August 2011 LIP of the Month

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The Siberian Large Igneous Province and the End-Permian Extinction: Coincidence and Causality

Linda T. Elkins-Tanton, Mitsui Associate Professor of Geology, MIT, Dept. Earth, Atmospheric, and Planetary Sciences, Cambridge MA 02139   USA; ltelkins@MIT.EDU

The ca 252 Ma Siberian large igneous province contains what may be the largest eruption of continental basaltic magma known on Earth, yet its cause, depth of origin, relationship to global tectonics, climate, and ecosystems remain incompletely understood. The possible relationship between the Siberian large igneous province and the end-Permian extinction, the most significant mass extinction in Earth history, has been discussed for more than a decade. Over this time detailed study of the end-Permian extinction has allowed construction of a high-precision timeline for extinction, recovery, and large-scale perturbations of the carbon cycle from ca 255-247 Ma. No definitive causal mechanism has been described, however, and exact temporal coincidence has not been demonstrated through geochronology and magnetostratigraphy, nor has a detailed timeline of the extinction has not been linked to the tempo of the igneous event.

There are several barriers to resolving the question of coincidence or causality under current science paradigms. First, reaching a satisfactory understanding of relations among large igneous events, climate, and life requires direct collaboration among disciplines to fully inform models and test hypotheses; no single scientist or set of scientists from a single field can bring sufficiently broad expertise to this problem. Second, extensive work done by Russian scientists is not always known to or easily accessible by western investigators. And third, the Siberian Traps are located in remote regions of Eastern and Arctic Siberia: Field work is expensive and complex. We have organized an international effort to try to overcome these difficulties; for more information, please see our website at siberia.mit.edu.

Context

The Siberian large igneous province contains what may be the largest known volume of terrestrial flood basalt, estimated between 2 to 5 ´ 106 km3 of igneous rock (Masaitis 1983, Reichow et al 2002, Renne & Basu 1991); though Dobretsov and Vernikovsky (2001) estimated a volume of 3 to 4 ´ 107 km3 for intrusives and extrusives together. The flood basalts (Fig. 1) are largely covered by taiga and by the West Siberian Basin sediments, but crop out in four notable areas: Noril’sk, Tunguska, Putorana, and Maimecha-Kotui. The Maimecha-Kotui is both the most complete and least studied of the sections.  Most of the traps erupted at ~251-252 Ma, although a detailed chronology is lacking (Basu et al 1995, Kamo et al 2003, Kamo et al 1996, Reichow et al 2002). This age coincides within error with the end-Permian mass extinction, which eliminated up to 96% of marine species – more than any other event in Earth history (Bowring et al 1998, Erwin 1994, Erwin 2006, Mundil et al 2004, Reichow et al 2009, Renne 1995).


Figure 1: Left: Bedrock map of the Siberian traps showing pyroclastic rocks and intrusions and locations of some planned field trips. After Svensen et al. (2010b). Right: Russia with area of left figure in black, and area of likely extents of basalts under sedimentary rocks and of related intrusive rocks outlined in red, after Reichow et al. (2002).

The eruption of this large igneous province was clearly associated with a massive transport of heat and mass in the mantle. Petrology and geochemistry indicate deep, hot mantle melting, above 1600°C (Arndt et al 1995, Elkins-Tanton et al 2007, Ryabchikov et al 2009) and subsequent interactions between magmas and the lithosphere (Basu et al 1995, Il'yukhina & Verbitskaya 1976, Lightfoot et al 1990). Even the general details of mantle flow and the mechanisms that triggered such a massive but short-lived eruption, however, are under dispute. Suggestions include a plume head impinging on the base of a stable lithosphere (Dobretsov et al 2008, Fedorenko et al 1996, Richards et al 1989, Van der Voo & Torsvik 2004, Zolotukhin & Al'Mukhamedov 1991), a less massive upwelling combined with a Rayleigh-Taylor instability (delamination) of the base of the lithosphere (Elkins Tanton & Hager 2000, Hales et al 2005), melting in small-scale convection created by lithospheric extension (Burke & Torsvik 2004, Torsvik et al 2006, White 1996), or a cratonic edge effect (King & Anderson 1995). (Hypothesized non-convective triggers include bolide impact (Basu et al 2003, Jones et al 2002, Kaiho et al 2001), the reversal from right-lateral shear to left-lateral shear between Baltica and Siberia, which caused sites of extension around the Siberian craton (Sengor et al 1993), and subduction-related hydration of the upper mantle (Ivanov & Balyshev 2005, Ivanov 2007).

Sleep (1997, 2006) suggested that the melt produced in a hypothetical plume may reach the surface at a distance from the plume axis by flowing up lithosphere-asthenospheric topography. This concept may be critical if the Siberian province was created by a plume, since lithospheric topography is expected between the West Siberian Basin and the Siberian Platform, and because seismic studies have not yet been completely used to map intrusive rocks and crustal and lithospheric structure. Ernst and Buchan (2001) demonstrated that dike swarms associated with large igneous provinces indicate the center of uplift and eruption, and find that the dikes in Siberia point to an area to the north of the craton. Some other geological data conflicts with the dike swarm indications: the region around Noril’sk appears to have subsided during eruption (Czamanske et al 1998), while the West Siberian Basin appears to have been uplifted (Saunders et al 2005), perhaps indicating that it held the eruptive center.

Interactions between crustal rocks and magma

The extrusive component of the Siberian large igneous province consists largely of tholeiitic basalt with associated high-titanium or high-potassium lavas (Fedorenko et al 1996, Wooden et al 1992) and up to ~25% of volcaniclastic sedimentary rocks (Fedorenko et al 1996, Ross et al 2005), significantly more than in other provinces (Fig. 1). The province also contains many largely alkaline intrusive bodies within and surrounding the lavas. The links between large igneous provinces and carbonatites, particularly, appear to be widespread and to be important clues to both mantle melting temperatures and compositions (Ernst & Bell 2010).  The Siberian large igneous province has scores of alkaline massifs that often include carbonatite magmas, including the giant Guli massif (e.g. Kogarko and Zartman (2011) and references therein; Fig. 2), as well as some of the world’s richest ore bodies, such as the well-studied Ni-Cu-platinoid deposits of the Noril’sk-Talnakh region (Arndt 2005, Dalrymple et al 1995, Lightfoot & Hawkesworth 1997, Wooden et al 1992, Wooden et al 1993).


Figure 2: Panorama of the calcic carbonatite stock in the Guli province. Maimecha River at left, approximately 70.88 N, 101.31 E. (Photo Credit: Elkins-Tanton)

Volatile fluxes associated with emplacement and extrusion of large igneous provinces are a possible trigger for global climate change and extinction (Beerling et al 2007, Campbell et al 1992). Three potential sources for volatile release during flood basalt formation are the eruption of the plateau lavas themselves, associated pyroclastic eruptions, and direct release from sedimentary country rocks heated by sills, dikes, and erupting magmas. While sulfur and halogen emissions have been measured in some younger large basaltic eruptions (Thordarson & Self 1996, Thordarson & Self 2003), the sulfur, halogen, and CO2 contents of flood basalts are less well known.

Some have suggested that basaltic eruptions are unlikely contain sufficient volatiles to drive the magnitude of climate change required for a major extinction (Berner 2002). Research on the Deccan flood basalts and Icelandic lavas indicate that each basaltic eruption of plateau lavas may last for up to a decade or more, pouring from fissures, and may carry as much as ~1 Gt of SO2 and CO2 per year. Sulfur dioxide is likely to be more important, and will cool the Earth through production of sulfate aerosols (Thordarson & Self 1996, Thordarson et al 2003). Carbon dioxide, conversely, would cause atmospheric warming through the greenhouse effect, but the quantity released from basalts may be significantly less than that already present in the atmosphere (Self et al 2005). Though water and nitrogen compounds (NOxs) may also be important, they have not been studied in these settings.

Though recent studies indicate that the volatile contents of the lavas may be more significant than previously thought, eruption of volcaniclastic rocks may hold a greater potential for climate change and toxicity.  The basaltic plateau lavas are underlain by as much as 700 m of rock produced by explosive volcanic eruption (Ross et al 2005), which are known to produce more substantial atmospheric volatile loads than do basalts, and to inject particulates and volatiles into the stratosphere where they remain for years with the potential for climate forcing. These rocks have not been well studied or reported on in the scientific literature. One notable exception is the work of Büchl and Gier (2003), who describe altered tuffs from the Putorana region. Their samples came from tuff layers of a few centimeters to as much as 40 m thickness interbedded with basalts in two sample areas southeast of Noril’sk. No carbon, sulfur, or fluorine analyses were made on these rocks.

In addition to the interlayered tuffs of the northern craton, the southern Siberian craton along the Angara River contains voluminous tuffs that may be connected to the explosive pipes of that region (Svensen et al 2010b). River outcrops there are in places in excess of 250 m thickness, with eroded tops and bottoms hidden beneath the river level (Fig. 3). These outcrops stretch about 200 km along the river and have been largely flooded in the last year by a new hydroelectric dam, making future sampling more difficult.


Figure 3: Cliff of tuff on the Angara River, approximately 58.737N, 102.721 E. (Photo Credit: Elkins-Tanton)

The Siberian large igneous province intruded into and erupted onto the thick sedimentary rocks of the Tunguska basin: 200 to 300 meters of middle Carboniferous to Permian siltstones, sandstones and coal which overlie approximately 4 km of Proterozoic to lower Carboniferous evaporates, marls, dolostones and limestones. These rocks contain abundant sulfur, fluorine, chlorine, and an estimated 21,000 Gt of carbon (Beerling et al 2007). Basaltic magmas interact readily with sedimentary rocks, with several important consequences (Svensen et al 2007, Svensen et al 2010b). When magmas encounter layered rocks, sills form more readily than dikes, and horizontal flow dominates (Kent et al 1992, McFarlane 1929); contact time with sedimentary rocks is maximized. Magmatic heat and perhaps volatiles are transferred to the sediments and create a contact metamorphic aureole (Finkelman et al 1998). The largest sills in the Noril’sk region are 100-400 m thick and tens of km in length, and have produced contact aureoles several times thicker than the sills themselves. Many sedimentary dikes intrude the sills of the Siberian craton, indicating a further degree of interaction between magmatic bodies and sedimentary fluidized products (Svensen et al 2010a). Sedimentary rocks may be assimilated, changing magma composition, and metamorphosed sedimentary rocks may release volatiles into the atmosphere (Svensen & Jamtveit 2010).

Significant chemical interactions between magmas and coals have been recognized worldwide (Goodarzi & Cameron 1990). Colorado coals heated to ~700°C by magmatic dikes were found to have high concentrations of Ca, Mg, Fe, Mn, and Sr, thought to be the result of CO and CO2 reaction during “coking” of the coal by hot fluids from the intrusion (Finkelman et al 1998, Thorpe et al 1998). Carbonates and sulfides were enriched in the region of reacted coal. Carbonates in coal vesicles, therefore, may be used in some cases to define the region of coal affected by magmatic intrusion. Finkelman et al. (1990) demonstrated that Br, Se, and Hg are volatilized from coal at temperatures as low as 150°C, and are completely devolatilized by 550°C. Chou et al. (1992) found that HCl was released starting at 250°C, and completely depleted in the coal by 600°C.

Coal and magma interactions, therefore, may be a rich and relevant area of study for volatile release during formation of the Siberian large igneous province (Beerling et al 2007, Il'yukhina & Verbitskaya 1976, Retallack & Jahren 2008). Recently Grasby et al. (2011) found evidence for coal fly ash in sediments from the Canadian high Arctic of end-Permian age, and they suggest these resulted from coal and magma interaction in the Siberian flood basalts. These are the first remote evidence for coal burning by magmas. In a number of location coal and magma interaction resulted in smelting iron in the magma to metal, and in other areas coals in the basin show evidence of volatile loss in aureoles around magmatic intrusions (preliminary data from our project). Widespread local evidence for coal and magma interaction in the Tungusska Basin does not appear to exist. The coal lies near the surface in the basin, and the largest opportunities for crustal rock interaction with magmas would be at depth, in the chambering areas.

Assimilation of anhydrite in evaporitic sediment is thought to have triggered the segregation of the Ni-Cu-(PGE) sulfides of the Noril’sk-Talnakh ore bodies. Volatiles expelled from the contact aureoles pass to the surface along hydrothermal pipes that host iron-oxide deposits (Von der Flaass 1997). Craters up to 1.6 km wide and 700 m deep suggest highly energetic eruptions. One possible cause of crater formation is volatile overpressure generated during metamorphism and melting of evaporates (Svensen et al 2010b).

Ganino et al. (2007) have shown that the amount of CO2 released by decarbonation of sedimentary rocks intruded by sills can be 25 times greater than the volume of magmatic CO2. A similar argument can be applied to SO2 release: in mafic magma the solubility of sulfur is limited to about 1000 ppm if dissolved as sulfide, or up to 1 wt% if dissolved as sulfate (Jugo et al 2005). Similarly, contact metamorphism of organic-rich shale may release an order of magnitude more carbon gas that from equivalent volumes of degassing lava (Svensen et al 2004). Evaporites may contain about 25 wt% sulfur and a sequence of dolomites and evaporites would contain 5 to 10 wt% sulfur. If the metamorphic aureole has the same thickness as the sill, the mass of sediment-derived SO2 is at least five times larger than the mass of magmatic  SO2. Assimilation of crustal sulfur is apparently required to create the rich ore bodies associated with the province (Keays & Lightfoot 2010), and attendant sulfur release would also be expected. The toxic results of baking Siberian country rocks rich in carbon, chlorine, fluorine, and sulfur may also be a release of organohalogens, depleting ozone and producing the palynomorphs in Permian rocks thought to be created through mutation (Visscher et al 2004).

Evidence for global chemical change

A central question remains whether the possible climatic and environmental effects of the large igneous province can account for the chemical and biological record of the extinction in sedimentary rocks.  More than 50% of marine animal genera became extinct in less (or much less) than 500,000 years, coincident or nearly coincident with the extinction of terrestrial vertebrates and plants.  In the oceans taxa characterized by low metabolic rates and heavy calcification were affected preferentially (Knoll et al 1996). 

On land, taxa with larger lung capacity may have survived preferentially (Huey & Ward 2005).  Redeposited soils and an influx of soil-derived organic material to marine environments have been interpreted as evidence for widespread deforestation and ecosystem collapse; this is observed in at least nine widely-spaced locations at ~252 Ma (Sephton et al 2005). Models must account for sedimentary (Isozaki 1997) and geochemical (Grice et al 2005, Summons et al 2006) evidence of widespread anoxia in deep water and within the photic zone near the Permian-Triassic boundary, as well as significant perturbations of the global carbon (Baud et al 1989, Holser et al 1986, Horacek et al 2007, Jin et al 2000, Payne et al 2004, Riccardi et al 2007) and sulfur cycles (Kajiwara et al 1994, Marouka et al 2003, Riccardi et al 2006), with carbon cycle disturbance persisting for nearly 5 My (Payne et al 2004).

Global anoxia in late Permian and Early Triassic deep oceans appears to be a critical link between geochemical and biological records (Cao et al 2009, Isozaki 1997, Kozur 1998, Wang et al 1994). In particular, this extended period of anoxic conditions may have set the stage for a climate crisis caused by the Siberian Traps, and the rapid build-up and flooding of the shallow oceans and atmosphere with H2S that has been hypothesized based on pyrite minerals in sedimentary rocks (Nielsen & Shen 2004, Wignall et al 2005). Modeling suggests marine anoxia may be the product of changing circulation and productivity (Kiehl & Shields 2005, Winguth & Maier-Reimer 2005), but falling levels of atmospheric oxygen during the Late Permian and Early Triassic (Berner 2005) may also have played a role in marine anoxia and terrestrial extinctions (Huey & Ward 2005). Low oxygen availability has also been widely hypothesized to explain both marine and terrestrial animal extinctions (Huey & Ward 2005, Wignall & Twitchett 1996) and regional marine anoxia is required for H2S release from the oceans (Kump et al 2005). However, because the development of marine anoxia is only partially controlled by atmospheric pO2 and because the respiratory physiologies of air- and water-breathing animals differ considerably, it is not clear that parallel extinction patterns are expected in the marine and terrestrial realms.  Moreover, on timescales longer than the mixing time of the oceans, effects of Siberian Traps eruptions need not be parallel for marine and terrestrial animals. (For the deep oceans, mixing times may be as long as a few thousand years, but for the surface ocean, where most invertebrates live, mixing times are substantially shorter.) Ultimately, the test of a time lag between terrestrial and marine extinction must come from geochronology.

Anoxic conditions, flood basalts, and production of H2S have all occurred throughout the geological record but only one massive and rapid extinction has occurred.  The new evidence of volatile emission from not just baked sedimentary rocks (Ganino & Arndt 2007, Grasby et al 2011, Retallack & Jahren 2008, Svensen et al 2010b) but also from the magmas themselves (Black et al 2011, Ryabchikov et al 2009) strengthen the arguments that the large igneous province could be a trigger to the extinction. Additionally, carbon cycling after the main pulse of the extinction and eruption is tied to ongoing volcanism (Meyer et al 2011, Payne & Kump 2007, Payne et al 2011).

Improved magnetostratigraphy will provide constraints on the age and duration of magmatism. New investigations of the Noril’sk and Maimecha regions indicate that the lavas were emplaced in about 25 pulses of length not more than about 8,000 years (Pavlov et al 2011); this level of detail is necessary for clear results from global climate models concerning volatile emissions and their effects on the environment. The Noril’sk volcanic stratigraphy is over 3,000 m thick, and the Maimecha-Kotui is  ~4,000 m thick. Correlating the Maimecha-Kotui and Noril’sk stratigraphies is essential to estimating the total volcanic volume and duration and can best be achieved by integrating geochronological and paleomagnetic records.

Summary

A critical link between the Earth and life on Earth may be the transfer of heat from the interior to sedimentary basins in the form of magma intruding crustal rocks and liberating gases into the atmosphere. Global climate change may be driven this way, by the periodic work of large igneous provinces. The Siberian large igneous province displays not just plateau lavas, but abundant alkaline provinces and thick tuffs, along with explosion pipes, magmatic ores, and baked sediments, all of which may be related to the interaction between mantle melts and crustal rocks.

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