Early to middle Miocene radiolarian stratigraphy of the Kerguelen Plateau, Southern Ocean

Early to middle Miocene radiolarian assemblages were examined at three sites (747, 748, and 751) that were cored during Ocean Drilling Program Leg 120 south of the present polar frontal zone on the Kerguelen Plateau (Indian sector of the Southern Ocean). The radiolarian biostratigraphic study relies on a radiolarian zonation recently developed on Leg 113 materials in the Atlantic sector of the Southern Ocean, which is correlated with the geomagnetic time scale. New radiolarian biostratigraphic data also considering the established geomagnetic polarity record were used to improve and emend the age calibration of some lower Miocene radiolarian zones and a redefined middle Miocene radiolarian zonation is proposed. Based on these results, a revised age assignment of the lower Miocene sections drilled at Leg 113 Sites 689 and 690 is proposed.

Late middle Eocene to late Oligocene radiolarian biostratigraphy of ODP Hole 113-689B of Maud Rise, Southern Ocean

We propose a new biostratigraphic scheme comprising the Eucyrtidium spinosum, Eucyrtidium antiquum (new), Lychnocanoma conica (emended), Clinorhabdus robusta (emended) and Stylosphaera radiosa (emended) Zones, in ascending order, in Eocene to Oligocene sediments drilled on Maud Rise in Southern Atlantic Ocean (Site 689, Ocean Drilling Program Leg 113). The bases of these zones are defined by the lowermost occurrences of E. spinosum, E. antiquum, L. conica, C. robusta and the uppermost occurrence of Axoprunum irregularis (?), respectively. From correlation to the magnetostratigraphic data, the E. spinosum, E. antiquum, L. conica, C. robusta and S. radiosa Zones are assigned to the late middle Eocene through late Eocene (Subchrons C17n2 to C13r), earliest Oligocene (C13n to C11n), late early Oligocene (C11n to C10n2), early late Oligocene (C10n1 to C8r) and latest Oligocene (C8r to C7An), respectively. The four boundary datum levels and supplementary datum levels such as the lowermost occurrences of A. irregularis (?), Dicolocapsa microcephala and Lithomelissa challengerae may be recognized in other ODP sites in the Southern Ocean. The first occurrence of E. antiquum approximates the Eocene-Oligocene boundary in Southern Ocean but the last occurrences of many species such as Periphaena decora, D. microcephala and the Lithomelissa sphaerocephalis group are commonly diachronous between high latitude sites. Two new species, Theocyrtis (?) triapenna and Spirocyrtis parvaturris, are described.

Benthic foraminifera across the Cretaceous/Paleogene boundary in the Southern Ocean

The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, biogeochemical and sedimentological changes in the oceans that have been intensively studied. Paleo-biogeographical differences in the biotic response to the impact and its environmental consequences, however, have been less well documented. We present a high-resolution analysis of benthic foraminiferal assemblages at Southern Ocean ODP Site 690 (Maud Rise, Weddell Sea, Antarctica). At this high latitude site, late Maastrichtian environmental variability was high, but benthic foraminiferal assemblages were not less diverse than at lower latitudes, in contrast to those of planktic calcifiers. Also in contrast to planktic calcifiers, benthic foraminifera did not suffer significant extinction at the K/Pg boundary, but show transient assemblage changes and decreased diversity. At Site 690, the extinction rate was even lower (~3%) than at other sites. The benthic foraminiferal accumulation rate varied little across the K/Pg boundary, indicating that food supply to the sea floor was affected to a lesser extent than at lower latitude sites. Compared to Maastrichtian assemblages, Danian assemblages have a lower diversity and greater relative abundance of heavily calcified taxa such as Stensioeina beccariiformis and Paralabamina lunata. This change in benthic foraminiferal assemblages could reflect post-extinction proliferation of different photosynthesizers (thus food for the benthos) than those dominant during the Late Cretaceous, therefore changes in the nature rather than in the amount of the organic matter supplied to the seafloor. However, severe extinction of pelagic calcifiers caused carbonate supersaturation in the oceans, thus might have given competitive advantage to species with large, heavily calcified tests. This indirect effect of the K/Pg impact thus may have influenced the deep-sea dwellers, documenting the complexity of the effects of major environmental disturbance.