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Dec 20, 1985 - Antarctic volcanic areas are indicated by circled n•mbers: 1, South Sandwich Is lands. 2, South Shetland Islands; 3, James Ross Island volcanic ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90,

VOLCANIC

NO. D7, PAGES 12,901-12,920,

DEPOSITS

IN

ANTARCTIC

Robert J. Delmas(1), Michel Legrand(l),

DECEMBER20,

1985

SNOW AND ICE

Alberto J. Aristarain(2),

and Fran•oise Zanolini (1) (1) Laboratoire de Glaciologie et G•ophysique de l'Environnement, St Martin d'H•res, France (2) Instituto Antarctico Argentino, Buenos Aires, and Instituto Argentino de Nivologia Glaciologia,

Mendoza, Argentina

Abstract. Major volcanic eruptions are able to spread large amounts of sulfuric acid all over the world. Acid layers of volcanic origin were detected for the first time a few years ago by Hammerin Greenland ice. The present paper deals with volcanic deposits in the Antarctic. The different

methods

that

can

be

used

to

find

Chichon have given a strong impetus to investigations concerning the atmospheric impact of large volcanic eruptions. An empirical approach has often been used in determining the climatic implications of eruptions, based on the comparison of historical volcanic records with available climate

data

compilations.

Such data

may have

volcanic acid deposits in snow and ice cores are compared: electrical conductivity, sulfate, and

various origins: ancient temperature measurements, glacial isotope records, tree ring records,

acidity measurements.Numeroussnow and ice

glacier variations, etc [0liver, 1976; Rampino et

samples collected

at several Antarctic

locations

were analyzed. The results reveal that the two

major volcanic 4 fallout in Antarctic ice events over recorded the last bYcH2SO entury are the

al.,

1979; Hammeret al.,

1981; Sch•nwiese, 1981;

Porter, 1981;La MarcheandHirschboeck, 1984].

Geochemical studies ofpolar icecores can play

eruptions of Krakatoa (1883) and Agung (1963), both located at equatorial latitudes in the southern hemisphere. The volcanic signals are found to be particularly well defined at central Antarctic locations apparently in relation to the

an important role in understanding the processes linking global climate to atmospheric chemistry since polar precipitation has recorded information about both past climates and past atmospheric compositions over several millenia. The two major questions dealt with in this

low snow accumulation rates demonstrated that volcanic

paper are (1) How much can ice

in these sulfuric

areas. It is acid in snow

records

tell

us about

the

is not even partially neutralized by ammonia. The possible influence of Antarctic volcanic activity on snow chemistry is also discussed, using the three recent eruptions of the Deception Island volcano as examples. Only one of them seems to have had a significant effect on the chemistry of

stratospheric sulfate burden? (2) How can ice records be used to reconstruct the history of the explosive activity of the various volcanic areas in the world? These represent two different interpretations of ice measurements: the former being mainly of

snow at a location 200 km from this volcano. It is concluded that Antarctic volcanic ice records are

interest to climatologists and the latter to volcanologists. To this we must add the usefulness

less complicated than Greenland records because of

of volcanic

the

climatic

limited

number of volcanos

in

the

southern

horizons

applications

for dating

ice cores.

The

are beyond the scope of the

hemisphere and the apparently higher signal to background ratio for acidity in Antarctica than in

present article. The Antarctic

Greenland.

low amount of aerosols. This is due to the remoteness of Antarctica (Figure 1) with respect to the most common atmospheric particle sources

1.

Introduction

Volcanic eruptions affect

atmosphere contains a remarkably

(ocean excepted) and to the considerable impediments to transport toward Antarctica, related in part to the cyclonic wind regime prevailing

atmospheric chemistry

around the continent and in part to the relatively

on a local or regional scale for moderateevents

high altitude of the Central PlateauEShaw,1979].

and on a global scale for more violent

The major portion of the trace impurities

eruptions.

In the latter case, volcanic gases and finely divided ashare injected into the stratosphere,

found in

the air [Shaw,1979, 1980]or in the snowEDelmas et al., 1982a; Legrand and Delmas, 1984] is

where they may remain for a period ranging from

secondary aerosol derived from the nucleation of

severalweeksto several years [Lamb,1970; Cadle

atmospherictrace gases. The low coastal areas

atmospheric transmissivityandconsequently affect

Langway, 1979;Warburton et al., 1981].

temperatures on the earth. Howeverthe responseof climate to stratospheric dust load modifications

The mechanismsinvolved in the removal of these particles over Antarctica and their deposition on

1981; Hammer et al.,

discussed the different

et al., 1976].Theaerosollayerformed canreduce onlyreceivelargeamounts of seasalt EHerron and

still remains a controversial topicESelfet al., 1982].

1981; Toon and Pollack,

The recent eruptions of MountSt. Helensand E1

the surface arestill widely debated. Shaw E1980•

is nowincreasing evidencethat the concentrations

of

the

tions

Copyright 1985by the American Geophysical Union. Papernumber 5DO453 O148-O227/85/OO5D-O453•O5.OO

possible processes

involvedat the air-snow interface.However there elements

in

the

in

air,

the

so it

ice

follow

appears

the

concentra-

reasonable

to

assume that the chemicalcompositionof trace

elements

in ancient

ice layers may be interpreted

in termsof atmospheric compositions,i.e., that past characteristics of Antarctic aerosol are 12,901

12,902

Delmas et al.

: Volcanic

Deposits

IFIC

ßscott

Fig.

1.

Map of Antarctica,

also showing surrounding continents.

Antarctic

and sub-

Antarctic volcanic areas are indicated by circled n•mbers: 1, South Sandwich Is lands 2, South Shetland Islands;

3, James Ross Island volcanic

Ellsworth

Land;

and McMurdo.

Andes

in

satisfactorily

and

5,

Hallett

Other

volcanic

group• 4, Marie Byrd Land and centers

are

located

in

the

New Zealand.

recorded

in the Antarctic

ice

originates

the major

portion

of

the volcanic

sheet [Pourchet et al., 1983; Lambertet al., productsfoundin the Antarctic (Figure 2). 1983]. This is particularly true whenthe temporal variations

of

a given

chemical

compound are

considered. The concentration variations

in snow

2.

layers most probably reflect the concentration variations in the atmosphere in which the snow was formed. The drilling of deep ice cores in Central

Atmospheric Aftermaths of Volcanic Eruptions The contribution

of

a volcanic

event

to the

Antarctica therefore makes possible the study of the terrestrial environment over several hundred thousand years. Over this time period, numerous volcanic

atmospheric aerosol loading depends on several factors that cannot always be estimated accurately. The strength of the eruption is obviously the parameter of primary importance, but the

eruptions

altitude

various

of

areas

large

magnitude

of the world.

have

We will

occurred

in

examine how

and to what extent the volcanic material from these eruptions might be stored in the Antarctic ice sheet and also from what region of the globe

the

of the crater

eruptive

summit and the duration

phase must

also

be

taken

of

into

account. The chemical composition (in particular the proportion of sulfur compounds)and the degree of fragmentation of the ejected material are other

Delmas et al.

: Volcanic Deposits

12,903

l SUBANTARCTIC Artbiles

REGIONS I

New Zealand Inclonesm

S ShetlandI$1dl

ChM,Argentma e g

Agun•

Fig.

MT TalK]he

e g Taupo

Tomboro

To rowera

Toba

Qulzopu

2.

e g Erel:ms

ß e. Decem,on

Krokatau

A schematic view of the various volcanic contributions to the chemistry of

Antarctic snow.Acidcompounds (mainlyH2SO 4) maybe transported by tropospheric or stratospheric pathways. Examples of past voIcanic eruptions corresponding to the four latitude belts of the southern hemisphere are given in the lower part of the drawing.

essential parameters.Several authors proposed a

gases. The coarsest dust settles rapidly,

but

generalclassificationof thedifferenttypesof reactivegases(suchasSO 2) apparently stratify

volcanic eruptions. For example, Berresheimand

at the altitude of injection forming the Junge

Jaeschke [1983]described eightcategories of layer,where SO 2 is subsequently converted into eruptions (A1 to A8), in agreementwith the fine sulfuric acϥaerosolEYue,1981;Keeseeand classification generally usedin the literature.

The phenomenologicalcategories A6 (Plinian), A7 (Pelean) and A8 (Krakatoan) are those of the most global concern. The authors discussed the

Castleman, 1982].

Finally, the transfer of volcanic products from the upper atmosphereto the ground must also be considered for a complete understanding of the

respective emission rates of gaseousS com•pounds influence of volcanism on the chemistry of into the atmosphere. NewhallandSelf E1982Jused Antarctic snowandice. a volcanic

explosivity

index (VEI)

as a scale to

categorize eruptions in relation to their potential for injecting dust and gas into the stratosphere.Lamb[1970] has also used the dust

3. Contamination of the AntarcticAtmosphere by VolcanicMaterial

veil index (DVI), a scale basedon the atmospheric 3.1. impact observed after

the eruption.

of the emitted material is, indeed, strongly dependent on the height reached by the volcanic cloud,

on the latitude,

and on the date of the

eruption as well as on the local meteorological conditionsandthe regional atmosphericcirculation ESelf et al., 1981]. All these factors explain

why

there

may be

Minor eruptions

The dispersion

some discrepancies

betweenthe various assessments of the strength of

For assessing whether or not the products emitted by a volcanic eruption may reach

Antarctica we should take into account the various

parametersdescribing the eruption. First of all wewill disregardminoreruptions(VEI < 3), which affect only the troposphere (the caseof Antarctic and sub-Antarctic

events

will

be discussed

in

subsection3.3) and have no chance to seriously

past major volcanic events. Moreover, when going contaminate the South Polar atmospherebecause back in time, eruption dates become increasingly (1) in the case of all eruptions with VEI < 3 uncertain. The most recent and widely used occurring in the northern hemispherethe equator compilations of volcanic records are those of Lamb is a very effective barrier for preventing the

E1970]andof Sirekinet al. E1981•.

The fraction of the emitted material

tropospheric long range transport of dust and

remaining

in the stratosphere is also a matter of discussion

gases

to the southern hemisphere,

as can be

observed for anthropogenicpollutants (except for

someauthorsassertingthat mostdust settles residence thosesuch as C02,whichhavea longatmospheric time)

within a few weeks, whereas others assign a long (>1 year) lifetime to volcanic products in the

stratosphere. It seems important to distinguish between the case of insoluble dust and that of

(2) the circumpolar circulation is such that the Antarctic continent is well protected against the penetration of air masses originating from

12,904

Delmas et al.

southern

mid-latitudes.

For

example

: Volcanic

the

Deposits

background

is

generally

considered

to

be

continental contributionto Antarctic aerosolhas negligible EHofmann andRosen,1981]. beenfoundto be very weakEShaw, 1979]. However, The numerous volcanos of the East and West we do not exclude, under favorable meteorological conditions, that upper tropospheric air masses from Patagonia or New Zealand (40-50øS) could reach the Antarctic continent. This could lead to occasional H SO. or dust tropospheric fallout from

Indies, Central America, Colombia, Ecuador, Papua New Guinea, New Hebrides, etc., all located between 20øN and 20øS, are therefore potential contributors to the Antarctic sulfate budget. The known eruptions of Tambora (1815), Krakatoa

HoweverThompson et al. [1975], Petit et al. [1981] and,morerecently, DeAngeliset al.E1984 •

of this low latitude volcanic activity [Rampino andSelf, 1982] and are presumably recordedin

volcanos loc2ate• in theseareas.

have shown that this situation may have been very different at the end of the last glaciation, some 15,OOO-20,OOO year B.P., meridional circulation having increased considerably at this time. This phenomenon is revealed by the microparticle content of three deep ice cores (Byrd Station, Dome C, and Vostok station), which was found to be 10 to 25 times higher in the ice layers corresponding to this time period than in recent snow.

This suggeststhat southernmid-latitude volcanic

eruptions at the end of the last glaciation could have contaminated the Antarctic atmosphere much more easily than now through tropospheric transport.

3.2.

Major eruptions

Major eruptions with VEI > 3 inject large amounts

of

their

products

directly

into

stratosphere wherebya global distribution fallout

is made possible.

northern

latitude

However, middle

eruptions

(1883),andAgung (1963)are well representative

Antarctic snow layers. There is no doubt that large eruptions occurring south of 20øS significantly affect the Antarctic atmospheric chemistry by stratospheric transport of volcanic debris, which in this case remains essentially in the southern hemisphere. Volcanic areas in these latitudes are scarce and are essentially in the Andes and New Zealand. From the beginning of the 16th century up to now,

Newhall and Self E1982] listed only six major

volcanic events (VEI > 4) in these latitudes. However, during the last 30 millenia, several cataclysmic eruptions are known to have occurred in New Zealand. Their ash bands are still visible in this country and in sea sediments in the vicinity of the islands. One of the most recent is

the eruptionof Taupoin 186 A.D. EWilsonet al., 1980].

the

of the

3.3. Antarctic and sub-antarctic eruptions

and high

of this type seem to

Eruptive

areas at southern polar latitudes

are

havelittle influenceon the southernhemisphere nowrelatively well known[Kyle et al., 1982]. [Lamb, 1970]. Recent examplesof sucheruptions They encompassseveral groups of volcanos support

this

assertion:

Katmai

(1912,

58øN)

spreading along an axis

from the Scotia

Arc and

examined in detail by Volz E1975•,and Bezymianny the Antarctic Peninsulato the Marie ByrdLandand (1956, 56øN), modelledby Cadle et al. E1976•. the McMurdo volcanic area. In East Antarctica, Hammer et al. [1981] assumed that the contribution only one small volcanic vent (Gaussberg,90øE, fromvolcanos south of 20øS is less than 1• in

Greenland

snow. The same type of assumption is

66øS) is knownand very recently two additional active volcanos were discovered near the Antarctic

proposed for the influence in Antarctica of

Peninsula[GonzalezFerran, 1982]. On the other

volcanos located north of 20øN. In fact it appears that only eruptions occurring in the 20øN-20øS latitude belt may concern both hemispheres. The wind patterns of the Hadley cells north and south

hand the history of the activity of these volcanos is very poorly documented, because of the late exploration of Antarctica in comparison with other parts of the world. The tephra layers found in the

of the equatorare essential in understanding the

Byrd Station ice core [GowandWilliamson,1971]

global spread of volcanic products originating from this zone. The direction taken by the

revealed that there was major volcanic activity from 14,OOO to 30,000 years B.P.on the Antarctic

eruption cloudmayvary according to the season

continent itself [Kyle and Jezek, 1978• Kyle et

ELamb,1970; Cadleet al., 1976]andalso to the

height of the top of the cloud.

Dyer and Hicks

al., 1981]. Theoccurrenceof these layers has

been correlated

with the climatic

isotope profile

E1968•found,for instance,that the dispersionof

obtainedfromthis ice core. It was deducedthat

the Agung debris

volcanic

was very

asymmetric,

whereas

atmosphericturbidity measurements in the years

activity

was not the cause of the last

glaciation [ToonandPollack, 1982], but it should

following the Krakatoa eruption (1883) have led to

be remembered that

the

contaminated equally[Lamb,1970]. In general,

regional impact and no significant

global climatic

effect. Mt Erebus,the most famous Antarctic

subtropical the major

eruptions do have a global influence, effect occurring, however, in the

volcano, volcanic

of the McMurdo were observed

hemisphere

of

during the last century in the Scotia

conclusion

that

the

both

injection.

hemispheres

The lifetime

were

.of

such eruptions

is the last active center group. Other eruptions

had only a

Arc and in

H2SO 4 aerosolbeinglarger thanthe meantransport the vicinity of the Antarctic Peninsula.The

time, strong volcanic eruptions at equatorial latitudes disperse significant amounts of sulfur compounds to the poles (this is, however, probably not the case for ash).

Finally, Castleman et al. 71974]suggested that

present volcanic activity considered as very limited on a global scale. At least it seems likely that most

south of 50øS may be and nearly negligible over the last millenia volcanic impurities

foundin Antarctic snoworiginate from north of

strong convective clouds occurring in the tropics are capable of lifting volcanic particles from low

50øS rather than from Antarctic high latitudes. However, it must be kept in mind that a relatively

altitudes kind of

small active volcano like Mt Erebus, which other parts of the world would be insignificant

up to the contribution

stratosphere. However, this to the stratospheric sulfate

in

Delmas et al.

: Volcanic Deposits

with respect to local atmospheric chemistry, could cause an imbalance in the regional budget of a few

12,905

northern latitudes). loading of radioactive

A maximum stratospheric products (such as Sr 90

tropospheric trace impurities, assuggested by summer •Krey et al.,1974]) 1974• and of wasvolcanic observedsulfate during be more complex than for metals, since the 1963-1964 in the southern hemisphere. Over

RadkeE19823.In fact the caseof sulfur compounds Castlemanet al., could

oxidation stepsleadingto the formationof H2S.O 4 the SouthPole, Agung'svolcanic dust wasalso in the atmosphere are slowed down in the clean air

first

observed in December 1963, i.e.

less than

conditionsprevailing over Antarctica. Thegaseous 1 year after the eruption [Dyer, 1966]. Moreover

sulfur compounds (mostlySO 2 E Radke,1982•) themaximum of grossbetaactivity in the at-

emitted by Mt Erebus could therefore

have time

to

mosphere (at

Dumont d'Urville,

Adelie Land)was

"escape"from the Antarctic atmosphere andto be

reachedonly 1 year later [Lambertet al.,

transformed

The years

and removed over

the

sub-Antarctic

1963 to

1965 therefore

1977].

represent

an

ocean regions [Delmas,1982]. Finally, it mustbe

excellent opportunityto investigate and compare

added that the low altitude of the tropopause over Antarctica favors easy access to the stratosphere for volcanic material. As emphasized by Kyle et

the transport mode and the fallout of both radioactive and volcanic products in the southern stratosphere. Moreover it has been demonstrated

al. [1981], relatively minoreruptionsare capable that radioactive products subside yearly from of injecting their products into the stratosphere, thus of distributing them over the whole continent.

their stratospheric reservoir to the Antarctic troposphere in December or January. Assuming that radioactive particles are attached to sulfate

4. Deposition of Volcanic Products on the Surface of the Ice Sheet

aerosols, it is also reasonable to assume that stratospheric sulfate falls out at the same time. After a period of intense nuclear testing, the radioactivity of snow increases considerably,

Dry deposition

of coarse volcanic

ash (for

since the natural radioactivity

level is extremely

examplefroman Antarcticvolcaniceruption) is a low •Pourchet et al., 1983]. In sucha casethe relatively sim•le processthat has beendiscussed identification of the exceptional radioactive by Kyle et al. •1981•. As at other latitudes the levels in snowis relatively easy. This is not the grain size is a critical parameter in the dispersion of the volcanic plume over the ice cap. On the other hand the behavior of volcanic aerosol fallout from the Junge layer (case of major eruptions) is similar to the case of polar tropospheric submicrometer particles. After their transfer to the polar troposphere, several

case for volcanic levels because the background sulfate content of snow is not zero. The volcanic contribution is superimposed on this background, which itself suffers natural variations. It can therefore be difficult to distinguish a volcanic sulfate increase from the natural fluctuations of the nonvolcanic sulfate content. The origin of

mechanisms maybe involved in the deposition of

this sulfate has beendiscussedelsewhere[Delmas,

suchvolcanic aerosol on the snowsurface(snow- 1982•. Its meanvalue in Antarctic snowvaries out, washout,dry deposition, impaction, etc..). relatively little, in the rangeO.5-1.3 •eq L--.

According to Shaw[1979] nucleation(snowout)and

On the other hand, results of' radioactivity

dry deposition are most likely the two primordial mechanisms by which the Antarctic atmosphere is

measurements strongly suggest that stratospheric sulfate deposition could vary significantly from

cleaned. Junge•1977] also madethe assumption one area to another on the Antarctic continent. that

undercloud

scavenging

is

unimportant

in

This would mean that

the ratio

of volcanic

sulfate

Antarctica. We shall see below how the respective importance of wet and dry deposition may be assessed in central Antarctic regions. The

to total sulfate deposition could also be different for a given volcanic event. In other words, some Antarctic sites would be more suitable

contribution

than others

of

stratospheric

sulfate

to

the

for use as volcanic

records.

Antarctic sulfur budgetmaybe significant in the Lambertet a1.•1983] pointedout that the lower years following major volcanic eruptionsEDelmas the snow accumulation, the higher the 90 Sr andBoutron,1980]. It was possible to estimate specific activity. In the central areas of

this contributionby using a comparison withthe Antarctica,2whe•e snow accumulation is very low fallout of artificial radioisotopes which are (2-10 g cm a-), the specific activity of 90 Sr indeed excellent tracers of the movements of tropospheric and stratospheric air masses.

is, on the average, 4 times higher than near the coast, and dry deposition is a preponderant

Radioisotopes are probably attached, in the

mechanism(~70•) for the deposition of stra-

stratosphere,

to the small sulfuric

acid droplets

knownto be the main constituents of volcanic productsEMartell, 1966;Volz, 1975]. To study the world wide dispersion of volcanic

debris, several authors•LazrusandGandrud,1974•

tospheric radioactive

particles

at the South Pole.

Pourchet et al. •1983] came to a similar conclusionon the basis of total beta radioactivity

measurements at 23 Antarctic

stations.

They established the relationship linking the

Volz, 1975• Cadle et al., 19763 have already usedor total depo•iti?n (in disintegrations per the analogy between an atmospheric nuclear test dph cm a ) oD• radioactive •rod•cts tohour the and a volcanic eruption, and it can be said that the interpretation

of volcanic

debris

and fallout has beenfurthered by the intensive studies that have followed the large nuclear test series over the last 30 years. In particular the

Agung eruption

snowaccumulationrate A (in g cm-

a- ):

trajectories

(March 1963, 8øS) occurred

just

DT = 9 + 0.97 A

(1)

If we assume that the same relationship applies to volcanic sulfate fallout, it may be calculated

after a sequenceof powerfulatmospheric nuclear that a volcaniceventwouldbe recordedin snow b_•

tests inof the northern hemisphere 1961 to a 5 times higher at Island Dome C (Acm_2 theend 1962 at high (USSR) or(October equatorial (USA) a-•ignal ) than at James Ross (A - 3•75 g g cm

12,906

Delmas et al.

-1

a ). Note, however, that equation (1) is

: Volcanic

empiri-

Deposits

5.2. Static Electrical

cal and that no satisfactory theoretical explanation has yet been given. Finally, Lambert et al.

Conductivity (SEC) Analysis

of Ice Cores

[1983] obtainedhigher deposition velocities for

This analysis was first developedby Hammer

radioactive particlesat theSouth {olethanat [1980]. Apairof electrodes is moved along a flat --

.

other Antarctic11ocations: 0.6 cms •nstead of O.17-O.37 cm sThese authors assumedthat

surface freshly cut parallel to the axisof the ice core. Hammer [1980] demonstratedthat the

strong stratospheric injections at this location enhance the deposition of stratospheric submicrometer particles.

current intensity traversing the electrodes depends on the acid impurity content of the studied ice layer and that the time variations of

,

this

5. 5.1.

Searchfor Volcanic Layers in Polar Ice:

parameter

in an ice core are strongly

related

to the chronologyof volcanic eruptions of global

Techniques

concern. Maccagnan et al. [1981]showed that this

method is indeed very useful

General

sulfate

must be used with caution.

Whenexamining an ice core, volcanic deposits are sometimes visible to the naked eye. In the

in detecting

volcanic

layers in ice but also pointed out that it Continuous SEC profiles

can be achieved easily and rapidly in the field. Note that this methodis not valid for superficial

Byrd Station ice core, for instance, Gowand

soft snow.Our (SEC)profiles of the ice core were

Williamson [1971] foundnumerous dirty and cloudy

obtained with the aid of an apparatusdesigned

layers, their different

aspect dependingon the

basically according to the method described by

presenceof visible ash in polar ice is generally

cold roomat -15øC.A detailed descriptionof this

size of the particles includedin the ice. The Hammer [1980]. Themeasurements are performed in a

explainedby nearbyvolcanic activity concerning technique wasgivenby Maccagnan E1981•. only a limited part of the ice sheet. The glacier

covering the volcano of DeceptionIsland has

Howeverour system presents somedifferences

with respect to the one usedby Hammer [1980].

recorded several recent eruptions and was used by

First

OrheimE1972•to reconstructthe eruptive history

diametrically opposedflat surfaces of the ice

Sheet or in Greenland.Ice impurities of volcanic

to the electrodes is low: 250 volts

of this volcano over the last 200 years. However, ash layers are rarely found in the Antarctic Ice

origin are mostcommonly very fine insolubleash particles

or

sophisticated discussed,

soluble

mineral

detection

such material

acids

methods. frequently

that

the two electrodes are moved along two

core. We therefore measurea volume rather than a surface conductivity. Second, the voltage applied

[1980]:1250volts).

(Hammer

require

As already transits

5.3. The Electroconductivity of Meltwater (ECM)

to

high latitudes (particularly in the case of Antarctica) via a stratospheric pathway. Volcanic debris arrives together with the other compounds forming the background aerosol, and it is often difficult to distinguish the pure volcanic

This parameter depends on all dissolved ionic species (particularly sea salt and secondary aerosol). As the specific conductivity of the proton is much larger than that of all other ions, conductimetric profiles obtained in central

contribution

Antarctica

in

the

deposited

matter,

since

analysis techniquesare generally not specific to

(where

the

sea

salt

contribution

represents only 15-20• of the bulk of deposited

volcanic products. matter EDelmas et al., 1982a•are very similar to Hammer E1977• andThompson andMosley-Thompson acidity profiles. In most coastal areas (at low [1981] tried to use the concentrationof insoluble altitudes only), conductivity mainly dependson particles horizons

in firn and ice cores to detect volcanic in Greenland and Antarctic deep ice

cores. In both cases it was concludedthat the identification of major volcanic events by this

sea salt volcanic

content. events,

For a rapid detection of major ECM measurements may be

sufficient •Hammer,1977], andthesecanevenbe performed in-the field relatively easily •Legrand,

method isgen•lr•7171 • demonstrated unsuccessful. On the other 19803. More accurate measurements (forinstance in for the first order to check the ionic balance) require the use time that layers of high electroconductivity may of a conductimetric method based on known

hand, Hammer

be found in the Greenland

ice

sheet

in relation

to

additions

of an acid

titrant.

The elimination

of

acid fallout of volcanicorigin. Delmas and the CO 2 effect is doneas proposed by Legrand Boutron [1978]detected anincrease of thesulfate E1980] sothat thevaluesgivenherecorrespond to

deposition in Antarctica which they attributed to the major volcanic eruption of Mt Agung in Bali

the soluble impurities of snow or ice without H CO- fomed after melting from dissolved CO_. Our 2

remotesensingtechniqueEMillar, 1981], there are

conductimeter (ModelCD78) in 6 m•.aliquots.

(1963). If weexcluderadar echoes,whichis a four methodsfor detectingvolcanicH2SO 4 fallout

in an Antarctic ice core: (1) electroconductimetry

of the ice (solid

E•M•ata wereobtained by usinga digital Tacussel

5.4.

Antarctic

Snow

or

static (SEC), (2) conductivity) electroconductimetry of the snowor ice

meltwater (ECM),

(3) titration of the acidityin meltwater, and (4) determination of the sulfate content in

This has gener•l_•y been found tobeacidic in for most locations studied.

the range 2-4 •eq

After a volcanic eruption, this parameter may be

stronglyenhanced for 1 or 2 years,as •uggested

by SECmeasurements, andexceed10 peqL-• for the

meltwater.

most powerful eruptions. However, the strong Agung

Oneof the aims of the present paper is to discuss

eruption (the strongest of the last 1OOyears, see

therespectivemeritsandlimitationsof these below)increased theacidityof centralAntarctic

methods,leading to the strongrecommendation that they must be used in conjunction with one another.

snowby only 3-4 peqL- . This is whya sophisti-

cated acid titration

method is needed to detect

Delmas et al.

: Volcanic

Deposits

12,907

TABLE 1. Geographical, Physical, and Glaciological Data for the Studied Antarctic Sites

Site

Coordinates

Elevation,

MeanAnnual

m above sea level South

Pole

90 øS

Dome C D 57 James Ross Is land (Dome Dalinger)

volcanic acid ments, acidity

fallout. values

74ø42'S, 68ø11'S, 64 ø13'S,

124ø04'E 137ø33'E 57 ø38'W

As opposed to ECM measureare insensitive to the sea

salt conten[ of thesolution (at leastwhen ENa•< 150 •eq L-

EPszennyet al.,

Temperature,øC

28OO

-

51

3240 2O5O 1640

-

54 32 14.5

5.5.

Sulfate

Snow Accumul•ti?n Rate, g cm a 8.5

3.6 3O 57

Determination

This is a useful methodto characterize

1982]) and so may volcanic products provided that the background

also be used at coastal locations. In our investigations, acidity is titrated in 6 mI. aliquots of snow or ice meltwater by a known

value of this compound at the studied location is known. Sulfate is assumed to arrive in Antarctic snow from three major sources: neutral sulfate

-addition methodas describedby Legrand et al.

(Na2SO 4) fromsea salt, excesssulfate (H2SO 4)

[1982]. Only strong acidity (pk. < 3.6) is determined.Accordingly,carbonicac• formedfrom

from marine biogenic activity, and volcanic sulfate. Sea salt sulfate is only importantat the

dissolvedCO 2 doesnot interferein theH+ border of thecontinent EDelmas et al., 1982a•. It

concentrations [resented in this paper.Accuracy is assumed that gasderivedH2S04generallythe is + 0.2 •eq L- .

most important contribution in Antarctica, is

However,acidity is not specificto H2S04,and relatively constantoverperiodsof a fewyears.

several natural strong acids may also contr'ibute to total acidity (particularly HNO_ and HC1).

Volcanic sulfate is best characterized by a sudden increase in the concentration in snow, lasting a

the fraction of acidity linked to H2SO 4.

va uesfoundfor background sulfate are 0.5-2 •eq

Sulfate determinations mustbe used•o evaluate fewyearsEDelmas et al., 1980].Thetypical •974 340

•O

•970

L-«atAntarctic inland locations EHerron, 1982• Delmaset al., 1982a, b]. At DomeC and South Pole, Delmasand Boutron[1978] founda sulfate

196õ Ye0rs{0a,} •0

--• ! ! I I I I I I I I .1•!-- eruption. increase of1-2 •eq for 2years after the Agung 5.6. MajorSoluble Anions (SO 4, NO 3, C1)and Cations (Na,NH4,K)

•'• •o •

These

42

are

measured

with

the

aid

of

an

ion

chromatograph (Dionex Model 10), except for a few metal determinations by flameless atomic

o

absorption. Ionchromatographic measurements are performed

without

a preconcentration

column by•

using 5 mLof meltwater. Accuracyis typically 10• for this techniqueELegrand et al., 1984].

• DEUTERIU•

6. Snow

and

Sampling Sites ice

samples

and Methods were

col lected

at



different geographical locations and at different

•-i

data on the sites and the sampling conditions. Firn samples were collected in pits by pushing



precleaned plasticvials (50mL"Accuvettes" from

depths. Table 1gives asummary ofthe various

•. •

2

Coulter) 1984).

••



2

3 De•h {m•

4

into

Special

the snow walls

(Legrand

et al.,

care was taken to avoid contamina-

tion by the operator.Plastic vials werekept frozen in double sealed plastic bags and allowed to melt

just

before

laboratory

analytical

work.

Fig. 3. South Pole station. Acidity profile of snow from 1 to 4 m depth. Dating of the snow layers is accurately known by stable isotope

Numerous blanks were included in the series of samples. They consisted of empty "Accuvettes" opened and closed in the field, transported along

(deuterium) measurements. This time series comprises the record of the Agung eruption (around 3 m depth). The origin of the subsequent

with the samples and filled with pure water in the laboratory. Firn and deep ice cores were taken by an

acidity peaks is discussed in the text. is given above (January of each year).

electromechanical drill. Ice cores were recored in the laboratory within a clean air bench by using a

Dating

12,908

Delmas et al.

: Volcanic

small thermal probe (• 4 ca), which permitted the

level

recovery

in

of

the clean

central

part

of the ice

Deposits

in 1970. A first explanation can be found

the

occurrence

of

five

important

volcanic

without any detectable contaminationfrom the surface. Meltwater was immediately analyzedor

eventsat low latitudes from1966to 1968ENewhall and Self, 1982•. However,an alternative explana-

refrozen. Some measurements were performed in the field: this was the case at D 57 (SEC) and at

tion can also be given here. Acidity is contributed not only by HASO. but also by the two mineral

Dome C (ECM, secular profile). Because of the three acids HNO 3 andH•i •Delmas et al., 1982a•. These influence of temperature, the field and laboratory acids were measured for another series of SEC measurements values. 7.

may give

different

absolute

Results and Discussions

7.1. TheLast20 Yearsandthe Agung TimePeriod

samples collected at South Pole. A very similar acidity profile was obtained. The chemical analysis of the anions revealed that the persistence of a relatively high level of acidity after

1965was only partly due to H_SO 4 andthat HNO 3

andHC1couldaccountfor mos{of the acidity

peaksbetween2.0 and 2.9 m depth [Legrandand

The relatively intense volcanic ofthe De lmas, 1984]. Nevertheless thesecondarYeH•S impact on theactivity stratosphere peak, occurring in 1968-1969 could be relat to

last 20 years and its

have been widely studied by various investigators. These years therefore form a time period that is particularly useful for calibrating the influence of major volcanic eruptions on the chemistry of Antarctic snow. At South Pole and at Dome C,

the eruptions of Deception Island (near the Antarctic Peninsula in December 1967, see Figures

Delmas and Boutton [1978, 1980] detected an

gives a well defined referencehorizon at a depth

layers corresponding to the years 1963-1967.

1964-1965,

increase

of the sulfate

concentration

in the snow

This

phenomenon was attributed to sulfate fallout linked to the eruption of Mt •Agung (Bali,

spring

6 and 8) and of Fernandina (OøS, June 1968). From our acidity measurementsof South Polar snow, we conclude that the maximum of Agung acid fallout corresponding

to

the

summers of

as formerly

1963-1964

and

proposed by Delmas and

BouttonE1978•on the basis of their sulfate data. The "ash" of

the Agung eruption

was first

1963). We have carried out detailed glaciochemical studies of these years in order to obtain a more thorough understanding of the Antarctic atmosphe-

detected over the South Pole, in the stratosphere, at the end of November 1963. The effect of this eruption on the local atmospheric transmittance

tic chemistrydisturbances causedby this major eruption. Several pits were dug, and firn cores

lasted three summers(until 1966 EViebrockand Flowers, 1968]). Furthermore, the radioactive

were drilled

at the South Pole, in the DomeC area

and on James Ross Island

(Antarctic

Samples were analyzed for acidity described

Figure

Peninsula).

by the method

above.

fallout

rementscarried out on 1OOsamplescollected in a

nuclear

to the powerful

tests

of

began to increase in the first attained

3 gives the results of acidity measu-

corresponding

hemisphere its

climax

the years

northern 1961-1962

months of 1964 and

in South Polar

snow during

the

same•ears, as already discussed. Jouzel et al.

E1983Jshowedthat at SouthPole the beta radio-

pit at the South Pole from 2 to 4 m depth with a frequency of one sample each 2 cm. Dating (from 1960 to 1970) was achieved by three independent methods (field stratigraphy, annual deuterium variations and total 8 radioactive horizons), leading to an accuracy better than +1 year. Dating accuracy is exceptionally good at the South Pole

activity increase appears clearly at the expected time (beginning of 1964). Our acidity results are therefore in excellent agreement with the optical measurements made over the South Pole and with the artificial radioactivity of snow determined in the same snow pit. Now let us look at the extent to which these

bec•use_•f the regularand relativelyhigh(~8.2g caa ) deposition rate of snow at this

excellent resultsare confirmed at otherAntarctic locations. The DomeC area has been extensively

location EJouzelet al., 1983]. studiedfrom1974-1975up to present (Table 2). This acidity profile exhibits regular seasonal Petit et al. E1982• showed importantspatial and

variations, even during periods of volcanic

temporal variationsin the snowacc_u•ula[ion rates

quiescence. The maximumacidity levels observed in summer may be due to various factors (e.g., meteorological, photochemical) and are not necessarily linked to the troposphere-stratosphere air

that vary between 2.5 last 15 years for the dating and definition much less accurate at

mass exchange ELegrand and Delmas, 1984].

Pole, except apparently in the Dome C 23 pit

in

each

Stratospheric sulfate fallout, addition

to

the

when it occurs,

summer maximum.

It

is

is

also

clear that the winter minimum nearly disappears in volcanic periods such as the post Agung years (see

Figure 3 at 3 m depth).

In this

figure

the snow

and 4.9 g cm a over the 19 stations studied. The of snow layers is therefore Dome C than at the South

(DC 23) sampled in detail annual

layer,

in January 1980, where

at least

for

the Agung period,

was clearly identified by the deposition of sea salt (minimum in summer). The acidity maximumis

reached simultaneously with the beta radioactivity

layers of 1964 and 1965, well defined by deuterium variations, have a relatively high acidity level

maximum (Figure 4a) at a depth of 1.50-1.55 corresponding to summer1964-1965.

(about 1.5 time the pre-Agung level). During the year 1964, there is a sharp increase in acidity

The acidity not significantly

m

level during summer1963-1964 is different from the background,

whichjumpsfrom~ 4.0 to ~6 •eq L-1 Twoyears whereas acidityhadalreadystartedto increase at ß

after, the acidity declines, but not to its pre-Agung level, which is only reattained in 1970. This last phenomenonagrees with stratospheric sulfate loading data for the southern hemisphere

this date at the SouthPole (Figure 3). An explanation could be that the snowaccumulationrate is

so low in the Dome C area that an annual peak may very well be missing in the volcanic acidity or radioactivity profiles, both parameters depending

•Dyer, 1974], which also fell to its pre-Agung on the brief stratosphere-troposphereexchange

Delmas et al.

TABLE 2.

Date of Sampling, January

: Volcanic Deposits

Shallow

Depth Interval, m

12,909

Sampling at Dome C

Sample Thickness, cm

Measurements of

Results in Figure

Pit

DC5

1975

O-2.2

DC5 DC 6

1978 1978

0-2.1 1.O-1.5

20

5 1

bC 20 DC20 DC23

ß1980

1980 1980

1. 2-20 . 0-2 ß6 1.3-1.8

6 6 1

H+ Na+ NH. + H+ Cond. • H+ Na+

4

DC20

1980

2.O-10.2

6

H+ Cond.

5

Firn

occurring

generally in

Decemberor January.

The

SO=

4

H•+ H

4 4 5 4

Core

be more severe at Dome C (taking

DC 23 as the

acidity increaselinkedto theAgung eruptionis

reference because of the regularity of the sno_•

other pits dug in this area, where dating is much more questionable and sampling much less detailed.

respectively). distinguishing

here ~4 •eq L

. This figure is confirmedin the

In pit DC5, sampled in 1978for a•idity, the pre-

Agungacidity level is-2.5

•eq L

(Fig. 4C),

in

excellent agreement with the 1980 measurements in pit DC 20. (Fig. 5, curve 1). In pit DC 5, the

layers) than at South Pole (3.4 and 2.5 •eq L This means that the possibility the stratospheric contribution

of from

the tropospheric "noise"or "background" wouldbe

better

at DomeC than at South Pole. As suggested

earlier, this observation could be linked to the snow accumulation rate, which is more than double

highestacidity value corresponding to the Agung at SouthPole compared to DomeC. The •pecific •

eruptionreaches5.5 •eq L - in January 1965.In

radioactivityof snow •s ~ 3500dphkg-- at Dome C

also observedaround1.5 mdepth (Figure4c) in

1965-1966 peakyears•Pourchetet al., 1983•. The

the samepit• a sulfate deposition maximum was

(DC) and 2500 dph kg-• at SouthPole (SP) for the

a profile obtained fromsamplescollected in 1975 ratio DC/SPis therefore 1.4 for beta radioactiat depth intervals of 20 cmEDelmas andBoutron, vity, whereasit reaches1.2-1.6 for acidity. The

1978n•a• Finally, 6 (Figur_• two values arethe therefore inexcellent agreement. of Agungin is pit alsoDC ~3.5 •eq L 4d),+the of H but net the Near the coast, snow accumulation rates are

sig

1965-1966 summerpeak of acidity is higher than the one of the preceding year (the inverse figure

is generally observed, see Figures 4a and 4c). The acidity profile of pit DC 20 (Figure 5) is

still higher than at the South Pole and it is of interest to determine whether the Agung peturbation is still visible in the acidity profiles obtained in such areas. The snow acidity deter-

morecomplicated. It exhibits anothermaximum at

minations carried ou_•o_• JamesRoss Island

1.8-1.9 m depth. This event is also visible in the conductivity profile. We interpret this secondary peak as a "ghost peak" linked to a reworking of

deposited snow rather

than to another volcanic

event. This view is supported by the existence of a'similar phenomenon in the beta radioactivity

profile site

Stratigraphic

have shown that

field observations at this this

explanation

is quite

(accumulation ~ 60 gcm a ), corresponding to a continuous sampling from 1963 until 1979 (Figure 6), do not exhibit really exceptional values during the years 1964, 1965, and 1966 (between 14

and 17 m depth). Peak values of the beta radioactivity profile during this period. are also -1 much lower than at DomeC (1OOOdph kg . at James Ross Island

compared to 3500 dph kg

at Dome C

reasonable. Weconclude that the "mgung level" EPourchet et al., 1•83•). TheAgung signalat (1965) in this

pit

most probably

starts

at 2 m

depth and endsaround1.3 m. The snowlayer of relatively

low acidity (near 1.7 m) was in½orpo-

rated in this peak, possibly by a reworking of the

DomeC being 4 •eq L

-1

1.1 •eq L Island

if

a value of 4 1OOO=

can be calculated

we assume that

3500

for James Ross

the snow accumulation

snow layers bywind. This example shows the limitseffect is of_l•ajor This contribution. of the temporal definition in central Antarcti• (1.1 •eq L is importance. not strong enough to be arraswith lowsnow accumulation rates(2 •S cm -*) during the Agung morepowerfulthan the Agungeruption, might be

yeass tothe background (20 g cm a ) in Greenlandthan at recordedin snowand ice layers by their H SO 4

central Antarctic s•tes (Dome •1: 3.7,the and South fallout. This which transited 2to itates dating Antarctica mostacid, probably byhas a stratospheric Pole, 8.5 g cm- a- ). This fac

of ice cores but, as explained above, hampers the identification of the stratospheric sulfate contribution within the excess sulfate tropospheric background variations. In this sense Greenland volcanic records can be compared to those that could be obtained at Antarctic sites

pathway, is essentially unneutralized. An excellent opportunity to test this assertion was afforded by the Agung eruption (1963), which affected mainly the southern hemisphere during the years 1964 and 1965. Our convergent measurements by conductimetry, acid titration, and ion chroma-

suchas James RossIslandor D 57 (AdelieLand) ß

tographyreveal significant H2S0.4fallout from

located north of 55øN in Alaska, Kamtchatka, and Iceland. Their products may very well be transpor-

Pole and Dome C but not at the coastal site of James Ross Island. It may be suggested that paleo-

ted by a tropospheric pathway to Greenland or the High Arctic, which are located in the same latitude band. On the other hand, very few

volcanic studies in ice cores will be most success ful at central Antarctic locations where snow accumulation rates are lower than not only coastal

(2)

Numerous

active

volcanic

centers

are

this

important

event in all

snow pits

dug at South

volcanosare locatedat high southernlatitudes, areasbut also Greenland.The strongest signa_• and the long range transport of tropospheric that •e foundin ice (at D 57) exceeded 10 •eq L sulfate of volcanic origin toward Antarctic latitudes seems to be rare, as discussed earlier.

of H but corresponded Antarctic or sub-Antarctic

TheIcelandic Laki eruption is a typical case. Hammer [1977] showed that it considerablyincrea-

that excesssulfate levels in central Antarct_if precipitation are in the range 1-2 •eq L

sed the acidity of snow all over Greenland for several months. This eruption was of the Hawaiian

volcanic fallout increase this backgroundlevel by a factor of less than 10 for a time period of

type [Thorarinsson, 1969], and the gaseous generally 1-3 years. emissions

occurred at low altitudes

(