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Iglesias J.,CSIC - Institute of Marine Sciences | Iglesias J.,University of Vigo | Ercilla G.,CSIC - Institute of Marine Sciences | Garcia-Gil S.,University of Vigo | Judd A.G.,Alan Judd Partnership
Geo-Marine Letters | Year: 2010

Pockmark-like seabed features located on the Landes Plateau, Bay of Biscay, are depressions up to 1 km across and 50 m deep according to multibeam echo sounder data. Seismic (airgun and TOPAS) profiles show that each feature comprises a stack of identical features which extend down to 300 ms (twt). Three types of depressions, elongate, irregular and circular, appear as non-truncating V-shaped features in the Plio-Quaternary sediments. These features are located above the Parentis Basin where deep faults, basement ridges and diapiric bodies extend upwards across the sedimentary cover, providing ideal migration pathways for any buoyant fluids. Initial inspection suggests that these are classic pockmarks; however, the absence of reflection truncation and the absence of indications of shallow gas beneath the features indicate that they were not formed by the removal of sediment. These are "pockforms" but not "pockmarks". This paper presents an explanation for the formation of these features, involving collapse and subsidence, sedimentary erosion, and only in some cases the erosion of seabed sediments by probable escaping fluids. These origins are mainly conducted through tectonic fluid dynamics which acted in the area up to the Late Miocene. It might be expected that these features would have been infilled by subsequent sedimentation, but their shape has been preserved because sedimentation in the area mainly comprised muds deposited from low-energy transportation (diluted gravity flows) and settling from hemipelagic suspension. © Springer-Verlag 2009. Source

Leifer I.,Bubbleology Research International | Judd A.,Alan Judd Partnership
Marine and Petroleum Geology | Year: 2015

The 22/4b blowout occurred in the UK sector of the North Sea in November 1990, but during a survey in September 2011 strong gas emissions still were occurring. This manuscript summarizes the findings of the 2011 survey and subsequent studies, considering them in the context of previous investigations, and the regional geologic and oceanographic environments.The seabed crater formed during the initial event is still there, as is a previously-undiscovered secondary crater. Seabed bubble flux estimates indicate methane emission rates of 90 L s-1 at the time; however, this methane's fate(s) remains unclear. The very strong thermocline that persists for more than half the year acts as an effective barrier to upward migration, despite the presence of strong upwelling flows around the bubble plume. Clearly a large proportion of the methane is advected away from the site to be either oxidized microbially in the water column, or released to the atmosphere as a result of normal sea:air gas exchange processes. Nevertheless, a significant atmospheric methane anomaly persists in the vicinity of the blowout site. This has been constrained to likely less than 0.72 Mscfd (5 kTon yr-1) and possibly less than 0.36 Mscfd (2.5 kton yr-1).During the late-fall to early spring months (when there is no thermocline), direct methane emissions to the atmosphere are expected to increase significantly. Also, long-term monitoring has shown that periodic eruptive events occur, which likely expel great quantities of methane. These demonstrate the dynamic nature of the system and suggest that migration pathways in and between the deep sub-seabed and seabed remain active.The 22/4b Study resulted in the development and adaptation of novel techniques that are applicable to other studies of seabed seepage and the development of a number of critical hypotheses with application to megaplume seepage by natural migration pathways or from the result of anthropogenic intervention. © 2015 Elsevier Ltd. Source

Judd A.,Alan Judd Partnership
Marine and Petroleum Geology | Year: 2015

Atmospheric methane concentrations were measured close above the sea surface during surveys in 2011 and 2012 using a Picarro cavity ring-down spectrometer. These surveys covered parts of the UK, Dutch, German, Danish and Norwegian sectors of the North Sea. The highest concentrations were recorded in block UK22/4b in a survey area around the site of blowout that occurred in 1990, and which still is known to be actively seeping gas. Variations in the methane concentration recorded during transits between ports and this and other survey areas (Sleipner and Juist) were examined in order to assess whether or not the 22/4b emissions are significant within the general context of the North Sea.The provenance of the air mass passing over the ship evidently influences the background methane concentration with additional methane introduced by local (natural and/or anthropogenic) sources; this is made apparent by the large discrepancy between pre- and post-survey measurements made in port. Background variations, estimated by smoothing the raw methane concentrations (taking the hourly average), are significant, and likely reflect both air mass provenance and local contributions. Residual values (the difference between raw and background) reflect deviations from the background, attributable to local sources. 15 residual anomalies were identified; some associated with natural sources, others with petroleum industry sources. Although these comprise lower methane concentrations than those associated with the 22/4b blowout site, they were recorded at transit speed rather than the slower survey speed, or whilst the ship was engaged in seabed operations. The speed and the chance encountering of these anomalies suggest that direct comparisons with the 22/4b methane concentrations are not valid. It is concluded that, whilst the 22/4b emissions are significant, they are not unique. Other methane sources, natural and anthropogenic, are present in the North Sea; the strength of the anomalies suggests that some may be potent. © 2015 Elsevier Ltd. Source

Vigliano P.H.,CONICET | Jones A.,Geoscience Australia | Judd A.,Alan Judd Partnership | Planas H.,National University of Comahue | Lippolt G.,CONICET
Revista de la Asociacion Geologica Argentina | Year: 2011

Gas emissions discovered on the Brazo Rincon sector of Lake Nahuel Huapi in Patagonia, are described and characterized through analysis of hydroacoustic targets rising from the bottom. Echograms target analysis allowed for the identification of two possible sources. The first one corresponding to multiple gas vents in a seep field in 30 m to 100 m water depths. The second consisting of gas, fluids, resuspended bottom debris, and sediments in near-shore shallow waters. The hydroacoustic characteristics of these plumes are described, and possible origins of gas emissions are considered in relation to geological characteristics associated to local volcanism and/or biological phenomena. Source

Munoz Sobrino C.,University of Vigo | Garcia-Moreiras I.,University of Vigo | Castro Y.,University of Vigo | Martinez Carreno N.,University of Vigo | And 4 more authors.
Quaternary Science Reviews | Year: 2014

Two sedimentary sequences (coastal and subtidal) were studied in San Simón Bay (Ría de Vigo), situated on the Atlantic coast of NW Iberia. The coastal record is a shallowing upward sequence which evidences a locally-developed low marsh, situated below the current beach, and dated at the second half of the 4th century. During the following decades this low marsh was progressively replaced by an alder swamp which formed on it. This suggests an apparent stabilisation or slow-down of the relative sea-level (RSL), in this site, at the beginning of the Dark Ages (DA). The subtidal sequence studied reflects the main changes in the landscape, the hydrological conditions, climate and RSL affecting this part of NW Iberia during the last 1250 years. Evidence of changing dinocysts content in the sediment reveals that two centennial or decadal-scale episodes existed of shelf marine waters more intensely penetrating inside the bay: between the 15th-18th centuries and at ca 1800-1930 AD. Besides, we related different proxies with the occurrence of four main climatic stages, namely the previously described Dark Ages (DA, ca350-750 AD), the Mediaeval Climatic Anomaly (MCA, ca750-1100 AD) and the Little Ice Age (LIA. ca1500-1930 AD); in addition we propose a regional MCA/LIA transition (ca1100-1500 AD) that it has not been previously described. Our environmental characterization indicates a persistent North Atlantic Oscillation (NAO) negative mode domain in Ría de Vigo during the MCA, but this became weaker during the LIA and, probably, also during the earlier DA. NAO mode become more irregular during the MCA/LIA transition, generally persisting in dominant negative mode except for a phase of minor upwelling intensification, at ca1150-1350 AD, which mainly affected the external parts of the ria. We postulate that an almost simultaneous phase (ca1100-1350 AD) of stronger continental contribution in the sediments may be related to increasing storm intensities, probably linked to a reinforcement of the Easter Atlantic (EA) pattern; and also that the intertidal/supratidal ecosystems inside San Simón Bay may have extended further in the past, at least towards the end of the 5th century, and between ca1050-1350 AD and ca1450-1750 AD. A number of local historical references are consistent with our palaeoecological data and so support the chronology proposed as well as many of the environmental changes reconstructed. This good agreement will help in the interpretation of other analogous sequences extending back in time. © 2014 Elsevier Ltd. Source

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