Paleoceanological Conditions on the Northern Segment of the Mohns Ridge (Norwegian Sea) at the End of the Middle Pleistocene Based on the Dinocyst Analysis

Abstract. The Arctic Front (AF) in the Norwegian Sea is an important biologically productive region which is well-known for its large feeding schools of pelagic fish. A suite of satellite data, a regional coupled ocean–sea ice data assimilation system (the TOPAZ reanalysis) and atmospheric reanalysis data are used to investigate the variability in the lateral and vertical structure of the AF. A method, known as “singularity analysis”, is applied on the satellite and reanalysis data for 2-D spatial analysis of the front, whereas for the vertical structure, a horizontal gradient method is used. We present new evidence of active air–sea interaction along the AF due to enhanced momentum mixing near the frontal region. The frontal structure of the AF is found to be most distinct near the Faroe Current in the south-west Norwegian Sea and along the Mohn Ridge. Coincidentally, these are the two locations along the AF where the air–sea interactions are most intense. This study investigates in particular the frontal structure and its variability along the Mohn Ridge. The seasonal variability in the strength of the AF is found to be limited to the surface. The study also provides new insights into the influence of the three dominant modes of the Norwegian Sea atmospheric circulation on the AF along the Mohn Ridge. The analyses show a weakened AF during the negative phase of the North Atlantic Oscillation (NAO−), even though the geographical location of the front does not vary. The weakening of AF during NAO− is attributed to the variability in the strength of the Norwegian Atlantic Front Current over the Mohn Ridge associated with the changes in the wind field.

In the absence of abiotic sources of CO2, variation in pCO2 and pO2 is expected to be inversely correlated in the water column due to biogenic processes. It has previously been suggested to use this correlation for leakage monitoring of offshore geological carbon storage (GCS) sites. In this study the aim is to investigate the extent of this correlation in ocean water masses with different origin and history in the Norwegian Sea, as well as in water masses in the vicinity of an active hydrothermal vent field at Mohn's Ridge, where pH is used as a proxy for pCO2. Over a hydrothermal vent site, a strong correlation between pH and pO2 is observed from 0 to 1700 m, whereas at depths >1700 m there is no correlation, likely due to CO2 emissions from the hydrothermal vents. However, at a reference site nearly 200 km from the hydrothermal vents, the intermediate Arctic water masses (700 – 1600 m depth) also show pH-pO2 correlations that are inconsistent with biogenic processes, but less pronounced compared to the hydrothermal vent site. These findings show that the suitability of this monitoring strategy will depend on a thorough site-specific evaluation of pH/CO2 and O2 relationships of relevant water masses.

End_Page 2488------------------------------As with all oceanic sea floor, the process of axial accretion along the crest of an active mid-oceanic ridge is of paramount tectonic importance to the geologic fabric of the northern Atlantic Ocean floor. The chronologic evolution of the Norwegian-Greenland Sea region has been derived from the analysis of Raff-Mason magnetic anomaly patterns by several investigators. These studies indicate that Norway and Greenland were severed when rifting commenced 60-70 m.y. ago, approximately along what are now the edges of their continental shelves. A half rate of 1.2 cm/year has been measured as the rate for this earliest sea-floor spreading. A second episode of spreading lasted from 40 to 18 m.y. ago, and was accompanied by a westward axial shift of the mid-oceanic ridge in the Norwegian Sea. At present, the axis of activity is along the Iceland-Jan Mayen and Mohns Ridges. Mohns Ridge apparently has been stable throughout the evolution of the region whereas the Iceland-Jan Mayen Ridge appears to be a very recent feature. In the Greenland Sea the Knipovich Ridge is apparently now acting as a trench which connects the mid-oceanic ridge branches in the Arctic and Norwegian Seas. Baffin Bay is enigmatic as to whether it is down-faulted continental or oceanic crust. The writers prefer the hypothesis that Baffin Bay was formed at the same time as the Labrador Sea (prior to 60 m.y. ago) in a proto-North Atlantic by the process of sea-floor spreading. The now-extinct Mid-Labrador Sea ridge would have extended via transform faults through Baffin Bay and perhaps even to the Alpha Ridge in the Arctic. This system then slowed down and became extinct in the Tertiary. End_of_Article - Last_Page 2489------------

The Greenland Sea intermediate waters transported via the Jan Mayen Channel (JMCh) are a significant source for the Iceland‐Scotland Overflow Water. Based on hydrographic data collected by a Norway‐China survey in 2015 and Argo floats, we classify the water masses of the Greenland Sea outflow and then reveal their distributions in the Norwegian Sea. The Atlantic‐origin water produced by density increase during winter in the northern Greenland Sea was an important component of the intermediate waters exported to the Norwegian Sea, accounting for about 30% of the total volume in 2015. The hydrographic data revealed that the major passage of outflow from the Greenland Sea was located in the southern Mohn Ridge in 2015, rather than in the JMCh, as generally recognized. The transport of intermediate waters from the Greenland Sea via the southern Mohn Ridge in the summer of 2015 is estimated to be about 0.8–1.7 Sv. This transport pattern provides a perspective that there exists a multi‐passage system of outflow in the JMCh and Mohn Ridge, promoting a stable supply of intermediate waters to the Norwegian Sea.

Cold and dense water from the Greenland Sea, which has been found in the Lofoten Basin in the Norwegian Sea, is an important contributor to the Greenland–Scotland Ridge overflow, which feeds the deep and bottom waters in the North Atlantic. These two basins are divided by the Mohn Ridge, but there is no clear current connecting them. The aim of this study is to investigate how the Greenland Sea water enters the Lofoten Basin. We deployed a mooring on the western flank of the Mohn Ridge to measure the potential transport across the ridge during two periods: 2016/17 and 2017/18. The observation results indicate that the water above 1500 m in the Greenland Sea can be intermittently transported to the Lofoten Basin. In addition, we observed periods of flow reversal, which indicate bidirectional exchange between the two basins across the ridge. Our data from three consecutive seasons indicate that such inflows in August–September are a typical feature of the exchange across the Mohn Ridge. Net exports during these two periods into the Lofoten Basin were eltimated to be 5.86 Sv and 3.00 Sv, exhibiting noticeable interannual variations. We propose two possible mechanisms that could be driving the export. One is due to passing cyclones, which lower the sea level height along the Mohn Ridge and drive outflow. The second is due to the sudden weakening of the wind in summer, which results in outflow from the Greenland Sea through temporary geostrophic deviation.

A swarm of earthquakes occurred on an oceanic spreading center north of Iceland in late 1995 and was recorded by U.S. Navy hydrophone arrays in the Norwegian Sea. About two dozen of these events on the Mohns Ridge were detected by onshore seismic arrays. Analysis of the hydrophone array data shows that 7000 events occurred; the greatest number (40/h) took place in a 3-day period in the middle of the 70-day duration of the swarm. Recorded arrivals include P waves, water-borne T waves, PT pairs, and P waves reflected at the seasurface. Separation in arrival times of P and T waves are used to determine relative locations of events and their spatial evolution throughout the swarm. The locus of activity shifts by 30–40 km during the swarm but steady migration of activity is not apparent. This suggests that surface breaks during dike injection did not occur or, at least, did not generate T waves, or that the swarm was not associated with a simple dike emplacement along the ridge. The time history of the activity, on the other hand, is quite similar to that seen associated with two known volcanic events on the Juan de Fuca and Gorda ridges.

Although oceanic spreading is often perpendicular to the ridge trends, in some cases the angle between these two directions can be significantly less than 90° (40°–50°). This occurs because of either a bend of the ridge trend or a change of the spreading direction. We here describe oblique spreading in the Mohns Ridge, resulting in deformation partitioning between the valley walls, which are dominantly affected by strike‐slip displacements, and the axial valley which is subject to nearly pure extension. The axial valley walls are characterized by en échelon normal faults affecting the walls, while the axial valley is affected by parallel faults grouped into oblique sets. These fault sets define different structures, horst or tilted blocks, that are regularly spaced inside the axial valley. Moreover some ridge segments mainly undergo pure extension, whereas others are affected by oblique extension. We explain this faulting pattern, including the along‐strike and transverse variations, as a consequence of depth variations of the brittle‐ductile transition.

The poleward flow of Atlantic Water in the Nordic Seas forms the upper limb of the meridional overturning circulation driving an important heat transport. The Norwegian Atlantic Front Current along the Mohn Ridge between the Greenland and Norwegian Seas is characterized for the first time, using repeated sections over 14 months from autonomous underwater gliders and two research cruises. The Norwegian Atlantic Front Current follows the 2,550‐m isobath with a width of 38 ± 2 km and absolute geostrophic velocities peaking at 0.56 ± 0.03 m s−1. The mean transport of Atlantic Water is 3.2 ± 0.2 Sv (equivalent to temperature transport of 71 ± 5 TW). Seasonal variability was observed with a magnitude of 0.8 Sv and maximum values in the fall. The deep currents at 1,000 m explained most of this seasonal variation and were anticorrelated with time‐integrated wind stress curl over the Lofoten Basin. Part of this flow might recirculate within the Lofoten Basin, while the rest continues toward the Arctic.

Continuous two-year studies of particle fluxes and associated environmental parameters in the axial zone of the Arctic segment of the Mid-Atlantic Ridge at the junction of the Mohns and Knipovich ridges (Norwegian Sea) have been carried out for the first time. Sediment traps were deployed from the R/V Akademik Mstislav Keldysh in June 2019 in the northeastern part of the Mohns Ridge with revision in August 2020 and lifting in July 2021. It is shown that the sedimentation of particles in the study area was affected by the global transport of water masses in the northeasterly direction, with Atlantic waters in the subsurface layer and water masses of Arctic origin recirculating deeper in the subsurface. A weak positive temperature trend at a depth of more than 2500 m (0.02°C over two years) is detected. The bimodal vertical distribution of fluxes and changes in the composition of sinking particles corresponded to the pattern of sediment recycling in the ocean. The annual course of fluxes in the subsurface layer is determined by the activity of Si-concentrating and calcifying plankton, while the maximum bloom period is also manifested in the deep layer. At the same time, the main significant source of the flux deeper than 2000 m is the supply of lithogenic matter from the near-bottom nepheloid layer.

Approximately 147,000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas. From these data inferences could be made about the geologic structure and evolution of the Arctic Ocean basin. The Alpha and Nansen Ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic that cross the Reykjanes Ridge and profiles in the Norwegian Sea that cross Mohns Ridge. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 m.y. ago, when the locus of rifting shifted to the Nansen Cordillera. The lack of magnetic disturbances associated with the Lomonosov Ridge is interpreted to be a section of the former Eu asian continental margin that was translated into the Arctic basin by sea-floor spreading along the Nansen Cordillera axis. Within the Canada basin there is a thickening of sediments from the Asia continental margin toward the Canada Arctic Archipelago. Sediment thickness in the Makarov basin is estimated to be 1-1½ km. There appears to be only about ½ km of sediment covering the younger Fram and Nautilus basins. The absence of large magnetic anomalies over these basins can be explained by a 10-km elevation of the Curie isotherm. End_of_Article - Last_Page 2512------------

The deep waters of the central and southern Greenland Sea are examined using Bartlett 1989 and 1990 summer data displayed at very high temperature and salinity resolutions. The mixing of Arctic Ocean Deep Water (AODW) with Greenland Sea Deep Water (GSDW) in proportions of about 1:1 to form “new” Norwegian Sea Deep Water (NSDW) is confirmed. However, the previously accepted route of this new NSDW through the ridge system into the Norwegian Sea, the Jan Mayen Channel, is shown to pass deep water only intermittently; a passage through Mohn's Ridge farther north, beginning near 72°06′N, 2°W, is identified as a more likely route. Part of the ultimate AODW‐GSDW mixture is shown to continue on along the Greenland continental slope into the Iceland Sea. A peculiar isothermal condition was found in the deeper parts of the GSDW. This is postulated to result from upward diffusion of temperature from a layer initially having the adiabatic temperature gradient. Above the isothermal layer is one of warming and decreasing salinity, the result of recent successive deep convections to different depths.

In a continuous effort to reduce cost and improve efficiency, the Oil and Gas industry has been trying for the last 10 years to develop methods to perform subsea Coiled Tubing (CT) operations from a vessel and without a riser. In September 2020 a large campaign of Riserless Coiled Tubing (RLCT) coring was successfully executed in the Norwegian Sea, on the Mohns Ridge, approximately 330 nautical miles from the coast. The campaign was performed from a small Anchor Handler Tug Supply vessel, the Island Valiant. A total of 14 open water gravity-fed RLCT runs were executed in water depths between 2780 and 3085 m. The system performed extremely well and proved to be very robust, efficient and effective for these innovative operations. This was the first time that RLCT coring operations were completed without the use of a subsea injector, in the so-called gravity-fed mode, and in such ultra-deep water. This paper describes the project in detail, including the system setup used, a summary of the operations and the actual results achieved, before discussing future improvements and applications of the RLCT technology.

AimWithin the intensively‐studied, well‐documented latitudinal diversity gradient, the deep‐sea biodiversity of the present‐day Norwegian Sea stands out with its notably low diversity, constituting a steep latitudinal diversity gradient in the North Atlantic. The reason behind this has long been a topic of debate and speculation. Most prominently, it is explained by the deep‐sea glacial disturbance hypothesis, which states that harsh environmental glacial conditions negatively impacted Norwegian Sea diversities, which have not yet fully recovered. Our aim is to empirically test this hypothesis. Specific research questions are: (1) Has deep‐sea biodiversity been lower during glacials than during interglacials? (2) Was there any faunal shift at the Mid‐Brunhes Event (MBE) when the mode of glacial–interglacial climatic change was altered?LocationNorwegian Sea, deep sea (1819–2800 m), coring sites MD992277, PS1243, and M23352.Time period620.7–1.4 ka (Middle Pleistocene–Late Holocene).Taxa studiedOstracoda (Crustacea).MethodsWe empirically test the deep‐sea glacial disturbance hypothesis by investigating whether diversity in glacial periods is consistently lower than diversity in interglacial periods. Additionally, we apply comparative analyses to determine a potential faunal shift at the MBE, a Pleistocene event describing a fundamental shift in global climate.ResultsThe deep Norwegian Sea diversity was not lower during glacial periods compared to interglacial periods. Holocene diversity was exceedingly lower than that of the last glacial period. Faunal composition changed substantially between pre‐ and post‐MBE.Main conclusionsThese results reject the glacial disturbance hypothesis, since the low glacial diversity is the important precondition here. The present‐day‐style deep Norwegian Sea ecosystem was established by the MBE, more specifically by MBE‐induced changes in global climate, which has led to more dynamic post‐MBE conditions. In a broader context, this implies that the MBE has played an important role in the establishment of the modern polar deep‐sea ecosystem and biodiversity in general.

Pliocene and Pleistocene sediments from Ocean Drilling Program Leg 151, Hole 911 A, drilled on the innermost Yermak Plateau (Eastern Arctic Ocean), were studied for their dinoflagellate cyst content. Three assemblage zones were tentatively defined, characterized by the predominance of few species. The composition of the assemblages changed markedly, even within single assemblage zones, during the last 2.6 to 2.8 m.y., reflecting the variable influence of warmer water from the Norwegian Sea, fluctuations in the influence of cold polar water masses, and the extent of sea-ice cover. Polar to subpolar surface water masses prevailed on the Yermak Plateau during the late Pliocene, when the eastern Arctic Ocean was probably isolated from the Norwegian-Greenland Sea. Intrusions of warmer water are recorded since the latest Pliocene, alternating with colder periods and a prolonged seasonal sea-ice cover. The composition of the dinoflagellate cyst assemblages has also changed considerably since the middle Pleistocene, reflecting the establishment of stronger fluctuations in surface water mass conditions than before at Yermak Plateau.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23060132