Just weeks after a mega-paper in Science concluded that the Chicxulub asteroid impact was responsible for the late Cretaceous mass extinction, opponents of that theory are at it again.
Dr. Michael Prauss has submitting some work on the Brazos section in Texas which according to him shows that the Chixculub impact was not the sole cause of the extinction.
The basic argument of the opponents has been:
In stratigraphic sections spanning the late Cretaceous to Paleogene north of the impact site in parts of Mexico, Texas and New Mexico, there is sediment between the impact layer and the geochemically and micro-paleontologically defined Cretaceous-Paleogene boundary. This sediment accumulated gradually according to them. They estimate that the impact occurred a good 300,000 years before the K-Pg boundary crisis and thus the asteroid impact could not have been the sole cause of the mass extinction.
Opposing this view, the proponents of the impact theory say that this sediment is an event deposit which formed within hours to days of the impact and therefore the impact layer is the K-Pg boundary.
In this recent study published in Palaeogeography, Palaeoclimatology, Palaeoecology (I haven't read the paper -behind pay-wall) and described in Science Daily Dr.Prauss has shown that in the sediment between the impact layer (he calls it ED for event deposit) and the K-Pg boundary there is a gradual increase to peak abundance in trilete spores. This he interprets as indicative of recolonization of damaged landscapes possibly after the asteroid impact by pioneering fern species but well before the Paleogene.
And below the ED in the entire studied section spanning upper Maastrichtian to lower Danian there seems to be signatures of long term sea-level fluctuations and also fluctuations in the oxygen and carbon isotopes matched by fluctuations in the types of dinocyst - a type of marine protist- assemblages.
He concludes that the data shows there to be a significant time span between the impact layer and the K-Pg boundary layer (as judged by the gradual increase in trilete spores) and that the K-Pg boundary was preceded by a long period of high frequency environmental changes and ecosystem degradation going back to a time even before the asteroid impact, a pattern inconsistent with a single catastrophic cause for the mass extinction.
Its really remarkable how two teams can come to conclusions which are such polar opposites. Supporters of the meteorite impact theory have interpreted the said sediment as an event deposit. Their opponents have read long term sea-level changes in the same section and have included the sediment in a sequence stratigraphy framework! There seemingly cannot be a reconciliation of these views. One reading of the data is completely wrong. Both sides will vociferously state that they are going strictly by the evidence but you just wonder how much of an influence does allegiance to a pet theory have on your reading of the data.
Earlier the opponents of the impact theory (Gerta Keller and her team) had based a lot of their conclusions on the presence of sedimentological features suggesting gradual deposition in a quiet environment and on the presence of late Cretaceous foraminifera in the sediment above the impact layer.
This line of reasoning was rejected by the supporters of the impact theory on the grounds that the foraminifera were not really foraminifera but recrystallized dolomite which looked like foraminifera! Alternatively some suggested that the presence of late Cretaceous foraminifera was due to the mixing of materials of different ages during the violent reworking and redeposition of sediment just after the asteroid impact.
All this is very confusing. The evidence collected in favor of the asteroid impact theory is massive. On the other hand it will be interesting to see how supporters of the impact theory explain the apparent preservation of correlated sedimentological, geochemical and palynological trends signifying long term change in environmental conditions.
And that pattern of gradual increase in spore content should not be preserved in a reworked event deposit.