Sedimentary diatom assemblages of Prydz Bay and Mac.Robertson Shelf, East Antarctica, and their use as palaeoecological indicators

• Main Author: Taylor, F
• Format: Thesis
• Language: English
• Published: 1999
• Subjects: Diatoms; Paleoecology; Antarctic; Antarctic Peninsula; East Antarctica; Prydz Bay; Ross Sea; Weddell; Weddell Sea; West Antarctica; Antarc*; Antarctica; Iceberg*; Sea ice
• Online Access: https://eprints.utas.edu.au/22030/; https://eprints.utas.edu.au/22030/7/Taylor_whole_thesis_ex_pub_mat.pdf; https://eprints.utas.edu.au/22030/1/whole_TaylorFiona1999_thesis.pdf

The use of diatoms as palaeoecological indicators is well established, particularly in polar marine and lake environments where the frustules are well preserved in, and often dominate, the sedimentary record. Until recently, marine studies have been concentrated primarily in West Antarctica, namely the Ross Sea, Weddell Sea and Antarctic Peninsula. In comparison, relatively little research has been conducted in East Antarctica, which is the focus of this study. Here, diatom assemblages preserved in over 100 surface sediment samples, collected from Prydz Bay and Mac.Robertson Shelf, have been analysed for diatom distribution and abundance. They are compared to the assemblages preserved in six sedimentary cores from the same area, as a means of reconstructing the natural variability of Holocene palaeoclimates on the East Antarctic continental shelf. The distribution and composition of four diatom assemblages are identified in the surface sediments of Prydz Bay and Mac.Robertson Shelf using multivariate statistical analyses. Multiple regression is used to identify the relationship between these assemblages and known environmental variables: The "coastal" assemblage; characterised by sea-ice species, is present in near-shore and shallow regions where seasonal sea ice breakout does not regularly occur. In the centre of the bay, and extending west along Mac.Robertson Shelf, the "shelf" diatom assemblage is characterised by sea-ice and ice-edge diatoms. Offshore of the continental shelf break zone, the "oceanic" assemblage is characterised by open water species. This assemblage also extends along the eastern margin of Prydz Bay, and it is suggested that they have been transported onshore by the slowly circulating, cyclonic Prydz Bay gyre. The fourth diatom assemblage contains both sea-ice and open water species. All are characteristically large and heavily silicified, and the assemblage is interpreted to represent one from which the smaller and more fragile diatom frustules have been removed by current winnowing. The diatom assemblages in cores from Prydz Bay and Mac.Robertson Shelf are analysed and compared to the surface assemblages using the same multivariate methods. Down core assemblages that are analogous to those in the surface sediment indicate periods during which the depositional regime and palaeocological conditions on the continental shelf were similar to that of those today. These assemblages dominate the cores, providing a useful indicator of sea ice concentration and extent since the Late Pleistocene. It is also suggested that they indicate changes in both the geographic position of the Prydz Bay gyre and strength in water currents exiting the bay. The presence of down core assemblages that have no modern analogue indicate periods of deposition that were significantly different from today. This is most evident in the depositional basins of Iceberg Alley and Nielsen Basin, on Mac.Robertson Shelf, where laminated core intervals are characterised by Chaetoceros resting spores or Corethron criophilum. Compared to similar assemblages observed in West Antarctica, the intervals suggest that, the during the Holocene, parts of the East Antarctic continental shelf experienced periods of decreased sea ice concentration and increased primary production. Chaetoceros resting spores may also be a useful proxy to record changes in the position of maximum summer ice retreat.