North Fiji Basin Research Articles

Using altimetry and seafloor pressure data to estimate vertical deformation offshore: Vanuatu case study

Measuring ground deformation underwater is essential for understanding Earth processes at many scales. One important example is subduction zones, which can generate devastating earthquakes and tsunamis, and where the most important deformation signal related to plate locking is usually offshore. We present an improved method for making offshore vertical deformation measurements, that involve combining tide gauge and altimetry data. We present data from two offshore sites located on either side of the plate interface at the New Hebrides subduction zone, where the Australian plate subducts beneath the North Fiji basin. These two sites have been equipped with pressure gauges since 1999, to extend an on-land GPS network across the plate interface. The pressure series measured at both sites show that Wusi Bank, located on the over-riding plate, subsides by 11±4mm/yr with respect to Sabine Bank, which is located on the down-going plate. By combining water depths derived from the on-bottom pressure data with sea surface heights derived from altimetry data, we determine variations of seafloor heights in a global reference frame. Using altimetry data from TOPEX/Poseidon, Jason-1, Jason-2 and Envisat missions, we find that the vertical motion at Sabine Bank is close to zero and that Wusi Bank subsides by at least 3mm/yr and probably at most 11mm/yr.This paper represents the first combination of altimetry and pressure data to derive absolute vertical motions offshore. The deformation results are obtained in a global reference frame, allowing them to be integrated with on-land GNSS data.

When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region

An unusual group of more than 100 earthquakes is located ∼600 km beneath the North Fiji Basin in the southwest Pacific Ocean. These earthquakes are attributed to seismicity within detached segments of the subducted Australian plate. One of these detached slab segments has collided with, and now impinges on, the subducted Pacific plate at a depth of ∼500 km. The region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large-magnitude (Mw >7.0) earthquakes for this depth. The folded shape of the Pacific slab beneath Fiji, as well as the abundance of earthquakes, is interpreted to result from deformation and deformation-enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity. Detachment of the slab segments is interpreted to have occurred at ca. 5 Ma, but collision between the eastern segment and the west-dipping Pacific slab is interpreted to have occurred at ca. 4 Ma, coincident with initial opening of the overriding Lau Basin.

The spatial scale of genetic subdivision in populations of Ifremeria nautilei, a hydrothermal-vent gastropod from the southwest Pacific

BackgroundDeep-sea hydrothermal vents provide patchy, ephemeral habitats for specialized communities of animals that depend on chemoautotrophic primary production. Unlike eastern Pacific hydrothermal vents, where population structure has been studied at large (thousands of kilometres) and small (hundreds of meters) spatial scales, population structure of western Pacific vents has received limited attention. This study addresses the scale at which genetic differentiation occurs among populations of a western Pacific vent-restricted gastropod, Ifremeria nautilei.ResultsWe used mitochondrial and DNA microsatellite markers to infer patterns of gene flow and population subdivision. A nested sampling strategy was employed to compare genetic diversity in discrete patches of Ifremeria nautilei separated by a few meters within a single vent field to distances as great as several thousand kilometres between back-arc basins that encompass the known range of the species. No genetic subdivisions were detected among patches, mounds, or sites within Manus Basin. Although I. nautilei from Lau and North Fiji Basins (~1000 km apart) also exhibited no evidence for genetic subdivision, these populations were genetically distinct from the Manus Basin population.ConclusionsAn unknown process that restricts contemporary gene flow isolates the Manus Basin population of Ifremeria nautilei from widespread populations that occupy the North Fiji and Lau Basins. A robust understanding of the genetic structure of hydrothermal vent populations at multiple spatial scales defines natural conservation units and can help minimize loss of genetic diversity in situations where human activities are proposed and managed.

HELIUM ISOTOPE COMPOSITION OF FLUID INCLUSIONS HOSTED IN MASSIVE SULFIDES FROM MODERN SUBMARINE HYDROTHERMAL SYSTEMS

Noble gas compositions of fluid inclusions hosted in sulfides from two modern submarine hydrothermal sites of contrasting tectonic settings have been analyzed. The JADE Field (central Okinawa Trough, Japan) is located in an active intracontinental back-arc basin whereas the SO 134 field is related to the central spreading ridge of the North Fiji Basin, which represents an active, highly mature intra-ocean back-arc system. Previous conventional studies of fluid inclusions have revealed compelling evidence for subcritical phase separation of the hydrothermal fluids at both sites. Elemental abundances of Ne, Ar, Kr, and Xe in fluid inclusions in all studied samples of chalcopyrite and sphalerite from both hydrothermal sites indicate that the heavier noble gases are derived from ambient seawater. In contrast, helium concentrations are 1 to 2 orders of magnitude higher than expected for equilibrium solution in seawater, indicating that helium is predominantly derived from a magmatic source beneath the hydrothermal systems. The 3 He/ 4 He ratios in sphalerite-hosted fluid inclusions from the JADE Field and the SO 134 field are similar to those reported from related vent fluids and show that fluid inclusions reliably record the helium isotope ratio of the original hydrothermal fluid. However, in the North Fiji Basin, earlier “black smoker” activity and the formation of Fe-rich “Kies-type”sulfides seems to have been related to a more MORB-like volcanism, whereas the younger recent “white smoker” hydrothermal activity in the North Fiji Basin can be attributed to a hot spot magmatism in the study area.

Probabilistic Seismic Hazard Model for Vanuatu

Abstract This study develops a preliminary probabilistic seismic hazard model for Vanuatu. The area of investigation, formerly referred to as the New Hebrides, lies in the center of a chain of partly vulcanologically active islands that mark the present-day boundary between the Australia-India plate and the microplate of the North Fiji basin. The seismicity of the Vanuatu arc is dominated by an east-dipping subduction zone, which shows striking structural anomalies in the central part between 14° and 18° S. Our historical catalog contains 7519 events within the Vanuatu region for the period from 1964 to 2003, drawn from the global teleseismic catalogs by the United States Geological Survey/National Earthquake Information Center (USGS/NEIC, see Data and Resources section) and Engdahl et al. (1998). As a measure of seismic hazard, we use horizontal peak ground acceleration (PGA) and horizontal spectral ground acceleration (SGA) at a period of 1 sec. The hazard estimates are based on a logic-tree approach to account for the epistemic uncertainties associated with our analysis. Our results suggest that the entire island arc experiences a high and uniform seismic hazard. Typical values for PGAs range from 0.65 g to 0.77 g with a 10% probability of exceedence in 50 yr. For Port Vila, the capital and largest city in Vanuatu, we additionally present a PGA hazard curve and a uniform hazard spectrum over the period range 0.1–2 sec.

Hydrothermal deposits at the central spteading axis of the North Fiji Basin(SW Pacific)

Hydrothermal deposits at the central spteading axis of the North Fiji Basin(SW Pacific)

A new species ofArchinome(Polychaeta: Archinomidae) from hydrothermal vents on the Pacific–Antarctic Ridge 37°S

Archinome storchisp. nov. is described as the second species of the family Archinomidae. The specimens were collected from a hydrothermal vent site in 2212 m depth on the Pacific–Antarctic Ridge at 37° S. It differs fromArchinome rosaceamainly in the position of the anus, the first appearance of branchiae and the length of the nuchal cirrus. Additional specimens ofA. rosaceacollected from hydrothermal vent locations in the North Fiji Basin and on the Mid-Atlantic Ridge were used for morphological comparison. A synoptic table of characters is given forA. rosaceaandA. storchisp. nov. together with a list of records for Archinomidae compiled from the literature.

Source of the 70-Day Mesoscale Eddy Variability in the Coral Sea and the North Fiji Basin*

Abstract Large-amplitude sea surface height (SSH) variations with a wave period of 70 days and a wavelength of about 700 km are detected in the Coral Sea centered along ∼16°S and in the North Fiji Basin along ∼18°S. These mesoscale eddy signals have a well-defined annual cycle with a maximum in December/January and a minimum in May/June. As the southeasterly trades intercept the island mountains of Vanuatu and Fiji, they generate localized wind stress curl dipoles, which in turn induce the eastward-flowing Coral Sea Countercurrent (CSCC) and Fiji Basin Countercurrent (FBCC) in the Coral Sea and the North Fiji Basin, respectively. The high eddy variability band along 16°S is hypothesized in this study as resulting from barotropic instability due to the lateral shear between the eastward-flowing CSCC and its neighboring westward-flowing North Caledonian and North Vanuatu jets (NCJ and NVJ). Based on stability analyses on the background NCJ–CSCC–NVJ system, it is found that the most unstable mode of the system has an equivalent barotropic structure and is insensitive to the specification of the background flow’s vertical shear. This barotropic instability hypothesis is also supported by observations; specifically, the Reynolds stresses inferred from the observed SSH data are tilted against the meridional shear of the background NCJ–CSCC–NVJ flow. As the intensity of the NCJ–CSCC–NVJ shear fluctuates with the season, the growth rate of instability changes, resulting in seasonal amplitude modulations in the observed 70-day eddy signals.

Cavascope: The Broadband Seismological Network of the New Hebrides Subduction Zone and Its Associated Data Base

Beginning in 1993, a seismological broadband network (named the Cavascope network) was set up in New Caledonia and Vanuatu in the Southwest Pacific, within a collaborative framework of researchers from the Institut de Recherche pour le Developpement, Noumea, New Caledonia (IRD); Ecole et Observatoire des Sciences de la Terre, Strasbourg, France (EOST); and the Department of Geology, Mines and Water Resources of Vanuatu (DGMWR). This seismological network, located in a particularly fast converging area, i.e. , the New Hebrides subduction zone, and surrounded by numerous other very active seismic zones (Papua New Guinea, Salomon Islands, Fiji, Tonga-Kermadec, and New Zealand) is of great interest to the scientific community. In this article, we give a short overview of the tectonic setting of the region and past seismological surveys, and then we present the instrumental characteristics of the Cavascope network and its associated database. The active margin of New Hebrides is associated with the subduction of the Australian plate under the North Fiji Basin. This extends 1,500 km from the Santa Cruz archipelago in the north up to 23 degrees south, including the entire Vanuatu archipelago (figure 1). This seismic zone is dipping with an angle of 60–70° down to a depth of 300 km. The plate convergence axis trends ENE, perpendicular to the trench, and the convergence rate reaches 12 cm/year in the south and 16 cm/year in the north, with a minimum of 4 cm/year observed near the central part of the margin (Pelletier et al. 1998; Calmant et al. 2003). These high convergence rates produce a high level of seismicity that each year yields more than 10 earthquakes of moment magnitude (Mw) higher or equal to 6.0. Despite the area's high seismic potential and volcanic hazards (Louat and Baldassari 1989; Eissen et al. 1991; Robin and …

Diversity of meiofauna and free-living nematodes in hydrothermal vent mussel beds on the northern and southern East Pacific Rise

The ecology and biogeography of meiofauna at deep-sea hydrothermal vents have historically received less attention than those of mega- and macrofauna. This study examines the composition of major meiofaunal taxa in beds of the mussel Bathymodiolus thermophilus at hydrothermal vents on the northern and southern East Pacific Rise (EPR) and presents the first comparison of species assemblages of the dominant taxon, the nematodes, among sites spanning 27 degrees of latitude. Meiofaunal samples were collected by submersible from three mussel beds at 9°N on the EPR and four mussel beds between 17 and 18°S in 1999. Estimated ages of the mussel beds at the time of sampling range from 4 to >20 years, enabling investigation of the influence of mussel bed age on meiofaunal assemblages. Overall, the meiofauna of the mussel beds was dominated by nematodes, with copepods constituting the second most abundant meiofaunal group. There was variation in the ratio of nematodes to copepods between sites, however, with copepods more abundant than nematodes in the youngest mussel beds. Apart from polychaete larvae, other meiofaunal groups were generally present at very low abundance (<1%) in the samples and restricted in diversity to gastropod larvae, acari, foraminifera, ostracoda and turbellaria. Seventeen nematode species from 14 genera and 11 families were found in the samples, with no evidence of endemicity to hydrothermal vents at the generic level. Four genera present were not previously recorded at hydrothermal vents. Nematode species richness, species:genus ratios and abundances were low compared with other deep-sea habitats, though the ecological relevance of comparisons with soft-sediment benthos is discussed. Nematode assemblages exhibited high dominance by a few species, with one species of Thalassomonhystera most abundant at five of the seven vent sites. Multivariate analysis of nematode assemblages reveals similarities among sites that do not match geographical proximity. The youngest mussel beds were most similar to each other and exhibited lower species richness than other sites, consistent with colonization of mussel bed habitat by nematodes over time. Similarity in the composition of nematode assemblages among sites separated by ~3000 km indicates that they lie within a single biogeographic province, consistent with that proposed for mussel bed macrofauna. At a generic level, samples exhibited some overlap with nematode assemblages at vents elsewhere on the EPR, on the Mid Atlantic Ridge and in the North Fiji Basin.

Hf–Nd evidence for the origin and distribution of mantle domains in the SW Pacific

Pb isotope systematics have already been used successfully to demonstrate that the lavas of the arc-basin terrains of the SW Pacific are derived from two mantle domains, one of Pacific-like character and the other of Indian-like character. However, the mobility of Pb during subduction and alteration has mainly restricted the fingerprinting of domains to fresh lavas of MORB composition. We demonstrate that the less alteration-sensitive Hf–Nd isotope projection also discriminates successfully between ‘Pacific’ and ‘Indian’ domains, and thus enables us to extend mantle domain fingerprinting to the back-arc basin basalts and boninites of the Lau and North Fiji Basins and the volcanic arc lavas of the Kermadec, Tonga and Vanuatu arcs. Fingerprinting is facilitated by the observation that the Hf isotope ratio is independent of subduction-input parameters, indicating that Hf has been essentially conservative during the subduction process. Subducted Nd has been added to the mantle source, but subtracting this numerically using the magnitude of negative Hf anomalies filters out the subduction effect. The data show that the ‘Indian’ domain provides the source for magmas erupted at ridges, and arcs near these ridges, that have propagated southwards following the 12 Ma collision of the Ontong-Java Plateau with the Vitiaz Trench. This indicates that the ‘Indian’ domain is actually derived from SOPITA mantle (South Pacific Isotopic and Thermal Anomaly) — mantle modified by the Samoa and other plumes outboard of the trench which only entered the SW Pacific arc-basin system after the Ontong-Java Plateau collision removed the slab barrier at < 12 Ma. In the west, mantle flows beneath the network of south-propagating ridges in the North Fiji and NW Lau Basins, undergoing progressive depletion until the final loss of plume components produces an N-MORB mantle (Indian MORB Mantle) composition in the south North Fiji Basin and Central Lau Spreading Centre. In the east, newly-depleted Samoan plume mantle provides the source for the boninites and depleted arc tholeiites of the northern Tonga arc.

Role of mantle flow at the North Fiji Basin: Insights from anomalous topography

The North Fiji Basin is a complex back‐arc system situated between the opposite‐verging Tonga and New Hebrides subduction zones. On the basis of two independent calculations, we find that the back‐arc region is anomalously high with topography residuals reaching up to 2000 m above what should be isostatically supported. Our study aims to test whether a hot buoyant upwelling is required beneath the North Fiji Basin to support the observed high elevation. The mantle thermal structure beneath the North Fiji Basin is inferred from the global tomographic model S20RTS, where seismic velocity anomalies are converted to density/temperature anomalies. This thermal structure is input into a three‐dimensional mantle convection code, CitcomS, and the flow‐induced surface topography is calculated and compared with the observed anomalous topography. In a series of experiments, we eliminate various portions of the mantle structure to consider what type of mantle flow can support the anomalous topography. The experiments suggest that the observed topography in the North Fiji Basin is consistent with the support of hot buoyant upper mantle upwelling, and specifically rising material in the depth range from 70 to 120 km is the most critical component. A relatively thin/weak lithosphere is required to produce the high‐amplitude and short‐wavelength topography residuals, and the presence of discrete breaks in the plate produces further localization of flow‐induced topography.

Source location of the 26 sec microseism from cross‐correlations of ambient seismic noise

Very strong, narrow band Rayleigh wave arrivals peaking near 26 sec are observed in cross‐correlations of long time sequences of ambient noise records on US, European, and African stations with apparent arrival times that are typically smaller than expected for propagation along the great‐circle linking the stations. Use of these apparent travel times to locate the source of narrow band arrivals shows that the waves originate off the west African coast in the Gulf of Guinea and propagate with an average speed of about 3.5 km/s; i.e., close to the expected group speed of Rayleigh waves at this period. Using data from different months demonstrate that the source location is temporally stable with amplitudes that maximize during the southern hemisphere winter. A microseism with similar features but with a slightly broader spectral peak is observed at stations in East Asia and in the western Pacific, and apparently originates in the North Fiji Basin. The nearly antipodal location of the two source regions leads to the hypothesis that the 26 sec signals are excited by a single source located in the Gulf of Guinea. The Pacific inter‐station cross‐correlations may be for major‐arc propagation.

A MORB source for low-Ti magmatism in the Semail ophiolite

We present the result of a trace element and Nd, Sr and Pb isotope study of a tholeiitic volcanic section in the Salahi massif, carried out in order to characterize the evolution from MOR-type (Geotimes) to the earliest stages of low-Ti arc-like (Lasail) magmatism in the Semail ophiolite. Because of the alteration of the extrusive section, as shown by the high 87Sr/ 86Sr ratios of the studied lavas, the two magmatic episodes have been characterized using trace elements considered to be immobile during alteration processes. Geotimes lavas have trace element compositions similar to N-MORB and Lasail lavas have lower trace element contents, light-rare-earth-element depleted patterns and low Zr/Hf ratios relative to Geotimes. Both magmatic units are characterized by a slight negative anomaly in Nb and Ta relative to Th and LREE on N-MORB normalized patterns. Similar to N-MORB, decompression melting of a convecting mantle can explain Geotimes chemical compositions with the exception of the slight Nb–Ta depletion. Further melting of the Geotimes mantle source and melt–rock interaction with shallow mantle, probably associated to intra-oceanic thrusting, accounts for the depleted trace element compositions of Lasail. Geotimes and Lasail highly incompatible element compositions overlap that of both MORB and back-arc lavas. Yet, Nd and Pb isotope data support a common mantle source for the two magmatic episodes and a Depleted MORB Mantle composition for their source reservoir. These compositions are consistent with the Semail ophiolite being formed either in a large back-arc basin, similar to present day North Fiji Basin, or at a mid-ocean ridge above an “abnormal mantle”, which has preserved older slab-derived signatures, the same as that sampled today by Indian Ocean MORB.

Formation of hydrothermal vents in the North Fiji Basin: Sulfur and lead isotope constraints

Hydrothermal sulfides were recovered from the 16°50′S triple junction area in the North Fiji Basin, at a water depth of ca. 1900 m. The chimney samples can be divided into three groups based on their major metal contents: 1) type 1 (Fe–Cu-rich), 2) type 2 (Fe–Cu-rich with minor Zn), and 3) type 3 (Zn-rich) chimneys. Type 1 chimneys are mainly composed of chalcopyrite and pyrite, and are enriched in elements commonly precipitated under high temperature conditions (> 300 °C), such as Cu, Co, Mo, and Se. Type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite, and are enriched in elements commonly associated with relatively low temperatures (150 to 250 °C), such as Zn, Cd, Pb, As, and Ga. Type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples. Type 2 chimneys appear to be the products of low-temperature replacement, which might be related to formation of type 3 chimneys, of type 1 chimneys. The trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources: N-MORB and E-MORB. Sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys. The type 1 sulfides ( δ 34S = 2.8 ± 1.2‰; 206Pb/ 204Pb = 18.082 to 18.132; 207Pb/ 204Pb = 15.440 to 15.481; 208Pb/ 204Pb = 37.764 to 37.916) are depleted in 34S and have lower Pb isotope ratios compared to types 2 and 3 chimneys ( δ 34S = 4.1 ± 0.9‰; 206Pb/ 204Pb = 18.122 to 18.193; 207Pb/ 204Pb = 15.475 to 15.554; 208Pb/ 204Pb = 37.882 to 38.150). The higher δ 34S values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater. The more radiogenic Pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to E-MORB volcanism that shows higher Pb isotope composition than N-MORB rocks. However, ferromanganese crust samples have radiogenic Pb isotope compositions that are identical to those of E-MORB. Therefore, input of hydrogenous Pb is also a possible source for radiogenic Pb isotopes in the types 2 and 3 chimneys.

A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes

A Cenozoic tectonic reconstruction is presented for the Southwest Pacific region located east of Australia. The reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for Pacific–Australia and Lord Howe Rise–Pacific relative plate motion. The reconstruction is based on a conceptual tectonic model in which the large-scale structures of the region are manifestations of slab rollback and backarc extension processes. The current paradigm proclaims that the southwestern Pacific plate boundary was a west-dipping subduction boundary only since the Middle Eocene. The new reconstruction provides kinematic evidence that this configuration was already established in the Late Cretaceous and Early Paleogene. From ∼ 82 to ∼ 52 Ma, subduction was primarily accomplished by east and northeast-directed rollback of the Pacific slab, accommodating opening of the New Caledonia, South Loyalty, Coral Sea and Pocklington backarc basins and partly accommodating spreading in the Tasman Sea. The total amount of east-directed rollback of the Pacific slab that took place from ∼ 82 Ma to ∼ 52 Ma is estimated to be at least 1200 km. A large percentage of this rollback accommodated opening of the South Loyalty Basin, a north–south trending backarc basin. It is estimated from kinematic and geological constraints that the east–west width of the basin was at least ∼ 750 km. The South Loyalty and Pocklington backarc basins were subducted in the Eocene to earliest Miocene along the newly formed New Caledonia and Pocklington subduction zones. This culminated in southwestward and southward obduction of ophiolites in New Caledonia, Northland and New Guinea in the latest Eocene to earliest Miocene. It is suggested that the formation of these new subduction zones was triggered by a change in Pacific–Australia relative motion at ∼ 50 Ma. Two additional phases of eastward rollback of the Pacific slab followed, one during opening of the South Fiji Basin and Norfolk Basin in the Oligocene to Early Miocene (up to ∼ 650 km of rollback), and one during opening of the Lau Basin in the latest Miocene to Present (up to ∼ 400 km of rollback). Two new subduction zones formed in the Miocene, the south-dipping Trobriand subduction zone along which the Solomon Sea backarc Basin subducted and the north-dipping New Britain–San Cristobal–New Hebrides subduction zone, along which the Solomon Sea backarc Basin subducted in the west and the North Loyalty–South Fiji backarc Basin and remnants of the South Loyalty–Santa Cruz backarc Basin subducted in the east. Clockwise rollback of the New Hebrides section resulted in formation of the North Fiji Basin. The reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere, a change in plate kinematics or slab–mantle interaction.

Host-symbiont relationships in hydrothermal vent gastropods of the genus Alviniconcha from the Southwest Pacific.

Hydrothermal vent gastropods of the genus Alviniconcha are unique among metazoans in their ability to derive their nutrition from chemoautotrophic gamma- and epsilon-proteobacterial endosymbionts. Although host-symbiont relationships in Alviniconcha gastropods from the Central Indian Ridge in the Indian Ocean and the Mariana Trough in the Western Pacific have been studied extensively, host-symbiont relationships in Alviniconcha gastropods from the Southwest Pacific remain largely unknown. Phylogenetic analysis using mitochondrial cytochrome c oxidase subunit I gene sequences of host gastropods from the Manus, North Fiji, and Lau Back-Arc Basins in the Southwest Pacific has revealed a new host lineage in a Alviniconcha gastropod from the Lau Basin and the occurrence of the host lineage Alviniconcha sp. type 2 in the Manus Basin. Based on 16S rRNA gene sequences of bacterial endosymbionts, two gamma-proteobacterial lineages and one epsilon-proteobacterial lineage were identified in the present study. The carbon isotopic compositions of the biomass and fatty acids of the gastropod tissues suggest that the gamma- and epsilon-proteobacterial endosymbionts mediate the Calvin-Benson cycle and the reductive tricarboxylic acid cycle, respectively, for their chemoautotrophic growth. Coupling of the host and symbiont lineages from the three Southwest Pacific basins revealed that each of the Alviniconcha lineages harbors different bacterial endosymbionts belonging to either the gamma- or epsilon-Proteobacteria. The host specificity exhibited in symbiont selection provides support for the recognition of each of the host lineages as a distinct species. The results from the present study also suggest the possibility that Alviniconcha sp. types 1 and 2 separately inhabit hydrothermal vent sites approximately 120 m apart in the North Fiji Basin and 500 m apart in the Manus Basin.

Growth increments and stable isotope variation in shells of the deep-sea hydrothermal vent bivalve mollusk Bathymodiolus brevior from the North Fiji Basin, Pacific Ocean

Bathymodiolus brevior [von Cosel, R., Métivier, B., Hashimoto, J., 1994. Three new species of Bathymodiolus (Bivalvia: Mytilidae) from hydrothermal vents in the Lau Basin and the North Fiji Basin, western Pacific, and the Snake Pit Area, mid-Atlantic ridge. Veliger 37, 374–392] a bivalve mollusk living at deep-sea hydrothermal vents, exhibits daily microgrowth structures in its shell. This interpretation is substantiated by various lines of evidence: (1) similar shell portions of contemporaneous specimens from the same locality contain almost the same number of microincrements; (2) the number of microincrements coincides with the expected number of days in which shell portions of Bathymodiolus spp. form; (3) the width of such microincrements compares well with daily growth rates estimated for the close relative B. thermophilus [Kenk, V.C., Wilson, B.R., 1985. A new mussel (Bivalvia, Mytilidae) from hydrothermal vents in the Galapagos rift-zone. Malacologia 26, 253–271]; (4) different specimens from the same site show similar microgrowth curves. In addition, we found support for tide-controlled shell growth. Daily shell growth rates fluctuate on a fortnightly basis. Some shell portions also revealed the typical tide-controlled microgrowth pattern commonly observed in intertidal bivalves. Based on the analyses of lunar daily growth increments, a growth curve for B. brevior was computed: X t =14 cm−(14–0.04 cm) e −0.26 t . This curve enables estimation of ontogenetic age from shell length. According to this equation, B. brevior reaches its maximum shell length of 14 cm at about age 18. Shell isotope analyses suggest that some major shell growth interruptions or retardations are related to extremely active hydrothermal venting activity. However, shell growth also stopped during periods of low venting implying physiological controls on shell formation. Results of the present study demonstrate that shells of B. brevior provide calendars and environmental data loggers that can complement or partly substitute for long-term observations of venting systems.

A new genus and species of alvinocaridid shrimp (Crustacea: Decapoda: Caridea) from hydrothermal vents on the North Fiji and Lau Basins, south-western Pacific

Nautilocaris saintlaurentae gen. nov., sp. nov., is described and illustrated from hydrothermal vents in the North Fiji Basin and Lau Basin in the south-western Pacific. This new taxon is assigned to the Alvinocarididae. It has a well-developed, dorsally dentate rostrum, carapace with pterygostomian spine present, eye broadly fused mesially, epipod–setobranch complex present above pereopods, and appendix internae on second to fourth pleopods greatly reduced. This combination of characters places it in an intermediate position between a group of species in the genus Alvinocaris with well developed, dorsally dentate rostrum, carapace with pterygostomian spine present, eyes narrowly fused mesially, epipod–setobranch complex absent and well-developed appendices internae on second to fourth pleopods, and Mirocaris fortunata with rostrum unarmed, carapace with rounded pterygostomian angle, eye broadly fused mesially, epipod–setobranch complex present above pereopods and appendices internae on second to fourth pleopods greatly reduced. The habitat of the new species is briefly described.

Petrogenetic variability along the North?South Propagating Spreading Center of the North Fiji Basin

Previous studies on propagating rifts suggested that segmentation of a spreading axis could represent the superficial mark of mantle behavior (Sinton et al., 1983; Nicolas, 1989; Gente et al., 1995). The study of North–South Propagating Spreading Center (NSPSC) from the North Fiji Basin (NFB) brings new insights to this debate. Basalts from the central part of the propagator have more variable incompatible and isotopic ratios then those from its northern tip. A model of dynamic partial melting of a thermally and slightly geochemically and isotopically heterogeneous mantle is proposed. Beneath the central segment, the partial fusion starts deeper (ca. 30 km) and reaches a higher rate (ca. 22%). Further open system differentiation occurs within shallow permanent magma reservoirs along most of the central segment. Below the segment closest to the tip of the propagator the partial fusion starts shallower (ca. 25 km) and stays at a lower rate (ca. 16%). The maximum of differentiation occurs close to the propagator, in small, periodically disconnected, magma bodies resulting in the production of ferrobasalts close to the tip. In order to explain these variations, the presence of an asthenospheric diapir focused beneath the central part of the NS-PSC is proposed. The petrogenetic processes of propagating spreading centers of mature oceanic basins or back-arc basins are identical.