Northern Mid-Atlantic Ridge Research Articles

New records of Echiura and Sipuncula in the North Atlantic Ocean, with the description of a new species of Jacobia

Data on Echiura and Sipuncula collected by the MAR-ECO expedition on the RV G.O. Sars to the northern Mid-Atlantic Ridge are presented. Eight specimens of echiuran and 13 specimens of sipunculan worms were obtained from eight stations. Two species of Echiura and two species of Sipuncula were identified. One echiuran is a new species of Jacobia.

Asteroid fauna of the northern Mid-Atlantic Ridge with description of a new species Hymenasterides mironovi sp. nov.

Asteroids collected on the north part of the Mid-Atlantic Ridge by the RV G.O. Sars in summer 2004, under the MAR-ECO programme, were studied. Asteroids were found at 17 stations, at depths ranging from 966 to 3509 m. The collection includes 32 species. One species of Hymenasterides is here described as new to science. This is the first record of the genus Hymenasterides Fisher, 1911a in the Atlantic Ocean. Previously the genus Hymenasterides included only one species, Hymenasterides zenognathus Fisher, 1911a, from the West Pacific. The new species differs from it by a larger number of paxillar spines (9–20 instead of five to seven) and by alternate adambulacral plates with one and two spines instead of one and three. An annotated check-list is given of the asteroids in the MAR-ECO collection.

Holothurians (Holothuroidea, Echinodermata) of the northern Mid-Atlantic Ridge collected by the G.O. Sars MAR-ECO expedition with descriptions of four new species

The material collected by the G.O. Sars MAR-ECO expedition to the Mid-Atlantic Ridge included 36 species of holothurians. Most belong to the order Elasipodida and the family Synallactidae (Aspidochirotida). Four species in the families Elpidiidae and Psychropotidae (Elasipodida) are new to science. Descriptions of Benthodytes gosarsi sp. nov., Penilpidia midatlantica sp. nov., Peniagone longipapillata sp. nov. and Peniagone marecoi sp. nov. are provided, along with the full list of identified species. Some descriptions are supplemented by observations in situ from dives of the Mir submersibles.

Deep-sea pelagic bioluminescence over the Mid-Atlantic Ridge

Abstract Measurements were made for the first time of the number of stimulated sources of bioluminescence from the subsurface layer to the seafloor at 14 stations over the northern Mid-Atlantic Ridge, 42°56′N to 53°18′N. Overall, light emissions decreased from typical means of 46 sources m−3 at 300–500 m down to 5 m−3 at 2800 m depth. In the vicinity of the Azores, higher mean abundance (29.4 m−3) occurred at 500–1500 m compared to 13.9 m−3 over the same depth range at a reference station east of the ridge. Compared with the reference station, elevated levels of bioluminescence (25 m−3) were also observed at depths of 700–965 m in the water column over the Faraday Seamount. North of the Subpolar Front there was little variability in the number of bioluminescent sources between the sampling stations. However, in the Frontal Zone high abundance of bioluminescent sources was detected down to 1500 m within an anticyclonic eddy. Smoothing techniques revealed that >90% of the observed variation in the distribution of bioluminescence over the ridge was explained by sample depth in the water column.

Distribution of net-collected planktonic cnidarians along the northern Mid-Atlantic Ridge and their associations with the main water masses

Abstract Planktonic cnidarians and ctenophores were sampled with a multiple opening–closing net (Multinet) as well as a non-quantitative plankton net along the northern Mid-Atlantic Ridge (MAR) between Iceland and the Azores. Sixty-four species or genera of planktonic cnidarians (38 siphonophora, 21 hydromedusae, 5 scyphomedusae) and one genus of ctenophore were collected. Of these, Leuckartiara adnata and Clausophyes laetmata were new records for the area. Multinet samples collected from depths of 0–100, 100–500, 500–1000, 1000–1500 and 1500–2500 m at 11 stations were compared. Multivariate analysis of the data indicated that species composition and abundance along the ridge varied with the dominant water masses, with changes in the cnidarian zooplankton assemblage observed with regard to geographic location as well as depth. The surface waters of the two northernmost stations characterized by modified North Atlantic Water (MNAW) as well as the three southernmost stations characterized by North Atlantic Central Water (NACW) exhibited relatively high abundances (3284–13,915 individuals·1000 m −3 ) in the upper 100 m. No such peak was evident at the middle stations characterized by Subarctic Intermediate Water (SAIW), where the abundances in the upper three depth strata were consistently lower (57–863 individuals·1000 m −3 ). Across the study area, the lowest abundances were found in the 1500–2500 m stratum (0–56 ind.·1000 m −3 ). The main divergence in the species composition and abundance of planktonic cnidarians was observed at the Subpolar Front (SPF), which marked the boundary for the distribution of many species. The divergence at the SPF was strongest in the upper 500 m but observable down to 1500 m. Profoundly different epipelagic species assemblages were observed in SAIW and NACW on opposite sides of the SPF, with the distribution of several species of calycophoran siphonophores confined to the southern NACW. At mid-water depths, the species composition north of the SPF was possibly influenced by Labrador Sea Water (LSW). The highest diversity of planktonic cnidarians was observed in the surface waters south of the SPF and in the 100–1000 m range north of the SPF.

Abundance, feeding and reproduction of Calanus finmarchicus in the Irminger Sea and on the northern Mid-Atlantic Ridge in June

Abstract Measurements of hydrography, chlorophyll a, abundance, egg production rates and gut fluorescence of Calanus finmarchicus were made on the northern Mid-Atlantic Ridge, along a north–south transect extending from Iceland to the Azores (∼60–44°N, 25–35°W) in June 2004. In addition, similar data were sampled in the Irminger Sea and on the Reykjanes Ridge in June 2003. The results show marked differences in abundance, phytoplankton ingestion rates and egg production rates from north to south. C. finmarchicus was most abundant in the central Irminger Sea (∼80,000–160,000 individuals m−2, 0–100 m), with abundance declining drastically (

Vertical structure, biomass and topographic association of deep-pelagic fishes in relation to a mid-ocean ridge system

Abstract The assemblage structure and vertical distribution of deep-pelagic fishes relative to a mid-ocean ridge system are described from an acoustic and discrete-depth trawling survey conducted as part of the international Census of Marine Life field project MAR-ECO 〈 http://www.mar-eco.no 〉. The 36-station, zig-zag survey along the northern Mid-Atlantic Ridge (MAR; Iceland to the Azores) covered the full depth range (0 to >3000 m), from the surface to near the bottom, using a combination of gear types to gain a more comprehensive understanding of the pelagic fauna. Abundance per volume of deep-pelagic fishes was highest in the epipelagic zone and within the benthic boundary layer (BBL; 0–200 m above the seafloor). Minimum fish abundance occurred at depths below 2300 m but above the BBL. Biomass per volume of deep-pelagic fishes over the MAR reached a maximum within the BBL, revealing a previously unknown topographic association of a bathypelagic fish assemblage with a mid-ocean ridge system. With the exception of the BBL, biomass per volume reached a water column maximum in the bathypelagic zone between 1500 and 2300 m. This stands in stark contrast to the general “open-ocean” paradigm that biomass decreases exponentially from the surface downwards. As much of the summit of the MAR extends into this depth layer, a likely explanation for this mid-water maximum is ridge association. Multivariate statistical analyses suggest that the dominant component of deep-pelagic fish biomass over the northern MAR was a wide-ranging bathypelagic assemblage that was remarkably consistent along the length of the ridge from Iceland to the Azores. Integrating these results with those of previous studies in oceanic ecosystems, there appears to be adequate evidence to conclude that special hydrodynamic and biotic features of mid-ocean ridge systems cause changes in the ecological structure of deep-pelagic fish assemblages relative to those at the same depths over abyssal plains. Lacking terrigenous input of allochthonous organic carbon, increased demersal fish diversity and biomass over the MAR relative to the abyssal plains may be maintained by increased bathypelagic food resources. The aggregation of bathypelagic fishes with MAR topographic features was primarily a large adult phenomenon. Considering the immense areal extent of mid-ocean ridge systems globally, this strategy may have significant trophic transfer and reproductive benefits for deep-pelagic fish populations.

Distribution, identity, and possible processes sustaining meso- and bathypelagic scattering layers on the northern Mid-Atlantic Ridge

Abstract As an element of comprehensive exploratory studies of the poorly known pelagic ecosystem associated with the Mid-Atlantic Ridge, acoustic data collected by R.V. G.O. Sars during the MAR-ECO expedition in June–July 2004 were used to describe vertical and geographical distributions of meso- and bathypelagic scattering layers. Scattering layers were observed in the entire study area and at all depths to 3000 m with a split-beam echosounder using the 18-kHz drop-keel mounted transducer. The spatial variation in density of surface to 1500 m area backscattering strength, and that of individually identifiable vertical layers, is described in relation to bathymetry and large-scale hydrographical features such as the Sub-polar Front. The inhabitants of scattering layers were identified by exploratory midwater trawling. A major pattern was the elevated backscattering associated with waters around the Charlie-Gibbs Fracture Zone (CGFZ) and the associated Sub-polar Front at and south of 52°N. In this section of the Mid-Atlantic Ridge, the interaction between topographical features, circulation and primary production appears to create favorable conditions for many taxa across all trophic levels. By comparison, the density was lower both to the south of the CGFZ towards the Azores and in particular northwards towards Iceland. Along the north–south gradient a peak in near-surface algal biomass was also found near the CGFZ, with lower values further north and south. We hypothesize that fish productivity is bottom-up regulated, i.e., limited by primary production within the ridge-associated system, supplemented by advection of allochthonous material, especially in the south.

Olivine-hosted melt inclusions and melting processes beneath the FAMOUS zone (Mid-Atlantic Ridge)

Major- and trace-element abundances have been obtained for olivine-hosted melt inclusions from a picritic basalt (ARP73–10–03) dredged in the FAMOUS locality along the Northern Mid-Atlantic Ridge. Isolated in homogeneous, highly primitive olivine phenocrysts (Fo 87.2–91.3), melt inclusions provide important information about the processes of melt generation beneath mid-ocean ridges. They contain near-primary magmas that display a large range of major- and trace-element compositions and form a primitive continuation of the FAMOUS whole rock suites. Both slightly depleted and enriched melt inclusions can be distinguished on the basis of their trace-element compositions, with (La/Sm) N ratios ranging from 0.58 to 1.52. Melt inclusions display chemical trends on both major- and trace-element variation diagrams, indicating that they constitute a suite of genetically-related, highly primitive magmas. The evolution of the melt inclusions is inconsistent with a model of mantle-melt chemical exchange during reactive transport, suggesting that the trapped melts are able to preserve pristine information about the melting process and/or source composition. Comparison of these trends with predicted curves for models of peridotite melting indicates that compositional variations are best reproduced by polybaric partial melting of a relatively homogeneous mantle source and subsequent mixing in various proportions of the melt batches produced at different degrees of melting and/or in different parts of the melting system. These observations require that transport of melts from the melting region to the site of olivine crystallization occurs without significant chemical exchange with the surrounding mantle.

A dual symbiosis shared by two mussel species, Bathymodiolus azoricus and Bathymodiolus puteoserpentis (Bivalvia: Mytilidae), from hydrothermal vents along the northern Mid‐Atlantic Ridge

Bathymodiolus azoricus and Bathymodiolus puteoserpentis are symbiont-bearing mussels that dominate hydrothermal vent sites along the northern Mid-Atlantic Ridge (MAR). Both species live in symbiosis with two physiologically and phylogenetically distinct Gammaproteobacteria: a sulfur-oxidizing chemoautotroph and a methane-oxidizer. A detailed analysis of mussels collected from four MAR vent sites (Menez Gwen, Lucky Strike, Rainbow, and Logatchev) using comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH) showed that the two mussel species share highly similar to identical symbiont phylotypes. FISH observations of symbiont distribution and relative abundances showed no obvious differences between the two host species. In contrast, distinct differences in relative symbiont abundances were observed between mussels from different sites, indicating that vent chemistry may influence the relative abundance of thiotrophs and methanotrophs in these dual symbioses.

Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone array

The seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002–09/2003), the northern ‘SIRENA’ network, deployed between latitudes 40° 20′N and 50° 30′N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0–5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35–4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of mainshock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.

Evidence of a recent magma dike intrusion at the slow spreading Lucky Strike segment, Mid‐Atlantic Ridge

Mid‐ocean ridge volcanic activity is the fundamental process for creation of ocean crust, yet the dynamics of magma emplacement along the slow spreading Mid‐Atlantic Ridge (MAR) are largely unknown. We present acoustical, seismological, and biological evidence of a magmatic dike intrusion at the Lucky Strike segment, the first detected from the deeper sections (>1500 m) of the MAR. The dike caused the largest teleseismic earthquake swarm recorded at Lucky Strike in >20 years of seismic monitoring, and one of the largest ever recorded on the northern MAR. Hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband (>3 Hz) intrusion tremor, suggesting a volcanic origin. Two submersible investigations of high‐temperature vents located at the summit of Lucky Strike Seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting. This magmatic episode may represent one form of volcanism along the MAR, where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long‐lived, discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems.

Seismic reflection imaging of an oceanic detachment fault: Atlantis megamullion (Mid-Atlantic Ridge, 30°10′N)

We present multichannel seismic reflection data collected over the Atlantis megamullion, at the eastern ridge–transform intersection of Atlantis fracture zone on the northern Mid-Atlantic Ridge, and over its conjugate crust. These data image for the first time the internal structure of a young, well-developed megamullion dome formed by tectonic extension across a long-lived oceanic detachment fault. The exposed, corrugated detachment-fault surface exhibits a sharp, coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor. At the termination of the megamullion the fault is imaged ∼13 km along-strike beneath a volcanic hanging-wall block at a sub-seafloor depth of 0.2–0.5 s two-way travel time, reaching north as far as 30°19′N. The eastward dipping of the fault beneath the hanging-wall block is estimated to be ∼6–14°. The corrugated fault surface is underlain by a continuous, strong, and relatively smooth reflection (D) at 0.2–0.25 s sub-bottom below the central axis of the dome. This reflection deepens up to 0.6 s sub-bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope. We suggest that Atlantis massif formed by sequential slip on two different detachment faults that merged at depth, with breakaways as little as ∼2 km apart. The initial detachment is represented by reflection D, and the second corresponds to the presently exposed fault surface. In this interpretation, much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity; it lies in contact with less altered, higher-velocity mantle below the first detachment, resulting in the strong, smooth character of reflection D. Mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism. In conjugate crust corresponding to termination of the megamullion, observed sub-bottom reflections are interpreted as base of seismic layer 2A. This layer is as thick as or thicker (∼570–900 m) than layer 2A in normal Atlantic crust, and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming.

Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise

Geochemical data from melt inclusions in olivine phenocrysts in a picritic basalt from the Siqueiros Transform Fault on the northern East Pacific Rise provide insights into the petrogenesis of mid-ocean ridge basalts (MORB). The fresh lava contains approximately 10% of olivine phenocrysts (Fo89.3 - 91.2) and rare, small (<1 mm) plagioclase phenocrysts with subhedral to irregular shapes with a range of compositions (An80-90, An57-63). Melt inclusions in olivine phenocrysts are glassy, generally rounded in shape and vary in size from a few to approximately 200 lm. Although most of the inclusions have compositions that are generally consistent with being representative of parental melts for the pillow-rim glasses, several inclusions are clearly different. One inclusion, which contains a euhedral grain of high-Al, low-Ti spinel, has a composition unlike any melt inclusions previously described from primitive phenocrysts in MORB. It has a very high Al2O3 (approximately 20 wt%), very low TiO2 (approximately 0.04 wt%) and Na2O (approximately 1 wt%) contents, and a very high CaO/Na2O value (approximately 14). The glass inclusion is strongly depleted in all incompatible elements (La =0.052 ppm; Yb =0.34; La/Sm(n) approximately 0.27), but it has large positive Sr and Eu anomalies (Sr/Sr* approximately 30; Eu/Eu* approximately 3) and a negative Zr anomaly. It also has low S (0.015 wt%) and relatively high Cl (180 ppm). We suggest that this unusual composition is a consequence of olivine trapping plagioclase in a hot, strongly plagioclase-undersaturated magma and subsequent reaction between plagioclase and the host olivine producing melt and residual spinel. Two other melt inclusions in a different olivine phenocryst have compositions that are generally intermediate between 'normal' inclusions and the aluminous inclusion, but have even higher CaO and Sr contents. They are also depleted in incompatible elements, but to a lesser degree than the aluminous inclusion, and have smaller Sr and Eu anomalies. Similar inclusions have also been described in high-Fo olivine phenocrysts from Iceland and northern Mid-Atlantic Ridge. We suggest that the compositions of these inclusions represent assimilation of gabbroic material into the hot primitive magma. The localised nature of this assimilation is consistent with it occurring within a crystal mush zone where the porosity is high as primitive magmas pass through earlier formed gabbroic 'cumulates'. In such an environment the contaminants are expected to have quite diverse compositions. Although the interaction of primitive melts with gabbroic material may not affect the compositions of erupted MORB melts on a large scale, this process may be important in some MORB suites and should be accounted for in petrogenetic models. Another important implication is that the observed variability in melt inclusion compositions in primitive MORB phenocrysts need not always to reflect processes occurring in the mantle. In particular, inferences on fractional melting processes based on geochemistry of ultra-depleted melt inclusions may not always be valid.

The plume and spreading associations of basalt and their reflection in the petrological and geophysical parameters of the northern Mid-Atlantic Ridge

The plume and spreading associations of basalt and their reflection in the petrological and geophysical parameters of the northern Mid-Atlantic Ridge

Implications for magmatic processes at Ontong Java Plateau from volatile and major element contents of Cretaceous basalt glasses

Major elements, Cl, CO2, and H2O were determined in unaltered tholeiitic basalt glasses from Drilling Program Sites 803D and 807C on Ontong Java Plateau (OJP), and Sites 462A in Nauru Basin (NB) and 802A in East Mariana Basin (EMB). Glasses are moderately evolved (Mg# = 53–60) with low Na2O, indicating they formed by large extents of melting. Extent of melting is less for glasses from OJP than from EMB, requiring faster or more focused mantle upwelling or a longer period of activity for OJP.Glasses within each hole define 1–3 different compositional types. Each compositional type is made up of many glass‒bounded flow units whose chemistry is identical within analytical uncertainty, suggesting that each compositional type formed during a single eruptive episode with multiple magma surges. The lack of diversity within each type also suggests that the magmatic system was large and well buffered. Major element chemistry also requires that significant shallow crystallization occurred during plateau formation and limits the amount of deep crystallization that might be responsible for the thick crust of the plateau. Chlorine contents are constant within each compositional type and are 580 ppm in type A and 850 ppm in types C–G from Hole 807C, and 360 ppm in glasses from Hole 803D. Glasses from 802A and 462A have ∼240 ppm and 1975 ppm Cl, respectively. Cl/K ratios range from 0.2 to 2.33, well above mid‐ocean ridge basalt (MORB) mantle values (0.07), suggesting that most liquids assimilated hydrothermally altered material as they ascended and crystallized. High Cl/K ratios also suggest that abundant hydrothermal activity may have accompanied plateau formation. Low dissolved CO2 (i.e., CO32−) contents in one OJP compositional type (54 ppm in 807C type A) are similar to those on shallow sections of the mid‐ocean ridge and are consistent with a maximum depth of eruption of 1320 ± 250 m, in contrast to the greater eruption depths inferred from fossils in the overlying sediments. Our proposed eruption depth falls in the MORB field on a plot of depth versus Na8.0 supporting a ridge or near‐ridge origin. The shallow depth estimate allows for more postformation subsidence and diminishes the importance but does noteliminate the need for additional processes such as prolonged underplating to account for OJP's shallow depth. Higher CO2 in other glasses, including types C–G from Hole 807C, suggests that some liquids were oversaturated with CO2 when emplaced and therefore did not travel far from their point of eruption.H2O contents are similar to MORB and range from 0.13 wt% for NB, to 0.25 wt% (807C, types C–G) to 0.49 wt% (807C type A). Ratios of H2O to rare earth elements (REE) are slightly higher than most MORB and are similar to those from northern Mid‒Atlantic Ridge (MAR), which has distinctively high H2O. H2O/Ce (constant in depleted through enriched MORB) ranges from 200 to 400, compared to 220–380 for northern MAR glasses and 150–250 for MORB elsewhere and &gt;500 for arc‒related glasses. The slightly higher H2O contents in the OJP mantle source could have contributed very little to the extensive melting that built the plateau, so a substantial temperature anomaly is still required.

Revised correlations of Early and Middle Miocene calcareous nannofossil events and magnetostratigraphy from DSDP Site 563 (North Atlantic Ocean)

Deep Sea Drilling Project Site 563, located on the west flank of the northern Mid-Atlantic Ridge, recovered a long Miocene section from which magnetostratigraphic and isotopic stratigraphy are available. Quantitative analyses of calcareous nannofossil assemblages have been performed in the Lower and Middle Miocene sediments from Site 563. The abundance patterns of the identified species allow us to determine several bioevents for this time interval. The recognized biohorizons, related to the available magnetostratigraphy, provide new data on the biostratigraphic value of many species and on the synchroneity of the events over a wide geographic area. Relations with the oxygen isotope stratigraphy are also reported. Sphenolith distribution is examined in particular detail due to their biostratigraphic importance in the Early Miocene. In particular the recently described species Sphenolithus procerus, Sphenolithus tintinnabulum and Sphenolithus multispinatus can be useful to subdivide the Lower Miocene zones NN2 and NN3. A large variety of Reticulofenestra pseudoumbilicus has been identified within zones NN6 and NN7.

Detection of hydrothermal plumes on the northern Mid-Atlantic Ridge: results from optical measurements

Several water column surveys conducted on the northern Mid-Atlantic Ridge have been instrumental in the discovery of three new hydrothermal venting sites, and seven other active segments. Hydrothermal sources have yet to be pinpointed in six of these segments. In this study, the observed frequency of high-temperature venting from 23°N to 41°N on the Mid-Atlantic Ridge (MAR), a distance of approximately 1900 km, is one venting system per 150 km of ridge. Combining these data with those of other investigators for the region from 12°N to 41°N leads to an approximate venting frequency of one site per 90 km of ridge, although venting frequencies on sections of the MAR within this region are as high as one site per 25 km. To support this frequency of venting, and considering the larger dimensions of the MAR rift valley, the ratio of axial to off-axis heat loss must be higher on the MAR than on faster spreading ridges.

Hydrothermal anomalies in the Lucky Strike segment on the Mid-Atlantic Ridge (37°17′N)

As part of the FARA (French-American Ridge Atlantic) project the northern Mid-Atlantic Ridge (MAR) was surveyed for hydrothermal activity between 33° and 40°N during the FAZAR cruise in August–October 1992. After detection of hydrothermal activity in the Lucky Strike segment (37°17′N) a detailed water column survey was undertaken in this region. During a follow-up cruise (with the DSRV ALVIN) several active high-temperature vents were found on the summit of an central seamount. Here we focus on the results of the water column survey undertaken during the FAZAR cruise. The influence of Mediterranean water and the site's shallow depth complicates the hydrography at Lucky Strike; background temperature and salinity gradients are usually too complex to indicate hydrothermal anomalies clearly. Light transmission anomalies within the Lucky Strike region were small, but were detected throughout the segment from mid-depth (∼ 1500 m) all the way to the valley seafloor (∼ 2300 m depth). The large depth range covered by these anomalies probably results from vertical mixing as well as the presence of multiple vent sources at various depths. Methane anomalies and a temperature anomaly observed deeper than the vents documented by ALVIN further support the idea of additional, deeper vents within this segment. Particle samples of the deep particle layer are enriched in Fe and trace metals.

Crenulated seafloor: Evidence for spreading-rate dependent structure of mantle upwelling and melting beneath a mid-oceanic spreading center

The availability of dense GEOSAT-GM satellite altimetry data south of 30°S [1] provides the first opportunity to compare at ∼ 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates. The axis of the slow-spreading Southwest Indian Ridge as well as the seafloor that it has generated has a distinctly crenulated appearance. This crenulated seafloor shows regularly spaced gravity lineations in the spreading direction, crenulations that record stationary centers of mantle upwelling over the life of the Southwest Indian Ridge. A similar pattern of crenulation is seen along sections of the slow-spreading northern Mid-Atlantic Ridge, but along the faster (intermediate) spreading southern Mid-Atlantic Ridge, while regularly spaced highs are apparent along sections of the spreading axis, stationary crenulation does not persist off the axis. This pattern implies that the faster spreading southern Mid-Atlantic Ridge has variable centers of time-dependent upwelling that migrate along the ridge. Crenulations are not visible near the axis of the fast-spreading Pacific-Antarctic Ridge. Crenulation amplitudes decrease strongly with increasing spreading rate. The absence of crenulation at high spreading rate could be due to a 3D/2D transition in flow structure or the effect of mechanisms to redistribute crust along axis. While crenulations are not ubiquitous, even at slowly spreading ridges, where they exist they provide a time record to observe the characteristics of mid-ocean ridge upwelling, melting and melt migration not only beneath active spreading centers but through a much longer spreading history.