Unlocking the secrets of the Crater of Doom.

Luis and Walter Alvarez at the K-T Boundary in Gubbio, Italy, 1981 (From Wikimedia Commons)

The noble and ancient city of Gubbio laid out along the ridges of Mount Ingino in Umbria, was founded by Etruscans between the second and first centuries B.C. The city has an exceptional artistic and monumental heritage which includes marvelous examples of Gothic architecture, like the Palazzo dei Consoli and the Palazzo del Bargello. The rich history of the city is recorded in those buildings. Outside the city, there are exposures of pelagic sedimentary rocks that recorded more than 50 million years of Earth’s history. In the 1970s it was recognized that these pelagic limestones carry a record of the reversals of the magnetic field. The  K-Pg boundary occurs within a portion of the sequence formed by pink limestone containing a variable amount of clay. This limestone, know as the “Scaglia rossa”, is composed by calcareous nannofossils and planktonic foraminifera.

In 1977, Walter Alvarez – an associate professor of geology University of California, Berkeley – was collecting samples of the limestone rock for a paleomagnetism study. He found that the foraminifera from the Upper Cretaceous (notably the genus Globotruncana) disappear abruptly and are replaced by Tertiary foraminifera. The extinction of most of the nannoplankton was simultaneus with the disappearance of the foraminifera (Alvarez et al., 1980).

Forams from the Upper Cretaceous vs. the post-impact foraminifera from the Paleogene. (Images from the Smithsonian Museum of Natural History)

At Caravaca on the southeast coast of Spain, Jan Smith, a Dutch geologist, had noticed a similar pattern of changes in forams in rocks around the K-T boundary. Looking for clues, Smith contacted to Jan Hertogen who found high iridium values at the clay boundary. At the same time, Walter Alvarez  gave his father, Luis Alvarez – an American physicist who won the  Nobel Prize in Physics in 1968 – a small polished cross-section of Gubbio  K-Pg boundary rock. The Alvarez gave some samples to Frank Asaro and Helen Michel, who had developed a new technique called neutron activation analysis (NAA). They also discovered the same iridium anomaly. The sea cliff of Stevns Klint, about 50 km south of Copenhagen, shows the same pattern of extinction and iridium anomaly. Another sample from New Zeland also exhibits a spike of iridium. The phenomenon was global.

Iridium is rare in the Earth’s crust but metal meteorites are often rich in iridium. Ten years before the iridium discovery, physicist Wallace Tucker and paleontologist Dale Russell proposed  that a supernova caused the mass extinction at the K-Pg boundary. Luis Alvarez realised that  a supernova would have also released plutonium-244, but there was no plutonium in the sample at all. They concluded that the anomalous iridium concentration at the K-Pg boundary is best interpreted as the result of an asteroid impact, which would explain the iridium and the lack of plutonium. In 1980, they published their seminal paper on Science, along with Asaro and Michel, and ignited a huge controversy. They even calculated the size of the asteroid (about 7 km in diameter) and the crater that this body might have caused (about 100–200 km across).

A paleogeographic map of the Gulf of Mexico at the end of the Cretaceous (From Vellekoop, 2014)

In 1981, Pemex (a Mexican oil company) identified Chicxulub as the site of a massive asteroid impact. In 1991, Alan Hildebrand, William Boynton, Glen Penfield and Antonio Camargo, published a paper entitled “Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico.” They had found the long-sought K/Pg impact crater.

The crater is more than 180 km (110 miles) in diameter and 20 km (10 miles) in depth, making the feature one of the largest confirmed impact structures on Earth. The  Chicxulub impact released an estimated energy equivalent of 100 teratonnes of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Model simulations suggest that the amount of sunlight that reached Earth’s surface was reduced by approximately 20%.This decrease of sunlight caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”. Cold and darkness lasted for a period of months to years.  Photosynthesis stopped and the food chain collapsed. This period of reduced solar radiation may only have lasted several months to decades. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species while freshwater environments shows low extinction rates, about 10% to 22% of genera. Additionally, the vapour produced by the impact  could have led to global acid rain and a dramatic acidification of marine surface waters.

The Chicxulub asteroid impact was the final straw that pushed Earth past the tipping point.  The K-Pg extinction that followed the impact was one of the five great Phanerozoic  mass extinctions. Currently about 170 impact craters are known on Earth; about one third of those structures are not exposed on the surface and can only be studied by geophysics or drilling. Now, a new drilling platform in the the Gulf of Mexico, sponsored by the International Ocean Discovery Program (IODP) and the International Continental Scientific Drilling Program, will looking rock cores from the site of the impact. The main object is learn more about the scale of the impact, and the environmental catastrophe that ensued.

References:

Alvarez, L., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction: Experimental results and theoretical interpretation. Science 208:1095–1108.

Alvarez, W. (1997) T. rex and the Crater of Doom. Princeton University Press, Princeton, NJ.

Hildebrand, A.R., G.T. Penfield, D.A. Kring, M. Pilkington, A. Camargo, S.B. Jacobsen, and W.V. Boynton. 1991. Chicxulub crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. Geology 19:867–71.

 

 

Darwin’s Fossil Mammals.

Portrait of Charles Darwin painted by George Richmond (1840)

When Charles Darwin arrived to South America, he was only 22 years old. He was part of the second survey expedition of HMS Beagle. During the first two years of his voyage aboard HMS Beagle, Darwin collected a considerable number of fossil mammals from various South American localities. He sent all the specimens, to his mentor John Stevens Henslow. The samples were deposited in the Royal College of Surgeons where Richard Owen began its study. Between 1837 and 1845, Owen described eleven taxa, including: Toxodon platensis, Macrauchenia patachonica, Equus curvidens, Scelidotherium leptocephalum, Mylodon darwinii and Glossotherium sp. Previous to this expedition, the first news of “fossils” in South American were reported by early Spanish explorers, and George Cuvier, in 1796,  published the first scientific work about Megatherium americanum, based on the specimen recovered by Fray Manuel Torres from Lujan, Buenos Aires, Argentina.

Darwin recovered his first fossil at Punta Alta (Buenos Aires Province, Argentina) on September 23, 1832, and continued collecting intermittently at this locality until October 16. Later, he went to Monte Hermoso and returned to Punta Alta between August 29 – 31, 1833. Then, he moved to Guardia del Monte (Buenos Aires Province); the Rio Carcarañá (Santa Fe Province, Argentina), and the Bajada Santa Fe (Paraná, Entre Ríos Province, Argentina). After a short stay in Uruguay, Darwin returned to Argentina and collected his last specimens at Puerto San Julián (Santa Cruz Province) in 1834. During his journey between Buenos Aires and Santa Fe he wrote “We may therefore conclude that the whole area of the Pampas is one wide sepulchre for these extinct quadrupeds” (Voyage of the Beagle, Chapter VII, Oct. 1833).

Fossil Toxodon on display at Bernardino Rivadavia Natural Sciences Museum.

Toxodon was named by Owen based on a large skull purchased by Darwin. He paid 18 pence for it. Darwin described it as “one of the strangest animals, ever discovered…” Owen bestowed the name because its upper incisors were strongly arched (Toxodon means “arched tooth”). He also recognized Toxodon as “A gigantic extinct mammiferous animal, referable to the Order Pachydermata, but with affinities to the Rodentia, Edentata, and Herbivorous Cetacea”. Toxodonts shares a number of dental, auditory and tarsal specializations. They had  short hippopotamus-like head with broad jaws filled with bow shaped teeth and incisors, a massive skeleton with short stout legs with three functional toes. The estimated weight is over a tonne. About the different groups that appeared to be related to Toxodon, Darwin stated:“How wonderfully are the different orders, at the present time so well separated, blended together in different points of the structure of Toxodon”

A recent phylogenetic analysis indicates that Toxodon is most closely related to perissodactyls, a group that includes rhinos, tapirs, and horses.

Macrauchenia patachonica by Robert Bruce Horsfall.

Macrauchenia, meaning “big neck,” was named by Owen based on limb bones and vertebrae collected by Darwin in January 1834 at Puerto San Julian, in Santa Cruz Province, Argentina. The bizarre animal had a camel-like body, with sturdy legs, a long neck and a relatively small head. Owen described as “A large extinct Mammiferous Animal, referrible to the Order Pachydermata; but with affinities to the Ruminantia, and especially to the Camelidae”. Macrauchenia is now considered among the more derived native South American litopterns. Darwin also made inferences about the environment which Macrauchenia lived: “Mr. Owen… considers that they form part of an animal allied to the guanaco or llama, but fully as large as the true camel. As all the existing members of the family of Camelidae are inhabitants of the most sterile countries, so we may suppose was this extinct kind… It is impossible to reflect without the deepest astonishment, on the changed state of this continent. Formerly it must have swarmed with great monsters, like the southern parts of Africa, but now we find only the tapir, guanaco, armadillo, capybara; mere pigmies compared to antecedents races… Since their loss, no very great physical changes can have taken place in the nature of the Country. What then has exterminated so many living creatures?…We are so profoundly ignorant concerning the physiological relations, on which the life, and even health (as shown by epidemics) of any existing species depends, that we argue with still less safety about either the life or death of any extinct kind” (Voyage of the Beagle, Chapter IX, Jan. 1834).

Scelidotherium leptocephalum, Muséum national d’Histoire naturelle, Paris (From Wikimedia Commons)

Darwin recovered fossil remains of at least five species of giant ground sloth. In a letter sent to John Stevens Henslow in November, 1832, Darwin listed the fossils collected, among which he emphasized “… the upper jaw & head of some very large animal, with 4 square hollow molars — & the head greatly produced infront. — I at first thought it belonged either to the Megalonyx or Megatherium.” Darwin decided in favor of Megatherium based on the presence of osteoderms collected in the same formation, but Owen (1838-1840) recognized the specimens assigned by Darwin to Megatherium as glyptodonts, toxodonts, and large ground sloths (Fernicola et al., 2009; Allmon 2015). Scelidotherium, was described by Owen on the basis of the only nearly complete skeleton found by Darwin at Punta Alta (Buenos Aires Province). Darwin considered the specimen as “allied to the Rhinoceros”. Scelidotherium is distinctive by an elongated, superficially anteater-like head. Another sloth, Mylodon was named by Richard Owen on the basis of a nearly complete lower jaw with teeth, which was found by Charles Darwin at Punta Alta (Buenos Aires Province). Owen (1839b) erected Mylodon for two species, Mylodon darwini and Mylodon harlani. The former species was based on a left dentary from Punta Alta (Buenos Aires Province), whereas the second was based on a cast of a mandible from North America.

Fossil mammals collected by Charles Darwin in South America during the voyage of H.M.S. Beagle (From Allmon, 2015).

Darwin also found fossil horse teeth assignable to the modern genus Equus. The two molars from Argentina were recovered from Punta Alta (Buenos Aires Province) and Bajada Santa Fe (Entre Rios Province), and represent the first fossil horses found in South America. He wrote: “Certainly it is a marvellous event in the history of animals that a native kind should have disappeared to be succeeded in after ages by the countless herds introduced with the Spanish colonist! (1839, p. 150).”

By the end of the expedition, Darwin was already earned a name as a geologist and fossil collector. He narrated his experiences in his book “Journal of Researches into the Geology and Natural History of the Various Countries visited by H.M.S. Beagle, under the Command of Captain FitzRoy, R.N. from 1832 to 1836″, published in 1839 and later simply known as “The Voyage of the Beagle”. When Darwin wrote his memories in 1858, he described the expedition in one strong and powerful sentence: “the voyage of the Beagle has been by far the most important event in my life and has determined my whole career”.

 

References:

Warren D. Allmon (2015): Darwin and palaeontology: a re-evaluation of his interpretation of the fossil record, Historical Biology, DOI: 10.1080/08912963.2015.1011397

Fernicola JC, Vizcaíno SF, de Iuliis G. 2009. The fossil mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle. Revista de la Asocición Geológica Argentina. 64(1):147–159.

Fariña, Richard A.; Vizcaíno, Sergio F.; De Iuliis, Gerry (2013). Megafauna. Giant Beasts of Pleistocene South America. Indiana University Press.