4.11 Extinction

Overview of Mass Extinction at the End of the Cretaceous

The final segments of this course delve into the significant event known as the End-Cretaceous mass extinction, characterized as a pivotal point in Earth's biological history. This extinction, commonly previously referred to as the K-T extinction (Kretaceous-Tertiary), has been reclassified to the Cretaceous-Paleogene (K-Pg) boundary, which denotes the transition from the Cretaceous to the Paleogene period. The event resulted in the eradication of approximately three-fourths of all existing species, with both terrestrial and marine ecosystems suffering extensive losses. The most notable victims were the non-avian dinosaurs, from which only the avian lineage survived.

Causes of the Mass Extinction

Meteorite Impact Hypothesis

Recent evidence strongly supports the hypothesis that a large meteorite impact was the primary catalyst for this mass extinction. In 1980, a team including Walter Alvarez and his father, Luis Alvarez, identified an iridium-rich clay layer corresponding to the K-Pg boundary in Italy. Iridium, a metal scarce at Earth's surface but abundant in meteorites, suggests that an impact event was responsible for the global distribution of this material. Normal iridium levels in Earth's crust are about 0.3 parts per billion (ppb), whereas samples from the K-Pg boundary showed concentrations up to 9 ppb, representing a thirtyfold increase.

Iridium-Rich Clay Layer

This iridium-rich clay, which has been discovered globally, aligns with significant changes in fauna and flora, revealing a dramatic shift in biodiversity following its deposition. Samples indicate a loss of many species preceding the clay layer, signifying a mass extinction event.

Additional Evidence from Ejecta

Accompanying the iridium-rich layer were microscopic ejecta that provide further evidence of an impact. These micro-spherules formed from vaporized rock during the impact event and can be identified through electron microscopy. Furthermore, the clay layer contains shocked quartz grains, which exhibit characteristic structural changes from extreme stress. Shocked quartz can be produced during violent volcanic eruptions, nuclear explosions, or meteorite impacts, adding weight to the argument for a meteorite impact.

Chicxulub Crater

Evidence has also pointed to the Chicxulub Crater on the Yucatan coast, which, while buried under sediment, has been characterized through seismic studies. This crater displays the requisite size and features indicative of a significant impact event. Wells drilled in the area have revealed shatter-cone textures, further confirming the site as an impact location.

Global Effects of the Impact

While the immediate effects of the meteorite impact would have been localized, its indirect consequences had a global reach. Significant disruptions occurred in Earth's systems, leading to climate change, obliteration of photosynthesis, and widespread wildfires caused by the heat generated from the impact. Seismic and evidence of catastrophic tsunamis caused by the impact were seen in deposits along coastlines, showcasing their immense scale.

Mechanism of Environmental Disruption

Meteorite Angle and Heat Generation

The angle of the meteorite's approach plays a crucial role in the destruction inflicted. A steep, roughly 60-degree angle would allow it to penetrate the atmosphere effectively, generating considerable heat through compressive forces rather than merely friction. This heat could ignite widespread forest fires across continents, drastically affecting climate by reducing sunlight and inhibiting photosynthesis.

Role of Sulfur and Acid Rain

Compounding the disaster was the impact's location over evaporate rock deposits containing vast amounts of sulfur. The vaporization of these rocks released sulfur into the atmosphere, where it combined with water vapor to produce acid rain, negatively impacting terrestrial and marine organisms, including calcareous plankton essential to ocean ecosystems. Ocean acidification, alongside terrestrial shifts in vegetation, resulted in a catastrophic decline in primary productivity worldwide.

Consequences of Ejecta and Soot

The falling hot ejecta from the impact possibly raised atmospheric temperatures to lethally high levels for large surface-dwelling species while subsequent cooling from sulfur aerosols reduced temperatures drastically. This dual effect may have resulted in an annual temperature drop of approximately 26° C, leading to further environmental crisis and extinction.

The Role of Volcanism

Deccan Traps Volcanism

The volcanic events in the Deccan Traps of India, predating the meteorite impact by a significant margin, coincided with the mass extinction event. These colossal lava flows contributed to atmospheric changes through the emission of gas and particles that further disrupted global biodiversity.

Connection Between Impact and Volcanism

While the exact relationship remains under investigation, the potential for the meteorite impact to have catalyzed an increase in the volcanic activity remains plausible. Studies suggest the impact may have accelerated the volcanic processes contributing to climatic and ecological deterioration during this time.

Aftermath and Evolutionary Radiation

Following the extinction event, a new evolutionary radiation occurred, leading to the predominance of mammals and birds in the subsequent epochs. Despite an initial loss of biodiversity, this period gave rise to the modern ecosystems we see today.

Implications for Human History

Impact on Mayan Civilization

Intriguingly, the consequences of the K-Pg impact extend into human history, influencing the development of the Mayan civilization. The cenotes, or sinkholes formed due to the impact-related fractures, provided essential water supplies to the Mayan people. This adaptation allowed them to thrive in a region characterized by substantial historical upheaval.

Climate and Sociopolitical Changes

However, the Mayan civilization also faced a significant crisis around 900 CE when a 200-year drought led to mass abandonment of cities in the southern lowlands. The northern regions benefitted from cenote systems allowing continued survival, illustrating how the aftermath of ancient geological events could have lasting impacts on human societies.

Conclusion

In summary, the End-Cretaceous extinction offers a compelling example of interconnectedness in Earth's evolutionary and climatic history. Both the immediate effects of the meteorite impact and the sustained volcanic activity combined to create a perfect storm of ecological devastation, reshaping life on Earth profoundly, emphasizing the intricate links between historical events and modern realities.