Celestine Jehanno

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

Abstract Magnetic spinels produced by oxidation of extraterrestrial objects in the atmosphere have a composition distinct from terrestrial spinels. They are characterized by a high iron oxidation state, arising from crystallization under high oxygen fugacities, and a high nickel concentration due to the relatively high abundance of this element in extraterrestrial material. The iron oxidation state increases from micrometeorites, to meteoroid ablation material and to impact-generated products. This reflects a progressive increase of the oxygen fugacity, corresponding to decreasing altitudes of crystallization. Spinels found at the Cretaceous-Tertiary boundary are similar to those that crystallized from meteoroid ablation material and impact-generated products, supporting the view that a collisional event did occur at the end of the Cretaceous.

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

Abstract Ni-rich spinels have been found throughhout the world at the Cretaceous-Tertiary boundary. These minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris, suggesting that the Cretaceous-Tertiary boundary spinels result from an extraterrestrial event. Their stratigraphic distribution, which has been observed at the Cretaceous-Tertiary boundary sites of El Kef (Tunisia), Caravaca (Spain) and Hole 761C (ODP Leg 122, Indian Ocean), are markedly different from site to site and are all much less extended than the Ir anomalies. Regional differences in Ni-rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions. The wider stratigraphic dispersion of Ir is attributed to postdepositional chemical processes. At El Kef, the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate, the 1–2 mm thick Ni-rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years. This upper limit is consistent only with the collision of an asteroid or a comet at the close of the Cretaceous.

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

Abstract The K-T boundary at Beloc in Haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite. A small fraction of these objects still contains well-preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the Caribbean area. However, Beloc glasses are markedly different from tektites and microtektites: their structure and composition suggest that they are not impact-generated products, but are likely of volcanic origin. On the other hand, in sections which have probably preserved their original stratigraphy, the globule bed, showing well-developed grading, appears first in the sedimentary column followed by the ubiquitous Cretaceous-Tertiary boundary (KTB) clay layer containing Ir- and Ni-rich spinels: granulometric considerations indicate that the globule bed and the Ir- and spinel-rich clay layer correspond to two distinct events. We conclude that the globules at the KTB depositional sequence of Beloc represent a local volcanic event slightly preceding the great KTB cosmic catastrophe. Our observations do not provide any unequivocal evidence that a cratering event occurred nearby.

The Cretaceous-Tertiary boundary at Gubbio revisited: vertical extent of the Ir anomaly

Abstract The classical Gubbio (Italy) Cretaceous-Tertiary boundary section has been resampled for both magnetostratigraphy and iridium. Paleomagnetic samples were taken over 7 m in the Maastrichtian and 6 m in the Paleocene. Previous results obtained a decade ago are confirmed. The reversal sequence is well defined and the individual reversals are somewhat more precisely located. A noticeable difference is the location of the position of the 29N/29R limit which may be lowered by 20–30 cm in the stratigraphic column. This would imply that the KTB occurs near the middle of chron 29R. Iridium measurements were made on samples from both shales and surrounding limestone beds from 2 m below to 3 m above the boundary: these measurements indicate that Ir is associated with clay minerals. Concentrations in the two types of samples are indeed compatible when reduced to a carbonate-free basis. Iridium concentrations stand above background over almost 3 m of section, corresponding to half a million years based on magnetostratigraphy. This is likely to indicate a protracted duration of the (external or internal) source of iridium, on top of which the main (short-lived) KTB anomaly proper stands.