Neue Einblicke in Fluidfluss- und Fluidaustrittsprozesse - Fallbeispiele aus dem Nordatlantik und vor Neuseeland

In this thesis, four fluid flow systems are studied in terms of their development, venting activity, and relation to past and current climate warming periods. The studies were accomplished by using a variety of geophysical methods, including 2D and 3D seismic, sidescan sonar, and sediment echosounder methods, as well as heat flow measurements, geochemical analyses, and numerical modelling. The first case study presents a fluid flow system early in its development, located north of the Knipovich Ridge on the western Svalbard margin. Gas hydrates in this region are more widespread than anywhere else in the eastern North Atlantic, indicating a substantial gas reservoir. The origin of the gas is discussed, as are the source rock potential on the Svalbard margin and the role of the Knipovich Ridge with respect to thermogenic gas production. Case study 2 focuses on a more developed fluid flow system, the Giant Gjallar Vent in the Norwegian Sea. This vent system is one of the largest in the North Atlantic and is characterised by two prominent conduits that terminate beneath the seabed. Based on new seismic data, the activity history of the vent is re-interpreted and implications on the future development of this structure are discussed. Case study 3 deals with a deviation from the classic fluid flow system: on the upper continental margin offshore Svalbard, fluid venting occurs without fluid conduits being present, as the reservoir crops out at the seabed. Seepage is linked to dissociation of gas hydrates, which is caused by seasonal fluctuations of bottom-water temperatures. The onset of periodic gas hydrate formation and dissociation is constrained from carbonate samples and discussed with respect to ongoing decadal-scale global warming. The fourth case study looks at cold seeps on the Hikurangi Margin offshore New Zealand, which represent a fully developed fluid flow system. Seafloor expressions imaged with sidescan sonar fall into four distinct types of backscatter pattern. These patterns are integrated with observations of seabed fauna and discussed with respect to cold seep development.In dieser Arbeit werden vier Fluidflusssysteme bezüglich ihrer Entwicklung und ihrer Gasaustrittsaktivität untersucht. Außerdem ist von Interesse, ob ein Zusammenhang mit der aktuellen Klimaerwärmungsphase sowie vergangenen Klimaereignissen besteht. Für die Untersuchungen wurden verschiedene geophysikalische Methoden wie 2D- und 3D-Seismik, Sidescan-Sonar- und Sedimentecholotmethoden eingesetzt, sowie Wärmeflussmessungen, geochemische Analyseverfahren und numerische Modellierung. Das erste Fallbeispiel zeigt ein Fluidflusssystem nördlich des Knipovich-Rückens am westlichen Kontinentalrand von Spitzbergen, welches sich in einem frühen Entwicklungsstadium befindet. Gashydrate sind in dieser Region stärker verbreitet als in jedem anderen Gebiet des Nordatlantiks, was auf ein enormes Gasreservoir hindeutet. Diskutiert werden die Herkunft des Gases, das Muttergesteinpotential entlang des Kontinentalrandes, sowie die Rolle des Knipovich-Rückens hinsichtlich thermogener Gasproduktion. Fallbeispiel 2 befasst sich mit einem weiter entwickelten Fluidflusssystem – dem Giant Gjallar Vent in der Norwegischen See. Dieses Fluidflusssystem ist eines der größten im Nordatlantik und besteht aus zwei Aufstiegskanälen, welche bis unterhalb des Meeresbodens reichen. Anhand von neuen seismischen Daten wird die Aktivitätsgeschichte des Systems neu interpretiert und Auswirkungen auf die zukünftige Entwicklung dieser Struktur werden diskutiert. Beim Fallbeispiel 3 handelt es sich um eine Abweichung vom klassischen Fluidflusssystem: am oberen Kontinentalrand von Spitzbergen tritt Gas aus dem Meeresboden aus, ohne dass Aufstiegskanäle vorhanden sind, denn das Gasreservoir reicht bis an den Meeresboden. Fluidaustritte sind bedingt durch die Auflösung von Gashydraten, was durch saisonale Schwankungen in der Bodenwassertemperatur hervorgerufen wird. Der Beginn periodischer Gashydratbildung und -auflösung wird anhand von Karbonatproben bestimmt und hinsichtlich andauernder Klimaerwärmung diskutiert. Das vierte Fallbeispiel behandelt Gasaustrittsstellen (Cold Seeps) am Hikurangi-Kontinentalrand vor Neuseeland. Hierbei handelt es sich um voll entwickelte Fluidflusssysteme. Sidescan-Sonar-Bilder der Meeresbodenstrukturen der Seeps zeigen vier verschiedene Rückstreuungsmuster. Diese Muster werden mit Beobachtungen von Meeresbodenfaunen verglichen und hinsichtlich der Entwicklung von Cold Seeps diskutiert

New insights into fluid flow and seep processes - Case studies from the North Atlantic and offshore New Zealand

In this thesis, four fluid flow systems are studied in terms of their development, venting activity, and relation to past and current climate warming periods. The studies were accomplished by using a variety of geophysical methods, including 2D and 3D seismic, sidescan sonar, and sediment echosounder methods, as well as heat flow measurements, geochemical analyses, and numerical modelling. The first case study presents a fluid flow system early in its development, located north of the Knipovich Ridge on the western Svalbard margin. Gas hydrates in this region are more widespread than anywhere else in the eastern North Atlantic, indicating a substantial gas reservoir. The origin of the gas is discussed, as are the source rock potential on the Svalbard margin and the role of the Knipovich Ridge with respect to thermogenic gas production. Case study 2 focuses on a more developed fluid flow system, the Giant Gjallar Vent in the Norwegian Sea. This vent system is one of the largest in the North Atlantic and is characterised by two prominent conduits that terminate beneath the seabed. Based on new seismic data, the activity history of the vent is re-interpreted and implications on the future development of this structure are discussed. Case study 3 deals with a deviation from the classic fluid flow system: on the upper continental margin offshore Svalbard, fluid venting occurs without fluid conduits being present, as the reservoir crops out at the seabed. Seepage is linked to dissociation of gas hydrates, which is caused by seasonal fluctuations of bottom-water temperatures. The onset of periodic gas hydrate formation and dissociation is constrained from carbonate samples and discussed with respect to ongoing decadal-scale global warming. The fourth case study looks at cold seeps on the Hikurangi Margin offshore New Zealand, which represent a fully developed fluid flow system. Seafloor expressions imaged with sidescan sonar fall into four distinct types of backscatter pattern. These patterns are integrated with observations of seabed fauna and discussed with respect to cold seep development

Prediction of seismic p-wave velocity using machine learning

Measurements of seismic velocity as a function of depth are generally restricted to borehole locations and are therefore sparse in the world's oceans. Consequently, in the absence of measurements or suitable seismic data, studies requiring knowledge of seismic velocities often obtain these from simple empirical relationships. However, empirically derived velocities may be inaccurate, as they are typically limited to certain geological settings, and other parameters potentially influencing seismic velocities, such as depth to basement, crustal age, or heatflow, are not taken into account. Here, we present a machine learning approach to predict seismic p-wave velocity (vp) as a function of depth (z) for any marine location. Based on a training dataset consisting of vp(z) data from 333 boreholes and 38 geological and spatial predictors obtained from publically available global datasets, a prediction model was created using the Random Forests method. In 60 % of the tested locations, the predicted seismic velocities were superior to those calculated empirically. The results indicate a promising potential for global prediction of vp(z) data, which will allow improving geophysical models in areas lacking first-hand velocity data

Gas hydrate distribution and hydrocarbon maturation north of the Knipovich Ridge, western Svalbard margin

A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially

Fluid dynamics and slope stability offshore W-Spitsbergen: Effect of bottom water warming on gas hydrates and slope stability - Cruise No. MSM21/4 - August 12 - September 11, 2012 - Reykjavik (Iceland) - Emden (Germany)

The main goal of MSM21/4 was the study of gas hydrate system off Svalbard. We addressed this through a comprehensive scientific programme comprising dives with the manned submersible JAGO, seismic and heat flow measurements, sediment coring, water column biogeochemistry and bathymetric mapping. At the interception of the Knipovich Ridge and the continental margin of Svalbard we collected seismic data and four heat flow measurements. These measurements revealed that the extent of hydrates is significantly larger than previously thought and that the gas hydrate system is influenced by heat from the oceanic spreading centre, which may promote thermogenic methane production and thus explain the large extent of hydrates. At the landward termination of the hydrate stability zone we investigated the mechanisms that lead to degassing by taking sediment cores, sampling of carbonates during dives, and measuring the methane turn-over rates in the water column. It turned out that the observed gas seepage must have been ongoing for a long time and that decadal scale warming is an unlikely explanation for the observed seeps. Instead seasonal variations in water temperatures seem to control episodic hydrate formation and dissociation explaining the location of the observed seeps. The water column above the gas flares is rich in methane and methanotrophic microorganisms turning over most of the methane that escapes from the sea floor. We also surveyed large, until then uncharted parts of the margin in the northern part of the gas hydrate province. Here, we discovered an almost 40 km wide submarine landslide complex. This slide is unusual in the sense that it is not located at the mouth of a cross shelf trough such as other submarine landslides on the glaciated continental margins around the North Atlantic. Thus, the most widely accepted explanation for the origin of such slides, i.e. overpressure development due to deposition of glacial sediments on top of water rich contourites, is not applicable. Instead we find gas-hydrate-related bottom simulating reflectors underneath the headwalls of this slide complex, possibly indicating that subsurface fluid migration plays a major role in its genesis

Giant depressions on the Chatham Rise offshore New Zealand – Morphology, structure and possible relation to fluid expulsion and bottom currents

Highlights • Large seafloor depressions with diameters of up 10 km across have been mapped on the southern Chatham Rise, New Zealand. • Seismic reflection data show scarce indications for vertical fluid flow but no clear link between fluid flow and depressions. • Methane gas or methane hydrates appear to be absent on the southern Chatham Rise. • Seismic evidence suggests that vertical fluid flow was likely fuelled by polygonal faulting and silica diagenesis • The depressions are the results of erosion and sediment drift deposition of bottom currents associated with the Subtropical Front. Abstract Several giant seafloor depressions were investigated on the Chatham Rise offshore New Zealand using mainly bathymetric and seismic data, supplemented by sediment cores and reported porewater geochemistry data. The depressions have diameters of up to 11 km and occur on the southern flank of the Chatham Rise in water depths between 600 and 900 m, i.e. roughly underneath the location of the strongest thermal gradients of the Subtropical Front (STF) and characterized by eastward flowing currents. With up to 150 m of relief the depressions cut into post-Miocene deposits. Some of the depressions are partially filled with drift deposits that have similar seismic characteristics as the surrounding sediments and consist of alternations of silty muds and silts. Seismic profiles also show completely filled depressions that no longer have a bathymetric expression. Despite several pipe structures indicating vertical fluid flow, neither active fluid seepage nor indications for past fluid seepage are present at the seafloor of the Chatham Rise. Also, both pore water geochemistry and geophysical data do not show indications for an existing or past gas hydrate system in the area. Instead, seismic data suggest widespread polygonal faulting and the presence of silica diagenetic fronts. The release of mineral-bound water during silica diagenesis or fluid expulsion during sediment compaction can explain the presence of vertical fluid flow features but not the giant depressions themselves. Instead, the depressions are interpreted as the result of scouring by strong bottom currents for which fluid venting may have created the nucleation points

ADRIA LITHOSPHERE INVESTIGATION ALPHA - Cruise No. M86/3, January 20 - February 04, 2012, Brindisi (Italy) - Dubrovnik (Croatia)

The Adriatic Sea and underlying lithosphere remains the least investigated part of the Mediterranean Sea. To shed light on the plate tectonic setting in this central part of southern Europe, R/V METEOR cruise M86/3 set out to acquire deep penetrating seismic data in the Adriatic Sea. M86/3 formed the core of an amphibious investigation crossing Adria from the Italian Peninsula into Montenegro/Albania. A total of 111 OBS/OBH deployments were successfully carried out, in addition to 47 landstations both in Italy and Montenegro/Albania, which recorded the offshore airgun shots. In the scope of this shoreline-crossing study, the aim is to quantify the shallow geometry, deep boundaries and the architecture of the southern Adriatic crust and lithosphere and to provide insights on a possible decoupling zone between the northern and southern Adriatic domains. Investigating the structure of the Adriatic crust and lithospheric mantle and analyzing the tectonic activity are essential for understanding the mountain-building processes that underlie the neotectonics and earthquake hazard of the Periadriatic region, especially in the vicinity of local decoupling zones

Seal bypass at the Giant Gjallar Vent (Norwegian Sea): indications for a new phase of fluid venting at a 56-Ma-old fluid migration system

Highlights: • The Giant Gjallar Vent is still active in terms of fluid migration and faulting. • The Base Pleistocene Unconformity acts as a seal to upward fluid migration. • Seal bypass in at least one location leads to a new phase of fluid venting. The Giant Gjallar Vent (GGV), located in the Vøring Basin off mid-Norway, is one of the largest (~ 5 × 3 km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56 Ma. It is fed by two pipes of 440 m and 480 m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV’s reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100 m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV