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The Ireviken Event was a minor extinction event at the Llandovery-Wenlock boundary. Whole event took place during regression, after the onset of global sea-level fall and finished before the maximum lowstand (Calner 2008). The mechanism responsible for the event originated most probably in the deep oceans, and made its way into the shallower shelf seas. Accordingly, shallow-water reefs were scarcely affected, while pelagic and hemipelagic organisms were hit hardest. Subsequent to the first extinctions, excursions in the 813C and 8180 records are observed; 813C rises from +1.4%o to +4.5%o, while 8180 increases from -5.6%o to -5.0%o (Munnecke et al. 2003). Here, identification of redox conditions was carried out using pyrite framboid diameters. The size distribution of framboids supplies information about ancient redox conditions at a fine scale resolution (Wignall & Newton 1998). Measurements of pyrite framboid diameters were carried out using Scanning Electron Microscope (SEM) in backscattered electrons (BSE). Twenty seven samples of sedimentary rocks from Wilków IG1 borehole were analysed. Based on graptolite stratigraphy, the Ireviken Event, in the described borehole, starts at a depth of 585 meters and finishes at ca. 581 meters. Almost 10 m before this Silurian event, framboids are not observed. It may suggest rather oxic conditions during sedimentation. The beginning of Ireviken Event is characterized by presence of tiny framboids with a mean diameter of 4.55 (j.m (minimum value of 2.77 (j.m, maximum value of 9.27 (j.m). After that euxinic pulse, anoxic conditions have changed gradually to more oxic/disoxic, which is expressed by larger pyrite diameters. The average values of framboid diameters for the central part of the extinction event are ranging from 5.35 jj.m to 8.7 (j.m. Euxinic conditions in water column returned at the end of Ireviken Event and were again characterized by small pyrite framboid diameters. The fluctuations during this extinction event are clearly seen and may suggest intensive sea level oscillations.
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