Have you ever thought about the extinction of Dinosaurs from the Earth?
We have seen many Dinos in the movie and cartoons but did we really see it? There is a long long story for the extinction of Dinosaurs. Let us begin with the general term known as “Mass extinction”.
What is mass extinction?
Mass extinction is the eradication of different groups of living organisms within a short time. Here, short time refers to several million years. As we know the age of Earth is 4.6 billion years, some million years will be a short time.
Mass extinction is also concluded as “Biotic Crisis”. Various Mass extinctions have occurred in the past and many will occur in the future. However, the accurate estimation of the mass extinction events is not possible. Because the majority of Earth’s biotic community is composed of microorganisms. Therefore to maintain an accurate record is not possible.
|Geological time scale|
Mass extinction events
In the Phanerozoic Eon, eight mass extinctions have taken place. Of which five are the major mass extinctions whereas other three are having lesser impact.
The major ones occurred during late Ordovician, Late Devonian, Late Permian, Late Triassic and Late Creatceous. The lesser ones occurred during Devonian-Carboniferous, Cenomanian-Turonian and Palaeocene-Eocene boundaries.
- Late Ordovician extinction
It occurred around 450-440 Ma. Two events were recorded from this period. The cause of such an event is “Global warming”. At that time Global warming was the consequence of volcanic activities and anoxia.
- Late Devonian extinction
The event occurred around 375-360 Ma. It continued for 20 million years.
- Late Permian or Permian-Triassic extinction
The Permian-Triassic extinction is the largest of all the major mass extinction events. It is a notable event because it is the reason behind the extermination of 60% organisms.
During the P/Tr boundary (Permian-Triassic boundary), the supercontinent Pangea was about to complete. The sea level has encountered a significant downfall which is evident from the widespread Evaporite deposits.
There can be many causes which show impacts together resulting in the mass extinction. The cause of P/Tr extinction is attributed to the “Siberian flood basalts”. Such an event occurred during 250土0.2 Ma.
Also, the cause includes CO2 and Greenhouse gases along with Global warming. The release of sulphate aerosols from the volcanoes have contributed to the “acid rain”. Sea level changes also play a crucial role. The terrestrial record of this period is poor. However, marine records are more acceptable.
In South China, complete succession of marine fossil records was achieved. The records indicate disappearance of 91% of Invertebrates which includes 98% ammonoids, 85% bivalves and 75% shallow water fusulinids.
Terrestrial records indicate the extermination of 80% reptile families and six of the nine amphibian families. Eight orders of insects of which four suffered serious decline and three got extinct during Triassic.
- Late Triassic extinction
The late Triassic extinction event occurred during 201.3 Ma.
- Late Cretaceous or K/T extinction
This is one of the most debated mass extinction events. This was the time for the extinction of Dinosaurs. This extinction was the life taker of Dinosaurs however lizards and alligators survived the event.
The mass extinction during the Late Creatceous is controversial and is still a mystery. Teeths of about 12 Dinosaur genera were recorded from Palaeocene time.
Tertiary Dinosaur remains are obtained from India, Argentina and New Mexico. The extinction wiped out the belemnites, pelecypods and rudistids. Benthic organisms have a low extinction rate. However, sharp extinctions were faced by calcareous phytoplankton and phytoplanktonic foraminifera.
|Belemnites: cigar shaped fossils|
Mass extinction causes
The causes of Mass extinction could be extra terrestrial, physical and biological. Extra terrestrial and physical causes are dominant than the biological cause.
The extra-terrestrial causes include increased production of cosmic rays from nearby stars, increased radiation during the reversal in the magnetic field, climate change caused by supernova and asteroid impact.
The physical causes include rapid change in the climate, reduction in oceanic salinity due to the evaporite deposition, fluctuations in atmospheric oxygen levels, changes in sea level, collision of continents and flood basalts.
Causes for the Late Cretaceous extinction
Three reasons are taken into consideration for the Late Cretaceous mass extinction.
- Ultimate cause – impact extinction
- Immediate cause – rapid change in environment
- Flood basalts
What would be the consequences of collision of an asteroid of 10 km diameter with the Earth?
The answer to this question will clarify the ultimate and immediate cause of the Late Cretaceous extinction. Following are the consequences:
- Darkness: due to collision of such an exhaustive asteroid, the upper atmosphere will face widespread dust particles and also soot. Such particles will not allow the sun radiation and thus block sunlight. As sunlight is the primary source of energy on the Earth, the food chain gets disturbed.
- Cold: as the sunlight gets blocked by the dust particles and soot, the temperature would fall down leading to cooling. The surface continental temperature would fall to -20° C.
- Increased greenhouse effect: if an Asteroid collides with the ocean, dust and water vapour would spread in the atmosphere. After some time period the dust particles will start settling but water vapour remains in the atmosphere and it enhances the greenhouse effect.
- Acid rain: the energy liberated by the collision will be capable of reacting the gases in the atmosphere. As a result, compounds like nitric acid and oxides of nitrogen would form. When moisture in the atmosphere reacts with such gases it results in acid rain which is harmful for all the organisms.
- Wildfires: the soot particles reported from the clay layers at the K/T boundary is the result of burning of coniferous trees. The wildfires must be from the ignition by infrared radiation.
What evidence was obtained that suggests the extinction through the asteroid?
- Iridium anomalies: generally iridium is a rare mineral in the Earth’s crust. But at the K/T boundary i.e. during 66.7 Ma, the clay layer is found to be enriched in iridium. Such an enrichment of iridium must be from the extra-terrestrial source.
- Glass spherules: glass spherules are glassy droplets of felsic composition i.e. it is rich in silica. Such spherules show analogy with tektites. Such an analogy suggests the extra-terrestrial impact as well as melting of crustal rocks followed by sudden chilling.
- Soot: soot is a carbonaceous material. Such a zone of soot in the clay layer suggests wildfires due to ignition effects.
- Shocked quartz: it is considered as one of the strongest evidence. Such a type of quartz is produced in a condition of high pressure. Due to high pressure shock lamellae were formed. Such shocked quartz can be found near nuclear sites and impact sites.
- Stishovite: stishovite is a high pressure polymorph of quartz. Such polymorph is formed due to the high pressure conditions.
The location of such an asteroid hit is considered in Carribean. The location is consistent with the evidence suggesting the hit. Presence of a 180 km Chicxulub crater in Yucatan, shocked quartz and spherules are the strongest records. Moreover, impact Breccia deposits are also widespread in Cuba and Haiti.
|Flood basalt sampling by scientist|
According to Courtillot and Cisowski, 1987 major flood basalt eruptions shows a periodicity of about 30 M.y with some peaks coinciding with some major extinction peaks.
The discovery of iridium in atmospheric aerosols from the eruption of kilauea in Hawaii has led to the notion of iridium concentration due to oceanic plume fed volcanic eruptions.
The distribution of glass spherules is thought to be around the eruptive centers. Such glass spherules can form during basaltic eruptions.
From the results of isotopic dating in India, the age of Deccan traps is 66 Ma i.e. during the late Cretaceous. Thus, it also suggests the flood basalts as reasons.
The acid rain may be the result of sulphate aerosols emitted by the volcanoes. Volcanoes can also add volcanic ash and CO2 in the atmosphere. Volcanic ash by blocking the Sun radiation can contribute to Global cooling.
Sixth mass extinction
Sixth mass extinction is the Holocene mass extinction. It is an ongoing process. The reason behind the ongoing extinction is considered as the Human activity.
Earth has lost 60% of vertebrates since the year 1970. Australia has lost more than 100 species of Vertebrates and has faced the worst latest mass extinction.