Seismic sequence stratigraphy of the Palaeogene offshore of Belgium, southern North Sea

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

Gas hydrate systems respond slowly to seafloor warming

In a marine environment, gas hydrates are stable at certain pressure (sea level) and temperature (bottom water temperature) conditions. Changes in these conditions may result in the destabilization of the gas hydrates. In this study we investigate the temporal response of a continental margin gas hydrate reservoir to changes in the pressure and/or temperature regime, considering the latent heat of hydrate dissociation and the long response times to conductive heat transport in submarine sediments. Gas hydrates and the surrounding sediments do not instantly respond to changing environmental conditions. A vertical subsoil column without gas hydrates needs more than 5,000 years to adapt its temperature profile to an increase in seafloor temperature. A vertical subsoil column containing gas hydrates has the same response time if the stability of the hydrates is not affected. Although, when gas hydrates stability is affected due to changes in their environment, the response time to these changes is extended. Destabilized gas hydrates will dissociate into methane gas and water. The dissociation process happens at a constant temperature and requires a lot of energy (heat). Dissociation of gas hydrates thus delays the response time of the surrounding subsoil; up to 100,000 years may pass before the temperature profile completely adapted to the changed environmental parameters. Because of this slow response to changes in environmental parameters, gas hydrate dissociation cannot be regarded as the trigger to global warming at the end of glacial and stadial periods and gas hydrate dissociation cannot be responsible for the high observed atmospheric methane concentrations in ice core records, as has been postulated in a number of high-profile publications

An 18,000-year multiproxy lacustrine record of climate variability in south-central Chile (40°S): Lago Puyehue, Chilean Lake District

An 11-m-long sediment core was collected in Lago Puyehue (40ºS, Lake District, Chile). The coring site had been selected on basis of a seismic-stratigraphic analysis that highlighted it as an area of relatively condensed, continuous and undisturbed sedimentation in this otherwise highly dynamic post-glacial lake. The 11-m core extends back to 17,915 cal yr BP. An age-depth model was established by 9 AMS 14C dates, constrained by 210Pb, 237Cs, 241Am measurements, by the identification of event-deposits related to earthquakes and/or volcanic eruptions, and by varve-counting for the past 600 yr. The core was submitted to a multi-proxy analysis, including sedimentology, mineralogy, grain-size, major geochemistry and organic geochemistry (C/N ratio, d13C), loss-on-ignition, magnetic susceptibility, diatom analysis and palynology. Along-core variations in sediment composition reveal that the area of Lago Puyehue was characterised since the Last Glacial Maximum (LGM) by a series of rapid climate fluctuations superimposed on a long-term warming trend. These rapid climate changes are: (1) an abrupt warming at the end of the LGM at 17,300 cal yr BP, (2) a short, relatively cold interval between 13,100-12,300 cal yr BP, (3) a second abrupt warming, possibly with increased precipitation, at about 12,300 cal yr BP, and (4) an increase in climate variability in the late Holocene at 5000-6000 cal yr BP. The timing of these rapid climate changes confirms previously reported climate trends from continental southern South America and their out-of-phase relationship with those from the northern hemisphere and from Antarctica

A new methodology for quantifying bubble flow rates in deep water using splitbeam echosounders: Examples from the Arctic offshore NW-Svalbard

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 (> 100m). 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

Seismic evidence of small-scale lacustrine drifts in Lake Baikal (Russia)

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 expected on such an isolated high, 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 includes migrating sediment waves and non-depositional/erosional incisions that strongly suggest 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 preferentially took place on the lee sides of obstacles to 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 at least 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

The use of acoustic seafloor backscatter measurements for quantitative and qualitative characterization of methane seep areas

During the 2003 and 2004 cruises of the EC project CRIMEA almost 3000 active methane seeps were detected with an adapted scientific split-beam echosounder in the Dnepr paleo-delta area in the NW Black Sea (Naudts et al., in press). The seeps are widely, but not randomly, distributed over the transition zone between the continental shelf and slope, in water depths of 66 to 825 m. The highest concentration of seeps occurs on the shelf, in water depths of 80 to 95 m. Here, the location of the seeps is controlled by the underlying geology (filled channels) and seepage is characterized by the presence of pockmarks and high acoustic seafloor backscatter, visible on both multibeam and side-scan sonar data.Since seep detection during the CRIMEA cruises was performed independently but simultaneously with the multibeam and side-scan sonar recordings, these datasets possess a great potential for quantitative and qualitative analyses of acoustic seafloor backscatter in relation to the seep locations. Our analyses are further sustained by visual observations, high-resolution 5 kHz seismic data and sediment samples from gravity and multi-coring.For this study we selected an area of 37 km2 on the shelf.Within this area the normalized multibeam backscatter values ranges from -28.32 dB to 20.42 dB. After eliminating high-backscatter values caused by high topographic gradients, all seep positions within this area correspond to backscatter values of more than -2.89 dB and have a standard normal distribution. Furthermore, no seeps occur at locations characterized by the highest backscatter values. Within the area, 99.3 % of the seeps correspond to backscatter values ranging between -1.39 and 4.60 dB.These data indicate that actively bubbling seeps do not necessarily correspond to the highest backscatter values as would be expected; they rather surround the highbackscatter areas. This is also clear from visual observations in which bubbles are seen to emanate at the perimeter of white Beggiatoa mats. Since Beggiatoa mats are commonly associated with the precipitation of authigenic carbonates formed via AOM, these carbonates are very likely to be the cause of the higher backscatter values. Sediment samples and visual observation also indicated that areas corresponding to higher backscatter values are characterised by more shell material in the first 5-10 cm of the seabed.Also pockmarks are characterised by typical backscatter patterns. Better evolved, deeper, pockmarks are characterised by higher backscatter values and the seep activity is lower than at shallow pockmarks, which are often active bubbling. This could be explained by some sort of self-sealing of these seeps, as postulated by Hovland (2002).All these observations at the seafloor are clearly a result of the underlying geology where fluid migration is focussed to the sides of filled paleo-channels. The seismic data show the presence of a distinct “gas front” that locally domes up to the seafloor. These areas of gas front updoming on the shelf are characterised by seeps, higher backscatter values, Beggiatoa mats and pockmarks

New evidence for important lake-level changes in Lake Baikal during the Last Glaciation

In recent years, a number of estimates have been proposed of fluctuations of the Baikal lake level caused by climate changes. They were mainly based on the interpretation of reflection seismic data from the Selenga delta area (eastern coast of Lake Baikal). These estimates range between 2 m [Colman, 1998] and 600 m [Romashkin et al., 1997]. Better-constrained values of lake-level changes during the last 100 ky were presented by Urabe et al. [2004]. According to their reflection seismic data from the Selenga delta area, the level of Lake Baikal was significantly lower than the present-day level during the two last cold stages (i.e. -45 m during MIS2 and -73 m during MIS4). To precise and verify these values, we carried out an additional high-resolution reflection seismic study in the area of Olkhon Gate (western shore of Lake Baikal). The maximum water depth in this area does not exceed 40 m. The seismic data were collected using two different types of seismic sources: i) a multi-electrode CENTIPEDE sparker with a frequency range of 350-1400 Hz, and ii) the “Sonic-2” seismic system with a frequency range of 2-5 kHz. They allow investigation of the sedimentary record with a resolution of about 1 m (to 300 m depth) and 15-20 cm (to 30 m depth), respectively.Interpretation of these new data allowed distinguishing several seismic units separated by unconformities (erosion surfaces) in the upper part of the seismic profiles. These unconformities could be traced across the entire study area. The uppermost two erosion surfaces are more sharply defined. In the deepest parts of the channel (at 37-40 m water depth) the uppermost unconformity occurs at 5-10 ms below the lake floor, and the second unconformity at 15-20 ms below the lake floor. Both unconformities are interpreted as subaerial erosion surfaces and thus mark a lowstand of the lake level during a prolonged time. For calculation of the thickness of these two units, we used the acoustic logging data from the BDP-98 borehole [BDP Members, 2000]. According these data p-wave velocities vary from 1.6 to 1.8 km/s. The thickness of our upper two seismic units can thus be converted to 4-8 m and 12-16 m, respectively. This implies that the uppermost unconformity occurs at 41-48 m, and the second unconformity at 52-64 m below present-day lake level, which is approximately at the same depth as the two unconformities in the Selenga delta area that were studied by Urabe et al. [2004] and attributed with the MIS2 and MIS4 cold periods, respectively.Our new data thus support the growing amount of evidence of a lowering of the Lake Baikal water level by 40-65 m during glacial/cold periods. The lowstands are probably caused by water redistribution in the Lake Baikal watershed due to climate changes (i.e. glaciation and atmospheric circulation). These data also allow making quantitative assessments of water balance and paleoclimate parameters in the past