The Ireviken event was a minor extinction event

Extinction event

An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

 at the Llandovery/Wenlock boundary (mid Silurian).
The event is best recorded at Ireviken, Gotland

Gotland

Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...

, where over 50% of trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

 species go extinct; 80% of the global conodont

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

 species also become extinct in this interval.

Anatomy of the event

The event lasted around 200,000 years, spanning the base of the Wenlock epoch.

It comprises eight extinction "datum points"—the first four being regularly spaced, every 30,797 years, and linked to the Milankovic

Milankovic

Milankovic can refer to*Milutin Milanković Serbian geophysicist famous for his description of Milankovic cycles*Milankovic crater...

 obliquity cycle. The fifth and sixth probably reflect maxima in the precessional cycles, with periods of around 16.5 and 19 ka. The final two data are much further spaced, so harder to link with Milankovic changes.

Casualties

The mechanism responsible for the event originated in the deep oceans, and made its way into the shallower shelf seas. Correspondingly, shallow-water reefs were barely affected, while pelagic and hemipelagic organisms such as the graptolites, conodonts and trilobites were hit hardest.

Geochemistry

Subsequent to the first extinctions, excursions in the δ¹³C and δ¹⁸O records are observed; δ¹³C rises from +1.4‰ to +4.5‰, while δ¹⁸O increases from −5.6‰ to −5.0‰.