CRETACEOUS-TERTIARY (K-T) EVENTS: A MULTI-CAUSAL SCENARIOThierry Adatte (*), Gerta Keller (**) and Wolfgang Stinnesberck (***)* Institut de Géologie, 2007 Neuchatel, Switzerland, Thierry.adatte@geol.unine.ch** Dep.t of Geosciences, Princeton University, Princeton, NJ 08544, gkeller@princeton.edu*** Geologisches Institut, Uni. Karlsruhe, 76128 Karlsruhe, Germany, wolfgang.stinnesbeck@bio-geo.uni-karlsruhe.deMass extinctions events occur in Earth history when environmental stresses exceed tolerance limits for organisms and consequently lead to their extinction. The magnitude of the extinction depends on the kind of stress, various environmental factors and the tempo at which stress is imposed, the latter determines whether a mass extinction is gradual or sudden. A multiproxies study is therefore of great interest for a better understanding of the causes which led to a mass-extinction.Paleontologic, climatic and ecologic data reviewed here thus provide strong evidence for a progressive, rather than sudden, mass extinction pattern that began during the last 500 k.y. of the Cretaceous and culminated at the K-T boundary. No single kill mechanism can be identified for this extinction pattern. Evidence for a likely multi-event killing mechanism includes a series of rapid and extreme climate fluctuations associated with sea-level changes, a period of major volcanic activity prior and across the K-T boundary and asteroid or comet impacts (comet showers). Thus, the mass extinction resulted from an addition of unfavourable conditions which includes long term perturbations (e.g. Deccan traps volcanism, cooling, sea-level fluctuations) and short term event as asteroid impact (s), the latter giving the final stroke to an already stressed biosphere.The stratigraphy and age of altered impact glass (microtektites, microkrystites) ejecta layers from the Chicxulub crater are documented in late Maastrichtian and early Danian sediments in Mexico, Guatemala, Belize and Haiti. In northeastern Mexico two to four ejecta layers are present in zone CF1, which spans the last 300 k.y. of the Maastrichtian. The oldest ejecta layer is dated at 65.27±0.03 Ma based on sediment accumulation rates and extrapolated magnetostratigraphy. All younger ejecta layers from the Maastrichtian and early Danian P. eugubina zone (Pla(l) may represent repeated episodes of reworking of the oldest layer at times of sea level changes and tectonic activity. The K/T boundary impact event (65.0 Ma) is not well represented in this area due to widespread erosion. An early Danian Pla(l) Ir anomaly is present in five localities (Bochil, Actela, Coxquihui, Trinitaria, Haiti) and is tentatively identified as a third impact event at about 64.9 Ma. A multi-impact scenario is most consistent with the impact ejecta evidence. The first impact is associated with major Deccan volcanism and likely contributed to the rapid global warming between 65.4-65.2 Ma, decrease in primary productivity and onset of terminal decline in planktic foraminiferal populations. The K/T boundary impact marks a major drop in primary productivity and the extinction of all tropical and subtropical species. The early Danian impact may have contributed to the delayed recovery in productivity and evolutionary diversity.
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