Large Igneous Provinces (LIPs) are geological events of volcanic activity that emplace huge volumes of magma into the shallow crust and lavas at the surface. Volcanic activity is one of Earth’s main sources of greenhouse gases such as CO2 and CH4 to the atmosphere. If emitted rapidly, and in large quantities, carbon gases can lead to significant climatic, environmental and biotic changes, through global warming and changes in the ocean chemistry. This is shown for a number of LIPs in Earth history1.
A large source of the carbon gases comes from magma intrusions emplaced into carbon-bearing sedimentary rocks (e.g. organic matter or carbonate rock) in the crust2. The heat from the intrusions leads to contact metamorphism of the surrounding rocks, which in turn produces thermogenic CO2 and CH4 that can be released to the atmosphere at the Earth’s surface. If the metamorphosed sediments are rich in organic material, which is preserved in shales and hydrocarbon reservoirs, the thermogenic carbon gases have an isotopically light signature (i.e., a higher ratio of 12C compared to 13C).
The emplacement of the Karoo LIP (KLIP) occurred at the same time as the Toarcian crisis (~183 million years ago)3, which was characterized by extinctions on land and in the oceans, ocean anoxia, global warming, and abrupt carbon cycle changes. Toarcian sedimentary records show an increase in the atmospheric CO2 concentrations (pCO2)4, indicating a large increase in the volume of carbon released to the atmosphere. There are also several negative carbon isotope excursions (CIEs)5, suggesting that the released carbon was isotopically light. Huge volumes of the KLIP magmas were intruded into organic-rich rocks in the Karoo sedimentary basin in South Africa (Fig. 1a), and it has been suggested (by using various methods) that several thousand billion tonnes of carbon gases were generated in the Karoo Basin6. The carbon gases were likely released into the atmosphere through explosion pipes, which can be found in large numbers within the Karoo Basin (Fig. 1b). It has therefore been suggested that thermogenic carbon released from the KLIP was responsible for the carbon cycle changes observed in the rock record3-4,6-7. However, the contact metamorphism from KLIP intrusions and its relation to the Toarcian crisis has not been fully explored.
In this new study was a collaboration between researchers at the Centre for Earth Evolution and Dynamics (CEED) at the University of Oslo and CNRS-Université Paul Sabatier (Toulouse, France). The team tested the effects of releasing carbon gases from the KLIP on the atmosphere and oceans by using the GEOCLIM carbon cycle model. The model simulations show that the release of ~ 20,500 billion tonnes of carbon, with a ~60% contribution of thermogenic carbon, can explain the observed proxy data for climate change in the Toarcian, such as pCO2 increases and negative CIEs. The study corroborates the hypothesis that thermogenic carbon gas release is especially important during episodes of rapid climate change and mass extinctions. By improving our understanding of climatic and environmental change of the past, we can improve our predictions of the future, now that we are causing a rapid warming event of our own through anthropogenic carbon gas emissions.
Contact: Thea Hatlen Heimdal, Centre for Earth Evolution and Dynamics (CEED), University of Oslo, PO Box 1028, Blindern, NO-0315 Oslo, Norway, +47 41548103, t.h.heimdal@geo.uio.no.
Publication details: Thea H. Heimdal, Yves Goddéris, Morgan T. Jones & Henrik. H. Svensen. Assessing the importance of thermogenic degassing from the Karoo Large Igneous Province (LIP) in driving Toarcian carbon cycle perturbations. Nature Communications (2021).
References
1Bond, D.P.G. & Wignall, P.B. (2014). Large igneous provinces and mass extinctions: An update, in Keller, G. & Kerr, A.C (eds.), Volcanism, Impacts, and Mass Extinctions: Causes and Effects, Geological Society of America Special Paper 505.
2Svensen, H.H. et al. (2004). Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, 429, 3, 542-545.
3Greber, N. D. et al. (2020). New high precision U-Pb ages and Hf isotope data from the Karoo large igneous province; implications for pulsed magmatism and early Toarcian environmental perturbations. Results in Geochemistry, 100005.
4McElwain, J. C. et al. (2005). Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature, 435, 479–482.
5Remírez, M. N. & Algeo, T. J. (2020). Carbon-cycle changes during the Toarcian (Early Jurassic) and implications for regional versus global drivers of the Toarcian oceanic anoxic event. Earth Science Reviews, 209, 103283.
6Galerne, C. Y. & Hasenclever, J. (2019). Distinct degassing pulses during magma invasion in the stratified Karoo Basin – new insights from hydrothermal fluid flow modeling. Geochemistry, Geophysics, Geosystems, 20, 2955-2984.
7Svensen, H. H. et al. (2007). Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming. Earth and Planetary Science Letters, 256, 554-566.
