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Size measurements of the calcareous nannofossil taxon Discoaster multiradiatus were carried out across the Paleocene–Eocene Thermal Maximum (PETM) in Ocean Drilling Program Holes 690B (Maud Rise, Weddell Sea) and 1209B (Shatsky Rise, Pacific Ocean). Morphometric investigations show that D. multiradiatus specimens are generally larger at ODP Site 1209 than at ODP Site 690. A limited increase in size of D. multiradiatus is recorded at ODP Site 1209, whereas significant enlargements characterize ODP Site 690. Preservation is comparable at both sites: nannofossils are moderately preserved with some evidence of etching/overgrowth in the PETM interval. Yet, D. multiradiatus variations do not correlate with preservation state and morphometric data most likely represent primary signals rather than diagenetic artifacts. There is a direct relationship between D. multiradiatus size and paleotemperatures: largest specimens are coeval with global warming associated with the PETM, inferred to result from excess atmospheric CO2 due to (partial) oxidation of massive quantities of methane. Size increases and largest specimens of D. multiradiatus occur at different stratigraphic levels within PETM at ODP Sites 690 and 1209. A marked shift in diameter size was observed at the onset and peak of the Carbon Isotopic Excursion (CIE) at ODP Site 690, but only at the end of CIE and initial recovery interval at ODP Site 1209. This diachroneity is puzzling, but indeed correlates well with reconstructed changes in surface and thermocline water masses temperature and salinity in the PETM interval at low and high latitudes. The presumed high concentrations of carbon dioxide seem to have not influenced the morphometry of D. multiradiatus. The major size increase of D. multiradiatus in the CIE of ODP Site 690 could represent the migration of larger-sized allochtonus specimens that moved from peri-equatorial/subtropical areas to higher latitudes during the warmest interval of the PETM, although no direct evidence of distinct populations/subpopulations has been obtained from the frequency diagrams. As a result, we infer that D. multiradiatus is a proxy of water masses stratification and might be used for deriving temperature–salinity–nutrient conditions in the mixed layer and thermocline and their dynamics.

The Paleocene–Eocene Thermal Maximum (PETM) was an abrupt global warming event at f55 Ma. This event had profound effects on the biosphere. Quantitative investigations of calcareous nannofossil assemblages were performed across the PETM, at Deep Sea Drilling Project Site 213, in the tropical Indian Ocean, and Site 401, in the Bay of Biscay, North Atlantic Ocean, at a temperate latitude. Calcareous nannofossil assemblages from both sites show major changes before, during, and after the PETM. Fluctuations at Site 213 are affected by selective dissolution of the least robust taxa. By contrast, assemblages in the lower part of the PETM at Site 401 show less diagenetic alteration and display high abundances of genera such as Discoaster, Fasciculithus, and Ericsonia. These taxa reflect relatively warm and, probably, oligotrophic surface waters. Abundances of the genus Chiasmolithus, an indicator of relatively cool and eutrophic waters, decreased dramatically. The upper portion of the PETM is characterized by an increase in the abundance of Toweius spp. and Zygrhablithus bijugatus, interpreted as a return to cooler and more eutrophic conditions. The elimination of Fasciculithus was probably due to competition with the holococcolith species, Z. bijugatus, which occupied the same ecological niche. The occurrences of long-armed and asymmetrical Discoaster species such as Discoaster araneus, Discoaster anartios, and Discoaster okadai, and Rhomboaster species, such as Rhomboaster calcitrapa and Rhomboaster cuspis, are characteristic of the PETM interval. D 2004 Elsevier B.V. All rights reserved.

Morphometric analyses of Aptian calcareous nannofossils, revealed the occurrence of differently sized specimens of Assipetra infracretacea and Rucinolithus terebrodentarius . The study was conducted on the Cismon core in the Tethys and DSDP Site 463 in the Pacific Ocean with good bio-, magneto- and chemostratigraphic control. Investigation was extended to selected samples from other low-latitude sections/cores to constrain the spatial and temporal distribution of the differently sized morphotypes. Normal- and large-sized specimens display the same ultrastructure, but are clearly separated when maximum diameter is measured. No transitional forms were observed. The large-sized specimens of Assipetra infracretacea and Rucinolithus terebrodentarius are here defined as new subspecies A. infracretacea larsonii and R. terebrodentarius youngii , respectively. Both taxa have FO close to the Barremian/Aptian boundary and LO close to the Aptian/Albian boundary in all investigated sections. They increase in abundance during the Early Aptian with maximum values during OAE 1a and then gradually decrease during the late Aptian. The distribution and abundance of A. infracretacea larsonii and R. terebrodentarius youngii are coeval with major Aptian igneous, geological, geochemical and biotic changes. These nannoliths might, therefore, result from nutrification events and/or physico-chemical reorganization of the mid-Cretaceous ocean.