The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental United States. The related bolide struck from the southeast at a low angle, potentially distributing ejecta downrange to the northwest across the Western Interior Cretaceous Seaway. Here, we (1) examine possible correlation of Manson impact horizons across the Cretaceous seaway to terrestrial formations of Montana, and (2) test a large hadrosaur bone bed from the Two Medicine Formation for evidence indicative of the Manson impact. The study includes geochronology; palynomorph, soot, and geochemical analyses; and physical searches for impact ejecta. The impact ejecta–bearing Crow Creek Member of the marine Pierre Shale can be correlated to the SB2 discontinuity in the Judith River and Two Medicine Formations of Montana based on radiometric dates, ammonite zonation, and an association with the onset of the Bearpaw transgression. A 40 Ar/ 39 Ar analysis of an associated bentonite bed dates the hadrosaur bone bed (TM-003) to 75.92 ± 0.32 Ma referenced to MMhb-1 at 523.1 Ma. This bentonite and associated lacustrine units suggest a potential correlation with the SB2 and the Crow Creek Member. However, our examination of the bone bed produced no definitive impact evidence. The combined analyses did reveal three unusual aspects: (1) an abundance of Ulmoideipites sp., (2) a high soot content, and (3) elemental and mineralogical changes suggestive of distinct geochemical units. A major wildfire followed by a postcatastrophe bloom dominated by Ulmoideipites sp. likely preceded the eventual debris flow that generated the bone bed. The SB2 discontinuity and the 33n.3r magnetic subzone represent traceable stratigraphic markers that could serve as guides in future exploration for Manson impact evidence in terrestrial formations west of the seaway.

Organic-rich shales of Late Jurassic age make up the main source rock for oil and gas in large parts of the Arctic. These sediments, which locally may contain more than 15% total organic carbon (TOC), covered the target area of the Mjølnir impact. We suggest that the extreme richness of organic matter and highly volatile components in the target rock resulted in colossal and intense fires in the impact area, both in the air and on the seafloor. This hypothesis is supported by numerical simulations and explains the large quantities of soot that have been found in samples associated with the Mjølnir impact.

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Carbon and nitrogen provide a chemical and isotopic record of the immediate environmental effects of the terminal Cretaceous impact. At Woodside Creek, New Zealand, kerogen carbon is enriched 15-fold, and total sediment nitrogen 20-fold in the basal layer of the boundary clay. Both elements show marked changes in their isotopic compositions on a millimeter scale, which may reflect the rapid sweep-out of plankton by ejecta fallout. The enrichment of nitrogen may reflect a contribution of HNO 3 acid rain from a shock-induced combination of N 2 and O 2 , but would not be expected from volcanism. Ground truth for such sudden fallout on the oceans is obtained from the 9-cm-thick Toba ash layer in the Indian Ocean. The accumulation rates of organic carbon and carbonate rise by ~3 orders of magnitude at the base of the ash layer, but then drop by 1 to 4 orders of magnitude after 3 to 4 cm. Apparently 3 to 4 g/cm 2 of the Toba ash efficiently scavenged and buried virtually all the CaCO 3 and much of the particulate organic material in the ocean up to 2,000 km from the site of the eruption. Presumably the 2 to 5 g/cm 2 of Cretaceous/Tertiary (K/T) ejecta were no less efficient, in view of their much finer grain size. It is significant that the amounts of marine and land biomass at the K/T boundary are approximately equal to the steady-state global inventory (one generation), as are the amounts of other enriched terrestrial elements (As, Sb, Zn). This is expected in a catastrophic but not a gradualist scenario.

A global charcoal and soot layer, coinciding with the Ir layer, is present at the Cretaceous/Tertiary (K/T) boundary, and apparently comes from a global fire. Soot is present even in the basal layer of the boundary clay, implying that the fire started soon after the impact. No comparable soot enrichments, let alone of global extent, occur in the latest Maastrichtian or in a wide range of other marine sediments. Much or all of the fuel was biomass, as indicated by the presence of retene (a hydrocarbon diagnostic of resinous wood fires) and by the carbon isotopic composition (δ 13 C = −25.8 ‰ ± 0.6 ‰), which resembles that of natural charcoal and atmospheric carbon particles from biomass fires. The mean amount of elemental C at 11 K/T boundary sites (11 ± 3 mg/cm 2 ) is very large, and requires that much of the Cretaceous biomass burned down and yielded a larger mass fraction of soot and charcoal than small fires. At one undisturbed site (Woodside Creek, New Zealand), soot in the boundary clay correlates tightly with Ir, As, Sb, and Zn. A possible reason for this correlation is that soot and Ir-bearing ejecta particles—containing some volatile chalcophiles from the target rock—coagulated in the stratosphere, and then scavenged additional chalcophiles from the hydrosphere. In view of this coagulation, the K/T fire would only slightly prolong the period of darkness and cold caused by impact ejecta. However, it would contribute other environmental stresses, e.g., a CO 2 greenhouse and a variety of pyrotoxins and mutagens. Because the total of recognized stresses has risen to 12, there is no basis for the contention that an impact cannot explain the observed selectivity of extinctions.