The response relationship between equivalent neutral wind speed anomaly (ENWSA) and sea-air temperature difference anomaly (SATDA) has been analyzed over four typical sea regions, i.e., the Kuroshio Extension, the Gulf Stream, the Brazil-Malvinas Confluence and the Agulhas Return Current. The results show that ENWSA is more sensitive to SATDA than sea surface temperature anomaly (SSTA), which implies that SATDA seems to be a more suitable parameter than SSTA to analyze the mesoscale air-sea interactions. Here, the slope of the linear relation between ENWSA and SATDA is defined as the air-sea coupling coefficient. It is found that the values of the coupling coefficient over the four typical sea areas have obvious seasonal variations and geographical differences. In order to reveal the reason of the seasonal variation and geographical difference of the coupling coefficient, the influences of some environmental background factors, such as the spatially averaged sea surface temperature (SST), the spatially averaged air temperature, the spatially averaged sea-air temperature difference and the spatially averaged equivalent neutral wind speed, on the coupling coefficient are discussed in detail. The results reveal that the background sea-air temperature difference is an important environmental factor which directly affects the magnitude of the coupling coefficients, meanwhile, the seasonal and geographical variations of the coupling coefficient.

A large sand bar develops in the inner Qiantang River Estuary, China. It is a unique sedimentary system, elongating landwards by about 130 km. Based on long-term series of bathymetric data in each April, July, and November since the 1960s, this study investigated the morphological behavior of this bar under natural conditions and the influence of a large-scale river narrowing project (LRNP) implemented in the last decades. The results show that three timescales, namely the seasonal, interannual and decadal timescales, can be distinguished for the sand bar evolution. The first two are related to the seasonal and interannual variations of river discharge. During high discharge seasons or years, erosion took place at the upper reach and sedimentation at the lower reach. Consequently, the bar apex shifted seaward. The opposite development took place during low discharge seasons or years. The decadal timescale is related to LRNP. Due to the implementation of LRNP, the upper reach has experienced apparent erosion and currently a new equilibrium state has been reached; whereas the lower reach has been accumulated seriously and the accumulation still continues. Nonlinear relationships for how the bar apex location and elevation depend on the river discharge over various stages of LRNP have been established. Compared with the earlier stage of LRNP, the bar apex at present has shifted seaward by about 12 km and lowered by about 1 m. The sand bar movement has significant feedback on the hydrographic conditions along the estuary and has practical implications for coastal management.

This study analyzes the signal quality and the accuracy of BeiDou 3rd generation Satellite Navigation System (BDS3) Precise Point Positioning (PPP) in the Arctic Ocean. Assessment of signal quality of BDS3 includes signal to noise ratio (SNR), multipath (MP), dilution of precision (DOP), and code-minus-carrier combination (CC). The results show that, 5 to 13 satellites are visible at any time in the Arctic Ocean area as of September 2018, which are sufficient for positioning. In the mid-latitude oceanic region and in the Arctic Ocean, the SNR is 25–52 dB Hz and the MP ranges from −2 m to 2 m. As the latitude increases, the DOP values show large variation, which may be related to the distribution of BDS satellites. The CC values of signals B1I and BIC range from −5 m to 5 m in the mid-latitude sea area and the Arctic Ocean, which means the effect of pseudorange noise is small. Moreover, as to obtain the external precise reference value for GNSS positioning in the Arctic Ocean region is difficult, it is hard to evaluate the accuracy of positioning results. An improved isotropy-based protection level method based on Receiver Autonomous Integrity Monitoring is proposed in the paper, which adopts median filter to smooth the gross errors to assess the precision and reliability of PPP in the Arctic Ocean. At first, the improved algorithm is verified with the data from the International GNSS Service Station Tixi. Then the accuracy of BDS3 PPP in the Arctic Ocean is calculated based on the improved algorithm. Which shows that the kinematic accuracy of PPP can reach the decimeter level in both the horizontal and vertical directions, and it meets the precision requirements of maritime navigation.

This research investigated eight stations in Clarion-Clipperton Fracture Zone (CCFZ) in the eastern tropical Pacific in 2017 to study the spatial distribution characteristics of nutrients and chlorophyll a (Chl a) concentration, and compared nutrient concentrations and molar ratios with those of other investigations 20 years ago in the same area. The study found that dissolved inorganic nutrient (N, P and Si) concentrations were lowest in the upper layer, and increased from surface to some depths, then they decreased a little to the bottom. N was the limited nutrient factor for the growth of phytoplankton community. Although nutrient concentrations and molar ratios have no obvious changes in 2017 comparing those in 1998−2003, supplemented from the equatorial Pacific, nutrient concentrations in the study area were higher than those in seamounts in the North Pacific and Station ALOHA. Furthermore, this study used Generalized Additive Models (GAMs) to infer the underlying bottom-up factors controlling phytoplankton abundance (Chl a concentration), showing that depth, salinity and ${\rm{PO}}_4^{3 - }{\text -}{\rm{ P}} $ concentration were major factors controlling the growth of phytoplankton community. Furthermore, this study can provide basic data and theoretical support for the development of polymetallic nodule area and its long-term impact assessment on the environment.

Submarine groundwater discharge (SGD) has received increasing attention by studies on coastal areas; however, its effects on biogeochemical zonation have not been investigated to date. The Huanghe River Estuary (HRE) is a world class river estuary with high turbidity, and heavy human regulation. This study investigated how SGD is related to the benthic biogeochemistry of the HRE. Based on the distribution of several parameters (e.g., salinity, temperature, dissolved oxygen (DO) levels, pH, radium isotopes, and nutrients), the HRE was subdivided into six different zones, and the SGD fluxes within each zone were quantified and compared. The highest SGD flux was found in the northwest nearshore zone, where it was more than one order of magnitude higher than in the offshore zone. High SGD resulted in low DO and pH, but high nutrient levels in the benthic boundary layer. The southeast nearshore zone was also characterized by high SGD flux, but benthic waters were more oxic because of the dominating inputs by the Huanghe River. These data suggest that such a zonation would help to understand benthic biogeochemical processes. High SGD may not only contribute to the estuarine nutrient budget, but may also contribute to the formation of hypoxia and acidification.

The Kongsfjorden is highly sensitive region to climate variability, however, the study of gamma-ray radionuclides in related areas is relatively scarce. In this study, the grain size, total organic carbon (TOC), ¹³C_org isotopes, and specific activities of seven gamma nuclides were analysed in surface sediments of the Kongsfjorden in the Arctic during the summer of 2017. The specific activities of ²¹⁰Pb_ex, ¹³⁷Cs, ²³⁸U, ²²⁶Ra, ²²⁸Ra, ²²⁸Th, and ⁴⁰K were 12–256 Bq/kg, 0–3.8 Bq/kg, 25–42 Bq/kg, 24–38 Bq/kg, 22–40 Bq/kg, 22–40 Bq/kg, and 354–738 Bq/kg, respectively, with average values of (121±94) Bq/kg, (2.0±1.2) Bq/kg, (34±6) Bq/kg, (32±4) Bq/kg, (32±6) Bq/kg, (33±6) Bq/kg, and (611±119) Bq/kg. This study observed a significant positive correlation (r=0.845, p<0.05) between TOC and ²¹⁰Pb_ex, highlighting the strong influence of organic matter on the distribution of ²¹⁰Pb_ex. The boundary scavenging of ²¹⁰Pb from the open sea contributed 27.5%–46.2% to the total ²¹⁰Pb_ex in the sediments of the outer Kongsfjorden. The grain size was an important factor affecting the activity distribution of several radionuclides (²³⁸U, ²²⁸Ra, ²²⁸Th, ²²⁶Ra, and ⁴⁰K). The specific activity of ¹³⁷Cs indicated the transport of terrestrial materials from the exposed area of the Kongsfjorden. The sediments in the Kongsfjorden were derived from various material contributions of glacial meltwater debris, glacial rivers, bare soil, atmospheric deposition, and marine sources. This study explains the source of the Kongsfjorden sediment and the distribution characteristics of radionuclides, and illustrateas the main factors affecting the distribution of radionuclides, which provides a reference for the behavior of polar radionuclides in future research.

Compound-specific stable isotope analysis of individual amino acids (CSIA-AA) has been widely used in ecological and biogeochemical studies. It has been proven to be powerful in tracing the diet sources and trophic interactions. However, assessing the N sources of mesopelagic fishes has been inconclusive because the mesopelagic fishes’ unique domain (water depth ranged from 0 to 1000 m) and unresolved nitrogen isotopes of various forms. This study proposes a new method for coupling instruments (ion chromatography and Precon-IRMS) and chemical method of oxidation-reduction of amino acids, and also combined δ¹⁵N of AAs with δ¹³C of fatty acids (FAs) to analyze the trophic interactions of mesopelagic fishes in the South China Sea (SCS). AAs were isolated by ion chromatography with high peak resolution and collected by an automated fraction collector. The chemical method then converted the AAs into N₂O with a robust oxidation yields and suitable molar ratio of NH₂OH to ${\rm {NO}}_2^- $. Finally, the δ¹⁵N of AAs at 20 nmol were measured with a reasonable precision (<0.6‰). With this method, this study report the first batch high precision δ¹⁵N of AAs and δ¹³C of FAs of mesopelagic fishes collected from SCS. Diaphus luetkeni, Chauliodus minimus and Bathygadus antrodes showed similar δ¹³C values of 20:4n-6 (~ −28‰), while Argyropelecus affinis and Stomias had similar values (~ −32‰). These results reflect that mesopelagic fishes had complex diet sources. An increase of 4‰ in δ¹⁵N of glutamic acid (Glu) was found between piscivorous and planktivorous fishes, which might suggest a trophic discrimination factor of mesopelagic fishes in the SCS. This study used δ¹³C of 20:4n-6 to reveal the diet sources of mesopelagic fishes and δ¹⁵N of Glu to clarify trophic level between piscivorous and planktivorous fishes. Thus, this combinative method could therefore ultimately be applied in a variety of deep-sea ecosystem.

A new high-resolution velocity model of the southern Kyushu-Palau Ridge (KPR) was derived from an active-source wide-angle seismic reflection/refraction profile. The result shows that the KPR crust can be divided into the upper crust with the P-wave velocity less than 6.1 m/s, and lower crust with P-wave velocity between 6.1 km/s and 7.2 km/s. The crustal thickness of the KPR reaches 12.0 km in the center, which gradually decreases to 5.0–6.0 km at sides. The velocity structure of the KPR is similar to the structures of the adjacent West Philippine Basin and Parece Vela Basin (PVB), indicating a typical oceanic crust. Isostatic analysis shows that some regional compensation occurs during the loading of the KPR, which implies that the KPR was built mainly by magmatism during the splitting of the Izu-Bonin-Mariana arc and the following back-arc seafloor spreading of the PVB during 30–28 Ma BP. The absence of the thick middle crust (6.0–6.5 km/s) and high velocity lower-crustal layers (7.2–7.6 km/s) suggest that arc magmatism plays a less important role in the KPR formation.

As an interoceanic arc, the Kyushu-Palau Ridge (KPR) is an exceptional place to study the subduction process and related magmatism through its interior velocity structure. However, the crustal structure and its nature of the KPR, especially the southern part with limited seismic data, are still in mystery. In order to unveil the crustal structure of the southern part of the KPR, this study uses deep reflection/refraction seismic data recorded by 24 ocean bottom seismometers to reconstruct a detailed P-wave velocity model along the ridge. Results show strong along-ridge variations either on the crustal velocity or the thickness of the KPR. P-wave velocity model is featured with (1) a crustal thickness between 6–12 km, with velocity increases from 4.0 km/s to 7.0 km/s from top to bottom; (2) high gradient (~1 s⁻¹) in the upper crust but low one (<0.2 s⁻¹) in the lower crust; (3) a slow mantle velocity between 7.2 km/s and 7.6 km/s in the uppermost mantle; and (4) inhomogenous velocity anomalies in the lower crust beneath seamounts. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, this study suggests the southern part of KPR is a thicken oceanic crust rather than a typical arc crust. The origin of low velocities in the lower crust and upper mantle may be related with crustal differentiation, which implies advanced crustal evolution from normal oceanic crust to partly thicken oceanic crust. High velocities in the lower crust are related to the difference in magmatism.

Subducted sediments play an important role in the magmatism at subduction zones and the formation of mantle heterogeneity, making them an important tracer for shallow crustal processes and deep mantle processes. Therefore, ascertaining the chemical compositions of different subduction end-members is a prerequisite for using subducted sediments to trace key geological processes. We reports here the comprehensive major and trace element analyses of 52 samples from two holes (U1414A and U1381C) drilled on the subducting Cocos Ridge segment at the Southern Central American (SCA) subduction zone during Integrated Ocean Drilling Program (IODP) Expedition 344. The results show that the SCA subducting sediments contain 51% (wt%) CaCO₃, 27% (wt%) terrigenous material, 16% (wt%) opal, and 6% (wt%) mineral-bound H₂O⁺. Compared to the global trenches subducting sediment, the SCA subducting sediments are enriched in biogenic elements (Ba, Sr, and Ca), and depleted in high field strength elements (Nb, Ta, Zr, Hf, and Ti) and alkali elements (K, Rb, and Cs). Meanwhile, the sediments in this area were affected by the carbonate crash event, which could have been caused by a ~800 m rise in the carbonate compensation depth at 11 Ma in the Guatemala Basin. The reason for the sedimentary hiatus at Hole U1381C may be the closure of the Panama Isthmus and the collision between the Cocos Ridge and the Middle America Trench. In addition, the sediments from the subducting Cocos Ridge segment have influenced the petrogenesis of volcanic lavas erupted in the SCA.