Degassing Patterns in the Laacher See Basin, Germany

Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-014-1133-3

ORIGINAL PAPER

Volcano-tectonic structures and CO2-degassing patterns in the Laacher See basin, Germany

Andreas Goepel · Martin Lonschinski · Lothar Viereck · Georg Büchel · Nina Kukowski

Received: 22 November 2013 / Accepted: 17 December 2014 © Springer-Verlag Berlin Heidelberg 2014

Abstract The Laacher See Volcano is the youngest are possibly related to volcano-tectonic faults. In the southern

(12,900 year BP) eruption center of the Quarternary East- part of Laacher See, CO2 vents occur in a high spatial den- Eifel Volcanic Field in Germany and has formed Laacher See, sity only within the center of the arc-shaped structure Barsch- the largest volcanic lake in the Eifel area. New bathymetric buckel possibly tracing the conduit of a tuff ring. data of Laacher See were acquired by an echo sounder system and merged with topographic light detection and rang- Keywords Laacher See Volcano · East-Eifel Volcanic ing (LiDAR) data of the Laacher See Volcano area to form Field · CO2 vents · Bathymetry an integrated digital elevation model. This model provides detailed morphological information about the volcano basin and results of sediment transport therein. Morphological anal- Introduction ysis of Laacher See Volcano indicates a steep inner crater wall (slope up to 30°) which opens to the south. The Laacher See The Quaternary East-Eifel Volcanic Field (EEVF) is basin is divided into a deep northern and a shallower south- located in the western part of Germany (Fig. 1). In a ern part. The broader lower slopes inclined with up to 25° region of about 400 km2, at least 103 volcanoes have change to the almost flat central part (maximum water depth been mapped (Ahrens 1932; Duda and Schmincke 1978; of 51 m) with a narrow transition zone. Erosion processes of Frechen 1971; Goepel 2008; Schmincke 1977; van den the crater wall result in deposition of volcaniclastics as large Bogaard and Schmincke 1990b; Viereck 1984), which deltas in the lake basin. A large subaqueous slide was identi- erupted in six major phases between >650,000 year BP and fied at the northeastern part of the lake. CO2-degassing vents about 12,900 year BP (Schmincke 1996; Schmincke et al. (wet mofettes) of Laacher See were identified by a single- 1999; van den Bogaard and Schmincke 1985, 1990a). Two- beam echo sounder system through gas bubbles in the water thirds of the generally K-rich magmas were of melilithic, column. These are more frequent in the northern part of the nephelinitic, leucititic and basanitic-tephritic composition, lake, where wet mofettes spread in a nearly circular-shaped whereas one-third had intermediate and phonolitic com- pattern, tracing the crater rim of the northern eruption center positions, respectively. The Laacher See Volcano, located of the Laacher See Volcano. Additionally, preferential paths close to Maria Laach Abbey (Fig. 1), explosively erupted for gas efflux distributed concentrically inside the crater rim about 6.3 km3 of phonolitic magma during a dominantly phreato-plinian series of eruptions at about 12,900 year BP (Harms and Schmincke 2000; Schmincke et al. 1999; Electronic supplementary material The online version of this van den Bogaard and Schmincke 1990a). First eruptions article (doi:10.1007/s00531-014-1133-3) contains supplementary material, which is available to authorized users. of the Laacher See Volcano started in the south and later moved toward the northern part of the Laacher See basin A. Goepel (*) · M. Lonschinski · L. Viereck · G. Büchel · Schmincke et al. (1999). As a result, the crater of Laacher N. Kukowski See Volcano was predominantly formed by vent erosion Institute of Geosciences, Friedrich Schiller University, Burgweg 11, 07749 Jena, Germany and collapse. Later, the crater was filled with water due to e-mail: andreas.goepel@uni‑jena.de groundwater inflow.

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Fig. 1 Topographic map of the East-Eifel Volcanic Field in Germany showing eruptive centers after Ahrens (1932), Duda and Schmincke (1978), Frechen (1971), Goepel (2008), Schmincke (1977), van den Bogaard and Schmincke (1990b) and Viereck (1984). The white rectangle marks the Laacher See Volcano with Laacher See, covering an area of 3.315 km2. Geographic coor- dinates are given in longitude and latitude (WGS84) as well as in UTM 32N

The recent lake level of 275.3 m (amsl) is maintained by a drainage system, controlled by an overflow through the manmade gallery Deliusstollen which was finished in 1845 AD (Grewe 1979). However, anthropogenic water management through the last centuries influenced Laacher See. His- torical records show that first attempts to lowering the lake level from 288 to 278 m (amsl) were realized in 1167 AD by finishing the drainage gallery Fulbertstollen (Grewe 1979). The Laacher See is holomictic (Scharf 1987), i.e., a complete vertical mixing of the water body occurs at least once a year, but an annual stratification is formed during the summer months. During our study in August and Sep- tember 2010, a lake stratification into epilimnion, metalim- Fig. 2 Water column profiles of temperature, electrical conductivity, dissolved oxygen and pH value in Laacher See on August 1, 2010, at nion and hypolimnion had been established (Fig. 2). 12:00 p.m. MEZ The Quaternary East- and West-Eifel volcanic fields are accompanied by numerous highly mineralized springs 1 1 (Na–Mg–Ca–K–HCO3−-type acidulous water) with a CO2 (5 mmol mol− ), CH4 (1 mmol mol− ) and noble gases. 1 concentration of more than 250 mg kg− (H2O) (Carlé Nevertheless, CO2 plumes in the lake have been men- 1975; May 2002). Discharge of CO2 gas (wet mofettes) tioned mainly at the northeastern part of the lake close to in the Laacher See has been known for many years (e.g., the shoreline. At this particular area on the eastern side of

Frechen et al. 1967; Giggenbach et al. 1991; Meyer 1994; the lake, numerous vents (dry mofettes) with CO2 contents Pfanz 2008). Giggenbach et al. (1991) analyzed the gas higher than 90 % in the soil gas are widespread (Pfanz 2008; 1 phase and found besides CO2 (990 mmol mol− ) also N2 Stoiber-Lipp et al. 2012). Dry mofettes are also known

1 3 Int J Earth Sci (Geol Rundsch) from a small area along the western lake side (Gal et al. (2004) studied the volcano-structural properties of the Bod-

2011). Hernández et al. (2010) estimated a CO2 efflux from rum peninsula (Turkey) by morphometric analysis of DEM 2 1 Laacher See to be about 14.04 t km− d− . This amount is images. DEM data are also very useful in the investigation relatively low in comparison with the CO2 emission of other of landslides (e.g., Guzzetti et al. 2012). volcanic lakes with neutral pH conditions worldwide (mean: The aim of our study was to map the spatial distribution 2 1 3 4 201.1 t km− d− , Pérez et al. 2011). Studies of He/ He of CO2-degassing vents within Laacher See and provide 13 ratios and δ C values of CO2 in the Laacher See indicate a additional information on volcanic and sedimentological mantle signature (Aeschbach-Hertig et al. 1996; Giggenbach features by joint analysis of bathymetric and DEM data. et al. 1991). The efflux of CO2 from the mantle could be linked to feeder dikes that serve as short cuts in the layered crust or to tectonic structures. Similar CO2-degassing pattern Methods in the West-Eifel Volcanic Field (WEVF) has been found to be related to tectonic structures (May 2002). Correlations Conductivity–temperature–depth probe (CTD) between CO2 efflux and the trending direction of local vol- measurements and hydrochemistry canic structures are described, for example, for the Cuicocha volcano in Ecuador (Padrón et al. 2008) and for the Miyake- To determine the temperature-related lake stratification, jima volcano in Japan (Hernández et al. 2001). conductivity, temperature and depth measurements in the

CO2 bubbles in lake water can be identified with an echo water column were conducted on August 1, 2010, at 12.00 sounder system. This method has been successfully applied p.m. MEZ. The measurements were performed using a in the previous studies, e.g., Cardigos et al. (2005) utilized CTD90M (Sea and Sun Technology), equipped with sen- an 28 kHz echo sounder to indicate vent bubble plumes close sors for pH and O2 acquisition, additionally. The CTD was to the Azores, Canet et al. (2010) found flares of gas bubbles used in a continuous mode with a sampling rate of 5 Hz. in the Gulf of California by an 18 kHz sounder, and Ber- The probe was moved down with a hand-operated winch 1 nard (2011) used an 50 kHz echo sounder for the investiga- with a speed of about 0.05 m s− . Acquisition of these tion of CO2 degassing at the Taal volcano, Philippines. Wes- parameters is essential to correct sonic wave velocity, sels et al. (2010) used various hydroacoustic tools, including which, e.g., depends on temperature. sidescan sonar and multibeam echo sounder, to study pock- Water samples were collected on August 14, 2010, in marks caused by methane degassing at the lake ground of five different depths: 1, 2, 12.5, 20 and 40 m in order to Lake Constance. In bubble release experiments performed at determine the hydrochemistry of Laacher See. Therefore, Lake Lucerne in combination with discrete bubble modeling, a FreeFlow water sampler (Hydro-Bios) with a sample vol-

McGinnis et al. (2011) confirmed that bubbles of CO2 dissolve ume of 5 L and a hand-operated winch with a mechanical rapidly and released CO2 tend to remain near the lake floor. counter was used to determine the water depth. Bathymetric surveys in volcanic lakes are a suitable tool Dissolved CO2 was determined directly after sampling to identify volcanic features to unravel the recent and his- by titration (with NaOH solution) using a Digital Titrator toric volcanic activity. For example, hydrothermal explo- Kit (Hach) and a portable device WTW pH320 (Xylem) sion craters, hydrothermal vents, sublacustrine landslide for pH measurements with pH probe (Pt-electrode and KCl deposits and submerged former shorelines in the Yellow- solution, Schott). stone Lake were identified based on a multibeam sonar Samples for anions (except for HCO3−) and elements mapping (Morgan et al. 2003). The lake morphology of the were filtered using glass fiber prefilters (Sartorius) and Kawah Ijen volcano crater lake in Indonesia was compared cellulose acetate filters (0.45 µm, Sartorius). Water sam- to historical morphological records to estimate changes ples for element analysis finally were acidified with HNO3 in the underwater volcanic activity to improve the predic- (65 %, subboiled) to pH < 2. All samples were kept at 6 °C tion of future eruptions (Takano et al. 2004). Anzidei et al. until they were analyzed.

(2008) recognized five underwater crater structures and for- HCO3− was analyzed at the day of sampling by titration mer submerged shorelines for the Colli Albani volcano in of total alkalinity (Titrino 716 DM, Metrohm USA Inc.), 2 Italy revealed by echo sounder surveys. Recently, joint dig- whereas Cl−, F−, NO3− and SO4 − were analyzed by ion 3 ital elevation models (DEM), bathymetric data and seismic chromatography (DX-120, Dionex). PO4 − was determined surveys are increasingly being used to perform morphomet- by spectrophotometer DR/4000 U (Hach). The elements ric data analysis (e.g., slope analysis) to discover concealed Ca, K, Mg, Na, Si and Sr were determined by inductively volcanic structures (e.g., Fornaciai et al. 2012; Grosse coupled plasma optical emission spectrometry (ICP-OES; et al. 2012; Ross et al. 2014). In addition, morphometric Varian 725 ES). Inductively coupled plasma mass spec- analysis is used to identify topographic attributes and sedi- trometry (ICP-MS; X-Series II, ThermoFisher Scientific) mentological structures as well. For example, Ulusoy et al. was used to analyze Fe and Mn.

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Fig. 3 I Tracks of the echo sounder mapping (total length: 235 km) Pfalz (LVermGeo)”). II Screenshots of the echo sounder signatures of are indicated by the red lines. The digital elevation model of Laacher gas bubble plumes (A–D). The scales on the images (right side) indi- See Volcano is superimposed by an aerial photo (data provided by cate water depth (m) and surface water temperature (°C) “Landesamt für Vermessung und Geobasisinformation Rheinland-

Bathymetry and topography in salt water (Wilson 1960) incorporating the temperature– depth curve of Laacher See from August 2010 (Fig. 2), Laacher See floor bathymetric data were acquired using an the hydrostatic pressure (depending on water depth) and a echo sounder GARMIN GPSMap 421s utilizing a 500 W constant salinity of 0.4 ‰. The sounder system has a ver- dual-frequency transducer single-beam. The transducer has tical measuring accuracy of 0.3 m (GARMIN 2012). Dur- a beam width of 10° at 200 kHz or 40° at 50 kHz, respec- ing data acquisition, Laacher See lake level was constant tively, and was mounted at the bottom of a float (0.05 m at 275.3 m (amsl) as measured by a Topcon GR5 Differen- below lake level). This device has a dimension of 1 m by tial Global Positioning System (DGPS). Bathymetric data 0.5 m and consists of a polypropylene foam core encap- (available in Supplement A) were interpolated to a raster on sulated by a plastic bottom. This setup minimizes wave- a 10-m regular grid using kriging algorithm (12 neighbors, induced roll and pitch movements of the transducer very search radius 100 m) of software package ArcGIS v. 9.3.1 effectively. Above the transducer, a GARMIN GA25 GPS (ESRI). Bathymetric volume calculations were also per- antenna was mounted to acquire the geographic position. formed using ArcGIS v. 9.3.1 (ESRI). Data were collected on profiles with an average speed of Terrestrial topographic data used in this study are 1 4 km h− (total length of 235 km, Fig. 3) and stored at a derived by airborne acquired high-resolution point cloud sampling rate of 1 Hz on a custom-made external data log- data (LiDAR). These data have a resolution of at least four ger. The transducer was operated at 200 kHz in single-beam dots per square meter with a position accuracy of 0.3 m and mode only, which results in a sweep cone with a maximum contain vegetation as well as ground points (Landesamt für diameter of approximately 18 m at a depth of 50 m. Since Vermessung und Geobasisinformation Rheinland-Pfalz, the sounder is calibrated to be used in salt water (wave LVermGeo). No ground filtering algorithms were applied 1 velocity: 1,500 m s− ; GARMIN 2012), a correction of to remove vegetation points. LiDAR data were gridded the raw depth data was necessary. To do so, we applied a to a raster with a cell size of 1 m using software package numerical model using the equation for the speed of sound ArcGIS v. 9.3.1 (ESRI) featuring the 3D Analyst Terrain

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toolset (ESRI). Slope analysis was performed using the 3D Table 1 Concentrations of dissolved CO2, elements and anions in Analyst toolsets (ESRI). different water depths of Laacher See Bathymetric (echo sounder) and topographic (LiDAR) Sampling 1 m 2 m 12.5 m 20 m 40 m data support different spatial resolutions. Our bathymetric depth (mg L–1) data have a maximum spatial resolution of 10 m, whereas LiDAR data provide a resolution which is 10 times higher. CO2 0 0 11.6 16.4 54.5 To permit a joint interpretation of both data sets, a new Ca 40 40.1 44.1 43.9 44.1 seamless 1 m spaced DEM was gridded whereby bathymet- Fe 0.0015 0.0018 <0.0008 <0.0008 0.001 ric data were extrapolated to an equal grid size. K 25.2 25.5 25.1 25.2 24.6 Mg 34.9 35 34.7 34.5 34.4 Mn 0.0015 0.0015 0.0008 0.0008 0.0016 CO2‑degassing vents (wet mofettes) Na 46.5 46.8 46.5 46.3 45.4 The echo sounder signals (specification identical to bathy- Si 0.45 0.42 1.06 1.07 1.67 metric measurements) reflect rising gas bubbles. Figure 3 Sr 0.52 0.52 0.53 0.53 0.52 displays examples of typical degassing spots at the lake F− 0.48 0.48 0.48 0.48 0.49 bottom forming nearly vertical flares. In this way, the Cl− 18.62 18.47 18.22 18.13 18.19 2 SO − 26.8 26.6 26.5 26.5 25.7 method is suitable to localize CO2 vents in the Laacher 4 See. All vents reported here (e.g., Fig. 7) were detected on NO3− 0.9 0.4 0.4 0.4 0.4 3 the monitor of the sounder, and the geographic position PO4 − <0.05 <0.05 <0.05 <0.05 0.25 was recorded. The measurements were taken in August and HCO3− 384.1 378.9 406.6 406.2 408.4

September 2010. The spatial distribution of the CO2 vents was stable during the entire time period of our study as observed from repeated survey along the same profiles. and anion concentrations are depicted in Table 1, but will not be discussed in detail since this study is not focused on this topic. Generally, most species have the same concen- Results tration along the entire water column with the exception of Ca and HCO3−, which are lower at 1 and 2 m water depth. Conductivity–temperature–depth probe (CTD) measurements and hydrochemistry Bathymetry and topography

During the entire time period of our study, Laacher See The crater of Laacher See Volcano is expressed as a lake showed three-layer temperature stratification; the epilim- basin with a maximum water depth of 51 m (Fig. 4). The nion was built down to 7.5 m water depth, the metalimnion recent water body of Laacher See, 0.103 km3 in volume, was found from 7.5 to 15 m water depth, and the water covering an area of 3.309 km2 with an 8.471 km long body below 15 m depth characterized the hypolimnion shoreline (Table 2). Laacher See basin is surrounded by (Fig. 2). Values of electrical conductivity increased from an elongated crater wall with a northeast–southwest exten- 1 1 0.69 mS cm− in the epilimnion to 0.74 mS cm− in the sion of 3,800 m and a northwest–southeast extension metalimnion and remained almost constant at this value of approximately 3,000 m (Fig. 5). The crater wall rim in the hypolimnion. Concentration of dissolved oxygen extends 160 m above the lake level of 275.3 m (amsl), at 1 was saturated at 10 mg L− in the epilimnion and reached a distance of 300–700 m from the shoreline. In the south- 1 maximum values of 20 mg L− at 10 m water depth in the west, the crater wall rim is strongly eroded. The inner cra- metalimnion. The hypolimnion shows oxygen contents of ter walls are very steep (with slopes up to 30°) and charac- 1 8 mg L− down to 35 m water depth and decreasing oxygen terized by a number of gullies as a result of intense erosion 1 concentrations to values lower 2 mg L− close to the lake (Fig. 6). The outer crater wall has an average slope of about bottom. The pH value was constant at 8.3 in the epilimnion 15° and thus shows a much flatter morphology. Due to ero- and dropped to 7.5 in the metalimnion. The hypolimnion sion, the crater wall is open to the southwest through the shows pH values decreasing with water depth, whereby valley of the creek Beller Bach (loc. A in Fig. 5), but also lowest pH values (6.8) were found close to the lake bottom. to the northwest (loc. C in Fig. 5), and southeast (loc. L

Concentrations of dissolved CO2 increased with water and K in Fig. 5). The lake basin is shaped like an hourglass 1 depth. Highest concentrations (54.5 mg L− ) were detected and divided into a shallower (down to 44 m water depth) 1 at 40 m water depth, 16.4 mg L− at 20 m water depth and southwest part and a deeper (down to 51 m water depth) 1 11.6 mg L− at 12.5 m water depth. No dissolved CO2 was northeast part (Figs. 4, 5). Generally, the lake bottom drops found at 1 and 2 m water depth (Table 1). Further element toward the central lake basin with a slope up to 25° (Fig. 6).

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Fig. 4 Simplified bathymetric map of Laacher See. Numbers besides the isobaths indicate the water depth (m) below lake level (275.3 m amsl)

Table 2 Area and volume calculation for various shorelines based on the digital elevation model Shoreline age in years AD Elevation in m (amsl) Area in km2 Volume in km3 Change based on present lake level Δ Area (%) Δ Volume (%)

1845a to present (see SL1 in Fig. 6) 275.3 3.3085 0.1029 no change no change 1167b (see SL2 in Fig. 6) 278.0 3.7593 0.1126 13.6 9.4 + + Thirteenth centuryc (see SL3 in Fig. 6) 283.0 4.2399 0.1326 28.2 28.9 + + Before 1167 (see SL4 in Fig. 6) 288.0 4.6331 0.1548 40.0 50.4 + + a 1845 AD—finished construction of gallery Deliusstollen (Grewe 1979) b 1167 AD—finished construction of gallery Fulbertstollen (Grewe 1979) c Accidental collapse of gallery Fulbertstollen (Grewe 1979)

In the central northern part of the lake, a flat area at about and 50–70 m width (location F in Fig. 5) in the southern 50 m water depth (Fig. 4, location I in Fig. 5) has been lake basin marks the so called Barschbuckel. The ridge identified. It is stretched over 1,000 m in northeast–south- extends approximately 20–25 m above the lake ground. In west direction and 500 m in northwest–southeast direction. the north of the Barschbuckel, three humps (up to 4 m high) Furthermore, an arc-shaped ridge of about 600 m length with diameters up to 60 m were identified (loc. D in Fig. 5).

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to 600 m, and the progradations into the lake reach from 100 to 150 m. A major subaqueous slide with a rough surface topography was identified in the northwestern part of the lake (loc. G in Fig. 5). It is spread over 700 m in width and up to 600 m into the lake. From local slope gradients (Fig. 6), former shorelines of the lake can be identified. SL1 (Fig. 6) marks the recent lake level at 275.3 m (amsl). SL2 (Fig. 6) is at 278.0 m (amsl) and runs above the entrance to the manmade gallery Deliusstollen (entrance: loc. E in Fig. 5). A further line is at 283.0 m (loc. SL3 in Fig. 6), and the oldest paleo-shoreline (loc. SL4 in Fig. 6) is located at an elevation of about 288.0 m (amsl).

CO2‑degassing vents (wet mofettes)

In addition to the acquisition of bathymetric data, rising gas bubbles (wet mofettes) were identified along profiles throughout the Laacher See basin (Fig. 7). The number of wet mofettes is considerably higher in the northeastern part

of the lake basin with a patchy distribution of CO2-degassing vents. Nevertheless, localized zones with a high density of vents were identified on the eastern shore close to the Jägerspitze (loc. J in Fig. 5), between the scoria cone Alte Burg (loc. M in Fig. 5) and the lava flow of Loren- zfelsen (loc. O in Fig. 5) as well north of Lorenzfelsen close to the lakeshore. In the immediate vicinity of the eastern shoreline (north of Lorenzfelsen) at a length of 1,000 m, degassing vents are directly visible as fields of gas bubbles. In the northwestern part of the lake, we found several areas with a higher density of wet mofettes at a distance from 150 m up to 500 m from the western shore (Fig. 7). There is a noticeable lack of degassing vents in the central deep part of the basin. Overall, wet mofettes in the northeastern part of the lake basin are spread in an almost circular- shaped pattern with a diameter of about 1.5 km (Fig. 7). We have not identified any wet mofettes in the southwestern lake basin with the exception of a major cluster of degassing vents with a diameter of about 100 m, approximately 130 m offshore from the southwestern shoreline. Fig. 5 Hillshade image (azimuth: 315°, elevation: 45°, vertical exag- geration (VE) 5) of digital elevation model including the bathymetric model of= the Laacher See basin. Cross sections S1–S1′ and S2–S2′ with VE 3. Denotations: A, valley of creek Beller Bach; Discussion A′, mouth of creek= Beller Bach; B, intermitting creek Im Unselt; C, Mohlenberg valley; D, humps; E, entrance gallery Deliusstollen; F, Barschbuckel; G, subaqueous slide, white line contours the slide body Physical and chemical properties (volume approximately: 3.65 106 m3); H, collapsed entrance gallery Fulbertstollen; I, deep lake plain; J, Jägerspitze; K, L, unnamed ero- Physicochemical measurements in the water column of sion valleys; M, scoria cone Alte Burg; N, erosion valley Im Unselt; Laacher See indicate a three-layer stratification with con- O, lava flow Lorenzfelsen stant values in the epilimnion, strong gradients within the metalimnion and a hypolimnion showing constant val- A number of deltas were deposited in the littoral zones ues for temperature and electrical conductivity, but values around the shoreline of Laacher See (locations: D1–D6 in for dissolved oxygen and pH decreasing with water depth

Fig. 6). The horizontal extents of the deltas range from 300 (Fig. 2). The pH value is strongly related to dissolved CO2,

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Fig. 6 Local slope gradients of the Laacher See Volcano basin. D1, delta of creek Beller Bach; D2, unnamed delta; D3, delta of Mohlenberg valley; D4, delta of creek Im Unselt; D5, D6, unnamed deltas; SL1, present lake level 275.3 m (amsl); SL2, lake level formed after first lowering in 1167 AD; SL3, lake level at thirteenth century; SL4, lake level prior 1167 AD

the highest concentrations of which were found close to the measurement. Water chemistry is dominated by Mg, Ca, Na, lake bottom (Table 1). CO2 concentrations decrease with K and HCO3− (Table 1), most likely due to interaction of decreasing water depth, which is mirrored in pH values. inflowing groundwater with tephra material, which is abun-

This most probably may be due to parts of the rising CO2 dantly outcropping at the floor of Laacher See. gas bubbles getting into solution and remaining near the lake bottom. This is consistent with the model of McGin- Bathymetry and topography nis et al. (2011), who stated that rising bubbles of CO2 in lake water should dissolve rapidly and subsequently sink to The bathymetric measurements reveal that the hourglass- the lake bottom. Most likely, not all CO2 bubbles in Laacher shaped basin of Laacher See is divided into a shallower and See get in solution during gas rise, but reach the lake sur- smaller southwestern part and a deeper and larger north- face. Nevertheless, concentration of dissolved CO2 in the eastern part. This is consistent with the eruptive history epilimnion is zero due to CO2 consuming photosynthesis of of Laacher See Volcano, during which the eruptive center algae and other organisms living in the epilimnion. Electri- moved from the southern part of the basin toward its north- cal conductivity of Laacher See water was approximately eastern part during volcanic activity (Schmincke 1999). 1 0.7 mS cm− and therefore higher than in maar lakes (Ulme- This implies that the products of the later explosive phase 1 1 ner Maar: 0.2–0.3 mS cm− ; Pulver Maar: 0.1 mS cm− buried the initial southern eruption center. The collapse of (Scharf and Oehms 1992) in the WEVF. A chemocline that the volcanic complex at the end of the eruptions led to the means a vertical hydrochemical gradient within the water very steep crater slopes. Due to post-eruptive sedimenta- column becomes not evident in Laacher See at the time of tion, the central part of the lake is almost flat. This deep

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Fig. 7 Spatial distribution of CO2 vents (mofettes) in the area of Laacher See Volcano. Wet mofettes (yellow dots) were identified by an echo sounder; dry mofettes (red dots) are soil sampling points with CO2 contents >90 vol%; blue crosses are soil sampling points with CO2 contents <90 vol%, respectively. Data of dry mofettes are published by Stoiber-Lipp et al. (2012)

flat has existed most likely already in an early stage of lake The underwater slide (loc. G in Fig. 5) at the northwest- formation, similar to maar lakes in the WEVF (Scharf and ern part of the lake was likely caused by instabilities of a Menn 1992). The bathymetric model also reveals differ- former delta. It is also possible that this slide originated ent morphological features like deltas, subaqueous slides, from subaerial source areas, such as debris flows into the the Barschbuckel ridge and a group of humps of unknown lake. Turbidities in lake sediment cores, described by Bah- composition and origin on the lake floor. rig (1985), may indicate that subaqueous slides occurred Erosion of the crater wall of Laacher See Volcano has within the lake. However, these records are not clear indi- led to the deposition of sediments in the lake basin. As a cations since turbidities could also be formed, e.g., by result, deltas occur offshore the littoral zones of the lake. extreme flood events. This is confirmed by sediment cores (Bahrig 1985). To The arc-shaped Barschbuckel structure (location F in determine the provenance of the sediments the DEM of the Fig. 5), conspicuously visible in the bathymetric model, has Laacher See area can be used. In two cases, the observed been previously known and interpreted as a ridge of Devo- deltas are related to recently existing creeks. The creek Bel- nian shale, which already existed before the eruption of ler Bach (Fig. 5) formed the delta located in the southwest the Laacher See Volcano (Bahrig 1985). The statements of (location D1 in Fig. 6), and the intermittent creek Im Unselt Bahrig (1985) are based on interpretations of seismic pro- is connected to location D4 (Fig. 6) in the northern part of files which do not reveal any obvious evidence for an iso- the lake. The flow direction of Beller Bach was altered in lated volcanic center at this location. Total magnetic field the last centuries; it now runs into the lake underground data (Lorenzen and Berckhemer 1976; Pucher 1992) do not between locations A (Fig. 5) and the present-day mouth indicate an anomaly related to a separated volcanic center (loc. A’ in Fig. 5). The remaining deltas (D2, D3, D5 and at this location, although the topography of the structure D6 in Fig. 6) originated from sediment input from dry val- is similar to a partly conserved (possibly palagonized) leys, e.g., Mohlenberg valley (loc. C in Fig. 5) or a name- tuff ring. Therefore, we suggest that the conservation of a less valley between the lava flow Lorenzfelsen (loc. O in preexisting arcuate Devonian ridge at this position seems Fig. 5) and the scoria cone Alte Burg (loc. M in Fig. 5). unlikely. The origin and lithological composition of the

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Fig. 8 Sketch of assumed volcano-tectonic elements inside the Laacher See Volcano basin. The northern (N) and southern (S) crater rims of Laacher See Volcano were first mentioned by van den Bogaard and Schmincke (1990a, b). Based on our newly acquired data of spatial distribution of CO2-degassing vents the run of the northern (N) crater rim of Laacher See Volcano was modi- fied. Preferential paths for gas rise indicate possible volcano- tectonic fault scarps inside the crater rim. The position of the southeastern crater rim of the scoria cone Alte Burg and the tuff ring Graue Ley (as tuff ring M1) is mentioned in Freundt and Schminke (1984). Based on morphological features and the spatial distribution of wet mofettes, Barschbuckel is considered as a partially preserved crater wall of a palagonized tuff ring

three humps (loc. D in Fig. 5) north of Barschbuckel is not Volcano, which could be identified in detail in this study unveiled through the data given here, and further investiga- for the first time. The crater rim is likely characterized by tions are under way. volcano-tectonic faults, being preferential flow paths for

Between the scoria cone Alte Burg (loc. M in Fig. 5) the uprising CO2. Based on seismic data, volcano-tectonic and the lava flow Lorenzfelsen (loc. O in Fig. 5), Freundt faults were also suggested by Bahrig (1985) for the east- and Schminke (1984) suggested a phreatomagmatic erup- ern part of Laacher See. Magnetic measurements (Loren- tive center (Graue Ley in Fig. 8) based on tuff ring deposits zen and Berckhemer 1976) indicate a northwest–southeast exposed at this area. Based on our bathymetric dataset, the elongated positive magnetic anomaly in the northern part tuff ring Graue Ley has to be considered as older than the of Laacher See which most likely is caused by a magmatic

Laacher See Volcano, due to the lack of morphological tuff body. The absence of CO2 vents in the central northern ring features within the basin of Laacher See Volcano. part of the lake thus may be a consequence of this magmatic intrusion, which may prevent upward gas flux. The

CO2‑degassing vents (wet mofettes) eastern part of the northern crater rim is located close to the eastern shoreline, which is evident by the large number

The spatial distribution of wet mofettes, in an approxi- of wet mofettes. In this area, the CO2 gas flux is increased mately ring-shaped pattern (Figs. 7, 8), traces the northern also onshore, where dry mofettes with CO2 concentrations crater rim (“N crater rim” in Fig. 7) of the Laacher See higher than 90 vol % in soil gas were found (Pfanz 2008;

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Stoiber-Lipp et al. 2012). The spatial distribution of dry a new lake level at 273 m (amsl). This level has remained mofettes (Fig. 7) is bound up to 30 m sub-parallel to the unchanged since then (loc. SL1 in Fig. 6). shoreline (Stoiber-Lipp et al. 2012) and marks the eastern crater rim. Between the lava flow Lorenzfelsen and the scoria cone Alte Burg, wet mofettes are widespread (Figs. 7, Conclusions 8). Preferential paths for the gas rise are most likely linked to a conjugated set of faults originating from post-erup- Laacher See Volcano is composed of two main craters, tive volcano-tectonic activity. Closer to the shoreline, wet overall elongated in northeast–southwest direction and with mofettes may be additionally associated with the older tuff its lowermost crater rims in the southern part. The general ring Graue Ley (Fig. 8), which is documented by deposits slope of the inner crater wall is twice as steep as the outer onshore (Freundt and Schminke 1984). Widespread clus- crater wall with average values of 15°. Inside the crater of ters of wet mofettes were also found in the western part of Laacher See Volcano, the lake basin has a maximum exten- the northern crater rim (Fig. 8). We suggest that the distri- sion of 2,200 m in north–northeast to south–southwest and bution of the mofettes is controlled by conjugated faults. 900 m in north–northwest to south–southeast directions. Wet mofettes were not identified at the northern lakeside, The lake basin has a steep sloping shoreline with slopes suggesting that the northern crater rim is located onshore up to 25° and a 1,000 m by 500 m wide almost flat cen-

(Fig. 8). Here, further measurements of CO2 in the soil gas tral part, which is down to 52 m deep. An arc-shaped ridge are under way to identify the northern crater rim. (Barschbuckel) of about 600 m length and 50–70 m width A southern crater rim (“S crater rim” in Fig. 8), repre- occurs in the southwestern part of the lake basin. We sug- senting the first stage of Laacher See volcano eruption, gest this structure is part of a palagonized tuff ring. About was already defined by van den Bogaard and Schmincke 125 m north of the Barschbuckel, three humps each 4 m (1990a, b). A number of wet mofettes close to the west- high on average with a diameter up to 60 m were identi- ern shoreline are likely associated with the western part fied. The origin of these structures is not yet understood of the southern crater rim (Fig. 8). Since the extent of this and under investigation. Several deltas occur in the littoral crater rim in the south and south-east is presently located zone of the lake basin and are linked to erosional processes onshore, wet mofettes cannot be related to the crater rim. along the crater wall. A major underwater slide occurred in Measurements to identify dry mofettes have not been suc- the northwestern section of the lake body (loc. G in Fig. 5). cessful up to now. It is conspicuous that in the southern part From horizontal gradients of elevation three former lake of the lake, CO2 vents in high spatial density only occur terraces can be distinguished and can be attributed to lake level in an area close to the delta of the creek Beller Bach (loc. regulations as water management measures in historical times. D1 in Fig. 6). This very small area of degassing approxi- At the time of our measurements in summer 2010, in the mately marks the geometric center of the circular structure water body of Laacher See, three-layer stratification was indicated by the Barschbuckel. The cause of this degassing established. Measured pH values are strongly dependent pattern thus may mirror the conduit of a possible but not on the concentration of dissolved CO2, which are highest yet confirmed Barschbuckel volcanic center. close to the lake bottom. That means that amounts of the

rising CO2 gas bubbles get dissolved and CO2 remains in Historical water management the bottom water. The number of CO2-degassing vents (wet mofettes) in the lake is considerably higher in the northern The run of the former shorelines of the lake on the land- part of the Laacher See basin. Overall, the vent clusters side (Fig. 6) is closely related to historical events of water form an almost circular-shaped (diameter of 1,500 m) pat- management at the lake (Table 2). Shoreline SL4 (Fig. 6) is tern, tracing the crater rim of the northern eruption center located at 288 m (amsl) and represents the lake level prior of the Laacher See Volcano. The concentrically arranged to 1167 AD, when the lake level was lowered for the first clusters inside the crater rim are zones of preferential gas time after completion of the drainage gallery Fulbertstollen flow along conjugated sets of faults, which are caused by (Grewe 1979). Since the lake was lowered by about 10 m post-eruptive tectonic processes. At the eastern lake basin, (Grewe 1979), the new shoreline after 1167 AD was at the crater rim of the northern eruption center runs close to

278 m (amsl), marked as line SL2 (Fig. 6). Between these the shoreline, where CO2-degassing vents occur on lake- shorelines, a further line (loc. SL3 in Fig. 6) was identified side as well as landside (dry mofettes). Since wet mofettes at 283 m (amsl). According to historical records, the Ful- do not occur at the northern lake, we suggest that the north- bertstollen had collapsed in the thirteenth century (Grewe ern part of the northern crater rim extends onshore. The 1979), and the lake level temporarily rose to 283 m (amsl). absence of degassing vents in the central northern lake is To lower the lake level again, the drainage gallery Deliuss- most likely related to the extent of a magma body below tollen was finished in 1845 AD (Grewe 1979), resulting in the lake, which is inhibiting gas rise. In the southern part of

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the lake, CO2-degassing vents occur in high spatial density Gal F, Brach M, Braibant G, Jouin F, Michel K (2011) CO2 escapes in only in a single area, close to the foot of the subaqueous fan the Laacher See region, East Eifel, Germany: application of natural analogue onshore and offshore geochemical monitoring. Int J of the creek Beller Bach. The cause of this isolated mofette Greenh Gas Control 5:1099–1118 cluster may be taken as proof for the conduit of the Barsch- GARMIN (2012) GPSMAP 400/500 series: owner’s manual, 78 pp buckel palagonitic tuff ring. Giggenbach WF, Sano Y, Schmicke HU (1991) CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Acknowledgments We appreciate the kind support of the Benedic- Dieng, Indonesia, and Mt. Gambier, Australia—variations tine Abbey of Maria Laach, “Struktur- und Genehmigungsdirektion on a common theme. J Volcanol Geotherm Res 45:311–323. Nord, Koblenz” and the logistical assistance of Ansgar Hehenkamp. doi:10.1016/0377-0273(91)90065-8 LiDAR data and aerial photos have been kindly provided by “Lande- Goepel A (2008) Natürliche Gläser und Glasuren in den Vulkaniten samt für Vermessung und Geobasisinformation Rheinland-Pfalz, der Westeifel. PhD thesis, Institute for Geosciences, Friedrich- LVermGeo.” We would also like to thank the numerous students of Schiller University, Jena, 499 pp the Institute of Geosciences (Friedrich Schiller University of Jena, Grewe K (1979) Der Fulbert Stollen am Laacher See. Eine Ing- Germany) who supported the data collection during several field cam- enieursleistung des hohen Mittelalters. Zeitschr Arch Mittelalt paigns. We would like to thank two anonymous reviewers for helpful 7:107–142 suggestions which significantly improved the manuscript. 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