CLIMATE × EARTH

Oceanic Anoxic Events

~120 to 94 million years ago

Phanerozoic · Mesozoic · Cretaceous

Oceanic anoxic events are several episodes of the Cretaceous, between 120 and 94 million years ago, during which the deep ocean lost its oxygen and organic matter stopped decaying on the sea floor.

An oceanic anoxic event is an interval during which the deep ocean was anoxic, that is, without oxygen. Several occurred during the Cretaceous. Without oxygen, organic matter ceased to decay on the sea floor and accumulated there. It formed black shale, a dark rock rich in organic matter, which was deposited at the same time across whole ocean basins. This black shale contains a large share of the world's oil, since organic matter that escapes decay is the raw material from which oil forms. Volcanism is the usual explanation for these events. Researchers still argue over which volcanic province, a vast region of volcanic rock, is responsible for each event.

Why it matters

Oceanic anoxic events are the closest analogue known from the geological past to what occurs when carbon dioxide enters a warm ocean rapidly. Zones depleted in oxygen are expanding in present-day seas. They belong to the same process, at a still limited scale. The anoxic events of the Cretaceous indicate the outcome of this process once it is no longer limited.

Dating & uncertainty

Several distinct oceanic anoxic events occurred, and the best dated falls around 94 million years ago. Which volcanic province was responsible remains disputed. For Walker-Trivett and colleagues (2024), mercury and neodymium, two metals, from the Indian Ocean point to Kerguelen. Isotopes of lead, the variants of that element, measured in Italy point to the Caribbean and Madagascar. The mechanism that links an eruption to a stagnant, deoxygenated ocean is accepted only in outline.

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