Monday, 9 June, 2025/ Edition 62

Oceans play a vital role in the habitability of Earth and record global environmental changes. Yesterday was World Ocean Day, celebrated since 1992, and this week, the United Nations Ocean Conference is taking place in Nice, France.

Let’s review marine records for five major mass extinction events, before turning to a new book and film, Ocean, by David Attenborough. 

Rasoul Sorkhabi

Editor, Core Elements

Marine Records and Diagnosis of Mass Extinction Events

Tami Freed/Shutterstock.com

Recent articles in Paleobiology, Treatise in Geochemistry-Third Edition, and Scientific American reviewed the “Big Five” mass extinction events during the past 550 million years and how they were related to abrupt geochemical and climate changes.

But before we look at those five events, let’s review how scientists gauge the geochemical indicators and environmental stresses of global mass extinctions.

Definition: “Mass extinctions are geologically short intervals of elevated taxonomic losses,” according to Matthew Clapham in this recent article. 

Casualty analysis: Extinction causes are categorized into primary or direct “triggers” and consequential “kill mechanisms” often in the form of environmental catastrophes.

The Deadly Trio: A 2013 paper suggests that the combined global warming, ocean acidification, and deoxygenation due to drastic and rapid climate changes have devastating impacts on ecosystems and species.

Scientists measure each of the “Deadly Trio” using proxies:

For paleotemperatures:

• Oxygen18/oxygen16 ratios or oxygen isotopic fractionation in biogenic calcium carbonate materials (shells) precipitated from sea water

• Magnesium/calcium ratios in carbonate minerals

• Precipitated organic molecules (tetraether index of 86 carbon atoms, TEX 86)

For ocean acidification:

• Calcium44/calcium40 isotope ratios, as decreased carbonate saturation during ocean acidification results in smaller isotope fractionation factor between seawater and precipitated mineral

• Boron11/boron10 isotope ratios indicate seawater pH, due to the acid-base chemistry of boron compounds in seawater.

For oxygenation (or deoxygenation):

• “Local evidence” inferred from sedimentological and geochemical data:

  • Examples include massive deposition of organic-rich black shales, iron speciation favoring reactive oxide and pyrite minerals, and high concentrations of redox-sensitive metals such as molybdenum and vanadium.

• Uranium 238/uranium 236 isotopes with negative uranium-238 shift in anoxic seawater

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Big Five Mass Extinction Events

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No. 1: Late Ordovician Mass Extinction (LOME)

Duration: 1.5 million years in two pulses: LOME-1 Katian-Hirnantian boundary (455.2 Ma) and LOME-2 during the later part of the Hirnantian, ending at 443.1 Ma

Cause: Probably glaciation with abrupt changes between greenhouse and icehouse climates

• Volcanism and a gamma-ray burst from space have also been suggested.

Biotic extinction: 85 percent of marine species

Environmental changes: Global anoxia (oxygen deficiency) and euxinia (sulfidic anoxia), abrupt sea level changes (eustasy), and toxic metals

Deadly trio: Warming (some), deoxygenation

No. 2: Late Devonian Mass Extinction

Duration: Four million years in two pulses, the Kellwasser event at the Frasnian-Famennian boundary (372 Ma) and the Hangenberg event during the later part of Famennian (ending at 359 Ma)

Causes: Volcanism, supernova explosion, meteorite impact, climate change, or soil weathering

Biotic extinction: 70 percent of mostly marine species

Environmental changes: Global anoxia and euxinia, abrupt sea level changes (eustasy), and toxic metals

Deadly trio: Deoxygenation

No. 3 End Permian or Permian-Triassic (PT) Extinction Event

Duration: 60,000 years at 251.9 Ma

Cause: Volcanism of Siberian Traps

Biotic extinction: 90 percent both marine and terrestrial

Environmental changes: Carbon dioxide and methane release, global warming, wildfires, aridification, ocean acidification, anoxia and euxinia, sea-level rise, ozone depletion, toxic metals

Deadly trio: Warming, acidification, deoxygenation

No. 4: End Triassic (Triassic-Jurassic) Event

Duration: 850,000 years at 201.4 Ma

Biotic extinction: 75 percent both marine and terrestrial

Cause: Volcanism of Central Atlantic Magmatic Province

Environmental changes: Carbon dioxide and methane release, global warming, wildfires, ocean acidification, anoxia and euxinia, sea-level rise, ozone depletion, and toxic metals

Deadly trio: Warming, acidification, deoxygenation

No. 5: End Cretaceous or Cretaceous-Paleogene (K-Pg) Event

Duration: 10,000 years at 66 Ma

Cause: Chicxulub meteorite accompanied by Deccan Trap volcanism

Biotic extinction: 75 percent both marine and terrestrial species

Environmental changes: Toxic metals, wildfires, sea-level rise, and ocean acidification

Deadly trio: Warming (some after the event), acidification, deoxygenation (some after the event)

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Ocean: Earth’s Last Wilderness

Altitude Films/ YouTube

A new documentary film entitled Ocean with David Attenborough premiered in London on May 8, celebrating David Attenborough’s 99th birthday.

Attenborough and Colin Butfield also published a book entitled Ocean: Earth’s Last Wilderness.

Here are some takeaways:

One Ocean:

• The so-called five or seven oceans are divided by imaginary lines. The ocean is an interrelated water body in constant interaction with the lands and the atmosphere.

• The ocean currently absorbs one-third of carbon dioxide from the atmosphere.

Coral reefs:

• Coral Reefs are fascinating shallow water features created by a delicate interplay of mineral (carbonate), animal (coral polyps, related to jellyfish), and plant (zooxanthellae, a type of algae).

• Coral reefs provide support for various marine life forms, but they are also sensitive to water temperature, acidity, and salinity. They require tropical waters, but not very warm waters.

Bleaching:

• Increased water temperatures “bleach” coral reefs to their original white color as the zooxanthellae are preferentially removed.

• Acidic seawater can dissolve carbonates in coral skeletons and shells of marine animals.

• Prolonged and severe bleaching and acidity lead to the death of a coral colony.

• “In my lifetime,” Attenborough writes, “the world has lost over half of its coral.”

Plastic pollution:

• Plastics dumbed into the ocean “now permeate every level of the marine food web.”

• A New Zealand study found that a mouthful of plankton eaten by baleen whales contained more than 24,000 microplastic fragments.

30×30 Resolution:

Attenborough supports the 2022 United Nations Biodiversity Conference of the Parties resolution to designate 30 percent of the ocean as protected areas (from fishing and other human activities) by 2030.

Go deeper: You can stream the film on Disney+ and Hulu.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.