Changes in beach deposit characteristics on Joinville and Livingston Islands, Antarctica
The sedimentary characteristics of raised beach deposits are a potential archive of past wave climate as well as processes acting on beaches. In this study I examine changes in the grain-size, grain roundness, and spatial density of ice-rafted debris from two sets of raised beaches on opposite sides of the Antarctic Peninsula (AP): Joinville Island along the Eastern AP (EAP), and Livingston Island along the Western AP (WAP). All beaches were labeled starting at the closest proximity to the modern shoreline. Overall, the 9 beaches on Livingston Island are stratified with poorly sorted clasts compared to the better sorted 21 stratified lower beaches and 15 unstratified upper beaches on Joinville Island. The dissimilarity likely reflects the difference in foreshore gradient between the two islands. The Joinville profile is steeper, allowing waves to break on the coastline with high energy while the Livingston profile is shallower, enabling wave attenuation before reaching the shoreline. Grains on the raised beaches of Joinville Island show an overall increase in roundness through time while grain size shows low variability. However, the roundness trend is interrupted at beaches 5, 13-15.5, and 28. Beach 5 exhibits less and beach 28 exhibits more rounding than the general trend. Less rounding of sediments within beach 5 could be explained by short open water seasons with an increase in sea ice while the opposite could hold true for beach 28. The transition between beaches 13 and 15.5 indicates a decrease in roundness over time, opposite the overall roundness trend. The ages of beaches 15.5-13 (~2.8-2.3 cal. kyr BP) coincides with the onset of the Neoglacial time period ~3 cal. kyr BP. The presence of sea-ice or increased glacial activity could hinder clast rounding or introduce less rounded materials during cooler periods associated with this Neoglacial time period. Grains within Livingston Island beach ridges also show an overall increase in roundness through time but no coherent trends in grain size. However, Livingston Island contains two types of beach deposits: strand plains and beach ridges, the latter of which are interpreted as storm ridges. Strand plains were deposited by normal swash processes and exhibit sub-angular to sub-rounded sediments. Typically, storm ridge sediments would be less rounded, characteristic of high energy storm deposits, than the strand plain deposits, roundness and grain size are uncorrelated. However, the beach ridges contain sub-rounded to rounded deposits while the strand-plain deposits are sub-angular to sub-rounded. Ground penetrated radar profiles through the beach ridge crests suggest they bury older strand-plain deposits. Therefore, I suggest the more rounded nature of the beach ridge deposits on Livingston Island is due to the recycling of older strand-plain deposits by storms.