Lundgreni Event
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The Lundgreni Event, also known as the Mid-Homerian Biotic Crisis,[1] was an extinction event during the middle Homerian age of the Silurian period. Evidence for the event has been observed in Silurian marine deposits in the Iberian Peninsula,[2] Bohemia,[1] and Poland.[3]
In the Kosov quarry in Bohemia, the extinction is observed during the latest lundgreni biozone and over the course of the flemingii biozone interval. The following parvus biozone, corresponding to the nassa, the parvus-nassa, or dubious-nassa biozones in other localities, represents a post-extinction interval, which is in turn followed by the frequens, praedeubeli-deubeli, and ludensis-gerhardi biozones that mark the period of recovery from the extinction.[1]
Causes
Eutrophication and anoxia coeval with abrupt ecological changes have been implicated as extinction mechanisms bringing about the Lundgreni Event. Immediately after the extinction event, geological records from Bartoszyce evidence a sharp slowdown of ocean mixing. The Lundgreni Event has been hypothesised to have occurred during a period of global marine transgression, a proposed explanation cohering with the relative lack of effect this biotic crisis had on benthic fauna due to the fact that anoxia would likely not have spread into shallow, epicontinental seas.[3]
Isotopic effects
The extinction is marked by the start of a double-peaked positive carbon isotope excursion beginning in the lundgreni graptolite biozone. The first peak spans from the uppermost portion of the lundgreni biozone all the way to the praedeubeli-deubeli graptolite biozone, with a particularly sharp trend towards increasingly positive δ13C values observed during the flemingii and parvus graptolite biozones, corresponding to the extinction interval. The second peak’s start is close to the base of the ludensis-gerhardi graptolite biozone, during the recovery interval.[1]