* Iridium is rare on Earth's surface: Iridium is a heavy metal that is very rare in the Earth's crust but is more abundant in extraterrestrial objects like asteroids and comets.
* Sedimentary layers preserve history: Sedimentary rocks form over time by the accumulation of layers. The presence of a distinct layer of iridium indicates a sudden, significant input of this element at a specific point in geological history.
* Worldwide distribution points to a global event: The fact that this iridium layer is found across the globe suggests that the event that deposited it was of immense scale and had a global impact.
The most famous example of this is the Chicxulub impact event:
* This event, which is believed to have led to the extinction of the dinosaurs, is associated with a global layer of iridium-rich sediment dated to about 66 million years ago.
* The impact of a massive asteroid or comet in what is now the Yucatan Peninsula is thought to have injected a tremendous amount of iridium into the atmosphere, which eventually settled on the Earth's surface.
Other possible explanations for a global iridium layer:
* Volcanic eruptions: While less common, massive volcanic eruptions can also release iridium, but the amount would likely be less significant than an impact event.
* Multiple events: It's possible that multiple events, potentially even spanning millions of years, could have contributed to a global iridium layer.
In conclusion: The presence of a worldwide iridium layer in sedimentary rocks of the same age is a strong indicator of a significant global event. While the exact cause can be debated, the most likely explanation is a catastrophic impact event like the Chicxulub impact.