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METAMORPHIC FLUIDS AND GLOBAL ENVIRONMENTAL CHANGES
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Henrik Svensen and Bjørn Jamtveit
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Physics of Geological Processes (PGP), University of Oslo, Norway.
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Email: hensven@fys.uio.no.
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Carbon dioxide is produced by metamorphic reactions in orogenic belts and high heat-flow
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systems. Part of this carbon is ultimately released to the atmosphere, but the long
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timescales of regional metamorphism implies that the short term effects on the
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environment are minor. Here we show that contact metamorphism around igneous sill
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intrusions in organic-rich sedimentary basins has the potential to generate huge volumes of
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CH4 and CO2, and that these gases are rapidly released to the atmosphere via vertical pipe
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structures. The high fluxes and volumes of greenhouse gases produced in this way,
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indicates that metamorphic processes may have first order influence on global warming
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and mass extinctions.
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Keywords: Metamorphic carbon degassing, contact metamorphism, Large igneous provinces,
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volcanic basins, rapid environmental changes
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INTRODUCTION
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Carbon-bearing fluids are constantly seeping from lithospheric sources to Earth’s surface and
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atmosphere (e.g., KERRICK, 2001; MORNER and ETIOPE, 2002). In some places, such as young
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mountain ranges and high heat-flow zones, the seepage is more intense than in others. In these
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settings, fluid expulsion at the Earth’s surface is a reflection of metamorphism taking place many
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kilometers below. Hot springs and mud pots are familiar surface manifestations of metamorphic
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fluid production. The processes behind metamorphic production of C-rich volatiles include
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reactions that transform carbonates, feldspar and quartz to amphibole, epidote, pyroxene, garnet,
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and CO2 (e.g., KERRICK et al., 1991). Key question include how much and how fast CO2 can be
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generated and released by metamorphic processes, and if the degassing can trigger rapid global
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warming and thus have an impact on life on Earth?
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Pioneer work by Derrill M. Kerrick and co-workers in the 1990’s attempted to link
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Cenozoic greenhouse events (see ZACHOS et al., 2001) to large scale CO2 degassing generated by
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prograde metamorphism of carbonate-bearing rocks (KERRICK et al., 1995; NESBITT et al., 1995).
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It was proposed that contact metamorphism around plutons in western North American in the
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Eocene released large amounts of metamorphic CO2 (0.167 Gt per km2 per m.y.) and that this
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degassing could have contributed to the Eocene greenhouse conditions if more than 106 km2
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underwent metamorphism (NESBITT et al., 1995; KERRICK and CALDEIRA, 1998). This much CO2
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degassing would exceed CO2 consumption by silicate weathering (KERRICK and CALDEIRA,
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1993). An Early Cenozoic climatic effect of CO2 degassing by Himalayan regional
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metamorphism was however discarded (KERRICK and CALDEIRA, 1999), but new data suggests
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that at least the present day regional metamorphism is a net supplier of CO2 to the atmosphere
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(BECKER et al., 2008; EVANS et al., 2008).
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The relatively short duration of Cenozoic greenhouse-gas driven events, which lasted less
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that 200,000 years relative to the long duration of orogenesis is a major challenge for the
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hypothesis that metamorphic CO2 affected climate. Several rapid global warming events and
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mass extinctions are known from the geological record, including the Paleocene-Eocene thermal
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maximum (PETM; 55 Ma), the Toarcian event (183 Ma), and the end-Permian (252 Ma) and the
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Triassic-Jurassic (200 Ma) extinctions (e.g., WIGNALL, 2001; COURTILLOT and RENNE, 2003).
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The triggering mechanisms of all these environmental events are poorly understood. Gas hydrate
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dissociation, sluggish ocean circulation and anoxia, lava CO2 degassing, and metamorphic CH4
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degassing are among the hypotheses that have been proposed (e.g., WIGNALL, 2001; COHEN et
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al., 2007; KNOLL et al., 2007). However, the correlation in time between these events, the
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formation of large igneous provinces (LIPs) and the contact metamorphism in sedimentary
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basins intruded by the LIP magmas (Table 1) suggests a possible causal relationship.
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VOLCANIC BASINS AND LIPs
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Contact metamorphism in sedimentary basins occurs when hot and partly molten igneous
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intrusions heat their country rocks. These basins are called volcanic basins, and the igneous
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intrusions are often basaltic and emplaced during LIP formation. Volcanic basins are common
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along volcanic rifted continental margins and on lithospheric cratons (Figure 1). The intrusions
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commonly form sheet-like sills and dikes, and sill thicknesses can exceed 350 meters,
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sandwiched between sedimentary sequences. As sedimentary basins are among the main crustal
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reservoirs of organic and inorganic carbon (e.g., IPCC, 2007), there is a huge potential for CH4
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and CO2 generation in volcanic basins. A link between contact metamorphism and the PETM
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was suggested in 2004 based on detailed seismic imaging and borehole studies in the Vøring and
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Møre basins, offshore Norway (SVENSEN et al., 2004), and this provided a new framework for
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investigating the environmental consequences of metamorphism. The cornerstone of this
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hypothesis is rapid gas generation from organic material and subsequent release of greenhouse
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gases to the atmosphere. Recent results have provided strong evidence that a similar explanation
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applies to other climate events including the Toarcian (Lower Jurassic), the Triassic-Jurassic, the
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end-Permian, and the end-Guadalupian (e.g., MCELWAIN et al., 2005; BEERLING et al., 2007;
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SVENSEN et al., 2007; RETALLACK and JAHREN, 2008; GANINO and ARNDT, 2009; SVENSEN et
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al., 2009). This hypothesis is supported by geological constraints (e.g., observed contact
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aureoles, sill intrusions, and vertical pipe structures) as well as the fact that metamorphism of
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organic carbon leads to generation of
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geochemical data than the emission of 12C -depleted mantle CO2 due to LIP lava degassing.
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C-enriched CH4 which can better explain the available
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CONTACT METAMORPHISM AND CARBON DEGASSING
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Vast volumes of sedimentary rocks are heated in volcanic basins following sill emplacement
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(Figure 1 and Table 2). Sill intrusions may extend laterally over several hundred kilometres,
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commonly have thicknesses in the 50-200 meter range, and dominate the geology of sedimentary
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basins (Figure 2A). Thus single sill intrusions may have volumes of 5,000-20,000 km3.
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Considering that the thicknesses of metamorphic aureoles are commonly about the same as the
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sill thickness (both below and above the sill), the volume of heated sediments could be as large
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as 10-40,000 km3. Because the inner contact aureoles reach peak metamorphic conditions
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(typically 400-500 °C) shortly after sill emplacement (10-500 years), the metamorphic reactions
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and associated fluid production are also very fast. If only 1 wt. % of the organic carbon in shale
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or siltstone is transformed into gaseous carbon compounds, the gas production potential
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associated with a 5,000-20,000 km3 sill intrusion is 230-920 Gt C (corresponding to 310-1200 Gt
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CH4). This means that a single melt batch injected into an organic-rich sedimentary basin can
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generate sufficient methane, rapidly enough, to cause global warming.
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In volcanic basins there is abundant evidence for rapid injection of the aureole generated
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gases into the atmosphere. This process is initiated by local overpressure within the sediments.
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When sedimentary host rocks are heated by magma intrusion, decomposition of organic matter,
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mineral dehydration (generating H2O and CO2), and pore fluid expansion or boiling occur on a
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timescale of years. The resulting overpressure may lead to hydrofracturing and the formation of
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breccia pipes and hydrothermal vent complexes (e.g., JAMTVEIT et al., 2004). In the Karoo Basin,
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South Africa, the hydrothermal vent complexes commonly crops out in the uppermost 400-500
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meters of the basin (Figure 2). In addition, numerous breccia pipes are rooted in black shale
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contact aureoles at deeper basin levels, demonstrating that high pore fluid pressures may also
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developed during rapid decomposition of organic matter to carbon gases (SVENSEN et al., 2007).
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Baked grey shales that have lost the organic carbon remain in the pipes (Figure 3). Vent
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structures and breccia pipes are characteristic features of many volcanic basins including the
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Vøring and Møre basins, the Faroe-Shetland Basin, and the Tunguska Basin (Figure 1; Table 1).
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In the Tunguska Basin in Siberia, spectacular pipes with up to 1.6 km wide subaerial explosion
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craters formed during the end-Permian. A schematic cross section of a volcanic basin with pipe
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structures is shown in Figure 4. In marine settings, like in the Vøring Basin, the craters can reach
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a size of more than 10 km in diameter (PLANKE et al., 2005).
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SEDIMENT COMPOSITION AND ENVIRONMENTAL EFFECTS
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The chemical composition of the sedimentary rocks heated by igneous intrusions has a profound
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influence on the metamorphic fluid composition (e.g., SVENSEN et al., 2004; GANINO and
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ARNDT, 2009; SVENSEN et al., 2009). An overview of the sediment-types heated in various
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volcanic basins is given in Table 2. For instance, organic-rich shale generates CH4 during contact
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metamorphism, whereas coal generates fluids with CO2. In addition, water is generated. Since
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many sedimentary basins contained hydrogen rich kerogen and oil and gas accumulations at the
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time of sill emplacement, petroleum-derived gases like CH4 and C2H6 may have dominated the
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metamorphic fluid. If limestones or dolostones are heated, the generated fluid will be dominated
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by
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evaporites with anhydrite and rock salt may generate SO2 and HCl, and if organic matter or
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petroleum is present, CH4 and halocarbons such as CH3Cl and CH3Br may also form. This was
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recently confirmed by experiments in which natural rock salt from the Tunguska Basin in East
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Siberia was heated to 275 °C in order to simulate contact metamorphism. Halocarbons were
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among the detected gases, and they were generated by reactions between petroleum-bearing fluid
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in inclusions and the host salt (SVENSEN et al., 2009).
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C-enriched CO2, or
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C-depleted CH4 if organic matter or graphite is present. Moreover,
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In East Siberia, a major sill emplacement event took place at the end of the Permian, and
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thick sills are present throughout the basin. Mass balance calculations suggest that 10,000-30,000
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Gt C could have been generated during contact metamorphism of organic matter, accompanied
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by 4,500-13,000 Gt CH3Cl (SVENSEN et al., 2009). The presence of hundreds of pipe structures
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rooted in evaporite lithologies suggests that the gases were released to the atmosphere. Thus the
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end-Permian global warming event could have been triggered by aureole degassing. Moreover,
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recent atmospheric modelling shows that >1000 Gt of methyl chloride are required to cause
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significant stratospheric ozone depletion over a 100,000 year time scale (BEERLING et al., 2007).
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This may explain some of the terrestrial consequences of the end-Permian mass extinctions, as
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mutated pollen are ascribed to ozone layer breakdown and damaging UV-B radiation (VISSCHER
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et al., 2004).
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CONTACT VS REGIONAL METAMORPHISM
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The 5-10 million year timescale of orogeny or crustal extension cannot explain rapid and
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transient climatic events, and thus regional metamorphism is unlikely to affect short term global
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climate changes. Moreover, other lines of evidence suggest that release of CO2 due to carbonate
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metamorphism played a minor role in Cenozoic global climate changes. The main reason is that
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metamorphism of carbonates generates
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shows that the carbon responsible for Eocene global warming was 12C-enriched in terms of 13C
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(e.g., DICKENS et al., 1997; ZACHOS et al., 2001). Contact metamorphism in volcanic basins is a
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far more likely explanation to the observed transfer of CH4 and CO2 from the lithosphere to the
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atmosphere on short timescales.
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C-depleted CO2 whereas available geological data
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Acknowledgments
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This study was supported by a Centre of Excellence grant to PGP, by a Young Outstanding
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Researcher grant and a PetroMaks grant to H. Svensen, all from the Norwegian Research
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Council. We thank Sverre Planke for introducing us to volcanic basins, and for providing figures
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1 and 2A.
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References
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158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
Becker, J. A., Bickle, M. J., Galy, A., and Holland, T. J. B., 2008. Himalayan metamorphic CO2
fluxes: Quantitative constraints from hydrothermal springs. Earth and Planetary Science
Letters 265, 616-629.
Beerling, D. J., Harfoot, M., Lomax, B., and Pyle, J. A., 2007. The stability of the stratospheric
ozone layer during the end-Permain eruption of the Siberian Traps. Philosophical
Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences
365, 1843-1866.
Cohen, A. S., Coe, A. L., and Kemp, D. B., 2007. The late Palaeocene-Early Eocene and
Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales,
associated environmental changes, causes and consequences. Journal of the Geological
Society 164, 1093-1108.
Courtillot, V. E. and Renne, P. R., 2003. On the ages of flood basalt events. Comptes Rendus
Geoscience 335, 113-140.
Dickens, G. R., Castillo, M. M., and Walker, J. C. G., 1997. A blast of gas in the latest
Paleocene: simulating first-order effects of massive dissociation on oceanic methane
hydrate. Geology 25, 259-262.
Evans, M. J., Derry, L. A., and France-Lanord, C., 2008. Degassing of metamorphic carbon
dioxide from the Nepal Himalaya. Geochem. Geophys. Geosyst. 9.
Ganino, C. and Arndt, N. T., 2009. Climate changes caused by degassing of sediments during the
emplacement of large igneous provinces. Geology 37, 323-326.
IPCC, 2007. Climate Change 2007. Fourth Assessment Report (AR4). Intergovernmental Panel
on Climate Change, Geneva, 2007.
Jamtveit, B., Svensen, H., Podladchikov, Y., and Planke, S., 2004. Hydrothermal vent complexes
associated with sill intrusions in sedimentary basins, Physical geology of high-level
magmatic systems. Geological Society Special Publication, London.
Kerrick, D. M., 2001. Present and past nonanthropogenic CO2 degassing from the solid Earth.
Reviews of Geophysics 39, 565-585.
Kerrick, D. M. and Caldeira, K., 1993. Paleoatmospheric consequences of CO2 released during
Early Cenozoic regional metamorphism in the Tethyan orogen. Chemical Geology 108,
201-230.
Kerrick, D. M. and Caldeira, K., 1998. Metamorphic CO2 degassing from orogenic belts.
Chemical Geology 145, 213-232.
Kerrick, D. M. and Caldeira, K., 1999. Was the Himalayan orogen a climatically significant
coupled source and sink for atmospheric CO2 during the Cenozoic? Earth and Planetary
Science Letters 173, 195-203.
Kerrick, D. M., Lasaga, A. C., and Raeburn, S. P., 1991. Kinetics of heterogeneous reactions.
Reviews in Mineralogy 26, 583-671.
Kerrick, D. M., McKibben, M. A., Seward, T. M., and Caldeira, K., 1995. Convective
hydrothermal CO2 emission from high heat flow regions. Chemical Geology 121, 285293.
8
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
Knoll, A. H., Bambach, R. K., Payne, J. L., Pruss, S., and Fischer, W. W., 2007. Paleophysiology
and end-Permian mass extinction. Earth and Planetary Science Letters 256, 295-313.
McElwain, J. C., Wade-Murphy, J., and Hesselbo, S. P., 2005. Changes in carbon dioxide during
an oceanic anoxic event linked to intrusion into Gondwana coals. Nature 435, 479-482.
Morner, N. A. and Etiope, G., 2002. Carbon degassing from the lithosphereSymposium on
Global Carbon Cycle and Its Changes over Glacial-Intergiacial Cycles, Strasbourg,
France.
Nesbitt, B. E., Mendoza, C. A., and Kerrick, D. M., 1995. Surface fluid convection during
Cordilleran extension and the generation of metamorphic CO2 contributions to Cenozoic
atmospheres. Geology 23, 99-101.
Planke, S., Rassmussen, T., Rey, S. S., and Myklebust, R., 2005. Seismic characteristics and
distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and
Møre basins. In: Dore, T. and Vining, B. Eds.)Petroleum Geology: North-West Europe
and Global Perspectives - Proceedings of the 6th Petroleum Geology Conference.
Geological Society Publishing House, London.
Polteau, S., Mazzini, A., Galland, O., Planke, S., and Malthe-Sorenssen, A., 2008. Saucer-shaped
intrusions: Occurrences, emplacement and implications. Earth and Planetary Science
Letters 266, 195-204.
Retallack, G. and Jahren, A. H., 2008. Methane Release from Igneous Intrusion of Coal during
Late Permian Extinction Events. The Journal of Geology 116, 1-20.
Svensen, H., Jamtveit, B., Planke, S., and Chevallier, L., 2006. Structure and evolution of
hydrothermal vent complexes in the Karoo Basin, South Africa. Journal of the
Geological Society 163, 671-682.
Svensen, H., Planke, S., Chevallier, L., Malthe-Sorenssen, A., Corfu, F., and Jamtveit, B., 2007.
Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming.
Earth and Planetary Science Letters 256, 554-566.
Svensen, H., Planke, S., Malthe-Sorenssen, A., Jamtveit, B., Myklebust, R., Rasmussen Eidem,
T., et al., 2004. Release of methane from a volcanic basin as a mechanism for initial
Eocene global warming. Nature 429, 542-545.
Svensen, H., Planke, S., Polozov, A. G., Schmidbauer, N., Corfu, F., Podladchikov, Y. Y., et al.,
2009. Siberian gas venting and the end-Permian environmental crisis. Earth and
Planetary Science Letters 277, 490-500.
Visscher, H., Looy, C. V., Collinson, M. E., Brinkhuis, H., Cittert, J., Kurschner, W. M., et al.,
2004. Environmental mutagenesis during the end-Permian ecological crisis. Proceedings
of the National Academy of Sciences of the United States of America 101, 12952-12956.
Wignall, P. B., 2001. Large igneous provinces and mass extinctions. Earth-Science Reviews 53,
1-33.
Zachos, J., Pagani, M., Sloan, L., Thomas, E., and Billups, K., 2001. Trends, rhythms, and
aberrations in global climate 65 Ma to present. Science 292, 686-693.
Figures
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Figure 1. Overview of large igneous provinces (basalts in red) and volcanic basins. The basalts
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are lava flows from the LIPs whereas the sub-volcanic part of the provinces, with sills and dikes,
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are outlined with heavy lines. The volcanic basins mentioned here are indicated with black
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outlines, and other major volcanic basins with a black circle.
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Figure 2. Sill intrusions and pipes in the Karoo Basin, South Africa. A) The Golden Valley sill
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complex with its saucer-shaped intrusions (see POLTEAU et al., 2008). The dolerite sills are about
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100 meters thick and are emplaced in sandstones and mudstones. Sill intrusions are associated
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with all the hills in the background. Photo: Sverre Planke. B) Breccia pipes from the
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Loriesfontein area in the western parts of the basin. The pipes are seen as black dots with light
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grey circular alteration haloes. A dirt road in the lower half of the image gives the scale. C) The
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Witkop III hydrothermal vent complex (SVENSEN et al., 2006) is a crater that formed at the
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Lower Jurassic paleosurface due to release of fluid from contact aureoles.
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Figure 3. Photograph of a drill core showing contact metamorphism of black shale from within a
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breccia pipe in the Calvinia area in western Karoo Basin (SVENSEN et al., 2007). The original
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shale was black in color and contact metamorphism resulted in transformation of the organic
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matter to gas and a change in color. The secondary bleaching of the sample is due to reactions
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with hydrothermal fluids within the breccia, and the precipitation of sulfides.
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Figure 4. Schematic cross section of a sill intrusion with associated contact aureole and
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hydrothermal vent complex.
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Table 1. Major volcanic basins, LIPs, and environmental changes.
Volcanic Basin
LIP
Sill age (Ma) Event
270
Vøring and Møre (Norway) North East Atlantic
55.6
PETM
Karoo (South Africa)
Karoo-Ferrar
182.6
Toarcian
Amazonas (Brazil)
CAMP
~200
Triassic-Jurassic boundary
Solimoes (Brazil)
CAMP
~200
Triassic-Jurassic boundary
Tunguska (Russia)
Siberian Traps
~252
Permian-Triassic boundary
Sichuan (China)
Emeishan Traps
~261
End-Guadalupian
Abbreviations: CAMP: Central Atlantic Magmatic Province; CIE: Carbon isotope excursion.
Pipe
structures
yes
yes
?
?
yes
?
CIE
yes
yes
yes
yes
yes
yes
Mass
extinction
minor
minor
major
major
major
major
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Table 2. Key facts about volcanic basins.
Volcanic Basin
Area with sills
1000 km2
Vøring and Møre (Norway) 85
Karoo (South Africa)
390
Amazonas (Brazil)
200
Solimoes (Brazil)
600
Tunguska (Russia)
2000
Sichuan (China)
200
Sills
in black shale
yes
yes
yes
yes
yes
?
Sills
in evaporite
no
no
yes
yes
yes
no
Sills
in carbonates
no
no
yes
yes
yes
yes
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