Mass Extinctions and Evolution

• Natural selection drives evolution, but extinction is equally vital in shaping life's history.
• Extinction clears ecological space for new species to emerge and diversify.
• Most species exist for a few million years before disappearing, highlighting the continuous turnover in the history of life.
• An estimated 99.9% of all species that ever existed are now extinct, largely due to background extinction.
• Mass extinctions occur when multiple lineages vanish simultaneously, suggesting a common cause.
• Mass extinctions are followed by adaptive radiation, where surviving species diversify to fill vacant ecological niches.
• Species geologic ranges, defined by their first and last appearances in the fossil record, help identify extinction events.
• Mass extinctions cause significant drops in the diversity and abundance of organisms across various environments.

The Big Five Mass Extinctions

• Historical geologists recognize five major mass extinction events in the fossil record.
• These events are marked by sharp declines in diversity and significant turnover in species.
• The "Big Five" are the end-Ordovician, late Devonian, end-Permian, end-Triassic, and end-Cretaceous extinctions.
• Each of these extinctions had unique characteristics, affected different organisms, and potentially had different causes.

End-Ordovician Extinction

• Occurred around 443 Ma, impacting marine invertebrates like mollusks, trilobites, graptolites, and corals.
• 52% of documented species disappeared at the Ordovician/Silurian boundary.
• Uniquely among the Big Five, this extinction was limited to marine organisms, as land was not yet colonized.
• Recovery of Earth's oceans took 50 million years.
• Global cooling, evidenced by glaciation in Gondwana, is the likely cause.
• Global seawater temperatures dropped by an estimated 5°C.
• Glaciation led to a drop in sea level, reducing shallow marine habitats for invertebrates.
• Potential causes of the cooling climate include Gondwana's position over the South Pole and a reduction in greenhouse gases.
• Lower greenhouse gas levels may have resulted from enhanced silicate weathering during the Taconian Orogeny or reduced sunlight due to meteorite impacts.
• The Taconian Orogeny caused increased weathering and erosion, consuming CO2 from the atmosphere.
• The Ordovician Meteor Event, marked by numerous meteorites, potentially introduced stratospheric dust that blocked sunlight.

Late Devonian Extinction

• Occurred over an extended period of 20-25 million years, with major die-offs at 374 Ma and 359 Ma.
• Resulted in the destruction of 99% of reefs and severe losses in fish diversity and marine groups.
• An estimated 40% of taxa were lost.
• Characterized by a profusion of black shales, indicating widespread oceanic anoxia.
• The evolution of trees and their root systems contributed to increased weathering and nutrient runoff into the oceans.
• Nutrient-rich runoff caused eutrophication, leading to algal blooms and subsequent oxygen depletion when the algae died and decomposed.
• The removal of organic carbon and burial in sediments led to a draw-down of atmospheric CO2, potentially causing global cooling.
• Evidence for glaciation in the Appalachian basin suggests colder conditions.

End-Permian Extinction

• The most extreme extinction event in Earth's history, known as "The Great Dying."
• 62% of marine genera and severe impacts on terrestrial biota became extinct.
• Only about 17% of species on Earth survived.
• Life had rebounded from the late Devonian extinction with diverse marine and terrestrial ecosystems.
• The Glossopteris seed fern dominated Gondwana.
• Reptiles, including synapsids (mammal-like reptiles), diversified on land.
• Two pulses of extinction occurred, the Capitanian at 260 Ma and the end-Permian at 252 Ma, both linked to global warming.
• Variations in oxygen isotopes indicate a 6°C warming of the global ocean.
• Two major large igneous provinces erupted during the late Permian: the Emeishan Traps in China and the Siberian Traps.
• The Siberian Traps represent one of the largest eruptive episodes in Earth's history, covering a vast area with lava.
• Volcanic eruptions released large amounts of CO2, with estimates reaching 3000-30,000 ppm, causing extreme warming.
• The Siberian Traps passed through a vast coal field, adding even more CO2 to the atmosphere from burning coal.
• Warm climates led to stagnant oceans and widespread anoxia, recorded by black sedimentary deposits.
• Anoxia promoted the growth of sulfur-reducing bacteria, releasing toxic hydrogen sulfide gas (H2S).
• H2S release may have caused chemical warfare, killing terrestrial animals.
• Ozone depletion due to chemicals released disrupted by the Siberian Traps eruption increased UV radiation, causing mutations and cancers.
• Clam, Claraia, adapted to low-oxygen conditions, flourished in the early Triassic seas.
• The end-Permian extinction marked a major reset of Earth's biosphere.

End-Triassic Extinction

• Opened the door for dinosaurs to dominate.
• About half of Triassic genera in the sea did not make it into the Jurassic.
• Marine reptiles got decimated.
• Conodonts (eel-like jawless fish) disappeared at 202 Ma.
• Due to (1) large volcanic province erupts, (2) CO2 levels spike, and (3) global warming ensues.
• Rifting split Pangaea and opened the Atlantic Ocean, crust thinned.
• Central Atlantic Magmatic Province (CAMP) resulted in basalt and gabbro production in North America, South America, Africa, and Europe.
• CAMP may have erupted more basalt than the Siberian Traps and came in four pulses over 600,000 years.
• The number which stomata are inversely correlates with atmospheric CO2 levels.
• Fossil leaves counted with a microscope to see how CO2 levels varied across the Triassic/Jurassic transition.
• Fossil leaves in Greenland and Sweden showed stomata density decreased dramatically, commensurate with atmospheric CO2 levels 7-8 times higher than today.
• A 3-4°C jump in temperature occurred.

End-Cretaceous Extinction

• Dinosaurs diversified and lived, there were giant reptiles, marine life
• Western Interior Seaway developed as the Sevier Orogeny produced an adjacent foreland basin.
• Deccan Traps. 1 million cubic kilometers of basalt, slathering half a million square kilometers of western India (it may have originally been much higher in volume and area - At the time of its eruption, India was its own small continent, surrounded by the Indian Ocean — i.e., prior to its collision with southern Eurasia.
• Eruptions triggered by the island continent’s drift over the Réunion hotspot, a long-lived welt in the Earth’s mantle that is still producing lava today, at namesake Réunion Island
• Walter Alvarez and Luis Alvarez published a paper that laid out the case for an extraterrestrial impact as the cause of the end-Cretaceous extinction
• Boundary between the Cretaceous and the Tertiary* = single layer of clay between two ~4 cm thick limestone strata.
• Alvarez found that it really came down to a single layer of clay between two ~4 cm thick limestone strata.
• Alvarez saw distinctive Cretaceous radiolarian fossils. Above it, none.
• His father, a Nobel-prize-winning physicist, suggested examining the clay layer from the perspective of the small but (presumed-) constant flux of micrometeorites to Earth.
• he elder Alvarez suggested the element iridium, which (he hypothesized) should have a more or less constant flux to Earth’s surface.
• amount of iridium measured were high.
• giant meteorite impact somewhere else in the world.
• Testing done by others found iridium anomaly, spherules and shocked quartz.
• Spherules are little glass droplets that form when rock, melted instantaneously by the tremendous heat of the impact, is flung into the air as billions of tiny droplets
• Shocked quartz deformed as the powerful shock wave of the impact passes through it, wrecking its previously precise crystal lattice but not destroying the integrity of the grain by melting or disintegrating it..
• K/Pg boundary layer thickened toward the Americas, and toward the Gulf of Mexico even more.
• 1981, geophysicists Glen Penfield and Antonio Camargo suggested Pemex mapped circular structure that was an impact crater and the one responsible for the end-Cretaceous extinction but no one noticed.
• The crater was centered on a small port town called Chicxulub structure is about 180 kilometers (110 miles) in diameter and 20 kilometers (12 miles) deep, but it is now mostly hidden from view underneath the subsequent 65 million years worth of sedimentation – limestone, mostly
• impactor would have been about ten kilometers across, roughly the diameter of San Francisco, or Mount Everest and energy released would have been something on the order of 100 million megatons of TNT.
• Jan Smit added tsunami deposits at the K/Pg boundary at sites in Mexico and Texas. Floretin Maurrasse also documented a substantial tsunami deposit on the island of Haiti
• Hell Creek is a widespread regional geological unit, Tanis holds an incredible concentration of fossils that appear to have been deposited within hours of the impact.
• the presence of meteorite impact evidence inside the organisms fossilized. Specifically, fish breathe with gills, and those gills strain oxygen from the water -
• At Tanis, about half of the fish fossils have impact spherules caught in their gills, suggesting they were alive and breathing as impact debris was raining down upon their waterway last day of the Cretaceous period.
• Various kill mechanisms: radiative heat of the impact, blunt force trauma from its shockwave, subsequent forest fires, violent tsunamis, ocean acidification, and finally a prolonged “nuclear winter” from small particles suspended in the stratosphere.
• Dinosaurs aren’t all gone… birds are with us today

Other Mass Extinctions

The Great Oxidation Event

• Changed the atmosphere's composition and caused a mass extinction of anaerobic organisms.
• Occurred from 2.4 Ga to 2.0 Ga, where Earth transitioned from an oxygen-free to an oxygen-rich atmosphere.
• Unusual chemical sedimentary rocks could only form in an ocean that is largely oxygen free (banded iron formations [BIFs]).
• Minerals like pyrite and uraninite present
• Middle of the Proterozoic, appearance of oxidized terrestrial sediments (red beds)
• Photosynthetic cyanobacteria are preserved from Archean and younger times as stromatolites
• Cyanobacteria was busy for billions of years pumping oxygen out into the oceans
• Oxygen reacted with carbon, silicon, or with iron dissolved in the seawater
• Sulfur-reducing bacteria, for instance (the ones implicated above in end-Permian euxinia) cannot survive in the presence of free oxygen.

Cambrian Explosion

• Cambrian Explosion is considered a great blossoming of diverse animal life forms, recorded in the geologic record by shells and other hard parts
• Soft bodied, sessile animals were widespread across the globe
• Ediacaran biota, a diverse group of organisms presumed to be animals
• Adaptive radiation, and therefore took place in an ecological space that had recently been cleared out of its previous inhabitants.
• Destroyed their ecosystem..
• It was a time of microbial mat-dominated ecospace
• Animals of the Cambrian could swim high above and burrow into the sediment

Eocene-Oligocene Transition

• Marked by a cooling climate and turnover in organisms.
• Separation of Antarctica from Australia and South America = new ocean circulation pattern developed the the circum-polar current, which wrapped around Antarctica like a doughnut.
• Deep sea ocean temperatures dropped globally by 4°-5°C.
• 3 substantial bolide impacts: Popigai in Siberia (35.7 ± 0.2 Ma), Toms Canyon offshore from New Jersey (~35 Ma), and Chesapeake Bay, Virginia (35.5 ± 0.3 Ma).
• Dust and debris thrown up to the stratosphere can screen out solar radiation and reduce temperatures.
• Spiky foraminiferida were replaced by forms with less surface area. Bivalves adapted to warm conditions died out,
• Grasslands increased, Mammals adapted to these cooler, drier conditions

Pleistocene Megafauna

• Unlike the preceding examples, the most recent mass extinction skipped sea life, skipped plant life, skipped small mammals, and skipped nocturnal animals
• Focused on large mammals and birds.
• dead include giant ground sloths, mammoths and mastodons, glyptodonts, huge bears, saber-toothed cats of several varieties, and (in Australia) marsupial equivalents of many of these.
• "megafauna,” were killed off and cause of extinction varies from continent to continent and tied to the arrival of the first humans to arrive there.
• "overkill” hypothesis = predatory habits of our own species with the loss of these giant beasts
• happened least was the one place where the megafauna had hundreds of thousands of years to get used to Homo sapiens: Africa.

The Modern Biodiversity Crisis

• Currently living through another mass extinction, brought on by humans’ severe impact on the natural world.
• Dubbed “the Sixth Extinction”
• Elizabeth Kolbert’s Pulitzer-prize-winning - Uniquely among books about the current environmental crisis, it is exceptionally well grounded in the geological context -> factors that contribute to the modern crisis:
  • Habitat destruction and fragmentation
  • Invasive species
  • Disease
  • Pollution
  • Climate change
• Rates of extinction documented in the modern day match or even exceed rates calculated from fossil turnover during the Big Five mass extinctions but have only been operating for a geologically brief amount of time (a few centuries).

The Last Life on Earth

• In 2.5 billion years from now life will cease due to plate tectonics and/ or The fate of our planet ultimately hinges on the future the Sun.
• warmer core means less liquid outer core through time, as the solid inner core grows larger-> When the core becomes completely solid, magnetic geodynamo of the outer core will cease losing atmosphere-protecting magnetic field
• the Sun will fry the Earth, evaporating away its oceans and sterilizing its surface.
• sun will evaporate Earth, reduce temp