RAPID CLIMATE CHANGE EVENT RECORDED BY SULFUR ISOTOPIC SHIFT IN SEDIMENT FROM LAKE FRYXELL, TAYLOR VALLEY, ANTARCTICA

The McMurdo Dry Valleys of Antarctica comprise the largest ice-free expanse on the Antarctic continent. Taylor Valley is the southernmost of the McMurdo Dry Valleys and contains several amictic, perennially ice-covered lakes. Sediments deposited in these lakes provide a unique record of local Holocene climate changes in Taylor Valley. Lake Fryxell, the largest lake in Taylor Valley, contains abundant microbial life, including sulfate-reducing bacteria. Sulfur isotope values (δ³⁴S), expressed as per mil (‰), of recovered sulfur minerals from these lake sediments were used to infer the relative water level history of Lake Fryxell. A sediment core from Lake Fryxell was obtained in November of 2002 and sequential chemical extraction was used to purify and concentrate pyrite and soluble sulfate from sediments between 30 and 130 cm depth. This depth range was analyzed since it contained the highest weight percentage of total sulfur (0.5% to 4%). Thin laminae of microbial mats were observed in the sediment core, indicating that microbial processes, including possible bacterial sulfate reduction, were active in Lake Fryxell during the time of sediment deposition. Yields of soluble sulfate in all samples were very low indicating little dissolved sulfate was present in porewater. Values of δ³⁴S for recovered pyrite display two distinct trends. The upper portion of the sediment core (30-104 cm depth) contains average δ³⁴S values of -21‰. This is interpreted to be the result of relatively high water levels in Lake Fryxell, allowing dissolved sulfate levels to exceed removal by sulfate-reducing bacteria. The lower portion of the sediment core (106 to 130 cm depth) contains average δ³⁴S values of -3‰. This is interpreted to be the result of relatively low water levels in Lake Fryxell, indicating bacterial sulfate reduction was depleting dissolved sulfate levels in the lake. The change in lake level, indicated by δ³⁴S values for pyrite, is interpreted to represent a cooling event in Taylor Valley that prevented glacial meltwater from draining into Lake Fryxell. Our conclusions regarding inferred lake levels and climate shifts obtained from δ³⁴S analysis correspond well with geochronology and sedimentological results from this same sediment core that are currently in press.