Amazon lakes
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ACIDIFICATION IN LAKES Thomas Hellström Thellva HB, Dalarö, Sweden
Definition A lake can be considered acid or slightly acid when pH is below neutral (50, million years ago (Barrett, 1999). A liquid water lake formed in this depression when the ice became sufficiently thick to reach the pressure melting point (3 C). The youngest possible age of lake water is the same as the ice at the base of the ice sheet, 420,000–1,000,000 years old. Some of the water could be as old as Lake Vostok itself if the lake is stratified (Siegert et al., 2003). It is thought that a subglacial lake could have existed in this location for as long as the ice sheet has been at a continental scale, which may be at least 15 million years (Summerfield et al., 1999). Sediments on the lake bed could be significantly older than the water if a subaerial lake existed prior to glaciation, provided that the advancing ice sheet did not scour them out of the basin.
ANTARCTIC SUBGLACIAL LAKE VOSTOK
Ice motion, subglacial melting, and freezing above Lake Vostok Ice flow above Lake Vostok is NNW to SSE in the northern part of the lake, and W to E in the southern part (Tikku et al., 2004). Ice velocity measured using GPS and repeat-pass synthetic aperture radar interferometry (InSAR) from the ERS-1 satellite ranges from 3.0 0.8 m a–1 to 4.2 m a–1 at Vostok Station (Bell et al., 2002; Kwok et al., 2000), such that ice would take approximately 15,000–20,000 years to cross the lake. The liquid water in Lake Vostok results from melting of the ice sheet in the north where the ice is sufficiently thick to reach the pressure melting point. Melting here is thought to be approximately balanced by lake refreezing to the underside of the ice sheet in the south, where the lake water is colder than the pressure melting point, so that the volume of water in the cavity does not change. Melt and freeze rates vary from a few millimeters to a few centimeters per year (Siegert et al., 2001; Bell et al., 2002). Residence time and water circulation Estimates of the residence time of water in the lake vary from 40,000 to 300,000 years (after Bell et al., 2002 and Siegert et al., 2001). Circulation of water in the lake can only be predicted from theoretical models until the lake is accessed directly. Two possible scenarios have been modeled: 1. If the lake water is fresh, melt water formed in the northern region will be colder and denser than the resident lake water so will sink to the lake floor. Background geothermal heat will raise the temperature of this water above that of the overlying layers, decreasing the density and producing plumes of water that rise to the ice–water interface before cooling and sinking again. 2. If the lake water is saline, the fresh glacial meltwater will be buoyant compared with the resident lake water. The meltwater from the north would probably move southward and upward along the ice–water interface and eventually enter a region where it is below the melting point and refreeze onto the ice sheet. Any circulation patterns are likely to be
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