The Amadeus Event (OAE1c) was an
oceanic anoxic event (OAE). It occurred 106 million years ago (Ma), during the
Albianage of the
Cretaceousperiod,[1] in a climatic interval known as the Middle Cretaceous Hothouse (MKH).[2]
Extent and duration
OAE1c lasted for approximately 567
kyr.[3] 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 were unaffected by anoxia.[4]
Causes
The MKH was one of the hottest intervals of the entire
Phanerozoiceon, with OAE1c occurring during a particularly warm time known as the Amadeus Thermal Maximum; these conditions were prime for generating anoxic waters.[2] 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.[5]Black shale deposition occurs during minima in the
axial precession cycle, when waters were less saline and/or warmer. Increased influxes of terrestrial
runoff would have induced
stratification of the
water column, inhibiting mixing of water masses at different depths. Peaks in carbonate deposition occurred during precession maxima, when
precipitation and
evaporation rates were lower and saline deep water bodies were able to form, preventing stratification.[1] The lack of
anomalousosmium enrichments of any significance rules out
large igneous provincevolcanism as a suspected causal factor, which is known to have caused other Cretaceous OAEs like
OAE1a and
OAE2.[6]
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 occurred as a result of OAE1c.[7]