⁹⁰Sr is recognized to be one of most important artificial radionuclides. A huge amount of radioactive substance (e.g., ⁹⁰Sr) was released into marine environment after the Fukushima Nuclear Accident (FNA). High ⁹⁰Sr activity was still observed in the treated wastewater which was stored on site in many tanks. However, ⁹⁰Sr was rarely investigated in marine environment due to its complicated and time-consuming analytical procedure after the FNA, constraining a comprehensive understanding of the fate of ⁹⁰Sr in marine environment. We discussed the ⁹⁰Sr activity and environmental half-life (EHL) in seas surrounding Japan (SSJ) and the South China Sea (SCS) on the basis of previous data and our ⁹⁰Sr data in seawater and marine biotas (e.g., sargassum, shrimp, oyster, mangrove, reef coral) collected from the SCS during 2015−2018. We found that the EHL of ⁹⁰Sr in the SSJ was 15.4 years during 1975−2010. ⁹⁰Sr in the SSJ was significantly elevated after the FNA. Radiation dose rate of ⁹⁰Sr on marine fish was increased by five orders of magnitude after the FNA relative to the baseline before the FNA. As the downstream basin of the North Pacific Subtropical Gyre, the SCS was not identified with noticeable ⁹⁰Sr-derived from the FNA. The EHL of ⁹⁰Sr in the SCS was calculated to be 26.7 years during 1984−2018 based on the compilation of historical ⁹⁰Sr data. The contrasting patterns of the EHL of ⁹⁰Sr and ¹³⁷Cs in the marginal seas and open oceans were attributed to the distinct sources (river input) and sinks (marine biological pump) of ⁹⁰Sr and ¹³⁷Cs. In the context of challenge of ⁹⁰Sr analytical method in marine environment, extremely high ⁹⁰Sr concentration factor (around 1000 L/kg) was observed in the reef coral skeleton after comparing with concentration factors of ⁹⁰Sr in more than 10 kinds of marine biotas. Additionally, reef coral is potential to be a reliable ⁹⁰Sr marine bioindicator with other advantages of easy availability in large quantity, growth at a fixed location, continue record with high resolution, and simple pretreatment. The study of ⁹⁰Sr in reef coral will not only help to reveal distinct sources and sinks of artificial radionuclides in marine environment, but also provide valuable insights to optimization and improvement of standards/guidelines of marine radioactivity monitoring program.

The Bering Sea and western Arctic Ocean, as high production areas, play a key role in the Arctic Ocean biological pump, and are vulnerable to abrupt climate change, especially sea water warming and sea ice melt. Alterations in biological pump can influence the sources and degradation of sedimentary organic matter, and thus can be indicated by fatty acid (FA) content and composition of sediment. FA analysis of surface sediments, collected during the 5^th and 6^th Chinese Arctic Research Expeditions, showed that the total FA of the Chukchi Shelf was exceptionally high ((97.15 ± 55.31) μg/g), while the Bering Basin was the lowest ((15.00 ± 1.3) μg/g), and the Canada Basin, the Chukchi Shelf and the Bering Shelf were intermediate ((88.65 ± 3.52) μg/g, (70.35 ± 11.32) μg/g and (38.28 ± 14.89) μg/g, respectively). Marine FAs (short chain saturated FA + unsaturated FA) accounted for the most abundant (86.82% ± 7.08%), terrestrial FAs (long chain saturated FA) as the second abundant (8.45% ± 6.62%), while bacterial FAs (odd FA) as the least (4.63% ± 2.24%); diatom index (16:1ω9/16:0) was high at the southern and northern Chukchi Shelf (> 0.82) and the Bering Shelf edge (> 0.65), while it was low at the rest areas. These results indicated that: (1) marine source was the major contributor of sedimentary organic matter of the Bering Sea and the western Arctic Ocean, while terrestrial one contributes minor; diatom predominates was the primary producers of the southern and northern Chukchi Shelf and the Bering Shelf edge; percentage of bacterial FAs was remarkably low, comparing with tropical and temperate seas, suggesting a suppressed bacterial activity under low temperature; (2) labile organic matter accumulation rate was extremely high at the Chukchi Shelf, and was extremely sensitive to sea water warming and sea ice melt; (3) chlorophyceae and prymnesiophyceae dominate phytoplankton community at the Canada Basin and the Chukchi Slope. In conclusion, FA of surface sediment can be used to indicate sources and degradation of organic matter in the Bering Sea and the western Arctic Ocean; further, combining with other samples and biomarkers, FA was viable to shed light on the response of biological pump under the abrupt Arctic climate change.

Biogenic elements are the basis of primary production in marine systems. Their cycles in marine environments are affected by various physical, chemical, and biological processes. Understanding the concentration distribution, structural characteristics, and influencing factors of biogenic elements are essential to understand the dynamics of marine ecosystems. In February 2019, a field investigation was conducted in the Shenhu Area of the northern South China Sea and seawater samples were collected to analyze dissolved inorganic nutrients. Combined with the hydrological environment parameters, such as temperature, salinity, chlrophyll a (Chl a), pH and dissolved oxygen (DO), the distribution and controlling factor of nutrient concentration and structure in the Shenhu Area were discussed. Concentrations of each nutrient in the seawaters from 0 m to 30 m were very low, and the nutrient concentrations gradually increased with the increase of depth. At the depth of about 3 000 m, the concentrations of dissolved inorganic nitrogen (DIN), phosphate and silicate reached 38.02 μmol/L, 2.71 μmol/L and 149.07 μmol/L, respectively. Temperature, pH and DO were significantly correlated with nutrients, indicating that environmental factors greatly affected the biogeochemical processes of nutrients. In addition, the concentration of nutrients in the northeast direction of the study area at a depth of 75 m was relatively low and showed a gradual increasing trend in the southwest direction, which may be related to the intrusion of Kuroshio water with high temperature, high salinity and low nutrients. Meanwhile, the difference between the conservative mixing concentrations calculated based on an end-member mixing model and the measured values of nutrients showed that, at 75 m depth, silicate and phosphate were consumed by biological activities, while nitrates were controlled by biological addition. With the increase of phosphate, the DIN at each site increased linearly, but the silicate increased with a power function, indicating that the regeneration rate and recycling efficiency were different for different nutrients. The ratios of N/P in the Shenhu Area showed opposite trends compared with the ratios of Si/N and Si/P. At 0−30 m, the N/P ratios were low and the Si/N and Si/P ratios were high. At 75 m, the N/P ratios increased along with the decrease of Si/N and Si/P ratios possibly because of different biological effects. Below 75 m, the N/P ratios gradually decreased to 14.44, while the Si/N and Si/P ratios gradually increased. All nutrient ratios remained stable below 1 000 m. The calculation results of the N−anomaly showed that nitrogen fixation in seawater above 300 m was stronger than denitrification, and denitrification below 300 m was enhanced. The distribution characteristics of the concentration and structure of nutrients in the Shenhu Area indicated that the Kuroshio intrusion and biological activities significantly affected the biogeochemical process of nutrients in this area.

Global climate change has a profound impact on the world fisheries. Recently, the composition of catch in Northeast Atlantic Ocean has changed significantly. Based on the catch data in the Northeast Atlantic Ocean from 1982 to 2016 provided by the United Nations Food and Agricuture Organization, the diversity of catch, mean trophic level and the variation of principal component were analyzed in time series. Finally, combined with climate and environmental factors (sea surface temperature, SST; sea surface salinity, SSS; sea surface height, SSH; Sea Ice Concentration, SIC; North Atlantic Oscillation, NAO; Arctic Oscillation, AO; Atlantic Multidecadal Oscillation, AMO) in the Northeast Atlantic Ocean, the relationship between catches composition and climate change was explored by generalized additive model (GAM). The results showed that the diversity generally presented a declining trend, it was at a relatively low level during 2002 to 2010. The mean trophic level decreased slowly before 2002, then began to rise sharply. The correlation analysis showed that the changes of these two indicators were related to the environmental factors. The principal component analysis of catch composition showed that the variance of the first principal component was 35.3%, and it also has a close relationship with climatic and environmental factors, which could characterize the change of catch structure under the influence of climate. The results of GAM analysis showed that the orders of the contribution to structure change of catches were in sequence of SST, SSH, SSS, SIC and NAO.

Jumbo squid Dosidicus gigas is the main fishing target by Chinese squid-jigging fisheries, which mainly distributes in the eastern Pacific Ocean. Its habitat can be greatly affected by climatic changes. In this study, we calculated the habitat suitability index (HSI) on the squid fishing ground in the Southeast Pacific Ocean off Peru based on data of sea surface temperature (SST), the anomalies of sea surface height (SSHA) and Niño3.4 index during 1950 to 2015. Spatio-temporal variations in the squid habitat were further examined under the El Niño, the normal and the La Niña conditions. The results showed that variations in SST anomalies (SSTA) and SSHA in the fishing ground were consistent with those of the Niño3.4 index. Cross correlation analysis indicated that both SSTA and SSHA were significantly positively correlated with the Niño3.4 index. However, the HSI values were significantly negatively correlated with the Niño3.4 index. We divided the years between 1950 and 2015 into three types, the El Niño years, the normal years and the La Niña years. During the El Niño years, water temperature became warm, and the SSH increased; these changes led to contracted areas of suitable SST and SSHA, therefore, the range of suitable habitats largely decreased. However, during the normal and the La Niña years, water temperature was cold, the SSH decreased, and ranges of suitable SST and SSHA enlarged; these changes resulted in the increases in suitable habitat for the squid. Furthermore, the latitudinal gravity centers of suitable HSI on the squid fishing ground were significantly positively correlated with the Niño3.4 index. The latitudinal gravity centers of suitable HSI shifted southward in the El Niño years. Our findings suggested that ENSO events significantly affected environmental conditions on the squid fishing ground, and thus affected ranges and latitudinal distribution of suitable habitat for the squid in the Southeast Pacific Ocean off Peru.

Dissolved oxygen is an indispensable element for the survival of marine organisms. With the increase of human activities, the situation of hypoxia in the global coastal waters has become exacerbated, which is considered as an important factor threatening the health of the marine ecosystem. The vertical distributions of water temperature, salinity and density were investigated in the offshore area of Qinhuangdao from May to September in 2017. Additionally, the average monthly oxygen consumption rate was evaluated, and hence the mechanism of hypoxia and acidification was discussed. The results showed that the water column in the studied area was vertically homogenous in May, and no obvious difference for dissolved oxygen (DO) between the surface and bottom layers, with the value larger than 8 mg/L. From June to August, the pycnocline presented in the mid of the water column. In this period, DO and pH in bottom water gradually decreased, and reached 2−3 mg/L for DO and less than 7.8 for pH in the end of August, suggesting significant hypoxia and acidification in this area. However, DO and pH in bottom water sharply increased in September, after the pycnocline disappeared. The result indicated that hypoxia and acidification of Qinhuangdao offshore waters are seasonal. DO was significantly correlated with chlorophyll a and pH, indicating that the hypoxic and acidified processes in the offshore area of Qinhuangdao were local. The evaluated oxygen consumption rate, based on the box model, in bottom water and sediment ranged from 951 mg/(m²·d) to 1193 mg/(m²·d) (mean: 975 mg/(m²·d)) from June to August in 2017. Comprehensive analysis showed that water stratification was the prerequisite for the occurrence of hypoxia and acidification in Qinhuangdao offshore waters, and DO consumption caused by organic matter decomposition was an important reason of hypoxia and acidification in bottom water.

Photochemical properties and photochemical degradation of chromophoric dissolved organic matter (CDOM) were investigated in the upper water of the western Pacific Ocean during the autumn in 2018. The results showed that the absorption coefficient a(320) of CDOM ranged from 0.025 m⁻¹ to 0.64 m⁻¹, with an average of (0.20 ± 0.08) m⁻¹. The a(320) showed relatively lower values in the surface water, which was mainly related to the photobleaching removal of the surface CDOM. The higher values of a(320) were observed in the 100−200 m water layers, which was mainly related to the biological production in the subsurface layer. The tyrosine-like component C1 and the marine humic-like component C2 were identified by FDOM excitation/emission matrix spectroscopy and a parallel factor analysis. The production of C1 was mainly originated from the production of phytoplankton and the degradation of microorganisms, while C2 was mainly originated from the input of the marine humic brought by Kuroshio. Moreover, the absorption loss spectra of CDOM indicated that the UV radiation was responsible for the photodegradation of CDOM. The tyrosine-like component was more susceptible to photodegradation than the marine humic-like component. The results also indicated that photodegradation was the important removal route of CDOM in the western Pacific Ocean.

In this study, we investigated clay mineral assemblages together with other provenance indicators of Core ARC7-LIC retrieved from the southern Alpha Ridge, in order to reveal the changes in sediment deposition, ocean circulation in the western Arctic Ocean, and the development of surrounding ice sheet through the mid-late Pleistocene (～MIS 29). Changes in clay mineral composition in Core ARC7-LIC suggest a transition of Siberian sourced material during MIS 29−13 towards a North American sourced material during MIS 12−1. This transition reflects the change of the circulation in the western Arctic Ocean before and after the Mid-Brunhes Event, which features an amplification of glacial-interglacial cycles in the post Mid-Brunhes Event. An exceptionally high smectite peak characterizes MIS 12, which is inferred to be from North American source. Laurentide ice sheet discharges icebergs and fine material in the western Arctic Ocean since MIS 16. The amplitude of ice sheet growth and decay increases since MIS 12. During MIS 6, 4 and 3, the asynchronous variations of Ca/Al and kaolinite suggest heterogeneous development of Laurentide ice sheet on the Canadian Arctic Archipelago and Alaska-Mackenzie sides.

As an important water quality parameter, the distribution and dynamic change of suspended sediment have a profound impact on the ecology, environment and material circulation of the estuary and the near shore. GF-4 satellite has the ability to observe at any time, can quickly provide a large number of observation data, and has the application potential in water color remote sensing. In order to explore the monitoring effect of GF-4 satellite on suspended sediment in water, takes the Hangzhou Bay as the research area in this paper, constructs suspended sediment concentration inversion model, and uses GOCI satellite to cross verify. The results show that the index model established by using the ratio of remote sensing reflectance of the 5th and 4th band of GF-4 as the remote sensing factor has a high inversion accuracy, with a determination coefficient of 0.92, a root mean square error of 273.6 mg/L and an mean relative error of 17.2%. The cross-validation results show that GF-4 satellite data, as a new remote sensing data source, is similar to the distribution of GOCI satellite inversion suspended sediment concentration in the low concentration region, but the difference increases with the increase of concentration in the high concentration region. The research shows that GF-4 satellite is suitable for high precision inversion in the waters with low suspended sediment concentration and can be applied in most marine areas of China.

In order to solve the problem of spatial distribution of fracture system at the end of methane leakage system, based on the high-resolution three-dimensional seismic data in the north of South China Sea, the spatial structure and distribution characteristics of fractures in bottom simulating reflector (BSR) distribution area are described by using visualization and coherent body technology. The geological genetic types of fractures are described. The relationship between fractures and other types of transport systems on methane gas accumulation is discussed. The fractures in the upper part of BSR interface are far less than those in the lower part of BSR interface, which makes the methane gas supply larger than that in the process of hydrate accumulation. It has a general indicating role in studying hydrate accumulation and detecting methane gas leakage. According to the development scale of fractures, the study area can roughly identify four types of fractures, they are each short fractures, long fractures, fracture bundles, and fracture groups, which enhance the leakage capacity of fluids in turn. These fractures often coexist in multiple types in the stratum, or form a leakage system together with other geological structures. These results and understandings are of great significance to improve the gas hydrate accumulation model and mechanism of methane leakage system in deep-water basin.