August 2008 LIP of the Month

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Corresponds to event #5 in LIP record database.

The North Atlantic volcanic province (NAVP) and the Paleocene-Eocene Thermal Maximum (PETM)

Henrik Svensen1 and Sverre Planke1,2

1: Physics of Geological Processes (PGP), University of Oslo, Norway. Email: hensven@fys.uio.no.

2: Volcanic Basin Petroleum Research, Oslo Research Park, Oslo, Norway. Email: planke@vbpr.no.

LIPs and the environment

The geological record shows that abrupt increases in the atmospheric concentration of greenhouse gases have occurred tens of times during the Phanerozoic (e.g., Jenkyns, 1988; Hesselbo et al., 2000; Kemp et al., 2005; McElwain et al., 2005). Release of several thousand gigatons of isotopically light carbon gases from sedimentary basin has been proposed to be the cause of warm periods at e.g. the Permo–Triassic boundary (~251 million years ago; Ma), in the Toarcian during the Early Jurassic (~183 Ma), and in the initial Eocene (~55.5 Ma). The resulting climate changes are extensively documented by chemical proxy data from sedimentary rocks, commonly demonstrating 5–10 °C global warming lasting a few hundred thousand years, accompanied by anoxic conditions in the oceans and extinctions.

A majority of the documented global warming events took place at the same time as a Large Igneous Province (LIP) was formed (e.g., Stothers, 1993; Wignall et al., 2001; Courtillot and Renne, 2003). Three of the most severe mass extinctions occurred simultaneously with the formation of continental LIPs; the end-Cretaceous extinction and the Deccan Traps (65 Ma), the end Triassic extinction and the Central Atlantic Magmatic Province (200 Ma), and finally the end Permian extinction and the Siberian Traps (251 Ma).

The exact relationship between LIP formation and climate perturbations is however poorly understood, and the geological processes responsible for rapid formation and transport of greenhouse gases from the huge sedimentary reservoir to the atmosphere are debated. The role of LIP lava degassing has been downplayed due to the long duration (>500 ky) of lava emplacement and the relatively “heavy” carbon isotopic composition of magmatic CO2 (e.g., Saunders, 2005). Melting of marine gas hydrates has for the last decade been the favored climatic trigger (i.e., carbon release mechanism) among many geoscientists, but recent studies of the present day hydrate reservoirs questions its possible contribution (Milkov, 2004). Other proposed mechanisms are related to changes in ocean circulation, orbital forcing, burning of peat, and comet impacts.

The strong temporal correlation between LIP formation and major climate events during the Phanerozoic has led us to study the dynamics of LIP emplacement to understand how volcanism may cause rapid climate change and release of isotopically light carbon gases. Our focus has particularly been on understanding the initiation of LIP volcanism in sedimentary basins. The aim is to test the theory proposed by Svensen et al. (2004) that intrusive volcanism in carbon-rich sedimentary basins at the end of the Paleocene led to the formation and release of sufficiently large volumes of isotopically light carbon to cause both the global warming and the negative carbon isotope excursion observed during the Paleocene-Eocene thermal maximum (Figure 1). The theory provides a new approach to understand the relationship between LIPs and global environmental changes, where the emplacement environment is a crucial factor.


Figure 1: Schematic cross section of a volcanic basin. Sills and dikes (red) form the sub-volcanic part of many LIPs. Contact metamorphism around the sills generate carbon gases from organic matter. The resulting pressure build-up leads to hydrofracturing and the formation of hydrothermal vent complexes, transporting greenhouse gases from the aureoles to the atmosphere.


Figure 2: Overview of the lavas of the NAVP, both present offshore (pink) and on land (black). The volcanic Vøring and Møre basins are located offshore mid-Norway.


Figure 3: Sills and dikes of the NAVP are exposed on Greenland. The picture shows a dike cutting sandstone and coal seams on western Greenland.

The volcanic basins offshore Norway

The Vøring and Møre basins offshore Norway are two prime examples of volcanic basins in the northeast Atlantic (Figures 2 and 3). A huge magmatic complex of dominantly subhorizontal sheets (sills) of basaltic composition intruded the Cretaceous Vøring and Møre basins before, and during, the northeast Atlantic continental break-up about 55 million years (Myr) ago (Skogseid et al., 1992; Berndt et al., 2000). This represents the second pulse of the NAVP (also known as the NAIP; the north Atlantic igneous province) for which the initial pulse occurred at about 61 Ma (e.g., Storey et al., 1998). Detailed seismic and borehole interpretations show that a sill complex covers an area of at least 80,000 km2 (Figures 4 and 5; Planke et al., 2005), but sills are also likely to be present in basin segments covered by lava flows that inhibit deep imaging. The actual thickness and number of sills is more difficult to map as sub-sill seismic imaging is often poor. However, both field and seismic data commonly reveal several levels of sill intrusions (Berndt et al., 2000; Brekke, 2000; Smallwood and Maresh, 2002; Planke et al., 2005; Hansen and Cartwright, 2006). Three sills in the Vøring Basin were drilled by the well 6607/5-2 (Utgard) with thicknesses of 2m, 91m, and >50m (Berndt et al., 2000; Brekke et al., 1999). The middle sill is very well imaged on seismic reflection data, and can be followed for >50 km into the Vøring Basin (Figure 4).


Figure 4: Seismic line from the Vøring Basin showing sill intrusions (green) and the paleosurface during the Paleocene-Eocene (yellow). The Utgard borehole (6607/5-2) penetrated three sill intrusions. The sill complex can be traced for >50 km into the basin towards the west. From Planke et al. (2005).

A conservative estimate of the volume of the sill complex in the Vøring and Møre basins is 0.9–2.5 x 104 km3 based on an intruded area of 85,000 km2 and an average vertical accumulated sill thickness of 100–300 m (Svensen et al., 2004; Planke et al., 2005). The area of the entire sill complex in the NAVP is probably at least five times greater than the sill complex in the Vøring and Møre basins. This estimate is based on the fact that the two studied basins comprise only 10% of the length of the northeast Atlantic volcanic margins, and a conservative estimate of the width of the NAVP sill complex being 50% of the width of the Vøring and Møre basins sill complex.

Hydrothermal venting of greenhouse gases

We have identified and characterized 735 hydrothermal vent complexes in the Vøring and Møre basins (Figure 5; Svensen et al., 2004; Planke et al., 2005). The total number of vent complexes in the Møre and Vøring basins is estimated to be at least 3–5 times greater, given the size distribution of the vent complexes and the seismic line coverage. This factor has been confirmed by comparing the number of vent complexes identified on two- and three-dimensional (2D and 3D) seismic data in the same region.

The hydrothermal vent complexes represent pipe-like structures with upper parts consisting of craters or mounds. They are interpreted to originate in contact aureoles around sill intrusions, and formed by explosive release of carbon-gases and sediments shortly (tens of years) after sill emplacement (Jamtveit et al., 2004; Svensen et al., 2006). This interpretation is supported by geochemical and petrographic data from boreholes in the Vøring Basin (Figure 6) and field studies in the Karoo basin in South Africa (Svensen et al., 2006; 2007).

The production potential of carbon gases in the Vøring and Møre basins has been estimated, based on the area with sills, the estimated sill (and aureole) thicknesses, and the transformation of 0.5 to 2 wt.% of organic matter to gas. The calculated production potential is up to 2,500 Gt of carbon (Svensen et al., 2004), which is likely sufficient to cause global warming.


Figure 5: Map of sill intrusions (in color) and hydrothermal vent complexes (black dots). The map is based on interpretation of more than 150,000 km of seismic lines as a collaboration effort with the Norwegian petroleum industry. Figure from Svensen et al. (2004).


Figure 6: Interpreted cross section through a hydrothermal vent complex that was drilled in the 1980’s in the Vøring Basin. The pipe structure is terminated in a crater at the paleo-surface. Seep carbonates are present in the strata above the crater and are evidence for flow of methane-bearing fluids within the conduit for a long time after the initial formation (Svensen et al., 2003).

Timing of sill emplacement and degassing

Both field and seismic data suggest that the major part of the sill complexes were formed in a short time span. Individual sills in the NAVP may be up to hundreds of kilometers long and must have formed very rapidly (tens of years) to avoid solidification. Seismic observations indicate that a small number of large-volume intrusive episodes have formed the entire intrusive complex of the Vøring and Møre basins. For comparison, single flows with volumes exceeding 2,000 km3 have been estimated in the Columbia River Flood Basalt Province (Hopper, 1997). Only 4 to 12 such events are required to form the entire Vøring and Møre basins sill complex. However, even larger individual events can be expected in the much more voluminous NAVP.

Constraints on the timing of magma emplacement, and thus vent complex formation, can be obtained from seismic interpretation and biostratigraphy. The seismic interpretation reveals that the sill emplacement volcanism occurred mainly before the main extrusive events because: (1) the Top Palaeocene horizon (the stratigraphic level where ~95% of the hydrothermal vent complexes terminate) continues beneath the extrusive pile; and (2) no hydrothermal vent complexes have been identified within or above the extrusive sequence. Careful interpretation shows that most vent complexes terminate at the same stratigraphic level, the Top Paleocene, except for 20 vent complexes in the Møre Basin that terminate within the Paleocene sequence (Planke et al., 2005).

New biostratigraphic dating of the hydrothermal vent complex drilled by 6607/12-1 (Figure 6) shows that this vent complex was formed during the TP5a palynozone (55.0 to 55.8 Ma; Svensen et al., 2004), most probably at the start of the PETM. We have recently done a re-sampling of the Utgard borehole (Figure 4) aimed at obtaining zircons from the two sills for dating. The zircons that were found in the cuttings were successfully dated to the Paleocene-Eocene boundary (Svensen et al., in prep), thus supporting the hypothesis of a casual relationship between the sills emplacement and the PETM. Similar conclusions were recently reached by Storey et al. (2007) based on Ar/Ar dating of lavas and tuffs associated with the PETM.

Relevance to other LIPs

Several other LIPs have extensive intrusive complexes in organic-rich sedimentary basins, e.g., the Siberian Traps, the Central Atlantic Magmatic Province, and the Karoo. We have conducted extensive field work both in the Karoo and Siberia, and find that both these LIPs share a similar geological development as the Vøring and Møre basins, including extensive sill complexes in organic-rich sedimentary sequences and hundreds of hydrothermal vent complexes rooted in metamorphic aureoles (Jamtveit et al., 2004; Svensen et al., 2006, 2007). There is a further good temporal correlation between the timing of intrusive volcanism and global warming events for these LIPs, supporting the theory that venting of greenhouse gases from metamorphic aureoles may lead to global warming (Svensen et al., 2007). Work in progress is aimed at quantifying several aspects of the theory, both with respect to contact aureole processes (Aarnes et al, 2008; Polteau et al., 2008), sill emplacement and timing, and the basin scale gas generation. We continue our work in the basins offshore Norway, in the Tunguska Basin in Siberia, and the Karoo Basin.

References

Aarnes, I. Svensen, H., and Polteau, S. (2008) Gas formation from black shale during contact metamorphism: Constraints from geochemistry and kinetic modelling. International Geological Congress 2008, Oslo, abstract. www.33igc.org.

Berndt, C., Skogly, O. P., Planke, S. and Eldholm, O. High-velocity breakup-related sills in the Vøring

Basin, off Norway. J. Geophys. Res. 105, 28443–28454 (2000).

Brekke,H., Dahlgren, S., Nyland, B.and Magnus, C. (1999) The prospectivity of the Vøring and Møre basins on the Norwegian Sea continental margin. In Petrol. Geol. NW Eur. Proc. 5th Conf. (eds Fleet, A. J. & Boldy, S. A. R.) 261–274.

Brekke, H. (2000) The tectonic evolution of the Norwegian Sea Continental Margin with emphasis on the Vøring and Møre Basins. Geol. Soc. Spec. Publ. 167, 327–378.

Hansen, D.M. and Cartwright, J. (2006) Saucer-shaped sill with lobate morphology revealed by 3D seismic data: implications for resolving a shallow-level sill emplacement mechanism. J. Geol. Soc. London 163, 509–523.

Hesselbo, S.P. et al. (2000) Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event, Nature 406, 392–395.

Hopper, P. R. (1997) The Columbia River flood basalt province: Current Status. AGU Geophys. Monogr. 100, 1–28.

Jamveit, B., Svensen, H., Podladchikov, J.J., and Planke, S. (2004) Hydrothermal vent complexes associated with sill intrusions in sedimentary basins. Geol. Soc. London, Spec. Publ. 234, 233-241. Download PDF (4.0 MB)

Jenkyns, H.C. (1988) The Early Toarcian (Jurassic) anoxic event: stratigraphic, sedimentary and geochemical evidence, Am. J. Sci. 288, 101–151.

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.

Milkov, A.V. (2004) Global estimates of hydrate–bound gas in marine sediments: how much is really out there? Earth-Sci. Rev. 66.

Planke, S., Rassmussen, T., Rey, S.S. & Myklebust, R. (2005). Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basins. In: Dore, A. & Vining, B. (eds) Petroleum Geology: North-West Europe and Global Perspectives. Proceedings of the 6th Geology Conference. Geological Society, London, 833–844. Download PDF (2.0 MB)

Polteau, S., Svensen, H., Planke, S., and Aarnes, I. (2008) Geochemistry of contact aureoles in the Karoo Basin and the implications for the Toarcian carbon isotope excursion. International Geological Congress 2008, Oslo, abstract. www.33igc.org.

Saunders, A.D. (2005) Large Igneous Provinces: Origin and environmental consequences. Elements 1, 259-263.

Skogseid, J., Pedersen, T., Eldholm, O. & Larsen, B. T. (1992) Tectonism and magmatism during NE Atlantic continental break-up: the Vøring Margin. Geol. Soc. Spec. Publ. 68, 305–320.

Smallwood, J. R. & Maresh, J. (2002) The properties, morphology and distribution of igneous sills: modelling, borehole data and 3D seismic from the Faroe-Shetland area. Geol. Soc. Spec. Publ. 197, 271–306.

Storey, M., Duncan, R.A., Pedersen, A.K., Larsen, L.M., Larsen, H.C. (1998) 40Ar/39Ar geochronology of the West Greenland Tertiary volcanic Province. Earth and Planetary Science Letters 160, 569–586.

Storey, M., Duncan, R.A., and Swisher III, C.C. (2007) Paleocene-Eocene Thermal Maximum and the opening of the Northeast Atlantic. Science 316, 587, 589.

Stothers, R.B. (1993) Flood basalts and extinction events, Geophys. Res. Lett. 20, 1399–1402.

Svensen, H., Planke, S., Jamtveit, B., and Pedersen, T. (2003) Seep carbonate formation controlled by hydrothermal vent complexes: a case study from the Vøring volcanic basin, the Norwegian Sea. Geo-Marine Letters 23, 351-358. Download PDF (1.3 MB)

Svensen, H.,  Planke, S., Malthe-Sørenssen, A., Jamtveit, B., Myklebust, R., Eidem, T., and Rey, S. S. (2004) Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, 429, 542-545. Download PDF (1.3 MB)

Svensen, H.,  Jamtveit, B., Planke, S., and Chevallier, L. (2006) Structure and evolution of hydrothermal vent complexes in the Karoo Basin, South Africa. J. Geol. Soc. London 163, 671-682. Download PDF (0.7 MB)

Svensen, H., Planke, S., Chevallier, L., Malthe-Sørenssen, A., Corfu, B., and Jamtveit, B. (2007) Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming. Earth and Planetary Science Letters 256, 554-566.  Download PDF (1.9 MB)

Appendix

Terminology:

Volcanic basins are sedimentary basin with a significant amount of primary emplaced volcanic rocks, most commonly in the shape of shallow level sill intrusions and lava flows. The sills are commonly sub-horizontal, but may locally have transgressive segments, i.e. segments that are cross cutting the surrounding sedimentary stratigraphy.

Contact metamorphic aureoles are the volume of rocks heated beyond 100ºC following the sill emplacement. As a rule of thumb, the thickness of the metamorphic aureole is comparable to, or greater, than the sill thickness on both sides of thick (>50 m) sills intruded into shales.

Hydrothermal vent complexes are the near-surface expressions of vertical piercement structures originating in contact aureoles around sill intrusions. They are associated with wide craters (to 10 km in diameter), and are filled with brecciated sedimentary rocks. The content of magmatic material is small, which distinguishes these complexes from other magmatic-dominated diatreme systems.

Breccia pipes are vertical chimney-like structures consisting of fragmented and brecciated thermally altered rocks located in a relatively un-deformed sedimentary sequence. They are filled by mixtures of magmatic and sedimentary rock fragments, and represent the deep parts of hydrothermal vent complexes.

The Paleocene Eocene thermal maximum (PETM)

A 200,000 period of global warming occurred in the initial Eocene, about 55 million years ago. Negative oxygen isotope excursions in marine and terrestrial sediments, and studies of paleosoil profiles, suggest that the Earth’s surface temperature in creased by 5-10°C in a geological instant (2-15,000 years), and that a warm climate prevailed for about 200,000 years (e.g., Kennett and Stott, 1991). The global warming has been explained by input of large quantities of greenhouse gases to the atmosphere. Several mechanisms have been proposed for the PETM, including melting of gas hydrates, changes in ocean circulation, extrusive volcanism, and contact metamorphism of organic-rich rocks followed by gas venting.