Evidence of deep circulation in two perennially ice‐covered Antarctic lakes

Abstract

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, we utilize chlorofluorocarbon (CFC) concentration profiles from Lakes Hoare and Fryxell in the McMurdo Dry Valley to determine the extent of deep vertical mixing occurring over the last 50 years. Near the ice-water interface, CFC concentrations in both lakes were well above saturation, in accordance with atmospheric gas supersaturations resulting from freezing under the perennial ice covers. Evidence of mixing throughout the water column at Lake Hoare was confirmed by the presence of CFCs throughout the water column and suggests vertical mixing times of 20-30 years. In Lake Fryxell, CFC-11, CFC-12, and CFC-113 were found in the upper water column; however, degradation of CFC-11 and CFC-12 in the anoxic bottom waters appears to be occurring with CFC-113 only present in these bottom waters. The presence of CFC-113 in the bottom waters, in conjunction with previous work detecting tritium in these waters, strongly argues for the presence of convective mixing in Lake Fryxell. The evidence for deep mixing in these lakes may be an important, yet overlooked, phenomenon in the limnology of perennially icecovered lakes.

Bacterial dissolved organic carbon demand in McMurdo Dry Valley lakes, Antarctica

“…We estimated a vertical diffusion coefficient of 1.6 ϫ 10 Ϫ5 m 2 d Ϫ1 for DOC based on the Wilke-Chang correlation (Bird et al 1960), which accounts for the molecular mass of the solute. Our diffusion coefficient is similar to the one used to predict chlorofluorocarbon (CFC) concentration profiles in Lakes Fryxell and Hoare (4.93 ϫ 10 Ϫ5 m 2 d Ϫ1 ; Tyler et al 1998) if the molecular mass of CCl 3 F (137 Daltons) is taken into account. Lake Fryxell sediment pore-water DOC is of a different composition than deep-water DOC and is a significant source of DOC to Lake Fryxell and presumably the other lakes McKnight et al 1993).…”

The Saline Lakes of the McMurdo Dry Valleys, Antarctica

“…However, there is also evidence in Lake Fryxell that density driven flow, initiated by evaporation of water from the ice-free ''moats'' that form around the lake edges, may transport small volumes of cold saline surface water to depth (Miller and Aiken 1996;Tyler et al 1998). In addition, because both Lake Fryxell and the west lobe of Lake Bonney directly contact a glacier, advection driven by this interaction is probably also significant.…”

Pressure‐driven, shoreline currents in a perennially ice‐covered, pro‐glacial lake in Antarctica, identified from a LiCl tracer injected into a pro‐glacial stream

“…Tyler et al . () attributed this behaviour to the advection of meltwater from the Canada Glacier, whereas Dowling et al . () argued mixing was caused by gravity currents generated by salt exclusion during the refreezing of moat ice in the autumn.…”