3. Wochenbericht AL483

Wochenbericht AL–483, 20.08. – 23.08.201

2. Wochenbericht AL483

Wochenbericht AL–483, 13.08. – 19.08.201

1. Wochenbericht AL483

Wochenbericht AL–483, 06.08. – 12.08.201

Submarine methane seepage in the Paleo Dnepr Area and Sorokin Trough and its influence on the Black Sea methane budget

Methane is a greenhouse gas that exerts a significant influence on the radiation budget and thereby the earth’s climate. Geological sources of methane, especially submarine, are particularly relevant in this regard because they emit significant quantities of methane into the atmosphere. However, it is difficult to estimate the magnitude of the global geological source because the extent of these methane-releasing areas and the processes, which influence the methane cycle, are not yet well understood. The Black Sea is a region characterized by numerous submarine methane sources. Methane emissions in the form of methane-containing fluids or gas bubbles have been documented here at all water depths, from the shelf down to the abyss. This marginal sea features a unique hydrographic structure resulting from the inflow of freshwater from rivers and the salty waters of the Mediterranean Sea over the Bosphorus. These inputs have very different densities, and this leads to a pronounced stratification of the water column that limits vertical mixing. High biological production in the surface waters leads to an intense consumption of oxygen in the upper water column, which can not be balanced by downward ventilation of oxygenated surface waters. The pronounced redox-gradient has in turn an important influence on the biochemical cycles in the water column and leads to anoxic conditions at depths below approximately 100 m. These particular conditions make the Black Sea the largest anoxic marine basin on earth. Chapters 2 and 3 of this work (as well as the articles in the appendix) explore the questions of what influence methane emissions have at different water depths on the methane cycle of the water column and what significance these submarine emissions have as sources of atmospheric methane. To this end, the zones of methane sources along the northwest shelf and continental slope (Paleo Dnepr Area) as well as in the northeastern deep sea region (Sorokin Trough) of the Black Sea were explored throughout 2003 and 2004. The observed depths span from between 60 and 2100 m and encompass the entire biogeochemical spectrum of the water column. The methane concentrations of the water column and the overlaying atmosphere were carefully recorded, as were the carbon isotope signatures of this methane. The physical description of the water column, the microorganisms involved in the methane cycle and their methane consumption rates were additionally documented. The area affected by the emission of methane from submarine sources was ascertained through extensive sampling campaigns. The dataset was used to model the methane flux at the sea/air interface and to evaluate the influence of methane sources, present in different water depth on the marine methane emission. In addition, the exploration of this area throughout two consecutive years provides insight into the seasonal variability of the source strength of these submarine emissions. Chapter 4 serves to combine the results of this investigation with those present in the literature for the purpose of constructing a methane budget that quantifies the significant sources and sinks of methane in the Black Sea. A transport-reaction model investigates the release of methane at discrete water depths and identifies those of particular interest. Various model scenarios were used to determine to what extent sizable methane emission events, for example from mud volcano eruptions or submarine landslides, affect the methane budget of the Black Sea and the associated methane emissions at the ocean surface. The following work cannot completely address the complex methane cycle of the Black Sea and its associated processes. This work serves rather to provide new insights, discuss relevant areas and point out still unanswered questions. The increase in research on methane in the Black Sea during recent years shows that its methane cycle is of great scientific interest

Cruise Report RV Alkor Cruise No. AL483

Dates of the cruise: from 06.08.2016 to 25.08.2016 Areas of Research: methane chemistry, molecular biology, planktology Dates and names of Ports of call: Ventspils (Latvia). 15-18 August 201

Dense bottom gravity currents and their impact on pelagic methanotrophy at oxic/anoxic transition zones

We show that inflows of oxygenated waters into sulfidic layers have a strong impact on biogeochemical transformation at oxic/anoxic transition zones. Taking the pelagic methane dynamics in the Gotland Basin as an example, we performed our studies when one of the largest inflows ever recorded entered the Baltic Sea in March 2015. An inflowing gravity current transported oxic waters into the sulfidic deep layers and freshly generated a near-bottom secondary redox interface. At the upper slope, where the inflowing water masses were vigorously turbulent and the main and secondary redox interfaces in close contact to each other, methane oxidation rates inside the transition zone were found to be higher compared to the weakly turbulent basin interior. At the main redox interface in the basin center, lateral intrusions of oxygenated waters into intermediate water depth may have stimulated the growth of the methanotrophic community and their activity

Fluid and gas fluxes from the Logatchev hydrothermal vent area

The Logatchev hydrothermal field at 14°45′N on the MAR is characterized by gas plumes that are enriched in methane and helium compared to the oceanic background. We investigated CH4 concentration and δ13C together with δ3He in the water column of that region. These data and turbidity measurements indicate that apart from the known vent fields, another vent site exists northeast of the vent field Logatchev 1. The distribution of methane and 3He concentrations along two sections were used in combination with current measurements from lowered acoustic Doppler current profilers (LADCP) to calculate the horizontal plume fluxes of these gases. According to these examinations 0.02 μmol s−1 of 3He and 0.21 mol s−1 of methane are transported in a plume that flows into a southward direction in the central part of the valley. Based on 3He measurements of vent fluid (22 ± 6 pM), we estimate a total vent flux in this region of about 900 L s−1 and a total flux of CH4 of 3.2 mol s−1

Discovery and quantification of a widespread methane ebullition event in a coastal inlet (Baltic Sea) using a novel sonar strategy

How much of the greenhouse gas methane is transported from the seafloor to the atmosphere is unclear. Here, we present data describing an extensive ebullition event that occurred in Eckernförde Bay, a shallow gas-hosting coastal inlet in the Baltic Sea, in the fall of 2014. A weak storm induced hydrostatic pressure fluctuations that in turn stimulated gas ebullition from the seabed. In a finely tuned sonar survey of the bay, we obtained a hydroacoustic dataset with exceptionally high sensitivity for bubble detection. This allowed us to identify 2849 bubble seeps rising within 28 h from the seafloor across the 90 km² study site. Based on our calculations, the estimated bubble-driven episodic methane flux from the seafloor across the bay is 1,900 μMol m-2 d-1. Our study demonstrates that storm-associated fluctuations of hydrostatic pressure induce bulk gas-driven ebullitions. Given the extensive occurrence of shallow gas-hosting sediments in coastal seas, similar ebullition events probably take place in many parts of the Western Baltic Sea. However, these are likely to be missed during field investigations, due to the lack of high-quality data acquisition during storms, such that atmospheric inputs of marine-derived methane will be highly underestimated

Bubble transport Mechanism: Indications for a gas bubble-mediated inoculation of benthic methanothrophs into the water column

Highlights • A new bentho-pelagic transport mechanism of microorganisms is hypothesized • A bubble transport hypothesis was tested using a new gas bubble-collecting device • Bubble-mediated transport rate of methanotrophs was quantified at a gas vent • The Bubble Transport Mechanism may influence the pelagic methane sink Abstract The importance of methanotrophic microorganisms in the sediment and water column for balancing marine methane budgets is well accepted. However, whether methanotrophic populations are distinct for benthic and pelagic environments or are the result of exchange processes between the two, remains an area of active research. We conducted a field pilot study at the Rostocker Seep site (Coal Oil Point seep field, offshore California, USA) to test the hypothesis that bubble-mediated transport of methane-oxidizing microorganisms from the sediment into the water column is quantifiable. Measurements included dissolved methane concentration and showed a strong influence of methane seepage on the water-column methane distribution with strongly elevated sea surface concentrations with respect to atmospheric equilibrium (saturation ratio ~17,000%). Using Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD FISH) analysis, aerobic methane oxidizing bacteria (MOB) were detected in the sediment and the water column, whereas anaerobic methanotrophs (ANME-2) were detected exclusively in the sediment. Critical data for testing the hypothesis were collected using a novel bubble catcher that trapped naturally emanating seep gas bubbles and any attached particles approximately 15 cm above the seafloor. Bubble catcher experiments were carried out directly above a natural bubble seep vent and at a nearby reference site, for which an “engineered” nitrogen bubble vent without sediment contact was created. Our experiments indicate the existence of a “Bubble Transport Mechanism”, which transports MOB from the sediment into the water column. In contrast, ANME-2 were not detected in the bubble catcher. The Bubble Transport Mechanism could have important implications for the connectivity between benthic and pelagic methanotrophic communities at methane seep sites