Tectonic Controls on Gas Hydrate Distribution off SW Taiwan

The northern part of the South China Sea is characterized by widespread occurrence of bottom simulating reflectors (BSR) indicating the presence of marine gas hydrate. Because the area covers both a tectonically inactive passive margin and the termination of a subduction zone, the influence of tectonism on the dynamics of gas hydrate systems can be studied in this region. Geophysical data show that there are multiple thrust faults on the active margin while much fewer and smaller faults exist in the passive margin. This tectonic difference matches with a difference in the geophysical characteristics of the gas hydrate systems. High hydrate saturation derived from ocean bottom seismometer data and controlled source electromagnetic data and conspicuous high‐amplitude reflections in P‐Cable 3D seismic data above the BSR are found in the anticlinal ridges of the active margin. In contrast all geophysical evidence for the passive margin points to normal to low hydrate saturations. Geochemical analyses of gas samples collected at seep sites on the active margin show methane with heavy δ13C isotope composition, while gas collected at the passive margin shows light carbon isotope composition. Thus, we interpret the passive margin as a typical gas hydrate province fuelled by biogenic production of methane and the active margin gas hydrate system as a system that is fuelled not only by biogenic gas production but also by additional advection of thermogenic methane from the subduction system

Groundwater dynamics in coastal gravel barriers backed by freshwater lagoons and the potential for saline intrusion: Two cases from the UK

“NOTICE: this is the author’s version of a work that was accepted for publication in Journal of Marine Systems. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Marine Systems, [VOL 123, (01.08.13)] DOI 10.1016/j.jmarsys.2013.04.004". The full text is under embargo until 01.08.15

New Rhizon in situ sampler for pore water studies in aquatic sediments: For example nutrient input from submarine groundwater discharge in costal areas.

To investigate coastal biogeochemical cycles, especially at the sediment/water interface,improved sampling methods are necessary. For this purpose, we developed apore water in situ sampler with miniature sampling devices, so called Rhizons. Rhizonsoil moisture samplers have been used as sampling devices in unsaturated soilsfor the last ten years. In aquatic science they have been rarely used to extract porewater from sediments. This study presents a new developed Rhizon In Situ Sampler(RISS) as a non-destructive and inexpensive tool for in situ pore water sampling. Fieldexperiments, tracer studies and numerical modeling were combined to assess the suitabilityof Rhizons for pore water sampling. Our investigations show that the RISS isa very suitable alternative to classical methods for in situ sampling. Combined withan in situ benthic chamber system the RISS allows studies of benthic fluxes and porewater profiles at the same location with negligible effect on the incubated sedimentwater interface. This allows improved calculation and modeling of transport and reactionprocesses. Results of nutrient and freshwater input into surface water derivedby in situ sampling of tidal flat sediments of the Wadden Sea (Sahlenburg/Cuxhaven,Germany) are presented. Long term deployments of the RISS and repetitive pore watersampling at the same location might support future studies of seasonal variation ofbenthic processes in sediments of the coastal zone and open ocean

Numerical Studies on Groundwater Flow in Coastal Aquifers.

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Models for wellhead protection in regional unconfined aquifers and stratified aquifers

En este trabajo son presentados los conceptos matemáticos usados para el desarrollo de la protección de cabezales de pozos y la metodología de zonificación. Ellos están basados en soluciones analíticas, en el flujo del agua subterránea y en la modelación en diferencias finitas del rastreo de partículas. Los límites de la protección de cabezales de pozos están en función no sólo de la geología local, de los parámetros del acuífero y de las características regionales del acuífero, también de los caudales de extracción (o de la productividad del acuífero). Verticalmente el análisis fue realizado usando un modelo de flujo simétrico a un eje asumiendo que el flujo de agua subterránea hacia el pozo es radial. Las herramientas matemáticas fueron desarrolladas para ser robustas y de uso amigable, facilitando la aplicación de la nueva legislación portuguesa relacionada con la protección del agua subterránea. La aplicación de la metodología es ejemplificada en dos estudios de caso, uno desarrollado para el acuífero costero de Bardez, en el estado de Goa, India y el otro en Ramahal, Portugal

Transport and consumption of oxygen and methane in different habitats of the Håkon Mosby Mud Volcano

The Hakon Mosby Mud Volcano is a highly active methane seep hosting different chemosynthetic communities such as thiotrophic bacterial mats and siboglinid tubeworm assemblages. This study focuses on in situ measurements of methane fluxes to and from these different habitats, in comparison to benthic methane and oxygen consumption rates. By quantifying in situ oxygen, methane, and sulfide fluxes in different habitats, a spatial budget covering areas of 10-1000-m diameter was established. The range of dissolved methane efflux (770-2 mmol m(-2) d(-1)) from the center to the outer rim was associated with a decrease in temperature gradients from 46 degrees C m(-1) to < 1 degrees Cm(-1), indicating that spatial variations in fluid flow control the distribution of benthic habitats and activities. Accordingly, total oxygen uptake (TOU) varied between the different habitats by one order of magnitude from 15 mmol m(-2) d(-1) to 161 mmol m(-2) d(-1). High fluid flow rates appeared to suppress benthic activities by limiting the availability of electron acceptors. Accordingly, the highest TOU was associated with the lowest fluid flow and methane efflux. This was most likely due to the aerobic oxidation of methane, which may be more relevant as a sink for methane as previously considered in submarine ecosystems

Rhizon in situ sampler (RISS) for pore water sampling from aquatic sediments.

Rhizon soil moisture samplers were successfully used for multi-level pore water sampling from in-situ sediments and sediment cores. By this method, high resolution pore water profiles may be sampled with minimum disturbance of both, the sediment structure and possible flow fields. To investigate biogeochemical cycles, especially at the sediment/water interface, we present a newly developed Rhizon In Situ Sampler (RISS) as a non-destructive tool for in-situ pore water sampling. Field experiments, tracer studies and numerical modeling were combined to assess the suitability of Rhizons for pore water sampling. A combination of a benthic chamber with the RISS allows studies of benthic fluxes and pore water profiles at the exact same location with negligible effect on the incubated sediment/water interface. This allows improved modeling of transport and reaction processes. Long term deployments of the RISS and repetitive pore water sampling will support future studies of the seasonal variation of benthic processes

The Håkon Mosby mud volcano: 330 000 years of focused fluid flow activity at the SW Barents Sea slope

Studying the morphology and subsurface geometry of mud volcanoes provides insights into their activity. This paper describes the internal structure of the Håkon Mosby mud volcano (HMMV) in the southwestern Barents Sea and presents a conceptual model of its evolution. The lack of a mud edifice and the profuse gas flares suggest that in the recent past the mud volcano evolution was predominantly controlled by venting of gas-rich fluids and free gas. However, the analysis of high-resolution single-channel seismic (SCS) data reveals for the first time the existence of a pseudo-mud chamber at the top of the 3 km deep central conduit. It was once created at the seabed and is now a buried expression that acts as mud chamber. The pseudo-mud chamber is situated approximately 300 m below the seafloor, directly above the 330 ka Bear Island Slide (BIS) scar reflection and below glacigenic debris flow deposits that constitute the sediment on top. The sediment profiler data indicates a younger mud deposit above the debris flows, which points to a reactivation of the mud volcano. The reactivation was most likely triggered by the contrast in density between the gas-rich mud chamber and the high-density debris flow deposits. Three stages, i.e. initiation, sealing and reactivation, and a second active period define the evolution of this young mud volcano. Both, the morphology and size of the conduit as well as in-situ temperature gradients point towards a focused and rapid fluid flow.<br/