Gas emissions due to magma–sediment interactions during flood magmatism at the Siberian Traps: Gas dispersion and environmental consequences

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Abstract

We estimate the fluxes of extremely reduced gas emissions produced during the emplacement of the Siberian Traps large igneous province, due to magma intrusion in the coaliferous sediments of the Tunguska Basin. Using the results of a companion paper (Iacono-Marziano et al., accepted for publication), and a recent work about low temperature interaction between magma and organic matter (Svensen et al., 2009), we calculate CO–CH4-dominated gas emission rates of 7×1015−2×1016 g/yr for a single magmatic/volcanic event. These fluxes are 7–20 times higher than those calculated for purely magmatic gas emissions, in the absence of interaction with organic matter-rich sediments. We investigate, by means of atmospheric modelling employing present geography of Siberia, the short and mid-term dispersion of these gas emissions into the atmosphere. The lateral propagation of CO and CH4 leads to an important perturbation of the atmosphere chemistry, consisting in a strong reduction of the radical OH concentration. As a consequence, both CO and CH4 lifetimes in the lower atmosphere are enhanced by a factor of at least 3, at the continental scale, as a consequence of 30 days of magmatic activity. The short-term effect of the injection of carbon monoxide and methane into the atmosphere is therefore to increase the residence times of these two species and, in turn, their capacity of geographic expansion. The estimated CO and CH4 volume mixing ratios (i.e. the number of molecules of CO or CH4 per cm3, divided by the total number of molecules per cm3) in the low atmosphere are 2–5 ppmv at the continental scale and locally higher than 50 ppmv. The dimension of the area affected by these high volume mixing ratios decreases in the presence of a lava flow accompanying magma intrusion at depth. Complementary calculations for a 10-yr duration of the magmatic activity suggest (i) an increase in the mean CH4 volume mixing ratio of the whole atmosphere up to values 3–15 times higher than the current one, and (ii) recovery times of 100 yr to bring back the atmospheric volume mixing ratio of CH4 to the pre-magmatic value. Thermogenic methane emissions from the Siberian Traps have already been proposed to crucially contribute to end Permian–Early Triassic global warming and to the negative carbon isotopic shift observed globally in both marine and terrestrial sediments. Our results corroborate these hypotheses and suggest that concurrent high temperature CO emissions also played a key role by contributing to increase (i) the radiative forcing of methane and therefore in its global warming potential, and (ii) the input of isotopically light carbon into the atmosphere that generated the isotopic excursion. We also speculate a poisoning effect of high carbon monoxide concentrations on end-Permian fauna, at a local scale.

Highlights

► Gas emissions due to magma intrusion in coaliferous sediments at the Siberian Traps. ► These calculated fluxes are 7–20 times higher than those of purely magmatic gases. ► Significant increase of CO and CH4 concentrations and lifetimes in the atmosphere. ► Methane radiative forcing at the end of the Permian results strongly increased. ► Major contribution of CO–CH4 emissions to the end-Permian C isotopic excursion.

Introduction

Large Igneous Provinces (LIPs) are enormous crustal emplacements (millions of km3) of predominantly mafic extrusive and intrusive rocks originating via processes other than “normal” seafloor spreading, e.g. continental flood basalts (or traps), volcanic passive margins, oceanic plateaus, and seamount groups (Coffin and Eldholm, 1994). Vogt (1972) noted that the end-Cretaceous mass extinction was synchronous with the eruption of the Deccan Traps. Since then the idea that LIP eruption times coincide with major mass extinctions has been broadened and several other cases have been documented (Courtillot and Renne, 2003). The extinctions have been ascribed to both short and long term effects of magmatic gas emissions (e.g. Vogt, 1972, Axelrod, 1981, Cockell, 1999, Retallack, 1999). As noted by Wignall (2001) and Ganino and Arndt (2009) among others, the main problem with such a hypothesis is that the volume of erupted magma is not correlated with the magnitude of the extinction, as measured by the number of taxa that became extinct. In addition, the erupted magmas are dominantly tholeiitic basalts, a category of magma believed to be volatile-poor (Saal et al., 2001). Sobolev et al. (2011) advocate an important contribution of CO2–HCl-dominated volatiles deriving from recycled oceanic crust in the magma source. A possible role of silicic magmas has also been invoked for sulphur emissions from LIPs, also in this case the volume of explosive products being not directly related to the importance of the environmental crisis (Scaillet and Macdonald, 2006).

To explain the lack of correlation with the volume of the magmatic products, the importance of extinctions has been linked to both the nature and the amount of thermogenic gases produced during the LIP emplacement, which are controlled by the type of rocks encountered by basaltic magmas during their ascent (Svensen et al., 2004, Svensen et al., 2007, Svensen et al., 2009, Erwin, 2006, Retallack and Jahren, 2008, Ganino and Arndt, 2009). A growing body of evidence indicates that interactions between country rocks and magmatic sills and dikes can be particularly important due to the long exposure to high temperatures: heating of carbonates, sulphates, salts, or organic-compounds (such as bituminous shales or coals) can inject into the atmosphere a quantity of volatiles that greatly exceeds the amount delivered by purely magmatic degassing (Svensen et al., 2004, Svensen et al., 2007, Svensen et al., 2009, Ganino et al., 2008, Iacono-Marziano et al., 2007, Iacono-Marziano et al., 2009). This may help explain why some traps coincide with major extinction events, while others leave almost no trace in the fossil record (Ganino and Arndt, 2009).

The Siberian Traps are the most voluminous subaerial large igneous province (LIP), which covers more than 60% of the Siberian Platform (1.5 million km2) (Reichow et al., 2002); the distribution of lavas suggests that they do not constitute a single continuous province but rather the amalgamation of several sub-provinces (Mitchell et al., 1994). The emplacement of the Siberian Traps has occurred contemporaneously to the end-Permian crisis, the most severe extinction affecting marine and continental biota (Wignall, 2001, Erwin et al., 2002), and to a strong perturbation of the Earth's carbon cycle, globally marked by a negative carbon isotope excursion recorded in marine and terrestrial carbonates and organic matter (Korte et al., 2010 and references therein). The relative timing of these events is not fully resolved, the beginning of the volcanic activity preceding the strongest isotopic excursion and the main extinction event, both, however, occurring before the main emplacement phase of the Siberian Traps (Korte et al., 2010). Siberian basaltic flows are underlain almost everywhere by terrigenous, coal-bearing sedimentary rocks of the Tunguska Basin (Fig. 1), which are absent only in a few, local-scale, tectonic structures (Czamanske et al., 1998). The 300-m-thick Late Carboniferous–Permian Tunguska coal basin is one of the largest in the world and contains numerous thick coal seams (Czamanske et al., 1998). The underlying Neoproterozoic to Carboniferous sedimentary sequence, through which basaltic magmas ascended, comprises carbonates, shales, sandstones and, concentrated in central and southern Tunguska, evaporites. Numerous petroleum levels with regional distribution and variable thicknesses (from a few metres to 80 m) are present in the Silurian, Ordovician, Cambrian, and Neoproterozoic strata (Svensen et al., 2009, Kuznetsov, 1997). The average organic matter content of the sedimentary rocks is 1–3 wt% and can locally reach 8–22 wt% (Kuznetsov, 1997).

Important interactions between magmas and carbonaceous sediments have already been proposed by several authors to have been the major cause of the environmental impact of the Siberian Trap emplacement (Visscher et al., 2004, RetallackPlease provide the location of the publisher for the following Ref.: Retallack and Krull (2006). and Krull, 2006, Payne and Kump, 2007, Retallack and Jahren, 2008, Svensen et al., 2009, RetallackPlease complete and update the reference given here (preferably with a DOI if the publication data are not known): Retallack (in press). For references to articles that are to be included in the same (special) issue, please add the words ‘this issue’ wherever this occurs in the list and, if appropriate, in the text.,). Recent analyses of terrestrial carbon in marine sediments from the Canadian High Arctic have brought further evidence for high temperature magma–coal interactions at the Siberian Traps (Grasby et al., 2011). This terrestrial carbon is in the form of char and is very similar to modern fly ash produced from coal-fired power plants (combustion temperatures between 1300 and 1600 °C), it has been found more than 20,000 km far from Siberia, suggesting a global dispersion of the ashes derived from the combustion of coaliferous sediments (Grasby et al., 2011). In a companion paper (Iacono-Marziano et al., accepted for publication) we show how the occurrence of graphite and native iron in several large intrusions belonging to the Siberian Traps indicates intense high temperature interactions between magma and coaliferous sediments, and implies extremely reduced gas emissions. The fate of these gases upon reaching the atmosphere is a crucial point to elucidate, in order to estimate their potential environmental impact.

In this paper we estimate the fluxes of unusually reduced gases that could have been produced during a single magmatic event of the Siberian Traps. To investigate the short and long term dispersion of these gases into the atmosphere we use atmospheric modelling coupling gas transport and chemistry, a methodology applied for the first time to volcanic gas emissions.

Section snippets

Quantification of gas emissions due to magma–sediment interactions

We use the gas compositions estimated by Iacono-Marziano et al. (accepted for publication) to characterize the degassing that may have occurred at the Siberian Traps during the emplacement of native iron-bearing intrusions. The quantification of these emissions requires the knowledge of the amount of magma involved in organic matter assimilation. Field evidence in the Northern Tunguska Basin (Ryabov and Lapkovsky, 2010) suggests that magma–coal interaction persisted for an entire intrusive

Gas production potential of a single intrusive event at the Siberian Traps

As Fig. 2 schematically illustrates, basaltic magmas of the Siberian Traps ascended through a thick Neoproterozoic–Permian sedimentary pile comprising carbonates, shales, sandstones, evaporites (concentrated in central and southern Tunguska), and coaliferous sediments (Svensen et al., 2009, Czamanske et al., 1998).

The composition of the gas phase is dictated by the presence of native iron that constrains the fO2 of the magma and coexisting gases to be <FMQ-6: these conditions preclude the

Impact on the environment and life

Although volatile emissions during the emplacement of Siberian volcanism/magmatism are still not univocally identified as the cause of the end-Permian crisis (e.g. Knoll et al., 2007, Brand et al., 2012), several hypotheses exist regarding the related kill mechanisms that impacted both marine and terrestrial environments in apparently distinct ways. Poisoning due to trace metals was suggested by Vogt (1972). Rapid and severe climatic changes and major modifications in marine and terrestrial

Conclusions

LIP eruptions coincide with major mass extinctions (Vogt, 1972, Courtillot and Renne, 2003). The possible causal link has been ascribed to both short and long term effects of volcanic gas emissions (Wignall, 2001, Scaillet, 2008). Here we show that the environmental consequences of magmatic activity are, inter alia, strongly dependent on magma–host rock interactions, in particular when the host rocks are composed of carbonates, sulphates, salts, or organic compounds (as already proposed by

Acknowledgements

G.I.M. was funded by the European Community's Seventh Framework Programme (Grant agreement no. PIEF-GA-2008-220926). F.G. is supported by the ERC contract no. 279790. The numerical simulations were performed on the cluster of the Centre de Calcul Scientifique en Région Centre. CATT-BRAMS is free software provided by CPTEC/INPE and distributed under the CC-GNU-GPL licence. We thank Roberto Salzano for his useful advice. We thank B. Marty for the careful handling of the manuscript, H. Svensen and

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