Contrasting sulfur isotopic characteristics for sulfate and sulfide in water and sediment profiles from three lakes in Taylor Valley, Antarctica

Perennially ice-covered lakes within Taylor Valley, Antarctica contain diverse communities of microbial organisms including sulfate reducing bacteria. Water column samples were collected from Lake Bonney, Lake Hoare and Lake Fryxell in November and December of 2003 and 2004. Sediment cores were collected from Lake Hoare in November of 1994. Bulk surface sediment and a sediment core were collected from Lake Fryxell in November of 2003. Dissolved sulfate was precipitated from all water column samples for sulfur isotopic analysis. Sequential extraction was used to separate different sulfur species from sediment samples. Dissolved sulfate from East and West Lobe of Lake Bonney shows delta 34S compositions ranging from 17.4‰ to 20.8‰. Sulfur isotopic compositions suggest past evaporative drawdown in East Lobe Lake Bonney and perennial ice cover for an extended time in West Lobe Lake Bonney. Dissolved sulfate from Lake Hoare shows delta34S compositions ranging from 13.9‰ below ice cover to 15.6‰ in bottom water. Pyrite from Lake Hoare sediments range from -4.5‰ to -5.3‰ in oxic sediment and from -1.9‰ to -21.8‰ in anoxic sediment. The overall trend of delta34S for pyrite from anoxic sediment indicates that the dissolved sulfate reservoir in the water column has fluctuated overtime. Values of delta34S for dissolved sulfate in the water column of Lake Fryxell range from 7.6‰ just below ice cover to nearly 52‰ in bottom water, indicating substantial removal of dissolved sulfate by bacterial sulfate reduction. Sulfate concentrations decrease with depth from 100 mg/L just below ice cover to 30 mg/L in bottom water. A Rayleigh Model suggests that nearly 70% of available sulfate in Lake Fryxell bottom water has been removed by bacterial sulfate reduction. Values of delta34S for pyrite from Lake Fryxell sediment ranges from -28‰ to 15‰. This wide range of delta34 S compositions indicates marked fluctuation in size and isotopic composition of the dissolved sulfate reservoir within Lake Fryxell over time.