Contribution of oil-cracked methane to climate change cannot be ignored

Methane emissions created by volcanic activity burning buried fossil fuels deposits could have played a major role in the global warming that triggered the largest biological crisis in Earth’s history.
Published in Earth & Environment
Contribution of oil-cracked methane to climate change cannot be ignored
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The late Permian mass extinction (LPME), also known as ‘the greatest dying’, happened around 250 million years ago, and wiped out more than 90% of Earth’s species. Previous studies have found evidence that the LPME was likely caused by devastating magmatic activities and related greenhouse gas release, including the Emeishan large igneous province (ELIP; ~260 Ma) and Siberian Traps large igneous province (STLIP; ~252 Ma). Large volumes of gas emissions released by Large Igneous Province (LIP), particularly CO2, were proposed to be the main driver of the global warming during the LPME. However, due to the absence of strong evidence, formation of CH4 in subsurface environment linked to LIPs has not been well-explored, hindering the understanding of the impact of LIP-induced CH4 emission on climate change and extinction event.

In our recent study, published in the journal Nature Communications, we provided evidence to show that huge volumes of oil-cracked methane triggered by massive volcanic activities may have also played a significant role in the LPME crisis. This research was led by scientists from Lancaster University along with researchers from the Chinese Academy of Sciences, the University of Manchester, and industrial partner PetroChina. Dr. Chensheng Chen from Guangzhou Institute of Geochemistry, Chinese Academy of Sciences is the first author, and professor Yunpeng Wang & Dr. Zheng Zhou are the corresponding authors from the Chinese Academy of Sciences and Lancaster University, respectively.

In this study, we present analytical results from natural gas samples from Anyue gas field in the central Sichuan Basin, South China. The gas field is sitting on the outer zone of the ELIP. Using a combination of clumped methane isotopes and noble gas isotopes together with basin modelling, we studied the origin of natural gas samples. We found key evidence of large-scale oil cracking induced by the ELIP event in the basin. Furthermore, we proposed a methane emission model associated with the ELIP induced oil-cracking in the subsurface formations of the Sichuan Basin, which generated large amounts of methane. We suggested that the release of oil-cracked CH4 into the atmosphere played a major role in the global warming and climate change during the end-Guadalupian mass extinction. This is a significant departure from previous assessments of the root cause of LIP-induced mass extinctions, which assume CO2 is the main contributor.

Using a ‘clumped methane isotope’ technique we calculated the formation temperature of oil-cracked methane to be around 260 ℃. This was higher than the temperatures normally experienced during the geological burial history of the gas reservoir, suggesting the involvement of an additional source of heating. In addition, analysis of noble gas isotopes in the samples identified the involvement of mantle derived fluids, confirming that mantle was involved in generating methane. The methane could subsequently rise to the surface and into the atmosphere through cracks and fissures along the geological formations.

Figure 1. This image shows a hand specimen of coarse veins sampled from the Sinian Dengying Formation, which is one of the main formations in the gas reservoir in our study. It mainly consists of quartz and pyrobitumen. It is obvious in this image that paleo-oil in the formation was heated and cracked by hydrothermal intrusion, leading to the formation of pyrobitumen and quartz-dominated hydrothermal minerals and recording the geothermal history associated with methane emissions.

Pyrobitumen (Figure 1), a byproduct of oil cracking into methane, is widely distributed in the ELIP (e.g., Sichuan Basin) and STLIP (e.g., Tunguska Basin) regions. Using a methane emission model based on pyrobitumen, we calculated that up to 1440 Gigatonnes of methane may have been released into the atmosphere from the entire Sichuan basin during the Late Permian era. In global warming terms, this is equivalent to more than 40,410 Gigatonnes of carbon dioxide — which is 1,000 times of the current annual global carbon emissions, because the global warming potential of methane is 28 times of that of carbon dioxide over a 100-year period. Also, we propose that this mechanism of methane emission could have happened in the STLIP region, where up to 10,000 Gigatonnes of methane might have been released into the atmosphere during end-Permian. Therefore, we suggest that large amounts of methane could have been generated when superhot volcanic “mantle plumes” heated underground coal and oil deposits, eventually making huge impact on the carbon cycle and change our planet.

Our new study suggests that methane emissions generated by volcanic activities can heat up buried fossil fuel deposits and have played a major role in the global warming that triggered the largest biological crisis in Earth’s history (Figure 2). Similarly, modern methane emissions (e.g., fossil energy industry and landfills) probably have also made a huge contribution to heat up our planet in addition to CO2 emissions. However, geological events contributed to the past climate change, while human activities are main causes for current crisis. What we can learn from this study is that methane could impact our planet as much as CO2 does, sometimes even in a deadly way. While cutting anthropogenic methane emissions is urgent to avoid warming our planet, real-time monitoring of geological methane emissions is another important task for understanding the future climate change.

Figure 2. This illustration shows geological methane emissions associated with mantle activities and their environmental impacts in the Late Permian. Before Permian, large amounts of organic matter were buried underground. Very large quantities of methane were generated by mantle plumes heating the organic matter. Methane was then released from oil, or other organic matter buried underground. These methane emissions could have happened on land and in ocean at the same time, making contributions to global climate change and biological crisis.

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