In Walt Disney's movie "Fantasia" (1940), dinosaurs were shown as dying in a hot and dry world, full of active volcanoes. Recent discoveries show that something like that might really have happened and that the idea that the dinosaurs were killed by an asteroidal impact appears to be incompatible with the available data. Rather, it seems that the dinosaurs died out because of the global warming resulting from the emission of large amounts of greenhouse gases from volcanoes. In several respects, it is not unlike what's happening today to us.
I know what you are thinking: these silly scientists; first they tell us that an asteroid killed the dinosaurs, now they tell us that it is not true. So, how can we believe them when they tell us that humans are causing global warming?
On this, I have to tell you something: science is a mighty truth-seeking juggernaut. Yes, individual scientists are not immune from mistakes, political biases, and human failures, but, on the whole, science manages to filter away bad ideas and keep the good ones. The case of the extinction of the dinosaurs is a beautiful example of how well the mechanism works.
As you will read in the article below, the non avian dinosaurs, it seems, went away not with an asteroidal bang, but with a volcanic whisper. They were killed over several tens of thousands of years by the global warming created by the emission of gases from the giant basaltic eruption known as the "Deccan Traps", today located on the Indian subcontinent. To be sure, the discussion is far from being settled and many scientists still favor the impact theory (e.g. Peter Ward and Joe Kirschvink in their recent book "A new history of life"). Personally, I am no specialist in these matters but, if I did my homework well (and I think I did), my impression is that the data overwhelmingly favor the volcanic hypothesis over the asteroidal one.
So, no asteroid killer? If that's the case, how could science make such a mistake? The answer is that there was no "mistake". There was just the gradual build-up of data and models that led to a better and better understanding of the mechanisms of mass extinctions in the earth's past and of the specific events that led to the so-called "K/T" mass extinction that involved the demise of the non-avian dinosaurs. So, it is true that there was a large asteroidal impact occurring approximately at the K/T boundary. But whether this was the actual cause of the mass extinction always remained a hypothesis. It was only the spectacular character of this hypothesis that led it to become so popular with the general public. But popularity in the media is not the same as scientific certainty and, after decades of work, science is gradually arriving at a consensus on this matter, just as it has arrived to a consensus on climate change. Science, unlike politics and fashion, doesn't go in cycles, it moves forward.
The real causes of the extinction of the dinosaurs
by Aldo PiombinoAldo Piombino is an independent researcher collaborating with the Department of Earth Sciences of the University of Florence.
It is widely accepted by the public opinion and by many scientists that the Dinosaurs went extinct because of a meteorite impact that occurred along the Southern Mexico coasts, along the coast of the Yucatan peninsula.
Well, this is not true. The “Impactists,” those who propose the impact theory, have been successful for a while in having a stronger voice than their opponents. But, in March 2013 a meeting at the Natural History Museum in London, (acts are published in the Geological Society of America Special publication n. 505) left no doubts: the killer of the dinosaurs was not the Yucatan impact, but the gases and other volatiles that came from the Deccan Traps activity, in which some hundreds of thousands of cubic kilometers of magma were produced in a few tens of thousands of years.
There is multiple proof that the Earth system has seen similar conditions causing mass extinctions well before the dramatic K/T event that killed the non-avian dinosaurs. A similar volcanic activity occurred (and it is blamed for) the mass extinction events of the late Devonian, the end Permian, end Triassic, end Cenomanian, end Paleocene, and others. From the end of the Devonian period, all the main boundaries of the Earth chronology correspond to similar volcanic activities, termed "large igneous provinces" (LIPs).
The first scientific ideas about the dinosaur extinction in the '30s were that they were wiped out because of a sudden warming of the Earth. This is well exposed in “Fantasia”, one of the masterpieces in the Walt Disney's production, in which we see great reptiles dying in a dry word, surrounded by a warm haze. In the 50's someone blamed a meteorite for the extinction: the cosmic fall would have triggered a sudden and violent atmospheric warming.
Global warming remained the best explanation since 1980, when the Berkeley team of Louis Alvarez found that all the K/T section known at that time (Gubbio in Italy, Stevns Klimt in Denmark and Woodside Creek in New Zealand, showed an anomalous spike in the Iridium content (5). So they proposed that a chondritic meteorite crashed on the Earth, triggering a long winter, a sort of “nuclear winter”. The Iridium, contained in the celestial body, had been released in the air and deposited on the ground and on the sea surface. This idea became immediately popular and scientists that didn't agree with the idea of the extraterrestrial origin of K/T event had troubles in having their voices being heard.
The 80's saw many scientists searching of the impact crater which was finally found in 1991, along the Yucatan coasts. The dimensions of the crater coincided with the hypothesis declared by the Berkeley team of a body with a 10 km diameter and the age of the collision was Late Cretaceous. But a few years later, Upper Maastrichtian sediments were found on top of the impact ejecta, thus refuting a precise K/T age of the event.
At the same time, scientists found that all the main extinction events, such as the End Permian and End Triassic extinctions, were simultaneous with the emplacement of huge basaltic series: the flood basalts, Large Igneous Provinces, and that the same activity occurred also at the K/T boudary (the large igneous province known as the Deccan Traps). Today, there are no doubts that the K/T extinction has been triggered by the gas emissions from the Deccan Traps. Recent studies of the palaeomagnetic declination registered in the lavas demonstrate that the emplacement of the second, and larger, phase of the activity lasted few tens of thousand years and not hundreds of thousands as it was supposed earlier on (1). The main elements that favor volcanic emissions as the cause of the mass extinction are the following
1. The Maastrichtian climatic evolution is clearly in tune with the pulses of the volcanic activity: the biotic crisis begun well before K/T and the impact occurred well after the beginning of the crisis.
2. According to the impact hypothesis, the K/T event has been a cold, dark moment because of the powders derived from the impact and of the fires ignited worldwide by hot ejecta. This cannot be true, because, instead, there exists compelling evidence that the last 50.000 years of the Cretaceous saw a sudden warming, triggered by the enormous CO2 emissions from Deccan traps; thus, it was not a cooling phase. It is true that, after the impact (between 150.000 and 100.000 years before the Mesozoic Era end) there was been a cooler stage, but this is an ordinary event in mass extinction dynamics, when they are triggered by huge volcanism, since they are always accompanied by strong sea level variation. In particular, the final stage is normally a marine transgression following a cooler period characterized by a huge sea level drop. These sea level drops are mainly triggered by the arrival of volcanic volatiles of the Large Igneous Province in the stratosphere, thus enveloping the entire Earth and preventing much of the solar rays to arrive in the lower atmosphere. So, the K/T was mainly characterized by a warm climate because of the high levels of atmospheric greenhouse gas.
3. The sudden extinction in microfossils such as Planktonic Foraminifera is reported where an upper Maastrichtian hiatus is not seen: the low sea level in the upper Maastrichtian before the latest 50.000 years of the stage in many areas (and above all in the Caribbean) resulted in a temporary stop of the sedimentation. Stratigraphic sections where marine sedimentation continued without a hiatus show a very gradual extinction pattern. This scenario fits very well with the volcanic trigger, like the other mass extinctions do, and does not fit with a punctual event like a meteorite impact.
4. The sea water acidity clearly originated by the high CO2 amount coming from the Deccan traps and the acidification began well before the impact. After the K/T acidity crisis, the system saw a partial recovery, but it was interrupted in early Danian, synchronous with a new, later, spike of volcanic activity in India. It is impossible that the asteroidal impact, alone, could have generated such a high amount of this gas; simply because it broke up rocks of the carbonate platform of Yucatan and, above all, the CO2 increase begun well before the impact
5. No one can say whether the dinosaurs were wiped off in a long or in a short time (3), but we must note that the youngest dinosaur fossils or footprints are almost 450.000 years older than K/T and now it is not known if this is due to a lack of fossils or if they became extinct well before the K/T.
6. The Iridium anomaly is probably best explained as the result of the Deccan volcanoes, generated by aerosol diffusion: similar anomalies occur in the volatiles of Kilauea and in Antarctica (2). These forms of volcanism are typical of intraplate volcanism. And, also, volatiles coming from the intraplate Piton de la Fournaise volcano, located over the mantle plume that originated Deccan Traps long ago, show the anomaly (4). Moreover the anomaly found by the Berkeley team in Gubbio begun well before the K/T boundary and vanishes for a long time interval before the final increase (5). How can Iridium came from the impact if his anomaly begins well before the event?
7. The microspherules that were found in the K/T sediments, for example in Denmark and in New Zealand, are of sedimentary origin and they are not the alteration of tektites coming from the impact. Moreover, also the occurrence of fullerenes doesn't necessarily imply fires triggered by the impact worldwide: charcoals are widespread in all upper Cretaceous sediments because of the occurrence of wildfires triggered by high levels of atmospheric Oxygen. The diffusion of the wildfires is one of the causes of the decline of Conifers and of the Angiosperm diffusion. During the latest Cretaceous period, wildfires increased at the highest level because of the worldwide warm and dry climate; these changes were triggered by volcanic emissions
8. Those who propose the impact theory say that a 3 meters thick level along the coast of the gulf of Mexico was deposited by the Tsunami triggered by the meteorite crash. This cannot be true: the level shows many hiatuses (demonstrated also by the occurrence of paleosoils showing bioturbation structures) and it has sedimented for a long time, some tens of hundreds years: it is the result of sedimentation during the low standing sea level before the earliest Maastrichtian transgression
9. Smectites are very common at the K/T boundary. They do not represent the alteration of the impact tektites, they show a huge volcanic signature and they have originated from Deccan traps. It's interesting that the smectite amount increases, replacing illite deposits, in 3 time intervals that are coeval with the 3 main phases of Deccan activity
10. It is evident that the epicenter of the geochemical and biotic crisis is placed in the indian region, as we can see in the Krishna - Godavari basin and in the Meghalaya area.
11. For the International Commission on Stratigraphy the K/T limit is defined if there is one of these characteristics: the Iridium spike, the extinction of all tipically Cretaceous planktonic foraminifera except the Guembelitria Cretacea (a high acidity and low Oxygen resistant form), the occurrence of the first Danian foraminifera and a particular excursion of δ13C. The ejeta from the Yucatan crated are not considered as a diagnostic character for the K/T boundary, because the impact occurred some time before.
12. The δ13C excursion demonstrates a huge perturbation in the carbon cycle; it is diagnostic for a large igneous province and occurred in a similar way at the end of Permian and at the end of Triassic.
13. The 1783 eruption of the Laki volcano can be seen as a small scale simulation of what can happen during the emplacement of a many thousands cubic kilometers lava flow like the ones of a Large Igneous Province. With the Laki eruption, only 17 cubic kilometers of lavas were produced, but the eruption saw the highest registered mortality level in a century and a dry fog enveloped all Europe with widespread damage to agriculture.
In conclusion, the Deccan Traps fit better than meteorites as the trigger of the K/T event for all the geochemical, sedimentary and micropaleontological characteristics.
References
(1) Chenet et al., (2009) Determination of rapid Deccan eruptions across the Cretaceous-Tertiary boundary using paleomagnetic secular variation: 2. Constraints from analysis of eight new sections and synthesis for a 3500-m-thick composite section. Journal of Geophysical Research, vol 114, no. B6, B06103, pp. 1-38., 0.1029/2008JB005644
(2) Archibald J.D., (2014), What the dinosaur record says about extinction scenarios. Geological Society of America Special Papers 505, 213–224
(3) Olmez et al., (1986), Iridium emissions from Kilauea Volcano. Journal of Geophysical Research – Solid Earth 91/B1, 653–663
(4) Toutain & Meyer (1989) Iridium‐bearing sublimates at a hot‐spot volcano (Piton De La Fournaise, Indian Ocean), Geophys. Res. Lett.16(12), 1391-1394
(5) Alvarez et al., 1980, Extraterrestrial causes for the Cretaceous - Tertiary extinction K/T Experimental results and theoretical interpretation. Science 268, 1095–1108
