Nansen Basin Research Articles

Origin of sediment pellets from the Arctic seafloor: sea ice or icebergs?

Sediment cores from the Norwegian and Greenland Seas and the Nansen Basin were studied to determine the origin of sediment pellets, centimetre-sized aggregations of clay to sandsized sediment occurring in the cores. By comparing the grain size, grain shape and composition of the pellet sediments to sediments collected directly from the surfaces of sea ice in the Nansen Basin and from icebergs in the Barents Sea, the pelleted sediment was found to be more similar to that in the icebergs than that on the sea ice. The pellets may be formed on, in or under a glacier or during transport on/in an iceberg. When icebergs overturn or melt, the pellets fall out and are consolidated enough to survive a drop of up to 4 km to the ocean bottom and to retain their integrity even after burial on the seafloor.

Distribution of dominant copepods in the Nansen Basin, Arctic Ocean, in summer

Zooplankton were collected from five depth strata (0-25-50-100-200-500 m) at 12 stations along two sections across the Nansen Basin of the Arctic Ocean in July/August 1987. Vertical and horizontal distribution of biomass and abundance of the four dominant copepod species Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa, together with their gonad maturity state, are presented. Egg production of C. finmarchicus, C. glacialis and Metridia longa was measured from concurrent Bongo net samples in the upper 80 m. Four biological provinces were distinguished, which closely agreed with the hydrographic regimes. In the south, the Barents Shelf slope and the Southern Transition were located in the boundary current system of Atlantic water flowing eastward along the Eurasian shelves. They were characterized by the dominance of Calanus finmarchicus. The Barents Shelf slope, representing the core of the boundary current, hosted large populations of C. hyperboreus and Metridia longa. Metridia longa, with its main occurrence in the depth of the Atlantic layer, was the biological marker of this province. The southern provinces were separated from the Central Nansen Basin and the Nansen-Gakkel Ridge by a sharp discontinuity in biomass, stage distribution and gonad maturity of the three Calanus species north of 83°N. This faunistic boundary coincided with a well defined hydrographic frontal zone. Apparently, zooplankton distribution followed closely the flow of Atlantic water below 60-200 m eastward, although the three Calanus species had their center of abundance in the surface layer consisting of polar water, which supposedly follows the Transpolar Drift westward.

Internal waves and mixing in the Arctic Ocean

The variability of internal wave shear levels in the eastern Arctic Ocean is explored using velocity profiler and CTD data from Fram Strait and the Nansen Basin. Shear levels are consistently low over the abyssal plains and higher over rougher topography. Applying the parameterization of GREGG (1989, Journal of Geophysical Research, 94, 9686-9698) to these data gives diapycnal diffusivities that vary from about 10 -6 to above 10 -4 m s -2. Extrapolating these diffusivities to the entire Arctic Basin suggests that internal wave mixing could play a major role in transporting heat from the warm intermediate water to the surface. Internal wave generation by the barotropic tide on rough topography may explain the higher shear levels found there.

Tracing upper waters of the Nansen Basin in the Arctic Ocean

Measurements of chemical tracers obtained during the 1987 cruise of F.S. Polarstern across the Nansen Basin of the Arctic Ocean are used to suggest the general flow of upper waters in the Arctic Ocean. In surface water, total carbonate concentrations distinguish between a northern freshwater component whose origin is river run-off and a southern freshwater component whose origin is sea ice meltwater. Below the surface layer, sub-surface water formed in the Barents-Kara Seas with low concentrations of “NO” must divide, some flowing with the Atlantic layer to the east and north before flowing in the same direction as the Transpolar Drift to Fram Strait, and some flowing to the west and south. The former characterizes the lower halocline water observed at the CESAR and LOREX ice camps in the central Arctic Ocean. The latter can be traced in a general way to Fram Strait where the two meet and the two low NO waters merge.

Arctic deep-sea research: the Nansen Basin Section

Arctic deep-sea research: the Nansen Basin Section

Combined high-resolution magnetostratigraphy and nannofossil biostratigraphy for late Quaternary Arctic Ocean sediments

A detailed paleomagnetic and calcareous nannofossil study has been carried out on long sediment cores taken during the 1987 ARK IV/3 expedition of R.V. Polarstern in the eastern Arctic Ocean. The Brunhes-Matuyama boundary was not recovered in any of the cores. The magnetostratigraphies reflect the complex behaviour of the geomagnetic field during the Brunhes Chron of predominent normal polarity. Several short excursions and polarity events of the earth's magnetic field are documented in the cores. Their regional correlation was possible on the basis of the characteristic downcore magnetic susceptibility patterns and other physical properties of the sediments. Using the calcareous nannofossil chronostratigraphic framework, seven geomagnetic events of short duration were identified for the last 200 ky. Sedimentation rates derived from the stratigraphic data are highly variable for the Nansen-Gakkel Ridge area, ranging from a few mm ky -1 to several cm ky -1 . More uniform rates of 2-3 cm ky -1 were obtained for the central Fram Strait, the eastern Yermak Plateau, and the southern Nansen Basin and up to 9 cm ky -1 at the Svalbard continental slope and the northern edge of the Yermak Plateau. The chronostratigraphic scheme of geomagnetic events and coccolith abundance patterns developed for the Fram Strait area was applied to the sites of the ARK IV/3 expedition further north. It was then possible to relate the occurrence of coccoliths in the sediments to time. Towards the north, coccoliths occur later in the interglacial cycle, reflecting the influx of southerly currents carrying coccolith stocks to the Arctic Ocean.

Rare earth elements in the Arctic Ocean

In this paper we present the first data set for rare earth elements (REE) in the Arctic Ocean. The data are from unfiltered samples. The concentrations are La: 18.8–41.3 pM; Ce: 5.7–25; Pr: 1.4–9.9; Nd: 14.6–39.6; Sm: 2.7–7.3; Gd: 3.8–13.4; Dy: 3.7–12.3; Er: 3.6–9.6; and Yb: 2.9–8.1. The concentrations found in the deep water are lower than deep-water concentrations found elsewhere and are oceanographically consistent. Our data show that the distribution of REE are influenced by riverine input in the northern part of the investigated area. Transport of resuspended material into the water from the Barents Sea Shelf is suggested to influence the concentrations of REE in the southern part of the Nansen Basin. The covariation between REE and nutrients is opposite to that found in other oceans. The fresh water end member concentrations for the REE in the northern surface water have been calculated.

Internal wave observations from the Arctic environmental drifting buoy

A free‐drifting buoy serving as an ice anchor and flotation sphere for a 125‐m instrumented mooring line was deployed in the Arctic pack ice by the R/V Polarstern in August 1987 at the northernmost point achieved by the Arktis IV expedition. Current measurements were made from this Arctic environmental drifting buoy (AEDB) using a 150‐kHz acoustic Doppler current profiler (ADCP) attached to the mooring line at a depth of 16 m below the flotation sphere with the transducers facing downward. Velocity profiles spanning depths between 35 and 327 m with 15‐m resolution were collected at half‐hour intervals over a period of 216 days while the buoy drifted from its starting point near 86°N, 22°E to about 70°N, 16°W. The full experimental record was separated into three sections corresponding to periods of the drift spent in the Nansen Basin, over the Yermak Plateau, and in the Greenland Sea. Velocity time series and spectra were computed, and results for the three analysis sections were compared. Spectra in the Nansen Basin were found to have energy levels significantly less than those typical for mid‐latitudes, roughly one third of the canonical Garrett‐Munk (GM) level, and a spectral slope between ƒ and 2.5ƒ that was significantly flatter than that expected from the GM model. Between 83°N and 81°N, as the buoy passed over the Yermak Plateau, internal wave energy increased to levels comparable to GM. Isolated, energetic near‐inertial wave packets with upward group velocity were observed in this portion of the record, presumably generated by interaction of the barotropic tide with the bottom topography of the plateau. Estimates of the upward energy flux from these near‐inertial packets were comparable to the downward fluxes from typical surface sources. Interpretation of the Greenland Sea section was complicated by ADCP sampling problems and a lack of Argos fixes, but the available data showed internal wave spectral levels that were intermediate between those of the Nansen Basin and Yermak Plateau and no evidence of upward‐propagating near‐inertial energy.

Bromoform in the Nansen Basin in the Arctic Ocean

Bromoform (CHBr 3) has been measured throughout the water column on a transect across the Nansen Basin in the Arctic Ocean. The work was carried out on board the German R/V “Polarstern” in the summer of 1987. The analytical technique incorporated the extraction of the water samples with pentane and capillary gas chromatography with electron capture detection. Surface water concentrations varied from 7 ng l −1 in the southern part of the basin to 3 ng l −1 at the northernmost station. Bottom water concentrations in the interior of the basin were consistently around 0.7 to 0.9 ng l −1. The sources of bromoform in the environment are known to be largely natural, for example releases from marine algae. The results obtained in this investigation indicate that pelagic, as well as benthic algae, are capable of producing bromoform. The results also suggest that there is a flux of bromoform from the water to the atmosphere in the Arctic Ocean, possibly in the order of 10 6 kg year −1, and that marine algae thus are a significant source of atmospheric bromine in the Arctic.

Turbulent mixing near the Yermak Plateau during the Coordinated Eastern Arctic Experiment

During the ice camp component of the Coordinated Eastern Arctic Experiment (CEAREX) in March–April 1989, 1500 profiles of temperature, conductivity, and velocity shear microstructure were obtained near the Yermak plateau. The dominant signal in isopycnal displacements was the diurnal variability near the plateau, consistent with the enhanced diurnal currents found in this region previously from the Fram III and Fram IV ice camps. The dissipation rate of turbulent kinetic energy ε reached maximum values of about 10−6 W kg−1 in two distinct regions, the surface mixing layer and the pycnocline between 120 and 220 m. There was a significant component of diurnal variability in ε in each region. In the surface layer, ε was proportional to the cube of the ice‐relative current speed at 30 m below the ice, and it decayed approximately exponentially with increasing depth. Major mixing events in the pycnocline were correlated with large‐amplitude, short‐duration, isopycnal displacements. The lowest dissipation rates were recorded over the deep Nansen basin, north of the plateau, and in the subsurface core of a submesoscale baroclinic eddy in northern Fram Strait. The time‐averaged vertical eddy diffusivity near the slope of the plateau was about 2.5×10−4 m2 s−1 in the pycnocline above the Atlantic layer, implying an upward heat flux of 25 W m−2, although only a small fraction of this heat reached the base of the mixed layer. The results confirm that current interactions with steep topography are critical to the modification of Atlantic Water in the eastern Arctic Ocean.

Ventilation rates of the waters in the Nansen Basin of the Arctic Ocean derived from a multitracer approach

Tritium/3He−, 14C− and helium/neon data from a station located in the central Nansen Basin of the Arctic Ocean are reported and discussed. It is demonstrated that 3He is a valuable tracer for studies of the upper water column and, together with tritium, 3He provides a rough time scale of the ventilation of these waters. The apparent tritium/3He age is about 7 years in the surface water and about 14 years in the lower part of the halocline. Water of Atlantic origin defined by a temperature maximum shows tritium/3He ages between 7 and 9 years. This variation of ages indicates that the circulation of the individual water layers of the upper water column is largely decoupled and occurs on different time scales. Tritium/3He ages observed in the Arctic Intermediate Water suggest a ventilation on a time scale of 15 to 20 years (depth <1000 m). The mean age of the Eurasian Basin Bottom Water is estimated on the basis of 14C data to approximately 200 to 250 years. There is no indication of noble gas fractionation due to sea ice formation below the surface layer. From analysis of the 3He background it appears that there is a small primordial 3He component in Eurasian Basin Bottom Water.

Cesium 137 transport into the Arctic Ocean through Fram Strait

Cesium 134 and 137Cs activities were measured in water samples collected in March–April 1981 at the Fram 3 ice station located 200 km north of Fram Strait. The water depth profiles of these isotopes reveal the presence of two pathways for the entry of anthropogenic tracers into the Arctic Ocean through Fram Strait. Atlantic water passing across the Yermak Plateau at the 100–200 m level is labeled by elevated concentrations of 134Cs and 137Cs associated with inputs from the Sellafield reprocessing plant in the United Kingdom. The 134Cs/137Cs ratio is consistent with a transit time into the Nansen Basin from the Sellafield, United Kingdom, vicinity of 5–6 years. An elevated concentration of 137Cs at the 1500‐m level is associated with inputs of recently ventilated water from the Greenland‐Norwegian seas or as a consequence of brine formation and off‐shelf transport from the Barents Sea.

The first oceanographic section across the Nansen Basin in the Arctic Ocean

The first quasi-synoptic oceanographic section across a major deep basin of the Arctic Ocean reveals three different regimes: a narrow boundary current system along the northern Barents Shelf slope, a wide interior basin regime and a northern boundary curret regime with several distinct cores along the Nansen-Gakkel Ridge at 86°N. The southern boundary current cores are marked by high oxygen concentrations, high salinities and low temperatures that indicate sources on the shelf and in Fram Strait. The northern boundary current regime contains water mass signatures that are thought to come from the Amundsen Basin as well as from Fram Strait. The Nansen Basin interior is only slowly ventilated from the boundary currents and shelves, the deep water having an age of several decades. At 83°N characteristics change abruptly from those representative of the southern half of the section to those typical for the northern half and deep Arctic basins. Waters of the southern half of the basin, which have a strong melt-water component in their surface layers, largely originate directly in Fram Strait River runoff from the Siberian rivers and sea ice from the Laptev Sea contributes to the northern part of our section.

Arctic Ocean Ventilation Studied with a Suite of Anthropogenic Halocarbon Tracers

The chlorofluoromethanes (CFMs: CCl(2)F(2) and CCl(3)F), methyl chloroform (CH(3)CCl(3)), and carbon tetrachloride (CCl(4)) have been measured in deep waters of the Arctic Ocean. Oceanic and atmospheric inventories of these compounds result from known anthropogenic releases; because the CFMs and CCl(4) are also chemically nonreactive, they can be used as transient tracers of ocean circulation. The input history of CCl(4) is longer than that of any other transient tracer identified to date( approximately 70 years). This long input history, together with an e-folding time scale of increase(tau) of approximately 28 years, makes CCl(4) potentially the most useful tracer for calibrating models of the oceanic uptake of the fossil-fuel CO(2) transient(tau approximately 25 years). The bottom water of the Nansen Basin, Arctic Ocean, has detectable CCl(4) but undetectable CFM(s) and CH(3)CCl(3), which suggests either that the bottom water is approximately 50 years old, or that there is a small, nonanthropogenic component of atmospheric CCl(4)(<6 parts per trillion by volume).

Late Quaternary nannofossil record north of the Faeroe-Iceland Ridge

Over 60 short sediment cores from the Norwegian and Greenland Seas, the Fram Strait, the Nansen Basin and the continental shelf around Svalbard and Greenland have been analysed for calcareous nannofossils (coccoliths) (Fig. I). These photosynthetic planktonic algae provide information about the age of the sediment and the sea surface water environment.

Meiobenthic stocks and benthic activity on the NE-Svalbard shelf and in the Nansen Basin

High Arctic meiofaunal distribution, standing stock, sediment chemistry and benthic respiratory activity (determined by sediment oxygen consumption using a shipboard technique) were studied in summer 1980 on the NE Svalbard shelf (northern Barents Sea) and along a transect into the Nansen Basin, over a depth range of 240–3920 m. Particulate sediment proteins, carbohydrates and adenylates were measured as additional measures of benthic biomass. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigments (chlorophylls, pheopigments) were assayed. Pigment concentrations were found comparable to values in sediments from the boreal and temperate N-Atlantic. Meiofauna, which was abundant on the shelf, decreased in numbers and biomasses with increasing depth, as did sediment proteins, carbohydrates, adenylates and sediment oxygen consumption. Meiofaunal abundances and biomasses within the Nansen Basin were comparable with those observed in abyssal sediments of the North Atlantic. Nematodes clearly dominated in metazoan meiofauna. Protozoans were abundant in shelf sediments. Probably in response to the sedimentation of the plankton bloom, meiofauna abundance and biomass as well as sediment proteins, carbohydrates and adenylates were significantly correlated to the amount of sediment bound chloroplastic pigments, stressing the importance of food quantity to determine benthic stocks. Ninety-four percent of the variance in sediment oxygen consumption were caused by chloroplastic pigments. Benthic respiration, calculated per unit biomass, was 3–10 times lower than in the East Atlantic, suggesting low turnover rates in combination with a high standing stocks for the high Arctic benthos.

Inversion of refraction data from the Fram and Nansen Basins of the Arctic Ocean

As a part of the Fram 2 experiment (March–May 1980) six refraction lines were shot in the Pole Abyssal Plain or Fram Basin of the eastern Arctic Ocean. During the Fram 4 experiment (March–May 1982), four refraction lines were also shot in the Nansen basin just northeast of the Yermak Plateau. The Fram 2 lines at 86°N, 24°W cover both the abyssal plain region and the northern flank of the Morris Jessup Rise near magnetic anomalies 22 and 23. The Fram 4 lines near 83°N, 15°–20°E are on slightly younger crust at anomaly 12. In this paper we introduce new methods for processing and inversion of Arctic refraction data and contrast the results from these tectonically similar regions on opposite sides of the spreading center at the Arctic Mid-Ocean (Nansen) Ridge. In both experiments the data were recorded digitally on a two-dimensional array of 24 hydrophones located 93 m below the floe camps. These multi-channel data are processed by high-resolution array processing techniques to yield velocity spectra over the short 1 km apertures of the arrays. These velocity spectra are transformed to tau-slowness and offset-slowness parameterizations and are inverted by 1. (1) velocity-depth migration 2. (2) tau-sum 3. (3) extremal inversion techniques to obtain crustal velocity models down to the Moho. Without the ability of the array processing to discern late breaking primaries, multiples, and shear arrivals, the sparsely shot lines with 5–15 offsets over the 30–100 km line lengths would not be invertible by these methods. These results demonstrate the effectiveness of arrays in determining crustal structure under the constraints of Arctic ice pack environment.

Reinterpretation of morphology and crustal structure in the Central Arctic Ocean Basin

Ice station bathymetric profiles and submarine echograms reveal a steep escarpment at least 200 km long with up to 1700 m of relief subparallel to the Lomonosov Ridge. This feature, formerly called the Marvin Spur, is here interpreted as the Makarov‐facing flank of the Alpha Ridge. East of the 160°W meridian the intervening Makarov Basin is a narrow grabenlike trough between 25 and 60 km wide filled with horizontally stratified sediments that were deposited largely after the Marvin Seamounts were constructed along the basin axis. The association of basement relief with geomagnetic field variations within the trough plus evidence of normal faulting on the Lomonosov Ridge suggest that this segment of the Makarov Basin is an extensional feature produced by other than typical seafloor‐spreading processes. Gravity models indicate that the crust below the Makarov flank of the Alpha Ridge ranges in thickness between 16 and 20 km, considerably less than the 26 km known from seismic refraction work to exist below the Lomonosov Ridge. It is argued, on the basis of gravity data, that adjacent to the Lomonosov Ridge the Fram Basin crust, which is probably truly oceanic, is 3–4 km thinner than that of the Makarov Basin.

Inversion of refraction data from the Fram and Nansen basins of the Arctic Ocean : Duckworth, G.L. and A.B. Baggeroer, 1985. Tectonophysics, 114(1–4):55–102

Inversion of refraction data from the Fram and Nansen basins of the Arctic Ocean : Duckworth, G.L. and A.B. Baggeroer, 1985. Tectonophysics, 114(1–4):55–102

Crustal structure of the Lomonosov Ridge and the Fram and Makarov Basins near the North Pole

Reversed refraction surveys were conducted along and across the Lomonosov Ridge as part of the 1979 Lomonosov Ridge Experiment. Interpretation of the strike profiles indicates a 5‐km‐thick upper crustal layer with a velocity of 4.7 km/s overlying a 15‐ to 20‐km‐thick layer of 6.6 km/s material. An upper mantle velocity of 8.3 km/s is indicated by a few pn arrivals. High‐amplitude reflection events recorded from this boundary can be successfully modeled by a transition zone of rapidly changing velocity over a depth interval of 5 km. Ray trace modeling of the dip profiles suggests a root structure extending to about 28‐km depth flanked by crust thinning to a depth near 13 km beneath the Makarov Basin and a more gradual thinning to near 16 km beneath the Fram Basin. The similarity between the crust of the Lomonosov Ridge and that beneath the Barents and Kara seas supports the suggestion that the ridge is a slice rifted from the Baltic Shelf.