Discovery May Give Clues to Ice Shelf Collapse

["greenbase" <greenbas@gb.greenpeace.org>]

ANTARCTIC CHANNEL DISCOVERY MAY GIVE CLUES TO ICE SHELF
COLLAPSE  

ANTARCTIC PENINSULA, FEBRUARY 7 1997 -  Scientists aboard a
Greenpeace ship have discovered a deep underwater channel in
the Antarctic which will provide new insights into the
collapse of an Antarctic ice shelf and the effects of climatic
change in the Antarctic Peninsula.
 
The 1000 metre deep submarine channel was discovered this week
by Greenpeace scientist, Ricardo Roura, and Argentinean
Antarctic Institute researcher (IAA) Jorge Lusky as the MV
Arctic Sunrise was steaming through uncharted  waters formerly
occupied by the Larsen Ice A Shelf. The
sudden collapse of this ice shelf in January 1995 has been
linked to recent warming in the region.
 
Dr Rodolfo Del Valle, Head of the Geology Department at the 
IAA said: "From  a geological point of view, this is a
tremendous discovery which we were not expecting".  He added
that it will help scientists to understand how and why the
shelf collapsed. "It is possible to link the retreat of
glaciers and the ice shelf collapse to the greenhouse 
effect," he said.  
Dr. Del Valle speculates that the channel and other nearby
channels refrigerated the bottom of the ice shelf and delayed
thinning by warmer sea currents circulating underneath the ice
shelf. The scientists  crossed the channel along eight
transects (two along the length of the channel and six along
the width, roughly perpendicular to the channel's main axis)
five nautical miles apart.   
 
The submarine channel is located where the northern end of the
Larsen-A Ice Shelf used to be, offshore from the Drygalski
Glacier (the largest glacier on the Nordenskjold Coast),  and
is 15 nautical miles long and six to eight nautical miles
wide, approximately coinciding with the glacier's width.    
The MV Arctic Sunrise is on a five week "Polar Meltdown"  Tour
documenting impacts of climate change.  The channel is located
offshore the Drygalski Glacier, the  largest glacier on the
Nordenskjold Coast, Antarctic  Peninsula and roughly coincides
with the glacier's width. Argentinean scientists have asked
Greenpeace to collaborate in the naming of the channel.  
Contact: Jon Walter Greenpeace International on 3120-523 6222
or on board MV Arctic Sunrise - ph +874 (or 871) 130 25 77 -
Ricardo Roura, Greenpeace scientist (English, Spanish) Janet
Dalziell, Tour Coordinator (English)  Erwin Jackson, Climate
impacts specialist (English)
Martina Krueger, Campaigner (German, Dutch) Emiliano Ezcurra,
Campaigner (Spanish, Portuguese, French,  Italian)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Briefing notes on Antarctic Deep Submarine Channel Discovery  

1. The deep submarine channel is located where the northern
end of the Larsen-A Ice Shelf used to be, offshore from the
Drygalski Glacier (the largest glacier on the Nordenskjold
Coast),  and is
15 nautical long, approximately coinciding with the glacier's
width.  The maximum depth of the trench is 1000 m and the
average depth is 960 metres. .  
2.  The scientists (Ricardo Roura, Greenpeace, and Jorge
Lusky, Argentine Antarctic Institute) crossed the channel
along eight transects (six across the channel and
perpendicular to its main axis and two along its length). The
width of the channel was 6 and 8 nautical miles respectively. 

 
3. The channel becomes narrower and shallower towards the
east, away from the mainland.  The orientation of the main
axis is approximately 80 degrees (ENE) which is roughly
perpendicular to the coastline (but slightly to the N). The
location and shape suggest significant glacial influence
(glacial activity could have been superimposed over existing
geological features of tectonic or other origin) 
 
4.While the glaciology and geology of the area was well known,
there was no information about the topography of the sea
bottom other than speculations arising from geophysical
profiles of the ice shelf and surface observations. These
speculations were quite correct as far as confirming the
presence of underwater channels, but wrong in terms of the
magnitude of these.  
5.Up until now it was supposed that the ice shelf was mostly
grounded on the sea bottom at around 200 metres (sea bottom
depth at the ice shelf front). the depth of the sea bottom
from the ice shelf front to its source in the glaciers of the
Antarctic Peninsula was estimated to be on the 50-200 metres
range. The presence of a large empty space underneath reaching
depths of 1000 metres could have influenced the stability of
the ice shelf in two ways:  
- By channelling meltwater originated in glaciers upstream to
the underside of the ice shelf, thus mitigating the thinning
effect caused by warm sea currents.  
- Once the ice shelf was thin enough to allow the influx of
warm sea currents under the ice shelf, marine processes would
have taken over. In addition to the effect of warm sea
currents, tidal and wave action would have contributed to
destabilize the ice shelf.
 
These processes do not necessarily explain what was the
trigger that led to the final collapse of the ice shelf but
they are more likely contributing factors.   
6. The channel is marked by sudden drops in depths of 100-200m
on the northern side and more gradual 20-50m drops on the
southern side. The coordinates of these drops, which
approximately mark
the sides of the trench, are:
 
64deg 41.64'S      60deg 21.87'W
64deg 49.60'S       60deg 17.85 W
64deg 41.63'S       60deg 10.30'W
64deg 47.42'S       60deg 06.80'W
 
 
7.The northern front of the ice shelf (which runs E-W along a 
string of nunataks -rocky crops appearing above the ice -
which did not exist before the collapse of the barrier)
appears
to be grounded on relatively shallow, constant depths of
200-250m. This helps explains why the disintegration of the
ice shelf ended there: the grounding provides some stability. 
 
8. Channels were also noticed some 100m deep, running N-S
across the actual ice shelf front (i.e. perpendicular to the
front but across the ice shelf front before the collapse).
 
 
 
9. In seas already chartered to the east there is no evidence
of deep water channels or trenches near the Antarctic
Peninsula. In these areas, the seabed is relatively shallow 
(200-350m) and slopes gently to the east.  The only
interruption is a step and/or convexity where the ice shelf
front used to be. This is thought to be a morainic and/or
glacial delta deposit.  
10. Dr Del Valle has speculated that the deep water contained
in  the deep submarine channel refrigerated the bottom of the
ice shelf and delayed   the thinning effect of the warmer
currents circulating underneath the ice shelf. [Scientists had
speculated at the length of time the ice shelf had taken to
respond to the warming atmospheric and  sea temperatures in
the region - the cold water introduced by the underwater
channel appears to have slowed the collapse.]
 
11. Unfortunately there appears to be no data about currents
offshore Larsen A prior to its collapse.  This is not a new
theory arising from the finding of the channel (as it may have
seemed, due to the "accidental" nature of the finding) but a
theory Dr. Del Valle's has had for some time.  In fact our
working hypothesis when we started sounding the area
previously occupied by the ice shelf was that the depths were
of the range of 50-200m and that the sea bed was criss-crossed
by channels of glacial origin.  We were looking for those
hypothetical channels when we came across the deep channel.   
ENDS