FOUND: SMOKING METEOR SOLVES DINOSAUR DISAPPEARANCE; THE ULTIMATE COLD CASE
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| Artist image of the instant a six-mile wide meteor impacts with the Earth along the Yucatan coastline of Mexico. Scientists are accepting this collision as the leading cause of the dinosaur extinction more than 65 million years ago. |
GUEST
BLOG—By Wikipedia--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. The age of the Chicxulub
asteroid impact and the Cretaceous–Paleogene geological boundary (K–Pg boundary)
coincide precisely. The crater is more than 110 miles in diameter and 12 miles
in depth, making the feature one of the largest confirmed impact structures on
Earth; the impacting meteor that
formed the crater was at least six miles in diameter.
Antonio Camargo and Glen Penfield, geophysicists who had been
looking for petroleum in the Yucatán during the late 1970s, discovered the
crater. Penfield was initially unable to obtain evidence that the geological
feature was a crater, and gave up his search. Through contact with Alan
Hildebrand, 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.
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| GROUND ZERO |
The age of the rocks marked by the impact shows that this impact
structure dates from roughly 66 million years ago, the end of the Cretaceous
period, and the start of the Paleogene period. It coincides with the K-Pg
boundary, the geological boundary between the Cretaceous and Paleogene.
The impact associated with the crater is thus implicated in the
Cretaceous–Paleogene extinction event, including the worldwide extinction of
non-avian dinosaurs. This conclusion has been the source of controversy. In
March 2010, 41 experts from many countries reviewed the available evidence: 20
years' worth of data spanning a variety of fields. They concluded that the
impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.
OIL
EXPLORER’S BIG DISCOVERY.
In 1978, geophysicists Antonio Camargo and Glen Penfield were
working for the Mexican state-owned oil company Petróleos Mexicanos, or Pemex,
as part of an airborne magnetic survey of the Gulf of Mexico north of the
Yucatán peninsula. Penfield's job was to use geophysical data to scout possible
locations for oil drilling. In the data, Penfield found a huge underwater arc
with "extraordinary symmetry" in a ring 40 miles across.
He then obtained a gravity map of the Yucatán made in the 1960s.
A decade earlier, the same map suggested an impact feature to contractor Robert
Baltosser, but he was forbidden to publicize his conclusion by Pemex corporate
policy of the time. Penfield found another arc on the peninsula itself, the
ends of which pointed northward. Comparing the two maps, he found the separate
arcs formed a circle, 111 miles wide, centered near the Yucatán village
Chicxulub; he felt certain the shape had been created by a cataclysmic event in
geologic history.
Pemex disallowed release of specific data but let Penfield and
company official Antonio Camargo presented their results at the 1981 Society of
Exploration Geophysicists conference. That year's conference was underattended
and their report attracted scant attention. Coincidentally, many experts in impact
craters and the K–Pg boundary were attending a separate conference on Earth
impacts. Although Penfield had plenty of geophysical data sets, he had no rock
cores or other physical evidence of an impact.
He knew Pemex had drilled exploratory wells in the region. In
1951, one bored into what was described as a thick layer of andesite about 4,200
ft. down. This layer could have resulted from the intense heat and pressure of
an Earth impact, but at the time of the borings it was dismissed as a lava dome
— a feature uncharacteristic of the region's geology. Penfield tried to secure
site samples, but was told such samples had been lost or destroyed. When
attempts at returning to the drill sites and looking for rocks proved
fruitless, Penfield abandoned his search, published his findings and returned
to his Pemex work.
At the same time, scientist Luis Walter Alvarez put forth his
hypothesis that a large extraterrestrial body had struck Earth and, unaware of
Penfield's discovery, in 1981 University of Arizona graduate student Alan R.
Hildebrand and faculty adviser William V. Boynton published a draft
Earth-impact theory and sought a candidate crater. Their evidence included
greenish-brown clay with surplus iridium containing shocked quartz grains and
small weathered glass beads that looked to be tektites.
Thick, jumbled deposits of coarse rock fragments were also
present, thought to have been scoured from one place and deposited elsewhere by
a kilometres-high tsunami resulting from an Earth impact. Such deposits occur
in many locations but seem concentrated in the Caribbean basin at the K–Pg
boundary.
So when Haitian professor Florentine Morás discovered what he
thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it
could be a telltale feature of a nearby impact. Tests on samples retrieved from
the K–Pg boundary revealed more tektite glass, formed only in the heat of
asteroid impacts and high-yield nuclear detonations.
In 1990, Houston Chronicle
reporter Carlos Byars told Hildebrand of Penfield's earlier discovery of a
possible impact crater. Hildebrand contacted Penfield in April 1990 and the
pair soon secured two drill samples from the Pemex wells, stored in New
Orleans. Hildebrand's team tested the samples, which clearly showed shock-metamorphic
materials.
A team of California researchers including Kevin Pope, Adriana
Ocampo, and Charles Duller, surveying regional satellite images in 1996, found
a sinkhole (cenote) ring centered on Chicxulub that matched the one Penfield
saw earlier; the sinkholes were thought to be caused by subsidence of the
impact crater wall. More recent evidence suggests the actual crater is 190
miles wide, and the previous ring is in fact an inner wall of it.
The Chicxulub impactor had an estimated diameter of 10 km (6.2
mi) and delivered an estimated energy equivalent of 100 teratons of TNT
(4.2×1023 J).[21] By contrast, the most powerful man-made explosive device ever
detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017
J),[22] making the Chicxulub impact 2 million times more powerful. Even the
most energetic known volcanic eruption, which released an estimated energy
equivalent of approximately 240 gigatons of TNT (1.0×1021 J) and created the La
Garita Caldera,[23] delivered only 0.24% of the energy of the Chicxulub impact.
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| Artist view of Chicxulub impact area much of which is submerged in the Caribbean Sea and buried in the Yucatan Peninsula of Mexico |
IMPACT
SCENARIO.
The impact would have caused some of the largest megatsunamis in
Earth's history. A cloud of super-heated dust, ash and steam would have spread
from the crater as the impactor (meteor) burrowed underground in less than a
second. Excavated material along with pieces of the impactor, ejected out of
the atmosphere by the blast, would have been heated to incandescence upon
re-entry, boiling the Earth's surface and possibly igniting wildfires;
meanwhile, colossal shock waves would have triggered global earthquakes and volcanic
eruptions.
The emission of dust and particles could have covered the entire
surface of the Earth for several years, possibly a decade, creating a harsh
environment for living things. The shock production of carbon dioxide caused by
the destruction of carbonate rocks would have led to a sudden greenhouse effect.
Over a longer period, sunlight would have been blocked from reaching the
surface of the Earth by the dust particles in the atmosphere, cooling the
surface dramatically. Photosynthesis by plants would also have been
interrupted, affecting the entire food chain. A model of the event developed by
Lomax et al. (2001) suggests that net primary productivity (NPP) rates may have
increased to higher than pre-impact levels over the long term because of the
high carbon dioxide concentrations.
In February 2008, a team of researchers led by Sean Gulick at
the University of Texas at Austin's Jackson School of Geosciences used seismic
images of the crater to determine that the impactor landed in deeper water than
was previously assumed. They argued that this would have resulted in increased
sulfate aerosols in the atmosphere. According to the press release, that
"could have made the impact deadlier in two ways: by altering climate
(sulfate aerosols in the upper atmosphere can have a cooling effect) and by
generating acid rain (water vapor can help to flush the lower atmosphere of
sulfate aerosols, causing acid rain)."
In their 1991 paper, Hildebrand, Penfield, and company described
the geology and composition of the impact feature. The rocks above the impact
feature are layers of marl and limestone reaching to a depth of almost 3,300 ft.
These rocks date back as far as the Paleocene. Below these layers lie more than
1,600 ft of andesite glass and breccia. These andesitic igneous rocks were only
found within the supposed impact feature, as is shocked quartz. The K–Pg
boundary inside the feature is depressed to 2,000 to 3,600 ft. compared with the
normal depth of about 1,600 ft. measured 3.1 miles away from the impact
feature.
Along the edge of the crater are clusters of cenotes or
sinkholes, which suggest that there was a water basin inside the feature during
the Neogene period, after the impact. The groundwater of such a basin would
have dissolved the limestone and created the caves and cenotes beneath the
surface. The paper also noted that the crater seemed to be a good candidate
source for the tektites reported at Haiti.
OTHER
SUSPECTS.
In September 2007, a report published in Nature proposed an origin for the asteroid that created Chicxulub
Crater. The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný,
argued that a collision in the asteroid belt 160 million years ago resulted in
the Baptistina family of asteroids, the largest surviving member of which is
298 Baptistina. They proposed that the "Chicxulub asteroid" was also
a member of this group. The connection between Chicxulub and Baptistina is
supported by the large amount of carbonaceous material present in microscopic
fragments of the impactor, suggesting the impactor was a member of a rare class
of asteroids called carbonaceous chondrites, like Baptistina.
According to Bottke, the Chicxulub impactor was a fragment of a much
larger parent body about 110 miles across, with the other impacting body being
around 40 miles in diameter.
In 2011, new data from the Wide-field Infrared Survey Explorer
revised the date of the collision which created the Baptistina family to about 80
million years ago. This makes an asteroid from this family highly improbable to
be the asteroid that created the Chicxulub Crater, as typically the process of
resonance and collision of an asteroid takes many tens of millions of years.
In 2010, another hypothesis was offered which implicated the
newly discovered asteroid P/2010 A2, a member of the Flora family of asteroids,
as a possible remnant cohort of the K/Pg impactor.
The Chicxulub Crater lends support to the theory postulated by
the late physicist Luis Alvarez and his son, geologist Walter Alvarez, that the
extinction of numerous animal and plant groups, including dinosaurs, may have
resulted from a bolide impact (the Cretaceous–Paleogene extinction event). Luis
and Walter Alvarez, at the time both faculty members at the University of
California, Berkeley, postulated that this enormous extinction event, which was
roughly contemporaneous with the postulated date of formation for the Chicxulub
crater, could have been caused by just such a large impact.
This theory is now widely accepted by the scientific community.
Some critics, including paleontologist Robert Bakker, argue that such an impact
would have killed frogs as well as dinosaurs, yet the frogs survived the
extinction event.
Gerta Keller of Princeton University argues that recent core
samples from Chicxulub prove the impact occurred about 300,000 years before the
mass extinction, and thus could not have been the causal factor.
The main evidence of such an impact, besides the crater itself,
is contained in a thin layer of clay present in the K–Pg boundary across the
world. In the late 1970s, the Alvarezes and colleagues reported that it
contained an abnormally high concentration of iridium.
Iridium levels in this layer reached 6 parts per billion by
weight or more compared to 0.4 for the Earth's crust as a whole; in comparison,
meteorites can contain around 470 parts per billion of this element. It was
hypothesized that the iridium was spread into the atmosphere when the impactor
was vaporized and settled across the Earth's surface amongst other material
thrown up by the impact, producing the layer of iridium-enriched clay.
In recent years, several other craters of around the same age as
Chicxulub have been discovered, all between latitudes 20°N and 70°N. Examples
include the disputed Silverpit crater in the North Sea and the Boltysh crater
in Ukraine. Both are much smaller than Chicxulub, but are likely to have been
caused by objects many tens of metres across striking the Earth. This has led
to the hypothesis that the Chicxulub impact may have been only one of several
impacts that happened nearly at the same time. Another possible crater thought
to have been formed at the same time is the larger Shiva crater, though the
structure's status as a crater is contested.
The collision of Comet Shoemaker–Levy 9 with Jupiter in 1994
demonstrated that gravitational interactions can fragment a comet, giving rise
to many impacts over a period of a few days if the comet should collide with a
planet. Comets undergo gravitational interactions with the gas giants, and
similar disruptions and collisions are very likely to have occurred in the
past. This scenario may have occurred on Earth at the end of the Cretaceous, though
Shiva and the Chicxulub craters might have been formed 300,000 years apart.
In late 2006, Ken MacLeod, a geology professor from the
University of Missouri, completed an analysis of sediment below the ocean's
surface, bolstering the single-impact theory. MacLeod conducted his analysis
approximately 2,800 miles from the Chicxulub Crater to control for possible
changes in soil composition at the impact site, while still close enough to be
affected by the impact. The analysis revealed there was only one layer of
impact debris in the sediment, which indicated there was only one impact. Multiple-impact
proponents such as Gerta Keller regard the results as "rather
hyper-inflated" and do not agree with the conclusion of MacLeod's
analysis, arguing that there might only be gaps of hours to days between
impacts in a multiple impact scenario (cf. Shoemaker-Levy 9) which would not
leave a detectable gap in deposits.
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