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

Abstract

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 and Discoaster 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: LAD Helicosphaera sellii, 1.37 Ma (diachronous; earlier outside the equatorial zone); LAD Calcidiscus macintyrei, 1.45 Ma; LAD Discoaster brouweri, 1.88 Ma; LAD Discoaster asymmetricus, 2.2 Ma; LAD DisDiscoaster pentaradiatus, 2.35 Ma; LAD Discoaster surculus, 2.41 Ma; LAD Discoaster tamalis, 2.65 Ma; LAD Discoaster variabilis, 2.90 Ma; LAD Sphenolithus spp., 3.45 Ma; LAD Reticulofenestra pseudoumbilica, 3.56 Ma; LAD Amaurolithus delicatus, 3.66 Ma; FAD Ceratolithus rugosus, 4.62 Ma. The final 0.15 m.y. of the range of Discoaster 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.