Origin of crystalline, cold desert salts in the McMurdo region, Antarctica
J. R. (HARRY) KEYS and KAREN WILLIAMS
Department of Chemistry and AntarctIc Research Center.
Victoria Llniversity of Wellington.
New Zealand
The distribution
of crystalllne salt minerals in deposits in the McMurdo region of AntarctIca
has been examined to study the origin of these salts. Sulphate. chloride, sodium and calcium salts are
most frequently
encountered.
Salts containing
chloride and sodium ions become less common away
from the coast. Sulphate salts are more regularly distributed
but tend to be related isotopically
to sea
water sulphate. Salts containing
magnesium
ion tend to exist mainly on substrates
composed
of basic
igneous rocks. whereas calcium and carbonate
salts are present on all rock types in the regon.
These distributions
show that salts of marine origin are regionally and quantitatively
most important
but that chemical weathering
of mafic materlals In rocks and soils is also significant. However. biological, volcanic and hydrothermal
processes are or have been active contributing
to salts in local areas. that
include penguin rookeries and eastern Taylor Valley, the summit area of Erebus Volcano. and subsurface rocks. respectively
Abstract
INTRODUCTION
SAI TS in both
crystalline and dissolved forms, are
widespread
in the McMurdo
region of Antarctica.
This is an area comprising 3OOG4000 km2 of ice-free
terrain in McMurdo
oasis, South Victoria Land,
together with smaller ice-free areas on Ross Island
and around McMurdo Sound (Fig. I). Three east-west
trending valleys, Taylor, Wright and Victoria. constitute the main part of the oasis and lie between peaks
rising to elevations of 2000-40COm. Most of the flatter parts of the oasis are covered by glacial sediment
or unconsolidated
rock material. and ice-covered
saline lakes and ponds occupy the bottoms of many
internal drainage basins.
In the crystalline form. the salts exist in a variety of
compositions
and deposit types, from massive sublahc and probably subglacial deposits containing up
to 10’” kg of salt (HENDY rt crl., 1977; KEYS. 1979a)
down to traces in soil. rock. snow and ice. However. efflorescences and encrustations
on the surface
of the regolith.
and accumulations
just beneath
surface boulders.
cobbles
and pebbles,
are the
most common deposit types (KEYS, 1979b), typically
amounting to about 1--2Og of salt per deposit. More
than 30 different salt phases (minerals) have been
identified from the region (KEYS. 1979b) but only 10
of these arc widespread.
These 10 are thenardite
(Na,SO,). gypsum (CaS0,2H,O).
halite (NaCl), calcite (CaC03), darapskite
(Na3N0,S0,H20),
soda
nitre (NaNO,). mirabilite (Na2S0,10H20),
bloedite
(Na2Mg(S0,)24H20).
epsomite (MgS047H20)
and
hexahydrite (MpS0,6H20).
The presence of macroscopic
quantities
of such
crystalline salt phases is a characteristic
of desert or
semi-desert
areas. Antarctica is earth’s coldest and
driest continent;
in the McMurdo region the mean
annual air temperature
is about -20 C (SCHWERDT-
FIXER, 1970; THOMPSON et ul.. 1971). and the annual
precipitation
is less than 200 mm, mostly falling as
snow (KEYS. 1980a). Antarctica is often described as a
cold desert, unlike most other arid or semi-arid parts
of the world. The difference influences the composition and behaviour of salts because the general saltwater system is temperature dependent.
Much earth science research in Antarctica
is directed towards understanding
the climatic and glacial
history of the continent, and the salts are important
because they indicate prevailing arid conditions. The
salt content of surface sediment and saline lakes in the
McMurdo
region records environmental
conditions
and changes over the last several thousand
years
while deeper materials record conditions several million years ago (WILSON. 1964; WILSON et trl.. 1974;
FIELD, 1975; NAKA~ rt (I/.. 1975; HENDY or t/l.. 1977,
1979; MATSURAYA tv (II.. 1979; TORII ct rtl.. 1979).
Several absolute ages for specific glacial or limnological events have been obtained using salts, but conflicting dates and interpretations
have sometimes been
presented. This conflict is partially due to the difliculty in understanding
the processes by which saline
lakes and salts in crystalline deposits evolve. The ultimate origin(s) or source(s) of the salts is a fundamental part of these evolutionary processes.
Previous studies of salt origin in the region have
tended to concentrate on the saline lakes. specific salts
(or elements) or on specific localities. Such studies
cannot lead to valid generalizations
of origin because
the accumulation
of different soluble salts is both irregular and sporadic and subject to perturbing evolutionary processes including leaching, migration
of
salts and fractional
crystallization
from solution.
However. a variety of origins have been proposed:
either (I) a marine source, or (2) chemical weathering
of local rock material are favoured by an extensive
literature.
2299
.I R.
2300
(HARRY)
KI:YS and
The present
study is based on the distribution
oi
eight common salt phases, and their component ions,
in crystalline salt deposits of the McMurdo region.
Analytical results from over 300 deposits have been
subjected to statistical treatment. The resulting numerical data are graphically displayed to examine the
influence of either the marine source or the local rock
on salt distribution.
Stable isotope data produced by
other workers are also discussed. Other origins of salt
are briefly mentioned.
KAREN
WILI.IA~S
60 -
:
$
.
halite .thenvdite A_
-
gypsum&--CalClte.
40.
$
f
1
20 -
DATA
COLLECTION,
REDUCTION
PRESENTATION
AND
Areas for salt collection
were selected to study the regional distribution
of salt. Taylor and Wright Valleys of
McMurdo oasis (Fig. 1) were chosen as the main sampling
regions because these valleys offer convenient
access to a
variety of geographical,
geological and climatological
environments.
Sampling was performed during the summers
of 1972~13, 1913/14. 1974/75 and 1976,:77 in a number of
areas at increasing distance west (inland) from the coast of
McMurdo
Sound. In each area representative
samples
were taken from salt deposits that were encountered
during
foot traverses. Where possible, deposits were sampled from
substrates in which a specific rock type was present on its
own or could be assessed, semiquantitatively.
as being
dominant.
Topographic
obstacles
and permanent
ice accumulations,
as well as a naturally
sparse and sporadic
distribution
of salts, limited ability to sample in some
areas. Systematic
sampling was not performed
for this rcgional study but was for local distribution
studies reported
elsewhere (KEYS, 1980b).
After collection, the samples were sealed m plastic bags
which is normally
a sufficient precaution
against changes
in the hydration
state of most well known Antarctic salt
phases. Mirabilite
is the only one of the IO widespread
phases that is unstable in the laboratory
environment.
The
i_-LL_._..
IO
30
_.. .I-..
50
Fig. 2. Distribution
of halite. thenardrte,
gypsui’? .rnd <.;I;cite (number of occurrences
of these phases as ‘I pcrcentag!c
of the total number of occurrences
of the clghr cc)mtnou
phases) in deposits within successive 10 km inirrcalb f’rom
the coast up to 1000 m asl in T,ty!ot- and Wrighr V;c!lei:
efflorescence
(to thenardite)
01 this characterl>tic
ir’,lt;klucent to opaque mineral is slow enough for mlisbllitc
t?b
be readily identifiable.
Reported.
uncommon
pl~rcs th;r!
are unstable
outside the Antarctic
environment
include
dihydrohalite
(CRAIG et al., 1974) and hydrates of calcium
carbonate
salts (BROWNE. 1973; NEHIYAMA ,~rd KI:KASAWA. 1975); these alter to halite and calcite Irespectively
(PALACHE or ul., 1951). Furthermore.
antarcticitc
(TORII and
OSSAKA, 1965) deliquesces and dissolves. These instahilitie\
do not affect the interpretation
of the data prehcnted hen*
since the elements in the salts do not change.
The samples were analysed by X-ray diffraction
.md 1lx
results
have been published
elsewhere
(KAYj. 1079hl.
together with site environmental
details
K~YY !lQ79ht !X
WI- 3
ROSS ICE SHELF
Whlta
Island
\
Fig.
I The McMurdo
region
of Antarcuca,
-ic
Distance inland, km
showing
-
places mentioned
DVDP hole @
in Ihv [t‘\k
Origin
of crystalline,
soda rlllre ldarrqsldte o--
epsomiten .. . ....
bloedite0 ---
IO
Distance
inland, km
Fig. 3. Distribution
of soda nitre, darapskite,
epsomite and
bloedite (number of occurrences
of these phases, percent, as
in Fig. 2).
the mineral data base for the present, interpretive,
study.
Two reduction
treatments
were performed on the mineral
data for the present study; the results of these treatments
are displayed graphically
in Figs 2-5 and 6, respectively.
The first treatment
was designed to assess the marine
influence on salt deposits in McMurdo
oasis. The deposits
considered
were limited to those below 1000 m elevation
and between 25 and 75 map kilometres
inland in Taylor
and Wright Valleys, and were subdivided into five successive IO km intervals (Figs 2-5). Each time one of the eight
common
salt phases (thenardite,
gypsum, halite, calcite,
darapskite,
soda nitre, bloedite, epsomite) was present in a
deposit, one count (occurrence)
was made for that phase
for the distance interval in which the deposit was located.
Since there were different numbers of deposits sampled in
each interval, the resulting counts (frequency of encounter)
were normalized
by calculating
the number of occurrences
counted for each phase as a percentage
of the total counts
of all eight phases together in that interval. Linear regression was then used to fit straight lines to bivariate plots of
the number of occurrences
of each phase as a percentage of
the total number of occurrences,
versus distance inland
(Figs 2 and 3). The data for gypsum and calcite are not
fitted well by this procedure.
In order that no bias be
created towards
those widespread
salts represented
by
.-.--.
-.i
so.*
i/
r.5
-.
-zr_
2’:
H
cold desert salts
2301
more than one phase, the less widespread phases of sodium
sulphate (i.e. mirabilite) and magnesium sulphate (hexahydrite) were not considered
in this treatment.
These distribution
trends are more clearly defined when
the distributions
of specific anions and catlons are examined. Here a particular ion was counted once for each time
a phase containmg
it was present in a deposit and percentages calculated
as before. This was a presence or absence
test for one cation and one anion per salt. no account
being taken of stoichiometric
relationships.
Darapskite
was
classed as sodium nitrate and bloedite as magnesium
sulphate so that no bias was created towards double salts.
Straight lines were fitted as before (Figs 4 and 5) and the
confidence
level of the slope and correlation
coefficient
obtained by standard statlstical treatment
(FRI.u\~ PI (I/..
1960). In Figs 4 and 5 a solld line Indicates that a slgmticant correlation
is evident from regresslon
analysis. with
the confidence level of the slope being greater than 90”,,
and correlation
coefficient greater than 0.7.
The influence of the local rock on salt deposits
was
examined by a second treatment.
There are live dlffcrent
lithologies exposed in the region: alkaline volcanics. metasediments. acid plutonics, dolerite and Beacon Supergroup
sediments
(WARREN. 1969). Occurrences
of each of the
seven common elements or Ions in the salts (Na’. C’a’ +.
Mg’+. SO:-. Cl-, NO,. CO:-) were counted once for
each deposit in which they were present. for deposits on
substrates
consistmg primarily
of one lithology.
The frequency of occurrence
of each ion was then obtained
as a
percentage
of the total number of deposits considered
for
each of the five lithologies. Finally, the mean distance from
coast was determined for deposits considered In each lithology (Table I). Histograms
were drawn from this Table
(Fig. 6). The number of deposits listed for metasediments
and plutonics is not large because pure examples of these
types of slibstrates
were encountered
infrequently.
Salts
from the summit region of Erebus Volcano were excluded
from this treatment
because
those salts wcrc dcribed
mainly from volcanic gas (Krus. lOXOh).
THE
MARINE
SOURCE
The sea is an important
source
of salts in the
McMurdo region, the main lines of evidence being the
negative salt gradients inland in McMurdo oasis. The
clearest evidence is in the distribution
of chloride
because this is the major ion in sea water and a very
4-
-.-
Cl-
-d-y
60
\
CO,*
““-“‘-+-i._*
I
~....._,...___,,_.,_,__,_
_I_
‘;>k.?.,
-
NO;
IO
__
30
50
70
Diiance inland, km
Fig. 4. Distribution
of the eight common
salts as their
anions (number of occurrences
of phases containing
anion
X as a percentage of the total number of occurrences
of the
eight common phases). Data from Figs 2 and 3, except
darapskite
and bloedite are treated as sodium nitrate and
magnesium sulphate respectively. A solid best-fit line indicates that a significant correlation
exists between the population and distance parameters.
No significant
correlation
exists between these parameters
for sulphate (dashed line)
and carbonate
(dotted line) anions.
2ol
/
IO
Mg.‘_e_._y<
30
50
70
Distance inland, km
Fig.
5. Distribution
of the eight common
cations as in Fig. 4 above.
salts
as their
.I. R (HARRY) KEYS and KAKEN WILLIAMS
2302
lable 1.
Distribution of cations and anions as a percentage
of crystalline sale deposits that contain a
particular ion, for deposits on substrates con
sisting primarily of one lithology
I<ocktype
!0n
Volcanlcs* s,,Z~;,,, I'lutonicsDolerite s$~~~ts
Number ot
deposits, ND
7h
Mean dis
tance from
coast (km)
Zi:
Ii
7i
46
j:
i:
80
‘1
.i4
*Cape Bird, White and Black Island5 and Taylor Valley
,below 760m asl.
*Aright (8 samples) and Taylor Valleys.
A CONNORS
100
Volconics
Metasedinent
tao-
I
60-
40-
I
'H
d
.c
2o
=
2
6
E
1!
r
!
,
I
t
Dolerite
PbJuucs
I_rl
1
._
Nat Mg*Ca”
Na’Mg’+Ca’
-2.
Z+
Nd Mg’iCa’f
---..
--__
Metasedimena
Rukrllcs
1
Dolerlte
Mcm
._.-
sedinents
i-
60
~
:I- NO3-So’-CO’
,I-NO;SO:-CO:
!
4
Fig. 6. Distribution
of cations
substrates consisting primarily
I
Cl-
NO; SOgO'
and anions (percentage
of deposits containing
ion
of :i single lithologq.
arrvnged in order of increasing
Z) an depovth
OL:
Ji\t;mce 1nla11tl
Origin
of crystalline.
ion in acid plutonic, basic igneous and sedimentary rocks (GOLDSCHMIDT, 1954;TAYLOR, 1964).
Chloride
distribution
shows a marked decreasing
trend inland (Figs 4 and 6) due to a marine influence
that is strong near the coast but lessens inland. Similar decreasing tendencies can be seen for sodium (Figs
5 and 6) and halite and thenardite (Fig. 2). Similar
lessening influences with increasing elevation in the
oasis and distance south of Cape Royds on Ross
Island have been shown by KEYS (1980b) for these
ions and phases.
These findings can be related to a quantitative
study of soil salt distribution made by CLARIDGE and
CAMPHELL(1977). In the present study, sulphate and
nitrate phases tend to be encountered more frequently
towards the west whereas chlorides are less frequently
encountered. Thus, the ratios S04/Cl and N03iCl. for
relative distribution of phases, increase away from the
coast (Fig. 4). These patterns were observed also by
CI.ARIIX;E and CAMPBELL (1977) for relative concentrations of these ions in Antarctic soils, and attributed
to a marine influence lessening inland. This same lessening influence was believed to be partly responsible
for a decrease in soil pH inland in the oasis, from high
values of 9 in coastal areas, to less than 6 along the
edge of the continental ice sheet.
Gradients
of ion composition
and concentration
also exist in snow on the continental ice sheet. A general decrease inland in the concentration
of soluble
salts in surface snow has been demonstrated
(LORIUS
c’f trl.. 1969: BOUTRONet trl., 1972; DELMAS and Bouminor
cold desert salts
2.303
TRON, 1978). Chloride and sodium ions predomlpate
in the first few hundred kilometres from the coast but
sulphate predominates
in the interior of the continent.
Thus, the SO,.;CI ratio in the snow increases inland.
parallelling the trends in this ratio noted earlier.
It has been demonstrated
that marine aerosols in
the lower atmosphere
are the main source of saline
impurities in snow in Antarctica (MUROZUMI (‘r (I/..
1969; BOUTRONor (I/.. 1972: BOU’RON, 1979). In particular. most of the sulphate originally came from the
sea either as sulphate, sulphur dioxide or hydrogen
sulphide (NGUYEN er cl/., 1974: DELMASand BOIJTRON,
1978). Furthermore,
the pH of precipitation
is closely
related to atmospheric
sulphuric acid (LIKENS and
BORWUS. 1974) which accounts for a significant part
of this sulphate (CALXE t’f trl.. 1968). Therefore. a close
inverse correlation between pH and SO4 Cl ratio in
oasis soils. noted by CLARID(;E and CAMPBELL( 1977),
is due to an increasing contribution
inland of acidic
salts derived from atmospheric
marine aerosols. as
well as a lessening, more direct marine contribution.
Overall. a marine influence is present throughout.
Numerous analyses of cS”~S in crystalline and dissolved sulphate have been published and show that
sulphates of marine origin are widespread
in the
region (RAFTER and MIZUTANI, 1967; BOWSER c’t trl.,
1970: N.~KAI. 1974; NAKAI rt (I/., 1975. 1978: NISHIYAMA and NAKAI. 1975; LYON. 1978). Reports of
6180 in these same sulphates arc less numerous but
the two deviations plotted together are valuable indicators of sulphate source (Fig. 7). The depletion of
%
8%
0
d
I
25
0 LakeVando
.50 m ..$I.
I
t
I
llm1ts of 834 S
I” seowoter
j
Glacw
DVDP3,200-38lm
t _......___,.. .;/.
:
4m
Koettlltz
20
:
5011w Ice
Hobbs
Dromedary
Walcott
Glacier
Glacier
@
Glace~_
_ _
_
_
_ -,
\
I5
A
IO
Walcott
Glacier
I
@I
sulphate
5
I” ran‘:
:
’I
I
,
,
I
I
DVDP 3
IOO-200m
Fig. 7. Stable
Data
isotope ratios of sulphates:
63JS,,,
vs 6180sm\\..
(1967). Bowst~ et al. (1970). LYON (1978) and NAKAI
from
RAFTEK
et trl. (19781.
and
MI~UTAUI
2304
J.
R. (HARRY)
KEYSand KARENWILLIAMS
180(S0,) from marine-derived
sulphates in Fig. 7 has
been discussed by BOWSER et ul. (1970), NAKAI rt ul.
(1975). and LYON (1978), and attributed to microbiological activity. Similarly, the depletion of 34S(S04) in
some sulphates has been attributed
to atmospheric
transport of salt from open sea water and/or saltcovered sea ice (NAKAI PI (I/., 1975: NISHIYAMAand
NAKAI. 1975; KEYS. 1980b).
Strontium isotopes in salts of lakes and soils in
McMurdo
oasis have been studied by JONES and
FAURE (1978). *‘Sr/s%r ratios systematically decrease
from 0.7136 around Lake Bonney to 0.7090 at Lake
Fryxell. This decrease was attributed to the presence
of Sr of marine origin which becomes dominant in
eastern Taylor Valley.
Nitrate ion is believed to be derived mainly from
the sea via atmospheric
aerosols (CLARIIXE and
CAMPBELL, 1968). The ratio of nitrogen oxides to ammonia in aerosols generally increases inland over land
masses. partly due to oxidation of the ammonia which
is mainly of marine origin (LODGE and PATE, 1966).
The increasing trend in the ratio NO,CI
for relative
distribution of phases (Figs 4 and 6) is consistent with
the increasing trend in the atmosphere. The absence
of nitrate in coastal salt deposits (Figs 4 and 6) can be
attributed to a lack of nitrate in coastal snowfalls, and
to leaching and biological activity in this relatively
moist environment.
Some nitrate in the region may
have been fixed by ionization processes in the upper
atmosphere
(WILSON and HOUSE.. 1965), resulting in
very low values of 6”N (T. TORII, personal communication).
A full examination
of the pathways followed by
marine-derived
salts into the oasis is outside the scope
of this paper, but some discussion is pertinent. Most
salts of marine origin appear to have travelled by
relatively direct pathways. This is shown by (1) the
similarity between the pH of many oasis soils and
that of sea water (8-9), and (2) the similarity between
(jJ4S values of sulphate salts in the region and sea
water sulphate (Fig. 7). These salts have not undergone long contact with the acidifying and oxidizing
environment
of the atmosphere that was experienced
by salts further inland. Direct pathways include coastal snowfall (probably
the most widespread),
sea
spray, dry fallout (including salts blown off the sea
ice) and influx of sea water (KEYS. 1980bl.
CHEMICAL
WEATHERING OF LOCAL ROCK
MATERIAL
The clear association between magnesium ion and
substrates of basic igneous rocks (Fig. 6) is good evidence for a rock and soil source of magnesium salts.
The absence of magnesium
phases on the mainly
quartzose Beacon sediments indicates that fallout of
marine aerosols is not a significant source of Mg*’ in
McMurdo oasis. The presence of the ion in deposits
on volcanics, a substrate that is present near the
coast. is counter to the regional trends of Mg*’ in
McMurdo oasis (Fig. 5). Therefore, it can be concluded that the primary source of Mg’.’ is the dolerite and volcanic rocks in the region, similar to the
conclusion of CLARIDGE and CAMPBELL (lY77). The
Mg2 + trends inland (Figs 3 and 5) can be attributed
to the more frequent out-cropping of doleritc toward5
the west (Table 1).
Magnesium
is released by chemical weathering
probably by two main sets of reactions in Antarctica,
oxidation of pyroxenes and hydrolysis of other mafic
minerals.
Oxidation takes place in dolerite substrates, ‘1s in&
cated by the strong orange-brown
colour due to iron
oxides in old dolerite soils. Such soils are common rn
western parts of McMurdo oasis. Microscopic exam.
ination of weathered dolerite material from the Transantarctic Mountains
has shown that the pyroxenes
(hypersthene, augite, pigeonite) are almost completely
decayed while feldspars and other minerals are quite
fresh (CLARIDGE and CAMPBELL., 1977). In moister
areas of East Antarctica, GLAZOVSKAYA(195X) found
that individual
grains of ferromagnesian
minerals
show signs of iron-staining
even in relatively fresh
material, and accumulations
of secondary clay residues were noted in fractures in the minerals. ln
Antarctic soils thin liquid films of moisture with high
ionic strength surround
soil grains (U(w#.r~r and
ANDERSON, 1972), enabling atmospheric
oxygen and
carbon dioxide to be brought into close aqueous contact with ferromagnesian minerals. This allows chemical weathering to take place in the cold, arid environment. A reaction describing the oxidation ot pigconite, in mildly acid conditions in inland dolerlte soils.
can be represented as:
(ideal composition in example from dolerite graven by
DEER vf ul. (1962).
In general, however. the weathering of slltcatt’ mm-.
erals is mainly by hydrolysis (KRAIJSKOPF, 1967). Olivine, normally one of the most easily weathered maiic
minerals, is often abundant in basaltic volcanics un the
region (HASKELL et a/., 1965; Coot: (‘I (II., 1971 i. A
simplified reaction describing the hydrolysis of olivirtc
(as the end member forsterite) can be written (KRAIiSKOPF, 1967) as:
Mg2Si04(,, + 4C02(.,qI f 4H20 --+
f0rSl<rlt?
The equilibrium constant for reaction 2 at + 1’5 i‘ I:,
lo*.’ indicating a strong tendency to proceed to the
right. Reactions such as this are likely to bc more
important at old volcanic centres around McMurdo
Sound than in the oasis. Around the Sound, preclpitation and atmospheric relative humidities are greater
(KEYS. 1980a). and the moisture content i*f ioils IZ
Origin of crystalline, cold desert salts
higher (BERG and BLACK, 1966; CAMERON and CONROW, 1969), than in the oasis.
Several other studies have shown that chemical
weathering
of rock and soil minerals takes place,
albeit slowly, in the McMurdo region and has done
so for the lifetime of the region. In particular, the
amount of extractable
iron and the degree of hydration of micas. generally increase with time from
young soils to those older than four million years
(CLARIDGE, 1965; BEHLING, 1971; CAMPBELL and
CLARIDGE. 1975, 1978; JACKSONet ul., 1977). *‘Sr,@Sr
ratios in lake and river water are very similar to those
in soil, sediments and local rocks in Wright Valley
and around Lake Bonney (JONESand FAURE. 1978).
This implies that much of the strontium salts in these
areas have been derived by chemical weathering.
Besides Mg* +. chemical weathering is an important
source of Ca”, K * and probably carbonates (CLARIDGE and CAMPBELL, 1977). However, carbonate salts
are probably derived in part from atmospheric carbon
dioxide as well, via chemical weathering (reactions 1
and 2). The general abundance of Ca2+ and the less
widespread distribution of CO:- (as calcite) shown in
Fig. 6 can best be attributed to such processes.
2305
because it readily dehydrates
irreversibly to calcite
above O’C (PALACHE rr al., 1951).
A high titration alkalinity-to-~a2+
ratio in Lake
Fryxell waters (TORII et al., 1975) and a localized distribution of trona (NaHCO,Na,CO,H,O),
thermonatrite (Na2C0,H20)
and burkeite (Na,CO,(SO&)
(NISHIYAMA,1979) may also be partly due to biological activity in eastern Taylor Valley. Microbiological
anaerobic reduction of sulphate appears to have been
quite widespread in that area (NISHIYAMAand NAKAI,
1975) and bicarbonate
is usually formed as a byproduct of this process (BERNER, 1971). Loss of some carbon dioxide from a reduced sulphate brine either by
evaporation or photosynthesis
would produce sodium
carbonate and bicarbonate salts. Burkeite could then
be formed under specific conditions of concentration,
temperature (above about + 14°C at equilibrium) and
low partial pressures of CO2 (EUGSTER and SMITH,
l965), which are reasonable in eastern Taylor Valley
(Keys, 1980b). The absence of gypsum in eastern
Taylor Valley (NISHIYAMA, 1979) suggests that the
reaction:
CaCO,
+ SOa-
+ 2H20-
marhlc
OTHER SOURCES
Other sources of ions seem to have local importance in the McMurdo
region. Biological, volcanic
and hydrothermal
activity are probably most significant.
Biological processes are indicated by the presence
of phosphate
salts, some aragonite
deposits
and
monohydrocalcite.
Four documented
occurrences of
newberyite (MgHP043H20)
and calcium phosphate
(probably whitlockite, Ca3(P0&)
are all from penguin or skua rookeries situated on or near outcrops of
igneous
rocks, mainly
basaltic
volcanics
(KEYS,
1980b). Phosphates and uric acid in guano and other
organic material probably cause weathering of mafic
minerals in the substrates leading to the formation of
the phosphate salts of magnesium and calcium. Some
aragonite deposits in Taylor Valley have been formed
after depletion of carbon dioxide by photosynthesis
by blue-green
algae and bacteria in former lakes
(HENDY et al., 1979). Monohydrocalcite
has been
found in one locality, in eastern Taylor Valley (NISHIYAMA and KURASAWA, 1975). Other documented
occurrences
of this mineral from elsewhere in the
world have been reviewed by SKINNER et al. (1977).
All are believed to be the result of biological activity
possibly by specific organisms
and/or biochemical
reactions involving blue-green algae for example. If
monohydrocalcite
were formed as an intermediate
in
the dehydration
of hexahydrocalcite
(PALATHE rt ul.,
1951). which crystallizes
out of sea water below
-2.2 C (WEEKS, 1962), then. the phase would have a
more extensive distribution. Hexahydrocalcite
has not
been reported from Antarctic salt deposits, probably
is unimportant
there.
Activity associated with surface volcanism is an important source of salts in the summit area of Erebus
Volcano, but nowhere else in the region. The composition of Erebus salts is different from that of salts
studied elsewhere around McMurdo Sound. Table 2
compares Al, Si, Fe and F concentrations
in three
Erebus salts with those in one deposit in McMurdo
oasis. The Erebus salts were free of rock fragments,
but sand comprised about 10 percent (by volume) of
the sample from the oasis. The trace amounts of Al, Si
and Fe in the oasis salt can be attributed to this contamination.
Elsewhere in McMurdo
oasis, minerals
containing Al, Si, Fe and F may be present as trace
accessories in some salt deposits (HAMILTON et ul.,
1962; TORII et al., 1966). However, the Erebus salts
contain significant proportions
of aluminium phases
plus phases that may contain silicon, fluorine and
Table 2.
Concentrations (weight percent) of
aluminium, iron, fluorine and
silicon in Erebus salts and salt
from Pearse Valley, McMurdo oasis
Salt sample
Pear-se
Valley
Erebus
Element
24880
24885
24886
24848
Al
Si
Fe
F
10.4
2.0
1.9
<l
lZ.88
2:5
4.8
12.2
1.7
0.27
1.1
:.;
0:56
<l
Analyst: W. ZOLLER, University of Maryland,
U.S.A.; analysis by neutron activation
2306
Table 3.
Positively and tentatively identified crystalline salt
phases from the sumnit area of Mount Erebus
Positively
identified
___..
Halite
Gypsum
Alunite
Thenardite
Sylvite
Mirabilite
Calcite
Tentatively identified
___..__
___-___.
Chloraluminate AlC13.6H20
Alunogen
Alz(SOs)s,18H20
Jarosite
(K,Na)(Fe,Al)(SO4),(OH),
Malladrite
Na2SiF6
Sulphohalite
Na6C1F(S0,j,
Aluminium
trifluoride
AIF
Sodium aluminium
oxychloride*
NaAlu04C15
Hydromolyiite* FeC1,.6H20
NaMgAl(F,OH).H*O
Ralstonite*
*Tentatively identified by W. ROSE (Michigan Technological
Universitv. U.S.A.. oersonal communication),who also
tentative?,;identiiikd AlF,.
minor
iron. Table 3 lists minerals that have been posttively or tentatively
identified by X-ray diffraction
(KEYS. 1979b). Positive identification
of most Erebus
salt minerals by XRD is complicated by peak overlap.
post-sampling
changes in structural water. and probably by mutual substitution
of ions. Several of the
positively identified minerals in Table 3 are common
in the McMurdo region. However, sylvite is not widespread and alunite is not present away from Erebus
None of the tentatively identified minerals is present
outside Erebus.
Volcanic activity on Erebus has had little affect on
salts elsewhere in the region. Regional distribution
01’
salts has shown that most salt is derived from the sea
and by chemical weathering. Quantitative
analyses of
sulphur dioxide (P~I.IAN and LAMBEKT,lY7Y) and hydrogen chloride (ZOLLEK, personal communication)
in
the volcanic plume from Erebus lead to estimated
annual outputs of sulphate and chloride that arc
small (less than .5’!,,)compared to the Antarctic surface budgets of these species (POI.IAN and LAMBER’I.
1979; KEYS, 1980b). Therefore. it is concluded that
volcanic activity is. and has been. of local significance
only.
Hydrothermal
activity has produced some salt m
the region. The presence of zeolites and the unusual
mineral thaumasite (CaSiOjCaC03CaS041
5H20) in
Dry Valley Drilling Project (DVDP) holes l- 3 on
Ross Island indicates that hydrothermal
fluids have
circulated through these volcanic rocks after the)
were erupted (KNILL, 1960; BKOWNE. 1973, 1974). A
sequence of fluids, progressively
cooling and having
an increased partial pressure of carbon dioxide. is
indicated
by the secondary
mineral
assemblage
(BROWNE, 1973, 1974). BROWNL:(1974) also reported
siderite. a salt that is formed in reducing conditions
and either high partial pressure of CO2 or 1ow concentrations of S- (BERNER, 1971). It is not likely to be
formed under aerobic surface conditions.
This suggests that this and at least some of the other carbonates in these cores were formed from hydrothermal
fluids probably towards the end of the sequence of
fluid circulation.
Other phases have been tbrmcd hy hydr~~thcrm,ll
processes besides these carbonates.
Value\ i>! <ri”S
and 6’sO(SO.+) for sulphates between IOU and 200 m
depth in DVDP 3 (Fig. 7) indicate that hydrothermal oxidation of primary sulphide mincr,jls has
occurred, consistent with petrological evidcncc from
the volcanics (NAKAI. 1975; N,~KA! rt rrl.. i97icj. Some
calcite. aragonite, gypsum and ~eolites in l>\‘i)l’ 6 at
Lake Vida have also formed h) hydrothermal
prf)cesses (WATANGKI and MORIKA~A. 1975). I>olomitt*.
reported from one locality on Dias. Wrlghr ‘v’.tilc!, I:~
thought to be of hydrothermal
origm as indicated (7)
stable isotopes and association with ~.eohtu\ ! R;AKSI.
1Y74).
Variable. but generally small amuunt\ iii pyp\um
and calcite are derived from preglacial &posits ~-rl
these minerals. Lacustrine gypsum beds 1 1mar thick
have been noted in the Aztec Siltstone (Mc PHHWJU.
lY78) and Terra Cotta Siltstone. Devonian ;t.ged formations of the Beacon Supergroup. Small amount? tr!
gypsum are slowly leached from these bed\ and arc
locally abundant in surficial deposits arou!lc! them
(KEYS. 1979b). Similarly, calcite deposits att locally
abundant
around
rocks that contain
:,ignilicant
amounts of calcium carbonate. Marbles in ‘I‘.~kior ;mti
Victoria Valleys and near Koetthtc Glacier .ue inportant sources of calcite (Cl AKIDGLand c. ,\5!1’t11
1.1.
1977). Elsewhere. sedimentary and hydrothermal
GIIcite is of minor importance.
There is no evidence for other, more \tliuhie wit
minerals to have contributed to deposits III I Ihi \vi1>
Furthermore, it is unlikely that significant dlmrlunts ol
such salts could have survived what 1s h&c\~cd t.lj
have been a wet-based glaciation represcntmg
the
onset of glaciation in Antarctica (MAY~WSKI. 1976.5)
Late Oligocene to Early Miocene times (HA\ t s (‘1 $I/..
1975).
MIGRATION
AND SEPARATION
OF’ SALI‘S
individual salt deposits in the McMurdo rcg~on are
diverse in composition
and do not all contam salt
phases that are recognisably marme. dcsp~tc !he lm-
Origin
of crystalline,
portance
of the marine source of salts. This apparent
paradox is not caused by a predominance
of chemically weathering sources, since chloride anion is widespread and abundant. A possible cause of the paradox
is the complex set of processes that affect salt distribution on a local scale (i.e. O-1 km), a full discussion of
which is outside the scope of this paper.
KEYS (1980b) suggested that local distribution
is
controlled by processes that cause localized migration
and separation of salts, not by salt origin. Migration
is induced by water and wind, with soil brines moving
as thin liquid films (UGOLINI and ANDERSON,1972). by
capillarity and under the influence of gravity. Deflation of individual deposits and asymmetric accumulation of salt show that wind is important. Separation
of phases is a consequence of different physicechemical properties of salts. in particular eutectic temperatures. together with environmental
conditions. Separation seems to be achieved mainly by fractional dissolution and crystallization
processes; several deposits
containing
separated
(i.e. fractionated)
phases (e.g.
gypsum and thenardite)
have been found in the
region. Keys suggested that separation
processes,
together with salt migration, have operated over the
lifetime of the region (0 to >4Ma),
and have obscured the sources of the salts in the soils and saline
lakes.
CONCLUSION
This study has recognized that a variety of sources
exist for the salts in the McMurdo region, and have
contributed
to the salt system as a whole. Different
sources are important in different areas and no single
source can account for all the salts. Similarly, different
pathways exist for different sources.
Salts of marine origin are regionally and quantitatively most important.
This is indicated by: (1) the
abundance of chloride, sulphate and sodium salts. all
of which are derived mainly from the sea; (2) the
widespread distribution
of these same salts; (3) the
strong trends in the distribution
of chloride and
sodium salts; and (4) the alkaline pH of most soils in
the region.
Chemical weathering is the main process contributing magnesium, calcium and carbonate ions into the
system. Weathering
reactions can be described
in
terms of normal chemical decay of the most easily
weathered
rock and soil minerals.
Although
the
reactions are probably very slow in Antarctic conditions. they have operated for more than 4 Ma in
places, and thus the products of such weathering are
widespread.
Other sources of salts are locally important.
Biological activity is significant in eastern Taylor Valley,
and may have been underestimated
in the past.
Hydrothermal
and volcanic activity have produced
distinctive salts in subsurface (mainly volcanic) rocks,
and in the summit area of Erebus Volcano, but these
salts are not of regional or continental
significance.
cold desert salts
2307
Nor is there any evidence that significant amounts of
the salts present in the region were formed in times
pre-dating
the present
ice sheet.
Acknowleciyemenrs~This study was part of a Ph.D. study
by J. R. KEYS of salts and their distribution
in the
McMurdo
region. J. R. KEYS is indebted to Drs A. G.
FREEMAN and P. J. BARRETT for their supervision.
The
authors
are grateful to fellow expedition
members from
Victoria University as well as to numerous people from the
New Zealand Antarctic
Research Program.
Victoria University; Antarctic
Division. Department
of Scientific and
Industrial
Research, N.Z.; and Natlonal Science Foundation and VXE-6 Squadron.
U.S.A.. provided
logistic
support.
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