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.