Lake Hoare, Antarctica: sedimentation through a thick perennial ice cover

Abstract

Lake Hoare in the Dry Valleys of Antarctica is covered with a perennial ice cover more than 3 m thick, yet there is a complex record of sedimentation and of growth of microbial mats on the lake bottom. Rough topography on the ice covering the lake surface traps sand that is transported by the wind. In late summer, vertical conduits form by melting and fracturing, making the ice permeable to both liquid water and gases. Cross-sections of the ice cover show that sand is able to penetrate into and apparently through it by descending through these conduits. This is the primary sedimentation mechanism in the lake. Sediment traps retrieved from the lake bottom indicate that rates of deposition can vary by large amounts over lateral scales as small as 1 m. This conclusion is supported by cores taken in a 3 x 3 grid with a spacing of 1.5 m. Despite the close spacing of the cores, the poor stratigraphic correlation that is observed indicates substantial lateral variability in sedimentation rate. Apparently, sand descends into the lake from discrete, highly localized sources in the ice that may in some cases deposit a large amount of sand into the lake in a very short time. In some locations on the lake bottom, distinctive sand mounds have been formed by this process. They are primary sedimentary structures and appear unique to the perennially ice-covered lacustrine environment. In some locations they are tens of centimetres high and gently rounded with stable slopes; in others they reach approximately 1 m in height and have a conical shape with slopes at angle of repose. A simple formation model suggests that these differences can be explained by local variations in water depth and sedimentation rate. Rapid colonization of fresh sand surfaces by microbial mats composed of cyanobacteria, eukaryotic algae, and heterotrophic bacteria produces a complex intercalation of organic and sandy layers that are a distinctive form of modern stromatolites.

Sediment Melt-Migration Dynamics in Perennial Antarctic Lake Ice

“…These dimensions are similar to those in the ice covers of Lakes Bonney Priscu et al, 1998) and Hoare (Nedell et al, 1987;Squyres et al, 1991). Sediment coalescence into clumps and layers may enhance melt-migration by increasing shortwave absorption and thermally insulating grains from ice.…”

“…This flux will vary depending on sediment quantity, grain size, and ice microstructure. The flux we measured is within a factor of 3 of average sediment fall-out rates from ice at Lake Hoare (29-41 g m 22 yr 21 at 3/4 sites) (Simmons et al, 1986;Nedell et al, 1987;Squyres et al, 1991), and far exceeds an eolian sediment flux of 1 g m 22 yr 21 on Lake Fryxell (Lancaster, 2002), indicating that intergranular transport is an important sediment filtering mechanism on ice-covered lakes of the dry valleys. Fine sand around Lake Fryxell contains elevated levels of nitrogen and organic material (Fritsen et al, 2000), and fine sediment in the ice cover of Lake Bonney contains elevated levels of biologically required phosphorus , of which the lake plankton are depleted (Priscu, 1995;Dore and Priscu, 2001).…”

“…7), once the shear strength of ice is exceeded. This process may be accompanied by the formation of fractures, which in turn may play a role in sediment ''dumping'' (Simmons et al, 1986;Squyres et al, 1991). In any case, a sudden shift in piezometric level relative to the ice surface signals either an abrupt change in ice permeability, ice position, or combination thereof.…”

“…Transport to deeper ice requires the presence of fractures or conduits (Simmons et al, 1986;Nedell et al, 1987;Squyres et al, 1991). An equilibrium depth of 2 m compares well with published depths of accumulated sediment in the ice covers of Lakes Bonney and Hoare (Nedell et al, 1987;Squyres et al, 1991). However, this depth exceeds values observed at Lake Fryxell, where sediment accumulates in the upper 0.5 m of ice Priscu et al, 2005).…”

“…Much of the lake floor sediment in the dry valleys originated as eolian deposits that migrated through the ice covers by melting and settling along fractures and conduits Simmons et al, 1986;Nedell et al, 1987;Squyres et al, 1991). Lancaster (2002) reported an eolian sediment flux of 1 g m 22 yr 21 on the ice of Lake Fryxell, of which 89% (by mass) was mud and 11% was sand.…”