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| Titel |
Contact metamorphism of black shales: global carbon cycle and climate perturbations |
| VerfasserIn |
I. Aarnes, H. Svensen, S. Polteau, J. A. D. Connolly, S. Planke |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027632
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| Zusammenfassung |
| There is an increasing interest in improving the understanding of past climate changes, as it
can lead to a better understanding of future challenges related to global warming and
anthropogenic release of greenhouse gases. The formation of Large Igneous Provinces
(LIPs) and sill intrusions in volcanic basins correlate with global warming events
and mass extinctions, e.g. the Karoo Basin, South Africa (~183 Ma), the Møre
and Vøring Basins offshore Norway (~55 Ma), and the Tunguska Basin, Siberia
(~252 Ma). The proxy records from these events suggest that rapid release of large
amounts of isotopically 13C-depleted greenhouse gases (CO2 and methane) to the
atmosphere.
Organic matter stored in sedimentary rocks (e.g. black shale) represents a major carbon
source. Large volumes of greenhouse gases may form by contact metamorphism of
organic-rich sediments around sill intrusions associated with LIPs. The organic-rich Ecca
Group forms the base of the Karoo sedimentary succession and contains thousands of
degassing pipe structures rooted in contact aureoles around sill intrusions. Numerical and
analogue modelling show that these piercement structures form during violent eruptions
releasing the overpressure driven by dehydration and devolatilization metamorphic
reactions.
In this study we evaluate the aureole processes numerically in order to constrain the
amount of gases formed in contact aureoles around sill intrusions, and the isotopic
composition of those gases. The total organic carbon (TOC) in the shale and the intrusion
thickness are the most important parameters controlling the amount of carbon gas that can
trigger pipe formation and release into the atmosphere. .
We model thermal cracking using a general kinetic approach, while dehydration reactions
are modeled under the assumption of thermodynamic equilibrium. The theoretical
approach is tested against borehole data from the Karoo Basin in South Africa
(geochemical analyses, Rock-Eval pyrolysis, TOC, vitrinite reflectance and stable
isotopes).
Decreasing TOC content and increasing vitrinite reflectance with decreasing distance to
the intrusive contact are signatures of thermogenic hydrocarbon formation. During high
temperature metamorphism, formation of carbon gases is preferred over liquid hydrocarbons.
However, only limited isotopic fractionation is occurring in the released carbon gases during
increasing temperature.
Increasing veining towards the contact of a 10 meter sill suggests that hydrocarbon
formation in organic-rich aureoles leads to pressure buildup and fracturing of the aureole,
even with small volumes. Our numerical model also shows that sill thicknesses in the order of
100 m are necessary to produce the pressure buildup in the contact aureole and subsequent
venting. In addition, mineral dehydration and thermal stresses contribute to pore fluid
pressure increase.
We use our numerical model to predict the amount of fluids produced as response to thin
(~10 meter) and thick (~100 meter) sills. The model provides us with important
estimates of rate and duration of gas formation. The time-scale of subsurface gas
formation is well within the time scale indicated by the proxy data. Results from isotope
compositions demonstrate that the 2.8t/m2 of organic carbon escapes the contact aureole
during devolatilization processes involving the generation of light carbon gases. The
calculated isotopic composition of the carbon released is similar whether using
the batch devolatilization or the Rayleigh distillation model, and ranges from the
background values to 1-2 permil lighter values with decreasing distance from the
contact.
The extrapolation of our results to the portion of the sedimentary basin intruded by
magma suggests that contact metamorphism of organic-rich sediments triggered a potential
release of between 2000 to 10000 Gt of isotopically light carbon gas to the atmosphere. In
conclusion, the amount and composition of methane that can be produced and vented from
contact aureoles in the Karoo Basin during the Toarcian is within the same order of
magnitude as required to explain global carbon isotope excursion and hence global warming. |
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