CARBON ISOTOPIC COMPOSITION OF BENTHIC AND PELAGIC ORGANIC MATTER IN FOUR POLAR LAKES OF THE MCMURDO DRY VALLEYS, ANTARCTICA

Perennially ice-covered lakes in the McMurdo Dry Valleys of East Antarctica represent end-member ecosystems on Earth.  To study the relationship between pelagic and benthic organic matter (BOM), Lake Hoare, Lake Fryxell, East Lake Bonney and West Lake Bonney in Taylor Valley, were sampled for benthic microbial mat, sediment, and particulate organic carbon (POC) at various depths.  The BOM signal in Taylor Valley lakes ranges from carbon-limited environments with limited fractionation (-1to -14 ‰) at shallow depths, grading to more depleted carbon signatures (-30 ‰) in deep water.  In Lake Bonney, bottom sediment d¹³C signatures show highly depleted d¹³C values (-46 ‰) suggesting methanotrophy.  BOM can be derived either from in situ benthic production or from the contribution of fallout POC. The d¹³C POC and BOM profiles differ at the shallowest depths by 13-23 ‰, while at the deepest sites the POC and BOM signals overlap.  This suggests that at all but the deepest areas, the pelagic POC does not significantly affect the carbon isotopic composition of BOM.  Our isotopic data also suggest that anaerobic methane oxidation is an important process in the dry valley lakes, and that methane oxidizing bacterial biomass contributes to BOM in the deepest sections of the lakes.  Lake Fryxell shows an exponential increase in methane oxidation in the bottom two meters of the lake also suggesting anaerobic methanotrophy.