Journal qfGlaciology, Vo!. 43, No. 143, 1997

Some comments on cliIllatic reconstructions froIll ice cores drilled in areas of high Illelt

Roy M. KOERNER

Terrain Sciences Division, Geological SUTVey qfCanada, Ottawa, Ontario K1A OE8, Canada

ABSTRACT. Poor considerati on has been given in many Arctic circum-polar ice-core studies to the effect of summer snow m elt on chemistry, stable-isotope concentrations and time-sca les. Many of these cores are drilled close to the firn line where m elt is intense. Some come from below the Grn line where accumulation is solely in the form of super­imposed ice. In all cases, seasonal signa ls are reduced or removed and, in some, time gaps develop during peri ods of excessive m elting which situate the drill site in the ablation zone. Consequently, cross correlat ions of assumed synchronous events among the cores are invalid, so that time-scales along the same cores differ between authors by factors of over 2. Many so-called climatic signals a re imaginary ra ther than real. By reference to published a nalyses of co res from the superimposed ice zone on Devon Ice Cap (Koerner, 1970) and Meighen Ice Cap (Koerner a nd PatersOJ1, 1974), it is shown how m elt affects all the norm ally well-es ta blished ice-core proxies and leads to their misinterpretation. D espite these limitations, the cores can give valuable low-resolution records for all or part of the Holocene. T hey show that the thermal maximum in the circum-polar Arctic occurred in the early H olocene. This m ax imum, effected negative balances o n all the ice caps and removed the smaller ones. Cooler conditions in the second half of the H olocene have caused the rcgrowth of these sam e ice caps.

INTRODUCTION

M ajor contribution s to the study of climate change h ave been made over the past two decades by the stud y of ice co res from the world 's two great ice sheets of Greenland a nd Anta rctica (e.g. D ansgaard and others, 1985, 1993; Lorius and others, 1985; Barnola and others, 1987). These ice sheets provide areas where there is positive balance and no m elting. Consequently, co ntinuou s reco rd s of climate cha nge are preserved. Studies of cores from ice caps on Spitsbergen (e.g. Gordienko and others, 1981; Punning a nd Tyugu, 1991; Sin'kevich, 1992; Tarussov, 1992) the Severnaya Zemlya a rchipelago (e.g. Kotl yakov a nd others, 1991) and Meighen Ice Cap (e.g. Koerner and Paterson, 1974) in the Canadi an Arctic are less well known. With one exception, these records are restricted to the Holocene period. H ow­ever, recognition of a climatic signal a nd development of time-scales in some of these co res is difficult and, a t times impossible, due to heavy summer melting and the presence of time gaps in the record formed during long periods of negative balance at the drill site. Unfortunately, m a ny of these studies have not given due consideration to the mode of accumulation and melt at the drill-site locat ions and its effect on the derivation of time-scales, or the interpre tation of ice-core chemistry, ice texture and stable-isotope ratios in the cores. Many of these problems h ave a lready been dis­cussed and publi shed in work based on a core from M eigh­en l ee Cap (Koerner, 1968; Paterson , 1968; Koerner and others, 1973; Koerner a nd Paterson, 1974). I wish, therefore, to re-address the problem of ice-core ana lysis in cores taken from a reas of high melt la rgely by reference to that work. Benson's (1959; see a lso Paterson, 1994) Grn-facies terms will be used throughout.

90

DRILL-SIT E CHARACT ERISTICS

The location, elevation and surface cha rac te ri sti cs of the drill sites to be di scussed are shown in Table 1 and Figure I. Published results from these cores are shown in Figures 2-4. Apart from the results from M eighen and Agassiz Ice Caps, the values plotted in the fig ures a re derived from digitiza­tion of published diagrams referred to in thi s tex t. T ime­scale a re those determi ned by the authors of the va rious publications.

Canadian Arctic

Meighen Ice Cap is a small ice cap that an ice co re, drilled in 1964, showed to have acc umul ated a lm ost ent irely through the formation of superimposed ice (K oerner and Paterson , 1974). Mass-ba lance measurements have bee n made a lmost annually since 1959 and show a mean an nual balance for the ice cap of - 13.2 g cm 2 a \. The same period has effected a slightly negative balance of - 27 111m ice a - \ on the dome where the ice core was drilled. \80 results from this core are shown in Figure 5.

Six surface-to -bedrock cores have bee n drill ed from Agassiz Ice Cap in an area that li es within the percolation zone. Thi s paper uses the co mbined record of two cores drilled at the LOp of the Ilowline in 1984 and 1987 as a refer­ence far climatic change (Koerner and Fi sher, 1990; Fisher and Koerncr, 1994; Fisher and others, 1995) as the area does not suffe r the limitat ions for ice-core interpreta tion dis­cussed in this paper.

Svalbard

The cores on Sva lbard have been drilled by the former Soviet Union (3) and theJ apancse (1). TheJ apanese co re is from the

Koerner: Climatic reconstructions from ice cores in areas qf high melt

Table 1. Melt (in parentheses, column 5) is the j)roduct if melt j)er cent and annual accumulation, nol halance. H owevel; annual accumulation equals balance above theJirn line. The melt values are only directly comparable between the sites if the sites lie above the saturation Line. Below this, run-wocCllrs, i.e. melt may exceed 100% ifmeLting extends into underlying annual layers. The Iwo valuesjor balance on HfJghetta include the valuejor the last 20 years and (in parentheses) the valueJor the 1959- 63 period. The latter is the one used to date the core; Fujii and others (1990) identified recurrent layers qf this thickness at depth and considered them annual la)eTS . • Stiivenard and others (1996) believed the isotopic signature qf a 37- 60 cm layer if ice sandwiched between

Jro;::en sediments underlying a more recent Vavilov core rejJTesents Pleistocene ice. Somewhat contentious, it must at best represent ice that may have survived the early Holocene by ils burial underj)rotective debris. The "trend"colwnn rifers to whether tlte trend in either (j 18 0 or melt between 7 and 0.5 ka is significant

Site ELevation Ice depth klean (i'BO lee mel!/Huen! Balance nrn Pieis!ocene ice) Trend

Agassiz' M eighcn 2

3 H oghctta

Fr/Gron' LomonsovS

Austfon na6

Vav ilm,7 Akad. :\'auk '

ma.s. 1.

1730 268

1200

450

1000 700 720 810

m

127 121 85.6

213 220 566 +67 761

/JffWlt

o 7 ka

27.1 21.2

- 11.3

- 1+.2 17.0

20.0 - 21.0

thickness .,

gem - mmw.e. a m

2.9 (6) 98 60 YeS Yes 100 (172) 0 0 :\'0 No 100 (200) 47 0 No No

(200) 750 + No )lo

820 27 30 1\0 )10

67 (532) 794 30+0 :':0 Yes 41 (205) 113 < 10 No' No cl·7 (154) 315 Ycs Yes

I: Fisher and others, 1995; 2: Koerner and Paw'son, 1974; 3: Fuj ii a nd Olhcrs. 1990; 4 : Punning and 01 hers, 1980; 5: Gordicnko and others, 1981; 6: Tarussov, 1992; Zagorodnov and Arkhipov. 1989; 7: K o tl yako,' and olhers, 1991.

H oghetta ice dome on Spitsberge n (Fig. I). No di rect mea­surements of mass balance have been made in the vicinity of the d rill site but, as rhe ice core drill ed from there consists entirely of superim posed ice, the d rill site must lie within rhe superimposed ice zo ne (FLUii and others, 1990).

The Sovie t ice cores a re from Vestfonna (Punning and 1) ' ugu, 1991; Sin'kevich, 1992), Aust fonna (Zago rodno\' and A rkhipov, 1989; Ta rrusov, 1992), the ice divid e be twee n

G ro nfj ord and Frid tj ov Gl ac ie rs, w hi ch I will call th e Gro nG ord ice co re (Punni ng a nd o th ers, 1980) and Lo mo­nosO\' Ridge (Gordienko and others, 1981). While accumula­ti o n rates are high, mel ting in summ er soaks most or a ll of th e a nnual layer (L omonosov, Vestfo nna and Austfonn a ) or occasiona ll y removes the annual layer (G ronG ord ). Thus, th e d r ill sites are ge nerally in t he sa tu rati on zone w he re melt percolates deeper than the current ann ual layer and,

D

Fig. 1. D riLL sites rifened to in text. l. GRIP and GISP2; 2. Devon Ice Cap; 3. rlgassi;:: fee Cap; 4. Meighen Ice Caj); 5. Academii Nauk; 6. Vavilov lee Caj); 7. H @ghetta, GrfJrifjord, Lomonosov; 8. Allsifonna, Vestfonna; 9. Penny lee Call; 10. Fran;::] osif.

91

Journal qfGlaciology

Balance near equilibrium line

50 ... .... ..... ......... , .. . ......... ... : ................... : .... ... .. .. ........ ~ ................... : . . . . . . . . . . . . .. . ..

40 . .

30 ~

20 'ca ai 10 ~ 0 E -10 E -20 Q) () -30 c ca -40

: : r-----~----~-, ..... ... .... .. : .......... .... ..... ~ .. ...... .. ..... ; -- Devon 1050 m asl

(ij CD -50

-60 .. .. ........... j .............. ... ( .. .... . .. .... : - Meighen 268 m asl

......... ~ ..... , .. .......... . ~ .. ..... .... ... . " .. ~ .... .... ... .... .. .. : .................. . ,' , .. . . . . -70 -80

.. .. .. . ....... .. ( ... ..... , .. , ...... -1 -' .... .... ........ ~ ..... , ..... .... .... ~ . . .. . ... ...... ... ':' .. .. .... , ..... ... ~ .. ......... .. .... . .... ....... , .... ... ........... : .. .... .... .... ..... ; ........ ....... .. .

60 65 70 75 80 85 90 95

Year

Fig. 2. Stable isotopes on a depth scale: Meighen Ice Cap ( Koerner and Patmon, 1974), Lomonosov ( Gordienko and others, 1981) and GronJJord/ Fridtjqf ( Punning and others, 1980) ice cores. Values from the Soviet ice cores are from digitization cif the published records. M eighen and GmnJJord data sll1jace to bedrock, Lomonosov suiface to 20 m above the bed.

in the case of Gr0nfjord, occasionally in the ablation zone. Stable-i so tope d a ta from th e G ro nfj ord and Lo m onosov cores a re shown in Figure 2 a nd melt from Austfo nna is shown in Figure 4.

Severnaya Z ernlya

The Severnaya Zemlya archipelago has been the centre of deep drilling by the former Soviet Union since 1975 (Ko­tlyakov a nd others, 1991). Six boreholes on the Vavilov ice dome (460- 557 m ) and one on Academii Nauk ice dome (AN ) (761 m ) have been drill ed to bedrock. Both ice caps ri se above the firn line (Kotlyakov a nd others, 1991; Barkov and others, 1992) but the accumul a ti on undergoes substan­tial summer melt. The to ps of both ice caps probably li e within the saturation zone as the AN ice core consists, on average, of 42% melt ice and Vavilov ice core (from Kotlya-

J 0

co re

92

Stable Isotopes, Svalbard, Arctic Canada

-5 ,

-7 ................ .... ..

-9

-11 ... ;.

-13

-15

-1 7

-19

-21

-23 ......... ;.. . .. ,: ..

-25

o 40 80 120 160 200 240

Depth (m)

Fig. 3. Stable isotopes, Academii Nauk, Vavilov ( Kotlyakov and others, 1991) and Agassiz ( Fisher and others, 1983) Ice Caps. Valuesfol' Academii Nauk and Vavilov al'eJrom digiti­zation cif the published diagrams. R un-won Vavilov ( B al'kov and others, 1992) has probably afficted this record.

kov a nd others, 1991) 48% melt ice. M elt-ice percentage a nd stab le-isotope data fro m Soviet cores a re shown in Figures 2- 4.

EFFECTS OF MELT ON ICE-CORE ANALYSIS

Stable is otopes

Sta ble iso topes (6180 , 6D ) are used as proxy tem pera ture indicators in ice-cor e a nalyses. The tra nsfer functi on (6 / tempe rature) is based on empirical rela tionships drawn from globally di stributed data (D a nsgaard and other s, 1973)_ The most impo rtant effect is tha t of cooling of the wate r vapour as it moves from its ocean source to the loca­tio n where it condenses close to the ice-core site. While lim­ita tions of the transfer function have been well discussed in terms of ice cores from above the percola tion line (Pa terson, 1994), they are less well known for ice cores fro m a reas of ve ry heavy melt. The most importa nt (a nd additi ona l to those th at apply above the percola tio n line) have been dis­cussed by Koerner a nd others (1973) a nd a re as foll ows.

L E a rly summer m elt will cause melting of the very nega­tive (cold )-6 winter/spring snow which lies near the sur­face. The meltwa ter then percola tes down to refreeze within, or at th e base of, the current annual snowpack. Further melt may cause run-ofT o f less negati ve (wa r­m er)-8 snow deposited during the ea r ly winter/fa ll per i­od. A very negative (co ld)-8 snowpack remains.

2. L a te summer melt will cause m elting of surface snow depo ited during the early summer period. Thi s m ay p ercolate and refreeze in the annu a l snowpack. The ea r­ly spring/winter snow is then exposed at the surface a nd furth er melting m ay cause this snow to leave the ice cap as run-off. A less negative (wa rme r )-6 snow pack re­nla ms.

3. Cool summers, with very little melt but accumula tion of summer snow, will add a layer ofl ess negative wa rmer-6 snow at the surface. This will give a di sproportiona tely wa rm 8 value fo r the annual layer.

-20 -22 -24

~ -26 0 -28 Cl)

?o -30 -32 -34 -36 -38

0 2 4 6

Koemer: Climatic reconstructionsJrom ice cores in areas rifhigh melt

8 10 12

-17 ~

Cl)

o f

-25

Years before present (x 1000)

Fig. 4. Melt recordforAcademii Nauk ( Kotlyakov and others, 1991) Ausifonna ( "lCLrllSSOV, 1992) and Agassiz lee Caps ( Koemer and Fis/w; 1990). AI/el! expressed as gem 2 a I based on accumulation mtes alld melt jJelu ntages jmblished by the authors and, ill the case if Academii Nauk, and Ausifonna, digitizedJrom the published diagram. Time-scales are those if the authoTS. In the case qf Ausifonna, both a theoretical and stratigraj)hic time-scale aregiven ( .:(agorodnov and Arkhij)ov, 1989). I lIse the theoretical that gives the bed an age rif 6000- 7000 )lears. The stratigraphic gives a basal ice age qf 3000 4000 )Iears.

4. M elting of the snow cover and subsequent percolation through rec rysta lli zing snow causes isotopic fraction­ation a nd enrichment in 180 of the solid phase at the ex­p ense of the meltwater (Arnason, 1969).

Chemistry and wind-blown dust

Melting causes leaching of ions in the snowpaek. T his post­depositional eHect homogeni zes the snow chemistry, there­by red ucing the seasonal variation s often used to detec t annu a l layering and to date the core. Furth ermore, J ohan­nesse n a nd o th ers (1977), J ohan nessen a nd H enrik se n (1978), Goto-Az um a a nd others (1993) a nd Gj ess ing and othe rs (1993) among others have shown that some ions, such as sulphates, a re preferentially leach ed from the sno"v. This means that, ifmcltwater leave the drill site, the melt-

')'

E u Cl

~ ::E

')'

E u Cl

~ ::E

Melt

90

r 80 70 60 50 40 -l"

Austfonna

....... ... ...... __ ... .

..... . .. .... .. ..... ..... .

.... .. ...... .. ..•.. ...

25

20 Academii Nauk

15

10

5 ... .. .. ..... .... ! .......... .... ... !.... ·· ·· ····· · ... i-

0 .. - . .. .. ... :.

o 2000 4000 6000 8000 10000 12000

Years before present

Fig. 5. Annual mass balance at the drill site on j\!/eighen Ice Cal) and at a Slake sitl/ated close to the equilibrium line on the northwest side if Devon Ice Cap.

water is su Iph a te-enri ched a nd the remain ing ice co re is sulpha te-deple ted . We have found in work on Canad ia n High Arct ic ice caps tha t, whi le chlorides are much less affected than sulphates by th is process, intense melting will alter th e pos it ion and/or a mplitude of chloride spring peaks to rende r th em unusable fo r recogni zing a nnu a l laye rs; some inves ti gato rs have reli ed on the seasonal variations of chlorides to date cores (e.g. Pu nn ing and Tyug u, 1991). High­Iyacid layc rs, fo rmed from di stant volcanic e ruptions and used lo cl a te ice co res drill ed from above the p ercolat ion li ne (Fi sher a nd oth er , 1983), may a lso be a lte red uffi­cientl y in m agnitude to ma ke them unusable fo r dating pur­poses (Fujii a nd others, 1990).

Inso lu b le micropanicles te nd to co ll ec t at a melting surface. \lVith melting thi s produces a small pea k in con­ce ntrat io n s (K oe rner, 1977). H oweve r, non e of the core analyses cl isc ussecl here expl o its the use of micropart icles fo r dating purposes. [n . ome cases, thi s was clue to a lack of adequ a lc equipment (e.g. a Coulter counte r ) and in others (e.g. M eighen Ice Cap) because melting and ru n­off removecl th e seasona l cycles. Small er ice caps a re also likely to receive high inputs o f wind-blown dust from the surrou ndi ng ice-free terrai n. Th is is particul a rl y true dur­ing winters of very low snow accumul ation. Some of these parti cles may be soluble a nd wi ll affec t the ice chemistry during summ er melt. Part of the inc reas in g pH /t im e trend in the H oghetta co re (FlU ii and others, 1990) may be attr ibu table to a n increased input of buffering wind-blown dust with inc reas ing depth. K oerner and Paterson (1974) related dust-layer concentrations in the M eig hen core to the chang i ng size of the ice cap. Expan ion of the ice cap, for exam plc by loweri ng of the fi rn line to ice-free g rou nd beyond the ice-cap margins, inc reases the drill site-to-dust source di sta nce; dirt-I aye ,- concentrati on in the co re then dec reases (Fig. 5; Koerner, 1968). Thus, reduc tion of dust concentrations may be a reOec tion of increased g lac ieri za­tion of t he surrounding land.

The melt-layer transfer function and ice texture

The distribution of melt laye rs and the va riations of ice tex-

93

Journal qfGlaciology

ture in ice cores ca n be u sed as summe r-t empera ture proxies. The cl imate-related transfer func tion depends, in the case of ice layers, on wa rmer summers causing a higher percentage of mclt ice in the a nnual layer. The transfer func­tion is dependent on empirical relation hips found in the a rea of investigati on. Fo r exam pl e, K oe rner and Fi she r (1990) used an ice per cent/elevati on/temperature relation­ship in the Canadian High Arctic ice caps (Koerner, 1977) to calcul ate a 2°C temper ature change in the Holoce ne from the changing melt pe rcentage in the Agassiz co res . The transfer function has two thres holds. The upper thresh­old is crossed when the zone of superimposed ice ex tends into the drill-site a rea. In thi s zone, all summers causing comple te melting of th e annua l snow laye r will leave a 100 % mel t-i ce section, whether run-o ff occ urs or not. A summer with much Hill-off will be warmer than one with little or none. However, the ice-core record will show a fl a t, 100 % melt-ice record, regardl ess of how much run-off oc­curred. The lower threshold is crossed when the clima te causes the dry-snow line to drop below the drill site. In this case, no melt layers are formed and there is no melt record. Both situa tions can be seen in two of th e cores presented here; they will be discussed later. In fact, Alley and Anan­dakrishnan (1995) have extended thi s technique to dry-snow areas. By very careful examination of the GRIP Greenland core, they have developed a m elt record from a core with a n average of only one 2- 3 mm thick ice laye r every 200 years! Such a study demand a ver y high-quality ice core but ex­tends the applicability of the method.

In the second case, i. e. in the zone of superimposed ice, where there is 100% melt ice (Meighen a nd H0ghetta ), a cli mate transfer fun ction has to be developed from ice-tex ture changes and is useable in only very genera l terms. The tra ns­fer function is based on the dependency of g rain-size in the newly forming ice, on the degree of meltwater saturation of the snow cover and the temperature of the winterlspring snowpack a nd underl ying ice during the early stages of melting (Koerner, 1970; see a lso Waka ha m a and other s, 1976). At one end of the sp ectrum, meltwater standing o n the surface at the end of the melt season freezes slowly to form very coarse-grained ice. At the oth er end of the sam e spectrum, incompletely satura ted snow forms fine-grained , bubbly, ice. On Devon Ice Cap, there is a gradual transition in the superimposed ice zone, from fine-g rained, bubbly ice just below the firn line to coarser-grained , clea r ice just above the equilibrium line (K oerner, 1970). This cha nge is rela ted to the summer temperature/eleva ti on rel ati onship. A simple ice-core model, based on this rel a tionship equa tes fine-g rained bubbly ice with cold summers a nd coarse, clear ice with warmer summers. H owever, the warmest summers a re unrepresented as a negati ve balance removes all or pa rt of the a nnual accumulation. The transfer function therefore has limited application.

By simil a r processes, g ra in-size/bubble-concentra tion vari ations at one site each year may produce recognizable a nnual laye rs in the superimposed ice zone. These laye rs have been studied on Vestfonna by Palosuo (1987) and used by J onsson and H ansson (1990), toge ther with chemical and mi c ro pa rticl e sig na tures, to d ate a sh a llow core from Storoya ice cap (a sm all island off the east coast of Nord­Austlandet). The latter is the only work I a m aware of tha t has made a ca reful study of annual laye ring in superim­posed ice and applied it to the development of a ti mc-scale

94

in the superimposed ice zone. H owever, continuous suites of recognizable annual laye rs through periods of past climatic warming must be uncommon a t sites now situa ted near the urn line. Any period of wa rmer clim ate will c reate a se­quence with seasons and/or complete annua l layers miss­ing. Furthermore, if at any t im e during the hi story of the ice cap, the firn line com es close to the drill site, melting will also add a layer of ice to the continuous ice under the firn (6 m d ep th on the no rthwest side of D evon Ice Cap (Koerner, 1966)). In this se nse, the resolution of co re sec­tions deposited just above the firn line can never be better than about 6 m .

Koerner a nd Paterson (1974), although unabl e to detect continuous sequences of annua l layers, made qu antitati ve measurem ents of bubbl e conce ntrations in th e Meighen core to draw inferences about climatic and slope changes at the drill site. For example, the bubbly, fine-gra i ned ice tex­ture of sections of the ice core 22- 44 m from the surface sug­gested they form ed during years with very cold summers. They were a ttributed to the Little Ice Age, despite stable­isotope val ues that were less negati ve than the enclosing ice. Other studies have used qu alitativcly rather than quan­titati ve ly deri ved meas ure m e nts of ice tex ture to detec t climate sign als (e.g. Kotlyakov and others, 1991).

High variability of the accumulation rate

"Vhile the snow-accumulatio n rate may not var y much close to the firn line, the speciuc m ass balance does, due to fluc­lUations in the melt rate each summer. The 1961- 93 mass­balance records taken close to the mean equilibrium line on Devon a nd Meighen Ice Caps illustrate thi s (Fig. 5). O ver the sam e period, the a nnual equilibrium line on De­von Ice Cap has moved b e tween 1500 m a .s. l. (above the urn line) in 1962 and <500 m a .s.l. (below the ice cap) in 1964, 1965 a nd 1976. This g ives a 32 year range of > 1000 m. Any site located within this ra nge, including those in the lower reaches of the firn zone, will have exp erienced years of accumula tion by superimposed ice or firn, a nd yea rs of mass loss. This means tha t som e of the drill si tes cited in Table 1 will include c1iscontinuities, or time gaps.

TIME-SCALES AND DISCONTINUITIES

The main problems facing the derivation of time-scales in cores drilled close to the firn line may be summarized as follows:

1. Gaps in the reco rd produced during periods of negative balance a nd the high vari ability of the drill-site annual mass bala nce limit the use of flow modcls (e.g. Nye, 1960) and m ake detail ed cross-correlation between cores high­ly cO I!i ectural (e.g. Punning and others, 1980; Kotlyakov and others, 1991).

2. Detection of seasonal va ri a tions of ions, microparticles and sta ble isotopes b ecom es conjectura l due to the ef­fec ts of p e rco lat ing m e ltwater (e.g. G ordi enko a nd others, 1981; Punning a ncl Tyugu, 1991).

3. Cross-correlation of volca nic events with the well-dated Greenl a nd cores (H ammer, 1983; H ammer a nd others, 1985) is frustrated by m eltwater alteration of the origi­nal signal strength of these events (e.g. Fujii and others, 1990).

Koerner and Pa terson (1974, fi g. 2) drew attention to prob­abl e time gaps in the Meighen core and did not develop an overall time-scale. However, a general lack of understand­ing of the significance of time gaps in ice cores is common in some of the other studies. :1"o r exa mple, }<L~ii and others (1990), aware of the ex istence of time gaps in the H oghetta core, claimed tha t the co re i ncl uded a 4000 year gap be­tween the basal layer , where o rganic materi a l was dated a t c. 5000 yea rs BP and AD 1700 when they estimated acc u­mul ation recommenced. Dowdeswcll and others (1990) cor­rec tly obj ec tcd to such a time gap pa rtl y on lichenometric evidence from nearby a reas but a lso on the bas is that "a very fine bala nce between mass inputs and outputs would have to be maintained over a very long period of time (ap­proximately 4000 years) to preser ve a 35 m depth of pre­sum a bl y stag nant ice". Thi s is not the case. A tim e gap, while it represents a period of overall nega tive ba lance, need not represent onc of delicate ba lance between positive a nd negative ba la nces. The time gap may represent a lmost a ny combina tion of acc umula tion a nd ablation pe riods of va rying length, as long as the tota l ablation during that per­iod exceeds the tota l accumulation. The 4000 yea r time ga p co uld include a long period of accumulation that thickened the ice cap to greater than its present dimensions followed by (or interspersed with ) period s of ablation thinning the ice cap eventuall y to the 35 m level. The so le requirement is fo r a n overall negative balance persisting for a n un­kn own peri od o f tim e and reducing th e ice cap to 35 m thicknes , foll owed by an overall positive balance begin­ning at the 35 m level and continuing to the present. How­ever, no infere nces may be dra wn e ither abo ut clim a tic change during tha t overall positive balance period (unl ess it can be shown to be continuous) or the sequence of events fo rmi ng the ti me-gap di sconti nu i ty.

The Sovi e t co res from both S\"a lba rd and Severnaya Zeml ya suffer from questionabl e time-scales. A few years of recent acc umulati on, using nuclear fa ll-out laye rs as datum (e.g. Gordienko a nd others, 1981), have been ove r-extrapo­la ted a nd variations of chemica l signa tures have been inap­pro pri a te ly co ns id e red seaso nal (e .g. Va ikm yae a nd Pun ni ng, 1982). In additi on, the poss ibi I ity th a t sections m ay be miss ing from the co res (due to negati ve bal ance at the drill site) has not been conside red in the derivation of th eoreti ca I ti m e-sca les (e.g. Pu n n ing a nd o th e rs, 1980; K otl yakO\' and o thers, 1991).

The imprec ision of their flow-d a ting method s a re ap­pa rent when the basa l ice in Vav ilov Ice Cap has been da­ted at 9000 yea rs in one case (Kotlyakov and others, 1991) a nd >20000 yea rs in another (Va ikmyae and Punning, 1982). The Austfon na core has been g iven two time-scales, o ne theoreti ca l a nd the othe r based on a repetiti ve ice­tex ture signal which is conside red a nnual (Zagorodnov a nd Arkhipov, 1989; Tarrusov, 1992). The time-scales differ by a factor of 2 a t the bed.

DISCUSSION

Because of the defi ciencies of ea rlier a na lyses, and the criti­c isms and limita tions elaborated above, onl y a broad time fra me can be es tabli shed for these cores. However, impor­ta nt low-reso lution inferences about climatic change may still be made. As a first step, I refer to the combined reco rd of two Agassiz top-of-the-O owline co res (Fisher a nd others,

Koem er: Climatic reconstrllctionsJrom ice cores in areas (J/ Iziglz meLt

1995). The time-scale depends, first, on sea sonal signa ls a nd, secondl y, on fine-tuning by correla ti on of volcanic events with those in the Greenland eo res and is beli eved COITect to within ± 5% (Fisher a nd others, 1983). Secondly, I se t apart the Academii Nau k ice core from the other cores di sc ussed so fa r. This core shows a relatively la rge <5

IRO step to very negative values near its base (Kotlyakov a nd others, 1991). Th e step (Fig. 3) is ver y similar to the la rge glacial / inte r­g lacia l steps in all the Greenland, Anta rctic and Canadia n Arc ti c ice cores (Fig. I; Koerner, 1989; Cuffey and o thers, 1995). The <5 cha nges below thi s, in the Academii Nauk core, al so look simil a r to the Younge r Dryas, Allerod and BoIling events in th e Greenland and C a nadian cores (Fig. I; K oerner, 1989). The same conclusions have been drawn by Stievenard and o thers (1996). In gener a l terms, this sug­ges ts tha t the time-scale calculated by Kotl yakov and others (1991) may be reasonable for broad conclusions to be drawn about its Holocene record.

The AN stable-isotope and melt records a re shown with those from the 1984 Agassizcore in Fig ures 3 and 4. All four reco l-d s depict lo ng -pe ri od cooling tre nd s that p e rsis t thro ug h to the Littl e Ice Age. The Austfonna melt profile is a lso shown in Figure 4. This record shows a cooling trend beg inning about 3 ka ago. Howeve r, its earli er reco rd is "n a t" because melt reaches a regul a r 850/0 of th e annu al laye r there. At thi s leve l, the record is no longe r clim a ti­ca ll y se nsitive (warme r summers cause run-o ff). The a s­se mbl ed reco rds show a Holocene p e ri od with a the rm a l max imum early on a nd climate beginning to deteri o ra te be tween 8.5 and 9.0 ka. This deteriora ti on continued inte r­mitt e ntl y a nd has r ecove red onl y sli g htly in th e pas t 150 years.

Although not shown here, the Greenland melt profile of Alley a nd Ananda kri sh nan (1995) a lso shows a warmer per­iod prio r (0 4 ka. So too does the "reco nstructed" <5 180 re­co rd from the Holoce ne pa rt of the Gree nl and GRIP core (Jo h nse n and oth e rs, 1995). Thi s reco rd shows exce lle nt ag reem ent with the Agass iz and AN H olocene blSO a nd melt profil es. I should po int out , however, that no correc­ti ons of the GRIP typ e have been a ppli ed to any of these pro fi les.

No ne of the o th e r reco rds (Fig. 2) shows ev idence of eithe r Pl eistocene ice o r of a cooling trend. This indica tes tha t they all began g rowth during the H o locene, probably dUl-ing its second ha lf. Further support fo r thi s argum ent comes from 11 C-da ted materi al from nea r the base of the Hog hetta ice cap (Fujii a nd others, 1990) which puts the old­es t ice in thi s ice cap loose ly into th e second half of th e Holocene. The ice above thi s lTlust, like the Meighen core, represent il1lermittent g rowth since then with most growth occurring, if we use the Agassiz/Academii Nauk/Austfo nna records as guides, in the las t 2- 3 mill ennia (as Dowdeswell and o thers (1990) suggested ). Regrowth o f a ll the "Pleisto­ce ne ice-free" ice caps must have ta ken pl ace over the last few thousand yea rs with the small est of them beginning re­growth most recentl y.

The prese nt mass ba lances of three ice ca ps and two glac iers that have been measured for ove r 30 yea rs in Hig h Arc tic Can ada (Cog ley a nd othe rs, 1995; Koe rn er a nd Lundgaard, 1995) a re slightl y negative. The same is true o f those g laciers that have been measured in Sva lbard (H age n a nd Li es tol , 1990) a nd on Vav il ov Ice C a p (Ba rkov a nd othe rs, 1992). These negative balances have to be seen in the

95

JournaloJGlaciology

light of the melt and (iISO profiles from Agassiz, Academii Nauk and Austfonna Ice Caps (Figs 4 and 5) which show warmer conditions than present during the first ha lf of the Holocene. It mea ns that very n egat ive bala nces ex isted throughout the first half of the Holocene in the ci rcum-polar region. This was the period th at saw the fin al demise of the great Pleistocene ice sheets. I would suggest that highly nega­tive balances removed not only the great ice sheets but also many small ice caps in the ci rcum-polar region in the early Holocene. This would explain the absence of Pleistocene ice in most of the records discussed in this paper (see l ilble I).

There is other work which lends support to thi s argu­m ent. Li ch e nom etri c a nd lake-sed iment analyses by '''!erner (1988) in the west Spitsbergen area a re strongly sup­portive of late Holocene glacier regrowth in that region. Similarly, Bradley's (1990) synthesis of palaeoclimate stu­di es in the Canadian High Arctie also presents a st rong body of evidence for late Holocene cooling in that region. Kotlyakov and others (1991) cited evidence for restricted ice-cap extent in the Severnaya Zemlya region in the early Holocene, although they extended this period back into the late glacia l as well.

CONCLUSIONS

Ice cores drill ed in areas of hig h melt require g reat caution wh en attempting to derive pa laeocl imatic inform ation from them. Because melting can introduce time gaps and remove good seasonal signals, time-scales can be difficu lt or impos­sible to evalua te. However, I should stress tha t techniques for core analysis have improved enormously since many of these cores were studied. IlTlproved detection limits of ion­anal ysis equipment, the use of modern microparticle COLII1-

ters and a lso of the solid electrical conductivity method as­sociated with continuous pH, liquid conductivity and laser! microparticle measurements (originally discussed by Ham­mer (1980); H a mmer, 1983) co uld make the study of new cores from some of these areas a n importa nt addition to

the stud y of cl imatic change in the circum-polar area. The exceptiona lly careful work of Alley and Anandakrishnan (1995) has also shown that the "cold threshold" of melt-layer anal ysis can be pushed further down the temperature scale to include high-elevation Greenland cores, thereby provid­ing furth er opportunities for cross-correlat ion with abso­lutely dated co res from that area.

ACKNOWLEDGEMENTS

I should like to thank two anonymous reviewers and B. Le­fauconnier 10r very thorough rev iews of the m a nuscr ipt. Their suggestions have substa nti a ll y improved the paper. Thi s is Geological Survey o f Canada contribution No. 43895.

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MS received 13 November 1995 and accejJted ill revisedJorm 11 SejJtember 1996

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