The McMurdo Dry Valleys are a largely ice-free region of Antarctica
on the Ross Sea coast, discovered by Robert Scott in the early twentieth
century. The valleys contain a series of lakes, lacking surface connections and
often separated by glaciers. Most of the ground in the valleys is frozen,
though near-surface groundwater seeps and tracks are present in the upper 70 cm
of soil, which are typically extremely saline, enabling them to remain liquid
at temperatures below zero degrees centigrade. Deeper saline waterways are also
predicted in the Valleys, but several attempts at finding them with boreholes
by the Antarctic Dry Valley Drilling Project in the 1970s were unsuccessful.
The most famous feature of the Valleys is Blood Falls, an
intermittent outpouring of highly saline, iron-rich water from Taylor Glacier
into Lake Bonney, which stains the surface of the glacier a deep crimson. This
is thought to be caused to by the weight of the glacier pressing down on
deepsubsurface waters and forcing them to migrate upwards. The waters of Blood
Falls are known to host a rich microbial community, which probably help to
weather iron from the bedrock metabolically. The water of blood falls is
sufficiently saline to remain a liquid a -6˚C at surface pressures, and
therefore at cooler temperatures in the higher pressure conditions beneath the
glacier.
Blood Falls, McMurdo Dry Valleys, Antarctica.
In a paper published in the journal Nature Communications on 28
April 2015, Jill Mikucki of the Department of Microbiology at the University ofTennessee, Knoxville, Esben Auken of the Department of Geosciences at AarhusUniversity, Slawek Tulaczyk of the Department of Earth and Planetary Sciences at the
University of California, Santa Cruz, Ross Virginia of the Environmental StudiesProgram at Dartmouth College, Cyril Schamper of Sorbonne Universités, Kurt Sørensen,
also of the Department of Geosciences at Aarhus University, Peter Doran of the Departmentof Geology and Geophysics at Louisiana State University, Hilary Dugan of the Departmentof Earth and Environmental Sciences at the University of Illinois at Chicago
and Neil Foley, also of the Department of Earth and Planetary Sciences at the University
of California, Santa Cruz describe the results of a study of deep groundwater
in the McMurdo Dry Valleys using airborne electromagnetic systems to measure
the resistivity of deep sediment layers.
Mikuckiet al. used a
system called SkyTEM, which uses a high-powered transmitter loop carried by
helicopter to induce subsurface electromagnetic currants. Similar systems have
previously been used to map permafrost and buried ice features in the Alaskan
Arctic and on Livingstone Island off the coast of Anartica.
Map of Taylor Valley in Antarctica. (a) Map of major
lakes, glaciers and Antarctic Dry Valley Drilling Project (DVDP) boreholes in
Taylor Valley, Antarctica. Dotted red line indicates the location of the Lake Fryxell
DVDP geophysical survey. (b) Airborne electromagnetic flight lines in green
with survey lines for which data were processed shown in yellow. Terrain
surveyed in this paper is highlighted in red. Dashed line indicates regions
where higher-resolution surveys were conducted in the Bonney and Fryxell
Basins. Red circle indicates the location of the example SkyTEM sounding. Mikucki et al.(2015).
Mikucki et al. were able to
differentiate buried brine systems, which have very low resistivities, in the
order of 10–100 Ωm, from buried ice, which has a very high resistivity, in the
order of 500–20,000Ωm. This enabled them to detect two extensive brine systems
in the McMurdo Dry Valleys, one beneath Taylor Glacier and the upper Lake
Bonney Basin and one beneath Lower Taylor Valley and the Lake Fryxell Basin.
Resistivity cross-section for the length of the Taylor
Valley. Resistivity profile along the length of the Taylor Valley. Low
resistivities near McMurdo Sound to Lake Hoare interpreted as hydrological
connectivity of brine in sediments extending from the coastal margin inland and
beneath the Canada Glacier. To the west, resistivities increase below Suess
Glacier and again towards Lake Bonney. In the Bonney Basin, low-resistivity
patterns suggest connectivity of brine-rich sediments below the Taylor Glacier
with proglacial Lake Bonney at the glacier terminus and near the location of
Blood Falls. Mikucki et al.(2015).
The McMurdo Dry Valleys are thought to have formed as a fjord system
in the Miocene, when a wetter climate led to a more extensive Taylor Glacier
(glaciers are ultimately formed from snowfall, so a warmer wetter climate will
often produce a more extensive glacier than a cooler dryer one, as long as
temperatures at the glacier itself remain below freezing point for most of the
time) which extended during cooler periods and was intruded by seawater in
warmer spells. During this cycle seawater would have become trapped in the
valleys then turned to dense brines through cryoconcentation, a process in
which the majority of the water freezes but the dissolved salt, which does not
freeze, is concentrated in the remaining liquid. This would have led to a
build-up of salts in the sediments beneath the valleys.
In the 1980s it was theorized that during the Pleistocene glacial
episodes much of the McMurdo Dry Valleys would have been occupied by a glacial
lake (lake trapped behind a glacier), Lake Washburn. However more recent
studies have suggested that this may only have occupied the western part of the
Valleys, with the lower eastern part alternatively occupied by a raised and
intruding Ross Ice Shelf (sea ice floating on water) and a series of smaller
glacial lakes. The repeated forming of such lakes from trapped seawater would
have served to further raise the salinity of sediments in the lower Valleys by cryoconcentation.
Prior to this study the lakes of the McMurdo Dry Valleys were
assumed to all be separate bodies, however Mikuckiet al.’s results suggest that the lakes of the lower Valleys are in
fact connected by an extensive subsurface brine system which enables movement
of water from one lake to another down the Valleys, beneath the separating
glaciers.
Conceptual diagram depicting predicted hydrological
connectivity. Two distinct regions of subsurface brine were identified in the
MDV. The ‘?’ indicates the zone between Lake Bonney and Lake Hoare where no
connectivity was identified with the survey. Mikucki et al. (2015).
The waters of Lake Hoare are known to be less saline than those of
the other lakes in the Valleys. Previous studies of the system have postulated
that the lake dried up completely about 12 000 years ago, leaving extensive
evaporate salt deposits, which were then removed by aeolian transportation
(blown away by the wind). However Mikuckiet
al.’s findings suggest an alternative (and simpler) explanation; the waters
of Lake Hoare are less saline because it the headwater lake in an extensive groundwater
system, and water is continuously flowing from the Lake Hoare into Lake
Fryxell, and subsequently into the McMurdo Sound, carrying with it some of the
dissolved salt from the lake. Lake Fryxellis more saline than Lake Hoare as it
is receiving salt from it, but is in turn losing salt into the McMurdo Sound.
Lake Bonney is the most saline lake in the Valleys, as it is receiving brine
input from beneath Taylor Glacier, but has no outlet.
The weight of Canada Glacier may be causing enhanced discharge of
brine into Lake Fryxell in the same way that the weight of Taylor Glacier is
into Lake Bonney. However there is no visible surface discharge as seen at
Blood Falls, so if this is occurring it is entirely beneath the lake.
The waters of Blood Falls are known to host a substantial microbial
community, and similar communities have previously been identified from
subglacial waters and groundwater elsewhere in Antarctica. Mikuckiet al. therefore theorize that the
groundwaters of the lower Valleys are also likely to host such microbial
communities, as may other buried hypersaline groundwater systems elsewhere on
the continent.
It has also been theorized that Mars may host such buried brine
systems, the remnants of ancient oceans once present on the planet’s surface,
and that such brine systems could also be host to microbial life. While Mikucki et al.’s study cannot shed any direct
light upon this idea; it does produce further evidence that such systems might
at least be possible, and presents a viable method for searching for such
buried briny waters on Mars.
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