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The Serendipitous Discovery of an Extraterrestrial Iridium Anomaly at the Cretaceous-Palaeogene Boundary in Gubbio and the Rise of a Far-Reaching Theory

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The Serendipitous Discovery of an Extraterrestrial Iridium Anomaly at the Cretaceous-Palaeogene Boundary in Gubbio and the Rise of a Far-Reaching Theory TO L O N O G E I L C A A P I ' T A A T L E I I A Bollettino della Società Paleontologica Italiana, 58 (1), 2019, 77-83. Modena C N O A S S. P. I. The serendipitous discovery of an extraterrestrial iridium anomaly at the Cretaceous-Palaeogene boundary in Gubbio and the rise of a far-reaching theory Alessandro Montanari & Rodolfo Coccioni A. Montanari, Osservatorio Geologico di Coldigioco, Contrada Coldigioco, I-62021 Apiro, Italy; [email protected] R. Coccioni, Dipartimento di Scienze Pure ed Applicate, Università degli Studi di Urbino Carlo Bo, Piazza della Repubblica 13, I-61029 Urbino, Italy; [email protected] KEY WORDS - Alvarez Hypothesis, mass extinctions, paradigm shift, Italy. ABSTRACT - It is not so frequent that in a scientific investigation using a deductive approach, an unforeseen result leads to a new hypothesis, which grows stronger through the same deductive approach of investigation, and eventually becomes a theory leading to a paradigm shift. The theory that a catastrophic impact of an extraterrestrial object, a comet or an asteroid, on the Earth caused a global mass extinction at the end of the Cretaceous Period strongly challenged the Lyellian paradigm of gradualism/uniformitarianism. In the 1980s, a nonconformist theory sprouted from the serendipitous discovery of an iridium anomaly in an inconspicuous clay layer marking the Cretaceous-Palaeogene boundary in the pelagic succession of Gubbio and opened the way to forty years of heated debates and thousands of scientific publications in the most disparate fields of Earth and planetary sciences. With this paper, our intention is to recount the scientific background, which led to the formulation of this theory. RIASSUNTO - [La fortuita scoperta dell’anomalia dell’iridio al limite Cretaceo-Paleogene presso Gubbio e l’origine di una teoria paradigmatica] - Non è frequente che in un’indagine scientifica, condotta con approccio deduttivo, un risultato imprevisto conduca alla formulazione di una nuova ipotesi che si rafforza attraverso lo stesso approccio deduttivo dell’indagine originaria fino a divenire una solida teoria in grado di generare un cambio di paradigma. La teoria secondo la quale l’impatto catastrofico di un oggetto extraterrestre - una cometa o un asteroide - sulla Terra abbia causato un’estinzione di massa globale alla fine del periodo Cretaceo ha fortemente sfidato il paradigma Lyelliano del gradualismo/uniformitarismo. Negli anni ‘80, questa teoria anticonformista scaturì dalla scoperta fortuita di un’anomalia dell’iridio in un sottile strato di argilla che segnava il confine tra Cretaceo e Paleogene nella successione pelagica di Gubbio. La scoperta aprì la strada a quattro decenni di accesi dibattiti e migliaia di pubblicazioni scientifiche nei più disparati settori delle scienze della Terra e planetarie. Con questo articolo intendiamo raccontare il background scientifico che portò alla formulazione di questa teoria. INTRODUCTION very few words, Alvarez and co-workers communicated the discovery of an anomalously high concentration of The theory that the impact on the Earth of a large the platinum group element iridium in the thin clay layer extraterrestrial object, probably a C-chondritic asteroid, that marks the K/Pg boundary in the pelagic limestone caused a global mass extinction at the end of the section of the Bottaccione Gorge, near Gubbio, as well as Cretaceous Period, is today accepted by a vast majority in other sections around the Umbria-Marche Apennines of the scientific community (e.g., Schulte et al., 2010, of central Italy, and in the type locality for the Danian and references therein). Nevertheless a small minority Stage at Stevns Klint in Denmark. Being an extremely of scientists supports the hypothesis that the extinction rare element in crustal rocks (concentrations in the order was gradual and caused by a climate change triggered of 0.02 ppb [parts per billion] or less), and four orders of by an intense eruptive activity in the igneous province magnitude more abundant in stony meteorites, which are of the Deccan Traps (west-central India), which initiated made up of undifferentiated solar matter, this Ir anomaly several hundreds of thousand years before the impact, and was interpreted by Alvarez and co-workers as the signature apparently increased during the Cretaceous-Palaeogene of an en masse accretion of undifferentiated solar matter (K/Pg) boundary time interval (e.g., Burgess, 2019, and on the surface of the Earth following the impact of a large references therein). extraterrestrial object, probably a chondritic asteroid. The impact theory sprouted from an article by Alvarez Iridium abundances in Italian and Danish sections as et al. (1980), which appeared in Science magazine, bluntly compared to chondritic meteorites lead Alvarez and co- titled “Extraterrestrial Cause for the Cretaceous-Tertiary workers to estimate the diameter of the impactor to be Extinction”. It is a 14-pages paper including two tables of in the order of 10 km. Assuming a velocity of 30 km/sec data and 13 figures, mostly graphs illustrating the results (pretty much an average velocity for objects travelling in of neutron activation analyses (NAA) of trace elements the inner Solar System), the total energy released by the in marine carbonate rocks, so a rather long technical impact would have been equivalent to that of 100 million paper for a standard Science magazine’s article. Seen tons of TNT. With these order-of-magnitude estimates, in retrospect, this paper is a masterpiece of scientific Alvarez et al. (1980) predicted that the crater left on the communication for its clarity and simplicity of expression, surface of the Earth would have had a diameter in the order and a great example of deductive scientific method. In a of 200 km. This prediction was fulfilled eleven years later ISSN 0375-7633 doi:10.4435/BSPI.2019.11 78 Bollettino della Società Paleontologica Italiana, 58 (1), 2019 with the discovery of the 180-km-diameter Chicxulub One thing these theories have in common is the fact that crater buried under the northwestern tip of the Yucatán they have caused paradigm shifts: plate tectonics with the Peninsula in Mexico (Hildebrand et al., 1991). passage from a “fixist” (or “verticalist”) to a “mobilist” After deducing the size of the bolide, the potential (or “horizontalist”) vision of global tectonics, and the energy of the impact explosion, and ultimately the impact theory with the passage from an uniformitarian predictable size of the crater, Alvarez and co-workers and gradualistic vision of the evolution of life to the vision went on with their dissertation by causally linking the of a single catastrophic, non uniformitarian event that impact event to the terminal Cretaceous mass extinction, caused an instantaneous mass extinction at a global scale, thus postulating their hypothesis that the mass extinction therefore an interruption of a gradualistic evolution of life. observed in the stratigraphic record at the K/Pg boundary However, while the idea of spreading ocean floors and was caused by the hypothesised giant impact event. Today continental drift came from an intuition of Alfred Wegener this is referred to as the Alvarez Hypothesis. in 1912, and became an universally accepted theory After having estimated the mass of target rock fifty years later with the advent of modern geophysical material, whether vaporised, molten, or pulverised, investigation techniques such as rock palaeomagnetism, ejected from the crater into the stratosphere, Alvarez seismic imaging of plate margins, and magnetic and and co-workers proceeded with an inductive approach to bathymetric ocean floor surveying, the impact hypothesis, propose a mechanism for the mass extinction. They did as postulated by Alvarez and co-workers in 1980, had all that by scaling from the famous 1883 Krakatoa volcanic the prerequisites to effectively be considered a testable eruption(s) in Indonesia, which released a total energy theory inevitably leading to a paradigm shift, inasmuch equivalent to 200 megatons of TNT (i.e., six orders of it spelled out with precision and clarity the evidence for a magnitude less or a million-fold smaller than the K/Pg giant meteorite impact through a deductive approach, and impact explosion), and which produced an ejecta cloud proposed the mechanisms that lead to the consequential that altered the transparency of the atmosphere at a global global mass extinction in a more inductive way. scale causing a drop in temperatures for weeks to come. Alvarez and co-workers came up with the scenario that the whole planet would have been enveloped by the impact BACKGROUND OF A RESEARCH ejecta blocking almost completely sunlight for several years. Not only this would have driven temperatures Of the five big mass extinctions, which punctuated below freezing even at tropical latitudes, but also it would the evolution of life on this planet, 1) at the end of the have suppressed photosynthesis with the consequence of Ordovician; 2) in the Late Devonian; 3) at the Permian- cutting off the food chains at their origin by eliminating Triassic boundary; 4) in the Late Triassic; and 5) at the plants in terrestrial environments and phytoplankton in Cretaceous-Palaeogene boundary, the latter has always the marine realm. ben by far the most popular one. This may be because despite it was not the most severe of the five in terms of percentage of species getting extinct, it involved the PARADIGM SHIFT sudden and definitive disappearance of iconic animals such as the dinosaurs and the ammonoids. It is curious After its publication, the Alvarez Hypothesis was the fact that none of the authors in Alvarez et al. (1980) welcome by an ever-increasing number of Earth scientists is a biologist, an ecologist, or a palaeontologist with, one as an attractive chance for testing the impact hypothesis would think, some scholarly background to rightfully following a deductive approach in the most disparate say something about an important biotic event such as physical science disciplines such as geochemistry, the mass extinction at the end of the Cretaceous Period.
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