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.

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 (96%) are

Here we present a detailed description of different geomorphic features to complement the Maps of Geohazard Features of the Ionian Calabrian Margin produced by the Magic project (Marine Geohazard along Italian Coasts). Some of the most striking features we imaged are sources of widespread and recurrent geohazards. These include multiple coastal landslides, failure scars along open slopes, shelf-indenting retrogressive canyon headwalls and active fluid venting structures, that we investigated by integrating regional high-resolution multibeam sonar and sub-bottom profiling data. The main triggers and predisposing factors for the marine geohazards that we identify in our study area include frequent seismic activity, the rapid uplift of the margin since 1 Ma and the presence of Messinian evaporites at depth. Large-scale gravity-driven movements and the incipient retrogressive canyon headwalls are of particular concern, as they are located just a few hundred meters from the coast, where critical infrastructures and densely populated urban centers are situated, and also where high-resolution geophysical data are often lacking. Overall, our study provides a key reference for more detailed follow-up studies to foster a better understanding of marine geohazard occurrences. The insights provided are critical for planning monitoring programs and for the protection of coastal settlements and marine infrastructures along the Calabrian Ionian margin.

Here, we explore the complex seabed morphologies of the Gulf of Taranto in southern Italy including their connection to the geodynamic evolution of the region that began during the Neogene period when the Adria plate started subducting beneath the retreating Calabrian arc. We compiled the first Maps of the Geohazard Features of the Gulf of Taranto through comprehensive and collaborative high-resolution seabed surveys, integrating regional high-resolution multibeam sonar and sub-bottom profiling. Our findings indicate that the most significant marine geohazards identified are (i) the headwall of the shelf-indenting retrogressive canyon near Cir & ograve; Marina, situated close to the harbour, (ii) multiple landslide scarps on the steep slopes of intra-slope basins, along with buried stacked debris flow deposits at their base, indicating repeated mass movements, (iii) large-scale landslide scarps eroding the Apulian slopes (some controlled by faults). We propose that seismicity and tectonic tremor associated with slow slip events, represent potential triggers for geohazards in the Gulf of Taranto. The distinctive physiography of the Gulf creates a natural laboratory for studying and monitoring coastal and marine geohazards. Our study offers a resource for improving the understanding of marine geohazards along the Ionian Calabrian and Apulian margins in the Gulf of Taranto crucial for safeguarding coastal communities and marine infrastructures.

Fluid migration in sedimentary basins has profound effects on a range of geological processes, including the methane cycle, tectonic and sedimentary geohazards, and microbial communities in the oceans. The Alboran Sea is a tectonically active basin characterized by contourite drifts that host migrating fluids, expressed in places by pockmarks and mud volcanoes, the latter associated with seafloor methane seepage. In this study, we examine the composition and origin of near-seafloor fluids in the Alboran Sea using sediment cores (up to 20 m long) from a pockmark field (site CL06), a nearby background area (site CL04) and a fault zone (site CL55).We use halogens (Cl, Br, and I) dissolved in interstitial water to understand the origin of fluids in the Alboran Sea. Chlorine is considered a conservative ion in interstitial water geochemistry, its concentration changing with pore water salinity. Iodine has a strong biophilic character and is incorporated in organic matter deposited with sediments, which during burial decomposes in response to geothermal heat or microbial activity to produce methane. Iodine and methane concentrations are strongly correlated and highly concentrated compared to seawater, so that iodine has been used as a methane tracer. Bromide also has a weak biophilic character and behaves similarly to iodine.Interstitial water was extracted aboard ship using Rhizon samplers. Chloride concentration was determined by ion chromatography (ICS-1600, DIONEX) at the Tokyo University of Marine Science and Technology; iodine and bromine concentrations were determined by Inductively coupled plasma mass spectrometry (ICP-MS Agilent 7500) at Micro Analysis Laboratory, Tandem accelerator (MALT), University of Tokyo.The results reveal halogen profiles that differ between the pockmark and fault sites, providing evidence of different modes of fluid migration within the contourite drifts of the Alboran Sea:(1)Pockmark and background sites: surprisingly, halogen profiles are similar at these two sites. Cl concentration decreases with depth from 610 to 590 mM over the 15 m length of the cores, a trend indicating fresher water is present in deeper sediments. I and Br concentrations increase with depth (I: 0 to 70 µM, Br: 760 to 820 µM). I and Br are strongly enriched (up to 8% and 60%, respectively) by a deep fluid source, which may relate to high TOC or evaporated seawater in deeper sediment.(2)Fault zone site: in contrast to the other two sites, Cl concentration increases with depth from 600 to 610 mM over the 16 m length of the core 55, a trend indicating saline water is dominant in deeper sediments. I and Br concentrations increase with depth (I: 35 to 70 µM, Br: 800 to 830 µM). I and Br concentrations in near-seafloor sediments are usually less strongly affected by organic decomposition, with concentrations as low as seawater; however, at site 55, I and Br are strongly enriched in near-seafloor sediments. This observation suggests vertical fluid migration is active and reaches the seafloor to maintain high I and Br concentrations.

O presente capítulo faz uma introdução geral sobre o tema hidratos de gás naturais, i.e., que ocorrem nos sedimentos, apresentando os tipos existentes, como se formam, os principais gases envolvidos, a sua importância econômica e ambiental (incluindo mudança climática e como geohazards), além de suas ocorrências naturais no mundo. A seguir é apresentado um breve histórico dos estudos e da exploração de hidratos de gás no Brasil, seguido de uma descrição das ocorrências naturais confirmadas no país, no leque do Amazonas (Bacia da Foz do Amazonas) e no Cone de Rio Grande (Bacia de Pelotas). Por fim é apresentada uma breve comparação entre as duas ocorrências e uma discussão sobre o desenvolvimento futuro da exploração de hidratos de gás no país. Palavras-chave: metano, leque do Amazonas, Cone de Rio Grande, recurso energético, mudança climática, geohazard. Abstract This chapter starts with an introduction of the topic of natural gas hydrates, presenting the main existing types, how they form, the main gases involved in their formation, their economic and environmental (including climate change and as geohazards) importance, in addition to their worldwide natural occurrences. Also included is a brief history of the study and exploration of gas hydrates in Brazil, and a description of the two confirmed occurrences in the country: The Amazon fan (Foz do Amazonas basin) and the Rio Grande Cone (Pelotas Basin). The chapter ends with a brief comparison between the two occurrences and a discussion about the future development of the exploration for gas hydrates in the country. Keywords: methane, Amazon fan, Rio Grande Cone, energy resource, climate change, geohazard.

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.

Submarine canyons are a key element of the growth of continental margin depocentres such as the Niger Delta, the progradation of which is commonly influenced by syn-sedimentary processes of gravity-driven tectonism. In such setting, canyons commonly constitute combined stratigraphic-structural traps because of: (i) their clayey infilling and, (ii) the incision of marine reservoirs. Nevertheless, most of the time, their drainage network and/or the external factors leading to their formation are poorly characterized, which is also the case in other settings when the resolution of geophysical dataset is less accurate. Here we use 3D seismic reflection data from the onshore eastern Niger Delta to study the morphology of a buried canyon system in relation to extensional tectonic structures, which are E-W trending growth-faults and roll-over anticlines. We map the Afam Incision Surface (AIS), which cuts up to 1000 m into subjacent Eocene-Miocene deltaic and continental series. In plan form, the AIS is organized in 6 stream orders. The Afam Canyon which has the highest stream order is globally NNW-SSE oriented and is locally controlled by E-W oriented grabens, which give it right-angle changes of direction. It perpendicularly captures 5th and 4th order tributaries, each of which has an asymmetric crosssectional profile controlled by one major growth-fault. In the southern area, the western tributaries all include an upstream part that drains in the opposite direction to the regional slope, leading to bayonet-shaped paths in planform. Whatever the stream order of the tributary considered; its increase is controlled by growth-faults. Therefore, the AIS planform network has the characteristics of a rectangular pattern. Erosion of the AIS as a submarine canyon system is consistent with its more than one-kilometer depth, preserved knickpoints within the thalweg of the main canyon; and the many erosional terraces boarded by arcuate scarps located along the main canyon and its tributaries. The formation of the AIS is correlated to the 50 m Tor2 sea-level fall at 9.5-9.2 my, and may have been initiated by by-pass currents that acted in tandem with retrogressive erosion. The latter greatly contributed to deepening and widening of the canyon and its tributaries whatever the orientation of the local slope. Erosion of the AIS was favored by the high-density of growth-faults. During the final excavation phases, the channelization of longshore drift currents by the canyon and its tributaries probably contributed to the destabilization of their upper margins.

The offshore Tumbes-Guayaquil forearc basin in the accretionary prism of Northern Peru-Southern Ecuador shows evidence of gravity-driven large-scale deformation systems active during the Late Neogene-Quaternary period. Subsurface data and the construction of eight structural cross-sections show that the similar to 8 km-thick Oligocene-Quaternary sedimentary infill is detached seaward and completely decoupled from the underlying inner accretionary prism systems. The Corvina decollement in the Tumbes basin and the Posorja decollement in the Guayaquil basin constitute two thin-skinned gravity tectonic systems associated with kilometer-scale, updip "raft" extensional structures paired with downdip fold-thrust systems (Barracuda and Domito thrust systems). Although many previous studies have described the structural and stratigraphic architecture of the Tumbes-Guayaquil forearc basin, no model explicitly accounts for this anomalous large-scale gravity tectonics. We propose that this gravity tectonic style, more commonly observed in passive continental margins, is primarily controlled by the combination of tectonostratigraphic features, including crustal-scale transtensional deformation related to oblique convergence along the Northern Andean margin, basal decollement slope tilting, strong sediment accumulation, and the presence of overpressured shales.