Mass balance of the Lambert Glacier basin, East Antarctica

Mass balance of the Lambert Glacier basin, East Antarctica

Vol. 45 No. 9
SCIENCE IN CHINA (Series D)
September 2002
Mass balance of the Lambert Glacier basin, East Antarctica
REN Jiawen ()1, Ian Allison2, XIAO Cunde ()1
& QIN Dahe ()1
1. Laboratory of Ice Core and Cold Regions Environment, Cold and Arid Regions Environmental and Engineering
Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China;
2. Antarctic CRC and Australian Antarctic Division, GPO Box 252-80, Hobart, Tasmania 7001, Australia
Correspondence should be addressed to Ren Jiawen (email: [email protected])
Received February 14, 2002
Abstract Since it is the largest glacier system in Antarctica, the Lambert Glacier basin plays an
important role in the mass balance of the overall Antarctic ice sheet. The observed data and shallow core studies from the inland traverse investigations in recent years show that there are noticeable differences in the distribution and variability of the snow accumulation rate between east
and west sides. On the east side, the accumulation is higher on the average and has increased in
the past decades, while on the west side it is contrary. The ice movement measurement and the
ice flux calculation indicate that the ice velocity and the flux are larger in east than in west, meaning that the major part of mass supply for the glacier is from the east side. The mass budget estimate with the latest data gives that the integrated accumulation over the upstream area of the investigation traverse route is larger than the outflow ice flux by 13%, suggesting that the glacier
basin is in a positive mass balance state and the ice thickness will increase if the present climate is
keeping.
Keywords: mass balance, Antarctic Ice Sheet, Lambert Glacier.
The Antarctic ice sheet is the largest solid reservoir with an area of 13.5×106 km2 and a volume of nearly 30×106 km3, which contains around 90% of the total ice and more than 70% of the
fresh water on the earth[1,2]. Since even a little variation in its volume will cause the significant
environmental effect (for instance, 1% of its volume can result in 0.7ü 0.8 m change in the global
sea level), recent interest in global change has focused attention upon the potential of the ice sheet
to grow or shrink in the near future.
The volume change of the ice sheet is in general expressed in terms of the mass balance because it describes the budget status between the mass income (mainly as the precipitation, the
proxy is the snow accumulation rate) and the mass loss (mainly as the iceberg calving and then the
melting). The positive balance means the mass income exceeds the mass loss in amount and as a
result the ice sheet volume will increase, and contrariwise it will decrease.
The Lambert Glacier basin is the largest ice stream system of the Antarctica, and thus plays
an important role in the mass balance of the overall Antarctic ice sheet. In the past several decades,
some attempts have been made on the estimation of the mass balance state in the basin from the
observations within very limited areas. Since 1990, Australian National Antarctic Research Expe-
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dition (ANARE) carried out the inland traverses over the basin along a route with about 2500 m
elevation, but the emphasis was put in west part of the basin. Chinese National Antarctic Research
Expedition (CHINARE) has taken an inland ice sheet investigation along the transect from the
Zhongshan Station to Dome A in east part of the basin (fig. 1). Moreover, some Chinese scholars
also participated in the ANARE’s work. These long distance traverse investigations have covered
the main interior region of the basin. From in situ measurement of the accumulation rate and shallow core studies, some results of the accumulation rate distribution and variability in partial sections of the investigation routes have been presented, such as in west part of the basin by Goodwin
et al.[3] and Ren et al.[4
ü6]
. Besides, Higham et al.[7,8] reported the data on the accumulation meas-
urement along the ANARE traverse route, and Fricker et al.[9] discussed the ice flow flux and the
mass balance along the route. In the present paper, we would like to make an attempt at presenting
a comprehensive discussion on the distribution and variations of the surface accumulation rate
over the basin, the ice flow flux across the investigation sections and the total integrated accumulation and the mass balance state over the upstream area using all available data from traverses
along both ANARE and CHINARE routes. Nevertheless, we lay the emphasis upon the calculation and discussion of the present state of the mass balance.
1 Research area and field work
The Lambert Glacier lies in a deep rift valley in the East Antarctica. In directions of west,
south and east within a large extent ice flows convergently into the glacier and then drains into the
Amery Ice Shelf. So the Lambert Glacier basin contains three parts: Amery Ice Shelf, Lambert
Glacier and the upstream area (fig. 1). The total area of the basin is 1550000 km2, including 69000
km2 of floating ice in the Amery Ice Shelf [9]. Since there are many exposed mounts in the Lambert
Glacier, it is rather difficult to make the mass balance measurement. The alternative method is the
investigation of the accumulation and the ice flux over the upstream area.
As shown in fig. 1, the Lambert Glacier basin traverse route (hereafter referred to as the LGB)
starts at LGB00 about 130 km from the coast (the Mawson Station) on the west side and ends at
LGB72 about 70 km from the coast (the Zhongshan Station) on the east side, with a total distance
of 2014 km and an elevation of around 2500 m. The Zhongshan-Dome A traverse route (hereafter
referred to as the DT) starts near Zhongshan , coincides with LGB within the initial 300 km and
then extends from LDB65 toward Dome A almost along 77eE with a total distance of 1100 km.
Along LGB route five time traverses were made during 1990ü1994. Of them two are for the
whole route and three limited in the west side. Along the DT route three traverses were made from
1997 to 1999, but only the third one covers the whole route and the other two arrived at 300 km
and 500 km from Zhongshan, respectively.
The snow accumulation rate was measured by means of bamboo canes at 2 km spatial intervals for both LGB and DT routes. Along the LGB route more than twenty shallow cores with
depth range of 10ü60 m were drilled, but the relatively detailed analysis was made for those
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drilled with depth of 10ü27 m in the west side. Along the DT route the core drilling was made at
four stations (DT001, DT085, DT263 and DT401) with depths of 50ü100 m.
Fig. 1. A map of the Lambert Glacier basin showing the LGB and DT traverse routes and the division of the upstream area of
the LGB route for the mass balance calculation.
2 Distribution and variability of accumulation rate
2.1 The distribution pattern
As an example, fig. 2(a) displays the measured net accumulation rate along the initial 460 km
section of the DT route (Zhongshan to DT085) on the east side during 1998ü1999. From the figure, it can be seen that in general the accumulation rate is high near the coast and decreases gradually toward the inland, consistent with that in the other regions. The average accumulation rate
over the 460 km distance is 171 kggm−2ga−1 (equal to 419 mm snow) in 1998ü1999 and over
the 300 km distance is 124 kggm−2ga−1 in 1994. On the west side, the average over the section
of LGB00 to LGB16 (130ü610 km from the coast) is 94 kggm−2ga−1. This suggests that more
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moisture reaches the east side than west side, consistent with the moisture transport modeling results[10,11].
2.2
Recent variations
Fig. 2(b) shows the accumulation rate variations in the past 250 years derived from a 50-m
core at DT001 on the east side of the basin, about 2 km from LGB65 and 300 km from the coast.
The dating of this core has been discussed elsewhere[12,13]. Further inland at DT085 near 500 km
from the coast, the primary results of a 50-m core show a similar tendency. It can be seen from the
figure that the accumulation rate has generally been in a continuous increase during the past 250
years. In the last decades, the increase tendency is relatively obvious, although the 1960s is a low
value period. However, it is contrary on the west side, where, from studies of the 10ü27 m
cores[6], the accumulation rate has been in decrease in past 30 ü 60 years, with the decrease
amount ranging from 16% at LGB16 (610 km from the coast) to 37% at MGA (about 40 km from
LGB00 and 180 km from the coast), and has been in increase since the late 1980s.
Fig. 2. Distribution of the accumulation rate from bamboo cane measurement at 2 km intervals in the east part (a) and variations of the accumulation rate in past 250 years derived from the DT001 core (b). The thick line is a 11-point running average in
(a) and a 7-point running average in (b).
3
Ice movement velocity and flux
Ice movement measurement was made along both LGB and DT traverse routes. Totally, seventy-three stations were established for ice movement measurement along LGB route with a
spacing interval of approximately 30 km. At each measurement station, the surface velocity magnitude and azimuth were determined by means of static global positioning system (GPS) observations made in two or more separate years. On the west side (LGB00üLGB35) four sets of this
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data were obtained, and on the east side (LGB35üLGB72) only one set was obtained. The accuracy is better than 0.3 mga−1 for 60% of the measured velocities and better than 1 mga−1 for the
rest[9,14]. Fig. 3 shows the average horizontal velocity components along LGB route. It is indicated
that on the average the velocity is lower than 20 mga−1 on the west side and relatively higher on
the east side, meaning that the major mass supply is from the east side of the basin. Near the coast
a very rapid increase in velocity may be related to the steep slope of the coast and the
off-obstruction of the edge of the ice sheet.
Fig. 3. The horizontal velocities measured along the LGB route (from Kiernan[14]).
Along the DT route, the surface velocity data were obtained at eight stations between LGB72
and DT085 in the period of 1997ü1999. The measured velocities are very consistent with those
along the LGB route and thus do not need to be mentioned again here.
For calculation of ice flow flux, the ice thickness is another necessary parameter. During the
traverses, the radio-echo sounding was employed for ice thickness measurement. The thickness
data along the LGB route has been introduced elsewhere[15], and along the DT route the data are
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847
still in processing but the rough sketch of ice thickness along the route can be determined.
Using the data of ice velocity and thickness along the LGB route, Fricker and others calculated the ice flow flux across the route. According to their result, the ice fluxes over the west
(LGB00ü32, about 1000 km), east (LGB72ü56, about 500 km) and stream (LGB32ü56, about
500 km) segments are 13.6, 8.6 and 21.8 Gtga−1, respectively. If the route is divided into only the
west and east segments, the stream segment will be completely incorporated into the east segment
and hence the ice flux in the east is much larger than in the west. Along the DT route, the average
thickness is around 1600 m and the average velocity is about 15.69 mga−1 over 500 km distance
toward the inland from LGB72. And consequently the ice flux over the traverse route section (500
km) is estimated roughly at 10 Gtga−1, higher than that along LGB.
4 Mass balance state
It is well known that if the ice flow flux across a section is equal to the net accumulation over
the upstream area in a time interval, the area is in the balance state, meaning that the thickness
keeps constant, and if not, it is in the imbalance and ice thickness will change. For the Lambert
Glacier basin, we have made an attempt at estimating the total integrated accumulation over the
upstream area of the LGB route (the area between LGB route and the dividing line). The upstream
area was divided into three parts: the east, the west and the stream. And then the east and west
parts are further divided into I and II, as shown in fig. 1. The division basis is the topographic
characteristics on large scale, shape feature of traverse routes and observation data. For instance,
over LGB00ü16 section the available data are the most abundant, and LGB52 is one of the turning points of the LGB route and can be compared with the DT263 where the stratigraphy of
snow-pit and core was studied. So the three segments along the LGB route are not the same as
those proposed by Fricker et al.[9].
In the calculations, input accumulation rates were mainly based on the observed data along
the traverse routes and the average values were adopted if possible. In the east I, since contours
and traverse routes are all basically extending in a north-south direction and the distances between
them are not large, we supposed no change in the accumulation from traverse route to the dividing
line. In the east II, the variability between LGB and DT was extrapolated to the divide. In the west
part, there is an auxiliary investigation route 50 km away from the LGB route[7] and thus the variability between the two routes was extrapolated to the divide. In the stream part, the observed data
are relatively sparse except those near the east part and so we extrapolated the variation pattern
along the DT route ( mainly determined from snow-pit observations) toward west.
The calculated accumulations over individual areas are listed in table 1. It is seen that the
accumulation is 12.7 Gtga−1 for the east area, 21.6 Gtga−1 for the stream area and 15.4 Gtga−1
for the west area, respectively. This yields the total integrated accumulation over the upstream area
of 49.7 Gtga−1, 13% higher than the flux of 44 Gtga−1 across the LGB traverse route.
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Table 1 Calculated accumulations and outflow fluxes (Gtga−1) over the upstream area of the LGB route
4
2
Area×10 /km
Accumulation
Outflow flux
Imbalance (%)
5
I
6.2
6.3
East
II
10.7
6.4
Sum
16.9
12.7
8.6
47.7
Stream
41.8
21.6
21.8
−0.9
I
7.8
7.3
West
II
13.9
8.1
Sum
21.7
15.4
13.6
13.2
Total
80.4
49.7
44.0
13.0
Discussion
Based on the measurements in the Amery Ice Shelf, Budd et al.[16,17] believed that the inflow
ice flux is three times of the outflow flux in the ice shelf. Morgan et al.[18] also suggested that the
accumulation is higher than the outflow ice in the interior area according to the investigation in
the 1970s in an area from the Mawson station to 2000 m elevation. Allison[19] firstly made an estimate from observations along a section with elevation of 1500ü2000 m in the central valley of
the Lambert Glacier and got a positive mass balance result. But McIntyre[20] raised a query on it
with interpretation of satellite imagery. Some researchers paid attention mainly to the variations of
surface accumulation rate, such as the result of Goodwin et al.[3] on it near the coast on the west
side and that of Ren et al.[4,6] for the west part of the LGB route. Moreover, we can also find some
results on the mass balance state of the basin from the moisture transportation modeling[21,22], but
their conclusions are not defined because the scale is usually so large that it can give only an outline of the accumulation rate distribution pattern, and additionally the data on ice flux are lack.
Compared with the other regions in Antarctica, the data obtained from LGB and DT route
investigations on the accumulation rate and ice movement in the Lambert Glacier basin are very
plentiful in both spatial and temporal. Even though, the estimate of the total accumulation in the
upstream area of the LGB route has some uncertainty because the observed data are mainly on the
investigation routes and are sparse near the divides so that errors arise in calculation of the average value over an area. Fricker et al.[9] used different patterns of the accumulation distribution
drawn from observed data along the LGB route and from the moisture transportation modeling
respectively and showed quite a large difference between the results because the calculated total
accumulation is highly sensitive to the accumulation distribution pattern but no observed data
could be used at altitudes higher than 2500 m. In the present paper, we add the latest data obtained
along the DT route. This not only enriched the observed data in the east part of the basin but also
extended the data up to 3900 m altitude. Therefore, our estimate of the mass balance is relatively
reliable compared with that of Fricker et al.[9]. However, analysis of value range in the data extrapolation and the average determination shows that error of the final value for the mass balance
still could be as high as 10%.
In table 1, the large difference in the imbalance values is mainly caused by two factors.
Firstly, the area division in calculation of the ice fluxes is different from that in calculation of the
accumulations. And secondly, the data used in the east part are from the in situ measurements in
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the period from 1997 to 1999 and that in the west part from 1990ü1994. Therefore, in view of
that the accumulation rate has obviously increased in recent years, the positive value may be
higher than 13%.
6 Conclusions
In the last decade, the inland traverse investigations over the Lambert Glacier basin made by
CHINARE and ANARE provided quite plentiful data on the surface accumulation, ice flow velocity and ice thickness and the shallow core study results, so that it is possible for us to understand
comprehensively the distribution and variability of the accumulation rate and to make an estimate
on the mass balance state for the largest glacier basin in the Antarctic.
The in situ measurements of the accumulation rate and shallow core studies show that there
are noticeable differences in distribution and variability of the snow accumulation rate between
east and west sides. On the east side, the accumulation is higher on the average and has generally
been in a continuous increase in the past 250 years, and the increase is more obvious in the last
several decades. On the west side, it is lower and has an decrease tendency in the past decades,
and for a longer period it is not clear due to core depth limitation.
The ice movement measurements and the ice flux calculation indicate that the ice velocity
and the flux are larger in the east than in the west, meaning that the major part of mass supply for
the glacier is from the east side.
The mass budget estimate with the latest data shows that the integrated accumulation over the
upstream area of the investigation traverse route is larger than the outflow ice flux by 13%, suggesting that the Lambert Glacier basin is in a positive mass balance and the ice thickness will increase under the present climatic condition, although the uncertainty still exists since the observed
data near the divide are relatively sparse.
Acknowledgements This work was supported by the Chinese Academy of Sciences (Grant No. KZCX2-303), the Ministry of Science and Technology of China (Grant No. 2001DIA50040) and the National Natural Science Foundation of China
(Grant Nos. 49971021 and 40071025). We thank all the persons who participated in ANARE LGB and CHINARE DT traverses
and those who made a contribution to the data used in this paper.
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