Philippine Basin Research Articles

Complicated thermo-chemical heterogeneity of the mantle transition zone beneath the Philippine Sea Plate revealed by SS precursors investigation

The Philippine Sea Plate (PSP), a region renowned for its intricate history of multi-stage subduction and back-arc extension, encounters difficulties in elucidating its deep seismic structure, primarily due to the sparse distribution of seismic stations. This study uses over 44,086 traces of SS precursors collected from >1,000 seismic stations over 10–20 years period, to image the mantle transition zone (MTZ) beneath the PSP. With the curvelet denoising technique, we provide high-resolution maps of the depths of the 410 km (D410) and 660 km (D660) discontinuities and the thickness of the MTZ. Notably, we demonstrate a 10–35 km MTZ thickening extending from the West Philippine Basin to the Shikoku Basin, which may be associated with the thermal effect and dehydration of stagnated slabs in the MTZ. Furthermore, the reflection gap and multiple reflectors were both observed in D410 and D660 beneath the Mariana Trench, which suggests that the vertically subducted Pacific plate transports substantial water and non-olivine components into the MTZ in this area. Additionally, a ∼10–25 km MTZ thinning with an abnormally shallow D660 has been observed beneath the Parece Vela Basin and Caroline Plate in the southern PSP, suggesting the potential existence of thermal upwelling from a secondary plume, which may be possibly the tree branch of the Caroline mantle plume in MTZ. Our results provide new seismological constraints on present and past mantle dynamics in and around the PSP.

Unraveling the link between magma and deformation during slow seafloor spreading

Detachment faulting related to oceanic core complexes (OCCs) has been suggested to be a manifestation of slow seafloor spreading. Although numerical models suggest OCCs form under low magma supply, the specific interaction between magmatism and tectonic faulting remains elusive. This paper examines seismic observations detailing the spatiotemporal interactions between magmatism, high-angle faulting, and detachment faulting at a slow-spreading mid-ocean ridge in the West Philippine Basin. We identified a magma-rich spreading phase, indicated by a magmatic top basement and oceanic crust with shallow-penetrating high-angle normal faults. An axial valley reveals an along-strike transition from magmatically-dominated to highly tectonized oceanic crust over a distance of 70 km. Two older OCCs with concave-down fault geometries and a younger OCC with steep-dipping faulting suggest sequential detachments with the same polarity. Our findings suggest: (1) slow seafloor spreading alternates between high-angle faulting with a relatively high magma supply and detachment faulting with a limited magma supply; (2) sequential development of younger detachments in the footwall of its predecessor leads to an asymmetric split in the newly accreted crust; and (3) the life cycle of OCC ends with high-angle faults that overprint the detachment and act as magma pathways, sealing the OCC. Our study captures the dynamic interaction between high-angle and detachment faults and their concurrent and subsequent relationship to magmatic systems. This reveals that strain distribution along strike is critical to OCC formation, thus enriching our understanding beyond conventional considerations such as spreading rates and melt budgets at mid-ocean ridges.

A pressure-triggered isobaric sequential liquid sampling system for full-ocean-depth: Design and field deployment

Collecting pressure-retaining deep-sea liquid samples is important for the research on in-situ environment and life. A pressure-triggered isobaric sequential liquid sampling system (PTISLSS) was developed for the collection of seawater at target depths. The sampling system comprises six independent seawater collection modules, each containing a titanium sample container and a pre-charged accumulator. A motor-driven sampling mechanism was designed to open and close the fluid channels at depth. Theoretical studies have indicated that the mechanism can generate a contact pressures of up to 160 MPa to ensure the sealing of the samples. The relationship between the pressure of pre-charged gas and the volume of fluid sample has been derived considering the effects of seal friction. The sampling system was built for full-ocean-depth sampling operation and the reliability of the system was tested at a pressure of 120 MPa. The field test was conducted in the West Philippine Basin on a recent scientific research cruise. Pressurized seawater from depths of 2000 to 4500 meters was recovered, and the pressure-retaining rate exceeded 93.5%. The developed sampling system presents satisfactory performance and high operational efficiency, and it will have a wide range of applications in the field of large-scale deep-sea sampling.

Geochemical constraints on ridge-plume interaction in the West Philippine Basin

The West Philippine Basin is not only a back-arc basin resulting from the subduction of the paleo-plates, but it has also undergone modification due to intraplate processes. However, the interaction between intraplate processes and back-arc spreading processes remains unclear. In this study, we present geochemical compositions of basaltic lavas from various tectonic sites in the West Philippine Basin. Lavas from the Benham Rise and its adjacent seamount, as well as basement crust samples from the West Philippine Basin, are similar to oceanic island basalts and mid-oceanic ridge basalts, respectively. The lavas from the Central Basin Fault lie between the two aforementioned types. Based on the mixing modeling of geochemical data, morphological structure of the West Philippine Basin, and age data for lavas, we developed a conceptual model for the three-stage tectonic evolution, with the simultaneous influence of back-arc spreading and mantle plume processes.

Origin of Philippine Sea Basins During Subduction Initiation in the Western Pacific

AbstractUnderstanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific.

Along-Strike Morphologic Variations of the Kyushu-Palau Ridge at 13°–17° N and Their Tectonic Implications

The paper provides a detailed description of the topographical distribution characteristics, along-strike morphologic variations, and tectonic implications of the Kyushu-Palau Ridge (KPR) in the 13°–17° N regions. The description is based on multibeam bathymetric data that was recently collected in the mid-southern segment of KPR and its surrounding areas. The findings indicate that the KPR in the studied region comprises several discontinuous seamounts and seamount chains. Along the ridge running from east to west, there is significant variation in the topography of the seafloor. Visible on this ridge are numerous deep-sea depressions and apparent topographical discontinuities. Geological tectonic processes such as plate subduction and seafloor spreading largely determine the topography of the seafloor. During the beginning of the Parece Vela Basin (PVB) spreading, the pre-existing volcanic arcs fractured under east-west extensional stress, resulting in the development of prominent seamounts, sharp ridges, relief alternations, and rift basins in the KPR. The east-west stress was subsequently transmitted to the weak and inactive mid-ocean ridge in the West Philippine Basin (WPB), which was reactivated and resulted in an episode of magma-poor spreading during 30–26 Ma. This episode caused the KPR to move eastward, stretched, and destroyed its pre-existing structure.

Depth and predation regulate consumption of dolphin carcasses in the hadal zone

Natural whale falls and falls of smaller-sized food have been reported for more than 30 years and are known to be important sources of nutrients and organic matter for the seafloor community. However, the associated species composition and ecological processes during such events in the hadal zone were unknown. Therefore, we aimed to determine the impact of depth and predation on the hadal community during the early stages of a food-fall event. This is the first study to investigate the preliminary stages of the two deepest artificial dolphin-fall events in the Philippine Basin (PB) (∼7729 m) and Mariana Trench (MT) (∼8200 m). A total of nine dives were conducted over a period of 86 days (PB) and 50 days (MT) using the deep-sea manned submersible “Fendouzhe.” Our observations in the PB indicated that the first stage (mobile-scavenger) was controlled by the feeding ecology of hadal amphipods and snailfish. In contrast, the absence of predatory snailfish in the MT enhanced the degradation rate of the carcass compared to that in the PB. Most soft tissues were entirely consumed by scavenging amphipods within days of the event in the MT, whereas in the PB, amphipods were observed taking a feeding hiatus on Day 10 to escape snailfish predation. The second stage (enrichment-opportunist), which hosted few grazing faunas, began at different times in each location and overlapped with the mobile-scavenger stage. Dolphin carcasses, being smaller than those of full-grown whales, can only sustain a large community of scavenging amphipods, and indirectly, predatory snailfish. After the first stage, the dispersed organic matter and limited lipid content in dolphin bones were likely insufficient to sustain an active grazing community or the chemosynthetic community that typically follows. We concluded that water depth influences the successional stages and decomposition rate of food falls in the hadal zone by controlling the dynamic relationship between prey and predators. Our study elucidates the ecology of food-fall events in the hadal zone and highlights the key differences in food-fall events at different depths.

Refining the final phase of the seafloor spreading process in the West Philippine Basin through the recalibration of magnetic isochrones

The evolution of the West Philippine Basin (WPB), the largest back-arc basin in the northwestern Pacific margin, remains unclear, mainly due to the lack of accurate magnetic chronological constraints. We integrate a newly acquired shipborne magnetic grid and EMAG2 data to investigate the final phase of seafloor spreading and post-spreading magma-poor extension in the WPB during the Eocene and Oligocene. An ∼800 km-long transect located in the eastern part of the WPB was modeled to recalibrate the magnetic isochrones. The results show that the seafloor spreading direction of the WPB rotated counterclockwise from ∼NE–SW to ∼NNE–SSW at ∼44 Ma (Isochrone C20r) and from ∼NNE–SSW to ∼N–S at ∼39 Ma (Isochrone C18n.1n). After the latter rotation, the spreading became unstable, decreased significantly, fluctuated, and finally ceased at ∼33 Ma (Isochrone C13n). The post-spreading magma-poor extension along the Central Basin Rift (CBR) driven by the initial opening of the Parece Vela Basin (PVB) began ∼4 Myr after the cessation of spreading. In the area east of ∼132°E, the CBR inherited the fossil spreading center, but to the west, it manifests a newly formed rift valley intersecting the segmented fossil spreading center. The destabilization of the spreading process in the marginal basin during its extinction phase was predominantly controlled by the dynamic process and stress field imposed by the adjacent plate boundaries.

Influence of Lee Wave Breaking on Far‐Field Mixing in the Deep Ocean

AbstractBreaking of internal lee waves generated by flow‐topography interaction is an important driving mechanism for abyssal mixing. By assuming that lee‐wave‐driven mixing is a local process, direct measurements in the past mainly focused on the water column over rough topography. In this study, a non‐local role of lee waves on remote mixing is investigated by using mooring observations in the northwestern Philippine Basin and a series of 2‐dimensional high‐resolution numerical simulations. We find that the breaking of lee waves can generate near‐inertial waves (NIWs) which can propagate away from their generation sites. The NIWs have strong vertical shear which can significantly elevate turbulent dissipation and mixing over remote uneven seafloor. Based on the topography perturbations numerical experiments, we find that for the remote gentle topography with inverse Froude number (Fr−1) between 0 and 0.5, the arrival of upstream NIWs can elevate the near‐bottom layer (0–1,000 m above bottom) dissipation rate and vertical diffusivity by 9.2 and 10.4 times, respectively at 30–50 km away from the NIWs source. For the remote rough topography with Fr−1 between 0 and 4, the corresponding increases are 3.5 and 4.4 times, respectively. The mechanism of upstream NIWs to elevate the downstream dissipation and mixing is through strengthening the shear instability over the far‐field topography, which is associated with NIWs' interactions with the topography and the generated lee waves therein. The results of this study will provide a foundation for improving the mixing parameterizations associated with current‐topography interactions.

Unstable geomorphologic fabric impacted by excess magmas and propagating rifts in the central West Philippine Basin

The central West Philippine Basin (WPB) comprises three contrasting geomorphologic provinces, the mantle-plume type Benham Rise in west, the volcanic arc-type Kyushu-Palau Ridge (KPR) at the east limit, and the in-between central rift hosting the deepest (∼7874 m) rifted seafloor. However, their morphic transit and dynamic interactions remain elusive owing to the insufficiency of relevant data. By incorporating new and previous multibeam bathymetry data fully enveloping the central rift, we synthesized a 3-band, 3-segment seafloor fabric of the central WPB and deduced its formation mechanisms. The bathymetry documents prominent across- and along-axis variations and variable landform assemblages, including: 1) overall trends of abyssal hill lineation swing counter-clockwise from ∼N100/105°E at a distance of 100 km from the fossil rift valley to ∼N85/95°E at a distance of ∼30/50 km, then clockwise to ∼N100/140°E in the rift valley, which recognizes three irregularly bands (I, II, and III ripping into II) and a rotatory spreading fabric; 2) the rift valley narrows westward from ∼85 km wide near the KPR to ∼25 km at Centric Deep, and then fades out to further west, synchronously from magmatic to amagmatic then to failed, indicating a northwestward propagating stirred by the KPR volcanism. Moreover, it is found in the middle segment that three major transform faults bend southwesterly into Southern Band I with their concaves pointing to the Benham Rise. Behaving as pseudo-faults, they are likely triggered by the former magmatism of the Benham Rise. Lying just at the intersection of a relict nodal basin and the western tip of the later PR, the Centric Deep might have resulted from overprinting amagmatic extension on former pull-apart. Thus, the geomorphic fabric featuring high spreading instability reflects strong magma-tectonic intervention by excessive magmatism of mantle plume and subduction arc.

Multi‐channel seismic reflection study of tectonic–sedimentary features and subduction initiation in the middle Kyushu–Palau Ridge and adjacent basins

AbstractThe Kyushu–Palau Ridge and adjacent basins are ideal locations for investigating the formation and evolution of marginal seas and initiation of plate subduction. In this study, the tectonic–sedimentary features and crustal structure of the Kyushu–Palau Ridge and adjacent basins were investigated using newly obtained deep seismic reflection and borehole data. The initial mechanism of subduction in the West Philippine Sea and its tectonic evolution are discussed. Two sets of sedimentary strata with different provenances occur in the eastern Philippine Basin. The thickness of the lower strata is variable, and most of the sediment was sourced from island arc volcanism on the Kyushu–Palau Ridge. These strata thicken towards the Kyushu–Palau Ridge, and volcaniclastic rock aprons are developed near the foot of the Kyushu–Palau Ridge. The upper strata have a relatively uniform thickness and comprise fine‐grained, deep‐water marine sediments. The crustal thickness of the West Philippine and Parece Vela Basins is 6–7 km, which is similar to the global average thickness of oceanic crust. The Moho beneath the West Philippine Basin has a broad and undulating form that mimics the thickness of the oceanic crust beneath the sediments. The depth to the Moho decreases towards the Central Basin Spreading Center. The Moho beneath the Parece Vela Basin is planar, which is in contrast to the undulating nature of the oceanic crust base in this area. The West Philippine Basin may have been located at the northern margin of Australia in the Southern Hemisphere during the Mesozoic. It developed gradually in a continental margin arc as a result of Palaeogene inter‐arc and oceanic extension and contains fragments of continental crust. Seismic profiles and drillhole data in the West Philippine Basin reveal that compression occurred during the Eocene. Subduction along the paleo‐Izu–Bonin–Mariana arc may have been caused by the far‐field effects of the India–Asia collision. Subduction was accompanied by lateral propagation and a compressive stress field until the break‐up of the island arc at ca. 30 Ma.

Internal Lee Wave Generation from Geostrophic Flow in the Northwestern Pacific Ocean

Abstract Among the global mapping of lee wave generation, a missing piece exists in the northwestern Pacific Ocean (NPO), which features complex topographies and energetic circulations. This study applies Bell’s theory to estimate and map internal lee waves generated by geostrophic flows in the NPO using Mercator Ocean reanalysis data and the full topographic spectra obtained from the latest synthetic bathymetry product. Unlike the dominant contributions from abyssal hills in the Southern Ocean, multiple topographies, including ridges, rises, and continental margins, result in an inhomogeneous lee wave generation with multiple hotspots in the NPO. The generation rate is generally higher in the Philippine basin and lower in the central Pacific seamounts. Over ridges, the rough topography creates a high potential for triggering lee waves. Over rises and continental margins, the stronger currents at the shallow depths are favorable for lee wave generation. In the Kuroshio extension region, the rough topography and strong currents cause the strongest lee wave generation, with an energy flux reaching 100 mW m−2. By mean–eddy decomposition, it is found that the lee wave hotspots contributed by mean flow are concentrated in specific regions, while those by geostrophic eddies are widely distributed. Geostrophic eddies are the primary contributor to lee wave generation, which account for 74.6% of the total energy transferred from geostrophic flow to lee waves. This study also reveals that tides suppress the lee wave generation by 14%, and geostrophic flow can cause an asymmetric generation of internal tides.

Experimental study of self-locking seal structure based on full ocean depth sediment pressure retaining sampler

The full ocean depth environment is characterized by high pressure, low temperature, and high corrosiveness, making it difficult for the current deep-sea sampling equipment to obtain pressure-retaining sediment samples from the seafloor. A new self-locking seal structure is designed with an automatic centering plus floating valve core structure to ensure that the long needle valve core only carries the axial load and autonomously fits the sealing cone of the pressure retaining device. The novel self-locking seal design makes the sampler seal more reliable under high pressure. We conducted a series of experiments to verify the sealing performance of the self-locking seal structure, and the test results show that the self-locking seal structure has better sealing performance when the pressure difference between the inside and outside is larger. This new self-locking seal structure has been used for the full ocean depth sediment sampler during the TS-21 cruise in 2021 and was tested in situ at a depth of 7700 m in the West Philippine Basin, retrieving pressure-retaining sediment samples with 94 % pressure retention.

Distinctive Features of Monsoon-TC Joint Rainfall over Western North Pacific and its Relationship with the Maritime Continent Thermal Condition

ABSTRACT The East Asian summer monsoon (EASM) and tropical cyclones (TCs) in the Western North Pacific (WNP) are both responsible for the East Asian summer rainfall, yet most studies only examine their rainfall separately. In this study, the East Asian summer rainfall for the past 39-years (1983-2021, May to September) is divided into three categories: monsoon rainfall without TCs’ influence (monsoon-only rainfall), TC rainfall independent of monsoon (TC-only rainfall) and monsoon-TC joint (MS-TC) rainfall. Compared with the other two categories, MS-TC rainfall exhibits distinctive features. During strong MS-TC years, a distinct cyclonic anomaly centre prevails over tropical WNP with anomalous southeasterlies extending from the tropics to the subtropics. Large rainfall centres are located at the west edge of the northern Philippines, the Philippine Basin, and the Korean Peninsula. The WNP Subtropical High (WNPSH) withdrawals eastward, with an eastward extension of the monsoon trough. These circulation configurations provide favourable environmental conditions for more northward movements of TCs, including low-level positive relative vorticity and enhanced vertical motion in WNP. Observational and theoretical analysis results show that anomalous thermal conditions in the southern Maritime Continent (MC) (97.5°−112.5°E, 8°−18°S) in early spring (March to April) can be a precursor for anomalous MS-TC rainfall. Strong MS-TC rainfall is usually preceded by depressed convection and anomalous westerlies near the MC. These anomalies can persist through the following summer and induce the positive feedback of cooling sea surface temperature (SST) in MC and cyclonic anomalies in tropical WNP through the wind-evaporation-SST (WES) effect and local Hadley circulation anomalies. The linear baroclinic model (LBM) experiments demonstrate that enhanced convection in tropical WNP further maintains the anomalous cyclone through Gill's response, which in turn modulates the distribution and amount of MS-TC rainfall.

The vertical structure and intraseasonal variability of the deep currents in the Southern Philippine Basin

Based on the four-mooring array measurements along 130°E over 2015–2020, the vertical structure and variability of deep currents in the central Southern Philippine Basin (SPB) are revealed. The deep currents below 1500 m in the central SPB are characterized by alternating westward and eastward zonal jets with strong intraseasonal variability (ISV). These zonal jets have a weak mean velocity of less than 1.5 cm s−1 but vary with a relatively large amplitude of 10 cm s−1 at the intraseasonal period of 20–120 days. The full-depth zonal velocity in the central SPB has three typical vertical structures that are mainly controlled by the first baroclinic mode, the second and higher baroclinic modes, and the barotropic mode in descending order of occurrence frequency. These structures are vertically coherent below ∼2000–2500 m, below ∼1200 m, and over the whole water column, respectively. Further analysis of numerical ocean modeling results demonstrates that the ISV of deep currents is induced by the deep mesoscale eddy. The deep ISV intensifies when a deep eddy passes by and keeps at a low value when there is no deep eddy activity around. The deep eddy extends to the bottom with the upper boundaries at ∼1500 m below the subsurface layer or at ∼300 m including the subsurface layer. These deep eddies arise primarily from barotropic instability and secondarily from baroclinic instability.

Abyssal Circulation From the Yap‐Mariana Junction to the Northern Philippine Basin

AbstractThe lower deep branch of the Pacific Meridional Overturning Circulation (L‐PMOC) is a crucial element of the ocean's climate and biogeochemical systems through carrying the Lower Circumpolar Water (LCPW). For the first time, the pathway and volume transport of L‐PMOC from the Yap‐Mariana Junction (YMJ) to the Northern Philippine Basin (NPB) are revealed by a six‐mooring array measurement over 2019–2021. The L‐PMOC seasonally intrudes into the western Pacific at the YMJ. Then, it is directed into the West Mariana Basin (WMB) through YMJ‐Northern Channel with 1.41 ± 1.43/0.26 (mean ± standard deviation/total rms error) Sv, and further into the NPB through Kyushu‐Palau Ridge (KPR) Channel with 0.75 ± 0.53/0.18 Sv. Their difference 0.65 ± 1.35/0.28 Sv is the net lateral flux of LCPW into the WMB over the 2.5 yr. Analyses of a data‐assimilative ocean model solution suggest that the L‐PMOC transports through the deep channels are consistent with deep pressure gradients forced mainly by upper ocean processes.

A novel submersible-mounted sediment pressure-retaining sampler at full ocean depth

High-efficiency pressure-retaining sampling technology for obtaining seabed sediments is required for studying marine geological history, the survival principles of marine microorganisms, and the evolution of earth life. In this paper, a novel submersible-mounted sampler capable of collecting pressure-retained samples at a full ocean depth is designed. The structure scheme is first presented, including the sampling unit, pressure-retaining unit, and pressure-compensation unit. The sampling kinematics model is then established, and the influences of pressing velocity, and the length and inner diameter of pressure pipe on the pressing force of the mechanical arm are determined, providing important guidance for the design of the sealing structure. The maximum working depth of the sampler is 11,000 m, the coring diameter of the sampler is 54 mm, the maximum coring depth is 500 mm, and obtained samples can keep close to in-situ pressures. The sampler can be mounted on a submersible and operated using a single mechanical arm. During cruise TS-21 from August to October 2021, the sampler was deployed 4 times at depth of 7700 m in the West Philippine Basin, and the high pressure sediment samples were successfully collected. The pressure change of the samples remained within ±6%, which verified the rationality of the design and the feasibility of this novel submersible-mounted pressure-retaining sampler.

Zunongwangia pacifica sp. nov., isolated from surface seawater of the Western Pacific Ocean.

Zunongwangia is a group of marine bacteria with important industrial application potential and ecological functions. In this study, a Gram-stain-negative, rod-shaped, non-motile, strictly aerobic and bright yellow pigmented bacterial strain within this genus, designated C2-37M9T, was isolated from a surface seawater sample from the Philippine Basin in the Western Pacific Ocean. Strain C2-37M9T grew at 10-44 °C (optimum, 28-30 °C), pH 6-9 (pH 7) and in the presence of 0-12 % NaCl (w/v; 2-3 %). Phylogenetic analysis based on 16S rRNA gene sequences indicated that it belonged to the genus Zunongwangia and had 95.7-98.7 % sequence similarity to all type strains of this genus, with the highest value corresponding to Zunongwangia profunda (98.7 %). Digital DNA-DNA hybridization, average nucleotide identity and average amino acid identity values between strain C2-37M9T and all valid type strains were 27.5-32.3, 83.8-86.7 and 86.9-89.0 %, respectively. The principal fatty acids (>5 %) were iso-C15 : 0, iso-C17 : 0 3-OH, anteiso-C15 : 0, summed feature 9 (C16 : 0 10-methyl and/or iso-C17 : 1 ω9c), iso-C15 : 1 G and summed feature 3 (C16 : 1 ω6c and/or C16 : 1 ω7c). The predominant respiratory quinone was MK-6. The polar lipids were one phosphatidylethanolamine, two unknown glycolipids, three unidentified aminolipids and six unidentified lipids. The genomic DNA G+C content of strain C2-37M9T was 36.7 mol%. Based on phylogenetic results and genomic-based relatedness indices, as well as phenotypic and genotypic characteristics, strain C2-37M9T represents a novel species within the genus Zunongwangia, for which the name Zunongwangia pacifica sp. nov. is proposed. The type strain is C2-37M9T (=MCCC M21534T=KCTC 82852T).

Critical metal enrichment in atypical hydrogenetic ferromanganese nodules: A case study in the Central Basin Ridge of the West Philippine Basin

Typical hydrogenetic ferromanganese nodules, which are formed by colloidal aggregation of hydrated FeMn oxides and scavenge multiple critical metals from seawater, exhibit high Co contents (>0.4 wt%) and low Mn/Fe ratios (<2.5). Based on a systematic study of the microstructures, mineralogy, and elemental, Nd and Hf isotope compositions of 49 nodules from the Central Basin Ridge of the West Philippine Basin, we identified a subtype of hydrogenetic ferromanganese nodules exhibiting lower Co (<0.4 wt%) and Ti contents, higher Pb and REY contents, and higher Fe/Co and Fe/Mn ratios than typical hydrogenetic nodules. These nodules have elemental, Nd and Hf isotope compositions of hydrogenous origin. Geochemical analysis and regional mapping show that such subtype nodules are widely distributed off the continental margin of the adjacent western Pacific and are labeled “atypical hydrogenetic ferromanganese nodules” in this study. Nevertheless, their significant selective enrichment of metal elements in seawater is similar to that of typical hydrogenetic ferromanganese nodules. They both have relatively consistent REY and PGE patterns; although Nd and Os isotopes indicate changes in material sources in historical periods. They also control scavenged-type elements in seawater, such as Co, Mn, Ce, and Pb. Notably, the mineral compositions of Fe-vernadite (43.6 ± 15.5 wt%), 10 Å vernadite (17.2 ± 10.1 wt%) and the abundant detritus in the nodules indicate an oxidized and strong bottom current environment. The lower Co and Ti contents of the nodules can be attributed to dilution caused by higher growth rates or detrital contents. Importantly, Fe, Pb and REY with short-lived residence times can be well recharged through the Asian continent to provide the metal fluxes favorable for the atypical hydrogenetic ferromanganese nodules. Coincidentally, the strong bottom currents hinder further accumulation of detrital components, making the ridge a favorable place for nodule growth since the mid-Miocene.

Abyssal Circulation from the Yap-Mariana Junction to the Northern Philippine Basin

Abyssal Circulation from the Yap-Mariana Junction to the Northern Philippine Basin