The Amadeus Event (OAE1c) was an

oceanic anoxic event An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved Oxygen, oxygen (O2), creating toxic, Euxinia, euxinic (anoxic waters, anoxic and wikt:sulfidic, sulfidic) waters. Although anoxic events have no ...

(OAE). It occurred 106 million years ago (Ma), during the

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period, in a climatic interval known as the Middle Cretaceous Hothouse (MKH).

Extent and duration

OAE1c lasted for approximately 567

kyr A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more exa ...

. Environmental conditions across much of the globe facilitated the formation of dysaerobic waters, although OAE1c varied in its nature and magnitude depending on region. Shallow shelf environments in the

Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...

were unaffected by anoxia.

Causes

The MKH was one of the hottest intervals of the entire

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

eon, with OAE1c occurring during a particularly warm time known as the Amadeus Thermal Maximum; these conditions were prime for generating anoxic waters. Orbital forcing is considered the most likely cause of OAE1c, as most geochemical changes observed across the OAE1c interval were in lockstep with

Milankovitch cycles Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after the Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he pr ...

Black shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

deposition occurs during minima in the

axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In parti ...

cycle, when waters were less saline and/or warmer. Increased influxes of terrestrial runoff would have induced stratification of the

water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...

, inhibiting mixing of water masses at different depths. Peaks in carbonate deposition occurred during precession maxima, when

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

and

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

rates were lower and saline deep water bodies were able to form, preventing stratification. The lack of anomalous

osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...

enrichments of any significance rules out

large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive ( sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The format ...

volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

as a suspected causal factor, which is known to have caused other Cretaceous OAEs like OAE1a and OAE2.

Effects

Unlike many other OAEs across the MKH, the black shales deposited during OAE1c were made up of type III kerogen and consisted primarily of terrestrial organic matter, in contrast to the type II kerogen made up of marine organic matter that the black shales of OAE1a, OAE1b, OAE1d, and OAE2 were composed of. Also unlike other MKH OAEs, no significant radiations or extinctions of

radiolarians The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ecto ...

occurred as a result of OAE1c.

References

{{reflist Albian Stage Anoxic events

Extent and duration

Causes

Effects

See also

References