Gtadas-Oceatt-AtmomteK Interactions/Proceedings of the International Symposium held at St Petersburg, September 1990). IAHS Publ. no. 208,1991. The last déglaciation in Antarctica; evidence of a "Younger Dryas" type climatic event J. JOUZEL (1, 2), J.R. PETIT (1, 2), J. CHAPELLAZ (1), J.M. BARNOLA (1) 1. Laboratoire de Glaciologie et Géophysique de l'Environnement, BP 96, St-Martin-d'Hères 38402, Cedex, France 2. Laboratoire de Géochimie Isotopique, CEA - CEN, Saclay 91191, Cedex, France N.I. BARKOV, V.N. PETROV Arctic and Antarctic Research Institute, Beringa 38, St Petersburg 199226, USSR ABSTRACT The Younger Dryas was a cold event which occurred during the last climatic transition, following the warming trend of the Bôlling-Allerod and spanning approximately a millennium from 11 to 10.2 kyr B.P. Isotopic Dome C results have shown that the transition is a two-step process with two warming trend periods interrupted by a slightly colder period estimated to have taken place from about 13.2 to 11.7 kyr B.P. This cooling event is also well recorded in the Vostok record but again during a time interval preceding the Younger Dryas by about 1 kyr. Recent measurements of methane and dust concentration in the Vostok core are discussed as useful information for linking Northern and Southern Hemisphere observations. INTRODUCTION The Younger Dryas is a climatic stage which took place during the second half of the last déglaciation and was originally defined for a pollen zone in Europe (Jensen, 1938; Iversen, 1954) . It corresponds to a cold event spanning approximately a millennium from ~ 11 to 10 kyr B.P. which followed the warming trend of B0lling and Allerôd interstadials. Available data on the Younger Dryas were recently compiled by Rind e_t al. (1986) both for Europe where this event is best recognized, and for the whole world. They comprise terrestrial pollen records which indicate that trees, which have started growing in response to the climatic warming of the déglaciation, were suddenly replaced by shrubs and herbs, and characteristics of a glacial regime, and isotope records from the Camp Century and Dye 3 Greenland ice cores (Dansgaard e_t al. , 1973; Oescheger et al. 1984) and isotope records from lake 269 /. Jouzel et al. 270 sediments in France (Eicher et al., 1981) and Switzerland (Oeschger et. al. 1980) . -38 -36 -34 -32 -30 -28(%o) FIG. 1 Detailed 5180 profile along a 120 m increment of the Dye 3 deep core containing ice deposited during the entire Pleistocene to Holocene transition (middle curve) . CO 18 2 concentration in air bubbles and 5 0 for lime sediments in the Swiss lake Gerzen are shown on the right and left curves respectively (adapted from Dansgaard & Oeschger, 1989). This is illustrated in Fig. 1 which shows the remarkable correlation between lake Gerzensee (Switzerland) and Dye 3 (Greenland) climatic records, as deduced from 5 0 profiles, over this period. At Dye 3 the cooling corresponding to the Younger Dryas is estimated to be ~ 7 C and recent studies (Dansgaard et al.. 1989) have shown that this event ended very abruptly and possibly in less than 20 years with a transition characterized by a sudden shift in nearly all parameters studied in this core (deuterium-excess; chemical trace elements; acidity and continental dust). The deuterium and oxygen 18 concentrations are expressed in S units (<5D and 5 0) , expressed in per mill versus SMOW (the Standard Mean Ocean Water). Observed clearly in Greenland and often in European records the Younger Dryas is a well defined climatic event corresponding to very significant changes in climatic and environmental conditions with respect to the preceding Bôlling-Allerôd period with a particularly abrupt transition towards the conditions prevailing during the 271 The last déglaciation in Antarctica Holocene. On the other hand, the change in C02 concentration shown on Fig. 1 has to be confirmed because it may be due to the presence of melt layers (Dansgaard & Oeschger, 1989). -180 -120 -60 0 60 120 FIG. 2 This map adapted from Rind et. al. (1986) shows the worldwide assessment of the published results indicating the presence (Y), absence (N) or possible (?) paleoclimatic indication of a large-glacial oscillation which may or may not be correlative with the Allerôd-Younger Dryas. Also included is the location of the North Atlantic polar front for different time periods (Ruddiman & Mclntyre, 1981). Deglacial retreat was interrupted by the readvance from 11-10 kyr B.P. During the Younger Dryas a major cooling of the North Atlantic ocean took place, resulting in the North Atlantic polar front advancing to the South and East to a position about 5°C poleward of its full glacial position (Ruddiman & Mclntyre, 1981). Indeed, the close link between climatic conditions in the North Atlantic and the Younger Dryas cooling is well recognized (Rind e_t al. 1986; Broecker e_t al. 1985, 1989; Fairbanks, 1989; Jansen & Veum, 1990) although (Shackleton, 1989) the mechanisms involved and their relation with the complex melting history of the Northern Hemisphere ice sheet are largely debated. Another indication of the specific role of the North Atlantic is that, for North America, only regions adjacent to the Atlantic report unequivocal evidence of a /. Jouzel et al. 272 climatic oscillation (Rind e_t al. 1986; Peteet e_£ al. 1990). This pattern of a cooling being more intense over Greenland, Europe and the eastern part of North America generally agrees with a climate simulation in which the sensitivity of global climate to a colder North Atlantic sea surface temperature was investigated (Rind e_t al. 1986) . These authors noted that, outside these regions, under the influence of North Atlantic evidence for a Younger Dryas cooling was not convincing. For the Southern Hemisphere there is some indication of a late-glacial climatic oscillation in South America, South Africa, South Georgia, and New Zealand (Rind e_£ al. , 1986) . Heusser & Rabassa (1987) established the correspondence between the late-glacial cooling recorded in southernmost South America and the Younger Dryas. In a recent study, Labracherie et al. (1989) also showed that the last déglaciation in the southern ocean was interrupted by a cooling phase similar to the Younger Dryas. However, detailed accelerator mass spectrometry (AMS) dating shows that this cooling took place between 12 and 11 kyr B.P., therefore preceding the Younger Dryas by about 1 kyr. Antarctic ice cores give a unique opportunity to obtain high resolution of the last déglaciation giving access to many parameters relevant to climate and environmental changes over this period. In an article dealing with the Antarctic ice record during the late Pleistocene, Jouzel ejt al. (1987a) reviewed the isotopic results available for Byrd and Dome C, stating that the warming associated with the last climatic transition was a two-step process with two warming trend periods interrupted by a cold reversal. These authors already noted that this cold reversal may have preceded the Younger Dryas by 1 kyr or more. The objective of the present article is to extend this study of the last déglaciation to Antarctica by examining the isotopic records, and two other types of relevant, parameters, namely the concentration in trace gases (and in particular in methane), and the dust fallout. We will successively examine isotope, trace gases, and dust records. THE ISOTOPE RECORDS We will focus on the three Antarctic deep ice cores: Byrd, Dome C and Vostok. The first record was obtained at Byrd station in West Antarctica (elevation 1530 m mean annual temperature -28°C, accumulation 16 g cm" year- ) . The drilling reached the bedrock at a depth of 2163 m in 1966. The 905 m Dome C core was drilled during the 1978 summer season in East Antarctica (3240 m elevation, -53°C and 3.4 g cm_2year~J) . At Vostok station (3490 m, -55.5°C and 2.3 g cm year ) , a série of drilling was performed over the last two decades with, at the moment (Barkov e_t al. 1977; Lorius e_t al. 1985; Jouzel et al. 1987b), four cores 273 The last déglaciation in Antarctica covering the last climatic transition (from ~ 15 to 10 kyr B.P.) which corresponds to the depths from 265 to 355 m. Due to higher accumulation, the same time interval corresponds to greater depths at Dome C (from 365 to 500 m) and at Byrd (from 935 to 1235 m) but in both cases they are sufficiently far from the bedrock and thus at depth where the ice sequences are essentially undisturbed by flow conditions. DOME-C -T~r -r~r~r Age (kyrBP.) FIG. 3 Isotopic ice core climatic records over the 8-18 kyr B.P. period from Dome C, Vostok and Byrd. The deuterium profiles are shown for Dome C and Vostok (Left scale). For Byrd, we have for the sake of comparison of the three isotopic curves, the <5180 results of Johnsen e_t al. (1972) translated in a <5D curve using the relationship 5D = 7.9 <5180 established by Epstein et al. (1970). The Byrd, Dome C and Vostok isotopic profiles are reported in Fig. 3 with respect to time for the period /. Jouzel et al 21A from 18 to 8 kyr B.P. with an additional smoothed curve in which much of the high frequencies oscillations are filtered out (see dating below). For Dome C, we have reported the deuterium data obtained along with 5 0 on the 905 m core (Lorius e_t al. 1979; Jouzel e_t al. , 1982, 1987b) . For Vostok, the deuterium profile published in Jouzel e_t al. (1987b) was obtained on the 3 F core below 138 m and on a new adjacent core (4 T) from the surface down to 279 m, using one-meter ice increment.