Southern Norwegian Sea Research Articles

Reconstruction of hydrographic changes in the southern Norwegian Sea during the past 135 kyr and the impact of different foraminiferal Mg/Ca cleaning protocols

AbstractThe shallow subsurface hydrography in the southern Norwegian Sea during the past 135,000 years was investigated using parallel measurements of Mg/Ca and δ18O in shells of the planktic foraminiferal species Neogloboquadrina pachyderma. Two cleaning methods were applied prior to Mg/Ca analysis, “Mg cleaning” and “full cleaning” methods. Different results were obtained, which are most likely due to a more efficient removal of Mn‐contaminant coatings of the shells, when the “full cleaning” procedure was applied. We further combined Mg/Ca and B/Ca from the “full cleaning” method with δ18O values to constrain the calcification temperature and seawater‐δ18O (δ18Osw) in the past. During Heinrich Stadial (HS)1 (∼18.5–15 ka) N. pachyderma constituted >80% of the planktic foraminiferal population, δ18Osw decreased by ∼1.5‰, and shallow subsurface temperature increased by ∼1.5–3°C, suggesting strong stratification in the upper water column and a possible subsurface inflow of Atlantic water below a well‐developed halocline during the calcification seasons of N. pachyderma. Similar decreases in δ18Osw are also recorded for other Heinrich stadials (HS2, 3, 4, 6, and 11). Our temperature estimates confirm previous observations of the delay of the last interglacial “Eemian” warm peak in the eastern Nordic Seas compared to the North Atlantic, and a late warming coinciding with the summer insolation minimum at 60°N. In addition, relatively high values of δ18Osw during the early Eemian suggest a shallow subsurface inflow of Atlantic water below a thin layer of Arctic surface water.

Opportunistically recorded acoustic data support Northeast Atlantic mackerel expansion theory

Abstract Fisheries independent monitoring of widely distributed pelagic fish species which conduct large seasonal migrations is logistically complex and expensive. One of the commercially most important examples of such a species in the Northeast Atlantic Ocean is mackerel for which up to recently only an international triennial egg survey contributed to the stock assessment. In this study, we explore whether fisheries acoustic data, recorded opportunistically during the English component of the North Sea International Bottom Trawl Survey, can contribute to an improved understanding of mackerel distribution and provide supplementary data to existing dedicated monitoring surveys. Using a previously published multifrequency acoustic mackerel detection algorithm, we extracted the distribution and abundance of schooling mackerel for the whole of the North Sea during August and September between 2007 and 2013. The spatio-temporal coverage of this unique dataset is of particular interest because it includes part of the unsurveyed summer mackerel feeding grounds in the northern North Sea. Recent increases in landings in Icelandic waters during this season suggested that changes have occurred in the mackerel feeding distribution. Thus far it is poorly understood whether these changes are due to a shift, i.e. mackerel moving away from their traditional feeding grounds in the northern North Sea and southern Norwegian Sea, or whether the species' distribution has expanded. We therefore explored whether acoustically derived biomass of schooling mackerel declined in the northern North Sea during the study period, which would suggest a shift in mackerel distribution rather than an expansion. The results of this study show that in the North Sea, schooling mackerel abundance has increased and that its distribution in this area has not changed over this period. Both of these findings provide, to our knowledge, the first evidence in support of the hypothesis that mackerel have expanded their distribution rather than moved away.

On the flow of Atlantic water and temperature anomalies in the Nordic Seas toward the Arctic Ocean

AbstractThe climatic conditions over the Arctic Ocean are strongly influenced by the inflow of warm Atlantic water conveyed by the Norwegian Atlantic Slope Current (NwASC). Based on sea surface height (SSH) data from altimetry, we develop a simple dynamical measure of the NwASC transport to diagnose its spatio‐temporal variability. This supports a dynamical division of the NwASC into two flow regimes; the Svinøy Branch (SvB) in the southern Norwegian Sea, and the Fram Strait Branch (FSB) west of Spitsbergen. The SvB transport is well correlated with the SSH and atmospheric variability within the Nordic Seas, factors that also affect the inflow to the Barents Sea. In contrast, the FSB is influenced by regional atmospheric conditions around Svalbard and northern Barents Sea. Using a composite analysis, we further relate anomalous strong SvB flow events to temperature fluctuations along the core of Atlantic water. A warm composite anomaly is found to propagate northward, with a tendency to amplify enroute, after these events. A roughly 12 months delayed temperature signal is identified in the FSB. However, also in the Lofoten Basin interior a delayed temperature signal is found, which appears to originate from the NwASC. This study suggests that hydrographic anomalies both upstream from the North Atlantic, and locally generated in the Norwegian Sea, are important for the oceanic heat and salt transport that eventually enters into the Arctic.

Upper Cretaceous agglutinated foraminifera from a red sediment interval in the southern Norwegian Sea

Foraminiferal assemblages from a red sediment interval of the uppermost Santonian–middle Campanian Nise Formation, offshore Norway were examined for taxonomy and their paleoenvironmental implications using morphogroup analysis. The relatively highly diversified assemblages are composed only of deep-water agglutinated forms, and in total 61 taxa were identified. The taxonomic and morphogroup composition of the assemblages resemble both the slope flysch-type biofacies in containing robust tubular forms and rzehakinids, and assemblages described from Cretaceous Oceanic Red Beds (CORBs) in the North Atlantic and the Carpathians in the high abundance of infaunal forms in supposedly environments with low flux of organic matter. The intermediate nature of the assemblages in the red sediment interval is probably related to the shallower bathyal depositional settings and higher flux of organic matter due to the proximity to land in a narrow seaway inferred frommorphogroup analysis than abyssal settings of other CORBs. The foraminifera from the red sediment may be regarded as the background deep-water assemblage which reflectswell-oxygenated and low-nutrient environments only with small terrigenous input in the Norwegian Sea which was tectonically active during the Cretaceous.

Downhole Gauges Meet Challenges of Operating Beside ESPs in HT Reservoir

Technology Update Talisman Energy needed to enhance its continuous downhole monitoring capability at the Gyda field offshore Norway with a system that could perform in high-temperature (HT) conditions and operate beside electric submersible pumps (ESPs) in two oil production wells. Situated in the southern Norwegian Sea 270 km from Stavanger, Gyda is a mature field in which production peaked in 1995. To address the increasing water production and decreasing reservoir pressure, the company installed ESPs in the completion of new wells A-19A and A-26 in 2011. Estimated field production for 2011 was 8,000 BOPD, according to the Norwegian Petroleum Directorate. The field’s production license was recently extended to 2028. Challenges for New Monitoring System The Gyda field has an extensive permanent downhole monitoring network that tracks information on individual well conditions, such as temperature and pressure. Through sensors, gauges, and other downhole operating equipment, Talisman can monitor and, if necessary, maintain pressure; control each well; and optimize field production. However, adding monitoring systems to the A-19A and A-26 wells presented special challenges. The systems, which would need to monitor well zone conditions and the ESPs to ensure their function at the optimum points of their pump curves, would face reservoir temperatures as high as 160°C (and pressures as high as 300 bar). The HT environment can put enormous stress on instrumentation and impair its ability to generate reliable data. The standard gauges designed for the ESPs were not qualified to operate at the higher temperature levels found in the Gyda reservoir, thus further justifying the need for enhanced permanent monitoring. Another challenge was that the new downhole monitoring systems would have to operate effectively next to the ESPs. These pumps require large amounts of power, which is transferred downhole on a three-phase cable. This cable runs alongside the ESP cable and is susceptible to noise disturbance from the pumps’ motors and variable speed drive (VSDs). The VSDs—at 12 and 24 pulse, for example—can cause total harmonic distortion. This can be induced onto the downhole gauge control lines and interfere with downhole communication and gauge electronics, particularly during an ESP startup when the drive current can be six to eight times the normal operating current.

Variability of the northeast Atlantic sea surface Δ 14C and marine reservoir age and the North Atlantic Oscillation (NAO)

We compiled new 14C analyses of mollusc shells (bivalves and gastropods) of known age from the collection of the Muséum National d’Histoire Naturelle de Paris and previously published data to investigate changes in the sea surface Δ 14C and reservoir age in the northeast Atlantic sector (NEA) between 1823 and 1952 AD. The mollusc shells are mainly located off the Atlantic margin between 45°N and 60°N downstream of the North Atlantic Current (NAC). We show that the temporal variability of the NEA Δ 14C is independent of the mollusc species, depth habitat, diet and latitudinal distribution. The quasi-null difference between mollusc Δ 14C and the marine model indicate that the mollusc Δ 14C reflects the Δ 14C values of open marine conditions. Between 1823 and 1850 AD, the pre-anthropogenic mean of Δ 14C is −45 ± 5‰, corresponding to a reservoir age of 380 ± 60 years and a Δ R value of −7 ± 50 years, in agreement with previous estimates. The Δ 14C values show a significant long-term decrease of ∼12‰ from 1823 to 1952 AD attributed to changes in 14C production between 1823 and 1900 AD and the Suess effect between 1900 and 1952 AD. Between 1885 and 1950 AD, Δ 14C fluctuations of ∼10‰ up to 18‰ occurred in the northeast Atlantic, corresponding to reservoir age variations of ∼90 years up to 170 years. These fluctuations are very similar to changes of Δ 14C in the southern Norwegian Sea. Spectral analyses of the NEA Δ 14C exhibit quasi-periodic cycles of about 7.4 years, almost equivalent to the quasi-periodic cycles of the winter index of the North Atlantic Oscillation (NAO) with a period around 6.5 years. We find that changes of NEA Δ 14C cannot be attributed to changes in river runoff or the precipitation/evaporation budget. The Δ 14C lows (or high reservoir ages) correspond to the more intense phase of the winter NAO, with a time lag of ∼1–3 years. Such a time lag may reflect the eastward transit time of upstream changes originating in the Labrador Sea, then entrained within the NAC along with the subpolar gyre into the northeast Atlantic sector. During these events, we estimate that about 70% of the total transport in the NAC was due to water originating in the Labrador Sea.

OIL‐PRONE LOWER CARBONIFEROUS COALS IN THE NORWEGIAN BARENTS SEA: IMPLICATIONS FOR A PALAEOZOIC PETROLEUM SYSTEM

In this study, we assess the oil generation potential of Lower Carboniferous, liptinite‐rich coals in the Tettegras Formation on the Finnmark Platform, southern Norwegian Barents Sea. Oil from these coals has been expelled into intercalated sandstones. The coals may have contributed to petroleum recorded in well 7128/4–1 on the Finnmark Platform and may constitute a new Palaeozoic source rock in the Barents Sea.The Tettegras Formation coals contain up to 80 vol.% liptinite (mineral matter free base) and have low oxygen indices. Hydrogen indices up to 367 mg HC/g TOC indicate liquid hydrocarbon potential. In wells 7128/4–1 and 7128/6–1, the coals have vitrinite reflectance Ro= 0.75–0.85 %. Compared to shale and carbonate source rocks, expulsion from coal in general begins at higher maturities (Ro= 0.8–0.9% and Tmax= 444–453°C). Thus, the coals in the wells are mostly immature with regard to oil expulsion. The oil in well 7128/4–1 most likely originates from a more mature part of the Tettegras Formation in the deeper northern part of the Finnmark Platform. Wide variations in biomarker facies parameters and δ13C isotope values indicate a heterogeneous paralic depositional setting. The preferential retention by coal strata of naphthenes (e.g. terpanes and steranes) and aromatic compounds, compared to n‐alkanes and acyclic isoprenoids, results in a terrigenous and waxy oil. This oil however contains marine biomarkers derived from the intercalated shales and siltstones. It is therefore important to consider the entire coal‐bearing sequence, including the intercalated shales, in terms of source rock potential.Coals of similar age occur on Svalbard and Bjørnøya. The results of this study therefore suggest that a Lower Carboniferous coaly source rock may extend over large areas of the Norwegian Barents Sea. This source rock is mature in areas where the otherwise prolific Upper Jurassic marine shales are either immature or missing and may constitute a new Palaeozoic coal‐sourced petroleum system in the Barents Sea.

An isopycnal view of the Nordic Seas hydrography with focus on properties of the Lofoten Basin

Few basins in the world exhibit such a wide range of water properties as those of the Nordic Seas with cold freshwaters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature patterns on four upper ocean specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. These saline waters come from the Norwegian Atlantic Slope Current by means of a very high level of eddy activity in the Lofoten Basin. The isopycnal analyses further show that the principal water mass boundary between the waters of Arctic origin in the west and Atlantic waters in the east aligns quite tightly with the Jan Mayen, Mohn, Knipovich Ridge system suggesting little cross-ridge exchange. Instead, the main routes of exchange between the eastern and western basins appear to be limited to the northern and southern ends of ridge system: Atlantic waters into the Greenland Sea in the Fram St and Artic waters into the southern Norwegian Sea just north of the Iceland-Faroe Ridge. Analysis of a representative isopycnal in the main pycnocline shows it to be stable over time with only small variations with season (except where it outcrops in winter in the Greenland and Iceland Seas). However, two very cold winters, 1968–1969, led to greater than average heat losses across the entire Lofoten Basin that eroded away much of the Lofoten eddy and induced the greatest temperature anomaly in the entire 50-year record. Interannual variations in isopycnal layer temperature correlate with the NAO index such that waters in the Iceland Sea become warmer than average with warming air temperatures and conversely in the Lofoten Basin.

An objective climatology of the dynamical forcing of polar lows in the Nordic seas

AbstractA new objective climatology of polar lows in the Nordic (Norwegian and Barents) seas has been derived from a database of diagnostics of objectively identified cyclones spanning the period January 2000 to April 2004. There are two distinct parts to this study: the development of the objective climatology and a characterization of the dynamical forcing of the polar lows identified.Polar lows are an intense subset of polar mesocyclones. Polar mesocyclones are distinguished from other cyclones in the database as those that occur in cold air outbreaks over the open ocean. The difference between the wet‐bulb potential temperature at 700 hPa and the sea surface temperature (SST) is found to be an effective discriminator between the atmospheric conditions associated with polar lows and other cyclones in the Nordic seas. A verification study shows that the objective identification method is reliable in the Nordic seas region.After demonstrating success at identifying polar lows using the above method, the dynamical forcing of the polar lows in the Nordic seas is characterized. Diagnostics of the ratio of mid‐level vertical motion attributable to quasi‐geostrophic forcing from upper and lower levels (U/L ratio) are used to determine the prevalence of a recently proposed category of extratropical cyclogenesis, type C, for which latent heat release is crucial to development. Thirty‐one percent of the objectively identified polar low events (36 from 115) exceeded the U/L ratio of 4.0, previously identified as a threshold for type C cyclones. There is a contrast between polar lows to the north and south of the Nordic seas. In the southern Norwegian Sea, the population of polar low events is dominated by type C cyclones. These possess strong convection and weak low‐level baroclinicity. Over the Barents and northern Norwegian seas, the well‐known cyclogenesis types A and B dominate. These possess stronger low‐level baroclinicity and weaker convection. Copyright © 2008 Royal Meteorological Society

Steric height variability in the Nordic Seas

The variability of steric height in the Nordic Seas is analyzed on seasonal, interannual, and decadal timescales using a comprehensive data set of temperature and salinity observations for the second half of the twentieth century. Results from a regional Ocean General Circulation Model (OGCM) are used to assess the reliability of the averaging and the temporal interpolation of the inhomogeneous distributed observation data. The annual cycle explains only a minor part of the monthly variability for most of the region. The analysis on interannual to decadal timescales confined to the Norwegian Sea displays a clear rising trend starting at the end of the 1960s with particularly strong changes along the Barents Sea opening (6 to 7 cm). Moreover, a general freshening is found for the entire Norwegian Sea. In addition, a north‐south dipole of the thermal component of the steric height variability is identified. This dipole elevates the general rising trend along the Barents Sea opening and reduces it in the southern Norwegian Sea. The bulk of the interannual variability in steric height is governed by variations in the local meridional wind stress that determine the relative distribution of Atlantic and Arctic waters in the Norwegian Sea. In addition, reduced heat loss to the atmosphere strongly correlates with the North Atlantic Oscillation winter index. This, in turn, may in particular explain the large steric height increase found at the Barents Sea opening.

Meltwater and ice rafting in the southern Norwegian Sea between 20 and 40 calendar kyr B.P.: Implications for Fennoscandian Heinrich events

The timing of meltwater release in the southern Norwegian Sea in relation to millennial‐scale climate variability is studied from core MD99‐2283 based on down‐core analysis of stable oxygen and carbon isotopes, calcium carbonate and ice‐rafted debris (IRD). Between 20 and 40 calendar (cal) kyr B.P., strong Dansgaard‐Oeschger cyclicity is expressed in increased carbonate content and reduced total organic carbon during warm interstadials and IRD marking the end of cold stadials. The planktonic δ18O record of core MD99‐2283 compared to available isotopic records in the region confirms the existence of multisourced, synchronized meltwater anomalies during Heinrich (H) events 2 to 4. It was found that the sudden release of meltwater occurs near major ice streams and that no significant increase in IRD was associated with the peaks of the meltwater events in the southern Norwegian Sea, suggesting meltwater discharges from ice‐dammed lakes. Significant meltwater events not related to the H events were also observed between 33 and 35 cal kyr B.P., indicating that the release of meltwater is not necessarily connected with major cooling and enhanced IRD delivery. The simultaneous release of fresh water during H events in the Nordic Seas, through icebergs and ice‐dammed lakes, is thought to be the result of sea level increase. The meltwater input to the Nordic Seas provides a significant additional contribution to global sea level rise associated with H3 and H4.

Laminated sediments preceding Heinrich event 1 in the Northern North Sea and Southern Norwegian Sea: Origin, processes and regional linkage

Rapidly deposited laminated sediments dated between 18 and 21 cal. ka BP have been observed at several core locations in the Northern North Sea and the Southern Norwegian Sea. Sedimentation rates range from 20 cm/ka up to 2000 cm/ka, compared to a normal background sedimentation rate in the area of 5–10 cm/ka. Dropstones, in situ paired bivalves and grain size distribution indicate a hemipelagic origin for the laminae in core MD99-2291, from the upper continental slope of Southern Norway. AMS radiocarbon dating and cyclic changes in the number of planktonic foraminifera and lithological parameters suggest that the laminations in the core are annual varves. The laminated sediments are interpreted to be suspension plume deposits and two phases of plume deposition have been dated to 21.5 and 18.6 cal. ka BP. Large fluxes of meltwater were generated during this period, as reflected in oxygen and carbon isotopes records from cores in the area. The meltwater and the laminated plume deposits are interpreted to be associated with the presence and disintegration of the Norwegian Channel Ice Stream. Observations from other regions demonstrate that laminated high rate deposition is very widespread on glaciated continental margins. Around 18.6 cal. ka BP increases in sediment fluxes also occurred on the Barra Fan, the North Sea Fan, the Lofoten contourite drift and the Svalbard margin, preceding increased sediment and meltwater fluxes on the Greenland margin and the Labrador Sea, associated with Heinrich event 1. Warm water inflow to the Norwegian Sea together with rising sea level probably triggered the destabilisation of the ice streams of the Fennoscandian Ice Sheet, while a similar response of the Laurentide ice sheet was delayed by 500–1000 yr.

Latitudinal fractionation of polychlorinated biphenyls in surface seawater along a 62 degrees N-89 degrees N transect from the southern Norwegian Sea to the North Pole area.

Surface seawater concentrations of PCBs, relative congener abundance, and possible effects of cold condensation were studied along a transect from the southern Norwegian Sea to the central Arctic Ocean (62 degrees N-89 degrees N). Large volume samples were collected from an ice breaker using a stainless steel surface seawater intake connected online to an ultra-clean laboratory. Concentrations of all studied PCB congeners, except for trichlorinated PCB 18, decreased with latitude. For instance, PCB 52 decreased from 470 fg L(-1) at 62 degrees N to 110 fg L(-1) at 89 degrees N and PCB 180 from 110 to 12 fg L(-1). Concentrations in the central Arctic Ocean were on the order of 10-100 fg L(-1) for the most abundant congeners. The relative contribution oftrichlorinated PCBs to the total PCB concentration increased with latitude, the tetrachlorinated contribution to the total PCBs did not show any correlation to latitude, and the relative contribution of heavier congeners decreased with latitude. This study establishes the occurrence at very low abundances of PCBs in seawater in the central Arctic Ocean and demonstrates a northward concentration decrease. The latitudinal shift in congener pattern is reflecting the relative propensity of the PCB congeners to undergo long-range transport in the Arctic and is consistent with their relative vapor pressures.

Polycystine radiolarians in surface sediments in the Arctic Ocean basins and marginal seas

The polycystine radiolarian faunas in the Arctic Ocean, including the adjacent Arctic seas, have been studied in more than 377 surface samples. The deep basins in the Arctic Ocean have revealed 31 polycystine radiolarian species (11 spumellarians and 20 nassellarians). The polycystine radiolarian fauna in the southern Norwegian Sea was introduced from the north Atlantic Ocean ca. 13 400 yr BP. The shallow low Arctic marginal sea sediments (Iceland, Barents, and Chukchi Seas) have a strong dominance of Cannobotryidae (79%, 77% and 74%, respectively), while the deep Arctic Ocean basins (Nansen, Amundsen, and Makarov Basins) have a dominance of Actinommidae (90%, 70%, and 75%, respectively). The arctic polycystine radiolarian faunas have a strong affinity with the Norwegian Sea fauna, which supplies the Arctic Ocean with faunal elements that can live, thrive and reproduce there. In the Chukchi Sea the Pacific faunal influence is obvious (41 species), but none of the Pacific species are found in any abundance in the Amerasian Basin. No radiolarian species was found to be endemic to the Arctic Ocean except for a morphotype of Actinomma l. leptoderma, with a round inner sphere. This species is a possible endemic candidate for the Arctic Ocean. Eight species are to be found in all of the central Arctic basins, and can therefore be referred to as circumpolar species: Actinomma leptoderma leptoderma, A. l. leptoderma (with round middle sphere), A. l. longispina, A. boreale, Spongotrochus glacialis, Pseudodictyophimus platycephalus, Cycladophora davisiana, and Tholospyris(?) gephyristes, while Amphimelissa setosa probably is the only true arctic radiolarian species.

Greenland–Scotland overflow studied by hydro-chemical multivariate analysis

Hydrographic, nutrient and halocarbon tracer data collected in July–August 1994 in the Norwegian Sea, the Faroe Bank Channel (FBC), the Iceland and Irminger Basins and the Iceland Sea are presented. Special attention was given to the overflow waters over the Iceland–Scotland Ridge (ISOW). The Iceland–Scottland overflow water (ISOW) was identified along its pathway in the Iceland Basin, and entrainment of overlying water masses was quantified by multivariate analysis (MVA) using principal component analysis (PCA) and Partial Least Square (PLS) calibration. It was concluded that the deeper portion of the ISOW in the FBC was a mixture of about equal parts of Norwegian Sea Deep Water (NSDW) and Norwegian Sea Arctic Intermediate Water (NSAIW). The mixing development of ISOW during its descent in the Iceland Basin was analysed in three sections across the plume. In the southern section at 61°N, where the ISOW core was observed at 2300 m depth, the fraction of waters originating north of the ridge was assessed to be 54%. MVA assessed the fractional composition of the ISOW to be 21% NSDW, 22% NSAIW, 18% Northeast Atlantic Water (NEAW), 11% Modified East Icelandic Water, 25% Labrador Sea Water (LSW) and 3% North East Atlantic Deep Water. It may be noted that the fraction of NEAW is of the same volume as the NSDW. On its further path around the Reykjanes Ridge, the ISOW mixed mainly with LSW, and at 63°N in the Irminger Basin, it was warmer and fresher ( θ=2.8°C and S=34.92) than at 61°N east of the ridge (θ=2.37° C, S=34.97) . The most intensive mixing occurred immediately west of the FBC, probably due to high velocity of the overflow plume through the channel, where annual velocity means exceeded 1.1 m s −1 . This resulted in shear instabilities towards the overlying Atlantic waters and cross-stream velocities exceeding 0.3 m s −1 in the bottom boundary layer. The role of NSAIW as a component of ISOW is increasing. Being largely a product of winter convection in the Greenland Sea when no Greenland Sea Deep Water (GSDW) is formed, it spreads above the older and denser deep water in the Nordic Seas. Little or no GSDW, which earlier was considered to be the principal overflow water, has been formed since 1970. This shows that the Iceland–Scotland overflow may also be maintained with intermediate waters as the principal overflowing component. Decadal variability in ISOW properties has not been insignificant, as since the early 1960s there has been a decrease in salinity and temperature, by 0.06 and up to 0.5°C, respectively. Such a trend applies also to the LSW, particularly in the Irminger Basin, where it was warmer, saltier and less dense in the late 1950s and early 1960s (θ≈3.5° C, S≈34.9, σ 1.5≈34.64 kg m −3) than in 1994 (θ≈2.9° C, S≈34.86, σ 1.5≈34,69 kg m −3) . CFC tracers were used to assign apparent ages of water masses, showing that the NSDW had an apparent age of about 30 years and that the age of Iceland Sea Deep Water exceeded 25 years. NSAIW observed in the southern Norwegian Sea was estimated to be 6–16 years old. An upper age limit of LSW in the Iceland Basin was found to be 18–19 years. It was further concluded that the products of the onset of intense wintertime convection in the Labrador Sea in the late 1980s were not yet observed in the northern central part of the Iceland Basin. The LSW in the Irminger Basin was found to be significantly younger. Two layers were found there. A shallower layer at a depth of 1000– 1500 m depth was older than the layer beneath by about 4 years, while the deeper layer at 1500– 1800 m depth was assessed at an apparent age ranging between less than 1 (formed during the previous winter) and 4 years.

Norwegian sea‐surface palaeoenvironments of marine oxygen‐isotope stage 3: the paradoxical response of dinoflagellate cysts

AbstractHigh‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ18O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ18O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ18O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.

Interannual variability in water masses in the Greenland Sea and adjacent areas

Oceanographic data covering the period 1950–1998 are used to determine interannual variations in the convection intensity and water mass structure in the Greenland Sea and adjacent areas. Extremely cold winters throughout 1965–1970 assisted intensification of the water vertical exchange in the Greenland and Norwegian seas. As a result, cold and fresh Greenland Sea Deep Water (GSDW) production was extremely high in the central Greenland Sea while in the southern Norwegian Sea warm and salty water spread downwards. The recent rapid warming in the Greenland Sea Gyre interior from 1980 originates, we argue, from an increase in the Atlantic Water (AW) temperature due to the advection of warm waters into the region with the Return Atlantic Current. The negative water temperature and salinity trends in the upper 300 m layer of the Atlantic Water in the Norwegian Sea prevailed during 1950–1990, whereas during 1980–1990 the water temperature trends are indicative of warming of that layer. Observation series obtained onboard the Ocean Weather Ship Mike confirmed the existence of layers with advectiondriven high oxygen concentrations in intermediate and deep layers. The depth of oxygen maxima and the values of oceanographic parameters at this horizon can be regarded as indicators of the convection intensity in the Arctic domain. A simultaneous rise in NAO index and GSDW temperature points to a link between atmospheric and thermohaline circulation. Weakening in water exchange with the North Atlantic could be the reason for the Polar Water recirculation increase within the Nordic seas.

Statolith shape and microstructure as indicators of ontogenetic shifts in the squid Gonatus fabricii (Oegopsida, Gonatidae) from the Norwegian Sea

Statolith shape and microstructure were studied in 151 specimens of the common arctic squid Gonatus fabricii (7.3–322 mm pen length) collected in the southern Norwegian Sea. Statolith development and growth both comprised two main periods, which corresponded with the epipelagic and meso-bathypelagic ontogenetic periods of G. fabricii. During the epipelagic period (pen length range from 3 to 50–60 mm), statoliths quickly developed from the droplet-like form in early paralarvae to the pre-definite stage in juveniles (30–50 mm pen length). Paralarval and juvenile statoliths grew with high growth rates, and their microstructure contained narrow first-order growth increments. Three main growth zones (Z1, Z2, Z3) developed during this period, being well distinguished from each other by specific patterns of microstructure and separated from each other by distinct checks. During the meso-bathypelagic period (from 50–60 to 322 mm pen length), statoliths hardly changed their shape and grew very slowly. Only one growth zone (Z4) was formed within the statolith microstructure, characterized by disappearance of the first growth increments and formation of specific second-order bands. Each second-order band consisted of approximately seven first-order increments. If the assumption “one increment-one day” is true for G. fabricii, the squid would then be a slow-growing animal with a life span for both sexes not exceeding 2 years.

Distribution and abundance of overwintering Calanus finmarchicus in the Faroe–Shetland Channel

The distribution and abundance of overwintering Calanus finmarchicus in the Faroe–Shetland Channel was studied during December 1994 and January 1995. Maximum abundance of animals in the Channel was approximately 50 000 m−2, with a peak concentration of 627 m−3 at a depth of 930 m. Copepodite stages IV and V accounted for > 98% of the population. A clear association was found between the horizontal and vertical distribution of animals and the Arctic water masses in the bottom of the Faroe–Shetland Channel. The Wyville–Thomson Ridge formed a barrier to the southern distribution of Arctic waters and the abundance of overwintering C. finmarchicus was 25 times lower to the south of the Ridge than to the north. Spatial variability in lipid content and composition indicated that overwintering C. finmarchicus in the southern Norwegian Sea were in poorer condition with respect to wax ester content, and in a more advanced state of emergence from overwintering, than the animals within the Channel. The overwintering stock in the Channel probably originated from the Norwegian Sea or from north of the Faroe Islands. The Faroe–Shetland Channel is an important source of animals advected into the northern North Sea in the spring (March/April). The population abundance in the Faroe–Shetland Channel was estimated to be 4.5 × 1014 individuals, which is more than adequate to account for the mean concentration of adult stages observed in the northern North Sea in April.

Benthic δ18O records in the North Atlantic over the Last Glacial Period (60–10 kyr): Evidence for brine formation

Four high‐resolution benthic isotope records from North Atlantic and south Norwegian Sea cores are compared for the past 60 kyr. The benthic records show light δ18O peaks simultaneously to or following the Heinrich events. The amplitude of these light peaks varies between 1‰ for the most northern core ENAM93‐21 and 0.35‰ for the deep North Atlantic cores. The high δ18O gradient between the North Atlantic cores and the Pacific core V19‐30 suggests a regional influence of δ18O‐depleted meltwater to deeper depth during periods of iceberg discharges. The most likely process for transferring δ18O‐depleted meltwater to depth is sea ice formation induced by brine formation. During Heinrich events 4 and 3 this mecanism was of major importance (compared to open ocean convection) in the southern Norwegian Sea and at the northern boundaries of the Atlantic, leading to intermediate to deep water formation. Other δ18O excursions appearing in all the records point to the contribution of both brine formation and global changes in the δ18O of seawater associated with the Heinrich event periods.