Anoxic event
Periods of widespread ocean oxygen depletion linked to mass extinctions.
An anoxic event is a time when large areas of the ocean lost their dissolved oxygen, turning into toxic, sulfidic waters. These events haven't occurred in millions of years, but the geologic record shows they happened repeatedly in the past. They often lined up with mass extinctions and may have helped cause them—some of these extinctions are used by geobiologists as markers for dating rock layers. However, not all anoxic events led to extinctions; for example, widespread black-shale deposits from the mid-Cretaceous show anoxic conditions without associated mass die-offs. Many geologists link oceanic anoxic events to a slowdown in ocean circulation, climate warming, and higher greenhouse gas levels. Researchers point to increased volcanism, which releases CO₂, as the main external trigger for the development of euxinia (anoxic, sulfidic conditions).
In the current Holocene epoch, human activities like nutrient runoff from farms and sewage create smaller-scale dead zones. Oceanographer Andrew Watson notes that a full-scale ocean anoxia would take thousands of years to develop. The idea that modern climate change could trigger such an event is called Kump's hypothesis.
The concept of the oceanic anoxic event was first proposed in 1976 by Seymour Schlanger and geologist Hugh Jenkyns, based on discoveries from the Deep Sea Drilling Project in the Pacific. They found black, carbon-rich shales in Cretaceous sediments on submarine volcanic plateaus like the Shatsky Rise and Manihiki Plateau. These deposits were the same age as similar cores from the Atlantic Ocean and outcrops in Europe—especially in the limestone-dominated Apennines of Italy. This led to the realization that these widespread, distinct layers recorded unusual, oxygen-depleted conditions in the world’s oceans during several separate geological periods.
Modern studies of these organic-rich sediments often show fine laminations undisturbed by bottom-dwelling animals, indicating anoxic seafloor conditions, likely with a layer of poisonous hydrogen sulfide (H₂S). Detailed organic geochemistry has also found biomarkers from purple and green sulfur bacteria—organisms needing both light and free H₂S—showing that anoxic conditions reached high into the sunlit upper water column. This understanding has been pieced together over the last three decades.
- Proposed by
- Seymour Schlanger and geologist Hugh Jenkyns in 1976
- Associated mass extinctions
- several, including some used as time markers in biostratigraphic dating
- Typical duration
- less than a million years
- Mean surface temperature during events
- probably in excess of 25 °C (77 °F)
- Current quaternary mean surface temperat
- 13 °C (55 °F)
- Depth of hypothesized hypersaline interm
- 500 to 1,000 m (1,640 to 3,281 ft)
- Temperature of that layer
- 20 to 25 °C (68 to 77 °F)
Lore & Background
The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger and geologist Hugh Jenkyns, arising from discoveries made by the Deep Sea Drilling Project in the Pacific Ocean. The finding of black, carbon-rich shales in Cretaceous sediments on submarine volcanic plateaus, coupled with identical-age deposits from the Atlantic Ocean and known outcrops in Europe—particularly in the otherwise limestone-dominated Apennines chain in Italy—led to the observation that these widespread strata recorded very unusual, oxygen-depleted conditions spanning several discrete geological periods. Modern sedimentological investigations reveal fine laminations undisturbed by bottom-dwelling fauna, indicating anoxic conditions on the seafloor believed to coincide with a low-lying poisonous layer of hydrogen sulfide (H2S). Detailed organic geochemical studies have revealed biomarkers from purple sulfur bacteria and green sulfur bacteria, illustrating that anoxic conditions extended high into the photic upper-water column.
Anoxic events with euxinic (anoxic, sulfidic) conditions have been linked to extreme episodes of volcanic outgassing. Volcanism contributed to the buildup of CO2 in the atmosphere and increased global temperatures, causing an accelerated hydrological cycle that introduced nutrients into the oceans, stimulating planktonic productivity. These processes potentially acted as a trigger for euxinia in restricted basins where water-column stratification could develop. Under anoxic to euxinic conditions, oceanic phosphate is not retained in sediment and could be released and recycled, aiding perpetual high productivity. Two durable hypotheses explain the short-period events: one suggests enhanced preservation of organic matter under restricted, poorly oxygenated conditions due to basin geometry; the other proposes a major increase in ocean fertility leading to an expanded oxygen minimum zone. Geochemical data indicate all major oceanic anoxic events were associated with thermal maxima, and the ages of three large igneous provinces correlate well with major Jurassic and Cretaceous anoxic events, suggesting a causal link.
Reader's Guide
The significance of anoxic events lies in their profound impact on Earth's history and their relevance to understanding modern environmental changes. The article notes that anoxic events coincided with several mass extinctions and may have contributed to them, including some used as time markers in biostratigraphic dating. However, not all anoxic events are associated with mass extinctions; widespread black-shale beds from the mid-Cretaceous indicate anoxic events without such extinctions. Many geologists believe oceanic anoxic events are strongly linked to the slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the 'central external trigger for euxinia.' The article also describes how human activities in the Holocene epoch, such as the release of nutrients from farms and sewage, cause relatively small-scale dead zones, but British oceanologist Andrew Watson says full-scale ocean anoxia would take 'thousands of years to develop.' The idea that modern climate change could lead to such an event is referred to as Kump's hypothesis. The legacy of anoxic events includes their role in producing worldwide bands of black shale that are sources of the world's oil reserves, and they serve as a natural example of Earth's response to excess carbon dioxide injection into the atmosphere and hydrosphere.
Did You Know?
- Anoxic events have not happened for millions of years but occurred many times in the past.
- Biomarkers from purple and green sulfur bacteria show anoxic conditions extended into the photic upper-water column.
- The ages of three large igneous provinces correlate with major Jurassic and Cretaceous oceanic anoxic events.
Frequently Asked Questions
What exactly is an anoxic event in ocean history?
It refers to a stretch of time when vast stretches of the ocean lost their dissolved oxygen, turning into toxic, sulfidic waters. These episodes show up repeatedly in the geologic record, though none have occurred in the last several million years.
Who originally put forward the anoxic-event hypothesis?
Seymour Schlanger and geologist Hugh Jenkyns proposed the idea back in 1976, linking widespread ocean oxygen depletion to the extinction pulses visible in the fossil record.
How long do anoxic events typically last?
They are geologically brief, generally lasting less than a million years from onset to recovery.
What were surface temperatures like during an anoxic event?
Mean surface temperatures are estimated to have exceeded 25 °C (77 °F), far above the roughly 13 °C (55 °F) average we see in the current Quaternary period.
Does every anoxic event trigger a mass extinction?
No—several episodes, such as the mid-Cretaceous black-shale deposits, produced widespread anoxic conditions without any associated mass die-off. Only a subset of these events coincided with the extinctions that geobiologists now use as biostratigraphic time markers.
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