Supporting Information. Methane bubble growth and migration in aquatic sediments observed by X-ray µct

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1 Supporting Information Methane bubble growth and migration in aquatic sediments observed by X-ray µct Liu Liu, 1,* Tim De Kock, 2 Jeremy Wilkinson, 1 Veerle Cnudde, 2 Shangbin Xiao, 3 Christian Buchmann, 1 Daniel Uteau, 4 Stephan Peth, 4 and Andreas Lorke 1 1 Institute for Environmental Sciences, University of Koblenz-Landau, Landau, Germany 2 PProGRess-UGCT, Department of Geology, Ghent University, Krijgslaan 281/S8, 9000 Ghent, Belgium 3 College of Hydraulic & Environmental Engineering, China Three Gorges University, Yichang, China 4 Department of Soil Science, University of Kassel, Witzenhausen, Germany * Corresponding author - address: liu@uni-landau.de; Tel: +49 (0) Total Pages: 14 Number of Text: 3 Number of Figures: S1

2 Figure S1 Natural sediments taken from rivers in Germany. (a) CT images for a 29 cm long sediment core taken from an impoundment at River Main. On the left side, the two layers (5 and 18 cm thick) between the white horizons marked homogenized rapid sedimentation during strong flood events; on the right side, 3D visualization of sediment bubbles is shown (bubbles are tinted blue). (b) Another example on Germersheim, River Rhine, where clayey sediment for the experiments was collected. Sediment was well-mixed with presence of leaf matter and shells. (c) An example of organic matter (mainly leaf matter and woody debris) removed from natural sediments. S2

3 Figure S2. Grain size distributions of clay and sand. The dotted lines show grain size distributions of the sediment after removal of organic matter. To examine the effects of organic matter (OM) and particle aggregation, particle size distributions were measured in untreated samples, as well as samples treated with hydrogen peroxide (for complete removal of OM). The grain size distribution for each group was estimated as the average of nine measurements S1. CT image processing and segmentation Scans were reconstructed and subsequently stacked using ACQUILA software (XRE, Belgium). Data analysis was performed using Octopus Analysis (XRE, Belgium). Prior to image analysis, anisotropic diffusion filtering 1 was applied to the raw reconstructed images to reduce noise. To eliminate beam hardening, the outer 0.6 cm was excluded from analysis. Images were then segmented according to image intensity distribution curve by carefully choosing thresholds for solid sediment, water and gas, respectively (Figure S3). To allow for meaningful comparisons of S3

4 46 47 the scan time-series, consistent thresholds were chosen for each sediment type. Following segmentation, further noise reduction was achieved by removing isolated foreground voxels Figure S3. Examples of filtered µct images at day 14 of the incubation experiment (upper panels: full column scans; lower panel: region of interest (ROI) scans). Dark regions mark gas bubbles, white regions show solid material. The panels on the right side show histograms of grayscale image intensity distributions of the two sediments. The red dashed lines show the thresholds for segmentation. Two phases (gas and wet sediment) can be segmented for Clay, Sand and Clay_ROI; all three phases (gas, water and sediment) can be identified for Sand_ROI. S4

5 Figure S4. Depth profiles of volumetric water content (θ w ), solid fraction (θ s ) and gas content (θ g ) for clay and sand at day 20 of incubation Figure S5. Wet bulk density (ρwet) depth profiles for clay and sand at different incubation days. S5

6 Figure S6. Sediment yield shear strength (SYS) measurement by vane shear test at different solid volume fraction (θs). S2. Sediment compressibility test Sediment compressibility was measured by using an oedometer (model 08.67, Eijkelkamp, The Netherlands) at University of Kassel. Prior to the experiment, sediments were sterilized by using an autoclave for 2 h to prevent sediment methane production. Then they were filled into two 10 cm diameter plastic tubes a week before compression test for natural settlement, which allowed to reproduce compactness of sediment during the initial stage of bubble formation. The sediment columns were sub sampled every 3 cm and carefully transferred to stainless steel rings (3 cm thick, 10 cm diameter). The rings were closed with 5 mm thick sintered metal filters on both sides to allow for porewater drainage. Porewater pressure was measured by a ceramic tensiometer at the bottom of the sediment sample. The loading apparatus was programmed to apply successive vertical stresses at fixed time intervals (2 h for clay and 0.5 h for sand). The incremental loads were 5, 10, 20, 30, 40, 50 and 60 kpa for clay and 5, 10, 50, 100, 250, 400, 600 kpa for sand. At the end of each test, a controlled unload (10 kpa) was applied. Sediment settlement (vertical displacement) in response to applied vertical stress was measured by a micrometer. Two examples of compression test are shown in Figure S7. S6

7 Figure S7. Examples of compression test using the odometer for clay (θs = 26.2%; maximum load = 60 kpa) and sand ((θs = 68.7%; maximum load = 600 kpa). The black lines show the stepwise increase of vertical stress; the red lines show sediment settlement (vertical displacement) in response to applied vertical stress; the light blue lines show porewater pressure change during compression test measured by tensiometer. S7

8 Figure S8. Volumetric deformation in response to 5 and 10 kpa vertical stress, respectively at different solid volume fraction (θs). S8

9 Figure S9. Overview of gas content development. (a) Total volumetric gas content (θ g ) in clay and sand; (b) The ratio of daily ebullition (E b ) to daily gas production (P); (c) The fraction of gas stored by capillary invasion (θ cap ); (d) Methane (CH 4 ) flux from incubated sediments; (e) and (f) show gas content depth profiles calculated from time-lapse CT scans in clay and sand, respectively. S3. Sediment methane (CH 4 ) and CO 2 production CH 4 and CO 2 mass (mmol) in gas bubbles was accounted for both ebullition and sediment gas bubbles. The former can be calculated from measured headspace gas concentrations and daily S9

10 ebullition volume; the latter from daily sediment gas storage and gas concentrations of sediment bubbles. CH 4 and CO 2 concentrations of sediment bubbles were estimated by subtracting the headspace concentrations at early stage of incubation from steady stage of ebullition (after day 5 in clay and day 10 in sand), assuming released gas bubbles were the main cause of the sharp increase in headspace gas concentrations. The dynamics of CH 4 :CO 2 ratio in headspace and gas bubbles is shown in Figure S S10

11 Figure S10. (a) CH 4 :CO 2 ratio of headspace concentration in clay and sand; (b) CH 4 :CO 2 ratio in gas bubbles Figure S11. Excess dissolved gas pressure (EDGP) in porewater during the incubation experiment in clay (upper panel) and in sand (lower panel). The dotted lines show the hydrostatic pressure (H) at the depths where EDGP sensors were installed. Note that the measured EDGP is offset by ambient atmospheric pressure. S11

12 Figure S12. Pore and bubble size distributions in region of interest scans of clay and sand. The probability density of bubble number was normalized by the analyzed sediment volume. S12

13 Figure S13. Gas bubble growth in the two sediments tracked by µct scans. The images show gas bubbles (white) growth on the selected vertical cross sections of sediment columns over time Figure S14. Bubble size distribution at different depths of clay at day 20. The peak diameter at different depths are marked out. S13

14 References (1) Weickert, J. Anisotropic diffusion in image processing; B.G. Teubner: Stuttgart, Germany, S14

Looking at pore scale processes in geomaterials using timeresolved 3D imaging and multi-scale imaging

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