CHEMICAL AND ISOTOPIC EVOLUTION OF THE DRY VALLEY LAKES: TIMING OF LATE HOLOCENE CLIMATE CHANGE IN ANTARCTICA

To understand the long-term climate history of Antarctica, we investiged the lakes that occupy the Taylor Valley, Southern Victoria Land (78°S).  Helium isotope ratios and He, Ne, Ar and N₂ concentration data obtained from hydrocasts in the East (ELB) and West (WLB) Lobe of Lake Bonney provided important constraints on the Holocene evolution of the lake system.  Based on the very low concentrations of Ar and N₂ in the bottom waters of ELB that are consistent with atmospheric solubility at this salinity, ELB was free of ice (i.e. in contact with the atmosphere) until 200±50 years ago. Low salinity water, flowing over the sill that separates the West and East lobes of the lake below 13 meters depth, covered ELB and formed a perennial ice cover, inhibiting the exchange of gases with the atmosphere.  A simple diffusion model of the He, Ne, Ar, and N₂ into the deep water of the East Lobe constrains the age of this event at 200±50 years.  In contrast to the East Lobe, the West Lobe retained an ice cover through the Holocene (and potentially for a much longer time).  The deep brines (~ 4x seawater Cl) of the WLB have N₂ and Ar gas concentrations indicating a meteoric origin that has not been significantly modified by atmospheric exchange or ice formation.    By fitting a diffusional loss to the model to the ³He/⁴He, helium (max=0.28cc/liter), and Cl profiles, we calculate a time of ~3000 years for the initiation of flow over the sill separating the East and West Lobes.

Lakes Hoare and Fryxell have more recent histories and much more rapid throughput than Bonney. The low salinity waters show extreme Ar enrichment with Ne and He depletion, consistent with a multi stage ice formation process. The gas chemistry of Lake Hoare shallow water responds rapidly to the influx of fresh water as water collected after the recent “flood” event shows only limited Ar enrichment.