Search

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. © 2023, The Author(s).

The BRITICE‐CHRONO Project has generated a suite of recently published radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically stimulated luminescence ages from adjacent onshore sites. All published data are integrated here with new geochronological data from Wales in a revised Bayesian analysis that enables reconstruction of ice retreat dynamics across the basin. Patterns and changes in the pace of deglaciation are conditioned more by topographic constraints and internal ice dynamics than by external controls. The data indicate a major but rapid and very short‐lived extensive thin ice advance of the Irish Sea Ice Stream (ISIS) more than 300 km south of St George's Channel to a marine calving margin at the shelf break at 25.5 ka; this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel. The release event between 25 and 26 ka is interpreted to have stimulated fast ice streaming and diverted ice to the west in the northern Irish Sea into the main axis of the marine ISIS away from terrestrial ice terminating in the English Midlands, a process initiating ice stagnation and the formation of an extensive dead ice landscape in the Midlands. © 2021 The Authors Journal of Quaternary Science Published by John Wiley & Sons, Ltd.

The Amazon continental shelf hosted one of the world's largest mixed carbonate-siliciclastic platforms from the late Paleocene onwards - the Amapá carbonates. The platform architecture, however, remains poorly understood and causes and timing of the cessation of carbonate deposition are still controversial. Here we present a stratigraphic analysis of the Neogene succession of the Amapá carbonates, based on a grid of 2D/3D seismic data correlated to revised micropaleontological data from exploration wells. The results provide improved constraints on the age of the transition from predominantly carbonate to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf (NW, Central and SE). Four Neogene evolutionary stages of carbonate deposition could be defined and dated with reference to the new age model: (1) between ca. 24 and 8 Ma a predominantly aggrading mixed carbonate-siliciclastic shelf prevailed across the entire region carbonate production gave way to siliciclastic sedimentation across the Central and SE shelves; (2) between 8 and 5.5 Ma carbonate production continued to dominate the NW shelf, as deposition was able to keep up with base level oscillations; (3) between 5.5 and 3.7 Ma (early Pliocene), sediment supply from the paleo-Amazon River promoted the progressive burial of carbonates on the inner NW shelf, while carbonates production continued on the outer shelf (until 3.7 Ma). Longer-lasting carbonate sedimentation on the NW shelf can be explained by a lesser influx of siliciclastic sediments due to the paleo-geography of the Central shelf, characterized by a 150-km-wide embayment, which directed most terrigenous sediments sourced from the paleo-Amazon River to the continental slope and deep ocean; (4) from 3.7 Ma onwards, when the Central shelf embayment became completely filled, continuous sediment supply to the NW shelf resulted in the final transition from carbonate to siliciclastic-dominated

An integrated geophysical, geological, and geochemical investigation of seabed fluid venting was carried out in upper Bantry Bay, a large marine inlet on the southwest coast of Ireland. The results provide evidence of the seafloor venting of gas rich fluids, resulting in the formation of a pockmark field identified here for the first time. The pockmarks occur in an area where sub-bottom profiles provide evidence of chimney-like features interpreted to record upward gas migration through Quaternary sediments to the seafloor. Three vibrocores up to 6 m long were acquired in water depths of 24–34 m, two from the pockmark field and one from outside. Methane of predominantly biogenic origin was quantified in all three cores by headspace analysis of sediment sub-samples. Well-defined sulfate methane transition zones (SMTZs) were observed in two of the cores, the shallowest (1.25 m below sea floor (mbsf)) inside the pockmark field and the other (3.75 mbsf) outside. It is likely that an SMTZ occurs at the location of the third core, also within the pockmark field, although deeper than the samples obtained during this study. Gas migration towards the seafloor is suggested to involve both diffuse pore fluid migration across wide areas and focused flow through the pockmarks, together driven by methanogenesis of pre-glacial lacustrine sediments preserved in a bedrock basin, and possible gas release from the Owenberg River Fault. Analysis of phospholipid fatty acids (PLFAs) and archaeal isoprenoid hydrocarbons was used to investigate the microbial ecology of these sediments. Anaerobic oxidation of methane (AOM) may play a role in controlling release of CH4 to the water column and atmosphere in this shallow gas setting, potentially mediated by syntrophic sulfate reducing bacteria (SRB) and anaerobic methanotrophic archaea (ANME).

Two-dimensional flexural backstripping and thermal modelling (assuming uniform stretching and cooling) is applied to four interpreted, depth-converted seismic profiles across the Rockall, Faroe–Shetland and Vøring basins, along 1600 km of the Atlantic continental margin of NW Europe. The results reveal a significant discrepancy between the modelled palaeo-depths for the base of the Cenozoic succession and those proven by geological evidence at control points (subaerial conditions or depositional depth ranges in wells). The discrepancy is of Rm-scale, much larger than the possible range of parameter error determined by sensitivity tests (up to 0.5 km). Assuming a Cretaceous rift episode (100 Ma), the discrepancy is at least 1.7 km in the Rockall Basin, up to 2.1 km in the Faroe–Shetland Basin and at least 1 km in the Vøring Basin (which also contains evidence of kilometre-scale uplift of the inner margin). Assuming (unproven) a second rift in the early Cenozoic (60 Ma), the discrepancy remains of kilometre-scale in the Rockall and Faroe–Shetland basins. The restorations also provide evidence of uplift, both above compressive structures and across the modelled profiles as seaward rotations of palaeo-bathymetric records. The palaeo-bathymetric discrepancy corresponds to an anomaly in subsidence that is the cumulative product of all the tectonic episodes that have affected the NW European margin, and may incorporate both permanent effects of the last episode of lithospheric extension and transient responses to the interaction of the margin with mantle convective flow. Any explanation must accommodate both the large magnitude of anomalous subsidence along the margin and evidence of its episodic character.

Submarine hydrocarbon seeps are geologically driven “hotspots” of increased biological activity on the seabed. As part of the HERMES project, several sites of natural hydrocarbon seepage in the European seas were investigated in detail, including mud volcanoes and pockmarks, in study areas extending from the Nordic margin, to the Gulf of Cádiz, to the Mediterranean and Black seas. High-resolution seabed maps and the main properties of key seep sites are presented here. Individual seeps show ecosystem zonation related to the strength of the methane flux and distinct biogeochemical processes in surface sediments. A feature common to many seeps is the formation of authigenic carbonate constructions. These constructions exhibit various morphologies ranging from large pavements and fragmented slabs to chimneys and mushroom-shaped mounds, and they form hard substrates colonized by fixed fauna. Gas hydrate dissociation could contribute to sustain seep chemosynthetic communities over several thousand years following large gas-release events.

The sediment-undersupplied eastern margin of the Rockall Trough, west of Ireland, is incised by numerous canyons and slope failure features. Swath multibeam bathymetry has been integrated with 2D seismic profiles to constrain the Neogene evolution of the slope and its canyons. The morphology varies along the margin, with canyon heads located at mid-slope depths in the south but extending onto the shelf in the north. West of Porcupine Bank, slope gullies connect with a distributative channel system on the trough floor, while north of Porcupine Bank the basin floor is flat and featureless. Draped fault-blocks and deep structures exerted an important influence on slope gradients, canyon extent and geometry. A ‘bottom driven’ upslope–retrogressive slope failure mechanism is inferred for canyon formation. They were initiated by failure localisation following widespread slope rotation and instability linked to differential subsidence that produced a latest Eocene–early Oligocene (C30) regional unconformity. In the NE Rockall, where the greatest density of canyons occurs, a large mass failure wedge directly overlies the C30 surface and the seabed canyons have incised the upper part of the wedge. Axial profile data indicate that canyons in the NE Rockall Trough formed in Mid-Cenozoic times but were locally reutilised as sediment conduits during Plio-Pleistocene slope progradation.

The lower Neogene stratigraphy of the NW European Atlantic margin, from the Vøring to the Porcupine basins, is interpreted to record a discrete phase of compressional tectonism that spanned at least 8 Ma from the earliest to the early mid-Miocene. This compressional tectonism may be coeval with a local reorganisation of the NE Atlantic plate system with the transfer of the Jan Mayen micro-plate from Greenland to Europe. The compressional tectonics has resulted in a number of stratigraphic sequences of complex character bounded by regional base Neogene and intra-Miocene unconformities. These are traceable across a range of depths and record distortion of the basin margins and changes in deep-water circulation patterns. This episode of compressional tectonics has also resulted in the creation of a number of anticlinal domes along the Norwegian, Faroese and UK Atlantic margins. The stratigraphic and structural evidence are interpreted to record two stages in the development of the margin: the first being characterised by a prolonged period of regional flexure in response to the build-up of compressive stresses; the second stage is the development of anticlinal structures that led to a rapid release of stress. In the Wyville–Thomson–Faroes region, compressional deformation influenced the creation of the present-day deep-water conduit of the Faroe Bank Channel, which is interpreted to be an early Neogene syncline. Together, the Faroe Bank and Faroe–Shetland channels represent the deepest water passageway across the Greenland–Scotland Ridge. The early Neogene development of this conduit is considered to mark the onset of deep-water exchange across this oceanic gateway.

Sequence stratigraphic analysis is now carried out routinely on Mesozoic–Cenozoic sedimentary basins with a view to understanding their post-rift stratal geometries and predicting facies architecture. The conventional method embodies two concepts; the physical division of strata into unconformity-bounded units and their genetic interpretation on the assumption that hierarchical changes in sea-level represent the primary control on sedimentary architecture. The genetic model is reflected in a complex descriptive terminology and derives from the development of the method in shallow-water basins, where the results of analyses typically accord with geological data constraints. Applied to deep-water underfilled basins, however, the sea-level assumption yields results that are unsatisfactory and in many cases misleading, reflecting the influence of additional controls, including tectonic movements, deep-sea currents and climate (e.g. glaciation) on stratal geometries and facies architecture. This is illustrated by comparative analyses of examples from the deep-water Cenozoic basins of the NW European Atlantic passive margin, which contain a set of seismic megasequences, bounded by unconformable to conformable surfaces, correlative from shelf to slope to basinal settings. The deep-water successions contain regional, locally diachronous, submarine unconformities that were generated in response to changes in deep-water current circulation and slope processes, rather than subaerial erosion. These bound aggradational contourite drifts that include upslope accretion against basin margins. The deep-water successions interfinger at two levels with seaward-prograding shelf-slope wedges that reflect differential tectonic movements, involving coeval uplift and subsidence. The youngest wedges formed in the last 4 Ma and record seaward progradation of the margin by up to 100 km, overprinting high-amplitude variations in sea-level and coeval with changes in deep-ocean currents. Sedimen

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have driven major changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganisation (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (<1°) of the margin from the early Pliocene, in which km-scale uplift and erosion was accompanied by increased offshore subsidence, resulting in a major seaward progradation of the shelf–slope wedge as well as deep-marine erosion during a reorganisation of bottom current patterns. The large amplitude of tilting cannot be accounted for by intra-plate stress variations, but is consistent with a dynamic topographic response to upper mantle convection, in particular edge-driven flow beneath the continental margin. Sedimentary a

The present paper provides an overview of glacial related seabed features and sedimentary sequences found along the formerly glaciated NW European margin and compare it with those found on contemporary glaciated margins from both the Southern and Northern Hemispheres. A brief review of the seabed physiography and strata architecture of the margins under consideration is followed by comparison of the most relevant similarities and differences. Comparison of the present-day bathymetric setting of both former and contemporary glaciated margins reveals no clear link to the effect of neither ice sheet or sediment load. Three different types of glacially eroded shelf transverse troughs have been identified, while marginal troughs seem connected to similar geological settings everywhere. Beyond the shelf edge interaction between downslope and alongslope processes has occurred resulting, amongst others, in the formation of large sedimentary mounds on the rise. More frequent large-scale mass wasting occurs on the former glaciated NW European margin than the Greenland and Antarctic margins in the latest Neogene to recent times. A two-stage evolution of the shelf prograding wedges is observed on all margins under consideration, which may reflect a general development of an ice cover from an initial phase of non- to restricted glaciation, evolving to a mature stage of expansive glaciation.

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to r