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Impact and extinction in remarkably complete Cretaceous-Tertiary boundary sections from Demerara Rise, tropical western North Atlantic

Kenneth G. MacLeod^,1, Donna L. Whitney^,2, Brian T. Huber^,3 and Christian Koeberl^#,4

¹ Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
² Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
³ Department of Paleobiology, Smithsonian National Museum of Natural History, Washington, DC 20013-7012, USA
⁴ Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

Ocean Drilling Program (ODP) Leg 207, on the Demerara Rise in the western tropical North Atlantic, recovered multiple Cretaceous-Paleogene boundary sections containing an ejecta layer. Sedimentological, geochemical, and paleontological changes across the boundary closely match patterns expected for a mass extinction caused by a single impact. A normally graded, 2-cm-thick bed of spherules that is interpreted as a primary air-fall deposit of impact ejecta occurs between sediments of the highest Cretaceous Plummerita hantkeninoides foraminiferal zone and the lowest Paleogene P0 foraminiferal zone. There are no other spherule layers in the section. In addition to extinction of Cretaceous taxa, foraminiferal abundance drops from abundant to rare across the boundary. Ir concentrations reach a maximum of 1.5 ppb at the top of the spherule bed, and the Ir anomaly is associated with enrichment in other siderophile elements. We attribute the unusually well-preserved and relatively simple stratigraphy to the fact that Demerara Rise was close enough (4500 km) to the Chicxulub impact site to receive 2 cm of ejecta, yet was far enough away (and perhaps sheltered by the curve of northern South America) to have been relatively unaffected by impact-induced waves.

Key Words: Cretaceous-Paleogene boundary • mass extinction • impact • Demerara Rise • Ir anomaly • ODP Site 1259 • ODP Leg 207

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