The topography of sub-Antarctic Marion Island during the Last Glacial Maximum: evidence from bathymetry data

Abstract

Sub-Antarctic Marion Island (46°54'5 37°45'E) is a young (0.45 Ma) volcanic island [1]. Up to eight volcanic, and five glacial episodes, have been inferred from K-Ar dating of striated outcrops, till, fluvioglacial deposits and glaciogenic deposits [2]. However, an evaluation of glacial extent during the Last Glacial Maximum remains speculative [3, 4]. Sea levels during the Last Glacial Maximum (circa 21 000 B.P.) were depressed by approximately 125 m [5] and, thus, the glacial margins and associated sedimentary evidence extends beyond the current coastal margins. In addition, the now submerged topography of Marion Island below 120 m during the Last Glacial Maximum has been poorly mapped. Integration of data from General Bathymetric Chart of the Oceans (GEBCO), South African Naval bathymetry data and echo sounder data (120kHz) collected on the SA Agulhas II (2013-2016) has, for the first time, enabled the mapping of sea floor around the Prince Edward Islands. Preliminary delineation of the extent of the Prince Edward islands during the Last Glacial Maximum indicates that the subaerial extent of the Marion and Prince Edward Islands were approximately 430.5 km2 and 94.3 km2, respectively. Interpolation and three-dimensional modelling of the bathymetry data depicts a shallow narrow bench surrounding Marion Island at a depth of roughly 40 m, particularly on the western coastline. The data also depicts a relatively flat submarine plateau at a depth of 80-120 m which extends approximately 2 500 m offshore from Repettos Hill in the north to Crawford Bay in the south-east. The bathymetry data not only changes ones spatial perspective of Marion Island during the Last Glacial Maximum but, more importantly, may indicate the geographic locations of offshore evidence of glaciations. In addition, the potential of the bathymetry data makes a case for the continuation of data collection but also emphasises the need for better resolution data and highlights opportunities for researchers from various disciplines to work together.