Selli Event

The negative δ¹³C excursion representing the onset of OAE1a was rapid, taking only 22,000-47,000 years.^[3] The recovery of the global climate from the injection of large amounts of isotopically light carbon lasted for over a million years.^[4] The end of OAE1a is characterised by a positive δ¹³C excursion,^[5] which had a magnitude of +4 to +5%.^[6] The OAE lasted for about 1.1 to 1.3 Myr in total;^[7] one high-precision estimate put the length of OAE1a at 1.157 Myr.^[8]

OAE1a ensued during a hot climatic interval, with the global average temperature being around 21.5 °C.^[9] The Tethys Ocean experienced an increase in humidity at the beginning of OAE1a, while conditions around the Boreal Ocean were initially dry and only humidified later on during the OAE.^[10]

The increase in global temperatures that caused OAE1a was most likely driven by large igneous province (LIP) volcanism.^[2] The negative δ¹³C excursion preceding the OAE, occurring in the C3 isotopic interval,^[11] is believed to reflect volcanic release of carbon dioxide into the atmosphere and its consequent warming of the Earth.^[12] Enrichments in unradiogenic osmium, which is primarily derived from alteration of oceanic crust by hydrothermal volcanism, further bolster volcanism as the driver of OAE1a.^[13][14] Multiple LIPs have been implicated as causes of the rapid global warming responsible for the onset of OAE1a, including the High Arctic Large Igneous Province (HALIP),^[15] the Kerguelen Plateau,^[2] and the Ontong Java Plateau.^[12][16][17] The rate of greenhouse gas emissions leading up to OAE1a was relatively slow, causing the anoxic event to only generate a minor extinction event, in contrast to the severe LIP-induced Capitanian, Permian-Triassic, and Triassic-Jurassic mass extinctions and the ongoing Holocene extinction caused in part by anthropogenic greenhouse gas release, each of which were or are characterised by a very high rate of carbon dioxide discharge.^[2] Despite a much smaller methane clathrate reservoir relative to the present day, the degassing of methane clathrate deposits may have nonetheless significantly exacerbated volcanic warming.^[16] Following OAE1a, δ¹⁸O values increased, indicating a drop in temperatures that coincided with a δ¹³C_org decline,^[18] which began in the C4 isotopic phase of the interval.^[11]

OAE1a coincided with a peak in a 5-6 Myr periodicity cycle in the accumulation of phosphorus in marine sediments. During such peaks, the short-term positive feedback loop of increased biological productivity caused by an abundance of phosphorus that caused decreased oxygenation of seawater that then caused increased regeneration of phosphorus from marine sediments dominated, but it was eventually mitigated by a long-term negative feedback loop caused by an increase in atmospheric oxygen that resulted in enhanced wildfire activity and diminished phosphorus input into the oceans.^[19] An increase in the ratios of organic carbon to reactive phosphorus species and of total nitrogen to reactive phosphorus confirms leakage of sedimentary phosphorus back into the water column occurred during OAE1a, with this process likely being accelerated by the increased global temperatures of the time.^[20]