Nansen Basin Research Articles

Crustal structures of the Canada basin near Alaska, the Lomonosov ridge and adjoining basins near the North Pole

Seismic refraction surveys conducted in 1976 and 1979 over the broken ice surface of the Arctic Ocean, reveal distinctly different crustal structures for the Fram, Makarov and Canada basins. The Canada Basin, characterized by a 2–4 km thick sedimentary layer and a distinct oceanic layer 3B of 7.5 km/s velocity has the thickest crust and is undoubtedly the oldest of the three. The crust of the Makarov Basin has a thin sedimentary layer of less than 1 km and is about 9 km in total thickness. The Fram Basin has a similarly thin sedimentary layer but is 3–4 km thicker than the Makarov as it approaches the Lomonosov Ridge near the North Pole. The ridge itself is cored by material with a velocity of 6.6 km/s and may be a metagabbro similar to oceanic layer 3A. This ridge root material extends to a depth of about 27 km, where a change occurs to upper-mantle material with a velocity of 8.3 km/s. The core is overlain by up to 6 km of material with a velocity of about 4.7 km/s which could be oceanic layer 2A basalts or continental crystalline rocks with some sedimentary material. The Fram Basin probably began to open contemporaneously with the North Atlantic about 70 m.y. ago, by spreading along the Nansen-Gakkel Ridge. Although not yet dated, the Makarov Basin is probably no older than the initiation of the Fram Basin and may be much younger. The Alpha Ridge may once have been part of the Lomonosov Ridge, splitting off to form the Makarov Basin between 70 and 25 m.y. ago and possibly contributing to the Eurekan Orogeny of 25 m.y. ago, evident on Ellesmere Island. In contrast, the likely age of the Canada Basin lies in the 125–190 m.y. range and may have been formed by the counter-clockwise rotation of Alaska and the Northwind Ridge away from the Canadian Arctic Islands. The Lomonosov Ridge emerges from this scenario as a block resulting from a strike-slip shear zone on the European continental shelf, related to the opening of the Canada basin (180-120 my) and then becomes an entity broken from this shelf by the opening of the Eurasia Basin (70-0 m.y.).

The residence time of the freshwater component in the Arctic Ocean

The time function of bomb tritium concentrations in river runoff to the Arctic Ocean has been reconstructed from published data on tritium in precipitation 1959–1975. Tritium measurements on oceanic samples through the halocline exhibit strong linear relationships between tritium concentrations (TU values) and salinity. These waters thus look like binary mixtures of Atlantic source water and freshwater runoff. Combining these data, the vintage of the freshwater component in the Arctic Basin has been determined assuming no other major tritium source. The relation indicates the average age of the freshwater component to be 11±1 years in the Nansen Basin and the outflow and somewhat higher in the Canada Basin. According to tritium/salinity data, a surface layer of 10–60 m is affected by sea ice melting and freezing in the Nansen Basin, and the thickness of this layer increases to 150–170 m toward the Canada Basin. There is tritium also in the deeper waters, the unmixed Atlantic water, which points at residence times for that water not to exceed 17 years.The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐012; $1.00, payment must accompany order.

The Lomonosov ridge experiment: “Lorex 79”

In the spring of 1979 the Department of Energy, Mines and Resources (EMR) undertook a large‐scale, multidisciplinary project to study the nature and origin of the Lomonosov Ridge. The scientific program was planned and coordinated by the Earth Physics Branch, and the logistic support was provided by the Polar Continental Shelf Project/Scientists from other branches of EMR, from the Department of Fisheries and Oceans (DFO), and from a number of universities in Canada and the U.S.A. took part in the project code named Lorex 79.The Lomonosov Ridge is an aseismic submarine mountain range that bisects the Arctic Ocean into the Amerasia and Eurasia Basins (Figure 1). It rises sharply to a height of some 3 km above the adjacent abyssal plains, the Makarov Basin to the west, and the Fram Basin to the east. It is close to 200 km wide where it approaches the North American and Eurasian continental shelves but narrows to 25 km at its midpoint near the North Pole. Its relatively flat top, the steepness of its flanks, and results from previous geophysical measurements indicate that it may be a sedimentary structure of continental origin rather than a structure of oceanic origin [Sweeney et al., 1978]. In this context it has been suggested on several occasions that it is a rafted fragment split off the Eurasian continental shelf at the time of the opening of the North Atlantic and the Eurasia Basin in Early Tertiary times. However, since the available geophysical, geological, and bathymetric data are relatively sparse and often of doubtful quality (because of the enormously difficult operating conditions), the continental nature of the Lomonosov Ridge has never been unambiguously demonstrated.

A seismic and gravity profile across the Arctic Ocean Basin

Gravity and seismic profiles were obtained from ice island ARLIS-II as it drifted across the central Arctic Basin and out into the Greenland Sea. The drift track begins over the Fletcher Abyssal Plain (Makarov Basin) and crosses the Lomonosov Ridge, Pole Abyssal Plain (Fram Basin), Morris Jessup Rise and ends on the edge of the Lena Trough. Hyperbolic reflectors are observed in the Fletcher Abyssal Plain that are interpreted as buried volcanic structures. Up to 1 km of seismic penetration of largely undisturbed sediments were obtained over the Lomonosov Ridge, which is also undercompensated with a + 50 mGal gravity high. This evidence supports the hypotheses of the ridge's origin as a former sliver of the Eurasian continental margin translated into the Arctic Basin by spreading along the Nansen Ridge axis. Fram Basin contains thick sequences of turbidite deposits. Deep reflecting horizons under the Morris Jessup Rise are interpreted as bedded sequences of rhyolitic lava and tuffs.

Rankine memorial Movement

Sediment samples collected during expedition ARK XIII/2 in summer 1997 with the German ice-breaker R/V POLARSTERN were investigated to estimate benthic microbial activity and total biomass of the smallest sediment-inhabiting organisms (size range from bacteria to meiofauna) from the Yermak Plateau northwest of Spitsbergen and adjacent deep-sea areas. Stations covered water depths from about 500 m north of Svålbard and on top of the Yermak Plateau to 3250–4250 m in the Fram Strait and the Nansen Basin. The area of investigations is located in a region with permanent ice-coverage, but in summer 1997 stations along the southern transect crossing the Yermak Plateau at about 81°N lay 50–100 km from the ice-edge. The hydrography of the area is characterized by the inflow of relatively warm, nutrient-rich and particle-laden Atlantic Water into the Arctic Ocean. The input of organic matter from primary production was estimated by measuring concentrations of sediment-bound chloroplastic pigments. Benthic activities and biomasses were evaluated by analyzing a series of biogenic compounds (i.e. bacterial exoenzymes, total adenylates, phospholipids, particulate proteins) in the sediments. Bacterial numbers and biomasses, meiofauna abundances, and nematode biomasses were determined for direct comparison with biochemical parameters. Faunal and biochemical data suggest a high current-driven lateral input of particulate organic matter from the South associated with increased sedimentation rates along the western slope of the Yermak Plateau.