| A new age model for the upper Miocene to Pliocene sedimentary section from Ocean Drilling Program (ODP) Hole 695A enables extrapolation of absolute ages of diatom datums and provides better understanding of paleoenvironmental conditions in the Weddell Sea. Four hiatuses in the sedimentary record, spanning <5.8 to >5.1 Ma, <4.9 to >4.8 Ma, <4.2 to 1 > 3.8 Ma, and 3.0 Ma to recent, are present. Sediment accumulation rates of 400 m/m.y. between 6.4 to 4.9 Ma, and 166 between 4.8 to 3.0 Ma suggest the presence of a poly-thermal ice sheet in Antarctica during the early Pliocene. Peaks in abundance of Thalassionema nitzschioides and T. nitzschioides var. parva reflect fluctuations in the strength of latitudinal thermal gradients between the Antarctic and Subantarctic sectors of the Weddell Sea, and are correlated to intervals of climatic warming noted in the Kerguelen Plateau region of the Southern Ocean. The first occurrence of Southern Ocean endemic diatom taxon Fragilariopsis kerguelensis and late Pliocene hiatuses reflect increased thermal isolation of the Southern Ocean, cold-polar ice sheets on Antarctica, and increased bottom water erosional activity after ∼3.25 Ma. This sedimentary section, assigned to the Fragilariopsis reinholdii through Thalassiosira orbiculata-Thalassiosira vulnifica diatom biostratigraphic zones, contain amongst other diatom taxa, Cyclomedius maccollumii n. gen., n. comb., Fragilariopsis acutalis n. sp., Fragilariopsis extensa n. sp., Fragilariopsis serpentina n. sp., Fragilariopsis pingua n. sp., Synedropsis rostrata n. sp., Synedropsis bicapitata n. sp., Synedropsis capula n. sp., Denticulopsis rhombiformis n. sp., and Rouxia quartera n. sp. Renaming of diatom taxa Thalassiosira insigna sensu Harwood and Maruyama as Thalassiosira orbiculata n. sp. and Actinocyclus fasciculatus Harwood and Maruyama as Actinocyclus bohatyi n. sp., elevation of Thalassiosira oliverana var. sparsa to species level as Thalassiosira sparsa n. stat., and amendment of the morphological description of Thalassiosira striata clarifies their respective systematic positions. Additionally, Thalassiosira sparsa, Thalassiosira fasciculata, and Thalassiosira tumida groups, defined herein, provide insight into the evolutionary history of the genus Thalassiosira. |
