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The Cretaceous record of the diatoms presented here outlines our understanding of their early morphological development. Recent documentation of well preserved Lower Cretaceous (Aptian/Albian) diatom assemblages provides a window into the early history of the diatoms and serves as a base for comparing their subsequent morphological changes. A brief review of diatom biology and morphology is provided to introduce this paper and that of Barron and Baldauf (this volume). Additional background information on the diatoms can be found in the works of Schrader and Schuette (1981), Tappan (1980), Bach and Burkhardt (1984), Barron (1985a, 1993), Fenner (1985), Ricard (1987), Bradbury (1988), Round et al. (1990), and Picket-Heaps et al. (1990).

New information and discussions regarding Mesozoic diatoms presented over the last decade advanced our knowledge of their origin and early history. The oldest confirmed centric diatom fossils are presented here from the earliest Cretaceous, and araphid and raphid pennate diatoms now date from the Late Cretaceous; all from terrestrial sediments. Molecular sequencing helped clarify relationships between diatom lineages, and verify the position of diatoms within the heterokontophytes. Molecular clock approaches estimate a diatom origin near ~135 Ma, but not before 240 Ma. Biomarkers in marine sediments are able to trace a diatom presence back to the mid-Cretaceous, even when siliceous fossils are absent. Seasonal growth and encystment cycles in Late Cretaceous planktonic marine diatoms are now well documented. A biostratigraphic framework for the Late Cretaceous Arctic will aid regional and global biostratigraphic correlations. The systematic position of many new taxonomic groups is now included within a more natural classification scheme that better reflects phylogenetic relationships evident in molecular data and affirmed by biostratigraphic micropaleontology. Discussions regarding the impact of diatoms on several global systems are maturing, as more information becomes available. Four stages in diatom evolution are proposed to explain the history of radiation, extinction, and expansion into new environments and habitats during the Mesozoic.

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A multi-scale approach to brittle fracture of irradiated RPV steels has been developed within the EURATOM FP6 Integrated Project PERFECT, and the EC-sponsored ISTC Project 3072: “Modelling of Brittle and Ductile Fracture and Prediction of Irradiation Damage Effects on Fracture Toughness Properties of Steels for Reactor Pressure Vessels on the Basis of Local Approach”. In the present paper, the physical and mechanical aspects of the irradiation embrittlement phenomenon are analysed on the basis of a multi-scale approach that includes the physical mechanisms of irradiation damage, the physical mechanisms of nucleation and propagation of cleavage microcracks, formulation of a local approach criterion and calculation of fracture toughness. It has become clear from the study performed that irradiation embrittlement is not only due to the hardening of material as traditionally considered. Damage features such as irradiation-induced lattice defects, element precipitation and impurity segregation also affect cleavage microcrack nucleation, although they do not appreciably change the critical stress for microcrack propagation. It is shown that irradiation embrittlement of RPV steels is connected both with the increase of yield stress (mechanical factor) and the decrease of the critical stress for microcrack nucleation (physical factor). This finding allows an interpretation of the experimental observations, showing that for some cases there is no simple one-to-one relationship between the brittle-to-ductile transition temperature shift ΔTtr and the corresponding increase in yield stress ΔσY. For example, this explains why irradiation-induced phosphorus segregation may significantly increase the transition temperature shift ΔTtr but does not result in a significant increase in yield stress. Calculation of the KJC(T) curves has shown that the contributions of mechanical and physical factors in irradiation embrittlement may differ for various steels. So, for low-alloy, low-strength steel the mechanical factor predominates, while for RPV steels both of these factors control irradiation embrittlement. The results obtained have been interpreted with reference to a local cleavage fracture criterion.