DIATOM DISTRIBUTION IN THE WEDDELL GYRE REGION DURING LATE WINTER

Diatoms from the water column and sea-ice were analyzed in a data set collected from the Soviet icebreaker Mikhail Somov in late austral winter (October-November) 1981 in the Weddell Gyre region. Two major diatom assemblages are apparent in teh water column and have distribution patterns that are influenced by surface and near-surface water hydrography (0-100 m). An assemblage dominated by Nitzschia kerguelensis is associated with typical Weddell Deep Water (WDW), while most warm-water cells within the WDW are associated with an assemblage dominated by Thalassiothrix antarctica. Sea-ice cores usually contain a surface assemblage dominated by Cylindrotheca closterium, a neritic form underlain by N. kerguelensis- and T. antarctica-dominated asemblages. The diatom stratigraphy in these cores and the nature of the assemblages suggests that the surface form, C. closterium, has grown in place while the N. kerguelensis- and T. antarctica-dominated assemblages were frozen into the ice. It is hypotehsized that, as the winter progressed, these latter two assemblages were sequentially frozen into sea-ice, thus leaving a record of gross oceanographic changes that occurred in surface waters beneath the ice during the austral winter.