Volume articles

To reveal the spatial and temporal distribution patterns of internal wave-induced mixing in global ocean and investigate its influencing factors, this study employs an internal wave fine-scale parameterization method to statistically analyze Argo temperature and salinity data at 250-500 m depth from 2006 to 2021. The analysis characterizes the spatial and temporal features of internal wave mixing and identifies the impact of wind-induced near-inertial energy flux on mixing across global oceans under varying seasonal conditions. In space, there is strong wind-induced near-inertial energy flux in the four seasons of the North Atlantic and Southern Ocean in the whole year, resulting in significant internal wave mixing. However, in the western Pacific and the north of 40°N in the North Pacific, the spatial distribution of internal wave-induced mixing are inconsistent with the wind-induced near-inertial energy flux. Instead, it follows the spatial distribution of eddy kinetic energy since internal wave-driven mixing can be also regulated by eddies. In terms of time, the strongest internal wave-induced mixing of global occurs from December to February, followed by September to November and March to May, and June to August. This is consistent with the seasonal variation of global wind-induced near-inertial energy flux. In the northern hemisphere, the wind-induced near-inertial energy flux and mixing are the strongest in winter, while the weakest in summer. In the southern hemisphere, the variation of wind-induced near-inertial energy flux and mixing over four seasons is inconsistent. However, the seasonal cycles of mixing and wind-induced near-inertial energy flux in the northern and southern hemispheres are roughly consistent, especially in the North Atlantic, where the wind-induced near-inertial energy flux and mixing match well.

In December 2023, the project “Construction of Regional Deep-Argo Observation Network” sponsored by Laoshan Laboratory deployed a domestically-produced HM4000 profiling float with the maximum profiling depth of 4 000 m (the World Meteorological Organization number is 2902895) in the Philippine Sea, which was equipped with an RBRargo³ deep 6k Temperature-Conductivity-Depth (CTD) sensor produced by RBR, Canada. It was found that the salinity observation data reported by the float exhibited a systematic deviation compared to the shipboard CTD and climatological salinity. In order to correct the salinity data of the float, the conductivity slope of the RBR CTD was calculated by using bottle salinity measured by the Autosal 8400B salinometer and salinity measurements from the shipboard CTD cast. Salinity profiles of the float were then calibrated, and the calibrated salinity was found to be basically consistent with the nearby float and the climatological data. With the implementation of the “Construction of Regional Deep-Argo Observation Network” project, an increasing number of domestically-produced deep Argo floats will be deployed. Compared to the Core Argo floats that measure temperature and salinity profiles in the upper 2 000 m of the ocean, Deep-Argo requires higher accuracy to resolve smaller variations in deep waters. Currently, technical problems are still found in deep CTD sensors, and improper handling and operation during storage, transportation, and usage of some floats and sensors are inevitable, resulting in large errors in the observations, especially the salinity data. Therefore, this study proposes a method of calibrating Deep-Argo floats’ observation data using in-situ shipboard CTD cast, which can provide essential technical support for quality control of the Deep-Argo floats.

The anaerobic oxidation of methane (AOM) is a pivotal component of elemental cycling within cold seep sediments. This process is usually performed by anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB), which usually exist as symbionts. However, pure cultures of ANME have not yet been obtained, and their slow metabolism hinders further exploration and research into their metabolic characteristics and collaborative mechanisms. In this study, we utilized hybridization chain reaction-fluorescence in situ hybridization (HCR-FISH) technology and high-throughput 16S rRNA gene sequencing to investigate the composition and state of ANME communities at different depths of the sediments in the black microbial mat area of the South China Sea Formosa cold seep. The results showed that ANME-1 and ANME-2 were the dominant groups in the sampled Formosa cold seep sediments. Specifically, ANME-2 was found to form consortia with SRB, while no such associations were detected for ANME-1. This observation suggested that ANME-2 and SRB primarily engage in symbiotic AOM processes, highlighting potential differences in physiological roles and methane metabolism pathways between ANME-1 and ANME-2. Furthermore, in sediment samples of all layers, the diameters of ANME-2/SRB consortia were predominantly concentrated between 3-10 μm. Correlation analysis indicated a significant link between the distribution of consortium diameters and environmental factors such as sulfate concentration in the sediment, underscoring the impact of environmental factors on the growth of ANME/SRB consortia. Additionally, using HCR-FISH, we further discovered the presence of multiple consortium clusters in the Formosa cold seep sediment, characterized by orderly connected and uniform-sized consortium, implying possible connections or cooperative relationships among consortia. This study revealed the presence and distribution patterns of ANME groups and sizes of symbiotic microbial consortia in sediment samples from different depths of the Formosa cold seep, laying the foundation for further understanding methane metabolism mechanisms and ecological functions of different ANME groups in situ cold seep sediments.

Deep-sea sediments and polymetallic nodules are rich habitats for microorganisms. Exploring their community structure and functionality is crucial for understanding microbial genetic resources and their role in mineral formation. Current research on the bacterial diversity and structure within the nodules and surrounding sediments is limited, especially regarding microbial contributions to nodule formation. Using full-length 16S rRNA sequencing, we analyzed the bacterial composition of various nodule types and surrounding sediments in the Pacific Ocean. Scanning electron microscopy and energy dispersive spectroscopy revealed bacterial-like microsphere structures and metal element distribution on their surfaces. The bacterial community composition varied among different nodules and sediments, with Proteobacteria and Bacteroidetes dominating. Functional groups like Shewanella and Colwellia, known for metal oxidation-reduction and biofilm formation, may contribute to nodule formation. These microsphere structures promoted metal aggregation, potentially serving as mineral precipitation sites. This study enhanced our understanding of microbial functions and mineral interactions, crucial for insights into deep-sea biogeochemical cycles and microbial mineralization.

The Zengmu-Beikang Basin, located in the southern South China Sea, was formed under a complex geological background, with a large number of oil and gas reservoirs developed, and various types of fluid flow structures widely distributed. Seismic data indicate that the fluid flow system composed of gas chimneys, faults, tubular channels, mud volcanoes, and mud diapirs in the southern South China Sea may be related to the accumulation of gas hydrates. Seabed seepage and bottom simulating reflection (BSR) indicate the possible existence of gas hydrates. The formation of gas chimneys originates from hydraulic fracturing caused by deep oil-gas accumulation, which transports fluids to shallow areas. The gas chimneys are related to BSR, indicating the enrichment of gas hydrates. Faults developed in deep and were connected to potential source rocks or reservoirs, thus accumulating a large amount of shallow gas and gas hydrates around the faults. Pockmark is also an indicative structure for seabed seepage and an area where cold seepage gas hydrates are usually enriched. The formation of mud volcanoes and mud diapirs not only leads to vertical fluid migration, but also triggers the shallow strata deformation and fault development. Therefore, the development areas of mud volcanoes and mud diapirs are also potential areas for gas hydrate enrichment. In addition, this article uses the volume method to estimate the gas hydrate resources in the Zengmu-Beikang Basin in the southern South China Sea. The results show that the gas hydrate resources in the Zengmu-Beikang Basin are approximately 1.62×10¹³ m³. The Zengmu-Beikang Basin has strong potential for gas hydrate resources and is a region worthy of attention for future gas hydrate exploration activities.

The construction of artificial island will inevitably cause damage to the marine ecological environment while satisfying the land demand. Therefore, evaluating the effects of marine ecological restoration around artificial islands is a key and challenging aspect of island and coastal ecological restoration work. Based on pressure-state-response (PSR) model, the evaluation index system of ecological restoration effect in the sea area around artificial islands was constructed, and the best-worst method (BWM) was used to assign weights to the evaluation indexes, and combined with Bayesian network (BN) to evaluate the ecological restoration effect of the sea area around Riyue Island in Hainan. The results indicated that from 2016 to 2019, under the restoration strategy “natural recovery as the main and artificial restoration as the auxiliary”, the marine ecological environment around Riyue Island had shown some restoration effectiveness. The expected values of the ecological environment quality in the tourism and leisure area, agriculture and fisheries area, and reserve area increased by 32.6%, 31.7%, and 22.7% respectively. Although water environmental pressure and sediment environmental pressure significantly decreased, there was no improvement in the biological conditions. The results of sensitivity analysis showed that the ecological environment quality of the three marine functional zones was less sensitive to sediment indicators, while it was most sensitive to the density of benthic organisms. Therefore, future restoration measures should focus on improving biological ecological indicators. This study provides valuable insights for the evaluation of marine ecological restoration effects.

Based on the satellite altimeter data from January 1993 to December 2021, the least squares method and the ensemble empirical mode decomposition (EEMD) were used to analyze the long-term changes of the sea level in Zhoushan and the adjacent East China Sea and its influencing factors. The study found that the sea level in the study area was generally on an upward trend, and the upward trend was more obvious in the coastal waters on the east side of the Zhoushan Islands. The average linear rate was 0.36±0.10 cm/a, and the upward trend had been somewhat mitigated since 2018. The sea level in the study area showed obvious seasonal differences. Its linear rate was the largest in autumn (0.37±0.12 cm/a), followed by in winter, and slightly smaller in spring and summer (approximately 0.34±0.10 cm/a). The nonlinear change trend over the past 30 years showed that the upward rates in summer and autumn had almost remained unchanged, the upward rate in winter had shown a slowdown trend, and the upward trend in spring had been accelerating. There was a trend of increasing annual amplitude of the sea level in the study area. The long-term changes of the sea level were closely related to the seawater thermal expansion effect caused by temperature and the water increase-decrease effect caused by changes in wind stress.

Coastal estuaries are influenced by terrestrial inputs and usually act as sources of atmospheric carbon dioxide (CO₂), whereas mangrove ecosystems generally serve as sinks of atmospheric CO₂. Therefore, accurately measuring the CO₂ emissions at mangrove estuaries is of great significance for constructing regional and global carbon budgets. Dongzhai Harbor locates in the northeastern of Hainan Island, and connects to the Qiongzhou Strait outward, surrounding by 5 major small rivers. Mangroves are mainly distributed in the west and south of Dongzhai Harbor. This study conducted four field surveys in Dongzhaigang, the surrounding major rivers and the adjacent sea areas in December 2022 (dry season), December 2023 (dry season), May 2022 (wet season) and August 2023 (wet season) respectively. The results show that the surface water partial pressure of CO₂ (pCO₂) presents a decreasing trend from rivers to inner and outer harbor. Temperature, river-sea mixing, and biological respiration jointly affect the spatial distributions of pCO₂ in the dry and wet seasons. The CO₂ flux in wet season (8.8±8.2 mmol·m^-2·d^-1) is greater than that in dry season (3.4±3.6 mmol·m^-2·d^-1), and the annual CO₂ flux (6.1±6.3 mmol·m^-2·d^-1) is lower than that in other tropical mangrove estuaries around the world. This study estimates that the estuarine CO₂ emission could offset about 10.4%~21.9% of the carbon sequestration by plants in Dongzhai Harbor.

Based on the field survey in May 2023 along with data obtained from indoor culture experiments, the distribution characteristics of seawater partial pressure of carbon dioxide (pCO₂) and its main control mechanisms in Daao Bay (coral reefs region) in spring were explored. The pCO₂ in Daao Bay ranged from 412.9 to 555.7 μatm in spring, and the study area acted as a source for atmospheric CO₂ with the average efflux of 0.53±0.90 mmol·m^-2·d^-1. During the survey period, the horizontal distribution of pCO₂ was generally higher in nearshore area than that in offshore zone, which was mainly controlled by biological activities (net respiration) and coastal terrestrial input. In addition, pCO₂ showed significant diurnal variation with a maximum difference of 168 μatm. Diurnal differences in biological activities (photosynthesis and respiration) were the main factors leading to changes in pCO₂, contributing 89.4% and 66.4% of pCO₂ in the reef and non-reef areas, respectively. In comparison, physical processes (temperature and tidal effects) had a weak effect on the pCO₂ dynamic, and the temperature effect contributed 12.7% and 21.5% of pCO₂, which was much lower than that of biological processes. Furthermore, the metabolic activity of corals might increase the pCO₂ in the local (reef area) of Daao Bay and enhance the CO₂ source properties of the sea area.

Global warming has led to rising sea levels and weakened the ability of natural barriers such as coral reefs to withstand extreme disasters like hurricanes and tsunamis. Therefore, artificial barriers, such as seawalls or submerged structures, need to be deployed near coasts to effectively protect shoreline areas. This study aims to investigate the hydrodynamic effects of submerged artificial structures on the propagation and deformation of solitary waves over reefs through numerical simulation. A high-precision wave numerical flume was established using the non-hydrostatic model NHWAVE, and the model was validated with experimental data. The study focused on analyzing the impacts of factors such as incident wave height, reef flat water depth, slope of artificial structure, peak width of artificial structures, and slope of the fore reef on hydrodynamic characteristics of solitary waves. Results show that the presence of submerged artificial structure increases wave reflection coefficients and induces vortex formation between waves and water, and the complex flow field can effectively dissipate part of the incident wave energy, which has a mitigating effect on the amplitude and climbing height of the solitary wave. The results of this study can provide a valuable reference for the design of submerged artificial structures.