One of the theories as to why the dinosaurs became extinct is that a huge comet struck the earth and caused the sun to be blocked out because of all of the dust and debris generated by the object’s impact on the earth. What I don’t understand is that there is such a thing as gravity and that such debris would fall back to the ground in a matter of days. So why would the sun be blocked out? This is something I have just never been able to ‘get’. How does this theory work? Thanks for any info.
Depending on how high the debris was distributed, it could take well more than a few days. Dust is fairly light and can easily be carried by the wind and not fall to earth for quite some time. Krakatoa’s explosion, for example, caused sunsets to be shaded different colors for well more than a few days.
Zev Steinhardt
Many of the particles are so small that atmospheric circulation will keep the aloft for months, or even years. Debris from the explosion of Mt. Krakatoa lingered for a year or so and actually lowere average earth temperature for a time.
Here’s a really short version of theKrakatoa explosion. I’m sure Wikipedia and other sites have a lot more if you search.
From Weather Events: The Year Without A Summer:
*What Made 1816 So Cold?
The meteorological facts of life during 1816 have been laid out. The period March to September was marked by a series of strong and frequent invasions of dry arctic air across New England. While the movement of arctic air masses through this region is not uncommon in other seasons, their appearance in the summer as cold and frequent as in 1816 is indeed unusual. The question arises, why? Various theories have been put forward.
The most likely cause was volcanic influences. Proponents note that a number of major volcanic eruptions preceded 1816: Soufriére and St. Vincent in 1812: Mayon and Luzon in the Phillippines during 1814; Tambora in Indonesia during 1815. The volcanic theory of climatic influence relates increased volcanic activity with decreased temperatures due to the increased reflection of solar radiation from volcanic dust blown and trapped high in the atmosphere. The Tambora eruption has been estimated to be the most violent in historical times. The explosion is believed to have lifted 150 to 180 cubic kilometres of material into the atmosphere. For a comparison, the infamous 1883 eruption of Krakatau ejected only 20 cubic kilometres of material into the air, and yet it affected sunsets for several years after.*
A comet or asteroid could produce the same effects many times over, depending on it’s mass.
Related question: wouldn’t blocking out the sunlight hit smaller critters hardest? It’s my understanding that mostly big critters were wiped out, hence the theory that land bridges were forming, allowing big animals to carry their diseases over to other continents. Or has that theory been discredited?
This particular view as to why most dinosaurs* went extinct is not entirely on the mark, for the exact reason you mention - gravity gets in the way of a good, long-lasting debris cloud. It’s not entirely wrong either, but it is very likely only one factor among several. (For the purposes of this post, “dust” includes both particulate matter and aerosols.)
For the record, the debris thrown into the atmosphere by a meteorite impact or large volcanic eruption can influence global climate for a period of years, but there are certain conditions that need to be met:
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The particle size must be small enough for the dust particles to remain easily adrift on air currents. That does mean that chunks of debris big enough for you to see easily with the naked eye are not going to stay aloft very long, owing to that pesky gravity thing.
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The particles need to be injected into the stratosphere, where they are not subjected to the vagaries of normal, everyday weather patterns. If a dust particle is drifting along somewhere within the troposphere, it will eventually get “rained out” (i.e., it will adhere to a raindrop or other form of precipitation and be carried to the ground). The stratosphere does not have weather patterns like the troposphere’s (no precipitation), and there is little direct exchange between these two layers of the atmosphere, so a particle that’s up there will tend to stay up there for some time.
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Whether or not there is global distribution of dust generated from a point source (meteorite crater or volcano) is going to be dependent upon the latitude of the source. Mid- to high-latitude sources will not have their dust distributed very effectively, because the circulation of higher altitude winds tends to be zonal (following along bands of latitude). Thus a volcano in Iceland, say, could have its dust widely distributed across northern Europe and the north polar region, but the dust won’t make it very far south. HOWEVER, a low-latitude dust source DOES have the potential to have a global impact because of the Intertropical Convergence Zone (ITCZ), an area of low atmospheric pressure and strong updrafts as well as a break in the boundary (tropopause) between troposphere and stratosphere. (See this diagram for an idea of how the atmosphere circulates from the equator to the poles.) Dust can be carried through the break in the tropopause, where it would then be picked up and distributed through the stratosphere both north and south of the equator by jet streams.
The mechanism by which atmospheric dust is supposed to affect global climate is through an increase in the Earth’s albedo - that is, the high-altitude dust would reflect more of the sun’s energy back into space than usual, resulting in cooling of the Earth’s surface. The more dust, the more severe the cooling ought to be.
Back to the dinosaurs -
In the past, it was thought that the volume of dust accompanying a large meteorite impact, volcanic eruption, sufficiently large nuclear bomb blast (the “nuclear winter” scenario) or maybe even large-scale oil field fires (as in Gulf War I) could lead to a significant disruption in global climate that in the very least would lead to widespread famine. This was the prevailing view at the time that Luis Alvarez and colleagues first proposed that the K-T impact event led to the extinction of the dinosaurs (in 1980), so it’s not surprising that the idea is still associated with dinosaur extinction among many geologists and lay people today.
However, climatologists who spent some time reviewing historic records of large volcanic eruptions, such as the ones lieu mentioned, began to realize that while dust (particulates and aerosols) did indeed influence climate, the story was rather more complicated. The climatic effects could be contradictory (e.g., dark particulates on snow decrease albedo and increase heat absorption at the surface, whereas aerosols in the stratosphere reflect sunlight and promote cooling), and the impacts were nowhere near as severe on a global basis as they would have thought. In fact, none of the eruptions studied appeared to have climatic effects that lingered more than a decade or so. (In contrast, for the more recent and relatively less spectacular eruption of Mount Pinatubo in 1991, the atmospheric effects lingered for just 2 years.)
In the case of the K-T boundary impact, a recent study suggests that the debris thrown into the atmosphere by the impact eventually fell out of the atmosphere in 60 years or less, with climatic effects lasting for roughly the same length of time. That is significantly longer than the largest historic eruptions we are familiar with, but there are complicating factors that make it difficult to know just how the climate system responded. For one, the impact wasn’t located on land, but rather a shallow marine carbonate platform (think of something on the order of the Great Barrier Reef off the coast of Australia). The Chicxulub crater is on the order of 200 km across, and the carbonate platform was about 3 km thick, so a huge volume of limestone (mostly CaCO[sub]3[/sub]) and salts (mostly gypsum, CaSO[sub]4[/sub]) were vaporized on impact, along with the overlying sea water. Vaporized limestone yields carbon dioxide, which along with water vapor constitutes two of the most powerful natural greenhouse gases.
However, vaporized (aerosolized) sulfur dioxide from the gypsum has a cooling effect on climate (just like the volcanic gasses emitted during eruptions). So, which competing climate forcing held more sway - the cooling dust and aerosols in the stratosphere, or the warming greenhouse gases in the troposphere? Most current climate models are not capable of calculating the radiative effects of more than about 20 times the modern level of atmospheric carbon dioxide, so for the moment we don’t really have an answer.
Additional possible complications that we can’t assess very well right now include the possible impact of significant amounts of acid rain destroying ecosystems (carbon dioxide mixing with precipitation to form carbonic acid), large scale fires and their resultant soot and ash contributions (mostly within North America), and perhaps pre-existing environmental stresses that were already forcing a decline in biodiversity before the impact (e.g., volcanic outgassing that accompanied the eruption of the Deccan Traps basalt flows in India).
The bottom line is that there appears to be no simple answer to the story of the end of the dinosaurs. Maybe more than you really wanted to know, 
but that’s the situation as it now stands.
- Many paleontologists now accept that birds are the direct descendants of one line of dinosaurs, so in one sense the dinosaurs did not go completely extinct at the K-T boundary.
To answer your questions:
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A significant blocking of sunlight/cooling climate would have the greatest impact on herbivores (plant-eaters) since plants would obviously take a hit. Large animals are at a greater disadvantage because of their larger foods requirements, but small herbivores could suffer badly as well. Small critters that didn’t have a selective diet (omnivores) would probably be in the best position to ride out a major environmental disruption.
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I think you’re confusing the extinction of the dinosaurs (65 million years ago) with the extinction of Pleistocene mega-fauna (circa 10-12,000 years ago). Many large mammals unique to the Americas seem to have gone extinct within a very short time, and it’s been suggested that migration of new critters, including humans, across the Bering land bridge that contributed to that extinction, either through overhunting or the importation of Old World diseases to the New World. Both the hunting and disease hypotheses are considered controversial.
excellent post, sunfish, I just have one thing that always bothers me:
It seems pretty obvious to me that the Deccan and Siberian traps were both caused by major impacts. ( and probably the Columbia river ones as well).
Both major volcanic eruptions happened on the exact opposite side of the planet as a major impact, at about the same time. It boggles my mind as to why the majority of the scientific community is so resistant to the idea, simply because they can’t model it well enough to get the same result. Obviously the shockwaves that went around and through the earth from the impact all converged at these points and had a major effect on the local geologic activity.
I’ve seen that the two events differ slightly in dates, with the major volcanism events sometimes shown to procede the impacts, but the error bars give plenty of room for them to have happened at the same time.
The formation of the Deccan traps would have certainly added to the destruction caused by the impactor , and are probably also another result of it, IMO.
Of course the opening up of the Atlantic ocean would have no effect on the climate.
Thanks for the compliment, Thaumaturge. I worry that sometimes I get too long-winded for my own (and anyone else’s) good.
With regard to the hypotheses linking the K-T impact directly to the Deccan Traps: I know where you’re coming from, as the issue was being pretty hotly debated not all that long ago (certainly while I was still a grad student). But in this case, it’s fair to say that the age constraints we now have in hand ([sup]40[/sup]Ar/[sup]39[/sup]Ar radiometric dating and magnetostratigraphy) rule out a direct link between the two, and the eruption of the Deccan Traps did in fact begin before the K-T impact. Courtillot et al. (2000) sum up the whole situation; the following is an excerpt from their abstract, with some footnotes or italicized comments added by me to explain some of the details:
[sup]1[/sup] Chrons are time intervals that reflect how long the Earth’s magnetic field polarity was oriented in a direction either the same as at present (a normal chron) or reversed (a reversed chron). A tightly dated framework of these polarity intervals is available to constrain rock ages for the most recent 180 million years or so; the intervals are numbered rather than named, with lower numbers being more recent.
[sup]2[/sup] Since the element iridium (Ir) is much more abundant in meterorites relative to the Earth’s crustal rocks, an increase in the amount of iridium in a sedimentary layer is interpreted as a contribution via meteorite impact.
[sup]3[/sup] The most precise date for the K-T impact that I’m aware of is 65.2 +/- 0.4 Ma (Sharpton et al., 1992), and the paleomagnetic data from Chicxulub impact debris coincides with the magnetic field orientation for chron C29R (Urrutia-Fucugauchi et al., 1994), which makes it clearly younger than the oldest part of the Deccan Traps.
As for the Siberian Traps, I believe that the jury is still out on the topic. At this point in time, the age constraints aren’t so good, and since the rocks are rather older (late Paleozoic), the hunt for appropriate data is a bit more difficult.
References:
Courtillot, V., Gallet, Y., Rocchia, R., Feraud, G., Robin, E., Hofmann, C., Bhandari, N., and Ghevariya, Z.G., 2000, Cosmic markers, [sup]40[/sup]Ar/ [sup]39[/sup]Ar dating and paleomagnetism of the KT sections in the Anjar area of the Deccan large igneous province. Earth and Planetary Science Letters 182:137-156.
Sharpton, V.L., Dalrymple, G.B., Marin, L.E., Ryder, G., Schuraytz, B.C., and Urrutia-Fucugauchi, J., 1992, New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary. Nature 359:819-821.
Urrutia-Fucugauchi, J., Marin, L., and Sharpton, V.L., 1994. Reverse polarity magnetized melt rocks from the Cretaceous/Tertiary Chicxulub structure, Yucutan Peninsula, Mexico. Tectonophysics 237:105-112.
Over the (really) long haul it surely did. However, since the opening of the Atlantic had been underway already for well over 100 milion years at the time of the K-T impact, I think it’s safe to say that it was not the proximal cause of the mass extinction at 65 Ma.
Thank you, my people. I understand much better, now.
Actually, wasn’t the original name "KrakatAO?
In one of his essays S.J. Gould addresses the issue. It seems reasonable to assume that both the Deccan Traps and the Chicxulub impact were contributing factors to dinosaur decline and eventual extinction. However, it’s also worth noting that other factors came into play. For example, while most families of dinosaurs that survived into the Late Cretaceous to begin with lasted until the K-T boundary, species diversity was very much on the decline in the last stages of the Cretaceous.
E.g., to set modern examples, Proboscidea and Perissodactyla are orders that are reasonably stable – but the overwhelming majority of variant proboscideans and perissodactyls are now extinct. The two species of elephant (in two genera) are the only surviving proboscideans, out of perhaps 20 different genera in the Miocene and Pliocene. Among perissodactyls, there is one genus of equids, with perhaps seven species, three or four species of tapirs (two genera IIRC), and five species in four genera of rhino. And several equids and nearly all the rhinos have two feet in the grave and the other two on banana peels.
To relate this to dinosaurs, there were two genera of ceratopsians, I believe only one genus of sauropod, between one and three genera of large carnosaurs (it’s debatable whether two of the genera are synonymous with Tyrannosaurus), only one species each of nodosaur and ankylosaur, and significant declines in the numbers of coelurosaur and hadrosaur genera. So just as it would take very little pressure to cause rhinos and elephants to go extinct, so too would most of the major dinosaurs be easily driven to extinction.
But the key point to all this is the idea that no one single cause is the reason the dinosaurs went extinct. As well might we say that Chicago Bridge Corp. drove the ivory-billed woodpecker into extinction – it’s true, but only in the sense that that company lumbered off their last refuge after rampant timber harvesting had cleared the rest of their habitat areas. So to say “X made the dinosaurs extinct” is merely to say, “After causes A-M had driven the dinosaurs into near-irretrievable decline, X caused the last dinosaurs to die.”
I would even go so far as to say, “in any evolutionary meaningful sense, dinosaurs did not go extinct, nor are they now extinct.” Phylogenetically, birds can be traced from reptiles through dinosaurs - specifically, through Saurischia (which also includes the giant sauropods), on through Theropoda (all of the non-saurpodian saurischians: the bipedal, mostly “meat-eaters”), continuing on through Tetanurae (“stiff-tailed theropods”), then Coelurosauria (this is where they then branch with theropods such as Tyrannosaurus), through Maniraptora (allying them with the likes of Deinonychus and Velociraptor), through Ornithurae, and finally Ornithothoraces (the non-avian members of which are still remarkably bird-like, e.g., Confuciusornis, and are thus often referred to as “enantiornithine birds”, even though they are not members of Aves proper).
Because birds are dinosaurs, and because birds escaped the fates of their larger, more well-known (as dinosaurs, anyway) brethren, it cannot be reasonably said that dinosaurs are, in fact, extinct. And to deny that birds are dinosaurs at this point is akin to denying that humans are mammals. This obviously requires some mental tinkering, as most of us have been taught from a very early age that “dinosaurs are extinct”. Such an idea itself is largely the result of the decidely non-evolutionary classification scheme created by Linnaeus, which separated birds completely from the likes of Reptilia, thereby allowing whole branches of Reptilia to be wiped out at the end of the Cretaceous without so much as a gentle breeze blowing through the avian family tree, much less a complete shakedown of it.
Aside from that fact, we also have another problem associated with “dinosaur extinction” that is all-too-often glossed over in favor of the more exotic tyrannosaurs and ceratopsians and such: at least five major groups ate it at the end fo the Cretaceous. In a comprehensive survey[sup]1[/sup] of the western North American fauna at the K-T boundary (or, more accurately, the 10 milion years or so preceding the boundary), it was found that the following groups were represented (numbers represent # of species present in that fauna during that time):
Elasmobranchii (sharks and kin): 5
Actinopterygii (ray-finned fish): 15
Lissamphibia (true amphibians): 8
Multituberculate mammals: 10
Eutherian mammals: 6
Metatherian mammals (marsupials): 11
Testudines (turtles): 17
Squamate reptiles (lizards): 10
Champsosaurs: 1
Crocodilia: 5
Dinosauria (non-avian): 19
Of those groups, none of the elasmobranchs survived past the boundary; 9 of the actinopterygians did, as did all 8 of the amphibians (a fact which is itself somewhat peculiar, as amphibians today are notably sensitive to changes in environment and the extinction or endangerment of many species is indicative of some sort of environmental “trouble”); half of the multituberculates died out, as did all but one of the marsupials; all 6 eutherians survived, as did 15 of the 17 turtles; only 3 of the 10 squamates survived, and so did the lone champsosaur species; 4 of the 5 crocodilians made it, but none of the 19 dinosaur species did. So the groups that took it the hardest from this sample were the sharks, lizards, marsupials, and dinosaurs. This peculiar pattern of extinction and survival makes finding the cause quite problematic, as no single explanation seems to cover all bases. Indeed, the single best explanation which would account for the losses as well as the survivors is that of seaway regression (some may recall that much of central North America was a vast inland sea during much of the Cretaceous. This seaway began disappearing toward the end of the Cretaceous) - all, except the lizards, that is. There is no reason to suspect that regression alone would have resulted in such a significant body count among lizards. I do not think that a similar comprehensive faunal census has occurred for other regions, so it is largely unknown if the specific North American pattern is repeated across the globe, or if it represents a special case (obviously, sharks and such did survive somewhere, for example, but other faunas may have had their own peculiarities of survival and extinction; though we do know that we have yet to find a non-avian dinosaur specimen above the boundary so it is a reasonable conclusion that that pattern, at least, is repeated). Also note that other groups not mentioned are also known to have bought the proverbial farm at this point as well, including the marine reptiles (mosasaurs and plesieosaurs; ichthyosaurs went extinct sometime during the mid-Cretaceous), as well as pterosaurs.
So at the end of the Cretaceous, there are three major events happening: massive volcanism, resulting in the formation of the Deccan Traps; seaway regression of a vast inland sea in North America; and a significant impact event in the Yucatan peninsula. It is likely that all three of these contributed in some way to the demise of the various groups, and it is possible that some groups weathered one or more of the events only to be done in by another. One problem we have with determining the ultimate killer is that we lack fine-enough detail in the fossil record. To determine, for example, if the impact itself was the ultimate cause, we would need to know what groups were still around within about a hundred years on either side of the K-T boundary (this is because the impact event surmises a sudden, catastrophic extinction, rather than a long, slow decline). And, unfortunately, we just don’t know right now. So, contrary to popular belief, it is not a “done deal” that asteroid impact wiped out the dinosaurs (or anything else, really) at this time.
1 J.D. Archibald and L.Bryant, 1990. Differential Cretaceous-Tertiary extinctions of nonmarine vertebrates: evidence from Northeastern Montana, Geological Society of America Special Paper 247, pp. 549-562
I’m glad you stepped in with the specifics, Darwin’s Finch. Paleobiology is more of a hobby for me, and it’s usually Neoproterozoic at that. I haven’t dared to stick my head too far into the room while the vertebrate paleontologists sort out who is related to whom. 
By the way, didn’t someone argue once that they had found “classic” dinosaur fossils (IIRC Triceratops) above the K-T boundary layer? I would assume that since I heard about it only once about 10 years ago that something was remiss with the interpretation, but since it’s not my field I lost track of the discussion.
There are indeed quite valid dinosaur fossils (and not artefacts) from Paleocene and Eocene sedimentary rock. I recall reading specifically about carnosaur teeth from the Eocene. But the general view of paleontologists is that these are fossils which weathered out of Cretaceous (or earlier) sediments and were redeposited in early Cenozoic strata. There are ways, in which I am not expert, to identify when this sort of redeposition is likely to have occurred. (Other than the fact that besides such disarticulated fossils there is no other evidence for continued survival of the species.)
As a sedimentary geologist by training, I’m well familiar with reworked sediments and the things that can wind up in them. No, in the case of the alleged post K-T fossil I read about, I seem to recall the scientist involved arguing that his fossil lay in strata a few hundred thousand years younger than the impact. Since it was reported in a journal like Science or Nature, I would have hoped that the reviewers would have ruled out a reworked fossil before it was published, but I suppose there’s always room for error.
There have been a number of unsubstantiated claims for non-avian dinosaur fossils above the K-T boundary - most of these are in the form of teeth. There is, however, also this little tidbit from 2000 (.pdf document) - an isolated hadrosaur bone found in Paleocene rock. I haven’t seen a critique of this paper, though, so I have no idea what the current standing is on it.
(I am, by the way, but an amateur in these things as well. My goal, way back when, was to become a professional paleontologist one day, but things didn’t turn out that way. So now it’s just a hobby )
OK - I did some digging around (heh) and found that a Robert Sullivan (et al.) published a critique[sup]1[/sup] of the paper I mentioned above. Based on Sullivan’s findings, Fasset’s sample had become contaminated with Paleocene pollen, thus lending the appearance of those rocks being Paleocene in age. Sullivan was unable to identify similar pollen in his own sampling of the area - which was actually Late Campanian (81-72 million years old) in age. So, it looks like once again, the possibility of Paleocene dinosaurs has been shot down.
1 Sullivan, Robert M., Spencer G. Lucas, and Dennis Braman, 2002, Paleocene Dinosaurs? A critique of ages assigned to the upper Kirtland Formation, San Juan Basin, New Mexico: Journal of Verebrate Paleontology, 23 (Supplement to Number 3), Abstracts of Papers, Sixty-second Annual Meeting, p. 122A