The acoustic turbid layer in muddy sediments of Eckernfoerde Bay, Western Baltic: methane concentration, saturation and bubble characteristics

MARINE

GEOLOGY

INT£RNATIONAL JOURNAl. OF MAAINE

GEOLOGY, GEOCHEMISTRY AND GEOPHYSICS

ELSEVIER

Marine Geology 137 (1997) 137-147

The acoustic turbid layer in muddy sediments of Eckernfoerde Bay, Western Baltic: methane concentration, saturation and bubble characteristics Friedrich Abegg", Aubrey L. Andersonb a

Geologisch-Paldontologisches Institut der Universitdt Kiel, Olshausenstrasse 40, D-24118 Kiel, Germany b Department of Oceanography, Texas A & M University, College Station, TX 77843-3146, USA Received 5 December 1994;revision 27 February 1996; accepted 21 August 1996

Abstract In Eckernfoerde Bay in the western Baltic, an enclosed basin with organic-rich mud, acoustic turbidity has been observed since 1952. To investigate the relationships between acoustic properties and free gas we developed a method for rapid sub-sampling of gravity cores to determine the total methane content. From temperature measurements, determinations of the salinity of the pore water and data from the literature, saturation limits were calculated for each core. It is demonstrated that there is a certain amount of free gas in the mud of Eckernfoerde Bay but subbottom depths of the oversaturation and of the acoustic turbidity do not correspond. The existence of free gas implies the existence of bubbles in the sediment which are regarded to be responsible for the acoustic turbidity. Bubble sizes observed by X-ray computed tomography while still under in situ pressure and temperature vary between 1 and 10 mm equivalent diameter. The distribution of bubbles is discontinuous, occurring in depth zones from 2 to 20 em thickness.

Keywords: acoustic turbidity; methane saturation; sediment gas bubbles; Eckernforde Bay

1. Introduction Like many other regions in the world, Eckemfoerde Bay is well known for a widespread acoustically impenetrable horizon in the seafloor. During the first investigations with a depth sounder the occurrence of this horizon was related to shells and gas bubbles (Schuler, 1952). Later investigations with various sound sources, using an air gun and higher frequencies from 3 to 30 kHz, were unable to provide information about layers below the so called "Becken Effekt" (Hinz et a1., 1969, 1971). First measurements of methane concen0025-3227/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PI! S0025-3227(96)00084-9

trations were conducted by Whiticar (1978). He showed that at some sites in Eckemfoerde Bay the saturation limit for methane was exceeded. From these measurements he deduced the existence of gas bubbles in the sediment (Whiticar, 1982). These bubbles were regarded to be responsible for the acoustic anomalies (e.g. Hampton and Anderson, 1974; Schubel, 1974; Anderson and Hampton, 1980). We will present new total-methane plots and will demonstrate the relationship between saturation limit, based on temperature and salinity determinations from the same core, and the acoustically

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impenetrable horizon. Although attempts to demonstrate bubble characteristics have been made (e.g. Schubel, 1974; Martens and Klump, 1980), the problem of degassing and bubble growth during core retrieval caused by hydrostatic pressure release has not been solved previously. One of the purposes of this paper will be to demonstrate the in situ gas bubble characteristics using samples which have not suffered from these problems.

2. Area description Located at the southwestern edge of the Kiel Bight, Eckernfoerde Bay is an elongated inlet with water depths up to 29 m and an opening to the northeast. The opening to Kiel Bight is divided into two channels by the Mittelgrund (Fig. 1). The deeper northern channel, called the Boknis Channel, is connected to the channel system of the

Western Baltic Sea. The southern channel reaches water depths of only 20 m. The basin of the Bay was excavated by glaciers during the Pleistocene. After melting of the ice there was a limnoterrestrial development before the marine influence began after the Litorina Transgression (Seibold et al., 1971). During the Litorina Transgression, sea level in the Kattegat rose and connected the fresh water of the Ancylus Lake, in the bed of the present Baltic Sea, resulting in a brackish state called Litorina Sea (Rumohr et al., 1987). In general the sediments inside Eckernfoerde Bay can be divided into four areas. Zone one, near the coastline, contains lag sediments with sand sheets. Zones two and three are characterised by sands from medium to fine grain sizes, and reach a depth of 22.5 m. The 4th zone, the central part of the Bay below 22.5 m, is filled with mud (Wefer et al., 1974). It is a very soft mud with water contents as high as 500% in the topmost em, decreasing to

10° 00'

10° 10'

Fig. I. Map of Eckernfoerde Bay. A= site K 137, B= site K 139, C= site K 140, D= site K 141, E= core 315, F= core 339.

F. Abegg, A.L. Anderson/ Marine Geology 137 (1997) 137-147

250% and more stiff material at a depth of 4 m below the seafloor surface. The topmost section is black and has a strong H 2S odour. About 1 m below the seafloor the odour disappears and the colour changes to a dark olive-green. Organic carbon content on an average is at 4-5% and may reach values of8% (Whiticar, 1978; Abegg, unpublished data). The organic carbon is the source for bacterial methane production under anoxic conditions below the sulfate reduction zone (e.g. Martens and Berner, 1974; Reeburgh and Heggie, 1974; Crill and Martens, 1986).

3. Methods 3.1. Sampling technique and total methane content

Based on the techniques described by Devol (1983) and Jones et al. (1986) we use a 5 m long gravity corer with a PVC-liner. The liner has predrilled holes which are sealed with tape. Upon core retrieval the PVC-liner is pulled out of the core barrel and the ends are closed with caps. The covering tape is cut away from the sample holes one at a time. After the tape is cut off, sub samples are taken with a syringe of 2 ml which has its tip cut off. 1 ml of sediment material is transferred into small glass vials and immediately sealed with gastight rubber septums. For the measurement of methane content we use a Gas Chromatograph with a Porapak Q-Column. After heating the vial up to 65°C, a certain amount of the head space gas in the vial is sent into the column. The amount of methane is detected with a Flame Ionisation Detector. Results are expressed as millimol per litre (mmoljl) pore water. Many efforts have been made to control the gas loss during sampling including efforts to subsample the cores under in situ pressure in a hyperbaric chamber. One result was that the cores are subsampled from bottom to top. Each sample is taken immediately after the tape is cut away from the pre-drilled hole in the core liner. Secondly, we learned that a small amount of methane is lost during subsampling (Abegg, 1994). From our latest investigations we have learned that this amount depends on the sampling speed. For this

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reason we have reduced the sampling time of a 5-m long core to 35 minutes. 3.2. Methane saturation

The total methane content does not provide much information regarding the acoustic properties of the sediment. These properties are controlled by free gas, and bubble formation requires methane-over-saturated conditions. The probable amount of free gas may be calculated from the difference between the saturation limit and the total methane content. Our calculations of the saturation limit are based on the Bunsen solubility coefficients ~ worked out by Yamamoto et al. (1976). They developed a formula which is based on experimental measurements of the quantity of dissolved methane in distilled and sea water. In a table, ~ values are given for a temperature range from - 2 to 30°C and salinity values from 0 to 40 ppt. For accurate determinations of the in situ solubility at the seafloor-pressure, corrections according to Henry's Law were added. Henry's Law indicates that, at these shallow depths (max. 35 m), the amount of dissolved methane is proportional to increasing water depth. A main source of error in these calculations might be the lack of solubility values for a three-phase system. Determinations of Yamamoto et al. were made for the two phase system of water and gas. The influence of solids and other gases in the system on the solubility of methane is still unknown. This simple model was first used to look at the different influences of salinity, temperature and pressure changes on methane solubility in the seafloor using data from Eckernfoerde Bay for all three parameters. Curves in the plot of Fig. 2 show the saturation limit for salinity values from 0 to 26 ppt at 6°C. The left curve represents solubility at the sea surface, the right curve at a water depth of 25 m. Differences of 0.4 mmoljl at sea surface and 1.3 mmoljl at 25 m indicate that the influence of salinity on the solubility of methane is nonlinear. The non-linearity is more obvious in Fig. 3. Here we plotted a temperature range from 0 to nearly 15°C. Again the left curve represents the result at the sea surface and the right at 25 m

F. Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

140 Methane (mmol/l) 0 25

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50 em increments. These data were transferred to the computer together with the water depth at the core site, and the resulting saturation limit was plotted together with the total methane content. Examples will be given in section 4.1., Fig. 4-7.

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3.3. Determination ofbubble characteristics

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Fig. 2. Saturation limit versus salinity at a temperature of 6°C, left curve at sea surface, right curve at a water depth of 25 m. Methane (mmolll) 3

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water depth 0 m, .allnlty 20 ppt Saturation limIt va temperature, water depth 25 m, ..lInlty 20 ppt

\ Fig. 3. Saturation limit versus temperature at a salinity of 20 ppt, left curve at sea surface, right curve at a water depth of 25 m.

water depth and a salinity of 20 ppt. The differences are 0.7 mmol/l and 2.5 mmol/l, respectively. The publications of Whiticar and Werner (1981) and Hovland and Judd (1988) indicate that the salinity of the pore water in Eckernfoerde Bay sediments can vary significantly. For this reason and to include seasonal temperature variations, these parameters were recorded in parallel with the methane measurements. Sediment temperatures were measured immediately after core recovery in 50 em intervals. Determination of the salinity was made with a refractometer using pore water obtained from centrifuged sub samples, also in

To maintain the in situ conditions of the seafloor samples, a technique has been developed using pressure tight aluminum pressure transfer chambers (aptc). At the seafloor divers cut the coring tubes, obtained either from a 5 m gravity corer or taken by the divers themselves, into 1 m long sections. These sections are sealed into the aptc at the seafloor. The aptc have a total length of 1200rnm, an outer diameter of 150 mm and a wall thickness of 5 mm. To determine the inside pressure a pressure gauge is located at the end of the aptc. Upon aptc retrieval the cores are transported to an x-ray computed tomography scanner, maintaining the in situ temperature until the cores are scanned. X-ray computed tomography (CT) was mainly developed for medical use (Hounsfield, 1973) but has also been applied to soil sciencesand to analyse marine sedimentary structures (e.g. Anderson et al., 1988; Kenter, 1989; Holler and Kogler, 1990). A brief description of theory and measurement techniques is given by Orsi et al. (1992). There are several advantages in using a CT-scanner. First, it is a non-destructive technique. No sample preparation, with all the problems of damaging the sample through tearing, dewatering, drying or expansion by degassing, is required. Second, an x-ray beam is able to penetrate the wall of the aptc and provide images of high quality from the sediments inside. Third, CT-scanning has high resolution capabilities. Attenuation contrasts down to 0.1% (Kenter, 1989) can be detected, and the minimum slice thickness may go down to 1 mm, depending on the scanner. Every slice is represented by a matrix of digital values, which is used to reconstruct the image. These digital values may be stored on a suitable medium for subsequent processing. The digital values are directly related to the x-ray attenuation within a small volume element of the scanned sample (volume element =

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F Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

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a voxel = pixel x slice thickness). Although several factors influence this x-ray attenuation, for our samples a primary control is bulk density of the

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Fig. 5. Top: total methane concentration and saturation limit at site K 139. Bottom: 3.5 kHz profiler record from site K139 (courtesy FWG).

material in a voxel. Thus the matrix of digital values, with suitable calibration, is interpreted as a matrix of bulk density values.

F. Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

142

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For the measurements described here we used an Elscint CT Twin scanner. This scanner is equipped with a double dynamic-focus x-ray

Fig. 7. Top: total methane concentration and saturation limit at site K 141. Bottom: 3.5 kHz profiler record from site K141 (courtesy FWG).

system which allows two slices to be taken at the same time. This reduces the scanning time for one slice to I second. Scanning parameters for the

F. Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

cores from Eckernfoerde Bay have been set to a scan circle diameter of 180mm, a picture element (pixel) matrix of 512 x 512 and slice thicknesses of either 1 or 2.5 mm. The x-ray tube was set to 120kV and 220 or 300 mAs. The data are stored on erasable optical disks with 1 Gbyte capacity. Subsequent processing of the data includes both generation of imagery and determination of quantitative characteristics such as free gas content and bubble size, which is possible because of the great bulk-density contrast between free gas and other constituents of the sediment. Two and three dimensional images are generated with Spyglass Transform and Dicer Software. The software for the determination of free gas distribution and bubble size was developed at Texas A & M University as part of the study reported here. 3.4. Site locations During a cruise in April 1994 we investigated four sites with different depths of the acoustically turbid horizon both with shallow seismic profiling and coring. Acoustic records were taken with a 3.5 kHz sub bottom profiler by the Federal Armed Forces Underwater Acoustics and Marine Geophysics Research Institute (FWG). Three sites are located inside Eckernfoerde Bay west of the Mittelgrund (site K 137, K 139 and K 140, A + B, and C in Fig. 1). For comparison we have chosen one site which is located outside Eckernfoerde Bay in the Kiel Bight, five miles northeast of the Boknis Channel (site K 141, indicated as D). Core 315 (E in Fig. 1), which has been used for the CT-scans, was obtained from a site in the centre of the Bay. The other scanned core (339, F in Fig. 1) was taken inside a pockmark close to the northernmost coastline.

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contents higher than 7 mmoljl first occur at a depth of 180 em below seafloor. Core K 137 has a peak concentration of 12.7 mmoljl at 140em below seafloor. Seafloor surface temperatures ranged from 3.8 to 4.8°C. At a depth of 180cm, below seafloor, temperatures were from 5.8 to 6.7°C, and the highest temperature (7SC) was measured in core K 140 at a depth of 310 em, All four sites showed the same porewater salinity of 20 ppt at the surface. In cores K 137 and K 141 a decrease with depth was measured; values were 14 ppt and 18 ppt, respectively, at core ends. K 139 kept a salinity of 20 ppt until the end, and in core K 140 fresh conditions were reached 220 em below seafloor. Methane concentrations and saturation limits are shown in Figs. 4-6. At all sites inside the bay oversaturated conditions are reached within the top 50 cm. The shallowest point of oversaturation (poo) is reached, at site K 140, at only 15 em below seafloor. At site K 137, poo is 35 em below seafloor and at site K 139 it is located at a depth of 50 em. At site K 141, outside the bay (Fig. 7), oversaturation is reached at a depth of 175 cm. The lower panels of Figs. 4-7 show the corresponding 3.5 kHz records. A comparison between the depth of poo and the depth of the acoustic turbidity horizon shows that the two do not coincide. At all sites acoustic turbidity occurs below the poo, and there is a tendency for a shallow poo to be combined with a smaller difference between poo and the depth of the turbidity horizon. At site K 140 the turbidity starts 50 em below seafloor with a resulting difference between poo and turbidity of 35 em. Outside the Bay, at site K 141, the turbidity starts 250 em below seafloor, thus having a difference of 75 em. At site K 137, with a turbidity depth of 75 em, the difference is 40 em, and at site K 139, where the turbidity begins 100 em below seafloor, the difference is 50 em.

4. Results

4.2. Bubble characteristics 4.1. Methane saturationand acousticscattering depth In Eckernfoerde Bay, methane contents during the 1994 cruise reached values from 7 to 8 mmoljl within the topmost meter. Outside the Bay,

Several cores from the floor of Eckernfoerde Bay with a maximum length of up to 5 m have been investigated with the CT-scanning technique. In Fig. 8 the free gas volume concentration as determined for each CT "slice" of core 315 is

F. Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

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Gas Concentration ("!o) 0.0

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plotted versus depth. At subseafloor depths shallower than 60 em no free gas was measured. The profile demonstrates an important characteristic of the gas bubble depth distribution. It is obvious that the bubble distribution is not uniform. Bubbles exist within certain depth intervals which are separated by intervals with no gas bubbles. The thickness of these bubble intervals varies from about 2 em to 11 em in this core. In other cores, thicknesses of up to 20 em have been noted. Fig. 9 demonstrates a single slice with I rom thickness from a depth of 135 cm in core 315. Clearly visible is the aptc (outer white circle) and the core liner (inner white circle). Sea water, printed in light grey, is trapped between the aptc and the liner. The sediment (dark grey) inside the core liner contains a tube which is elongated in one direction and filled primarily with free gas printed in black. The air outside the aptc is also printed in black. For Fig. 10 a set of 50 single slices was used as a data base. This 3D image covers the interval from 131 em to 136em in core 315. The image, representing 5 em of an 11 ern continuously scanned gassy interval of the core, shows that the

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Fig. 9. Single slice from core 315, depth -135 em below seafloor.

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gas bubbles fill sediment fractures with two long and one short dimension. Remarkably in different depth zones of this and other cores, one of the

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long dimensions is vertical, the other is horizontal. This results in "coin"-shaped gas filledvoids which are standing in a vertical or near vertical orientation. Another type of bubble is located at a depth of 80 em in core 315. These have a more spherical or near spherical shape. A third type of gas accumulation has been detected 150 em below seafloor. Here free gas is entrapped in large shells. The minimum gas bubble dimension which is resolved in these measurements is about 1 mm and several bubbles of this small size were identified. The largest dimension measured for the gas filled fractures was about 2.5 cm. Calculation of equivalent diameters (diameter of a sphere with volume equal to actual volume of bubble) shows a range from 1 to 10 mm. As indicated by the gas concentration profile of Fig. 8, most of the gassy depth zones had volume concentrations below about 0.5%. The highest free gas volume concentrations have been measured in a pockmark from Eckernfoerde Bay. Fig. 11 is a single slice from 38 cm sub seafloor depth of core 339, a pockmark location close to the northern shore of Eckernfoerde Bay.

This is the scan with the highest gas content of all our cores. The largest gas volume concentration measured in all samples occurs here and is 4.5%. Again in this image of 2.5 mm thickness, the 2-D sections of the bubbles show their non-spherical shape. Fig. 12 demonstrates a 2-D reconstruction of a vertical section of core 339 on the left, containing portions of 351 single slices. In the top few ems the slump of sediment is visible, but the main section of the core is undisturbed as can be seen from the stratification. The right panel of Fig. 12 shows the corresponding free gas concentration. In the slumped part of the core the concentrations remain below 0.2%. From that point on the intervals contain more and more free gas until the sub seafloor depth around 38 em, which has a maximum of 4.5% gas as discussed before. Below 38 em the gas concentration is more comparable with the results of core 315.

GASCONTENT (%) 0.0

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Fig. 12. Left: 2D data base of core 339. Right: corresponding gas content of core 339.

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5. Discussion

6. Conclusions

As mentioned earlier several examples of bubbles or bubble-like structures in sediments are given in the literature, but most authors discuss a bubble growth dependent on the pressure release during core recovery. Forstner et al. (1968) showed voids in thin slides from the Bodensee (Germany) and demonstrated that migration of bubbles is able to destroy the stratification. Schubel (1974) recognised, besides bubble-like voids, elongated fissures in conventional x-ray radiographs of an acoustically turbid site from Chesapeake Bay. He postulates that these fissures might be caused by coring. Milkert (1993) recognised similar threadlike vertical cracks in cores from Eckemfoerde Bay. She discusses them as being caused by degassing after core retrieval. Martens and Klump (1980) demonstrate open tubes in a radiography from Cape Lookout Bight sediments, which they describe as being caused by ebullition of methane bubbles. Besides these tubes there are small bubbles with diameters from 1 to 2 mID, regarded as being caused by pressure release. Larger 5-20 mID vertically elongated white streaks were regarded as naturally occurring bubbles. An example of a bubble like void in the um-scale is given by Yuan et al. (1992). During their investigation in the western Irish Sea these authors found micro bubbles using scanning electron micrographs. Investigating gas-charged sediments, especially to determine bubble size and distribution, it is obvious that the very sensitive influence of pressure and temperature requieres that samples be preserved at in situ values of these two parameters. Using the methods described here a first step has been made. It is understood that the use of diver cores is limited to a certain depth. To obtain pressure tight cores from greater water depth, a modified piston corer is under construction at Texas A & M University. Until now there is no answer to the question of what is causing the depth-difference between the point of oversaturation and the beginning of the acoustic turbidity. It might be that in this area only a few very small bubbles are formed and these do not affect the acoustic properties of the sediment significantly.

It is evident that free gas influences the acoustic properties of soft mud through scattering, resonance and reverberation. Depending on the environment, chemical conditions may allow biogenic methane production and retention in the sediment. We have demonstrated that there are significant free methane contents in muddy sediments of Eckemfoerde Bay and that we have a clearly identifiable acoustic turbid horizon. But the acoustic turbidity does not occur at the same depth within the sediment column as the point of over-saturation as derived from calculations. As demonstrated by four examples, it seems that the difference varies with sub seafloor depth and is probably caused by methane production, consumption and accumulation processes. Although production and consumption are well investigated topics, there is a lack of information about gas accumulation processes. A very special problem related to our investigations is the behaviour of the amount of free gas between the poo and the point where the amount of free gas affects the acoustic properties or may be measured by the CT-scanning technique. This includes questions of bubble growth and migration. Measurements of cores with the CT -scanning method described here, give us confidence that bubbles of free gas exist in situ. Accurate determinations of bubble sizes and distributions are possible with these data and they will allow simulations to provide better understanding of the acoustic properties of a marine environment. These data can be a basis for further development of acoustic models of the seafloor.

Acknowledgements Very special thanks are directed to Prof. Dr. R. Koster, Univ. of Kiel. We also would like to thank the Federal Armed Forces Underwater Acoustics and Marine Geophysics Research Institute (FWG), especially Mr. I. Stender, for the help before and during the field campaigns. Special thanks go to Mrs. H. Fiedler for providing the acoustic records, the captains and crews of PLANET and W. PULLWER and of course to the divers for their tremendous efforts. We also wish to thank the Radiologische Gemeinschaftspraxis Pruner Gang

F Abegg, A.L. Anderson / Marine Geology 137 (1997) 137-147

for allowing use of the CT-Twin and Mrs. Grenda for her technical support in using the scanner. Support was given by Fraunhofer Gesellschaft fur Angewandte Forschung, the Naval Research Laboratory and the Office of Naval Research Marine Geology and Geophysics Program. This work has been done as a part of the Coastal Benthic Boundary Layer Special Research Program (CBBL SRP) and the Joint High Frequency Backscatter Experiment (JOBEX).

References Abegg, F., 1994. Methoden zur Untersuchung methanhaltiger mariner Weichsedimente. Meyniana, 46: 1-9. Anderson, A.L. and Hampton, L.D., 1980. Acoustics of gasbearing sediments I. Background. J. Acoust. Soc. Am., 67: 1865-1889. Anderson, S.H., Gantzer, CiL, Boone, J.M. and Tully, R.J., 1988. Rapid nondestructive bulk density and soil-water content determination by computed tomography. Soil Sci. Soc. Am. J., 52: 35-40. Crill, P.M. and Martens, e.S., 1986.Methane production from bicarbonate and acetate in an anoxic marine sediment. Geochim. Cosmochim. Acta, 50: 2050-2061. Devol, A.H., 1983. Methane oxidation rates in the anaerobic sediments of Saanich Inlet. Limnol. Oceanogr., 28: 738-742. Forstner, U., MUller, G. and Reineck, H.-E., 1968. Sedimente und Sedimentgefiige des Rheindeltas im Bodensee. Neues. Jahrb. Min. Abh., 109: 33-62. Hampton, L.D. and Anderson, A.L., 1974. Acoustics and gas in sediments: Applied Research Laboratories (ARL) experience. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments. Plenum Press, New York, pp. 249-273. Hinz, K., Kogler, F.C., Richter, I. and Seibold, E., 1969. Reflexionsseismische Untersuchungen mit einer pneumatischen Schallquelle und einem Sedimentecholot in der westlichen Ostsee. Teil I: MeJ3methoden, Instrumentarium, Interpretation. Meyniana, 19: 91-102. Hinz, K., Kogler, F.e., Richter, I. and Seibold, E., 1971. Reflexionsseismische Untersuchungen mit einer pneumatischen Schallquelle und einem Sedimentecholot in der westlichen Ostsee. Teil II: Untersuchungsergebnisse und geologische Deutung. Meyniana, 21: 17-24. Holler, P. and Kogler, F.-e., 1990. Computer tomography: A nondestructive, high-resolution technique for investigation of sedimentary structures. Mar. Geol., 91: 263-266. Hounsfield, G.N., 1973. Computerized transverse axial scanning (tomography): Part I. Describition of system. Br. J. Radiol., 46: 1016-1022. Hovland, M. and Judd, A.G., 1988. Seabed Pockmarks and Seepages:Impact on Geology, Biology and the Marine Environment. Graham and Trotman, London, 293 pp. Jones, G.B., Floodgate, G.D. and Benell, J.D., 1986. Chemical

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and microbiological aspects of acoustically turbid sediments: Preliminary investigations. Mar. Geotech., 6: 315-332. Kenter, J.A.M., 1989. Applications of computerized tomography in sedimentology. Mar. Geotechnol., 8: 201-211. Martens, C.S. and Berner, R.A., 1974. Methane production in the interstitial water of sulfate-depleted sediments. Science, 185: 1167-1169. Martens, C.S. and Klump, J.V., 1980. Biogeochemical cycling in an organic-rich coastal marine basin-I. Methane sediment-water exchange processes. Geochim. Cosmochim. Acta, 44: 471-490. Milkert, D., 1993. Auswirkungen von Sturmen auf die Schlicksedimente der westlichen Ostsee. Rep. Geol-Palaontol. Inst. Univ. Kiel, 66, pp. 153. Orsi, T.H., Anderson, A.L., Leonard, J.N., Bryant, W.R. and Edwards, C.M., 1992. Use of x-ray computed tomography in the study of marine sediments. Civ. Eng. Oceans, 5: 968-981. Reeburgh, W.S. and Heggie, D.T., 1974. Depth distribution of gases in shallow water sediments. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments. Plenum Press, New York, pp. 27-45. Rumohr, J., Walger, E. and Zeitschel, B., 1987. Seawater-Sediment Interactions in Coastal Waters (Lect. Notes Coast. Estuar. Stud., 13). Springer, Berlin, 338 pp. Schubel, J.R., 1974. Gas bubbles and the acoustically impenetrable, or turbid, character of some estuarine sediments. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments. Plenum Press, New York, pp. 275-298. Schuler, F., 1952. Untersuchungen uber die Machtigkeiten von Schlickschichten mit Hilfe des Echographen. Dtsch. Hydrol. Z., 5: 220-231. Seibold, E., Exon, N., Hartmann, M., Kogler, F.-C., Krumm, H., Lutze, G.F., Newton, R.S. and Werner, F., 1971.Marine geology of Kiel Bay. In: G. MUller(Editor), Sedimentology of Parts of Central Europe. Guidebook Excurs. 8th Int. Sedimentol. Congr., Heidelberg, pp. 209-235. Wefer, G. and Tauchgruppe Kiel, 1974.Topographie und Sedimente im "Hausgarten" des Sonderforschungsbereichs 95 der Universitat Kiel (Eckernforder Bucht, West!. Ostsee). Meyniana, 26: 3-7. Whiticar, M.J., 1978.Relationship of interstitial gases and fluids during early diagenesis in some marine sediments. Rep. SFB 95, 35, Kiel, 152 pp. Whiticar, M.J., 1982. The presence of methane bubbles in the acoustically turbid sediments of Eckernforde Bay, Baltic Sea. In: K.A. Fanning and F.T. Manheim (Editors), Dynamic Environment of the Ocean Floor. Lexington Books, MA, pp.219-235. Whiticar, M.J. and Werner, F., 1981. Pockmarks: Submarine vents of natural gas or freshwater seeps? Geomar. Lett., 1: 193-199. Yamamoto, S., A1causkas, J.B. and Crozier, T.E., 1976.Solubility of methane in distilled water and seawater. J. Chern. Eng. Data, 21: 78-80. Yuan, F., Bennell, J.D. and Davis, A.M., 1992. Acoustic and physical characteristics of gassy sediments in the western Irish Sea. Cont. Shelf Res., 12(10): 1121-1134.