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Projects: NEO Earth Expeditions

1996 Italy Expedition: Rationale

Why Italy? If the great impact that set off the chain of events that led to the extinction of the dinosaurs occurred in Central America near Belize, why did we travel halfway around the globe to investigate the event?

If the theory is correct, then the impact at Chicxulub caused a planet-wide chain of events. Thus, the dust and soot from the impact would have settled all over the world. Indeed, evidence of iridium has been found in the K/T layer on every continent. Still then, why Italy?

There are three reasons: The mountains, the history of the area, and Coldigioco (pronounced kol-dee-szho-coh). First, the mountains.

The Apennine mountain range is ideal for this type of study because it was created by a relatively recent (roughly 6 million years ago) thrust-and-fold event. Prior to that, the area lay undisturbed (by major geological events) in deep water for around 200 million years. During this long period of time, sediment from marine life (animal and plant) accumulated in layer after layer. When Sardinia migrated and rotated eastward towards Italy and created the Apennines, these layers were thrust up above sea level and are now exposed. The areas around Ancona, Frontale, and especially Gubbio are particularly suitable for viewing and study. The following paragraphs about the outcrops in Gubbio are excerpted from a field guide prepared by Alessandro Montanari and Rodolfo Coccioni for the 4th International Workshop of the ESF Scientific Network on "Impact Cratering and Evolution of Planet Earth" which took place in Ancona, May 12-17, 1995.

The Bottaccione Gorge

The Bottaccione gorge is the classic outcrop site for the Upper Cretaceous-Paleogene pelagic sequence of the Umbria-Marche Apennines. This nearly continuous and complete road exposure offers the rare opportunity to intercalibrate the calcareous plankton biostratigraphy with the sequence of paleomagnetic polarity reversals through a span of 80 million years. Alvarez et al. (1977) proposed it as the Type Section for the Upper Cretaceous-Paleocene Magnetic Polarity Sequence, and were able to correlate this sequence with the marine magnetic anomalies recorded across oceanic plates, thus providing a new, precise means for dating the expansion and evolution of the world's oceans.

The Contessa Sequence

The Contessa valley is the twin sister of the Bottaccione Gorge, both in a morphological and stratigraphic sense. While the exposure of the Cretaceous Scaglia Rossa is perhaps better in the classic Bottacione section, and mostly covered by retention steel nets along the Contessa highway, the Maiolica, Marne a Fucoidi, and Scaglia Bianca sequence are nowhere in the region as well exposed as in the huge and spectacular quarries of the Barbetti Cement Company. Here one can have a good look at the Bonarelli Level (K/T boundary) as well as a large exposure of a soft-sediment slump at the boundary between the Maiolica and the Marne a Fucoidi (upper Aptian boundary) which is marked, in undisturbed sections throughout the Umbria-Marche basin, by the so-called Selli Level, a black, bituminous horizon remarkably similar to the Bonarelli Level (Coccioni et al., 1987).

The second reason to come to Italy is that many of the recent discoveries about the K/T boundary were made here in the Apennines, in particular at the Bottaccione Gorge and the surrounding area. The entire area has great historical significance. Again, excepts from the field guide:

The K/T boundary is well exposed at 347.6 meters, in the upper part of magnetic zone Chron 29r. This was the first site where an iridium anomaly was found in the clay layer marking the boundary (Alvarez et al., 1979), suggesting the hypothesis that the mass extinction at the end of the Mesozoic Era was triggered by the impact against the Earth’s surface of a 10 km extraterrestrial object (i.e. an asteroid or a comet; Alvarez et al., 1980). A high-resolution iridium profile from the same outcrop was produced by Alvarez et al. in 1990, which confirmed an anomaly of 3,000 parts-per-trillion iridium in the boundary clay, but revealed also several minor peaks between 50 and 100 parts-per-trillion just above and below the boundary clay. Alvarez et al. (1990) suggested that these regularly spaced minor peaks may reflect Milankovich cycles.

Soon after the discovery of iridium, Montanari et al. (1982 and 1993) reported the presence of flattened microspherules, about 500-800 mm in diameter, made of K-feldspar, glauconite, and iron oxide, and interpreted them as quenched droplets of basaltic melt produced by an impact. The whitish K-feldspar spherules showed similar textural, crystallographic, and chemical characteristics to the "sanidine" spherules found by Smith and Klaver (1981) in the K/T boundary clay in Spanish sections. In the following years, similar spherules associated with an iridium anomaly were found in the K/T boundary clay in numerous sections and deep sea cores throughout the world, strongly supporting the Alvarez hypothesis of a large impact at the end of the Cretaceous.