| Popis: |
At 53° S, pans, shallow lakes, lacustrine terraces, perched dunes and paleosols are most of the typical landforms and features of the Fuegian steppe conditioned by climate. They are in a cold, semi-arid, oceanic climate highly affected by the Southern Westerlies winds (SWW).During spring and early summer, rainfall, snow melting, and runoff promote the infill of the pans that become in brackish, shallow lakes. When the systems are filled with water, the permanent, high-speed SWW generates waves on the lakes' coasts; whereas the shallow lakes dry up again, bottom sediments are deflated. Wind erosion on the coastal rocky cliffs and deflation contribute to the perched dunes formation.Since the Late Glacial, multi centennial wet/dry cycles allowing the development of specific landforms and features are also recorded by proxy studies. During the dry intervals, erosion removed fine sediments from the marine sandstones and limestones cliffs while deflation deepened the dried basins contributing with bottom lake sediments, diatoms and organic matter remnants in the dune deposits. Both processes provide sediments which accumulate on the top of the cliffs forming perched dunes. Accumulation rates varied through different time intervals allowing the perched dunes to grow and increase their heights, while the spreading of halophytes around the lakes evidence that its area diminished. The algae and ostracod assemblages suggest mesohaline or slightly alkaline conditions, higher conductivity, and enhanced evaporation whereas there was a remarkable change in diatom assemblages from planktonic to common species in terrestrial and dry settings. All these evidences suggest a lowering in lake water levels. Conversely, during the wet intervals, lake water levels were relatively higher and probably deeper than today. Several lacustrine terraces extend around the lake’s margins more than 10 m above the present lake levels. During these intervals, grasses expanded, and halophytes retracted. The algae and ostracods assemblages indicated freshwater supply, high nutrient loading, oligohaline and deeper water body. At this time, diatom assemblages were more diverse including planktonic species. Also, the presence of sedges and hygrophilous taxa pointed to the development of moist environments surrounding the lake. Dune formation was interrupted and pedogenetic processes occurred on their upper layers until renewed aeolian deposition, a dry cycle, buried them again. Paleosol structures varied over time according to the availability of environmental humidity, with a less developed pedological structure towards the Middle and Late Holocene. These paleosols showed the dominance of local herbaceous and/or shrub vegetation along with the presence of arbuscular mycorrhizal fungi.These wet-dry cycles were conditioned by the latitudinal migration of the SWW core. When it migrates northwards, the intensity of winds diminishes and the entrance of humid air masses from the Atlantic is favoured. By the contrary, dry cycles result from the position of the SWW core in latitudes close to 50°S, allowing the wind to reach the region with high speed and frequency, preventing entry of moist air from the Atlantic and promoting a rain-shadow effect in the landscape. |
