Volcanic gases drive volcanic eruptions. They determine whether volcanic eruptions are effusive or explosive and can also influence the volcanic ‘plumbing system’ during dormant periods [ Girona et al. , 2015]. During these dormant periods, significant amounts of carbon dioxide (CO 2 ) can be released into the air through passive escape of volcanic gases (i.e., without an eruption) [ Carn et al. , 2016]. These emissions can, in turn, influence the amounts of CO 2 in the atmosphere and oceans over a much larger area [ Aiuppa et al. , 2019].
We spent three days, sometimes with four boats at the same time, sailing across a lake whose surface gave no indication of the huge magma reservoir beneath the volcanic crater.
The direct observation of CO 2 emissions from subaerial volcanoes (those that are not underwater) is technically difficult because of the high CO 2 abundance in the atmosphere. In contrast, the detection of CO 2 emitted by underwater volcanoes is relatively straightforward using acoustic instruments.
The escape of CO2 from the lake bed was the subject of an unusually focused experiment carried out by a large group of researchers from various disciplines last August on the picturesque Laacher See in the Eifel region. On the shore of the 2-kilometer-wide, tree-lined lake lies the venerable 900-year-old Benedictine Abbey of Maria Laach. The lake itself fills the crater of a volcano that is believed to have last erupted at the end of the Pleistocene. Our multinational team of 13 volcanologists, sedimentologists, oceanographers and hydrogeophysicists tried to prove connections between these gas releases, the surrounding sediment structures and volcanic activity. We spent three days, sometimes with four boats at the same time, sailing across a lake whose surface gave no indication of the huge magma reservoir beneath the volcanic crater. During our field studies, we were only interrupted by fishermen and vacationers on pedal boats.
From outbreak to blister formation
Gas vents near volcanoes that are submerged in the sea or in lakes can provide information that cannot be obtained from subaerial volcanoes. The lakebed of Lake Laach has numerous such gas vents and is therefore an attractive choice for our study.
The Laacher See volcano produced a Plinian eruption about 12,900 years ago, spewing ash as far as Greece. It ejected 20 cubic kilometers of rocks known as tephra, the dense rock equivalent of 6.3 cubic kilometers of magma [ Schmincke et al. , 1999], making the eruption as intense as the Pinatubo eruption in 1991. Since that massive eruption, a lake has formed in the crater, and the volcano has been dormant. In recent years, however, significant plumes of gas bubbles have been observed in the lake [ Goepel et al. , 2015] and high concentrations of dissolved CO 2 have been measured.
Although we did not investigate the compositions of the bubbles in the present study, recent geochemical analyses indicate that the CO 2 contained in the lake is of magmatic origin, and the CO 2 content of the soil air ranges from atmospheric values (0.03%) to 100% [ Gal et al. , 2011; Giggenbach et al. , 1991]. A recent study documented deep, low-frequency earthquakes, presumably caused by magma movements beneath the volcano [ Hensch et al. , 2019]. However, there are currently no signs of volcanic activity in the near future.
Sound, Sediments and Stratification
We divided our team of 13 into several groups to investigate different aspects of the lake and its gas vents. Two groups focused on the bottom of Lake Laach and the gas vent in the water column, using the iXblue Seapix 3-D and Norbit multibeam echo sounders to characterise the bubble eruptions. Although echo sounders are usually used to map the terrain underwater [ Morgan et al. , 2003], they can also record gas bubbles in the water. The sound waves emitted by the echo sounder are reflected by the gas bubbles and registered in the water column as high acoustic backscatter values. This backscatter can then be used to track the bubbles as they rise [ Greinert et al. , 2010] and to quantify the corresponding gas flux [ Ostrovsky et al. , 2008].
Four scientific groups also investigated whether there are relationships between the locations where the fuel is escaping and certain sediment structures on the lake floor, such as trenches, small depressions and pockmarks. In addition to the echo sounder profiles, we acquired reflection seismic profiles using the Innomar parametric echo sounder and the iXblue Echoes 10000 Chirp sub-backside profiler to image the upper 35 meters of sediment fill in the crater lake with a vertical resolution of about 8 centimeters. In addition, the groups used a remotely operated underwater vehicle to provide optical images of the lake floor. Initial results from these studies show clear relationships between the gas leak sites and the morphology of the lake floor, such as possible pockmarks and trenches, and provide insight into how these landforms influence volcanic degassing in this area.
A team tested new devices that could be used to monitor underwater volcanoes, like the seismic and infrasound instruments on the Earth’s surface.
A team tested new equipment that could be used to monitor underwater volcanoes, like the seismic and infrasound instruments on the surface. For example, we used a hydrophone – essentially a microphone submerged in water – to record the sound of bubbles emerging from the lake floor for several hours. As has been observed for bubbles in other volcanic lakes [ Vandemeulebrouck et al. , 2000], the gas bubbles in Lake Laach emitted energy below 5 kilohertz; this similarity suggests that the technique could also be suitable for monitoring various volcanic regions underwater.
Ein anderes Staff suchte nach neuen Möglichkeiten zur Erkennung bevorstehender limnischer Eruptionen – eine typische Gefahr, die mit Vulkanseen einhergeht. Diese plötzlichen Gaseruptionen, die nicht unbedingt vulkanischen Ursprungs sind, können infolge der Stratifizierung des Seewassers entstehen, wenn eine Abfolge stabiler Wasserschichten, die von der Oberfläche zum Seeboden absteigend immer kühler werden, sich nicht vermischen. CO2, das in kühles Druckwasser in Seebodennähe dringt, löst sich rasch auf und kann stark anwachsen – wie Kohlensäure in einer gekühlten Flasche Sprudelwasser. Wenn eine Störung, wie ewa ein Erdrutsch, das Wasser aufwühlt und die Schichtung durcheinander bringt (wie beim Schütteln einer verschlossenen Sprudelwasserflasche), kann das CO2 zur Oberfläche aufsteigen und zu einer Eruption führen. Dabei bildet sich in der Atmosphäre eine sauerstoffarme Schicht, in der Menschen und Tiere ersticken können. Limnische Eruptionen forderten in der Vergangenheit zahlreiche Todesopfer: Bei der Nyos-Tragödie 1986 in Kamerun entwich CO2 explosionsartig aus dem Nyos-See und kostete rund 1.700 Menschen das Leben [Kling et al., 1987].
Dieses Staff untersuchte die Möglichkeiten der elektrischen Impedanz-Tomografie und Methoden der Transienten-Elektromagnetik zur Erkennung der thermischen Stratifizierung des Sees von der Seeoberfläche aus. Erste Ergebnisse zeigen, dass diese berührungslosen Verfahren eindeutig eine Stratifizierung des Laacher Sees ausmachen konnten. Damit sind diese Methoden vielversprechend für die kontinuierliche Überwachung von Seen in der Zukunft, wo limnische Eruptionen die umliegenden Gemeinden gefährden könnten.
Das Verständnis der spezifischen und räumlichen Dynamik von vulkanischen Gasblasen bildet den Eckpfeiler in der Entwicklung einer neuer Era von Frühwarnsystemen, die für die Risikobewertung von Vulkanen gebraucht werden. Die Feldstudie am Laacher See von 2019 lieferte eine Momentaufnahme der vulkanischen Gasaustritte unter Wasser, ihres Zusammenhangs mit den Sedimentstrukturen und ihrer möglichen Nutzung für die Überwachung von Gasaustritten und thermischer Stratifizierung. Die Analyse der gesammelten Daten sowie künftige Experimente werden in Entscheidungen einfließen, welche Rolle die methodisch breit gefächerten hydroakustischen Daten bei der laufenden wissenschaftlichen Arbeit zur Erhöhung der Vorhersagbarkeit vulkanischer und limnischer Eruptionen spielen werden.
Danksagung
Die Wissenschaftler dieser Expedition vertraten das Institut des Sciences de la Terre (Frankreich), die Firma iXblue, das Flanders Marine Institute und die Universität Gent (Belgien), die Technische Universität Wien (Österreich), das GFZ Helmholtz-Zentrum Potsdam und das Landesamt für Bergbau, Energie und Geologie (LGB; Deutschland). Wir danken Thomas Vandorpe, Robin Houthoofdt, Koen De Rycker, Anouk Verwimp, Philipp Högenauer und Johannes Hoppenbrock herzlich für ihre Hilfe vor Ort und bei der Datenverarbeitung.
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Creator Info
Corentin Caudron (corentin.caudron@univ-smb.fr), Institut des Sciences de la Terre (ISTerre), Grenoble, France; Marc De Batist, Ghent College, Belgium; Guillaume Jouve and Guillaume Matte, iXblue, La Ciotat, France; Thomas Hermans, Ghent College, Belgium; Adrian Flores-Orozco, Vienna College of Know-how, Austria; Wim Versteeg, Vlaams Instituut Voor de Zee, Oostende, Belgium; Zakaria Ghazoui, GeoForschungsZentrum, Potsdam, Germany; Philippe Roux and Jean Vandemeulebrouck, ISTerre, Grenoble, France; and Bernd Schmidt, Landesamt für Geologie und Bergbau, Mainz, Germany
