Mass Extinctions

The Oxygen Isotope Paleothermometer

When Earth’s climate is warm, the ocean becomes isotopically light (low δ18O). When the climate is cold, isotopically light water is trapped in ice, making the ocean isotopically heavy (high δ18O).

What do high δ18O values indicate?

Colder climate.

What do low δ18O values indicate?

Warmer climate.

Stable Carbon Isotopes

Used to reconstruct the ancient carbon cycle; photosynthetic organisms prefer light carbon (12C), and changes in δ13C reflect major shifts in the carbon cycle.

What does an increase in δ13C mean?

More burial of organic carbon; ocean becomes isotopically heavy.

What is a mass extinction?

A period when extinction rates rise dramatically above background levels, exceeding new species origination.

Background extinction

Normal, constant rate of species loss through time (>99% of all species that ever lived are extinct).

Sepkoski Curve

Compilation of marine fossil data used to identify the 5 major mass extinction events.

The Big Five Mass Extinctions

Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic, and Cretaceous-Paleogene.

Largest mass extinction

Permian-Triassic (~252 million years ago), known as “The Great Dying,” with ~90% of species lost

Ordovician-Silurian Extinction

Occurred ~443 million years ago; ~85% of marine species lost; possible causes: evolution of land plants, global cooling, glaciation, sea level drop, and habitat loss

Causes of the Permian-Triassic Extinction

Massive Siberian Trap volcanism, methane release, intense global warming, ocean anoxia (lack of oxygen), and ocean acidification.

Siberian Trap Volcanism

Large-scale volcanic eruptions releasing CO₂, warming the planet, and causing environmental collapse.

Extinction vulnerability factors

Small geographic range, specialized diet/habitat, low reproductive rate, and physiological limitations increase extinction risk.

Why do mass extinctions happen?

Major environmental shifts (climate change, volcanism, sea-level changes, asteroid impacts) disrupt ecosystems beyond recovery.

The Permian-Triassic Extinction sequence of events

Initiating event: Siberian Trap volcanism → release of CO₂ → global warming → ocean anoxia and acidification → collapse of marine and terrestrial ecosystems