Polar beach-ridges as climate archives (Quaternary of King George Island, South Shetland Islands, Antarctica) (Polar Beach)
Zusammenfassung der Projektergebnisse
The architecture of polar gravel-beach ridges is presented, and it is shown that ridge internal geometries reflect past wave-climate conditions. Ground-penetrating radar data obtained along the coasts of Potter Peninsula (King George Island) show that beach ridges are sediment accumulations unconformably overlying strata of the prograding beach plain. Development of individual ridges is seen as the result of multiple storms in periods characterized by enhanced storm frequency and/or less annual sea-ice coverage. Beach-plain progradation, on the other hand, is the result of swash sedimentation at the beach-face under enduring calm conditions. The age framework for this study is based on radiocarbon dates, obtained from mollusk shells, seaweed, and organic sediment, revealing that the sedimentary record covers about the last 4300 years. The frequency of ridge building ranges from decades in lowenergy settings to more than 1500 years under high-energy conditions. Four distinct ridge levels were identified in the study area: up to 4 m, 5.5 m, 7.5 m, and 10 m above the present day storm beach. In sheltered settings, two beach ridge types can be distinguished based on their internal architecture: Type I ridges show seaward-dipping beds, interpreted to be the result of swash deposition, Type II ridges are characterized by an aggrading bedding pattern as the result of wave overtopping. At exposed beaches large composite ridges develop, classified as Type III, with washover fans on their landward side. Distinct ridge types are interpreted to reflect different storm-wave runup heights; consequently, different ridge types in one beach-ridge system are interpreted to reflect variations in storm climate on a multi-decadal to centennial time scale. The number of individual ridges that are preserved from a given time interval varies along the coast depending on the morphodynamic setting: Sheltered coasts are characterized by numerous small ridges, whereas at exposed beaches fewer but larger composite ridges develop. For the South Shetland Islands, periods with increased storminess and/or less sea-ice coverage are assumed for around 4300 cal BP, c. 3100 BP, 1900 cal BP, and for sometimes after 650 cal BP, most likely around c. 350 cal BP. Periods in between are characterized by reduced storminess and/or longer lasting annual sea-ice.
