Potential for Abrupt Changes in Atmospheric Methane

Methane (CH4) is the second most important greenhouse gas that humans directly influence, carbon dioxide (CO2) being first. Concerns about methane’s role in abrupt climate change stem primarily from (1) the large quantities of methane stored as solid methane hydrate on the sea floor and to a lesser degree in terrestrial sediments, and the possibility that these reservoirs could become unstable in the face of future global warming, and (2) the possibility of large-scale conversion of frozen soil in the high- latitude Northern Hemisphere to methane producing wetland, due to accelerated warming at high latitudes. This chapter summarizes the current state of knowledge about these reservoirs and their potential for forcing abrupt climate change

Ecology and biogeography of free-living nematodes associated with chemosynthetic environments in the deep sea: A review

Background: Here, insight is provided into the present knowledge on free-living nematodes associated with chemosynthetic environments in the deep sea. It was investigated if the same trends of high standing stock, low diversity, and the dominance of a specialized fauna, as observed for macro-invertebrates, are also present in the nematodes in both vents and seeps.Methodology: This review is based on existing literature, in combination with integrated analysis of datasets, obtained through the Census of Marine Life program on Biogeography of Deep-Water Chemosynthetic Ecosystems (ChEss).Findings: Nematodes are often thriving in the sulphidic sediments of deep cold seeps, with standing stock values ocassionaly exceeding largely the numbers at background sites. Vents seem not characterized by elevated densities. Both chemosynthetic driven ecosystems are showing low nematode diversity, and high dominance of single species. Genera richness seems inversely correlated to vent and seep fluid emissions, associated with distinct habitat types. Deep-sea cold seeps and hydrothermal vents are, however, highly dissimilar in terms of community composition and dominant taxa. There is no unique affinity of particular nematode taxa with seeps or vents.Conclusions: It seems that shallow water relatives, rather than typical deep-sea taxa, have successfully colonized the reduced sediments of seeps at large water depth. For vents, the taxonomic similarity with adjacent regular sediments is much higher, supporting rather the importance of local adaptation, than that of long distance distribution. Likely the ephemeral nature of vents, its long distance offshore and the absence of pelagic transport mechanisms, have prevented so far the establishment of a successful and typical vent nematode fauna. Some future perspectives in meiofauna research are provided in order to get a more integrated picture of vent and seep biological processes, including all components of the marine ecosystem

Ecology and Biogeography of Free-Living Nematodes Associated with Chemosynthetic Environments in the Deep Sea: A Review

Background: Here, insight is provided into the present knowledge on free-living nematodes associated with chemosynthetic environments in the deep sea. It was investigated if the same trends of high standing stock, low diversity, and the dominance of a specialized fauna, as observed for macro-invertebrates, are also present in the nematodes in both vents and seeps. Methodology: This review is based on existing literature, in combination with integrated analysis of datasets, obtained through the Census of Marine Life program on Biogeography of Deep-Water Chemosynthetic Ecosystems (ChEss). Findings: Nematodes are often thriving in the sulphidic sediments of deep cold seeps, with standing stock values ocassionaly exceeding largely the numbers at background sites. Vents seem not characterized by elevated densities. Both chemosynthetic driven ecosystems are showing low nematode diversity, and high dominance of single species. Genera richness seems inversely correlated to vent and seep fluid emissions, associated with distinct habitat types. Deep-sea cold seeps and hydrothermal vents are, however, highly dissimilar in terms of community composition and dominant taxa. There is no unique affinity of particular nematode taxa with seeps or vents. Conclusions: It seems that shallow water relatives, rather than typical deep-sea taxa, have successfully colonized the reduced sediments of seeps at large water depth. For vents, the taxonomic similarity with adjacent regular sediments is much higher, supporting rather the importance of local adaptation, than that of long distance distribution. Likely the ephemeral nature of vents, its long distance offshore and the absence of pelagic transport mechanisms, have prevented so far the establishment of a successful and typical vent nematode fauna. Some future perspectives in meiofauna research are provided in order to get a more integrated picture of vent and seep biological processes, including all components of the marine ecosystem

Distribution and fate of methane released from submarine sources - Results of measurements using an improved in situ mass spectrometer

Methane (CH4) is the most frequent organic compound in the atmosphere and its influence on the global climate is subject of currently conducted scientific discussion. Despite its limited content in the atmosphere (1787 ppbv in 2003), it contributes to ~15 % of the global warming as a result of its 20 to 40 times higher global warming potential compared to carbon dioxide (CO2) on a 100 year timescale. One source of atmospheric methane is the release of biogenic and/or thermogenic CH4 from the oceans seafloor, which is currently one of the research priorities of the marine geosciences. These submarine sources are characterized by rising gas bubbles or diffusive methane flux into the water column. It is estimated that these point sources release a total of ~30 Tg CH4 per year into the ocean, and after its biological oxidation or dissolving in the water, ~10 Tg CH4 are released into the atmosphere per year. Additionally, due to the warming of the oceans, an increasing release of methane can be expected as a result of the melting of permafrost and gas hydrates. Steep gradients over very short distances (< 20 m) and high time-based variability (few hours) are known from dissolved methane concentrations in the water column above these submarine CH4 sources. Due to the limited number of samples taken by conventional ex situ methods, an accurate quantification of the methane distribution could hardly be estimated. Nevertheless, one objective of the present thesis was the detailed spatial representation of the dissolved CH4 in the water column originates from submarine seeps as well as the study of relevant pathways such as vertical or horizontal transport, dilution and its microbial oxidation. Therefore, the first part of the dissertation deals with the optimization and establishment of a novel underwater mass spectrometer (UWMS, Inspectr200-200, Applied Microsystems Limited ) designed for inline, real time and in situ sampling in high frequency. Analysis and evaluation of several thousand samples per day take place in one step, so that one obtains the measurement result in situ and, unlike using conventional methods, without delay, and thus the sampling strategies can be adapted to the existing environment. Additionally, through the use of this novel analytical tool, potential sources of errors that occur during sampling or transport to the laboratories are eliminated. In order to be able to use the potential of this mass spectrometer for scientific research questions, it was necessary to optimize the detection limit for the trace gases that were to be determined. For this purpose, a Stirling cooler was applied, which serves as a trapping system for water vapour and thus leads to optimized conditions for the analysis. Within the framework of this thesis two gas ebullition areas were studied in detail. While one, which is located in the continental shelf northwest of Spitsbergen, is in the center of scientific attention, the gas ebullition area that was studied in the North Sea has not yet been examined until now with regard to the methane release into the water column and its subsequent pathways. With the help of the optimized mass spectrometer it became possible for the first time to obtain distribution patterns of dissolved CH4 in the water column in high resolution. With respect to the geochemical functionality of these increasingly important methane sources, the research conducted in this dissertation contribute to improve our knowledge of the entry of CH4 into the water column as well as its fate. Therefore, the applied novel technique can contribute to revolutionize our understanding of the behavior of seep plumes as suggested by Judd and Hovland (2007)

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

Increased fluid flow activity in shallow sediments at the 3 km Long Hugin Fracture in the central North Sea

The North Sea hosts a wide variety of seafloor seeps that may be important for transfer of chemical species, such as methane, from the Earth's interior to its exterior. Here we provide geochemical and geophysical evidence for fluid flow within shallow sediments at the recently discovered, 3-km long Hugin Fracture in the Central North Sea. Although venting of gas bubbles was not observed, concentrations of dissolved methane were significantly elevated (up to six-times background values) in the water column at various locations above the fracture, and microbial mats that form in the presence of methane were observed at the seafloor. Seismic amplitude anomalies revealed a bright spot at a fault bend that may be the source of the water column methane. Sediment porewaters recovered in close proximity to the Hugin Fracture indicate the presence of fluids from two different shallow (<500m) sources: (i) a reduced fluid characterized by elevated methane concentrations and/or high levels of dissolved sulfide (up to 6 mmol L−1), and (ii) a low-chlorinity fluid (Cl ∼305 mmol L−1) that has low levels of dissolved methane and/or sulfide. The area of the seafloor affected by the presence of methane-enriched fluids is similar to the footprint of seepage from other morphological features in the North Sea

Hydrocarbon Seepage at Campeche-Sigsbee Salt Province, Southern Gulf of Mexico (Detection, Mapping, and Seafloor Manifestation)

Hydrocarbon seepage is a process during which hydrocarbon fluids are emitted from the seafloor into the water column. This phenomenon has been observed globally from continental margins to the deep abyssal. Hydrocarbon seepage has significant impacts on the marine environment such as (a) influence on sediment composition and dynamics at the seafloor, (b) increasing the habitat heterogeneity on seep biodiversity and (c) contributes to the global carbon cycle. However, the occurrence, distribution, and dynamics of hydrocarbon seepage in the marine environment, especially in the deep ocean remains unclear due to limited investigation. The northern Gulf of Mexico is a well-known prolific petroleum-producing region where numerous gas and oil emissions, associated with salt tectonism, have been observed. The Campeche-Sigsbee salt province in the southern GoM is considered to be an analog to the salt province in the northern GoM, but there has been very little research conducted in this region. Based on the occurrence of natural oil slicks on the sea surface resolved by satellite images, previous studies suggested that there is a widespread distribution of oil seeps in the Campeche-Sigsbee salt province. However, there is still a lack of direct evidence for the presence and the distribution of gas emissions. In addition to gas and oil seepage, Chapopote asphalt volcanism, a novel type of hydrocarbon seepage was first introduced in 2003. Since then, submarine asphalt deposits have attracted considerable research interest. This study aims to have a comprehensive understanding of the hydrocarbon seepage system and dynamics in the southern GoM. The objectives are to investigate the distribution of gas emissions and to understand the controlling factors on the distribution. Furthermore, detailed investigations were carried out at Challenger Knoll and Mictlan Knoll to gain a better understanding of the diverse hydrocarbon seepage system including gas and oil emissions, as well as asphalt deposits. Consequently, the research questions about the fate of the methane bubbles and the quantity of gas bubble released from gas emission site are finally addressed in this study. During research cruise M114 of R/V METEOR, a multidisciplinary approach was conducted, including hydroacoustic surveys, visual seafloor observations, and sampling of gas bubbles. Ship-based multibeam echosounder was used for seafloor bathymetry, backscatter and water column flare mapping in the Campeche-Sigsbee salt province. In addition, multibeam echosounder mounted on Autonomous Underwater Vehicle (AUV) was utilized to obtain high-resolution seafloor bathymetry, backscatter, and water column data at Mictlan Knoll. Remotely Operated Vehicle (ROV) and TV-sled were applied for investigating and documenting seafloor manifestations of hydrocarbon seepage at the seafloor. Gas bubbles were collected by pressure-tight gas bubble samplers operated by ROV at the seafloor of Mictlan Knoll for gas analyses, quantification of gas bubble emissions, and finally gas flux calculation. In summary, gas emissions are found in large numbers in the Campeche-Sigsbee salt province. Their distributions are controlled by the present geological structures. The case study in the Sigsbee Knolls suggests that the edges of flat-top knolls might provide an effective migration pathway for hydrocarbons. As there is no direct evidence for the presence of current oil seepage in the Sigsbee Knolls, we suggested that oil seepage occurs intermittently. Gas, oil seepage and asphalt volcanism are found to occur close together at the Mictlan Knoll, indicating that this diverse hydrocarbon seepage system might be a common phenomenon in the Campeche Knolls. This thesis shows the complex association between the dynamics of diverse hydrocarbon seepage and the geological controls in the southern GoM

Methane oxidation in permeable sediments at hydrocarbon seeps in the Santa Barbara Channel, California

A shallow-water area in the Santa Barbara Channel, California, known collectively as the Coal Oil Point seep field, is one of the largest natural submarine hydrocarbon emission areas in the world. Both gas and oil are seeping constantly through a predominantly sandy seabed into the ocean. This study focused on the methanotrophic activity within the surface sediments (0–15 cm) of the permeable seabed in the so-called Brian Seep area at a water depth of ~10 m. Detailed investigations of the sediment biogeochemistry of active gas vents indicated that it is driven by fast advective transport of water through the sands, resulting in a deep penetration of oxidants (oxygen, sulfate). Maxima of microbial methane consumption were found at the sediment-water interface and in deeper layers of the sediment, representing either aerobic or anaerobic oxidation of methane, respectively. Methane consumption was relatively low (0.6–8.7 mmolm−2 d−1) in comparison to gas hydratebearing fine-grained sediments on the continental shelf. The low rates and the observation of free gas migrating through permeable coastal sediments indicate that a substantial proportion of methane can escape the microbial methane filter in coastal sediments

Remote sensing and GIS-based analysis of hydrocarbon seeps: Detection, mapping, and quantification

The thesis aims to elucidate the transport and fate of hydrocarbon emissions from deep-sea seeps through the water column towards the atmosphere. An array of hydroacoustic, satellite, and optical imaging techniques was employed to detect, map, and quantify such seeps and accompanying oil and gas emissions. The major finding is that gas transport via bubbles is the overwhelming mechanism, to transfer hydrocarbons to the hydrosphere. However, only at seeps that discharge oil and gas (oily gas bubbles) these emissions might reach the sea surface and atmosphere. At other sites gas dissolves in the water column, thus not representing a primary source of atmospheric methane and higher hydrocarbon concentrations. Therefore it is suggested to focus research on oil seeps when aiming to study the potential effect of marine hydrocarbon seeps on the present climate