Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record

Climate Dynamics (1992) 6:169-177

Holocene change

Climate Uynumiu

© Springer-Verlag 1992

Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record* P Ciais I , JR Petit 2' 1, j Jouzel 1,2, C Lorius 2, NI Barkov 3, V Lipenkov 4, and V Nicola'iev 4

1 Laboratoire de G~ochimie Isotopique, CEA/CEN Saclay, DSM, F-91 191 Gif sur Yvette cedex, France 2 Laboratoire de Glaciologie et de G~ophysique de l'Environnement, BP 96, F-38402 Saint Martin d'H~res cedex, France 3 Arctic and Antarctic Research Institute, Beringa Street 38, Leningrad 199226, USSR 4 Institute of Geography, Academy of Science of the USSR, Staromonetny 29, Moscow 109107, USSR

Received August 26, Accepted September 10, 1991

Abstract. In the interpretation of the Antarctic deep ice-core data, little attention has been given to the Ho- locene part of the records. As far as translation of the stable isotope content in terms of temperature is concerned, this can be understood because expected temperature changes may be obscured by isotopic noise of various origins and because no 14C dating has yet been available for this type of sequence. In this article, we focus on the Dome C and Vostok cores and on a new 850-m long ice core drilled out at Komsomolska'ia by the Soviet Antarctic Expeditions. These three sites are located in East Antarctica, on the Antarctic plateau, in a region essentially undisturbed by ice-flow conditions, so that their detailed intercomparison may allow us to identify the climatically significant isotopic signal. Our results compare well with the proximal records of Southern Hemisphere high latitudes and support the existence of a warmer "climatic opt imum" between 10 and 6 k a y BP. Maximum temperatures are reached just at the end of the last deglaciation, which confirms pre- vious observations at high latitudes, in contrast with later dates for the Atlantic and hypsithermal optima in Europe and North America.

Introduction

The period since the last deglaciation has been characterized by relatively stable climatic conditions with fluctuations in the global average temperature that never exceeded 2 ° C. It is not clear, however, whether those fluctuations were global or not. Central and west- ern Europe, China, eastern North America and Japan have experienced higher temperatures (at least in summer) during the mid-Holocene approximatively around

* Contribution to Clima Locarno - Past and Present Climate Dy- namics; Conference September 1990, Swiss Academy of Sciences - National Climate Program

Offprint requests to. P Ciais

6 ka BP (Huntley and Prentice 1988; Webb 1985; Yo- shino and Uruchibara 1978; Zhang and Wang, submitted). The curve presented in Fig. 1 displays the general trends in global temperature over the last 10 4 years. Re- versely, if we consider high latitudes in the Canadian Arctic, as well as in southern Patagonia, Tasmania and New Zealand, there is no convincing evidence for an Holocene thermal optimum centered around 5-6 k a y BP (Macphail 1979; Markgraf 1989; Newnham, personal communication; Ritchie et al. 1983), but instead during the early Holocene simultaneously with the maximum summer solar radiation in the Northern Hemisphere. Lastly, there is no very clear indication of a climatic optimum in the two Greenland deep ice cores (Camp Century and Dye 3), but this may be due to the change in the origin of the ice owing to the fact that these two cores are not drilled on a summit (S. Johnsen, personal communication).

Antarctic ice cores offer us the opportunity to study Holocene climatic variations on the basis of continuous, high-resolution records. The isotopic content of ice layers (deuterium or 180) allows access to important climatic parameters such as temperature and, more in- directly, precipitation rates or moisture conditions in the water vapor source regions (Dansgaard 1964; Jou- zel et al. 1982, 1989a, b; Petit et al. 1991). Nevertheless, because of various sources of noise in the isotopic sig-

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Holocene maximum Little ice age

i i 1 I I I 1 I 10,000 8,000 6 ,000 4 ,000 2 ,000 0

Years before present

Fig. 1. Global average temperature variations during the Holo- cene (Folland et al. 1990)

170 Ciais et al.: Evidence for an early

nals (Fisher et al. 1985; Robin 1983), a detailed study of the Holocene period must be undertaken cautiously. In this paper we present and discuss principally deep ice- core data from central East Antarctica (Fig. 2) for

~o 6o~/

-90w 9OK"

Fig. 2. Map of Antarctica showing location of the drilling sites

Holocene climatic optimum in the Antarctic deep ice-core record

which, at least over the last 10 ka y, neither ice flow conditions nor changes in the ice thickness are likely to bias the structure of the climatic records. On the other hand, the very low accumulation rates in this region (a few centimeters per year) has the effect of enhancing the isotopic noise, such as that linked with the relative distribution of winter and summer snow, and prevents dating by counting well-defined year-to-year variations.

We will therefore need to consider the translation of isotopic contents in terms of temperature and the choice of an adequate chronology for our records; the approach proposed here is based on glaciological models, which are unfortunately highly dependent on the poorly known local accumulation data. The aim of the discussion itself is to provide a coherent picture of the Antarctic Holocene climate, to compare our results with those available from pollen palaeodata in the nearest terrestrial areas, and to discuss them in a more global perspective.

The isotopic profiles

Figure 3 shows the deuterium data with respect to depth for the three East Antarctic sites discussed in this

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Fig. 3. Isotopic profiles versus depth of the four Antarctic deep ice cores. Deuterium data are reported for Vostok, Dome C, and Kom- somolskaia. 1sO data from Johnsen et al. (1972) are represented for Byrd ice core. (Isotopic ratios expressed vs SMOW)

Ciais et al.: Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record 171

article long with the oxygen 18 profile measured along the Byrd ice core in West Antarctica (Johnsen et al. 1972) for the period covering the Holocene and part of the last deglaciation.

The 905-m-long ice core obtained at Dome C, an ice divide region (elevation: 3240 m; annual temperature; - 53.5 ° C and mean accumulation: 3.4 g cm -2 y-a) has previously been studied both concerning the full record (Jouzel et al. 1982; Lorius et al. 1979) and over the last two millenium (Benoist et al. 1982). The excellent core recovery has made possible isotopic measurements on a quasi-continuous basis. The Holocene period covers the top 400 m.

At Vostok station, located at 72 ° 28'S, 106 ° 48'E, the present mean temperature is -55.5° C with a snow accumulation rate of about 2.3 g cm -2 y-1. A drilling program initiated two decades ago permitted a depth of 2546 m to be reached in 1989. The climatic record for the top 2083 m, which spanned a whole climatic cycle over the last 160000 years, has been presented elsewhere (Lorius et al. 1985, 1987, 1989a, b). Four cores cover at least the Holocene period, corresponding to depths from the surface to 300 m. We will now focus our attention on the deuterium data from core 3 F, only available below 138 m, with some 8 m of ice missing during the transition, and on the new adjacent core 4F. The latter has been sampled continuously on a 1-m basis thin strip parallel to the core, from the surface down to 281 m, and every 50 cm in the interval 281-400, corresponding to the last deglaciation. The profile shown in Fig. 3 results from a combination of those two data sets. An intercomparison between those records is proposed (see section 'isotopic temperature noise') in order to estimate the noise in the temperature-isotope relationship that affects our sequences at a given site.

The new deep ice core presented here has been ex- tracted using the thermal drilling technique by the So- viet Antarctic Expeditions at the summer camp of Komsomolskaia in 1983, and it extends to 850 m. The site location is 74 ° 05'S, 97 ° 29'E with an annual temperature of - 52.6 ° C and an average accumulation rate at least twice higher than at Vostok station (about 5 g cm -2 y-1 according to Young et al. 1982). We car- ried out deuterium measurements in 2-m-long sections, selected every 5 m along the core. Ice is missing between 77 and 120 m and the resolution is reduced to 1 sample every 25 m below 725 m. Note that an 180 profile was obtained in parallel at the Institute of Geogra- phy in Moscow (Nikolaiev et al. 1988); a detailed intercomparison of the two isotopic profiles (riD and fi180) and of the variation in deuterium-excess (d = r D - r 1 8 0 ) will be done elsewhere.

l e e c o r e c h r o n o l o g y

Although various methods may be used to provide dating for ice cores, no absolute dating is available in the case of our records. Owing to the very low accumulation values in high elevation regions of the ice sheet, both the removal process at the surface and the diffu-

sion of isotopic species in firn may smooth out the seasonal variations of the isotopic signal (Johnson 1977, 1983; Whillans and Grootes 1985). The method that is employed here is thus based on specific ice flow models. In central East Antarctica where the three cores ori- ginate, the ice sheet velocities do not exceed a few meters per year (Budd et al. 1971), and the Holocene period covers depths sufficiently distant from the bedrock so that the sequences are quasiundisturbed by ice-flow conditions. We will first recall here how the Dome C and Vostok time scales were established.

For the Dome C core, located in an ice dome region, a simple unidimensionnal vertical thinning model assuming constant vertical strain rate was used (Lorius et al. 1979). An attempt was made to account for the fact that accumulation was lower during the glacial than during the Holocene period because the amount of precipitation over the Antarctic plateau is essentially con- trolled by the mass of water vapor available in the inversion layer and thus by the temperature of this layer (Robin 1977). Lorius et al. (1979) simply assumed that the accumulation during the glacial part of the record was 75% of that during the Holocene, with a linear increase toward the Holocene value during the climatic transition. This variable accumulation approach was further refined by Benoist (1986) in assuming that the accumulation rate is directly proportional to the deriva- tive of the saturation vapour pressure is formulated by Eq. (1.1), in which a (z) is the accumulation (z being the depth), a (0) its present value and Ty the temperature of formation of the precipitation.

a (z) = a (0) (tiPs~fiT)To(z) (1.1) ( rPs / rT )Ts (o )

This improvement had the noticeable result of ex- tending the Dome C chronology at the bottom of the core from 32 to 39 kay BP (Benoist 1986; Jouzel et al. 1989a, b). For the upper part of the record we are dis- cussing here, the difference between the two chronologies never exceeds a few hundreds of years. For exam- ple, for z=450 m, the ages are respectively 12.1 kay BP (Lorius et al. 1979) and 12.5 kay BP (Benoist 1986).

The chronology of' Vostok is based on a two-dimensional steady-state glaciological model, in which the known bedrock and surface elevations along the flow line are introduced (Lorius et al. 1985; see Ritz, to be published, for a full description). In this dating method, the saturation vapor pressure approach was used based on the discontinuous oxygen 18 profile then available. This dating was further slightly modified using the continuous deuterium profile (Jouzel et al. 1989) with no noticeable change in the Holocene part of the record. Remarkably, these Dome C and Vostok datings with common use of the saturation vapor pressure approach put the a°Be peaks discovered at Vostok around 600 m and that at Dome C around 820 m (Raisbeck et al. 1985) practically in phase, so that those datings with a further minor adjustment (3% for Dome C) were adopted by Jouzel et al. (1989).

We have reported in Fig. 4 the upper part of the Dome C and Vostok smoothed deuterium records

172 Ciais et al.: Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record

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Fig. 5. H o l o c e n e c l imat ic ice cores records o f D o m e C, Vostok, a n d Komsomolska ' / a . T e m p e r a t u r e scales are exp re s sed as a dis- t ance to m e a n va lues at each site over the las t 5000 y. (Vos tok translated 1.5°C and Komsomolskaia of 3 ° C.) Byrd: right scale in ~sO x 7.9 for the sake of comparison with &D. The upper figure shows the average temperature curve obtained from the three East Antarctic ice cores

-500

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Fig. 6. I so top ic prof i les ve r sus dep th f rom ad jacen t cores 3 F a n d 4F at vostok station. The common part of the two bore holes un- der study extends between 138 and 280 m. (Core 4Fvertical scale translated 23%0)

(down to 15 k a y BP) using these time scales. These two isotopic profiles show a typical cool oscillation that occurred during the second half of the last climatic transition, starting apparently 1 ka y earlier at Dome C than at Vostok. Jouzel et al. (1991, in press) fully discussed this part of the Antarctic records and the possible link between this cold revearsal and the well-documented Northern Hemisphere Younger Dryas event. We give here only their conclusions as far as dating is concerned. First, the non-synchronousness between the Dome C and Vostok cold revearsal is not real but due to dating uncertainties. This is supported by the dust records and may indeed be accounted for by uncertainties in the assumptions used for estimating the accumulation rate. As noted in Jouzel et al. (1989a, b), the accumulation at Dome C is about 10% less from 1815 to the present than over the last 30 years. Note that com- pared to that, the uncertainty about the accumulation associated with the glaciological models themselves is quite negligible. Secondly, there is no firm argument that would permit us to know accurately the absolute age of this Antarctic cold reversal.

In view of these results, we have chosen to put the Dome C and Vostok cold reversals in phase by simply decreasing the Vostok modern accumulation rate by 12% and keeping the Dome C chronology unchanged (see Fig. 5). This choice is quite arbitrary but is provi- sionally adopted because it has the advantage of put- ting the beginning of the cold reversal in phase with that seen in the 1sO record of the Byrd ice core in West Antarctica (also reported in Fig. 5) and dated indepen- dently by H. B. Clausen and C. U. Hammer (personal communication, 1988). Still, we have to keep in mind that in terms of absolute chronology, the accuracy is probably no better than ~ 10% (i.e., 1 k a y at the beginning of the Holocene) both for the Dome C and Vostok c o r e s .

F o r Komsomolska~a, the glaciological model applied to Dome C may be used in first approximation, but we face the problem of having a very poor estimate

Ciais et al. : Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record 173

of the modern accumulation, the representativeness of available data, 4.6 to 5.3 g cm -2 y-a, being limited to 4 years of accumulation (Young et al. 1982). Using 5 g cm -2 y - I as modern accumulation leads to an age of 18 ka y BP at the bottom of the core, which is quite unrealistic given that the isotopic profile indicates that this bottom ice was deposited during the climatic transition. Indeed, this profile suggests that the ice between 700 m and 765 m corresponds to the cold reversal, and we have assumed that this event is in phase with that recorded at Vostok and Dome C to obtain the time scale of Fig. 5. This corresponds to take a value of 6.6 g cm -2 y-1 for the present accumulation rate.

Temperature reconstruction

The ~ value-temperature relationship

Isotopic fractionation, whose effect is to impoverish the vapor relative to the condensed phases, occurs at each phase transition during the atmospheric cycle of the three molecules, H20, HDO and H2aao (in the follow- ing sections, their isotopic ratios will be expressed in the 6 scale versus the SMOW standard). This phenome- non is due to the slight differences in their saturation vapor pressures and molecular diffusions, the latter leading to an additional kinetic fractionation. Because the relative importance of this last effect is weaker for deuterium than for oxygen, 6D will be preferred to 8lsO for estimating temperatures values. The expected consequence of successive fractionations is a general decrease of the isotopic ratios with the inversion layer temperature: Ty, which corresponds to the formation of precipitations over the ice sheet. In the real world, Tf is linearly related to the surface snow temperature, T~, and modern polar precipitations isotopic ratios obey ef- fectively well the linear relationship:

6=aTs+b (2.1)

Both temperature gradient a and intercept b have been defined by observations in central East Antarctica where climatological conditions are relatively simple and used to reconstruct temperatures quantitatively (Lorius and Merlivat 1977). Our confidence in this type of reconstruction is strenghtened by the fact that the measured gradients (6%0 for 6D and 0.75%0 for 6asO) agree correctly with those inferred from one-dimensional models, including a physically detailed description of fractionation during the history of moist air masses (Jouzel and Merlivat 1984; Merlivat and Jouzel 1979; Petit et al. 1991). However, a straightforward translation of our isotopic profiles in terms of palaeo- temperatures must be made with caution because: 1. Ice-flow dynamics, implying an upslope origin of depth layers or the occurrence of possible changes in ice thickness, may systematically bias the structure of the isotopic profiles. 2. There is no strong climatic argument for using the modern relationship (2.1) in order to interpret temporal variations of the isotope content as past temperature changes.

In the present study concerning the Holocene parts (i.e., the few first hundred meters) of cores located in the high East Antarctic plateau, point 1 may not be taken into account. Note that the situation may be sligthly different for Byrd and the generally increasing trend in the isotopic content during the Holocene (Fig. 3) may be linked with a concurrent decrease of the ice- cap thickness over this part of west Antarctica. More- over, incorporating stable water isotopes into atmospheric general circulation models suggests that the isotope-temperature gradient may be conservative for climates as different as full glacial conditions (Joussaume 1987; Joussaume and Jouzel, submitted). Nevertheless, the intercept b has to be corrected for past changes in the isotopic composition of sea water. For this purpose, we used the marine 6180 series obtained by Labeyrie et al. (1987), multiplied by 8 for the sake of comparison with 6Ds.

Isotopic temperature noise

At Vostok station, the recovery of the two adjacent cores 3F and 4 F gives us the possibility of evaluating the 'noise' on the isotopic series, which is defined by the uncorrelated part of the two signals. In central high-elevation regions of Antarctica, the climatic conditions governing the precipitation process are relatively stable, and no surface melting is likely to occur throughout the year. 'The predominant source of isotopic noise can therefore be attributed to the irregulari- ties of the snow deposition, a process which is magnif- ied by the very low accumulation rates encountered in the region. The method proposed by Fisher et al. (1985) consists of separating each Si sequence between a common part and random noise, so that:

5i(t) =5i c(t)+N~(t)

In the case of two adjacent holes, where the snow deposition conditions are equivalent, it appears reasonable to make the hypothesis that the variance of noise is unchanged, which reads: Var (Ni (t)) = Var (Nj (t)). One can hereafter show that the variance of the noise is given by formula (3.1), where r~j designs the correlation coefficient between the two series.

Var (N (t)) = (1 - rij) × Var (6 (t)) (3.1)

If we use (3.1) for the Holocene period (0-260 m), with the original data of 3 F and 4F, for which one deuterium measurement represents an approximately 50 years on average, the correlation coefficient is 0.72. The resulting noise variance represents 20%o. On the other hand, when we consider 3F and 4F smoothed profiles, in which the high-frequency component has been par- tially removed so that only variations on time scales longer than 0.5 kay remain, it is also possible to calcu- late the variance of the decorrelated parts of the two signals. This represents the true 'noise' that should be taken into consideration when we present and discuss such isotopic curves. We applied the method described above to 3 F and 4F smoothed profiles using the same


spline function presented in Fig. 5. We focused our attention on the common parts of the two bore holes, according to the fact that 4 F is quasi-continuous over the Holocene, while the 3 F available data start from 138 m and present some gaps. The correlation coefficient is 0.95 and we obtain the noise variance for the smoothed data (designed by a prime symbol):

(a~vs) 2 = 3.2%0 (3.2)

In the empirical relationship (2.1), 6 = a T+ b, the intercept b can be expressed by b = b ' + 6 °, where 6 ° represents the eventual deviation from the present days SMOW standard of the oceanic moisture source ratio. The variance of the random part of the 60 record in 180 is mainly due to sampling and measurement errors (La- beyrie, personal communication) and may be estimated as

(o-~o) 2 --- 0.1%0 (3.3)

For D /H ratio, this is equivalent to a value of 0.8%o. This latter 6 ° noise being independent of the 6 noise on our records, we can therefore evaluate from (2.1), the variance of the random part of our reconstructed temperature curve called aNT:

a~8 + credo (3.4) O'2T ~ a2

At Vostok station, using (3.2) and (3.3), we find aNT = 0-32 ° C. This value can be understood as the minimum temperature variation significant from a climatic point of view, when using curves smoothed exactly like those of Figs. 3, 4 and 6 in this work. It should be pointed out that this estimation concerns the site with the lowest accumulation rates or, in other words, the strongest deposition noise. It then appears reasonable to believe that the value of 0.32 ° C constitutes an upper limit of the noise variance in the case of Dome C and Komsomolskaia smoothed temperature profiles presented hereafter.

Holocene climate

Average temperature trends

Temperature series derived from isotopic profiles of Dome C, Vostok and Komsomolskaia ice cores are presented in Fig. 5. The three curves have been smoothed in order to avoid the high-frequency component of the records. The surface-temperature variations are given with respect to the modern values, defined as averages over the past 5 kay. For the sake of comparison, the West Antarctic Byrd ice-core oxygen profile is also shown on the same figure. During at least the first half of the Holocene, the three temperature sequences dis- play similar trends and, despite originating from a distinct geographical area where the glaciological regime is very different, the Byrd profile also presents some common characteristics, which probably reflect the same climatic events. Owing to the fact that the Byrd core originates from a region with a different glaciolog-

ical regime, however, we prefer to focus the discussion on the three East Antarctic cores. For the sake of this discussion, we have derived a mean surface temperature profile from the three East Antarctic records reported in the upper part of Fig. 5.

The average temperature exhibits a rapid and important positive shift from cool to warm conditions during the interval 12.5-11 kay BP. This warming represents approximately 3.5 ° C, which is about one-third of the total deglaciation amplitude. The surface temperatures registered during the 9-11 kay BP interval, I°C above present days values, are the warmest in the whole Holo- cene, although a short cool oscillation occurred around 10.5 kay BP. After 10 ka y BP, a progressive general temperature decrease occurred, marked by less defined oscillations and lasted up to 6-7 kay BP. During that interval, minimum temperatures are respectively encountered around 8 kay BP at Dome C (absolute Holo- cene minimum) and Komsomolskaia, and around 7 kay BP at Vostok. The second half of the Holocene, since 6 kay BP, reveals a succession of warmer spikes of weaker amplitude, lasting about 1 kay. The events that do not appear to be clearly correlated and lead to nearly unchanged average temperature, are discussed elsewhere in the framework of late Holocene climate, together with better resolution results obtained from coastal sites.

Comparison with Southern Hemisphere records

As far as comparison with Southern Hemisphere paleoclimatic data is concerned, we will briefly review here the general trends of the Holocene, limiting ourselves to regions in proximal relationship with the Antarctic continent. According to Markgraf (1989), pollen data from South America appear strongly marked by latitu- dinal differences and may exhibit different and even opposite vegetational history (Heusser 1983). However, if we only consider sequences obtained from Tierra del Fuego (51 to 54 ° S), the semi-quantitative patterns of temperature trends, based on pollinic diagrams, indi- cate warmer climates for the early Holocene, between 10 k a y BP and 7 ka y BP. A colder and dryer interval replaced those conditions by 6 kay BP, while since 5 ka y BP, no significant changes from modern environmental conditions have occurred (Markgraf 1983). In the vicinity of Tierra del Fuego, the hydroclimatic conditions of lake Cardiel in Chile recently studied by Stine (1990) also suggest a wetter period (and probably warmer according to the authors), lasting from 10 kay BP to 8 k a y BP, in fair agreement with our temperature curve.

In southern Tasmania (42 ° S-43 ° 30'E), Macphail (1979) indicates that rapid warming has occurred since 11.5 kay BP, progressively succeeded by minor in- creases in temperature and precipitation, and con- cludes that a warmer than present episode extended from 8 kay BP to around 5 kay BP. However, it should be pointed out that different authors seem to favor earlier dates (Goede and Hitchman 1983). As regards New

Ciais et al.: Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record 175

Zealand, a review proposed by Salinger (1983) shows off a reduction of the westerly circulation between 18 kay BP and 9 ka y BP, accompanied by milder and more equable climates around 9 kay BP. This latter point appears corroborated by three pollen series located in Northern Iceland and recently studied by Newnham (to be published) using principal component analysis to reconstruct temperature. By 10 kay BP, for instance, the area was experiencing I°C warmer temperatures than present.

Turning now to the regions closer to Antarctica, in Kerguelen subantarctic islands (49 ° S, 69 ° E), Young and Schofield (1973) say that a warming trend began by about 12 ka y BP and culminated by 10 ka y BP, associated with maximum glacial retreat. However, they speak in favor of a wanner optimum in the area extend- ing around 8 kay BP, characterized by temperatures considerably warmer than the present, only followed by a return to cooler conditions after 5 ka y BP. Glacier fluctuations and paleoecological changes in south Georgia (54 ° S, 37 ° E) are found to be globally in agreement with Antarctic data on the Holocene, but the maximum warmth seemed instead to have occurred at 6 kay BP (Clapperton et al. 1989). In the Antarctic peninsula, geomorphological evidence accounted for a glacial retreat 8 ka y ago (Sugden and Clapperton 1980). Few data allow us to document precisely the temperature changes of subantarctic surface waters during the Holocene, but core RC 11-120 (41 ° 31'S, 79 ° 52'E), sit- uated between the subtropical convergence and the Antarctic polar front, indicates maximum temperatures around 10-9 kay BP (Hays et al. 1976), a feature also observed in two cores located in the Indian sector of the southern ocean: MD 73-025 and MD 84-527 (43 ° 49'S, 51 ° 19'E) (Labracherie et al. 1989).

One may see in Fig. 5 that the Antarctic temperature trends agree well with palaeoclimatic evidence from the surrounding regions. As previously mentioned in the case of long-term climatic changes (Jouzel et al. 1989a, b), this suggests large spatial representativeness of Ant- arctic ice cores climatic archives. Despite the discre- pancies between the chronologies, as well as the influence of regional conditions, this rapid overview strongly suggests the existence of a well-defined thermal maximum in Antarctica during the first millenia of the Holocene, lasting from 11.5 ka y BP to 9 k ay BP and substantially preceding the Northern Hemisphere hypsithermal period centered around 6 kay BP. It should be stressed that with respect to our ice-core common chronology, the beginning of the Holocene marked by a maximum, after which the temperatures remain close to the present values, is positioned at 11 kay BP.

Discussion and comparison with model predictions

If we try to place the Antarctic early Holocene warming, confirmed by high southern latitudes records into a more general context, one may clearly see that the temperature rise associated with deglaciation is considera-

bly delayed in the continental areas in the Northern Hemisphere, owing to the presence of the extensive re- maining ice sheets. The Fennoscandian ice sheet, for instance, disappeared around 7 kay BP, while in the Northwest United States, the Laurentide ice sheet re- mained until 6 kay BP (Lamb 1977). This is probably the reason why maximum Holocene warming is reached at about 6 kay BP, corresponding to the Hyp- sithermal in America and Atlantic period in Europe (Flint 1965; Lamb 1977; Huntley and Prentice 1988). In contrast, recent data concerning the high northern latitudes (Queen Elizabeth islands and Canadian Arctic, as well as northwestern Canada) speak in favor of the warmest conditions, at least in summer,during the beginning of the Holocene (Bradley 1990; Koerner and Fisher 1990; Ritchie et al. 1983). In tropical regions, the situation is less clearly defined, as possible temperature changes fall within the limits of paleoclimatic data (Crowley 1990).

Palaeoclimatic records of high latitudes in both hemispheres do not confirm the existence of a wide- spread mid-Holocene 'climatic optimum', which suggests that the general picture of the Holocene proposed in Fig. 1 should therefore be considered cautiously and apparently better reflects European and North Ameri- can conditions.

The apparently similar behavior between the high latitudes in the south and north may illustrate the fact that the southernmost areas have responded synchron- ously with maximum summer solar radiation in the Northern Hemisphere. Short and Mengel (1986), and more recently Short (1990), approached this problem, using a two dimensional energy balance model and pre- dicted no phase lag between the occurrence of the warmest Antarctic summer temperatures and the Northern maximum solar radiation. Of the plausible explana- tions, one may keep in mind that the continental re- sponse of the Boreal Hemisphere plays an important role in determining the main component of global climatic changes. In addition, in contrast to the preceding forcing (20 kay period), during low-excentricity peri- ods, the obliquity forcing (41 kay period) remains in phase between the two hemispheres and may also account for the synchronism observed (Crowley 1990). It is therefore interesting to note that the EBM temperature shift, due only to orbital insolation forcing between 9-10 ka y BP and the present (+0.3 ° C), is con- sistent with the Antarctic data of the present work. On the other hand, orbital forcing variations cannot account for the occurrence of minimum temperatures observed at 6-7 kay BP in our records.

Comparison of the Antarctic Holocene climate with predictions of more sophisticated models like GCMs should bring more detailed information (Kutzbach and Gallimore 1988; Kutzbach and Guetter 1986; Mitchell 1990; Mitchell et al. 1988). At 9 kay BP, for instance, Kutzbach and Guetter (1986), although predicting a global increase over land in the Southern Hemisphere (+ 0.8°C above present), point out 2.5°C cooler conditions over Antarctica in January and July.

Ice cores have demonstrated their potential for pro-


viding access to trace gases, in particular CO2, t rapped in air bubbles in the ice. The continuous CO2 record of Vostok station (Barnola et al. 1987) has clearly shown the close similarity between CO2 levels and long-term climatic variations. In the Holocene, it is not easy to tell whether CO2 is also linked with shorter tempera ture variations that do not exceed one-tenth of the deglaciation ampli tude, because very few accurate data are available. At Byrd station, Neftel et al. (1988) recorded a CO2 minimum, dated at 8 k a y BP, and preceded by higher early Holocene levels. At Vostok, the Holocene is not sufficiently documented, so that the two values 10 p p m v beyond early Holocene levels observed at 6.8 and 7.5 ka y BP, in phase with a cooler interval (see Fig. 5), may not allow us to draw precise conclusions. It should be noted, however, that CH4 data also displayed a marked negative shift at that t ime (Chappelaz et al. 1990). Although an apparent correlation of CO2 and CH4 concentrat ions and temperature changes must yet be conf i rmed during the Holocene, it should constitute an interesting element for understanding the CO2 cycle behavior in the face of short-durat ion climatic shifts.

Conclusions

The present study concerns a period during which climatic f luctuations are far weaker and never exceed - 2 ° C. The ' isotopic thermometer ' allows us to detect such variations, so that reconstructing past tempera ture changes on the basis o f three distinct isotopic profiles offers us the possibility of picturing the climatic conditions prevail ing over Antarctica. As previously outlined for glacial t imes (Jouzel et al. 1989a, b), a close similarity is observed between the records of distant sites: Byrd, on the one hand, and Vostok, Dome C and Kom- somolska'ia on the other. The general good concor- dance of our results with terrestrial paleoclimatic data f rom the Southern high latitudes also favors the wide geographical significance of the Antarctic climate record.

Our data show that the warmest conditions of the Holocene occurred during the period 10-7.5 k a y BP. At that time, temperatures were higher by about 1 ° above the present values, a trend that seems conf i rmed over surrounding areas. By 6 ka y BP, the progressive return to cooler climates had been achieved. Although the reasons for this op t imum are still unclear, analysis o f chemical impurities of trace gases contained in the ice may prove to be very helpful in reconstructing other climate-related parameters (Barnola et al. 1987; De An- gelis et al. 1987; Legrand and Delmas 1987; Petit et al. 1981). In order to document the Holocene per iod better, bore holes in coastal regions where accumulat ion rates are much more higher than on the Antarctic plateau, constitute an appropr ia te solution for obtaining more detailed climatic records, even if in the latter case other factors like ice flow or sheet ice thinning must be taken into account.

Acknowledgements. We thank all Soviet and French participants for doing the drilling, field work and ice sampling. We acknowl- edge the Soviet Antarctic Expeditions and the US division of Po- lar Programs for logistic support. We thank G. Mondet, P. Doira and R. Chiron for isotopic determinations. This work was supported in France by the EPF (Expeditions Polaires frangaises), PNEDC (Programe National d'&udes de la Dynamique du Cli- mat) and TAAF (Terres Australes et Antarctiques Fran~aises) and in the USSR and Vostok and Komsolmoskaia by Soviet Antarctic Expeditions.

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Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record