WP3 - deliverable No. 3: Detection and monitoring of methane ebullition
Deliverable
Jens Schneider v. Deimling, Wanda Gülzow, Marina Ulyanova, Zygmunt Klusek, and Gregor Rehder.
Extended summary
The main goal in WP3.3 was to detect and monitor the ebullition of methane gas in the Baltic Sea. In this regard various acoustic and geochemical surveys have been conducted and new methods have been developed for single-beam, next generation multibeam, and infrared-based methane measurement applications.
With the advent of the next generation multibeam systems, the water column can be remotely investigated in regard to gas bubbles therein. Deploying such a next generation multibeam prototype sounder allowed for decent imaging of rising methane gas bubbles and we successfully developed a method to sense the respective rise pattern of individual gas bubbles released from the seafloor (Fig. 1).
- Figure 1a and 1b
Figure 1: (a) Successive echo-image frames recorded during water column imaging with SB3050 showing Rosette (RWS) downcast, contact with gassy sediments, and induced bubble escape into the water column. (b) “Beam-Slice” presentation with the x-axis representing the ping times in seconds where the y-axis is two-way-travel time [s]. Horizontal features represent non-buoyant microbubbles (I) where to the right some ascending bubbles occur (II). Adapted from Schneider von Deimling and Papenberg (2011).
For the first time low frequency multibeam echosounder were operated to image shallow gas buried a few meters below the seafloor. The respective surveys were accompanied by subbottom profiling and geochemical core analysis.
- Figure 2a and 2b
Figure 2: The blue and red line indicate the two sediment types ‘mud’ and ‘mud hosted with shallow gas’ (a) Backscatter amplitude chart of EM120 with a transition zone between bluish/no gas and yellowish/shallow gas areas; the inlet shows amplitude data gathered from the 95 kHz system not showing any transition (b) PARASOUND subbottom data recorded along the blue and red line in (a) starting at 08:15 UTC. The transition zone between shallow gas (right) and no shallow gas (left) plots exactly at the same time as seen in the multibeam data (a) (Schneider et al., 2010).
Both geochemical and subbottom data demonstrate that the sensitivity of the used multibeam sounder is high enough to detect free gas even 5m beneath the seafloor with high reliability (Fig. 2). Given the wide coverage of multibeam swath systems we attribute this method a great potential for efficient subsurface mapping of very shallow gas, especially in combination with subbottom profiling.
At the beginning of BALTIC GAS the baseline methane concentration of the Baltic Sea was assessed and this shed light into the distribution of methane in the Baltic Sea (Schmale et al. 2010). A strong correlation between the vertical density stratification, the distribution of oxygen, hydrogen sulfide, and methane has been identified.
One extraordinary surface methane concentration value could be measured in the surface water of the Arkona Basin by the newly developed ferrybox methane measurement system (Gülzow et al., 2011a, Gülzow et al., 2011b). This exceptional event of high methane concentrations in the surface water was accompanied by a longer period of strong wind with a resulting sea water level shift of almost 1.5m within a day (Fig. 3). As a first hypothesis a strong pressure release might have caused degassing/expansion of the shallow gas methane deposits, that occur close to the seafloor in this area (Thiessen et al., 2006). This process might occur in all regions, were shallow gas is influenced by significant hydrostatic pressure changes. Alternatively, the extraordinary peak was produced by mixing processes bringing deeper and methane-rich water to the sea surface.
Figure 3: Three months time series gathered by the ferrybox system in the Arkona Basin showing sea surface methane concentration and water level changes.
Even though many areas showing free methane gas in the seafloor have been identified in the Baltic Sea, only a few indications for gas bubble ebullitions into the water column are visible from our data. Close to the coast several gas ebullition sites were identified occasionally accompanied by pockmark structures (Pimenov et al. 2010). Free gas ebullitions were found at deeper water than 70m only at two localities and we hypothesis, that natural gas emissions off the coasts of the Baltic Sea only play a minor role in regard to the overall methane fluxes from the seafloor into the water column and atmosphere.
Authors: Schneider v. Deimling, J.; Gülzow, W.; Ulyanova, M.; Klusek, Z.; Rehder, G.
References
Gülzow, W., G. Rehder, B. Schneider, J. Schneider v. Deimling, and B. Sadkowiak (2011a), A new method for continuous measurement of 1 methane and carbon dioxide in surface waters of the Baltic Sea using off-axis integrated cavity output spectroscopy (ICOS), Limnology Oceanography Methods.
Gülzow W., Schneider von Deimling, J., Rehder G., Schneider B. (2011b), Seasonal and spatial distribution of methane in the surface water of the Baltic Sea, BSSC, St. Petersburg.
Pimenov, N., M. Ulyanova, T. Kanapatsky, E. Veslopolova, P. Sigalevich, and V. Sivkov, (2010), Microbially mediated methane and sulfur cycling in pockmark sediments of the Gdansk Basin, Baltic Sea, Geo-Marine Letters, 30(3), 439-448, doi:10.1007/s00367-010-0200-4.
Schmale, O., Schneider v. Deimling, J., Gülzow, W., Nausch, G., Waniek, J. and G. Rehder (2010), Distribution of methane in the water column of the Baltic Sea, Geophys. Res. Lett., 37(L12604), doi:10.1029/2010GL043115.
Schneider v. Deimling, J., Weinrebe, W., Bürk, D., Thot, Z., Endler, R., Fossing, H., Spiess, V., and Rehder, G. (2010), Subbottom mapping of shallow gas in the Baltic Sea using medium to low frequency multibeam sounders, AGU, San Francisco.
Schneider von Deimling, J., and C. Papenberg (2011), Technical Note: Detection of gas bubble leakage via correlation of water column multibeam images, Ocean Sci. Discuss., 8(4), 1757-1775, doi:10.5194/osd-8-1757-2011.
Thiessen, O., M. Schmidt, F. Theilen, M. Schmitt, and G. Klein (2006), Methane formation and distribution of acoustic turbidity in organic-rich surface sediments in the Arkona Basin, Baltic Sea, Continental Shelf Research, 26(19), 2469-2483.