We present data pertaining to mineral assemblages and composition of the Central Indian Ocean Basin (CIOB) pumices. Eight groups of pumices were identified considering the presence of phenocrysts of plagioclase, clinopyroxene, orthopyroxene, hornblende and biotite together with the occurrence of quartz and glass. Pigeonite, fayalite and ulvospinelare reported for the first time from these pumices. In the eight groups, the modal percentage of the constituents are phenocrysts 3% to 19% (avg 9.6%), silicic glass 33% to 54% (avg 43%) and the rest is vesicles. Based on the above factors we have identified the possible sources of the CIOB pumices. The mineral compositions of plagioclase, pyroxenes, and biotite of the CIOB pumices were compared with those of Krakatau and Toba. Most of the plagioclase and pyroxene compositions resemble the Haranggoal Dacite Tuff of Toba and Krakatau. Considering the mineral assemblages and compositions, there are pumices which do not correlate to any of the above eruptions and are probably from yet unidentified source/sources. These sources could either be from nearby terrestrial volcanoes or intraplate seamounts present in the CIOB. In a global context, it is viable that petrological characteristics could be used as initial criteria to determine the source of pumices that occur at abyssal depths in the world ocean.

Before the implementation of offshore oil and gas exploitation, it is essential to understand the various factors that influence the stability of submarine sediments surrounding the project. Considering the factors such as cost and operability, it is not feasible to assess the physical-mechanical properties of sediments covering the entire region by borehole sampling. In this study, the correlation between near seafloor seismic amplitude and the mean shear strength of shallow sediments was explored using seismic and core testing data from the northern continental slope area of the South China Sea. Results showed that the mean water content of sediments in the layer up to 12 m below the seafloor (mbsf) gradually increased with increasing water depth, and the mean shear strength tended to decrease rapidly near the 1 000 m depth contour. The near seafloor seismic amplitude could reflect the mean shear strength of sediments in the 12 mbsf layer under seismic frequency of 65 Hz and wave velocity of 1 600 m/s. When the mean shear strength was greater than 10 kPa or the water depth was less than 1 000 m, there was a significant linear positive correlation between mean shear strength and near seafloor seismic amplitude. Otherwise, there was a significant linear negative correlation between mean shear strength and near seafloor seismic amplitude. On the basis of these correlations, the pattern of shear strength was estimated from near seafloor seismic amplitude and mapped. The mean shear strength of sediments above 12 mbsf gradually decreased with increasing water depth in the continental slope area, whereas little change occurred in the continental shelf and the end of the canyon. Within the canyon area, the mean shear strength of sediments was characterized by larger values in both sides of the canyon walls and smaller values in the canyon bottom, which was consistent with the infinite slope stability theory. The study provides a method for using near seafloor seismic amplitude data to guide sediment sampling design, and presents a continuous dataset of sediment strength for the simulation of regional sediment stability.

We observed a subthermocline eddy (STE) with a cold and fresh core during an observation cruise along a transect of 10°S in the southeastern tropical Indian Ocean (SETIO) in December 2017. The vertical scale, speed radius, and maximum swirl velocity of the STE were about 200 m, 55 km, and 0.5 m/s, respectively. The mean Rossby number and Burger number of the STE were then estimated to be about −0.7 and 2.4, indicating the STE was a submesoscale coherent vortex. The STE core water had characteristics of the Indonesian Throughflow (ITF) water and was distinct from that of surrounding areas. By examining Argo float data, another STE was well captured by five successive profiles of the same Argo float. Both STEs showed significant temperature and salinity anomalies at the σ₀=26.0–26.5 kg/m³ surfaces. With the assumption that the low-salinity ITF water parcels could be carried only by surface eddies and the STEs, the Argo profiles, which detected low-salinity ITF water and were located outside a surface eddy, were believed to be inside an STE and were used to analyze the distribution, origin, and generation mechanism of the STE. The results suggested that the STEs carrying ITF water may be generated under topography-current interaction at the eastern coastal waters or under front-induced subduction in the area away from coastal waters. Those STEs may be widely distributed in the SETIO and may play a role in ITF water parcel transport.

Taking the advantage of sun glitter (SG) observed from high-resolution satellites Gaofen-1 (GF-1) and Gaofen-6 (GF-6), a new method named Shield and Vortex 1-2 (SAV1-2) for extracting spatial texture information from sun glitter was established. Sea surface texture detail information around Diaoyu Dao and its affiliated islands was extracted using SAV1-2. Meanwhile, the dynamic characteristics of the water environment were analyzed by combining the water color, temperature and dynamic data. The results show the following information. (1) Stable westward shield effect and eastward Karman vortex streets, extracted from SG, appear upstream (westward) and downstream (eastward) of the Diaoyu Dao and its affiliated islands respectively. (2) The dominant direction of the Kuroshio Current in the Diaoyu Dao and its affiliated islands area is east, the Diaoyu Dao and its affiliated islands interacting with Kuroshio, inducing western shield and eastern shelter and wake. Furthermore, wave intervals of the shield in front of the island range from hundreds to thousands of metres, and extend wider than the island itself. (3) Combining with high-resolution sea surface temperature (SST) and chlorophyll a (Chl a) concentration, the directional changes of water color elements are mutually supported and proved with the stable shield (wave packet) effect in the west and vortex street effect in the east. The change of SST upstream of the island (western shield area) is not significant, the temperature in the downstream shelter area decreases slightly, and the SST in the downstream wake area shows a general trend of obvious decrease. The wake area is dominated by Karman vortex streets in shape, supplementing by vortices. The concentration of Chl a in the west shield of island is as low as that in downstream shelter area, while it increases significantly in the downstream wake area with shape in eddy or ribbon. (4) The SAV methods of extracting sea surface texture detail using SG can be widely used in different sea areas and water quality. This paper can provide reference for the protection and development of Diaoyu Dao and its affiliated islands and its surrounding waters. It is suggested that some kind of current buffer, such as marine wind farm, should be established in the western waters of Diaoyu Dao and its affiliated islands to protect the islands, and marine pasture should be developed in the downstream of eastern waters of Diaoyu Dao and its affiliated islands.

Total dissolved nitrogen (TDN) is an important parameter for assessing the nutrient cycling and status of natural waters. The accurate determination of TDN in natural waters is essential for assessing its contents and distinguishing different forms of nitrogen in the water. The TDN in various systems has been largely documented, and the concentrations of TDN are usually obtained using high-temperature catalytic (HTC) or persulfate oxidation (PO). However, the accuracy of these methods and their suitability for all types of natural waters are still unclear. To explore both methods in-depth, assorted samples were tested, including eight solutions composed of nitrogen-containing compounds (3 dissolved inorganic nitrogen fractions: ${{\rm {NO}}_3^-}$, ${{\rm {NO}}_2^-} $ and ${{\rm {NH}}_4^+} $; 5 organic compounds: EDTA-2Na, vitamin B1, vitamin B12, amino acids, and urea) and 105 natural waters which were collected from an open ocean (Northwest Pacific Ocean, 28), a marginal sea (Yellow Sea, 34), an estuary (Huanghe River mouth, 31), rivers (Huanghe River, 4; Licun River, 4), and precipitations (4 samples). The results showed that heterocycles and molecular dimensions had certain effects on the oxidation efficiency of the PO method but had little effect on HTC. There was no significant difference between the two methods for natural waters, but HTC was more suitable for deep-sea samples with low TDN concentrations (less than 10 μmol/L) and low organic activity. Overall, HTC has a relatively simple measurement process, a high degree of automation, and low error. Therefore, HTC can be recommended to determine the TDN of samples in freshwater and seawater.

The migration and transformation of nitrogen (N) in sediments play an important role in regulating the N concentration and nutrient structures in shallow seas. However, studies of sedimentary N dynamics are rarely focused on carbonate sediments, although these account for about 40% of the continental shelf area. Thus, the regulation mechanisms of the N dynamics in the carbonate sands of coral reefs are not clear. Taking the coral reef area of Weizhou Island, which has a relatively high N concentration, as the research object, we conducted a series of flow-through reactor experiments to investigate the fluxes of different N forms at the interface of sediment and seawater and their regulation mechanism by environmental factors. The fluxes of dissolved inorganic and organic N (DIN and DON) at different stations were −0.39–0.12 mmol/(m²·h) and −0.18–0.39 mmol/(m²·h), respectively. Denitrification (0.11–0.25 mmol/(m²·h)) was closely coupled to nitrification, which was limited by the availability of organic matter and its degradation product (i.e., ${\rm{NH}}_4^+ $). Thus, the excessive ${\rm{NO}}_3^– $ might be reduced to ${\rm{NH}}_4^+ $ by dissimilatory nitrate reduction to ammonium, rather than to N₂ by denitrification. ${\rm{NO}}_3^– $ reduction peaked at intermediate advection rates (96 L/(m²·h)) and flow path lengths (10 cm), but the release of DON also peaked at the same condition. In addition, climate warming would significantly affect sedimentary N dynamics at Weizhou Island. These results may help address the broader issue of the N cycle in coral reef ecosystems under the dual pressure of climate warming and anthropogenic activities, and these results are beneficial to coral reef protection and local ecological management.

Complete mitochondrial genomes (mitogenomes) can provide useful information for phylogenetic relationships, gene rearrangement, and molecular evolution. In the present study, two newly sequenced mitogenomes of Ocypodoidea (Cleistostoma dilatatum and Euplax sp.) were reported for the first time, which are 15 444 bp and 16 129 bp in length, respectively. Cleistostoma dilatatum is the first species in the family Camptandriidae whose complete mitogenome was sequenced. Each mitogenome contains an entire set of 37 genes and a putative control region, but their gene arrangements are largely different. Tandem duplication and random loss model is proposed to account for their gene arrangements. Comparative genomic analyses of 19 mitogenomes clustering in one branch reveal that 18 of them shared the same gene rearrangement, while that of C. dilatatum mitogenome was consistent with the ancestral gene arrangement of Brachyura. The dN/dS ratio analysis shows that all PCGs are evolving under purifying selection. Phylogenetic analyses show that all Macrophalmidae species cluster together as a group, and then form a sister clade with Camptandriidae. Moreover, the polyphyly of three superfamilies (Ocypodoidea, Eriphioidea, and Grapsoidea) is reconfirmed. These findings help to confirm the phylogenetic position of Camptandriidae, as well as provide new insights into the phylogeny of Brachyura.

Acanthogobius ommaturus, which belongs to the family Gobiidae, is a euryhaline and demersal fish that is widely distributed in the coastal areas, harbors, and estuaries of China, D. P. R. Korea and Japan. In this study, the genetic diversity and genetic structure of five geographical populations of A. ommaturus was assessed using the mitochondrial hypervariable region gene and microsatellite markers. The results of the two genetic markers indicated that the A. ommaturus populations had a high level of genetic diversity. The mitochondrial marker detected weak genetic differentiation among populations, and the Neighbor-Joining tree showed that there was no obvious pedigree branches and geographic structure as well. However, population of Zhoushan showed significant genetic differentiation with other populations by microsatellite markers. The population of A. ommaturus has not experienced bottleneck effect recently. We speculated that the Pleistocene climate change and juvenile fish dispersal played an important role in the population differentiation of A. ommaturus.

The quantification of transparent exopolymer particles (TEP) by colorimetric method is of large error and low repeatability, one major reason of which is related to the absence of clear definition and evaluation for part steps of the original method. It is obscure that the 80% sulfuric acid solution, acted as the extraction solution in the determination of TEP, is prepared based on a volume ratio or mass ratio. Furthermore, the change of solubility of recently available Gum Xanthan (GX) from the market means that the original protocol is no longer applicable, and the grinding of GX stock solution with a tissue grinder is replaced by shaking with a rotating shaker in the study to prevent the excessive dissolution of GX. We found that different preparation techniques could result in the varied concentrations of 80% H₂SO₄. The duration of shaking during the preparation of standard solution significantly affected the slope of the calibration curve, which caused different correction results of TEP. The impacts of different extraction solution concentrations and shaking time of GX solution on the quantification of TEP were investigated based on the field sampling and laboratory analysis. The extraction capacities of H₂SO₄ with different concentrations for Alcian Blue were distinct, but had limited effect on the final measuring result of TEP. The change of the standard curve slope came along with the variation of shaking time, which markedly altered the detection limit and calibration result, and the extended shaking time was in favor of the determination of low-concentration TEP. It was suggested that the extraction solution concentration, shaking time and filtration volume of standard solution are required to be well controlled and selected to obtain more accurate results for TEP with different concentrations.

In this study, the nitrogen and oxygen isotope compositions of nitrite in the upper 150 m water column of the Amundsen Sea in the summer of 2019 and 2020 were measured to reveal the distribution and transformation of nitrite in the euphotic zone of the Southern Ocean. We found that primary nitrite maxima (PNMs) are widely present in the Amundsen Sea, where the depth of occurrence deepens from east to west and nitrite concentrations increases. Evidence from dual isotopes suggests that the formation of PNMs in all regions of the Amundsen Sea is dominated by ammonia oxidation. More importantly, the nitrogen and oxygen isotope compositions of nitrite in the Amundsen Sea mixed layer are abnormal, and their depth profiles are mirror symmetrical. Isotopic anomalies exhibit spatial variations, with central surface water having the lowest nitrogen isotope composition (−89.9‰±0.2‰) and western surface water having the highest oxygen isotope composition (63.3‰±0.3‰). Isotopic exchange reaction between nitrate and nitrite is responsible for these isotope anomalies, as both nitrogen and oxygen isotopes have large isotopic fractionation and opposite enrichment effects. This proves that isotopic exchange reaction operates extensively in different regions of the Amundsen Sea. Our study highlights the unique role of dual isotopes of nitrite in deepening the understanding of nitrogen cycle. Further studies on ammonia oxidation and isotopic exchange between nitrate and nitrite are warranted in the future to understand their roles in the nitrogen cycle in the Southern Ocean.