Jan Backman

Quantitative biochronology of Pliocene and early Pleistocene calcareous nannofossils from the Atlantic, Indian and Pacific oceans

Semi-quantitative methods have been used to refine the precision with which Pliocene and early Pleistocene nannofossil datums may be applied for biostratigraphic purposes. Using these methods, the datums may be applied with a precision of between 0.01 m.y. (e.g.Calcidiscus macintyrei andDiscoaster brouweri) and 0.2 m.y. (e.g.Discoaster asymmetricus). The age of each datum is estimated by interpolation between magnetic reversals, so that the uncertainty in each age estimate is a function of proximity to the nearest reversal as well as of uncertainties in the reversal chronology but is probably better than 0.1 m.y. for all the datums studied here. The following ages are estimated for biostratigraphically useful datums: LADHelicosphaera sellii, 1.37 Ma (diachronous; earlier outside the equatorial zone); LADCalcidiscus macintyrei, 1.45 Ma; LADDiscoaster brouweri, 1.88 Ma; LADDiscoaster asymmetricus, 2.2 Ma; LAD DisDiscoaster pentaradiatus, 2.35 Ma; LADDiscoaster surculus, 2.41 Ma; LADDiscoaster tamalis, 2.65 Ma; LADDiscoaster variabilis, 2.90 Ma; LADSphenolithus spp., 3.45 Ma; LADReticulofenestra pseudoumbilica, 3.56 Ma; LADAmaurolithus delicatus, 3.66 Ma; FADCeratolithus rugosus, 4.62 Ma. The final 0.15 m.y. of the range ofDiscoaster brouweri is characterised by a high (about 20%) proportion of the triradiate form, which is a useful pointer for the extinction of this species even in the presence of considerable reworking.

Eocene bipolar glaciation associated with global carbon cycle changes

The transition from the extreme global warmth of the early Eocene ‘greenhouse’ climate ∼55 million years ago to the present glaciated state is one of the most prominent changes in Earths climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica ∼34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the worlds oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse–icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from ∼42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to ∼1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse–icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.

Late Paleocene to middle Eocene calcareous nannofossil biochronology from the Shatsky Rise, Walvis Ridge and Italy

Abstract The precision of late Paleocene to middle Eocene nannofossil datums is investigated by means of quantitative methods and correlated to the magnetic polarity stratigraphy, using sequences from the Northwest Pacific, Southeast Atlantic and Italy. It is the rule rather than the exception to find tails of very reduced abundances prior to, or after, a range of consistent and higher abundances. The absolutely first or final occurrence of a species, therefore, seldom provides a synchronous datum when material from different geographic areas are compared. On the other hand, synchroneity is often confirmed when the initial sharp rise or the final sharp decline in abundance is used as datum level. The use of datums not employed in the two principal existing nannofossil zonal schemes can substantially improve the biostratigraphic resolution. Two established zonal markers show abundance patterns making them unsuitable as datums: the first occurrences of Ellipsolithus macellus (base NP4, diachronous) and Tribrachiatus nunnii (base NP10 and Paleocene/Eocene boundary, too rare and too short range in open ocean sections). The first occurrence of either Fasciculithus spp. or Sphenolithus spp. is a better marker near the base of NP4. The first occurrence of Discoaster diastypus at 56.6 Ma represents a suitable replacement for recognition of the Paleocene/Eocene boundary.