Harbor, most icebergs under 2 m in diameter will likely melt
within 24 hours if exposed to wave attack. Under these circumstances, most ice-rafted debris will be transported only short
distances from calving tidewater glaciers in subpolar regions.
This work is supported by National Science Foundation
grant OPP 92-18495.
References
Budd, W.F., T.H. Jacka, and V.I. Morgan. 1980. Antarctic iceberg melt
rates derived from size distribution and movement rates. Annals
of Glaciology, 1, 103-112.
Dowdeswell, J.A., and T. Murray. 1990. Modelling rates of sedimentation from icebergs. In J.A. Dowdeswell and J.D. Scourse (Eds.),
Glacimarine environments: Processes and sediments (Geological
Society Special Publication No. 53). London: Geological Society.
Martin, S. 1994. Personal communication.
Martin, S., E. Josberger, and P. Kaufman. 1978. Wave-induced heat
transfer to an iceberg. In A.A. Husseiny (Ed.), Iceberg utilization.
New York: Pergamon Press.
Russell-Head, D.S. 1980. The melting of free-drifting icebergs. Annals
of Glaciology, 1, 119-122.
Weeks, W.F., and W.J. Campbell. 1973. Icebergs as a fresh-water
source: An appraisal. Journal of Glaciology, 12(65), 207-233.
Sedimentation at a subpolar tidewater glacier, Maar Ice
Piedmont, Anvers Island, Antarctic Peninsula
GAIL M. ASHLEY, Department of
Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903
NORMAN D. SMITH, Department of Geological Sciences, University of illinois at Chicago, Chicago, Illinois 60680
MATTHEW C. GOSS and PETER C. SMITH, Department of Geological Sciences, Rutgers University, New Brunswick,
New Jersey 08903
2-month intensive study of sedimentation processes was
arried out in austral summer 1993-1994 near a tidewater
portion of the Maar Ice Piedmont in Arthur Harbor (adjacent
to Palmer Station) (figure 1). Data consist of conductivitytemperature-turbidity-depth (CTTD) profiles, water samples,
and sediment-trap catches to study processes and patterns of
sediment dispersal and sedimentation; bottom cores and
grabs to document the record of recent glacial marine sedimentation; and video surveys of the ice terminus and the iceproximal sea bottom with a remotely operated vehicle (ROV).
Anvers Island is a 70-kilometer (km) by 35-km island
composed of tonalite. A mountain chain runs up the center of
the island; the west side is an extensive low, gently sloping
piedmont. The island is ice covered, and the ice cap thickens
inland, reaching 600 meters (m). Iceflow is toward the coast
with highest velocities greater than 200 m per year (m/yr) in
ice streams over bedrock valleys and 10-15 m/yr between the
valleys (Rundle 1973). Iceflow rates above the station range
from 20 to 50 m/yr feeding into the ice cliff at Arthur Harbor
(Rundle 1973). The nearly vertical ice cliff ranges between 20
and 60 m high (above sea level) and is grounded at depth up
to 45 m below sea level. The retreat rate of the glacier has
been about 10 m/yr since 1965 (figure 1). We interpret the
shallow (10-30 m) sill that parallels the modern ice front at a
distance of 0.7 km to represent a former ice-front position (a
paleogrounding line). The basal debris layer is thin (less than
1 m) (figure 2).
Mean annual temperature is -3°C; peak daytime temperature in summer may reach 6°C-7°C (winter averages -10°C).
The tides are mixed—mainly diurnal ranging from 1.9 m
(springs) to 0.6 m (neaps). Summer water temperatures range
from -1°C to 1.4°C; the harbor is ice covered in winter. Salinity
is 32-34%o depending upon the contribution of glacial melt-
water or the presence of melting icebergs. Turbidity ranges
from 2-4 milligrams per liter (mgIL) to 35 mg/L near the glacier margin.
An intensive program of daily conductivity-temperatureturbidity profiles in Arthur Harbor at both proximal and distal
locations to the ice front indicates that little meltwater is
coming directly from the glacier. On two occasions during
austral summer 1994, meltwater- generated surface plumes
(raised turbidity, lowered salinity) were present. These
plumes followed days of unusually warm air temperatures
(4°C-6°C) which likely produced higher surface ablation and
increased run-off. More frequently, however, ephemeral cold,
high-turbidity zones appear within the water column at iceproximal locations (figure 3). The origin of these horizons
termed stream tubes (MacAyeal 1985, pp. 133-143) or cold
tongues (Domack and Williams 1990, pp. 71-89) requires further study, but they appear to be related to meltwater injected
into an already density-stratified water column.
The water column in Arthur Harbor in summer is stratified, with less dense (less saline) water overlying more dense
(more saline) water. The interface between the two layers is at
approximately 30 m and a broad (approximately 13-m thick)
pycnocline composes the lower portion of the less dense surface layer. The interface between the two layers is coincident
with and possibly related to the prominent bathymetric high
(approximately 30 m) that likely affects oceanographic circulation in the embayment (figure 1).
Colder, more turbid water occasionally occurs within the
broad pycnocline. Fluorometer analyses to measure chlorophyll did not indicate high phytoplankton levels in these
zones. The cold tongues are best developed during times of
ebb tide. They are also best developed close to the submerged
glacier terminus and become less well-defined away from the
ANTARCTIC JOURNAL - REVIEW 1994
94
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Figure 1. Bathymetric map of study site, Arthur Harbor, Anvers Island, Antarctic Peninsula.
ice front, suggesting a glacial source. The water in these
ephemeral turbid horizons is 0.3 0C-0.40C colder and measured turbidity is up to 8-10 mg/L higher than surrounding
water. Fresh, but turbid, glacial water emanating at the base
and front of the glacier would slowly rise adjacent to the ice
face, become more saline (by mixing) and less turbid (by settling), and move away from ice on horizon(s) of equal density.
In Arthur Harbor, the horizon is apparently within the pycnodine, where offshore flow may be aided by
ebb tide currents. High-turbidity layers also
occur on the seafloor in ice-proximal locations (figure 3). These contain up to approximately 20-30 mg/L and are likely due to disturbance (resuspension) of bottom sediments
due to frequent calving.
Sediment traps located about 200 m
from the ice and 2 m and 30 m from the
seabed collected significant sediment (mineral and organic matter, including fecal pellets)
during repeated 2-4 day deployments.
The most important sources of sediment
for Arthur Harbor appear to be the melting of
icebergs (calved glacial ice) and probably
direct melting of the ice front. Melting experiments with brash ice reveal that melting rates
vary considerably, with agitation being a
major factor. Most calved icebergs melted
Figure 2. Photograph of basal debris layer at subaerial margin of Maar Ice Piedmont
within a kilometer of the ice front. Icebergs
ANTARCTIC JOURNAL - REVIEW 1994
95
Arthur Harbor 27 January 1994
Figure 3. Sea Cat CTTD profile
located 200 m from ice front.
Ephemeral high turbidity layer (12-22
m depth) occurs within the pycnodine and corresponds to a zone of
cooler temperature. The turbid zone
at the base is likely due to resuspension of bottom sediments disturbed
by calving. Backscatterance of 15 is
equal to approximately 17-20 mg/L.
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trapped in thick brash jams melt 12-25 times more slowly
than isolated icebergs subjected to currents and wave action
(Smith and Ashley, Antarctic Journal, in this issue).
Grain-size analysis of bottom grab samples from Arthur
Harbor reveals that sediments are coarsest (20 percent sand,
60 percent silt, 20 percent clay) in shallow (less than 30 m
deep) areas of Arthur Harbor and finest in the deep areas
(more than 50 m deep) (5 percent sand; 45 percent silt; 50
percent clay). X-rays of 30-cm long cores reveal extensive bioturbation and minor occurrences of stratified sediments within 300 m of the ice cliff.
An ROV deployed from Zodiacs provided images of a sea
bottom covered by life (tunicates, algae, mysids, brittle stars,
tube worms) and krill immediately adjacent to the submerged
glacier margin. The ice front appeared highly fractured with
vertical cracks and horizontal ledges. The surface of the ice was
scalloped similar to subaerial "suncups." Thick clouds of sediment were stirred from the ledges by movement of the vehicle.
Bouldery morainal deposits occur at the grounding line.
We appreciate help by Dean Kirkham and Herb Baker in
data collection. This research was supported by National Science Foundation grant OPP 92-18485.
References
Domack, E.W., and C.R. Williams. 1990. Fine structure and suspended
sediment transport in three antarctic fjords. In C.R. Bentley (Ed.),
Contributions to antarctic research I (Antarctic Research Series,
Vol. 50). Washington, D.C.: American Geophysical Union.
MacAyeal, D.R. 1985. Evolution of tidally triggered meltwater plumes
below ice shelves. In S.S. Jacobs (Ed.), Oceanology of the Antarctic
Continental Shelf (Antarctic Research Series, Vol. 43). Washington,
D.C.: American Geophysical Union.
Rundle, A.S. 1973. Glaciology of the Maar Ice Piedmont, Anvers Island,
Antarctica (Institute of Polar Studies Report No. 47). Columbus:
Ohio State University.
Smith, N.D., and G.M. Ashley. 1994. Observations on the melting rates
of brash ice, Arthur Harbor, Antarctic Peninsula. Antarctic Journal
of the U.S., 29(5).
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