The ODP Leg 171B drilled a transect of four sites at Blake Nose in the NW Atlantic providing an excellent record of the K-T boundary. At the deepest, Site 1049, the boundary is marked by a 9–17 cm thick layer formed mostly of green spherules composed of Fe-rich smectite resulting from the diagenetic alteration of tektites and impact glasses. Minor amounts of authigenic zeolites and palygorskite also occur. This association represents a notable break in the clay mineral composition of Cretaceous and Tertiary sequences. The clay mineral assemblages of the Cretaceous and Tertiary sediments are dominated by inherited clays. Aluminium-rich smectite of pedogenic origin is abundant in both Cretaceous and Tertiary sediments, indicating a relatively warm and hydrolysing climate across the K-T boundary. At Hole 1049A, where the oldest sediments of the interval were analysed, an increase in kaolinite and smectite down core suggests tectonic rejuvenation and distal transport of illite and kaolinite, probably accompanied by more hydrolysing conditions during the late Maastrichtian.

The proposed impact event at the end of the Cretaceous resulted in mass extinctions and subsequently significant variations in the geochemical and mineralogical composition of the sediments marking the K/T boundary. The impact-generated material derived from target rocks produced the ejecta layer deposits around Chicxulub crater, which were subsequently diagenetically altered to mainly smectite in marine sections and to kaolinite in continental sections. The fireball layer represents the cosmic dust dispersed and deposited globally and contains smectite derived from the alteration of microkrystites and the finest fraction. The lowermost Danian clay layer, recognized in marine sections, resulted from the sudden decrease in ocean productivity and represents a reduced sedimentation deposit. Its clay mineral associations depend on local environmental conditions and diagenetic processes. Overall, the diagenetic alteration of the boundary materials resulted in a significant modification of original signatures. The composition of the clay mineral phases can, however, still be evidence of the nature of the precursor materials providing evidence for an extraterrestrial impact event.

The Chicxulub impact is commonly believed to have caused the Cretaceous–Tertiary boundary mass extinction and a thin impact spherule layer in the North Atlantic and Caribbean is frequently cited as proof. We evaluated this claim in the seven best North Atlantic and Caribbean Cretaceous–Tertiary boundary sequences based on high-resolution biostratigraphy, quantitative faunal analyses and stable isotopes. Results reveal a major Cretaceous–Tertiary boundary unconformity spanning most of Danian subzone P1a(1) and Maastrichtian zones CF1–CF2 (~400 ka) in the NW Atlantic Bass River core, ODP Sites 1049A, 1049C and 1050C. In the Caribbean ODP Sites 999B and 1001B the unconformity spans from the early Danian zone P1a(1) through to zones CF1–CF4 (~3 Ma). Only in the Demerara Rise ODP Site 1259B is erosion relatively minor and restricted to the earliest Danian zone P0 and most of subzone P1a(1) (~150 ka). In all sites examined, Chicxulub impact spherules are reworked into the early Danian subzone P1a(1) about 150–200 ka after the mass extinction. A similar pattern of erosion and redeposition of impact spherules in Danian sediments has previously been documented from Cuba, Haiti, Belize, Guatemala, south and central Mexico. This pattern can be explained by intensified Gulf stream circulation at times of climate cooling and sea level changes. The age of the Chicxulub impact cannot be determined from these reworked impact spherule layers, but can be evaluated based on the stratigraphically oldest spherule layer in NE Mexico and Texas, which indicates that this impact predates the Cretaceous–Tertiary boundary by about 130–150 ka.

In estimating the biotic effects of large terrestrial impacts we are reliant upon apparent crater diameter as a proxy for impact magnitude. This underlies the ‘kill-curve’ approach which graphs crater diameter directly against likely percentage losses of taxa. However, crater diameter is a complex product of syn- and post-impact processes that can be site-dependent. Furthermore, location (global positioning) and timing (moment in geological history) also strongly influence biotic effects. We examine four of our largest and best-documented Phanerozoic impacts to explore this more holistic size–time–place relationship. Only the c. 180 km end-Cretaceous Chicxulub crater (Mexico) links to any substantial immediate extinction and some of the worst effects stem from where it struck the planet (a continental margin carbonate platform site) and when (a time of high regional and global biodiversity). Both the c. 100 km late Triassic Manicouagan crater in NE Canada (arid continental interior, low regional and world biodiversity) and the c. 35 Ma 100 km Popigai crater, Siberia (continental arctic desert) provide much less damaging scenarios. However the c. 90 km Chesapeake Bay crater, Eastern USA (also c. 35 Ma) marks a far more sensitive (Chicxulub-like) site but it also proved relatively benign. Here the rheologically varied shallow marine target site produced an anomalously broad crater, and the scale of the impact has evidently been overestimated. We offer a new approach to the graphical prediction of biotic risk in which both crater diameter and a generalised time/place factor we term ‘vulnerability’ are variables.

High levels of an exotic amino acid (Aib) and enhanced levels of iridium are evident in sediments pre-dating the Chicxulub impact by several tens of millennia. The source is thought to be debris from the break-up of a giant comet or trans-Neptunian body, a large fragment of which was the 10 km sized impactor that caused the famous iridium spike identified with the K/T boundary. In this paper it is argued that the Aib is not extra-terrestrial but the indicator of exotic pathogenic microfungi that flourished through this era. Its abundance implies a significant role for the fungi in the ecology, in species extinction and in driving evolution as the Tertiary period got underway. Microfungi containing the complex of genes that underlie the synthesis of Aib peptides flourished early on in the K/T transition and attacked species – including dinosaurs – that lacked counter immune mechanisms. Species (including mammals) that possessed or developed effective defence mechanisms won through in the early Tertiary-period flowering of new species. The genetic coding for Aib peptide synthesis might have evolved by natural selection. However, the coincidence in the boundary record between Aib peptides and the rise of iridium is indicative of the Aib blueprint arriving from space, in some carrier-organism or in microfungi themselves.