I had a great conversation this morning with a group of oceanography students from Soap Lake High School in Washington (thanks to teacher Matt Brewer for facilitating). They’d done their homework and asked some really good questions. Among other things they asked me to draw some comparisons between Soap Lake (just down the road from their school) and the environments we are studying in Antarctica. The comparison is interesting enough that it’s worth expanding upon here.
It's not soapy, it just looks that way. Soap Lake, an alkaline lake in the state of Washington. Photo from http://en.wikipedia.org/wiki/File:Soap_Lake.jpg.
Soap Lake isn’t a normal lake, it’s one of several saline lakes in the eastern part of Washington state. In fact Soap Lake falls into a special category of salt lakes; it’s an alkaline or “soda” lake. This means it has an unusually high pH brought on by the presence of certain minerals (primarily carbonate). Other famous soda lakes include Mono Lake in California.
As I’ve mentioned before in this blog frost flowers, and anything else at the sea ice surface, tend to be very salt. So how similar are these two salty environments? If we based our answer purely on the amount of salt present they are pretty similar. Frost flowers contain up to 15 % salt (ocean water is 3.5 %). The most saline water in Soap Lake, which forms a permanent layer at the bottom of the lake, is also close to 15 %. But quantity of salt isn’t everything. The composition of the salt probably has a bigger impact on these ecosystems.
Precipitation of salts from seawater during sea ice formation. Each "inflection" in the horizontal lines is a point where the composition of the ions in sea ice is changing due to the precipitation of a specific salt. Some, but not all, of the salts produced are listed. Figure from Assur, 1958.
Frost flowers are basically frozen seawater but they don’t contain the same normal ions, or charged elements, found in seawater; sodium, chloride, magnesium, sulfate, and carbonate among others. Mix a bunch of different ions together in water and evaporate the water, the ions will pair off to form the salt left in the bottom of the container. For example sodium might pair off with chloride to form halite, or table salt. A similar thing happens when water freezes. The ions will pair off at specific temperatures, the resulting salt will leave the water and settle to the bottom. In seawater calcium joins up with carbonate to form the salt calcite at only -2.3 C, not long after freezing started. As the temperature keeps going down other salts are lost. Sodium comes together with sulfate to form the salt mirabilite at -7.6 C. So before frost flowers even get a chance to form they are already deficient in calcium, sulfate, and carbonate (there is so much sodium present in seawater that little is lost during the production of mirabilite).
Soap Lake has an excess of carbonate, frost flowers are depleted in it. This is a critical difference. No one has yet come up with a way to measure the pH on the miniscule amounts of water in frost flowers, if we could we might find that these brines are more acidic than seawater (pH of 8.6) for the same reason that Soap Lake is more basic. This difference in pH could have a substantial impact on biology in the ice.
Researchers at Lake Vanda in the McMurdo Dry Valleys. Photo from http://cyanobacterialadventures.blogspot.com/2010/12/out-and-about.html.
There are however, some Antarctic environments that could be very much like Soap Lake. In the nearby McMurdo Dry Valleys (which we are scheduled to visit toward the middle of the month) there are many saline lakes including Don Juan Pond, possibly the most saline lake in the world*. These lakes aren’t alkaline lakes. Instead of carbonate salts these lakes are dominated by the bizarre calcium chloride mineral antarcticite. They do share one important feature with Soap Lake however; high concentrations of sulfur in a layer of deep water that never comes to the surface. We don’t yet know a whole lot about the microbiology of these lakes, but these two things – a high concentration of sulfur and a permanent layer of deep water – have produced an interesting microbial ecosystem in Soap Lake. This ecosystem includes an incredibly dense community of bacteria capable of making their own organic carbon from carbon dioxide and found, as of yet, in no other lake on Earth (see article here). Of course studying Soap Lake and similar exotic bodies of water in our own backyards yields a wealth of information and techniques that make it possible to understand the ecology of lakes in the Dry Valleys and other places where access is limited.
*By the measure of water activity, the most biologically useful measure for how salty something is. Antarcticite is extremely hygroscopic, meaning it has a high affinity for water. As a result adding a few grams of antarcticite to a test tube containing cells and water would have a much bigger effect than adding a few grams of sodium chloride, as the cells would not be able to “compete” effectively for the water.
