Younger Dryas" Type Climatic Event

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. We took the opportunity of the new 4 F drilling for getting a more detailed (0.5 m) profile during the last déglaciation. It was also noted that in 3 V there was some missing ice (8 m) during this déglaciation at the time of the cold reversal which occurred during the transition. The new data set now available from 280 to 400 m which extends the 4 F record down to 400 m allowed to fill this gap and to obtain a fully continuous record during the last déglaciation. The record showed in Fig. 3 combines these new data (from 280 to 400 m) and that previously published for the same core (Jouzel e_i al. . 1987b) above. For Byrd, we have used the S 0 record of Johnsen e_t al. (1972) but reported a <5D record obtained using the relationship 5D = = 7.9 5180 established by Epstein (1970). Obtaining an accurate time scaling for these Antarctic ice cores proved difficult because for large regions of the Antarctic Plateau such as at Dome C and Vostok, the annual accumulation rate is too low for a clear annual isotopic signal to form (Lorius e_t al. . 1979; Jouzel et al., 1987a,b). At Byrd, seasonal interpretation is difficult for the isotopic signal because of irregularly distributed accumulation and obliteration at depth (Johnsen et al.. 1972). However, there is a well preserved acidity signal thought to be of seasonal character (Hammer et al. . 1985) , which has been used by Clausen & Hammer (personal communication) to construct a Byrd time scale. For Dome C we have used the new time scale proposed by Jouzel e_t al. (1989) which puts the bottom of this core at 39 kyr B.P. (instead of 32 kyr B.P. in Lorius e_t al. . 1979) and for Vostok the time scale of Lorius et al. (1985) established in both cases using glaciological models and accounting for the variation of accumulation with time. Note that the Byrd redating suggested by Jouzel et al. (1989), using the possible correspondence of climatic events recorded in the Byrd, Dome C and Vostok cores, does not affect the part of the Byrd record reported in Fig. 3. We only briefly recall here the generally well accepted climatic interpretation of these ice core isotopic records. Isotopic fractionation processes which occur during the atmospheric water cycle lead to a general decrease of the ôs with temperature. For modern polar precipitation there is a well explained linear relationship between the annual averages of the 5s and the surface temperature T (Dansgaard e_t al. , 1973; Lorius & Merlivat, 1977; Jouzel & Merlivat, 1984) with a slope of 275 The last déglaciation in Antarctica

6%„ /°C for the deuterium and of 0.75%o/°C for the oxygen 18 in East Antarctica (Lorius & Merlivat, 1977) . Correspondingly, it is assumed that at a given site snow which fell under glacial conditions is isotopically impoverished with respect to modern snow (Fig. 3). Before being translated into a temperature scale, isotopic profiles must be corrected for the isotopic changes of sea water leading to an estimate of AT of ~ 9°C between glacial and interglacial conditions at Vostok and Dome C and to AT ~ 12°C at Byrd. This interpretation refers to the temperature change at the precipitation site; it does not account for the possible changes of elevation either due to the motion of the ice from its origin or the general changes in the thickness of the ice sheet which could explain the higher Byrd AT value (the difference between East and West Antarcticasmay also be real). Note at least that due to the presence of a strong inversion over Antarctica, the temperature change in the atmosphere above the inversion, AT , is lower than at the surface (AT - 2/3 AT , Lorius etaal., 1979; Jouzel et al.. 1987b). As previously discussed (Jouzel e_t al. . 1987a) , at Dome C, the transition is a two-step process with two warming trends interrupted by a cold oscillation. The two warming trends took place from 17.2 to 13.3 and from 11.7 to 10.2 kyr B.P. respectively with the colder period lasting - 1.5 kyr from 13.2 to 11.7 kyr B.P. (these figures are different from those of Jouzel e_t al. . 1987a, because to the Dome C redating) . The S amplitude of the cold reversal defined peak to peak as illustrated in Fig.3 is of ~ 10%o in 5D corresponding to AT of ~ 1.5°C. This cold reversal also took place at Byrd sfrom ~ 13.2 to 12.5 kyr B.P. with an amplitude comparable to that of Dome C but on a shorter period (note however that the relative importance of the reversal with respect to the déglaciation signal is less at Byrd than at Dome C). This cold reversal is now well seen in the Vostok 4 Y isotopic record (Fig. 3). It lasts from ~ 12.3 to 11 kyr B.P. starting about 1 kyr later than at Dome C or Byrd, a difference which raises the question of dating accuracy (see Section DISCUSSION), with an amplitude (again defined peak to peak) above 20%» in 5D (which would correspond to a AT of 3 to 4°C). One very likely reason of this large amplitude is indeed the higher time resolution of the Vostok record (one sample represents about 25 years at Vostok instead of 100 years at Dome C). A significant part of this amplitude is related to the presence of two spikes at around 12.5 and 11 kyr B.P. which are possibly missed at Dome C due to the coarser time resolution.

THE CO AND CH RECORDS 2 4 CO and CH measurements performed in Greenland and Antarctic deep ice cores over the last ten years revealed substantial changes between glacial and interglacial /. Jouzel et al. 276

concentrations with increases during the last climatic transition in CO from ~ 200 to 280 ppmv (Delmas ei al., 1980; Berner e± al., 1980; Neftel e£ al.. 1982; Barnola et al. . 1987) and in CH from ~ 350 to 650 ppbv (Stauffer et al.. 1988; Chappelaz4et al.. 1990). The question of how this increase occurred is interesting for our present purpose as much as available data suggest a different behavior for CO or CH during the déglaciation. For this period, CO data have been obtained for the three deep Antarctic cores: Dome C, Byrd (Neftel e_t al. . 1988), and Vostok. Data available from Dome C (Delmas et al.. 1980; Raynaud & Barnola, 1985) and Vostok (Barnola et al. , 1987; Barnola et al., 1991) suggest that a CO increase from about 200 to 280 ppmv occurs in two steps separated by relatively constant level of about 230 ppmv, including the Younger Dryas cold reversal. The very detailed data set from Byrd (Neftel et, al. . 1988) has unfortunately a gap during the most interesting part of the record in view of a Younger Dryas study. Nevertheless, the interpolation of the data during the gap shows a slower CO increase during this period, and thus does not contradict the data from Dome C and Vostok. Furthermore, even if more measurements are needed to know the detailed CO increase during the last déglaciation, it is important to note that none of the three Antarctic records presently available shows a strong drop in CO concentration associated with the cold reversal revealed by the isotopic records.

-650 > ^600 Q. Li. -550 — -500 CD C >-450 CO 400 350 CD Age(kyrB.R) FIG. 4 Methane concentration (squares) at Vostok betwee n - 9 and 17 kyr B.P. (scale on the right) along with the smoothed deuterium profile (left scale)

The situation is better for CH in which case the last déglaciation was relatively (10 levels) well documented at 277 The last déglaciation in Antarctica

Vostok in a recent study by Chappelaz e_t_ al. (1990). A striking feature (Fig. 4) is the significant CH4 drop at the end of the last déglaciation documented by the analysis of two levels (11.1 and 10.5 kyr B.P.). After a fast increase from 450 to 650 ppbv between 13 and 12 kyr B.P., CH concentration decreases by ~ 170 ppbv (12-11.1 kyr B.P.) and then reaches again the high value of - 650 ppbv. This is confirmed by duplicate measurements at several levels (seven for the "low" value at 11 kyr B.P., six samples for the peak at 12 kyr B.P.). A low Younger Dryas value (477±25 ppbv) was previously measured at Dye 3 (one level dated at 10.8 kyr B.P., Stauffer et al.. 1988). As noted in Chappelaz et. al. (1990), CH records should provide useful information for linking information in the Northern and Southern Hemispheres and for understanding the coupling between such abrupt climatic change and atmospheric CH changes (see further discussion in Section DISCUSSION). 4 Chappelaz et al. (1990), estimate the cooling effect associated with this CH drop to 0.3°C.

THE DUST RECORDS As for trace gases, the interest of examining how dust concentration has varied during the last déglaciation in ice cores is twofold; first large changes in the dust atmospheric loading (which are recorded in ice) may have a potential climatic role (Petit e_t al. . 1981; Royer e_t al. . 1983; Harvey, 1988; Broecker & Denton, 1989); second, dust events may serve as stratigraphie markers between ice core and other dust records (Petit e_t al. . 1990) . One common and striking feature of the Dome C and Vostok dust records is the large dust fallout increase recorded in both cores during the Last Glacial Maximum with respect to Holocene values. The 18 to 8 kyr B.P. part of these two records is reported in Fig. 5 (along with the smoothed isotopic records) including part of this dust peak. This figure shows that the dust fallout values close to those corresponding to the Holocene are reached at the end of the first warming trend and thus prevail during the cold reversal. There is thus no indication that a change in the aerosol loading has then played a climatic role in this cooling. As far as the interest for the dust records as stratigraphie markers is concerned, we note that the end of the dust peak although occurring at the same isotopic level in the two cores is, referring to the adopted chronology, dated 1 kyr earlier at Dome C (13.8 kyr B.P.) than at Vostok (12.8 kyr B.P.). This suggests, but would have to be confirmed through higher resolution dust records, that one at least of the Dome C and Vostok time scales would have to be revised for this déglaciation part of the records. /. Jouzel et al. 278

10 12 14 16 18 Age(kyr B.R) FIG. 5 Comparison of the Dome C and Vostok dust records over the 8 to 18 kyr B.P. period along with the smoothed deuterium records (curves (a) and (c) respectively). _ The dust fallout is expressed in mg m year assuming a particle density of 2. The two arrows indicate the point for which "Holocene" dust concentrations are reached.

DISCUSSION The results presented in this article fully confirm the assumption of Jouzel et. al. (1987a) that the last déglaciation was in Antarctica a two step process with two warming trends interrupted by a cold reversal. Evidence of this reversal previously based essentially on Dome C data (and to a lesser degree on the Byrd record), now includes the Vostok isotope profile based on a new, relatively high resolution (0.5 m i.e. - 25 years) record. Owing probably to this high resolution, the shape of this 279 The last déglaciation in Antarctica reversal is now better documented and Vostok data suggest the existence of two spikes (one at the beginning and the other at end of this period). This would, if confirmed by even more detailed records from Vostok or from other Antarctic sites, correspond to a temperature amplitude of up to 3 to 4°C. It is however only half of the amplitude of the Younger Dryas as recorded in Greenland cores (7°C). Other features associated with this cold reversal that we have examined, are a significant drop in CH concentration and the absence of a dust signal, the dust fallout being quite similar to the low Holocene values. CO concentrations are too poorly documented in Antarctic cores over this period to draw any conclusion about the existence of significant changes during this Antarctic reversal parallel to that suggested by Greenland data (see Fig. 1). A key question to be solved in determining if and how this Antarctic cold reversal is related with the Northern Hemisphere Younger Dryas is that of dating. Unfortunately, the datings neither of the cold Antarctic reversal nor of the Northern Hemisphere Younger Dryas are firmly established. The discrepancy between Dome C and Vostok datings for this part of the isotope profile was noted whereas the dust records suggest that they are in phase. Even if the youngest Vostok dates for the reversal (from 12.3 to 11 kyr B.P.) were accepted, this is still older than the currently accepted dates for the Younger Dryas (from 11 to 10 kyr B.P.). However, Bard et al. (1990), recently questioned the dating of the Younger Dryas in proposing an age of 11 kyr B.P. for the Preboreal/Younger Dryas boundary instead of 10 kyr. This is based on a new calibration of the C time scale using U-Th ages from Barbados corals. Bard et al. (1990) noted the agreement with obtained dendrochronological estimates (a 11.3 kyr B.P.) and if confirmed this would practically cancel the time lag between the cold Antarctic reversal and the Younger Dryas (the corresponding boundary, i.e. the end of the cold reversal is dated at 11 and 11.7 kyr B.P. at Vostok and Dome C respectively) . Also it is pointed out (Bard e_t al. . 1990) that the Younger Dryas climatic event may have been as long as 1 kyr in agreement with the qualitative estimates of Oeschger et al. (1980) and, thus except for Byrd, in agreement with the duration of the cold reversal as recorded in Antarctic ice (1.5, 1.3 and 0.7 kyr B.P. at Dome C, Vostok and Byrd respectively). This new calibration of the 14C time scale indicates that the Younger Dryas and the Antarctic reversal may be in phase. However, this is contradicted by two other results. First, the proposed Younger Dryas redating is in disagreement with that deduced independently (annual counting) for Greenland cores which places the Younger Dryas/Preboreal boundary at - 10 kyr B.P. Second, Labracherie et al. (1989) study of the last déglaciation indeed shows a ~ 1 kyr lag of the Younger Dryas compared to the sea surface temperature in the Southern Ocean. This /. ]ouzel et al. 280 suggests that the Southern and Northern Hemispheres may be out of phase at least for the déglaciation - a conclusion which is not affected by the new C calibration. As far as the correspondence between the cold Antarctic reversal and the Younger Dryas is concerned, we thus face a complex situation. Fortunately, CH measurements constitute one very promising way to link Greenland and Antarctic ice cores and thus to clarify this situation. More measurements in both polar ice sheets in well chosen sites (i.e. with relatively high accumulation) are required to establish accurately such a Northern/Southern Hemisphere link through CH well marked variations. More generally and this will be our conclusion, a very detailed and more comprehensive study of the cold reversal period in Antarctic cores would be beneficial for a global interpretation of the last déglaciation. Additional measurements could include deuterium-excess which gives access to the conditions prevailing in the moisture source regions, Be (as a proxy of accumulation), acidity (for volcanic events) and other trace gases and chemical compounds.

ACKNOWLEDGEMENTS We thank all Soviet and French participants in drilling, field work, and ice sampling at Dome C and Vostok and the U.S. Division of Polar Program for logistic support. We thank R. Chiron and P. Doira for isotope determinations (4 r Vostok core) and C. Hammer and H. Clausen for providing us with their unpublished Byrd chronology. We are very indebted to C. Lorius for his continuous support in this study and for very fruitful discussions. This work is supported in France by Programme National d'Etudes de la Dynamique du Climat (PNEDC), Terres Australes et Antarctiques Françaises (TAAF), and the Commission of the European Communities and in the USSR by the Soviet Antarctic Expeditions.

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