Lau event

The Lau event was a mass extinction towards the end of the Silurian period, and is marked by an excursion in geochemical isotopes and a change in depositional regime.

Biotic impact

The Lau event started at the beginning of the late Ludfordian, a subdivision of the Ludlow stage, about 420 million years ago. It is best exposed in Gotland, and takes its name from the parish of Lau. Its base is set at the first extinction datum, in the Eke beds, and despite a paucity of data, it is apparent that most major groups suffered an increase in extinction rate during the period of the event; major biotal changes are observed worldwide at correlated rocks, with a "crisis" observed in populations of conodonts and graptolites (Urbanek 1993). Local extinctions may have played a role in many places, especially the increasingly enclosed Welsh basin; the event's relatively high severity rating of 6.2 (Jeppsson 1998) does not bely the fact that many taxa became re-established shortly after the event, presumably surviving in refugia or unpreserved environments. Despite taxa persisting, community structure was permanently altered by the event, with many taxa failing to regain the niches lost in the event (Jeppsson & Aldridge 2000).

Isotopic effects

A peak in δ¹³C, accompanied by fluctuations in other isotope concentrations, is are often associated with mass extinctions. Some workers have attempted to explain this event in terms of climate or sea level change — perhaps arising due to a build-up of glaciers (Lehnert et al. 2006); however, such factors alone do not appear to be sufficient to explain the events (Sambleten et al. 1996). An alternative hypothesis is that changes in ocean mixing were responsible, with the driver for the density increase causing water to downwell changing from hypersalinity (due to ice formation and evaporation) to temperature (due to water cooling) (Jeppsson & Aldridge 2000).

Anacrhonism

The event is also associated with an anachronism, marked by a return of flat-pebbled conglomerates in the Eke beds, which is further evidence of a major blow to ecosystems of the time — such deposits can only form in conditions similar to those of the early Cambrian period, when life as we know it was only just becoming established. Indeed, stromatolites and other microbially controlled sedimentary structures, which rarely form in the presence of abundant higher life forms, are observed during the Lau event and, occasionally, in the overlying Burgsvik beds (Calner 2005); such features are common to the larger end-Ordivician and end-Permian extinctions.

References

• Calner, M. (2005-04-01). "A Late Silurian extinction event and anachronistic period". Geology. 33 (4): 305–308. doi:10.1130/G21185.1. {{cite journal}}: Check date values in: |date= (help)CS1 maint: date and year (link)

• Jeppsson, L. (1998). "Silurian oceanic events: summary of general characteristics". Silurian Cycles: Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic and Tectonic Changes. James Hall Centennial Volume. New York State Museum Bulletin. Vol. 491. pp. 239–257. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)

• Jeppsson, Lennart (2000-11-01). "Ludlow (late Silurian) oceanic episodes and events". Journal of the Geological Society. 157 (6): 1137. Retrieved 2007-06-26. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Lehnert, O. (2006). "The Ludlow Lau Event-another Glaciation In The Silurian Greenhouse?". Geological Society of America Abstracts with Programs. 2006 Philadelphia Annual Meeting. Vol. 38. p. 183. Retrieved 2007-06-26. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Samtleben, C. (1996). "The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells" (PDF). International Journal of Earth Sciences. 85 (2): 278–292. Retrieved 2007-06-26. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• Urbanek, A. (1993). "Biotic crises in the history of Upper Silurian graptoloids: a palaeobiological model". Historical Biology. 7: 29–50.