A mass extinction is an event in the fossil record, a fossilised disaster if you like, in which a massive, globally widespread and geologically rapid loss of species occurred from numerous environments. The “Big five” extinctions of the Phanerozoic (that time since the beginning of the Cambrian period, 541 million years ago) are those in which, in each instance, over half of known species disappeared from the fossil record.
How did they happen? The causes of such events, with a truly global reach, have been a well-known bone of contention within the Earth Sciences community over many decades. The popular media likes to portray such things as Hollywood-style disasters, in which everything gets wiped out in an instant. But in the realms of science, things have changed. The critically important development has been the refinement of radiometric dating, allowing us to age-constrain events down to much narrower windows of time. We can now, in some cases, talk about the start and end of an event in terms of tens of thousands (rather than millions) of years.
Such dating, coupled with the other time-tools of palaeomagnetism and the fossil record, have made it possible to develop a much clearer picture of how mass-extinctions occur. That picture is one of periods of global-scale pollution and environmental stress associated with large perturbations to the carbon cycle, lasting for thousands of years. Such upheavals are related to unusual episodes of volcanic activity with an intensity that is almost impossible to imagine. The geological calling-cards of such events are known as Large Igneous Provinces (LIPs). Bringing environmental and climatic changes at rates similar to the ones we have been creating, they have been repeat-offenders down the geological timeline. This introductory piece examines LIPs in the framework of more familiar volcanic activity: it is the only way to get a handle on their vastness.
For those readers already familiar with LIPs, you may want to skip this and go straight to Part Two, which covers the biggest extinction of them all, at the end of the Permian period, 252 million years ago (Ma). With more than 90% of all species wiped out, it was the most severe biotic crisis in Phanerozoic history. The extinction was global: almost all animals and plants in almost all environmental settings were affected. An idea of the severity can be visualised by considering that the time afterwards was marked by the beginning of a coal gap lasting for ten million years: coal-forming ecosystems simply did not exist for that time. Likewise, Howard Lee has recently considered the relationship between the end-Cretaceous extinction - the one that got the dinosaurs - and LIP volcanism here. But for those who are new to LIPs, it is recommended that you read this post first.
Let's start by contextualising that volcanicity, starting with an especially well-known example. Mount St. Helens is one of a number of volcanoes in the Cascade Range of the north-western United States. In early 1980, it began a period of activity with earthquakes and clouds of steam billowing forth: by the middle of spring its northern side was starting to bulge ominously, a sure sign of magma and pressure build-up. On May 18th, following another earthquake, its entire northern side collapsed, depressuring the magma and volatiles beneath in an instant. The resulting blast destroyed everything in a 600 square kilometres zone around the northern flank of the volcano. A huge cloud of hot ash shot skywards, reaching over twenty kilometres in height. Ash and debris, mixed with great volumes of meltwater, brought major flash-flooding and mudflows into local rivers. The energy released has been estimated to be equivalent to a 24-megaton nuke and in total this nine-hour eruption spewed out some 2.79 cubic kilometres of felsic lava, ash, gases and debris. Remember that last figure.
The famous eruption of Krakatoa on August 26th-27th 1883 reached its climax on the 27th: the largest explosion, at 10:02 A.M, was heard 3,110 km (1,930 mi) away in Perth, Western Australia. The eruption and the tsunamis associated with it killed over 36,000 people according to official figures. This incredibly violent and destructive eruption, with an energy-release likened to a 200-megaton nuke, produced an estimated 21 cubic kilometres of eruptive products. Again, remember that figure.
Now, contrast those deadly eruptions with the mostly late Permian Siberian Traps LIP. The province contains what may be the largest known volume of terrestrial flood basalt (dark-coloured, iron and magnesium-rich lava) in the world. How much? At least three million cubic kilometres. That's enough to bury an area the size of the United Kingdom beneath a layer of basalt some 12 kilometres thick.
Fig. 1: We're gonna need a bigger graph! Volumes of well-known volcanic eruptions compared to LIPs. Geologists may argue that comparing single eruptions of various standard volcanoes and LIPs is like comparing apples to oranges. Actually, that's the point!
A large igneous province is defined as a vast accumulation, covering an area of at least a hundred thousand square kilometres, of igneous rocks episodically erupted or intruded within a few million years. The majority of erupted products may in some cases accumulate within much shorter time-spans of tens of thousands of years or less. Total eruption volumes are at least a hundred thousand cubic kilometres. Erupted products are dominated by repeated flows of basaltic lava ("flood-basalts"): weathering and erosion of these stacked basalt sheets often gives the countryside where they occur a hilly, stepped topography. Such areas are often referred to today as "Traps" because of this distinctive landscape: the term, as used in "Siberian Traps" or "Deccan Traps" is based on a Swedish word for stairs. Rocks intruded beneath the surface in LIPs include ultramafic (dense, iron and magnesium rich) and alkaline (sodium and potassium-rich) bodies, plus uncommon types such as the carbonate-rich carbonatites. LIP events are infrequent along the geological timeline, with an average of one such event every twenty million years.
Fig. 2: Plate tectonics 101: oceanic crust is erupted at mid-ocean ridges and tens of millions of years later it is consumed at subduction zones. Graphic: jg.
Plate tectonics has over the years been particularly involved with what goes on at existing plate boundaries such as subduction zones and mid-ocean ridges (fig. 2), where magmatism is highly focussed. However, LIPs reflect another set of processes altogether, where vast amounts of mantle-derived magma make it to the surface within plates. They have played a significant role in the development of the hypothesis of great plumes of hot rock and magma occurring deep in Earth's mantle, which create localised "hotspots" that occur irrespective of tectonic plate boundaries and are the sites of major, within-plate eruptions over millions of years. That there is still much lively (and at times acrimonious) debate concerning the Plumes Hypothesis, including postulated alternative formation-mechanisms for LIPs, need not concern us here. That LIP events occurred and how they affected the biosphere is our focus.
The Oxford English Dictionary definition of pollution is as follows: the presence in or introduction into the environment of a substance which has harmful or poisonous effects.
Harmful or poisonous effects depend on the physical and chemical properties of any one substance. Substances are widely variable in their toxicity in terms of concentration. Carbon dioxide, essential to photosynthetic plantlife, has other properties which, at higher concentrations, make it dangerous. As a strong greenhouse gas, any substantial increase in its atmospheric levels over a matter of a few centuries make it a pollutant because of the impacts of rapid climate change. At much higher levels it becomes an asphyxiant - a gas that kills by displacing air, thereby causing suffocation, as tragically evidenced in 1986 at Lake Nyos, in Cameroon. Here, the magma underlying the floor of an old volcanic crater-lake gives off carbon dioxide, with which the lake water becomes super-charged. At depth, the pressure of the water-column above keeps the gas stably dissolved in the water. However, any triggering mechanism that suddenly forces a lot of that deep water upwards to shallow levels where that confining pressure is absent can cause it to explosively degas. In the 1986 event, a large cloud of carbon dioxide burst forth from the lake. Due to its relative density, it rolled along the ground, displacing the air as it did so. Over 1,700 people and 3,500 livestock died from asphyxiation in nearby communities. Like many substances, carbon dioxide is best taken in moderation.
All subaerial volcanic eruptions blast out gases and ash into the troposphere and in some cases the stratosphere. The most important volcanogenic gases are water vapour, carbon dioxide, sulphur dioxide and halogen compounds such as hydrogen chloride and hydrogen fluoride.
Of these, only carbon dioxide can contribute to global warming over a geological timescale because of its centuries-long atmospheric residence time (the time it takes natural processes to remove most of it again). At present, global volcanogenic carbon dioxide emissions are calculated to be up to 440 million tonnes a year. This can usefully be compared to human carbon dioxide emissions of (in 2014) 32.3 billion tonnes a year – ours are presently two orders of magnitude greater than those from volcanoes. LIP eruptions are another matter: the entire Siberian Traps LIP eruptive cycle is estimated to have produced thirty thousand billion tonnes of carbon dioxide. Bearing in mind the residence time of carbon dioxide, if eruptive events are continuous or closely-spaced enough to keep recharging the atmosphere with it, a long-lived warming effect would occur.
Sulphur dioxide's greenhouse gas abilities are somewhat stunted as it tends to form sun-blocking sulphate aerosols (suspensions of fine solid particles or liquid droplets in a gas) that have a net cooling effect. Unless an eruption is powerful enough to inject a lot of the gas up into the stratosphere (where sulphur compounds may also cause damage to the ozone layer), the cooling effect is short-term – just a year or two, by which time the sulphate has mostly returned to the surface, dissolved in rainwater and thereby giving a short-term acid rain effect where that rain falls. Stratospheric sulphate aerosols have effects lasting for a few more years, but unless they are continuously supplied then the system recovers to its pre-eruption state. Additionally, because of the way that Earth's airmasses interact with one another as a result of the planet's rotation, gases have to be injected into the stratosphere from a relatively low latitude if they are to be spread on a truly global basis. So a pattern emerges of a steady global warming due to increasing carbon dioxide with shorter, often more regional punctuations along the way in the form of sulphate-induced cooling.
Water vapour quickly cycles back to the surface as rain, bringing with it (in addition to the sulphate) the volcanic ash out of the troposphere. The halogen compounds likewise acidify that rainfall and at higher local concentrations make it directly toxic. Halogen compounds injected into the stratosphere also cause ozone layer damage.
Historically, there are several good examples of problems caused by major eruptions causing short-term atmospheric pollution. A good example is the eight-month long fissure-eruption of Laki, which began in June 1783 in Iceland (a mere 15 cubic kilometres event). Apart from vast amounts of lava, Laki released an estimated 122 million tonnes of sulphur dioxide, fifteen million tonnes of hydrogen fluoride and seven million tonnes of hydrogen chloride. The effect was to leave parts of the Northern Hemisphere shrouded in an unpleasant fog for several months. The acidic, halogen-rich haze and resulting toxic rains were highly damaging to terrestrial life in Iceland, Europe and North America. Livestock mortality in Iceland was over 50% and a quarter of the island's population perished in the resultant famine.
Fig. 3: Extinction magnitude through the past 400 million years plotted against the age and estimated original volume of large igneous provinces. Continental flood basalt LIPs are shown as black bars, while oceanic plateau basalt provinces are shown as gray bars. Abbreviations: D = Devonian; C = Carboniferous; P = Permian; Tr = Triassic; J = Jurassic; K = Cretaceous; T = Tertiary; CAMP = Central Atlantic magmatic province. Figure is adapted from Bond & Wignall, 2014.
The geological timeline of the Phanerozoic (part of which is shown in fig. 3) is marked by a number of LIP events. A few seem to have had little impact on planetary life, especially the oceanic plateau basalt provinces (perhaps underwater eruptions have different outcomes?); some are linked to moderate extinctions and some are linked to major mass-extinctions. Why this variability and what makes a LIP event a killer?
Fig. 4: potential kill-mechanisms associated with Large Igneous Provinces. Graphic: jg.
Several factors are clearly critical in determining the outcome of a LIP event. The state of the biosphere and climate prior to an eruption - how stressed the systems are - must be important. Any occurrence of other global-scale events coincident with a LIP eruption - such as a large asteroid impact - would only make things worse. But the most important factor must surely relate to the "three D's" - the distribution, duration and degree of pollution.
Distribution, duration and degree of a pollution event depends on frequency and intensity of eruptive events (the pollutant supply) and the residence times of the pollutants involved. Continuous or very frequent intense events over tens of thousands of years would not only provide sufficient pollutants but give them adequate time to be spread globally at dangerous levels. On the other hand, continuous low-intensity eruption over a similar time may not raise levels of pollutants to harmful values, or perhaps only do so on a regional basis. Low frequency LIP eruptions occurring over longer timespans may still yield vast volumes of lava, but the low frequency allows ecosystem recovery in between eruptions. Therefore, it is possible for some LIPs to have had little more than local effects whereas in others the global ecosystem has been almost completely overwhelmed.
Now we have had an overview of LIPs and their effects, we can look at a specific example in Part Two with the end-Permian mass-extinction, how it occurred and its links to the Siberian Traps LIP - and its significance compared to the pollution caused by modern-day human activities.
The following paper, available online, is an excellent overview of LIPs and their role in specific extinction events - it also has an exhaustive list of references for further reading.
Bond, D.P.G. and Wignall, P.B. (2014): Large igneous provinces and mass extinctions: An update. In: Keller, G., and Kerr, A.C., eds., Volcanism, Impacts, and Mass Extinctions: Causes and Effects: Geological Society of America Special Paper 505.
Posted by John Mason on Thursday, 19 March, 2015
 |
The Skeptical Science website by Skeptical Science is licensed under a Creative Commons Attribution 3.0 Unported License. |