Conclusions regarding the brine ponds
After returning home I correlated the new discoveries with my previous observations on the salt flat. I had some presumptions even before, but now I realized the big picture. Let’s analyze the puzzle in parts:
When I first visited the area in January 2011 I found a decent sized pond, which I observed/ selected beforehand using a detailed satellite image. The imagery’s date is 2010 May 27, so it was less than a year old, when I went there. The circular lake I found was more than 10 meters in diameter and (as it was only partially frozen) I estimated the maximum depth to be more than 2 meters. The temperature of the upper layer was -21 degrees Celsius. I returned here about 10 days later and found the pond frozen, except a small portion near one shore, where I measured -22 degrees Celsius close to the surface.
Next winter, In January 2012 I revisited this place and found this lake completely frozen, covered with a thick layer of salty ice. The ice was so massive I could safely walk on it and dig holes on its surface to study the brine beneath. The temperature under the ice was -23 degrees Celsius. This was an expected value as the solution beneath the ice became even more dense, conversely has even lower freezing point than before. But I was a little surprised to not found a single place where the depth exceeded 130-140 cm. I thought maybe it’s caused by more salt deposition on the bottom as the brine became even more denser during the freezing-point depression.
This particular pond wasn’t visited in the following years, I returned here only this winter, in February 2019. There was no lake, not even a shallow, frozen one. I rechecked the coordinates in my GPS at home and correlated with the imagery, but it was no doubt: that water body disappeared.
In January 2014 I was back to the salt flats. This time I accidentally discovered a new lake, which I recognized to be one of the bigger ponds visible on the same 2010 May 27 imagery. The 8 meter diameter circular pond was unfrozen and I measured below -23 degrees Celsius close to the surface. The shores were steep, the bottom at the deepest portion wasn’t clear because of the considerable depth, which was probable more than 2 meters.
In February 2016 I returned to see this lake. This was an exceptionally cold winter with -46 degrees recorded at Zavkhan weather station at the end of January. The lake was now covered with a thick layer of salty slush, but its temperature was only -18 degrees Celsius, a few degrees warmer than before, unfrozen. Maybe because of the changing in the surface layer’s composition? Or because of the higher solar angle in late February? Not sure. I checked the depth of the lake using the poles of my tent and found out that the ground was very uneven on the bottom. In some places it was 1.5 m, in others (just a little farther) less than 1 m. But far from 2 meters at any point.
In January 2014 I searched for eutectic frozen brine, so I was looking for very shallow ponds. However I was surprised not to found a deeper lake in one particular area, as I remembered from the imagery there is one. There were some shallow frozen ponds, but none of them had considerable depth (less than 0.5 meters) as I was expecting.
This winter, in February 2019 I discovered this new, 2 meters deep brine lake (where I’ve accomplished the dive), which I thought to be the one I couldn’t found in 2014. But after analyzing the exact coordinates I figured out that this lake is not exactly on the same place, where the deeper pond on the imagery appears. Also the shape is different. And the deeper pond (according to the imagery) is no longer there. This lake has the slush temperature of around -22 degrees Celsius, with the incredible bottom temperature of +2 degrees. As this sensor with the 3 m long cable is a new one, I have no infos regarding the bottom temperature of the other ponds in the former research years, but I suppose, that in the case of the deeper ones, the stratification can be similar.
This winter, in February 2019 I discovered this partially unfrozen, new pond of around 4 meters in diameter, with the surface temperature of -24 degrees Celsius. After returning home I found out that at those coordinates there is no lake on the 2010 imagery, not even a tiny one. The imagery has a 0.5 meters resolution, so even smaller features are identifiable (like camels or bovines). It’s clear: on that date this pond didn’t existed.
Taking into account the above mentioned facts, regarding the geomorphological/ hydrological aspect my major conclusion is that the lifespan of these brine ponds is small (usually less than a decade), they are constantly forming by the action of the springs under the salt crust (underground erosion/ dissolution) and disappearing by the formation of new salt deposits on the bottom of the water bodies (colmatation with evaporites).
Regarding the water temperatures my observation is that the surface layer generally reaches the slush state between -20 and -25 degrees Celsius, in most cases around -21, -22 degrees. There are some smaller ponds, where I saw liquids even close to -29 degrees Celsius. As the water bodies are not far from each other (sometimes it’s only a few meters between them), I consider that the general ratio between the solution’s components must be (almost) the same and these fluctuations between the freezing points are mainly caused by general concentration and not ratio changes between the constituents.
Another thing I could mention is that when the surface layer reaches the slush state at a certain temperature, the deeper layers must have an even lower freezing point as in the case of salt water the density rises constantly with the temperature decrease, so if the deeper, unfrozen layer is still heavier than the upper one, its freezing point is certainly lower. This also means that the surface layer’s slush can never reach the maximum possible concentration of the solution.
Note: After I get the water sample analysis results from Budapest (hopefully soon) I will update this section.