Which observation in a rock layer would provide scientists with evidence that a mass extinction event had occurred?
A. A gradual shift in the structures of fossilized organisms and the levels of uranium isotopes
B. A rapid change in the features of fossilized species, and the discovery of new isotopes in fossils
C. Fossilized structures that are similar to those found in current organisms
D. High levels of iridium (a metal found in asteroids) and a sudden lack of fossils of many species
The correct answer and explanation is:
The correct answer is D. High levels of iridium (a metal found in asteroids) and a sudden lack of fossils of many species.
A mass extinction event, such as the one that occurred at the end of the Cretaceous period (which wiped out the dinosaurs), is often marked by a sudden and dramatic decrease in biodiversity. The most prominent evidence of such an event is the presence of iridium in rock layers. Iridium is a rare metal on Earth but is commonly found in asteroids. Scientists hypothesize that a large asteroid impact may have caused widespread environmental changes, leading to a mass extinction. This impact would have released massive amounts of energy, causing wildfires, climate shifts, and a “nuclear winter” effect that drastically altered the planet’s conditions.
In the rock layers associated with these extinction events, there is often a noticeable spike in iridium levels, which serves as a key marker of asteroid impacts. This sudden increase in iridium is often found at the boundary between different geological periods, such as the Cretaceous-Tertiary (K-T) boundary. Along with this, there is a sudden and widespread disappearance of many species’ fossils. The extinction of these species occurred relatively rapidly in geological time, often within a few thousand years or less.
The disappearance of so many species and the presence of iridium are strong indicators that a catastrophic event, like an asteroid impact, led to the mass extinction. This contrasts with gradual changes, such as those suggested in options A and B, which typically reflect slower, evolutionary processes. Option C is not correct because fossilized structures that are similar to current organisms suggest continuity, not mass extinction.