The Cretaceous (about 1.45〜0.66 hundred million years ago) was one of the “supergreenhouse” periods in Earth history. Atmospheric CO₂ concentrations reached 4〜8 times present levels, and ocean environments also differed greatly from those of today. Research on this period is attracting attention as a natural reference for ongoing global warming.
Basic Knowledge
Earth during the Cretaceous was a typical example of a greenhouse climate. Average temperatures were about 6〜10℃ higher than today, there was no ice even in the polar regions (ice-free period), and sea level was about 70〜200m higher than today. Atmospheric CO₂ concentrations are estimated at approximately 500〜2000 ppm (about 420 ppm today), with large-scale volcanic activity (large igneous provinces: LIP) considered a major carbon source.
Oceanic Anoxic Events (OAE) were periods recorded several times during the Cretaceous (including OAE1a and OAE2) when the seafloor over broad areas became oxygen-deficient (anoxic or hypoxic). Large amounts of organic matter were not decomposed and accumulated on the seafloor, remaining as black shale rich in organic carbon. A proposed cause of OAE is a cascade of large-scale volcanic activity → increased CO₂ → warming → ocean stratification → interruption of oxygen supply to the deep ocean.
Organisms adapted to the warm Cretaceous environment, such as ammonites, mosasaurs, and plesiosaurs, depended on warm oceans and are thought to have been unable to survive the sudden cooling at the K–Pg boundary (the impact winter caused by a meteorite collision). Organic-carbon-rich black shale from the Cretaceous is one of the main sources of present-day petroleum resources.
Explain the mechanism by which an oceanic anoxic event (OAE) occurs, following the sequence volcanic activity → warming → changes in ocean circulation.
Answer: An OAE occurs through the following cascade: large-scale volcanic activity (LIP eruptions) → increased atmospheric CO₂ → warming → reduced density difference between surface and deep water as seawater temperature rises (stratification) → weakened oxygen supply to the deep ocean (thermohaline circulation) → oxygen-deficient (anoxic) bottom water.
- Cretaceous: greenhouse climate, no polar ice, sea level +70〜200m
- Cretaceous CO₂: about 500〜2000 ppm (1〜5 times the present level of about 420 ppm)
- Main source of CO₂: large-scale volcanic activity (LIP)
- OAE (oceanic anoxic event): occurred several times during the Cretaceous; bottom water became anoxic
- Product of OAE: black shale (rich in organic carbon) → source of modern petroleum resources
- Cause of OAE: LIP eruptions → warming → ocean stratification → deep-ocean oxygen depletion
- OAE research provides a reference for present-day warming and ocean deoxygenation
Points to Note
① Cretaceous warming occurred at a fundamentally different speed from present-day human-caused warming (millions of years on a geological scale versus about 150 years since the Industrial Revolution). ② Black shale from OAE is black and rich in organic carbon because organic matter was preserved without decomposing (if it decomposed, ordinary mudstone would form). ③ Sea-level rise during the ice-free period also included thermal expansion as well as ice-sheet melting.
Practice
- Explain the characteristics of the Cretaceous climate in terms of “ice-free period,” “sea level,” and “CO₂.”
- What characteristic rock was deposited during an OAE?
- State the effect of large igneous province (LIP) eruptions on the Cretaceous climate.