Lau event

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The Lau event was the last of three relatively minor mass extinctions (the Ireviken, Mulde, and Lau events) during the Silurian period. [4] It had a major effect on the conodont fauna, but barely scathed the graptolites, 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 taphonomic reasons. [5] It coincided with a global low point in sea level caused by glacioeustasy and is closely followed by an excursion in geochemical isotopes in the ensuing late Ludfordian faunal stage and a change in depositional regime. [6] [5]

Contents

Biological impact

The Lau event started at the beginning of the late Ludfordian, a subdivision of the Ludlow stage, about 420  million years ago. Its 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 conodonts and graptolites. [7] More precisely, conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion. [6] 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 does not change the fact that many life-forms became re-established shortly after the event, presumably surviving in refuge or in environments that have not been preserved in the geological record. [8] 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. [9]

Isotopic effects

A peak in δ13C, 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 – perhaps arising due to a build-up of glaciers; [10] however, such factors alone do not appear to be sufficient to explain the events. [11] 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). [9] 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. [12] 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. [6]

The δ13C 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. [6]

A positive excursion of δ34S in pyrite coincides with the positive δ13C excursion following the Lau event, likely related to the expansion of euxinic conditions and enhanced pyrite burial. [5] [13]

Sedimentological impact

Profound sedimentary changes occurred at the beginning of the Lau event; these are probably associated with the onset of sea level rise, which continued through the event, reaching a high point at the time of deposition of the Burgsvik beds, after the event. [14]

These changes appear to display anachronism, 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 – 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, which rarely form in the presence of abundant higher life forms, are observed during the Lau event and, occasionally, in the overlying Burgsvik beds; [15] microbial colonies of Rothpletzella and Wetheredella become abundant. This suite of characteristics is common to the larger end-Ordovician and end-Permian extinctions.

See also

Further reading

Related Research Articles

<span class="mw-page-title-main">Silurian</span> Third period of the Paleozoic Era, 443–419 million years ago

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

In the geologic timescale, the Aeronian is an age of the Llandovery Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that began 440.8 ± 1.2 Ma and ended 438.5 ± 1.1 Ma. The Aeronian Age succeeds the Rhuddanian Age and precedes the Telychian Age, all in the same epoch.

The Andean-Saharan glaciation, also known as the Early Paleozoic Ice Age (EPIA), the Early Paleozoic Icehouse, the Late Ordovician glaciation, the end-Ordovician glaciation, or the Hirnantian glaciation, occurred during the Paleozoic from approximately 460 Ma to around 420 Ma, during the Late Ordovician and the Silurian period. The major glaciation during this period was formerly thought only to consist of the Hirnantian glaciation itself but has now been recognized as a longer, more gradual event, which began as early as the Darriwilian, and possibly even the Floian. Evidence of this glaciation can be seen in places such as Arabia, North Africa, South Africa, Brazil, Peru, Bolivia, Chile, Argentina, and Wyoming. More evidence derived from isotopic data is that during the Late Ordovician, tropical ocean temperatures were about 5 °C cooler than present day; this would have been a major factor that aided in the glaciation process.

<span class="mw-page-title-main">Ludfordian</span> Seventh stage of the Silurian

In the geologic timescale, the Ludfordian is the upper of two chronostratigraphic stages within the Ludlow Series. Its age is the late Silurian Period, and within both the Palaeozoic Era and Phanerozoic Eon. The rocks assigned to the Ludfordian date to between 425.6 ± 0.9 Ma and 423.0 ± 2.3 Ma. The Ludfordian Stage succeeds the Gorstian Stage and precedes the Pridoli Epoch. It is named for the village of Ludford in Shropshire, England. The GSSP for the Ludfordian is represented as a thin shale seam, coincident with the base of the Leintwardine Formation, overlying the Bringewood Formation in England.

<span class="mw-page-title-main">Gorstian</span> Sixth stage of the Silurian

In the geologic timescale, the Gorstian is an age of the Ludlow Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that is comprehended between 427.4 ± 0.5 Ma and 425.6 ± 0.9 Ma, approximately. The Gorstian Age succeeds the Homerian Age and precedes the Ludfordian Age. The age is named after Gorsty village southwest of Ludlow. The base of the age is marked by Graptolites tumescens and Graptolites incipiens. The type section is located in a quarry in the Elton Formation at Pitch Coppice, Shropshire, United Kingdom.

<span class="mw-page-title-main">Homerian</span> Fifth stage of the Silurian

In the geologic timescale, the Homerian is an age of the Wenlock Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that is comprehended between 430.5 ± 0.7 Ma and 427.4 ± 0.5 Ma, approximately. The Homerian Age succeeds the Sheinwoodian Age and precedes the Gorstian Age.

<span class="mw-page-title-main">Sheinwoodian</span>

In the geologic timescale, the Sheinwoodian is the age of the Wenlock Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon that is comprehended between 433.4 ± 0.8 Ma and 430.5 ± 0.7 Ma, approximately. The Sheinwoodian Age succeeds the Telychian Age and precedes the Homerian Age.

<span class="mw-page-title-main">Telychian</span> Third stage of the Silurian and last stage of the Llandovery

In the geologic timescale, the Telychian is the third and final age of the Llandovery Epoch of the Silurian Period of the Paleozoic Era of the Phanerozoic Eon. The Telychian Age was between 438.5 ± 1.2 million years ago (Ma) and 433.4 ± 0.8 Ma. The Telychian Age succeeds the Aeronian Age and precedes the Sheinwoodian Age. The name of the interval is derived from the Pen-lan-Telych Farm near Llandovery, Powys, Wales. The GSSP is located within the Wormwood Formation.

In the geologic timescale, the Rhuddanian is the first age of the Silurian Period and of the Llandovery Epoch. The Silurian is in the Paleozoic Era of the Phanerozoic Eon. The Rhuddanian Age began 443.8 ± 1.5 Ma and ended 440.8 ± 1.2 Ma. It succeeds the Himantian Age and precedes the Aeronian Age.

In the geological timescale, the Ludlow Epoch occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, England.

In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.

<span class="mw-page-title-main">Burgsvik Beds</span> Sequence of limestones and sandstones found in Sweden

The Burgsvik Beds are a sequence of shallow marine limestones and sandstones found near the locality of Burgsvik in the southern part of Gotland, Sweden. The beds were deposited in the Upper Silurian period, around 420 million years ago, in warm, equatorial waters frequently ravaged by storms, in front of an advancing shoreline. The Burgsvik Formation comprises two members, the Burgsvik Sandstone and the Burgsvik Oolite.

The Ireviken event was the first of three relatively minor extinction events during the Silurian period. It occurred at the Llandovery/Wenlock boundary. The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species became extinct; 80% of the global conodont species also became extinct in this interval.

<span class="mw-page-title-main">Fröjel Formation</span>

The Mulde event was an anoxic event, and marked the second of three relatively minor mass extinctions during the Silurian period. It coincided with a global drop in sea level, and is closely followed by an excursion in geochemical isotopes. Its onset is synchronous with the deposition of the Fröel Formation in Gotland. Perceived extinction in the conodont fauna, however, likely represent a change in the depositional environment of sedimentary sequences rather than a genuine biological extinction.

<span class="mw-page-title-main">Pridoli Epoch</span> Final Series (Epoch) of the Silurian

In the geologic timescale, the Přídolí Epoch is the uppermost subdivision of the Silurian Period, dated at between 423 ± 2.3 and 419.2 ± 3.2 mya. The Přídolí Epoch succeeds the Ludfordian Stage and precedes the Lochkovian, the lowest of three stages within the Lower Devonian geological epoch. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb, Slivenec, in the Czech Republic. The GSSP is located within the Požáry Formation, overlying the Kopanina Formation. Přídolí is the old name of a cadastral field area.

The Wenlock is the second epoch of the Silurian. It is preceded by the Llandovery Epoch and followed by the Ludlow Epoch. Radiometric dates constrain the Wenlockian between 433.4 and 427.4 million years ago.

The Lundgreni Event, also known as the Mid-Homerian Biotic Crisis, 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, Bohemia, and Poland.

The Šilalė Event was an extinction event affecting conodonts during the Přídolí, the final stage of the Silurian period.

References

  1. ↑ Jeppsson, L.; Calner, M. (2007). "The Silurian Mulde Event and a scenario for secundo—secundo events". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 93 (02): 135–154. doi:10.1017/S0263593300000377.
  2. ↑ Munnecke, A.; Samtleben, C.; Bickert, T. (2003). "The Ireviken Event in the lower Silurian of Gotland, Sweden-relation to similar Palaeozoic and Proterozoic events". Palaeogeography, Palaeoclimatology, Palaeoecology. 195 (1): 99–124. doi:10.1016/S0031-0182(03)00304-3.
  3. ↑ "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  4. ↑ The Ireviken, Mulde, and Lau events, were all closely followed by isotopic excursions.
  5. 1 2 3 Frýda, Jiří; Lehnert, Oliver; Joachimski, Michael M.; Männik, Peep; Kubajko, Michal; Mergl, Michal; Farkaš, Juraj; Frýdová, Barbora (September 2021). "The Mid-Ludfordian (late Silurian) Glaciation: A link with global changes in ocean chemistry and ecosystem overturns". Earth-Science Reviews. 220: 103652. doi:10.1016/j.earscirev.2021.103652 . Retrieved 16 October 2022.
  6. 1 2 3 4 Loydell, D.K. (2007). "Early Silurian positive d13C excursions and their relationship to glaciations, sea-level changes and extinction events". Geol. J. 42 (5): 531–546. doi: 10.1002/gj.1090 . S2CID   128822111.
  7. ↑ Urbanek, A. (1993). "Biotic crises in the history of Upper Silurian graptoloids: a palaeobiological model". Historical Biology. 7: 29–50. doi:10.1080/10292389309380442.
  8. ↑ Jeppsson, L. (1998). "Silurian oceanic events: summary of general characteristics". In Landing, E.; Johnson, M.E. (eds.). 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.
  9. 1 2 Jeppsson, Lennart; Aldridge, Richard J. (2000-11-01). "Ludlow (late Silurian) oceanic episodes and events". Journal of the Geological Society. 157 (6): 1137. Bibcode:2000JGSoc.157.1137J. doi:10.1144/jgs.157.6.1137. S2CID   129255983 . Retrieved 2007-06-26.
  10. ↑ Lehnert, O.; Joachimski, M.M.; Fryda, J.; Buggisch, W.; Calner, M.; Jeppsson, L.; Eriksson, M.E. (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. Archived from the original on 2008-03-17. Retrieved 2007-06-26.
  11. ↑ Samtleben, C.; Munnecke, A.; Bickert, T.; Pätzold, J. (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. Bibcode:1996IJEaS..85..278S. doi:10.1007/bf02422234. S2CID   129446078 . Retrieved 2007-06-26.[ dead link ]
  12. ↑ Kozłowski, Wojciech; Sobień, Katarzyna (1 July 2012). "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". Palaeogeography, Palaeoclimatology, Palaeoecology . 339–341: 74–97. doi:10.1016/j.palaeo.2012.04.024 . Retrieved 26 December 2022.
  13. ↑ Bowman, Chelsie N.; Lindskog, Anders; Kozik, Nevin P.; Richbourg, Claudia G.; Owens, Jeremy D.; Young, Seth A. (1 September 2020). "Integrated sedimentary, biotic, and paleoredox dynamics from multiple localities in southern Laurentia during the late Silurian (Ludfordian) extinction event". Palaeogeography, Palaeoclimatology, Palaeoecology. 553: 109799. doi:10.1016/j.palaeo.2020.109799. S2CID   219438899 . Retrieved 16 October 2022.
  14. ↑ Calner, M.; Eriksson, M.J. (2006). "Evidence for rapid environmental changes in low latitudes during the Late Silurian Lau Event: the Burgen-1 drillcore, Gotland, Sweden". Geological Magazine. 143 (1): 15–24. Bibcode:2006GeoM..143...15C. doi:10.1017/S001675680500169X. S2CID   129946754.
  15. ↑ Calner, M. (2005-04-01). "A Late Silurian extinction event and anachronistic period". Geology. 33 (4): 305–308. Bibcode:2005Geo....33..305C. doi:10.1130/G21185.1.
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