Methane bubble growth and migration in aquatic sediments observed

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Methane bubble growth and migration in aquatic sediments observed by X-ray µCT Liu Liu, Tim De Kock, Jeremy Wilkinson, Veerle Cnudde, Shangbin Xiao, Christian Buchmann, Daniel Uteau, Stephan Peth, and Andreas Lorke Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06061 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Environmental Science & Technology

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Methane bubble growth and migration in aquatic sediments observed by X-ray µCT

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Liu Liu,1,* Tim De Kock,2 Jeremy Wilkinson,1 Veerle Cnudde,2 Shangbin Xiao,3 Christian

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Buchmann,1 Daniel Uteau,4 Stephan Peth,4 and Andreas Lorke1

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Institute for Environmental Sciences, University of Koblenz-Landau, 76829 Landau, Germany

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PProGRess-UGCT, Department of Geology, Ghent University, Krijgslaan 281/S8, 9000 Ghent,

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Belgium

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Yichang, China

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4

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College of Hydraulic & Environmental Engineering, China Three Gorges University, 443002

Department of Soil Science, University of Kassel, 37213 Witzenhausen, Germany

* Corresponding author - Email address: [email protected]; Tel: +49 (0)6341 280-31584

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ABSTRACT

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Methane bubble formation and transport is an important component of biogeochemical carbon

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cycling in aquatic sediments. To improve understanding of how sediment mechanical properties

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influence bubble growth and transport in freshwater sediments, a 20-day laboratory incubation

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experiment using homogenized natural clay and sand was performed. Methane bubble

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development at high-resolution was characterized by µCT. Initially, capillary invasion by 1 ACS Paragon Plus Environment


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microbubbles (< 0.1 mm) dominated bubble formation, with continued gas production (4 d for

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clay; 8 d for sand), large bubbles formed by deforming the surrounding sediment, leading to

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enhanced of macropore connectivity in both sediments. Growth of large bubbles (> 1 mm) was

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possible in low shear yield strength sediments (< 100 Pa), where excess gas pressure was

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sufficient to displace the sediment. Lower within the sand, higher shear yield strength (> 360 Pa),

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resulted in a predominance of microbubbles where the required capillary entry pressure was low.

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Enhanced bubble migration, triggered by a controlled reduction in hydrostatic head, was

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observed throughout the clay column, while in sand mobile bubbles were restricted to the upper 6

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cms. The observed macropore network was the dominant path for bubble movement and release

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in both sediments.

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INTRODUCTION

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Anaerobic organic matter decomposition in aquatic sediments produces methane, a potent

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greenhouse gas. Low solubility and slow diffusive transport cause sediment gas bubble

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accumulation. Stationary gas voids can reduce vertical solute transport , but provide a shortcut for

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gas transport.1 Once sediment gas storage capacity is exceeded, gas exits the sediment by

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bubbling.2 In inland waters, ebullition is an important pathway for methane release to the

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atmosphere.3-6 Bubble release can also enhance solute transport across the sediment-water

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interface.7-9 Thus, understanding methane bubble development and movement in sediment

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contributes to understanding biogeochemical cycling, emission dynamics and controlling factors

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in aquatic systems.

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The primary control of methane bubble growth is sediment gas production rate.10-12 Methane

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production in-excess of diffusional transport, is necessary to cause porewater supersaturation

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leading to bubble formation. In marine sediments, methane bubble accumulation only starts 2 ACS Paragon Plus Environment

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below the surface layer where sulfate is present.13, 14 In freshwater sediments methane production

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in the superficial surface layer was greatest, and decreases exponentially with depth.15, 16 Since

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methane production generally declines with increasing depth, and bubble formation requires

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production to exceed diffusive transport away from the source, new bubble formation can

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generally only be expected in the upper sediment layers. In a recent study ebullition correlated

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well with sediment methane production in a riverine impoundment, where sediment depth > 1 m

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was considered to contribute little to total ebullition.17

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Where methane production supports bubble formation, sediment mechanical properties become

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important.18-20 Experiments and modelling both demonstrate the dependence of bubble growth by

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capillary invasion on grain size.18, 21 Microbubble (< grain size) formation is favored in coarse

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sediments, and in fine-grained sediments, bubbles (gas voids) grow larger than sediment grain

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size by elastic/plastic deformation.22-24 Bubble growth in cohesive marine sediments has been

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explained by linear elastic fracture mechanics (LEFM).25-27 Both size and shape of such

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bubbles/voids is predicted well by sediment tensile fracture toughness (KIC),20, 25, 28 which was

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also considered important for bubble migration.

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Efficient gas transport by bubble migration can dominate over diffusion in sediments,22, 29 and is

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controlled mainly by sediment mechanics.30, 31 In weak slurry-like sediments bubble movement

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can be driven by buoyancy.32 In strong fine-grained sediments, initial bubble was controlled by

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elastic fracturing, and bubble release was facilitated by vertical/sub-vertical fracture

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propagation.25, 31 Such fracture formation and propagation can be considered as sediment tensile

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failure, in line with LEFM for bubble growth in cohesive sediments.25, 31 These fracture/conduit

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structures were responsible for persistent bubble release in sediment.32-35

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While sediment bubble growth and migration behavior have been well studied in marine

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sediments and reconstituted artificial sediments, understanding of gas storage capacity and 3 ACS Paragon Plus Environment


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sediment gas bubbles responses to external disturbances in freshwater sediments is still limited.

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In freshwater sediments low ionic strength makes flocculation and coagulation of fine sediment

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particles more difficult than in marine sediments.36,

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riverine impoundments and reservoirs, may also be strongly affected by hydrologic and

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hydrodynamic conditions,38 thus sediment grain size distribution, organic matter (OM) content

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and sediment compaction may vary widely within and between locations, which may limit the

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applicability of sediment mechanical properties from one system to another.

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Here, we focus on natural sediments (sand and silty-clay) from impounded rivers often with rapid

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sedimentation and particulate OM input derived from predominantly forested catchments.

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Laboratory incubated sediment were characterized for bubble growth dynamics by high-

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resolution X-ray computed microtomography (µCT),39, and time-lapse µCT scanning on

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completion of incubations enabled characterization of bubble movement. Sediment mechanical

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properties, shear yield strength (SYS) and compressibility, were characterized to explain

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observed patterns of bubble formation and migration. The experiments provide a basis for

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mechanistic modelling of freshwater sediments methane bubble storage and release.

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Freshwater sediments, particularly in

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METHODS

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Instead of intact cores, homogenized sediments were used for two purposes: 1) to mimic methane

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bubble growth in riverine impoundments with rapid sedimentation (30 cm yr-1) and efficient OM

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burial such as River Saar, Germany;17 2) to avoid substrate limitation on methane production and

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hence bubble formation, given that gas production in freshwater sediments often declines sharply

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with depth and age.15, 16

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Definitions 4 ACS Paragon Plus Environment

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To simplify terminology relating to the use of “bubble” and “gas void”, we define “bubble” as a

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volume of free gas enclosed in a liquid or solid. A void can be a quasi-static feature within the

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sediment, a cavity that may contain gas and/or water at any given time. In soils, macropores are

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defined as structures that provide preferential pathways for water flow (bypassing the soil matrix)

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irrespective of their size, and the measured size strongly depends on the observational methods.40,

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41

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spatial resolution of CT scans, here macropores are defined as pores > 100 µm in equivalent

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diameter.

The reported equivalent diameter of macropores in soil is usually > 50 µm.40 Limited by the

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Sediment Collection, Processing and Characterization

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In June 2016, clayey and sandy sediments were sampled from a sidearm of the Rhine River in

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Germersheim (49.221735°N, 8.382457°E) and a stream in Hochstadt (49.24678°N, 8.22675°E),

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respectively. Riverine sediments can have rapid sedimentation and thick homogenized layers

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(e.g., Figure S1a, b). Natural freshwater sediments generally include particulate OM, comprised

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of leaf or woody debris, and its presence changes the particle size distribution by increasing the

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fraction of coarse material in our clayey sediment (Figure S2). To promote homogeneous

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enhanced gas production, we removed large pieces of OM (by sieving (> 2 mm) Figure S1c), and

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amended sediments with powdered air-dried alder leaf (10 g L-1 wet sediment). Leaf matter

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amendment raised sediment OM content (estimated by loss on ignition at 550 ºC) to 12.5% in

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clay, and 2.1% in sand (increases of 1.3% and 0.7%, respectively). Sediment particle size

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distribution was determined by laser diffraction with a particle size analyzer (Mastersizer 3000,

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Malvern, UK) (Figure S2): the median particle size (D50) of clay was 21 µm and 352 µm for sand.

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Experimental Setup 5 ACS Paragon Plus Environment


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For each sediment, paired incubation experiments in 60 cm tall, 2 mm thick transparent 6 cm

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(inner) diameter Plexiglas tubes filled with approximately 30 cm well-mixed sediment and

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topped-up with 15 cm tap water, sealed with rubber stoppers, were set-up. A 1.5 L inflatable gas

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bag was fitted to the top each tube to measure total gas volume produced (P, mL). P, water level,

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hw, and the position of the sediment-water interface (SWI) hs, in each tube were monitored daily.

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The difference between P and the daily change in total sediment gas storage gives the ebullition

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Eb. We express sediment gas content (θg), not as a volume, but based on the height and height

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change within the tubes, assuming that the mass (and volume) of (gas free) sediment and water

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remain relatively constant throughout the experiments. The θg relates to the hw and hs in the tube,

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θg = (∑∆hw)/hs; gas storage by capillary invasion θcap = θg - (∑∆hs)/hs. ∆hw and ∆hs are the daily

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change in hw and hs, respectively. 10 mL gas was extracted daily from gas bags to track methane

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and CO2 concentrations (measured with a greenhouse gas analyzer (Los Gatos, US)). The

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methane and CO2 flux was calculated from P and concentration measurements. 2 mL sediment

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porewater samples were taken at the end of the experiment using Rhizon tubes and vacuumed 10

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mL glass vials at different depths of the sediment columns. Porewater dissolved methane and

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CO2 were estimated from headspace concentrations.

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One of each paired incubated columns was used for µCT scanning and the other for excess

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dissolved gas pressure (EDGP) measurements. Porewater EDGP was measured with 3 vented

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pressure sensors (resolution 0.001 kPa, SENECT, Germany; 10 s sampling interval) in contact to

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the porewater through gas-permeable membranes (Contros, Germany) via the tube side wall. Two

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EDGP sensors were mounted in clay, 15 and 25 cm below the SWI, respectively; the third

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measured at 20 cm below the SWI in sand.

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To avoid transportation disturbance of the columns, they were stored dark adjacent to the µCT

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scanner at constant temperature (24.5 ± 0.8 °C). After incubation, cores were cut into 2 cm slices

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and the material stored in 50 mL water-tight plastic vials awaiting sediment water content (θw)

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and dry particle density measurement (ρdry: clay: 2553.7 kg m-3; sand: 2557.3 kg m-3).

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X-ray Computed Microtomography (µCT)

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A custom-made high-energy CT system HECTOR27 (Centre for X-ray Tomography, Ghent

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University: www.ugct.ugent.be) was used to scan the sediment columns. Using 7 vertical

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overlapping scans a voxel (volumetric pixel) size of 64.1 µm for the full column scan was

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achieved. A 1 mm Cu filter reduced beam hardening and the X-ray tube was operated at 60 W,

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190 kV for clay and 55 W, 190 kV for sand. Each full column scan was followed by a high-

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resolution scan (voxel size of 19.7 µm), in a central cylindrical region of interest (ROI) (19.7 mm

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diameter, 19.7 mm height), without a filter (15-16 W, 190 kV). With scans at incubation day 1, 4,

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8, 14 and 20, the progression of bubble growth was followed. The sand had one additional ROI

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scan in the upper 5 cm layer at day 20, but was skipped for clay because bubbles were adequately

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captured by column scans. Followed the day 20 scans, sediment column water level was dropped

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8 cm to trigger bubbling, and a further column scan performed after 8 hours. In between scans,

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the columns were left undisturbed to allow for accurate spatial tracing of bubble movement.

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CT Data Analysis

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Following image processing and segmentation (Text S1), gas bubble and pore parameters

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(bubble/pore volume, equivalent spherical diameter (Deq), connectivity, bubble shape, orientation

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and θg) were extracted from the final images using Octopus Analysis (formerly Morpho+).42

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Macropore density (ρpore) is simply the number of macropores (N) per unit analyzed sediment 7 ACS Paragon Plus Environment


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volume. The overall macropore connectivity (at voxel size 64.1 µm) was quantified using Euler–

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Poincaré characteristic (E)43, and decreasing E/N indicates increasing macropore connectivity.

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Bubble sphericity (0-1) was characterized as the ratio of the largest inscribed sphere diameter to

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Deq. Bubble orientation (0-180o) was characterised by the angle of the principal axis of the

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equivalent ellipsoid to the vertical (z-axis).

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Sediment column gas bubble migration was demonstrated by generating difference images for the

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last two day 20 µCT scan images (processed using DataViewer: Bruker microCT, Belgium). The

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resulting images were analyzed in Octopus applying column-specific threshold values; a low

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threshold for newly formed bubbles originally filled by water, a high threshold for those that

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disappeared due to movement.

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Sediment Rheology and Compressibility

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Using measured θw at day 20 (Figure S4) and ρdry, gas-free sediment wet bulk density (ρwet) was

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calculated. For each scan, volumetric fraction of solid sediment particles (θs) was computed from

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measured CT scan θg profiles and θcap data (14.9-22.8% for clay, and 44.2-70.4% for sand).

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Profiles of ρwet including gas phase for all scans were also calculated (Figure S5).

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Shear yield strength was measured using a rheometer (Anton Paar, MCR 102, Austria) involving

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a four-blade vane (22 mm diameter) inserted into a cup (29 mm diameter) containing gas free

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homogenized sediment samples (with θs ranging from 9.0-24.7% for clay and 37.4-61.6% for

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sand; Figure S6). Strain was measured while increasing shear stress from 0 Pa in logarithmically

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distributed increments until sediment failure; the SYS value was taken at the break point from

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initial linearity.44 The clear exponential dependence of SYS on θs enabled back calculation of


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SYS depth profiles for each CT scan (assuming θs changes due to θg development resulted in

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similar SYS change).

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Sediment compressibility was tested using an oedometer (model 08.67, Eijkelkamp®, the

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Netherlands), on samples with a range of θs (clay, 20.1-26.2%, and sand 52.6-68.7%), under

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static incremental loads (clay 5, 10, 20 … 60 kPa, and sand 5, 10, 50 … 600 kPa). From this the

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pre-compression stress of the sediments could be related to their θs (for details refer to Text S2

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and Figure S7). At vertical stress 5 and 10 kPa (lying in the range of measured sediment EDGP),

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sediment volumetric deformation and θs had a strong linear correlation (Figure S8) enabling the

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prediction of θg depth profiles (at 5 and 10 kPa EDGP) from the initial depth θs for both

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sediments.

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RESULTS

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During incubation, θg in clay increased sharply, reaching a maximum ~20% at day 4 (Figure S9a),

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and stabilized thereafter ~18.4%. In sand, θg development was slower and stabilized at ~15.2%

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after 8 days. In accordance with θg development, ebullition was less intense in the first 2 days.

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The occurrence of steady state, i.e. gas production equals ebullition (Eb/P = 100%), after day 5

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for clay and day 9 for sand, indicated that sediment gas storage capacity had been achieved

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(Figure S9b). The observed methane flux was minimum (< 0.01 mmol day-1) at the initial stage of

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θg development (until day 3 in clay and day 5 in sand), and then was enhanced dramatically at the

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steady state (~2 orders and 1 order higher for clay and sand, respectively) (Figure S9d), which

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highlights the importance of ebullition in methane transport. Compared to the change of

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methane:CO2 ratio in headspace (which was consistent to the change of Eb/P) (Figure S10a), this

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ratio experienced an initial peak at the early stage and was stabilized at the steady state (~1.7 for 9 ACS Paragon Plus Environment


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clay and ~0.3 for sand) (Figure S10b. This can be explained by the greater solubility of CO2

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relative to methane and more CO2 went into solution at the early stage before saturation was

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reached. By the end of incubation, ~3% and ~13% methane was found in solution in clay and

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sand, respectively, while ~50% for CO2. The estimated methane:CO2 ratio of the total production

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during the incubation experiment was ~1 and ~0.3 in clay and sand, respectively.

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Bubble growth by capillary invasion

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Capillary invasion dominated gas content development in both sediments (Figure S9c), although

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more so for sand than clay. In clay, θcap/θg dropped sharply from the initial 100% to 47.6% in the

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first 4 days finally stabilizing at ~67%; in sand, θcap/θg was 100% in the first 5 days decreasing

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linearly to ~80% by day 12. Capillary invasion was also evidenced by EDGP dynamics (Figure

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S11), decreasing sharply by 10.0 kPa in the surface layer, and 8.0 kPa at lower depths during the

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first 4 days; in sand, EDGP decreased by 5.0 kPa from an initial 3.6 kPa during day 2-8. This

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sediment EDGP reduction was in accordance with bubble growth dynamics by capillary invasion

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(Figure S9c).

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The sand microbubble size distribution (from µCT scans) peaked at 60 µm diameter (Figure S12).

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Indirect evidence from EDGP measurements suggest microbubbles in clay with a diameter of 30

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µm (estimated from the 10.0 kPa EDGP decrease), fall into the size range of capillary pores. This

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confirms the initial occurrence of capillary invasion for bubble formation in both sediments and

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is consistent with Reed et al.45 whose µCT scans found a similar size range (diameter > 60 µm) of

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bubbles in reconstituted clay.

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Dependence of large bubble growth on sediment mechanical properties


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Both gas content profiles (Figure S9e and f) and 2D vertical CT slices (Figure S13) showed the

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development of large depth gradients of gas content, and bubble size distribution, in clay and

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sand, these are attributable to sediment mechanical properties.

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Significant changes in bubble size distribution in both sediments were shown by the µCT scans

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(Figure 1), these can be categorized to two general modes of bubble growth: 1) bubble density

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(ρbub, number of bubbles per sediment volume) decrease associated with increasing bubble

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volume; 2) ρbub increasing over time without bubble volume increase. The bubble growth mode 1

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was observed throughout the clay column, but only in the surface layer of sand, where ρbub

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decreased by two orders of magnitude. Initially, the bubble size distribution was log-normal at all

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depths, becoming bimodal towards the end. Bubble growth in the sand mid-layer was

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characterised mode 2: the log-normal bubble size distribution persisted over time with an

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increasing ρbub of microbubbles, while the peak bubble size was decreasing, suggesting bubble

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growth by invading smaller pores. These modes of bubble growth were confirmed by 3D

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visualization (Figure 1b), which also enabled detection of a change not apparent from bubble size

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distribution statistics alone: the bubbles in the clay surface layer were significantly less at day 20

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compared to day 8 in both density and volume.

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The maximum gas storage capacity of the sediments could be related to sediment compressibility.

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Expected clay θg at 5 kPa EDGP was 19-24.9% (Figure 2a), comparable to θg at steady state

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bubble growth (except for the upper 4 cm). The measured maximum clay θg was in the range

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predicted for 10 kPa EDGP. In sand, predicted θg at 5 kPa load only explained half the gas

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content formation. Increasing sand EDGP from 5 to 10 kPa led to < 1% volume expansion,

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demonstrating poor sand compressibility. Porewater drainage from sand was much faster (5 min)



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than for clay (90 min) (see compression curves, Figure S7), indirectly confirming that capillary

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invasion in sand is easier compared to water displacement in clay.

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SYS increased exponentially with θs for both sediments (Figure S6), and was affected by the

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counteracting processes, sediment compaction and gas content development, the former

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increasing shear strength, and the latter weakening it. Both effects were both minor in clay. All

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clay SYS were < 15 Pa, whereas, in sand, compaction enhanced SYS over the entire depth (day

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1-4), and gas content development decreased sediment strength (from day 4); most apparent in

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the surface layer (SYS ranging between 99-376 Pa at day 4, compared to 25-94 Pa at day 20).

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As with gas content, sediment compressibility explained bubble size distribution change over

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depth in clay well (e.g. the clay surface layer bubble size distribution peak decreased from 1.7

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mm to 1 mm in the bottom layer, Figure S14). Bubble sphericity and orientation (in addition to

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gas content and bubble size) were closely related to depth gradients in sediment mechanical

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properties. In surface clay, bubbles were near spherical (sphericity = 0.6), but at lower depths

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were elongated (sphericity < 0.4) and ~50% of bubbles were horizontally-oriented. This pattern

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was stronger in sand where bubble shape changed from spherical (sphericity = 0.6) above 3.5 cm

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to being horizontally-oriented elongated bubbles (sphericity = 0.3, ~34% bubbles horizontally-

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oriented) at 3.5-6 cm depth.


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Figure 1. (a) Bubble size distribution in surface (0-6 cm) and mid (13-19 cm) layers of sediment

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columns at different days of incubation. The probability density of bubble volume was

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normalized by the analyzed sediment volume, i.e., the integration of each distribution equals the

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total volumetric gas content of the analyzed sediment layer. (b) 3D visualization of gas voids

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(golden color with shading) in a control volume (1 cm3) at selected sediment column depths.

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Figure 2. Change of sediment mechanical properties due to bubble growth. (a) measured θg depth

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profiles (thick black lines) at incubation day 20 and predicted θg for initial sediment at different

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EDGP (5 and 10 kPa; represented by red solid and dotted lines, respectively) from sediment

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compressibility test; (b) sediment shear yield strength (SYS) for clay (left side, red lines) and

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sand (right side, black lines) at different incubation days calculated from θs depth profiles.

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Development of sediment macropores

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Sediment gas content development not only changed sediment strength, but also altered

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macropore structure due to plastic sediment volumetric deformation (Table 1). In response to

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methane bubble growth, ρpore decreased at all depths in both sediments, and the total volume of

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macropores (macro-porosity) increased. In clay, macro-porosity doubled over the entire depth 14 ACS Paragon Plus Environment

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during the period of incubation; in sand, initial macro-porosity was high and the increase in pore

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volume less pronounced (6% in the mid layer, 10.6% in the surface layer) due to the dominance

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of bubble growth by capillary invasion. The reduction in ρpore and increase in macro-porosity,

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enhanced macropore connectivity in both sediments, was reflected by changes in E/N. Despite

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high macropore connectivity (E/N < 1) in both sediments at steady state gas content development

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(suggesting a well-developed macropore network) E/N was initially more variable. A large

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decrease in clay E/N (0.6 to -0.6 in the surface layer, and 1.1 to -3.6 in the mid layer) was

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observed over the first 4 incubation days. Conversely, an increase in E/N was seen in sand (+5.0

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surface, and +2.3 mid layer) during the first 4 days, this however reversed from day 4-8.

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Following the initial changes in E/N (day 4 clay, and day 8 sand), a steady increase was observed

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in both sediments.

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Macropore connectivity development was closely related to large bubble growth (Figure 3). In

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surface clay, isolated small bubbles with relatively weak pore connections were observed at day 1

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(Figure 3a). As bubbles grew larger and maximum sediment gas storage reached (day 4, Figure

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1b and Figure S9a), macropores (mainly gas-filled) were enlarged and their connectivity was

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significantly enhanced. From day 4 in clay, intense bubble release (Figure S9b) led to the

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formation of water-filled macropores (Figure 3a). This was also observed in sand but with a

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further 4 day delay. Macropore connectivity decrease in days 1-4 was associated with slight

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initial settling, and microbubble formation by capillary invasion did not contribute to the change

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in sediment macropore connectivity (Figure 1b and Figure S9c). The great increase in macropore

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connectivity during day 4-8 coincided with large bubble growth by sediment matrix deformation

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(Figure S9c).

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Table 1. Macropore connectivity (E/N), macropore density (ρpore) and macro-porosity at two

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selected depths in two sediment columns over time. Macropore connectivity increases with

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decreasing E/N (macropores were isolated when E/N ≥ 1); ρpore is the number of pores

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normalized by the analyzed sediment volume. Sediment type

Clay

Sand

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Day 1 4 8 14 20 1 4 8 14 20

Surface 6 cm layer Macroρpore E/N -1 porosity % (mL ) 0.60 -0.60 -0.30 0.10 0.10 -5.00 0.02 -3.60 -3.10 -2.10

314 63 164 203 192 491 698 292 354 349

13.9 37.2 34.7 30.0 30.2 29.0 22.7 41.6 39.9 39.6

Bu


Mid 13-19 cm layer Macroρpore E/N -1 porosity % (mL ) 1.10 -3.60 0.10 0.60 0.60 -1.80 0.50 -16.70 -14.70 -11.60

365 25 114 160 172 645 716 150 179 195

15.0 41.0 33.4 29.0 28.6 26.7 20.2 32.8 31.8 32.7

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Figure 3. (a) 3D visualization of macropore and gas voids in a control volume (1 cm3) in the

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surface sediment layer; (b) 3D visualization of macropore and gas voids in surface layer of clay

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and sand at day 20. Water-filled macropores are colored blue/green and gas voids red/yellow.

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Bubble mobility

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At incubation day 20, 8 cm water-level drawdown experiments were conducted. Each sediment

326

column was scanned before and after the water-level change enabling bubble mobility

327

characterization. Sediment pores in the clay upper layer remained stable over 8 h and vigorous

328

bubble movement was observed (Figure 4a and b), with gas movement clearly apparent in the

329

difference image of the two scans (Figure 4c). While gas movement occurred over the entire clay

330

column, it was restricted to the uppermost 6 cm in sand (Figure 4d). The estimated gas bubble

331

flux from clay (125.2 mL d-1) was ~5.8 times greater than that from sand (21.6 mL d-1). 17 ACS Paragon Plus Environment


332

These bubble mobility differences in clay and sand are related to macropores, which enhance

333

pore connectivity and decrease capillary pressure. In sand, this was observed in the surface layer,

334

but not the mid layer. Yet, the diameter of all mobile bubbles was in the millimeter range (0.2-6.1

335

mm in clay, 0.2-3.5 mm in sand), comparable to the pore sizes (Figure 5).

336

The shape of mobile bubbles in both sediments showed no strong pattern. In clay mean mobile

337

bubble sphericity (Figure 4(c)) was 0.4, smaller but comparable to bubbles before triggering

338

movement (sphericity = 0.6, e.g. Figure 4(a)). In addition, no preferential orientation of mobile

339

bubbles was observed in either sediment.

340 341

Figure 4. (a) - (b) time-lapse scans of the surface 6 cm of clay. (c) difference between images (a)

342

and (b), bubble and porewater movement are shown as white and black patches, respectively. The

343

black circles identify a newly formed bubble at 10pm; the red circles highlight the disappearance

344

of a bubble originally trapped in a pore. The height and diameter of analyzed volume are marked


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345

by yellow axes. (d) Mean vertical profiles of bubble movement. The black lines show the changes

346

in volume fraction of gas phase.

347 348

Figure 5. Sediment bubble and pore size distributions. The probability density of bubble volume

349

was normalized by the analyzed sediment volume. Black and red lines show pore and bubble size

350

distributions in the upper 6 cm of the sediments before water-level change, respectively; and blue

351

lines are for mobile bubbles.

352 353

DISCUSSION

354

The role of sediment mechanical properties in bubble growth

355

Methane bubbles in soft marine sediments have been characterized as disk-like in shape and with

356

a vertical/sub-vertical orientation.26, 46 Their growth can be well explained by LEFM theory,19, 25

357

i.e., these bubbles grow by elastic fracture of the sediment. Disk-like bubble development was

358

not observed in this study, instead, sediment displacing bubbles both in clay and sand were either 19 ACS Paragon Plus Environment


Page 20 of 30

359

close to spherical or were elongated bubbles with a horizontal orientation, suggesting the control

360

of bubble growth by elastoplastic deformation of sediment matrix rather than elastic fracturing.

361

In clay, compressibility, explains the growth of larger bubbles, due to sediment matrix

362

deformation, although a quantitative compressibility to bubble size relationship remains elusive.

363

The predicted θg at 5 and 10 kPa EDGP was consistent with observed θg despite a few percent

364

overestimation (Figure 2); whereas in sand this discrepancy was > 10%. In clay, where pore

365

drainage is slow, applied compression load is effectively taken by the porewater due to the high

366

water content and the visco-plastic fluid-like sediment can be easily deformed (Figure S7).

367

Conversely, in sand, the free drainage of porewater means the applied stress is taken by the

368

sediment matrix, in accordance with the dominance of bubble formation by capillary invasion

369

(θcap/θg > 80%), hence the underestimation of θg in sand is likely due to porewater displacing

370

microbubbles.

371

In clay, the underestimation of θg by sediment compressibility (in response to measured EDGP)

372

may be due to sediment gas storage capacity and controlled by buoyancy-induced instability. In

373

clay, the measured EDGP ranged from 8.1-10 kPa, corresponding to an expected θg of 29.6%, 9.9%

374

higher than the θg measured by µCT. High gas content, however, results in ρwet < water density,

375

leading to buoyancy-induced instability where the excessive gas content is released upwards.32

376

The critical θg for the initial gas free ρwet for clay (1270 kg m-3) was 25.5%. Depth profiles of ρwet

377

indicate the presence thin gas-charged layers below 5 cm depth with ρwet < 1000 kg m-3 (Figure

378

S5), and hence low mechanical stability zones where buoyancy can limit gas storage capacity.

379

Such a limitation on maximum gas storage did not apply in sand because ρwet > 1200 kg m-3.

380

One study found that bubble size in weak sediment (SYS < 10 Pa) was limited to 9 mm

381

(equivalent spherical diameter) above which bubbles leave the sediment, but in strong sediment 20 ACS Paragon Plus Environment

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382

(SYS of a few hundred Pa) gas release was facilitated by fractures.47 In the present experiment,

383

sand SYS (~102-104 Pa) would be sufficient to stabilize fractures,32 and clay was weak (SYS < 15

384

Pa) and continuous long fractures could not form, whereas short fractures/macropores that

385

connect macropores can be stabilized (Figure 3b). Depth gradients in bubble shape in sand

386

correspond with strong vertical SYS gradient; bubbles changed from spherical to horizontal

387

elongated shapes below 3.5 cm depth where SYS was ~94 Pa and doubled to ~192 Pa at 6 cm

388

depth, as observed elsewhere.47 In clay, the large SYS depth gradient was absent, but similar

389

change in bubble shape was observed. The formation of horizontal bubbles may be explained by

390

sediment compactness increase with depth (Figure S4), i.e., sediment was more compressible

391

horizontally than vertically, as previously reported for gas dome formation in cohesive

392

sediments.48

393 394

Macropore networks: a framework for bubble migration

395

Macropore transport appeared as the dominant form of gas bubble movement in this study. The

396

development of connected macropore structures, where bubbles could accumulate and move, was

397

revealed by µCT scans. These structures are analogous to macropores in soil, which serve as

398

preferential pathways for air and water movement.40, 41, 49 The most apparent and intense bubble

399

migration (Figure 4, 5) occurred in sediment layers containing large macropores, in the mm to cm

400

scale, produced by sediment deformation during bubble formation. The pre-existing sediment

401

macropores in sub-mm range (e.g. in sand mid layer) were largely occupied by microbubbles

402

(~50%) (Figure S11). Gas transport in this layer was facilitated by direct breakthrough due to

403

high inter-connectivity, which was evidenced by the sharp increase of EDGP in sand prior to day

404

8. The extent of bubble movement depended on the stage of macropore development, which, with

405

concurrent ebullition, increasing during sediment expansion in both clay and sand (Figure S1b 21 ACS Paragon Plus Environment


406

and c). Bubbles occurred with a relatively uniform spatial pattern at discrete distance intervals

407

within the existing macropore network, this general pattern persisted, and was similar to

408

observations of rising bubbles in glass bead columns.50 This is also supported by the videos in

409

previous experiments2, 25 where bubble chain movement in pores was observed. Macropores are

410

important for solute transport in soils,51, 52 and for bubbles in this study - where macropores were

411

connected by fractures. The upward moving bubbles can be facilitated by vertically/sub-vertically

412

oriented fractures opened by the passage of previously migrated bubbles (Figure 3b and Figure

413

4a/b), which was previously explained by viscoelastic fracturing.31

414

Potential alternative bubble migration mechanisms to macropore transport include, buoyant

415

migration (as discussed above) and fluidization32,

416

transport are very small (< 0.1 mm), and immobile bubbles in our experiments had Deq > 2 mm at

417

steady state, and since smaller bubbles moved freely within macropores our observations do not

418

support fluidization as a relevant bubble transport mechanism. Indeed, Johnson et al.47 observed

419

no fluidization, even in extremely low-strength sediments (SYS = 7 Pa). Regarding buoyancy, as

420

already discussed above, only large bubbles Deq > 9 mm have been found to migrate, and only in

421

very weak sediments.47 In our study, larger bubbles (Deq > 2 mm), tended to be stationary (Figure

422

5), only small bubbles (Deq < 2 mm) were dominantly mobile, and this mobility was strongly

423

influenced by the relative size of bubbles to pores. So, despite low sediment strength in our clay

424

column (SYS < 15 Pa), we found no evidence to suggest that buoyant migration was an important

425

bubble movement mechanism, and we propose that macropore transport is dominant in feeding

426

ebullition in highly methane productive freshwater sediment systems.

47

. Critical bubble diameters in fluidized

427 428

Implications


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Page 23 of 30


429

In our experiments, small scale, but high resolution examination of bubble formation and

430

transport mechanisms in freshwater sediment was achieved. The small diameter columns were a

431

limitation, and to investigate larger-scale structure development, incubation vessels should be

432

several meters wide.

433

structure in the surface layer of clay (Figure 4a and b). Although initially homogenized natural

434

sediments were used, sediment porosity in clay was 80.8-81.6% at incubation day 20 when

435

steady-state macropore structure was reached. This is consistent with porosity of freshwater

436

sediments previously reported in lakes and river impoundments (e.g., Lake Kinneret, Israel - 70-

437

87%55 and Saar River, Germany 80 ± 0.4%5). The addition of leaf matter was also reasonable,

438

and stimulated methane production consistent with levels observed in natural river cores.17 The

439

relatively high methane:CO2 ratio (~1.7) in gas bubbles produced from incubated natural river

440

sediments suggests the potential for greenhouse gas emission mitigation by harvesting methane

441

and burning to CO2, as proposed elsewhere for large tropical reservoirs. 56, 57

442

We found that the depth range, where gas bubbles can be mobilized by changing hydrostatic

443

pressure differed considerably between sediments, highlighting the importance of the pore

444

structure for ebullition dynamics in sediments. We believe that the observations shown here are

445

applicable to natural systems with commonly occurring, strong flood depositional events,

446

resulting in thick well-mixed sediment layers rich in OM (e.g. Figure S1), and that extrapolation

447

to freshwater systems with gradual sedimentation (e.g. lakes) is not appropriate.

448

Direct high-resolution observations from µCT revealed the importance of macropores in methane

449

bubble migration in freshwater sediment, and sediment SYS was a key physical property in

450

determining the depth of the surface zone exhibiting macropore network development. Sediment

451

gas content could be predicted by sediment compressibility, which is an estimation of sediment

452

elastic-plastic deformation under applied loads. An improved model, which incorporates these

53, 54

Despite this limitation, µCT captured a well-developed macropore



453

physical characteristics, such as depth-dependent macro-porosity and pore size distribution, can

454

be expected to better capture bubble storage and release dynamics in aquatic sediments.

455 456

ACKNOWLEDGEMENTS

457

The authors would like to thank Marijn A. Boone (XRE) for his contribution to X-ray CT 3D

458

image reconstructions and Inka Meyer for helping with sediment grain size analysis. Thanks to

459

Jeroen Van Stappen for his help during experiment preparation and to Björn Krüger for his

460

assistance with sediment compressibility test. This study was financially supported by the

461

German Research Foundation (grant LO 1150/5). Tim De Kock is a postdoctoral fellow of the

462

Research Foundation - Flanders (FWO) and acknowledges its support.

463 464

Supporting Information

465

Additional figures on examples of natural sediments taken from River Rhine, sediment grain size

466

distributions, examples of filtered µCT images, depth profiles of sediment volumetric water

467

content (θw), solid fraction (θs) and gas content (θg), sediment wet bulk density (ρwet) depth

468

profiles, sediment yield shear strength (SYS) at different solid volume fraction (θs), examples of

469

compression test using the odometer, volumetric deformation in response to 5 and 10 kPa vertical

470

stress, respectively at different solid volume fraction (θs), overview of gas content development,

471

CH4:CO2 ratio in headspace and gas bubbles, excess dissolved gas pressure (EDGP) in porewater,

472

pore and bubble size distributions in region of interest scans, vertical µCT slices of gas bubble

473

growth, bubble size distribution at different depths of clay at day 20; additional text on CT image

474

processing and segmentation, sediment compressibility test, sediment methane (CH4) and CO2

475

production.

476 24 ACS Paragon Plus Environment

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477

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Methane bubble growth and migration in aquatic sediments observed

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