ABSTRACT More than four decades have passed since Walter Alvarez helped to bring mass extinctions to the attention of a broad audience and inspired extensive multidisciplinary research on a wide variety of topics ranging from the Cretaceous/Paleogene (K/Pg) and other impact events to astronomy, climate modeling, and the centuries-long debate on the extent to which apparent extinctions are a real phenomenon or due to incompleteness of the fossil record. Many questions about ecosystems in the aftermath of extinctions remain, and we summarize knowledge about an integral part of this discussion, i.e., oceanic productivity after the K/Pg mass extinction. We compiled new and published benthic foraminiferal data across the K/Pg boundary globally, at geographically and bathymetrically diverse sites, to contribute to the understanding of environmental consequences of the K/Pg impact through analysis of extinction patterns in Earth’s largest habitat: the deep seafloor. We find no significant links between the severity of extinction of benthic foraminiferal species or their global decrease in diversity and factors such as the distance from the Chicxulub crater, paleo-water depth, and paleolatitude. Benthic foraminiferal populations show strong post-impact variability in space and time, supporting the hypothesis of heterogeneous oceans with extensive, local-to-regional plankton blooms, but we suggest that the apparent geographic variability may at least in part be due to incompleteness of the geological record at high time resolution. Additional high-resolution studies are necessary to enable us to evaluate the rates of past extinctions and compare these to the rates of present and future extinctions.

ABSTRACT Along with the origin of life, the quest for the ultimate cause of the end of the dinosaurs and ~72% of other species is one of the most publicized questions in the history of our planet. So, it probably should not have come as a surprise that when Walter Alvarez and his team launched the impact-extinction theory, the opposition and the resistance against the theory was strong from the beginning and continues up to the present day. This paper follows the winding road around the roadblocks that were set up against the theory and how both the opposition against and accumulation of new data, e.g., the finding of the Chicxulub impact structure and extraterrestrial Cr isotope ratios to further develop the theory, went hand in hand. Often the roadblocks were overcome, but new ones were set up, and in the struggle to surmount these, the proponents were forced to look back on their arguments, to carefully re-formulate their viewpoints, and to check whether tunnel-vision had developed that might prevent seeing the data available in a different light. However, looking back on the competition among proponents and opponents 40 years later, the impact-extinction theory is stronger than ever before. It has survived and matured from a hypothesis into a well-established theory, although many questions remain to be solved.

Abstract The Paleocene/Eocene Thermal Maximum (PETM) is characterized by pronounced global warming and associated environmental changes. In the more-or-less two decades since prior regional syntheses of Apectodinium distribution at the PETM, extensive biological and geochemical datasets have elucidated the effect of rising world temperatures on climate and the biome. A Carbon Isotope Excursion (CIE) that marks the Paleocene/Eocene Boundary is associated with an acme of marine dinocysts of the genus Apectodinium in many locations. Distinctive foraminiferal and calcareous nannofossil populations may also be present. For this updated, dinocyst-oriented view of the PETM, data from worldwide locations have been evaluated with an emphasis on stratigraphic and sedimentological context. What has emerged is that a change in lithology is common, often to a distinctive siltstone or claystone unit, which contrasts with underlying and overlying lithotypes. This change, present in shallow marine/coastal settings and in deep-water turbidite deposits, is attributed to radical modifications of precipitation and erosional processes. An abrupt boundary carries the implication that some time (of unknowable duration) is potentially missing, which then requires caution in the interpretation of the pacing of events in relation to that boundary. In most instances an ‘abrupt’ or ‘rapid’ CIE onset can be attributed to a data gap at a hiatus, particularly in shallow shelf settings where transgression resulted from sea-level rise associated with the PETM. Truly gradational lower boundaries of the PETM interval are quite unusual and, if present, are poorly known so far. Gradational upper boundaries are more common, but erosional upper boundaries have been reported. Taxonomic changes have been made to clarify identification issues that have adversely impacted some biostratigraphic interpretations. Apectodinium hyperacanthum has been retained in Wetzeliella , its original genus. The majority of specimens previously assigned to Apectodinium hyperacanthum or Wetzeliella ( Apectodinium ) hyperacanthum have been reassigned to an informal species, Apectodinium sp. 1. Dracodinium astra has been retained in its original genus as Wetzeliella astra and is emended.

ABSTRACT Quantitative insights into the geochemistry and petrology of proximal impactites are fundamental to understand the complex processes that affected target lithologies during and after hypervelocity impact events. Traditional analytical techniques used to obtain major- and trace-element data sets focus predominantly on either destructive whole-rock analysis or laboratory-intensive phase-specific micro-analysis. Here, we present micro–X-ray fluorescence (µXRF) as a state-of-the-art, time-efficient, and nondestructive alternative for major- and trace-element analysis for both small and large samples (up to 20 cm wide) of proximal impactites. We applied µXRF element mapping on 44 samples from the Chicxulub, Popigai, and Ries impact structures, including impact breccias, impact melt rocks, and shocked target lithologies. The µXRF mapping required limited to no sample preparation and rapidly generated high-resolution major- and trace-element maps (~1 h for 8 cm 2 , with a spatial resolution of 25 µm). These chemical distribution maps can be used as qualitative multi-element maps, as semiquantitative single-element heat maps, and as a basis for a novel image analysis workflow quantifying the modal abundance, size, shape, and degree of sorting of segmented components. The standardless fundamental parameters method was used to quantify the µXRF maps, and the results were compared with bulk powder techniques. Concentrations of most major elements (Na 2 O–CaO) were found to be accurate within 10% for thick sections. Overall, we demonstrate that µXRF is more than only a screening tool for heterogeneous impactites, because it rapidly produces bulk and phase-specific geochemical data sets that are suitable for various applications within the earth sciences.

ABSTRACT A combined petrographic and chemical study of ejecta particles from the Cretaceous-Paleogene boundary sequence of El Guayal, Tabasco, Mexico (520 km SW of Chicxulub crater), was carried out to assess their formation conditions and genetic relation during the impact process. The reaction of silicate ejecta particles with hot volatiles during atmospheric transport may have induced alteration processes, e.g., silicification and cementation, observed in the ejecta deposits. The various microstructures of calcite ejecta particles are interpreted to reflect different thermal histories at postshock conditions. Spherulitic calcite particles may represent carbonate melts that were quenched during ejection. A recrystallized microstructure may indicate short, intense thermal stress. Various aggregates document particle-particle interactions and intermixing of components from lower silicate and upper sedimentary target lithologies. Aggregates of recrystallized calcite with silicate melt indicate the consolidation of a hot suevitic component with sediments at ≳750 °C. Accretionary lapilli formed in a turbulent, steam-condensing environment at ~100 °C by aggregation of solid, ash-sized particles. Concentric zones with smaller grain sizes of accreted particles indicate a recurring exchange with a hotter environment. Our results suggest that during partial ejecta plume collapse, hot silicate components were mixed with the fine fraction of local surface-derived sediments, the latter of which were displaced by the preceding ejecta curtain. These processes sustained a hot, gas-driven, lateral basal transport that was accompanied by a turbulent plume at a higher level. The exothermic back-reaction of CaO from decomposed carbonates and sulfates with CO 2 to form CaCO 3 may have been responsible for a prolonged release of thermal energy at a late stage of plume evolution.

ABSTRACT During hypervelocity impacts, target rocks are subjected to shock wave compression with high pressures and differential stresses. These differential stresses cause microscopic shear-induced deformation, which can be observed in the form of kinking, twinning, fracturing, and shear faulting in a range of minerals. The orientation of these shear-induced deformation features can be used to constrain the maximum shortening axis. Under the assumption of pure shear deformation, the maximum shortening axis is parallel to the maximum principal axis of stress, σ 1 , which gives the propagation direction of the shock wave that passed through a rock sample. In this study, shocked granitoids cored from the uppermost peak ring of the Chicxulub crater (International Ocean Discovery Program [IODP]/International Continental Drilling Project [ICDP] Expedition 364) were examined for structures formed by shearing. Orientations of kink planes in biotite and basal planar deformation features (PDFs) in quartz were measured with a U-stage and compared to a previous study of feather feature orientations in quartz from the same samples. In all three cases, the orientations of the shortening axis derived from these measurements were in good agreement with each other, indicating that the shear deformation features all formed in an environment with similar orientations of the maximum principal axis of stress. These structures formed by shearing are useful tools that can aid in understanding the deformational effects of the shock wave, as well as constraining shock wave propagation and postshock deformation during the cratering process.