Chicxulub Catastrophism, or: The Eocene-Oligocene Example

The Scaglia Variegata and Scaglia Cinerea are two formations of marl deposited during the late Eocene and early Oligocene (36 and 33 million years), during a period characterized by profound climatic changes and a biological crisis of global scale, and outcropping in the Italian Province of Marche.
In less than a million years about 20% of the genera of marine organisms became extinct. The causes of this mass extinction are not yet entirely clear, one hypothesis invokes the slow drift of continents and the resulting climatic and environmental changes, the alternative hypothesis invokes the impact of an extraterrestrial mass as the main cause of the biological change.

Fig.1. The typical scenery of the proposed Cretaceous-Paleogene transition (figure from MONTANARI & COCCIONI).

From the Eocene there are known at least two large impact craters: Chesapeake Bay (North-East America) and Popigai (northeast Siberia). With a diameter of 100km, the Popigai is one of the largest impact craters on our planet (and the largest in the last 65 million years), followed by the 85km of Chesapeake Bay. Smaller craters of the upper Eocene can be found in Canada and Australia.

According to the proposed scenario these impacts would have altered the climate and the environments on a worldwide scale. The impact of a large extraterrestrial object could liberate such a quantity of gas, vapour and dust from the vaporized cover and obscure the entire planet. Photosynthesis would be blocked or reduced, and the climate would become dark and cold for years. The entire food chain would be affected, resulting in the disturbance of the ecological balance in the various marine and terrestrial environments.

The sedimentological record and the biological crisis that affected the late Eocene are well documented in the quarry of Massignano, a small village near Ancona.

Fig.2. The Eocene-Oligocene transition in the quarry of Massignano (Marche), near the top of the quarry, above bed 17 (click on image to enlarge).

At the base of the outcrop with the Scaglia Variegata and Scaglia Cinerea two thin layers have been identified that contain abnormal amounts of the element iridium and the isotope helium-3, components rare on earth but found concentrated in extraterrestrial material. The layers also contain spherules of spinel and quartz grains with a lamellar structure, typical mineralogical products of high temperatures and pressure during an impact.

However the relationship between the cosmic impacts and the Eocene crisis has not yet been finally clarified. It's seems certainly that impacts by cosmic objects occurred during the transition Eocene-Oligocene, but it's seems also that they effects on the terrestrial ecosystems was limited. Because of the uncertainties between the age of the impacts and the pattern of extinctions, the observed slow but inexorable terrestrial and climatic changes, like the tectonic isolation of Antarctica and subsequent cooling of the planet, must also be considered in trying to explain the observed biological crisis.
According to some dating efforts the largest impacts do not coincide with the Eocene extinction phase (37-38 Ma) or the Eocene-Oligocene boundary (35.5-36.0Ma), but happened 1 to 2 million years before and respectively after these ages.

Contrary to what the general media claims, mass extinctions of diverse entity have been numerous in the history of our planet, but there is no agreement on how, why and when they occur.

Considering the meteor hypothesis, there still remains important question - how many impacts happened and what was really their role in the evolution of life on earth?

Fig.3. "Raup´s Kill Curve". The "kill curve" (dashed line) of RAUP 1991 was originally fit to the C/Pg impact data (60% of species wiped out with a crater of about 180 kilometers in diameter), and it predicted that much smaller impacts should cause significant extinctions. However, when the late Eocene impacts (which caused almost no extinctions) are plotted, the "kill curve" takes a different, S-like shape, and suggests that only the largest impacts have the potential to cause mass extinctions. (figure from PROTHERO 2006).

Bibliography:

PROTHERO, D. (2006): After the Dinosaurs: The Age of Mammals (Life of the Past). Indiana University Press: 384

Online Ressources:

Scientific project by A. Montanari and R. Coccioni :Geological routes of the park of Conero.