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1. Material Properties of Sediments Steer Burrowers and Effect Bioturbation

   https://doi.org/10.1029/2022JG007269  

   Dorgan, Kelly M.; Arwade, Sanjay R. (June 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Infaunal organisms mix sediments through burrowing, ingestion and egestion, enhancing fluxes of nutrients and oxygen, yet the mechanisms underlying bioturbation remain unresolved. Burrows are extended through muddy sediments by fracture, and we hypothesize that the cohesive properties of sediments play an important but unexplored role in resisting bioturbation. Specifically, we suggest that crack branching, tortuosity, and microcracking are important in freeing particles from the cohesive matrix, and that the sediment properties that affect these processes are important predictors of bioturbation. We use finite element modeling and simplified, mechanics‐based models to explore the relative importance of sediment mechanical properties and worm behaviors in determining crack propagation paths. Our results show that crack propagation direction depends on variability in fracture toughness, and that applying more force to one side of the burrow wall, simulating “steering” behavior, has surprisingly little effect on crack propagation direction. Burrowers instead steer by choosing among crack branches. Paths created by burrowing worms in natural sediments are mostly straight with some crack branching, consistent with modeling results. Crack branching also requires sufficient stored elastic energy to drive two cracks, and worms can exert larger forces resulting in more stored energy in stiffer sediments. This implies that more crack branching and consequently more particle mixing occurs in heterogeneous sediments with low fracture toughness relative to stiffness. Whether sediments with greater potential for crack branching also experience higher bioturbation remains to be tested, but these results indicate that material properties of sediments may be important in resisting or facilitating bioturbation. 

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2. Fracture toughness of a metal–organic framework glass

   https://doi.org/10.1038/s41467-020-16382-7  

   To, Theany; Sørensen, Søren S.; Stepniewska, Malwina; Qiao, Ang; Jensen, Lars R.; Bauchy, Mathieu; Yue, Yuanzheng; Smedskjaer, Morten M. (May 2020, Nature Communications)

   Abstract Metal-organic framework glasses feature unique thermal, structural, and chemical properties compared to traditional metallic, organic, and oxide glasses. So far, there is a lack of knowledge of their mechanical properties, especially toughness and strength, owing to the challenge in preparing large bulk glass samples for mechanical testing. However, a recently developed melting method enables fabrication of large bulk glass samples (>25 mm³) from zeolitic imidazolate frameworks. Here, fracture toughness (K_Ic) of a representative glass, namely ZIF-62 glass (Zn(C₃H₃N₂)_1.75(C₇H₅N₂)_0.25), is measured using single-edge precracked beam method and simulated using reactive molecular dynamics.K_Icis determined to be ~0.1 MPa m^0.5, which is even lower than that of brittle oxide glasses due to the preferential breakage of the weak coordinative bonds (Zn-N). The glass is found to exhibit an anomalous brittle-to-ductile transition behavior, considering its low fracture surface energy despite similar Poisson’s ratio to that of many ductile metallic and organic glasses. 

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3. Spatially and Temporally Variable Impacts of Hurricanes on Shallow Sediment Structure

   https://doi.org/10.1029/2023JC020820  

   Clemo, W_C; Dorgan, K_M; Wallace, D_J; Dzwonkowski, B. (July 2024, Journal of Geophysical Research: Oceans)

   Abstract Sediment dynamics are fundamental to understanding coastal resiliency to climate change in the coming decades. Tropical cyclones can radically alter shallow sediment properties; however, the uncertain and destructive nature of tropical cyclones make understanding and predicting their impacts on sediments challenging. Here, grain size sampling in conjunction with continuous hydrodynamic data provided an unprecedented perspective of the impacts of two tropical cyclones, including Hurricane Sally (2020), in which the inner core of the storm passed directly over the field sites, on shallow coastal sediments in Alabama (USA). Sampling directly before and after Sally as well as out to ∼7 months after the second storm event, Hurricane Zeta, showed that the changes in sediments following storm events exhibited notable site‐to‐site variability. This variability during the first storm event was consistent with low sand supply and flow interactions driven by local bathymetry that led to sand transport and deposition at some previously‐muddy sites, near‐surface mud loss at some sandy sites, or little change at others. Post‐Sally impacts to grain size were well preserved 8 months after the storm, despite passage of Zeta as well as seasonal winds and riverine inputs during winter and spring. Overall, high temporal‐resolution sampling over a relatively large area (<500 km²) revealed relatively small‐scale spatial variability (on the order of 5–10 km) of hurricane impacts to sediment structure. These observations demonstrate a critical limitation for accurately predicting changes to coastal sediment dynamics in the face of a changing climate and its impact on tropical cyclones. 

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4. The Effects of Engineered Aeration on Atmospheric Methane Flux From a Chesapeake Bay Tidal Tributary

   https://doi.org/10.3389/fenvs.2022.866152  

   Lapham, Laura L.; Hobbs, Edward A.; Testa, Jeremy M.; Heyes, Andrew; Forsyth, Melinda K.; Hodgkins, Casey; Szewczyk, Curtis; Harris, Lora A. (August 2022, Frontiers in Environmental Science)

   Engineered aeration is one solution for increasing oxygen concentrations in highly eutrophic estuaries that undergo seasonal hypoxia. Although there are various designs for engineered aeration, all approaches involve either destratification of the water column or direct injection of oxygen or air through fine bubble diffusion. To date, the effect of either approach on estuarine methane dynamics remains unknown. Here we tested the hypotheses that 1) bubble aeration will strip the water of methane and enhance the air-water methane flux to the atmosphere and 2) the addition of oxygen to the water column will enhance aerobic methane oxidation in the water column and potentially offset the air-water methane flux. These hypotheses were tested in Rock Creek, Maryland, a shallow-water sub-estuary to the Chesapeake Bay, using controlled, ecosystem-scale deoxygenation experiments where the water column and sediments were sampled in mid-summer, when aerators were ON, and then 1, 3, 7, and 13 days after the aerators were turned OFF. Experiments were performed under two system designs, large bubble and fine bubble approaches, using the same observational approach that combined discrete water sampling, long term water samplers (OsmoSamplers) and sediment porewater profiles. Regardless of aeration status, methane concentrations reached as high as 1,500 nmol L⁻¹in the water column during the experiments and remained near 1,000 nmol L⁻¹through the summer and into the fall. Since these concentrations are above atmospheric equilibrium of 3 nmol L⁻¹, these data establish the sub-estuary as a source of methane to the atmosphere, with a maximum atmospheric flux as high as 1,500 µmol m⁻²d⁻¹, which is comparable to fluxes estimated for other estuaries. Air-water methane fluxes were higher when the aerators were ON, over short time frames, supporting the hypothesis that aeration enhanced the atmospheric methane flux. The fine-bubble approach showed lower air-water methane fluxes compared to the larger bubble, destratification system. We found that the primary source of the methane was the sediments, however,in situmethane production or an upstream methane source could not be ruled out. Overall, our measurements of methane concentrations were consistently high in all times and locations, supporting consistent methane flux to the atmosphere. 

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5. Anisotropic fracture toughness of Poorman Schist rocks from EGS Collab Experiment 1 Site

   Ruplinger, C.; O’Connell, R.; Sone, H.; Trzeciak, M.; Wang, H. (January 2020, 54th US Rock Mechanics/Geomechanics Symposium)

   We investigate the mode 1 fracture toughness and its anisotropy of Poorman Schist rocks recovered from the Enhanced Geothermal Systems Collaboration (EGS Collab) Experiment 1 site. The EGS Collab team is conducting a series of intermediate (10-20m) scale stimulation and inter-well flow tests with comprehensive instrumentation and characterization at the Sanford Underground Research Facility to validate existing theories and description of hydraulic fractures propagation and associated fluid flow. An important parameter to constrain is how the fracture toughness varies depending on the orientation of the fracture and the direction of fracture propagation, which may have controls on hydraulic fracture propagation. Fracture toughness relative to foliation orientation was determined through the utilization of Cracked Chevron Notched Brazilian Disk (CCNBD) samples in three different orientations (Divider, Arrester, and Foliation Splitting/Short Transverse). Each sample group contains at least three 25.4 mm diameter and 12.7 mm thick CCNBD samples, one of each sample type. Arrester and Foliation Splitting samples were obtained from the same sub-core while Divider samples were obtained from a separate sub-core obtained in close proximity. We found fracture toughness to be weakest in the Foliation Splitting orientation and strongest in the Divider orientation, similar to findings from anisotropic fracture toughness measured in shale rocks. Our findings on the influence of foliation orientation on fracture toughness are presented here. 

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