Honeycomb development on Alexander Island, glacial history of George VI Sound and palaeoclimatic implications (Two Step Cliffs/Mars Oasis, W Antarctica)
Résumé
Abstract Combining the analyses of three generations of glacial deposits and of a range of geomorphic features including widespread honeycombs and tafonis at Two Step Cliffs / Mars Oasis (71°52'S, 68°15'W) provides new insights into the geomorphological evolution of West Antarctica, with special respect to alveolar weathering. At Two Step Terrace, indicators of the inherited character of cavernous weathering were found, such as 97 % non flaking and varnished backwalls, and 80 % tafoni floors that are till-covered and/or sealed by lithobiontic coatings. Based on the NE predominant aspect of the alveolized boulder faces, tafoni initiation is attributed to coastal salt spray weathering by halite coming from the George VI Sound during the 6.5 ka BP open water period. The present-day activity of these inherited cavities is restricted to roof flaking attributed to a combination of processes involving thermal stresses. This 6.5 ka BP phase of coastal alveolization is the first step of a six-stage Holocene geomorphological scenario that includes alternatively phases of glacial advance or stationing, and phases of vegetal colonization and/or rock weathering and aeolian abrasion on the deglaciated outcrops. This geomorphic scenario is tentatively correlated with the available palaeoenvironmental record in the Antarctic Peninsula region, with two potential geomorphic indicators of the Holocene Optimum being identified: 1. clusters of centimetric honeycombs facing the sound (marine optimum at 6.5 ka BP); 2. salmon-pink lithobiontic coatings preserved inside cavities and at the boulder surface (terrestrial optimum at 4-3 ka BP). Keywords : Antarctic Peninsula - Ice shelf fluctuations - Alveolar weathering – Lithobiontic film –Geomorphic and vegetational response to Holocene climate fluctuations