View source for Earth:Lau event

{{short description|Relatively minor mass extinction during the Silurian period}}
{{Silurian graphical timeline}}
The '''Lau event''' was the last of three relatively minor mass extinctions (the [[Earth:Ireviken event|Ireviken]], [[Earth:Mulde event|Mulde]], and '''Lau''' events) during the [[Earth:Silurian|Silurian]] period.<ref>The Ireviken, Mulde, and Lau events, were all closely followed by isotopic excursions.</ref> It had a major effect on the [[Biology:Conodont|conodont]] fauna, but barely scathed the [[Biology:Graptolite|graptolite]]s, though they suffered an extinction very shortly thereafter termed the Kozlowskii event that some authors have suggested was coeval with the Lau event and only appears asynchronous due to [[Earth:Taphonomy|taphonomic]] reasons.<ref name ="LudfordianGlaciation">{{cite journal |last1=Frýda |first1=Jiří |last2=Lehnert |first2=Oliver |last3=Joachimski |first3=Michael M. |last4=Männik |first4=Peep |last5=Kubajko |first5=Michal |last6=Mergl |first6=Michal |last7=Farkaš |first7=Juraj |last8=Frýdová |first8=Barbora |date=September 2021 |title=The Mid-Ludfordian (late Silurian) Glaciation: A link with global changes in ocean chemistry and ecosystem overturns |url=https://www.sciencedirect.com/science/article/abs/pii/S0012825221001537 |journal=Earth-Science Reviews |volume=220 |page=103652 |doi=10.1016/j.earscirev.2021.103652 |access-date=16 October 2022}}</ref>  It coincided with a global low point in sea level caused by glacioeustasy and is closely followed by an excursion in [[Chemistry:Geochemistry|geochemical]] [[Physics:Isotope|isotope]]s in the ensuing late Ludfordian faunal stage and a change in depositional regime.<ref name=loydell07>{{cite journal | first=D.K. | last=Loydell | year=2007 | title=Early Silurian positive d<sup>13</sup>C excursions and their relationship to glaciations, sea-level changes and extinction events. | journal=Geol. J. | volume=42 | pages=531–546 | doi=10.1002/gj.1090 | issue=5 | s2cid=128822111 | doi-access=free }}</ref><ref name ="LudfordianGlaciation" />

==Biological impact==
The Lau event started at the beginning of the late [[Earth:Ludfordian|Ludfordian]], a subdivision of the Ludlow stage, about {{Ma|420}}.  Its [[Earth:Stratum|strata]] are best exposed in Gotland, Sweden, taking its name from the parish of Lau.  Its base is set at the first extinction datum, in the Eke beds, and despite a scarcity of data, it is apparent that most major groups suffered an increase in extinction rate during the event; major changes are observed worldwide at correlated rocks, with a "crisis" observed in populations of [[Biology:Conodont|conodont]]s and [[Biology:Graptolite|graptolite]]s.<ref>{{cite journal
 | author = Urbanek, A.
 | year = 1993
 | title = Biotic crises in the history of Upper Silurian graptoloids: a palaeobiological model
 | journal = Historical Biology
 | volume = 7
 | pages = 29–50
 | doi =10.1080/10292389309380442
 }}
</ref>  More precisely, conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion.<ref name=loydell07/>  Local extinctions may have played a role in many places, especially the increasingly enclosed Welsh basin; the event's relatively high [[Biology:Extinction event|severity rating]] of 6.2 does not change the fact that many life-forms became re-established shortly after the event, presumably surviving in [[Biology:Refugium (population biology)|refuge]] or in environments that have not been preserved in the geological record.<ref>{{cite book
 | author = Jeppsson, L.
 | year = 1998
 | chapter = Silurian oceanic events: summary of general characteristics
 | title = Silurian Cycles: Linkages of Dynamic Stratigraphy with Atmospheric, Oceanic and Tectonic Changes. James Hall Centennial Volume. New York State Museum Bulletin
 | volume = 491
 |editor=Landing, E. |editor2=Johnson, M.E.
 | pages = 239–257
}}</ref>  Although life persisted after the event, community structures were permanently altered and many lifeforms failed to regain the niches they had occupied before the event.<ref name=jeppsson00>{{cite journal
 | author = Jeppsson, Lennart
 |author2=Aldridge, Richard J.
  | date = 2000-11-01
 | title = Ludlow (late Silurian) oceanic episodes and events
 | journal = Journal of the Geological Society
 | volume = 157
 | issue = 6
 | pages = 1137
 | doi = 10.1144/jgs.157.6.1137| url = http://jgs.geoscienceworld.org/cgi/content/abstract/157/6/1137 
 | accessdate = 2007-06-26
 | bibcode = 2000JGSoc.157.1137J|s2cid=129255983
 }}</ref>

==Isotopic effects==
A peak in {{delta|13|C|link}}, accompanied by fluctuations in other isotope concentrations, is often associated with mass extinctions. Some workers have attempted to explain this event in terms of climate or sea level change&nbsp;– perhaps arising due to a build-up of glaciers;<ref>{{cite conference |author=Lehnert, O. |author2=Joachimski, M.M. |author3=Fryda, J. |author4=Buggisch, W. |author5=Calner, M. |author6=Jeppsson, L. |author7=Eriksson, M.E. |year=2006 |title=The Ludlow Lau Event-another Glaciation In The Silurian Greenhouse? |conference=2006 Philadelphia Annual Meeting |book-title=Geological Society of America Abstracts with Programs |volume=38 |number=7 |pages=183 |url=http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_113842.htm |accessdate=2007-06-26 |archive-url=https://web.archive.org/web/20080317032410/http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_113842.htm |archive-date=2008-03-17 |url-status=dead }}</ref> however, such factors alone do not appear to be sufficient to explain the events.<ref>{{cite journal |last1=Samtleben |first1=C. |last2=Munnecke |first2=A. |last3=Bickert |first3=T. |last4=Pätzold |first4=J. |year=1996 |title=The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells |journal=International Journal of Earth Sciences |volume=85 |issue=2 |pages=278–292 |doi=10.1007/bf02422234 |url=http://www.springerlink.com/index/E6857763723729P2.pdf |accessdate=2007-06-26 |bibcode=1996IJEaS..85..278S|s2cid=129446078 }} </ref> An alternative hypothesis is that changes in ocean mixing were responsible. An increase in density is required to make water downwell; the cause of this densification may have changed from hypersalinity (due to ice formation and evaporation) to temperature (due to water cooling).<ref name=jeppsson00/> A different hypothesis attributes the carbon isotope fluctuations to methanogenesis caused by the increased influx of iron-bearing dust and consequent disruption of limiting nutrient ratios.<ref>{{cite journal |last1=Kozłowski |first1=Wojciech |last2=Sobień |first2=Katarzyna |date=1 July 2012 |title=Mid-Ludfordian coeval carbon isotope, natural gamma ray and magnetic susceptibility excursions in the Mielnik IG-1 borehole (Eastern Poland)—Dustiness as a possible link between global climate and the Silurian carbon isotope record |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018212002283 |journal=[[Earth:Palaeogeography, Palaeoclimatology, Palaeoecology|Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=339-341 |pages=74–97 |doi=10.1016/j.palaeo.2012.04.024 |access-date=26 December 2022}}</ref> Loydell suggests many causes of the isotopic excursion, including increased carbon burial, increased carbonate weathering, changes in atmospheric and oceanic interactions, changes in primary production, and changes in humidity or aridity. He uses a correlation between the events and glacially induced global sea level change to suggest that carbonate weathering is the major player, with other factors playing a less significant role.<ref name=loydell07/>

The {{delta|13|C}} curve slightly lags conodont extinctions, hence the two events may not represent the same thing. Therefore, the term ''Lau event'' is used only for the extinction, not the following isotopic activity, which is named after the time period in which it occurred.<ref name=loydell07/>

A positive excursion of [[Biology:Δ34S|{{delta|34|S}}]] in pyrite coincides with the positive {{delta|13|C}} excursion following the Lau event, likely related to the expansion of euxinic conditions and enhanced pyrite burial.<ref name ="LudfordianGlaciation" /><ref>{{cite journal |last1=Bowman |first1=Chelsie N. |last2=Lindskog |first2=Anders |last3=Kozik |first3=Nevin P. |last4=Richbourg |first4=Claudia G. |last5=Owens |first5=Jeremy D. |last6=Young |first6=Seth A. |date=1 September 2020 |title=Integrated sedimentary, biotic, and paleoredox dynamics from multiple localities in southern Laurentia during the late Silurian (Ludfordian) extinction event |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018220302443 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=553 |page=109799 |doi=10.1016/j.palaeo.2020.109799 |s2cid=219438899 |access-date=16 October 2022}}</ref>

==Sedimentological impact==
Profound sedimentary changes occurred at the beginning of the Lau event; these are probably associated with the onset of [[Earth:Sea level rise|sea level rise]], which continued through the event, reaching a high point at the time of deposition of the Burgsvik beds, after the event.<ref>{{cite journal
 | author = Calner, M.
 |author2=Eriksson, M.J.
  | year = 2006
 | title = Evidence for rapid environmental changes in low latitudes during the Late Silurian Lau Event: the Burgen-1 drillcore, Gotland, Sweden
 | journal = Geological Magazine
 | volume = 143
 | issue = 1
 | pages = 15–24
 | doi = 10.1017/S001675680500169X
 | bibcode = 2006GeoM..143...15C
 |s2cid=129946754
 }}</ref>

These changes appear to display [[Anachronism|anachronism]],<!--this special usage, not covered at the link, needs dab here: footnote?--> marked by an increase in erosional surfaces and the return of flat-pebbled conglomerates in the Eke beds.  This is further evidence of a major blow to ecosystems of the time&nbsp;– such deposits can only form in conditions similar to those of the early [[Earth:Cambrian|Cambrian]] period, when life as we know it was only just becoming established. Indeed, [[Earth:Stromatolite|stromatolite]]s, which rarely form in the presence of abundant higher life forms, are observed during the Lau event and, occasionally, in the overlying Burgsvik beds;<ref>{{cite journal
 | author = Calner, M.
 | date = 2005-04-01
 | title = A Late Silurian extinction event and anachronistic period
 | journal = Geology
 | volume = 33
 | issue = 4
 | pages = 305–308
 | doi = 10.1130/G21185.1
 |bibcode = 2005Geo....33..305C }}</ref> microbial colonies of ''[[Biology:Rothpletzella|Rothpletzella]]'' and ''[[Biology:Wetheredella|Wetheredella]]'' become abundant.  This suite of characteristics is common to the larger end-Ordovician and end-Permian extinctions.

==See also==
*[[Earth:Anoxic event|Anoxic event]]

==Further reading==

* [https://pubs.geoscienceworld.org/gsa/geology/article-abstract/573358/linking-the-progressive-expansion-of-reducing Linking the progressive expansion of reducing conditions to a stepwise mass extinction event in the late Silurian oceans]

==References==
{{reflist|30em}}

{{ExtEvent_nav}}

[[Category:Extinction events]]

{{Sourceattribution|Lau event}}