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)

DISTRIBUTION AND FATE OF METHANE RELEASED FROM SUBMARINE SOURCES – CHALLENGES AND RESULTS OF MEASUREMENTS BY 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. One source of atmospheric methane is the release of CH4 from the oceans seafloor. These submarine sources are characterized by rising gas bubbles or diffusive methane flux into the water column. Due to the limited number of samples taken by conventional ex situ methods, an accurate quantification of the methane distribution could hardly be estimated. With the help of an optimized mass spectrometer (9 years of ongoing engineering) it became possible to obtain distribution patterns of dissolved CH4 in the water column in high resolution. In this talk I will present the challenges of the work with the Inspectr200-200 during the last 9 years and, from the scientific point of view, the detection and mapping of submarine released methane as well as the study of relevant pathways and its potential contribution to the atmospheric methane budget

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 Summary 11 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. In particular for CH4, the detection limit could be decreased from more than 100 nmol L-1 to 16 nmol L-1. 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. The global climatic change in the Arctic regions can be increasingly monitored. Thus years an increase in ocean temperature was observed in the last 30, which potentially leads to destabilization of gas hydrates and reduced methane storage capacity of the sediments. In the studied area with a size of 175 km² and an average water depth of 245 m, 10 gas ebullition locations, which were possibly induced by these climatic changes, were detected by using hydroacoustic methods. In this study, the release of dissolved CH4 into the water column and its subsequent lateral and vertical transportation, microbial oxidation and dilution were investigated. For this purpose, methane concentrations, isotopic ratios and oxidation rates were determined and compared with hydroacoustic and oceanographic data. The study area is influenced by the northwards flowing West Spitsbergen current (WSC), which leads to a stratification (about 20 m above ground) of the water column due to salinity differences. With the help of detailed sampling and a subsequent modeling, we determined that ~80 % of the methane contained in the ascending gas bubbles is dissolved under the pycnocline into the surrounding water and leads to a locally increased CH4 concentration. Even though rise heights of up to 50 m under the sea surface were detected by means of hydroacoustics, a direct transport of methane into the atmosphere via gas bubbles can be excluded due to the fast dissolution of the gaseous CH4 into the ambient water. Summary 12 Therefore, the vertical transport of dissolved CH4 was studied in more detail as a potential source for atmospheric methane. The observed pycnocline represents a limitation for the vertical transportation of dissolved CH4. Therefore, dissolved methane is mostly laterally transported underneath the pycnocline to greater depths and is, as shown by the determinations of the oxidation rates, microbiological oxidized over time. As long as there is a stratification of the water column, the transport of methane in the gas bubbles (about ~20 % of the entire amount of CH4) into the water masses above the pycnocline and the subsequent dissolution of the gas bubbles represent the only potential pathway of methane into the atmosphere. The investigations at the gas seepage area in the Netherland economic zone in the North Sea reveal significant differences regarding to the fate of dissolved CH4 in the water column. It has been shown in previous studies that most probably the gas source is a gas reservoir at a sediment depth of 600 m, which reaches the sediment surface through fractures (gas chimneys) in the sediment structure. Our video observations indicated 113 gas streams with an estimated release of 35.3 + 17.65 t CH4 yr-1. The dissolved methane concentration in various depths was measured in high resolution with the optimized in situ mass spectrometer. This data set, consisting of 11.900 single measurements sampled in between 24 hours, represents the dissolved CH4 concentration above a gas seepage in up to 750 times higher temporal and spatial resolution than would be possible by conventional methods. Highest methane concentrations (~3.5 μmol L-1) were detected in the surrounding water of the gas bubble streams and the inventory in the entire water column was calculated to a total amount of ~6.4*105 μmol. Due to the unique measurements, these results are not yet comparable with other gas seep areas. During the time of measurement the oceanographic data reveal a pycnocline, which we indicated as the main forcing factor for the pathway of methane at the gas seepage in the shelf area offshore West Spitsbergen. However, due to the short distance to the seabed (~10 m), a high grade of gas bubble dissolution takes place in the mixed layer above the pycnocline, and we consider that ~40 % of the total seabed released methane could enter the Summary 13 atmosphere via this indirect pathway. Additionally, we measured direct CH4 transport via gas bubbles of ~25 % by discrete gas bubble sampling at the sea surface, which emphasize, that 65 % (23 + 11.5 t CH4 yr-1) of the entire seabed released CH4 potentially contributing to the atmospheric methane budget, which is far above most studied gas seepages. 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)

In Situ Mass Spectrometry in Marine Science; Distribution and Quantification of Submarine Methane Sources

We present the in situ use of an underwater mass spectrometer (UWMS) for the detection and quantification of active submarine methane seeps. The key benefit of using this novel technique is the rapid instrumental response, which allows an up to 750 times higher sampling frequency of dissolved gas concentrations compared to other methods. Among other major and trace gases, the increased data density and accuracy allows for the improved determination of methane point-sources. Methane is the most abundant organic compound in the atmosphere and its influence on global climate is the subject of current scientific discussion. One source of atmospheric methane with variable strength is the ocean’s seafloor e.g. through the destabilization of gas hydrates. These submarine methane sources are characterized by rising gas bubbles and/or the diffusive flux of gas into the water column. While ex situ sampling methods are limited by the time and costs, the primary limitation is the difficulty in capturing concentrations that are highly dynamic in time and space. Using an optimized UWMS (significantly improved in detection limit by a cryo-trap system) in the North Sea (Atlantic Ocean), we have obtained high-resolution 3D distribution patterns of dissolved methane in the water column. In addition to presenting novel methodologies using in situ mass spectrometry to detect, map and calculate the methane inventory above submarine sources, we will present a new configuration of instruments to determine the diffusive transportation rates at the sediment-water-transition-zone

First year of routine measurements at the AWI MICADAS 14C dating facility.

In November 2016, the first Mini-Carbon-Dating-System (MICADAS) manufactured by Ionplus AG was delivered and installed at the Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research (AWI), Germany. After one year of establishing the instrument and preparation methods, we started routine operation for scientific purposes in January 2018. The new facility includes a graphitization unit (AGE3) connected to an elemental analyser (EA) or a carbonate handling system (CHS), and a gas inlet system (GIS). The facility at AWI focuses on analysing carbonaceous materials from samples of marine sediments, sea-ice, and water to investigate various aspects of the global carbon cycle. A particular emphasis will be on sediments from high-latitude oceans, in which radiocarbon-based age models are often difficult to obtain due to the scarcity of carbonate microfossils (e.g., foraminifera). One advantage of the MICADAS is the potential to analyse samples as CO2 gas, which allows radiocarbon measurements on samples containing as little as 10 µgC. For example, it is possible to determine 14C ages of foraminifera from carbonate-lean sediments allowing paleoclimate reconstructions in key locations for the Earth’s climate system, such as the Southern Ocean. Likewise, compound-specific 14C analyses receive growing attention in carbon cycle studies and require handling of small samples of typically <100 µgC. The wide range of applications including gas analyses (e.g., foraminifera and isolated compounds), and graphite targets require establishing routine protocols for various methods including sample preparation and precise blank assessment. We report on our standard procedures for dating organic matter from sediments or water including carbonate removal, combustion and graphitization using the AGE3 coupled to the EA, as well as on the methodology applied for carbonate samples using the CHS system and the GIS. We have investigated different sample preparation protocols and present the results using international standard reference materials (e.g., IAEA-C2 F14C = 0.4132 + 0.0052 (n= 14); Ref = 0.4114 + 0.0003). Additionally, we present the first results of process blanks for sediments (Eocene Messel shale F14C= 0.0007; equivalent to an conventional 14C age of > 52000yr (n=29)), as well as Eemian foraminifera (F14C = 0.005; equivalent to an conventional 14C age of >42700yr (n=98)). We are also presenting results of samples processed and analysed as graphite and directly as gas showing a good reproducibility irrespective of the method used

Radiocarbon dating of methane

Methane (CH4) is the most abundant organic compound in the atmosphere and its influence on the global climate is subject to widespread and ongoing scientific discussion. Two sources of atmospheric methane are the release of methane from the ocean seafloor, as well as from thawing permafrost. In recent years the origin, sediment and water column processes and subsequent pathways of methane have received growing interest in the scientific community. 13C/12C ratio measurements can be used to determine the methane source (biogenic or thermogenic), but potential formation/alteration processes by microbes are not yet fully understood. Radiocarbon analysis can help to understand these carbon cycling processes. The presented method is a novel approach for the radiocarbon age determination of methane. A modified PreConn is used to separate methane from other gases such as CO2 in a gaseous sample. Afterwards, the purified methane is transferred to a furnace and oxidized to CO2. Subsequently, produced CO2 is concentrated on a custom-made zeolite trap, which can be connected to a novel sampling unit implemented into the GIS system (by Ionplus AG) for direct CO2 measurements on a MICADAS. The zeolite trap has ¼“ quick-fit connectors (Swagelok) that allow to detach the trap from the oxidation unit and to re-attach it in the GIS. Initial testing showed minimal blank carbon incorporation associated with sample storage, transfer and handling of the custom-build zeolite trap. Here we will present the setup of the method, first results of the blank determination as well as precision of common standard gases

Establishment of routine sample preparation protocols at the newly installed MICADAS 14C dating facility at AWI

In November 2016, the first Mini-Carbon-Dating-System (MICADAS) manufactured by Ionplus AG was delivered and installed at the Alfred-Wegener-Institute (AWI), Germany. The new facility includes a graphitization unit (AGE3) connected with an elementar analyser (EA), a carbonate handling system (CHS), and a gas inlet system (GIS). The main goal for the facility at AWI will be the precise and independent dating of carbonaceous materials in marine sediments, sea-ice, and water to address various processes of the global carbon cycling. A particular focus will be on sediments from the high latitude oceans, in which radiocarbon-based age models are often difficult to obtain due to the scarcity of carbonate microfossils. One advantage of the MICADAS is the potential to analyse samples, which contain only a small amount of carbon as CO2 gas. For example, it will be possible to determine 14C ages of samples of foraminifera from carbonate-lean sediments, allowing for paleoclimate reconstructions in key locations for Earth’s climate system such as the Southern ocean. Likewise, compound-specific 14C analyses receive growing attention in carbon cycle studies and require handling of small samples of typically <100µg carbon. The wide range of applications encompassing gas analyses of foraminifera and compound-specific analysis as well as analyses of graphite targets requires establishing routine protocols of various methods of sample preparation, as well as thorough assessment of the respective carbon blanks. We report on our standard procedures for samples of organic matter from sediments or water including carbonate removal, combustion and graphitization using the AGE3 coupled to the EA, as well as on the methodology applied for carbonate samples using the CHS system and the GIS. We have investigated different sample preparation protocols and present the initial results using materials of known age. Additionally, we present the first results of our assessment of process blanks

Membrance interface evaluations for underwater mass spectrometers.

A component that has enabled the development of underwater mass spectrometry is a mechanically supported membrane interface probe. Our two research groups have used metallic porous frits that support polydimethyl siloxane (PDMS) membranes embedded in a heated membrane probe assembly, allowing the deployment of the underwater membrane introduction mass spectrometer (MIMS) instruments to ocean depths of 2000 meters. The fabrication of such frits has consisted of shaping larger Hastalloy C porous frits to the size required to support a PDMS capillary of 0.64 mm ID and 1.19 mm OD using a diamond‐coated wheel and Dremel tool. This procedure is time‐consuming and cumbersome, and the porosity of the final frits is likely not reproducible. To facilitate the fabrication of the membrane assembly, we report on the use of new porous metallic structures. Frits with diameters of approximately 3.0 mm (1/8”) and known porosities (48.3 % and 32.5%) were produced by the Fraunhofer Institute in Dresden, Germany, using powder metallurgical processes. We used these frits to fabricate new membrane interface assemblies. Using a new custom‐heated membrane probe with the new porous frits, we performed calibrations relating dissolved methane concentrations to mass spectrometer response (m/z 15) using linear least‐squares fitting procedures. Both the limit of detection (methane concentration in the tens of nanomolars) and the sensitivity (on the order of 10‐1 pico‐amps/nanomole of methane) were found to be comparable with those obtained with the previously fabricated Hastelloy C frits. The calibration parameters for the new assembly were also found to be a function of the flow rate, temperature, and sample hydrostatic pressure

Utilization, release, and long-term fate of ancient carbon from eroding permafrost coastlines

About 34% of global coast lines are underlain by permafrost. Rising temperatures cause an acceleration in erosion rates of up to 10s of meters annually, exporting increasing amounts of carbon and nutrients to the coastal ocean. The degradation of ancient organic carbon (OC) from permafrost is an important potential feedback mechanism in a warming climate. However, little is known about permafrost OC degradation after entering the ocean and its long term-fate after redeposition on the sea floor. Some recent studies have revealed CO2 release to occur when ancient permafrost materials are incubated with sea water. However, despite its importance for carbon feedback mechanisms, no study has directly assessed whether this CO2 release is indeed derived from respiration of ancient permafrost OC. We used a multi-disciplinary approach incubating Yedoma permafrost from the Lena Delta in natural coastal seawater from the south-eastern Kara Sea. By combining biogeochemical analyses, DNA-sequencing, ramped oxidation, pyrolysis and stable and radiocarbon isotope analysis we were able to: 1) quantify CO2 emissions from permafrost utilization; 2) for the first time demonstrate the amount of ancient OC contributing to CO2 emissions; 3) link the processes to specific microbial communities; and 4) characterize and assess lability of permafrost OC after redeposition on the sea floor. Our data clearly indicate high bioavailability of permafrost OC and rapid utilization after thawed material has entered the water column, while observing only minor changes in permafrost OC composition over time. Microbial communities are distinctly different in suspended Yedoma particles and water. Overall, our results suggest that under anthropogenic Arctic warming, enhanced coastal erosion will result in increased greenhouse gas emissions, as formerly freeze-locked ancient permafrost OC is remineralized by microbial communities when released to the coastal ocean

An optimized membrane inlet system (MIS) for underwater mass spectrometry (UWMS)

The pressure resistance in the deep ocean is most important for in-situ measurements. In case of the underwater mass-spectrometry (UWMS) especially the requirements of the combination of high permeability for fast low detection limits and stable structures for pressure resistance in the membrane-inlet-systems (MIS) are challenging. In the third funded project “SensorEplus” a MIS is redesigned and optimizedto get high pressure resistance with high gas permeability. The specific details in this project are pressure stability of up to water depth of 4000 m (400 bar), and a high porosity of the membrane supporting structure for gases to get low limits of detection for several gases and chemicals. While comparing these requirements with natural structures of diatoms in the ELISE department of the AWI-Bremerhaven (http://elise.de/en/), it was found a design solution: a kind of tree stabilization inside of the “German Frit” with a very porous surrounding surface. The designed component has a cylindrical form with a diameter of 1/8” at a length of 13 mm. To realize the production of a small and complex component like this, the manufacturing process of micro-printing as a generative manufacturing is used. Additionally a new heating management is adapted in the redesigned MIS. By heating the membrane a constant temperature is achieved and potentially energy can be saved. Also the use of a heat exchanger enables a constant temperature while saving energy. To check the stability of the MIS the components are tested in a high pressure tank at the AWI facilities to prevent a failure of each component. The permeability of the new developed MIS will be tested with the AWI-UWMS to get a comparison of the old MIS supported by a spring and the new structure. Here, we will present the evolution process and the structure of the new “German Frit”