Qingdao Institute of Marine Geology

Fluid seepages and seabed pockmarks are widely observed on continental margins worldwide in hydrate- and non-hydrate-bearing sediment. Subsurface gas chimneys connecting seafloor pockmarks to underlying gas reservoirs are commonly revealed by seismic reflection data, indicating pathways of past and present fluid migration. Fluid seepage occurs when the seal of a gas reservoir is breached, allowing fluids to migrate upward and vent at the seafloor, forming pockmarks. In hydrate-bearing settings, gas reservoirs beneath hydrate layers typically consist of coexisting water and gas phases. However, quantitative constraints on gas saturation in free-gas zones beneath hydrates inferred from pockmark morphology remain limited. In this study, a two-phase pockmark model was developed to investigate gas-chimney growth and pockmark formation, and to estimate gas saturation in free-gas zones below hydrates using pockmark depth and gas-zone thickness as key parameters. The model was applied to the Storegga Slide region off Norway, where hydrates, pockmarks, and chimney-like seismic anomalies have been documented. Here, the application is intended to represent localized near-threshold (pre-seepage) conditions leading to pockmark initiation, rather than the present-day post-venting state. Model results for the initiation (near-threshold, pre-venting) stage indicate that the effective gas saturation in the free-gas reservoir beneath the hydrates was approximately 1.36–1.58% for gas-zone thicknesses of 50–100 m, and that the corresponding chimney-propagation timescale during initiation was on the order of ~200 years. These estimates represent threshold conditions required for seal breach and pockmark formation rather than present-day seepage states. During venting, methane gas may form hydrates within the chimney inside the hydrate stability zone, while authigenic carbonates precipitate in pockmarks and shallow sediments. These secondary hydrates and carbonates eventually seal the chimney, leaving behind a residual gas chimney in the subsurface sediment.

Electrical Resistivity Tomography (ERT) has proven to be a highly sensitive geophysical method for characterizing the dynamics of seawater intrusion. This study uses tank experiments to simulate seawater intrusion, utilizing electrical resistivity tomography to monitor real-time changes in groundwater resistivity during the intrusion process. The objective is to quantitatively reveal the development and evolution mechanisms of seawater intrusion wedges in sandy aquifers, thereby establishing a real-time resistivity monitoring method for groundwater distribution and migration characteristics. This study improves resistivity imaging data processing methods, enhancing both efficiency and accuracy. The refined cross-hole ERT technique is applicable not only to meter-scale indoor experiments; its optimized forward and inverse algorithms can also be directly transferred to regional-scale field monitoring. Experimental results show that the average resistivity in the study area continuously decreases from 57 Ω·m in the initial freshwater state to 1.1 Ω·m at the intrusion stabilization point. Areas with resistivity values below 20 Ω·m corresponded exactly to the brine intrusion zone. The evolution of the freshwater-saltwater interface unfolded in three stages: Initially, the density difference (0.025 g/cm3) dominated, with the saltwater intrusion depth at the aquifer base reaching 0.45 m, significantly exceeding the 0.04 m penetration at the upper section. During the intermediate stage, the interface morphology differentiated into an “upper triangular, lower arc-shaped” configuration. The bottom intrusion distance increased to 1.65 m, and the thickness of the brackish-freshwater mixing zone expanded from 0.1 m to 0.3 m. In the final stage, the interface stabilized and began intruding toward the surface, establishing a new hydrodynamic equilibrium. In addition, the migration rate of saline water at the aquifer base gradually decreased from 6.25 × 10−4 cm/s initially to 1.16 × 10−5 cm/s at steady state. These results reflect the dynamic coupling process between seepage and dispersion and demonstrate that this method enables effective real-time monitoring of seawater intrusion development and conditions, as well as early warning capabilities.

Based on high-resolution spatial analysis of 463 surface sediment samples from the East China Sea (ECS) inner shelf, this study reveals that hydrodynamic sorting and sediment transport dynamics, rather than source strength alone, are the primary drivers governing polycyclic aromatic hydrocarbons (PAHs) distribution and compositional patterns, challenging emission-centric paradigms. Sedimentary PAHs exhibit extreme spatial heterogeneity, ranging from 19.38 to 10,047ng/g. Notably, semi-enclosed bays (e.g., Sanmen Bay) serve as significant pollution hotspots, contrasting sharply with lower levels found in adjacent open-shelf areas. We identified a novel "congener fractionation" phenomenon along the estuary-shelf continuum: low-molecular-weight PAHs (LMW; 2-3 rings) dominate proximal nearshore zones due to preferential deposition within coarse sediment fractions under strong tidal regimes. Conversely, high-molecular-weight PAHs (HMW; 4-6 rings) exhibit a stronger affinity for fine-grained particles and undergo offshore transport via prevailing currents (e.g., Zhejiang-Fujian Coastal Current-ZFCC), leading to significant enrichment within distal mud belts (HMW PAHs constituting up to 90%). This fractionation stems from molecular-weight-dependent variations in adsorption capacity, transport potential, and susceptibility to degradation, creating compositional gradients that mimic weathering signatures and confound conventional source diagnostics. Source apportionment identified combustion sources (contributing 75-92% of the total PAHs) as dominant across most study areas, except within Xiangshan Bay where petrogenic sources were predominant (80.8%). Hydrodynamic constraints and topographic bottlenecks (e.g. semi-enclosed bays) significantly amplify legacy accumulation by sequestering contaminated fine sediments. Seasonal currents (e.g., monsoon-driven flows) effectively facilitate the along-shelf transport of particle-bound HMW PAHs. Ecological risk assessment, based on calculated benzo[a]pyrene toxic equivalent quotients (TEQ: 0.84-2063.60ng/g), correlates with sediment texture, highlighting fine-grained depocenters (especially semi-enclosed bays) as primary high-risk sink zones. Consequently, effective pollution mitigation strategies should integrate hydrodynamic controls and sediment management alongside targeted source reduction measures.

Amid rising global temperatures and accelerating carbon-neutral initiatives, the efficient valorization of greenhouse gases has emerged as a central focus of contemporary research. Microbial metabolism enables the low-cost transformation of methane, which has evolved into a strategic technological reserve for a green and low-carbon future. Methanotrophs, widely distributed across diverse habitats, utilize methane as both a carbon and an energy source. Through key enzymes in their central metabolic pathways, these microorganisms sequentially oxidize CH₄ into methanol, formaldehyde, formate, and ultimately to CO₂. In synthetic microbial consortia comprising methanotrophs and methylotrophs, inter-species cross-feeding effectively alleviates the accumulation of inhibitory metabolites, improving overall methane conversion efficiency. Beyond regulating the source-sink balance of atmospheric greenhouse gases, methanotrophic consortia also drive the high-value resource utilization of high-concentration CH₄ and CO₂. Type I, II, and X methanotrophs possess distinct carbon fixation pathways and are capable of synthesizing high-value products such as methanol, single-cell protein (SCP), and polyhydroxyalkanoate (PHA). Investigating their mechanisms and efficient cultivation strategies is conducive to further exploring the potential of methanotrophs in carbon cycling and biomanufacturing. However, the practical application of methanotrophs still faces several challenges, including difficulties in process control, ineffective suppression of byproduct formation, and potential safety concerns associated with the synthesized products. Addressing these bottlenecks is imperative to unlock their full potential for large-scale industrial applications in greenhouse gas mitigation and sustainable biomanufacturing.