PUBLICATIONS. Journal of Geophysical Research: Biogeosciences. The role of sediment structure in gas bubble storage and release

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1 PUBLICATIONS Journal of Geophysical Research: Biogeosciences RESEARCH ARTICLE Key Points: Gas storage capacity in aquatic sediments is affected by sediment grain size distribution A gas-enriched upper layer with varying thickness is formed in different types of homogenous sediment Ebullition responds linearly to hydrostatic head drop, and release potential is determined by gas storage capacity Supporting Information: Supporting Information S1 Movie S1 Correspondence to: L. Liu, liu@uni-landau.de Citation: Liu, L., J. Wilkinson, K. Koca, C. Buchmann, and A. Lorke (2016), The role of sediment structure in gas bubble storage and release, J. Geophys. Res. Biogeosci., 121, , doi: / 2016JG Received 14 APR 2016 Accepted 10 JUL 2016 Accepted article online 14 JUL 2016 Published online 28 JUL American Geophysical Union. All Rights Reserved. The role of sediment structure in gas bubble storage and release L. Liu 1, J. Wilkinson 1, K. Koca 1, C. Buchmann 1, and A. Lorke 1 1 Institute for Environmental Sciences, University of Koblenz-Landau, Landau, Germany Abstract Ebullition is an important pathway for methane emission from inland waters. However, the mechanisms controlling methane bubble formation and release in aquatic sediments remain unclear. A laboratory incubation experiment was conducted to investigate the dynamics of methane bubble formation, storage, and release in response to hydrostatic head drops in three different types of natural sediment. Homogenized clayey, silty, and sandy sediments (initially quasi-uniform through the depth of the columns) were incubated in chambers for 3 weeks. We observed three distinct stages of methane bubble formation and release: stage I microbubble formation-displacing mobile water from sediment pores with negligible ebullition; stage II formation of large bubbles, displacing the surrounding sediment with concurrent increase in ebullition; and stage III formation of conduits with relatively steady ebullition. The maximum depth-averaged volumetric gas content at steady state varied from 18.8% in clayey to 12.0% in silty and 13.2% in sandy sediment. Gas storage in the sediment columns showed strong vertical stratification: most of the free gas was stored in an upper layer, whose thickness varied with sediment grain size. The magnitude of individual ebullition episodes was linearly correlated to hydrostatic head drop and decreased from clayey to sandy to silty sediment and was in excess of that estimated from gas expansion alone, indicating the release of pore water methane. These findings combined with a hydrodynamic model capable of determining dominant sediment type and depositional zones could help resolve spatial heterogeneities in methane ebullition at medium to larger scales in inland waters. 1. Introduction Ebullition is an important pathway for the emission of biogenic methane from inland waters. Its contribution to the total methane flux can reach more than 80 90% in lakes and reservoirs [Keller and Stallard, 1994; Casper et al., 2000; DelSontro et al., 2010; Zhu et al., 2016]. While methane formation in the sediments is a relatively steady process, methane fluxes associated with ebullition are stochastic and often characterized by hot moments, when large amounts of gas are released in short time periods [Wilkinson et al., 2015]. The intermittency of ebullition in space and time makes the accurate quantification and upscaling of methane fluxes from inland waters difficult [Bastviken et al., 2011; Holgerson and Raymond, 2016]. Ebullition is often triggered by atmospheric or hydrostatic pressure changes [Chanton et al., 1989; Casper et al., 2000; Varadharajan and Hemond, 2012; Maeck et al., 2014]. In water bodies with frequent water-level fluctuations, the temporal dynamics of ebullition is mainly controlled by hydrostatic pressure changes. Ebullition can be triggered either by increase or decrease of pressure through two different mechanisms. The former causes shrinkage of bubble volume, which allows bubbles to gain mobility to migrate upward [Rosenberry et al., 2006]; the latter affects the balance between capillary force (or resistance to fracturing [Algar et al., 2011]) and buoyancy due to bubble expansion [Clayton and Hay, 1994]. In inland waters, a large number of studies emphasized the effect of pressure drops on ebullition [Chanton et al., 1989; Varadharajan and Hemond, 2012; Maeck et al., 2014; Wilkinson et al., 2015], compared to the relatively smaller effect of pressure increase when triggering ebullition [Rosenberry et al., 2006]. Although generally linear relationships between the amount of gas released by ebullition and the magnitude of the pressure drop were observed in field data, exceptions have also frequently been observed [Varadharajan and Hemond, 2012; Maeck et al., 2014]. This uncertainty was linked to sediment characteristics and gas storage in the sediment [Varadharajan and Hemond, 2012]. Grain size and sediment mechanical properties are recognized as two important factors controlling gas bubble formation, storage, and transport. Boudreau et al. [2005] found that the shape of bubbles changed from elongated in soft muddy sediment to spherical in sandy sediment. Both models and experiments [Jain and Juanes, 2009; Choi et al., 2011] have shown that, depending mainly on grain size, gas transport in sediment LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1992

2 was controlled by two mechanisms: capillary invasion and fracturing. Tensile fracture toughness, a parameter that controls bubble growth and initial rise in soft cohesive sediment [Johnson et al., 2002], was found to vary with grain size [Johnson et al., 2012]. After their initial rise, bubbles migrate by following existing paths [Algar et al., 2011]. Field observations have also identified pockmark structures on lakebeds formed by bubble plumes released from individual conduits [Scandella et al., 2011; Bussmann et al., 2013]. In lakes and reservoirs, sediment grain size distribution is affected by hydrodynamic setting and can show strong spatiotemporal heterogeneities [Ostrovsky and Tęgowski, 2010], resulting in localized hot spots for methane ebullition [DelSontro et al., 2011; Maeck et al., 2013]. However, little is known about the processes and sediment properties which control the magnitude and dynamics of gas bubble storage and release. Previous studies (above mentioned) have been limited to the examination of the transient processes of individual bubble formation and transport. Laboratory experiments mainly focused on the short-term dynamics of bubbles in sediments and did not include the processes of gas buildup and release. The mechanisms which trigger ebullition are relatively well known, but the release potential (i.e., what percentage of stored free gas can be released) and the size of the gas reservoir are largely unknown. To improve the mechanistic understanding of methane bubble formation, storage, and release in aquatic sediments, we conducted extended laboratory incubation experiments to study methane bubble formation and release dynamics in natural sediments with different grain size distributions. 2. Materials and Methods 2.1. Sediment Sample Collection and Processing Eighty liters each, of three types of natural sediment, was collected from different locations in southwest Germany. Sandy sediment was collected on 21 October 2015 from a creek (depth 20 cm) in Hochstadt ( N, E). Silty sediment was collected on 25 November 2015 from a shallow pond (depth 30 cm) in Eusserthal ( N, E) using a spade. Clayey sediment was collected from the nearshore of a sidearm of the River Rhine (depth 1 m) at Germersheim ( N, E) on 25 November 2015; the surface cm layer sediment was collected using a grab sampler. The sediments were carefully sieved (mesh size: 2 2 mm) to remove fauna, large pieces of woody debris, and shells. In order to enhance methane production, air-dried autumn-fall leaf powder (10 g in 1 L wet sediment) was added to the sediments immediately prior to commencing the experiments Sediment Characterization Sediment physical characteristics including particle size distribution and initial pore size distribution were measured in the laboratory. The sequential wet sieving method was adopted to evaluate the particle size distribution for each sediment. Particles smaller than 63 μm were measured by laser diffraction with a particle size analyzer (LISST-100X, Sequoia Scientific, USA). A subsample of each processed sediment was placed into a 56 ml plastic tube to measure the initial pore size distribution by nuclear magnetic resonance relaxometry using a Bruker Minispec MQ20 1 H-NMR Relaxometer (Bruker, Karlsruhe, Germany). For each water-saturated sediment sample, the measured transverse magnetization decay of water protons was converted into the respective relaxation time distributions, and the pore size distribution was determined following a calibration procedure developed by Jaeger et al. [2009] and Meyer [2015] (see more detailed description in Text S1 in the supporting information). Total organic matter content of the amended sediment was measured by thermogravimetry (STA Netzsch F3 449 Jupiter, Germany) connected with Netzsch Aëolus QMS (evolved gas analysis) [Kucerik et al., 2016]. Methane production rates of the amended sediments were measured by anaerobic incubation of 9 ml sediment samples and weekly headspace methane concentration measurements (for method, refer to Wilkinson et al. [2015]) Experimental Setup Each sediment was incubated in a cylindrical chamber (Figure 1). The gastight chambers were made of transparent acrylic plastic (wall thickness 1 cm) with diameter 18.5 cm and height 49 cm, with a funnel-shaped lid and 22 cm long upper cylinder (inner diameter 5.2 cm). To allow for controlled changes of hydrostatic head during the experiments, a 2 m long vertical PVC tube (inner diameter 16 mm) was added on the top of each cylinder. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1993

3 Figure 1. Schematic experimental setup. 1 Oxygen sensor, 2 pressure sensor, 3 port and tap for water-level control, 4 gas bag, 5 light source, and 6 camera. An inflatable gas bag (maximum volume 1.5 L) was attached to the end of the tube to measure the total volume of gas production (P tot ). One port on the chamber was connected to an external water bucket which was used as a reservoir for regulating hydrostatic head in the chamber. Two vented pressure sensors (BCM Sensor Technologies, Belgium; accuracy ± 0.1 mm) were installed to measure the hydrostatic pressure in the water column above the sedimentwater interface (SWI) and within the sediment (5 cm above the chamber floor) with 2 s logging interval. To monitor dissolved oxygen, optical oxygen sensors (FirestingO2, Pyroscience, Germany) were installed in the water column above the sediment surface for clay and silt. A pressure and temperature logger (TDR 2050, RBR, Canada) was used to measure temperature and atmospheric pressure in the room with 10 s sampling interval. The experiment was performed from 30 December 2015 to 19 February 2016 in a dark room at a temperature of 19.4 ± 1.3 C. After thorough mixing to displace existing gas bubbles, the sediments were filled into each chamber taking care to avoid introducing air bubbles. The initial sediment depth in the three columns was approximately 30 cm. After the sediment preparation, the chambers were filled with tap water up to the upper cylinder to allow for water-level (WL) measurement. The gas volume in the gas bags and the SWI level in each chamber were measured daily. At the same time, side-view pictures were taken to track bubble development in each column Gas Content in Sediment Columns Total daily gas entrapment in the sediments (S tot ) was measured by water-level change, daily ebullition volume (E b ) was calculated as the difference between P tot and S tot, and the volume of free gas trapped by sediment expansion (S exp ) was calculated from daily SWI change (accuracy ±1 mm). Total volumetric gas content (θ tot ) and the fraction of gas trapped by sediment expansion (θ exp ) were calculated from S tot and S exp, respectively (by dividing by the sediment volume). The effect of WL change on the volume of gas stored in the sediments was negligible and thus not included for gas content calculation. To compare the ebullition volume to the volume of free gas stored in the sediments, the ebullition volume was corrected to in situ hydrostatic pressure. We traced bubble outlines from photos of the chamber sidewalls (taken from the side view of the upright chamber, approximately one third of total chamber wall area). This provided a representation of the area of visible gas voids on the sidewalls and was used to scale θ tot to give the volumetric gas content θ i at 1 cm depth intervals as follows: θ i ¼ θ tot θ exp þ θexp A i = X A i where A i is the gas void area in each sublayer i. Contour plots showing spatial and temporal dynamics of gas content in sediments were created (Surfer 9, Golden Software, USA) based on daily gas content profiles. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1994

4 Table 1. Particle Size Distribution (Mass Percent) for the Three Types of Sediment Particle Size Distribution (%) <20 μm μm μm μm μm 2000 μm Clay Silt Sand Tracking Sediment Structure by X-ray Computed Tomography The accuracy of the gas content estimation is potentially biased because of wall effects. To validate this method, six additional sediment cores (two for each type of sediment; diameter 60 mm, length 60 cm filled up to 30 cm) were prepared and kept at the local hospital for X-ray computed tomography (CT) scanning (Simons AS, Germany; 120 kv, exposure time 1 s). The cores were stored in an air-conditioned room at a constant temperature of 19.3 C. Of the six cores, three (one for each type of sediment) were filled 12 h before, and the remaining three cores were filled 6 days before the first scan to obtain sediment structure information by CT at two different days with a single scanning effort. CT scanning was performed three times, and time series of CT images of 0, 6, 7, 13, 28, and 34 days were obtained for each type of sediment. Individual CT images were obtained for vertical cross sections with spatial resolution of 1 mm/pixel (slice thickness 4 mm). The CT images were analyzed by using the thresholding method [Dufour et al., 2005]. The gas content profiles were calculated by dividing the number of pixels which correspond to gas voids by the total number of pixels at each sublayer (sediment column was divided to 30 sublayers with equal thickness of 1 cm). Averages were taken by processing multiple images Ebullition During Hydrostatic Head Changes After the 3 week incubation, hydrostatic head was stabilized at 210 cm level. Controlled reductions in hydrostatic head were applied by lowering the WL to investigate their effect on ebullition. For each chamber WL drops were performed 12 times. The amplitude of WL reductions were recorded by pressure loggers, and the volume of ebullition was measured after 4 h. WL was restored to the initial value after 4 h in preparation for the next drop. To allow the sediment to recover from gas loss, the time window between two subsequent WL drops was set to at least 1 day, depending on the previous gas loss and the observed gas production rate. Sediment pore size distributions were estimated from sediment sidewall pictures taken before the hydrostatic head change experiments. The equivalent spherical diameter (D pore ) of gas filled pores was obtained from the gas void area as described above. The size distribution of the gas bubbles released from the different sediments in response to the pressure drops were estimated from video observations of rising bubbles in the cylinder using a GoPro camera (HERO 3, GoPro Inc., San Mateo, USA). A white light-emitting diode panel was used as a diffuse light source to illuminate the cylinders from behind. Videos were recorded for 15 min with a frame rate of 30 frames/s during each experiment. After correction for lens distortion, frames showing clearly distinguishable individual bubbles were selected visually to estimate equivalent spherical diameters (D bubble ). Bubble size distributions were obtained from more than 900 individual bubble observations for each type of sediment. Multiple individual bubble trajectories were followed to evaluate the uncertainties caused by changes in distance and bubble volume expansion during bubble rise (detailed descriptions are provided in Figures S1 and Text S2). 3. Results 3.1. Sediment Characteristics The initial total organic matter content in sediments was 7.1%, 6.3%, and 2.8% for clay, silt, and sand, respectively. The addition of leaf material increased the organic matter content by 1.5%, 0.9%, and 0.6% accordingly. Methane production rates of the sediments increased from the initial 1.6, 4.3, and 0.8 g m 3 d 1 to 21.8, 21.0, and 13.5 g m 3 d 1 for clay, silt, and sand, respectively. The composition of the sediments was dominated by 72.6% fine particles (<63 μm and 49.2% below 20 μm) for clay, 64.7% of medium-sized particles ( μm) for silt, and 89.3% of coarse particles ( 200 μm) for LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1995

5 Table 2. Pore Size Distributions (Volume Percent) for the Three Types of Sediment a Pore Size Distribution (%) Fine Pores Medium Pores Coarse Pores Narrow Wide <0.2 μm μm >10 μm μm >50 μm Clay Silt Sand a The columns narrow and wide provide a further refinement of the distribution of coarse pores. sand, respectively (Table 1). The 1 H-NMR relaxometry measurements showed pore size distribution in the initial sediments, i.e., before methane bubble formation. The influence of sediment composition on the initial pore size distribution was obvious: while the relative contribution of both fine pores (<0.2 μm) and medium pores ( μm) decreased from clay to silt and to sand, the opposite order was observed for the fraction of coarse pores (>10 μm), which was decreasing from 79.2% in sand to 51.3% in silt and to 30.8% in clay (Table 2). Among the coarse pores, only a minor fraction ( 2.5%) were larger than 50 μm in clay and silt compared to a much larger fraction (32.2%) in sand, indicating a predominance of large pores in sand Dynamics of Gas Bubble Storage Oxygen in the water column overlying the experimental sediments was consumed within 7 h from the start of the experiment, dropping rapidly from an initial value of 6.5 to 0 mg L 1. Intense gas production started immediately after setup of the incubation. The dynamics of gas formation and storage during the first 3 weeks of incubation could be divided into three distinct stages (Figure 2). During the first 6 days (7 days for sand), bubbling was minimal (E b /P tot < 20% except for the first day in silt). The second stage was characterized by a steady Figure 2. (a) Total volumetric gas content (θ tot ). (b) Daily ebullition (corrected to in situ hydrostatic pressure) to daily gas production (E b /P tot ). The bold horizontal line indicates 100%. The two bold dashed vertical lines separate the three stages of gas content development described in the text. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1996

6 Figure 3. The percentage of gas content due to sediment expansion (θ exp ) out of total gas content (θ tot ) in the three sediments over time. increase of E b /P tot from <20% to around 100% (one exception is day 11 for clay, where ebullition was accidentally triggered by a WL drop). During the third stage (after 13 days, 14 for sand), the fraction of ebullition remained at around 100%, indicating a steady state with constant gas storage in the sediment. The maximum column-averaged volumetric gas content in the sediments reached 18.8% (188 L gas per m 3 wet sediment) in clay, 12.0% (120 L gas per m 3 wet sediment) in silt, and 13.2% (132 L gas per m 3 wet sediment) in sand. Figure 4. Spatial and temporal dynamics of the volumetric gas content in the three sediment columns. The white dashed lines indicate the onset of vertical gradients in gas content. In silt and sand, the initial increase of volumetric gas content was not associated with sediment expansion during the first 3 and 5 days, respectively. This indicates that bubbles formed initially by capillary invasion, i.e., by displacing mobile water from pores, in these two types of sediment. The ratio of volumetric sediment expansion to total volumetricgas storage (θ exp /θ tot ) started to increase for about 1 week before reaching constant values (Figure 3), suggesting an increasing importance of sediment expansion, i.e., the displacement of sediment particles by growing bubbles, for gas storage. In contrast to silt and sand, sediment expansion was observed in clay starting from the first day of incubation and the ratio θ exp /θ tot increased sharply to nearly 100% during the first 6 days (Figure 3). Hence, capillary invasion was of minor importance for gas storage in clay. Following the initial increase, θ exp /θ tot was relatively constant in LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1997

7 Figure 5. Examples for the outlines of gas voids (areas filled black) at the walls of the three incubation chambers at different stages. The numbers in the left bottom corners show the percentages of gas void coverage. all three sediments with mean values of 83.7% for clay, 55.6% for silt, and 25.5% for sand. Ebullition was mainly observed during sediment expansion, while gas storage by capillary invasion was not accompanied by ebullition (Figure 2b). The first 6 day rapid expansion of clay was characterized by the development of a dome-shaped surface (Figure S2). Gas accumulation reached a maximum at day 6, the end of the stage II (Figure 3), characterized by a rapid linear increase and decrease in θ exp /θ tot before and after day 6. At the same time, the dome-shaped sediment surface ruptured, leading immediately to increased ebullition and visibly demonstrating the failure in sediment cohesive strength (Figure S2). Although the formation and rupture of the gas dome were not observed in silt and sand, a clear turning point in θ exp /θ tot was observed in sand at day 11 (Figure 3). The volumetric gas content in all three sediment columns varied strongly with depth (Figure 4). In clay, gas was highly concentrated in 5 25 cm depth, where the greatest gas content was 46.8%. In sand, in contrast, most of the gas was stored in the uppermost 10 cm with a smaller maximum gas content of 25.4%. In silt, both the thickness of the gas-charged layer (12 cm) and the maximum gas content (35.3%) of this gas storage zone were in between those for clay and sand. In relation to the overall gas storage, the gassy zones each contained 83.7% (clay), 55.6% (silt), and 25.5% (sand) of the total free gas. Examples of changes in gas void distribution at the walls of the incubation chambers (Figure 5) demonstrated that gas void coverage increased significantly from the initial stage to the later stages for each type of sediment. At stage I, gas bubbles were small and sparse in all three columns. Large bubbles emerged at stage II in clay and silt, while bigger and more closely packed bubbles were observed in sand. During stage III the number of large bubbles increased and elongated conduits were observed in all three columns. The conduits tended to maintain their shape and location once formed (see the attached Movie S1 in the supporting information). LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1998

8 Figure 6. Gas content development in three sediments tracked by X-ray CT scanning. (left) The X-ray CT images show the selected longitudinal cross sections of sediment cores taken at different days (gas bubbles are black on these images). (right) The corresponding average gas content profiles are shown. Gas content development and depth stratification in the three sediments were confirmed by X-ray CT imaging (Figure 6). Compared to the initial stage which was almost bubble free (0.3%, 0.1%, and 0% for clay, silt, and sand, respectively), CT imaging demonstrated significant gas bubble growth both in number and volume from day 7 in clay and silt and from day 13 in sand. The CT profiles showed characteristics that were consistent with the three main experimental chambers (Figure 4), with elevated gas-content zones between 5 and Figure 7. (a) Changes in gas content due to ebullition (Δθ tot /θ tot ) in relation to reduction in hydrostatic head (Δh). (b) The relationship between Δθ tot /θ tot and the magnitude of SWI decrease (ΔSWI). The volume of gas loss due to ebullition was corrected to initial in situ hydrostatic pressure before the hydrostatic head reductions. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 1999

9 Figure 8. Probability density distribution of bubble size (D bubble ) during the ebullition events. The x axis scaling is logarithmic. PD/PD max represents the normalized probability density by the maximum for the three sediments; the bold lines show the fitted normal distributions for the observational data with mean values (μ) and standard deviations (σ) indicated in each plot. 25 cm in clay, above 12 cm in silt, and above 8 cm in sand. The maximum gas content values (27.2% in clay, 20.0% in silt, and 13.9% in sand) were smaller than in the main chambers; this was expected because gas bubbles smaller than 1 mm in diameter could not be resolved by the X-ray CT imaging system used Ebullition Due to Drops in Hydrostatic Head Reductions in hydrostatic head (Δh) in the range of cm triggered 12 intense ebullition events. The maximum volume of released gas varied from 494 ml in clay and 284 ml in sand to 258 ml in silt, corresponding to the largest Δh of 184.5, 166.0, and cm, respectively, for each type of sediment. By normalizing ebullition by the total volume of gas stored in the sediments prior to the drop in hydrostatic head, we found strong linear relationships (R , p < 0.001) between the fraction of stored gas in the sediments, which was released by ebullition (Δθ tot /θ tot ) and Δh for all three sediments (Figure 7a). The gas release was always accompanied by a reduction in sediment volume, and the sediment surface height dropped by up to 0.8 cm in clay and cm in silt and sand. Δθ tot /θ tot was linearly related to the magnitude of drop in SWI level (R 2 > 0.8, p < 0.001; Figure 7b), suggesting a direct contribution from the gas zones formed by sediment expansion during ebullition. The size distribution of the released gas bubbles differed among sediment types (Figure 8). The size of the most abundant bubbles decreased from 8.9 mm in clay to 6.1 mm in silt and 4.8 mm in sand. The largest bubbles were observed from clay (18.2 mm) and silt (17.4 mm), while all the bubbles from sand were smaller than 10.8 mm. The bubble size distribution was lognormal and most narrow for sand with a single peak for silt and sand and a bimodal distribution for clay. 4. Discussion 4.1. The Role of Sediment Physical Properties in Gas Storage By analyzing gas content development in water-saturated sediment columns, we identified three distinct stages of gas bubble formation, accumulation, and release: stage I gas accumulation, by capillary invasion resulting in water displacement without sediment expansion (stage I was missing for clay); stage II void formation and increasing ebullition, by sediment particle displacement leading to sediment expansion and bubble release; and stage III constant gas storage and steady state between gas production and ebullition rates. These three development stages can be related to the physical properties of the respective sediments. The gas bubble formation at stage I was controlled by the initial pore size distribution in the sediments. In sand, coarse pore spaces provided preferential pathways for rapid capillary invasion and displacement of LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 2000

10 mobile water. Such displacement requires much less force than the displacement of sediment particles [Jain and Juanes, 2009]. Our results further indicate that the extent of this mechanism and duration of stage I is determined by the fraction of coarse pores in the sediment. In sand, characterized by a large fraction of coarse pores (79.2% of pores >10 μm) at the initial state, capillary invasion continued for 5 days, in contrast to silty (3 days) and clayey sediments (0 day), each with a decreasing proportion of coarse pores (51.3% and 30.8%, respectively). The transition from capillary invasion to sediment expansion leading to stage III can also be explained by sediment properties. At the end of this stage, most of free gas (74.8%) was held in coarse pores in the sand, in contrast to the small percentage held in the clay (16.8%). The observed differences can be related to the different grain size distributions of the studied sediments: almost 90% of sediment particles >200 μm for sand compared to only 6.8% for clay. The strong correspondence between sediment particle size and initial pore size distribution supports our argument that grain size acts as a primary control for gas accumulation and storage capacity in sediments. The importance of sediment structures in determining gas storage characteristics has recently been emphasized in studies of peat soils [Ramirez et al., 2015; Chen and Slater, 2015]. Other studies linking methane bubble storage to sediment characteristics [Ostrovsky and Tęgowski, 2010; Hilgert, 2014] could only identify the spatial variability of the in situ steady state in gas content in lakes and reservoirs, whereas our laboratory experiments indicate a strong dependence of gas accumulation on sediment textures. Because organic matter content is generally related to sediment grain size [Bergamaschi et al., 1997], this finding can help resolve the spatial variability of gas content distribution in sediments. In addition, the upscaling of local measurements of methane ebullition from lakes and reservoirs could be improved by taking spatial variations of sediment grain size distributions into account Gas Content Distribution in Sediment Column: Link to Sediment Mechanical Properties Tracking in situ volumetric gas content depth profiles in aquatic sediments is challenging. In our experiments, the gas content profile was determined by monitoring WL and SWI changes in addition to observing the fraction of gas bubble area visible on the sidewall of the chambers. Our initial working hypothesis that capillary invasion results in uniform depth distributions of gas during stage I of sediment development in silt and sand was validated by time-lapsed X-ray CT imaging. The observations further revealed the presence of a high gas content upper layer in the sediment columns, which was unexpected in homogenized sediments where the methane production rate can be considered as being independent of depth. In lake and reservoir sediments, active methane production typically occurs in the top few centimeters and declines exponentially with increasing depth, in relation to temperature, availability of oxygen, substrate limitation, and microbial activity [Falz et al., 1999; Sobek et al., 2009; Wilkinson et al., 2015]. With homogenized sediments these factors were excluded from our experiments, and we propose the following possible reasons for the development of gas content stratification, which are related to the mechanical properties and behavior of the sediments. On one hand, for sediment displacing bubbles to grow, they must overcome both the hydrostatic and lithostatic loads which both increase with depth. Thus, there may be an optimum depth where production is sufficient to overcome the loads. In addition, low lithostatic loads near the sediment surface may be insufficient to compensate buoyancy forces and bubbles are released immediately. On the other hand, the diffusive upward flux of methane from greater depths may contribute to bubble growth in the midportion of the sediment. In addition, we assume that methane-enriched pore water transported upward by capillary invasion would also contribute to the local gas accumulation in the surface layer. Another important control on the development of a gassy layer is related to sediment elastic properties. In situ profiling of tensile fracture toughness and measurements of Young s modulus in cohesive sediments has shown relatively smaller values in the surface layers than at increasing depth and could be related to compactness and grain size distribution [Barry et al., 2012a; Johnson et al., 2012]. With our homogenized sediment columns, the presence of the gassy layer could not be due to grain size stratification but was related to the development of bulk density stratification, i.e., sediment compaction and the gas-charging process in the upper sediment layers. With the progress of the experiments, the weight and settlement of wet bulk sediment caused changes in sediment compactness from the initial quasi-uniform state to a typical exponential reduction from bottom to surface [Robbins and Edgington, 1975]. An analytical LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 2001

11 model [Katsman, 2015] has suggested that as a consequence of the minimal sediment fracture toughness, the formation of larger gas bubbles is restricted to a near-surface layer. By taking into account the above-mentioned reasons, the thickness of the gassy layer could be determined by the relationship between dissolved gas pressure and sediment fracture toughness plus ambient static pressure. The observed formation and rupture of a dome-shaped sediment surface in our experiments is consistent with Barry et al. s [2012b] observations that elastic thin-plate mechanics explains gas-dome formation in clay. Sediment surface rupture did not occur in the silt or sand; however, the different shapes of gas voids in the three sediment columns also indicate that the sediment expansion process was affected by sediment mechanical properties. At gas content development stage III, vertically oriented conduits were observed in the upper sediment layers (Figure 5), which were the result of fracture failure. This observation is in accordance with theoretical considerations for clayey sediment, which suggest that initial bubble rise is controlled by fracturing [Algar et al., 2011]. Other laboratory experiments in water-saturated noncohesive granular media also found that gas transport occurred along channels after the initial breakthrough [Gostiaux et al., 2002; Stöhr and Khalili, 2006]. In our experiments, the development of conduits (Figure 5) coincided with the turning point in θ exp /θ tot at the end of stage II (Figure 3), consistent with the finding of gas transport in granular materials. From these conduits intense bubble plumes were observed when ebullition was triggered (see the attached Movie S1) Bubble Release and Size Distribution Ebullition dynamics differed between the three stages of gas content development. Spontaneous ebullition was associated with bubble formation but was negligible during stage I (Figure 2b); it increased during stage II associated with sediment expansion and then remained at constant high levels during stage III. This indicates that the rate of gas transport in coarse pore spaces (associated with capillary invasion) was lower than that associated with stage II sediment expansion. The former is determined by the balance between capillary force and buoyancy and the latter by sediment mechanical properties. At stage III, the stable fraction of sediment expansion to daily total gas storage (Figure 2b) indicates that the transport capacity of the established sediment pore structure have reached equilibrium with the overall rate of gas production, and the gassy layer may act as a gas accumulation and buffering zone. Linear relationships (R 2 = ) between ebullitive fluxes and the magnitude of pressure change (<10 cm) were observed in peat soils [Comas and Wright, 2012; Chen and Slater, 2015]. We applied a much wider range of hydrostatic head changes ( cm) and found a stronger correlation between ebullition and reduction in hydrostatic head (R , p < 0.001). The drop in SWI level was also linearly correlated to ebullition volume, which may indicate a direct response from the gassy layer. However, the ebullition potential was estimated from the total gas storage rather than the storage in the gassy layer alone. In fact, the total volume of gas released was more than 100% greater than that estimated from volumetric expansion alone, and the associated ebullition continued for an extended period. This prolonged intense ebullition implies a sudden expansion loss from the gassy layer (may act as an intermediate buffer for gas transport), a slower release of gas from voids at greater depth, and the gradual dissolution and release of pore water methane (in a time scale of h/d) [Algar and Boudreau, 2009]. Modeling in peat soils [Ramirez et al., 2015] has also shown prolonged ebullition resulting from hydrostatic head reduction due to delayed bubble release from the denser lower layer. The modified porosity profile resulting from the development of a gas-enriched upper zone observed in our experiments is assumed to have a similar effect. The size of bubbles emerging from sediments and the distance they must travel to reach the air-water interface determines how much methane reaches the atmosphere [McGinnis et al., 2006]. In shallow waters, large bubbles can dominate the delivery of methane by ebullition to the atmosphere [DelSontro et al., 2015]. We present quantitative data on size distributions of bubbles emerging from natural sediments (albeit amended with organic matter and under laboratory conditions). The results show a transition from a narrow size distribution of bubbles in coarse sediment to a wider distribution and larger bubbles in fine sediment. The bubble size distributions were lognormal, which is consistent with the study of gas bubble transport in granular materials [Geistlinger et al., 2014], who found a lognormal distribution of bubble sizes. The bimodal size distribution of bubbles from the clayey sediment may be related to a difference in gas storage mechanisms compared to the silt and sand sediment columns. Geistlinger et al. [2014] found a good agreement between bubble and pore sizes (both diameter 1 mm) during percolation through a coarse and rigid (nondeformable) porous medium (1 mm glass LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 2002

12 beads). However, in our experiments large gas voids (maximum D pore = 10.2, 14.1, and 6.4 mm in clay, silt, and sand, respectively; Figure S3) formed due to sediment expansion. As a consequence, a bimodal pore size distributionwasobserved forclay andsilt,whileitwasless significantforsand(figures3). Thepore sizedistribution of the three sediments in general was well correlated to the size distribution of released bubbles, although the bubbles were larger than the pores. This finding could be due to the uncertainty of the pore size estimation from two-dimensional sidewall pictures. Another plausible explanation for this bimodal size distribution is that bubbles might merge as they rise, as observed in a bubble injection experiment in gelatin by Algar et al. [2011] Limitations and Future Work In recent years, high methane ebullition rates from reservoirs and impounded waters with high sedimentation rates and carbon burial have been reported [DelSontro et al., 2010; Maeck et al., 2013; Wilkinson et al., 2015]. A significant characteristic of such man-made water bodies is the frequent occurrence of water-level fluctuations, e.g., by reservoir operations or ship locking [Maeck and Lorke, 2014], which act as triggers for intense ebullition events. Our experiments may reflect sediment structure, methane production, and bubble dynamics in such systems (which often have high rates of sedimentation [DelSontro et al., 2010; Sobek et al., 2012; Maeck et al., 2013]). Indeed, our 30 cm thick homogenized sediment may have approximated the situation in the Saar River, where the highest reported sedimentation rate was 0.29 m yr 1 [Maeck et al., 2013; Wilkinson et al., 2015], and the methane formation rates of the amended sediments in our experiment were also comparable to those in the Saar [Wilkinson et al., 2015]. However, the extension of our findings to inland waters with much lower sedimentation rates may be limited, where the gas-producing sediment layer is mainly limited to the upper few centimeters [Sobek et al., 2009] and declines sharply with increasing depth [Wilkinson et al., 2015]. In our experiments, the sediments were amended by leaf matter to enhance methane production. The methane formation rates in the main incubation chambers (20.1, 16.6, and 10.5 g m 3 d 1 at steady state) were comparable to those from the bottle incubations (21.8, 21.0, and 13.5 g m 3 d 1 ). This demonstrates that the strategy to promote methane production by adding a natural organic carbon source was successful and shows the importance of carbon quality for methane formation; i.e., only very few percent of organic carbon increase led to a more than tenfold increase in methane production. In our experiments, sufficient spatiotemporal resolution for following individual bubble migration in sediments (i.e., at time scale of subseconds and spatial resolution of micrometers) could not be achieved. A further limitation is related to the estimation procedure for volumetric gas content based on bubble observations at the chamber wall, and the dynamics of bubbles next to the walls can be expected to differ from that in the interior because of wall effects. These differences may either concentrate or exclude bubbles, thus altering the gas void area. Further quantitative studies of effect of sediment structure on bubble growth dynamics would be better achieved with a high-resolution X-ray CT scanner. In addition, we had not expected the development of strong gas content stratification with depth in the sediment columns. Although we propose plausible mechanisms, further experiments are needed to validate these hypotheses quantitatively. Further examination of how ebullition relates to sediment structure should include quantitative tracking of the dynamics of sediment structure development and gas bubble migration in different sediments. 5. Conclusions Extended incubation experiments demonstrated that both sediment structure development-related gas storage and ebullition behavior in aquatic sediments were strongly influenced by grain size. The gas storage capacity varied strongly with the grain size distribution of the sediments: in fine-grained sediment, free gas was mainly stored by sediment expansion, whereas in coarser sediments, capillary invasion of the sediment matrix dominated gas storage. A gas-enriched upper layer with a thickness which varied among sediment types developed in the sediment columns. Although the gas stored in this layer was an important source of gas for rapid ebullitive release, gas stored deeper within the columns resulted in prolonged ebullition. The total ebullition was well in excess of the potential release due to expansion alone, indicating the dissolution and release of pore water methane. The dependence of gas storage on grain size distribution provides promise for resolving spatial heterogeneities in methane ebullition by coupling sediment transport models with biogeochemical models for gas formation. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 2003

13 Acknowledgments The authors would like to thank Daniel McGinnis, Sabine Flury, and Andreas Maeck for providing the incubation chambers and suggestions to the experimental setup. We thank Jiri Kucerik for helping with the total organic matter analysis of sediment samples. We thank the staff of the clinical center Landau for helping with the X-ray CT scanning of the sediment samples. Thanks to Christoph Bors for his help during sample collection in the field. The authors are also grateful to Bernard Boudreau and one anonymous reviewer for their constructive comments. This study was financially supported by the German Research Foundation (grant LO 1150/5). Data are available from the corresponding author upon request. References Algar, C. K., and B. P. Boudreau (2009), Transient growth of an isolated bubble in muddy, fine-grained sediments, Geochim. Cosmochim. Acta, 73(9), Algar, C. K., B. P. Boudreau, and M. A. Barry (2011), Release of multiple bubbles from cohesive sediments, Geophys. Res. Lett., 38, L08606, doi: /2011gl Barry, M. A., B. D. Johnson, and B. P. Boudreau (2012a), A new instrument for high-resolution in situ assessment of Young s modulus in shallow cohesive sediments, Geo Mar. Lett., 32(4), Barry, M. A., B. P. Boudreau, and B. D. Johnson (2012b), Gas domes in soft cohesive sediments, Geology, 40(4), Bastviken, D., L. J. Tranvik, J. A. Downing, P. M. Crill, and A. Enrich-Prast (2011), Freshwater methane emissions offset the continental carbon sink, Science, 331(6013), 50. Bergamaschi, B. A., E. Tsamakis, R. G. Keil, T. I. Eglinton, D. B. Montluçon, and J. I. Hedges (1997), The effect of grain size and surface area on organic matter, lignin and carbohydrate concentration, and molecular compositions in Peru Margin sediments, Geochim. Cosmochim. Acta, 61(6), Boudreau, B. P., C. Algar, B. D. Johnson, I. Croudace, A. Reed, Y. Furukawa, K. M. Dorgan, P. A. Jumars, A. S. Grader, and B. S. Gardiner (2005), Bubble growth and rise in soft sediments, Geology, 33(6), Bussmann, I., E. Damm, M. Schlüter, and M. Wessels (2013), Fate of methane bubbles released by pockmarks in Lake Constance, Biogeochemistry, 112(1-3), Casper, P., S. C. Maberly, G. H. Hall, and B. J. Finlay (2000), Fluxes of methane and carbon dioxide from a small productive lake to the atmosphere, Biogeochemistry, 49(1), Chanton, J. P., C. S. Martens, and C. A. Kelley (1989), Gas transport from methane-saturated, tidal freshwater and wetland sediments, Limnol. Oceanogr., 34(5), Chen, X., and L. Slater (2015), Gas bubble transport and emissions for shallow peat from a northern peatland: The role of pressure changes and peat structure, Water Resour. Res., 51, , doi: /2014wr Choi, J. H., Y. Seol, R. Boswell, and R. Juanes (2011), X-ray computed-tomography imaging of gas migration in water-saturated sediments: From capillary invasion to conduit opening, Geophys. Res. Lett., 38, L17310, doi: /2011gl Clayton, C. J., and S. J. Hay (1994), Gas migration mechanisms from accumulation to surface, Bull. Geol. Soc. Den., 41(1), Comas, X., and W. Wright (2012), Heterogeneity of biogenic gas ebullition in subtropical peat soils is revealed using time-lapse cameras, Water Resour. Res., 48, W04601, doi: /2011wr DelSontro, T., D. F. McGinnis, S. Sobek, I. Ostrovsky, and B. Wehrli (2010), Extreme methane emissions from a Swiss hydropower reservoir: Contribution from bubbling sediments, Environ. Sci. Technol., 44(7), DelSontro, T., M. J. Kunz, T. Kempter, A. Wüest, B. Wehrli, and D. B. Senn (2011), Spatial heterogeneity of methane ebullition in a large tropical reservoir, Environ. Sci. Technol., 45(23), DelSontro, T., D. F. McGinnis, B. Wehrli, and I. Ostrovsky (2015), Size does matter: Importance of large bubbles and small-scale hot spots for methane transport, Environ. Sci. Technol., 49(3), Dufour, S. C., G. Desrosiers, B. Long, P. Lajeunesse, M. Gagnoud, J. Labrie, P. Archambault, and G. Stora (2005), A new method for threedimensional visualization and quantification of biogenic structures in aquatic sediments using axial tomodensitometry, Limnol. Oceanogr. Methods, 3(8), Falz, K. Z., C. Holliger, R. Grosskopf, W. Liesack, A. N. Nozhevnikova, B. Müller, B. Wehrli, and D. Hahn (1999), Vertical distribution of methanogens in the anoxic sediment of Rotsee (Switzerland), Appl. Environ. Microbiol., 65(6), Geistlinger, H., S. Mohammadian, S. Schlueter, and H. J. Vogel (2014), Quantification of capillary trapping of gas clusters using X-ray microtomography, Water Resour. Res., 50, , doi: /2013wr Gostiaux, L., H. Gayvallet, and J. C. Géminard (2002), Dynamics of a gas bubble rising through a thin immersed layer of granular material: An experimental study, Granul. Matter, 4(2), Hilgert, S. (2014), Improvement of the understanding and detection of spatial and temporal heterogeneities of methane emissions by correlating hydro-acoustic with sediment parameters in subtropical reservoirs, PhD thesis, the Karlsruhe Inst. of Technol., Karlsruhe. Holgerson, M. A., and P. A. Raymond (2016), Large contribution to inland water CO 2 and CH 4 emissions from very small ponds, Nat. Geosci., 9, Jaeger, F., S. Bowe, H. Van As, and G. E. Schaumann (2009), Evaluation of 1 H NMR relaxometry for the assessment of pore-size distribution in soil samples, Eur. J. Soil Sci., 60(6), Jain, A. K., and R. Juanes (2009), Preferential mode of gas invasion in sediments: Grain-scale mechanistic model of coupled multiphase fluid flow and sediment mechanics, J. Geophys. Res., 114, B08101, doi: /2008jb Johnson, B. D., B. P. Boudreau, B. S. Gardiner, and R. Maass (2002), Mechanical response of sediments to bubble growth, Mar. Geol., 187(3), Johnson, B. D., M. A. Barry, B. P. Boudreau, P. A. Jumars, and K. M. Dorgan (2012), In situ tensile fracture toughness of surficial cohesive marine sediments, Geo Mar. Lett., 32(1), Katsman, R. (2015), Correlation of shape and size of methane bubbles in fine-grained muddy aquatic sediments with sediment fracture toughness, J. Struct. Geol., 70, Keller, M., and R. F. Stallard (1994), Methane emission by bubbling from Gatun Lake, Panama, J. Geophys. Res., 99, , doi: / 92JD Kucerik, J., M. S. Demyan, and C. Siewert (2016), Practical applications of thermogravimetry in soil science. Part 4: Relationship between clay, organic carbon and organic matter contents, J. Therm. Anal. Calorim., 123, Maeck, A., and A. Lorke (2014), Ship-lock-induced surges in an impounded river and their impact on subdaily flow velocity variation, River Res. Appl., 30(4), Maeck, A., T. DelSontro, D. F. McGinnis, H. Fischer, S. Flury, M. Schmidt, P. Fietzek, and A. Lorke (2013), Sediment trapping by dams creates methane emission hot spots, Environ. Sci. Technol., 47(15), Maeck, A., H. Hofmann, and A. Lorke (2014), Pumping methane out of aquatic sediments: Ebullition forcing mechanisms in an impounded river, Biogeosciences, 11(11), McGinnis, D. F., J. Greinert, Y. Artemov, S. E. Beaubien, and A. Wüest (2006), Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere? J. Geophys. Res., 111, C09007, doi: /2005jc Meyer, M. (2015), Determination of quantitative pore size distributions of soils with 1 H-NMR relaxometry Development and validation of a universal calibration curve, Diploma thesis, Univ. of Koblenz-Landau, Landau. LIU ET AL. SEDIMENT STRUCTURE AFFECTS EBULLITION 2004