EVIDENCE OF DEEP CIRCULATION IN 2 PERENNIALLY ICE-COVERED ANTARCTIC LAKES

The perennial ice covers found on many of the lakes in the McMurdo Dry Valley region of the Antarctic have been postulated to severely limit mixing and convective turnover of these unique lakes. In this work, w e utilize chlorofluorocarbon (CFC) concentration profiles from Lakes H oare and Fryxell in the McMurdo Dry Valley to determine the extent of deep vertical mixing occurring over the last 50 years. Near the ice-wa ter interface, CFC concentrations in both lakes were well above satura tion, in accordance with atmospheric gas supersaturations resulting fr om freezing under the perennial ice covers. Evidence of mixing through out the water column at Lake Hoare was confirmed by the presence of CF Cs throughout the water column and suggests vertical mixing times of 2 0-30 years. In Lake, Fryxell, CFC-11, CFC-12, and CFC-113 were found i n the upper water column; however, degradation of CFC-11 and CFC-12 in the anoxic bot tom waters appears to be occurring with CFC-113 only p resent in these bottom waters. The presence of CFC-113 in the bottom w aters, in conjunction with previous work detecting tritium in these wa ters, strongly argues for the presence of convective mixing in Lake Fr yxell. The evidence for deep mixing in these lakes may be an important , yet overlooked, phenomenon in the limnology of perennially ice-cover ed lakes.