Dispersed methane flux to the water column from natural gas bubble streams at the Black Sea shelf

Gas bubble streams are detected in the water column by the presence of strong, flare shaped backscatter signals recorded during hydroacoustic single beam echosounder surveys (flares). Some of these flares even reach the sea surface. In motion bubbles get into an evolutionary process caused by a variety of effects, including gaseous exchange with surrounding water. Simplistically, kinetics of such a gas exchange can be described by the Fick’s law; the direction of the transfer of any given gas through the bubble depends on partial pressure of respective the gas in bubble, Henry’s law constants and the concentration of dissolved gas in the water. In general, methane gradually dissolves during the lifetime of a bubble, while other gases enter the bubble. Consequently, bubbles cause a vertical transfer of methane from the sea bottom to upper water layers and can be considered as sources of dispersed methane flux to the water column. In present work an attempt is made to trace the methane gas phase exhaustion in flares trough the water column at the Black Sea shelf.Our approach is based upon acoustic observations and measurements carried out in 2003 and 2004 with the scientific echosounder EK-500 onboard RV Vodianitskiy as part of the EU funded project CRIMEA. For the estimation of bubble size distributions our data from direct measurements of acoustic cross-section of single bubbles were used. Data for the relation between rising speed and shrinking rate vs. bubble size were obtained by tracking of single bubbles. Modelling was used to evaluate features of the gas transfer process induced by rising bubbles. Having initial bubble size, gas composition and water depth as starting conditions the model produces series of time based values of bubble size, gas composition, rising depth and rising speed. Acoustic observations were utilized to verify the chosen model parameters.For seeps detected at 90 - 95 m water depth hydroacoustically measured bubble sizes ranged from 1.3 to 11.3 mm in diameter. This bubble size range was confirmed by visual observations during video and submersible inspections.We assumed a gas content of 99% methane and small amounts of nitrogen and oxygen as initial gas composition according to geochemical analyses of gas bubbles sampled by submersible just above the sea floor. To determine the entire free methane flux from the sea floor into the water column and maybe into the atmosphere we run our model for several bubble sizes classes. Then simulation data were summed up with weighting coefficients according to the respective amount of bubbles per class. As a result, vertical profiles of molar content (mkmol) and methane flux (mkmol/s m) per average statistic bubble vs. depth were obtained. To get methane flux from the whole seepage simple multiplication is required by average statistic initial number of bubbles above the bottom per unit height. Depending on the spatial extension of the seep area, point or volume backscattering methods were used to quantify the bubble amount. Of great importance for both methods is the averaging of a high amount of data in space and time. We detected an average of 400 bubbles at high intensity seep sites within a water volume of 1m thickness above the bottom. The hydroacoustically determined amount of bubbles is again in very good agreement with direct visual observations. With an average initial rising speed of 0.25 m/s, 400 bubbles escaping from the sea floor cause a methane flux of 3.45 mmol/s using average bubble methane content of 34.5 mkmol (400 x 0.25 x 34.5 = 3.45 mmol/s). As final methane content of average bubble at the sea surface is 6.5 mkmol, only 6.5/34.5*100 = 18.8 % of methane can reach the atmosphere due to the methane flux into the water column on the way up to the sea surface

Observation of microbial carbonate build-ups growing at methane seeps near the upper boundary of the gas-hydrate stability zone in the Black Sea

Extensive dredge sampling carried out in May-June 2004 in the deeper part of the Dnepr paleo-delta area (NW Black Sea) yielded for the first time chimney-shaped carbonate microbial build-ups, which occur at methane seeps close to upper boundary of the gas-hydrate stability zone (~ 700 m). Carbonate samples taken with a benthic trawl represent fragments of the uppermost, middle and lowest parts of the build-up; they are similar morphologically to those found previously at the shallower and deeper methane seeps in the Black Sea. At the same time, the perforated, plate-like carbonates in the lowest parts of the build-up provide first indications that gas channels are formed during the earliest growth phase of these microbial structures. Stable carbon isotope analyses of the carbonates from the uppermost fragments gave the 5I3C values ranging from -33.7 to -36.6 %o, while the 813C values of the lowest fragments are significantly lighter, varying between -42.0 and -44.6 %o. Oxygen isotopic values also show differences between the samples from the uppermost part of the build-ups, which are composed of a mixture of aragonite and Mg-calcite (5180 = 0.7 to 0.94 %o), and the only Mg-calcite cemented thin slabs of lowest carbonates (5180 = 1.35 to 1.57 96o). The isotope data for carbon and oxygen suggests that carbonates formed as a result of anaerobic microbiological oxi­dation of methane supplied as a shallower-sourced fluid component from below. The difference in 513C and 5I80 values found in the upper and lowest parts of the build-ups may indicate that more carbon derived from seawater and less hydrate water are involved to the chimney formation during its growth, but this may be also a record of the long-term changes in the near-bottom environments related to evolution of salinity, temperature and anoxic conditions in the Black Sea

Microbial carbonate build-ups at methane seeps near the upper boundary of the gas-hydrate stability zone in the Black Sea: results of EU project CRIMEA

Extensive dredging carried out in May-June 2004 in the deeper parts of the Dnepr paleo-delta area (NW Black Sea) yielded for the first time chimney-shaped carbonate microbial build-ups, which occur at methane seeps close to upper boundary of the gas-hydrate stability zone (~ 700 m). Carbonate samples taken with a benthic trawl represent fragments of the uppermost, middle and lowest parts of the build-up, which are similar to those found previously at the shallower and deeper methane seeps in the Black Sea. At the same time, the holed, plate-like carbonates in the lowest parts of the build-up provide first indications that gas channels are formed during the earliest growth phase of these microbial structures. Stable carbon isotope analyses of the carbonates from the uppermost fragments gave the d13C values ranging from -33.7 to -36.6 pro mil, while the d13C values of the lowermost fragments are significantly lighter, varying between -42.0 and -44.6 pro mil. Both these types of carbonates indicate that a major portion of the carbonate carbon originates from bacterial oxidation of the seeping methane. Oxygen isotopic values also show differences between the more irregular and porous samples from the uppermost part of the build-up, which are composed of a mixture of aragonite and Mg-calcite (d18O = 0.7 to 0.94 pro mil, and the only Mg-calcite cemented thin slabs of lowermost carbonates (d18O = 1.35 to 1.57 pro mil. The difference in d13C/d18O ratio found in the upper and lower parts of the build-up may reflect the changing of the water temperature and salinity during the chimney growth

Investigation of the natural sand transport on the Belgian Continental shelf: BUDGET (<u>B</u>eneficial <u>u</u>sage of <u>d</u>ata and <u>g</u>eo-<u>e</u>nvironmental <u>t</u>echniques)

On the Belgian continental shelf (BCS), a variety of sediment dynamical studies have been performed both by governmental organisations and research institutions. Each study proposed to achieve a better insight in the sediment dynamical processes taking place on a specific spatial scale and during a particular time period. However, all these studies contain a piece of information, which contribute to the global sediment dynamical behaviour of the sediments of the BCS.In the course of the project, an overview has been produced of all these studies. Most of the data has been re-evaluated and the results were compiled in a synthesis map to characterise the natural sand transport on the Belgian continental shelf. The map indicates the general nature of the surficial sediments superimposed with the occurrence of larger bedforms. Additionally, areas are indicated where the thickness of the quaternary deposits is less than 2.5 mas these sediments might take part in the sediment transport process. To illustrate the hydrodynamics of the BCS, current ellipses have been selected based on modelling results on a 750 mgrid resolution and locations were indicated where current meter or other hydrodynamic data has been collected. Towards the directions of sediment transport, a variety of arrows are drawn whereby a distinction is made between transport vectors based on geo-environmental methods and those based on in-situ sediment transport measurements and on modelling results. If available, quantities are added uniformised in tonnes/m/day.The study also included a critical analysis of the data and methods used. The deduction of residual transport directions was evaluated on the basis of the asymmetry of bedforms, tracer experiments, sediment differentiation, current and suspended sediment concentration measurements and based on numerical sediment transport modelling. Evaluation criteria were set-up regarding the different space and time scales involved. The influence of hydro-meteorological conditions on the sediment dynamics was discussed.The results allowed defining gaps in the present knowledge and including recommendations for future research and propositions for an integrated research programme on the Belgian continental shelf. Main emphasis is put on an efficient mapping of the seafloor including the set-up of an automated characterisation of seabed sediments albeit combined with a suitable sampling strategy. Regarding hydrodynamical and sand transport measurements, the development of a multi-sensor bottom frame is recommended including a realistic quantification of sediment fluxes through the water column.To enhance the efficiency and practical use of seabed data, the set-up of an overall Geographical Information System (GIS) is highly recommended including guidelines and protocols on the prerequisites of mapping and sampling projects since this would largely facilitate the set-up and evaluation of environmental impact assessments. The project largely benefited from contributions from foreign researchers from France, England and the Netherlands

Gas and hydrate occurences in the western Black Sea

Here we present the preliminary results of the ongoing project ¸SAssessment of the gas-bearing sediments in the north-western Black Sea?T supported through a EC Marie Curie Fellowship. The project aims to integrate information from geophysical and geomorphological data in order to define a general characterization of the gas accumulations in the recent sediments of the Black Sea. Investigation is based mainly on high-resolution seismic reflection data and swath bathymetry acquired during the BlaSON and BlaSON 2 surveys by IFREMER (France) in cooperation with GeoEcomar (Romania). Sub-bottom profiles (3,5 KHz) acquired by GeoEcoMar have been also used, as well as seismic and bathymetric data from the CRIMEA EC project.The western margin of the Black Sea contains evidence of abundant subsurface gas accumulations that appear in a complete suite of settings from the coastal zone to the deep basin. Considering the variation of their features from one setting to another, we defined several distinct gas provinces:- the coastal zone in front of the Danube Delta and along the Bulgarian shore is marked by the presence of a shallow gas accumulation that causes a widespread acoustic mask up to 1-3 m depth below the seafloor.- on the shelf the typical gas facies is represented by acoustic columns from a deep source (below the penetration limit of our profiles), either reaching the seafloor or being buried under the upper shelf deposits.- the shelf-edge zone (including the external shelf and the upper slope down to about 750 m water depth) shows significant gas discharge and numerous gas seeps have already been mapped. Gas escape zones are commonly associated with subsurface acoustic turbidity and acoustic columns.- on the slope below 750 m depth, bottom simulating reflectors (BSR) were detected and indicate the probable presence of gas hydrates. A remarkable BSR pattern consisting of multiple BSR-type reflectors occurs in the Danube fan.Zones of significant gas accumulation (especially the coastal zone and the shelf edge) seem to be associated with high sedimentation rates, which may indicate a possible biogenic origin for this gas. Nevertheless, there is evidence that deep thermogenic gas sources could have contributed as well

Seismic and hydro-acoustic evidence for subsurface controls of methane seepage in the Dnepr paleo-delta, Black Sea

The Dnepr paleo-delta in the NW Black Sea is characterized by an abundant presence of methane gas seeps. The presence of these seeps was first registered by Polikarpov et al. in 1989. During the CRIMEA 2003 expedition 1062 new plumes were detected using a SIMRAD EK 500 echosounder. All newly found plumes are located on the transition zone of the continental shelf and continental slope in water depths between 70 and 725 meters. A further exploration of the area included the collection of hydro-acoustic and geophysical datasets. For this presentation we have used these new data to get a better insight in the relation between the presence of gas seeps, seafloor morphology and subsurface structures.From the integration of the new datasets (multibeam bathymetric map, side-scan sonar mosaic, seep locations and several sparker, GI-gun and sub-bottom profiles) we could make some interesting observations:seeps are often located at crests of sediment ridges and at edges of canyons;seeps are located in areas characterised by high sea-floor backscatter values;the depth limit for the presence of seeps (725 m) almost coincides with the stability boundary of pure methane hydrate;there is a clear correlation between the depth of gas fronts within the sediments and the presence of seeps at the seafloor.These observations and the apparent absence of major faults on our seismic data suggest that the subsurface controls on methane seepage in our study area are mainly stratigraphic/sedimentary in nature.Whether the presence of seeps at the crest lines of the sediment ridges is due to the presence of more resistant coarse sediments (i.e. location of seeps is controlled by morphology) or whether these crest lines stand out due to enhanced methane-derived carbonate cementation (i.e. the morphology is controlled by the location of the seeps) is still unclear. The exact cause of the high backscatter values in the seep areas (i.e. higher degree of cementation, presence of free-gas bubbles, slightly coarser sediment due to blow-out of the finer fraction by the gas escape) is at present still unclear and awaits further ground-truthing

La depresión submarina de Guaracayal, estado Sucre, Venezuela: Una barrera para la propagación de la ruptura cosísmica a lo largo de la falla de el pilar

La depresión de Guaracayal, en el golfo de Cariaco, estado Sucre, Venezuela, fue inicialmente reconocida a partir de un levantamiento batimétrico realizado en la década de los ochenta. Un levantamiento de sísmica somera de alta resolución adquirido en el golfo de Cariaco a bordo del B/O Guaiquerí II en enero 2006 reveló que esta depresión resulta ser una cuenca en tracción activa (“active pull-apart basin”) sobre la traza activa submarina de la falla dextral de El Pilar, por su geometría y lo fresco y prominente de los escarpes de fallas que la limitan. Esta cuenca, con una profundidad de aguas de ~15m mayor que el fondo plano ubicado a unos -80m, mide aproximadamente 8km de longitud en dirección este-oeste y unos 2km transversalmente. La cuenca se forma en un relevo dextro, es decir transtensivo, de la traza submarina de la falla de El Pilar, que secciona en dos porciones lo propuesto anteriormente como un único segmento de falla con extensión entre Cumaná y Casanay-Guarapiche. Esta separación entre ambas trazas de 2km parece ser suficiente barrera para la propagación lateral de la ruptura sísmica, tal como lo evidencia la sismicidad contemporánea e histórica. El tramo de falla Cumaná-Casanay, de unos 80km de longitud, ha requerido en dos ocasiones de la conjunción de dos sismos contiguos en dirección oeste-este (1797-1684 y 1929-1997) para romperse en su totalidad. No obstante, no se excluye la posibilidad de un evento que rompa toda la extensión del segmento, a pesar de este comportamiento sísmico reiterado

The 600 yr eruptive history of Villarrica Volcano (Chile) revealed by annually laminated lake sediments

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 <= 1.7%. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region, and it highlights how lake records can be used to significantly improve historical eruption records in areas that were previously uninhabited