Relationship of mass extinctions with volcanism was reaffirmed

Putorana plateau in Taimyr formed of basaltic lava and is part of the Siberian trap province, formed about 250 million years ago with the rise of "mantle plume" — the flow of hot rock from the mantle. Photos of the synopsis to the discussion paper in Nature

The largest mass extinction in Earth's history occurred about 250 million years ago, at the turn of the Permian and Triassic periods. About the same time began the formation of the Siberian Traps — the grand thickness of igneous rocks of a few million cubic kilometers. The assumption of the connection between the two events is not confirmed by the available models, which showed that the amount of volcanic gases released from the magma, was insufficient for a global environmental catastrophe. Russian geologists have shown that mantle "plume" (upstream), responsible for the formation of the Siberian Traps, which contain a considerable admixture of recycled oceanic crust of solid rock. From calculations made with this in mind, that the CO2 and HCl in the early formation of the Siberian trap province was several times bigger than previously thought, and it could cause a mass extinction.

All volcanic eruptions, which have witnessed humanity during his short story — a mere trifle compared to what is actually capable of this planet. The most grandiose manifestation of volcanic earth — is education trap provinces (see Trapps) — huge strata of lava and tuff, covering hundreds of square kilometers in a few regions of the world.

One of the biggest trap provinces are Siberian Traps, the total amount of which is estimated at several million cubic kilometers of volcanic rocks (volcanic — erupted to the surface and intrusive — frozen deep in the earth's crust, see: Igneous rocks).

Siberian Trap Province started to form a little over 250 million years ago, which is quite exactly the same as abroad Paleozoic and Mesozoic eras, when there was the largest in the entire history of the Earth's mass extinction (see: Mass Permian extinction). Not surprisingly, many geologists and paleontologists have long assumed the existence of a causal relationship between these events. Thus far, however, this hypothesis has faced several serious challenges.

Trap magmatism must be accompanied by the release into the atmosphere of a large number of volcanic gases such as CO2, SO2
and HCl. This is in principle can drastically affect the climate and the carbon cycle and lead to a global environmental catastrophe — the whole question only on the scale and speed of the gases emitted.

The previous calculations were based on the known quantities of gases from volcanic igneous rocks and a known volume of Siberian Traps.

Given that the formation of traps is not instantaneous, and for quite a long time (more than a million years), we see that the average emissions of CO2 and other gases were comparable to those observed in our days as a result of human activity. This is insufficient to provoke a catastrophe similar to the great Permian-Triassic extinction.

The previous model had other flaws. The cause of trap magmatism considered grandiose updrafts superheated mantle material — so-called mantle plumes (see Mantle plume). With the rise of mantle material is heated and expands, resulting in a decrease in density. Calculations showed that the pressure of the plume on the lithosphere should lead to a rise in the earth's crust. This must happen before the start of the main phase of trap magmatism that is earlier than the molten magma will break through the crust and on its surface. In the case of the Siberian trap province had formed a grand mound height of 2 km, but no geological evidence of such an increase was observed.

These contradictions resolve geologists and Alexander Stephan Sobolev and their colleagues from various institutions in Russia, Germany and France, whose paper is published in the latest issue of the journal Nature.
A detailed chemical analysis of samples of the Siberian basalts, the authors concluded that the magma from which formed the Siberian Traps, there was a significant (10 to 20 percent) the impurity species recycled oceanic crust. As you know, oceanic crust into the mantle at subduction zones, confined to the deep trenches.

The oceanic crust is different from typical mantle rocks on the structure and density. It is heavier and contains more volatile compounds, which may be released when heated. Therefore, the fact of such an impurity requires a review of existing models of the formation of the Siberian trap province.

The model developed by the authors on the basis of new data, shows that there is no lift earth's surface should not be, because the top of a mantle plume to an impurity species of oceanic crust had a higher density than previously thought, and the mechanism of its interaction with the lithosphere was different. Plume is not lifted lithosphere, like a monstrous bubble, and gradually "eaten away" from below by erosion that occurred in the area of contact of the molten material from the top of the plume hard rocks of the upper mantle (which make up the lower part of the lithosphere). As a result, only a few hundred thousand years plume "eaten" their way to the lower layers of the Earth's crust at a depth of about 50 km.

Reconstruction of the initial stages of formation of the Siberian Traps. The vertical axis— Depth (km). Different colors shows the temperature of the rock. Up: Through 0.15 million years after the beginning of the process (the initial position shown in the top of a mantle plume dotted semicircle) Plume approaches the lower boundary of the lithosphere (solid black line) And "spread" under it. Down: After 0.5 million years of the plume peak by erosion (seen plunging into the depths of the wreckage of the lithosphere) have made their way through the mantle to the crust. This corresponds to the beginning of the main phase of trap magmatism. Image of the article under discussion in Nature

The model also showed that the atmospheric emission of CO2 and HCl, released from the molten rock, was, first, several times more extensive than previously thought, and secondly — not gradual, but rapid. Most of the gas was to break through the atmosphere early in the process, even before the molten magma rose to a depth of 50 km, that is, before the main phase of trap magmatism. Estimate of the time of this event — about 252.4 million years ago — very accurately matches the modern dating of the time of mass extinction.

Thus, the new model supports the hypothesis that the cause of the great extinction was trap magmatism, and provides answers to some of the controversial issues related to the mechanism of formation of trap province.

The rate of extinction of genera of animals and plants (blue areas) And the formation of large trap provinces (groups). Size of the circles indicates the size of trap province (million cubic km.). The horizontal axis— Time in millions of years ago. The largest extinction coincides with the formation of the Siberian Traps (Siberia, large red circle). Mass extinction at the turn of the Triassic and Jurassic (200 million years ago), coincides with the formation of the Central Atlantic Magmatic Province (CAMP). Figure out of the article in Nature

Whether the findings are applicable to other mass extinctions and other episodes of trap magmatism? It is known that the periods of some other trap province also coincide in time with the major extinctions. However, this dependence is clearly evident only to the middle of the Mesozoic era (slightly less than 200 million years ago). Subsequent episodes of trap magmatism, usually do not cause mass extinctions.

Exception — the Deccan Traps in India (Deccan traps), the formation of which coincides with the mass extinction at the turn of the Mesozoic and Cenozoic 65.5 million years ago (when the dinosaurs). But at this time fell to the ground and still Chiksulubsky meteorite that may have become the leading cause of extinction.

Why then to the middle of the Mesozoic trap magmatism led to major environmental disasters, and in the next era of biosphere response to such events become less dramatic? According to the authors, the cause may be occurring right in the middle of the Mesozoic changing biogeochemical cycling of calcium carbonate in the oceans (Ridgwell, 2005). This event is sometimes called the "Mesozoic marine revolution." At this time, have evolutionary success and extraordinarily multiplied planktonic unicellular organisms with calcareous skeletons (foraminifera, coccolithophores).

For the construction of the skeleton they take CO2 of water. Then the skeletons sink, carrying neutralized greenhouse gas to the bottom of the ocean. As a result, the ocean has become an effective buffer capable to smooth (within limits, of course) fluctuations in CO2
in the atmosphere and hydrosphere. Thus, it is possible that planktonic foraminifera and coccolithophores several times to save the Earth from global disasters such Permian-Triassic extinction.

Source: Stephan V. Sobolev, Alexander V. Sobolev, Dmitry V. Kuzmin, Nadezhda A. Krivolutskaya, Alexey G. Petrunin, Nicholas T. Arndt, Viktor A. Radko, Yuri R. Vasiliev. Linking mantle plumes, large igneous provinces and environmental catastrophes / / Nature. 2011. V. 477. P. 312-316.

Alexander Markov


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