Stream Boundary Detection of a Hyper-Arid, Polar Region Using a U-Net Architecture: Taylor Valley, Antarctica

Convolutional neural networks (CNNs) are becoming an increasingly popular approach for classification mapping of large complex regions where manual data collection is too time consuming. Stream boundaries in hyper-arid polar regions such as the McMurdo Dry Valleys (MDVs) in Antarctica are difficult to locate because they have little hydraulic flow throughout the short summer months. This paper utilizes a U-Net CNN to map stream boundaries from lidar derived rasters in Taylor Valley located within the MDVs, covering ∼770 km2. The training dataset consists of 217 (300 × 300 m2) well-distributed tiles of manually classified stream boundaries with diverse geometries (straight, sinuous, meandering, and braided) throughout the valley. The U-Net CNN is trained on elevation, slope, lidar intensity returns, and flow accumulation rasters. These features were used for detection of stream boundaries by providing potential topographic cues such as inflection points at stream boundaries and reflective properties of streams such as linear patterns of wetted soil, water, or ice. Various combinations of these features were analyzed based on performance. The test set performance revealed that elevation and slope had the highest performance of the feature combinations. The test set performance analysis revealed that the CNN model trained with elevation independently received a precision, recall, and F1 score of 0.94±0.05, 0.95±0.04, and 0.94±0.04 respectively, while slope received 0.96±0.03, 0.93±0.04, and 0.94±0.04, respectively. The performance of the test set revealed higher stream boundary prediction accuracies along the coast, while inland performance varied. Meandering streams had the highest stream boundary prediction performance on the test set compared to the other stream geometries tested here because meandering streams are further evolved and have more distinguishable breaks in slope, indicating stream boundaries. These methods provide a novel approach for mapping stream boundaries semi-automatically in complex regions such as hyper-arid environments over larger scales than is possible for current methods.

Antarctic Permafrost Degassing Revealed By Extensive Soil Gas and Co2 Flux Survey in Taylor Valley

McMurdo Dry Valleys comprise 10% of the ice-free soil surface areas in Antarctica. Permafrost stability plays an important role in C-cycle as it potentially stores considerable quantities of greenhouse gases. While the geomorphology of the Dry Valleys reflects a long history of changing climate conditions, comparison with the rapidly warming Northern polar region suggests that future climate and ecosystems may change more rapidly from permafrost degradation. In Austral summer 2019/2020 a comprehensive sampling of soil gases and CO2 flux measurements was undertaken in the Taylor Valley, with the aims to identify potential presence of soil gases in the active layer. The results obtained show high concentrations of CH4, CO2, He and an increasing CO2 flux rate. We identify the likely source of the gas to be from dissolved gases in deep brine moving from inland (potentially underneath the Antarctic Ice Sheet) to the coast at depth beneath the permafrost layer.

An Analysis of the Temperature Profiles of Some Antarctic Lakes

<p>The temperature profiles of certain lakes in Taylor Valley, Victoria Land, Antarctica, are shown to be consistent with the hypothesis that these lakes were formerly cold brine pools; that their levels were raised by the addition of fresh water; and that they have since been heated principally by the absorption of sunlight. The temperature profile of a lake in Wright Valley, Victoria Land, is shown to be consistent with the hypothesis that this lake was formerly warm and stable, as are those Taylor Valley lakes which were analysed; that the addition of a further large quantity of fresh water caused instability and limited convection; and that the heat source is again absorbed sunlight. The study of this lake requires an understanding of convection in the presence of a gradient of solute concentration. A survey of existing knowledge of this type of convection shows that it is inadequate for the task. Experiments which provide the necessary information are described.</p>

Salts and Their Distribution in the McMurdo Region, Antarctica

<p>Salts are widespread in the cold, arid McMurdo region of Antarctica. They exist in a variety of deposit types from massive subglacial and sub-lake deposits containing up to 1010 kilograms of salt, down to traces in soil, snow and ice. However, deposits on rock and soil typically amount to a few grams of salt. At least 30 salt phases are known but only 10 of these are widespread. These 10 are thenardite, gypsum, halite, calcite, darapskite, soda nitre, mirabilite, bloedite, epsomite and hexahydrite. The distribution of salts has been examined on two scales, local and regional. The local scale extends from individual deposits to areas of a few square kilometres. The regional scale covers McMurdo oasis, McMurdo Sound and Ross Island, though areas in McMurdo oasis, and particularly Taylor Valley receive most attention. Local distribution is controlled by salt migration and separation. Migration is induced by water and wind, with soil brines moving as thin liquid films, by capillarity and under the influence of gravity. Deflation and asymmetric salt accumulation provide evidence that wind is important. Separation of phases is a consequence of different physico-chemical properties of salts, and environmental conditions, including site aspect, ambient temperature and humidity. Eutectic temperature is a fundamental salt property but solubility is also important. Several salt deposits containing separated (fractionated) phases have been found in the region. Separation is achieved mainly by fractional dissolution and crystallization and the most evolved product of the general separation sequence is calcium chloride. The separation processes, together with salt migration, obscure the sources of the salts. Regional distribution of salts has been characterized by determining the relative frequency at which specific phases are encountered at increasing distance from the coast and above sea level. Chloride and sodium phases decrease, whereas magnesium phases increase in frequency away from the coast. Sulphates-to-chloride and nitrates-to-chloride ratios increase with increasing distance. Calcium and carbonate show little change except in Taylor Valley where a marked decrease is apparent. This regional distribution is mainly dependent on the sources of the salts. The marine source is most important, contributing almost all of the chloride, sodium, sulphate and probably nitrate ions that are present. Chemical weathering is the predominant source of magnesium, calcium and carbonate ions probably via reactions of mafic, ferromagnesian minerals in local rocks and regolith. Biological and volcanic activity are locally significant at eastern Taylor Valley and in the summit area of Erebus Volcano, respectively. The salts have accumulated over the lifetime of the region, that is over less than the last 20-25 Ma or so. There is no evidence that they are relics from earlier, preglacial times, except for very minor amounts of gypsum and calcium carbonate. There has been a recent influx of sea water into Taylor Valley perhaps between 50,000 and 20,000 years ago, and evaporation of this water has preceded advance of Taylor Glacier over part of the resulting salt deposit. The continuing interaction between glacier and salt is causing basal ice to melt and producing aperiodic discharges of up to a few thousand cubic metres of salty water from the terminus of the glacier.</p>

Salts and Their Distribution in the McMurdo Region, Antarctica

<p>Salts are widespread in the cold, arid McMurdo region of Antarctica. They exist in a variety of deposit types from massive subglacial and sub-lake deposits containing up to 1010 kilograms of salt, down to traces in soil, snow and ice. However, deposits on rock and soil typically amount to a few grams of salt. At least 30 salt phases are known but only 10 of these are widespread. These 10 are thenardite, gypsum, halite, calcite, darapskite, soda nitre, mirabilite, bloedite, epsomite and hexahydrite. The distribution of salts has been examined on two scales, local and regional. The local scale extends from individual deposits to areas of a few square kilometres. The regional scale covers McMurdo oasis, McMurdo Sound and Ross Island, though areas in McMurdo oasis, and particularly Taylor Valley receive most attention. Local distribution is controlled by salt migration and separation. Migration is induced by water and wind, with soil brines moving as thin liquid films, by capillarity and under the influence of gravity. Deflation and asymmetric salt accumulation provide evidence that wind is important. Separation of phases is a consequence of different physico-chemical properties of salts, and environmental conditions, including site aspect, ambient temperature and humidity. Eutectic temperature is a fundamental salt property but solubility is also important. Several salt deposits containing separated (fractionated) phases have been found in the region. Separation is achieved mainly by fractional dissolution and crystallization and the most evolved product of the general separation sequence is calcium chloride. The separation processes, together with salt migration, obscure the sources of the salts. Regional distribution of salts has been characterized by determining the relative frequency at which specific phases are encountered at increasing distance from the coast and above sea level. Chloride and sodium phases decrease, whereas magnesium phases increase in frequency away from the coast. Sulphates-to-chloride and nitrates-to-chloride ratios increase with increasing distance. Calcium and carbonate show little change except in Taylor Valley where a marked decrease is apparent. This regional distribution is mainly dependent on the sources of the salts. The marine source is most important, contributing almost all of the chloride, sodium, sulphate and probably nitrate ions that are present. Chemical weathering is the predominant source of magnesium, calcium and carbonate ions probably via reactions of mafic, ferromagnesian minerals in local rocks and regolith. Biological and volcanic activity are locally significant at eastern Taylor Valley and in the summit area of Erebus Volcano, respectively. The salts have accumulated over the lifetime of the region, that is over less than the last 20-25 Ma or so. There is no evidence that they are relics from earlier, preglacial times, except for very minor amounts of gypsum and calcium carbonate. There has been a recent influx of sea water into Taylor Valley perhaps between 50,000 and 20,000 years ago, and evaporation of this water has preceded advance of Taylor Glacier over part of the resulting salt deposit. The continuing interaction between glacier and salt is causing basal ice to melt and producing aperiodic discharges of up to a few thousand cubic metres of salty water from the terminus of the glacier.</p>

An Analysis of the Temperature Profiles of Some Antarctic Lakes

<p>The temperature profiles of certain lakes in Taylor Valley, Victoria Land, Antarctica, are shown to be consistent with the hypothesis that these lakes were formerly cold brine pools; that their levels were raised by the addition of fresh water; and that they have since been heated principally by the absorption of sunlight. The temperature profile of a lake in Wright Valley, Victoria Land, is shown to be consistent with the hypothesis that this lake was formerly warm and stable, as are those Taylor Valley lakes which were analysed; that the addition of a further large quantity of fresh water caused instability and limited convection; and that the heat source is again absorbed sunlight. The study of this lake requires an understanding of convection in the presence of a gradient of solute concentration. A survey of existing knowledge of this type of convection shows that it is inadequate for the task. Experiments which provide the necessary information are described.</p>

Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

First measurements of 222Rn and 220Rn activities in soil in Taylor Valley, Antarctica.

&lt;p&gt;Warming global climate threatens the stability of the polar regions and may result in cascading broad impacts. Studies conducted on permafrost in the Arctic regions indicate that these areas may store almost twice the carbon currently present in the atmosphere. Therefore, permafrost thawing has the potential to magnify the warming effect by doubling the more direct anthropogenic impact from burning of fossil fuels, agriculture and changes in land use. . Permafrost thawing may also intensify the Rn transport due to the increase of fluid saturation and permeability of the soil. A detailed study of &lt;sup&gt;222&lt;/sup&gt;Rn and &lt;sup&gt;220&lt;/sup&gt;Rn activity levels in polar soils constitutes a starting point to investigate gas migration processes as a function of the thawing permafrost. Although several studies have been carried out in the Arctic regions, there is little data available from the Southern Hemisphere. The Italian &amp;#8211; New Zealand &amp;#8220;SENECA&amp;#8221; project aims to fill this gap and to provide the first evaluations of gas concentrations and emissions from permafrost and/or thawed shallow strata of the Taylor Valley, Antarctica. Taylor Valley is one of the few Antarctic regions that are not covered by ice and therefore is an ideal target for permafrost investigations. Results from our first field observations highlight very low values for &lt;sup&gt;222&lt;/sup&gt;Rn (mean 621 Bq m&lt;sup&gt;-3&lt;/sup&gt;, max value 1,837 Bq m&lt;sup&gt;-3&lt;/sup&gt;) and higher values for &lt;sup&gt;220&lt;/sup&gt;Rn (mean 11,270 Bq m&lt;sup&gt;-3&lt;/sup&gt;, max value 27,589 Bq m&lt;sup&gt;-3&lt;/sup&gt;), suggesting a shallow source. These measured activity values are essentially controlled by the radionuclide content in the soil, by the permeability and porosity of the soil, and by the water content. This dataset also represents an important benchmark for future measurements to track the melt progress of Antarctic permafrost.&lt;/p&gt;