Ridge Flank Research Articles

Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons

AbstractTo close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing.

The Asymmetric Distribution of Heat Flow along Mid-Oceanic Ridges

Observations of heat flow along nine geotraverses across mid-oceanic ridges in the Atlantic, Pacific, and Indian oceans are analyzed statistically. It is found that there is significant heat flow distribution asymmetry: opposite sides of the axis of the ridges differ in the average heat flow. Geotraverses in the Southern Hemisphere of the Earth exhibit higher average heat flow on the western flanks of the mid-oceanic ridges, while geotraverses in the Northern Hemisphere are characterized by higher average heat flow along the eastern flanks. Various tectonic factors are taken into account when explaining this phenomenon; however, it is suggested that the Coriolis force must be considered the main factor responsible for this asymmetry, causing redistribution of magmatic material in the asthenospheric reservoir.

Long‐Term Evolution of Nontransform Discontinuities at the Mid‐Atlantic Ridge, 24°N–27°30′N

AbstractWe studied long‐term evolution of nontransform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0‐ to ~20‐ to 25‐Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly (RMBA), and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (<2 Myr) propagation is driven by reduced magma supply (increased tectonic extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (≲5 km). Propagation at larger offsets generally is slower and longer term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) gross volume and distribution of magma supplied to ridge segments as controlled by 3‐D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 Myr) RMBA asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.

The role of marine sediment diagenesis in the modern oceanic magnesium cycle

The oceanic magnesium cycle is largely controlled by continental weathering and marine authigenic mineral formation, which are intimately linked to long-term climate. Uncertainties in the magnesium cycle propagate into other chemical budgets, and into interpretations of paleo-oceanographic reconstructions of seawater δ26Mg and Mg/Ca ratios. Here, we produce a detailed global map of the flux of dissolved magnesium from the ocean into deeper marine sediments (greater than ∼1 meter below seafloor), and quantify the global flux and associated isotopic fractionation. We find that this flux accounts for 15–20% of the output of magnesium from the ocean, with a flux-weighted fractionation factor of ∼0.9994 acting to increase the magnesium isotopic ratio in the ocean. Our analysis provides the best constraints to date on the sources and sinks that define the oceanic magnesium cycle, including new constraints on the output flux of magnesium and isotopic fractionation during low-temperature ridge flank hydrothermal circulation.

Structure and evolution of the Eastern part of the Southwest Indian Ridge

The structure and evolution of eastern part of the Southwest Indian ridge is discussed. Based on geological-geophysical data and cartographic materials, analysis of spatial and temporal changes of ridge morphostructure was performed. Analysis of the data made it possible to recognize the stages of ridge evolution and delineate its sections differing in tectonics, structure, morphostructural segmentation of the rift zone and the ridge flanks. It is shown that crust in the axial zone of the ridge formed under repeatedly changing spreading kinematics, degree of hotspot activity, and underlying mantle along-axis temperatures. The temporal variations in these characteristics along the rift zone are established, with consideration of their influence on its segmentation and tectonic structural features.

ДОЛГОВРЕМЕННАЯ ИЗМЕНЧИВОСТЬ ТЕЧЕНИЙ В ЗОНЕ СУБАРКТИЧЕСКОГО ФРОНТА АТЛАНТИЧЕСКОГО ОКЕАНА

Based on time series of near-bottom current velocities and temperatures obtained in the period June, 2016 to July, 2017, at three points in the Atlantic Subarctic Front, along with the use of multi-year (since 1993 up to now) satellite ocean surface sounding data, multi-scale fluctuations of ocean surface and near-bottom flows over the western and eastern flanks of the Reykjanes ridge, as well as near Hatton Rise, on the Rokoll plateau, are studied. Hydrological profiles were carried out from the ocean surface to the bottom with readings every 10 m, when setting and retrieving the buoy stations. Using data from the Bank of hydrological stations (WOD13), SST satellite arrays (Pathfinder), long-term sea level and geostrophic velocities time series (AVISO), and bottom topography (model ETOPO-1), features of longterm cyclical fluctuations of SST, sea level, geostrophic currents on the ocean surface were defined in the sub-polar North Atlantic. It is shown that, in accordance with the large-scale thermohaline structure of the Subarctic front, two branches of the North Atlantic Current are detected on the ocean surface.One is directed from the Hatton towards the Icelandic-Faroese Rise, and the other – alomg the western flank of the Reykjanes Ridge toward Iceland. For the first branch, which is the main continuation of the North Atlantic Current, the average (for 25 years) water drift at a speed of 9.1±0.1 cm/s is determined to the northeast. The second branch, which forms the eastern part of the Subarctic cyclonic gyre, has the average water drift at a speed of 4.0±0.1 cm/s is directed north-northeast, along the western flank of the Reykjanes Ridge. In the intermediate waters of the frontal zone, an average water flow is observed at a speed of 2.7±0.1 cm/s to the north-northeast, along the eastern slope of the Reykjanes ridge.Due to the multy-scale components of the total variability, the average kinetic energy densities(KED) of total currents (109, 45, 97, (±3) erg/cm3, at station points from east to west) are much greater than the mean drift KED. The near-bottom flows on the Reykjanes ridge flanks are opposite to the direction of the North Atlantic Current. Outside the Subarctic gyre, the direction of average transport is maintained from the ocean surface to the bottom. The average (per year) KED of near-bottom currents are 31, 143, 27 (±3 erg/cm3), for three stations from east to west, respectively. In the intermediate waters of the frontal zone, above the eastern slope of the Reykjanes Ridge, there is a powerful reverse (relative to the North Atlantic Current) near-bottom water flow to the south-west, with a high average speed of ~ 15 cm/s. The KED of the currents during the year varies widely from zero to ~ 600 erg/cm3. The overall variability is due to cyclical variations and intermittency (“flashes”) of currents. Perennial cycles, seasonal variations, synoptic fluctuations with periods in the range of 30–300 days, as well as inertial oscillations and semi-diurnal tidal waves are distinguished. The intermittency of oscillations is partly due to changes in low-frequency flows, which can lead to a dopler frequency shift in the cyclic components of the spectrum. The amplitude of temperature fluctuations in the bottom layer for the year was (0.07–0.10) ± 0.01°C by the standard deviation. The seasonal changes of the bottom temperature are not detected. However, a linear trend with a warming of ~ (0.10–0.15) ± 0.01°С per year is noticeable.

Asymmetric crustal structure of the ultraslow-spreading Mohns Ridge

ABSTRACTNew analysis of the geophysical data of the ultraslow-spreading Mohns Ridge and its off-axis structure reveals a distinctive asymmetric structure. We calculate residual bathymetry (RB) and residual mantle Bouguer gravity anomaly (RMBA) and decompose the anomalies into symmetric and asymmetric components between the ridge conjugates. The western flank of the Mohns Ridge at crustal age of ~50–15 Ma is characterized by a broad zone of elevated RB and more negative RMBA, which we term the Vesteris Plateau (VP). The VP anomaly has a surface area of ~1.12 × 105 km2 and an excess crust volume of ~2.33 × 105 km3, making it a significant anomaly comparable to other anomalies such as the Bermuda Rise. Extending north of the Kolbeinsey Ridge for more than 500 km, the VP lies above an anomalous upper mantle region of low shear-wave seismic velocity, indicating that the VP might represent the northernmost reach of the Iceland-Jan Mayen mantle anomaly. In addition, the western ridge flank of the Mohns Ridge at crustal age of 6–0 Ma is associated with higher RB and more positive RMBA relative to the eastern conjugate, indicating tectonic uplift and associated exposure of lower crust and upper mantle near the ridge axis.

Evidence for Low‐Temperature Diffuse Venting at North Pond, Western Flank of the Mid‐Atlantic Ridge

AbstractDuring expedition MSM37 on the German RV Maria S. Merian, bottom water temperature and sediment temperature profiles were measured in the vicinity of North Pond (western flank of Mid‐Atlantic Ridge) during exploratory dives with Remotely Operated Vehicle Jason II. In addition, push cores were taken at locations with high sediment temperature gradients. We could identify two locations where sediment temperature gradients exceed 1 K/m and bottom water temperatures showed an anomaly of up to 0.04 °C above background. We interpret these observations as clear indication of low‐temperature diffuse venting of fluids that have traveled through the uppermost crust. We can safely assume that the observed phenomena are widespread at ridge flank settings where sediment cover is thin or absent, and hence, we can explain the efficient heat mining on ridge flanks. Due to the difficulties of locating diffuse low‐temperature discharge sites and due to the fact that discharge can occur through thin sediment cover as well as through sediment‐free basement outcrops, it will be very difficult to quantify fluxes of energy and mass from low‐temperature diffuse venting in ridge flank settings; however, thermal anomalies may be used to locate sites of discharge for geochemical, microbial, and hydrologic characterization.

Tidal Conversion and Dissipation at Steep Topography in a Channel Poleward of the Critical Latitude

AbstractIn high-latitude fjords and channels in the Canadian Arctic Archipelago, walls support radiating internal tides as Kelvin waves. Such waves allow for significant barotropic to baroclinic tidal energy conversion, which is otherwise small or negligible when poleward of the critical latitude. This fundamentally three-dimensional system of a subinertial channel is investigated with a suite of numerical simulations in rectangular channels of varying width featuring idealized, isolated ridges. Even in channels as wide as 5 times the internal Rossby radius, tidal conversion can remain as high as predicted by an equivalent two-dimensional, nonrotating system. Curves of tidal conversion as a function of channel width, however, do not vary monotonically. Instead, they display peaks and nulls owing to interference between the Kelvin waves along the wall and similar waves that propagate along the ridge flanks, the wavelengths of which can be estimated from linear theory to guide prediction. Because the wavelengths are comparable to width scales of Arctic channels and fjords, the interference will play a first-order role in tidal energy budgets and may consequently influence the stability of glaciers, the ventilation of deep layers, the locations of sediment deposition, and the fate of freshwater exiting the Arctic Ocean.

Does intermediate spreading-rate oceanic crust result from episodic transition between magmatic and magma-dominated, faulting-enhanced spreading?—The Costa Rica Rift example

Ocean-bottom seismograph and multichannel streamer wide-angle seismic data are jointly analysed and compared with reflection images, bathymetric maps and potential field data, to reveal the detailed structure of layer 2 of the oceanic crust formed at the intermediate spreading Costa Rica Rift (CRR). Separate modelling of each wide-angle data set independently reveals a gradual increase in P-wave velocity with distance (hence crustal age) from the ridge axis, with a model derived from their joint inversion, in turn, displaying a pattern of shorter-wavelength structural complexity in addition to a background flow-line trend. Normalizing against a ridge-located reference velocity–depth model reveals that, off-axis, velocity perturbations are correlated with trends in basement roughness and uplift; regions of rougher and uplifted basement correlate with slower layer 2 velocity, <0.5 km s−1 faster than at the ridge axis, and thinner sediment cover, while smoother basement and locations where sediment cover forms a continuous seal over the oceanic basement, are mirrored by regions of relatively higher velocity, 1.0–1.4 km s−1 faster than at the CRR. These velocity variations are interpreted to reflect periodic changes in the degree of magma supply to the ridge axis. Using a combination of global and shipboard magnetic data, we derive a spreading history model for the CRR which shows that, for the past 5 Ma, spreading has been asymmetric. Comparing the seismic model structure with variations in full spreading rate over this period, reveals a correlation between periods of slower spreading and slower layer 2 velocity, basement roughness and uplift, and faster spreading, higher velocity and smoother basement structure. Zones of slower velocity also correlate with lows in the residual mantle Bouguer anomaly, interpreted as most likely reflecting corresponding regions of lower density in the lower crust or upper lithospheric mantle. Using ODP borehole 504B as ground-truth, we show that periods of faster spreading are associated with phases of magmatic accretion, interspersed by phases of increased asymmetric tectonic extension that likely facilitates fluid flow to the deeper crust and results in metamorphic alteration, manifest as the modelled density anomalies. Overall, our study shows that the mode of CRR crustal formation is sensitive to relatively small changes in full spreading rate within the range of 50–72 mm yr−1, that tips the balance between magmatic and magma-dominated crustal formation and/or tectonic stretching, as characterized by significant variation in the fabric and physical properties of layer 2. We further hypothesize that this inherited structure has a direct influence on the subsequent evolution of the crust through secondary alteration. We conclude that descriptive phrases like ‘ocean crust formed at an intermediate-spreading rate’ should no longer be used to describe an actual crustal formation process or resulting crustal structure as, over the full range of intermediate spreading rates, a fine tipping-point dictates an episodic transition between primarily magmatic accretion and magma-dominated crustal formation coupled with enhanced faulting, with their asymmetry recorded in either ridge flank.

Growth Kinetics, Carbon Isotope Fractionation, and Gene Expression in the Hyperthermophile Methanocaldococcus jannaschii during Hydrogen-Limited Growth and Interspecies Hydrogen Transfer.

Hyperthermophilic methanogens are often H2 limited in hot subseafloor environments, and their survival may be due in part to physiological adaptations to low H2 conditions and interspecies H2 transfer. The hyperthermophilic methanogen Methanocaldococcus jannaschii was grown in monoculture at high (80 to 83 μM) and low (15 to 27 μM) aqueous H2 concentrations and in coculture with the hyperthermophilic H2 producer Thermococcus paralvinellae The purpose was to measure changes in growth and CH4 production kinetics, CH4 fractionation, and gene expression in M. jannaschii with changes in H2 flux. Growth and cell-specific CH4 production rates of M. jannaschii decreased with decreasing H2 availability and decreased further in coculture. However, cell yield (cells produced per mole of CH4 produced) increased 6-fold when M. jannaschii was grown in coculture rather than monoculture. Relative to high H2 concentrations, isotopic fractionation of CO2 to CH4 (εCO2-CH4) was 16‰ larger for cultures grown at low H2 concentrations and 45‰ and 56‰ larger for M. jannaschii growth in coculture on maltose and formate, respectively. Gene expression analyses showed H2-dependent methylene-tetrahydromethanopterin (H4MPT) dehydrogenase expression decreased and coenzyme F420-dependent methylene-H4MPT dehydrogenase expression increased with decreasing H2 availability and in coculture growth. In coculture, gene expression decreased for membrane-bound ATP synthase and hydrogenase. The results suggest that H2 availability significantly affects the CH4 and biomass production and CH4 fractionation by hyperthermophilic methanogens in their native habitats.IMPORTANCE Hyperthermophilic methanogens and H2-producing heterotrophs are collocated in high-temperature subseafloor environments, such as petroleum reservoirs, mid-ocean ridge flanks, and hydrothermal vents. Abiotic flux of H2 can be very low in these environments, and there is a gap in our knowledge about the origin of CH4 in these habitats. In the hyperthermophile Methanocaldococcus jannaschii, growth yields increased as H2 flux, growth rates, and CH4 production rates decreased. The same trend was observed increasingly with interspecies H2 transfer between M. jannaschii and the hyperthermophilic H2 producer Thermococcus paralvinellae With decreasing H2 availability, isotopic fractionation of carbon during methanogenesis increased, resulting in isotopically more negative CH4 with a concomitant decrease in H2-dependent methylene-tetrahydromethanopterin dehydrogenase gene expression and increase in F420-dependent methylene-tetrahydromethanopterin dehydrogenase gene expression. The significance of our research is in understanding the nature of hyperthermophilic interspecies H2 transfer and identifying biogeochemical and molecular markers for assessing the physiological state of methanogens and possible source of CH4 in natural environments.

Dissolved organic carbon in basalt-hosted deep subseafloor fluids of the Juan de Fuca Ridge flank

Marine dissolved organic carbon (DOC) is highly depleted in radiocarbon and thus inferred to be largely refractory to removal processes that operate on less than millennial timescales. However, a growing number of reports have shown that a large fraction of marine DOC can be effectively removed during circulation through submarine hydrothermal systems. What is not clear, however, is whether the DOC that remains in hydrothermal fluids is remnant non-reactive DOC from recharged seawater, or DOC that has been largely produced in the subsurface. We collected and characterized warm (∼65°C) hydrothermal fluids from deep (18, 40, 73, 200 m) within the basalt-hosted basement of the Juan de Fuca Ridge flank in the Northeast Pacific Ocean. DOC concentrations in hydrothermal fluids were 9 to 18 μM, much lower than measured in local deep seawater (37.5 μM). DOC Δ14C values of −683‰ to −856‰ were much lower than the Δ14C-dissolved inorganic carbon (DIC) values of −880‰ to −918‰, while DOC δ13C values of −23.6‰ to −27.0‰ were much heavier than that of the particulate organic carbon (POC) pool (∼−34‰), suggesting that biological production in the subsurface is not a primary source of DOC. Rather, our data suggest that isotopically enriched DOC are selectively removed from recharged seawater, leaving DOC that is very isotopically depleted in the basaltic basement fluids. Despite the removal of 50–75% of DOC in the subsurface, nuclear magnetic resonance (NMR) functional group analyses indicate that aromatic compounds were added to basaltic basement fluids during passage through the deep subseafloor and may partly contribute to the depleted 14C DOC in the ridge-flank basement fluids.

Carboxydotrophy potential of uncultivated Hydrothermarchaeota from the subseafloor crustal biosphere

The exploration of Earth’s terrestrial subsurface biosphere has led to the discovery of several new archaeal lineages of evolutionary significance. Similarly, the deep subseafloor crustal biosphere also harbors many unique, uncultured archaeal taxa, including those belonging to Candidatus Hydrothermarchaeota, formerly known as Marine Benthic Group-E. Recently, Hydrothermarchaeota was identified as an abundant lineage of Juan de Fuca Ridge flank crustal fluids, suggesting its adaptation to this extreme environment. Through the investigation of single-cell and metagenome-assembled genomes, we provide insight into the lineage’s evolutionary history and metabolic potential. Phylogenomic analysis reveals the Hydrothermarchaeota to be an early-branching archaeal phylum, branching between the superphylum DPANN, Euryarchaeota, and Asgard lineages. Hydrothermarchaeota genomes suggest a potential for dissimilative and assimilative carbon monoxide oxidation (carboxydotrophy), as well as sulfate and nitrate reduction. There is also a prevalence of chemotaxis and motility genes, indicating adaptive strategies for this nutrient-limited fluid-rock environment. These findings provide the first genomic interpretations of the Hydrothermarchaeota phylum and highlight the anoxic, hot, deep marine crustal biosphere as an important habitat for understanding the evolution of early life.

Landslide Hazard and Risk Assessment for a Natural Gas Pipeline Project: The Case of the Trans Adriatic Pipeline, Albania Section

The paper focuses on the assessment of landslide hazard and risk along or across the Trans Adriatic Pipeline (TAP) natural gas pipeline project in Albania. TAP is a natural gas pipeline that will transport gas from the Caspian Sea to Europe, crossing Northern Greece and Southern Albania. It has long been recognised that landsliding is a major factor for TAP’s pipeline route selection in mountainous regions, especially the challenging area of central Albania. Experience from similar major pipelines has shown that hazard avoidance is generally the most cost- and time-efficient strategy to minimise the landslide risk since geohazard-related decision-making is usually risk-based. For landslides, the risk profile is expected to be dominated by the upslope expansion of existing landslides, resulting in a loss of ridge crest (where the Right of Way (RoW) is usually located), possibly leading to pipeline rupture. However, it is still possible that new landslides could develop under static and/or seismic conditions, especially on steep ridge flanks along the route. An expert determination approach was adopted to define a consensus for the estimate of the risk (i.e., chance of rupture) for the pipeline at eighty-two (82) identified landslide sites in Albania, to identify “hot spots” along the route, where risk-reduction measures could be prioritised. Ten landslides were characterised as “High Risk”, fifteen as “Medium Risk” and nineteen as “Low Risk”. Following this risk assessment, two large re-routings, as well as several local re-routings, were considered. Further investigation was required to identify the site-specific geotechnical conditions and probable remedial measures in cases where landslides could not be avoided by rerouting.

Hydrothermal scavenging of 230Th on the Southern East Pacific Rise during the last deglaciation

Thorium-230 (230Th) is a fundamental tool for estimating sediment fluxes in the open ocean. Because 230Th is rapidly scavenged by particles falling through the water column, the flux of 230Th to underlying sediments is typically equal to its water column production rate. However, recent surveys suggest hydrothermal plumes are unusually efficient scavengers of 230Th. Here we show that hydrothermal scavenging on the Southern East Pacific Rise (SEPR) resulted in 230Th fluxes several times higher than the water column production rate during the last deglaciation. Elevated fluxes likely require diffusive transport of dissolved 230Th from the ridge flanks towards the ridge crest. Depending on the length-scale of 230Th transport, the resulting deficits in 230Th may yield overestimates of sediment flux to ridge flank sediments. We also show that Fe fluxes at 19°S on the SEPR lag those at 11°S and 6°S by several thousand years, inconsistent with a signal driven by changes in deep water pH and oxygen levels. Instead, variable hydrothermal activity is the simplest explanation of the observed signals in the Pacific, Indian, and Atlantic basins.

Divergent methyl-coenzyme M reductase genes in a deep-subseafloor Archaeoglobi

The methyl-coenzyme M reductase (MCR) complex is a key enzyme in archaeal methane generation and has recently been proposed to also be involved in the oxidation of short-chain hydrocarbons including methane, butane, and potentially propane. The number of archaeal clades encoding the MCR continues to grow, suggesting that this complex was inherited from an ancient ancestor, or has undergone extensive horizontal gene transfer. Expanding the representation of MCR-encoding lineages through metagenomic approaches will help resolve the evolutionary history of this complex. Here, a near-complete Archaeoglobi metagenome-assembled genome (MAG; Ca. Polytropus marinifundus gen. nov. sp. nov.) was recovered from the deep subseafloor along the Juan de Fuca Ridge flank that encodes two divergent McrABG operons similar to those found in Ca. Bathyarchaeota and Ca. Syntrophoarchaeum MAGs. Ca. P. marinifundus is basal to members of the class Archaeoglobi, and encodes the genes for β-oxidation, potentially allowing an alkanotrophic metabolism similar to that proposed for Ca. Syntrophoarchaeum. Ca. P. marinifundus also encodes a respiratory electron transport chain that can potentially utilize nitrate, iron, and sulfur compounds as electron acceptors. Phylogenetic analysis suggests that the Ca. P. marinifundus MCR operons were horizontally transferred, changing our understanding of the evolution and distribution of this complex in the Archaea.

Mineralogical and Geochemical Signatures of Metalliferous Sediments in Wocan-1 and Wocan-2 Hydrothermal Sites on the Carlsberg Ridge, Indian Ocean

In this paper, we conduct a comparative study on the mineralogy and geochemistry of metalliferous sediment collected near the active hydrothermal site (Wocan-1) and inactive hydrothermal site (Wocan-2) from Wocan Hydrothermal Field, on the Carlsberg Ridge (CR), northwest Indian Ocean. We aim to understand the spatial variations in the primary and post-depositional conditions and the intensity of hydrothermal circulations in the Wocan hydrothermal systems. Sediment samples were collected from six stations which includes TVG-07, TVG-08 (Wocan-1), TVG-05, TVG-10 (Wocan-2), TVG-12 and TVG-13 (ridge flanks). The mineralogical investigations show that sediment samples from Wocan-1 and Wocan-2 are composed of chalcopyrite, pyrite, sphalerite, barite, gypsum, amorphous silica, altered volcanic glass, Fe-oxides, and hydroxides. The ridge flank sediments are dominated by biogenic calcite and foraminifera assemblages. The bulk sediment samples of Wocan-1 have an elevated Fe/Mn ratio (up to ~1545), with lower U contents (&lt;7.4 ppm) and U/Fe ratio (&lt;~1.8 × 10−5). The sulfide separates (chalcopyrite, pyrite, and sphalerite) are enriched in Se, Co, As, Sb, and Pb. The calculated sphalerite precipitation temperature (Sph.PT) yields ~278 °C. The sulfur isotope (δ34S) analysis returned a light value of 3.0–3.6‰. The bulk sediment samples of Wocan-2 have a lower Fe/Mn ratio (&lt;~523), with high U contents (up to 19.6 ppm) and U/Fe ratio (up to ~6.2 × 10−5). The sulfide separates are enriched in Zn, Cu, Tl, and Sn. The calculated Sph.PT is ~233 °C. The δ34S returned significant values of 4.1–4.3‰ and 6.4–8.7‰ in stations TVG-10 and TVG-05, respectively. The geochemical signatures (e.g., Fe/Mn and U/Fe ratio, mineral chemistry of sulfides separates, and S-isotopes and Sph.PT) suggest that sediment samples from Wocan-1 are located near intermediate–high temperature hydrothermal discharge environments. Additionally, relatively low δ34S values exhibit a lower proportion (less than 20%) of seawater-derived components. The geochemical signatures suggest that sediment samples from Wocan-2 has undergone moderate–extensive oxidation and secondary alterations by seawater in a low–intermediate temperature hydrothermal environments. Additionally, the significant δ34S values of station TVG-05 exhibit a higher estimated proportion (up to 41%) of seawater-derived components. Our results showed pervasive hydrothermal contributions into station TVG-08 relative to TVG-07, it further showed the increased process of seafloor weathering at TVG-05 relative to TVG-10.

Geochemical Investigations of Fe-Si-Mn Oxyhydroxides Deposits in Wocan Hydrothermal Field on the Slow-Spreading Carlsberg Ridge, Indian Ocean: Constraints on Their Types and Origin

We have studied morphology, mineralogy and geochemical characteristics of Fe-oxyhydroxide deposits from metal-enriched sediments of the active (Wocan-1) and inactive (Wocan-2) hydrothermal sites (Carlsberg Ridge, Northwest Indian Ocean). Fe-oxyhydroxide deposits on the Wocan-1 site are reddish-brownish, amorphous and subangular. They occur in association with sulfides (e.g., pyrite, chalcopyrite and sphalerite) and sulfate minerals (e.g., gypsum and barite). The geochemical composition shows enrichment in transition metals (Ʃ (Cu + Co + Zn + Ni) = ~1.19 wt. %) and low (&lt;0.4 wt. %) values of Al/(Al + Fe + Mn) ratio. The Wocan-2 samples show poorly crystallized reddish brown and yellowish Fe-oxyhydroxide, with minor peaks of goethite and manganese oxide minerals. The mineral assemblage includes sulfide and sulfate phases. The geochemical compositions show two distinct types (type-1 and type-2). The type-1 Fe-oxyhydroxides are enriched in transition metals (up to ~1.23 wt. %), with low values of Fe/Ti vs. Al/(Al + Fe + Mn) ratio similar to the Wocan-1 Fe-oxyhydroxides. The type-2 Fe-oxyhydroxides are depleted in transition metals, with Al/(Al + Fe + Mn) ratio of 0.003–0.58 (mean value, 0.04). The ridge flank oxyhydroxides exhibit an extremely low (mean value ~ 0.01) Fe/Mn ratio and a depleted concentration of transition metals. Our results revealed that the Wocan-1 Fe-oxyhydroxides and type-1 Fe-oxyhydroxides of the Wocan-2 site are in the range of Fe-oxyhydroxides deposits that are precipitated by mass wasting and corrosion of pre-existing sulfides. The type-2 Fe-oxyhydroxides are precipitated from sulfide alteration by seawater in an oxygenated environment relative to type-1. The association of biogenic detritus with the oxyhydroxides of the ridge flanks and the low Fe/Mn ratio suggests hydrogenous/biogenic processes of formation and masked hydrothermal signatures with distance away from the Wocan hydrothermal field.

Mantle melting variation and refertilization beneath the Dragon Bone amagmatic segment (53°E SWIR): Major and trace element compositions of peridotites at ridge flanks

The evolution of the mantle melting regime and the process of refertilization beneath cold lithosphere remain ambiguous at ultraslow spreading ridges. Few previous studies were concerned with the temporal variations of mantle melting indicated by peridotites on the flank of a single ridge segment. Here, we present in-situ major and trace element analyses of harzburgites from the Dragon Bone ridge segment both from near the spreading axis and from rift mountains up to tens of kilometers from the ridge axis. We propose that the Dragon Bone mantle had been depleted anciently at higher pressures, prior to the recent ascent under the ridge at lower pressures, and only a limited volume of melt was generated during recent melting beneath the ridge. This interpretation is consistent with the high degree of depletion of the Dragon Bone harzburgites with little trapped melt, and thin discontinuous igneous crust on the seafloor. Comparison of peridotites from near-ridge locations with those further out on the flanks indicates that the melting regime has progressively shrunk over time creating at present a nearly amagmatic segment. Peridotites from segment ends are seldom affected by late-stage refertilization, while those from the segment center are highly depleted mantle residues (resembling those from fast spreading ridges) that sustained syn-melting metasomatism.

Strong impact of ion filtration on the isotopic composition of chlorine in young clay-rich oceanic sediment pore fluids

A collection of 34 pore fluids from young clay-rich sedimentary piles of the Shikoku basin on the Philippine oceanic plate at the front of the Nankai subduction zone, southeast of Japan, were analyzed for their chloride content and chlorine stable isotope composition (35Cl and 37Cl, reported as δ37Cl). These pore fluids are samples from IODP Sites C0011 and C0012, which penetrate deeper than 500 meters below the seafloor. At both sites, the δ37Cl values display a regular decrease with depth from about 0‰ (seawater value) at the seafloor to −8.5 and −6.5‰ respectively in hydrogeologically isolated, low permeability, sediment levels. At some depths, local deviations of up to 6‰ in δ37Cl interrupt the overall decreasing trends. They may be interpreted as the injection of external fluids into the sedimentary piles as a result of sedimentary levels with higher permeability. At the bottom of the sedimentary piles, the injected fluids, probably originating from the altered oceanic crust, systematically have δ37Cl values closer to seawater. Literature data indicate the display of downward trends of decreasing δ37Cl values of the pore fluids in many other young clay-rich sedimentary piles (Nankai Trough and the Japan Trench accretionary prisms, Black Ridge and Juan de Fuca Ridge flank). Here, we use conservation equations to demonstrate that these δ37Cl depth profiles can be modeled using the effect of the compaction of growing clay–rich sedimentary piles on the water and chlorides (including 35Cl and 37Cl isotopes). This modeling shows that a small isotope fractionation of chlorine isotopes, α37Cl/35Clexpulsed fluid-residual fluid, in the range of 1.006 to 1.001, may explain the observed negative δ37Cl chlorides at the bottom of the sedimentary piles. The complementary positive δ37Cl chlorides must accumulate at the top but are very likely erased by dilution with seawater chlorides across the seafloor interface. This range of chlorine isotope fractionation values is in agreement with the theory of ion filtration of Phillips and Bentley (1987) in which the mobility of chlorides through semi-permeable clay membrane is determined by ion repulsion. Based on this study, we show that young pore fluids in clay-rich sediments of the oceanic crust are 37Cl-depleted chloride reservoirs. They might be the source of 37Cl-depleted chloride fluids advected at mud volcanoes on the seafloor but do not seem to contribute largely to subduction zone magmatic, tectonic, metamorphic products where no such 37Cl-depleted chlorine is documented. This in turn is a strong argument to propose that most of the clay-rich sediment’s pore waters are released back into the ocean rather than subducted.