Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 249

Although Re and Os content is much higher in chondrites and iron meteorites
than the mantle, the Re/Os ratio is indistinguishable from that of meteorites. In
crustal rocks, the abundance of Os is much lower than in the mantle and the
Re/Os ratio is significantly different between mantle rocks and meteorites. Thus,
differences in the Re/Os ratio of meteoritic and crustal rocks lead to differences
in the increase of the radiogenic isotope 187 Os. The 187 Os/ 188 Os ratio of mete-
orite and mantle rocks changes relatively slowly with time, whereas the crustal
187 Os/ 188 Os ratio changes relatively rapidly with time due to the high Re con-
centrations. These features result in an identification tool for meteoritic rocks.
Sudden decreases in 187 Os/ 188 Os ratios have been found at the K--T boundary and
other deposits following extraterrestrial impacts. It seems reasonable that this
method can be used to provide a better understanding of the mixing processes
that occur between target rocks and impactors. Used with care, the Re--Os dating
method can provide a diagnostic tool for establishing an extraterrestrial origin
similar to that of shock metamorphism (Koeberl and Shirey, 1997).
MacDougal (1988) combined precise documentation of the strontium iso-
topic composition of ocean water through time and an understanding of the
atmospheric geochemical effects that accompany large extraterrestrial impacts
to analyse events at the K--T boundary. MacDougall suggested three possibili-
ties might explain the anomalously high strontium concentrations at the K--T
boundary. First, it is possible that the elevated 87 Sr/ 86 Sr ratio was a result of dis-
solution of the bolide in seawater. Second, it is possible that the impact ejecta
(both vapour and solid) were a major source of strontium, although it is not
likely as meteorites would not have a sufficiently high strontium composition to
produce the recorded anomaly. Third, the atmospheric chemical effects of the
impact from a large projectile could result in the production of large amounts
of nitrogen oxides due to shock heating. This would result in extremely acidic
precipitation near the impact centre and also globally over a more extended
time span. Acid rain is likely to increase continental weathering with the result
that continental strontium to the oceans is increased. MacDougal (1988) sug-
gested that the latter mechanism is the most probable cause of the enhanced
87 Sr/ 86 Sr ratio discovered at the K--T boundary. Volcanism can also potentially
produce acid rain and result in the same processes. However, this is likely to be
along-term effect, rather than a distinguishable peak as is the case at the K--T
boundary.
Isotopic chemostratigraphy and chronostratigraphy on sites in Antarctica and
Australia were usedbyRetallack et al. (1998)todetermine whether the Permian--
Triassic (P--T) boundary, which is associated with one of Earth's greatest oceanic
mass extinction events, was associated with an extraterrestrial impact. Micro-
spherules, spinels and iridium anomalies have been reported from this time