Isoprenoidal and branched glycerol dialkyl glycerol tetraethers (GDGT) are analyzed in microbial mats, sediments and a granite endolith from perennially ice-covered lakes of the McMurdo Dry Valleys (Lake Fryxell, Lake Hoare, Lake Chad and Lake Bonney). Except for Lake Chad, all other lake sediments contain Crenarchaeol, the GDGT with a cyclohexyl ring, an adaptation of archaeal (hyper)thermophilic ancestors to cold waters (Sinninghe Damsté et al., 2002). Lake Chad microbial mat contain up to 95% branched GDGT, a marker for terrestrial bacteria. Lake water temperatures calculated using the tetraether index TEX86
, an index based on isoprenoidal GDGT ratios, are overestimates of measured temperatures in the lakes. Estimates of annual mean air temperatures from branched GDGTs, however, are close to the measured values. Although the TEX86
values are overestimates, the decreasing trend upvalley is consistent with a decrease in the measured temperatures from coastal Lake Fryxell to continental Lake Bonney conditions. The granite endolithic community, which is a planetary analog for astrobiological studies, only contain isoprenoidal GDGTs. Environmental conditions and patterns of GDGT distribution in microbial mats and surface sediments are compared with sediments of a Lake Fryxell core for paleolimnological studies. Archaeal and bacterial lipid distribution are consistent with the formation of a large freshwater lake in the Pleistocene. Lake level decreased and formed small hypersaline ponds during the Holocene that filled-up and formed the Taylor Valley lakes today.
Sinninghe Damsté J.S., Schouten S., Hopmans E.C., van Duin A.C.T., Geenevasen A.J., 2002. Crenarchaeol: the characteristic core glycerol dibiphytanyl glycerol tetraether membrane lipid of cosmopolitan pelagic crenarchaeota. Journal of lipid research, 43, 1641-1651.