Revision of the early-middle pleistocene calcareous nannofossil biochronology (1.75-0.85 Ma)

The extant nannofossil biostratigraphic and biochronologic framework for the early-middle Pleistocene time interval has been tested through the micropaleontological analysis of globally distributed high-quality low- to mid-latitude deep-sea successions. The quantitative temporal distribution patterns of relative abundances of selected taxa were reconstructed in critical intervals, and the following biohorizons were defined: First occurrence of medium-sized Gephyrocapsa spp. (bmG), last occurrence of Calcidiscus macintyrei (tCm); first occurrence of large Gephyrocapsa spp. (blG): last occurrence of large Gephyrocapsa spp, (tIG); first occurrence of Reticulofenestra asanoi (bRa); re-entrance of medium-sized Gephyrocapsa spp. (reemG) and last occurrence of Reticulofenestra asanoi (tRa). The detailed patterns of abundance change at these biohorizons were used to generate a detailed biostratigraphy, and the biostratigraphic data were transformed into a precise biochronology by means of correlation to isotope stratigraphies and astronomical timescales. The degree of isochrony or diachrony of the biohorizons was evaluated. Biohorizons tIG and tRa are isochronous occurring close to marine isotope stages (MIS) 55 and MIS 22, respectively, and bmG and blG are slightly diachronous on the order of 30-40 kyr, whereas biohorizons tCm, reemG and bRa are confirmed as diachronous on the order of 100, 80 and 60 kyr, respectively. Some of the events are clearly controlled by environmental conditions, e.g. the last occurrence of R. asanoi, related to significant environmental changes Associated with the first large-amplitude glaciation of the late Quaternary, MIS 22. (C) 2002 Elsevier Science B.V. All rights reserved.