacial-interglacial cycle, even in climatically warm periods when the marginal areas may have suffered considerable thinning and retreat (Letr6guilly et al., 1991). Therefore, changes in ice-sheet surface elevations will only make a minor contribution to the variation of the oxygen-18 record, and will be neglected. The deviations between the measured 8t80 record and the steady-climate reference curve can then be translated into changes in temperature with respect to the present temperature by means of the conversion factor 0.62%0 8 t s O / K established for the Central Greenland ice sheet (Dansgaard et al., 1973). In Fig. 8, the isotopic temperature record is plotted on a timescale determined by assigning ages to the various isotopic stages in accordance with the timescale established
ICE-AGE OXYGEN-18
RECORD
FROM ICE-SHEET MARGIN
IN CENTRAL
for the Vostok ice-core (Lorius et al., 1985). Figure 8 shows that temperatures in Central Greenland were 10-12 K lower than at present during the last glacial maximum (EIS 2), 2 - 3 K lower at the beginning and end of EIS 3, with 6 - 1 0 K lower temperatures in the middle of this stage. EIS 4 is characterized by temperatures between 6 and 10 K lower than at present, whereas temperatures in EIS 5 show large variations from 3-5 K warmer in substages 5c and 5e to 2 - 8 K cooler in substage 5d. The temperature variations in EIS 5 differ substantially from the temperatures deduced by means of the 8(D) record from the deep ice core drilled at Vostok, East Antarctica (Lorius et al., 1985; Jouzel et al., 1987), which is also shown in Fig. 8. The Vostok record suggests that a rather cold climate prevailed during most of EIS 5, with substage 5e as the only period displaying an interglacial-type climate, somewhat warmer than now. On the other hand, the derived Greenland temperatures are in agreement with the climate variations inferred from recent glacial-geological studies in Northwest Greenland (Funder, 1990). These studies conclude that during the Q a m a t interstade (TL-dated to between 114 ka and 69 ka B.P., i.e. EIS 5(a,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 0 interstade (TL-dated to between 136 ka and 120 ka, i.e. EIS 5e), it is concluded that marine conditions were similar to those of the present or warmer (Feyling-Hansen and Funder, 1990). The Q a m a t and Saunders 0 warm episodes were separated by a glacial advance (the Narssarsuk stade) TL-dated to 114 + 1 0 ka B.P. This also agrees with our ice-margin isotopic temperature record, showing a cold climate during EIS 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 EIS 5, marine and glacial events in the Baffin Bay region were in phase. Moreover, a 140 ka climate reconstruction based
WEST GREENLAND
381
on European pollen records indicates warm interglacial-type climates not only in EIS 5e, but also in EIS 5a and 5c when temperatures were similar to present temperatures (Guiot et al., 1989). This evidence supports the climate history derived from our ice-margin isotopic record, and suggests that the climate in parts of the northern hemisphere behaved differently from that of the southern hemipheres during the last glacial/interglacial cycle, with major differences occurring in mid and late EIS 5. An interglacial type climate in the northern hemisphere in EIS 5e, 5c, and 5a would cause the continental ice sheets in North America and Eurasia to disappear or at least retreat significantly, and thus sea level should have been close to the present level in these stages. The H u o n coral-reef sea-level studies indicate 6 m higher, 12 m lower, and 18 m lower sea-level stands in EIS 5e, EIS 5c, and EIS 5a, respectively (Chappel and Shackleton, 1987). Revisions of the continental ice-volume records derived from marine isotopic records (Shackleton, 1987) confirm the high sealevel in EIS 5e, and support the moderate sea-level lowering in EIS 5c and 5a. However, a 12-18 m lower sea level indicates that an amount of ice equivalent to 2 - 3 times the volume of the present Greenland ice sheet was stored on the continents in excess of the present ice sheets and glaciers. Even this rather moderate amount of extra ice seems to be high for a period with an interglacial type climate. So, there seems to be a disagreement between our climate reconstruction based on northern hemisphere continental records, and the marine environmental records for the middle and late EIS 5.
Camp Century and Dye3 If the deviations between the measured 6lSo record and the steady-climate reference curve for C a m p Century (see Fig. 7) are translated into changes in temperature in the same manner as it was done for the Central Greenland record, a climate 16-18 K colder than at present will result for the last glacial maximum. This is about 6 K colder than indicated by the Central Greenland record.
382
Furthermore, a literal translation of the 61SOdeviation in EIS 5c to a temperature deviation suggests 10 K warmer temperature than now i.e. about 3 times warmer than indicated by the Central Greenland record. The relatively large shift of 6~SO in the C a m p Century isotopic record at the termination of the last glacial has been a matter of discussion (Raynaud and Lorius, 1973; Budd and Young, 1983; Reeh, 1984; Dansgaard et al., 1984). Since an explanation in terms of large surface elevation changes seems unlikely (Reeh, 1984; Herron and Langway, 1987) we are left with the explanation that either the temperature shift was anomalously high in Northwest Greenland, or the source area for the vapour forming the precipitation changed dramatically at the end of the last glacial (Dansgaard et al., 1984). However, even by using a similar explanation, it is hard to refer more than half of the 6%o 8180 deviation in EIS 5c to climate change. The other half is most likely due to a substantial lowering of the ice surface caused by thinning and retreat of the Northwest Greenland ice sheet in the warm EIS 5c stage. During the slightly warmer EIS 5e sub-stage, the retreat of the Northwest Greenland ice sheet was probably even larger, and the Camp Century area might have been ice free (Koerner, 1989; Reeh, 1990; Letr~guilly et al., 1991). This explains why the CC ice core record seems to begin at the transition to EIS 5d (see Fig. 7). Transferring the Camp Century time scale suggested in this work (Fig. 7) to the Dye3 record by means of the correlation illustrated in Fig. 6, results in the suggestion that the Dye3 record begins at the transition between EIS 5c and EIS 5b. This also indicates that the South Greenland ice sheet may have been subject to substantial disintegration in the warm EIS 5c, and therefore, probably also in sub-stage 5e (Koerner, 1989: Reeh, 1990; Letr~guilly et al., 1991). A total disappearance of the Greenland ice sheet in EIS 5e (the Eemian) is contradicted by the presence of ice referred to EIS 6 (the previous glacial) in the ice-margin isotopic profile. Since this ice was originally deposited in the Summit region of the ice sheet, a Central Greenland ice sheet must have existed during the last interglacial. This is supported by ice dynamic model stud-
N. R E E H I~T At
ies (Letreguilly et al., this issue) which conclude that only 1.5-2 m of the 6 m higher sea-level stand in EIS 5e was caused by increased melting of the Greenland ice sheet. Conclusions A temperature record for Central Greenland over the past 150 ka has been derived from an ice-margin isotopic record. The record was dated by first interpreting the record in terms of Emiliani isotopic stages and next assigning ages to the transitions between the stages in accordance with the time scale established for the Vostok deep ice core, East Antarctica. The time scale for the Central Greenland 6180 record implies that re-interpretation is necessary for the C a m p Century and Dye3 deep ice core records which are suggested to begin at the EIS 5 e / 5 d transition and 5 c / 5 b transition, respectively, and not in EIS 5e as suggested by Dansgaard et al., (1982). The new climate record shows that temperatures in Greenland were at least as warm as at present not only in EIS 5e (the Eemian), but also in EIS 5c and 5a. This is in contrast to the ice-core climatic record from Vostok, East Antarctica which shows that a long period with a climate colder than at present persisted from the termination of the Eemian to the beginning of the Holocene. Our study therefore indicates that climate behaved differently in the northern and southern hemispheres during the last glacial/interglacial cycle, with major differences occurring in mid and late EIS 5. Another important result of the ice-margin isotopic study is that the Summit region of the Central Greenland ice sheet seems to have survived the warm climate in the previous interglacial. This means that the ongoing deep-core drilling programs G R I P and GISP2 in this region are likely to provide ice-core records reaching back through the previous interglacial. The records may even be longer than our Central Greenland ice-margin record which may have been truncated as a result of basal melting at or near the ice margin. Basal melting is less likely to have taken place in the central region of the ice sheet.
ICE-AGE OXYGEN-18 RECORD FROM ICE-SHEETMARGIN IN CENTRALWESTGREENLAND
References Beer, J., Siegenthaler, U., Bonani, G., Finkel, R.C., Oeschger, H., Suiter, M. and Wolfli, W., 1988. Information on past solaractivity and geomagnetisme from 10Be in the Camp Century ice core. Nature, 331: 675-679. Budd, W.F. and Young, N.W., 1983. Application of modelling techniques to measured profiles of temperature and isotopes. In: G. de Q. Robin (Editor), The Climatic Record in Polar Ice Sheets. Cambridge Univ. Press, Cambridge, pp. 150-179. Chappel, J. and Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature, 324: 137-140. Dansgaard, W., Johnsen, S.J., Clausen, H.B. and Langway Jr., C.C., 1971. Climatic record revealed by the Camp Century ice core. In: K.K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale Univ. Press, New Haven, CN, pp. 37-56. Dansgaard, W., Johnsen, S.J., Clausen, H.B. and Gundestrup, N., 1973. Stable isotope glaciology. Medd. Gronl., 197 (2), 53 pp. Dansgaard, W., Clausen, 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. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N., Hammer, C.U. and Oeschger, H., 1984. North Atlantic climate oscillations revealed by deep Greenland ice cores (Geophys. Monogr., 29). Am. Geophys. Union, Washington, DC, pp. 288-298. Feyling-Hanssen, R.W. and Funder, S., 1990. Fauna and flora. In: S. Funder (Editor), Late Quaternary Stratigraphy and Glaciology in the Thule Area, Northwest Greenland. Medd. Gronl. Geosci., 22: 19-32. Funder, S. (Editor), 1990. Late Quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland. Medd. Gr~nl. Geosci., 22, 63 pp. Funder, S. and Houmark-Nielsen, M., 1990. Local events and regional correlation. In: S. Funder (Editor), Late Quaternary Stratigraphy and Glaciology in the Thule Area, Northwest Greenland. Medd. Gronl. Geosci., 22: 40-45. Guiot, J., Pons, A., Beaulieu, J.L. de and Reille, M., 1989. A 140,000-year continental climate reconstruction from two European pollen records. Nature, 338: 309-313. Herron, S.L. and Langway Jr., C.C., 1987. Derivation of paleoelevations from total air content of two deep Greenland ice cores. IAHS Publ., 170: 283-295. Hooke, R. LeB. and Hudleston. P.J., 1978. Origin of foliation in glaciers. J. Glaciol., 20: 285-299.
383
Jouzel, J., Lorius, C., Petit, J.R., Barkov, N.I., Kotlyakov, V.M. and Petrov, V.M., 1987. Vostok ice core. A continuous isotopic temperature record over the last climatic cycle (160,000 years). Nature, 329: 403-408. Koerner, R.M., 1989. Ice core evidence for extensive melting ofthe Greenland ice sheet in the last interglacial. Science, 244: 964-968. Letrrguilly, A., Reeh, N. and Huybrecbts, P., 1991. The Greenland ice sheet through the last glacial-interglacial cycle. Palaeogeogr., Palaeoclimatol. Palaeoecol.(Global Planet. Change Sect.), in press. Lorius, C. and Merlivat, L., 1977. Distribution of mean surface stable isotope values in East Antarctica. observed changes with depth in the coastal area. IAHS Publ.,ll8: 127-137. Lorius, C., Jouzel, J., Ritz, C., Merlivat, L., Barkov, N.I., Korotkevich, Y.S. and Kotlyakov, V.M., 1985. A 150,000year climatic record from Antarctic ice. Nature, 316: 591596. Raisbeck, G.M., Yiou, F., Bourles, D., Lorius, C., Jouzel, J. and Barkov, N.I., 1987. Evidence for two intervals of enhanced 10Be deposition in Antarctic ice during the last glacial period. Nature, 326: 273-277. Raynaud, D. and Lorius, C., 1973. Climatic implications of total gas content in ice at Camp Century. Nature, 243: 283-384. Reeh, N., 1984. Reconstruction of the Glacial ice covers of Greenland and the Canadian Arctic Islands by three-dimensional perfectly plastic ice-sheet modelling. Ann. Glaciol., 5: 115-121. Reeh, N., Thomsen, H.H. and Clausen, H.B., 1987. The Greenland ice-sheet margin--a mine of ice for paleo-environmental studies. Palaeogeogr., Palaeoclimatol., Palaeoecol., 58: 229-234. Reeh, N., 1990. Past changes in precipitation rate and ice thickness as derived from age-depth profiles from ice sheets; application to Greenland and Canadian Arctic icecore records. In: U. Bleil and J. Thiede (Editors), Geological History of the Polar Oceans. Arctic versus Antarctic. Kluwer, Dordrecht, pp. 255-271. Robin, G. de Q., 1983. Profile data, Greenland Region. In: G. de Q. Robin (Editor), The Climatic Record in Polar Ice Sheets. Cambridge Univ. Press, Cambridge, pp. 98-112. Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea level. Quat. Sci. Rev., 6: 183-190. Thorning, L. and Hansen, E., 1987. Electromagnetic reflection survey 1986 at the Inland Ice margin of the Pakitsoq basin, central West Greenland. Geol. Surv. Greenl. Rep., 135: 87-95.
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Elsevier Science Publishers B.V., Amsterdam
A new, detailed ice-age oxygen-18 record from the ice-sheet margin in central West Greenland Niels Reeh, H a n s Oerter, A n n e Letr6guilly, H e i n z Miller a n d H a n s - W o l f g a n g H u b b e r t e n Alfred-Wegener-lnstitutfiir Polar- und Meeresforschung, Postfach 120161, D-2850 Bremerhaven, Germany (Revised version accepted February 28, 1991)
ABSTRACT 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. Palaeogeogr., Palaeoclimatol., Palaeoecol.(Global Planet. Change Sect.), 90: 373-383. A new detailed oxygen-18 record measured on surface-ice samples from a West Greenland ice-margin location reveals the hitherto longest climatic record from the Greenland ice sheet, spanning the last c. 150,000 years. The new record implies that the Greenland deep ice-core records from Dye3 and Camp Century need to be re-interpreted. A comparison with the deuterium record from the Vostok deep ice core, Antarctica indicates that climate behaved differently in the northern and southern hemispheres during the last glacial/interglacial cycle, with major differences occurring in Emiliani isotopic stage 5.
Introduction
The large ice sheets in Greenland and Antarctica are rich sources of information about climate and environmental changes during the past c. 150,000 years (150 ka) and probably much more, as demonstrated by the results of deep ice-core drilling programmes (e.g. Dansgaard et al., 1982; Lorius et al., 1985; Jouzel et al., 1987). However, the old ice found at depth in the central regions of the ice sheets can also be retrieved from the surface of the ice-sheet margins, where the ice is found in a sequence with the oldest ice nearest to the ice edge (Lorius and Merlivat, 1977; Reeh et al., 1987) Here, we report on a detailed oxygen-18 record measured on surface-ice samples collected from the ice margin at Pakitsoq, Central West Greenland ( 6 9 ° 2 6 ' N , 50°16'W). The record spans a period of at least 150 ka covering the early part of the present interglacial (the Holocene), the last glacial, the last interglacial, and also a part of the previous glacial. The Pre-Holocene part of the record was originally deposited in the Summit 0921-8181/91/$03.50
region of Central Greenland (Fig. 1), and thus represents the climate in this region where deep ice-core drilling projects are yet being undertaken. The record has been translated into a Greenland temperature record covering the past 150 ka on the time scale derived for the deep ice core from Vostok, East Antarctica (Lorius et al., 1985). The derived isotopic temperatures indicate large temperature variations in Emiliani's isotopic stage 5 (EIS 5, approximate age 130-75 ka B.P.), with a climate warmer than at present not only in substage 5e, but also in substage 5c and during a short period of substage 5a. This is in contrast to the temperature record derived from the Vostok deuterium record, which shows colder-than-present temperatures during most of EIS 5 with substage 5e as the only exception. Our study, therefore, suggests that the climate was different in the northern and southern hemispheres during a large period of the last glacial-interglacial cycle. A correlation of the ice-margin oxygen-18 record with the corresponding records from the Greenland deep ice cores from Camp Century (CC) and Dye3 indicates that neither of these
© 1991 - Elsevier Science Publishers B.V. All rights reserved
374
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records reach back to the previous glacial, p r o b ably due to substantial thinning a n d retreat of the ice sheet in N o r t h and South G r e e n l a n d in the w a r m p e r i o d s d u r i n g EIS 5 ( K o e r n e r , 1989; Reeh, 1990; Letr~guilly et al., 1991).
/ 75°N
Flow pattern in an ice sheet
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The b a c k g r o u n d for the ice-margin isotopic study is the following: An ice sheet can be considered a s e d i m e n t a r y d e p o s i t consisting of sequences of layers d e p o s i t e d a n n u a l l y in the a c c u m u l a t i o n zone (i.e. the region of positive mass i n p u t ) as snow accumulation. The snow layers sink into the ice sheet subject to c o n t i n u o u s thinning, initially as a result of densification, by which snow is t r a n s f o r m e d into ice, but then mainly due to flow-induced vertical c o m p r e s s i v e strain. In this process the layers are stretched h o r i z o n t a l l y until they are advected by the ice m o t i o n into the a b l a t i o n zone, where the ice is r e m o v e d b y melting or calving of ice bergs. In areas where a b l a t i o n by melting is p r e d o m i n a n t , the layers move u p w a r d s relative to the surface where they eventually melt away. The ice flow pattern, illustrated in Fig. 2, shows particle p a t h s ill a cross section of an ice sheet. It a p p e a r s from the figure that the higher the ice is originally d e p o s i t e d in the a c c u m u l a t i o n area, the d e e p e r into the ice sheet it will sink, a n d the nearer to the ice-sheet m a r g i n it will r e a p p e a r
70 /
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~65 65---
~60°N
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40
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Fig. 1. The Greenland ice sheet. Sampling location on the ice margin at Pakitsoq, and deep drilling locations at Camp Century, and Dye3, and Summit are indicated. The flow line from Summit to Pakitsoq illustrates that Pre-Holocene ice now found at the ice-sheet surface at Pakitsoq was originally deposited in the Summit area of Central Greenland.
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Fig. 2. Flow in a cross section of an ice sheet.
300
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400
500
ICE-AGE OXYGEN-18
RECORD
FROM ICE-SHEET MARGIN
IN CENTRAL
at the surface in the ablation zone. Consequently the oldest ice is found near the base and along the margin of the ice sheet. In principle, a complete sequence of all the deposited layers can be obtained either by deep core drilling from the surface to the base in the accumulation zone or by surface sampling from the equilibrium line (the line separating the accumulation and ablation zones) to the ice margin. The layers become progressively older with depth, respectively with distance from the equilibrium line.
Surface ice sampling and reproducibility Nearly 1500 surface-ice samples were collected in a profile on the ice margin at Pakitsoq, c. 40 km N E of Jakobshavn, West Greenland (for location see map in Fig. 3). The profile extends 750 m in from the margin, and runs approximately parallel
375
WEST GREENLAND
to the main ice-flow direction. The sampling site is located at 380 m elevation, and is a region of strongly divergent flow (stagnation zone). The ice thickness decreases from about 200 m one kilometre inland to probably less than 10 m at the margin (Thorning and Hansen, 1987). Over the same distance, the ice-flow velocity at the surface decreases from 12 m / y r to nearly zero. The annual ablation rate is 2-2.5 m / y r of ice. At the time of the sampling (August, 1988) which was late in the ablation season, the surface generally consisted of bubbly glacier ice. Cryoconite holes of varying diameter (from a few millimetres to about one metre) and depth (0.1-0.5 m), and surface irregularities with wavelengths and waveheights of typically 2 - 5 m and 0.5 m, respectively are of common occurrence. Blue bands, 1 - 5 0 cm wide, running roughly parallel to the ice margin, often over distances of several hundred metres, are
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Fig. 3. Map of ice-sampling location at Pakitsoq, West Greenland. Inserts show maps of the Disko Bugt region and Greenland.
376
also c o m m o n . They consist of clear coarse-grained ice with 6~SO values several %e higher than those of the s u r r o u n d i n g ice. A n e x p l a n a t i o n of this 6~SO a n o m a l y is not yet known, but, to j u d g e from d e t a i l e d s a m p l i n g across the bands, they d o not seem to break the c o n t i n u i t y of the 6~SO record. A p a r t from the blue b a n d s which cause the large scale linear foliation p a t t e r n seen on the p h o t o graph in Fig. 4, strong foliation is found only in a b a n d between 25 and 137 m from the margin. However, as a p p e a r s from Fig. 4 there is no indication of folding at the ice surface. F r o m an oxygen-I 8 profile s a m p l e d in 1985 at the same location, it was k n o w n that ice of Pleistocene origin constitutes a c. 600 m wide b a n d parallel to the ice margin (Reeh et al., 1987). However, the previous s a m p l i n g at spacings of 2 - 5 m was not detailed enough to convincingly resolve the details of the Pleistocene 6~SO record. In the new profile, s a m p l i n g distances are 0 . 5 1 m, a n d a 170 m section through the Pleistocene
N
REEH lzl AL
Holocene transition has even been s a m p l e d continuously as 20-cm samples. The two 81SO-records are shown in Fig. 5. Due to a small difference in the o r i e n t a t i o n of the profile lines, a n d the fact that the foliation (as indicated by the blue b a n d s ) forms an angle of a p p r o x i m a t e l y 60 ° with the profile lines, there is a small difference of the length scales along the profiles. In o r d e r to facilitate the c o m p a r i s o n , the length of the 1985 profile has a c c o r d i n g l y been reduced by 9% relative to the length of the 1988 profile. W i t h this a d j u s t m e n t , the two records are nearly identical, considering the low resolution and the degree of r a n d o m n e s s i n t r o d u c e d in the 1985 profile due to spot s a m p l i n g at intervals of 2 - 5 m. It thus a p p e a r s that the surface 8~SO-record is r e p r o d u c i b l e (at least within d i s t a n c e s of the o r d e r of 50 m) in spite of the irregular, sometimes rather chaotic, a p p e a r a n c e of the ice surface. Part of the e x p l a n a t i o n is that the nearer to the ice margin a layer is found, the steeper is its dip, so
Fig. 4. Photograph of the sampling location at the ice-margin. The photograph is taken from the southeast. The lake on the righthand side of the photograph is the lake labeled 326 on the map in Fig. 3. The length of the ice margin contact with the mountain slope is approximately 1.5 km. The photograph illustrates the large-scale linear foliation (blue bands) parallel to the margin. The irregular pattern on the ice surface is caused by numerous melt streams. A rather large stream is seen in the foreground to the left.
ICE-AGE OXYGEN-18
RECORD
FROM ICE-SHEET MARGIN
IN C E N T R A L
377
WEST GREENLAND
were filled in plastic bottles which were closed with tight covers. The ice samples eventually melted to fill more than half the bottle, thus limiting isotopic fractionation due to evaporation to insignificant amounts. The samples were transported as water samples from Greenland to the Alfred Wegener Institute, Bremerhaven, West Germany, where they were stored at - 30 ° C until analysis in a Finnigan mass spectrometer with a precision of 0.2%~ 8180.
that the original near-horizontal stratification found in the interior accumulation areas ends up as a near-vertical stratification (foliation) at the ice margin (Hook and Hudleston, 1978). This justifies using the term "horizontal core" for the surface-ice records, since the layers are tilted to nearly 90 o and are sampled progressively, as are the horizontal layers cut by vertical coring in the interior regions of ice sheets. As regards the reproducibility of ice margin records over longer distances, a 3]80 record from an ice margin location c. 5 km away reveals broadly similar variations as the records shown in Fig. 5. The sampling procedure was as follows: The upper 10-20 cm thick weathered surface layer was cut away by means of an ice axe. Then, ice was sampled by means of a chisel along a 20-cm section of the freshly exposed ice. The samples
The oxygen-18 record As can be seen from Fig. 5, the ice-margin oxygen-18 record shows large variations. However, generally the variations display a degree of continuity, the transitions from valleys to peaks and vice versa being supported by several values. There
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Fig. 5. The two 8]80 records from the ice margin at Pakitsoq, 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 isotopic stages is shown in the figure.
378
are exceptions however, in the form of single-point spikes, all showing higher isotopic values than those of the adjacent ice. These spikes can all be explained as containing blue-band ice, which generally has a 81SO value which is several %o (from 1 to 16%o) higher than that of normal glacier ice, see discussion above. In very broad terms, the 1988 ice-margin record is characterized by relatively high 6180 values (between -31%o and -33%o) from 750 m to c. 580 m, and relatively low 81SO values (between -38%o and - 41%~,) from 580 m to the ice margin. Since the 1985 record shows that the high 61~O values continue for at least 250 m farther inland, it was concluded that the marked 8180 shift in the 1988 record at 580 m marks the HolocenePleistocene transition (Reeh et al., 1987). Along several sections of the Pleistocene record, 81SO reaches values similar to those of the Holocene ice. This is the case at 560, 210 and 160 m. High 6~SO values, not quite reaching Holocene levels, however, are found at 520, 350, 270, and 235 m. A tentative interpretation of the Pleistocene ~ S O variations in terms of Emiliani isotopic stages is shown in Fig. 5. The Allerod/Younger Dryas oscillation at the Pleistocene-Holocene transition is also indicated in the figure. According to this interpretation, the record reaches back at least into EIS 6, i.e. into the previous glacial. However, as far as the outermost 137-m section of the record is concerned, one should be cautious about interpreting the details of the record. Along this section, the ice surface displays a strongly foliated structure entirely different from the bumpy irregular ice surface along the rest of the profile. The reason for the change in surface structure and the possible influence on the 8180 profile is not yet understood.
Comparison with ice-core records The deep sections of the oxygen-18 records from Camp Century (CC) and Dye3 show a high degree of correlation (Dansgaard et al., 1982), see Fig. 6. The record from CC seems to reach furthest back in time. It is displayed in Fig. 7 together with the 1988 ice-margin record from Pakitsoq. The " b l u e - b a n d " spikes in the ice-
N. R E E H E 7 At.
170(
-- 1100 DepthI 180(
1200 Last glaciation 190C
1300 2000 m
2
b
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-25 ~'~
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Fig. 6. 8180 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 simultaneous deposition. Modified from Dansgaard et al. (1982).
margin record have been left out, and the 20-cm samples from the continuously sampled section have been averaged over 1-m increments. Correlation of undated time series is certainly a delicate matter, and it must be agreed that there is more than one way of correlating the two 6lSO-records shown in Fig. 7. However, the most plausible correlation of the main trends of the two records is indicated in the figure by arrows pointing at similar features. Since the ice-margin record was not sampled continuously, little importance has been attached to correlate the high frequency oscillations. A consequence of the correlation is that the unit from 13 to 38 m above the bottom in the CC record displaying relatively high 8a80 values must
ICE-AGE
OXYGEN-18
RECORD
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ICE-SHEET
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be referred to EIS 5(a,b,c), as originally suggested by Dansgaard et al. (1971) and not to EIS 5e as later suggested by Dansgaard et al. (1982). If at all present in the CC record, EIS 5e (the Eemian) seems to be represented only by the high 8180 value of -23%0 at the bottom of the core. This interpretation of the CC isotopic record, (which is more or less a return to the original suggestion by Dansgaard et al. (1971)) is supported by correlat-
-,0 1
Pakifsoq
O
o.
379
WEST GREENLAND
ing two distinct peaks in the 1°Be-concentration profile measured along the CC core (Beer et al., 1988) (see Fig. 7) with two similar peaks in the t°Be profile from the well-dated deep ice core drilled at Vostok, East Antarctica (Raisbeck et al., 1987). Raisbeck et al. (1987) concluded that the two distinct peaks at c. 35 ka and 60 k a in the Vostok t°Be concentration record are most likely due to production-rate changes in the atmosphere
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Fig. 7. Comparison of the 8180 records from the Central Greenland ice-margin (a) and the C a m p 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 8180 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 ka and 35 ka of the distinct peaks 75 and 155 m above the bottom are obtained by correlation with similar peaks in the l°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 top and bottom of the figure. The C a m p Century 8180 profile is adapted from Robin (1983). The ]°Be-profile is redrawn from Beer et al. (1988).
380
caused by extraterrestrial disturbances. Therefore, the 35 ka and 60 ka events should be detectable also in Greenland l°Be records. As shown in Fig. 7, there are, in fact, two distinct l°Be-concentralion peaks also in the CC record at distances of c. 75 m and 155 m above the bottom. If ages of 60 ka and 35 ka are assigned to these levels, the 13-38 m unit in the CC record with high 8lSO-values obviously must be of mid-late EIS-5 age. One might argue that the less distinct peak shown in Fig. 7 at c. 187 m above the bottom could be the 35 ka l°Be-peak. However, if this peak and the peak at 155 were taken to represent the 35 and 60 ka events, then the accumulation rate in the C a m p Century area must have been 4-+5 times lower than at present in the period 60-10 ka B.P. as shown by ice-dynamic model calculations (Reeh, 1990). If, on the other hand, the peaks at 155 m and 75 m are used, model calculations indicate that the accumulation rate in the last glacial was reduced by only a factor between 2 and 3 in respect to the present value (Reeh, 1990). This agrees better with the generally 2--3 times higherthan-present I°Be-concentration level during the last glacial, see Fig, 7, and with most other estimates of the accumulation rate in Greenland during this period, see discussion by Reeh (1990). We take this as support for the chronology in terms of isotopic stages shown in Fig. 7.
The ice-age climate The dashed lines in Fig. 7 represent the steadyclimate 81SO-trend lines, i. e. the expected alSO variation in the case that past temperatures had always been like those of the present. The steadyclimate reference lines have been determined by ice-dynamic model calculations, considering changes in past ice-flow velocities caused by changing accumulation rates, but neglecting the effect of ice-thickness changes. The decreasing trends back in time are due to the fact that the older the ice, the farther inland (i.e the higher) was the ice originally deposited (see Fig. 2). The decreasing trends simply reflect the present low 6J80 values at these higher deposition sites. Deviations of the oxygen-18 records from the steady-climate reference curves will have to be
N . R F . E H h I" A I
50
~O0
150
g 5-
2
//~ !o 2;+
mn -10.
E ®
I+ ,,o ~'+
r
50
1O0 Age
+
15C1
iko)
Fig. 8. Comparison of derived isotopic temperature deviations with respect to present temperatures for Central Greenland and Vostok, Antarctica. The Vostok temperature curve is from Jouzel et al. (1987).
explained in terms of either climatic changes or changes in ice-sheet surface elevations. Calculations based on a time-dependent ice-dynamic model using the climate history derived from the ice-margin oxygen-18 record (Fig. 8) as climate forcing, indicate that the changes in the position and elevation of the '+summit" of the Central Greenland ice sheet were moderate during the last glacial-interglacial cycle, even in climatically warm periods when the marginal areas may have suffered considerable thinning and retreat (Letr6guilly et al., 1991). Therefore, changes in ice-sheet surface elevations will only make a minor contribution to the variation of the oxygen-18 record, and will be neglected. The deviations between the measured 8t80 record and the steady-climate reference curve can then be translated into changes in temperature with respect to the present temperature by means of the conversion factor 0.62%0 8 t s O / K established for the Central Greenland ice sheet (Dansgaard et al., 1973). In Fig. 8, the isotopic temperature record is plotted on a timescale determined by assigning ages to the various isotopic stages in accordance with the timescale established
ICE-AGE OXYGEN-18
RECORD
FROM ICE-SHEET MARGIN
IN CENTRAL
for the Vostok ice-core (Lorius et al., 1985). Figure 8 shows that temperatures in Central Greenland were 10-12 K lower than at present during the last glacial maximum (EIS 2), 2 - 3 K lower at the beginning and end of EIS 3, with 6 - 1 0 K lower temperatures in the middle of this stage. EIS 4 is characterized by temperatures between 6 and 10 K lower than at present, whereas temperatures in EIS 5 show large variations from 3-5 K warmer in substages 5c and 5e to 2 - 8 K cooler in substage 5d. The temperature variations in EIS 5 differ substantially from the temperatures deduced by means of the 8(D) record from the deep ice core drilled at Vostok, East Antarctica (Lorius et al., 1985; Jouzel et al., 1987), which is also shown in Fig. 8. The Vostok record suggests that a rather cold climate prevailed during most of EIS 5, with substage 5e as the only period displaying an interglacial-type climate, somewhat warmer than now. On the other hand, the derived Greenland temperatures are in agreement with the climate variations inferred from recent glacial-geological studies in Northwest Greenland (Funder, 1990). These studies conclude that during the Q a m a t interstade (TL-dated to between 114 ka and 69 ka B.P., i.e. EIS 5(a,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 0 interstade (TL-dated to between 136 ka and 120 ka, i.e. EIS 5e), it is concluded that marine conditions were similar to those of the present or warmer (Feyling-Hansen and Funder, 1990). The Q a m a t and Saunders 0 warm episodes were separated by a glacial advance (the Narssarsuk stade) TL-dated to 114 + 1 0 ka B.P. This also agrees with our ice-margin isotopic temperature record, showing a cold climate during EIS 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 EIS 5, marine and glacial events in the Baffin Bay region were in phase. Moreover, a 140 ka climate reconstruction based
WEST GREENLAND
381
on European pollen records indicates warm interglacial-type climates not only in EIS 5e, but also in EIS 5a and 5c when temperatures were similar to present temperatures (Guiot et al., 1989). This evidence supports the climate history derived from our ice-margin isotopic record, and suggests that the climate in parts of the northern hemisphere behaved differently from that of the southern hemipheres during the last glacial/interglacial cycle, with major differences occurring in mid and late EIS 5. An interglacial type climate in the northern hemisphere in EIS 5e, 5c, and 5a would cause the continental ice sheets in North America and Eurasia to disappear or at least retreat significantly, and thus sea level should have been close to the present level in these stages. The H u o n coral-reef sea-level studies indicate 6 m higher, 12 m lower, and 18 m lower sea-level stands in EIS 5e, EIS 5c, and EIS 5a, respectively (Chappel and Shackleton, 1987). Revisions of the continental ice-volume records derived from marine isotopic records (Shackleton, 1987) confirm the high sealevel in EIS 5e, and support the moderate sea-level lowering in EIS 5c and 5a. However, a 12-18 m lower sea level indicates that an amount of ice equivalent to 2 - 3 times the volume of the present Greenland ice sheet was stored on the continents in excess of the present ice sheets and glaciers. Even this rather moderate amount of extra ice seems to be high for a period with an interglacial type climate. So, there seems to be a disagreement between our climate reconstruction based on northern hemisphere continental records, and the marine environmental records for the middle and late EIS 5.
Camp Century and Dye3 If the deviations between the measured 6lSo record and the steady-climate reference curve for C a m p Century (see Fig. 7) are translated into changes in temperature in the same manner as it was done for the Central Greenland record, a climate 16-18 K colder than at present will result for the last glacial maximum. This is about 6 K colder than indicated by the Central Greenland record.
382
Furthermore, a literal translation of the 61SOdeviation in EIS 5c to a temperature deviation suggests 10 K warmer temperature than now i.e. about 3 times warmer than indicated by the Central Greenland record. The relatively large shift of 6~SO in the C a m p Century isotopic record at the termination of the last glacial has been a matter of discussion (Raynaud and Lorius, 1973; Budd and Young, 1983; Reeh, 1984; Dansgaard et al., 1984). Since an explanation in terms of large surface elevation changes seems unlikely (Reeh, 1984; Herron and Langway, 1987) we are left with the explanation that either the temperature shift was anomalously high in Northwest Greenland, or the source area for the vapour forming the precipitation changed dramatically at the end of the last glacial (Dansgaard et al., 1984). However, even by using a similar explanation, it is hard to refer more than half of the 6%o 8180 deviation in EIS 5c to climate change. The other half is most likely due to a substantial lowering of the ice surface caused by thinning and retreat of the Northwest Greenland ice sheet in the warm EIS 5c stage. During the slightly warmer EIS 5e sub-stage, the retreat of the Northwest Greenland ice sheet was probably even larger, and the Camp Century area might have been ice free (Koerner, 1989; Reeh, 1990; Letr~guilly et al., 1991). This explains why the CC ice core record seems to begin at the transition to EIS 5d (see Fig. 7). Transferring the Camp Century time scale suggested in this work (Fig. 7) to the Dye3 record by means of the correlation illustrated in Fig. 6, results in the suggestion that the Dye3 record begins at the transition between EIS 5c and EIS 5b. This also indicates that the South Greenland ice sheet may have been subject to substantial disintegration in the warm EIS 5c, and therefore, probably also in sub-stage 5e (Koerner, 1989: Reeh, 1990; Letr~guilly et al., 1991). A total disappearance of the Greenland ice sheet in EIS 5e (the Eemian) is contradicted by the presence of ice referred to EIS 6 (the previous glacial) in the ice-margin isotopic profile. Since this ice was originally deposited in the Summit region of the ice sheet, a Central Greenland ice sheet must have existed during the last interglacial. This is supported by ice dynamic model stud-
N. R E E H I~T At
ies (Letreguilly et al., this issue) which conclude that only 1.5-2 m of the 6 m higher sea-level stand in EIS 5e was caused by increased melting of the Greenland ice sheet. Conclusions A temperature record for Central Greenland over the past 150 ka has been derived from an ice-margin isotopic record. The record was dated by first interpreting the record in terms of Emiliani isotopic stages and next assigning ages to the transitions between the stages in accordance with the time scale established for the Vostok deep ice core, East Antarctica. The time scale for the Central Greenland 6180 record implies that re-interpretation is necessary for the C a m p Century and Dye3 deep ice core records which are suggested to begin at the EIS 5 e / 5 d transition and 5 c / 5 b transition, respectively, and not in EIS 5e as suggested by Dansgaard et al., (1982). The new climate record shows that temperatures in Greenland were at least as warm as at present not only in EIS 5e (the Eemian), but also in EIS 5c and 5a. This is in contrast to the ice-core climatic record from Vostok, East Antarctica which shows that a long period with a climate colder than at present persisted from the termination of the Eemian to the beginning of the Holocene. Our study therefore indicates that climate behaved differently in the northern and southern hemispheres during the last glacial/interglacial cycle, with major differences occurring in mid and late EIS 5. Another important result of the ice-margin isotopic study is that the Summit region of the Central Greenland ice sheet seems to have survived the warm climate in the previous interglacial. This means that the ongoing deep-core drilling programs G R I P and GISP2 in this region are likely to provide ice-core records reaching back through the previous interglacial. The records may even be longer than our Central Greenland ice-margin record which may have been truncated as a result of basal melting at or near the ice margin. Basal melting is less likely to have taken place in the central region of the ice sheet.
ICE-AGE OXYGEN-18 RECORD FROM ICE-SHEETMARGIN IN CENTRALWESTGREENLAND
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