Gas‐Driven Tensile Fracturing in Shallow Marine Sediments.

The flow of gas through shallow marine sediments is an important component of the global carbon cycle and affects methane release to the ocean and atmosphere as well as submarine slope stability. Seafloor methane venting is often linked to dissociating hydrates or gas migration from a deep source, and subsurface evidence of gas‐driven tensile fracturing is abundant. However, the physical links among hydrate dissociation, gas flow, and fracturing have not been rigorously investigated. We used mercury intrusion data to model the capillary drainage curves of shallow marine muds as a function of clay content and porosity. We combined these with estimates of in situ tensile strength to determine the critical gas saturation at which the pressure of the gas phase would exceed the pressure required to generate tensile fractures. Our work showed that fracturing is favored in the shallowest 38 m of sediment when the clay‐sized fraction is 0.2, but fracturing may be possible to a depth of 132 m below seafloor (mbsf) with a clay‐sized fraction of 0.5 and to a depth of nearly 500 mbsf with a clay‐sized fraction of 0.7. Dissociating hydrate may supply sufficient quantities of gas to cause fracturing, but this is only likely near the updip limit of the hydrate stability zone. Gas‐driven tensile fracturing is probably a common occurrence in the upper 10–20 mbsf regardless of clay‐sized fraction, does not require much gas (far less than 10% gas saturation), and is not necessarily an indication of hydrate dissociation. Plain Language Summary: Gas bubble emissions from discrete locations on the seafloor are observed at many locations worldwide. Bubble emissions are often linked to hazards such as submarine landslides and may contribute to ocean acidification and release of methane to the atmosphere. Observations indicate that the gas tends to move through fractures or cracks, but the role of gas in potentially forming those fractures and the overall process of gas accumulation and flow in shallow marine sediments are not well understood. Using a new model based on laboratory data to predict how much gas is needed to generate fractures in shallow marine sediments, we show that gas can easily generate fractures near the seafloor, particularly when the sediments contain a significant amount of clay. Our results demonstrate that gas‐driven fracturing is probably a common occurrence near the seafloor, does not require very much gas, and may not necessarily be an indication of gas hydrate melting caused by ocean temperature increase. Key Points: Gas‐driven tensile fracturing can occur in the upper 40 m below seafloor if the clay‐sized fraction is greater than 20%If the clay‐sized fraction exceeds 60%–70%, gas saturations <10% can generate tensile fractures in sediments as deep as 2 km below seafloorHydrate dissociation can cause fracturing and venting where the base of the hydrate stability zone intersects the seafloor [ABSTRACT FROM AUTHOR]