Dec 20, 1985 - Antarctic volcanic areas are indicated by circled nâ¢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
(
