espanolLa plataforma continental del Algarve en el margen septentrio-nal del Golfo de Cadiz ha sido investigada mediante el analisis de una red de perfiles sismicos de alta resolucion. Se presentan las prin-cipales estructuras que afectan a la cobertera sedimentaria super-ficial. Una interpretacion sismoestratigrafica y correlaciones a nivel regional han permitido diferenciar dos intervalos separados por una discontinuidad principal, anterior y posterior al Pleistoceno Medio (PM). Los depositos del Plioceno y Cuaternario estan deformados por estructuras de deformacion tales como cabalgamientos ENE-OSO a NE-SO, pliegues N-S a NO-SE de escala kilometrica y fallas subverticales NNE-SSO a NNO-SSE, que indican actividad neotec-tonica. Las estructuras anteriores al PM se generan en relacion con el regimen transpresivo (NO-SE) dominante a fInales del Cenozoico en esta region y la reactivacion de las estructuras diapiricas mesozoicas observadas en el margen. En cambio, las estructuras posteriores al PM producen una ligera deformacion de la cobertera sedimentaria y actividad durante el ultimo maximo glaciar, lo que concuerda con un moderado levantamiento tectonico observado en la zona conti-nental del margen portugues. EnglishThe eastern Algarve continental shelf in the northern margin of the Gulf of Cadiz has been investigated by analysing a set of high-re-solution seismic reflection profiles. We examine the major structural elements affecting the upper sedimentary units. A seismo-strati-graphic interpretation and regional correlation allowed the identi-fication of a major discontinuity that separates two main intervals, pre-Middle Pleistocene (MP) and post-MP. In addition, a number of deformation features such as ENE-WSW to NE-SW thrusts, kilome-tric-scale N-S to NW-SE folds and NNE-SSW to NNW-SSE sub-ver-tical normal faults were identified affecting the Pliocene-Quaternary sedimentary record, indicating neotectonic activity. The structures pre-dating MP are in agreement with a dominant transpressive re-gime along the continental shelf, under the Late Cenozoic NW-SE oblique convergence and related to the reactivation of Mesozoic dia-piric structures. The structures post-dating MP induce a lower defor-mation of the sedimentary cover but were still active during the last glacial maximum, in agreement with the low uplift recognized in the Portuguese mainland

During the last 500 000 years, ice sheets occupied parts of the North Sea during three major glaciations. The existence of these ice sheets was accompanied by a large fall in sea level, causing the Southern North Sea to emerge and become isolated from the Atlantic. In this area a complex drainage system was created by river water of the West-European rivers (e.g., Thames, Rhine, Meuse and Scheldt) and glacial meltwater. Furthermore, most offshore studies support the idea of the formation of large proglacial lakes in front of these ice sheets, which may have caused high-magnitude outburst floods at the end of each glacial period. The existence of such a proglacial lake is used in the argument that glacial outburst floods during the Elsterian (500-450 ka) created erosional features still preserved nowadays in the Dover Strait.A remnant of this large, complex fluvial and glacial drainage system is the (North) Axial Channel, a prominent geomorphological feature seen on the present-day sea floor of the Southern North Sea. Its formation and evolution, however, are still uncertain. Previous studies state that the Axial Channel forms the northern extension of the Lobourg Channel, located in the Strait of Dover, which was formed during Middle Miocene times. Further erosion is assumed to have occurred during the Pliocene and Pleistocene, as sediments within the Murray Pit (located in the Axial Channel, about 100 km northeast of the Lobourg Channel) are assumed to be Early Pliocene, and no Quaternary infilled sediments have been identified. A series of NE-SW oriented scarps are identifiable from bathymetric and seismic reflection data and have been attributed to different Pleistocene incisional events. However, currently only a relative chronology of potential events has been established, with large uncertainties. Understanding the paleogeographic changes that affected the region also increases the knowledge on how early humans may have settled in and/or migrated through the region. In the framework of the WALDO project (“Where are All the (proglacial) Lake seDiments in the NOrth Sea Basin?”), a survey has been conducted in October 2023 during which high-resolution geophysical data (multibeam bathymetry and backscatter, acoustic and seismic data) combined with ground-truth data (vibrocores) have been acquired. One of the reflection-seismic grids was conducted ~40 km east of the East of England coast, over the western edge of the North Axial Channel, where also four sediment cores were taken. Here, we present the first interpretation of these new data, which allow us to evaluate, update and improve the relative chronology of the formation of the (North) Axial Channel.

Natural gas hydrates (NGHs) have recently been recognized as a promising source of relatively clean alternative energy and a significant factor in triggering marine geohazards. This paper presents a numerical method for calculating the transient excess pore pressure associated with hydrate dissociation in submarine sediments with THC (Thermo-Hydro-Chemical) coupling. Then, the dynamic stability of submarine slopes experiencing gas hydrate dissociation is evaluated based on limit equilibrium analysis considering the real evolution of excess pore pressure. Finally, this work is applied to investigate the dynamic responses of typical hydrate slopes in the Shenhu Sea area, South China Sea (SCS), under two different timescales: 1) Case I: gradual temperature increases at the seafloor due to climate warming and 2) Case II: sharp temperature increases in the interior of the hydrate deposit due to hydrate extraction. In Case Ⅰ, the timescale of hydrate dissociation is millennial. Due to the long-term temperature rise, the hydrate will dissociate slowly, which allows the generated free gas to migrate upwards and gradually accumulate at the transition zone between a porous layer and an overlying low-permeability layer. Eventually, the slow accumulation of free gas may lead to disc-shaped failure of the hydrate-bearing slope. In contrast, in Case Ⅱ, the temperature rises sharply over a short period of time, which leads to the drastic dissociation of the hydrate. The timescale of hydrate dissociation is decadal. As a result, the excess pore pressure accumulates rapidly. Under the influence of excess pore pressure, the sediment will deform dramatically, which may cause a penetration failure of the hydrate-bearing slope. These findings are relevant to the long-term (millennial) safety of human beings and short-term (decadal) utilization of energy resources.

New high-resolution seismic data (Sparker) and very-high-resolution parametric echosounder (PES) data acquired in an area of the southern North Sea (the Flemish Bight) reveal its Quaternary seismic stratigraphy in unprecedented detail. The identified seismo-stratigraphic units and geomorphological features have been examined with the view to better understand the Quaternary evolution of the southern North Sea. Seven acoustic units were recognised, including Lower Pleistocene deltaic sediments, Eemian to lower Weichselian shallow marine to coastal (lagoonal) clay-silt-sands, and Holocene coastal peat layers overlain by intertidal and marine sediments. Four erosional events were identified, two of which can be traced as regionally occurring surfaces, and two occurring as localised incisions. Mapping of geomorphological features revealed potential Elsterian moraines in the UK sector, an Elsterian ice-pushed ridge in the Dutch sector and possible permafrost-related structures (probably dating to MIS3). Seven newly dated peat samples, acquired near a tidal sand ridge known as the Brown Bank from depths between 31 m and 34 m below sea level and dating to between 9.5 and 10.9 cal ka BP, indicate that this area was terrestrial during the early Holocene. The results form the basis to further improve the regional Quaternary stratigraphic framework of the area, to better understand the region’s (de)glacial history, to enhance sea-level reconstructions and to examine the area’s geographical importance for human occupation during Prehistory.

Seismic hazards in subduction settings typically arise from megathrust, intraslab and crustal earthquake sources. Despite the frequent occurrence of intraslab earthquakes in subduction zones and their potential threat to communities, their long-term recurrence behaviour is barely studied. Sedimentary sequences in lakes may register ground shaking from different seismic sources. This study investigates two long sediment cores (13 m and 16 m) from Skilak Lake, a proglacial lake in south-central Alaska, to evaluate whether different seismic sources leave a distinct imprint. The sedimentary record shows a continuously varved sediment sequence, occasionally interrupted by turbidites, slump deposits and tephra beds. Turbidites and slump deposits were objectively identified using a statistical outlier analysis on varve thickness. The earthquake origin of these deposits was ascertained by resemblance with deposits induced by instrumentally recorded earthquakes (for example, 1964 ce M-w 9.2 megathrust and 1954 ce M-w 6.4 intraslab earthquakes) and correlation with multiple coeval landslide deposits on sub-bottom profiles. The Skilak Lake record chronicles 19 earthquakes with moderate to very high confidence level in the past 1350 years. The sedimentary evidence of instrumentally-recorded intraslab and megathrust earthquakes within the past 70 years demonstrates that not only megathrust earthquakes, but also past intraslab events are recorded. Although reported seismic intensities at Skilak Lake are comparable for the 1964 ce megathrust and the 1954 ce intraslab earthquakes, the long duration and low frequency content of seismic ground motion during megathrust earthquakes facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites. In contrast, the shorter duration and higher frequency source spectrum of intraslab earthquakes may only induce surficial slope remobilization and the generation of thinner turbidites. This study demonstrates that the sedimentary record of Skilak Lake has the potential to decipher multiple seismic sources, which opens possibilities for a comprehensive seismic hazard analysis for south-central Alaska.

A series of polygon networks has been discovered on the most recent LiDAR (Light Detection and Ranging) DEM (Digital Elevation Model) of Flanders (Belgium) available in a resolution of 1 m(2). They are located in the sandy Campine area (northern Belgium) and resemble thermal contraction crack polygon networks from presentday permafrost regions. Different network types were observed, ranging from orthogonal to hexagonal and various combinations of these. The inter-polygon troughs are typically several decimeters deep and up to several meters wide. The average polygon size is ca. 3000 m(2), which is equivalent to a diameter of ca. 60 m if the polygon shape is approximated with a perfect circle, or a side of ca. 55 m length if it were to be approximated by a perfect square. The average size is (much) larger than any of the studied present-day analogues, and also larger than fossil networks in the western part of Flanders, Poland and France. In contrast to the Campine polygons presented here, the fossil analogues in these countries were detected using satellite imagery and orthophotos, which may partially explain the observed size differences. The morphometric analysis of the Campine networks shows relationships between polygon type and local geomorphological position as orthogonal networks seem to have a preference to develop near shallow valley slopes. In addition, GPR (Ground Penetrating Radar) radargrams were acquired across polygon boundaries to investigate subsoil disturbances related to the former position of ice wedges or sand wedges. However, the evidence is not unequivocal due to the low dielectric contrast between the host and wedge material. It is not clear yet whether smaller 2nd and 3rd order cracks did develop but without leaving a topographical imprint. The observed polygon networks in the Campine area are interpreted as first order networks that developed during a time span of several thousands of years, up to 10 kyr at most, in a former Late Weichselian permafrost climate. (C) 2020 Elsevier B.V. All rights reserved.

On continental margins, the upper slope to shelf break environment forms a critical region where sediment supply, hydrographic activity and gravitational processes determine how and when sediments are partitioned between the shallow- and deep-marine realm. On the SW Iberian margin, relatively few studies have addressed the dynamics of this region, although it holds key information regarding the link between the sedimentary evolution of the continental shelf and the contourite depositional system on the middle slope. This work therefore presents a high-resolution analysis of the morphological and stratigraphic expressions of late Quaternary (dominantly last glacial and present-day interglacial) sedimentary processes on the upper slope and shelf margin sector between 7° and 7°30’ W.The integration of seismic, bathymetric and hydrographic data reveals the presence of alongslope processes and products (a bottom current-related plastered drift, moat, erosional surface and terrace, an internal wave-/tide-controlled sediment wave field), downslope (gravitational) processes and products (an upper slope – shelf margin valley system, slumps, debrites, gullies), neotectonic elements (diapirs) and fluid flow features (pockmarks, bright spots). The spatial distribution of these features indicates that the study area becomes increasingly alongslope-dominated towards the W, and oppositely, more downslope-dominated towards the E, because sediment supply to the latter area is enhanced under the dominant eastward dispersal of fluvially supplied sediments on the shelf. In addition, glacial-interglacial variations in the amount of sediments supplied to the shelf edge and the intensity of oceanographic processes in the study area also generate a distinct temporal variability, with glacial and interglacial intervals respectively recording principally downslope- and alongslope-controlled morphological elements. Finally, regardless of these overall spatial and temporal patterns, diapirism and fluid flow are inferred to locally destabilize sediments and induce small-scale mass wasting in the study area. These findings are not only relevant to the northern Gulf of Cadiz, but also to the general understanding of sedimentary dynamics and controls in mixed downslope- and alongslope-controlled upper slope to shelf margin settings worldwide.

Over the past decades, the northern Gulf of Cadiz has been the focus of a wide range of late Quaternary seismic and sequence stratigraphic studies, either addressing the slope contourite depositional system (CDS), or the development of the continental shelf. Yet, high-resolution seismic data bridging between these domains and age information have remained sparse. This study, based on new high-resolution reflection seismic profiles calibrated to IODP Expedition 339 sites U1386/U1387, now presents an updated stratigraphic framework, that integrates (for the first time) the late Quaternary records of the northern Gulf of Cadiz middle slope to shelf off the Guadiana River. Seismic stratigraphic analysis of the stacking, depocenter distribution, stratal architecture and facies of the seismic (sub-)units reveals the influence of similar to 100 kyr sea-level variations paced by Milankovitch (eccentricity) cycles, tectonics (manifesting as two pulses of uplift and margin progradation), sediment supply and bottom current activity. This work furthermore contributes to the application and understanding of high-resolution, late Quaternary sequence stratigraphy. Firstly, the proposed sequence stratigraphic interpretation shows that adaptations to the basic models are required to integrate the shelf and slope record, and to account for the presence of a significant alongslope (bottom current-controlled) component. Secondly, the results confirm that the sequences are dominantly composed of regressive deposits, whereas the preservation of transgressive to highstand deposits is more irregular. Significantly, the common assumption that successive major glacial lowstands are consistently recorded as well-marked, shelf-wide erosional unconformities, is demonstrated to be occasionally invalid, as tectonics can obliterate this one-to-one relationship.

3rd Deep-Water Circulation Conference (3DWC), Multidisciplinary Perspective, 14-16 September 2017, Wuhan - Wu Han

Gas hydrate reservoirs store large quantities of gas in sediments on continental margins, in deep lakes, and in continental and relic sub-shelf permafrost. The gas hydrate structure is only stable at sufficiently low temperature and high pressure, and may therefore collapse under changing climatic conditions. If a temperature rise or pressure drop (e.g. through falling sea level) is effective enough to dissociate hydrate deposits, methane (the most common gas component in hydrates and a potent greenhouse gas) is released from the hydrate structure and may eventually enter into the atmosphere. This may generate a positive feedback effect, as resulting enhanced greenhouse gas levels would additionally warm the atmosphere and hence maintain or reinforce hydrate dissociation. The significance of this mechanism has been debated over the past decades, often within the framework of geologically rapid Quaternary climatic oscillations and present-day climate warming. An extensive set of studies has addressed the climate-sensitivity of gas hydrate reservoirs in various study areas and geological settings, and by means of various approaches. No real consensus has yet been reached on the matter. In this study, we seek to evaluate the sensitivity of gas hydrate reservoirs to changes in global climate from a more general perspective, by firstly reviewing the available literature, and secondly developing a new numerical model to quantify gas hydrate destabilization under changing environmental conditions. Qualities of the model include the wide applicability to both marine and permafrost-related hydrate reservoirs and the integrative approach, combining existing hydrate formation models with a dissocation model that accounts for the consumption of latent heat during hydrate dissociation. To determine which settings are most vulnerable, and to acquire insight into the extent, fashion and rates of hydrate dissociation, we apply the model to four distinct types of hydrate reservoirs across a hypothetic high-latitude continental margin under two specific cases of climate change: the last deglaciation following the Last Glacial Maximum and present-day climate warming. The simulations indicate that hydrates on the upper continental slope and in association with thin, sub-shelf permafrost are most sensitive to the imposed climatic variations, whereas deepwater and onshore permafrost-related reservoirs react in a more stable way. However, the deep (i.e. at several tens to hundreds of meters subsurface depth) stratigraphic-type hydrates considered in this study constitute by far the largest fraction of the global gas hydrate volume, but dissociate on slow timescales of thousands to hundreds of thousands of years, even in the most sensitive environments. In contrast, shallow (i.e. at, or a few meters below the surface or seafloor) structural-type hydrates are able to respond to climatic variations on sub-millennial timescales, but the volumes of gas they may release are probably insignificant to the global carbon cycle and climate. Quaternary and present-day climate change do affect the stability of gas hydrate reservoirs, but at long timescales where hydrate volumes are large, and on short timescales where hydrate volumes are small. Consequently, gas hydrates dissociate to an extent that is too small or at a pace that is too slow to create a strong positive feedback effect.While the release of methane from the disintegration of gas hydrates is observed on different margins today, it is not likely to have played a leading role in Quaternary climatic variations or to become a significant process in the coming centuries as a result of present-day rising temperatures.