Search

Based on an extensive bathymetric data set obtained using modern high-precision swath bathymetry instruments, a Digital Terrain Model (DTM) of the lake floor of the Southern Baikal Basin has been created. In this work we present the morphological analysis based on this DTM. We were able to determine that tectonic activity was directly or indirectly involved in the formation of the main characteristics of the lake floor morphology. Modern tectonic processes are fixed by tectogenic scarps and paleoseismic dislocations that strongly affected the slope surfaces of the depression with detached blocks in the friable strata of the ancient delta front of the Selenga River and landslides on the southeastern shore of the basin. Underwater gravitational processes developed on the slope surfaces and the formation of canyons are also directly related to the tectonic movements. The Miocene-Pliocene sedimentary surface of the Tankhoy field, the bulk of which is below the lake level, currently experiences tectonic uplifts near the shoreline. These uplifts initiated significant underwater erosion, and underwater gravitational processes, which led to the formation of the modern delta fronts of the largest Lake Baikal tributaries and associated canyons.

Lake Baikal is the only freshwater basin containing sediments with gas hydrate accumulations, some of which are associated with mud volcano activity. Twenty-two mud volcanoes have already been identified in different areas of Lake Baikal, but the formation process and source depth remained unknown due to a lack of conclusive evidences. Here we discuss a set of geological and geophysical data to report the discovery of the hydrate-bearing Akadem mud volcanic complex (AMVC) on the Academician Ridge in central Lake Baikal. The obtained results allowed for the first time to concretely estimate the source depth of the mobilized fluids and sediments. Analysis of diatom skeletons present in the mud breccia revealed that the oldest diatom specimen is Cyclotella Iris et var. This species is characteristic for a short age interval ranging between 4.8 and 5.6 Ma. The same diatom was also detected between 230 and 310 m below the lake floor (mblf) in the borehole BDP-98 drilled near the AMVC. Combining biostratigraphic correlation and seismostratigraphy, it is estimated that the same interval is located at 200–300 mblf below the mud volcanic field. The elevated heat flow measured at AMVC indicates that the original base of gas hydrate stability (regionally located at ∼212 mblf) is currently shifted upwards of ∼100 m. The acquired data are consistent with a scenario envisaging the rise of warm fluids throughout the mud volcano complex zone. We suggest that migration of deep fluids could have initiated the gas hydrate dissociation and, in turn, rapidly generated overpressured shallow mud chambers. The ultimate piercing and triggering of the mud volcanoes activity resulted in the eruption of mud breccia and formation of densely packed crater sites in the study area. The depicted scenario can be applied to many mud volcanoes in Lake Baikal where similar anomalous heat flow conditions have been measured. These findings also emphasize that the genetic association between gas hydrate dissociation and the initiation of eruptive activity explains numerous peculiarities of the “Baikal” sedimentary volcanism (e.g. lack of lithified rocks among mud breccia clasts, gas hydrates, moderately elevated heat flow). This type of mud volcanism differentiates from the typically deeply rooted piercements observed worldwide in mature (marine) sedimentary basins. Ultimately our findings open a new prospective for mud volcano research worldwide, emphasizing that gas hydrates are not just one of the common features for sedimentary volcanism, but may have an active role as a triggering mechanism for the process itself.

Subaqueous landslides are common features at active and passive ocean margins, in fjords and lakes. They can develop on very gentle slope gradients (

International audience; Prominent landforms, either buried or preserved at the seafloor, provide important constraints on the processes that led to the opening and present-day configuration of the Dover Strait. Here, we extend previous investigations on two distinct landform features, the Fosse Dangeard and Lobourg Channel, to better understand the poly-phase history of their formation and inferences for the opening and Pleistocene evolution of the Dover Strait. The Fosse Dangeard consist of several interconnected palaeo-depressions. Their morphology and spatial distribution are interpreted to be the result of plunge-pool erosion generated at the base of north-eastward retreating waterfalls. Their infills comprise internal erosional surfaces that provide evidence for the occurrence of several erosional episodes following their initial incision. The Lobourg Channel comprises various sets of erosional features, attesting to the occurrence of several phases of intense fluvial and/or flood erosion. The last one of these carved a prominent inner channel, which truncates the uppermost infill of the Fosse Dangeard. The morphology of the Lobourg inner channel and the erosional features associated with its incision strongly resemble landforms found in megaflood-eroded terrains, indicating that this valley was likely eroded by one or several megafloods. Our study therefore corroborates the existence of waterfalls in the Dover Strait at least once during the Pleistocene Epoch. It also provides evidence of the occurrence of multiple episodes of fluvial and flood erosion, including megafloods. Finally, this study allows us to establish a relative chronology of the erosional/depositional episodes that resulted in the present-day morphology of this region. © 2018 Elsevier Ltd

High-resolution seismic profiles, combined with the integration of published drilling data, provide a detailed paleoenvironmental history of Lake Yamanaka (Fuji Five Lakes, Japan). This study presents a detailed analysis of the different depositional stages of the area currently occupied by Lake Yamanaka (floodplain wetland, river and lake). From ca. 5500 cal yr BP to ca. 5050 cal yr BP, the Yamanaka basin was occupied by floodplain wetlands. During that period, the landscape was very stable and erosion on northeastern flank of Mt. Fuji was relatively limited. From ca. 5050 cal yr BP to ca. 3050 cal yr BP, the water level increased and the floodplain wetlands became a lake. From ca. 3050 cal yr BP to ca. 2050 cal yr BP, the water level progressively decreased, leading to a reduction in lake extent. During this lowering of the lake's water level, a 1 km2 mass-transport deposit modified the physiography of the lake floor. From ca. 2050 cal yr BP to ca. 1050 cal yr BP, the lake disappeared and a river flowing towards the northwest occupied the depression. Ponds occupied morphological lows formed by mass-transport deposits. From ca. 1050 cal yr BP to the present day, the lake water level rose again, connecting the ponds with the main lake. Since then, the lake water level has continued to rise to the current level. Lake water level fluctuations are the results of several factors that could be interconnected: (i) changes in precipitation rates; (ii) margin destabilization (the Yamanaka mass-transport deposit), (iii) changes in river inlets and therefore variation in water supplies, (iv) volcanic eruptions (scoria fall-out and lava flows) and (v) changes in vegetation cover. This study highlights the importance of coupling sediment cores and high-resolution seismic reflection profiling to identify lateral variation and modification of sedimentary inputs through time.

An 8000-year lacustrine sediment record from Lake Motosu (Fuji Five Lakes) records several eruptions, including potentially unreported events, of the active Mt. Fuji volcano, which receives approximately 47 million annual visitors. A high-fidelity age model is constructed from tephra ages and high-density radiocarbon dating of terrestrial macrofossil and bulk organic matter. Variability in lake reservoir age is constrained by modern lake water radiocarbon measurement and reverse calibration of tephra calendar ages. We present more accurate ages for known eruptions, detect a wider distribution of ejecta for several eruptions, including the most recent summit eruption, and potentially identify previously undetected flank eruptions. There are closely spaced scoria-fall layers that may be difficult to differentiate as separate events in land-based surveys. These results demonstrate the utility of lacustrine sediments as powerful tools for understanding characteristics of volcanic eruptions.

This paper demonstrates that the Belgian Continental Shelf and coastal plain occupy a key position between the depositional North Sea Basin and the erosional area of the Dover Strait as it is an area where erosional landforms and fragmented sedimentary sequences provide new evidence on northwest European landscape evolution. The study area hosts 20–30 m thick penultimate to last glacial sand-dominated sequences that are preserved within the buried palaeo-Scheldt Valley. Here, we build on the results of previous seismo- and lithostratigraphical studies, and present new evidence from biostratigraphical analysis, OSL dating and depth-converted structure maps, together revealing a complex history of deposition and landscape evolution controlled by climate change, sea-level fluctuations and glacio-isostasy. This study presents strong new supportive evidence on the development of the incised palaeo-Scheldt Valley landform that became established towards the end of the penultimate glacial period (MIS 6; Saalian) as a result of glacio-isostatic forebulge updoming, proglacial lake drainage and subsequent collapse of a forebulge between East Anglia and Belgium following ice-sheet growth, disintegration and retreat in areas to the north. The majority of the incised-valley fill is of estuarine to shallow marine depositional context deposited during the transgression and high-stand of the last interglacial (MIS 5e: Eemian). A thin upper part of the valley fill consists of last glacial (MIS 5d-2: Weichselian) fluvial sediments that show a gradual decrease and retreat of fluvial activity to inland, upstream reaches of the valley system until finally the valley ceases to exist as the combined result of climate-driven aeolian activity and possibly also glacio-isostatic adjustment. Thus, strong contrasts exist between the palaeo-Scheldt Valley and estuary systems of the penultimate glacial maximum to Last Interglacial (Saalian, Eemian), the beginning of the Last Glacial (Weichselian Early Glacial and Early-Middle Pleniglacial), and the Last Glacial Maximum to Holocene.

In the framework of Multi-phase Gas Hydrate Project, near-surface gas hydrates were retrieved from the southern and central Baikal basins. In this report, we summarize characteristics of hydrate-bound hydrocarbons retrieved at Lake Baikal and focus on hydrate-bound ethane and its gas origin. The information of molecular composition of volatile hydrocarbons and their stable isotopes showed that the origin of hydrate-bound gas distributes in the fields of microbial, thermogenic, and their mixed-gas. As for stable isotope of ethane, 13C and D distribute from 69 to 23 V-PDB, and from 313 to 185 V-SMOW, respectively. Light ethane in 13C also depleted in D, indicating that the isotopically light ethane depleted in 13C and D is generated by a microbial process, and the origin of hydrogen in ethane molecules is thought to be lake water, same as methane.

The objective of our study is to clarify the formation process of natural gas hydrates in the sublacustrine sediments of Lake Baikal, and we are now compiling data on stable isotope ratios of hydrocarbons in hydrate-bound and sediment gases, which have been collected in the framework of collaboration between Russia, Belgium and Japan for 15 years. The results obtained in the VER19-03 cruise at the southern and central Baikal basins were summarized in this report. Comparison of stable isotope ratios of hydrate-bound and sediment gases at the same core and depth was performed. The methane carbon isotope ratio of the hydrate-bound gas was 1 to 2 smaller than the sediment gas. The methane hydrogen isotope ratio was similarly about 5 smaller. These results can be explained generally by the isotopic fractionation of methane. In contrast, some sediment cores from pockmarks in the center of the mud volcano showed small or reversed differences in methane carbon isotope ratios, as well as small differences in methane hydrogen isotope ratios, suggesting that dissociation of gas hydrate is occurring at that point.

LakeBaikalis the only freshwater reservoir on Earth with gas-hydrate accumulations in its bottom sediments, partly due to the activity of mud volcanoes. This paper describes a group of mud volcanoes recently discovered on the slope of the Academician Ridge between the northern and central LakeBaikalbasins. Our analysis of diatom skeletons in the mud breccia sampled from the study area shows a high abundance of Cyclotella iris et var. These extinct species were also discovered in a core sample from BDP-98 borehole. Based on the biostratigraphic and seismostratigraphic correlations, the age of the mud breccia in the studied mud volcanoes ranges from the Late Miocene to the Early Pliocene (4.6 to 5.6 Ma). The correlations suggest that the material originated from a depth of less than310 m below the lake bottom.

A new high resolution shallow marine seismic survey provides further insights on the fault source of the October 29th, 1900 earthquake, which affected a large region of North–central Venezuela, both offshore islands and mainland. This Mw 7.6 to 7.7 seismic event has been indistinctly ascribed to either the La Tortuga or San Sebastian fault, which has installed a live debate among different authors. These two faults allegedly run roughly east–west offshore, along the Coastal range. The San Sebastian fault (SSF) bounds this range to the north, being responsible for its linearity and the steep slope of its northern flank, whereas the La Tortuga fault (LTF) would do about the same, but some 20–30 km farther north, based on acoustic surveys carried out in the late 70's and early 80's. We herein bring strong evidence to support that the SSF is the source fault of this major offshore earthquake, mainly based on the freshness of its submarine scarp and recent fault throw, preserved at the Chuspa bay (close to Cabo Codera, eastern end of the Coastal range in central Venezuela), as depicted by one of the profiles. In addition, it is shown that most of the previous extent ascribed to LTF is not so. The LTF, besides exhibiting mainly normal slip, is much shorter than originally proposed. This precludes that LTF may have the seismic potential for an earthquake as big as the 1900 event. Finally, available reflection seismic data reveal the existence of a major fault system bounding the La Guaira shelf on the north, which exhibits tectonic inversion of pre existing extensional faults, shortening and strong syn-tectonic deformation during the Plio-Quaternary deposition. Sedimentation in the Bonaire basin during the Plio-Quaternary time is strongly controlled by this system activity.

Lake sediments contain valuable information about past volcanic and seismic events that have affected the lake catchment, and they provide unique records of the recurrence interval and magnitude of such events. This study uses a multilake and multiproxy analytical approach to obtain reliable and high-resolution records of past natural catastrophes from similar to 600-yr-old annually laminated (varved) lake sediment sequences extracted from two lakes, Villarrica and Calafquen, in the volcanically and seismically active Chilean Lake District. Using a combination of micro-X-ray fluorescence (mu XRF) scanning, microfacies analysis, grain-size analysis, color analysis, and magnetic-susceptibility measurements, we detect and characterize four different types of event deposits (lacustrine turbidites, tephra-fall layers, runoff cryptotephras, and lahar deposits) and produce a revised eruption record for Villarrica Volcano, which is unprecedented in its continuity and temporal resolution. Glass geochemistry and mineralogy also reveal deposits of eruptions from the more remote Carran-Los Venados volcanic complex, Quetrupillan Volcano, and the Huanquihue Group in the studied lake sediments. Time-series analysis shows 112 eruptions with a volcanic explosivity index (VEI) >= 2 from Villarrica Volcano in the last similar to 600 yr, of which at least 22 also produced lahars. This significantly expands our knowledge of the eruptive frequency of the volcano in this time window, compared to the previously known eruptive history from historical records. The last VEI >= 2 eruption of Villarrica Volcano occurred in 1991. Based on the last similar to 500 yr, for which we have a complete record from both lakes, we estimate the probability of the occurrence of future eruptions from Villarrica Volcano and statistically demonstrate that the probability of a 22 yr repose period (anno 2013) without VEI >= 2 eruptions is

International audience; We investigated the sedimentary processes that were active during the Holocene in the Gulf of Corinth, using high-resolution seismic reflection profiles and gravity cores. Seismic reflection data clearly show the presence of shallow-water sediment drifts at the western end of the Gulf, close to the Rion sill that links the Gulf to the Ionian Sea. Short cores indicate that drifts are composed of homogenous bioturbated mud in their upper part. The drift deposits flank a wide central area where the seafloor is eroded and where pre-Holocene deposits locally outcrop. The seafloor morphology in this area is marked by furrows oriented in different directions and by a depression attributed to the action of bottom-currents. The magnetic fabric of sediment samples from the drift, shelves, sub-basins and from the basin floor shows a significant anisotropy and a similar orientation of K max axes along core. The largest anisotropy (P = 1.043 ± 0.007) is observed in the drift and is interpreted as resulting from the action of bottom currents. The similar orientation of K max axes in the other cores, collected from areas east of the drifts, suggests that bottom currents also affect sediment deposition in the rest of the study area, even if seismic profiles and core analyses demonstrate that gravitational processes such as submarine landslides and turbidity currents exert the main control on sediment transport and deposition. Average K max axes for four cores were reoriented using the declination of the characteristic remanent magnetization. K max axes show variable orientations relatively to the slope of the seafloor, between along-slope and roughly parallel to the contour lines.

A total of 84 seismic profiles mainly from the western and eastern deltas of Lake Issyk-Kul were used to identify lake-level changes. Seven stratigraphic sequences were identified each containing a series of delta lobes that were formed during former lake-level stillstands. Lake-level has experienced at least four cycles of stepwise fall and rise of 400 m or more. These fluctuations were mainly caused by past changes in the atmospheric circulation pattern during the past. During periods of low lake levels, the Siberian High likely was strong, bringing dry air masses from the Mongolian steppe. The strong Siberian High blocked the mid-latitude Westerlies. During periods of high lake levels, the Siberian High must have been weaker or displaced, and the mid-latitude Westerlies could bring moister air masses from the Mediterranean and North Atlantic regions.

Overdeepened valleys and troughs on the northern and southern margins of the central European Alps have been shaped during the Quaternary glaciations (Preusser et al. 2010). These valleys host a number of fjord-type lakes, remnants of formerly larger water bodies, which show various morphological features that reflect their glacial origin, similar to marine fjords. Among the most prominent of these features are large-scale transverse moraine ridges in Lake Lucerne, a perialpine lake situated in a branched valley system on the northern margin of the Alps (Fig. 1b, c). Fig. 1. Swath-bathymetric data and reflection-seismic profiles from Lake Lucerne, central Switzerland. ( a , d ) Shaded-relief images of the Nase Moraine and Chindli Moraine. Colour represents depth; greyscale for areas outside the lake. Acquisition system GeoAcoustics GeoSwath Plus. Frequency 125 kHz. Grid-cell size 1 m. Greyscale derived from swisstopo swissALTI3D elevation model. Labels: ch, chutes and channels; fa, fan-shaped deposit; cr, secondary crest. ( b ) Location of study area (red box; map from GEBCO_08). ( c ) Overview of Lake Lucerne; areas shown in (a) and (d) are marked by red rectangles. ( e , f ) Reflection-seismic profiles across the moraine ridges. VE×4. Acquisition system RCMG Centipede sparker, SIG single-channel streamer, IXSEA Delph acquisition unit. Vertical scale …

The morphometric characteristics of 96 sublacustrine landslide complexes were measured on dense grids of high-resolution seismic reflection data from several lakes worldwide and were statistically analyzed. This analysis reveals that the morphology of the sublacustrine slopes exerts a strong control on the size of slope failures that occur on them, as the location of the headscarp and frontal ramp of these landslides are mainly determined by changes in the slope gradient. Our dataset also shows that the height drop of the failing slope section and the subsurface depth of the basal shear surface are the main parameters that determine whether a landslide will propagate in a frontally confined or frontally emergent manner. These parameters respectively represent the gravitational potential energy (driving force) of the sliding mass and the potential energy required (a resisting force) for it to emerge at its frontal ramp. These observations open perspectives for predicting the frontal emplacement style of future sublacustrine and submarine landslides and their associated natural hazards (e.g., tsunamis, dense flows). Although the investigated sublacustrine landslides have smaller dimensions than most submarine landslides, our data reveal mostly comparable inter-parameter correlations and relationships. However, frontally emergent landslides in lakes (and fjords) generally have a larger mobility and underwent a larger disintegration than what would have been expected by extrapolation of empirical relationships derived for ocean margin landslide datasets. This can be explained by the highly-unconsolidated material usually involved in the shallowly excavated slope failures in lakes and fjords.

Laguna Potrok Aike is a bowl-shaped maar lake in southern Patagonia, Argentina, with a present mean diameter of ~ 3.5 km and a maximum water depth of ~ 100 m. Seismic surveys were carried out between 2003 and 2005 in order to get a deeper knowledge on the lake sediments and the deeper basin geometries. A raytracing model of the Laguna Potrok Aike basin was calculated based on refraction data while sparker data were additionally used to identify the crater-wall discordance and thus the upper outer shape of the maar structure. The combined data sets show a rather steep funnel-shaped structure embedded in the surrounding Santa Cruz Formation that resembles other well-known maar structures. The infill consists of up to 370 m lacustrine sediments underlain by probably volcanoclastic sediments of unknown thickness. The lacustrine sediments show a subdivision into two sub-units: (a) the upper with seismic velocities between 1500 and 1800 m s- 1, interpreted as unconsolidated muds, and (b) the lower with higher seismic velocities of up to 2350 m s- 1, interpreted as lacustrine sediments intercalated with mass transport deposits of different lithology and/or coarser-grained sediments. The postulated volcanoclastic layer has acoustic velocities of > 2400 m s- 1. The lake sediments were recently drilled within the PASADO project in the framework of the International Continental Scientific Drilling Program (ICDP). Cores penetrated through lacustrine unconsolidated sediments down to a depth of ~ 100 m below lake floor. This minimal thickness for the unconsolidated and low-velocity lithologies is in good agreement with our raytracing model.

The Hazar Basin is a 25 km‐long, 7 km‐wide and 216 m‐deep depression located on the central section of the East Anatolian Fault zone (eastern Turkey) and predominantly overlain by Lake Hazar. This basin has been described previously as a pull‐apart basin because of its rhombic shape and an apparent fault step‐over between the main fault traces situated at the southwestern and northeastern ends of the lake. However, detailed structural investigation beneath Lake Hazar has not been undertaken previously to verify this interpretation. Geophysical and sedimentological data from Lake Hazar were collected during field campaigns in 2006 and 2007. The analysis of this data reveals that the main strand of the East Anatolian Fault (the Master Fault) is continuous across the Hazar Basin, connecting the two segments previously assumed to be the sidewall faults of a pull‐apart structure. In the northeastern part of the lake, an asymmetrical subsiding sub‐basin, bounded by two major faults, is cross‐cut by the Master Fault, which forms a releasing bend within the lake. Comparison of the structure revealed by this study with analogue models produced for transtensional step‐overs suggests that the Hazar Basin structure represents a highly evolved pull‐apart basin, to the extent that the previous asperity has been bypassed by a linking fault. The absence of a step‐over structure at the Hazar Basin means that no significant segmentation boundary is recognised on the East Anatolian Fault between Palu and Sincik. Therefore, this fault segment is capable of causing larger earthquakes than recognised previously.

Prokopenko and Kendall (J Paleolimnol doi:10.1007/s10933-008-9219-1, 2008) criticise the work presented in Fedotov et al. (J Paleolimnol 39:335–348, 2008), and instead propose an alternative interpretation for the grain-size evolution recorded in the KDP-01 core, retrieved from the central part of Lake Khubsugul. Their interpretation is based (i) on a seismic-stratigraphic re-interpretation of sparker seismic profile khub012 (which they copied from Fedotov et al. (EOS Trans 87:246–250, 2006)), (ii) on the presupposition that changes in lake level are the dominant control on facies distribution in Lake Khubsugul, and (iii) on the invalidation of our age-depth model. In this reply to their comment, we demonstrate that they interpreted seismic artefacts and geometries caused by changes in profile orientation as true stratigraphic features and that the lake-level reconstruction they derive from this interpretation is therefore incorrect. We also demonstrate that their grain-size predictions, which they consider to be predominantly driven by changes in lake level, are inconsistent with the measured sulphate concentration, which is a demonstrated proxy of lake level in Lake Khubsugul, and with the measured grain-size record. Finally, we point out that even if there would be a problem with the age-depth model, this problem would not affect the part of the sedimentary sequence discussed in Fedotov et al. (J Paleolimnol 39:335–348, 2008).

The Barcelona continental shelf is part of a 6–20 km wide canyon-bounded shelf located off the city of Barcelona (NE Spain). This study integrates newly acquired high-resolution single channel seismic reflection profiles and available lithological information of the Barcelona shelf to describe its late Quaternary architecture and the role of global glacio-eustatic fluctuations and local factors in its development. Four major sequence boundaries are identified throughout the seismic dataset. They are tentatively correlated with the four last 4th order (100–120 ka) high-amplitude late Quaternary glacio-eustatic lowstands. Most of the stratigraphic record is formed by falling stage systems tracts with forced-regressive deposits, which confers a general progradational vertical stacking pattern. Transgressive systems tracts are represented by continuous units confined directly off river sources and on previously eroded surfaces. Stillstand deposits are difficult to recognize in the available dataset. The uppermost seismic unit corresponds to the Holocene shore-parallel Besos and Llobregat joint prodelta affected by the prevailing south-westward circulation. The general architecture of the late Quaternary Barcelona shelf deposits is determined by global glacio-eustatic cycles. However, the lateral variability of the seismic units depends largely on local factors such as (i) the position of sediment sources (essentially the Llobregat River); (ii) differential subsidence and, especially, sediment compaction; (iii) erosive processes such as canyon incision, mass wasting or wave base dynamics; and (iv) underlying geomorphic restrictions.

The Barcelona continental shelf, off the city of Barcelona (NE Spain), is a relatively narrow canyon-bounded shelf in the northwestern Mediterranean Sea. Three medium-size rivers (Tordera, Besos and Llobregat) and several ephemeral rivulets flow into this margin. Two main domains have been recognized in the Barcelona shelf: (i) a modern, river-influenced area, and (ii) a relict, sediment depleted area, both affected by a variety of human impacts. A detailed geomorphologic study based on multibeam bathymetry and backscatter data, high resolution seismic profiles, and surface sediment samples allowed mapping and interpreting the main distinctive seafloor features on the Barcelona shelf. Modern sedimentary features reveal that the Llobregat River is the main sediment source of the Barcelona prodeltaic shelf. High-discharge fluvial events result in the formation of suspended sediment plumes and sediment waves on the shelf floor. Relict (late Pleistocene–Holocene) sedimentary features reflect that an important shift occurred in the seashore direction between MIS 4 and MIS 2, and that recent neotectonic reactivation has created a set of seafloor faults. The Barcelona inner and middle shelf is severely impacted by anthropogenic activities such as the enlargement works of the Port of Barcelona, sewage pipes, dredging, anchoring and trawling.

International audience; Recognising ice-age catastrophic megafloods is important because they had significant impact on large-scale drainage evolution and patterns of water and sediment movement to the oceans, and likely induced very rapid, short-term effects on climate. It has been previously proposed that a drainage system on the floor of the English Channel was initiated by catastrophic flooding in the Pleistocene but this suggestion has remained controversial. Here we examine this hypothesis through an analysis of key landform features. We use a new compilation of multi- and single-beam bathymetry together with sub-bottom profiler data to establish the internal structure, planform geometry and hence origin of a set of 36 mid-channel islands. Whilst there is evidence of modern-day surficial sediment processes, the majority of the islands can be clearly demonstrated to be formed of bedrock, and are hence erosional remnants rather than depositional features. The islands display classic lemniscate or tear-drop outlines, with elongated tips pointing downstream, typical of streamlined islands formed during high-magnitude water flow. The length-to-width ratio for the entire island population is 3.4 ± 1.3 and the degree-of-elongation or k-value is 3.7 ± 1.4. These values are comparable to streamlined islands in other proven Pleistocene catastrophic flood terrains and are distinctly different to values found in modern-day rivers. The island geometries show a correlation with bedrock type: with those carved from Upper Cretaceous chalk having larger length-to-width ratios (3.2 ± 1.3) than those carved into more mixed Paleogene terrigenous sandstones, siltstones and mudstones (3.0 ± 1.5). We attribute these differences to the former rock unit having a lower skin friction which allowed longer island growth to achieve minimum drag. The Paleogene islands, although less numerous than the Chalk islands, also assume more perfect lemniscate shapes. These lithologies therefore reached island equilibrium shape more quickly but were also susceptible to total erosion. Our observations support the hypothesis that the islands were initially carved by high-water volume flows via a unique catastrophic drainage of a pro-glacial lake in the southern North Sea at the Dover Strait rather than by fluvial erosion throughout the Pleistocene

Quantifying marine methane fluxes of free gas (bubbles) from the seafloor into the water column is of importance for climate related studies, for example, in the Arctic, reliable methodologies are also of interest for studying man-made gas and oil leakage systems at hydrocarbon production sites. Hydroacoustic surveys with singlebeam and nowadays also multibeam systems have been proven to be a successful approach to detect bubble release from the seabed. A number of publications used singlebeam echosounder data to indirectly quantify free gas fluxes via empirical correlations between gas fluxes observed at the seafloor and the hydroacoustic response. Others utilize the hydroacoustic information in an inverse modeling approach to derive bubble fluxes. Here, we present an advanced methodology using data from splitbeam echosounder systems for analyzing gas release water depth (>?100?m). We introduce a new MATLAB-based software for processing and interactively editing data and we present how bubble-size distribution, bubble rising speed and the model used for calculating the backscatter response of single bubbles influence the final gas flow rate calculations. As a result, we highlight the need for further investigations on how large, wobbly bubbles, bubble clouds, and multi-scattering influence target strength. The results emphasize that detailed studies of bubble-size distributions and rising speeds need to be performed in parallel to hydroacoustic surveys to achieve realistic mediated methane flow rate and flux quantifications.

On 1580 April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait (Pas de Calais). The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.0, has been located in the offshore continuation of the North Artois shear zone, a major Variscan tectonic structure that traverses the Dover Strait. The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD 1580 earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait. These data have revealed a broad structural zone comprising several subparallel WNW–ESE trending faults and folds, some of them significantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins. This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD 1580 earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity.

d SONIC, St. Petersburg, Russia Abstract: High- and very-high-resolution seismic data, side- scan sonar mosaics and piston cores from three isolated highs in Lake Baikal (Vydrino Shoulder, Po solsky Bank, Continent Ridge) have allowed to document in unprecedented detail the depositional, re-depositio nal and tectonic processes and to characterise the overal sedimentary environment on such isolated highs. Our data show that Vydrino Shoulder actually represent s a turbidity- or underflow-sourced slope fan, with a d istinct channel-levee morphology, and affected by t ectonics and stratigraphic unconformities. Sediment cores co ntain up to 90% of terrigenous material. Posolsky B ank is a major tilted fault block, clearly affected by tecto nic activity, by stratigraphic unconformities and b y mass wasting along its margins. Sedimentation on its cre st is undisturbed, but average sedimentation rates are high due to the influence of the nearby Selenga River. Conti nent Ridge is also affected by active faulting, str atigraphie unconformities and by mass-wasting along its flanks . The area receives only 30% of terrigenous input. There are no signs of sediment reworking due to bottom-currents. Our study thus shows that in comparison with ot her environments in Lake Baikal, the isolated highs pro bably hold the best potential to contain a high-qua lity, continuous, undisturbed and undiluted sedimentary r ecord suitable for paleoclimate research.

Swath bathymetry and backscatter data, side-scan sonographs, high-resolution (HR) and very high resolution (VHR) reflection seismic profiles, and sediment cores reveal the complexity of the seafloor and subseafloor features resulting from a large debris flow event that affected the Ebro continental slope and base-of-slope at the beginning of the Holocene. The BIG'95 debris flow, as it has been named, disturbed more than 2200 km2, including a 26-km3 deposit of remobilised sediment covering ~2000 km2. Swath bathymetry and backscatter imagery allow to distinguish four main areas within the debris flow: the source area, the proximal depositional area, the intermediate blocky depositional area and the distal depositional area. In the source area, a sinuous headwall scar, 20 km long and up to 200 m high, and several other secondary scars have been identified. Sediment released from the source area flowed southeastwards to the proximal depositional area, which is the main depocentre of the BIG'95 debris flow deposit, with accumulations over 90 m thick. Large slabs of sediment detached from this area and from the headwall scar and moved southeastwards embedded in a more mobile matrix, thus forming the intermediate blocky depositional area. Only the looser matrix reached the distal depositional area, finally freezing in and partially burying the Valencia Channel, a mid-ocean type channel. Digitisation of VHR seismic reflection profiles, where the debris flow deposit is shown as a body of mainly transparent seismic facies, has allowed the construction of isobath and isopach maps showing the thickness distribution of the deposit and its interplay with preexisting canyons and channels. Dating of two sediment cores gives a minimum age of 11,000 calendar years BP for the BIG'95 debris flow. A set of triggering mechanisms, including seismicity and oversteepening of the slope due to the existence of a volcanic structure underneath the main headwall, is invoked.

The north-western corner of Lake Le Bourget is situated along an active fault zone and accommodated a large sediment supply from the Rhone River until the end of the Late Glacial period. On the delta slope, the Holocene sheet drape that covers the largest buried mass wasting deposit (the HDU) shows undulations, small fractures and discontinuities that are attributed to downslope creep. Evidence for episodes of vigorous fluid expulsion is found in association with these discontinuities. All these features are rooted at the top of the HDU and occur along two specific isobaths. These observations indicate a close link between fractures and focused fluid flow. We suggest that focused fluid flow triggered by earthquakes facilitates the formation of small-scale faults that accommodate part of the downslope movement and eventually link up to form a headscarp of a large slide (c .1 0 7 m 3 ).

A large submarine slide - the Gebra Slide - has been discovered on the continental margin of Trinity Peninsula, Central Bransfield Basin, Antarctic Peninsula. The slide scar is clearly expressed in the bathymetry and is cut into the toe of the glacial-period slope-prograding strata on the lower continental slope. Seismic data give evidence of an associated debris-flow deposit embedded in the interglacial-period basin-fill strata of the basin floor. The total volume of sediment involved in the mass movement is about 20 km3. Indirect dating of the mass-wasting event, based on seismic-stratigraphic relationships of the slide scar and associated debris-flow deposit with underlying glacial-period slope units and the overlying interglacial-period basin-floor units, suggests that it took place at the transition between the last glacial period and the present-day interglacial. The initiation of the Gebra Slide is attributed to a combination of several factors, such as high sedimentation rates during the last glacial period, the unloading effect of a retreating ice sheet during deglaciation, pre-existing tectonic fabric and high seismicity in the area. This is the first recent submarine slide of this size identified on the glacial, continental margins of Antarctica. In morphology and general characteristics it is quite similar to the well-known large-scale submarine slides from the northern hemisphere glacial margins, although it is smaller. Its most striking characteristic is its lower-slope position (at 1500-2000 m of water depth), which remains up to now difficult to explain.

Lake Teletskoye in the northeastern part of the Altai mountain range has attracted the attention of geo-scientists for a long time, because it fills an impressive tectonic depression. The lake is 77 km long and 4 km wide, and it has a maximum water depth of 325 m. The vertical offset of the basement surface is up to 3000 m. A multidisciplinary study of the Teletsk graben was carried out during the last few years, including satellite image and air photo analysis, bathymetric-, structural- and geomorphological mapping, high-resolution seismic profiling and seismic refraction. The structural study revealed that reactivation of preexisting weak basement zones is important in controlling the basin formation. These zones separate different tectonic terranes at the contact of which the Teletsk graben developed. This study identifies the significance of the basin in the regional neotectonic context. It shows that the major vertical movements are restricted to the basin itself, but do not characterize the whole region. Outside of the basin, recent tectonic structures have the same pattern as adjacent areas of Northeast Altai and West-Sayan. Quaternary glaciations have had no major influence on the basin formation. Two stages of faulting are identified. First, transpressive movements restricted to discrete (reactivated) fault zones controlled the opening of the basin. In the second stage, normal faulting is dominant and is responsible for the modern basin outline. An echo-sounding survey led to the recognition of several morphological characteristics of the lake bottom. In the southern part, the uppermost sediments seem slightly disturbed, whereas further north, transverse ridges and slope breaks are increasingly common. The deepest part of the lake is located in a highly disturbed zone of normal fault-bounded blocks. The structural difference between the southern and northern subbasins is supported by the interpretation of a deep seismic refraction profile which indicates a substantial increase of basement isochores in the area where the reactivated Teletsk (Paleozoic) shear zone crosses the lake. Correlation of high-resolution seismic profiles suggests that the Teletsk graben started to evolve during the Pleistocene, and that its present shape was formed in two stages. The first stage was responsible for the opening of the southern basin. It probably started in the Middle Pleistocene. A second kinematic stage induced by a sinistral reactivation of the NE striking West-Sayan fault initiated the opening of the different segments of the northern subbasin due to opposite movements between the reactivated Teletsk and West-Sayan faults. This second stage was active after the end of Late Pleistocene glaciations and during the Holocene. The recent lateral extension and the related N–S-trending normal faults result from a change in tectonic regime, with related extensional movements along the main reactivated fault zones. These recent movements result in the lateral escape of the lake borders and the collapse of the area between them.

High resolution, single-channel seismic sparker profiles across the Akademichesky Ridge, an intra-basin structural high in Lake Baikal (Russia), reveal the presence of small sediment mounds and intervening moats in the upper part of the sedimentary cover. Such features interrupt the generally uniform and even acoustic facies and are not consistent with the hemipelagic sedimentation, which is expected on such an isolated high and which would produce a uniform sediment drape over bottom irregularities. The influence of turbidity currents is excluded since the ridge is an isolated high elevated more than 600-1000 m above adjacent basins. The mounded seismic facies, including migrating sediment waves and non-depositional/erosional incisions, strongly suggest that sediment accumulation was controlled by bottom-current activity. We interpret the mounds as small-scale (< few tens of km2 in area) lacustrine drifts. Four basic types of geometry are identified: 1) slope-plastered patch sheets; 2) patch drifts; 3) confined drifts; 4) fault-controlled drifts. The general asymmetry in the sedimentary cover of the ridge, showing thicker deposits on the NW flank, and the common location of patch drifts on the northeast side of small basement knolls indicate that deposition took preferentially place at the lee sides of obstacles in a current flowing northward or sub-parallel to the main contours. Deep-water circulation in the ridge area is not known in detail, but there are indications that relatively cold saline water masses are presently flowing out of the Central Basin and plunging into the deep parts of the North Basin across the ridge, a process that appears to be driven mainly by small differences in salinity. We infer that the process responsible for the observed bottom-current-controlled sedimentary features has to be sought in these large-scale water-mass movements and their past equivalents. The age of the onset of the bottom-current-controlled sedimentation, based on an average sedimentation rate of 4.0 cm/ky, is roughly estimated to be as least as old as 3.5 Ma, which is generally regarded as the age of the onset of the last major tectonic pulse of rift basin development in the Baikal region.

New high-resolution seismic reflection data from the central part of Lake Baikal provide new insight into the structure and stratigraphy of Academician Ridge, a large intra-rift accommodation zone separating the Central and North Baikal basins. Four seismic packages are distinguished above the basement: a thin top-of-basement unit; seismic-stratigraphic unit X; seismic-stratigraphic unit?A; and seismic-stratigraphic unit?B. Units A and B were cored on selected key locations. The four packages are correlated with a series of deposits exposed on the nearby western shores: the Ularyar Sequence (Oligocene); the Tagay Sequence (Lower to Middle Miocene); the Sasa Sequence (Upper Miocene to Lower Pliocene); the Kharantsy Sequence (Upper Pliocene); and the Nyurga Sequence (Lower Pleistocene). Based on stratal relationships, sedimentary geometries, distribution patterns and principal morphostructural elements – both onshore and offshore – we propose a new palaeogeographic evolution model for the area. In this model progressive tectonic subsidence of the Baikal basins and successive pulses of uplift of various segments of the rift margins lead to: (a) formation of the ridge as a structural and morphological feature separating the Central and North Baikal basins during the Middle to Late Miocene; (b) gradual flooding of the main parts of the ridge and establishment of a lacustrine connection between the two rift basins during the Late Miocene; and (c) total submergence of the top parts of the crest of the ridge during the latest Pleistocene. This new model helps to better constrain numerous phases in the structural evolution of the Baikal Rift, in which the Academician Ridge as an accommodation zone plays a crucial role.

A high-resolution seismic survey of the Rukwa Rift has imaged the upper 300 m of the Pliocene–Holocene section. The sediments are characterized by aggradational deposition in shallow lacustrine conditions that episodically dried out. Overall the reflection package expands northeastwards towards the Lupa Fault (half-graben bounding fault). Isopach maps for nine intervals (6 m to 65 m thick) show four periods of marked expansion of the section towards the Lupa Fault, interpreted to represent times of fault activity or infilling of fault-created topography during the early stage of fault quiescence. There are three periods of little or much reduced expansion interpreted to represent infilling of the accommodation space between the lake floor and the lake surface during periods of fault inactivity. Cyclicity of such high frequency has not been widely reported for faults which penetrate the crust. The periods of activity show remarkably similar displacement patterns with no lateral migration of the main depocentre, implying that seismic and aseismic slip has persistently clustered in the same parts of the fault. Identification of high-frequency fault cyclicity has implications for sequence stratigraphy in tectonically active areas, since there is a tendency to assume faults do not have cyclic effects on the scale of tens of thousands of years.

Lake Le Bourget is a deep, peri-alpine glacial lake originating from the last glacial, the Wurm. The lake has amassed over 250 m of sediment since the Last Glacial Maximum. Two seismic sparker surveys on the lake revealed the glacial and post-glacial sedimentary infill, the depth and the morphology of the Wurm erosion surface and penetrated the Riss–Wurm interglacial deposits as deep as the underlying Riss erosion surface. The density of the seismic grid and the high resolution of the data, even at greater depths, allowed a detailed study of the sedimentary processes and of their evolution throughout the deglaciation. Five main seismic facies characterise the basin fill. They are interpreted as glacial deposits, glacio-lacustrine sediments, proglacial lacustrine fans, alluvial fan deltas and authigenic lacustrine drape. These occur as a suite of deposits associated with the major lake tributaries and are the result of major changes in sediment supply and style of discharge into the lake. The three-dimensional facies associations within the basin fill document the history of the basin and its catchment area since the onset of the deglaciation.

Very high-resolution seismic data from the Kaiser-I-Hind sand bank (southern Celtic Sea) recently highlighted the internal structure of the enigmatic Celtic Banks, which are among the deepest and largest shelf sand ridges. The main body of the bank is made up of 4 seismic/depositional units which reflect a transgressive evolution. New data on the detailed architecture of two of these units allow discussion of bank growth in terms of either (1) a channel–levee system preserved both by lateral migration and aggradation of the channels, or (2) a package of large offshore tidal sediment bodies (bar chains and/or giant dunes). Careful geometrical observations of seismic discontinuities make the second hypothesis more likely. The unit architecture is analysed in terms of long- to short-term processes of build-up. Long-term processes are evinced by the landward stacking of erosive sub-units in response of the last post-glacial sea-level rise, whereas short-term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the Western Channel Approaches.

Lake Annecy is a peri-alpine lake of glacial origin, remnant of the last glaciation. The lake basin accumulated over 150 m of sediment since the Last Glacial Maximum. A dense grid of high-resolution seismic data, acquired during a 1993 sparker survey, revealed the glacial and post-glacial sedimentary infill, the depth and morphology of the Wurm erosion surface and penetrated the Riss-Wurm interglacial deposits as deep as the underlying Riss erosion surface. This paper focuses on the interpretation of the high-resolution seismic facies of the infilling sediments of the lake basin in correlation with the results of the Annecy core. The quality and density of the seismic data allow, in this case, detailed analysis of the seismic facies in their three-dimensional organisation and permit reliable interpretation in terms of depositional process and palaeo-environmental setting. The study reveals that the geometry and seismic facies of the infilling deposits are controlled essentially by the basin morphology and the characteristics of the sediment supply. The sediment supply to the lake changes throughout the deglaciation from subglacial drainage, to proglacial meltwater streams, to alluvial streams and finally to the present-day interglacial condition where authigenic sedimentation prevails. The depositional patterns change accordingly from chaotic outwash deposits and glacio-lacustrine muds at the base, over axially aggrading lacustrine fans, prograding alluvial fan deltas at the lake margins to, finally, a drape of lacustrine chalk and marl. The interpreted sedimentary environment and deduced character of the sediment source are used to reconstruct the changing palaeo-environment in the catchment area.

This paper presents the results of high-resolution reflection seismic surveys carried out between 1989 and 1996 along rivers and canals in northern Belgium. The seismic data penetrate down to 900 m in the sedimentary cover or to the Paleozoic basement. The reflection response of the acoustic basement provides clear indications with regard to the top of the Paleozoic: crystalline basement and Lower Paleozoic metasediments and volcanics of the London-Brabant Massif and NE-dipping Devonian and Carboniferous strata. The subhorizontal Mesozoic and Cenozoic sedimentary cover comprises 20 unconformity-bound seismic units: 5 in the Cretaceous and 15 in the Cenozoic. Based on borehole information, these units are correlated with lithostratigraphically defined formations or groups. Some of the unit-bounding unconformities are of regional importance. They are attributed i) to eustatic sea-level changes causing regional flooding during the Late Cretaceous or incision of deep valleys during the Late Oligocene and Late Miocene, ii) to regional tectonic tilting between Late Eocene and Early Oligocene, or iii) to a combination of eustasy and tectonics causing valley incisions during the Lutetian. Faults of the Roer Valley Graben have offset different stratigraphic levels by sometimes considerable amounts (up to 230 m in the Oligocene to Quaternary succession). Although the main tectonic phase took place during the Miocene, the activity has varied considerably through time, and also from fault to fault. Most faults seem to have a 10 to 30-m displacement since the Late Pliocene.

The Quaternary history of the southern Baltic is marked by a complex sequence of glacial, lacustrine and marine phases. Consequently, a large number of unconformities and erosional and depositional surfaces are present in the stratigraphic record. Over 600 km of high-resolution, single-channel reflection seismic profiles were collected offshore Bornholm. A sequence stratigraphic interpretation of the seismic data, in combination with some available core data, has led to the identification of two Quaternary depositional sequences unconformably overlying the late Mesozoic deposits. The sequences contain up to nine different seismic-stratigraphic units, and are separated by sharp boundaries representing strong erosional events. The lower depositional sequence contains five units (A-E) of glacial and late-glacial origin. The unconformity, at its base is marked by a southward flowing channel drainage pattern, which may be related to the mid-Weichselian Alnarp River system. Units A and B contain till and moraine deposits, probably of middle or early-late Weichselian age. Units C-E consist of glaciolacustrine clays, and most likely represent different Baltic Ice Lake phases of the late Weichselian. The upper depositional sequence contains four units (F-I) of postglacial lacustrine and marine origin. Its sharp erosional base possibly indicates subaerial exposure. Unit F consists of backstepping delta or beach deposits, which are partially interfingered with the lacustrine clays of unit G. Both units can most likely be correlated with the Ancylus Lake. The marine mud deposits of units H and I represent the widespread Litorina transgression.

We present here the initial results of a high-resolution (sparker) reflection seismic survey in Northern Lake Tanganyika, East African Rift system. We have combined these results with data from earlier multichannel reflection seismic and 5-kHz echosounding surveys. The combination of the three complementary seismic investigation methods has allowed us to propose a new scenario for the late Miocene to Recent sedimentary evolution of the North Tanganyika Basin. Seismic sequences and regional tectonic information permit us to deduce the palaeotopography at the end of each stratigraphic sequence. The basin history comprises six phases interpreted to be responses to variations in regional tectonism and/or climate. Using the reflection seismic-radiocarbon method (RSRM), the minimum ages for the start of each phase (above each sequence boundary) are estimated to be: ~7.4 Ma, ~1.1 Ma, ~393-363 ka, ~295-262 ka, ~193-169 ka, ~40-35 ka. Corresponding lowstand lake elevations below present lake level for the last five phases are estimated to have been: ~650-700 m, ~350 m, 350 m, ~250 m and ~160 m, respectively. The latest phase from ~40-35 ka until the p-re sent can be subdivided into three subphases separated by two lowstand periods, dated at 23 ka and ~18 ka. From the late Miocene until the mid Pleistocene, large-scale patterns of sedimentation within the basin were primarily controlled by tectonism. In contrast, from the mid Pleistocene to the present, sedimentation in Lake Tanganyika seems to have responded dramatically to climatic changes as suggested by repeated patterns of lake level fluctuations. During this period, the basin infill history is characterized by the recurrent association of three types of deposits: \\\'basin fill\\\' accumulations; lens-shaped \\\'deep lacustrine fans\\\'; and \\\'sheet drape\\\' deposits. The successive low-lake-level fluctuations decreased in intensity with time as a consequence of rapid sedimentary filling under conditions of declining tectonic subsidence. The climate signal has thus been more pronounced in recent sedimentary phases as tectonic effects have waned.

A fine-scale seismic stratigraphic model has been developed for the Palaeogene of the southern North Sea on the basis of interpretation of a dense high-resolution reflection seismic grid, covering the Belgian sector of the continental shelf and the adjacent parts of the Dutch, French and UK sectors. Classical seismic stratigraphic criteria have allowed up to 13 major units to be defined; the geometry and seismic facies characteristics of each have been analysed in detail. The seismic stratigraphy has been compared with the results of four offshore boreholes. ‘Events and trends’ identified on seismic sections and in outcrops in northern Belgium have been correlated, and offshore seismic facies have been tentatively matched with onshore lithofacies. The geological history of the study area is discussed in terms of eustatic sea level changes and regional tectonic events, and the main characteristics of the offshore Palaeogene deposits are evaluated in a sequence stratigraphic context.