Determining the causes of climate variability in the McMurdo Dry Valleys of Antarctica.

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. Mean annual air temperature on the valley floor ranges from −14.8 °C to −30.0 °C, and annual precipitation is less than 50 mm water equivalent. 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. Despite the apparently inhospitable nature of this area, it is not completely lifeless, with a microbially-dominated ecosystem whose functioning is controlled by available energy from sunlight. 

As in other areas, the amount of sunlight reaching the ground in the McMurdo Dry Valleys varies over time, driven by variations in the rotational and orbital cycles of the Earth, variations of the sunspot cycle and the atmospheric optical depth, i.e. the amount of light absorbed or refracted away by water vapour, cloud cover, and other aerosols in the atmosphere. In most areas on Earth, this variability is driven largely by the hydrological cycle, i.e. water entering and leaving the atmosphere, but in the cold, dry climate of the McMurdo Dry Valleys this is less likely to be an important factor.

In a paper published in the journal Scientific Reports on 22 March 2018, Maciej Obryk of the Cascades Volcano Observatory of the U.S. Geological Survey, Andrew Fountain of the Department of Geology at Portland State University, Peter Doran of the Department of Geology and Geophysics at Louisiana State University, Berry Lyons of the Byrd Polar Research Center of the Ohio State University, and Ryan Eastman of the Department of Atmospheric Sciences at the University of Washington, publish the results of a survey of solar radiation reaching the ground in the McMurdo Dry Valleys between 1987 and 2015, and speculate about the causes of the variations in this radiation.

Map of McMurdo Dry Valleys, Antarctica. Lake Hoare Station is located in Taylor Valley. Map generated in ArcGIS 10.1; Antarctica insert generated in Matlab. Obryk et al. (2018).

Obryk et al. used data from the meteorological station at Lake Hoare in Taylor Valley, which is is located 77.1 m above mean sea level and 15 km inland from the coast. The radiation sensor at Lake Hoare is located three meters above ground and receives direct and diffuse radiation for about 7 months a year. It is the longest running  of eight meteorological stations record solar radiation using LiCOR radiometers, chosen for this study because of the length of data available, though where data from the other stations was available it correlated very closely with the Lake Hoare data.

In 1987 the Dry Valleys received an average annual shortwave radiation of 78 watt per square metre. This rose slightly from 1987-1990, fell sharply in 1991 then slowly rose to 103 watt per square metre in 2001, then fell again to 86 watt per square metre by 2015. This was compared to the cloud cover records from McMurdo Station (on Ross Island, about 100 km from the study site), excluding low clouds such as nimbostratus, which occur around the Antarctic coast but are not thought to extend as far inland as the McMurdo Dry Valleys (the area is also potentially affected by the Antarctic Ozone Hole, but this allows extra ultra-violet light to reach the ground, wavelengths that were not studied).

Next Obryk et al. considered the possible impact of emissions from  Mount Erebus, a 3794 m volcano on Ross Island, which is continuously degassing, releasing potentially climate effecting sulphur dioxide particles, as well as Human outposts in the area, which produce a small amount to diesel fumes etc. However both of these sources are fairly constant, and even though Mount Erebus did undergo an increase in emission levels between 1984 and 1992, cannot explain the pattern of shortwave radiation variation seen in the Dry Valleys.

Aerial view of Mount Erebus in December 2000, showing fumarole activity near its craters. Josh Landis/National Science Foundation/United States Antarctic Program/Wikimedia Commons.

Next Obryk et al. considered global sources of emissions which might have influenced the climate of the Dry Valleys. Most such emissions of sulphate aerosols, whether volcanic or Human in origin, only enter the troposphere, which severely restricts their ability to reach the poles, which are protected by low-level currents which tend to keep such emissions at roughly the same latitudes. However emissions which reach the stratosphere can become much more global in distribution, and could have such an influence. Most volcanic eruptions do not influence the stratosphere, but very large eruptions close to the equator can produce material that rises this high. 

The last two eruptions likely to have achieved this were the El Chichón eruption of 1982, likely to have been two early to have had any influence on the study period, and the Mount Pinatubo eruption of 1991, which may have influenced the low shortwave radiation levels of the early 1990s. The 1990s also saw significant wildfires in the forests of Southeast Asia, Australia and North America, which are also likely to have input significant particulate material into the stratosphere, which may have added to the impact of the Pinatubo eruption, and slowed the recovery afterwards.

Air pollution over Southeast Asia in 1997, following a series of forest fires in Indonesia and neighbouring countries, considered to have been the largest forest fires in recorded history. White represents the aerosols (smoke) that remained in the vicinity of the fires. Green, yellow, and red pixels represent increasing amounts of tropospheric ozone (smog) being carried to the west by high-altitude winds. Made using the Total Ozone Mapping Spectrometer system. NASA/Wikimedia Commons.

Obryk et al. further note that from the early 2000s onwards there has been a marked increase in anthropogenic sulphur dioxide emissions, combined with a number of smaller volcanic eruptions, which may have contributed to the fall in shortwave radiation reaching the McMurdo Dry Valleys over the later part of the study.

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Groundwater systems beneath the McMurdo Dry Valleys of Antarctica.

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. Peter Rejcek/National Science Foundation/Wikipedia.

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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The Antarctic Summer Monsoon.

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Russian scientists prepare to break through into Lake Vostok.

Lake Vostok is a subglacial lake under the East Antarctic Ice Shelf. It is about 500 m below sea-level, but is buried under 4 km of ice. The lake is approximately 250 km long, by 50 km wide and has an average depth of 344 m. Lake Vostok was covered by the ice between 14 and 25 million years ago...

 

Drilling for Lake Ellsworth.

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