Changing Oligocene climate recorded by palynomorphs from two glacio-eustatic sedimentary cycles, Cape Roberts Project, Victoria Land Basin, Antarctica
Abstract Two marine palynomorph assemblages recovered from two glacio-eustatic sedimentary cycles from the Cape Roberts Project (CRP), which recovered over 1500 m of Oligocene and Miocene strata from the western margin of the Victoria Land Basin, are investigated. We present results from one cycle dated as early Oligocene and the other dated as late Oligocene. Modest numbers of marine palynomorphs were recovered, with an average abundance of 56 specimens per gram. The assemblages recovered from both cycles are broadly similar, and dominated by acritarchs and the phycoma of prasinophyte algae. These include Leiosphaeridia sp. 2, Leiosphaeridia sp. 3, and the prasinophyte alga, Cymatiosphaera sp. 2. In detail, however, the lower Oligocene assemblage is different from that of the upper Oligocene. The lower Oligocene cycle includes Impagidinium cf. dispertium and Impagidinium cf. elegans. Cymatiosphaera sp. 6 and Pyxidinopsis sp. are abundant. The upper Oligocene cycle is characterized by an absence of Cymatiosphaera (?) invaganata, Cymatiosphaera sp. 8, and Pterospermella sp. A. It has very low numbers of Acritarch sp. b, Impagidinium and Pyxidinopsis. It contains Tasmanites sp. and Brigantedinium pynei, and it has higher abundances of Lejeunecysta sp. 1 and Sigmopollis than the lower Oligocene cycle. The environmental interpretations of the CRP core from other workers using sedimentological and paleontological indicators are used to infer environmental conditions that are likely to have influenced the marine palynomorph assemblages. Paleontological and sedimentological analysis suggests that both cycles were deposited in cold conditions, with sea ice and melt water influence, although the upper Oligocene assemblage is inferred to have existed in colder marine conditions, with more prolonged sea ice cover and less associated fresh water input than the assemblage of early Oligocene times. The application of modern analogues for some of the marine palynomorph taxa identified suggest the presence of sea ice, and occasionally lowered salinity in both cycles. This is broadly consistent with the findings of the other paleoenvironmental studies on the CRP cores.