Gravity complexes as a focus of seafloor fluid seepage: the Rio Grande Cone, SE Brazil

Seafloor methane emissions can affect Earth's climate and ocean chemistry. Vast quantities of methane formed by microbial decomposition of organic matter are locked within gas hydrate and free gas on continental slopes, particularly in large areas with high sediment accumulations such as deep-sea fans. The release of methane in slope environments has frequently been associated with dissociation of gas hydrates near the edge of the gas hydrate stability zone on the upper slope, with discharges in greater water depths less understood. Here we show, using data from the Rio Grande Cone (western South Atlantic), that the intrinsic, gravity-induced downslope collapse of thick slope sediment accumulations creates structures that serve as pathways for gas migration, unlocking methane and causing seafloor emissions via giant gas flares in the water column. The observed emissions in the study region (up to 310 Mg year(-1)) are three times greater than estimates for the entire US North Atlantic margin and reveal the importance of collapsing sediment accumulations for ocean carbon cycling. Similar outgassing systems on the Amazon and Niger fans suggest that gravity tectonics on passive margins is a common yet overlooked mechanism driving massive seafloor methane emissions in sediment-laden continental slopes.

Automated summary

Seafloor methane emissions, caused by the release of methane from gas hydrate and free gas through the collapse of sediment accumulations on continental slopes, have significant effects on Earth's climate and ocean chemistry. This study highlights the importance of collapsing sediment accumulations as pathways for gas migration, resulting in seafloor emissions. The observed emissions in the study region are three times greater than estimates for the entire US North Atlantic margin, indicating the significance of collapsing sediment accumulations for ocean carbon cycling. Similar outgassing systems exist on other sediment-laden continental slopes.