The Barents Sea ice sheet - a sedimentological discussion

Sediment sampling and shallow seismic profiling in the western and northern Barents Sea show that the bedrock in regions with less than 300 m water depth is unconformably overlain by only a thin veneer (<10 m) of sediments. Bedrock exposures are probably common in these areas. The sediments consist of a Holocene top unit, 0.1–1.5 m in thickness, grading into Late Weichselian glaciomarine sediments. Based on average sedimentation rates (¹⁴C-dating) of the Holocene sediments, the transition between the two units is estimated to 10,000–12,000 B.P. The glaciomarine sediments are commonly 1–3 m in thickness and underlain by stiff pebbly mud, interpreted as till and/or glaciomarine sediments overrun by a glacier. In regions where the water depth is over 300 m the sediment thickness increases, exceeding 500 m near the shelf edge at the mouth of Bjørnøyrenna. In Bjømøyrenna itself the uppermost 15–20 m seem to consist of soft glaciomarine sediments underlain by a well-defined reflector, probably the surface of the stiff pebbly mud. Local sediment accumulations in the form of moraine ridges and extensive glaciomarine deposits (20–60m in thickness) are found at 250–300m water depth, mainly in association with submarine valleys. Topographic highs, probably moraine ridges, are also present at the shelf edge. Based on the submarine morphology and sediment distribution, an ice sheet is believed to have extended to the shelf edge at least once during the Pleistocene. Spitsbergenbanken and the northern Barents Sea have also probably been covered by an ice sheet in the Late Weichselian. Lack of suitable organic material in the glacigenic deposits has prevented precise dating. Based on the regional geology of eastern Svalbard, a correlation of this younger stage with the Late Weichselian is indicated.     

Dynamics of plankton growth in the Barents Sea: model studies

1-D and 3-D models of plankton production in the Barents Sea are described and a few simulations presented. The 1-D model has two compartments for phytoplankton (diatoms and P. pouchelii) , three for limiting nutrients (nitrate, ammonia and silicic acid), and one compartment called "sinking phytoplankton". This model is coupled to a submodel of the important herbivores in the area and calculates the vertical distribution in a water column. Simulations with the 3-D model indicate a total annual primary production of 90-120g C m⁻² yr⁻¹ in Atlantic Water and 20-50g C m⁻² yr⁻¹ in Arctic Water, depending on the persistence of the ice cover during the summer.
The 3-D model takes current velocities, vertical mixing, ice cover, and temperature from a 3-D hydrodynamical model. Input data are atmospheric wind, solar radiation, and sensible as well as latent heat flux for the year 1983. The model produces a dynamic picture of the spatial distribution of phytoplankton throughout the spring and summer. Integrated primary production from March to July indicates that the most productive area is Spitsbcrgenbanken and the western entrance to the Barents Sea. i.e. on the northern slope of Tromsøflaket.     

The structural and sedimentological evolution of early synrift successions: the Middle Jurassic Tarbert Formation, North Sea

ABSTRACT This study addresses the complex relationship between an evolving fault population and patterns of synrift sedimentation during the earliest stages of extension. We have used 3D seismic and well data to examine the early synrift Tarbert Formation from the Middle–Late Jurassic northern North Sea rift basin. The Tarbert Formation is of variable thickness across the study area, and thickness variations define a number of 1- to 5-km-wide depocentres bounded by normal faults. Seismic reflections diverge towards the bounding faults indicating that the faults were active contemporaneous with the deposition of the formation. Many of these faults became inactive during later Heather Formation times. The preservation of the Tarbert Formation in both footwall and hangingwall locations demonstrates that, during the earliest synrift, the rate of deposition balanced the rate of tectonic subsidence. Local space generated by hangingwall subsidence was superimposed upon accommodation generated due to a regional rise in relative sea-level. In basal Tarbert Formation times, transgression across the prerift coastal plain produced lagoons and bays, which became increasingly marine. During continued transgression, barrier islands moved landward across the drowned bays. In the southern part of our study area, shallow marine sediments are erosionally truncated by fluvial deposition. These fluvial systems were constrained by fault growth monoclines, and flowed parallel to the main faults. We illustrate that stratal architecture and facies distribution of early sedimentation is strongly influenced by the active short-lived faults. Local depocentres adjacent to fault displacement maxima focused channel stacking and allowed the aggradation of thick shoreface successions. These depocentres formed early in the rift phase are not necessarily related to Late Jurassic – Early Cretaceous depocentres developed along the major linked normal fault systems.     

Surface temperatures of the Barents Sea

Temperature conditions in the Barents Sea are determined by the quality and quantity of the inflowing Atlantic water from the west and by processes taking part in the Barents Sea itself, in particular as a consequence of winter cooling and ice formation. The field of inflow to the Barents Sea during the period 1977-1987 has been studied. The surface winter temperatures within the Barents Sea vary in parallel with variations in the deeper layers of the inflowing water masses, whereas the surface temperatures in summer have a different variation pattern which is most likely dependent on the summer heating process.     

Calanus in North Norwegian fjords and in the Barents Sea

The Physical environment of a North Norwegian fjord and of the Atlantic and Arctic domains of the Barents Sea are described. The seasonal variation of primary production and biomass of the most important copepod species are described in order to contrast regional differences in the timing of the plankton cycles. Analysis of the seasonal variation in the biomass of six different copepod species in Balsfjorden clearly demonstrate the importance of Calanus finmarchkus as a spring and early summer form, whereas Pseudoculanus acuspes , the most important smaller form, reaches the highest biomass later during the productive season. In the Atlantic part of the Barents Sea, C. finmarchkus is the dominant herbivorous form. The next most important species, Pseudocalanus sp. and M. longa , play a less important role here than in Balsfjorden. In the Arctic domain, the smaller copepod forms appear to have been replaced in trophodynamic terms by the youngest year-group (C-CIII) of C. glacialis , which prevails during the Arctic summer and autumn periods. The coupling between primary producers and Calanus on a seasonal basis is addressed through the grazing and the vertical organisation of the plant-herbivore community. The productivity of these two Calanus species is considered in relation to the seasonal and inter-annual variation in climate; although different mechanisms are utilised, cold periods tend to lower Calanus productivity both in the Arctic and the Atlantic domains of the Barents Sea. Interannual variations in Calanus biomass and productivity are discussed in the perspective of endemic and advective processes.     

Sea ice algae in the White and Barents seas: composition and origin

To examine algae populations, three expeditions (in March 2001, April 2002 and February 2003) were conducted in the Guba Chupa (Chupa Estuary; north-western White Sea), and one cruise was carried out in the open part of the White Sea in April 2003 and in the northern part of the Barents Sea in July 2001. Sea ice algae and phytoplankton composition and abundance and the content of sediment traps under the land-fast ice in the White Sea and annual and multi-year pack ice in the Barents Sea were investigated. The community in land-fast sea ice was dominated by pennate diatoms and its composition was more closely related to that of the underlying sediments than was the community of the pack ice, which was dominated by flagellates, dinoflagellates and centric diatoms. Algae were far more abundant in land-fast ice: motile benthic and ice-benthic species found favourable conditions in the ice. The pack ice community was more closely related to that of the surrounding water. It originated from plankton incorporation during sea ice formation and during seawater flood events. An additional source for ice colonization may be multi-year ice. Algae may be released from the ice during brine drainage or sea ice melting. Many sea ice algae developed spores before the ice melt. These algae were observed in the above-bottom sediment traps all year around. Three possible fates of ice algae can be distinguished: 1) suspension in the water column, 2) sinking to the bottom and 3) ingestion by herbivores in the ice, at the ice-water interface or in the water column.     

Primary production of the northern Barents Sea

The majority of the arctic waters are only seasonally ice covered; the northern Barents Sea, where freezing starts at 80 to 81°N in September, is one such area. In March, the ice cover reaches its greatest extension (74-75°N). Melting is particularly rapid in June and July, and by August the Barents Sea may be ice free. The pelagic productive season is rather short, 3 to 3.5 months in the northern part of the Barents Sea (north of the Polar Front, 75°N), and is able to sustain an open water production during only half of this time when a substantial part of the area is free of ice. Ice algal production starts in March and terminates during the rapid melting season in June and July, thus equalling the pelagic production season in duration.
This paper presents the first in situ measurements of both pelagic and ice-related production in the northern Barents Sea: pelagic production in summer after melting has started and more open water has become accessible, and ice production in spring before the ice cover melts. Judged by the developmental stage of the plankton populations, the northern Barents Sea consists of several sub-areas with different phytoplankton situations. Estimates of both daily and annual carbon production have been based on in situ measurements. Although there are few sampling stations (6 phytoplankton stations and 8 ice-algae stations), the measurements represent both pelagic bloom and non-bloom conditions and ice algal day and night production. The annual production in ice was estimated to 5.3 g Cm^-2, compared to the pelagic production of 25 to 30 g Cm^-2 south of Kvitøya and 12 to 15 g Cm^-2 further north. According to these estimates ice production thus constitutes 16% to 22% of the total primary production of the northern Barents Sea, depending on the extent of ice-free areas.     

The onset of Pleistocene glaciation in the Barents Sea: implications for glacial isostatic adjustment

In this paper the effect of a delayed onset of glaciation in the Barents Sea on glacial isostatic adjustment is investigated. The model calculations solve the sea-level equation governing the total mass redistributions associated with the last glaciation cycle on a spherically symmetric, linear, Maxwell viscoelastic earth for two different scenarios for the growth phase of the Barents Sea ice sheet. In the first ice model a linear growing history is used for the Barents Sea ice sheet, which closely relates its development to the build-up of other major Late Pleistocene ice sheets. In the second ice model the accumulation of the Barents Sea ice sheet is restricted to the last 6 ka prior to the last glacial maximum.
The calculations predict relative sea levels, present-day radial velocities, and gravity anomalies for the area formerly covered by the Weichselian ice sheet. The results show that observed relative sea levels in the Barents Sea are appropriate for distinguishing between the different glaciation histories. In particular, present-day observables such as the free-air gravity anomaly over the Barents Sea, and the present-day radial velocities are sensitive to changes in the glaciation history on this scale.
A palaeobathymetry derived from relative sea-level predictions before the last glacial maximum based on the second ice model essentially agrees with a palaeobathymetry derived by Lambeck (1995). The additional emerged areas provide centres for the build-up of an ice sheet and thus support the theory of Hald, Danielsen & Lorentzen (1990) and Mangerud et al. (1992) that the Barents Sea was an essentially marine environment shortly before the last glacial maximum.     

A study of the climatic system in the Barents Sea

The climatic conditions in the Barents Sea are mainly determined by the influx of Atlantic Water. A homogeneous wind-driven numerical current model was used to calculate the fluctuations in the volume flux of Atlantic Water to the Barents Sea which are caused by local wind forcing. The study period is from 1970 to 86. When compared with observed variations in temperature, ice coverage, and air pressure, the results show remarkably good agreement between all three parameters. The climate system of the Barents Sea is discussed with emphasis on the interrelations and feedback mechanisms between air, sea, and ice.     

Biomass and respiratory ETS activity of microplankton in the Barents Sea

The activity of the respiratory electron transport system (ETS) of microplankton was measured in the Central Barents Sea during summer 1988. In vitro ETS activity increased with assay temperature between 0 and 2°C, as reported for other enzyme systems in plankton. The higher in situ activities were observed near the surface (upper 10-25 m) and were associated with chlorophyll a maxima. Respiratory activity in the upper 60 m accounted for 40-60% of the total column respiration. The activities (0-100 m) were lower than oxygen consumption rates reported in the Canadian Arctic, mainly due to lower phytoplankton biomass. They were higher than ETS activity measured in the Weddell Sea (Antarctic Ocean). A high detrital versus total microplankton mass accounted for the low activity related to particulate organic carbon (POC). In general, the levels of respiratory ETS activity were in the range reported for temperate oligotrophic oceanic regions.     

Aerial strip surveys of polar bears in the Barents Sea

Aerial strip surveys of polar bears in the Barents Sea were performed by helicopter in winter 1987. The number of bears within 100 m on each side of the helicopter was counted. A total of 263.6 km² was surveyed and 21 bears were counted. Most of the bears were found in the southern part of the area, which indicates that the southwestern ice edge area in the Barents Sea is a very important winter habitat for polar bears.     

Distribution and life history of krill from the Barents Sea

Krill from the Barents Sea were studied on six cruises from 1985 to 1989. Thysanoessa inermis and T. longicaudata were the dominant species, while T. raschii and Meganyctiphanes norvegica were rarer in the studied areas. The two dominant species T. inermis and T. longicaudata are mainly found in the Atlantic. Water and they do not to a large extent penetrate into Arctic water masses in the northern Barents Sea. M. norvegica is a more strict boreal species that does not occur as extensively in the Barents Sea as do the Thysanoessa species. The mean population abundance ranged from 1 to 61 individuals m⁻² for T. inermis and from 2 to 52 ind. M⁻² for T. longicaudata . The mean dry weight biomass of these two species ranged from 14 to 616 and from 19 to 105 mg⁻². Length frequency distributions indicate a life span of just over two years for T. inermis and T. longicaudata . Growth took place from about April to autumn with no apparent growth during winter. Maturation and spawning seem to occur after two years for T. inermis and one year for T. longicaudata . Main spawning occurred from May to June coinciding with the spring phytoplankton bloom. Captive spawners of T. inermis (total length 17-22 mm) shed 30-110 eggs per female in a single batch.     

Surface wave tomography of the Barents Sea and surrounding regions

The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1°× 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions.     

Photosynthesis-irradiance relationships in natural phytoplankton populations of the Barents Sea

An analysis is made of the photosynthesis-irradiance relationships in natural phytoplankton populations in the Barents Sea. The data set comprises 232 experiments carried out during a 10-year period, both in open and ice-covered waters. The variability on the P-I parameters is discussed and examined in relation to the variation in a variety of environmental conditions. The results suggest that in the Barents Sea, as in other Arctic areas, phytoplankton photosynthesis is mainly controlled by physical variables. However, control of the phytoplankton stock, i.e. by zooplankton grazing, seems also to have a considerable indirect influence on P-I parameters, especially after the spring bloom and the depletion of winter nutrients.     

Nitrogen uptake rates in phytoplankton and ice algae in the Barents Sea

Uptake rates of NH₄⁺, NO₃⁻ and dissolved organic nitrogen (urea) were measured in phytoplankton and in ice algae in the Barents Sea using a ¹⁵N-technique. NO₃⁻ was the most important nitrogen source for the ice algae (f-ratio = 0.92). The in situ irradiances in the subsurface chlorophyll maximum and in the ice algal communities were low. The in situ NO₃⁻ uptake rate in the ice algal communities was light-limited The in situ NO₃⁻ and NH₄⁻ uptake rates in the subsurface chlorophyll maximum were at times light-limited. It is hypothesised that NH₄⁺ may accumulate in low light in the bottom of the euphotic zone and inhibit the in situ NO₃⁻ uptake rate.     

The Northeast Arctic cod stock's place in the Barents Sea ecosystem in the 1980s: an overview

Just after World War II the size of the Northeast Arctic cod ( Gadus morhua ) stock was about 6 million tonnes, but at the beginning of the 1980s the stock had been reduced to 1 million tonnes, due mainly to the excessively high fishing-mortality. Nevertheless, the stock produced strong year classes at the 0-group stage in the relatively warm period 1983-1985. At the same time, individual growth in the cod stock was good, and in 1986 the stock size increased to over 1.5 million tonnes.
However, the cod preyed increasingly on the capelin ( Mallotus villosus ) present, and by the end of 1986 the capelin stock was seriously depleted. The cod compensated for the loss of capelin by preying more intensively on other food items, including smaller cod. Cannibalism increased by a factor of three from 1984 to 1986, and this is one important reason why the 1984 and 1985 year classes did not recruit to the fisheries as expected. Individual growth was dramatically reduced, and the average fish weight decreased by about 50% in most age groups. Because the quotas are in tonnes, more fish than expected were caught. This resulted in serious management problems and led to reductions both in stock size and quotas compared to the optimistic prognosis of the mid-1980s.