Climatic and Environmental history of Isla de los Estados

PONCE, J. F.; FERNÁNDEZ, MARILÉN

Springer Verlag

Lugar: Dordretch; Año: 2014 p. 128

978-94-007-4362-5

Isla de los Estados (Staaten Island, 54°45?S, 64°15?W) due to its geographical location at the southeasternmost tip of Isla Grande de Tierra del Fuego, is excluded from the subantarctic region and is isolated from the southern end of South America. It represents a specially sensitive area for the paleoclimatic records and therefore, is specially interesting for paleoenvironmental studies of Late Cenozoic of South America. Last Glacial Maximum-Holocene environmental conditions were interpreted in Isla de los Estados from geomorphological, geological, palaoegeographical, palynological and diatomological analysis. A glacial model from Isla de los Estados based on the geomorphological analysis was made. This glacial model would have consisted of valley glaciers supplyed by cirque glaciers and little local ice sheets. Several of these valley glaciers have been eroded along tectonic alignments, such as faults, folds and intraformational contacts. In the southern sector of the island, glaciers would have been in contact with the sea, ending in a process of ablation of "calving" type, that is, through the physical removal of ice blocks from the glacial front in contact with the sea. The glaciers of the northern coast would have flowed into steep valleys to reach a vast plain located northward of the island. They ended like piedmont glaciers, in a simple ablation process. The development of rise sea level digital models for Isla de los Estados for the period spanning the Last Glacial Maximum (approximately 24,000 calibrated years B.P.) to the Middle Holocene allowed to estimate when the Le Maire strait was opened. This event would have happened at about 15,000 cal B.P. during the Late Glacial times. The deglacial times lasted from 16 to 11 cal yr B.P. in the diatom record from the central part of the island, when local glaciers had retreated due to a gradually warmer climatic trend. The first lake stage was characterized by fresh, oligothrophic and tycoplancktonic diatom species, common in postglacial sediments. Fluctuation on the diatom species association indicate a climatic set back at 15 cal yr B.P. Aulacoseira spp decrease its frequency values probably showing a lake level oscillation. From 14,5 to 12,8 cal yr B.P., when ACR took place, there was probably a higher precipitation rate on the island suggested by higher values of planktonic and freshwater diatoms. At around 12,8 yr cal B.P. the arid phase started. Warmer and less wind activity made possible the higher peat accumulation and the set of forest vegetation. Onset of peat formation in the western coastal area began prior to 12,600 cal B.P. in response to these warmer conditions. The pollen data in this area indicates at 12,600 cal B.P. initial treeless herbaceous and paludal vegetation with scarce Empetrum/Ericaceae type heaths and scrubs as a result of plant invasion and short-term succession vegetal communities spreading over the shoreline areas under locally more humid conditions. The vegetation between 10,300 and 8,300 cal B.P. included dwarf shrub heaths, scrubs, cushion plants and grasses with scattered trees, under warmer and drier climate conditions than today. After 8,300 cal BP, more humid conditions allowed the expansion of an open Nothofagus forest associated with dwarf shrub heath communities. It was followed at 6,700 cal B.P. by a gradual closed forest development in association with Drimys winteri and shrub and herb vegetation indicative of Subantarctic Evergreen Forest-Magellanic Moorland vegetation transition under cold and wet conditions. After 5,500 cal B.P., the rate of evergreen beech forest greatly increased with the development of almost pure Subantarctic Evergreen Forest communities. These vegetation changes accompanied a modification of the climate toward colder and wetter conditions. After 2,700 cal B.P., the closed forest was replaced by an open Nothofagus forest indicative of warm and dry conditions. The minimum of Nothofagus pollen registered between 1,000 and 500 cal B.P. may correspond to the Medieval Climate Anomaly (MCA) period. All these vegetation changes are in turn related to the positioning and intensity of the Southern Westerlies wind belt, sea-ice Antarctic extent and sea level changes. The palaeoenvironmental situation of Isla de los Estados could be used as an explanation for the absence of cultural remains and also for the scarce archaeological record for Península Mitre during the middle Holocene. However, during the late Holocene, milder conditions might have favored seasonal travelling to the outer islands of the Fuegian Archipelago, though no definitive or permanent settlements have been found yet. DOI 10.1007/978-94-007-4363-2