Southwest Indian Ridge Research Articles

Crust-scale reactive porous flow revealed by the brown amphibole in the IODP hole U1473A gabbros, Southwest Indian Ridge

Mid-ocean ridge magma reservoirs are mush-dominated systems, where reactive porous flow is a dominant post-cumulus process. However, the length scale of this process is poorly constrained, especially in ultraslow-spreading ridges. Here, we seek to address this question by presenting a comprehensive brown amphibole dataset of gabbros in IODP Hole U1473A drilled on the Atlantis Bank ocean core complex, Southwest Indian Ridge (SWIR). The results suggest a magmatic origin of the brown amphiboles. Intra-grain chemical variations and compositional zoning of these amphibole grains suggest they were crystallized in a closed system. Nevertheless, geochemical modelings indicate the remarkable Zr/Nd ratio variations in brown amphibole can only be explained by assimilation-fractional crystallization processes triggered by the reactive porous flow. Thus, the brown amphibole in olivine gabbros was crystallized from trapped melts residual after earlier melt-mush interaction. In this light, the reactive porous flow should have ceased prior to the brown amphibole precipitation. This, in turn, suggests the stratigraphic variations in amphibole trace elements reflect the final-stage reactive melt migration. The stratigraphic variations in amphibole trace elements reflect a major episode of reactive melt migration through an ∼800-m section upwardly. This implies that the porosity of the entire gabbroic section remained connected, providing a minimum estimate of the size of the melt-present zones in the lower oceanic crust during the spreading of the 57-60°E magmatic segment of SWIR. Our study reveals that large-scale reactive porous flow can occur in the magmatic segments of ultraslow-spreading ridges, and has important implications for the thermal and rheological evolution of the lower oceanic crust.

Assessment of RANS Turbulence Models in Prediction of the Hydrothermal Plume in the Longqi Hydrothermal Field

In this paper, the numerical models are selected to simulate the hydrothermal plume based on the water temperature observation data of the Longqi hydrothermal field in the Southwest Indian Ridge (SWIR). Then, the unsteady Reynolds-averaged Navier–Stokes equations are solved to evaluate the performance of the Realizable k-ε (rke) model and the SST k-ω (sst) model in hydrothermal plume simulation. By comparing the calculated results with the Conductivity Temperature Depth (CTD) observation data and the literature results, the difference in prediction performance between the two models is evaluated. Before the numerical simulation, the optimal mesh parameters are determined by considering the grid independence test. The results show that the relative difference of the maximum plume height calculated by the two models is within 5%. Compared with the CTD 05-2, the rke model calculates the root mean square error of the velocity is 0.5081, which is smaller than that of the sst model. In terms of turbulent viscosity, the rke model is in good agreement with reference value in predicting turbulent viscosity. Therefore, the turbulent viscosity distribution calculated by the rke model is more consistent with the plume development process than that calculated by the sst model. In addition, the two models have the same effect on the prediction of turbulent kinetic energy and plume temperature.

Fe-Cu-Zn Isotopic Compositions in Polymetallic Sulfides from Hydrothermal Fields in the Ultraslow-Spreading Southwest Indian Ridge and Geological Inferences

Submarine hydrothermal sulfides from the ultraslow-spreading Southwest Indian Ridge (SWIR) were sampled from three hydrothermal fields, and the Fe-Cu-Zn isotopic compositions were analyzed in this study. The Fe isotopes ranged from −0.011‰ to −1.333‰. We believe the processes controlling the Fe isotope variability in the hydrothermal systems include the sulfide precipitation process, the initial isotopic composition of the hydrothermal fluid, and the temperature during precipitation. Among these factors, the sulfide precipitation process is the dominant one. The Cu isotope compositions of the sulfides varied from −0.364‰ to 0.892‰, indicating that the hydrothermal fluid preferentially leached 65Cu in the early stages and that hydrothermal reworking led to decreases in the Cu isotopes in the later stages. In addition, because mass fractionation occurred during sulfide precipitation, the Zn isotope variations ranged from −0.060‰ to 0.422‰. Combined with the S isotopic compositions, these results also implied that different Fe-Cu-Zn isotopic fractionation mechanisms prevailed for the different sample types. Based on these results, we are sure that the metallic elements, including Fe, Cu, and Zn, were derived from the mantle in the SWIR hydrothermal field, and the Fe-Cu isotope results indicated that these metallic elements were provided by fluid leaching processes. Using the isotopic fractionation and sulfide results, we calculated that the Fe-Cu-Zn isotopic compositions of the hydrothermal fluid in this field were δ56Fe(fluid): −0.8~0.0‰; δ65Cu(fluid): 0.3~1.3‰; and δ66Zn(fluid): 0~0.48‰.

The complex Rodrigues triple junction migration since ca. 8 Ma: A response to episodic Amsterdam–St. Paul hotspot tail capture by the Southeast Indian Ridge?

AbstractThe mutual intersection of three plate boundaries, so-called triple junctions, has long been recognized as crucial boundaries for unraveling the spatiotemporal motion of tectonic plates. Yet, the dynamic and tectonic processes ruling their migration remain enigmatic. At the Rodrigues triple junction, the Southwest Indian Ridge lengthens northeastward in response to the unsteady linearity of the Southeast Indian Ridge–Central Indian Ridge dueling ridge system. This mechanism is episodically compensated by the southeastward propagation of the Central Indian Ridge and alternating stages of recession and progression of the Southeast Indian Ridge. This has led to an apparent length constancy of the first Southeast Indian Ridge segment over the past ca. 8 Ma. These multiple ridge propagation episodes, from northwest, southwest, and southeast, result in a net northeastward migration of the Rodrigues triple junction. Here, we suggest that its migration since ca. 8 Ma is a consequence of short-term readjustments of its plate boundaries induced by transitory motion changes of the Capricorn plate, driven by episodic push forces exerted from ephemeral captures of the Amsterdam–St. Paul plume tail by the Southeast Indian Ridge at the southeastern part of the Capricorn plate (77.3°E–78.6°E).

Seismic observation of an active detachment faulting system beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets.

Evolution of enhanced magmatism at the ultraslow spreading Southwest Indian Ridge between 46°E and 53.5°E

The Indomed-Gallieni supersegment (46-52°E) of the Southwest Indian Ridge (SWIR) has been of interest as a melt-rich endmember of ultraslow spreading mid-ocean ridges, attributed to enhanced magmatism since 11 Ma. This provides an excellent opportunity to study the evolution and the impact of enhanced magmatism at mid-ocean ridges. Here we combine shipboard bathymetry and gravity data over the SWIR 46-53.5°E. We illustrate that the enhanced magmatism initiated in the western part of the Indomed-Gallieni supersegment with decreasing crustal productions and no propagation. In the eastern part, the enhanced magmatism propagated to the east at decreasing propagating velocities, producing an increasingly thick crust. The present propagation tip is at 51.9°E, ∼30 km to the west of the Gallieni transform fault (TF). The most magmatically-robust area of the propagation has been located near 50.5°E since ∼6 Ma. We hypothesize that a mantle melting anomaly with heterogenous upwelling rates and/or geometry was responsible for the change of the enhanced magmatism. The enhanced magmatism plays significant roles in ridge segmentation and seafloor spreading modes. The degree of along-axis melt focusing was lowered, causing the segmentation to be typical of slow spreading ridges. As magmatism dramatically decreases over 160 km along the ridge axis from the most robust (producing a 9.5-km-thick crust at 50.5°E) to nearly-amagmatic near the Gallieni TF, this ridge section exhibits the full spectrum of ultraslow spreading modes, changing from magmatically-controlled (intense volcanism and almost no faulting) to tectonically-dominated (smooth ridges and extensive peridotites outcrops) modes.

Sulfide metallogenic model for the ultraslow-spreading Southwest Indian Ridge

Sulfide metallogenic model for the ultraslow-spreading Southwest Indian Ridge

Hydroacoustic Study of a Seismic Swarm in 2016–2017 near the Melville Transform Fault on the Southwest Indian Ridge

ABSTRACT Hydroacoustic monitoring has become particularly efficient for studying the low-magnitude seismicity occurring at mid-ocean ridges. In 2016–2017, a seismic swarm occurred near the Melville transform fault of the ultraslow-spreading Southwest Indian Ridge in the Indian Ocean. It comprised 258 events in the land-based International Seismological Centre catalog, extending from June 2016 to March 2017. We examined this seismicity using hydroacoustic records from three to nine hydrophones moored in the southern Indian Ocean from the Hydroacoustic Observatory of Seismicity and Biodiversity in the Indian Ocean (OHASISBIO) temporary network and the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. We detected 27,624 hydroacoustic events spanning 298 days (1 June 2016–25 March 2017) with lower localization and origin time errors. These include several energetic, short-duration impulsive events, which we interpret as due to lava–water interactions on the seafloor. The spatio-temporal distribution of all the detected events does not show clear tectonic mainshock–aftershock sequences and suggests a magmatic origin of the swarm with bursts of seismicity likely caused by dike emplacements.

Alteration enrichment of nitrogen in the gabbroic oceanic crust: Implications for global subducting nitrogen budget and subduction-zone nitrogen recycling

Uptake of nitrogen (N) during hydrothermal alteration of oceanic crust makes the altered oceanic crust (AOC) a significant N reservoir in parallel to seafloor sediments. While N enrichment in the upper (basaltic) oceanic crust during low-temperature alteration has been widely observed, the N characteristics of moderate- to high-temperature altered gabbroic oceanic crust, which accounts for ∼70 vol% of the crustal material in subducting slabs, have been rarely studied. The lack of these data resulted in large uncertainties in calculating the N input flux of global subducting slabs and modeling the N quantities released through the arc and subducted into the deep mantle. To fill this gap, we examined the N concentrations and isotope compositions of 38 altered gabbroic rocks recovered by ODP/IODP drillings from three oceans, i.e., Hole 735B at the Atlantis Bank in the Southwest Indian Ridge, Hole 1309D at the Atlantis massif in the Mid-Atlantic Ridge, and Hole 1415P at the Hess Deep in the East Pacific Rise. All the gabbroic samples show significant N enrichment with mostly positive δ15N values (in average: 7.4±3.8 ppm and +0.6±2.0‰ for Hole 735B, 5.3±2.1 ppm and +1.4±1.3‰ for Hole 1309D and 7.9±1.9 ppm and +2.0±1.8‰ for Hole 1415P), which are comparable to those of altered basalts from global oceanic crust. The N concentrations and δ15N values of these gabbroic rocks can be readily explained by mixing between minor inherited mantle N and mainly secondary N derived from seawater. The secondary N-hosting minerals in altered gabbroic rocks are likely plagioclase, amphibole and chlorite formed during moderate- to high-temperature alteration stages. Using these new data, we obtained a global nitrogen input flux of 20.6-1.3+0.9 × 109 mol·yr−1 to 29.9±2.2 × 109 mol·yr−1 for subducting AOC, in which 50% – 64% is contributed by the altered gabbroic oceanic crust. Integrating with previous estimate of N input flux from seafloor sediments, a total N input flux of 74.9-1.3+0.9 – 84.2±2.2 mol·yr−1 was estimated for global subducting slabs. Mass balance between this input N flux and various estimates of N output flux in global arcs gave a large range (from 47% – 53% to 10% – 20%) for the fraction of slab N to be subducted beyond the sub-arc depth to the deeper mantle. The upper end (47% – 53%) of this range is more consistent with the results from other constraints, such as thermal structural control and comparison between altered gabbros and meta-gabbros.

Correction to: shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

Correction to: shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

Geographical subdivision of Alviniconcha snail populations in the Indian Ocean hydrothermal vent regions

The hairy snails of the genusAlviniconchaare representative deep-sea hydrothermal vent animals distributed across the Western Pacific and Indian Ocean. Out of six known species in the genusAlviniconcha, only one nominal species ofA. marisindicawas found in the Indian Ocean from the Carlsberg Ridge (CR), Central Indian Ridge (CIR) to the northern part of Southwest Indian Ridge (SWIR) and Southeast Indian Ridge (SEIR). Recently, theAlviniconchasnails were found at three new vent fields, named Onnare, Onbada, and Onnuri, in the northern CIR, which promotes a more comprehensive phylogeographic study of this species. Here, we examined the phylogeography and connectivity of theAlviniconchasnails among seven vent fields representing the CR and CIR based on DNA sequence data of a mitochondrialCOIgene and two protein-coding nuclear genes. Phylogenetic inferences revealed that theAlviniconchasnails of the newly found in the northern CIR and two vent fields of Wocan and Tianxiu in the CR were divergent with the previously identifiedA. marisindicain the southern CIR and mitochondrialCOIdata supported the divergence with at least greater than 3% sequence divergence. Population structure analyses based on the three genetic markers detected a phylogeographic boundary between Onnuri and Solitaire that divides the whole snail populations into northern and southern groups with a low migration rate. The high degree of genetic disconnection around the ‘Onnuri’ boundary suggests that theAlviniconchasnails in the Indian Ocean may undergo allopatric speciation. The border may similarly act as a dispersal barrier to many other vent species co-distributed in the CIR. This study would expand understanding the speciation and connectivity of vent species in the Indian Ocean.

Arc signatures in abyssal peridotites and its implications

Investigating mass exchanges between the subduction zones and mid-ocean ridges provides essential clues to understanding recycling process and mantle convection in the Earth's mantle. In this study, we found evidence of recycled subduction signals in the abyssal peridotites from mid-ocean ridges through their extremely low bulk (Dy/Yb)N and bulk Al2O3, which could not be explained by normal decompression melting at mid-ocean ridges, but required a hydrous melting history. We show here that abyssal peridotites from the fast-spreading Pacific Oceanic Ridges (East-Pacific Rise and Pacific-Antarctic Ridge), to the ultraslow-spreading Southwest Indian Ridge and to the slow-spreading Mid-Atlantic Ridge, require increasing amount of slab-derived fluid in their mantle sources. Therefore, the subduction influence on abyssal peridotites is independent of the spreading rates of the overlying ridges. Locations, such as the Fifteen-Twenty Fracture Zone, the Rainbow Massif from the MAR and the Hess Deep from EPR, have abyssal peridotites overlap well with forearc peridotites, suggesting that the prominent characteristics of arc mantle wedge could be preserved in abyssal peridotites when the traveling distance or time is short for the recycled arc mantle migrating in the mantle convection flow. The well-preserved subduction-related signature in ridge areas that are relatively proximal to modern subduction zones imply that mass exchanges between the subduction zones and mid-ocean ridges may be more efficient than previously thought.

Quantifying the influence of magmatism and tectonism on ultraslow-spreading-ridge hydrothermal activity: Evidence from the Southwest Indian Ridge

Hydrothermal activity in mid-ocean ridges (MORs) is an important intermediary for the mass and heat exchange between the ocean and lithosphere. The development of hydrothermal activity on MORs is primarily controlled by coupled magmatic and tectonic activities. In ultraslow-spreading ridges, deep-dipping low-angle normal faults with large offsets, typically detachment faults in the inside corners of ridge offsets, favor the formation of tectonic-related hydrothermal activities, whereas volcanic-related hydrothermal fields are typically developed in neovolcanic zones in this category of the ridge system. However, whether tectonic or magmatic activity is dominant and to what extent they control the formation of hydrothermal activities on ultraslow-spreading ridges remain unclear. Segments in the west and east of the Gallieni transform fault (TF) located in the ultraslow-spreading Southwest Indian Ridge (SWIR), namely, western area (WA) and eastern area (EA), exhibit distinct magma-supply conditions that provide favorable conditions for examining the influence of magmatic and tectonic activities. We generated prediction models for these areas using the spatial analysis of the water depth, minor faults, large faults, ridge axis, nontransform discontinuity (NTD) inside corners, TF inside corners, Bouguer gravity anomaly, magnetic anomalies, and seismic activities. By employing the weights of evidence method, we reported that the formation of seafloor hydrothermal systems in SWIR was primarily correlated to the NTD inside corner, ridge axis, and minor fault (i.e., contrast values (C) of 4.186, 3.727, and 3.482 in WA and 4.278, 3.769, and 3.135 in EA). Furthermore, EA was significantly affected by the TF inside corner (C = 3.501), whereas WA was influenced by large faults (C = 4.062). Our results demonstrated that tectonism was the primary controlling factor in the development of hydrothermal activities in the study area, and the contribution of magmatism was secondary, even in WA, which has a relatively robust magma supply. We delimited prominent prospecting areas at each side based on posterior probability. Our results provided insights into the formation mechanisms of hydrothermal activities and support prospecting in MORs.

A hydrogenotrophic Sulfurimonas is globally abundant in deep-sea oxygen-saturated hydrothermal plumes

Members of the bacterial genus Sulfurimonas (phylum Campylobacterota) dominate microbial communities in marine redoxclines and are important for sulfur and nitrogen cycling. Here we used metagenomics and metabolic analyses to characterize a Sulfurimonas from the Gakkel Ridge in the Central Arctic Ocean and Southwest Indian Ridge, showing that this species is ubiquitous in non-buoyant hydrothermal plumes at Mid Ocean Ridges across the global ocean. One Sulfurimonas species, USulfurimonas pluma, was found to be globally abundant and active in cold (<0−4 °C), oxygen-saturated and hydrogen-rich hydrothermal plumes. Compared with other Sulfurimonas species, US. pluma has a reduced genome (>17%) and genomic signatures of an aerobic chemolithotrophic metabolism using hydrogen as an energy source, including acquisition of A2-type oxidase and loss of nitrate and nitrite reductases. The dominance and unique niche of US. pluma in hydrothermal plumes suggest an unappreciated biogeochemical role for Sulfurimonas in the deep ocean.

Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.

Geochemical Characteristics of Iron in a Sediment Core at 63°40′ E, Eastern Southwest Indian Ridge: Implications on Regional Hydrothermal Activities and Source Origin

Iron is a limiting nutrient in the marine biogeochemical cycle, and hydrothermal processes at mid-ocean ridges are well-known as one of its sources to the water column. However, a major portion of the hydrothermal iron is precipitated near the source and plays an essential role in oceanic elemental cycling. Here, we carried out a detailed study on the geochemical characteristics of Fe, using a sequential chemical extraction protocol, in a short sediment core collected from the eastern Southwest Indian Ridge (SWIR) to understand the iron association in individual mineral phases. Major and trace and rare-earth element concentrations, positive europium anomaly, and rare-earth fractionation show that the source components in the sediment core are composed of biogenic, local mafic, ultramafic, and hydrothermal origin. Solid-phase Fe speciation results indicate that &gt;60% of Fe is associated with the Fe-oxides phase and indicate the hydrothermal plume particulates settled from the water column. A relatively low concentration of Fe associated with the pyrite and silicate (FeRes) phase suggests an erosion of sulphide and silicate minerals from the nearby vent field. The down-core variation reflects the transformation of primary ferrihydrite to more stable oxide mineral goethite/hematite and, to some extent, the formation of silicate minerals.

Two new deep-water species of squat lobsters (Crustacea: Anomura: Galatheoidea) from the Central and Southwest Indian Ridge

The high diversity and endemism of benthic species on seamounts are extensively documented. Although the number of squat lobster species from seamount habitats has increased, species from the Central Indian Ridge (CIR) and Southwest Indian Ridge (SWIR) systems remain largely unknown. The hydrothermally active Indian Ridge system harbours potential sites for future Seafloor Massive Sulphides (SMS) mining. Since seabed mining may create a long-lasting impact on the deep-sea ecosystems, it could harm the unique deep-sea life associated with mineral deposits. This study identified two undescribed squat lobster specimens: one from the genus Munidopsis Whiteaves, 1874 from the CIR system in water depths ranging from 1981 to 2033 m, and one from the genus Typhlonida Macpherson & Baba in Machordom, Ahyong, Andreakis, Baba, Buckley, Garcia-Jimenez, McCallum, Rodriguez-Flores & Macpherson, 2022 from the SWIR system in depths ranging from 2070 to 2404 m. Both new species may be distinguished morphologically and genetically based on the partial mitochondrial (mtCOI) gene. To generate baseline environmental and biodiversity data, efforts for accurate sampling and correct identification of biological species are required.

Microseismicity and lithosphere thickness at a nearly-amagmatic oceanic detachment fault system

Oceanic detachment faults play a central role in accommodating the plate divergence at slow-ultraslow spreading mid-ocean ridges. Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow spreading Southwest Indian Ridge (SWIR) at 64°30’E accommodate ~100% of plate divergence, with mostly ultramafic smooth seafloor. Here we present microseismicity data, recorded by ocean bottom seismometers, showing that the axial brittle lithosphere is on the order of 15 km thick under the nearly-amagmatic smooth seafloor, which is no thicker than under nearby volcanic seafloor or at more magmatic SWIR detachment systems. Our data reveal that microearthquakes with normal focal mechanisms are colocated with seismically-imaged damage zones of the active detachment fault and of antithetic hanging-wall faults. The level of the hanging-wall seismicity is significantly higher than that documented at more magmatic detachments of slow-ultraslow ridges, which may be a unique feature of nearly-amagmatic flip-flop detachment systems.

Biogeography and potential ecological functions of prokaryotes in the hydrothermal and non-hydrothermal field sediments of the Indian Ocean Ridges

A consensus is emerging on marine microbial biogeography. However, knowledge of the biodiversity and biogeographic patterns of deep-sea microbes and the drivers is far from adequate. In this study, we investigated the diversity and biogeography of prokaryotes at the Indian Ocean Ridges (IOR) using 16S rRNA gene and dsrB sequencing combined with bioinformatics analyses. A total of 39 hydrothermal field sediment (HFS) and non-hydrothermal field sediment (NHFS) samples were collected from the Carlsberg Ridge (CR) and Southwest Indian Ridge (SWIR). Seventy-four prokaryotic phyla were detected in the samples, of which 13 phyla were distributed across all samples. The composition of prokaryotic communities diverged between HFSs and NHFSs, but not between the CR and SWIR. Sixteen phyla were differentially enriched between HFSs and NHFSs. The community structure was significantly correlated to calcium, sulfur, total phosphorus, and total nitrogen, the first three of which were the main distinguishing factors of HFSs and NHFSs. Moreover, a weak distance–decay pattern was observed among the NHFS communities. PICRUSt predictions revealed functional discrepancies in carbon and nitrogen metabolism between the HFS and NHFS communities, and dsrB sequencing analysis indicated higher diversity of sulfate reduction genes and taxa in HFSs than in NHFSs. The diversity and relative abundance of corresponding functional taxa and genes suggested that the nitrogen cycle might be centralized in an energy-generating manner to support the microbial community in NHFSs while the sulfur cycle is more important for the HFS community. These findings provide new insights into microbial biogeography and metabolic differentiation along the IOR that are influenced by hydrothermal activity.

Overlapping and fragmentation in the protection and conservation of the marine environment in areas beyond national jurisdiction

Marine areas beyond national jurisdiction (ABNJ) are under the growing threat of cumulative anthropogenic impacts including fishing, shipping, energy extraction, certain forms of marine scientific research, and the imminent deep seabed mining that prefigure a critical scenario in terms of biodiversity loss and environmental degradation. This article offers a contribution to the discussion on the best approaches to effectively implement environmental protection and conservation in ABNJ, also in the light of ongoing intergovernmental negotiations on the conclusion of an agreement implementing the United Nations Convention on the Law of the Sea on the conservation and sustainable use of biological diversity in ABNJ. The paper first analyzes the current legal gaps in the protection and conservation of ABNJ and the tools developed by some regional and universal regimes to preserve vulnerable marine ecosystems. It then presents two case studies, relating to hydrothermal vent fields of the Mid Atlantic Ridge (Lost City) and the South-West Indian Ridge (Longqi field) to discuss the fragmentation of the legal regimes applicable to ABNJ as well as the difficult cooperation among the regional, global and sectoral frameworks involved in their governance. The case studies show that a coordination mechanism, based on mutual recognition of the protection and conservation measures taken by each competent organization in a specific field, is of utmost urgency. Only a more structured system of cooperation among States and international organizations, that the new implementation agreement will hopefully develop, will allow for the identification of the most appropriate tools for the protection of a given marine area from the cumulative impacts of human activities.