Mid-Atlantic Ridge Research Articles

Representation and Annual to Decadal Predictability of Euro‐Atlantic Weather Regimes in the CMIP6 Version of the EC‐Earth Coupled Climate Model

Representation and Annual to Decadal Predictability of Euro‐Atlantic Weather Regimes in the CMIP6 Version of the EC‐Earth Coupled Climate Model

Modelling the Dispersion of Seafloor Massive Sulphide Mining Plumes in the Mid Atlantic Ridge Around the Azores

It is increasingly recognised that deep-sea mining of seafloor massive sulphides (SMS) could become an important source of mineral resources. These operations will remove the targeted substrate and produce potentially toxic plumes from in situ seabed excavation and from the return water pumped back down to the seafloor. However, the spatial extent of the impact of deep-sea mining is still uncertain because few field experiments and models of plume dispersion have been conducted. In this study, we used three-dimensional hydrodynamic models of the Azores region together with a theoretical commercial mining operation of polymetallic SMS to simulate the potential dispersal of plumes originating from different phases of mining operations, and to assess the magnitude of potential impacts. Although the model simulations presented here were subject to many caveats, they did reveal some important patterns. The model projected marked differences among sites making generalisations about plume-dispersal patterns in mid-ocean ridges difficult. Nevertheless, the models predicted large horizontal and vertical plume-dispersals above the thresholds adopted. Persistent plumes (temporal frequency >50%, i.e., 6 months out of 12 months) were projected to disperse an average linear distance of 10 to 20 km, cover an area of 17 to 150 km2, and extend more than 800 m in the water column. In fact, the model projected that plumes may disperse beyond the licensed mining areas, reach the flanks and summits of nearby topographic features, and extend into the bathypelagic, mesopelagic, and epipelagic environments. Modelled plume-dispersal overlaps with the predicted distribution of cold-water corals and with existing fishing activities. These potential impacts would be of particular concern in regions such as the Azores, where local populations are highly dependent on the sea for their livelihoods. The findings of this study are an important initial step towards understanding the nature and magnitude of deep-sea mining impacts in space and time.

Dynamics of eddying abyssal mixing layers over sloping rough topography

Abstract ABSTRACT: The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed watermass transformations are dominated by rough topography “hotspots”, where the bottom-enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger up-welling in a thin Bottom Boundary Layer (BBL). These watermass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downwards diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model which approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the non-local coupling to the basin-scale circulation.

Reproductive Traits of the Vent Crab Segonzacia mesatlantica (Guinot, 1989) From the Mid-Atlantic Ridge

Characteristics of the life-history biology of hydrothermal vent species are a prerequisite to understanding the dispersal, population connectivity, and ecology of these insular populations. The vent crab Segonzacia mesatlantica (Guinot, 1989; Brachyuran: Bythograeidae) is one of the most dominant endemic predators at deep-sea hydrothermal vents along the Mid-Atlantic Ridge (MAR). However, the biological life-history characteristics remain poorly understood for this species. The objective of this study was to reveal relevant biological characteristics of the reproductive ecology of S. mesatlantica, contributing to a better understanding of its ecology and the importance of the maintenance of healthy populations. The reproductive ecology of S. mesatlantica from the Broken Spur vent field such as the in situ behavior of brooding female crabs, gametogenesis, fecundity, and the embryonic and larvae development was studied. In contrast to non-brooding crabs, brooding crabs of S. mesatlantica were observed at the vent periphery exhibiting a sequence of specific brooding behaviors, suggesting that they might be providing oxygen to their embryo mass. Results from histological analysis reveal the presence of oocytes in different stages of development within the gonad. Once extruded, the mature oocytes form an egg mass underneath the abdomen. Similar to other bythograeid crabs, the egg mass of S. mesatlantica was composed of a high number of relatively small eggs in different stages of embryonic development. The eggs hatch as a first zoea that undergoes at least four stages of development.

The submarine Azores Plateau: Evidence for a waning mantle plume?

The submarine Azores Plateau in the Central Northern Atlantic has generally been considered to represent a large igneous plateau formed some 10 Ma by widespread volcanism, however a lack of age progression amongst the younger submarine and subaerial volcanism, an irregular distribution of platform-related magmas east and west of the Mid-Atlantic Ridge, a strong tectonic stress regime, and a lack of abundant tholeiitic compositions that reflect initial, high degrees of melting is not easily explainable in the framework of a classic, long-lived mantle plume model. Here, we present new bathymetric and seismic data from the submarine Azores Plateau obtained during cruises M113/1 and M128 with the German R/V Meteor. Our new data combined with prior geochemical and petrological studies indicate that the majority of the western Azores Plateau may indeed have formed during the arrival of a short-lived mantle melting anomaly at 10 Ma. However, our new data also indicate that volcanism <2 Ma is extremely localized and along structural features that result from plate reorganisation in the vicinity of the diffuse triple junction of the Eurasian, Nubian and North American plates, and that much of the easternmost Azores Plateau is magmatically inactive. Our model implies not only that the actual flood basalt province of the Azores is only some 30% of what has been previously estimated but may not have formed from a typical large-scale, long-lived mantle plume. Instead, excess melting as evident from the anomalously shallow water depth and the presence of islands in the Azores may be the result of a volumetrically small-scale melting anomaly in the upper mantle which is in its waning stages as evident from the localised distribution of magmatic activity at <1 Ma. Our model also solves the conundrum of a potential asymmetry of the recent submarine and subaerial volcanism both east and west of the Mid-Atlantic Ridge which instead is the result of an interacting upper mantle anomaly and the complex plate tectonic domain in the vicinity of the triple junction of the Eurasian, Nubian and North American plates.

Systematic variations in trace element composition of pyrites from the 26°S hydrothermal field, Mid-Atlantic Ridge

Variations in trace element composition of pyrites in inactive chimneys from the SMAR 26°S hydrothermal field provide important insights into the genesis and evolution of volcanic-hosted hydrothermal mineralization. Petrographic observations of zoned chimney samples integrated with in-situ LA-ICP-MS analysis of morphologically different pyrites allowed the assessment of fluid evolution during the chimney growth. Pyrite varieties precipitated with increasing temperatures from the outermost chimney wall (zone A) through the intermediate zone (zone B) to the inner zone (zone C). At the late mature stage, pyrite precipitates as interstice pore fillings. The distribution of trace elements in pyrites across the chimney indicates a strong dependence on time, temperature, and associated sulfide minerals. Variations in the trace elements composition of the different pyrite varieties indicate that the hydrothermal system evolved from low-temperature low-chloride liquid-dominated fluids (enriched in Zn, Cd, Tl, Ag, Pb, Mn, Mo, and V) to higher temperature, vapor-dominated fluids (Cu, Au, Te, and Bi), and then to high-temperature fluids (Co and Se). In the waning stage of the hydrothermal system, circulating hot fluids in auxiliary conduits were depleted in most trace elements. The systematic variations in trace element compositions of hydrothermal fluids induced by changes in physicochemical conditions are properly governed by the law of normal distribution of trace elements. The LA-ICP-MS time-depth analysis indicate that Cu, Zn, Cd, Tl, Ag, Te, and Bi are related to micro-/nano-inclusions, whereas Co, Se, and Mo may occur as substitutions in pyrite lattice. The As, Pb, Au, and Sb show both modes of occurrences as substitutions and as inclusions. Adsorbed films on pyrites control the distribution of V and Mo. The incorporation processes of trace elements in pyrites change with fluid evolution during the chimney growth: trapping of micro-/nano-inclusions and surface adsorption is seen more frequently at the low-temperature stage, and lattice substitution mainly at the elevated temperature stages.

Transition Paths of North Atlantic Deep Water

Abstract Recently introduced in oceanography to interpret the near-surface circulation, transition path theory (TPT) is a methodology that rigorously characterizes ensembles of trajectory pieces flowing out from a source last and into a target next, i.e., those that most productively contribute to transport. Here we use TPT to frame, in a statistically more robust fashion than earlier analysis, equatorward routes of North Atlantic Deep Water (NADW) in the subpolar North Atlantic. TPT is applied on all available RAFOS and Argo floats in the area by means of a discretization of the Lagrangian dynamics described by their trajectories. By considering floats at different depths, we investigate transition paths of NADW in its upper (UNADW) and lower (LNADW) layers. We find that the majority of UNADW transition paths sourced in the Labrador and southwestern Irminger Seas reach the western side of a target arranged zonally along the southern edge of the subpolar North Atlantic domain visited by the floats. This is accomplished in the form of a well-organized deep boundary current (DBC). LNADW transition paths sourced west of the Reykjanes Ridge reveal a similar pattern, while those sourced east of the ridge are found to hit the western side of the target via a DBC and also several other places along it in a less organized fashion, indicating southward flow along the eastern and western flanks of the Mid-Atlantic Ridge. Naked-eye inspection of trajectories suggest generally more diffused equatorward NADW routes. A source-independent dynamical decomposition of the flow domain into analogous backward-time basins of attraction, beyond the reach of direct inspection of trajectories, reveals a much wider influence of the western side of the target for UNADW than for LNADW. For UNADW, the average expected duration of the pathways from the Labrador and Irminger Seas was found to be of 2–3 years. For LNADW, the duration was found to be influenced by the Reykjanes Ridge, being as long as 8 years from the western side of the ridge and of about 3 years on average from its eastern side.

Transit Time Distributions and Ventilation Pathways Using CFCs and Lagrangian Backtracking in the South Atlantic of an Eddying Ocean Model

Abstract Transit time distributions (TTDs) for the Antarctic Intermediate Water (AAIW) in the South Atlantic Ocean are estimated from an eddying ocean model with a twofold scope: validation of the TTD method and identifying pathways of the AAIW. The TTDs are inferred both from Lagrangian particle backtracking and the modeled CFC-11 concentrations, under the assumption that the TTDs can be described with an inverse Gaussian function. A bimodal distribution is obtained for the Lagrangian TTDs with four major subduction regions identified: near the Agulhas retroflection, south of New Zealand, west of the Drake Passage (smallest mean age Γ = 13 years), and in the Argentine basin (largest mean age Γ = 25 years). With the Southern Ocean as source region, the inverse Gaussian is a reasonable representation for the TTDs in the eastern Atlantic basin (40°–35°S, 0°–20°E), whereas the fit for region west (40°–35°S, 60°–40°W) of the mid-Atlantic ridge is not as good and overestimates the TTDs for transit times &lt; 15 years. Mean ages from the modeled CFC-11 are mostly larger (up to 12 years) in the eastern Atlantic basin, and they are mostly smaller than the Lagrangian mean ages in the west. Both methods yield mean ages smaller in the western than in the eastern Atlantic basin and an aging of AAIW from the 1990s to the 2000s that is consistent with reduced flow velocities. The Antarctic Circumpolar Current appears to be the prime determinant of the transit times. The results suggest that the inverse Gaussian, despite assuming 1D advection–diffusion with constant mean flow and diffusivity, is a surprisingly good fit. Significance Statement In this article, we assess the transit time distribution method, often used to estimate anthropogenic carbon uptake in the ocean from observations, thereby exploring particle pathways from the surface into the ocean interior in the South Atlantic Ocean. We track thousands of particles in a model from their point of origin near the surface to the ocean interior. The tracking reveals multiple routes and gives the actual travel time of these particles, which we compare with the travel times predicted by theory. Thus, this research deepens our understanding of the routes and travel times of the water particles, which is important for the ocean circulation, and provides insights to improve the methods to infer anthropogenic carbon uptake, storage, and transport.

Mineralogical and geochemical characteristics of near-vent metalliferous sediments: Implications for hydrothermal processes along the southern Mid-Atlantic ridge (12°S–28°S)

In this study, we present mineralogical and geochemical analyses of 50 unconsolidated submarine surface sediments collected from separate locations within six active hydrothermal fields (Zouyu, Taij, Caifan, Deyin, Xunmei, and Tongguan) along the southern Mid-Atlantic Ridge (SMAR) at 12°S–28°S. The samples consisted of 33 metalliferous sediments (MS) and 17 non-metalliferous sediments (non-MS). The study area was divided into a northern segment that contained the Zouyu, Taij, Caifan, and Deyin hydrothermal fields and a southern segment containing the Xunmei and Tongguan hydrothermal fields. The presence of chalcopyrite, octahedral pyrite, and cubic pyrite without streaks on the surface of the sediments reflected a stable high temperature mineralization environment with a small temperature gradient in the Xunmei hydrothermal field. Cubic pyrite with growth striations co-existing with pentagonal dodecahedon-form pyrite indicated an unstable ore-forming environment with large temperature gradient in the early stage and a more stable ore-forming environment in the late stage in the Tongguan hydrothermal field. Major and trace element geochemistry data indicated that the most important material source for the abiogenic component of the sediments along SMAR 12°S–28°S was hydrothermal materials, the second most important source was lithogenic components, and the third source was elements scavenged from seawater. The Xunmei and Tongguan hydrothermal fields may have recorded phase separation, a process that would have resulted in the formation of two separate hydrothermal fluids, thereby forming two types of sulfide-rich MS with rare earth element (REE) patterns with positive and absent Eu anomalies, respectively. The Si-Fe-Mn oxide-rich MS had positive Eu anomalies, which also indicated that these oxides in sediments were directly precipitated from initial hydrothermal fluids at high temperature. The chlorite-rich MS exhibited nearly flat REE patterns similar to the volcanic material-rich samples and mid-ocean ridge basalt (MORB), which reflected the origin of the hydrothermal alteration of volcanic materials. In the northern segment, most of the MS were derived from plume fall-out Fe oxide particles. In addition, the chlorite-rich MS was likely derived from hydrothermal alteration. In the southern segment, the hydrothermal materials in the MS were primarily derived from the collapse of sulfide-bearing chimneys and Si-Fe-Mn oxide debris directly precipitated from hydrothermal fluids.

Geochemical Analogy Viscosity of Mid-Ocean Ridge Basalt as an Indicator for Determining the Location of Seafloor Hydrothermal Fields?

Many achievements have been made in the exploration of seafloor polymetallic hydrothermal vents, however, it needs to further study how to discovery more potential new hydrothermal or massive sulfide fields. In this study, we collected 9818 major-element geochemical data sets of basalt samples from the Mid-Atlantic Ridge (MAR, 78°N–53°S) and the seafloor topographic data for eleven known hydrothermal vents (e.g., TAG, Snake Pit, MAR 22°30′N, Lilliput). Based on the different spatial distances between basalts and hydrothermal vents, we classified all the basalts into three categories: 1) basalt near known vent (focal objects of the research, 29 samples), 2) basalt around known vent (observed objects, 155 samples), and 3) other basalt of the MAR (as background basalt). Meanwhile, we used major element components of basalts to calculate the analogy viscosity and analyzed the possible correlation between analogy viscosity and the spatial distribution of seafloor hydrothermal vents. We found that the analogy viscosity of basalt near known vent fall into a narrow medium value range (∼46.8–68.5 Pa s) contrasted with the range of background basalt values range (∼4.0–356.0 Pa·s). It infers that these basalts on the Mid-Atlantic Ridge with the medium values of analogy viscosity favor the formation of hydrothermal vents. However, due to the complexities of magmatic activity and fault structure, as well as the multi-stage water-rock reaction, whether the analogy viscosity of basalts can be used as an indicator for determining the location of seafloor hydrothermal field or not needs further verification. Hence, this paper provides a possible rule (the variation of analogy viscosity estimated by major element components in the basalts) for exploration the location of hydrothermal vents rather than a fully developed method.

Off-rift Axis Channelized Melt and Lithospheric Metasomatism along Mid-ocean Ridges—A Case Study from Iceland on the Limits of Melt Channelling

AbstractMagmatism in Iceland is classically explained by the interaction of the mid-Atlantic ridge with the Iceland plume. The growth of Iceland through time is the result of volcanic activity at the rift axis. However, Holocene volcanism (0–11.5 ka) is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snæfellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The rift and off-axis postglacial volcanic zones are separated by a gap of ~60–80 km. While the volcanic activity of the SFZ seems correlated with the actual location of the Iceland plume, the plume relation to the SVB is uncertain. To address the origin and relationships between rift and off-rift magmas, we present new data from two transects perpendicular to the Reykjanes rift. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than Mid-Ocean Ridge Basalt (MORB). Rift and off-rift Iceland lavas are all characterized by positive Ba and Nb anomalies, particularly in alkaline rocks. Tholeiitic and alkaline lavas show distinct differentiation sequences, with the main difference being the delayed crystallization of plagioclase in the fractionating assemblage of alkaline magmas. We apply these sequences to calculate primary magma compositions, which are then used to constrain melting conditions. Geochemical modelling indicates that Iceland rift and off-rift magmas can be produced from a peridotitic mantle source if lithospheric processes are involved. We demonstrate that recycled crust in the form of pyroxenite is not required to generate Snæfellsnes alkaline lavas. The low solidus temperature and high productivity of pyroxenite favour early and more extensive melting producing primary magmas that are not sufficiently enriched in incompatible trace elements to explain the compositional variation of Snæfellsnes magmas. An alternative mechanism to involve pyroxenite in the source of Snæfellsnes lavas relates to the hybridization of recycled oceanic crust with peridotite, but such reacted pyroxenite requires specific compositions to reproduce the Ba and Nb anomalies. As an alternative, we suggest that Snæfellsnes alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing amphibole ± phlogopite-bearing hydrous cumulates. Incongruent melting of these cumulates via renewed magmatic activity and melt-peridotite reaction can reproduce the alkaline compositions observed in the SVB, including the Ba and Nb anomalies. Numerical simulations of melt extraction below mid-ocean ridges suggest that low-degree melts produced as far as ~65 km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels at the lithosphere–asthenosphere boundary and acquire specific Ba and Nb anomalies. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of RZ lavas. Alkaline lavas observed in the SFZ show numerous analogies with the Snæfellsnes magmas, suggesting that similar lithospheric processes control their chemistry. The high thermal regime in Iceland and thick lithosphere explains the difference between Icelandic tholeiite and typical MORB. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.

Generation and evolution of the oceanic lithosphere in the North Atlantic

Half a century ago, our view of the Earth shifted from that of a Planet with fixed continents and ancient stable ocean basins to one with wandering continents and young, active ocean basins, reviving Wegener’s Continental Drift that had rested dormant for years. The lithosphere is the external, mostly solid and relatively rigid layer of the Earth, with thickness and composition different below the oceans and within the continents. We will review the processes leading to the generation and evolution of the Earth’s lithosphere that lies beneath the oceans. We will discuss how the oceanic lithosphere is generated along mid-ocean ridges due to upwelling of convecting hot mantle. We will consider in particular lithosphere generation occurring along the northern Mid Atlantic Ridge (MAR) from Iceland to the equator, including the formation of transform offsets. We will then focus on the Vema fracture zone at 10°–11° N, where a ~ 300 km long uplifted and exposed sliver of lithosphere allows to reconstruct the evolution of lithosphere generation at a segment of the MAR from 25 million years ago to the Present. This axial ridge segment formed 50 million years ago, and reaches today 80 km in length. The degree of melting of the subridge mantle increased from 16 million years ago to today, although with some oscillations. The mantle presently upwelling beneath the MAR becomes colder and/or less fertile going from Iceland to the Equator, with “waves” of hot/fertile mantle migrating southwards from the Azores plume. Scientific revolutions seem to occur periodically in the history of Science; we wonder when the next revolution will take place in the Earth Science, and to what extent our present views will have to be modified.

Anatomy and Symbiosis of the Digestive System of the Vent Shrimp Rimicaris Exoculata and Rimicaris Chacei Revealed Through Imaging Approaches

The shrimp Rimicaris exoculata and Rimicaris chacei are visually dominant fauna co-occurring at deep-sea hydrothermal sites of the Mid-Atlantic Ridge (MAR). Their co-existence was related to contrasted life-history traits, including differences in their diet and reliance on chemoautotrophic symbionts at the adult stage. Both species of shrimp are colonized by diversified chemosynthetic symbiotic microbial communities in their cephalothoracic cavity. Symbiotic association with bacteria was also evidenced in their digestive system, and the major lineages were identified through sequencing (with Mycoplasmatales in the foregut and Deferribacteres in the midgut) but their precise distribution within each host species was not assessed. For the first time, we used Fluorescence in situ Hybridization (FISH) to visualize these lineages and describe their association with digestive structures of their host. The aim of the study was to identify possible differences between host species that could be related to their different life-history traits. For this purpose, we developed new specific FISH probes targeting Deferribacteres and Mycoplasmatales lineages identified in the digestive system of these shrimp. Our FISH results showed a partitioning of the bacterial lineages according to the digestive organ corroborating sequencing data, and highlighted their association with specific anatomical structures. Despite morphological differences between the foreguts of R. exoculata and R. chacei that could be related to the adult diet, our FISH results showed overall similar distribution of digestive symbionts for the two host species. However, a more comprehensive study is needed with specimens at different life or molt stages to reveal potential host specific patterns. Such comparisons are now possible thanks to our newly designed FISH probes. The tools used in our study are valuable for tracking symbiont lineages in the environment, allowing a better understanding of their relationship with their host along its life cycle, including their acquisition mechanisms.

Population structure and reproduction of the alvinocaridid shrimp Rimicaris exoculata on the Mid-Atlantic Ridge: Variations between habitats and vent fields

The shrimp Rimicaris exoculata is the most conspicuous component of vent communities developing around hydrothermal fluid emissions below 2000 m on the northern Mid-Atlantic Ridge (nMAR). Its high genetic connectivity suggests a remarkable ability to produce dispersing larval stages. However, so far brooding females have been rarely observed and reproduction remained enigmatic. Spatially complex population structures related to the heterogeneity of local habitat conditions are described for many vent species, this information being fundamental to gain a better understanding of their life history. Here our aim was to assess such complexity along with reproductive development in R. exoculata populations within two vent fields, TAG and Snake Pit (3620m and 3470m depth respectively). We compared samples collected in January–February 2014 among visually distinct assemblages with different degrees of exposure to vent fluids. Dense aggregations located near active venting included mostly females and immature individuals, while inactive peripheries harbored low density assemblages of large males. Small juveniles gathering around low temperature diffusions belonged to another species, Rimicaris chacei. One third of the sexually mature females were ovigerous at the two vent fields during our sampling period, with lower fecundities and egg sizes in the TAG population. Overall, the observed shrimp distribution patterns were consistent across both vent fields, although a high degree of heterogeneity in population structure was observed locally within dense aggregations, probably reflecting micro-scale variations in environmental conditions. Our results thus highlight spatially complex population structures where R. exoculata females brood eggs within dense aggregations exposed to vent fluids, while peripheral inactive areas may be important mating grounds for adults. In addition, we suggest temporal variability in reproductive activity, increasing in the winter season, which questions potential seasonality in a deep-sea species.

SWE_of_Bathymetry.m: A geomorphometric tool to automate discrimination between detachment and magmatic seafloor at slow-spreading ridges from shipboard multibeam bathymetry

The shapes and directionality of the oceanic crust at slow-spreading ridges are key to understanding its magmatic or tectonic emplacement. At slow-spreading ridges, magmatic terrain is marked by linearly fault-bounded abyssal hills, while a more tectonic emplacement termed detachment terrain is marked by long-lived detachment faults forming Oceanic Core Complexes (OCCs). However, the quantitative description of the magmatic and detachment regimes is still limited. We develop a novel geomorphometric technique to automate terrain classification based on the parameterisation of the shape, directionality, and curvature of the seafloor. The algorithm consists of two steps: (1) characterising the pattern observed in the horizontal axes by computing the horizontal eigenvalues of the slope vectors at each multibeam cells and (2) building a weight matrix derived from the computed slopes. The eccentricity of the horizontal eigenvalues defines the dipping pattern in the horizontal axes, hence the term slope-weighted eccentricity (SWE). The technique is applied through a moving window and is tested at 12.5°–15.5° N on the Mid-Atlantic Ridge (MAR), where the two distinct modes of spreading occur. The application of this novel geomorphometric technique yields results consistent with published qualitative interpretation and the distribution of seismicity observed from the peak amplitudes of the tertiary waves (T-waves) in the study area. Using the established algorithm, we found that 41% of the seafloor in our study area experienced detachment faulting (up to 28% are identified as OCCs), 25% experienced typical magmatic accretion, and a buffer zone termed extended terrain affects 34% of the seafloor, where the morphology shows a transition from detachment to magmatic spreading or vice versa. These findings provide new insights into seafloor classification based on the observed morphology and the potential to automate such mapping at other slow-spreading ridge regions.

Bathymetry Based Modeling of Subaxial Magma Flows Under the Mid-Atlantic Ridge, 0 to 30° N

Fracture patterns of the Mid-Atlantic Ridge (MAR) provide evidence of tectonic forces related to divergence and magma upwelling at the ridge axis. In this study, we focus on the MAR from 0 to 30° N, where the N-S ridge exhibits slow spreading rates (2-4 cm/yr) and pronounced axial topography. Ridge segments and transform faults identified in bathymetry data were analyzed for strike orientation and axial depth profiles. Azimuths of transform faults and ridge segments exhibit increasing clockwise rotation with latitude, and all have left lateral displacement. Bathymetric sampling along ridge segments occurred at 9 km intervals with 20 km sampling radii, producing axial lithostatic pressure gradients. One-dimensional magma flows parallel to the ridge axis at 10 and 50 km depths were modeled using Darcy’s law based on published parameters and calculated gradients. Subaxial magma velocities of up to 4 cm/yr were predicted for horizontal flow at depth and are comparable in magnitude to upwelling rates in published literature. Average flow magnitudes (n = 422) within the melt generation region are predicted at 0.8 and 0.2 cm/yr for 10 and 50 km depths respectively. Flow velocities up to five times higher are expected with this model in the high-porosity boundary layer below the solidus. The Coriolis parameter would affect the movement of the flows predicted by our model and may be linked to rotational patterns observed at the MAR. Future research of magma migration below divergent margins would benefit from incorporating axial lithostatic load variations as a driver of flow.

Influence of Chemoautotrophic Organic Carbon on Sediment and Its Infauna in the Vicinity of the Rainbow Vent Field

Hydrothermal vents have been recognised as isolated islands of productivity in the deep sea, but very little is known about the transport and export of chemoautotrophic produced organic carbon to vent surrounding habitats. Here, we investigate vent carbon export and its influence on benthic food webs in sediments at the Rainbow (RB) vent field at the Mid Atlantic Ridge in ~2200 m water depth. Two sites were considered along the dispersal direction of the RB vent plume for sample collection: the close vicinity at about ~30-100 m (“near-vent”); and at 4 km distance (“off-vent”) to the nearest venting area. At both sites, box corers were used to sample sediments and their fauna. A turbidity sensor mounted on a CTD/Rosette provided data on the presence of a vent fluid plume in the water column. Niskin bottles were used to sample suspended particulate organic matter (SPOM) from the plume, as well as from surface waters at 75 m depth. SPOM, sediments and fauna were analysed for nitrogen and carbon stable isotopes. Carbon derived from in situ chemoautotrophy, characterised by depleted δ13C, was taken up by infauna close to the venting area as the main nutrition source, while fauna at the off-vent site showed less depleted δ13C with a signature more typical for photosynthetic- derived material. Nematodes were the most abundant faunal taxon. Their abundance and biomass were variable and not different at the two studied sites. In situ derived organic matter was traced back in the food web in the close vicinity of the venting area. The connectivity of vent and non-vent habitats should be taken appropriately into account when designing future spatial management plans with regard to deep-seabed mining at hydrothermal vents.

Lower oceanic crust formed by in situ melt crystallization revealed by seismic layering

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s−1 velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process.

Arrival of New Great Salinity Anomaly Weakens Convection in the Irminger Sea

AbstractThe Subpolar North Atlantic is prone to recurrent extreme freshening events called Great Salinity Anomalies (GSAs). Here, we combine hydrographic ocean analyses and moored observations to document the arrival, spreading, and impacts of the most recent GSA in the Irminger Sea. This GSA is associated with a rapid freshening of the upper Irminger Sea between 2015 and 2020, culminating in annually averaged salinities as low as the freshest years of the 1990s and possibly since 1960. Upon the GSA propagation into the Irminger Sea over the Reykjanes Ridge, the boundary currents rapidly advected its signal around the basin within months while fresher waters slowly spread and accumulated into the interior. The anomalies in the interior freshened waters produced by deep convection during the 2017–2018 winter and actively contributed to the suppression of deep convection in the following two winters.

Calcification, Dissolution and Test Properties of Modern Planktonic Foraminifera From the Central Atlantic Ocean

The mass of well-preserved calcite in planktonic foraminifera shells provides an indication of the calcification potential of the surface ocean. Here we report the shell weight of 8 different abundant planktonic foraminifera species from a set of core-top sediments along the Mid-Atlantic Ridge. The analyses showed that near the equator, foraminifera shells of equivalent size weigh on average 1/3 less than those from the middle latitudes. The carbonate preservation state of the samples was assessed by high resolution X-ray microcomputed tomographic analyses of Globigerinoides ruber and Globorotalia truncatulinoides specimens. The specimen preservation was deemed good and does not overall explain the observed shell mass variations. However, G. ruber shell weights might be to some extent compromised by residual fine debris internal contamination. Deep dwelling species possess heavier tests than their surface-dwelling counterparts, suggesting that the weight of the foraminifera shells changes as a function of the depth habitat. Ambient seawater carbonate chemistry of declining carbonate ion concentration with depth cannot account for this interspecies difference. The results suggest a depth regulating function for plankton calcification, which is not dictated by water column acidity.