Supercooling of Seawater Near the Glacier Front in a Fjord

We analyze seawater temperature and salinity in the immediate vicinity of the Paulbreen front in Spitsbergen. The CTD-measurements were carried out from ice in winter and from a boat in summer. ADCP profiling was performed near the glacier front from the ice in winter. In winter, we found water with lower salinity than the surrounding water in the fjord at a distance of 15 m from the glacier front and recorded a low upward water flux near the glacier. Relatively fresh water was found at a depth of 2-4 m near the glacier front in the place where the sea and glacier bed have local depression up to 17 m. Supercooling of the freshened water reached 0.35°C. We link this phenomenon to a flow of freshwater from under a polythermal glacier. This water becomes overcooled in the seawater with significantly lower temperature and higher salinity.


Introduction
In this paper, we analyze the measurements of water properties close to the front of Paulabreen (Paula Glacier) in Spitsbergen and examine the deviation of seawater temperature from its freezing point.Three mechanisms exist that lead to supercooling of water: (1) removal of heat, (2) differences in the diffusion rate between heat and salt, and (3) rapid pressure decrease.The processes that occur in the water near glaciers frequently lead to water supercooling.Some decades ago, the special term "glaciohydraulic supercooling" was introduced (Röthlisberger, 1972).The author reports that glaciohydraulic supercooling is a process that occurs when the pressure melting point of water ascending the adverse slope of a subglacial overdeepening rises faster than the water is heated by viscous dissipation resulting in water remaining liquid below 0°C.In other words, glaciohydraulic supercooling occurs when water rapidly ascends to smaller depths and cools to the temperature needed for freezing.Upwelling and cooling of water with salinity lower than that in the surrounding waters is a particular case of this phenomenon.Transport of water from the region of high pressure to the region of low pressure without equalization of the water's internal energy can lead to freezing (Creyts and Clarke, 2010).Cook et al. (2006) analyze the processes in the basement of glaciers and state that glaciohydraulic supercooling allows water to remain liquid at temperatures lower than the freezing point.They write: Glaciohydraulic supercooling is a process that allows water at the base of a glacier to remain liquid at a temperature below its freezing point in response to the geometry of water flow and subglacial pressure.
Conditions for seawater supercooling arise when water is cooled to the freezing point in situ due to the contact with ice in deep water.If the water then ascends to the depths where the local freezing temperature is higher, the water appears at a temperature below the freezing point (Stevens et al., 2010) because the freezing temperature decreases when pressure decreases.It has been shown that floating glaciers over a shelf can be sources of supercooled water (Debenham, 1965;Jeffries and Weeks, 1992).Such a phenomenon was found in the data from CTD casts (Stevens et al., 2010, Fig. 6) in seawater close to a glacier: the seawater was 0.01°C cooler than its freezing point under the floating ice of a glacier tongue in Antarctica.We think that in this case two mechanisms are responsible: fast cooling and fast upward motion to lower pressure.
If there are no nucleation centers, the supercooled water remains liquid and then platelets and frazil ice crystals are formed.Observations of this phenomenon are described in (Dmitrenko et al., 2010;Robinson, 2010).On the basis of laboratory studies, Brewster and Gebhart (1994) report strong supercooling of water with oceanic salinity 35 psu.This water supercooled by as much as 5.0°C before it began to freeze.Moreover, this supercooled water was found far from the cooling surface.Lawson et al. (1998) describe field measurements made near the Matanuska Glacier in Alaska and suggest a model wherein glaciohydraulic supercooling is an important mechanism for glacier ice growth.In their model, ice crystals accrete from supercooled water, which flows through the drainage system of the glacier.Such water contains suspended filaments: ice spicules.Supercooling is frequently observed in polynyas (Skogseth et al., 2009;Dmitrenko et al., 2010).The water can be supercooled by as much as 0.01-0.02°C.Supercooling results in ice formation that occurs initially throughout the surface water layer in the form of small millimeter-scale crystals, called frazil ice crystals that float slowly to the surface.
Laboratory experiments on frazil ice formation in the upper low saline water layer cooled from below by a more saline cold water layer are described in (Zatsepin and Golovin, 2001).The authors report that frazil ice production increases when both layers are in the turbulent motion.A similar effect of ice freezing below the water surface was described by Nansen (1897) in the sea when he observed a freshwater layer in the sea over cold saline sea waters at a temperature close to freezing.Błaszczyk et al., (2009) present a classification of Svalbard glaciers and the current state of tidewater glaciers (having contact with the ocean) including Paulabreen.Their classification is an extension of the inventory by Hagen et al., (1993).Jiskoot et al., (2000) analyzed the distribution of surge-type glaciers in Svalbard.They show that a polythermal regime and fine-grained bed facilitate the surge potential of Svalbard glaciers.
Several papers are dedicated to the water properties near a vertical ice wall.Foldvik and Kvinge (1974) considered a thermohaline convection of conditional instability in the ocean near the ice shelf released by the formation of ice crystals in the ascending water.A paper by Huppert and Turner (1980) considers a laboratory experiment when water originating from ice melting in stratified fluid forms a step structure.A similar structure is formed when water with a vertical gradient of salinity is cooled at a vertical wall.Laboratory experiments reported in (Josberger and Martin, 1981) study the convection generated in polar oceans when a fresh-water wall melts in salt water of different uniform far field temperature and salinity.Field measurements near the Erebus Glacier tongue in Antarctica were considered in (Jacobs et al., 1981).A vertically stable step structure was observed in the water column 400 m thick from the surface to the bottom.
The objective of this paper is to describe the measurements that recorded supercooling of water in the fjord near the front of Paulabreen in Spitsbergen and give our interpretation of the results.These measurements represent the case when the surrounding water near an ice wall is cooler than fresh water from the glacier, which flows into almost freezing surrounding water.To our knowledge, these are the first records of such a strong supercooling phenomenon in seawater measured in the field experiments very close to the glacier front.

Data of Measurements Near the Glacier Front
Measurements of water properties were carried out in Spitsbergen in Rinders Bay near Paulabreen in 2010-2015 (Fig. 1a).High resolution satellite image of Paulabreen made in 2011 provided by the Norwegian Polar Institute is shown in Fig. 1b.Yellow points in Fig. 1b show the locations of CTD profiling in August 2011 performed from the research vessel "Viking Explorer".A 3-meter draft of the boat did not allow us to approach closer to Paulabreen front.Points of sea depth measurements near the front of Paulabreen performed from sea ice in March 2013 are shown in Fig. 2 with red dots.Locations of the red dots relative to the glacier front in Fig. 2 can differ from their actual positions since the satellite image was adjusted to the map manually, while the point coordinates on the map correspond to their GPS coordinates measured in the field.The sea depth along the glacier front is shown in Fig. 3.The greatest concavities of the glacier front were found in the deepest places along the front of Paulabreen.       ) S e and salinity of small water hows that the w higher than th  Internal melting is observed in polythermal glaciers, which have a mixed basal thermal regime consisting of both warm ice (at 0°C) and cold ice (below 0°C).These glaciers consist of an upper layer with cold ice and a lower layer of warmer ice that also contains water.In Spitsbergen there are many such glaciers that descend all the way to the sea (Macheret, 2006;Macheret andZhuravlev, 1982, Kristensen andBenn, 2012).The mean water content in the lower warmer layer of such glaciers can exceed 0.1%.Some of the accumulated fresh water in this layer can freeze at the boundary between the cold and warm ice, while some of can flow into the sea even in the winter period (Macheret and Glazovsky, 2000).Schemes and explanation of freshwater flows from polythermal glaciers are given in (Hambrey and Glasser, 2012) and (Moorman, 2003).
The greatest concavities of the glacier wall are found in the deepest places along the front of Paulabreen.This makes us think that the glacier is most subjected to the loss of its mass in these places.Water flux from under the glacier can influence the sediment transport from the glacier bed, make the cohesion between the ice and soil weaker and contribute to the formation of local ice stresses and crevasses.
Mixing of freshened water at the freezing point with colder sea water can influence the formation of supercooled water if the sea water temperature is close enough to its freezing point.The weather in February and March 2013 was very cold with air temperatures close to -25 --30 o C, and therefore entire water column below the ice was almost at the freezing point in relatively shallow places in Spirsbergen fjords.This weather factor is important for the explanation of the observations in March 2013.
The measurements in 2013 were performed at a distance of 10-15 m from the glacier wall, while the measurements in 2011 and 2014 were performed at a distance of about 100 m from the glacier wall.The value of supercooling depends on the proportion of sea and fresh water in a unit volume of mixed water.Therefore the value of supercoiling is higher in the immediate vicinity of the glacier wall where the concentration of cold freshened water at the freezing point is higher.
Freshwater freezing in seawater develops in the form of ice crystals, which flow to the surface (Tweed et al., 2005).In the natural conditions such crystals reach the lower surface of the ice cover and increase the ice thickness.Measurements of the ice thickness show that ice thickness increases closer to the glacier.In addition, when we observe the water surface in the holes, which we drill in the ice to lower our instruments, ice crystals very quickly appear at the water surface in the holes drilled near the glacier (provided that the hole is initially cleared of snow and remains of the drilled ice).In 15-20 minutes the water surface in the hole can be covered with a layer of wet crystals approximately one centimeter thick.This phenomenon is not observed far from the glacier.

Conclusions
We found supercooled water near the front of Paulabreen in Spitsbergen.Analysis of vertical profiles of seawater temperature and salinity close to the front of Paulabreen in Rinders Bay demonstrated the presence of supercooled water with lower salinity than the surrounding water in the fjord.Such water was found at a depth range of 2 to 5 m, while the total depth near the glacier was 17 m.The water was supercooled up to 0.35°C at 3 m depth, an effect related to glaciohydraulic supercooling.We attribute this to the fact that meltwater that contains in the polythermal glacier flows from the glacier in the places of depressions of glacier bed into the seawater and cools by the surrounding water of lower temperature to the temperatures lower than the freezing temperature.

Figure
Figure 1.L dots indica Figure 5 Figure F

Table 1
Science Research 100 is laid off alon n in Fig.2with southwest pment we used distance of 10 ements were p s where CTD p e (Fig.2).The he glacier retre