Southeast Indian Ridge Research Articles

Microbial ecosystem assessment and hydrogen oxidation potential of newly discovered vent systems from the Central and South-East Indian Ridge.

In order to expand the knowledge of microbial ecosystems from deep-sea hydrothermal vent systems located on the Central and South-East Indian Ridge, we sampled hydrothermal fluids, massive sulfides, ambient water and sediments of six distinct vent fields. Most of these vent sites were only recently discovered in the course of the German exploration program for massive sulfide deposits and no previous studies of the respective microbial communities exist. Apart from typically vent-associated chemosynthetic members of the orders Campylobacterales, Mariprofundales, and Thiomicrospirales, high numbers of uncultured and unspecified Bacteria were identified via 16S rRNA gene analyses in hydrothermal fluid and massive sulfide samples. The sampled sediments however, were characterized by an overall lack of chemosynthetic Bacteria and the presence of high proportions of low abundant bacterial groups. The archaeal communities were generally less diverse and mostly dominated by members of Nitrosopumilales and Woesearchaeales, partly exhibiting high proportions of unassigned Archaea. Correlations with environmental parameters were primarily observed for sediment communities and for microbial species (associated with the nitrogen cycle) in samples from a recently identified vent field, which was geochemically distinct from all other sampled sites. Enrichment cultures of diffuse fluids demonstrated a great potential for hydrogen oxidation coupled to the reduction of various electron-acceptors with high abundances of Hydrogenovibrio and Sulfurimonas species. Overall, given the large number of currently uncultured and unspecified microorganisms identified in the vent communities, their respective metabolic traits, ecosystem functions and mediated biogeochemical processes have still to be resolved for estimating consequences of potential environmental disturbances by future mining activities.

Lithospheric architecture beneath the Amsterdam-St. Paul plateau, Southern Indian Ocean using the integrated gravity, magnetic and seismological study

Evolution of the Amsterdam-St. Paul (ASP) plateau is debated as a consequence of the Kerguelen mantle plume interaction with the South-East Indian Ridge (SEIR) or related with a weaker ASP plume origin. We performed an integrated geophysical approach using gravity and magnetic modeling along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion data to determine the crustal architecture, and upper mantle structure below the plateau. The gravity-magnetic model revealed that the plateau is associated with three crustal layers as basaltic layer, gabbroic layer and underplated material underlain by sedimentary strata. Our models further suggests that beneath the ASP plateau the Moho depth varies from ∼6.6 to 18.0 km. Joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve suggested the Lithosphere-Asthenosphere Boundary at depth of 50 km below the Amsterdam Island. Further seismological result suggests evidence of a Low Velocity Zone below the Moho at depth of 20–36 km which may represent a magma chamber. Our model suggests that the evolution of the ASP plateau is guided by the Amsterdam and St. Paul fracture zones, which acted as a thermal boundary confining the horizontal movement of the ASP plume. We propose that the ASP plateau is not linked with the Kerguelen plume and emphasize origin of the ASP plateau by the interaction of SEIR with the ASP plume.

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).

Structure Formation of the Southeast Indian Ridge at the Early Stages of Development: Physical Modeling

Structure Formation of the Southeast Indian Ridge at the Early Stages of Development: Physical Modeling

Structure Formation of the Southeast Indian Ridge at the Early Stages of Development: Physical Modeling

The authors of the article studied the features of the structure formation of the earth’s crust in the early period of the formation of the Southeast Indian Ridge, associated with the separation of Australia from Antarctica and the advancement of the rift zone to the west within the ancient oceanic lithosphere towards the large magmatic province of Kerguelen, formed by the activity of the plume of the same name. The separation of Australia and Antarctica covered a long period of continental rifting (~160‒80 Ma), which then turned into ultra-slow spreading (~80‒45 Ma), then into slow spreading (~45‒40 Ma) and then into stationary spreading at average rates (after 40 Ma). The advance of the rift zone towards the ancient oceanic lithosphere gave way to the accretion of young oceanic crust on the emerging Southeast Indian spreading ridge. The early stages of development of the young spreading ridge are captured in the modern structural plan of the study region. The advance of the rift zone from the continent into the boundaries of the ancient oceanic lithosphere led to the formation of the Naturalist Plateau and the Bruce Bank near the Antarctic margin. The split of the ancient oceanic lithosphere and the formation of a young crust on the Southeast Indian Ridge led to the formation of conjugated Diamantina and Labuan suture zones, fixing the position of the initial rift split. The transition from ultraslow spreading at the initial stage of oceanic crust formation to stationary spreading with medium velocities is clearly recorded in the change in the irregularity of the accretionary relief. The Southeast Indian Spreading Range westward collided with a large igneous province during the formation of the Kerguelen Plateau and separated the Broken Range from the plateau. The authors carried out physical modeling of the conditions for the occurrence of rifting and spreading processes, as well as structure formation in the region of the Southeast Indian Ridge.

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.

Late Miocene to recent tectonic evolution of the Macquarie Triple Junction

Abstract The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of Macquarie Island. Morphobathymetric, magnetic, and gravity data have been used to understand the evolution of the three accretionary/transform boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault triple junction. The long life (>6 m.y.) of this configuration can be attributed to a rapid increase in spreading asymmetry along the Southeast Indian Ridge segment as it approaches the MTJ, and to transtension along the southernmost strand of the Macquarie–Pacific transform boundary. A major change in plate motion triggered the development of the Macquarie plate at ca. 6 Ma and makes clear the recent evolution of the MTJ, including (1) shortening of the Southeast Indian Ridge segment; (2) formation of the westernmost Pacific-Antarctic Ridge, which increased its length over time; and (3) lengthening of the two transform boundaries converging in the MTJ. The clockwise change of the Pacific-Antarctic motion (ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to the east of the triple junction, with fragmentation of the long-offset Emerald transform fault and its replacement over a short time interval (1–2 m.y.) with closely spaced, highly variable transform offsets that were joined by short ridge segments with time-varying asymmetries in the spreading rates.

Plume–MOR decoupling and the timing of India–Eurasia collision

The debatable timing of India–Eurasia collision is based on geologic, stratigraphic, kinematic, and tectonic evidence. However, the collision event disturbed persistent processes, and the timing of disturbance in such processes could determine the onset of India–Eurasia collision precisely. We use the longevity of Southeast Indian Ridge (SEIR)—Kerguelen mantle plume (KMP) interaction cycles along the Ninetyeast ridge (NER) as a proxy to determine the commencement of India–Eurasia collision. The geochemical signature of the KMP tail along the NER is predominantly that of long-term coupling cycles, that was perturbed once by a short-term decoupling cycle. The long-term coupling cycles are mainly of enriched mid-ocean ridge basalts (E-MORBs). The short-term decoupling cycle is mostly derived from two distinct sources, MOR and plume separately, whereas the KMP is still being on-axis. The onset of India–Eurasia collision led to continental materials recycling into the mantle; hence the abrupt enrichment in incompatible elements at ca. 55 Ma, the MOR–plume on-axis decoupling, and the abrupt slowdown in the northward drift of the Indian plate was induced by the onset of India–Eurasia collision, thereafter MOR–plume recoupled.

Turbulent Mixing Variability in an Energetic Standing Meander of the Southern Ocean

Abstract This study presents novel observational estimates of turbulent dissipation and mixing in a standing meander between the Southeast Indian Ridge and the Macquarie Ridge in the Southern Ocean. By applying a finescale parameterization on the temperature, salinity, and velocity profiles collected from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper 1600 m, we estimated the intensity and spatial distribution of dissipation rate and diapycnal mixing along the float tracks and investigated the sources. The indirect estimates indicate strong spatial and temporal variability of turbulent mixing varying from O(10−6) to O(10−3) m2 s−1 in the upper 1600 m. Elevated turbulent mixing is mostly associated with the Subantarctic Front (SAF) and mesoscale eddies. In the upper 500 m, enhanced mixing is associated with downward-propagating wind-generated near-inertial waves as well as the interaction between cyclonic eddies and upward-propagating internal waves. In the study region, the local topography does not play a role in turbulent mixing in the upper part of the water column, which has similar values in profiles over rough and smooth topography. However, both remotely generated internal tides and lee waves could contribute to the upward-propagating energy. Our results point strongly to the generation of turbulent mixing through the interaction of internal waves and the intense mesoscale eddy field.

Contribution of Viscoelastic Stress to the Synchronization of Earthquake Cycles on Oceanic Transform Faults

AbstractEarthquake clustering can be promoted by local, regional, and remote triggering. The interaction between faults by static and dynamic stress transfer has received much attention. However, the role of quasi‐static stress interaction mediated by viscoelastic flow is still poorly understood. Here, we investigate whether the tight synchronization of moment‐magnitude 6 earthquakes every about 6 years on distant asperities in the Gofar‐Discovery fault system of the East Pacific Rise may be caused by mechanical coupling within the lithosphere‐asthenosphere system. We build a three‐dimensional numerical model of seismic cycles in the framework of rate‐ and state‐dependent friction with a brittle layer overlaying a viscoelastic mantle with nonlinear rheology to simulate earthquake cycles on separate asperities. The brittle section of the West Gofar fault consists of two frictionally unstable 20 km‐long by 5 km‐wide asperities separated by a velocity‐strengthening barrier, consistent with seismic observations, allowing stress transfer by afterslip and viscoelastic relaxation. We find that viscoelastic stress transfer can promote the synchronization of earthquakes. Even if the asperities are separated by as far as 30 km, synchronization is still possible for a viscosity of the underlying mantle of 1017 Pa s, which can be attained by dislocation creep or transient creep during the postseismic period. Considering the similarities in tectonic and structural settings, viscoelastic stress transfer and earthquake synchronization may also occur at 15’20 (Mid‐Atlantic Ridge), George V (Southeast Indian Ridge), Menard and Heezen transform fault (Pacific‐Antarctic Ridge).

Abyssal vent field habitats along plate margins in the Central Indian Ocean yield new species in the genus Anatoma (Vetigastropoda: Anatomidae)

New species in Anatoma Woodward, 1859 (Anatomidae, Vetigastropoda) are found in abyssal hydrothermal vent field habitats on the oceanic plate margins in the Indian Ocean. Six species are identified using molecular sequence analyses of which four species are described as new based on their morphological characters: Anatoma discapex sp. nov., Anatoma declivis sp. nov., Anatoma laevapex sp. nov. and Anatoma paucisculpta sp. nov. Inadequate material was available for a morphological description of the other two species with genetic identification, but it is likely that all six species are new to science and endemic to the Indian Ocean. The northern Central Indian Ridge localities are dominated by Anatoma declivis sp. nov.; its closest relative is Anatoma discapex sp. nov. which occurs in the central area near the Rodriguez Triple Junction. Anatoma laevapex sp. nov. and Anatoma paucisculpta sp. nov. as well as a fifth undescribed species are mainly found on the Southeast Indian Ridge.

Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica.

DNA Barcoding of Scavenging Amphipod Communities at Active and Inactive Hydrothermal Vents in the Indian Ocean

Hydrothermal vent areas have drawn increasing interest since they were discovered in 1977. Because of chemoautotrophic bacteria, they possess high abundances of vent endemic species as well as many non-vent species around the fields. During the survey conducted by the Bundesanstalt für Geowissenschaften und Rohstoffe (Federal Institute for Geosciences and Natural Resources, BGR) to identify inactive polymetallic sulfide deposits along Central and Southeast Indian Ridges, the INDEX project studied the scavenging amphipod community at three newly discovered hydrothermal fields. A sample consisting of 463 representatives of Amphipoda (Malacostraca: Crustacea) was collected by means of baited traps in active and inactive vents of three different sites and subsequently studied by both morphological and genetic methods. Molecular methods included the analysis of two mitochondrial (cytochrome c oxidase subunit I [COI] and 16S rRNA) and one nuclear (18S rRNA) genes. By six delimitation methods, 22 molecular operational taxonomic units (MOTUs) belonging to 12 genera and 10 families were defined. The existence of potential species complexes was noted for the representatives of the genus Paralicella. The inactive site, where 19 species were found, showed higher species richness than did the active one, where only 10 taxa were recorded. Seven genera, Ambasiopsis, Cleonardo, Eurythenes, Parandania, Pseudonesimus, Tectovalopsis, and Valettiopsis, were observed only at inactive sites, whereas Haptocallisoma, was collected exclusively at active ones. The species Abyssorchomene distinctus (Birstein and Vinogradov, 1960), Hirondellea brevicaudata Chevreux, 1910, and Hirondellea guyoti Barnard and Ingram, 1990, have been previously reported from vent sites in the Atlantic or Pacific oceans. The present study provides the first report of Eurythenes magellanicus (H. Milne Edwards, 1848) and five other already described species in the Indian Ocean. The addition of 356 sequences strongly increases the number of amphipod barcodes in reference databases and provides for the first time COI barcodes for Cleonardo neuvillei Chevreux, 1908, Haptocallisoma abyssi (Oldevig, 1959), Hirondellea guyoti, Tectovalopsis fusilus Barnard and Ingram, 1990, and the genera Haptocallisoma, Pseudonesimus, and Valettiopsis.

Tying policy to system: Does the Ross Sea region marine reserve protect transport pathways connecting the life history of Antarctic toothfish?

A central objective of the Ross Sea region Marine Protected Area (MPA) is to protect areas important to the life cycle of Antarctic toothfish (Dissostichus mawsoni), a top fish predator and by far the region’s most important commercial species. Juvenile toothfish predominate in deep basins along the inner continental shelf, whereas adults are found mostly along the continental slope and spawning areas on the Pacific-Antarctic Ridge. The inner basins connect to the continental slope via glacial troughs and predictable transport along each trough results in exchange with the Antarctic Slope Current as it flows westward. From the slope, two transport pathways, an eastern one from Iselin Bank and a western one that turns cyclonically along the flank of the Southeast Indian Ridge, connect northward to the Pacific-Antarctic Ridge, where the northern arm of the Ross Gyre and the Antarctic Circumpolar Current flow eastward. Using a circulation model to compare transport pathways connecting toothfish life history areas, we consider which inshore basins are likely most important in contributing to adult spawning aggregations; how transport pathways from each may be expected to influence distributions along the continental slope and Pacific-Antarctic Ridge; and how zonal transport pathways may promote export to areas downstream of the marine reserve. Although the MPA protects some critical life history pathways for toothfish, others remain vulnerable to commercial fishing, and we argue that those in adjacent areas along the Iselin Bank, Pacific-Antarctic Ridge and the Amundsen Sea might usefully be protected, discussing the range of policy instruments available. We also recommend consideration of transport pathways in deliberations for a proposed network of Southern Ocean MPAs, introducing a system-based tool using chemical tracers in otoliths that can test for toothfish movement between areas connected along the Antarctic Slope Current and Antarctic Circumpolar Current.

Unusual aliphatic hydrocarbon profiles at hydrothermal vent fields of the Central and Southeast Indian Ridges and Mid-Indian Basin

The hydrocarbon (n-alkane and n-alkene) distribution in deep seawaters of the Mid-Indian Basin (MIB) and hydrothermal vent sites of the Central (CIR) and Southeast Indian Ridges (SEIR) were investigated to understand the impact of hydrothermal activities in deep seawater organic matter. The distinguishing feature of the detected hydrocarbons was in a bimodal distribution with a strong predominance of even-numbered n-alkenes (C16:1-C34:1) maximizing at n-C16 or n-C18, and a series of n-alkane hydrocarbons ranging from C16–C35 with slightly high content in long chain par but without an even/odd carbon number preference. The mono-unsaturated aliphatics were generally 1–4 times higher than the saturated counterpart and exhibited more complex vertical variabilities. The higher content of aliphatics in deeper water, i.e., the signals at 2500 m from the CIR and the increasing tendency towards the bottom from the SEIR in the near vent field sites, indicate a potential hydrothermal contribution. Sulfate-reducing microorganisms of bacteria and archaeon in seawater and the hydrothermal system may be responsible for the distribution feature of the saturated or unsaturated hydrocarbons, but the possibility of abiogenic sources cannot be excluded. In addition, the hydrocarbon profiles recorded the organic debris mineralization in the subsurface coinciding with dramatically hydrographic variability. The present aliphatic profiles feature reflected the complex bio-geochemical process, a minor impact of hydrothermal activity on the hydrocarbon distributions and deep water circulations in the Indian Ocean.

Megafauna of the German exploration licence area for seafloor massive sulphides along the Central and South East Indian Ridge (Indian Ocean).

BackgroundThe growing interest in mineral resources of the deep sea, such as seafloor massive sulphide deposits, has led to an increasing number of exploration licences issued by the International Seabed Authority. In the Indian Ocean, four licence areas exist, resulting in an increasing number of new hydrothermal vent fields and the discovery of new species. Most studies focus on active venting areas including their ecology, but the non-vent megafauna of the Central Indian Ridge and South East Indian Ridge remains poorly known.In the framework of the Indian Ocean Exploration project in the German license area for seafloor massive sulphides, baseline imagery and sampling surveys were conducted yearly during research expeditions from 2013 to 2018, using video sledges and Remotely Operated Vehicles.New informationThis is the first report of an imagery collection of megafauna from the southern Central Indian- and South East Indian Ridge, reporting the taxonomic richness and their distribution. A total of 218 taxa were recorded and identified, based on imagery, with additional morphological and molecular confirmed identifications of 20 taxa from 89 sampled specimens. The compiled fauna catalogue is a synthesis of megafauna occurrences aiming at a consistent morphological identification of taxa and showing their regional distribution. The imagery data were collected during multiple research cruises in different exploration clusters of the German licence area, located 500 km north of the Rodriguez Triple Junction along the Central Indian Ridge and 500 km southeast of it along the Southeast Indian Ridge.

The megalopal stage of the hydrothermal vent crab Austinograea rodriguezensis Tsuchida amp; Hashimoto, 2002 (Decapoda: Bythograeidae): a morphological description based on CLSM images.

The Bythograeidae is unique amongst brachyuran crab taxa as it is the only family where all 6 genera and 16 species are endemic to hydrothermal vents. During the research conducted by German Federal Institute for Geosciences and Natural Resources to identify inactive polymetallic sulphide deposits along Central and Southeast Indian Ridges, the INDEX project collected from hydrothermal vent fields 6 Bythograeidae megalopae. Entire specimens and dissected appendages were stained, mounted on slides and examined using Light Microscopy and Confocal Laser Scanning Microscopy. Additional molecular analysis using mtCOI confirmed the identification of the specimens as Austinograea rodriguezensis Tsuchida Hashimoto, 2002. The megalopal stage of A. rodriguezensis shows similarities and distinctions between the characters of two other bythograeid megalopae, Bythograea thermydron Williams, 1980, and B. microps Saint Laurent, 1984. Unlike other brachyuran megalopae, B. thermydron and A. rodriguezensis lack long serrate setae and stout serrate spines on the dactylus of the fifth pereiopod. In both species no coxal spines on the pereiopods were observed. The elliptical carapace of B. thermydron is broader than long vs longitudinally rectangular in A. rodriguezensis. This carapace shape resembles B. microps more than B. thermydron, however, the dorsal surface of B. microps carapace is densely covered in short setae vs only covered with short setae on the anterior 1/4 and the posterior 1/6 length of carapace in A. rodriguezensis. Furthermore, the amount of plumose natatory setae on the pleopods in B. microps is in total larger and more variable, than in A. rodriguezensis. Bythograeid megalopae seem quite generalized and miss specific features that reveal them distinctively as endemic vent crab. A distinction from other species is possible by observing the unique combinations of certain characters. This is the first megalopal stage description of Austinograea and the fourth within the Bythograeidae.

The subantarctic lithospheric mantle

Abstract We present a summary of peridotite in the Subantarctic (46–60° S) surrounding the Antarctic Plate. Peridotite xenoliths occur on the Kerguelen Islands and Auckland Islands. The Kerguelen Islands are underlain by a plume, whereas the Auckland Islands are part of continental Zealandia, which is a Gondwana-rifted fragment. Small amounts of serpentinized peridotite has been dredged from fracture zones on the Southeast Indian Ridge, Southwest Indian Ridge and Pacific Antarctic Ridge, and represent upwelled asthenosphere accreted to form lithosphere. Suprasubduction-zone peridotite was collected from two locations on the Sandwich Plate. Peridotites from most subantarctic occurrences are moderately to highly depleted, and many show signs of subsequent metasomatic enrichment. Os isotopes indicate that subantarctic continental and oceanic lithospheric mantle contains ancient fragments that underwent depletion long before formation of the overlying crust.

Magnetic Constraints on Off‐Axis Seamount Volcanism in the Easternmost Segment of the Australian‐Antarctic Ridge

AbstractThe Australian‐Antarctic Ridge (AAR) is an intermediate‐spreading rate system located between the Southeast Indian Ridge and Macquarie Triple Junction of the Australian‐Antarctic‐Pacific plates. KR1 is the easternmost and longest AAR segment and exhibits unique axial morphology and various volcanic structures. We identified three asymmetric seamount chains positioned parallel to the seafloor spreading direction, which were indicative of prevalent off‐axis volcanism in the vicinity of segment KR1. Two‐dimensional magnetic modeling was used to predict the magnetization polarity of the seamounts, as well as to constrain their formation time and duration. The magnetic modeling revealed that the majority of the examined seamounts were formed over a period of less than ∼600 kyrs. The seamount formation primarily occurred during two distinct volcanic pulses from 0.16–1.14 to 1.58–2.69 Ma. A temporal gap of 200–650 kyrs between the formation time of the seamounts and seafloor was estimated for certain seamounts that were formed much later than their underlying seafloor and at a distance of 10–20 km from the KR1 axis. Typically, such off‐axis seamount activity is related to axial mantle convection caused by excessive magma supply near the ridge crest. Considering the scale of off‐axis volcanism and thickening lithosphere ∼20 km away from the axis with intermediate‐spreading rates, small‐scale upwelling made feasible by the fertile mantle heterogeneity is proposed as the mechanism for the seamount formations at off‐axis distances, and the geochemically enriched compositions of the seamounts support this alternative explanation.

Geomagnetic Field Paleointensity Spanning the Past 11 Myr From Marine Magnetic Anomalies in the Southern Hemisphere

AbstractGlobal variations in geomagnetic intensity can provide dating markers for sedimentary records, which is particularly helpful in precisely reconstructing climate changes. However, there are few continuous relative paleointensity (RPI) records derived from sediments spanning the past 11 Myr, and those that exist generally bear a high degree of uncertainty. Therefore, independent evidence from other continuous paleointensity records is needed to verify the existing information. Here, we analyzed magnetic anomaly profiles from the East Pacific Rise and the Southeast Indian Ridge and discovered correlative tiny wiggles in the stacked profiles from three different areas. A comparison with a previous magnetic study on the South Atlantic Ridge and the synthetic profile from RPI records generated for the past ∼8 Myr confirmed that the small wavelength anomalies result from global geomagnetic intensity fluctuations. The newly calibrated timescales from selected marine magnetic anomalies could provide a potential dating reference for sedimentary strata.