Seafloor evidence of structurally-controlled fluid expulsion from the upper Amazon deep-sea

The Amazon River culminates in one a deep-sea fan up to 10 km thick, a dynamic setting in which the rapid deposition of organic-rich sediment drives linked processes of methanogenesis, fluid migration and venting, gas hydrate formation, and large-scale slope instability. Growth of the fan over the last 8 Ma has been accompanied by its gravitational collapse on shale detachments to form extensional and compressional belts across the shelf and upper slope (<2250 m water depth), and by recurrent slope failure to form fan-wide megaslides. The upper slope compressional belt contains a ‘leaky’ gas hydrate system characterised by elongate bottom-simulating reflection (BSR) patches that are aligned with the crests of thrust-fold anticlines, and in places rise towards sub-circular seafloor fluid vents. Ongoing fluid venting from the fan is indicated by sea surface oil slicks reported on the shelf and upper slope, and water column gas flares observed on multibeam imagery obtained in 2016 across part of the thrust-fold belt. The extent of degassing across the vast fan area in water depths of 2500-4500 m is unknown due to a lack of water column data below the compressional front. The 2023 AMARYLLIS-AMAGAS I campaign acquired acoustic data (multibeam imagery, Chirp profiles) along multiple transects of the fan in water depths of 100-4200 m, and cores and heat flow data from sites in the thrust-fold belt. Here we present information on fluid expulsion from the Amazon fan based on seafloor data both from the campaign, and 3D seismic datasets on the upper slope (ANP Brazil). Multibeam imagery reveal hundreds of water column gas flares in water depths of 100-1900 m, with a peak in abundance near the upper limit of the MHSZ (565 ± 65 m water depth). Gas is observed to rise from areas of smooth seafloor in places, but mainly from sub-circular mounds and depressions. Bathymetric grids from multibeam and 3D seismic (4-50 m resolution) were used to capture sub-circular seafloor morphologies for morphometric analysis using a semi-automated training approach. Over 500 features were identified in water depths of 275-2265 m, identified as domes (59%), complex forms (28%) and depressions (13%); the vast majority (>96%) are <50 m in relief (mean 16 m) and <1 km wide (mean 500 m). Cores of alternating lighter hemipelagic and darker muds interpreted as mud extrusion were recovered both from domes and depressions; gas hydrates were cored in several domes with gas flares. Subbottom data reveal chaotic facies defining structures deeply-rooted in thrust-folds. We interpret the seafloor features as differing expressions of relatively small-scale mud volcanism, many actively venting gas. Our results indicate widespread fluid expulsion from the Amazon fan within the extensional and compressional belts, and a lack of evidence for venting in greater water depths. The primary control on degassing of the fan appears to be gravity tectonism, which provides pathways for fluid escape within and above the MHSZ. This is a contribution to studies of gas hydrate dynamics and slope stability in the context of the MEGA project (ANR-22-CE01-0031).