CLIMATE × EARTH

When the Ocean Ran Out of Oxygen

~120 to 94 million years ago

Phanerozoic · Mesozoic · Cretaceous

Several times during the Cretaceous the deep ocean lost its oxygen, and organic matter stopped rotting on the seafloor. What settled instead is black shale, laid down across whole ocean basins at the same moment, and it holds a large share of the world's oil. Volcanism is the usual suspect; which province did it is argued province by province.

Why it matters

These are the nearest thing the record holds to what happens when carbon dioxide enters a warm ocean quickly. The oxygen-poor zones spreading in today's seas are the same process at a scale that is still small, and the Cretaceous shows where it goes when it is not.

Dating & uncertainty

Several distinct events rather than one, the best dated at about 94 Ma. Volcanism is the usual suspect, but which province is argued: mercury and neodymium from the Indian Ocean point to Kerguelen, lead isotopes from Italy to the Caribbean and Madagascar. The mechanism linking eruption to a stagnant, oxygen-free ocean is agreed only in outline.

Sources

1. Jenkyns (2010), Geochemistry, Geophysics, Geosystems · Geochemistry of oceanic anoxic events
2. Kuroda et al. (2007), Earth and Planetary Science Letters · Contemporaneous massive subaerial volcanism and late Cretaceous oceanic anoxic event 2
3. Walker-Trivett et al. (2024), Nature Communications · Oceanic Anoxic Event 2 triggered by Kerguelen volcanism

Earth history as one day

23:21:5638 min 4 s before midnight

if Earth’s 4.54 billion years were compressed into a single day