The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. What could have caused the K-Pg mass extinction? (G)

The K-Pg mass extinction event, also known as the Cretaceous-Tertiary (K-T) extinction event, occurred approximately 66 million years ago and marks the end of the Cretaceous Period and the beginning of the Paleogene Period. During this event, approximately 75% of all species on Earth became extinct, including the non-avian dinosaurs.

When attempting to determine the cause of the K-Pg mass extinction event, we should consider the evidence available in the geological record, such as sediment deposits and fossil evidence.

Based on the available evidence, several possible causes for the K-Pg mass extinction event have been proposed, including: 1. Asteroid impact: A layer of iridium-rich clay in the geological record is consistent with a massive asteroid impact. The location of the impact is typically associated with the Chicxulub crater in Mexico. Such an event would have released enormous amounts of energy, causing widespread fires, tsunamis, and a "nuclear winter" effect, reducing sunlight and drastically altering climate conditions. 2. Volcanic activity: Extensive and prolonged volcanic activity, such as the Deccan Traps in India, could have led to severe climate change and acid rain, leading to the mass extinction. 3. Climate change: Gradual climate change could have made the environment increasingly inhospitable for many species, resulting in their extinction.

Based on the current evidence, the most widely accepted theory for the cause of the K-Pg mass extinction is the asteroid impact. The presence of the iridium-rich layer and the Chicxulub crater supports this hypothesis and suggests that the impact released enough energy and debris to cause the catastrophic environmental consequences that led to the mass extinction event.

In conclusion, the K-Pg mass extinction event was likely caused by a massive asteroid impact that occurred around 66 million years ago. The resulting environmental changes, such as reduced sunlight and global cooling, led to the extinction of approximately 75% of all species on Earth, including the non-avian dinosaurs. However, it is essential to acknowledge that multiple factors might have contributed to this mass extinction event, and ongoing research will continue to shed light on the exact causes and the complex interplay of factors involved.