Analysis of diatom assemblages in sediments from the Amundsen Sea, Antarctica

Abstract

The Amundsen Sea Polynyas, located off the western Antarctica Peninsula is the most productive region (per unit area) in the Antarctic. Here, we present temporal variabilities in diatom assemblages and geochemical properties in a sediment core ANA08B-33 in order to examine their capability to record oceanographic changes in the western Antarctic polynya environments. A 38-cm long box core was recovered from the Amundsen Sea polynya during the ANA08B cruise on the Icebreaker ARAON in 2019 (73°16'46.26''S-114°56'59.64''W, water depth: 827m). Overall, the siliceous microfossil assemblages in core ANA08B-33 consisted mainly of diatoms (97%), followed by silicoflagellates (2%), with a minor contribution and sponge spicules and radiolarians. The diatom concentrations (valves g-1) and opal contents (%) show a strong positive relationship (R2=0.74, n=12, p<0.01). The diatom assemblages can be classified into five ecological groups: nutrient-enriched water assemblage (Group A), open water assemblage (Group B) and sea-ice related assemblage (Group C), with a minor contribution (< 1%) of marine benthic assemblage (Group D) and stratified water assemblage (Group E). The Group C accounts for 72% (Fragilariopsis curta, F. cylindrus and Thalassiosira antarctica (T1, morphotype of cold waters)), followed by the Group A (Chaetoceros resting spores, 19%) and Group B (F. kerguelensis and Thalassiosira tumida, 8%). A particular note is for a significant transition in the marine sea-ice proxy (F. curta + F. cylindrus/F. kerguelensis) characterized by lower values below 24cm (before ca. 1850 years BP) and a two-fold higher values above 20cm (after ca. 1540 years BP), corresponding well with geochemical proxy variations. These results suggest an apparent oceanographic change in the Amundsen Sea, tentatively associated with extension of polynya environments forced by changes in an ocean current regime (e.g. the Circumpolar Deep Water).