Recent Lake, Playa, or Surface Deposits

This chapter discusses the occurrence of sodium sulfate deposits from the surface of several lakes and playa in the world. One of the world's most unusual occurrences of sodium sulfate is in the scattered large and small deposits and numerous surface crusts of mirabilite(Na 2 S0 4 • 10H 2 O) and thenardite (Na 2 S0 4 ) found in Antarctica. Seawater appears to be the obvious source for most of the sodium sulfate in Antarctica because its δ 34 S values are the same as in seawater, and essentially all of the sodium sulfate in the massive amounts of seawater that freezes each year is crystallized. Also, there are discontinuous mirabilite beds up to 1 m thick in a 1250 X 250 m area on the Ross Ice Shelf, called the Cape Spirit beds. They are underlain by up to 8 cm of marine mud (containing marine diatoms) and have non marine algal mats above them. By far, the most common occurrence of mirabilite and thenardite in the Antarctic is in the form of efflorescences and encrustations on the surface, or accumulations just beneath it, of the area's boulders and rocks.