EPOW - Ecology Picture of the Week

The Thrombolites of Sarmiento

Thrombolite Shoreline, Lago Sarmiento
Torres del Paine National Park, Chile

Explanation:  Here we are at the end of another year.  In the northern hemisphere it is winter, and what better way to celebrate this season than this photo of snow on the beach of a lake?

But ... this is not snow.  Then what is it?

We are on the shores of Sarmiento Lake in beautiful Torres del Paine National Park in southern Chile.  To understand the white edging around the shore, here is a major clue.  The region belongs to the geological Cerro Toro Formation of the Upper Cretaceous Period, comprised of deep, dense, turbid marine sediments called turbidites.  Turbidites form from terrestrial sediments that once washed into the ocean by rivers, and then compressed in massive quantities by underwater avalanches.  Eventually, the area became uplifted by mountain-forming tectonic pressures.  

OK so far.

The beautiful land and landscape around Sarmiento Lake is a major national park.
But only the immediate shoreline is white ... why?

Here's another clue.  Sarmiento Lake has a pH of about 9, which is as alkaline as baking soda and phosphate detergents.  The lake is also hypersaline -- extremely salty.  These are clues that the lake acts like a playa, that is, having no outlet, so that the only loss of water is through ground percolation and evaporation which serves to concentrate the salts and increase the salinity levels.

So now we can reveal the reason for the white shoreline.  It is from the work of blue green algae called cyanobacteria.  These microorganisms use the high saline content of the lake during their photosynthesis and produce a precipitate of calcium carbonate that has a coral-like hard "skeleton" structure known as microbialites.  As the microbialites increase in density and form, the minerals precipitate, creating white macro-structures called thrombolites which appear like rounded rocks covered in snow ... but they are neither rocks nor are they covered in snow.  They are masses of calcium carbonate covered by a thin surface layer of living cyanobacteria still doing their job of creating yet more microbialites and keeping the shoreline white.  

The white formations around the shoreline are therefore thrombolities that, here, have formed since the last glacial period that ended about ten thousand years ago.  

The most common cyanobacterium at Sarmiento Lake is Rivularia.  

The cyanobacteria can proliferate here because the alkaline and hypersaline waters are inhospitable to snails and other creatures that would otherwise feed on them and keep them in check.  

And now you know the amazing geological and biological secret behind the "white snow" of Sarmiento Lake!  

It is fitting, in this turn of another page on our calendar as we bid farewell to the old and welcome in the new, that we have explored this land of ancient geology and new life.  

Happy New Life, Happy New Year everyone!
  

In the distance rise the three "towers" of Torres del Paine National Park.