The compositional variability of ocean island basalts (OIBs) is thought to reflect partial melting of a lithologically-heterogeneous mantle source dominated by either pyroxenite or peridotite. The Pohnpei Island in Micronesia, which is associated with the Caroline hotspot, is suggested to have been generated from partial melting of a pyroxenite-rich mantle. To examine this hypothesis, we present new major- and trace-element compositions of olivine phenocrysts in basalts from the island. The olivines exhibit large systematic inter- and intra-crystalline compositional variability. In Sample DS1, olivines record compositional zonation, in which cores have relatively high Fo (77–85), Ni (550×10⁻⁶–2 392×10⁻⁶), and Fe/Mn ratios (66–82), whereas rims have lower Fo (71–78), Ni (526×10⁻⁶–1 537×10⁻⁶), and Fe/Mn ratios (51–62). By contrast, olivines within other samples preserve no clear compositional zonation, exhibiting similar or slightly lower Fo values (66–78), Ni contents (401×10⁻⁶–1 268×10⁻⁶), and Fe/Mn ratios (53–69) as the rims of zoned crystals. The distinct chemical contrast between the two different types of olivine suggests they formed in magma chambers at different depths. Analysis using forward petrological modeling and multi-element indicators (Fe/Mn, Zn/Fe, FC3MS (FeO^T/CaO−(3×MgO/SiO₂)), Mn/Zn, and Ni/(Mg/Fe)) of whole-rock samples and high-Fo olivines is inconsistent with a pyroxenite-rich mantle source. We suggest these inconsistencies reflect an influence on the partition coefficients of Ni and Mn between olivine and liquid during melting at variable pressures and temperatures. In addition, magma recharge and mixing within the magmatic plumbing system can change the composition of olivine. We suggest that identification of the mantle source of OIBs in volcanic islands such as the Pohnpei Island using olivine geochemistry should be treated with caution.

The thermocline-sea surface temperature (SST) feedback is the most important component of the Bjerknes feedback, which plays an important role in the development of the air-sea coupling modes of the Indian Ocean. The thermocline-SST feedback in the Indian Ocean has experienced significant decadal variations over the last 40 a. The feedback intensified in the late twentieth century and then weakened during the hiatus in global warming at the early twenty-first century. The thermocline-SST feedback is most prominent in the southeastern and southwestern Indian Ocean. Although the decadal variations of feedback are similar in these two regions, there are still differences in the underlying mechanisms. The decadal variations of feedback in the southeastern Indian Ocean are dominated by variations in the depth of the thermocline, which are modulated by equatorial zonal wind anomalies. Whereas the decadal variation of feedback in the southwestern Indian Ocean is mainly controlled by the intensity of upwelling and thermocline depth in winter and spring, respectively. The upwelling and thermocline depth are both affected by wind stress curl anomalies over the southeastern Indian Ocean, which excite anomalous Ekman pumping and influence the southwestern Indian Ocean through westward propagating Rossby waves.

Oceanic vertical mixing of the lower halocline water (LHW) in the Chukchi Borderland and Mendeleyev Ridge was studied based on in situ hydrographic and turbulent observations. The depth-averaged turbulent dissipation rate of LHW demonstrates a clear topographic dependence, with a mean value of 1.2×10^–9 W/kg in the southwest of Canada Basin, 1.5×10^–9 W/kg in the Mendeleyev Abyssal Plain, 2.4×10^–9 W/kg on the Mendeleyev Ridge, and 2.7×10^–9 W/kg on the Chukchi Cap. Correspondingly, the mean depth-averaged vertical heat flux of the LHW is 0.21 W/m² in the southwest Canada Basin, 0.30 W/m² in the Mendeleyev Abyssal Plain, 0.39 W/m² on the Mendeleyev Ridge, and 0.46 W/m² on the Chukchi Cap. However, in the presence of Pacific Winter Water, the upward heat released from Atlantic Water through the lower halocline can hardly contribute to the surface ocean. Further, the underlying mechanisms of diapycnal mixing in LHW—double diffusion and shear instability—was investigated. The mixing in LHW where double diffusion were observed is always relatively weaker, with corresponding dissipation rate ranging from 1.01×10^–9 W/kg to 1.57×10^–9 W/kg. The results also show a strong correlation between the depth-average dissipation rate and strain variance in the LHW, which indicates a close physical linkage between the turbulent mixing and internal wave activities. In addition, both surface wind forcing and semidiurnal tides significantly contribute to the turbulent mixing in the LHW.

We report on a hexactinellid sponge new to science, Walteria demeterae sp. nov., which was collected from the northwestern Pacific seamounts at depths of 1 271–1 703 m. Its tubular and basiphytous body, extensive lateral processes, numerous oval lateral oscula which are irregularly situated in the body wall, the presence of microscleres with oxyoidal, discoidal and onychoidal outer ends, and the absence of anchorate discohexasters, indicate it belongs to the genus Walteria of family Euplectellidae, which is also supported by molecular phylogenetic evidence from 18S, 28S, 16S rRNA and cytochrome c oxidase subunit I (COI) gene sequences. The unique morphotype, which is structured by a thin and rigid framework of body wall and lateral processes consisting of diactins, characterizes it as a new species. Local aggregations of individuals of this new species coupled with their associated macrofauna in the Suda Seamount are reported, highlighting its functional significance in providing biogenic microhabitats in the deep sea.

The effects of biological heating on the upper-ocean temperature of the global ocean are investigated using two ocean-only experiments forced by prescribed atmospheric fields during 1990–2007, on with fixed constant chlorophyll concentration, and the other with seasonally varying chlorophyll concentration. Although the existence of high chlorophyll concentrations can trap solar radiation in the upper layer and warm the surface, cooling sea surface temperature (SST) can be seen in some regions and seasons. Seventeen regions are selected and classified according to their dynamic processes, and the cooling mechanisms are investigated through heat budget analysis. The chlorophyll-induced SST variation is dependent on the variation in chlorophyll concentration and net surface heat flux and on such dynamic ocean processes as mixing, upwelling and advection. The mixed layer depth is also an important factor determining the effect. The chlorophyll-induced SST warming appears in most regions during the local spring to autumn when the mixed layer is shallow, e.g., low latitudes without upwelling and the mid-latitudes. Chlorophyll-induced SST cooling appears in regions experiencing strong upwelling, e.g., the western Arabian Sea, west coast of North Africa, South Africa and South America, the eastern tropical Pacific Ocean and the Atlantic Ocean, and strong mixing (with deep mixed layer depth), e.g., the mid-latitudes in winter.

A new species of Psychropotidae holothuroid, Benthodytes palauta sp. nov., was collected from the Kyushu-Palau Ridge at a depth of 2 666 m. This new species is characterized by a leathery body wall, red-violet skin, five pairs of dorsal papillae, nineteen pairs of tube feet, and a narrow brim. The internal organs include one Polian vesicle, two tufts of gonads, and no respiratory trees. Ventral ossicles are large and spinous, with crosses of four arms with central bipartite apophyses. Papillae ossicles are crosses with four arms with bipartite apophyses. The dorsal ossicles were few and large, and cross-shaped with four arms and central bipartite apophyses. Tentacle ossicles were large and rod-shaped or slender rods. Gonad ossicles were primary crosses of four arms and brim ossicles were cross-shaped with spines. The phylogenetic analyses of this species support that B. palauta sp. nov. belongs to Benthodytes. Furthermore, the paraphyletic relationships were confirmed; however, a revision of the genus Benthodytes is needed to resolve its phylogenetic relationship.

Seamounts are vulnerable ecosystems in the deep sea and can be heavily impacted by human activities, such as bottom fishing and deep-sea mining. The species composition and distribution patterns of benthic fauna is key information for the designation of marine protected areas and environmental management plans. Three contracts for cobalt-rich crust exploration have been granted to China, Japan and Korea in the Northwest Pacific Ocean by the International Seabed Authority. However, our knowledge of benthic biodiversity in this area is extremely insufficient. During 2013–2020, eight Chinese Ocean Mineral Resources R&D Association (COMRA) cruises were conducted to investigate the benthic assemblages of nine seamounts in this region. In this study, 191 ophiuroids collected from seamounts in the Northwest Pacific were identified into 29 species in 11 families. Ophiacanthidae and Euryalidae were the two most dominant families with 12 and 6 species, respectively. Ophiotomidae and Ophiopyrgidae were represented by two species each, while seven families were represented by only one species. Four species were widely distributed among 4–5 seamounts, and 17 species were found only at a single site. An integrated regional taxonomic dataset of Ophiuroidea was generated and analyzed. A total of 23 and 14 species were obtained from the Magellan Seamount Chain (MSC) and the Marcus-Wake seamounts (MWS), respectively, with 8 species shared between the two seamount groups. The individual-based rarefaction curves did not reach an asymptote, suggesting that the sampling effort was inadequate for either the entire region or each single seamount. Most species distributed in a narrow depth range, and the species composition was different between water depths above and below 2 000 m. Our results greatly improve the understanding of megafaunal biodiversity from seamounts in the Northwest Pacific Ocean, and highlight the necessity of further surveys to provide more robust information for environmental protection and management in this region.

Including significant warming trend, Arctic climate changes also exhibit strong interannual variations in various fields, which is suggested to be related to El Niño and Southern Oscillation (ENSO) events. Previous studies have demonstrated the different impacts on the Arctic of central Pacific (CP) and eastern Pacific (EP) ENSO events, and suggested these impacts are largely of opposite sign for ENSO warm and cold phases. Our results illustrate asymmetrical changes for the cold and warm ENSO events, especially for the La Niña events. Compared to the past frequent basin-wide cooling La Niña events, since the 1980s the cooling center for the La Niña event has strengthened and moved westward along with the increasing frequency for the canonical and CP La Niña events. Contrary to the basin-wide cooling and canonical La Niña events, the frequent CP La Niña events induce significant warming from the Beaufort Sea to Greenland via the convection center moving northward over the western Pacific. Observation analysis and numerical experiments both suggest that the changes in La Niña type may also accelerate Arctic warming.

Sea surface cooling induced by tropical cyclones (TCs) is an important component of air-sea interactions. Using coordinate transformation and composite analysis methods, we examined the interannual variability in TC-induced sea surface cooling (TCSSC) in the South China Sea (SCS). The frequency of surface cooling cases was over 86% and that of surface warming cases was less than 14%. The magnitude of TCSSC was defined as the absolute value of TCSSC. The maximum magnitude of TCSSC occurred on the right side of the TC track, and the mean magnitude of TCSSC decreased by 0.04°C/a from 2006 to 2018. The interannual variability in TCSSC was highly correlated with the TC translation speed and pre-TC mixed layer depth. Notably, TCSSC got enhanced in El Niño years of 2007, 2010, and 2015. The El Niño types were suggested to determine the occurring periods of strong TCSSC via controlling the positions of SCS anticyclones, which brought pre-TC shallow mixed layer and caused strong TCSSC via vertical mixing process during El Niño events. To quantify how the anticyclone influences TCSSC, we need to use mixed layer heat balances model in the next study.

The general features of the Great Whirl (GW) off the Somali Coast in 2017 and its influences on chlorophyll a (Chl a) concentration were studied by using satellite data and model outputs. Results show that GW, which initiated at 7°N, 53°E on June 13, had a lifetime of 153 d with an average amplitude of 16 cm and an average radius of 205 km. After the formation of GW, the concentration of Chl a in the interior of GW showed a downward trend throughout its life cycle, except in early July and mid-October. In early July, the Chl a blooms in the interior of GW were attributed to the combined effect of three processes. They are eddy horizontal transportation, the deepening of the mixed layer caused by the monsoon and eddy pumping, and the upward transportation of nutrients caused by eddy-induced Ekman pumping. In October, the Chl a blooms were probably due to the weakening of GW. During the period, water exchange occurred more frequently across the eddy, thus phytoplanktons were imported into the interior of GW.