Long-term observation of fluid venting features in the Amazon Fan

Discoveries of gas venting from the deep seafloor attract growing attention from the scientific community and the energy industry, given their implications for the energy transition and greenhouse gas emissions. Understanding the dynamics of gas hydrate systems and associated exudation processes is essential for assessing their potential environmental and economic impacts. The Amazon River culminates in one of the world’s largest deep-sea fans, offering a natural laboratory to study gas migration and expulsion within a rapidly-deposited and gravitationally collapsing depocentre. Gas venting has been documented within an upper slope compressional belt during a decade-long observational study involving campaigns in 2013 and 2023, which acquired hydroacoustic data and core samples that included gas hydrates. This study integrates these datasets with exploration 2D and 3D seismic data to investigate seafloor gas venting features and their connections to active fault systems. Over the 10-year observation period, within the same area of 1549 km² (water depths 900-1800m), water column gas flares increased in number, with 34 new flares identified in 2023; 17 flares observed in 2013 disappeared, while 13 remained active in 2023. The flares rise from seafloor mounds, and in some cases depressions, interpreted as mud volcanoes and possibly pockmarks. These seafloor vents are commonly associated with acoustically chaotic subsurface vertical zones interpreted as fluid escape conduits. In the case of mud volcanoes, conduits of kilometric vertical extent rise from anticlines and are associated with deformation of surrounding layers and extrusion of material onto the seafloor. Most venting structures lie above and pass through bottom simulating reflection (BSR) patches that cross-cut the tops of buried or seafloor anticlines; the BSR in places exhibits ‘pluming’ behavior, rising toward seafloor vents. The seafloor with the upper slope compressional belt is offset by both normal faults, observed above the crests of buried anticlines, and by thrust-faults within the anticlines which extend downward to shale detachments in upper Miocene and older formations. Bright spot reflections, often observed adjacent to faults, highlight zones of gas migration along these structures. Our findings underscore the widespread distribution of upper slope fluid vents linked to complex subsurface geological structures including active folds and faults. The temporal variability of gas venting, characterized by the emergence, persistence, and disappearance of gas flares, highlights the dynamic nature of these processes and their significance for understanding methane cycling and its implications.