Text and illustration by Frances Cooke
Methane is a powerful greenhouse gas. We know that at least a quarter of today’s warming is driven by methane from human actions but what we are unable to quantify with certainty is the natural methane release from both terrestrial and marine environments. Here we focus on the release of methane from beneath the seafloor in the eastern Fram Strait, located west of Svalbard. Our study site Vestnesa Ridge – a boomerang shaped sedimentary drift formed by deposition of suspended current material where the north Atlantic current splits, slows down and is diverted westwards along the ridge. At Vestnesa Ridge, methane gas is stored beneath the surface, at a minimum depth of ~170m where it first collected and reached the seafloor at around 2 million years BP, coinciding with the onset of northern hemisphere glaciations. Above the minimum depth of the free-flowing gas (extending upwards to the seafloor), methane is stored as gas hydrate (frozen methane).
The Arctic is a very sensitive region to climate change and little is known about the mechanisms that control the release of ‘free’ methane through ‘mud-cracks’ beneath the seafloor, imaged using state of the art high resolution seismic acoustic technology. We apply edge-detection filters (much like in photography editing but instead in a 3-D volume) to extract specific details such as fractures or upwards/downwards concavity in the morphology of the seismic image. Here we identify the size and location of mud-cracks and buried fluid-release craters using such filters (also referred to in seismic data as ‘attributes’) and interpret from these structures the history of methane release during the last 1.2 million years (the most recent glacial periods). This paper investigates the amount of fracturing and resulting pockmarks (i.e. small 20m craters) beneath the seafloor, west of the ridge. We identify 2 major fractured events as leakage-prone intervals. We consider that highly fractured zones (using a ‘fracture density’ attribute) are linked to potentially multiple significant climatic events in the recent glacial past. Significant climatic events impact the system by either increasing ocean bottom temperatures or fracturing the sediments related to the repeated growth and retreat of icesheets – cycling between exerting/releasing pressure on the Earth’s crust. The mechanisms proposed, destabilize the system and dissociate frozen methane into fluids (i.e. water and ‘free’ gas). The fluids become mobile in the sediments forming pathways via the mud-cracks. Methane gas previously trapped in the sediment is released into the ocean which in turn releases pressure. The ‘mud-cracked’ sediments become self-sealing, eventually blocking the flow of fluids until the next climatic event. We also indicate in our study that after a significant release the system takes time to collect gas back into the system and to become pressurized once again. In summary, using information from the past, our study has implications for understanding how a changing Arctic in the future might affect sediment stability and carbon transport to the oceans and atmosphere.
Published in Frontiers in Earth Science in May 2023 (https://www.frontiersin.org/articles/10.3389/feart.2023.1188737/full)
Figure: Map of the study area (top), example of how the sediment looks like underneath the seafloor, and map of the mud cracks (black and white inset) revealed by mathematical relationships between seismic traces. HDI stands for highly deformed (sedimentary) intervals. These intervals correspond to time periods where pressure and temperature changes had a larger impact on the sediments below the seafloor than any other period of time.