JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. C4, PAGES 2693-2703, MARCH 20, 1983

Climatic Information Over the Last Century Deduced From a Detailed Isotopic Record in the South Pole Snow

J. JOUZEL AND L. MERLIVAT

Laboratoire de Geochirnie Isotopique, Centre d't•tudes Nucl•aires de Saclay, 91191 Gif-sur-Yvette Cedex, France

J. R. PETIT AND C. LORIUS

Laboratoire de Glaciologie, 38031 Grenoble Cedex, France

A continuous and very detailed deuterium profile was obtained from the analysis of about 900 firn samples from the south pole representing the snow accumulated over the last 100 years. Seasonal variations are well preserved in the tim, allowing the samples to be dated back to 1887 with an accuracy probably better than 5 years. This dating is consistent with the results of other independent methods, such as stratigraphic observations (back to 1927) and artificial/3 radioactivity and tritium measurements (back to 1955). The resulting mean annual accumulation over the 1887-1978 period is equal to 9.2 g cm -2. The value between 1887 and 1957 (9.5 g cm -2) is significantly higher (30% and 40%) than values previously obtained at the same site over comparable time spans. A decrease in annual snow accumulation is deduced from a comparison of the 1887-1930 and 1930-1978 periods (from 10.0 to 8.5 g cm-2). From a thorough examination of data relative to the very well documented 1957-1978 period, mean annual and maximum deuterium values and annual deuterium amplitude are shown to be correlated with the corresponding mean annual and summer temperatures and the annual temperature amplitude. The degree of correlation depends on the tropospheric level considered. On the other hand, winter temperature and deuterium minima are poorly correlated, which is attributable to the large temperature inversion during winter. Experimental temperature-isotope (D and •80) gradients are consistent with theoretical gradients, taking into account that snow formation is a nonequilibrium process. As a whole, this work is very encouraging for the use of stable isotopes as a tool for past temperature reconstruction over periods for which only weak variation are expected. This is applied over the 1887-1978 period to the mean annual, winter, and summer temperatures along with the annual temperature amplitude.

1. INTRODUCTION

On a worldwide scale, it is observed that the deuterium or oxygen 18 content of precipitation generally decreases with the mean air temperature at the sampling site as a result of the fractionation processes occurring at each phase change of the atmospheric water cycle [Dansgaard, 1964]. Hence the determination of the deuterium or oxygen 18 content of paleoprecipitation is commonly used to reveal paleoclimato- logical information. This method is particularly suitable in polar regions where snow layers successively deposited over the ice caps can be analyzed. Moreover, a detailed dating of these layers and thus a determination of the snow accumula- tion is possible from various techniques such as examination of stable isotope profiles, visible stratigraphy studies, micro- particle determinations, and radioactivity measurements of artificial (gross/• and tritium) or natural (lead 210) origin. An annual dating from D or •80 profiles can be obtained in sites where seasonal variations are preserved during the firnifica- tion processes [Hammer et al., 1978; Jouzel et al., 1979].

We recently obtained a continuous and very detailed deuterium profile from the analysis of about 900firn samples representing the snow accumulated at south: pole station over the last 100 years, with the twofold objective to obtain information about the variations of the snow accumulation

and of the temperature over this period at this site. First, a detailed snow chronology obtained from four

Copyright 1983 by the American Geophysical Union.

Paper number 2C 1717. 0148-0227/83/002C- 1717505.00

independent methods (visible stratigraphy and three isotopic methods based on deuterium, tritium, and/3 determinations) is presented. A year-by-year dating is proposed back to 1887, and the deduced accumulation values are compared with similar data previously obtained in the same area.

A second aspect deals with the study of the temperature isotope relationship which must be known to make a quanti- tative evaluation of temperatures from the D or •80 composi- tion of historic precipitation as recorded in polar ice cores or other paleoclimatological sources. Over the last 2 decades, this relationship has been extensively studied in polar areas mainly for the cases in which relatively large isotopic effects are involved, including spatial, seasonal, and long-term variations.

1. On the basis of investigations by Dansgaard [1964], Lorius et al. [1969], Dansgaard et al. [1973], and Lorius and Merlivat [1977], the spatial variations appear very well documented in polar snow. As a general observation, the mean annual isotope content of precipitation is linearly related to the mean annual air temperature at ground level. However, all measurements reveal that it is only possible to establish a temperature isotope relationship in regions with relatively uniform meteorological conditions.

2. The seasonal variations have been investigated on individual Antartic snow precipitation events, taking into account the temperature prevailing in clouds when the precipitation is formed, by Picciotto et al. [1960] and Aldaz and Deutsch [1967], who have shown that the isotope content is linearly dependent on this parameter.

3. Long-term changes occurring between glacial and interglacial periods are recorded in deep ice cores [Dans-

2693

2694 JOUZEL ET AL' CLIMATE INFORMATION FROM SOUTH POLE SNOW

Prevailin• wind (40"E Grid)

Bur•ed old Pole Station

Geographical

South Pole

1956

26.9 m snow mine

( Giovl netto, 1960)

Run way Memorial

- - • .. - Pole

3Kin

New Pole Station

• Clean air facilities ••[ (Moryland University) I101 m core(under the dome) 'õm p,t TlOm pit

(E. Mosley -Thompsont1980) 8m core , ). • ,• 4Km •_•

Fig. 1. Schematic representation of the bases and sampling points in the south pole area.

gaard et al., 1969; Epstein et al., 1970; Barkov et al., 1977; Lorius et al., 1979].

On the other hand, the year-to-year temperature isotope comparison is, in polar areas, limited to a few studies [Dansgaard et al., 1974; Budd and Morgan, 1977; Aristar- ain, 1980]. Moreover, as no temperature record exists in the studied sites, the temperature at the nearest stations where these data are available is used. These different works tend

to encourage the use of stable isotopes as a tool of recon- struction of temperature changes even at short time scales (at least in polar areas). However, owing to the sparsity of the data and to the fact that the isotopic composition of the precipitation is controlled not only by its temperature of formation but also by the temperature of the source, the micrøphysical processes occurring in clouds (see theoretical section) and by the transportation processes [Kato, 1978] as shown for coastal precipitation by the influence of sea-ice extent [Brornwich and Weaver, 1981], the isotope tempera- ture comparison has to be thoroughly investigated to give confidence in the application of this method at short time scales.

Since 1957, meteorological data have been regularly re- corded at the south pole (Amundsen-Scott station) from one or two daily surface and upper air observations. As snow is very reliably dated and its isotopic content is not significant- ly altered (see section 6), this offers a unique opportunity in a polar region to compare isotope data with temperature values at the same site and at different atmospheric levels over a very well documented and relatively long period (21 years).

2. EXPERIMENTAL RESULTS

During the 1977-1978 Antartic field season, successive snow samples (referenced 1978 samples) were taken in a 10- m-deep pit (10.2 m) excavated 4 km from the new south pole base (Figure 1). Snow sampling was also carried out from 9.75 m to 17.9 m on a snow core drilled at the bottom of the

pit. Thus between 9.75 and 10.2 m, snow samples were obtained both from the pit and the core, providing a check on the effect of the change in sampling procedure.

Note that snow samples covering a shorter period of time were previously taken from a pit located a few hundred meters from the 1978 one (Figure 1) during the 1974-1975 field program (1975 samples). The same type of procedure was followed, a core being drilled down to 12.5 m at the bot-tom of a 5-m-deep pit. A first sequence of samples taken

on the wall of this pit provided, in particular, a continuous record of the artificial tritium fallout at the south pole since 1954 [Jouzel et al., 1979]. Our initial objective was to continue detailed isotopic studies on the corresponding core, but we observed a relatively rapid smoothing of the stable isotope signal after a 2-year period of conservation of this core in cold rooms in Grenoble. This process is probably identical to that occurring under natural conditions during the firnification of snow and attributed to water diffusion in

the vapor phase [Dansgaard et al., 1973; Johnsen, 1977]. As a result, the seasonal isotopic variations were obliterated in the core. Furthermore, an isotopic shift was observed be- tween the mean values relative to the bottom of the pit and to the summit of the core, representing the same depth incre- ment. The 1975 isotopic data, as a whole, appear to be unsuitable for deducing reliable past climatic data over the entire period studied, and the present article will be mainly devoted to a detailed interpretation of the 1978 results.

To prevent such isotopic signal modification in the 1978 samples, they were all prepared directly in the field. The deuterium content was measured on all the samples (501 from the pit and 389 from the core). It is expressed in/SD %0 with respect to SMOW (standard mean ocean water with a D/H ratio equal to 155.76 ppm) and is determined with a precision of 0.5 %0 (1 •r). The results are reported in Figure 2 as a function of the sample depth expressed in meters of snow.

The results relative to the depth increment, for which samples were obtained both from the core and from the pit, are given at an enlarged scale in the inset of Figure 2. The mean deuterium values calculated from the first minimum to

the last maximum are equal within the limits of measurement error (-395.3 %o for the pit and -394.9 %o for the core). This comparison validates the sampling methods used in the field and the conservation of the sample characteristics until their analysis. The slight shift observed between the depth of the corresponding extrema is attributable to the nonhorizonta- lity of the snow layers.

In addition, the oxygen 18 content was determined on 271 samples from the upper half of the pit and 14 from the core. The results are expressed in •SO %o versus SMOW (with a •SO760 ratio equal to 2005.2 ppm) and have a 0.15 %o precision (1 or). They are presented in a classical •D-•SO diagram (Figure 3) and will be further discussed in section 5; •D-•SO values were also determined on 34 precipitation samples collected between March 1978 and January 1979

JOUZEL ET AL.' CLIMATE INFORMATION FROM SOUTH POLE SNOW 2695

(Figure 3). (We are very indebted to S. Barnard for the collection of snow precipitation samples.)

Continuous tritium and/3 profiles were obtained down to a depth corresponding to the first arrivals of radioactive debris in the southern hemisphere following the first important nuclear tests. The results are, respectively, expressed in tritium units (1 TU is equivalent to T/H = 10 -18) calculated at the date of precipitation and in disintegrations per hour and per kilogram of water (dph kg-1) (Figure 4). Information deduced from this data concerning the/3 and tritium trans- port and fallout mechanisms is fully discussed elsewhere [Pourchet et al., 1982; Jouzel et al., 1979]. Here, we will only use it as a dating tool in the upper part of the pit.

3. SNOW CHRONOLOGY

The Dating of the Pit

We first used visual stratigraphic observations made on the wall of the pit. This dating method, based on stratifica- tion of the snow layers during the annual cycle, was previ- ously demonstrated very reliable in the south pole snow [Gow, 1965; Jouzel et al., 1979]. Fifty-one stratigraphic indexes corresponding to 'summer layers' were identified in the 1978 pit, the bottom of the pit therefore corresponding to the 1926-1927 austral summer. The depth of each of these stratigraphic indexes is indicated in Figure 2 by the vertical lines drawn in the upper part of the first two profiles.

The general feature of a summer maximum and of a winter miminum was shown in the oxygen 18 content of the south pole precipitation by Aldaz and Deutsch [1967], confirming that the stable isotope profiles can be taken to date the snow layers at this site. There is, in fact, in Figure 2 an excellent agreement between deuterium maxima and summer strati- graphic indexes over the 1927-1928 period. However, note

-45O

.z•50

•50

-350

-350

-•$0

-$50 ,35O

-,450'

A9• and •epta••,O •.• • •" •" •o Fig. 2. Deuterium content as a function of depth expressed in

meters of snow. The two upper profiles correspond to the pit, the two lower ones to the core, and the enlarged inset to their common part.

$OUTII POLL' '-360 .•. ':

.-$80

,•. , ß œd'n sam/o/ms --400 ',• .-"

ß '-•20

d/O%olWOW ." '-55 -53 -5'1 -49 -47 -,• -43 -4! I I I I I I I : -

Fig. 3. The {SD and {5180 content of firn and precipitation samples together with the meteoric water line ({SD = 8{5•80 + 10).

that some of these deuterium maxima appear more as humps than as distinctive peaks (1934, 1936, 1940, 1961, 1968, and 1974). The existence of an unusually high isotopic signal corresponding to the snow deposited during the 1957-1958 summer has been previously determined [Epstein et al., 1963, 1965; Picciotto et al., 1966; Jouzel et al., 1979]. Such an unusually high peak, at the expected date in the deuteri- um profile, thus definitely validates the annual dating be- tween 1958 and 1978.

An independent check on the validity of the stratigraphic method in this pit can be obtained from tritium and • profiles. The annual pattern of the precipitation tritium content exhibits a main annual peak occurring in August or September in the southern hemisphere [Taylor, 1968]. Each tritium peak is well marked between 1963 and 1973 (Figure 4). The corresponding samples have a low deuterium content indicating snow that fell during the Antarctic winter. On the other hand, the artificial • activity values in the Antarctic snow are characterized by two well-defined reference levels, one at the beginning of 1955 [Picciotto and Wilgain, 1963] and the other from the end of 1964 to the beginning of 1965 [Crozaz, 1969], attributed, respectively, to the arrival in the Antarctic area of radioactive debris from the Castle series and the 1962 nuclear tests. These two reference levels

appear clearly at the expected periods (with, however, a slight shift of the second/3 maximum toward the middle of 1964). In addition, a seasonal pattern with a broad summer maximum is displayed in this • profile and some of these

Fig. 4. Tritium content (lower curve) and total beta activity (upper curve) as a function of depth.

2696 JOUZEL ET AL.' CLIMATE INFORMATION FROM SOUTH POLE SNOW

Density (Kg. n• 3) 300 400 500 600

5 --

I0 --

o

15 --

20 0 5 I0 15

Water equivalent (m)

Fig. $. Density curve (curve b) with indication oœ the experimen- tal points averaged œor I m depth increments. Curve a gives the equivalence between snow depth and water equivalent.

summer/3 peaks were dated taking into account the preced- ing chronology (Figure 4).

Several processes such as prolonged sublimation [Cow, 1965] or, more generally, the lack of accumulation during one season, could cause the absence of seasonal indexes both in stratigraphy and isotopic profiles leading to an absence of certain years. Close examination of accumulation data relative to three successive summers deduced from

observation of 35 stakes (old snow stake field) effectively shows ablation during this season in approximately half the cases. However, datings from reference levels such as /3 peaks demonstrate that there is no missing year in the 1978 pit between 1955 and 1978 (as previously shown in the 1975 pit). Therefore, visual stratigraphy provides an unambiguous dating at this site over this period. This give a high level of confidence in the chronology e•ctended by this method back to 1927.

Dating of the Core

The dating of the core is not as straightforward, since we only have the deuterium profile. First, we examined if the isotopic signal was altered by diffusion processes in the core. There is no decrease in the deuterium variability between the lower part of the pit from 1927 to 1949 and the core (•rsr• equals 16 and 14.5 %0, respectively). Since datings deduced from the deuterium profile and from the stratigraphic index- es compare well in this part, a reliable core chronology can be proposed from the deuterium profile alone. We have estimated that the maximum possible error applying this procedure is equal to the number of humps (poorly defined peaks) in the core deuterium profile. This number was deduced assuming that the frequency of humps is the same in

the bottom of the pit as in the core. This procedure leads to a maximum possible error of 5 years on the date attributed to the bottom of the core (1887).

4. ACCUMULATION DATA

Before discussing accumulation data, it is interesting to precise that the total ice phase precipitation occurring at the south pole consists of several components [Smiley et al., 1980] including ice.precipitation or snow from clouds, fallout of ice crystals formed in clear air (diamond dust) as studied by Miller and Schwerdtfeger [1972] and Kikuchi and Hogan [1979], and surface ice accumulated by deposition from the vapor state (hoarfrost). According to Dalrymple et al. [1966], this last mechanism contributes only a small fraction of the total. On the other hand, annual crystal fallout is estimated between 1 and 3 gcm -2 [Miller and Schwerdtfeger, 1972; Schwerdtfeger, 1969] and then contributes significantly to the total accumulation at the south pole. The above pro- posed chronology (Figure 2) allows us to deduce the net annual accumulation corresponding to the distance between two successive summer levels and thus to a calendar year. Since the only level which is estimated for each of the years between 1887 and 1978 is the deuterium maximum, this level was chosen to determine the net annual accumulation.

Each annual accumulation value was calculated by using the density measurements given in Figure 5, which also gives the equivalence between meters of snow and water. • The annual values lie between 3.9 and 15.4 g cm -2 with a mean of 9.2 over the 91-year period. This set of 91 value• s is presented in Figure 6 as a function of time as a curve smoothed by using a filtering technique.

A general decrease of annual accumulation is observed from 1887 to 1978. For example, its mean value is equal to 10.0 g cm -2 between 1887 and 1930 and to 8.5 g cm -2 between 1930 and 1978. As stated above, the maximum possible error on the core dating is 5 years, corresponding to a maximum error of 1.2 g cm -2. Even when this maximum error value is used, the annual snow accumulation after 1930 stillappears to decrease. Note, at last, a significant increase over the last decade.

Comparison With Other Data

A great many studies concerning the determination of snow accumulation rate in the south pole vicinity (Figure 1) have been conducted since 1958. The various results were

summarized by Mosley-Thompson [1980]. We have complet- ed this summary with the recent determinations and indica-

$OUTII POLE raft • .q era '•

Fig. 6. Accumulation data versus time. Raw values were smoothed with a filter function. Curve a shows our data deduced

&om Figure 2 (1887-1978), curve b is the Giovinetto-Schwerdtfeger se6es (1887-1957), and curve c is the Mosley-Thompson series (1887-1956).

JOUZEL ET AL..' CLIMATE INFORMATION FROM SOUTH POLE SNOW 2697

TABLE 1. Net Annual Accumulation in the Vicinity of South Pole Station From Various Sources

Net Accu-

Time mulation, g Sampling Method Interval cm -2 a- • site Source

Gross/3 1975-1955 8.54 1975 pit Lambert et al. [1977] Pb 210 1955-1921 12.5 1975 pit Sanak and Lambert [1977]

1921-1889 7.0 1975 pit*

Stratigraphy,/3, deuteri- 1975-1950 8.2 1975 pit um, tribium

Stratigraphy,/3, deuteri- 1978-1960 8.3 1978 pit um, tribium

Stratigraphy and deuteri- 1978-1930 8.5 1978 pit um

Deuterium 1930-1887 10.3 _ 1.2 1978 pit

•Oxygen 18 1963-1958 7.0 old base Stratigraphy 1965-1955 7.5 old base /3 1962-1955 6.5 _ 0.5 old base Pb 210 1963-1850 6.1 _ 1.0 old base Stratigraphy 1957-1760 6.6 old base

Microparticles 1957-1760 7.16 new base Microparticles !974-1955 8.74 new base /3 1974-1955 9.03 new base

Jouzel et al. [1979]

Jouzel et al. [1979]

this work

this work

Epstein et al. [1965] Picciotto et al. [1964] Picciotto et al. [1964] Crozaz et al. [1964] Giovinetto and $chwerdtfeger

[1966]

Mosley-Thompson [1980] Mosley-Thompson [1980] /3 level found at 4.35 m

[Mosley-Thompson, 1980]

*Assuming constant 21øpb initial concentration.

tions of the sampling site (Table 1). Note that the values obtained for our two pits, located 300 m apart, are the same between 1955 and the sampling date (8.35 g cm -2 a-l).

From 1887 to 1957, our results can be compared with the stratigraphic and microparticles series obtained by Giovin- etto and Schwerdtfeger [ 1966] and Mosley-Thompson [ 1980], respectively. For this comparison, the three smoothed curves obtained by using the same filtering technique are given in Figure 6. They show a good agreement between the stratigraphic and microparticles series despite a slight shift in the mean values, and they show differences between these two series and our own results both for their mean value and their variation with time. Our estimation of the mean annual

accumulation over the 1957-1887 period (9.5 g cm -2) is, respectively, 30% and 40% higher than that given by Giovin- etto and Schwerdtfeger (7.35 g cm -2) and by Mosley- Thompson (6.85 g cm-2).

The 2•øpb method [Sanak and Lambert, 1977] also shows high values with a mean of 9.8 g cm -2 a -1 between 1889 and 1955. These investigators state that their estimation would change if the initial 2•øpb concentrations in the fresh snow vary with time. However, they point out that an accumula- tion rate lower than 8 g cm -2 a -1 seems inconsistent with the higher values measured in the superficial layers. Note, however, that the temporal changes in our series are differ- ent than in the 21øpb results depending upon the chosen length of time (Table 1).

Larger accumulation values in the older parts of the series could be explained by the obliteration of some of the deuterium indexes in the core. This casts doubt on the

significance of the decrease we have estimated between the successive periods 1887-1930 and 1930-1978. This possible higher frequency of systematic errors could lead to a larger error variance [Giovinetto and Schwerdtfeger, 1966]. To test this hypothesis, we examined the distribution of our annual accumulation data both in the pit and in the core. The relative variabilities are exactly the same in the two cases

(28%). The number of years with deviations higher than 1 tr are practically identical (33% in the pit and 36% in the core). This strongly suggests that there is no predominance of systematic errors in the core and indicates the validity of our chronology. A higher value of the mean annual accumulation at the 1978 pit site than previously estimated for the south pole area (at the old base) thus appears well founded.

A question arises about the differences between our series and that previously published: are they the result of dating uncertainties or of local changes in snow accumulation? Without discussing their validity, it can be pointed out that the Giovinetto-Schwerdtfeger and Mosley-Thompson series are based on a unique dating technique, whereas our results are deduced from four (back to 1955) and then two (1927- 1955) independent techniques and thus deserve a very high level of confidence. On the other hand, the 30% difference is not unusually high given the distance between the two sites as is shown by stake data. So results from the new snow stake field (centered at the old base and having the form of a cross 7 miles in length for each leg, with 70 stakes 0.2 miles apart) show similar variability (30%). Note, however,•that this figure applies only to a 2-year period (1962-1963), which is not comparable to the one in question (91 years).

To sum up, a local effect cannot be completely ruled out to explain the well-established higher accumulation value at the 1978 site and our series cannot be considered as repre- sentative of the entire south pole area until other series of this type are obtained.

5. THE TEMPERATURE ISOTOPE RELATIONSHIP

OVER THE 1957--1978 PERIOD

As pointed out in the introduction, we have at the south pole a unique opportunity to compare isotopic and tempera- ture data both at ground and atmospheric levels over a very well documented 21-year period. We will first examine how the isotopic signal of the precipitation is preserved in the snow.

2698 JOUZEL ET AL' CLIMATE INFORMATION FROM SOUTH POLE SNOW

TABLE 2. t•180 Data for 1957-1958 (Minimum and Maximum Values and Total Amplitude) as Observed at Different Times

This

Pit 5(16) Pit 3(16) Pit 4(16) Pit 1(16) Pit 2(16) 1975 Pit Work

Sampling year 1958 1963 1963 1964 1964 1975 1978 •180 maximum -39.3 -44.7 -41.8 -41.7 -44.1 -42.8 -41.85 /5180 minimum -55.3 -47.0 -55.3 -56.4 -57.3 -55.6 -55.75 /5180 amplitude 16.0 2.3 13.5 14.7 13.2 12.8 13.9

Conservation of the Isotopic Signal

Three processes can cause modification of the isotopic signal initially contained in precipitation: (1) at the snow surface, wind mixes snow of different seasonal or even yearly origins; (2) at the top of the snow column, evaporation and condensation processes lead, for instance, to depth hoar formation which can modify the snow isotopic content as suggested by Epstein et al. [1965]; and (3) further smoothing of the seasonal variations is due to diffusion in the vapor phase during firnification [Dansgaard et al., 1973; Johnsen, 1977]. We will successively discuss the influence of process- es 3, 2, and 1 for the south pole isotopic record in question.

Process 3

Taking advantage of the unusually high isotopic signal corresponding to 1958, the influence of diffusion processes can be estimated [Jouzel et al., 1979]. This isotopic signal has been determined at different times by various investiga- tors [Epstein et al., 1963, 1965; Picciotto et al., 1966]. It has been observed in the 1975 pit [Jouzel et al., 1979] and now in the 1978 pit (Figure 2). Its amplitude is given in Table 2, which shows that no significant decrease with time is ob- served between the signal recorded in pits 1, 2, 4, and 5 (14.3% on the average) and that registered in the 1978 pit (13.9%), indicating that on the 20-year time scale there is no significant smoothing of the isotopic signal in south pole snow.

The very well defined 1966 tritium peak (Figure 4), gives additional proof of the weak influence of diffusion processes which would affect tritium in the same way as deuterium. Its value (2800 TU) is high compared with that registered in the southern hemisphere at this time [Taylor, 1968], eliminating the possibility of a large modification during the firnification process.

Process 2

Combined •D-•180 measurements provide information concerning water vapor transport at the top of the snow column. Sublimation would change •D and •180 in such a way that the •D-•80 slope and the deuterium excess value (/iD-8/i•80) would decrease [Moser and Stichler, 1975]. Comparing precipitation and firn samples (Figure 3), there is no/iD-/i180 slope change (7.61 _+ 0.18 compared with 7.69 _+ 0.06) and only a very weak deuterium excess decrease (11.1 -+ 0.4 compared with 9.1 _+ 0.2). Keeping in mind that precipitation was sampled over less than 1 year, this compar- ison shows that neither condensation nor sublimation signifi- cantly altered the isotopic signal at the top of the snow column.

Process 1

There is some indication that removal processes due to wind transport and deposition can affect the annual ampli-

tude of the isotopic signal initially contained in precipitation. This can be seen from the comparison of the $•80 data obtained by Aldaz and Deutsch [1967] over the period November 1964 to October 1965 in precipitation and that obtained from firn samples for the same year. The same minimum value is obtained (-54.7 %o for the precipitation and -54.6 %o for the firn), but the maximum is significantly different in the firn (-50.7 %o compared with -43.1%o in the precipitation). Some of this change can be attributed to firn sampling, but the large-amplitude decrease observed (from 11.5 to 4 %o) strongly suggests that removal processes played a role for this particular year. On the other hand, the existence of very well marked tritium peaks between 1963 and 1973 shows that mixing of snow of different seasonal origin owing to wind action is probably not very important for the period studied as a whole.

To sum up, all these points dealing with specific aspects of deuterium, tritium, and oxygen 18 measurements lead to the conclusion that for a 20-year time scale, the isotopic signal registered in the firn is well representative of that initially contained in precipitation.

The Isotope-Temperature Comparison

At the south pole, the determination of the temperature at which the snowfall originates is difficult even when consider- ing individual precipitation events [Aldaz and Deutsch, 1967]. As an estimate, these investigators took the observed range of temperature between the ground and the 500 mbar levels. This level was chosen as the upper boundary of the precipitating layer because the contribution of the remainder of the troposphere should be negligible owing to its low moisture content.

For the present study, we will follow the same basic idea and compare deuterium values with temperatures recorded at the ground and at selected tropospheric levels up to the 500 mbar level. Temperature profiles (Figure 7) are charac- terized, except for the two summer months by a temperature

½00,

Fig. 7. Mean temperature profiles over south pole during winter (April-September) and summer (December and January).

JOUZEL ET AL' CLIMATE INFORMATION FROM SOUTH POLE SNOW 2699

TABLE 3. Deuterium Versus Temperature Correlations at South Pole Over the 1957-1978 Period With the Correlation Coefficient and the Confidence Level

Corre- Confi-

diSD/dr, lation dence Atmospher- (%døC) +- Coetfi- Level,

Studied Parameter ic Level 1 •r cient %

Deuterium maxima and summer

temperatures (December-January)

Deuterium minima and winter

temperatures (April-September)

Mean annual deuterium values and mean annual

temperatures

Winter to summer amplitude and annual temperature amplitude

ground 11.6 +-- 2.4 0.75 >99.9 650 mbar 11.9 +-- 3.7 0.62 99.7 600 mbar 11.5 +-- 4.0 0.57 99.3 550 mbar 12.0 +-- 4.2 0.56 99.3 500 mbar 12.9 +-- 4.5 0.57 99.3

ground 1.1 +- 3.9 0.06 650 mbar 3.4 +- 2.4 0.31 600 mbar 7.8 +- 4.4 0.37 550 mbar 9.2 +- 4.8 0.40 500 mbar 9.2 +- 5.1 0.38

91 94

93

ground 20.3 +- 8.7 0.47 97 650 mbar 8.0 +- 3.9 0.43 96 600 mbar 17.0 +- 6.1 0.54 99.1 550 mbar 17.8 +- 6.6 0.52 99 500 mbar 16.9 +- 7.4 0.46 97 T maximum 17.1 +- 5.7 0.57 99.3

ground 9.8 +- 2.6 0.67 99.9 650 mbar 8.3 +- 3.3 0.52 98.5 600 mbar 14.1 +- 4.2 0.63 99.7 550 mbar 15.1 +- 4.4 0.64 99.7 500 mbar 15.1 + 4.6 0.63 99.7

inversion due to the formation of cold air up to several hundred meters thick over polar ice sheets due to loss of heat from the surface layers by radiation [Robin, 1977]. This investigator indicated that mean temperatures above the inversion are within 4øC of the effective precipitation tem- perature formation. To account for this relationship, we will compare deuterium data to maximum tropospheric tempera- tures, which can be taken as the best representation of the temperature of inversion on an annual scale.

For these different comparisons, we have used the month- ly means of rawinsonde data compiled at the U.S. National Climatic Center in Asheville. Annual isotope data was obtained by averaging deuterium values between two successive deuterium maxima which are assumed to be due

to December to January precipitation. A basic feature of the south pole climate [Schwerdtfeger, 1977] is the existence of a sharp transition between a long winter (April to September) and a short summer (December and January) during which are expected, respectively, the isotopically poorer and richer precipitation. It thus appears worthwhile to compare deute- rium maxima with the corresponding December to January temperatures and deuterium minima with those of April to September. Hereafter, these two periods will be called summer and winter, respectively.

The results are presented in Table 3 with, in each case, the correlation coefficient and the level of confidence. The

correlation is good between the deuterium maxima and the summer temperatures, for all the atmospheric levels consid- ered. The best correlation is observed with ground level temperatures. On the other hand, the deuterium minima and winter temperatures appear uncorrelated (ground and 650 mbar) or poorly correlated (600, 550, 500 mbar). As a result, the mean annual deuterium and temperature values remain significantly correlated at any level. The best correlation (r = 0.57) is obtained when the maximum tropospheric tem- peratures are used, tending to justify that precipitation is

formed just above the inversion [Miller and Schwerdtfeger, 1972; Robin, 1977].

In addition, we have compared the annual deuterium amplitude with the correspondng summer-winter tempera- ture differences. These two parameters appear well correlat- ed at any level, showing that the annual temperature ampli- tude can be reconstructed reliably from that of deuterium.

Discussion of Experimental Results

First of all, note that an unavoidable noise is introduced in the above comparisons owing to the irregularity of precipita- tion over the year. Monthly stake data obtained from a 50- stake network over 9 entire years (1958-1964, 1976-1977) shows a higher monthly mean value during winter than during summer (2.2 compared with 1.35 cm of snow per month). Despite this irregularity, Table 3 shows correlation between annual means of deuterium content and tempera- ture.

Comparison of summer and winter temperature profiles at the south pole (Figure 7) provides a possible explanation of the weakness of the deuterium-temperature correlation dur- ing winter. Owing to the large inversion (up to 20øC) between the ground and 650 mbar levels during winter, it is impossi- ble to accurately estimate a mean temperature of precipita- tion formation from that existing at any tropospheric level. Furthermore, due to this strong inversion, there is no correlation between ground temperature and upper air tem- perature [Picciotto et al., 1966]. As an example, the correla- tion coefficient between the ground and 650 mbar levels during winter is only 0.3 (over the 1957-1978 period).

On the other hand, the temperature difference between these same levels (ground and 600 mbar) is very weak during December and January. Moreover, during these months the surface temperature correctly reflects the air temperature aloft [Picciotto et al., 1966] and the variations over the studied period are well correlated for any pair of levels

2700 JOUZEL ET AL.: CLIMATE INFORMATION FROM SOUTH POLE SNOW

between ground and 500 mbar (the correlation coefficient is generally higher than 0.9 with a minimum value of 0.83 for the ground-500 mbar pair). Therefore the temporal variation of the average temperature of precipitation formation can be estimated from atmospheric data recorded at any level during summer months.

The strong winter inversion which prevents an estimation of the temperature of precipitation formation from available data thus appears to be the basic reason for the low deuterium temperature correlation observed during winter. On the other hand, there is practically no noise introduced during summer months.

From their study of precipitation samples collected over 1 year (November 1964 to October 1965) at the south pole, Aldaz and Deutsch [1967] tentatively deduced a linear tem- perature versus •3•80 relationship with a slope of 1.4 %o equivalent to a deuterium-temperature gradient of 10.8 %døC. This figure compares well with that determined for the 21 successive summers, lying between 11.6 and 12.9 %døC depending on the tropospheric level considered (Table 3). Note that this consistency between seasonal and interannual temperature isotope gradients must be expected if there are no large seasonal or interannual changes in the characteris- tics (• and 00, see below) of the air masses at their origin. In this respect, the observed consistency is an interesting result of the present study.

To sum up, the most important result is the experimental evidence of a relationship between the temperature of forma- tion of precipitation and its stable isotope content from a 21- year snow sequence. This evidence is mainly based on the comparison between summer temperatures and maximum deuterium values (Table 3). There is no physical reason that such a relationship is not also valid for winter precipitation, but it is impossible to check this point from the present set of data, as the temperature of formation is indeterminable from that recorded at any tropospheric level. On a yearly scale, there remains a correlation between the mean values of

deuterium content and temperatures for all levels. This correlation is sufficiently high to justify the use of isotopic data to reconstruct past tropospheric temperatures even over short time periods for which only small variations are expected.

Theoretical Considerations

From a theoretical viewpoint, it is generally accepted that the variation of the isotopic content of snow can be de- scribed by a Rayleigh process, assuming that condensation of the vapor proceeds at isotopic equilibrium with respect to the fractionation coefficient relative to the vapor solid phase change and with immediate removal of the condensate after its formation [Dansgaard, 1964; Aldaz and Deutsch, 1967; Robin, 1977]. In this model the/i values for the condensate, •3½, will change according to

d•ic (1 da (a- 1) dmv) dO = (1 + •ic) + a dO m•, dO

where a refers to the isotopic fractionation coefficient (D or 180) at the condensation temperature 0; m•, is the water vapor content of the air mass. Integration of this equation shows that for given initial condensation temperature 00 and isotopic composition of the water vapor 80, the 8D and 8•80 contents and the condensation temperature can be consid-

ered as linearly related over the range of precipitation formation temperatures at a given site. About this point, it can be noticed that south pole precipitation is, at least for ice crystals, mainly formed in air masses originating from the half circle between 225 ø and 45 ø which includes the Weddel

Sea [Smiley et al., 1980]. It would be interesting to examine further if a part of the year-to-year variability existing in the isotopic signal is related to the condition of temperature and also of sea-ice extent [Bromwich and Weaver, 1981] existing in this possible source region.

To calculate the temperature-isotope gradient from the previous formula, we have assumed that the condensation is an isobaric process during its last phase, which takes place over the Antarctic Plateau. Taking the mean value of the monthly maximum tropospheric temperature as the mean condensation temperature (-35øC), -402 %o and -51.6 %o as mean deuterium and oxygen values (determined in the firn), and using the a determined by Merlivat and Nief [1967] for D and by Majoube [1971] for •80, we obtain diSD/dO = 14.9 %døC and dli180/dO - 2.35 %døC. These figures are sub- stantially higher than those previously quoted, particularly for/5•80. Moreover, a theoretical/tD-/t•80 slope equal to 6.3 is derived, significantly lower than the experimental one (7.6-7.7 with our data).

We are presently developing [Merlivat and Jouzel, 1979; Jouzel and Merlivat, 1981] a more general isotopic model taking into account the fractionation effects occurring at the air-sea interface and during the atmospheric processes lead- ing to formation of rain and snow. We propose the introduc- tion of a kinetic effect (similar to that existing at evaporation) at vapor deposition in supersaturated environment over ice. The effective fractionation coefficients between vapo.r and ice then depend on the supersaturation over ice and are decreased for both deuterium and oxygen 18. Miller and Schwerdtfeger [1972] pointed out that ice supersaturations of about 20% can be considered characteristic of the air advect-

ed from the warmer maritime regions over the Antarctic plateau in the lower troposphere above the surface inver- sion. With this value, the calculated temperature isotope gradients become 11.7 %døC for D and 1.48 %døC for •80 [Jouzel and Merlivat, 1981] leading to a •D-•180 slope of 7.9. These figures are very consistent with experimental results. This applies for the /5180 gradient of 1.4 %døC [Aldaz and Deutsch, 1967] as well as for our best estimated value obtained for the summer months (12 %døC in •D on the average between the ground and 500 mbar) and also for the •D-•180 slope (7.69 in the firn compared with 7.9). Thus our new model eliminates the contradictions existing when the classical Rayleigh model is applied for snow formation and leads us to propose a more convinvcing explanation of the experimental gradients determined at the south pole.

6. TEMPERATURE VARIATION ON A 100-YE^R

TIME SCALE AS DEDUCED FROM THE DEUTERIUM PROFILE

To derive the temperature variation at the south pole since 1887 from the complete deuterium profile presented in Figure 1, we first calculated each mean annual deuterium value, a year being defined by two successive deuterium maxima as above. These raw data were smoothed by using the filtering technique already mentioned (Figure 8). The temperature amplitude given in Figure 8 corresponds to the ground level gradient (i.e., 20.3 %døC). Note that this ampli- tude would be slightly higher for the maximum tropospheric,

JOUZEL ET AL.' CLIMATE INFORMATION FROM SOUTH POLE SNOW 2701

Fig. 8. Smoothed curve of mean annual temperature change versus time estimated from the deuterium profile of Figure 2, with indication of the estimated temperature amplitude at ground level and of the individual mean deuterium values of the 91 successive

years.

600,550, and 500 mbar levels, for which the most significant relationship (about 17 %døC) was obtained at the annual scale (Table 3).

The total temperature amplitude then derived is of 1.7 +- 0.6øC at tropospheric levels and 1.5 +- 0.6øC at ground level with, as a striking feature, significantly higher values for the period prior 1950 compared with the 1950-1978 one. The difference between the mean deuterium values (10.6 %0) corresponds to a temperature decrease of 0.5øC at ground level from the relatively warmer 1887-1950 period compared with the relatively colder 1950-1978 one. Over the first period (1887-1950), significiant events can be observed with maxima and minima centered around 1912, 1928, 1944 and 1900, 1921, 1936, respectively (Figure 8). Note the rapid temperature decrease between 1944 and 1965 followed by an equally rapid increase up until 1974.

The same filtering technique was applied to the minimum and maximum annual deuterium values and to its annual

amplitude. The first two curves presented in Figure 9 can be taken as representative of winter and summer temperatures. (The annual curve is included for comparison). Note, how- ever, that the representativeness of the curves is better for the summer curve than for the winter curve as deduced from

-t v /

$ O U TI-I POLl

i I i i "'---

Fig. 9. Smoothed curves of the minimum (curve a) and maxi- mum (curve b) annual values of deuterium versus time together with the mean annual curve c.

2O

Fig. 10. Smoothed curve of the annual amplitude of deuterium versus time.

Table 3. Prior to 1950, the three curves vary in the same way. Subsequently, the mean and minimum temperatures still appear well correlated but the maximum temperature shows a different variation with time, in particular after 1965. Note that the filtering technique was adapted to solve problems posed by end effects and the sharp increase of the maximum temperature after 1965 does not appear attribut- able to such effects.

Similarly, the annual temperature amplitude was recon- structed from the annual amplitude of deuterium values (Figure 10). This curve presents large fluctuations, in partic- ular over the last decades, with low values in the first part of the sequence (1887-1940), then higher values centered around 1950, followed by a rapid decrease until the middle of the sixties and a further increase. Due to the possible influence of vapor diffusion for time scales greater than 20 years, we have to be quite cautious in interpreting the first part of the curve (1887-1940). However, it is reasonable to conclude that the low values of deuterium amplitude before 1940 correspond, at least partly, to a decrease in the annual temperature amplitude over this period. Inversely, the fol- lowing part of the curve (1940-1978) cannot be misinterpret- ed and therefore clearly demonstrates that the larger mean annual temperature changes then observed are accompanied by important fluctuation in the temperature amplitude with low values in the colder periods (centered around about the beginning of the sixties) and high values during the warmer ones (the forties and the seventies).

In Antarctica, there have been only a few attempts to estimate past temperature conditions from isotopic data on the 100-year time scale. Such studies have been undertaken on two ice cores recovered in the Law Dome area (66øS, 113øE) reaching back about 240 and 450 years [Budd and Morgan, 1977], respectively. A common feature of these two profiles is the existence of two maxima around 1900 and 1950 with only one (but very pronounced) minimum during this time interval. Therefore there is no clear correlation with our

south pole profile. Recently, Petit et al. [1982] obtained a deuterium profile

covering the last 160 years from a core drilled in the Dome C area (74øS, 124øE). Despite a noise introduced in the 8 record by the removal processes at the surface, which are greater in the Dome C area than in other stations [Benoist et al., 1982], some resemblances can be observed between the two

smoothed profiles (even if there is a slight shift in the given

2702 JOUZEL ET AL.' CLIMATE INFORMATION FROM SOUTH POLE SNOW

TUNE ELITœMLIM œ11JV6E(øC) CHAIVE

0.2

OL)TN OLE

ture data (maximum tropospheric value) at south pole and in Antarctic stations over 4 successive pentads.

dates). For instance, the Dome C curve shows a well-marked maxima over the 1920-1940 period followed by a rapid decrease and then a further less pronounced maximum.

Due to the sparsity of the data, it is difficult to indicate if the lack of correlation when exists is mainly attributable to the method of reconstruction itself or to a geographical variability (the distances between the three sites are more than 1,000 km). Note, however, that the south pole recon- structed curve is by far the best documented one, with a precise dating and a calibration of the short-term isotopic record. Obviously, other profiles in the south pole area have to be obtained and many other sites need to be investigated to determine the spatial representativeness of the south pole curves (Figures 8-10).

Most of the meteorological evidence in Antarctica, and in particular for the interior of the continent, has been obtained since the beginning of the International Geophysical Year (1957). No longer record exists for the Antarctic continent itself [Schwerdtfeger, 1970]. There is only one place south of 55øS for which there is an uninterrupted record since 1903: the Argentine station Orcadas Del Sur (60øS, 44øW). Note that the annual temperatures are essentially determined by the winter conditions and are clearly related to the duration of the presence of ice [Schwerdtfeger, 1970], this probably limiting the spatial variability of the Orcadas series.

Examination of the spatial representativeness of the south pole curve can be undertaken only over a more restricted period. This has been done by using data presented by Damon and Kunen [1976], showing the mean surface air temperatures for 4 pentads between 1955 and 1974 as a function of the latitude south of 45øS. Results relative to the

six Antarctic stations (65øS-90øS) are presented in Figure 11 along with maximum tropospheric temperatures at the south

pole and deuterium values. These three curves compare quite well, but the number of stations and the period considered are very limited.

7. CONCLUSION AND PERSPECTIVES

The following are the main points of interest of this study based on the interpretation of a very detailed deuterium profile in the south pole snow spanning the 1887-1978 period.

1. Totally reliable dating was obtained from four differ- ent techniques over the 1955-1978 period (deuterium, triti- um, /3, and stratigraphy), and from deuterium and strati- graphic indexes back to 1927. From then back to 1887, only deuterium indexes were used.

2. A decrease in annual snow accumulation was deduced

from a comparison of the 1887-1930 and 1930-1978 periods (from 10.0 to 8.5 g cm -2) with a mean of 9.2 g cm -2 over the entire period (1887-1978). Between 1887 and 1957, our mean annual value (9.5 g cm -2) is, respectively, 30% and 40% higher than that previously estimated by Giovinetto and Schwerdtfeger [1966] and Mosley-Thompson [1980], and. the significant variations are not in phase with those shown in these two studies.

3. From a thorough examination of the data relative to the very well documented 1957-1978 period, mean annual and maximum deuterium values and annual deuterium am-

plitude were shown to be correlated with the corresponding mean annual and summer temperatures and the annual temperature amplitude. The degree of correlation depends on the tropospheric level considered (Table 3). On the other hand, winter temperature and deuterium minima are poorly correlated; this low correlation is attributed to the large temperature inversion during winter, which prevents a good estimation of the temperature of precipitation formation from available data.

4. Experimental temperature-isotope and •JD-•j180 gradi- ents are in good agreement with theoretical gradients, taking into account the fact that snow formation is a nonequilibrium process.

5. One hundred-year variations of mean annual, sum- mer, and winter temperature were reconstructed along with the annual temperature amplitude.

A question which has not been discussed is how long are ,:

the isotopic signals preserved beyond one century. Note' that the snow density at the bottom of the core (0.54 g cm -3) is close to the critical value (0.55 gcm -3) at which the mixing by vapor diffusion essentially ceases according to Johnsen [1977]. We may therefore be optimistic; either the seasonal isotopic variations are really preserved during all the firnifi- cation processes and, subsequently, in ice, or they can probably easily be reestablished by applying the deconvOlu- tion technique proposed by Johnsen [1977].

This detailed isotopic study of south pole snow, and particularly, the first successful comparison of the isotopic content of snow and temperatures (including tropospheric temperatures) at the same site, is very encouraging for the use of stable isotopes as a tool for past temperature recon- struction over periods for which relatively weak variations are expected. Furthermore, this study shows that the south pole is a good site to undertake such studies over a large part of the Holocene.

Acknowledgments. We are very grateful to M. Pourchet for his participation in field work (sampling and stratigraphic observations) and/3 determinations, to D. Mazaudier for tritium measurements,

JOUZEL ET AL.' CLIMATE INFORMATION FROM SOUTH POLE SNOW 2703

and to R. Chiron for stable isotope determinations and technical assistance. We thank J.P. Benoist for numerical data processing. This work was supported in the field by Terres Australes et Antarctiques Fran•aises, Expeditions Polaires Fran•aises, and the U.S. National Science Foundation (Office of Polar Programs).

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(Received March 3, 1982' revised September 15, 1982; accepted October 26, 1982.)