EARTH SCIENCES 4 – Notes from Lecture
Life undergoes great diversification 540 million years ago, but the fossil record identifies times of massive extinction (high extinction rates). Geologic time scale is a sequence of catastrophic mass extinctions causing changes in assemblages of life through time. We can name different times in relative order based on what suite of fossils exists in a given rock layer.
The last of the dinosaurs went extinct 65 million years ago. In fact nearly all life died, about 3/4 of all families. To account for this, about 90 percent of life on the planet had to end.
What could do this?
Before 65 MYA Cretaceous period (last age of dinosaurs). After 65 MYA Tertiary period. Boundary at 65 MYA is called the K-T boundary (K is for Cretaceous in German).
Walter Alvarez, UCB geologist studied the Gubbio, Italy rocks deposited from late Cretaceous into the early Triassic. Can find the boundary based on changes in microfossils of marine organisms (big change in shape, diversity, species). At the boundary there is a layer of clay a few centimeters thick. Wanted to date how long it took to deposit. Took samples of rocks across the boundary home and his dad, Nobel Laureate Luis Alvarez measured the rare metals in the rocks/clay. Looking to use presence of Iridium to date how long the clay took to deposit. Iridium is rare in Earth’s surface rocks (most went into the core of the Earth upon melting), but falls in dust from space at a continuous rate. But, in the clay they found a huge spike (Iridium Spike) with large concentration of the rare metal between 30 and 10,000 time the background level. The clay could not have taken a huge time to deposit (at normal rates, this much Iridium would need to accumulate over several millions of years, and little normal sediment would have to have been deposited so that the thin layer could be enriched; this is implausible, so the Iridium had to be concentrated by rapid infall from space. Postulated that a 10 km diameter meterorite hit the Earth and spread its Iridium over the surface. Consideration of the effects of a 10 km meteor hitting quickly recognized global catastrophe involving huge immediate explosion, rain of fiery debree that would heat the atmosphere to 450 deg. F., Dust and darkness for months would disrupt photosynthesis and lead to collapse of food chain. Acid rain.
One 10 km diameter meteorite:
Impact releases energy equal to 100 million megations of TNT (5 Billion Hiroshima bombs)
100 trillion tons of debris put into the atmosphere (impact winter, broiling re-entry)
Tremendous sea wave (tsunami) if at sea. Acid rain if on sulfates
The Moon tells us this should happen about every 50-100 million years.
Cretaceous-Tertiary Event Impact Hypothesis
10 km asteroid hit and caused global environmental perturbation that prompted extinction of surface and shallow ocean life forms.
Evidence for Meteroite:
1980 Iridium Anomalies (global)
1984 Shock Quartz and Microspherules
1988 Mega tsunami deposits
(thick around Caribbean)
1989 Candidate impact crater found in Yucatan at Chicxulub. 180 km diametr. Was in shallow oceanrocks were carbonates and sulfates.
Direct explosion (5 billion Hiroshimas)
100 trillion tons of dust
Global forest fires (soot layer)
Acid rain (hit on sulfate platform)
Atmospheric Broiling (as debris falls in air, heats atmosphere to 450 deg. F.
Frequency of impactors
Pea-size meteroites 10 per hour
Walnut size 1 per hour
Grapefruit size 1 every 10 hours
Basketball size 1 per week
Car size 1 per month (1 kiloton equivalent)
30m rock that would flatten an area size of New Jersey – 1 per 100 years
1-km asteroid 1 per 100,000 years
2 km asteroid 1 per 500,000 years
10 km asteroid 1 per 100,000,000 years
What can be done?
Nuke the Meteor
ADDITIONAL READING ON THE K-T EXTINCTION
“The geological record is extremely imperfect.”
-- Charles Darwin
The major boundaries in the geological record are defined primarily by major transitions in the life assemblages as recorded in fossils. Some of these transitions appear to be relatively gradual, while others are abrupt and dramatic. But, is the apparent abruptness an artifact of incomplete geological preservation of the history of time and life? A strong requirement of the Huttonian theory of geological evolution is that large gaps in time exist in the rock record, from the moment at which a region stops receiving new rock deposition and is elevated and eroded before sinking again and being overlain by rock. The gap in time may be immense, and almost must be immense, for the rates of vertical motion are presently observed to be relatively slow (1 cm/thousand years is pretty fast). The nature of unconformities predicts that there will be huge gaps in the rock, and hence in the fossil records.
From the work of Charles Lyell (Principles of Geology, 1833) to Charles Darwin (Origin of Species, 1859), the notion of great gouts of time, incomplete preservation of the rock record, and Uniformitarianism became so ingrained in Earth Science thought that Gradualism dominated the field for 150 years. Invoking uniformity of Law, Process and Rate, there was a strong dogma that no special catastrophic processes need be introduced to understand either geological history of evolution.
This was invoked repeatedly by Darwin, to explain the absence of gradual transitions in evolutionary history. The many ‘missing links’ between successive fossil lifeforms was attributed to an incomplete geological record. Darwin assumed that the environment changed only very gradually, and that competition drives the process of speciation and evolution. In a uniform environment, each organism will thrive by widening its niche, at the expense of other organisms sharing a common pie of life. This requires mutation and gradual adaptation which leads to the concept of survival of the fittest.
However, in the 1970’s this dogma came under scrutiny by paleontologists, who were looking at much more comprehensive rock and fossil records than available to Lyell or Darwin. Steven Gould articulated the notion of Punctuated Equilibrium in a series of works from 1972-1977, arguing that speciation occurs in subpopulations, largely in response to competition with a changing environment, rather than under uniform conditions. While accepting some aspects of gradualism, this notion invokes external triggers which change the environment abruptly to perturb the system, allowing some organisms to flourish and others to abruptly go extinct, by an accelerated natural selection.
Presently, Earth Sciences involve a merging of the ideas of Gradualism and Punctuated Change, recognizing that both play a role in both geological processes and evolution of life. In particular, mass extinctions prompted by rapid environmental changes have modulated the history of life.
Let us now consider one of the most intensively studied mass extinctions. This is the Cretaceous/Tertiary (K-T) extinction event which took place about 65 million years ago. This is long ago, but only a tiny fraction of the Earth’s lifetime, and the rock records are quite good for this time interval. As with any extinction event, the first issue to address is what lifeforms were involved in the extinction process that defines the transition. In this case, the K-T event involved extinction of 64% of all species on Earth, and included organisms in both Marine and Terrestrial environments:
mollusks: e.g., Belemnites, ammonites
plankton: coccoliths and forams with calcium carbonate shells
bryozoans, brachiopods and many corals
Dinosaurs were zeroed out
Marsupials were decimated,
Equally important are the survivors: small mammals, crocodiles
These facts suggest a global extinction that was not confined to just oceans or land, and which involved a wide range of organisms with different life needs.
The next issue is how abrupt was the extinction process? This involves the imperfection of the rock record. Precision in timing requires a well-documented, continuous record. Given the statistical vagaries of fossil preservation to begin with, sparse sampling of a gradual extinction may give the false impression of a catastrophic abrupt extinction.
For the K-T boundary, there has been extensive analysis of extinctions before the boundary, which show about 5-8 million years of reducing diversity in some groups such as the Rudists and Inoceramids. These well-preserved fossil records clearly indicate that there was a period of slow environmental change preceding the boundary which was responsible for some extinctions. The general cause is believed to have been the gradual lowering of sea level and recession of the huge interior seaways, such as had inundated much of North America. The question then arise; were the gradual changes responsible for some run-away mechanism that caused more abrupt extinction events at the K-T boundary? Or are there two independent processes operating?
The way to address the abruptness of any of the extinction events is to improve the record quality for the time interval of interest. One seeks appropriate age rock formations that were deposited in sites with continuous sedimentation (no intervals of uplift and erosion), which points toward marine environments. In addition one wants high sedimentation rates, so that a thick layer of rocks is deposited across the time interval. The faster the rate of sedimentation, the better the resulting time resolution. This points toward marine environments adjacent to continents, where there is good sediment supply.
This search to find a good record section led Walter Alvarez, a U.C. Berkeley geologist to the Gubbio rock formation in Italy. In this formation, a high rate of continuous sedimentation straddles the K-T boundary, and there were good characteristic fossils deposited throughout the section. At the ‘top’, or most recent part of the Cretaceous, the Gubbio formation involved pelagic marls, or carbonate rich sedimentary rocks, with stable Cretaceous age marine planktic (floating) foraminifera. This marl is disrupted by an layer of clay, dated right at 65 million years, indicating a major change in sediment type, that slowly was restored to pelagic marls, but now with Tertiary assemblages of foraminifera. The clay layer is very distinctive, and being bracketed by Cretaceous fossils and Tertiary fossils, it is taken to lie right at the boundary. What caused the change in sediment? Is it related to the change in lifeforms?
Walter asked the question, is there anything unusual about this clay? He benefited from the fact that his father, a professor of Physics at Berkeley, Luis Alvarez, had set up an analysis procedure to determine the composition of heavy metals in substances. Some of the boundary clay was tested, and revealed an unusually high concentration of the Platinum group element IRIDIUM. Iridium is a heavy metal very sparsely found on Earth, and with no known mechanism for concentrating it in a geological process. This suggested an extraterrestrial origin, as many iron-rich meteorites are much enriched in Iridium relative to the Earth’s rocks (most of the Earth’s Iridium is concentrated into its core). Thus, the Alvarez father and son advanced the idea that a large meteorite, enriched in Iridium by its own prior history of metal separation hit the Earth and the debris deposited a global layer.
This was immediately testable by examining the K-T transition at other regions with continuous sediment deposition across the boundary. Again and again, the materials, often involving a clay layer, but sometimes not, showed enriched Iridium relative to the background level of normal rock. In detail, the abundance of the Iridium relative to other heavy metals was very close to that for meteorite samples. Systematic mapping of this Iridium anomaly ensued. Further examination of K-T record sections revealed an equally unusual attribute in 1984. This was the discovery of quartz grains with shock lamella, or bands. Shocked quartz requires very high pressures and temperatures (it was previously only observed near underground nuclear explosions), and is very difficult to account for by anything other than a large impact. The K-T sections also revealed a large amount of microspherules, small glass beads, which appear to be drops of molten rock that cooled rapidly while flying through the air.
Mapping of the Iridium anomaly, shocked quartz, and microspherules, indicated a worldwide event at the end of the Cretaceous. Additional field studies revealed very thick, complex deposits around the Caribbean, some in Haiti, enriched in microspherules and quartz, and some in Texas and Louisiana which involved jumbled piles of debris that appears to be the deposit of a large tsunami wave. These thick deposits suggest that the impact took place in the Gulf of Mexico. In 1989, subsurface imaging revealed the existence of a 180 km diameter crater underlying the northern Yucatan peninsula, buried by the last 65 million years of sediment. The rock type there is rich in carbonates and evaporites and there is a thick deposit of broken rock overlying the subsurface crater. This is the Chicxulub crater, now the most likely candidate for the primary impact of the K-T period (there is actually evidence for two impacts, perhaps one year apart). Other candidate craters for later impacts include the smaller Manson crater in Iowa and the Popigai crater in Siberia, both of which have ages of 65 million years. Perhaps the asteroid broke up and their were multiple impacts, just as happened with the Shoemaker-Levy comet that hit Jupiter recently.
If we accept this evidence for a large impact, we must ask the question, how plausible is such an event? Is it a likely or highly improbable event? Studies of the crater density on Earth and the Moon provide some estimate of the rate of impact of different size objects. The results are startling:
Diameter Frequency Energy (TNT)
several meters 1/yr 20 Kilotons (Hiroshima=14Kt)
10’s of meters 1/100yr several Megatons (1908 Tunguska)
>1 km 1/1000000yr 1 million Megatons
There should be a 100 m object hitting the Earth about every 10,000 years, which is the size of impactor that is associated with Meteor Crater in Arizona. A 10 km object, such as needed to account for the volume of Iridium at the K-T boundary should hit Earth about once every 50-100 million years. Thus, in a way, meteor falls are common phenomena, and are indeed to be expected with time.
Could such an impact actually kill so many forms of life, as observed at the K-T boundary? What are the kill mechanisms? Several are postulated:
Direct Hit: The explosion produced by impact of a 10 km diameter projectile is about 100 million megatons of TNT, or 5 billion times the strength of the Hiroshima bomb. It would produce a 100+ km crater, like that found in the Yucatan. Over 100 x 10exp(12) tons of pulverized rock would be ejected into the atmosphere.
Since the impact site was under water, there was a huge wave generated.
The Dust in the atmosphere would produce a darkening for at least one year, sometimes called the Impact Winter scenario. This is long enough for most plants to die, disrupting the food chain on both land and sea.
The fireball from the impact would set the world’s forests burning, and indeed in the carbon isotope record for the K-T boundary there is evidence for massive fires.
The impact site was rich in carbonates, and this would vaporize to produce carbonic acid, with a short duration of acid rain.
Carbon dioxide released from the impact would add to a short-term Greenhouse effect, causing planetary warming.
These effects could have combined to eliminate many of the species that became extinct at the end of the Cretaceous.
“Your chance of dying in a global impact catastrophe is 1/10,000.”
-- Thorne Lay
The impact hypothesis for the K-T boundary event can account for the global environmental change that would account for the widespread extinction that occurred. However, is it a unique explanation? It provides an independent punctuation of the relatively gradual decrease in sea level and resulting reduction in shallow water environments that had been occurring for about 5 million years. It can explain many observations, such as the global presence of a thin Iridium-enriched clay layer, shocked quartz, microspherules of glassy materials, large wave deposits, and the presence of a 180km diameter crater in the Yucatan. As a scientific hypothesis, it is viable and testable. But that does not ensure that it is a unique interpretation, nor does it necessarily establish a causal link between the impact and global extinctions.
Another clear geological record of the late Cretaceous is massive layers of basalts, in the form of Flood Basalts, found in several places. Immense deposits of basalt flows are found in India, in the Deccan Traps, and in other countries including Siberia, Brazil, and the western U.S.. At the end of the Cretaceous India had not yet collided with Eurasia, and the Deccan Traps formed rapidly by massive volcanic extrusion over the present position of the island of Reunion. It is speculated that the dust arising from these prolonged and extensive eruptions could have opaqued the atmosphere, much as an Impact Winter would, causing global disruption of the food chain and massive extinction. There is no clear explanation for why the Cretaceous finished with massive volcanism, but presumably deep mantle dynamics and turn-over were involved. The onset of basalt flows in the Deccan Traps appears to have slightly preceded the end of the Cretaceous, but it is striking to plot the position of India at the time, relative to the impact site in the Yucatan. They are almost on opposite sides of the Earth, which has prompted the speculation that the impact sent shock waves through the Earth that accentuated the volcanic activity on the far side of the planet. In this way, the catastrophic effects of the impact and volcanism could be linked.
From our consideration of the frequency of impacts, it appears that a large meteorite or comet may strike the Earth about every 50 million years on average. Is this the explanation for the many dramatic extinctions in the geological record? Some extinctions were in fact much more extensive than the K-T event. 250 million years ago, at the boundary between the Permian and Triassic periods, 75% of all genera and 95% of all oceanic species went extinct rather abruptly. In fact, life on Earth was almost wiped out. This again suggests a global catastrophe. Other mass extinctions took place 360 and 435 million years ago, and lesser extinctions have speckled geological history.
Following the Alvarez hypothesis of a large impact, paleontologists began to scrutinize the record of extinctions for statistical properties. If one plots the rate of genus level extinctions as a function of time over the past 250 million years, a statistically significant periodicity of 26 million years is found. The same is true if one considers extinctions of entire families of taxa. This is extraordinary, and suggests some sort of regularity in what might be assumed to be a totally random process of environmental perturbations.
If one considers terrestrial craters that have 140-200 km diameters and ages over the past 2 billion years (well preserved in some parts of the continents, particularly long stable regions such as in North American, Europe and Australia, a total of 130 are found. Many smaller craters are also found, and if those with a diameter greater than 10 km are considered, there proves to be a 32 million year peak in the time between impacts, which is quite close to the 26 million year extinction number. Variations in low sea level show peaks with periods of 21 and 33 million years, while changes in plate creation (sea-floor spreading) peak at 18 and 34 million years. Are these processes independent or linked? This is very hard to establish, but one can ask the question, What in the Solar System has a 26 million year period? The answer is that nothing does, so any explanation for an extraterrestrial cause must invoke a larger scale phenomenon.
One idea that emerged in the last decade is that there may be a 26 million year perturbation of the Oort Cloud, which is a vast halo of comets that lies at large distances from the sun. Based on the number of comets that penetrate into the inner solar system, Oort proposed in 1950 that there are around 10exp18 comets about 1 light year away from the sun, the relic of the original solar nebula which formed the solar system. These are in a spherical shell of orbits, and some process is needed to periodically perturb them.
Two ideas for how to perturb the Oort Cloud have been proposed:
(1) A dark sister sun called Nemesis: Our sun formed as a binary, with a secondary star that never began to shine (too small for fusion). Interaction of the orbit of the two stars could regularly cause gravitational perturbation of the Oort Cloud. Systematic search for Nemesis has not yet revealed such a dark star.
(2) Galactic Plane Oscillation: Our solar system is not static within the galaxy, but oscillates up and down through the main symmetry plane of the Galaxy with about a 60 million year periodicity. Each passage through the plane could lead to the surrounding Oort cloud being perturbed by increased interstellar mass in the galactic plane. This would release a hail of comets, some of which would strike the Earth. The timing is not quite perfect, but it is on the right scale. We are now pretty much in the middle of a cycle, so would not expect another mass extinction for millions of years.
But, there are many objects in space that could fall to Earth at any time, and they need not all originate by some perturbation of the Oort cloud. By the best estimate now, your chance of being killed in a global catastrophic impact is considered to be 1 in 10,000. This is the same as your chance of dying in an airplane crash. It is about 1/60th of the chance that you will die in an auto accident.