Darkness and Body Size Shaped End-Cretaceous Marine Extinction Patterns
This study was conducted mainly to find out about the after asteroid impact which kill the dinosaurs it found that darkness due to you to dash particles and suit which resulted from the asteroid crash block the sunlight making it the primary reason that Marine organisms died and larger organisms were more likely to go extinct since the needed more energy from the sun to survive.
This study is of high importance because scientists have known for decades that an asteroid hit Earth 66 million years ago and caused the extinction of about 75% of species but the question remained was why did some marine organisms die while others survived? For example, many plankton species disappeared and most planktonic foraminifera went extinct. Meanwhile, small algae survived and organisms living near the poles survived better than tropical species. Scientists knew what happened, but not exactly why these specific groups were affected differently. This paper attempts to solve that mystery.
Important background
When the asteroid struck Mexico, huge amounts of dust entered the atmosphere. Along with this, it caused widespread wildfires, which eventually released soot and sulphur aerosols into the atmosphere. Sulphur aerosols are microscopic particles that mainly contain sulphur compounds. As these particles spread globally, they blocked sunlight, eventually causing Earth to enter a state known as an impact winter, a prolonged period of global cooling caused by a large asteroid striking Earth, blocking sunlight and disrupting ecosystems.
The main question researchers wanted to clarify was which of the environmental changes resulting from the asteroid impact actually killed marine organisms. Four theories have been proposed.
The first is the darkness theory, which states that after the asteroid struck Earth, soot and sulphur particles covered the atmosphere, blocking sunlight for months to years and creating an impact winter. Since photosynthetic organisms require sunlight to produce food, their growth slowed, eventually leading to a decline in their populations. This disrupted the entire food web, as many organisms depended on these primary producers for energy.
The second is the cooling theory, which states that atmospheric dust and sulphur aerosols reflect incoming sunlight back into space, causing global temperatures to fall rapidly. As a result, surface temperatures dropped significantly. Organisms adapted to warmer temperatures were unable to survive the sudden cooling because they lacked sufficient insulation and physiological adaptations to withstand such low temperatures. In addition, the colder temperatures slowed metabolic processes and reduced growth rates, making survival much more difficult for many marine species.
The third theory is ocean acidification. The asteroid impact triggered massive wildfires and volcanic activity, releasing large amounts of carbon dioxide into the atmosphere, which then dissolved into seawater. According to the chemical reaction, carbon dioxide combines with water to form carbonic acid, making seawater more acidic. This made it difficult for marine organisms to build calcium carbonate shells and skeletons, which they use for protection and structural support. Not only did existing shells begin to dissolve, but the formation of new shells also became much more difficult. This weakened organisms, slowed their growth and reproduction, and ultimately increased their mortality rate.
The final theory is the nutrient changes theory. The asteroid impact released large amounts of minerals such as phosphorus and iron into the oceans through erosion and the deposition of ash from volcanic activity. This led to excessive algal growth, and when the algae died and decomposed, they consumed large amounts of dissolved oxygen. As a result, many marine animals were unable to survive due to the lack of oxygen. The rapid changes in nutrient availability also disrupted ecosystem balance by favouring some species while placing others at a disadvantage.
Investigation
Instead of studying fossils alone, they built a massive computer simulation. In this the model recreated late cretaceous oceans, ocean currents, climate, plankton ecosystems and food webs. They simulated 112 different plankton functional types with different body sizes, different feeding strategies, different environmental conditions. Then they hit the model Earth with the asteroid's effects and watched what happened.
This model is different because most ecosystem models allow species to recover immediately once conditions improve. Real life doesn't work that way. If a species falls below a certain population size, it becomes extinct. The researchers introduced a new rule which came to be known as extinction threshold. If population biomass becomes too small, the species is considered extinct permanently. Even more importantly bigger organisms need higher biomass as in a large organism requires more food, more energy and larger populations to survive.Therefore bigger organisms had higher extinction thresholds. This was a key innovation in the study.
What Happened After the Impact?
Earth became much colder as average ocean surface temperature dropped from 26.4°C to 12.3°C. This cooling happened within about 3 years. The ocean changed completely. Normally, warm water stays near the surface and cold water stays deeper. After impact, the cooling mixed the ocean much more intensely. This disrupted marine ecosystems. Secondly, sunlight almost disappeared. This was the most important change. With little sunlight, photosynthesis collapsed so primary producers struggled and food webs broke down.
Main Results
78% of plankton types went extinct. The model predicted 78% loss of plankton diversity and 99.5% reduction in primary productivity which meant that the base of the marine food web almost disappeared.
Why Did Small Organisms Survive?
This is the central finding of the paper. Large organisms need lots of fuel, lots of food and constant energy. When food becomes scarce, it dies quickly. Small organisms need little food and little energy so it can survive longer during a crisis. The asteroid impact created a huge energy shortage. Therefore, small organisms survived better than large organisms. This is why body size became such an important predictor of extinction.
Why Did High-Latitude Species Survive Better?
Scientists had noticed species near the poles survived more often. The model explains why. Polar organisms were already used to low light. Before the impact, polar plankton experienced long winters and weaker sunlight so they had adaptations for surviving with less light. Tropical organisms were not prepared as they evolved under bright sunlight. When darkness arrived, they were affected much more severely.
The researchers tested temperature separately. Cooling had only a limited effect on extinction patterns. Even though temperatures fell dramatically, it was not the primary reason organisms died. Moreover, many scientists previously argued that more CO₂ → acidic oceans → death of shell-forming organisms. However, the model successfully reproduced extinction patterns without needing acidification effects. This suggests acidification may have contributed, but was not the dominant cause.
Ultimately, the main killer was darkness. Specifically, less sunlight led to less photosynthesis which decreased food production and availability causing starvation throughout the foodweb, leading to mass extinction. The study concludes that this chain of events explains most observed extinction patterns.
The Author’s big idea: Energy Balance
The researchers summarize everything using a simple concept “Survival = Energy gained − energy needed”. Darkness reduces energy available. Large body size means more energy is needed. Therefore large organisms are most vulnerable during a period of darkness while small organisms survive because they need less energy. This single framework explains body-size effects, foodweb collapse, survival of mixotrophs, survival at high latitudes and extinction of foraminifera all at the same time.
Conclusion
The asteroid did not kill most marine organisms directly through heat, cooling, or ocean acidification. Instead, it filled the atmosphere with dust that blocked sunlight, causing years of darkness. This shut down photosynthesis, created a severe food shortage, and led to mass starvation throughout marine food webs. Small organisms and species adapted to low-light conditions survived because they required less energy, while larger organisms with higher energy demands were far more likely to go extinct.
Bibliography
Hansini Dantuluri | Writer | The STEM Review
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