Cretaceous-Tertiary boundary at Blake Nose (Ocean Drilling Program Leg 171B):: A record of the Chicxulub impact ejecta

The Ocean Drilling Program (ODP) included as one of its Leg 171B objectives the recovery of a detailed record of the Cretaceous-Tertiary (K-T) events at Blake Nose (northwest Atlantic). This aim was successfully achieved with sections across the K-T boundary recovered at Sites 1049, 1050, and 1052, and a thick spherule bed recovered at ODP Site 1049. This spherule bed varies from 7 to 17 cm in thickness at the three different holes drilled at Site 1049, and occurs at the biostratigraphic boundary between the Cretaceous and the Paleocene. Mineralogical and geochemical analyses of the Blake Nose spherule bed reveal that it is mainly composed of smectite derived from the alteration of a precursor material, mostly glass. Also present in minor proportions are dolomite, quartz, zeolites, and trace amounts of rutile and some lithic fragments. Different types of spherules, dark green, pale yellow, and light green, that can be related to different precursors were observed in the Blake Nose spherule bed. Transmission electron microscope observations showed that smectite directly replaced the original material and that dark green spherules originated from a Si-rich precursor, whereas pale yellow spherules originated from a more Ca-rich precursor. The chemical composition of the spherule-bed material at Blake Nose shows little evidence for a significant extraterrestrial contribution, suggesting that the spherule-bed material was mainly derived from the alteration of target-rock-derived material from Chicxulub crater. In addition, rare earth element C1-normalized patterns also suggest that this material was derived from upper crustal rocks.