Southwest Indian Ridge Research Articles

Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

Several ODP-IODP expeditions drilled oceanic core complexes (OCCs) interpreted as exhumed portions of lower crust close to the ridge axis, and provide the community with invaluable sampling opportunity for further constraining magmatic processes involved in the formation of the slow-spreading lower oceanic crust. ODP Hole 735B presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section that was previously interpreted as a single crustal intrusion. We combined detailed structural and petrographic constraints with whole rock and in situ mineral analyses of this section in order to precisely determine the processes of emplacement, crystallization and melt migration within the lower crust. The lower half of the unit is comprised of alternating olivine gabbros and troctolites showing intrusive contacts, magmatic fabrics and crystal-plastic fabrics. Such structures and primitive lithologies are lacking in the upper half, rather uniform, gabbroic sequence. Whole rock compositions highlight the cumulative character of both lower and upper units and a great compositional variability in the lower sequence, whereas the upper sequence is homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. We show that the whole section, and related geochemical unit, constitutes a single magmatic reservoir, in which the lower unit is formed by stacked primitive sills formed by repeated recharge by primitive melts and melt-present deformation. Recharge led to partial assimilation of the crystallizing primitive cumulates, and hybridization with their interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual hybrid melts differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward reactive porous flow, and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the community with the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust that can potentially be applied to other magmatic lower crust sections.

Trophodynamics at the Longqi hydrothermal vent field and comparison with the East Scotia and Central Indian Ridges

The Longqi vent field, situated on the Southwest Indian Ridge, is ecologically distinct among known hydrothermal vents fields. It hosts a combination of previously unknown species and those shared at species or genus level with other hydrothermal vents on the Central Indian Ridge (CIR) and East Scotia Ridge (ESR). We investigate the size-based and trophodynamics of consumers at Longqi vent field and compared these with ESR and CIR vent fields using stable isotope analysis. Intra-specific variability in δ13C and δ15N values in relationship to shell length was observed in Gigantopelta aegis but absent in Chrysomallon squamiferum. A model-based clustering approach identified four trophic groupings at Longqi: species with the lowest δ13C values being supported by carbon fixed via the Calvin–Benson–Bassham cycle, the highest δ13C values being supported by the reductive tricarboxylic acid cycle and intermediate values potentially supported by a mix of these primary production sources. These clusters were driven by potential differences in resource partitioning. There were also differences in the spread of stable isotope values at the vent field level when comparing Bayesian stable isotope ellipse areas among Longqi, CIR and ESR vent fields. This was driven by a combination of the range in δ13C value of macrofauna, and the negative δ15N values which were only observed at Longqi and CIR vent fields. Many of the shared species or genera showed inter-vent field differences in stable isotope values which may be related to site-specific differences in food sources, geochemistry or potential intra-field competition. This study provides important information on the trophic ecology of hydrothermal vent macrofauna found within an area of seabed that is licensed for seabed mining exploration.

Enormous Lithium Isotopic Variations of Abyssal Peridotites Reveal Fast Cooling and Melt/Fluid‐Rock Interactions

AbstractFast diffusing Li isotopes provide important insights into the “recent” transient events or processes for both modern and ancient times, but questions remain concerning the large Li isotopic variations of mantle peridotites, which greatly hampers their usage as a geochemical tracer. This study investigates in situ Li content and isotopic profiles of the constituent minerals of abyssal peridotites from the Gakkel Ridge and Southwest Indian Ridge. The complicated and large variations of Li isotopic profiles in Clinopyroxene (Cpx) and Orthopyroxene (Opx) indicate Li isotopic disequilibrium at millimeter scale. The negative correlations of a wide range of Li contents (0.5 to 6.5 ppm) and δ7Li values (−10 to +20‰) of olivine, Opx and Cpx grains/relicts, trace element zoning of Cpx, the occurrence of plagioclase, olivine serpentinization along cracks, together with numerical modeling demonstrate the observed Li characteristics to be a manifestation of high‐temperature mineral‐melt Li diffusion during melt impregnation overprinted by low‐temperature mineral‐fluid Li diffusion during dissolution and serpentinization. The preservation of the Li isotopic diffusion profiles requires rapid cooling of 0.3–5°C/year after final‐stage melt impregnation at the Moho boundary, which is consistent with the low temperature at very slow spreadin g ridges caused by conductive cooling. Compared with the well‐studied melt‐rock interaction process, our study indicates that low‐temperature fluid‐rock interaction can induce Li diffusion even in the visibly unaltered mineral relicts of partially altered rocks.

Enhanced hydrothermal activity on an ultraslow-spreading supersegment with a seismically detected melting anomaly

Seafloor hydrothermal venting fields occur on all ocean spreading ridges (OSRs) regardless of spreading rates. However, the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow-spreading OSRs are poorly known. Chinese Dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system, with an array of turbidity sensors and a near-bottom camera, along the ultraslow-spreading Southwest Indian Ridge. Here we report the discovery of multiple hydrothermal plumes overlying segments 28, 29, and 30 between the Indomed and Gallieni fracture zones. From these data, and earlier explorations in segments 25–27, we identify nine active venting sites. The spatial density (Fs, sites/100 km) of active sites along the 394 km of ridge axis in our study area is thus 2.8, nearly 3× higher than predicted by the global trend of Fs for ultraslow OSRs in the InterRidge database. Previous studies concluded that an enhanced magma supply to the central Indomed–Gallieni supersegment 11–8 Ma is now limited to segment 27. Our results indicate that although hydrothermal activity may be most concentrated in segment 27, the discoveries of active venting in segments 25–30 implies the presence of additional magma bodies across a broad extent of the Indomed–Gallieni supersegment.

Petro-geochemical characteristics and origin of the quartz in the lower oceanic crust, example from IODP-Hole U1473A

IODP-Hole U1473A was drilled on the summit of Atlantis bank, Southwest Indian Ridge recovered large amounts of gabbroic rocks including mainly olivine gabbro. Felsic rocks are minor, approximately 1,5% of the total volume, which are comprising significant amount of quartz in some samples. The Ti concentrations and the estimated temperatures of the quartz in veins are relatively high, ranging from 30÷130 ppm and 540÷7000C, coupled with the myrmekitic textures in some veins are unambigeous evidence for the late magmatic origin. In addition to the crystallization mechanism in free spaces, such as crack/ fracture systems during the penetration of SiO2 - saturated magmas; the quartz is also formed by re-precipitation process at the same location leaving behind after the previous olivine in the host gabro has been dissolved.

Subcellular metal distribution in two deep-sea mollusks: Insight of metal adaptation and detoxification near hydrothermal vents

In this study, we determined the concentrations of Cu, Zn, Ni, Cd, Pb and As and their subcellular distributions within the tissues of mussels (Bathymodiolus marisindicus) and snails (Gigantopelta aegis) from two hydrothermal vent regions, i.e., Tiancheng and Longqi, at Southwest Indian Ridge. Mussels collected from the two venting regions showed comparable concentrations for Ni and Pb, but Cu, Zn, Cd and As concentrations were significantly different in mussel gills between the two vent regions. Similar ranges of metal concentrations were found in the snails as those in the mussels, but most of the metals were mainly accumulated in the viscera, except for Ni. Similar subcellular partitioning of Cu, Zn and Cd was documented in different mussel tissues, with cellular debris (50%) being the predominant fraction, followed by equivalent values in other fractions. Lead was distributed in both cellular debris and metal-rich granules (MRG) fraction, whereas Ni was predominantly distributed in MRG (90%). Arsenic was mainly partitioned in cellular debris and metallothionein-like protein. However, deep-sea snails displayed elevated subcellular partitioning of Cu in the organelles (up to 60%) and may be more susceptible to Cu stress than the mussels. Our results demonstrated the metal-specificity of detoxification strategies in these deep-sea hydrothermal vent mollusks, and the mussels may be more adaptable to high metal exposures than the snails at hydrothermal vent.

Complete genome sequence of Alteromonas pelagimontana 5.12T, a marine exopolysaccharide-producing bacterium isolated from hydrothermally influenced deep-sea sediment of eastern Southwest Indian Ridge.

The whole genome of Alteromonas pelagimontana 5.12T, a psychrotolerant deep-sea bacterium isolated from the sediment sample of eastern Southwest Indian Ridge, was sequenced and analysed for understanding its metabolic capacities and biosynthesis potential of natural products. The circular genome contained 4.3Mb with a GC content of 42.6mol%. Genomic data mining revealed a gene cluster for heavy metal resistance (czcABC, acrB, arsR1, copA, nikA, mntH, mntP), exopolysaccharides (EPS; epsCDEFHLM) and polyhydroxyalkanoates (PHA; phbC) production, as well as genes involved in complex polysaccharide degradation. Genes that could allow strain 5.12T to cope with acid stress (ibaG) and heat shock (ibpA, hslR) were observed along with ten chaperone-encoding genes which could possibly play vital role in adaptability of this strain to the hydrothermally influenced environment. Gene clusters for secondary metabolite production such as bacteriocin and arylpolyene were also predicted. Thus, genome sequencing and data mining provided insights into the molecular mechanisms involved in the adaptation to hydrothermally influenced deep-sea environment that could promote further experimental exploration.

Variations in faulting style of the Southwest Indian Ridge (46°–53.5°E): Implications for crustal accretion process at ultraslow-spreading ridges

The variations in faulting style at the Southwest Indian Ridge (SWIR) 46°–53.5°E segment, correspond to different patterns of crustal accretion under different magma supply in an ultraslow-spreading ridge. We use multi-beam and filtered bathymetry to identify the fault characteristics and measure the fraction of magmatic accretion M and axial valley depth D. Four styles of faulting, namely, mode-C, mode-E, one-side detachment, and flip-flop detachment faults, are identified in the study area. The heave of these four faulting styles tends to increase gradually, and these faults can be progressive evolve in order from symmetrical to asymmetrical spreading. The reducing M significantly affects the faulting styles from mode-C to mode-E faults by largening fault heave, resulting in a deeper D at mode-E faults. However, there was no significant difference in M and D from mode-E to one-side detachment faults with the increase of heave. We consider that more magma may preferentially intrude into the asthenosphere at one-side detachment faults. Moreover, the flip-flop detachment faults are developed at the amagmatic segments, constrained by the extreme low M, talc- serpentinization, and dense transform faults.

Chemical composition and petrogenesis of plagioclases in plagioclase-phyric basalts from the Southwest Indian Ridge (51°E)

Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts (PUBs) erupted on the Southwest Indian Ridge (SWIR) (51°E) to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems. The phenocryst cores are generally calcic (An74–82) and are depleted in FeO and MgO. Whereas the phenocryst rims (An67–71) and the plagioclase in the groundmass (An58–63) are more sodic and have higher FeO and MgO contents than the phenocryst cores. The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt, but are likely to have crystallized from a more calcic melt. The enrichment in incompatible elements (FeO and MgO), as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass, are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized. Our results indicate that an evolved melt replenishing under the SWIR (51°E) drives the eruption over a short period of time.

Trace Element and Isotopic Evidence for Recycled Lithosphere from Basalts from 48 to 53°E, Southwest Indian Ridge

AbstractMantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge.

The isotopic (He, Ne, Sr, Nd, Hf, Pb) signature in the Indian Mantle over 8.8 Ma

The eastern Southwest Indian Ridge, between 61° and 67°E, has a very low melt supply and comprises several corridors of nearly amagmatic spreading that expose mantle-derived serpentinized peridotite. More volcanically active ridge portions separate these corridors. He, Ne, Pb, Nd, Hf and Sr isotopes were analyzed in basalt glasses dredged on two types of seafloor: volcanic and ultramafic. Basalts dredged on on-axis ultramafic seafloor tend to be slightly more depleted for heavy radiogenic isotopes and show slightly higher 3He/4He isotope ratios than basalts dredged on volcanic seafloor, with no systematic difference in neon isotope ratios. We propose that both types of basalts are derived from the same mantle source, but that the basalts dredged on ultramafic seafloor are more affected by melt/mantle reactions, which slightly modify their isotopic signatures. Our dataset also includes a few basalts, dredged on off-axis ultramafic seafloor, that range in age between 2.6 and 8.8 Ma. These few and widely-spaced off-axis samples, erupted at the ridge axis, are a rare opportunity to capture the potential geochemical variability of the mantle source in an ultramafic seafloor corridor over 8.8 Ma. This temporal variability appears to be minor compared to the overall range of isotopic variability of the on-axis lavas from the 61°-67°E region.

Anomalous phase association of REE in ferromanganese crusts from Indian mid-oceanic ridges: Evidence for large scale dispersion of hydrothermal iron

Ferromanganese crusts collected at selected locations along the ~7000 km long stretch of Carlsberg, Central Indian and Southwest Indian ridges are studied for bulk geochemical composition and distribution of rare earths and yttrium in major geochemical phases; exchangeable/carbonate, Mn-oxide, Fe-oxyhydroxide and residual silicate. The crusts are dominated by vernadite with traces of quartz and calcite. A high concentrations of major (Fe, Mn), trace (Co, Ni, Cu & Zn) and rare earth elements indicates that these crusts are hydrogenous in nature. However, the negative or no cerium anomalies in the shale normalized REE pattern indicates that some of these crusts are of mixed type (hydrogenous and hydrothermal). Irrespective of their nature (mixed or hydrogenous), the phase distribution of REE-Y shows 70–83% of total REE-Y are associated with Fe-oxyhydroxide phase, whereas only about 17–30% are associated with Mn-oxide phase. Such phase-distribution is expected in mixed type crusts because it is a combination of hydrothermal (where 90% of the total REE-Y is associated with only Fe-oxyhydroxide phase) and hydrogenous processes (where 50% of the total REE-Y is associated with only Fe-oxyhydroxide phase). Conversely, the hydrogenous crusts also shows similar phase-distributions (70–83% association with Fe-oxyhydroxide phase) which is certainly different from typical hydrogenous crusts where both Fe-oxyhydroxide and Mn-oxide phases host equal amounts (50% each). The large scale dispersion of hydrothermal particulate iron along the Indian Ocean ridges is likely to contribute an additional Fe-bound REE-Y pool to these crusts. The positive yttrium anomaly in Mn-oxide phase of all these crusts substantiates the hydrothermal contribution.

Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros

In this work, we examine the tectono-metamorphic evolution of gabbroic rocks of the Atlantis Bank oceanic core complex, South West Indian Ridge, focusing on multistage strain localisation associated with exhumation. We study a sample from the core complex footwall below the depth of seawater hydrothermal fluid-rock interaction. We identify a succession of increasing strain localisation where early solid-state deformation of an olivine gabbro is succeeded by two episodes of melt present deformation resulting in increasing localisation of both strain and melt flux into a narrowing zone.The early melt-absent solid-state deformation occurs dominantly in the dislocation creep regime, and localises strain at the tens of metre scale. This evolved into melt present deformation characterised by (i) grain size reduction by replacement reactions, and (ii) dislocation creep and incipient grain boundary sliding in coarse- and fine-grained parts, respectively. This is based on (1) microstructures indicative of the former presence of melt, (2) a shift to lower calcium plagioclase, and (3) reaction textures involving partial replacement of olivine and diopside by fine grained enstatite and hornblende exhibiting only minor internal deformation features.A narrow (2–3 mm) high strain zone which cuts the earlier shear zone foliation at ~30°, exhibits strong and near complete grain size reduction by melt-rock reaction and tightly spaced foliation within a millimetre-wide high strain zone. Here, strain localisation is contemporaneous with a second melt influx shown by (1) complete olivine replacement by fine grained hornblende and enstatite exhibiting no crystallographic preferred orientation, (2) microstructures indicative of the former presence of melt, and (3) changes in the mineral chemistry of diopside, plagioclase, enstatite and hornblende. We suggest deformation was dominated by melt assisted grain boundary sliding allowing higher volumes of melt flux, a high degree of weakening and deformation focused in the narrow shear zone than during the porous melt flow in the wider, early shear zone.This combined microstructural and microchemical study highlights that the recognition of changes in deformation regime, their time relationships and potential melt related deformation are critical for understanding, and therefore modelling, progressive strain localisation, melt flux and the rheological evolution of oceanic detachment faults.

Seawater versus mantle sources of mercury in sulfide-rich seafloor hydrothermal systems, Southwest Indian Ridge

The Southwest Indian Ridge (SWIR) is an ultraslow-spreading ridge where large hydrothermal fields (HFs) are widely distributed. The HFs differ in geological settings, basement rock compositions and mineral associations, but are commonly associated with massive sulfides and sulfide-rich hydrothermal vents that are rich in mercury (Hg). However, the source of Hg remains not well understood. This is a first report on the concentration and isotopic composition of Hg in sulfides from two large HFs, named Duanqiao and Yuhuang, in the SWIR. Sulfides from Duanqiao and Yuhuang showed elevated Hg concentrations, ranging from 3.5 × 102 to 8.1 × 103 ng/g and 4.4 × 102 to 4.4 × 104 ng/g, respectively, which suggest that seafloor mass sulfide deposits can be an important sink of Hg to the deep marine environment. In both HFs, pyrite (4.4 × 102–4.4 × 104 ng/g) and sphalerite (8.3 × 102–6.0 × 103 ng/g) show higher Hg concentrations than chalcopyrite (3.5 × 102 ng/g), suggesting that the replacement of Fe(II) and Zn(II) by Hg(II) is a major form of Hg incorporation in sulfides. Sulfides from Yuhuang show relatively larger δ202Hg and Δ199Hg ranges of −1.23 to −0.05‰ and −0.10 to 0.20‰, respectively, compared with those from Duanqiao (δ202Hg: −0.63 to −0.12‰; Δ199Hg: 0.02–0.10‰), suggesting that the Hg sources are different in the two HFs. The differences in Δ199Hg suggest the dominance of magmatic/mantle Hg in the Duanqiao HF, but a mixture of magmatic/mantle and seawater Hg to the Yuhuang HF. We thus propose Hg isotopes as a potential source tracer of Hg in HFs and infer that magmatism may not only serve as a direct Hg source in HFs, but may also drive seawater circulation and cause the precipitation of Hg from hydrothermally circulated seawater.

The oxidation of hydrocarbons by diverse heterotrophic and mixotrophic bacteria that inhabit deep-sea hydrothermal ecosystems

Hydrothermal activity can generate numerous and diverse hydrocarbon compounds. However, little is known about the influence of such hydrocarbons on deep-sea hydrothermal microbial ecology. We hypothesize that certain bacteria live on these hydrocarbons. Therefore, in this study, the distribution of hydrocarbons and their associated hydrocarbon-degrading bacteria were investigated at deep-sea hydrothermal vents at the Southern Mid-Atlantic Ridge, the Southwest Indian Ridge, and the East Pacific Rise. A variety of hydrocarbon-degrading consortia were obtained from hydrothermal samples collected at the aforementioned sites after low-temperature enrichment under high hydrostatic pressures, and the bacteria responsible for the degradation of hydrocarbons were investigated by DNA-based stable-isotope probing with uniformly 13C-labeled hydrocarbons. Unusually, we identified several previously recognized sulfur-oxidizing chemoautotrophs as hydrocarbon-degrading bacteria, e.g., the SAR324 group, the SUP05 clade, and Sulfurimonas, and for the first time confirmed their ability to degrade hydrocarbons. In addition, Erythrobacter, Pusillimonas, and SAR202 clade were shown to degrade polycyclic aromatic hydrocarbons for the first time. These results together with relatively high abundance in situ of most of the above-described bacteria highlight the potential influence of hydrocarbons in configuring the vent microbial community, and have made the importance of mixotrophs in hydrothermal vent ecosystems evident.

Progressive Dissolution of Titanomagnetite in High‐Temperature Hydrothermal Vents Dramatically Reduces Magnetization of Basaltic Ocean Crust

AbstractHydrothermal alteration at high‐temperature vents near mid‐ocean ridge is thought to produce pervasive magnetization lows on basaltic ocean crust, but the detailed alteration process is insufficiently documented. Here, we performed microscopic and magnetic analyses on a large set of hydrothermal‐related basaltic samples from the Southwest Indian Ridge. Fresh basalts were chloritized and brecciated during hydrothermal alteration, where titanomagnetite nanoparticle clusters hosted in interstitial glasses were dissolved in the first order, followed by large micron‐scale dendritic particles. Natural remanent magnetization was reduced from 100–101 A/m for fresh basalts to 10−3 A/m for fully altered basalts. Hydrothermal deposits acquired a chemical remanent magnetization of 10−2 A/m. Our results link direct magneto‐mineralogical observations to geophysical interpretations, which is important in understanding seafloor hydrothermal circulation and mid‐ocean ridge geodynamics.

Where Marine Protected Areas would best represent 30% of ocean biodiversity

The IUCN (the International Union for Conservation of Nature) World Conservation Congress called for the full protection of 30% of each marine habitat globally and at least 30% of all the ocean. Thus, we quantitatively prioritized the top 30% areas for Marine Protected Areas (MPAs) globally using global scale measures of biodiversity from the species to ecosystem level. The analysis used (a) Ecosystems mapped based on 20 environmental variables, (b) four Biomes (seagrass, kelp, mangrove, and shallow water coral reefs) plus seabed rugosity as a proxy for habitat, and (c) species richness within each biogeographic Realm (indicating areas of species endemicity), so as to maximise representivity of biodiversity overall.We found that the 30% prioritized areas were mainly on continental coasts, island arcs, oceanic islands, the southwest Indian Ridge, the northern Mid-Atlantic Ridge, the Coral Triangle, Caribbean Sea, and Arctic Archipelago. They generally covered 30% of the Ecosystems and over 80% of the Biomes. Although 58% of the areas were within countries Exclusive Economic Zones (EEZ), only 10% were in MPAs, and <1% in no-take MPAs (IUCN category Ia). These prioritized areas indicate where it would be optimal to locate MPAs for recovery of marine biodiversity within and outside country's EEZ. Our results thus provide a map that will aid both national and international planning of where to protect marine biodiversity as a whole.

Deep high-temperature hydrothermal circulation in a detachment faulting system on the ultra-slow spreading ridge

Coupled magmatic and tectonic activity plays an important role in high-temperature hydrothermal circulation at mid-ocean ridges. The circulation patterns for such systems have been elucidated by microearthquakes and geochemical data over a broad spectrum of spreading rates, but such data have not been generally available for ultra-slow spreading ridges. Here we report new geophysical and fluid geochemical data for high-temperature active hydrothermal venting at Dragon Horn area (49.7°E) on the Southwest Indian Ridge. Twin detachment faults penetrating to the depth of 13 ± 2 km below the seafloor were identified based on the microearthquakes. The geochemical composition of the hydrothermal fluids suggests a long reaction path involving both mafic and ultramafic lithologies. Combined with numerical simulations, our results demonstrate that these hydrothermal fluids could circulate ~ 6 km deeper than the Moho boundary and to much greater depths than those at Trans-Atlantic Geotraverse and Logachev-1 hydrothermal fields on the Mid-Atlantic Ridge.

Connectivity between seamounts and coastal ecosystems in the Southwestern Indian Ocean

Understanding larval connectivity patterns is critical for marine spatial planning, particularly for designing marine protected areas and managing fisheries. Patterns of larval dispersal and connectivity can be inferred from numerical transport models at large spatial and temporal scales. We assess model-based connectivity patterns between seamounts of the Southwestern Indian Ocean (SWIO) and the coastal ecosystems of Mauritius, La Réunion, Madagascar, Mozambique and South Africa, with emphasis on three shallow seamounts (La Pérouse [LP], MAD-Ridge [MR] and Walters Shoal [WS]). Using drifter trajectory and a Lagrangian model of ichthyoplankton dispersal, we show that larvae can undertake very long dispersion, with larval distances increasing with pelagic larval duration (PLD). There are three groups of greater connectivity: the region between the eastern coast of Madagascar, Mauritius and La Réunion islands; the seamounts of the South West Indian Ridge; and the pair formed by WS and a nearby un-named seamount. Connectivity between these three groups is evident only for the longest PLD examined (360 d). Connectivity from seamounts to coastal ecosystems is weak, with a maximum of 2% of larvae originating from seamounts reaching coastal ecosystems. Local retention at the three focal seamounts (LP, MR and WS) peaks at about 11% for the shortest PLD considered (15 d) at the most retentive seamount (WS) and decreases sharply with increasing PLD. Information on PLD and age of larvae collected at MR and LP are used to assess their putative origin. These larvae are likely self-recruits but it is also plausible that they immigrate from nearby coastal sites, i.e. the southern coast of Madagascar for MR and the islands of La Réunion and Mauritius for LP.

Nearest vent, dearest friend: biodiversity of Tiancheng vent field reveals cross-ridge similarities in the Indian Ocean.

Biodiversity of hydrothermal vents in the Indian Ocean, particularly those on the Southwest Indian Ridge (SWIR), are still relatively poorly understood. The Tiancheng field on the SWIR was initially reported with only a low-temperature diffuse flow venting area, but here we report two new active areas, including a chimney emitting high-temperature vent fluids. Biological sampling in these new sites doubled the known megafauna and macrofauna richness reported from Tiancheng. Significantly, we found several iconic species, such as the scaly-foot snail and the first Alviniconcha population on the SWIR. Tiancheng shares a high proportion of taxa with vents on the Central Indian Ridge (CIR) and lacks a number of key taxa that characterize other vents investigated so far on the SWIR. Population genetics of the scaly-foot snail confirmed this, as the Tiancheng population was clustered with populations from the CIR, showing low connectivity with the Longqi field. Unlike the previously examined populations, scales of the Tiancheng scaly-foot snail were coated in zinc sulfide, although this results only from precipitation. The close connection between Tiancheng and CIR vents indicates that the dispersal barrier for vent endemic species is not the Rodriguez Triple Junction as previously suggested but the transformation faults between Tiancheng and Longqi, warranting further studies on deep currents in this area to resolve the key barrier, which has important implications for biological conservation.