Natural and anthropogenic fluid migration pathways in marine sediments

Fluids are an important agent in nearly all geologic processes that shape the planet Earth. Fluid abundance and composition are governed by flow along permeable beds or natural and anthropogenic structures in the subsurface including faults, wells, and chimneys/pipes. Spatial and temporal variations in fluid flow activity modify total fluxes between geosphere, cryosphere, hydrosphere, and atmosphere. These fluxes have broad implications for geological processes including the formation of natural resources or the occurrence of geohazards including landslides, earthquakes and blowouts. They further play a crucial role for the global carbon cycles and the climate system. A qualitative and quantitative understanding of fluid flow in the subsurface is therefore important to assess the role of fluids in the Earth system and to quantify fluxes from the geosphere into the hydro- and atmosphere. In this Ph.D. thesis I use an integrated, interdisciplinary approach to study natural and anthropogenic fluid migration pathways in marine sediments in the North Sea, the convergent Hikurangi margin, and a section of the ancient Tethys margin which is now exposed near Varna, Bulgaria. The applied methods include conventional 3D seismic, high-resolution 3D seismic, and 2D seismic data as well as hydroacoustic, sedimentological, unmanned aerial vehicle-based photogrammetric and geochemical data. In each of the studied systems, natural and/or anthropogenic fluid migration pathways allow the transport of significant amounts of fluids through marine sediments towards the seafloor. Often the co-existence of multiple pathways enables the fluids to bypass permeability barriers within the Earth’s crust resulting in the formation of structurally complex flow systems. Focused fluid flow along normal faults in the Hikurangi margin likely plays an active role in the subduction drainage system, influences the slope stability and the morphotectonic evolution of the margin. Results from the Eocene Tethys margin show that focused fluid flow in marine sediments is possible in unconsolidated sands if seepage is focused at the top of faulted units and the flux rate is high enough. This stands in contrast to the general assumption that focused fluid flow in marine sediments is limited to low-permeable sediments. In the marine environment the term fluid flow is often used to exclusively refer to the flow of hydrocarbons. However, geochemical data from the North Sea and the Tethys margin indicate that the involved fluids are of different origin including compaction-related dehydration and submarine groundwater discharge. In each of the investigated cases, the temporal and spatial evolution of fluid flow is not fully addressed yet, especially with regard to vertical fluid conduits or the safety of subsurface drilling and storage operations. The results of my thesis highlight that the investigation of fluid migration pathways requires an interdisciplinary approach which may indicate the origin of the fluids, help understand the fluxes of fluids from the geosphere into the hydrosphere and atmosphere of the past, present and future and reveal the resulting consequences for the global carbon cycles and the climate system

Dense Cores in Galactic Cirrus Clouds

In this thesis I study the formation and evolution of dense cores in cirrus clouds. Cirrus clouds are diffuse and translucent molecular clouds widely spread within the galaxy. Dense cores in molecular clouds are the locations of the origin of star formation. The knowledge of how cores form and how they evolve is the key to understand the initial conditions of the star formation process. Especially the starting conditions are still poorly known, despite recent progress. Studies of cores in various regions with different physical conditions will help to assess the important processes within the formation and evolution of these cores. A fair number of investigations have already been made towards regions with known star formation. However, there the processes are often more complicated by the feedback actions of new-born or young stars. Translucent cirrus clouds, on the other hand, are relatively simple and quiescent objects, dominated mostly by turbulent gas motions. Due to the lack of active star formation they are thought to show a more simple behaviour than many of the dark molecular clouds. The investigation of cirrus cloud cores could therefore help to reveal the importance of particular conditions and events. Using the IRAM 30-m radio telescope and the bolometer arrays I observed a small sample of 5 dense cores in cirrus clouds in the thermal dust continuum. The dust continuum emission appears to be one of the best tracers of the H2 column density and is particularly suited to locate and map the core regions. However, it does not provide any kinematic information and hence no access to the kinetic energy in the cores. Additionally, I observed the cores with the FCRAO 14-m radio telescope in the CS (2 → 1) transition, and several other molecular lines with the IRAM 30-m telescope. Molecular line observations provide kinematic properties, but because of abundance variations they are often difficult to interpret. Hence, the gas chemistry in the core becomes important and has to be considered. Together, these data provide the possibility to obtain a more realistic view of the core properties. I calculate core parameters and analyse the physical conditions. A comparison of cores in cirrus clouds with cores in star-forming regions and dark clouds shows the similarities but also some important differences. One particular core is observed in even more detail using the Plateau de Bure and the OVRO interferometer in CS and HC3N. These data reveal most interesting insights into the core sub-structure and demonstrate the need for observations with high spatial resolution. The star-forming ability of the studied cores is discussed, together with the question if cirrus clouds are able to form stars at all.Dichte Kerne in galaktischen Zirruswolken In dieser Arbeit wurde eine kleine Auswahl von 5 dichten Kernen in galaktischen Zirruswolken genauer untersucht, um das generelle Potential dieser Wolken, Sterne oder braune Zwerge bilden zu klonnen, zu erforschen. Bisher ist es vollkommen unbekannt, inwieweit solche diffusen Wolken überhaupt diese Fähigkeit besitzen. Allerdings ist es unverzichtbar für unser Verständnis des Prozesses der Sternbildung im allgemeinen, da die Anfangsbedingungen bei der Entstehung Sterne niedriger Massen noch immer nur wenig begriffen sind. Die Bildung und Entwicklung eines molekularen Kernes ist dabei ein kritischer Hauptpunkt des ganzen Prozesses. In Regionen mit bekannter Sternentstehungsaktivität werden Untersuchungen von prä-stellaren Kernen oft durch die Aktivitäten nahegelegener junger Sterne behindert. Findet man Kerne, die Sterne niedriger Masse bilden können, in einem relativ ruhigen Umfeld, könnte das sehr helfen die Anfangsbedingungen der Sternentstehung genauer zu bestimmen. Es wurde die thermische Kontinuumsstrahlung des Staubes bei 1,2mm mit den MAMBO Arrays am IRAM 30m Teleskop beobachtet. Der Nachweis aller 5 Kerne kann hier berichtet werden, und es wurden die fundamentalen Eigenschaften der Kerne daraus abgeleitet. Wir folgern, daß Kerne in Zirruswolken grundsätzlich ähnlich zu Kernen in Sternentstehungsgebieten oder Dunkelwolken sind, allerdings im unteren Massen und Dichten Bereich angesiedelt sind. Dieses Ergebnis bestätigt auf perfekte Weise unsere Erwartungen. Aber, die Kerne zeigen keineswegs eine einfache, sphärische Geometrie, sondern sind größtenteils gestreckt und unterteilen sich in eine Vielzahl von Unterklumpen. Unsere Ergebnisse deuten daher auf die Entstehung dieser Kerne durch turbulente Prozesse hin. Wir können mit diesen Daten leider noch nicht eindeutig entscheiden, inwieweit sie durch Selbst-Gravitation beeinflußt sind. Desweiteren wurde die CS (2 → 1) Linienstrahlung mit dem FCRAO 14-m und dem IRAM 30-m Teleskop beobachtet. Auch hier konnten alle Kerne nachgewiesen werden, was die relativ hohen Dichten, abgeleitet aus den Staub-Kontinuum Beobachtungen, bestätigt. Obwohl die räumliche Auflösung bei den Staub und Molekfullinien Beobachtungen unterschiedlich ist, lassen sich doch erhebliche Unterschiede in der räumlichen Verteilung der beiden Indikatoren feststellen. Das ist nicht unbedingt überraschend, da die Staub Kontinuumsstrahlung lediglich ein integriertes Bild liefert. Allerdings zeigt die CS-Linienstrahlung nicht allzuviel Unterstruktur in der Geschwindigkeit, sondern erscheint vielmehr tatsächlich unterschiedlich räumlich verteilt zu sein. Wir haben außerdem nach weiteren Indikatoren für dichtes Gas Ausschau gehalten. So wurden die Kerne ebenfalls in der HC3N(10 → 9) Linie und dem CS (5 → 4) Übergang mit dem IRAM 30-,m Teleskop beobachtet. Bedauerlicherweise konnten wir diese Linien in keinem weiteren Kern detektieren, außer in dem bereits zuvor untersuchten Kern in MCLD123.5+24.9. Zum Teil wurde dies durch nicht ideale Wetterbedingungen während der Beobachtungen verursacht. Aber auch ein Nicht-Nachweis liefert bereits obere Grenzen für die Häufigkeit des Moleküls. Wir führten daher LVG und RADEX Analysen durch, um die Molekülhäufigkeiten in den Kernen zu bestimmen. Ein Vergleich mit einem die Zeitabhängigkeit berücksichtigendem chemischen Modell, ursprünglich entworfen für einen dichten Kern in Taurus, TMC1, und mit anderen Kernen in Sternentstehungsgebieten und Dunkelwolken zeigt, daß die Kerne chemisch jung sind. MCLD123.5+24.9 bildet dabei möglicherweise eine Ausnahme. Wir denken, daß wir diesen Befund mit einer sehr klumpigen Struktur der Zirrus-Kerne erklären können, verursacht durch ihre Entstehung durch turbulente Prozesse. Die dichten Kerne in den galaktischen Zirruswolken fragmentieren in eine große Anzahl kleiner Klumpen, die in einem etwas weniger dichten Zwischenklumpen-Medium eingebettet sind. Allerdings können wohl nur die langlebigeren, etwas größeren und dichteren Klumpen höhere Häufigkeiten von Molekülen wie HC3N oder NH3 ausbilden. Das CS-Molekül, andererseits, wird sehr schnell gebildet, sofern die kritische Dichte erreicht ist und ist auch in dem Zwischenklumpen-Medium recht häufig. Es ist sogar möglich, daß CS in einigen der Klumpen bereits abgereichert wird. Dies geschieht ebenfalls ab einer bestimmten Dichte, z.B., durch ausfrieren der Moleküle auf Staubkörnern. Ein wichtiges Ergebnis ist die Entdeckung der systematisch größeren Linienbreiten des CS (2 → 1) Übergangs, verglichen mit anderen Molekülen wie C18O oder SO. Dies wird, sehr wahrscheinlich, durch die Überlagerung und Vermischung der Linien verschiedener Unterklumpen, zusätzlich verwischt durch die Strahlung des Zwischenklumpen-Mediums, verursacht. Insgesamt betrachtet müssen wir feststellen, daß sich das CS Molekül nicht sehr gut eignet, um die kinetische Energie dieser Kerne zu bestimmen. Das wird noch zusätzlich durch Effkte der Selbstabsorption des CS (2 → 1) Überganges erschwert. Infolge der Schwierigkeiten, bedingt durch die sehr klumpige Struktur der Kerne und die starken Häufigkeitsänderungen, konnten wir noch nicht entscheiden, inwieweit die Kerne in der Lage sind Sterne oder braune Zwerge zu bilden. Unsere Interpretation der Daten deutet auf eine Entstehung der Kerne aus rein turbulenten Prozessen hin. Daher ist es möglich, daß es sich um vorrübergehende Erscheinungen handelt, das heißt die Kerne lösen sich nach einiger Zeit wieder auf. Aber die weitere Entwicklung ist noch sehr unbestimmt, da es durchaus auch möglich ist, daß sich mehrere der kleinen Klumpen vereinigen und so gravitativ gebunden werden. Dieses Szenario wird von uns zumindest für den Kern in MCLD123.5+24.9, den am besten untersuchten, stark favorisiert. Hier tendieren wir dazu, die Bildung eines prä-stellaren Kernes zu folgern. Allerdings sind zusätzliche Beobachtungen mit der höchsten verfügbaren räumlichen Auflösung und ein spezielles, detailiertes Modell nötig um die Situation eindeutig zu klären. Diese Arbeit wird sehr zeitaufwändig sein, allerdings kann das Resultat wesentlich dazu beitragen die Grundlagen der Sternentstehung von Sternen niedriger Masse besser zu verstehen und so möglicherweise sogar zu einem besseren Verständnis des Erscheinungsbildes unserer Galaxie beitragen

An 1888 Volcanic Collapse Becomes a Benchmark for Tsunami Models

When volcanic mountains slide into the sea, they trigger tsunamis. How big are these waves, and how far away can they do damage? Ritter Island provides some answers

Spontaneously exsolved free gas during major storms as an ephemeral gas source for pockmark formation

Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre-existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm-induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear. Key Points - Storm-induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloor - This process is driven by pressure-sensitive phase instabilities - This mechanism could help explain elusive gas sources in recently observed pockmarks in the North Se

RV SONNE 241 Cruise Report / Fahrtbericht, Manzanillo, 23.6.2015 – Guayaquil, 24.7.2015 : SO241 - MAKS: Magmatism induced carbon escape from marine sediments as a climate driver – Guaymas Basin, Gulf of California

SO241 set out to test the hypothesis that rift-related magmatism is able to increase carbon emissions from sedimentary basins to the extent that they can actively force climate. To this end we investigated a study area in the Guaymas Basin in the Gulf of California which is one of very few geological settings where rift-related magmatism presently leads to magmatic intrusions into a sediment basin. During the cruise we collected 1100 km of 2D seismic lines to image the extent and volume of magmatic intrusions as well as the extent of metamorphic overprinting of the surrounding sediments and associated subsurface sediment mobilization. We selected three typical seep sites above magmatic intrusions for detailed geochemical studies using gravity corers, multicorers and TV grab. With these samples we will be able to determine the pore water composition to assess the amount and composition of hydrocarbon compounds that are released from these systems. Detailed ocean bottom seismometer measurements at a seep site in the center of the Guaymas Basin will provide further insights into effects of magmatic intrusions on carbon release and diagenetic overprinting of the sediments. It will be possible to reconstruct its long-term seepage history from big carbonate blocks that we have collected with a TV-grab. The northeastern margin of the Guaymas Basin is known for the presence of gas hydrates. During the cruise we collected several seismic lines, which show a clear but unusually shallow BSR indicating high heat flow in the region. Using the seismic data we discovered a previously unknown geological structure on the flank of the northern rift segment: a large mound that seems to consist entirely of black smoker deposits. It seems to be the result of a recent intrusion into the underlying sediments and changes the view how such systems function. The structure was investigated with a comprehensive geochemical, geothermal, and video surveying program which revealed at least seven vents that are active simultaneously. These vents inject methane and helium-rich vent fluids several hundred meters up into the water column. These findings suggest that large-scale magmatism, for example during the opening of an ocean basin under the influence of a hot spot, can be an effective way of liberating large amounts of carbon high up into the water column. The data collected during SO241 will allow us to constrain the amount of carbon that can escape into the atmosphere during LIP emplacement and their relevance on a global scale can be assessed. In addition to reaching the main objectives of the project we discovered a large landslide complex that was probably associated with a tsunami

Pockmarks in the Witch Ground Basin, central north sea

Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2‐D and 3‐D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km2. The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (>6 m deep, >250 m long, and >75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9–3.1 m deep, 26–140 m long, and 14–57 m wide) and are limited to the soft, fine‐grained sediments of the Witch Ground Formation and possibly sourced by compaction‐related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present‐day fluxes but require an analysis of the pockmark forming processes through geological time

Sector collapse kinematics and tsunami implications - SEKT, Cruise No. M154/1, April 3 - April 25, 2019, Mindelo (Cape Verde) - Point-á-Pitre (Guadeloupe)

Summary Deep-seated collapses of volcanic islands have generated the largest volume mass flows worldwide. These mass flows might trigger mega-tsunamis. The way in which these collapse events are emplaced is poorly understood, even though this emplacement process determines the scale of associated tsunamis. Key questions such as whether they are emplaced in single or multiple events, how they may incorporate seafloor sediment to increase their volume, and how they are related to volcanic eruption cycles and migration of volcanic centers, remain to be answered. This project forms a part of the comprehensive study of large volcanic island landslide deposits and is directly linked to IODP drilling campaign in the Lesser Antilles (IODP Leg 340). Unfortunately, Leg 340 only recovered material from a single site within the volcanic landslide deposits off Montserrat, and even at this site, recovery was not continuous. This single IODP site is insufficient to document lateral variation in landslide character, which is critical for understanding how it was emplaced. The main scientific goals of this project are to determine where the landslides are sourced from; to understand how these landslides are emplaced; and to understand the relationship between landslides, eruption cycles and initiation of new volcanic centres. Combining 3D seismology (Leg 1) and MeBo cores (Leg 2) provides a unique dataset of the internal structure, composition and source of material throughout a volcanic island landslide. The results will significantly contribute to understanding the emplacement of volcanic island landslides and they will allow us to assess the associated tsunami risk

Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies

Highlights • Gas release from wells may counteract efforts to mitigate greenhouse gas emissions. • An approach for assessing methane release from marine decommissioned wells. • This gas release largely depends on the presence of shallow gas accumulations. • Methane release from hydrocarbon wells represents a major source in the North Sea. Abstract Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000 m below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence interval = 0.7-4.2] kt yr−1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets

From gradual spreading to catastrophic collapse - Reconstruction of the 1888 Ritter Island volcanic sector collapse from high-resolution 3D seismic data

Volcanic island flank collapses have the potential to trigger devastating tsunamis threatening coastal communities and infrastructure. The 1888 sector collapse of Ritter Island, Papua New Guinea (in the following called Ritter) is the most voluminous volcanic island flank collapse in historic times. The associated tsunami had run-up heights of more than 20 m on the neighboring islands and reached settlements 600 km away from its source. This event provides an opportunity to advance our understanding of volcanic landslide-tsunami hazards. Here, we present a detailed reconstruction of the 1888 Ritter sector collapse based on high-resolution 2D and 3D seismic and bathymetric data covering the failed volcanic edifice and the associated mass-movement deposits. The 3D seismic data reveal that the catastrophic collapse of Ritter occurred in two phases: (1) Ritter was first affected by deep-seated, gradual spreading over a long time period, which is manifest in pronounced compressional deformation within the volcanic edifice and the adjacent seafloor sediments. A scoria cone at the foot of Ritter acted as a buttress, influencing the displacement and deformation of the western flank of the volcano and causing shearing within the volcanic edifice. (2) During the final, catastrophic phase of the collapse, about 2.4 km³ of Ritter disintegrated almost entirely and travelled as a highly energetic mass flow, which incised the underlying sediment. The irregular topography west of Ritter is a product of both compressional deformation and erosion. A crater-like depression underlying the recent volcanic cone and eyewitness accounts suggest that an explosion may have accompanied the catastrophic collapse. Our findings demonstrate that volcanic sector collapses may transform from slow gravitational deformation to catastrophic collapse. Understanding the processes involved in such a transformation is crucial for assessing the hazard potential of other volcanoes with slowly deforming flanks such as Mt. Etna or Kilauea