Irminger Basin Research Articles

Clay mineral signature of the NW Atlantic Boundary Undercurrent

Surface sediments were sampled along 2 transects, across the Iceland and Irminger basins and in the Labrador Sea. Clay mineral assemblages (deduced from X-ray analyses of the carbonate-free < 2 μm fraction) are largely dominated by smectites (about 60%) in the Iceland and Irminger basins. In the Labrador Sea, smectites are present along the Greenland Slope, but absent or rare in sediments from the Labrador Slope. They may, however, represent up to 50% of the clay fraction at depths between 2800 and 3400 m along both margins of the Labrador Sea, i.e., along the axis of the Western Boundary Undercurrent (WBUC). A detrital supply from the adjacent continents is unlikely. The WBUC is thought to be responsible for erosion and transport of fine particles from the smectite-rich Irminger and Iceland Basins, then for their redeposition in the Labrador Sea. These results suggest that clay minerals can be used as paleocurrent indicators in the Northwest Atlantic.

North Atlantic deep-sea sedimentation of Late Quaternary tephra from the Iceland hotspot

Piston cores recovered from the North Atlantic were used to study the sedimentation of Holocene and Pleistocene volcanic ash in the Irminger and Iceland Basins. Ash Zones 1 (≈ 11,100 yr B.P.), 2 (≈ 55,000 yr B.P.) and 3 (≈ 305,000 yr B.P.) were identified from their major element glass composition. The silicic and alkalic Ash Zones 1 and 2 originate from the Southeastern Volcanic Zone of Iceland, where they correlate with the Sólheimar ignimbrite from Katla volcano and the Thórsmörk ignimbrite from Tindfjallajökull volcano, respectively. The low-alkali composition of silicic Ash Zone 3 indicates a source from one of the silicic centers in the active rift system. Ash Zones 2 and 3 occur in the Irminger Basin as dispersed glass shards over a depth interval of several tens of centimeters. Their compositional and granulometric characteristics reflect an initial fallout on pack-ice north of Iceland, followed by ice-rafting sedimentation in the Denmark Strait, prior to bioturbation. δ 18O stratigraphy of foraminifera in the cores indicates that the ash zones were deposited during a cold interval, at the time when seas north of Iceland were ice-covered. Sedimentary features indicate that turbidity currents were also involved in the dispersal of Ash Zones 1 and 2 south of Iceland. The initiation of these gravity currents from the shelf can be attributed to either glacier bursts (jökulhlaups) carrying tephra, or the entrance of pyroclastic flows into the ocean.

An Isopycnic Model Study of the North Atlantic. Part I: Model Experiment

This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.

On the circulation of the northeastern North Atlantic

Along a section from Cape Farvel at the southern tip of Greenland to the Porcupine Bank off the Irish coast (WOCE section A1E/AR7E), obtained in September 1991, geostrophic velocities were calculated from CTD measurements and referenced to velocities recorded by an Acoustic Doppler Current Profiler (ADCP) in the upper 500 m. The mean accuracy of the absolute geostrophic velocities is estimated at±3.6 cm/s, which is comparable to that resulting from the assumption of a layer of no motion. The derived flow field is essentially columnar without a pronounced layer of no motion. Maximum velocities of 20–30 cm/s occur in the upper 1000 m of the East Greenland Current and the Porcupine slope current and at 2000 m depth in the Denmark Strait overflow. The meandering of the Irminger Current crossing the Reykjanes Ridge and an enhanced mesoscale variability along the Rockall Plateau and in the Rockall Trough are indicated. Compared to Schmitz and McCartney (1993) the estimated volume transports suggest an intensified meridional circulation with an increased supply from the subtropics to the Subpolar Mode Water (SPMW) and an increased entrainment of SPMW into the deeper layers. In the bottom layer with densities σ 0 ⩾ 27.80 kg/m 3 a reduced circulation of Iceland-Scotland Overflow Water (ISOW) in the Irminger Basin and a recirculation of ISOW in the Iceland and West European basins are found. About 12 Sv of bottom water are transported southwards and join the Deep Western Boundary Current.

Currents and mixing in the Irminger Sea and in the Iceland Basin

On the basis of a hydrographic section from the Faeroe Islands toward Greenland along 62°N and the deployment of about 20 satellite‐tracked buoys, it is shown that the Irminger Current branches at the Reykjanes Ridge. Its total transport amounts to 17–18 Sv. The eastern branch of the Irminger Current (8 Sv) turns east and enters the Norwegian Sea both through the Faeroe‐Shetland Channel (3 Sv) and by passing northeast of the Faeroe Islands. Maximum speeds of 70 cm s−1 are reached at the outer edge of the East and West Greenland Current. Very intensive mixing is observed between the four water masses that flow into the Irminger Basin. Stirring by eddies plays a dominant role for strong interleaving, subsequent salt fingering, and double‐diffusive layering. Conditions for these processes are optimal nearly everywhere in the Irminger Sea.

Drilling unearths “Fire and Ice” at Southeast Greenland Margin

Leg 152 of the Ocean Drilling Program, part of an ongoing campaign of continental margin drilling, investigated the southeast Greenland volcanic rifted margin (Figure 1) in October and November 1993. Drilling at six sites across the continental shelf and into the adjacent Irminger Basin provided a wealth of information about the tectonic and volcanic development of this margin, including the first penetration through the volcanic cover into underlying continental crust.Sediments recovered during the first deep drilling at the southeast Greenland margin give a detailed chronology of oceanographic and climatological events in the North Atlantic, including the surprising discovery that glaciation began in southeast Greenland as long ago as 7 Ma, far earlier than previously thought. This glaciation was preceded by an influx of cold, North Atlantic Deep Water into the Irminger Basin.

Recirculating components to the deep boundary current of the northern North Atlantic

The meridional overturning system of the North Atlantic is conventionally thought of as transporting warm water (≡ ≥4°C) to high latitudes and covd water to low latitudes. The northern cold water sources for this system are dense overflow from the Nordic Seas and less dense water from the labrador Sea (LSW). The overflow components are carried westward by a deep northern boundary current (DNBC) that is shown here actually to begin southeast of the overflow regions. The DNBC lies along the southern side of a system of islands and submarine ridges that divides the Nordic Seas from the subpolar basins. The overflows act to increase the transport of the DNBC, and this transport is further increased through the entrainment of warm waters. Once inside the Labrador Sea, the DNBC is joined by LSW before continuing south to low latitudes as a cold, deep western boundary current (DWBC). However, published transport estimates are larger than can be explained by warm water entrainment alone, thus indicating entrainment also of recirculating cold components, the subject of this paper. Their sources are shown to be the LSW and cold abyssal waters originating from the southern hemisphere. The LSW is entrained downward into the denser part of the DNBC and laterally into the upper part of the DWBC, whereas the cold abyssal waters are supplied by an eastern-intensified northward flow in the eastern Atlantic (that serves as the initial source of the DNBC) and by a similar northward flow in the western Atlantic. The meridional overturning system described includes recirculations and poleward transport of cold water in addition tothe components described by the conventional system. The cold abyssal waters in both the eastern and western basins have relatively low levels of oxygen and high concentrations of silicate reflecting their southern origin, but their influence in the northwestern Atlantic is somewhat obscurred by strong recirculating cyclonic gyres in the Newfoundland and Labrador Basins. This influence is detected by a deep silicate maximum extending poleward from the mid-latitude western Atlantic through the two gyres with eastern concentration and recirculating back (diluted) toward the south in the DWBC within the respective basins. Such a maximum also extends northward through the Irminger Basin into the DNBC, but there is an ambiguity as to whether this is a direct extension of the signal from the south in the western basin, or if it comes from the mid-latitude eastern basin by way of westward flow through the Charlie-Gibbs Fracture Zone; it is perhaps consequence of both. Estimates of the volume transports of the various recirculating cold components indicate that they supply about as much water to the deep boundary currents as do the combined cold water sources in the north.

Basement Structure and Sediment Distribution in Northwest Atlantic Ocean

We have interpreted all available single-channel and multichannel seismic reflection profiles to produce maps of structure contours on basement and isopachs of sediment thickness in the northwestern Atlantic Ocean, including the Labrador and Irminger basins. Where appropriate, we have modified and incorporated existing charts into the compilation. Contours are in meters, derived by applying velocity-regression equations based on sonobuoy results; this gives a more accurate representation of geologic relations than is possible in reflection-time mapping. Major structural relations and sediment distribution for the Mid-Atlantic Ridge, Mid-Labrador Sea Ridge, Davis Strait, and adjacent Greenland and eastern Canadian continental margins are clearly defined in the maps. Observ d basement structure is in good agreement with that expected from current plate-kinematic models of the region. The sediment-thickness patterns are controlled by interaction of such factors as age of underlying oceanic crust, crustal tectonic history, structural trends in basement, locations of sediment sources, and nature of the sedimentary processes delivering sediment to the basins.