Knipovich Ridge Research Articles

Paleoenvironments in the Fram Strait during Marine Isotope Stages 2–6 based on planktonic paleobiological and stable-isotope proxies and ice-rafted debris

Two sediment cores from the Knipovich Ridge in the Fram Strait have been studied to examine paleoceanographic and climatic changes in the Polar North Atlantic during the late Middle and Late Pleistocene. As inferred from the data on foraminifers, nannofossils, diatoms, IRD, oxygen isotope values, and AMS 14C ages, the recovered deposits span the interval of MIS 2–6. Several beds marked by a high productivity of microplankton and often by a great IRD content are recorded in MIS 2, MIS 5a, 5c, 5e, and MIS 6 suggesting a repeated occurrence of seasonally ice-free waters and strong meridional circulation in the eastern Fram Strait. The planktonic foraminifers Globigerinoides ruber, Globorotalia scitula, G. crassaformis, and pink-colored Globigerina rubescens, exotic for this latitude, were identified in three points of the section. A hiatus in one of the cores studied was revealed from comparison of IRD variations and the occurrence of Pullenia bulloides.

Origin of the unique geochemical anomaly in the continent/ocean transition zone between Spitsbergen and Knipovich Ridge

The paper is devoted to the interpretation of published data on the geochemical anomaly detected in interstitial water within a thick (about 1 km) Pliocene—Quaternary marine sequence recovered by Hole 986 (ODP Leg 162). Salinity of interstitial water in the core from this hole decreases downward the section and the value at the bottom becomes 1.5 times lesser, relative to the normal marine value at the top. Nature of this geochemical anomaly remains enigmatic so far. We examine this issue under a more general scope of the origin of the inverse interstitial and fissure waters in sedimentary rocks. We support the opinion of hydrogeologists who attribute the super-low mineralization of these waters to the influx of high-temperature CO2-rich fluids into the sedimentary cover from magma sources in the basement. This inference is confirmed by not only the distribution of TDS content in the interstitial water, but also its alkalinity, Li content, and Na/Cl ratio in the hole core. The obtained results should be considered in the analysis of the nature of the Knipovich Ridge, as well as the type, and age of the Earth’s crust in the vicinity.

Recent tectonics in the northern part of the Knipovich Ridge, Atlantic ocean

The walls of the Knipovich Ridge are complicated by normal and reverse faults revealed by a high-frequency profilograph. The map of their spatial distribution shows that the faults are grouped into domains a few tens of kilometers in size and are a result of superposition of several inequivalent geodynamic factors: the shear zone oriented parallel to the Hornsunn Fault and superposed on the typical dynamics of the midocean ridge with offsets along transform fracture zones and rifting along short segments of the Mid-Atlantic Ridge (MAR). According to the anomalous magnetic field, the Knipovich Ridge as a segment of the MAR has formed since the Oligocene including several segments with normal direction of spreading separated by a multitransform system of fracture zones. In the Quaternary, the boundary of plate interaction along the tension crack has been straightened to form the contemporary Knipovich Ridge, which crosses the previously existing magmatic spreading substrate and sedimentary cover at an angle of about 45° relative to the direction of accretion. The sedimentary cover along the walls of the Knipovich is Paleogene in age and has subsided into the rift valley to a depth of 500–1000 m along the normal faults.

Tectonic and sedimentary processes along the ultraslow Knipovich spreading ridge

2D multichannel seismic data and bathymetric records from the glaciated western Svalbard margin and the rift valley region of the ultraslow, and oblique-spreading, Knipovich Ridge are in this study interpreted to infer differences in seafloor spreading mechanisms and to identify sedimentary processes. Our results show that the rift flank geometry, the rift valley elevation and the active magmatism are closely linked. The inferred magmatic segments of the Knipovich Ridge exhibit high and steep rift flanks, whereas the rift flank heights of the proposed tectonic-dominated segments are lower and less steep. In addition, we observe significant rift flank asymmetry across the rift valley which can be partly explained by subsidence due to sediment loading. The identification of a huge sedimentary wedge on the western rift flank suggests that the oldest parts of these sediments have been transported from the western Svalbard margin and across the rift valley. However, we suggest that most of these sediments are glacimarine/hemipelagic sediments which have been deposited in the time period after the rift valley flanks had developed sufficiently to cut off the direct transport routes from the western Svalbard margin. We also observe thick current depositions on the western side, suggesting a strong along-slope influence of the West Spitsbergen Current during the Plio–Pleistocene time period.

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.

Seismic and density structure of the lithosphere−asthenosphere system along transect Knipovich Ridge−Spitsbergen−Barents Sea – geological and petrophysical implications

AbstractThis paper presents a study of the seismic P−wave velocity and density structure of the lithosphere−asthenosphere system along a 800 km long transect extending from the actively spreading Knipovich Ridge, across southern Spitsbergen to the Kong Karls Land Volcanic Province. The 2D seismic and density model documents 6-8 km thick oceanic crust formed at the Knipovich Ridge, a distinct continent−ocean−boundary (COB), the east− ern boundary of the dominantly sheared Hornsund Fault Zone, and the eastern boundary of the Early Cenozoic West Spitsbergen Fold−and−Thrust Belt. The crustal continent−ocean transitional zone has significant excess of density (more than 0.1 g/cm3in average), charac− teristic for mafic/ultramafic and high−grade metamorphic rocks. The main Caledonian su− ture zone between Laurentia and Barentsia is interpreted based on variations in crustal thickness, velocities and densities. A high velocity body in the lower crust is preferably in− terpreted in terms of Early Cretaceous magmatism channelled from an Arctic source south− wards along the proto−Hornsund zone of weakness. The continental upper mantle expresses high velocities (8.24 km/s) and densities (3.2 g/cm3), which may be interpreted in terms of low heat−flow and composition dominated by dunites. The lower velocities (7.85 km/s) and densities (3.1 g/cm3) observed in the oceanic lithosphere suggest composition dominated by primitive peridotites. The model of mantle allows for successful direct description of subcrustal masses distribution compensating isostatically uneven crustal load. The esti− mated low value of correlation between density and velocity in the mantle 0.12 kg·s·m−4suggests that horizontal density differences between oceanic and continental mantle would be dominated by compositional changes.

Why is Svalbard an island? Evidence for two‐stage uplift, magmatic underplating, and mantle thermal anomalies

AbstractSvalbard is an anomalous, subaerial part of the Barents Shelf, Northeast Atlantic Ocean. In this study, we performed both, one‐ and two‐dimensional subsidence analyses based on basin structure, water depth, and thermochronology, to quantify and date the phases of uplift affecting Svalbard during the Cenozoic. Svalbard has experienced two phases of uplift, from >36 to ~10 Ma, and since ~10 Ma, similar in timing to uplift phases identified in Greenland, Scandinavia, and the Barents Shelf. Total uplift across much of the Central Tertiary Basin of Svalbard is >1.5 km and exceeds 2.5 km in parts of the West Spitsbergen Foldbelt (WSFB). Uplift from >36 to ~10 Ma accounts for the greatest part of the vertical motion and like the younger phase reduces in magnitude towards the east. Flexural rigidity of the lithosphere is estimated to be low (Te ≈ 5 km), so that post‐36 Ma erosion of the WSFB contributes little to the uplift, whose permanent nature and proximity to the synchronous Yermak Plateau favors a link to regional magmatic underplating. Plume dynamic support and flexural unloading along the western transform plate margin can be ruled out as influences on vertical motions. Since ~10 Ma renewed uplift, generating the modern topography may be linked to thermal erosion of the mantle lithosphere under Svalbard. We suggest that a likely cause of much of the surface uplift is the northward propagation of the Knipovich Ridge to establish continuous seafloor spreading through the Fram Strait after ~10 Ma.

Seismic gap beneath Logachev Seamount: Indicator for melt focusing at an ultraslow mid‐ocean ridge?

AbstractUltraslow spreading mid‐ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis. There is little or no basaltic crust between isolated magmatic centers. The processes that focus melts to segments of robust magmatism are not yet understood. During a seismic survey of the ultraslow spreading Knipovich Ridge in the Norwegian‐Greenland Sea with ocean bottom seismometers, we discovered a seismic gap in the upper mantle beneath Logachev Seamount, where micro‐earthquakes clearly delineate a shallowing of the maximum depth of faulting. A topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers, but has never been confirmed observationally. Our results are the first geophysical evidence for an along‐axis variation of the lithospheric thickness at an ultraslow spreading ridge.

Peculiarities of the magmatism and tectonics of the Knipovich Ridge

Petrological-geochemical data were obtained for intrusive rocks (gabbroids) recovered on the eastern flank of the Knipovich Ridge by deep-sea site 344 (DSDP, Leg 238). It was found that these rocks are similar to basalts and basaltic glasses studied in the adjacent sections of the ridge rift zone [7, 8]. This indicates that the intrusive rocks and erupted lavas are comagmatic. The gabbroids, basalts, and their quenched glasses were derived by differentiation in different-depth chambers and feeder channels. The petrochemical features of the gabbros and basalts (low Fe content, oxidized magnetic minerals) caused their weak magnetic properties. Owing to the multidirectional movements of the oceanic blocks, the bodies of the intrusive and effusive rocks their lost strict linearity and produced the mosaic anomalous magnetic field of the Knipovich Ridge.

Tectonics and magmatism of ultraslow spreading ridges

The tectonics, structure-forming processes, and magmatism in rift zones of ultraslow spreading ridges are exemplified in the Reykjanes, Kolbeinsey, Mohns, Knipovich, Gakkel, and Southwest Indian ridges. The thermal state of the mantle, the thickness of the brittle lithospheric layer, and spreading obliquety are the most important factors that control the structural pattern of rift zones. For the Reykjanes and Kolbeinsey ridges, the following are crucial factors: variations in the crust thickness; relationships between the thicknesses of its brittle and ductile layers; width of the rift zone; increase in intensity of magma supply approaching the Iceland thermal anomaly; and spreading obliquety. For the Knipovich Ridge, these are its localization in the transitional zone between the Gakkel and Mohns ridges under conditions of shear and tensile stresses and multiple rearrangements of spreading; nonorthogonal spreading; and structural and compositional barrier of thick continental lithosphere at the Barents Sea shelf and Spitsbergen. The Mohns Ridge is characterized by oblique spreading under conditions of a thick cold lithosphere and narrow stable rift zone. The Gakkel and the Southwest Indian ridges are distinguished by the lowest spreading rate under the settings of the along-strike variations in heating of the mantle and of a variable spreading geometry. The intensity of endogenic structure-forming varies along the strike of the ridges. In addition to the prevalence of tectonic factors in the formation of the topography, magmatism and metamorphism locally play an important role.

Crustal structures of the Boreas Basin and the Knipovich Ridge, North Atlantic

The Boreas Basin is located in the Norwegian-Greenland Sea between Northeast Greenland and Svalbard. Towards the east, it is bounded by the ultraslow mid-ocean Knipovich Ridge. Here, we present a 340-km-long seismic refraction line acquired during the expedition ARK-XXIV/3 of research vessel Polarstern in 2009, using 18 ocean bottom seismometers. It crosses the central Boreas Basin from the Knipovich Ridge to the Northeast Greenland margin. Thus, the line provides the first reliable crustal structure information of this basin. In addition, the gravity data acquired parallel to the seismic refraction line are used to calculate a 2.5-D gravity model. The P-wave velocity model shows an unusual ∼3-km-thin oceanic crust with seismic velocities less than 6.3 km s−1, indicating the absence of a significant oceanic layer 3. Mantle velocities vary between 7.5 kms−1 in the uppermost mantle and 8.0 km s−1 at approximately 15 km depth. The low velocities within the upper mantle may be explained by 13 per cent serpentinisation, which is negligible at about 15 km depth. Furthermore, the S-wave velocity model shows low Vp/Vs ratios in the mantle, indicating a highly serpentinised mantle at shallow depths. The gravity model has crustal densities between 2.3 and 2.9 g cm−3, which also point towards the absence of a significant thick oceanic layer 3. The results of our seismic refraction line and other geophysical data indicate that the entire Boreas Basin opened at ultraslow spreading rates since at least ∼28 Ma. No evidence for an extinct spreading ridge in the centre of the Boreas Basin was found.

Kinematics and characteristic features of the morphostructural segmentation of the Knipovich Ridge

The rift zone’s relief, the spreading kinematics, and the experimental modeling of the Knipovich Ridge’s formation were analyzed. Its rift zone is formed in a transtension environment. Faulting is predominant in its northern part, while strike-slip is characteristic for the south. A system of short extension basins connected by deep strike-slip U-shaped troughs is observed in the south. A system of volcanic rises connected by short shallow basins is observed in the north. The rift valley is V-shaped. According to the experimental modeling data, these extension kinematics provide the formation of short extension basins connected by strike-slips and transtension faults. Their length and orientation depend on the spreading obliquity of each segment.

Crustal thickness and earthquake distribution south of the Logachev Seamount, Knipovich Ridge

During RV Polarstern cruise ARK‐XXIV/3 (2009), a geophysical study along the ultraslow spreading Knipovich Ridge was conducted. The survey, located in the rift valley south of the Logachev Seamount (∼76°36′N), provides a crustal thickness of ∼4.5 km in the amagmatic parts and 5.7 km underneath the seamount itself. The velocity‐depth function indicates the presence of a thick oceanic layer 2 (4.5 km), but no indication for a thick oceanic layer 3. The only exception is the area underneath the Logachev Seamount, where velocities higher than 6 km/s are detected. This indicates a stronger and focussed melt supply underneath the seamount. Local seismicity was analysed for two days. In total, 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ML = 2.6. At least 48 of them are located in the upper mantle (up to 18 km below sea level), supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply.

Potential serpentinization, degassing, and gas hydrate formation at a young (<20 Ma) sedimented ocean crust of the Arctic Ocean ridge system

Global assessment of methane must consider the role of mid‐ocean ridges. Fluids from serpentinized mantle and gabbro material are noteworthy on ocean ridges, although they are not very well understood. Only a few sedimented ocean ridges exist worldwide, and they may document past and ongoing serpentinization‐driven migration of gas‐rich fluids. This study is based on two multichannel reflection seismic profiles acquired across a sedimented segment of the ultraslow spreading Knipovich Ridge offshore NW Svalbard. Seismic data allow suggesting a potential link between inferred areas of serpentinization, transfer of carbon from the deep‐seated host rocks through the sediments above by diapirism, and methane capture within the gas hydrate stability zone at the eastern flank of the Knipovich Ridge. The origin of sediment remobilization features can be related to intrusions and the degassing process from mantle serpentinization. These disturbances in sediments overlying the oceanic crust can be observed in seismic data and are interpreted as diapirs. In shallower sediments, at the predicted base of the gas hydrate stability zone, the seismic data show a bright spot with all the characteristics of a gas hydrate related bottom‐simulating reflector (BSR), such as enhanced reflection amplitude, phase reversal relative to the seabed reflection, and crosscutting of sedimentary strata. The BSR occurs at about 200 ms two‐way time within a sequence of marine sediments. Two‐dimensional concentration models of methane hydrate using the differential effective medium theory predict saturations of up to 26% of methane hydrate in the pore space of sediments in the gas hydrate reservoir.

Sedimentary cover and Bouguer anomalies in the northern part of the Knipovich ridge

Sedimentary cover and Bouguer anomalies in the northern part of the Knipovich ridge

New data on heat flow in the North Atlantic Region

91 Fortyone new measurements of heat flow on the Arctic Sea bottom were obtained in the course of marine expeditions (2008 and 2010). The high geody� namic activity of the lithosphere in the North Atlantic Region west of the Svalbard Archipelago near Knipov� ich Ridge was confirmed in the course of numerical modeling of deep temperatures. In this region, the mantle material solidus isotherm (~ 1200°C) is at the depth of 15 km. This testifies to the fact that Knipov� ich Ridge has developed as a young oceanic rift having existed since the Miocene. Numerical modeling of heat transfer in the lithos� phere of the North Atlantic Region based on new measurements made it possible to estimate deep tem� peratures in that part of the ocean and to make a con� clusion on recent geodynamic activity near Knipovich Ridge. Geothermal investigations of the Arctic shelf mar� gin, continental slope, and adjacent water zones of the North Atlantic Region west of the Svalbard Archipel� ago were carried out under the scientific program of the Russian Academy of Sciences "Geological History and Lithosphere of the Polar Regions." The main objective of our investigations was to obtain new data on heat flow, to calculate deep temperatures in the oceanic lithosphere, and to study tectonothermal evo� lution at the newest stage of the region's development.

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.

From pull-apart basins to ultraslow spreading: Results from the western Barents Sea Margin

This paper describes results from a geophysical study in the area between the ultraslow Knipovich Ridge and Bear Island, western Barents Sea. The objective was to map the crustal structure along a profile crossing a pull-apart rifted continental margin and oceanic crust generated by ultraslow spreading. The results are based on modeling of wide-angle seismic and gravity data, together with interpretation of multichannel reflection data. Our results show a two layered oceanic crust in the western part of the profile. The thickness of the oceanic crust is variable in the western 130 km, ranging from 3.5 to 5.5 km. East of km 130 the crustal thickness is relatively constant, with values close to the global average for oceanic crust. The oceanic crust is buried by a thick package of Cenozoic sedimentary rocks. The continent–ocean transition (COT) is placed in the interval 207–255 km, between unequivocal oceanic crust and the foot of the westernmost fault in the Hornsund Fault Zone. It is not possible to conclude whether this interval is oceanic crust or thinned and intruded continental crust, but we favor the latter interpretation, at least for the eastern part of the COT. Stretched continental crust is observed between Hornsund Fault Zone and the Knølegga Fault. Here the sedimentary rocks have high velocities and are interpreted to be mainly of Mesozoic and Late Paleozoic age. In this interval Moho depths increase abruptly from 15 km in the west to 27 km in the east. Crystalline basement velocities are observed close to the seafloor east of the Knølegga Fault. We suggest that continental breakup north of Greenland–Senja Fracture Zone occurred around 33 Ma, after a period of pull-apart tectonics. The spreading rate of the earliest seafloor spreading may have been higher than the present day spreading, creating thicker oceanic crust close to the COT.

The Cenozoic western Svalbard margin: sediment geometry and sedimentary processes in an area of ultraslow oceanic spreading

The northeastern high-latitude North Atlantic is characterised by the Bellsund and Isfjorden fans on the continental slope off west Svalbard, the asymmetrical ultraslow Knipovich spreading ridge and a 1,000 m deep rift valley. Recently collected multichannel seismic profiles and bathymetric records now provide a more complete picture of sedimentary processes and depositional environments within this region. Both downslope and alongslope sedimentary processes are identified in the study area. Turbidity currents and deposition of glacigenic debris flows are the dominating downslope processes, whereas mass failures, which are a common process on glaciated margins, appear to have been less significant. The slide debrite observed on the Bellsund Fan is most likely related to a 2.5–1.7 Ma old failure on the northwestern Barents Sea margin. The seismic records further reveal that alongslope current processes played a major role in shaping the sediment packages in the study area. Within the Knipovich rift valley and at the western rift flank accumulations as thick as 950–1,000 m are deposited. We note that oceanic basement is locally exposed within the rift valley, and that seismostratigraphic relationships indicate that fault activity along the eastern rift flank lasted until at least as recently as 1.5 Ma. A purely hemipelagic origin of the sediments in the rift valley and on the western rift flank is unlikely. We suggest that these sediments, partly, have been sourced from the western Svalbard—northwestern Barents Sea margin and into the Knipovich Ridge rift valley before continuous spreading and tectonic activity caused the sediments to be transported out of the valley and westward.

New data on the structure of the Knipovich Ridge, North Atlantic

The Knipovich Ridge extends for 550–600 km between the Mohns Ridge and the demarcation Spitsbergen Fracture Zone. The structural features of this ridge are repeatedly mentioned in the literature; however, substantial discrepancies remain in the treatment of its tectonics. New data on the structure of this ridge presented in this paper are based on the results of continuous seismic profiling in the area studied by the expedition of the Geological Institute, Russian Academy of Sciences and the Norwegian Petroleum Directorate on the R/V Akademik Nikolaj Strakhov in 2006; 56 seismic lines allow us to depict zones differing in seismic records that provide insights into their internal tectonic structure. Interpretation of the seismic data makes it possible to compile maps of the acoustic basement surface and sedimentary cover thickness in the studied area. These maps expand our knowledge of the geological history and geodynamics of the Knipovich Ridge at the neotectonic stage of its evolution.