Late Devonian extinction
The Late Devonian extinction consisted of several
Although it is well established that there was a massive
By the Late Devonian, the land had been colonized by
The extinction event was accompanied by widespread oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter.[14][15] This, combined with the ability of porous reef rocks to hold oil, has led to Devonian rocks being an important source of oil, especially in Canada and the United States.[16][17][18]
Late Devonian world
Devonian graphical timeline | ||
Vertical axis scale: millions of years ago.
During the Late Devonian, the continents were arranged differently from today, with a supercontinent,
The biota was also very different. Plants, which had been on land in forms similar to mosses and liverworts since the
Extinction patterns
The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and had a greater effect on shallow warm-water organisms than on cool-water organisms. The Kellwasser event's effects were also stronger at low latitudes than high ones.[27] Large differences are observed between the biotas before and after the Frasnian-Famennian boundary, demonstrating the extinction event's magnitude.[28]
Reef destruction
The most hard-hit biological category affected by the Kellwasser event were the calcite-based reef-builders of the great Devonian reef-systems, including the
Marine invertebrates
Further taxa to be starkly affected include the
Vertebrates
Magnitude of diversity loss
The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous. A recent survey (McGhee 1996) estimates that 22% of all the 'families' of marine animals (largely invertebrates) were eliminated. The family is a great unit, and to lose so many signifies a deep loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species did not survive into the Carboniferous. These latter estimates[a] need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and sampling biases during the Devonian.
Duration and timing
Extinction rates appear to have been higher than the background rate for an extended interval covering the last 20–25 million years of the Devonian. During this time, about eight to ten distinct events can be seen, of which two, the Kellwasser and the Hangenberg events, stand out as particularly severe.[38] The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.[39]
The Kellwasser event, named for its type locality, the Kellwassertal in Lower Saxony, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary (372.2 ± 1.6 Ma). Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record and was the most severe of the extinction crises of the Late Devonian.[40] There may in fact have been two closely spaced events here, as shown by the presence of two distinct anoxic shale layers.[41][42][43]
There is evidence that the Kellwasser event was a two-pulsed event, with the two extinction pulses being separated by an interval of approximately 800,000 years. The second pulse was more severe than the first.[44]
Potential causes
Since the Kellwasser-related extinctions occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. From the end of the Middle Devonian (382.7±1.6 Ma), into the Late Devonian (382.7±1.6 Ma to 358.9±0.4 Ma), several environmental changes can be detected from the sedimentary record, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate. Possible triggers for the Kellwasser event are as follows:
Weathering and anoxia
During the
The relatively sudden input of nutrients into river water as rooted plants expanded into upland regions may have caused eutrophication and subsequent anoxia.[52][35] For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposition of dead organisms uses up all available oxygen, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatoporoids and (to a lesser degree) corals—organisms which only thrive in low-nutrient conditions. Therefore, the postulated influx of high levels of nutrients may have caused an extinction.[24][53] Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.[54] Evidence exists of a rapid increase in the rate of organic carbon burial and for widespread anoxia in oceanic bottom waters.[55][24] Signs of anoxia in shallow waters have also been described from a variety of localities.[56][57][58] Good evidence has been found for high-frequency sea-level changes around the Frasnian–Famennian Kellwasser event, with one sea-level rise associated with the onset of anoxic deposits;[59] marine transgressions likely helped spread deoxygenated waters.[2] Evidence exists for the modulation of the intensity of anoxia by Milankovitch cycles as well.[60][61] Negative δ238U excursions concomitant with both the Lower and Upper Kellwasser events provide direct evidence for an increase in anoxia.[62] Photic zone euxinia, documented by concurrent negative ∆199Hg and positive δ202Hg excursions, occurred in the North American Devonian Seaway.[63] Elevated molybdenum concentrations further support widespread euxinic waters.[64]
The timing, magnitude, and causes of Kellwasser anoxia remain poorly understood.
Global cooling
A positive δ18O excursion is observed across the Frasnian-Famennian boundary in brachiopods from North America, Germany, Spain, Morocco, Siberia, and China;[69] conodont apatite δ18O excursions also occurred at this time.[70] A similar positive δ18O excursion in phosphates is known from the boundary, corresponding to a removal of atmospheric carbon dioxide and a global cooling event. This oxygen isotope excursion is known from time-equivalent strata in South China and in the western Palaeotethys, suggesting it was a globally synchronous climatic change. The concomitance of the drop in global temperatures and the swift decline of metazoan reefs indicates the blameworthiness of global cooling in precipitating the extinction event.[71]
The "greening" of the continents during the Silurian-Devonian Terrestrial Revolution that led to them being covered with massive photosynthesizing land plants in the first forests reduced CO2 levels in the atmosphere.[72] Since CO2 is a greenhouse gas, reduced levels might have helped produce a chillier climate, in contrast to the warm climate of the Middle Devonian.[24] The biological sequestration of carbon dioxide may have ultimately led to the beginning of the Late Palaeozoic Ice Age during the Famennian, which has been suggested as a cause of the Hangenberg event.[73]
The weathering of silicate rocks also draws down CO2 from the atmosphere, and CO2 sequestration by mountain building has been suggested as a cause of the decline in greenhouse gases during the Frasnian-Famennian transition. This mountain-building may have also enhanced biological sequestration through an increase in nutrient runoff.
Volcanism
Recent studies have confirmed a correlation between Viluy traps in the
Coronene and mercury enrichment has been found in deposits dating back to the Kellwasser event, with similar enrichments found in deposits coeval with the Frasnes event at the Givetian-Frasnian boundary and in ones coeval with the Hangenberg event. Because coronene enrichment is only known in association with large igneous province emissions and extraterrestrial impacts and the fact that there is no confirmed evidence of the latter occurring in association with the Kellwasser event, this enrichment strongly suggests a causal relationship between volcanism and the Kellwasser extinction event.[86] However, not all sites show evidence of mercury enrichment across the Frasnian-Famennian boundary, leading other studies to reject volcanism as an explanation for the crisis.[63]
Another overlooked contributor to the Kellwasser mass extinction could be the now extinct
Impact event
Bolide impacts can be dramatic triggers of mass extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover.[4][91] The impact that created the Siljan Ring either was just before the Kellwasser event or coincided with it.[92][93] Most impact craters, such as the Kellwasser-aged Alamo, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction.[3] Although some evidence of meteoric impact have been observed in places, including iridium anomalies[94] and microspherules,[95][96][97] these were probably caused by other factors.[54][98][99] Some lines of evidence suggest that the meteorite impact and its associated geochemical signals postdate the extinction event.[100] Modelling studies have ruled out a single impact as entirely inconsistent with available evidence, although a multiple impact scenario may still be viable.[101]
Supernova
Other hypotheses
Other mechanisms put forward to explain the extinctions include
See also
Notes
References
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- Racki, Grzegorz, "Toward understanding Late Devonian global events: few answers, many questions" in Jeff Over, Jared Morrow, P. Wignall (eds.), Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events, Elsevier, 2005.
External links
- Late Devonian mass extinctions at The Devonian Times. An excellent overview.
- Devonian Mass Extinction
- BBC "The Extinction files" "The Late Devonian Extinction"
- "Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events: Towards an Integrated Approach Archived 2019-04-08 at the Wayback Machine": a Geological Society of America conference in 2003 reflects current approaches
- PBS: Deep Time