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.
The sedimentation mechanisms that occur in ice-covered Lake Hoare, Antarctica are examined, to determine how sediment enters the lake, and how the sedimentation pattern affects blue-green algal growth at the lake bottom. The 3 m-thick ice cover contains pebbly sand as much as 2 m below the surface. Sediment with similar texture and mineralogy is found at the lake bottom. Thisevidence, together with the lack of sediment in the inflowing stream and the markedly different texture of sediment from the other terrains around the lake suggest that most of the sediment at the lake bottom comes in through the ice cover. Sand grains intermittently migrate through porous ice on the surface, water-filled vertical gaschannels penetrating two-thirds of the ice cover, and possibly through cracks in the ice that act as conduits.The algae at the lake bottom are able to survive in part because sediment that comes through the ice cover does not obliterate them.
The interiors of the Valles Marineris canyon system contain plateaus of horizontally-layered material where individual layers are laterally continuous over tens of kilometers. Several workers have suggested that they were deposited in lakes that existed in these depressions early in Martian history. Recently, Nedell et al. (1987) conducted a detailed study of the Valles Marineris layered deposits using late Viking high-resolution images. These studies show that the deposits form thick sequences of rhythmically layered material whose bases are in the lowest elevations of the canyon floors and whose tops are commonly within a few hundred meters of the surrounding plateaus. Most of the deposits occupy the central canyons, which include Hebes, Ophir, Candor, and Melas chasmata. From stratigraphic relationships, Nedell et al. (1987) concluded that the layered deposits formed during roughly the same epoch in which the original tectonic canyons were enlarged by ground ice removal and collapse. Later, the deposits were eroded in some locations, producing their present geometry. This erosional episode may coincide with the formation of the large outflow channels that emanate from the east end of the Valles Marineris. Nedell et al. (1987) concluded that deposition in standing water was the only mechanism that could readily explain the distribution, lateral continuity, horizontality, great thickness, and rhythmic nature of the deposits.If standing bodies of water formed in the Valles Marineris, and an initial thick C02 atmosphere had thinned resulting in lower temperatures, these lakes would almost certainly have been icecovered. We suggest that a considerable fraction of the possible Martian paleolake sediment could be carbonate material that was precipitated in standing water under conditions of high atmospheric pressure of C02.One major problem with carbonate formation on early Mars is maintaining significant bodies of liquid water after the mean temperature fell below 273 K. According to the model of Pollack et al. (1987), this would occur when the atmospheric pressure drops below a value of a few bars. Ice-covered lakes fed by transitory surface melting in the Valles Marineris could have provided a stable body of liquid water that would have also had an enhanced level of atmospheric gases. This hypothesis is supported by our observations in the Antarctic dry valleys where perennially frozen lakes contain dissolved atmospheric gases at 200-300% above the equilibrium level and the mean annual temperature is 253 K. By analogy with these lakes, we suggest that lakes in the canyon system could have contained liquid water long after the mean temperatures on the surface were below freezing.
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