Arctic Ridge Research Articles

Drivers of pelagic and benthic microbial communities on Central Arctic seamounts

Seamounts are abundant features on the seafloor that serve as hotspots and barriers for the dispersal of benthic organisms. The primary focus of seamount ecology has typically been on the composition and distribution of faunal communities, with far less attention given to microbial communities. Here, we investigated the microbial communities in the water column (0-3400 m depth) and sediments (619-3883 m depth, 0-16 cm below seafloor) along the ice-covered Arctic ridge system called the Langseth Ridge. We contextualized the microbial community composition with data on the benthic trophic state (i.e., organic matter, chlorophyll-a content, and porewater geochemistry) and substrate type (i.e., sponge mats, sediments, basaltic pebbles). Our results showed slow current velocities throughout the water column, a shift in the pelagic microbial community from a dominance of Bacteroidia in the 0-10 m depth towards Proteobacteria and Nitrososphaeria below the epipelagic zone. In general, the pelagic microbial communities showed a high degree of similarity between the Langseth Ridge seamounts to a northern reference site. The only notable differences were decreases in richness between ~600 m and the bottom waters (~10 m above the seafloor) that suggest a pelagic-benthic coupling mediated by filter feeding of sponges living on the seamount summits. On the seafloor, the sponge spicule mats, and polychaete worms were the principal source of variation in sedimentary biogeochemistry and the benthic microbial community structure. The porewater signature suggested that low organic matter degradation rates are accompanied by a microbial community typical of deep-sea oligotrophic environments, such as Proteobacteria, Acidimicrobiia, Dehalococcoidia, Nitrospira, and archaeal Nitrososphaeria. The combined analysis of biogeochemical parameters and the microbial community suggests that the sponges play a significant role for pelagic-benthic coupling and acted as ecosystem engineers on the seafloor of ice-covered seamounts in the oligotrophic central Arctic Ocean.

"Candidatus Subterrananammoxibiaceae," a New Anammox Bacterial Family in Globally Distributed Marine and Terrestrial Subsurfaces.

Bacteria specialized in anaerobic ammonium oxidation (anammox) are widespread in many anoxic habitats and form an important functional guild in the global nitrogen cycle by consuming bio-available nitrogen for energy rather than biomass production. Due to their slow growth rates, cultivation-independent approaches have been used to decipher their diversity across environments. However, their full diversity has not been well recognized. Here, we report a new family of putative anammox bacteria, "Candidatus Subterrananammoxibiaceae," existing in the globally distributed terrestrial and marine subsurface (groundwater and sediments of estuary, deep-sea, and hadal trenches). We recovered a high-quality metagenome-assembled genome of this family, tentatively named "Candidatus Subterrananammoxibius californiae," from a California groundwater site. The "Ca. Subterrananammoxibius californiae" genome not only contains genes for all essential components of anammox metabolism (e.g., hydrazine synthase, hydrazine oxidoreductase, nitrite reductase, and nitrite oxidoreductase) but also has the capacity for urea hydrolysis. In an Arctic ridge sediment core where redox zonation is well resolved, "Ca. Subterrananammoxibiaceae" is confined within the nitrate-ammonium transition zone where the anammox rate maximum occurs, providing environmental proof of the anammox activity of this new family. Phylogenetic analysis of nitrite oxidoreductase suggests that a horizontal transfer facilitated the spreading of the nitrite oxidation capacity between anammox bacteria (in the Planctomycetota phylum) and nitrite-oxidizing bacteria from Nitrospirota and Nitrospinota. By recognizing this new anammox family, we propose that all lineages within the "Ca. Brocadiales" order have anammox capacity. IMPORTANCE Microorganisms called anammox bacteria are efficient in removing bioavailable nitrogen from many natural and human-made environments. They exist in almost every anoxic habitat where both ammonium and nitrate/nitrite are present. However, only a few anammox bacteria have been cultured in laboratory settings, and their full phylogenetic diversity has not been recognized. Here, we present a new bacterial family whose members are present across both the terrestrial and marine subsurface. By reconstructing a high-quality genome from the groundwater environment, we demonstrate that this family has all critical enzymes of anammox metabolism and, notably, also urea utilization. This bacterium family in marine sediments is also preferably present in the niche where the anammox process occurs. These findings suggest that this novel family, named "Candidatus Subterrananammoxibiaceae," is an overlooked group of anammox bacteria, which should have impacts on nitrogen cycling in a range of environments.

Water contents of nominally anhydrous orthopyroxenes from oceanic peridotites determined by SIMS and FTIR

This study presents new secondary ion mass spectrometry (SIMS) reference materials (RMs) for measuring water contents in nominally anhydrous orthopyroxenes from upper mantle peridotites. The enstatitic reference orthopyroxenes from spinel peridotite xenoliths have Mg#s between 0.83 and 0.86, Al2O3 ranges between 4.02 and 5.56 wt%, and Cr2O3 ranges between 0.21 and 0.69 wt%. Based on Fourier-transform infrared spectroscopy (FTIR) characterizations, the water contents of the eleven reference orthopyroxenes vary from dry to 249 ± 6 µg/g H2O. Using these reference grains, a set of orthopyroxene samples obtained from variably altered abyssal spinel peridotites from the Atlantic and Arctic Ridges as well as from the Izu-Bonin-Mariana forearc region was analyzed by SIMS and FTIR regarding their incorporation of water. The major element composition of the sample orthopyroxenes is typical of spinel peridotites from the upper mantle, characterized by Mg#s between 0.90 and 0.92, Al2O3 between 1.66 and 5.34 wt%, and Cr2O3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H2O and correlate well with Al2O3 contents (r = 0.80) and Cr#s (r. = -0.89). We also describe in detail an optimized strategy, employing both SIMS and FTIR, for quantifying structural water in highly altered samples such as abyssal peridotite. This approach first analyzes individual oriented grains by polarized FTIR, which provides an overview of alteration. Subsequently, the same grain along with others of the same sample is measured using SIMS, thereby gaining information about homogeneity at the hand sample scale, which is key for understanding the geological history of these rocks.

Spatiotemporal Relationship between Arctic Mid-Ocean Ridge System and Intraplate Seismicity of the European Arctic

AbstractIn this article, we investigate the influence of the Arctic mid-ocean ridge system (AMORS), including the Gakkel and Mohns ridges, and the Knipovich ridge–Lena trough (KL) segment, on seismicity of the Novaya Zemlya archipelago area (NZ) and the northernmost margin of the East-European Platform (EEP) for 1980–2019. For each individual area, the annual seismic energy was obtained by adding the energies of all earthquakes. To do this, we have converted various types of magnitude by different seismic networks into moment magnitude Mw. We compiled the updated catalog for the NZ, the northern EEP, and the northern part of the Ural fold belt (UFB). As a result, we constructed time distributions of annual seismic energy releases for each composing ridges of AMORS, NZ, and EEP combined with UFB. A model based on the Elsasser’s one describing the transfer of lithospheric stress disturbances in the horizontal direction was built, and quantitative calculations of the disturbance propagations from AMORS were performed. Results are in good agreement with the annual seismic energy time lags between rifts and NZ and EEP together with the UFB. We calculated correlation coefficients between the seismic energy releases over the time for the structures, enabling identification of the characteristic excitation cycles and estimation of the interval of disturbance transfer from AMORS. As a result, disturbances from the Gakkel ridge appear 3 yr later in NZ, from the KL segment in 4 yr, and from the Mona ridge in 8 yr. For the EEP + UBF combined area, we found the following disturbances spreading lags as 7 yr for the Mona ridge, 4 yr for the KL segment, and 5 yr for the Gakkel ridge. The obtained damping amplitudes of the disturbance spreading from the arctic ridges are sufficient to affect the intraplate seismic activity.

GIS-based Mineral Prospectivity Mapping of Seafloor Massive Sulfide on Ultraslow-spreading Ridges: A Case Study of Southwest Indian Ridge 48.7°–50.5° E

With the depletion of mineral resources on land, seafloor massive sulfide deposits have the potential to become as important for exploration, development and mining as those on land. However, it is difficult to investigate the ocean environment where seafloor massive sulfide deposits are located. Thus, improving prospecting efficiency by reducing the exploration search space through mineral prospectivity mapping (MPM) is desirable. MPM has been used in the exploration for seafloor deposits on regional scales, e.g., the Mid-Atlantic Ridge and Arctic Ridge. However, studies of MPM on ultraslow-spreading ridges on segment scales to aid exploration for seafloor massive sulfide have not been carried out to date. Here, data of water depth, geology and hydrothermal plume anomalies were analyzed and the weights-of-evidence method was used to study the metallogenic regularity and to predict the potential area for seafloor massive sulfide exploration in 48.7°–50.5° E segments on the ultraslow spreading Southwest Indian Ridge. Based on spatial analysis, 11 predictive maps were selected to establish a mineral potential model. Weight values indicate that the location of seafloor massive sulfide deposits is correlated mainly with mode-E faults and oceanic crust thickness in the study area, which correspond with documented ore-controlling factors on other studied ultraslow-spreading ridges. In addition, the detachment fault and ridge axis, which reflect the deep hydrothermal circulation channel and magmatic activities, also play an important role. Based on the posterior probability values, 3 level A, 2 level B and 2 level C areas were identified as targets for further study. The MPM results were helpful for narrowing the search space and have implications for investigating and evaluating seafloor massive sulfide resources in the study area and on other ultraslow-spreading ridges.

Frozen Charlotte by Susan de Sola

Reviewed by: Frozen Charlotte by Susan de Sola Midge Goldberg (bio) Susan de Sola, Frozen Charlotte (Able Muse Press, 2019), 126 pp. Susan de Sola writes in a whirl of color and imagination that leads us, surreptitiously, to deeper truths, both light and dark, about our own lives. Her use of rich language, rhyme and meter, and unusual topics makes reading her new collection, Frozen Charlotte, a dance through a unique landscape. The poem "The Tulips" reveals de Sola's delight in and mastery of language. In this blank verse ode, the flowers are brought home from a farmer's market and displayed in a vase on a table. De Sola captures their almost ridiculous beauty, describing them as "great goblets, plush concavities" and "blowsy, lipsticked interlocutors." Even though the flowers are technically dead, no one thinks of them this way at first: "What had this red exuberance / to do with death?" Once all the petals have fallen to the table, she reveals what's left: "Slim green stalks / with small white crowns stand bare. Abstract." De Sola's alliterative lines are a pleasure to the ear and a tribute to her imagination even when looking at the simplest of things: "A Dutch-bobbed slouching flapper of a flower, / so modernist and sleek, a silhouette. / A flower a cartoonist might invent." Many of her poems are metrical, and she sprinkles internal and end rhymes throughout, which adds a song-like quality. "Preserved Things," the first half of "Two-Part Song," is a heterometrical paean with echoes of Hopkins and Whitman: "I sing now of preserved things, of whorls in wood / and table rings; calico corn, gold and red, / like relic teeth in an ancient head." While the items are presented ultimately as "changeless, / outside flow. The answer to a riddle / I wished not to know," the sounds of the lines, as well as the specific examples, are playful and engaging—who doesn't have a jar of pearl onions in the back of the refrigerator and a bag of old peas in the freezer? To this, add things from the fridge, surviving on their arctic ridge,a jar of pearl onions on a shelf,a wishbone drying on a sill—probably it lies there still.Let us add the things that freeze:an ice age of meat and peas. A poem in anapestic dimeter, "Closely Observed Postman," immediately called up a childhood memory for me: Freddy the postman, who always gave me the thick red rubber bands that de Sola refers to in her poem. But the idea that he leaves them dangling from tree branches as a [End Page 290] sign of his very existence is marvelously conveyed in these simple anapestic dimeter ballad stanzas: Each round, openmouthed,elasticized spherespeaks for the postman:Look well, I was here. De Sola's meter isn't always song-like, though. "Bowl of Sea Glass" starts out in headless iambic pentameter, but an inversion in the third line introduces a ragged meter mimicking the action: "The sea batters them, as if a rough sculptor, / and not the conductor of impeccable measures." (I like the poet's joke at her own expense about the less-than-impeccable meter.) She continues with the pentameter, lulling us into the rhythm and the sonnet-like feel of the poem, until we are brought up short by a 15th line of six feet: "The sea lifts, pounds the glass. Insists on randomness." The poem reminds us not to become complacent and think the sea, or the poet, will behave as we expect. De Sola has fun using Cary Grant movies as a way of revealing deeper truths. In "The Light Gray Suit, North by Northwest," her light-hearted tone captures the style of 1940s movies, where men are roughed up but "the suit wears well," and women climb Mt. Rushmore while staying "nyloned all the reel." She gives a humorous nod to the film's need to remain decorous, all while maintaining the iambic pentameter: "Now Eve's in silk paj—CUT!—a scene suppressed!" But she goes beyond the story to gives us a glimpse inside the suit, to the man who, of course...

Trial By Ice

This article explores the experiments, use, and advantages of a next-generation ocean robot that would be used to explore oceans. Nereid Under Ice has been designed specifically for exploring the underside of Arctic sea ice, an area so far off-limits to scientists. As Nereid also must withstand extreme Arctic temperatures, its materials had to be tested even before it was assembled. In one flight, Nereid travelled over the fairly unexplored Arctic Gakkel Ridge system and its hydrothermal vents – fissures in sea floor that shoot up hot water – allowing scientists to survey life over these unique environs. Researchers hope it can also bring back fluid samples from the hydrothermal vents to offer insights into their ecosystems that sometimes host unique bacteria and life forms. The ability to explore these previously unreachable places opens new frontiers for polar science.

Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

We present new 238U–230Th–226Ra–210Pb, 235U–231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (∼5%) 230Th excesses to small 238U excesses (∼5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U–230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226Ra/230Th) ratios ranging from 3.07 to 3.65±3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th–226Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust.

A comprehensive analysis of the morphology of first-year sea ice ridges

Abstract A review of the morphological properties of over 300 full-scale floating first-year sea ice ridges has been made, including measurements from 1971 until the present time. Ridges were examined from the Bering and Chukchi Seas, Beaufort Sea, Svalbard waters, Barents Sea and Russian Arctic Ocean for the Arctic regions; and from the Canadian East Coast, Baltic Sea, Sea of Azov, Caspian Sea and Offshore Sakhalin for the Subarctic (or temperate) regions. Grounded ridges were excluded. A wide catalogue comprising the ridge thicknesses (sail, keel and consolidated layer), widths and angles as well as the macroporosity and the block dimensions is provided. The maximum sail height was found to be 8 m (offshore Sakhalin), and the mean peak sail height was 2.0 m, based on 356 profiles. The mean peak keel depth is 8.0 m, based on 321 profiles. The relationship between the maximum sail height, hs, and the maximum keel depth, hk, for all ridges is best described by the power equation hk = 5.11hs0.69. The correlation differs depending on the region. For Arctic ridges a linear relationship was found to be the best fit (hk = 3.84hs), while for the Subarctic ridges a power relationship (hk = 6.14hs0.53) best fit the data. The ratio of maximum keel to maximum sail is 5.17 on average (based on 308 values), and has also been calculated for each region mentioned above. Arctic ridges generally have a lower keel-to-sail ratio than those in Subarctic regions. The statistical distribution of keel-to-sail ratios is best represented by a gamma distribution. The average sail and keel widths were 12 and 36 m, respectively. The relationships between the sail and keel widths and other geometrical parameters were also determined. Variation of sail and keel thicknesses within individual ridges has been compared with the variability of all ridges. Ridge cross-sectional geometry can vary greatly along the length of a ridge, even over a short distance. A study was made on sail block thicknesses, and it was found that they correlate well with the sail height with a square root model. The typical macroporosity for a first-year ice ridge is 22% (based on 58 values) with an average sail macroporosity of 18% (based on 49 values) and average keel rubble macroporosity of 20% (based on 44 values). The average ridge consolidated layer thickness was 1.36 m based on 118 values. The variation of the consolidated layer was examined, and it was found that the layer tends to grow evenly with time over the width of the ridge cross section. A greater spacing between the measurements seemed to affect the variation, as it decreased with an increasing distance between each borehole. A statistical analysis based on 377 measurements of the consolidated layer of ridges in the Barents Sea showed that the gamma distribution well describes the distribution of the consolidated layer thicknesses in that area.

Teleseismic earthquake swarms at ultraslow spreading ridges: indicator for dyke intrusions?

Earthquakes at mid-ocean ridges reflect the active magmatic and tectonic processes that form new oceanic crust. Studies of large earthquakes observed on land and smaller earthquakes observed locally or regionally by ocean bottom seismometers or autonomous underwater hydrophones have greatly contributed to our understanding of the structure and active spreading processes at the mid-ocean ridges of the Atlantic and Pacific Ocean opening with velocities in excess of 25 mm yr–1. At spreading rates below 20 mm yr–1 full rate, the appearance and the accretion processes of mid-ocean ridges undergo fundamental changes as the melt supply is drastically reduced. The active spreading processes at these so-called ultraslow spreading ridges are still poorly known, as the main representatives, the Arctic Ridge System and the Southwest Indian Ridge, are poorly accessible and neither autonomous underwater hydrophone nor ocean bottom seismometer records of local seismicity are available. In an attempt to compare on a large scale the accretion style of ultraslow spreading ridge sections, I analyse the teleseismically recorded seismicity in 11 sections of the Arctic Ridge System and the Southwest Indian Ridge spanning altogether 7200 km. Epicentres located within 30–35 km of the rift axis were extracted from the catalogue of the International Seismological Centre for a time period of 35 yr. On the basis of a single-link cluster analysis, I identified 27 swarms with eight or more events. These swarms occur almost exclusively at centres of focussed magmatism suggesting that the swarms are probably initiated by magmatism. Normal faults along several tens of kilometres surrounding the volcanic centres react in large earthquakes (M > 5) to dyke emplacement. The routine generation of large earthquakes in the cold, brittle lithosphere of ultraslow spreading ridges makes the teleseismic record a valuable means to study ultraslow accretion processes and to provide a global framework for the interpretation of the limited local and regional seismicity studies.

Exitomelita sigynae gen. et sp. nov.: a new amphipod from the Arctic Loki Castle vent field with potential gill ectosymbionts

The newly discovered deep-sea hydrothermal vent field ''Loki's Castle'' at 2,350 m depth at 70N on the Knipovich Ridge north of the island Jan Mayen is the only known black smoker field from the Arctic Ridge system. This vent field holds a unique fauna clearly distinct from vent sites along the Mid-Atlantic Ridge south of Iceland. In addition to numerous maldanid and ampharetid polychae- tes one animal of particular interest at this vent site, is a new genus and species of melitid amphipod. This new species is found in cracks and crevices on vent chimneys as well as in areas where diffuse venting is common such as the base of vent chimneys. Here, we present a formal morphological description of this new melitid species and hypothesize about the main form of its nutrition using stable isotope and molecular data.

Topographic comparisons of uplift features on Venus and Earth: Implications for Venus tectonics

Venus and Earth display different hypsography. We use topographic profiles to search for well-understood terrestrial analogs to venusian features. Specifically, by using cross-correlation, we correlate average profiles for terrestrial rifts (slow and fast, “ultra-slow,” incipient and inactive) and also hotspots (oceanic and continental) with those for venusian chasmata and regiones, to draw inferences as to the processes responsible for shaping Venus’ surface. Correlations tend to improve with faster spreading rates; Venus’ correlations rank considerably lower than terrestrial ones, suggesting that if chasmata are analogous to terrestrial spreading centers, then spreading on Venus barely attains ultra-slow rates. Individual features’ normalized average profiles are correlated with profiles of other such features to establish the degree of similarity, which in turn allows for the construction of a covariance matrix. Principal component analysis of this covariance matrix shows that Yellowstone more strongly resembles Atla, Beta and W. Eistla regiones than it does the terrestrial oceanic hotspots, and that venusian chasmata, especially Ganis, most closely resemble the ultra-slow spreading Arctic ridge.

Rare gases in lavas from the ultraslow spreading Lena Trough, Arctic Ocean

Mid‐ocean ridge basalts (MORB) from the Arctic Ocean have been much less studied than those from the Indian, Atlantic, and Pacific due to the difficulty of access related to ice cover. In 2001 and 2004 the Arctic ridges (Gakkel Ridge and Lena Trough) were intensively sampled. In this study we present the first helium, neon, and argon concentrations and isotopic ratios in a suite of samples from the ultraslow spreading Lena Trough (∼0.75 cm/yr effective full rate). Central Lena Trough (CLT) lavas display 4He/3He between 89,710 and 97,530 (R/Ra between 7.4 and 8.1), similar to the mean MORB ratio of 90,000 ± 10,000 (R/Ra = 8 ± 1). In a three neon isotope diagram, the samples fall on the MORB line, without showing any excess of nucleogenic 21Ne. The 40Ar/36Ar ratios vary from 349 to 6964. CLT samples have a typical MORB He and Ne isotopic composition. Rare gases do not indicate any mantle heterogeneities or contribution of subcontinental lithospheric mantle, although this has been suggested previously on the basis of the Sr‐Nd and Pb isotopic systems. Based on noble gas systematics, a DUPAL‐like anomaly is not observed in the Arctic Ocean. We propose two possible models which reconcile the rare gases with these previous studies. The first is that the Lena Trough mantle has a marble cake structure with small‐scale heterogeneities (<1 km), allowing rapid diffusion and homogenization of rare gases compared to elements such as Sr, Nd, and Pb. The second model proposes that the recycled component identified by other isotopic systems was fully degassed at a recent date. It would therefore have a negligible mass budget of rare gases compared to other isotopic systems. This would suggest that the mantle enrichment beneath Lena Trough was generated by rift‐forming processes and not by recycling.

The role of currents and sea ice in both slowly deposited central Arctic and rapidly deposited Chukchi–Alaskan margin sediments

A study of three long cores from the outer shelf and continental slope north of Alaska in the Arctic Ocean indicate that localized drift deposits occur here with sedimentation rates of more than 1.5 m/kyr during the Holocene. Currents in this area average about 5–20 cm/s but can reach 100 cm/s and these velocities transport the sediment found in these cores primarily as intermittent suspended load. These high accumulation sediments form levee-like deposits associated with margins of canyons cutting across the shelf and slope. Unlike most textural investigations of Arctic sediment that focus on the coarser ice-rafted detritus (IRD), this paper focuses on the > 95% of the sediment, which is finer than 45 μm. The mean size of this fraction varies between 6 and 15 μm in Holocene sediments from the Chukchi–Alaskan shelf and slope with the higher values closer to shore. Analysis of detailed size distributions of these Holocene deposits are compared to 34 sediment samples collected from sea ice across the Arctic Ocean and to Holocene sediment from central Arctic Ocean cores and indicate that similar textural parameters occur in all of these sediments. Principal components of these size distributions indicate that sea ice is an important link between the shelves and the central Arctic. Factor scores indicate nearly identical components in the clay and fine silt size fractions but very different components in the coarse silt for sea ice sediment and central Arctic ridge sediments compared to shelf and continental slope deposits. Sea ice must contribute to sedimentation in both of these Arctic regions, but bottom currents dominate in the slope region, forming drift deposits.

A Note on Submarine Ridges and Elevations with Special Reference to the Russian Federation and the Arctic Ridges

The Russian Federation's claim to the 2,500,000 km2 area of the Arctic continental shelf is based mainly on the inclusion of the Lomonosov and Mendeleev Ridges as part of the Russian continental margin. The UN Convention on the Law of the Sea does not clearly define differences between oceanic ridges and natural components of the continental shelf. The main burden of interpretation rests with the Commission on the Limits of the Continental Shelf. How the Commission deals with Arctic ridges will influence the scope of claims from all the other coastal states with adjacent ridges and natural components of the continental shelf.

Decadal- to millennial-scale variability of Arctic–Subarctic oceans and adjacent lands: a contribution of the Polar Climate Stability Network of Canada to the International Polar YearThis article introduces a series of papers published in this Special Issue on the theme Polar Climate Stability Network. It is also in memorium of Dr. Roy M. (“Fritz”) Koerner (1932–2008)

The present special issue gathers contributions from the Polar Climate Stability Network (PCS-N), a network funded by the Canadian Foundation for Climate and Atmospheric Sciences. These contributions fall primarily under one of its four research themes, that of ‘‘The Arctic/North Atlantic Oscillation and the role of the Arctic Ocean in the Climate System: Paleoclimatological Perspectives’’. A companion special issue on ‘‘Polar Climate Stability’’, in the Journal of Climate (ams.allenpress.com/perlserv/?request=getcollectionc cf. also Koerner 1989) still deserves full recognition. The bridging of ice, land, and ocean studies at high latitudes encounters major difficulties, particularly associated with the uneven time control of relevant time series, but it is a focus point of the PCS-N group, and it will thus receive special attention at a later stage of our research activities. A first attempt has been made, though, during a dedicated meeting held two years ago at the GEOTOP research centre (Montreal, Quebec). During this meeting, ice-, land-, and oceanbased geoscientists tried to address the specific issue of seaice variability during the Holocene (Fisher et al. 2006). ‘‘Fritz’’ Koerner, who passed away just a few months ago, was part of this initiative, and the PCS-N group would like to acknowledge his exceptional contribution to Canadian paleoclimate studies by dedicating this special issue to his memory (see his obituary at www.igsoc.org/news/ fritzkoerner/index.html). On land, pioneering work by many Quaternary geologists started as early as the 18th century, but has steadily increased since the late 19th century (see Andrews (1985); or Dyke (1999) for an overview of benchmark papers and early work). For the obvious reason of their location, below the (previously) thick Arctic sea-ice cover, deep-sea records from the Arctic Ocean remained much less documented than land records, although here also, pioneering work started quite early (see the review by Weber and Roots (1990)). A major step forward was made in 2004, when IODP (Integrated Ocean Drilling Program) supported the ACEX (Arctic Coring Expedition) on Lomonosov Ridge (Moran et al. 2004), which yielded a discontinuous but critical sedimentary sequence illustrating major climate and environmental changes in the Arctic, since the Eocene, and then with the 2005 US–Swedish Healy Oden TRansArctic eXpedition (HOTRAX; Darby et al. 2005a, 2005b), which yielded a series of deep-sea cores from all Arctic ridges. Several papers from this special issue report on this freshly recovered material. However, a critical step in the planning of the work behind most of the present contributions was achieved earlier, during a GEOTOP meeting held in 2002 at the St-Michel-des Saints field station of the University of Quebec at Montreal. Several contributors of the present special issue attended this meeting, as well as, most importantly, two of the chief-scientists of HOTRAX, Leonid Polyak (Byrd Polar Research Center, the Ohio State University, Columbus, Ohio) and Dennis Darby (Old Dominion University, Norfolk, Virginia), thus helping to determine future coring targets and objectives (see Fig. 1). This special issue represents a first step in PCS-N achievements with respect to the setting of robust paleoclimate time series from ocean and land records. From this Received 1 September 2008. Accepted 1 October 2008. Published on the NRC Research Press Web site at cjes.nrc.ca on 14 January 2009.

Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply

High‐density, precisely located, dive and rock‐corer basalt samples from the 65‐km‐long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near‐continuum in the higher MgO range, the samples can be separated into two groups; one is typical N‐type mid‐ocean ridge basalt (MORB) (K2O/TiO2 < 0.09), and the other is a more enriched T‐MORB (K2O/TiO2 > 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N < 1 and Zr/Nb > 20 for N‐MORB and (Ce/Yb)N > 1 and Zr/Nb < 20 for T‐MORB. Samples collected from off‐axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)‐depleted N‐MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N < 1), although isotopic ratios are comparable to those on‐axis. Lavas erupted in the past, before the present‐day deep axial valley formed on this part of the ridge, were more uniform N‐MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE‐depleted N‐MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U‐shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N‐, T‐, and E‐MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E‐MORB (Zr/Nb < 10, (Ce/Yb)N >1.0–3.0). Arctic E‐MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

Morphology and small-scale strength of ridges in the North-western Barents Sea

Abstract Four medium to large first-year ridges have been examined with respect to geometry, porosity, morphology, and physico-mechanical properties in the Barents Sea close to the island Hopen in May 2002 to 2005. The geometry and morphology of the ridges compares well with the acknowledged literature, and ridges in the Barents Sea are expected to be similar to other Arctic ridges. The size of the pores increased with depth until about 1/3 to 1/2 of the keel depth, and decreased after about 2/3–3/4 of the keel depth. The porosity throughout the keels increased with depth. The salinity of the level ice and the consolidated layer was about 4 to 5 ppt and no systematic differences were found. Altogether 494 samples were tested in uniaxial compression for e˙nom = 10− 3 s− 1. Salinity, density and temperature was measured for each individual sample and the porosity (η) calculated. The strength (f) for different parts of the ridge and the level ice is presented and discussed as a function of η. The mean value and the coefficient of variation of f both decreased with increasing η. The strength dropped for relative air volumes (ηa) above 10% and fmax(ηa > 10%) = 2.60 MPa. The ratio of the vertical and horizontal strength of the consolidated layer (fclV / fclH) was about 1.1, and fcl was between the vertical and horizontal strength of level ice (fliV > fcl > fliH). This strength characteristics is due to the different ice texture in level ice and consolidated layer. The strength of the sail and the consolidated layer was comparable and higher than that of the rubble blocks. Brittle samples were (except for η

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

FIRE AND ICE: MANTLE MELTING BENEATH THE HIGH ARCTIC MID-OCEAN RIDGES

Microbial community diversity in seafloor basalt from the Arctic spreading ridges

Microbial communities inhabiting recent (⩽1 million years old; Ma) seafloor basalts from the Arctic spreading ridges were analyzed using traditional enrichment culturing methods in combination with culture-independent molecular phylogenetic techniques. Fragments of 16S rDNA were amplified from the basalt samples by polymerase chain reaction, and fingerprints of the bacterial and archaeal communities were generated using denaturing gradient gel electrophoresis. This analysis indicates a substantial degree of complexity in the samples studied, showing 20–40 dominating bands per profile for the bacterial assemblages. For the archaeal assemblages, a much lower number of bands (6–12) were detected. The phylogenetic affiliations of the predominant electrophoretic bands were inferred by performing a comparative 16S rRNA gene sequence analysis. Sequences obtained from basalts affiliated with eight main phylogenetic groups of Bacteria, but were limited to only one group of the Archaea. The most frequently retrieved bacterial sequences affiliated with the γ-proteobacteria, α-proteobacteria, Chloroflexi, Firmicutes, and Actinobacteria. The archaeal sequences were restricted to the marine Group 1: Crenarchaeota. Our results indicate that the basalt harbors a distinctive microbial community, as the majority of the sequences differed from those retrieved from the surrounding seawater as well as from sequences previously reported from seawater and deep-sea sediments. Most of the sequences did not match precisely any sequences in the database, indicating that the indigenous Arctic ridge basalt microbial community is yet uncharacterized. Results from enrichment cultures showed that autolithotrophic methanogens and iron reducing bacteria were present in the seafloor basalts. We suggest that microbial catalyzed cycling of iron may be important in low-temperature alteration of ocean crust basalt. The phylogenetic and physiological diversity of the seafloor basalt microorganisms differed from those previously reported from deep-sea hydrothermal systems.