Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 245

those of other major extinction boundaries; however, shocked quartz, microtek-
tites and other indicators of extraterrestrial impact are generally absent. Since
all the anomalies are found at redox and facies boundaries it is more likely that
they resulted from a sudden change in the palaeoredox conditions and facies.
This is supported by the fact that the black shales represent reducing and anoxic
conditions, where there is an absence of most living organisms, and by high
contents of organic carbon, chalcophile elements, sulphur and pyrite. Surround-
ing rocks associated with the shale are often well oxygenated and have normal
marine fossils. Ir anomalies are most commonly found at the basal and/or upper
boundary levels suggesting that redox changes at the boundaries played a more
important role than did lithologies in controlling the distribution of Ir anoma-
lies in these sections.
The Ir anomaly at the K--T boundary does contain many if not all of the other
diagnostic signatures for an extraterrestrial impact. However, one of the main
problems encountered when interpreting Ir and PGE abundances at the K--T
boundary is the assessment of the extent to which elemental mobility obscures
primary geochemical signatures. Although all PGE are mobile during weathering
and diagenesis, some are more resistant to remobilisation from the original
carrier phase. Evanc et al. (1993)found that the general order of PGE mobility
during weathering is Pd Os = Ir. Pt and Pd can form amine

and purine organic complexes so it is possible that they can form complexes
with free carboxylic acid and with soil organic matter. Since Ru and Ir are the
least organically associated PGE, and also most resistant to chemical influences
in nature, they are considered the most useful indication of the source for
anomalies associated with extraterrestrial impacts.
Ni-rich spinel and spherule layers suggest that there is little doubt that the
late Eocene was marked by a major extraterrestrial impact. Ir anomalies are
also present within late Eocene marker beds. Montanari et al. (1993)examined
Ir anomalies of two apparently complete and continuous late Eocene pelagic
sequences. Both sequences are calibrated by both planktonic biostratigraphy and
magnetostratigraphy. The core sample, from Italy, has also been calibrated by
radioisotopic dates on biotite using K/Ar, Rb/Sr and Ar/Ar methods and U/Th/Pb
dating on zircon and monazite from interbedded volcanic ash layers. Spherules
were not found in these samples, even though Ir anomalies were present, and
Montanari et al. (1993)suggest that the spherules were altered and obliterated
by diagenetic processes. The lack of spherules and other diagnostic indicators
show that the presence of Ir anomalies is probably insufficient evidence in itself
to diagnose an extraterrestrial impact event. In the case of the late Eocene,
however, the strong evidence elsewhere highlights that a bolide event probably
did occur at this time and that Ir anomalies in horizons of this age are a product

> Pt > Rh > Ru >