The extinction of the dinosaurs is a mystery that has stimulated the imagination of scientists, and of others as well, for decades. Why did these reptiles, which had dominated our planet uninterruptedly for nearly 180 million years, at a certain point completely disappeared? It may seem strange but the answer to this intriguing question is not contained just in those few continental deposits preserving rare dinosaur fossil remains, but rather in marine rocks, which represent the petrified deep ocean sediments of that time, and which are practically made of microfossils slowly and continuously deposited through geologic ages. At the end of the 1970’s, a team of scientists from the University of California at Berkeley, composed of physicist Luis Alvarez, his son Walter, a geologist, and nuclear chemists Helen Michel and Frank Asaro, made a shocking discovery. In a thin clay layer of the Scaglia Rossa Formation outcropping near Gubbio, in Umbria.
This clay layer marks the boundary between the Cretaceous and the Tertiary Periods (the so-called K-T boundary), these scientists detected an anomalous concentration of the element iridium. The important fact was that the iridium is a siderophile element that is very rare in terrestrial rocks but significantly more abundant in extraterrestrial bodies such as meteorites, asteroids, and the rocky nuclei of comets. In fact the mean concentration of iridium in terrestrial crustal ranges between 0.002 and 0.02 parts per billion, whereas in meteorites Ir concentrations are thousands or tens of thausands times higher. If a gigantic body such as an asteroid or the nucleus of a comet were to collide with the Earth, it would disintegrate in an instant, spewing all its iridium load into the atmosphere. The iridium-rich fallout would settle on land and at the bottom of ocean basins, creating a very thin layer of sediment enriched in extraterrestrial materials. In addition, such an event would be catastrophic: the impact against the Earth of an asteroid with a diameter of 10 kilometers, and travelling at a velocity of 30 kilometers per second, would free the energy equivalent to 100 million megatons of TNT, causing an explosion a billion times larger than that of the Hiroshima atomic bomb, and digging a crater 200 kilometers in diameter on the surface of the Earth. A cloud of vaporized, molten, and pulverized material from the asteroid and from the Earth’s crust involved in the impact would then spread through the stratosphere. At first, it would completely block the Sun’s light. Then, bit by bit, the heaviest particles would fall back to the surface of the Earth. The gases, still dispersed in the atmosphere, would cause a prolonged greenhouse effect. All this would cause a catastrophe involving the whole global ecosystem. The effects would be similar to a “nuclear winter” (darkness, suppression of photosynthesis, rapid and intense lowering of the temperature), followed by a “greenhouse effect” (rise in temperature) and persistent acid rains. Moreover, particles, which condensed in the explosion cloud and fell through the atmosphere, would heat due to air friction (as happens, for example, to a re-entering space shuttle or a shooting star).
This would have started enormous fires in many parts of the world, which would liberate other gases and ash particles further increasing the greenhouse effect and nuclear winter. In summary, the Alvarez hypothesis is that the anomalous concentration of iridium found in the thin clay layer of Gubbio is the result of an immense natural catastrophe, which caused the mass extinction at the end of the Cretaceous Period of the Mesozoic Era, the era of the Reptiles. The animal and plant species that survived the disaster became the ancestors of new forms of life. These repopulated the Earth in the following Tertiary Period, at the beginning of the new Cenozoic Era, the era of Mammals. In the sedimentary rocks of many parts of the world containing the K-T boundary, and in particular in the Scaglia Rossa formation of the Umbria-Marche Apennine, it is possible to see the hints of the catastrophe that caused the extinction not only of the dinosaurs but also of numerous other life forms, including those of the microscopic marine plankton which, in fact, constitute the limestones.
Also in this outcrop of Scaglia Rossa at Monte Cònero, with a good magnifying hand lens (at least 10 x), one can recognize the microfossils of planktonic Foraminifera contained in the limestone bed immediately below the K-T boundary clay layer. These unicellular microorganisms, which used to live in the open sea at shallow depths, are barely visible on a hand sample as tiny dark dots smaller than a millimeter. With a microscope, these tiny dots appear as shells with a variety of shapes and different dimensions indicating a rich and well diversified pre-impact open sea environment. On the other hand, in the first layer of the Tertiary, just above the boundary clay, the foraminiferal microfossils are so small that they cannot be seen with a hand lens. They can be seen with a powerful microscope and they appear as tiny rounded shells not bigger than one tenth of a millimeter. They are all more or less similar in shape, indicating an environment with scarce faunal diversity dominated by a few forms that survived the catastrophe including those that developed immediately after it.
The microplankton realm will take three million years after the catastrophe, through a slow and gradual evolutionary process, to regain the same biodiversity as it used to be in the Cretaceous. Is there a way to see the direct evidence of the impact fallout? Obviously, the iridium anomaly can be detected only with very sophisticated analytical instruments. However, anywhere in the world, and also in this outcrop at Monte Cònero, in the thin K-T clay layer one can find mineral microspherules (microkrystites) which were condensed from the impact cloud, and rare quartz grains exhibiting lamellae, which were produced by the impact metamorphism.
These particles, which are less than a millimeter in size, can be concentrated after having washed the clay with sieves and water, and observed with an optical microscope. The microkrystites in the K-T boundary clay at Monte Cònero, which at their origin had compositions similar to that of basic volcanic rocks such as basalt or andesite, were altered after their deposition on the sea floor. Today we found them transformed into minerals generated by low-temperature authygenesis (alteration), such as glaucony (green mineral), and sanidine (whitish mineral). In thin section or with a scanning electron microscope, these spherules exhibit internal structures characterized by dendrites and fibrous crystals, which indicate their origin as molten droplets of basaltic composition that quenched rapidly while flying through the atmosphere. The K-T boundary clay also contains tests of benthic Foraminifera, which are microorganisms that lived at great depths on the sea floor.
Thanks to the remote living environment, benthonic Foraminifera, unlike their planktonic cousins that lived in shallow waters (and thus they were more vulnerable to climatic changes), survived the biologic catastrophe provoked by the extraterrestrial impact almost unaffected.