Lau event
Encyclopedia
The Lau event was the last of three relatively minor mass extinctions during the Silurian
period, having a major effect on the conodont
fauna (but barely scathing the graptolite
s). It coincided with a global low point in sea level, is closely followed by an excursion in geochemical
isotope
s in the ensuing late Ludfordian faunal stage
and a change in depositional regime.
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 paucity of data, it is apparent that most major groups suffered an increase in extinction rate during the event; major biotal changes are observed worldwide at correlated rocks, with a "crisis" observed in populations of conodont
s and graptolite
s. More precisely, conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion. 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 bely the fact that many taxa became re-established shortly after the event, presumably surviving in refugia
or in environments that have not been preserved in the geological record. Despite taxa persisting, community structure was permanently altered by the event, with many taxa failing to regain the niches lost in the event.
However, the curve slightly lags conodont extinctions—hence the two events may not represent the same thing. Hence 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 occurs.
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.
, after the event.
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, 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; microbial colonies of Rothpletzella and Wetheredella become abundant. This suite of characteristics is common to the larger end-Ordovician
and end-Permian
extinctions.
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
period, having a major effect on the conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
fauna (but barely scathing the graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
s). It coincided with a global low point in sea level, is closely followed by an excursion in geochemical
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s in the ensuing late Ludfordian faunal stage
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
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 . Its strataStratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
are best exposed in Gotland
Gotland
Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, taking 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 event; major biotal changes are observed worldwide at correlated rocks, with a "crisis" observed in populations of conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
s and graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...
s. More precisely, conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion. Local extinctions may have played a role in many places, especially the increasingly enclosed Welsh
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
basin; the event's relatively high severity rating
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
of 6.2 does not bely the fact that many taxa became re-established shortly after the event, presumably surviving in refugia
Refugium
Refugium may refer to:* Refugium , an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply...
or in environments that have not been preserved in the geological record. Despite taxa persisting, community structure was permanently altered by the event, with many taxa failing to regain the niches lost in the event.
Isotopic effects
A peak in , 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; however, such factors alone do not appear to be sufficient to explain the events. 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).However, the curve slightly lags conodont extinctions—hence the two events may not represent the same thing. Hence 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 occurs.
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.
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 bedsBurgsvik beds
The Burgsvik beds are a sequence of shallow marine limestones and sandstones found near the town of Burgsvik in the southern part of Gotland, Sweden. The beds were deposited in the Upper Silurian period, around , in warm, equatorial waters frequently ravaged by storms, in front of an advancing...
, after the event.
These changes appear to display anachronism
Anachronism
An anachronism—from the Greek ανά and χρόνος — is an inconsistency in some chronological arrangement, especially a chronological misplacing of persons, events, objects, or customs in regard to each other...
, 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
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, when life as we know it was only just becoming established. Indeed, stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....
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; microbial colonies of Rothpletzella and Wetheredella become abundant. This suite of characteristics is common to the larger end-Ordovician
Ordovician-Silurian extinction events
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440...
and end-Permian
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
extinctions.