Éditeur(s) : Actes de colloques. Ifremer Brest [ACTES COLLOQ. IFREMER.]. 1986
Résumé : Seawater samples taken from shallow suspended particle maxima (20-57m) at or near the base of the surface mixed layer in various oceanic areas were tested for their potential to produce hydrogen (H sub(2)) gas. Results suggest that hydrogen can be bacterially produced through a fermentative process in microanaerobic niches within particulate matter. The organisms responsible for this process are ubiquitous within the temperate and tropical waters sampled. Enumeration of potential H sub(2)-producing bacteria using fluorescent antibody techniques yielded a maximum at the base of the surface mixed layer, the same depth where the greatest H sub(2) gas production potential was observed. Hydrogen-producing bacteria comprised up to 15% of the total bacterial direct count (AODC). Findings indicate that bacteria associated with shallow nepheloid layers are capable of producing gas and thus contributing to the observed supersaturated gas concentration in the marine environment.
Des échantillons d'eau de mer contenant des particules en suspension ont été prélevés en zone peu profonde (20 à 57 m) sur ou près de la couche brassée de surface en différents points de l'océan et ont été testés pour leur potentiel de production d'H2. Les résultats suggèrent que l'hydrogène peut être produit, par les bactéries, lors d'un processus fermentatif dans des microniches anaérobies de la matière particulaire. Les organismes responsables de ce processus sont ubiquistes dans les échantillons provenant des eaux tempérées el tropicales. Les numérations maximales de bactéries potentiellement productrices d'hydrocele, faites par des techniques d'anticorps fluorescents, sont obtenues à la base de la couche brassée de surface ; à cette même profondeur est observé le potentiel de production d'hydrogène le plus important. Les bactéries productrices d'hydrogène constituent jusqu'à 15 % des numérations bactériennes totales, effectuées par comptage direct (AODC). Ces observations indiquent que les bactéries associées aux couches nephéloïdes peu profondes sont capables de produire de l'hydrogène et contribuent ainsi à la sursaturation en gaz observée dans le milieu marin.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/1984/acte-962.pdf
http://archimer.ifremer.fr/doc/00000/962/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Excess gas phase in magmas erupting explosively is well known worldwide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the rate of gas accumulation is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreitic and trachytic composition whereas alkali basalt was emitted during preceding flank eruption. Tephra of the first explosive phase are composed of three glass types, alkaline rhyolite, mixed benmoreite, and basalt, which suggests that the basaltic magma intruded a pre-existing rhyolitic magma chamber, and ultimately triggered the eruption. The mixed benmoreitic tephra (erupted on 15 and 17 April 2010) had large 210Po in excess of 210Pb View the MathML source[(Po210/Pb210)0=1.88] at the time of eruption, and possibly a small 210Pb excess over its parent 226Ra. In contrast, the preceding flank eruption produced basalt with View the MathML source(Po210)0=0, upon eruption, and the final trachyte had lost most of its 210Po during open-system degassing. The 210Po excess in the first erupted benmoreites is interpreted to result from 210Po degassing of basaltic magma and the accumulation of 210Po-enriched gas, either in the upper part of the basaltic intrusion, below the rhyolite–basalt interface, or in the pre-existing residual rhyolitic magma chamber. From a simple model of radon and polonium accumulation in the rhyolitic reservoir, the ratio of the mass of basalt magma degassing over the mass of magma accumulating the excess gas decreased from 20 to 15 over 2 days, implying zoned magma reservoir, with the uppermost and gas-richest part erupting first. The duration of pre-eruptive gas accumulation in this model is approximately one year. This corresponds closely to the initiation of a seismic swarm beneath Eyjafjallajökull, early June 2009, which was the first pre-eruptive signal detected. The coincidence between initiation of gas accumulation at relatively shallow depth and deeper seismicity strongly suggests that the excess gas phase originated from a basalt magma batch intruded at depth, and that this gas phase reached the surface approximately a year later.
ISSN: 0012-821X

hal-01217142
https://hal.archives-ouvertes.fr/hal-01217142
DOI : 10.1016/j.epsl.2015.06.054

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A shallow field experimental site for CO2 injection was established at Maguelone (Languedoc, France), in order to test in an integrated manner a suite of surface and downhole hydrogeophysical monitoring methods. The objective is to improve monitoring of gas transport in the shallow subsurface and to determine the sensitivity of CO2 monitoring systems for leakage detection. The site offers a natural laboratory to study the processes associated with CO2 injection in a clastic and clay-rich context saturated with saline fluids. Prior to CO2 injection, three nitrogen (N2) injections were undertaken in 2012 to measure the site response to neutral gas injection. In 2013, a volume of 111 m3 (mass of 220 kg) of CO2 was injected during 3.5 h at 15 m depth. During each experiment, the gas plumes were successfully detected from pressure monitoring, time-lapse induction logging and downhole resistivity monitoring with dipole–dipole array. Increases in resistivity are attributed to free gas propagation (either N2 or CO2) whereas decreases in resistivity correlate with CO2 dissolution in the pore fluid. Chemical analyses confirm this hypothesis with a decrease in pH and an increase in the concentration of dissolved species in the latter case.
ISSN: 1750-5836

hal-01355943
https://hal.archives-ouvertes.fr/hal-01355943
DOI : 10.1016/j.ijggc.2015.12.005

Éditeur(s) : HAL CCSD
Résumé : The Karakorum Fault zone (KFZ), lying in the west of the Tibetan plateau, is an important dextral strike-slip fault and the important northern geological boundary of the west Himalaya. In the AyilaRi' gyu Range-Gai Basin area lying in the southeast of the KFZ, Ar-40/Ar-39 thermochronologic analysis was performed on the syn-tectonic minerals in the, metamorphic rocks in the Karakorum ductile shear zone, with the base of petrology and deformation structure researches. According to the microstructure features, the minerals in the shear zone recorded the continuous deformation from high ( > 600 degrees C) to low temperature ( < 250 degrees C) appeared as that the nearly horizontal dextral shearing has transformed into the oblique dextral normal slip, made the greenschist facies of deformation stack oil the early medium-high temperature deformation, indicated the uplift during the slip. The thermochronologic revealed the continuous shear process from the early Miocene to 4Ma at least, which containing three rapid cooling stage: from 25 similar to 22Ma to 21 similar to 18Ma, the deformation locally stopped or slower in the, shallow high temperature shear process; from 15Ma to 12Ma, the rapid uplift of the AyilaRi' gyu Range, the initial formation of the Gar Basin together with the main deep incision of river during the strip-slip of the KFZ; since 9Ma, the AyilaRi' gyu Range has increased the rapid uplift and the Gar Basin has formed finally. According to the different levels of deformation and the dextral displacement of the surface landform with different ages, the long term slip rate and the uplift rate indicated the intense dextral strike slip and uplift associated since the late Quarternary Period.
Acta Petrologica Sinica

hal-00411572
https://hal.archives-ouvertes.fr/hal-00411572

Éditeur(s) : Elsevier
Résumé : The Danube Canyon is a large shelf-indenting canyon that has developed seaward of the late Pleistocene paleo-Danube valley. Mechanisms of canyon evolution and factors that controlled it are revealed by analyzing the morphology and the sedimentary structure of the canyon, as well as the main features of the continental margin around the canyon. This is based on investigation by swath bathymetry in the canyon area combined with different types of seismic data. The canyon is a major erosional trough with a flat bottom cut by an entrenched axial thalweg. The thalweg path varies from highly meandering to fairly straight in relation to the local gradient. Segments of the canyon are characterized by specific morphology, orientation and gradient along the axial thalweg. We interpret these segments in terms of canyon maturity. The sedimentary structure of the canyon documents an older phase of erosion followed by partial infilling, and thus attests for repeated cycles of canyon development. Canyon morphology is interpreted as a result of erosive sediment flows along the entrenched axial thalweg that caused downcutting into the canyon bottom and instability of the canyon walls, and hence enlargement of the canyon and expansion by headward erosion. During the last lowstand level of the Black Sea the canyon was located in an area of high sediment supply close to the paleo-Danube River mouths. This is indicated by buried fluvial channels on the shelf and by a wave-cut terrace associated with a water level situated about - 90 m below the present level. We infer that erosive flows in the canyon resulted from hyperpycnal currents at the river mouths, probably favored by the low salinity environment that characterized the Black Sea during lowstand times. Other mechanisms could have contributed to trigger sediment failure along the canyon, such as instability related to the presence of shallow gas, or the effect of a deep fault.
Marine Geology (0025-3227) (Elsevier), 2004-05 , Vol. 206 , N. 1-4 , P. 249-265

Droits : 2004 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2004/publication-476.pdf
DOI:10.1016/j.margeo.2004.03.003
http://archimer.ifremer.fr/doc/00000/476/

Éditeur(s) : Elsevier
Résumé : Sediment cores acquired in 2004 off the Rhone prodelta show consistent anomalous methane concentrations of up to 87,440 ppm. Methane compositional and isotopic data support a biogenic origin, although there are a few sites that show strongly depleted delta C-13 values (-53%o PDB) suggesting a mixed source for the gas (biogenic and thermogenic). Anomalous methane concentrations (samples with more than 90 ppm) are discussed and integrated with organic carbon data, sedimentary rates and ADCP profiles. Highest gas concentrations were found directly off the river mouth (20-40 m water depth) and where the IFREMER models point to the thickest accumulation (> 2 m) in response to the Rhone flood event. In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met; in these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the sub-surface. We propose that in the Rhone prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake and leading to the potential for methanogenesis with burial.
Marine Geology (0025-3227) (Elsevier), 2006-12 , Vol. 234 , N. 1-4 , P. 215-231

Droits : 2006 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2006/publication-2396.pdf
DOI:10.1016/j.margeo.2006.09.005
http://archimer.ifremer.fr/doc/00000/2396/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
At the North-Ecuadorian convergent margin (1°S–1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake.This dataset highlights a subducted ∼30×40 km∼30×40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits.The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion.In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8–1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths.
ISSN: 0012-821X

hal-01256399
https://hal.archives-ouvertes.fr/hal-01256399
DOI : 10.1016/j.epsl.2015.10.043

Éditeur(s) : HAL CCSD Elsevier
Résumé : Adequate hydrogeophysical monitoring of CO2 geological storage remains a challenge as different parameters might be modified during storage. That implies to compare real-time measurements to an adequate baseline. At the Maguelone shallow experimental site a representative baseline for electrical resistivity was built from a large number of downhole geophysical measurements. At this coastal site this issue is particularly important due to the production of biogenic gas from the subsurface sediments. For this, a modified petrophysical model based on the Waxman-Smits model is proposed to estimate gas saturation found to vary from 2 to 7% within shallow sand layers.
ISSN: 1876-6102

hal-01412685
https://hal.archives-ouvertes.fr/hal-01412685
https://hal.archives-ouvertes.fr/hal-01412685/document
https://hal.archives-ouvertes.fr/hal-01412685/file/pezard2015.pdf
DOI : 10.1016/j.egypro.2015.07.910

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
To evaluate the performance of downhole and surface geophysical monitoring methods, a series of shallow gas injection-monitoring experiments has been performed in a coastal saline aquifer at Maguelone, France. The recorded data include pressure measurements with a Westbay multilevel completion and CO2 saturation at an observation well derived from electrical resistivity with a modified Waxman-Smits (MWS) model. In this work, the aim is to develop a simulation model capturing the gas transport behavior and consistent with field data. For this purpose, the simulation of the CO2 injection experiment is carried out with two conceptual models, a homogeneous model and a heterogeneous model treated with multiple realization Monte Carlo simulations. Numerical simulator TOUGH2 with the equation of state module EOS7C is used for the simulations. Comparison of the model results with field data suggests that the pressure responses are captured with relatively good accuracy. Similarly, the model also provides an overall reasonable agreement and correct order of magnitude for predicted gas saturation values. However, as the heterogeneity pattern in the field data remains largely unknown, the model predictions can only be used to capture the mean behavior as well as to provide insights into how heterogeneity can influence the system behavior, by means of sensitivity analyses of the influence of heterogeneities on individual realizations.
ISSN: 1750-5836

hal-01467240
https://hal.archives-ouvertes.fr/hal-01467240
DOI : 10.1016/j.ijggc.2016.09.006

Éditeur(s) : Springer
Résumé : The Håkon Mosby mud volcano is a 1.5-km-diameter geological structure located on the Southwest Barents Sea slope at a water depth of 1,270 m. High-definition seabed mapping of the mud volcano has been carried out in 2003 and 2006. A comparative analysis of the bathymetry and backscatter maps produced from the two surveys shows subtle morphological changes over the entire crater of the mud volcano, interpreted to be the consequence of mud eruption events. Mud temperature measurements point to a persistently warm mud at shallow depth in the crater. This is explained by upward fluid advection, rather than conductive cooling of mud flows. The small-scale spatial variability in the temperature distribution may be related to mud outflows or changes in the fluid flow regime. Furthermore, the locations of free gas venting observed in 2006 were found to differ from those of 2003. Our observations of overall similar topographic profiles across the mud volcano in 2003 and 2006 suggest that eruption events would have been modest. Nevertheless, the data bring evidence of significant change in activity even over short time intervals of only 3 years. This may be a characteristic shared by other submarine mud volcanoes, notably those considered to be in a quiescent stage.
Geo-marine Letters (0276-0460) (Springer), 2010-06 , Vol. 30 , N. 3-4 , P. 157-167

Droits : Springer-Verlag 2010
http://archimer.ifremer.fr/doc/00004/11527/10261.pdf
DOI:10.1007/s00367-010-0193-z
http://archimer.ifremer.fr/doc/00004/11527/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Urucum region (Mato Grosso do Sul, Brazil) contains Neoproterozoic–Cambrian manganese ore in the layers Mn-1, Mn-2, and Mn-3, which are interbedded with iron formations (IFs) from the Santa Cruz Formation (Jacadigo Group) and surrounded by carbonate rocks from the Bocaina and Tamengo Formations (Corumbá Group). These rocks fill an ancient graben, with sediments from the Santa Cruz Formation overlying fluvial deposits from the Urucum Formation and limestones from the Bocaina and Tamengo Formations deposited in the shallow marginal regions. As the graben deepened, it was flooded with microbe-rich seawater and hydrothermalism began. Iron and manganese precipitated within the pores in arkosic sandstones, thereby forming the clastic Lower Mn-1 ore layer. The remaining manganese was simultaneously deposited on the basin floor with clastic clay and silt from turbidity currents, thereby forming the Upper Mn-1 clastic–chemical ore layer. The sedimentation of Mn-1 and its host IFs was controlled by seawater saturation and homogeneous oxidation, which caused manganese and iron sedimentation and the death of microorganisms, generating mangano-ferriferous sediments with high P, Co and Ni contents. Mn-2 formed during periods of tectonic quiescence through water influxes that carried abundant microbes, Fe(II), Mn(II), and SiO2. Under UV radiation, these microbes oxidized Mn(II) and Fe(II) to form hydroxides and died, thereby forming biomass that was enriched in P, Co and Ni and minor carbonate adjacent to and below the oxycline. These microbes also formed kremydilites, oblate structures that resemble manganese stromatolites, within the biomass and produced CH4 and/or H2 gas bubbles. During early diagenesis, wormlike organisms inhabited the biomass and probably separated iron from manganese to produce cryptomelane microbialites, which agglutinated to form manganese micronodules that coalesced to form the massive layer Mn-2. This process ended with the disappearance of microorganisms and the sedimentation of the IFs, and a new water influx generated layer Mn-3 via the same process. The final seawater upwelling precipitated IFs in the basin depocenter, whereas limestone from the Tamengo Formation was deposited along the basin’s edges. The diagenesis initially transformed the manganese oxides into cryptomelane and then into hollandite.
ISSN: 0169-1368

hal-01684668
https://hal.archives-ouvertes.fr/hal-01684668
DOI : 10.1016/j.oregeorev.2017.09.018

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon using 3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 Integrated Ocean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations in pore water show that sulphate-methane transition zone, at 74 and 94 m below seafloor, are amongst the deepest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channel (Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, preventing normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic and focused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes at the seabed. This may also favour seawater circulation and we interpret the deep sulphate-methane transition zones as a result of high downward sulphate fluxes coming from seawater that are about 5-10 times above those measured in other basins. The results show that geochemical reactions within shallow sediments are dominated by seawater downwelling in the Mars-Ursa basin, compared to other basins in which the upward fluid flux is controlling methane-related reactions. This has implications for the occurrence of gas hydrates in the subsurface and is evidence of the active connection between buried sediments and the water column.
ISSN: 1468-8115

hal-00590501
https://hal.archives-ouvertes.fr/hal-00590501
DOI : 10.1111/j.1468-8123.2010.00304.x