Sedimentary and structural patterns on the Iberian continental margin: an alternative view of continental margin sedimentation

A long-range side-scan sonar (GLORIA) survey of the entire West Iberian slope and rise has provided the first overview of the interrelationship between structure and sedimentation patterns on a continental margin. The results emphasize the importance of slope-following contour currents as a depositional mechanism in fashioning this continental rise. Terrigenous sediments transported down-canyon by-pass the rise which does not consist of a series of coalescing fans. The sedimentation patterns identified on the sonographs can be interpreted in terms of facies models and caution must be exercised against over-emphasis of downslope processes in models for the construction of lower slopes and rises.     

Magnetic anomalies over the Central Levant continental margin

The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast.Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea.Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it.We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.     

Occurrence of pockmarks on the Ortegal Spur continental margin,Northwestern Iberian Peninsula

A total of 445 pockmarks were observed on the upper continental slope of the northwest corner of the Iberian Peninsula (the Ortegal Spur area) by swath bathymetric and ultrahigh resolution seismic data. The pockmarks are U-, V- and W-shaped and have terraces or indentations in cross-section, and are dish-shaped (circular to oval) in plan view. They occur on the surface of the seabed and buried within the Plio-Quaternary and Neogene sediments. Four types of pockmarks were identified and mapped on the basis of their plan-view and cross-section morphology: regular, irregular, asymmetric and composite. The concentration of pockmarks is attributed to seepage of fluids migrating up-dip from deeper parts of the sedimentary basin. A linear high-density concentration with a NNW to N, NE and ESE trend of pockmarks is observed above inferred basement faults that do not affect the Quaternary succession. These pockmarks are thus caused by seepage of thermogenic gas and/or other pore fluids from deeper Late Cretaceous units, and their distribution may help to improve our understanding of the fluid system and migration regime in this part of the Galicia continental margin.     

Spatial variability of nitrogen isotope ratios of particulate material from Northwest Atlantic continental shelf waters

Human encroachment on the coastal zone has led to concern about the impact of anthropogenic nitrogen (N) on estuarine and continental shelf waters. Western North Atlantic watershed budgets suggest that the export of human-derived N from estuaries to shelf waters off the east coast of the US may be significant; however, models based on water inputs and estimates of upwelling of deepwater nutrients to surface waters of the mid-Atlantic bight indicate that estuarine N may be a relatively minor component of the overall shelf N budget. Stable N isotope ratios could provide a means to assess the relative input of anthropogenic N to shelf waters, particularly since dissolved N from human sources has elevated δ¹⁵N values (range: 7–30‰). We collected particulate material from surface shelf waters off the US east coast from 2000 to 2005 at near-shore sample sites proximal to the mouth of six estuaries and corresponding sites farther offshore. Near-shore (mean 33.7 km from estuary mouth) δ¹⁵N values ranged from 5.5 to 7.7‰ Offshore values (mean 92.4 km from estuary mouth) were consistently lower than near-shore sites (average 4.7 ± 1.0‰ versus 6.8 ± 1.1‰), suggesting different N sources to near and offshore stations. Near-shore regions are often more productive, as mean monthly chlorophyll-a concentrations from the sea-viewing wide field-of-view sensor (SeaWiFS) were significantly higher at near-shore sites near the mouth of three of the six estuaries. A mass balance using a concentration-dependent mixing model with chlorophyll-a concentrations as a surrogate for dissolved inorganic nitrogen can account for all of the nitrogen at near-shore sites south of Cape Cod with estuarine nitrogen estimated to contribute 45–85% of the nitrogen to the near-shore surface particulate material. Our results support the hypothesis that estuarine nitrogen is influencing continental shelf ecosystems, and also provide preliminary evidence of the spatial extent of its influence on shelf waters in the mid-Atlantic bight.     

Tectonic development of the Mid-Norway continental margin

The area reviewed covers the Mid-Norway continental margin between latitudes 62°N and 68°N. Main structural elements, as defined at the base Cretaceous level, are the Tröndelag Platform, underlying the inner shelf, the Möre and Vöring Basins, located beneath the outer shelf and slope, and the Möre Platform and the Outer Vöring Plateau, forming a base of slope trend of highs. Sediments contained in the Mid-Norway Basin range in age from Late Palaeozoic to Cenozoic. The basement was consolidated during the Caledonian orogenic cycle. Devonian and Early Carboniferous wrench movements along the axis of the Arctic-North Atlantic Caledonides are thought to have preceded the Namurian onset of crustal extension. Rifting processes were intermittently active for some 270 My until crustal separation between Greenland and Fennoscandia was achieved during the Early Eocene. During the evolution of the Norwegian-Greenland Sea rift system a stepwise concentration of tectonic activities to its axial zone (the area of subsequent continental separation) is observed. During the Late Palaeozoic to Mid-Jurassic a broad zone was affected by tensional faulting. During the Late Jurassic and Cretaceous the Tröndelag Platform was little affected by faulting whilst major rift systems in the Möre and Vöring Basins subsided rapidly and their shoulders became concomitantly upwarped. During the latest Cretaceous and Early Palaeogene terminal rifting phase only the western Möre and Vöring Basins were affected by intrusive and extrusive igneous activity. Following the Early Eocene crustal separation and the onset of sea floor spreading in the Norwegian-Greenland Sea, the Vöring segment of the Mid-Norway marginal basin subsided less rapidly than the Möre segment. During the Early and Mid Tertiary, minor compressional deformations affected the Vöring Basin and to a lesser degree the Möre Basin. Tensional forces dominated the Late Palaeozoic to Early Cenozoic evolution of the Mid-Norway Basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements. The lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced. Massive dyke invasion of the thinned crust preceded its rupture.     

Probable gas hydrate/free gas model over western continental margin of India

Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.     

Regional gravity and magnetic studies over the continental margin of the central west coast of India

Gravity studies over the continental margin of the central west coast of India show a sediment thickness of 2–3 km on the shelf associated with deeper horst and graben structures, of 6 km in the shelf margin basin, and about 1 km in the deep sea. The upward trend in free-air gravity anomaly toward the deep sea region is interpreted as crustal thinning. Model studies indicate a 25-km-thick crust in the shelf region and a minimum of 18 km in the more offshore region. An abrupt magnetic signature change suggests differential basement depths in the shelf region. Major faulting in the region is confirmed in water depths of approximately 100–200 m.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.     

Geological interpretation of combined SEABEAM,GLORIA and seismic data from Porto and Vigo Seamounts,Iberian continental margin

Long-range sidescan sonar (GLORIA) data over Porto and Vigo Seamounts collected in 1978 has been re-interpreted in the light of SEABEAM bathymetric surveys conducted in 1982. The application of quantitative bathymetric information enables the interpreter to allow for artefacts inherent in the GLORIA data and to separate topography-related primary backscattering variations on the sonographs from more subtle changes that result from textural, slope and outcrop effects. The distinctions are made easier when slant-range corrected GLORIA data are available.Use of the combined survey data to precisely locate seismic profiling tracks and to identify likely areas of outcrop has allowed refined geological maps of the seamounts to be drawn and regional fault trends detected. The overall outline of the seamounts appears strongly fault-controlled.Porto and Vigo Seamounts are made up of the same geological formations and have had a similar structural history since their uplift as continental fault blocks in the Late Cretaceous to Middle Eocene period. Ravines that dissect the presumably lithified scarps bounding the seamounts may be relict features but still appear to control sediment input to gulley and channel systems in the surrounding topography. Sedimentary ridges associated with the seamounts represent anomalously thick sequences of post-Eocene material and probably result from interaction of downslope sedimentary processes and contour-following boundary currents.Contribution No. 274 from the Groupe d'Etude de la Marge Continentale (ERA 605).     

Macrobenthic community structure over the continental margin of Crete (South Aegean Sea, NE Mediterranean)

Macrobenthic faunal composition, abundance, biomass and diversity together with a suite of sedimentary environmental parameters were investigated on a seasonal basis in order to determine factors regulating faunal distribution over the oligotrophic continental margin of the island of Crete (South Aegean Sea, North Eastern Mediterranean). Macrofaunal species composition was similar to that of the western Mediterranean and the neighboring Atlantic having several common dominant species. Mean benthic biomass, abundance and diversity decreased with depth, with a major transition zone occurring at 540 m, beyond which values declined sharply. At comparable depths biomass and abundance values were considerably lower to those found in the Atlantic, high-lighting the extreme oligotrophy of the area. The continental margin of Crete was characterised by a high diversity upper continental shelf environment (dominated by surface deposit feeding polychaetes) and a very low diversity slope and deep-basin environment (dominated by carnivorous and filter feeding polychaetes). Classification and ordination analyses revealed the existence of four principle clusters divided by a faunal boundary between 200 and 540 m, as well as beyond 940 m depth. Significant correlations between macrofauna and sediment parameters led to the conclusion that besides depth, food availability (as manifested by the concentration of chloroplastic pigments) is the principle regulating factor in the system. Such being the case, the prevailing hydrographic features that structure the pelagic food web and are directly responsible for the propagation of organic matter to the benthos also affect its community structure.     

Foraminifer communities of the atlantic continental margin of Europe

Based on an analysis of the structures of the populations of benthic foraminifers at 639 stations located on the Atlantic continental margin of Europe, 33 communities were distinguished and mapped with respect to the abundance of the dominating species. The habitats of the communities are formed under the influence of environmental factors, which depend on the latitudinal and bathymetric zonations of the basin, the currents, the near-bottom upwellings, coastal water runoff, and topography of the boundaries between water masses. The communities recognized are confined to certain water masses, which helps one to trace the boundaries between the water masses and the regions of near-bottom upwellings.     

The bathymetry of the North Victoria Land continental margin

Abstract

The very unique continental margin of North Victoria Land, Antarctica, is characterized by complex bathymetry, reflecting control by glacial, tectonic, and marine processes. The abnormally shallow shelf can be divided into a deep, rugged, glacially dominated inner shelf and a smoother, shallower outer shelf, which is dominated by marine and glacial marine processes. Deep u‐shaped glacial troughs incise the shelf, while relict v‐shaped canyons incise the upper slope. Trending northwest‐southeast along the eastern edge of the area lies a rugged chain of seamounts representing the southern extension of the Balleny Fracture Zone. The continental slope is dominated by strong contour currents and gravity processes.     

Sediment slides on the northwestern continental margin of Africa

A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km² were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.     

Pleistocene glacial history of the NW European continental margin

In this paper new and previously published data on the Pleistocene glacial impact on the NW European margin from Ireland to Svalbard (between c. 48°N–80°N) are compiled. The morphology of the glaciated part of the European margin strongly reflects repeated occurrence of fast-moving ice streams, creating numerous glacial troughs/channels that are separated by shallow bank areas. End-moraines have been identified at several locations on the shelf, suggesting shelf-edge glaciation along the major part of the margin during the Last Glacial Maximum. Deposition of stacked units of glacigenic debris flows on the continental slope form fans at a number of locations from 55°N and northwards, whereas the margin to the south of this is characterised by the presence of submarine canyons. Glaciation curves, based primarily on information from the glacial fed fan systems, that depict the Pleistocene trends in extent of glaciations along the margin have been compiled. These curves suggest that extensive shelf glaciations started around Svalbard at 1.6–1.3 Ma, while repeated periods of shelf-edge glaciations on the UK margin started with MIS 12 (c. 0.45 Ma). The available evidence for MIS 2 suggest that shelf-edge glaciation for the whole margin was reached between c. 28 and 22 ¹⁴C ka BP and maximum positions after this were more limited in some regions (North Sea and Lofoten). The last glacial advance on the margin has been dated to 15–13.5 ¹⁴C ka BP, and by c. 13 ¹⁴C ka BP the shelf areas were completely deglaciated. The Younger Dryas (Loch Lomond) advance reached the coastal areas in only a few regions.     

Gravity results from the continental margin of south-western Africa

Gravity, magnetic and bathymetric data were collected over the continental margin of south-western Africa by H.M.S. Hecla in 1966. A study of the gravity measurements shows that the positive free-air anomalies of the continental edge effect are unusually large, and in excess of those calculated for an isostatic model of the Earth's crust. Taking into account the available seismic and magnetic evidence, a two-dimensional crustal model has been designed incorporating a body of relatively high density in the upper crust to account for the unusually large values.     

南极大陆边缘研究进展

本文综合国外有关资料,介绍了70年代以来南极大陆边缘地球物理调查、尤其是多道反射地震调查,深海钻探,地震地层研究,以及面积大、赋有油气远景的两个海区——威德尔海和罗斯海研究的进展情况。     

Along-slope oceanographic processes and sedimentary products around the Iberian margin

This contribution to this special volume represents the first attempt to comprehensively describe regional contourite (along-slope) processes and their sedimentary impacts around the Iberian margin, combining numerically simulated bottom currents with existing knowledge of contourite depositional and erosional features. The circulation of water masses is correlated with major contourite depositional systems (CDSs), and potential areas where new CDSs could be found are identified. Water-mass circulation leads to the development of along-slope currents which, in turn, generate contourite features comprising individual contourite drifts and erosional elements forming extensive, complex CDSs of considerable thickness in various geological settings. The regionally simulated bottom-current velocities reveal the strong impact of these water masses on the seafloor, especially in two principal areas: (1) the continental slopes of the Alboran Sea and the Atlantic Iberian margins, and (2) the abyssal plains in the Western Mediterranean and eastern Atlantic. Contourite processes at this scale are associated mainly with the Western Mediterranean Deep Water and the Levantine Intermediate Water in the Alboran Sea, and with both the Mediterranean Outflow Water and the Lower Deep Water in the Atlantic. Deep gateways are essential in controlling water-mass exchange between the abyssal plains, and thereby bottom-current velocities and pathways. Seamounts represent important obstacles for water-mass circulation, and high bottom-current velocities are predicted around their flanks, too. Based on these findings and those of a selected literature review, including less easily accessible ??grey literature?? such as theses and internal reports, it is clear that the role of bottom currents in shaping continental margins and abyssal plains has to date been generally underestimated, and that many may harbour contourite systems which still remain unexplored today. CDSs incorporate valuable sedimentary records of Iberian margin geological evolution, and further study seems promising in terms of not only stratigraphic, sedimentological, palaeoceanographic and palaeoclimatological research but also possible deep marine geohabitats and/or mineral and energy resources.     

Recent sediment transport and accumulation on the NW Iberian margin

Five transects across the NW Iberian margin were studied in the framework of the EU-funded Ocean Margin EXchange II (OMEX II) project, to determine and establish recent sediment and organic carbon transport and accumulation processes and fluxes.On the Galician shelf and shelf edge, resuspension of sediments resulting in well-developed bottom nepheloid layers was observed at all stations, but transport of suspended sediment appears largely confined to the shelf. On the continental slope, only very dilute bottom nepheloid layers were present, and intermediate nepheloid layers were only occasionally seen. This suggests that cross-slope transfer of particles is limited by the prevailing northerly directed shelf and slope currents.Optical backscatter and ADCP current measurements by the BOBO lander, deployed at 2152 m depth on the Galician slope, indicated that particles in the bottom boundary layer were kept in suspension by tidal currents with highest speeds between 15–25 cm s⁻¹. Net currents during the recording period August 6th–September 10th 1998, were initially directed along-slope toward the NNW, but later turned off-slope toward the SW.The separation of the water masses on the slope from the sediment-laden shelf water by the along-slope current regime is reflected in the recent sedimentary deposits of the Galician shelf and slope. Apart from compositional differences, shelf deposits differ from those on the slope by their higher flux of excess ²¹⁰Pb (0.57–5.37 dpm cm⁻²y⁻¹ versus 0.11–3.00 dpm cm⁻²y⁻¹), a much higher sediment accumulation rate (315.6–2295.9 g m⁻²y⁻¹ versus 10.9–124.7 g m⁻²y⁻¹) and organic carbon burial rate (1.01–34.30 g m⁻²y⁻¹ versus 0.01–0.69 g m⁻²y⁻¹).In contrast to the observations on the Galician margin, pronounced nepheloid layers occurred in the Nazaré Canyon, which extended to considerably greater water depths. This indicates that significantly greater transport of fine-grained particles in both the INL and the BNL was occurring within the canyon, as reflected in the exceptionally high ²¹⁰Pb excess flux (up to 34.09 dpm cm⁻²y⁻¹), mass accumulation rates (maximum 9623.1 g m⁻²y⁻¹) and carbon burial fluxes (up to 180.91 g m⁻²y⁻¹) in the sediment. However, radioisotope fluxes in the lower canyon were only slightly higher than at comparable depths on the Galician margin. This suggests that transport and rapid accumulation is focused on the upper and middle part of the canyon, from where it is episodically released to the deep sea. Compared to the Galician margin, the Nazaré Canyon may be considered as an important organic carbon depocenter on short time-scales, and a major conduit for particulate matter transport to the deep sea on >100 y time-scales.     

Models of near-bed dynamics and sediment movement at the Iberian margin

A three dimensional time-dependent baroclinic hydrodynamic model, including sediment transport and incorporating a turbulence energy sub-model, is used in cross sectional form to examine sediment movement at the shelf edge off North West Iberia at 42°40.5’N where measurements were made as part of the OMEX-II programme. These calculations are complemented by a simpler, in essence time-independent model, which is used to examine the sensitivity of the sediment distribution over the slope (from a shelf-break source) to changes in the specified values of horizontal and vertical diffusion coefficients. The philosophy of the paper is to use idealized tidal, wind and wind wave forcing to examine changes in sediment distribution resulting from these processes. Calculations with the time-dependent and steady state models give insight into both the role of events and long-term effects. The steady state model focuses on the off-shelf region, whilst the time-dependent model considers on-shelf events.Tidal calculations showed that for the stratification used here the internal tide in the OMEX region was primarily confined to the shelf edge and ocean. A mean on-shelf sediment transport in the surface layer and off-shelf transport at the bed was found. Across-shelf circulations produced by up-welling/down-welling favourable winds gave rise to on-shelf/off-shelf currents in the bottom boundary layer with an opposite flow in the surface layer. In the case of an up-welling favourable wind, sediment suspension was at a maximum in the near coastal region, with sediment being advected off shore in the surface layer. With a down-welling favourable wind, surface sediment was advected towards the shore, but there was offshore transport at the bed. Near the shelf edge any upwelling flow had the tendency to return this sediment to the surface layer from whence it was transported on-shore. So in essence the sediment was trapped within an on-shelf circulation cell. Wind waves effects increased the total bed stress and hence the sediment concentration and its transport, although its pattern was determined by tidal and wind forcing.The time independent model with increased/decreased lateral diffusivity gave an enhanced/reduced horizontal sediment distribution for a given settling velocity. As the settling velocity increases, the down-slope movement of sediment is increased, with a reduction in the thickness of the near-bed sediment layer, but with little change in its horizontal extent.     

Fluid escape structures in the north Sicily continental margin

High resolution and multichannel seismic profiles coupled with multibeam echosounder (seafloor relief) data, acquired along the northern Sicily continental margin (southern Tyrrhenian Sea), document the occurrence of mound and pockmark features, revealing fluid escape processes. Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by flat areas at a mean depth of 1500 m.Seismostratigraphic analysis tools and methods were used to identify fluid escape structures and to work out a classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. With the aim to evaluate the geochemical content of fluids, we collected a 2.3 m long sediment core in correspondence of a pockmark at a depth of 414 m. Pore waters were sampled every 10 cm and analysed in relation to their conductivity (EC) and composition (δ¹⁸O, δD, Li, Na, K, Mg, F, Cl, Br, NO₃, SO₄).The new data show the occurrence of different types of structures with highly contrasting seismic and morphologic signatures, both dome-type and concave-upward structures. The latter have a characteristic circular shape and are known as pockmarks. Morphobathymetric, stratigraphic and structural data suggest that these structures occur along fault planes, mainly associated with diagenetic carbonates and fluid venting activity. Pockmarks could be the result of both fault and landslide structures, as they appear aligned along a straight direction and occur in proximity of the slope, and are associated with slope instabilities. The structural features are possibly associated with the recent tectonics mapped on-land as well as the widespread seismicity of the margin.Geochemical features reveal that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater, while the distribution profiles of EC, ion concentration (Cl, SO₄, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be the destabilisation of gas hydrates.