Chicxulub crater (left) and his son
Walter Alvarez (right) at the K-Pg Boundary in
Gubbio, Italy, 1981 In 1980, a team of researchers led by Nobel prize-winning physicist
Luis Alvarez, his son, geologist
Walter Alvarez, and chemists
Frank Asaro and
Helen Vaughn Michel discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary contain a
concentration of
iridium hundreds of times greater than normal. They suggested that this layer was evidence of an impact event that triggered worldwide
climate disruption and caused the
Cretaceous–Paleogene extinction event, a
mass extinction in which 75% of plant and animal species on Earth suddenly became extinct, including all non-
avian dinosaurs. When it was originally proposed, one problem with the "
Alvarez hypothesis" (as it came to be known) was that no documented crater matched the event. This was not a lethal blow to the theory; while the crater resulting from the impact would have been larger than in diameter, Earth's geological processes hide or destroy craters over time. The Chicxulub crater is an
impact crater buried underneath the
Yucatán Peninsula in
Mexico. Its center is located near the town of
Chicxulub, after which the crater is named. It was formed by a large
asteroid or
comet about in diameter, the
Chicxulub impactor, striking the Earth. The date of the impact coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary), slightly more than 66 million years ago. The crater was discovered by Antonio Camargo and
Glen Penfield,
geophysicists who had been looking for
petroleum in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with
Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes
shocked quartz, a
gravity anomaly, and
tektites in surrounding areas. In 2016, a scientific drilling project drilled deep into the
peak ring of the impact crater, hundreds of meters below the current sea floor, to obtain
rock core samples from the impact itself. The discoveries were widely seen as confirming current theories related to both the crater impact and its effects. The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed right on the coast and would have caused gigantic
tsunamis, for which evidence has been found all around the coast of the Caribbean and eastern United States—marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact. The asteroid landed in a bed of
anhydrite () or
gypsum (CaSO4·2(H2O)), which would have ejected large quantities of
sulfur trioxide that combined with water to produce a
sulfuric acid aerosol. This would have further reduced the sunlight reaching the Earth's surface and then over several days, precipitated planet-wide as
acid rain, killing vegetation, plankton and organisms which build shells from
calcium carbonate (
coccolithophorids and
molluscs).
Deccan Traps Before 2000, arguments that the
Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 Ma and lasted for over 2 million years. However, there is evidence that two thirds of the Deccan Traps were created within 1 million years about 65.5 Ma, so these eruptions would have caused a fairly rapid extinction, possibly a period of thousands of years, but still a longer period than what would be expected from a single impact event. The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulfuric aerosols into the air which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the
greenhouse effect when the dust and aerosols cleared from the atmosphere.
Multiple impact event Several other craters also appear to have been formed about the time of the K–Pg boundary. This suggests the possibility of nearly simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the
Shoemaker–Levy 9 cometary impact with
Jupiter. Among these are the
Boltysh crater, a diameter impact crater in
Ukraine and the
Silverpit crater, a diameter proposed impact crater in the
North Sea Any other craters that might have formed in the
Tethys Ocean would have been obscured by erosion and tectonic events such as the relentless northward drift of Africa and India. A very large structure in the sea floor off the west coast of India was interpreted in 2006 as a crater by three researchers. The potential
Shiva crater, in diameter, would substantially exceed Chicxulub in size and has been estimated to be about 66 mya, an age consistent with the K–Pg boundary. An impact at this site could have been the triggering event for the nearby Deccan Traps. However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal. A severe regression would have greatly reduced the
continental shelf area, which is the most species-rich part of the sea, and therefore could have been enough to cause a
marine mass extinction. However, research concludes that this change would have been insufficient to cause the observed level of
ammonite extinction. The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth's
albedo and therefore increasing global temperatures. Marine regression also resulted in the reduction in area of
epeiric seas, such as the
Western Interior Seaway of North America. The reduction of these seas greatly altered habitats, removing
coastal plains that ten million years before had been host to diverse communities such as are found in rocks of the
Dinosaur Park Formation. Another consequence was an expansion of
freshwater environments, since continental runoff now had longer distances to travel before reaching oceans. While this change was favorable to
freshwater vertebrates, those that prefer
marine environments, such as
sharks, suffered.
Supernova hypothesis Another discredited cause for the K–Pg extinction event is cosmic radiation from a nearby
supernova explosion. An
iridium anomaly at the boundary is consistent with this hypothesis. However, analysis of the boundary layer sediments failed to find , a supernova byproduct which is the longest-lived
plutonium isotope, with a
half-life of 81 million years.
Verneshot An attempt to link volcanism – like the Deccan Traps – and impact events causally in the
other direction compared to the proposed Shiva crater is the so-called
Verneshot hypothesis (named for
Jules Verne), which proposes that volcanism might have gotten so intense as to "shoot up" material into a ballistic trajectory into space before it fell down as an impactor. Due to the spectacular nature of this proposed mechanism, the scientific community has largely reacted with skepticism to this hypothesis.
Multiple causes It is possible that more than one of these hypotheses may be a partial solution to the mystery, and that more than one of these events may have occurred. Both the Deccan Traps and the Chicxulub impact may have been important contributors. For example, the most recent dating of the Deccan Traps supports the idea that rapid eruption rates in the Deccan Traps may have been triggered by large seismic waves radiated by the impact. ==See also==