Mass Extinctions: A Comprehensive Overview

The History and Causes of Mass Extinctions on Earth

History of Extinction and Diversity Patterns

• Long-Term Perspective: The history of extinction is examined over the past $542$ million years, from the Cambrian period to the modern day, primarily focusing on marine organisms due to their robust fossil record.

• Extinction Intensity vs. Diversity:

  • Data is often plotted as extinction intensity (percentage of global marine genera destroyed) or biodiversity (number of surviving species/genera).

  • Extinction intensity shows a constant background level of extinction events, superimposed with peaks representing mass extinctions and troughs indicating relatively stable historical periods.

• Background Extinction Rate: There's a broadly constant background level of extinction, but the rate has shown a slight, gradual decline over geological time. This decline is interpreted as organisms becoming generally better adapted to life on Earth after surviving hundreds of millions of years of evolutionary events.

• Diversity Trends:

  • Species/Genera Diversity: The total number of species, particularly genera, has generally increased over time. Species-level diversity has increased exponentially, especially from the end of the Mesozoic to the present day.

  • Taxonomic Distinctness: Examining diversity at different taxonomic levels reveals different patterns.

    • Example Scenario: Consider $12$ species distributed among $3$ phyla (Yellow, Pink, Green). An extinction event reduces species count (e.g., from $12$ to $3$) and phylum count (e.g., Pink phylum completely lost). As surviving species diversify to fill available niches, the species count can rebound and even exceed pre-extinction levels (e.g., up to $13$ species).

    • Key Point: While species diversity might increase over time, the diversity of higher taxonomic ranks (like phyla) has actually decreased. All phyla seen today originated in the Cambrian Explosion; no new phyla have appeared since, but they can be lost in extinction events.

The Five Major Mass Extinction Events

1. End-Ordovician Extinction Event: affected marine organisms, with a rapid onset of glaciation and a resulting drop in sea levels

2. Late Devonian Extinction Event: A complex event involving at least $3$ distinct extinction pulses.

3. End-Permian Extinction Event: The largest and most severe extinction event on Earth, occurring at the end of the Paleozoic era. Sometimes combined with the End-Triassic event as the "Pervo-Triassic event" due to their catastrophic nature.

4. End-Triassic Extinction Event

5. End-Cretaceous Extinction Event: Famously known as the "dinosaur killer."

• Minor Spikes: Other smaller extinction spikes are noted, such as at the end of the Jurassic and the Eocene.

The K-T (Cretaceous-Tertiary) Boundary Event / End-Cretaceous Extinction

• Significance: This event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era, known for the extinction of dinosaurs, pterosaurs, mosasaurs, and many other groups.

• Dating: Well-dated at $65.5 ext{ Ma} ext{ (million years)} ext{ } ext{plus or minus } 0.3 ext{ Ma}$.

• Abruptness: Unlike some extended extinction events, the K-T extinction appears to have been very abrupt.

• Geological Marker: Globally, the event is marked by a distinctive change in lithology (rock type) in the geological record, for instance, a shift from Cretaceous chalk to different post-Cretaceous rock types.

• Alvarez's Discovery (1980):

  • Walter Alvarez published a pivotal paper in $1980$ focusing on the element iridium at the K-T boundary.

  • Iridium Anomaly: Iridium is a rare element in the Earth's crust (part of the platinum group, siderophilic/iron-loving), but it is much more concentrated in meteorites.

  • Alvarez discovered a significant spike in iridium concentration (up to about $3.5-4$ parts per billion, a dramatic tenfold or thousand percent increase) precisely at the K-T boundary layer in various global rock exposures.

  • Conclusion: This global iridium spike was attributed to the impact of an iron meteorite, estimated to be about $10$ kilometers in diameter.

Barringer Crater: An Example of an Impact Event

• Location: Northeastern Arizona, near Route $66$. Officially known as Barringer Crater, also advertised as Meteor Crater.

• Dimensions: $1.2$ kilometers in diameter and approximately $50,000$ years old. Its youth and arid location have preserved it well.

• Barringer's Miscalculation: In $1902$, Mr. Barringer bought the property, believing the crater was formed by a meteor that filled the entire hole (imagining a radius of $600$ meters). He calculated a volume of $9$ billion cubic meters, potentially yielding $1.5$ billion dollars in nickel (based on 10 ext{%} nickel content) from the impactor. This mining operation never happened.

• Actual Impactor Size: The object that formed Barringer Crater was only about $50$ meters in diameter.

• The Power of Kinetic Energy:

  • The discrepancy is explained by kinetic energy: $E = 0.5 imes m imes v^2$ (half the mass times the velocity squared).

  • Meteorites travel at extremely high velocities (e.g., $10$ to $70$ kilometers per second).

  • The Barringer impactor, weighing about $300,000$ tons and traveling at an estimated $12$ km/s, released energy equivalent to $2.6$ megatons (on an atomic weapon scale). For comparison, the Hiroshima atomic bomb was $13$ kilotons, meaning the Barringer impact was about $200$ times more powerful.

  • A relatively small object ($50$ m) moving at such high velocity delivers immense energy, capable of excavating a large crater.

The Chicxulub Crater and the Scale of the K-T Impact

• Energy Release: The K-T impact released an astounding $100$ trillion tons of TNT equivalent, about $2$ million times greater than the largest hydrogen bomb ever detonated (the $50$-megaton Tsar Bomb in $1962$).

• Global Iridium Anomalies: The geographic distribution of iridium anomalies (red dots on a Cretaceous world map) demonstrates that the impact material spread worldwide.

• Crater Discovery: The crater itself, named Chicxulub, was eventually located beneath younger rocks off the coast of the Yucatan Peninsula (near Cancun). Its initial discovery relied on seismic data from the oil and gas industry, later confirmed by drill cores.

• Crater Characteristics:

  • The Chicxulub crater is $110$ miles across and was originally about $12$ miles deep.

  • It formed in a shallow ocean environment.

  • Exhibits a classic double-wall structure characteristic of very large impact craters.

  • Formation Animation: Depicts an initial shockwave, ejecta being thrown out, rock melting, and later crustal rebound in the center with crater wall collapse.

• Devastating Global Effects of the Impact:

  • Tsunamis: Enormous tsunamis generated, with deposits found far inland (e.g., up to Houston, $1,100$ km away).

  • Ejecta: Debris (ejecta) spread globally. For example, at $1,000$ km distance, an estimated $3$ feet of fine, red-hot ejecta would have been deposited.

  • Earthquakes: Estimated to cause a magnitude $10.4$ earthquake at distances of $1,000$ km.

  • Thermal Radiation: Beyond $1,000$ km, thermal exposure lasted for hours, with radiation $153$ times greater than solar flux, causing third-degree burns and igniting trees.

  • Nuclear Winter: Massive amounts of dust and aerosols injected into the atmosphere caused a "nuclear winter" effect, significantly cooling the climate for decades and shutting down global photosynthesis, leading to ecosystem collapse.

• Frequency of Impacts:

  • Smaller meteorites: Annual impacts.

  • Tunguska-like (airburst over Siberia, early $20$th century): Every few centuries.

  • Barringer Crater-sized ($50$ m diameter): Approximately every $1,000$ years (the Earth is currently "overdue" for such an impact on land).

  • K-T event-sized ($10$ km diameter): Roughly every $100$ million years (the K-T event occurred $66$ Ma ago).

The Permo-Triassic Extinction (End-Permian)

• Severity: This was the most severe extinction event on Earth, with an estimated 95 ext{%} of all species going extinct.

• Marine Vulnerability: Marine organisms with calcareous (calcium carbonate) hard parts were most vulnerable, indicative of severe ocean acidification.

• Terrestrial Impacts: On land, evidence points to widespread hypoxia (low oxygen levels) and hypercapnia (high $CO_2$ levels).

• Specific Extinctions:

  • Nearly all calcareous foraminifera and radiolarians (97 ext{%} and 99 ext{%} respectively) disappeared.

  • Most sea anemones, corals (96 ext{%}), and brachiopods (96 ext{%}) were lost.

  • Deeper-water bivalves were less affected (e.g., 59 ext{%} loss).

  • Ammonoids (which later dominated the Mesozoic before the K-T extinction) nearly vanished, with only 3 ext{%} surviving.

  • All blastoids (100 ext{%}$) and trilobites were lost.

  • All sea scorpions and significant groups of fish also went extinct.

Volcanism and Mass Extinctions (Flood Basalts)

• Deccan Traps: A massive outpouring of basaltic rock in west-central India. This igneous province covers 1.2$million square kilometers and is over$1$kilometer thick.</p><ul><li><p><strong>Timing</strong>: Phases of eruption are precisely dated (e.g.,$67$million years ago), correlating very closely with the K-T extinction event.</p></li><li><p><strong>Extinction Mechanism</strong>: Flood basalts release enormous quantities of sulfur dioxide (causing ocean acidification) and carbon dioxide (leading to hypercapnia and global warming), which are believed to be major drivers of extinction.</p></li></ul></li><li><p><strong>Siberian Traps</strong>: An even larger flood basalt province, perfectly timed with the End-Permian extinction event.</p></li><li><p><strong>Correlation</strong>: There is a strong correlation between major extinction events (blue spikes) and large igneous provinces or flood basalt events (red bars) throughout geological history.</p></li><li><p><strong>Impact-Triggered Volcanism Hypothesis</strong>: Some evidence suggests that very large meteorite impacts might initiate flood basalt eruptions on the opposite side of the planet due to seismic shock waves, leading to a combined effect of impact devastation and prolonged volcanic environmental disruption.</p></li></ul><h4 id="596d6452-c725-495a-90ac-282ed7c1ba2f" data-toc-id="596d6452-c725-495a-90ac-282ed7c1ba2f" collapsed="false" seolevelmigrated="true">Other Causes of Extinction: Climate Change</h4><ul><li><p><strong>Late Quaternary Extinctions</strong>: Examinations from$56,000$years ago to$4,000$years ago show progressive extinctions of megafauna (mammoths, woolly rhinos, cave bears in Europe) as the climate progressively cooled into the Last Ice Age.</p></li><li><p><strong>Post-Glacial Warming Extinctions</strong>: Following the Late Glacial Maximum (around$20,000$years ago), as the climate warmed, another sequence of extinction events occurred, largely attributed to human presence and activities.</p></li></ul><h4 id="4596be56-e69b-4505-a018-ae69f6f9e692" data-toc-id="4596be56-e69b-4505-a018-ae69f6f9e692" collapsed="false" seolevelmigrated="true">Other Causes of Extinction: Human Impact (Mega-fauna Extinctions)</h4><ul><li><p><strong>Australia</strong>:</p><ul><li><p>During the last interglacial period ($120,000-125,000$years ago), Australia harbored diverse megafauna (e.g., giant kangaroos, wombats).</p></li><li><p>An initial extinction pulse around$107,000$years ago corresponded to climate cooling and increased aridification, leading to the conversion of much of Australia to desert.</p></li><li><p>A second, more widespread wave of extinctions ($70,000-80,000$years ago) coincided with the arrival of early humans (now dated to about$60,000-70,000$years ago), who spread fire across the continent and hunted large animals to extinction.</p></li></ul></li><li><p><strong>North America</strong>:</p><ul><li><p>During the Last Ice Age, a massive ice cap blocked migration from Siberia to North America.</p></li><li><p>An "ice-free corridor" opened around$12,600$years ago as the ice retreated, allowing Siberians (Clovis people) to migrate into North America.</p></li><li><p>The mass extinction of large mammals (e.g., mammoths) in North America largely coincides with this human migration.</p></li><li><p><em>Coastal Route Hypothesis</em>: More recent, though controversial, evidence suggests an earlier migration (perhaps$20,000$years ago) by Siberians using primitive boats along coastal routes. However, unequivocal radiocarbon dating for these pre-$13,000$-year-old sites is still lacking.</p></li></ul></li></ul><h4 id="de89b963-a68e-40b7-bdb7-7d94a3098ae9" data-toc-id="de89b963-a68e-40b7-bdb7-7d94a3098ae9" collapsed="false" seolevelmigrated="true">The Sixth Mass Extinction: Current Rates and Projections</h4><ul><li><p><strong>Current Event</strong>: Often referred to as the "sixth mass extinction event," it is currently ongoing and is comparable in scale to past major extinctions.</p></li><li><p><strong>Historical Trend</strong>: Species extinctions were low in the$1800$s but sharply increased starting around$1920$, reaching a peak of approximately$50,000$known species extinctions today.</p></li><li><p><strong>Primary Driver</strong>: The dominant cause of current extinctions is habitat loss, particularly in biodiversity hotspots like moist tropical forests.</p></li><li><p><strong>Current Extinction Rate</strong>: We are currently experiencing an estimated$15,000$$ extinctions per million species per decade.

• Projections:

  • If tropical forest loss continues at current rates, extinction rates are projected to rise further before eventually declining (as there will be less habitat remaining to destroy).

  • Optimistic scenarios, involving successful protection of biodiversity hotspots, suggest that extinction rates might have already peaked and could decline.

  • The reality is likely somewhere in between these projections.

• Magnitude: The current rate of extinction is immense, especially for insects. Hundreds of species are likely going extinct within short geological and historical timeframes.