New theory attempts to explain why some lived and others perished during the Cretaceous – Palaeogene extinction
66 million years ago some three-quarters of plants and animal species were wiped of the surface of the Earth. This event, known as the Cretaceous – Palaeogene boundary, marked the extinctions of both avian and non-avian dinosaurs and shallow marine organisms. The current hypothesis for the extinction is that an asteroid impact (Chicxulub crater, present day Mexico) formed condensed aerosols in the stratosphere. This blocked out vital sunlight needed for photosynthesis and created near-freezing temperatures around the globe.
Aerosols are minute particles suspended in the atmosphere, which when are in large quantities, can scatter and absorb sunlight.
While it is widely accepted in the science community that an asteroid impact caused the demise of three-quarters of all major organisms of Earth. A new study, published in Scientific Reports, suggests there is an alternative explanation as to why some animal and plant species survived and why others didn’t.
A group of scientists led by Kunio Kaiho, from Tohoku University in Sendai, has proposed a radical new theory whereby stratigraphic aerosols did not induce darkness and instead created milder cooling than previously thought. The study also suggests that latitude dependent climate changes caused by the presence of soot in the stratosphere was a prime driver in the survival of some species of animals and plants and not others.
Soot is a strong light-absorbing aerosol that can have significant impacts on climate. The minute size of soot also means it can stay in the stratosphere for years. The study postulated that the soot was sourced from the rocks Chicxulub, Mexico; as soot released from present-day wildfire emissions are ejected directly into the Troposphere and not the stratosphere
Up until now the amount of soot released in the Troposphere and Stratosphere by the impact has been unknown. The team of scientists estimate the amount of soot in the stratosphere to be between 500-2600 Tg ( 1Tg = 1.0E9). These values are based on the amount of soluble coronene, benzo(e)pyrene and benzo(g,h,i)pyrene which are components of soot.
The study, modelled the effects of climate change caused by the increase of soot in the stratosphere. The results revealed that the ejecta spread around the globe in several months and was slowly deposited on the surface over the following years. The black carbon (BC) soot in the stratosphere sufficiently absorbed the solar radiation leading to a hotter stratosphere and cooler tropospheric atmosphere.
The models suggested that the sudden cooling would have weakened the hydrologic cycles and reduced precipitation. When 500Tg measurements were used, the models estimated that cooling would have reduced the surface temperatures by 6-9 °C and would have lasted for 2-3 years. with the higher, 1500-2600 Tg, the cooling would have lasted 3-5 years and reduced the surface temperatures by 10-18 °C.
The changes in climate, based on the three Tg unit estimates, showed variations with latitude. Cooler temperatures were dominant in the mid-high latitude regions due to a higher presence of BC soot in the stratosphere in those regions. Contrastingly, there was milder cooling in the lower latitudes.
Ocean temperatures lowered due to the limited solar shortwave radiation reaching the surface. Between 0m -100m and 500m-1500m the temperatures lowered up to 4-5°C and 7-9°C respectively. The deepwater temperature reductions were less contrasting than those in the shallow depths.
Extinctions – Ocean and Land
The study concluded that dinosaurs and tropical plants present in the low latitude regions (where there was milder cooling) would have been able to survive. The best chances of survival would have been at the equatorial regions where BC soot compositions were approximately 1500Tg. Unlike reptiles and small mammals, dinosaurs could not hide underground (which would have been warmer) and therefore were limited to geographical localities. Like present crocodilians, the dinosaurs would have only been able to survive in temperatures >15°C. The habitats of dinosaurs, reptiles and small mammals therefore had a major role in whether they survived or became extinct.
Similarly, in the oceans, ammonites, inoceramid bivalves and planktonic foraminifera became extinct. These shallow living marine organisms likely suffered from a decrease in low seawater temperatures. The changes in temperatures would have disturbed the marine ecosystems, leading to their extinction.
The marine animals which lived in the deeper depths would have survived, as there were smaller changes in deepwater temperatures. These animals include nautilids, sepia and benthic(ground-dwelling) foraminifera. The study also postulated that the temperature minima in the oceans occurred 3-6 years after the impact and 1-2 years on land. Soot from wildfires was deposited in the oceans causing photosynthesis to cease. This would have affected the deepwater-dwelling ammonites which needed such processes to occur for their food.
The study proposes a new outlook on why some animals survived and why others became extinct. The study shows that of the three possible scenarios, the release of 1500 Tg of BC soot was the one most likely to cause the extinctions seen. The cooler climates in the mid-high latitudes led to the extinction of the dinosaurs. The cooling in the oceans and lack of solar radiation diminished photosynthetic processes and led to the extinction of many shallow marine organisms. Deepwater marine life which required photosynthesis to feed, also became extinct over longer periods of time.