North East Atlantic Deep Water Research Articles

230Th in the eastern North Atlantic: the importance of water mass ventilation in the balance of 230Th

The 230Th distribution in the water column on a north-south transect in the eastern North Atlantic was measured using thermal ionization mass spectrometry (TIMS). Water samples at three locations (L1, 33°N 21.6°W; L2, 47.4°N 19.5W; and L3, 54.4°N 21.1°W) were filtered and the dissolved and particulate fractions were analysed for their 230Th and 232Th contents. In the upper water column (< 1000 m water depth), the total 230Th ex distribution at the three locations could be explained by a reversible scavenging model. In the deep water column, however, 230Th ex concentrations decrease from south to north with relatively low and constant concentrations at stations L2 and L3. The major water mass below 2000 m is North East Atlantic Deep Water (NEADW), filling the deep water column at L1. The water column at L2 and L3 is influenced by Labrador Sea Water (LSW) down to 2000 m, whereas at L3, the deep water is strongly influenced by Iceland-Scotland Overflow Water (ISOW). The anomalous distribution of 230Th ex in the deep water column of stations L2 and L3 could be described after a ventilation term was added to the equations of the reversible scavenging model. For L3, a ventilation time (3–12 years) was found comparable to results derived from other tracers. However, the ventilation of the deep water (>2000 m) at L2 suggested by our model (8–25 years) is probably too rapid in comparison with previous findings based on chlorofluorocarbon data. Enhanced scavenging of 230Th would be an alternative explanation for the low concentration in NEADW at L2. With the scavenging-mixing model, we calculated an advective export flux of 230Th of up to 50% of the production in the water column in the eastern North Atlantic. Advective loss of 230Th has to be taken into account when using the 230Th flux into sediments for paleoceanographic studies in ocean basins where deep water residence time is comparable to the 230Th scavenging residence time.

Benthic foraminiferal assemblages in Labrador Sea sediments: relations with deep-water mass changes since deglaciation

Surface sediment samples from the Labrador Sea and the Irminger and Iceland basins have been analysed for their benthic foraminiferal content to define the relationship between benthic foraminiferal assemblages and bottom-water characteristics. On the shelf (301–530 m) and the upper slope (1364 and 1980 m) the distribution of assemblages is complex and appears related to diversified microhabitats. In the deep-sea domain (&gt; 2600 m) three main assemblages have been identified: the first, dominated by Epistominella exigua, is related to the North East Atlantic Deep Water (NEADW); the second, characterized by the co-dominance of Cibicides wuellerstorfi and E. exigua, seems to characterize the North West Atlantic Bottom Water (NWABW); the third, marked by the occurrence of Nuttalides umbonifera, is recorded at depths greater than 3500 m and is associated with the northern extent of the Antarctic Bottom Water (AABW).Microfaunal and isotopic analyses of two cores from the Greenland slope (90-013-011, 2800 m) and rise (90-013-013, 3300 m) provide insight into the changes in the deep-water mass characteristics of the Labrador Sea over the past 15 000 years. Prior to 8500 BP, sparse assemblages dominated by either Pullenia quinqueloba, Uvigerina peregrina, or Melonis pompiloides suggest changing environmental conditions. In particular, a peak of U. peregrina recorded just before the Younger Dryas event in the deepest core is associated with the northward advance of a relatively warm, oxygen-poor bottom-water mass from the Atlantic. After 8500 BP, the increasing proportion of E. exigua suggests the formation of a bottom-water mass similar to the modern NEADW. Finally, higher percentages of C. wuellerstorfi in late Holocene sediments (after 5500 BP) are associated with increased NWABW and indicate the development of modern bottom water.

Identification et distribution des grandes masses d'eau dans les mers du Labrador et d'Irminger

The main deep water masses present at the time of the CSS Hudson cruises in Labrador and Irminger seas in June 1990 and October–November 1991 have been identified using characteristic temperatures (T) and salinities (S). The purpose of this study was to establish the transfer functions between micropaleontological assemblages of top sediments and thermohaline characteristics of water masses. The water mass at the top of the Labrador Sea (Labrador Sea Water, LSW) is formed after intense movements of winter convection in the first 900-m depth of the water column. Below that depth, the LSW parameters reach a double minimum (S ≈ 34.80 and T ≈ 2.9 °C). Only the sediments located on the continental slopes of Greenland and Labrador between depths of 500 and 1500 m are in contact with the LSW. Below the LSW, the superior fraction of the North East Atlantic Deep Water (NEADW1) is characterized by a temperature maximum (≈ 3.3 °C) and, as such, is distinguishable from the inferior fraction (NEADW2). The latter is characterized by a maximum S (≈ 34, 90) when compared with other intermediary and deep water masses. In contrast to the NEADW1 that freely circulates over the Reykjanes Ridge, the NEADW2 must flow through the Charlie Gibbs Fracture Zone to go from the northeastern Atlantic to the Irminger Sea. The NEADW 1 and 2 respectively bathe the ridge section less than 2000 m deep and the European abyssal basins. On the contrary, the majority of the deep sediments of the Labrador and Irminger seas are in contact with the cold (T &lt; 2.6 °C) and salty (≈ 34.85) Denmark Strait Overflow Water. Although this water mass is normally found at depths exceeding 2700 m in pelagic environments, it can be found at less than 2000-m depth on the bottom of the continental slopes of Greenland and Labrador, where it is carried by the strong Deep Northern Boundary Current and Western Boundary Undercurrent. The presence of the NEADW 1 and 2 on the sediments is then restricted to narrow bands on the same continental slopes, between depths of 1800 and 2200 m.

Sedimentation and climate-induced sediments on Feni Ridge, Northeast Atlantic Ocean

Feni Ridge has been studied in detail in the area between 54 and 56°N and 12 and 16°W by using high-resolution profiling, water column sampling with transmissometer and CTD, and by piston and box core sampling. The crest of Feni Ridge is absent in the north of the study area, but on passing towards the south it begins to take shape and to increase in height; its distance from the Rockall slope also increases towards the south. On the lower southeastern flank a secondary ridge crest has locally developed, and sediment waves along the flanks have amplitudes of up to 45 m. The highest and steepest waves are found in the south, the waves in the north being somewhat lower. Bottom nepheloid layers of 175 m near the foot of Rockall Plateau and 125 m near the crest are attributed to a Norwegian Sea Overflow Water current with a core at 2100 m. Bottom nepheloid layers of 75–100 m on the lower flanks of the ridge are thought to have been caused by Northeast Atlantic Deep Water. Piston cores reflect alternations of glacial and interglacial periods. Interglacials have a high carbonate and C org content, a positive Eu anomaly and negative Ce anomaly, and a low Rb La ratio. They are dominated by a subpolar and transitional foraminiferal assemblage. Glacials show a low carbonate and C org content, a reduced Eu anomaly and a high Rb La ratio, suggesting increasingly continental sediment sources. These deposits are dominated by a polar planktonic foraminiferal assemblage characterized by N. pachyderma (s) and G. quinqueloba. Sedimentation rates reflect differential deposition. At crestal station NA 87-25 rates are 17.1 cm/k.y. for stage 1, 14.6 cm/k.y. for stage 2, 3.4 cm/k.y. for stage 3 and 5.4 cm/k.y. for stage 4. Stage 5a–e has a mean sedimentation rate of 6.1 cm/k.y. Over the last 125 k.y. the development of Feni Ridge has taken place under a relatively low energy current regime.

Glacial-Holocene stratigraphy, chronology, and paleoceanographic observations on some North Atlantic sediment drifts

Several sediment drifts in the deep North Atlantic have been studied using oxygen isotopes, percent CaCO 3, clay mineralogy, tephra content and Accelerator Mass Spectrometer (AMS) radiocarbon dating. Because enhanced deposition of fine-grained terrigenous sediment increases rates of sedimentation by as much as 100 times the regional mean, sediment drifts contain high-resolution records of climate and oceanographic change. The major results of this study are that: (1) patterns of sedimentation rate are regionally variable. In the Western North Atlantic (Bermuda Rise, Blake-Bahama Outer Ridges), rates of sedimentation have been relatively low during the Holocene, but were as high as 200 cm/1000 years during the latest glacial episode, reflecting a greater supply of sediment due to glaciation and sea level lowering. Deep circulation patterns were probably similar to today. In the northern North Atlantic (Gardar Drift), which is dominated today By Norwegian Sea Overflow Water (NSOW), sedimentation rates were lowest during the glacial, despite the availability of sediment, because NSOW production was stopped. Rates peaked during the early Holocene as NSOW resumed. Glacial and Holocene rates of sedimentation were roughly comparable in the northeast Atlantic (Feni Drift) because sedimentation at that location is influenced less by NSOW than by Northeast Atlantic Deep Water (NEADW), which has a large southern-source component. (2) Stable isotopic events of deglacial age are preserved with unprecedented clarity in the sediment of North Atlantic drifts. Where the Younger Dryas cooling and the Vedde Ash occurred together, at 56°N on Feni Drift, direct AMS dating suggests that the radiocarbon age difference between the surface ocean and atmosphere reservoirs in the northeast Atlantic was the same 10,500 years ago as it is today (about 400 years). A δ 18O maximum of Younger Dryas age indicates cooler or more saline surface waters above the Bermuda Rise at 33°N and as far south as 28°N on the Bahama Outer Ridge. Two δ 18O minima at 12,000 and 13,500 radiocarbon years BP on the Bahama Outer Ridge and the Bermuda Rise probably resulted from lowered surface water salinity. These low salinity events most likely originated as meltwater discharge down the Mississippi and into the Gulf of Mexico, followed by advection into the open North Atlantic via the Gulf Stream.

Temperature and salinity changes in the deep Labrador Sea, 1962–1986

Multi-year variations in the potential temperature (θ) vs salinity (S) curve of Denmark Strait Overflow Water and Northeast Atlantic Deep Water are demonstrated in time series of θ on density anomaly ( σ 2) surfaces, constructed from all deep observations obtained in the western Labrador Sea between 1962 and 1986. The temperature of the densest Overflow Water ( σ 2 = 37.16 kg m −3) increased in the early 1960s by ≌0.15°C, decreased by about the same amount in the latter part of the decade, dropped a further 0.15°C in the late 1970s or early 1980s and rose between July 1985 and August 1986 back to the 1962 value. The variations are coherent through the two superposed water masses, but the range increases from ≌0.13°C at θ 2 = 36.96 kg m −3 in the upper layers of the Deep Water, to ≌0.36°C at θ 2 = 37.16 kg m −3 near the bottom, suggesting that the Overflow Water is the source of the fluctuations. The temperature decreases in 1968 and 1981 may be caused by the large low salinity anomaly that first appeared in the Iceland Sea in 1968 and that was observed to travel from Denmark Strait, through the Labrador Sea, across the North Atlantic and into the Norwegian Sea between 1968 and 1979. The temperature increases in the Overflow Water probably occur when the outflow of low salinity water from the Arctic is decreased.

Neogene bottom water-masses and benthic foraminifera in the NE Atlantic Ocean

Modern benthic foraminiferal assemblages show a correlation with oceanic bottom water-masses. A study of the Neogene benthic foraminifera from seven sites to the south and two sites to the north of the Azores Ridge shows patterns of change which can be interpreted in terms of past water-masses. An Antarctic bottom water (AABW) influence has been present to the south since the mid-Miocene but to the north of the Azores Ridge, AABW was present only up to the late Pliocene, since when it has been absent. Overlying AABW is North Atlantic deep water (NADW) and North East Atlantic deep water (NEADW). The latter shows a spectacular increase in importance from the late Pliocene to Recent. The faunal changes can be correlated with changes in circulation in the late Pliocene which heralded the incoming of major glacial events from c. 2.4 Ma.

226Ra and Ba in northeast Atlantic deep water

226Ra and Ba data are presented for the northeast Atlantic below 2000 m depth from a section of stations 8°S to 46°N obtained by F.S. Meteor in 1981. The precision of the 226Ra data (±1%) allows a resolution of structures in the 226Ra distribution that is comparable to the resolution for Ba and nutrients. Linear property—property correlations in the upper parts of the depth range appear to be uniform within the observational error along the section for 226Ra, Ba [Ba(nmol kg −1) = 3.88 Ra (dpm 100 kg −1) + 12.3], and alkalinity, whereas the slopes of these properties versus Si increase northwards from about 10 to 25 × 10 −4 dpm Ra μmol Si −1. From this observation, and from studying the property distributions across the Romanche Fracture Zone, it is concluded that CaCO 3 dissolution must be an important process of 226Ra and barium regeneration, while opal dissolution appears to be of minor importance. The water overflowing eastward over the sill of the Romanche Fracture Zone has 17.2 ± 0.3 dpm 100 kg −1 226Ra, 79.5 ± 1.5·nmol kg −1 Ba and 2338 ± 4 μeq kg −1 alkalinity. Within 800–1400 m above the sea floor, excess 226Ra relative to the linear correlation with Ba further up in the water column is generally present, but there is also a reflection of 226Ra decay at intermediate depths at the northern stations of the section. The latter effect appears to amount to only 3 ± 1.5%, but this magnitude is consistent with the 14C distribution reported previously ( Schlitzer et al., Journal of Geophysical Research, 90, 6945–6952, 1985). From the excess 226Ra in the lowest 800–1400 m of the water column, the 226Ra flux from the sediments is estimated to be 7.7 ± 4 × 10 −21 mol Ra (m 2s) −1, a value in accordance with results by Cochran ( Earth and Planetary Science Letters, 49, 381–392, 1980).

Miocene to Recent bottom water masses of the north-east Atlantic: an analysis of benthic foraminifera

Summary The modern benthic foraminifera of the north-east Atlantic Ocean show a distribution pattern related to bottom water masses. Varimax factor analysis of data from depths &gt; 2000 m gives factors which correlate with North-East Atlantic Deep Water (NEADW), North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW); a fourth factor corresponds with the basal part of the Mediterranean Water. Miocene to Pleistocene benthic foraminiferal assemblages have been compared with the modern assemblages. On the assumption that species have not changed their environmental preferences through time, the fossil assemblages can be used to infer the past existence of water masses similar to those of today. The results presented here show that in the late Miocene, AABW penetrated to the southern flank of Rockall Plateau at the time of build-up of the East Antarctic ice sheet. During the early Pliocene the rate of production of NADW increased, perhaps as a result of a decrease in size of the Antarctic ice sheet, which in turn led to the southward retreat of the limit of penetration of AABW. Throughout much of the Pleistocene, as now, AABW has not penetrated north of the Azores Ridge. NEADW was the most extensive bottom water mass in the late Pliocene and Pleistocene.

14C in the Deep Water of the East Atlantic

The renewal of east Atlantic deep water and its large-scale circulation and mixing have been studied in observed distributions of temperature, silicate, ΣCO2, and 14C. 14C variations in northeast Atlantic deep water below 3500m depth are small. Δ14C values range from − 100‰ to −125‰. 14C bottom water concentrations decrease from Δ14C =−117‰ in the Sierra Leone Basin to Δ14C = − 123‰ in the Iberian Basin and are consistent with a mean northward bottom water flow. The characteristic of the water that flows from the west Atlantic through the Romanche Trench into the east Atlantic was determined by inspection of θ/Δ14C and θ/SiO2 diagrams. A mean potential temperature of θ = 1.50 ± .05°C was found for the inflowing water. A multi-box model including circulation, mixing, and chemical source terms in the deep water has been formulated. Linear programing and least-squares techniques have been used to obtain the transport and source parameters of the model from the observed tracer fields. Model calculations reveal an inflow through the Romanche Trench from the west Atlantic, which predominates over any other inflow, of (5 ± 2) Sv (potential temperature 1.50°C), a convective turnover of (150 ± 50) years and a vertical apparent diffusivity of (4 ± 1) cm2/s. Chemical source terms are in the expected ranges.

A meridional 14C and 39Ar section in northeast Atlantic deep water

14C, 39Ar, and complementary hydrographic and nutrient data are presented for deep water below 2500 m depth, from stations along a meridional section (8°S to 45°N) through the Romanche Trench and along the deep northeast Atlantic basins (F/S Meteor, cruise 56, leg 5). The large‐scale 14C distribution along the section is resolved at the 14C data precision of ±2‰. Bottom water Δ14C decreases by 6‰ from the equator to 45°N, and farther up there is a weak Δ14C minimum (−123‰) over much of the section. The 14C data are interpreted as giving a turnover time of about 30 years for the waters below the depth of the 14C minimum (∼4250 m). It is found that water of 1.50±0.05°C potential temperature enters the East Atlantic from the west through the Romanche Trench (sill depth about 4000 m), and a preliminary value for the inflow rate of 3.6×106 m3/s is deduced. This rate greatly exceeds estimated deep inflow rates through the Vema fracture zone or across the northern boundary of the East Atlantic. 39Ar data that cover an entire deep‐ocean circulation system are presented for the first time. The observed 14C and 39Ar distributions are mutually consistent. Transit times from the source regions to the equator for water from northern and southern deepwater sources are estimated to be about 170 and 105 years, respectively, and the 39Ar concentration of young Antarctic Bottom Water is deduced as 60±7% modern. The 39Ar‐14C correlation in the ocean appears to be affected by mixing of waters of different age and by more efficient raising of 39Ar in the deepwater formation processes.

A second Pallenopsis mollissima (Hoek), with other deep water pycnogonids from the Glasgow Museum

Three specimens of pycnogonid, from northeast Atlantic deep water samples taken off the Porcupine Bank and west of the Flannan Isles, are recorded to complete the analysis of material currently held at Glasgow Museum and Art Gallery, Scotland. One of the specimens is the second recorded individual of Pallenopsis mollissima, and allows the completion of the description of this rare species since the terminal leg segments are present. Dimensions of the chelae indicate close affinity between this species and P.paramollis, and the possibility of synonymy is discussed.

Determination of blanks in low-level tritium measurement

Tritium that was not present in the original water sample (“blank”) is observed when measuring tritium concentrations which are near analytical detection limit (in this laboratory about ±0.08TU, or ±0.26pCi/1 of water, 1σ). The analytical blank in this laboratory is determined to be 0.07 ± 0.03TU. Water samples stored in 1 1. gas bottles gain tritium at a rate of 0.01 TU per year (polyethylene bottles: 0.20 TU per year, ±15%), for an ambient tritium level of about 200 TU. An upper limit of 0.07 TU (2σ) is derived for the true tritium concentration of Northeast Atlantic deep and bottom water (∼40°N/15°W, 1972-74, depth 2000 m or more). This limit amounts to about 1% of the tritium concentration in North Atlantic surface water.

Morphology and Structure of the Southern Labrador Sea

Geophysical surveys since 1971 and sounding data reveal the morphology and structures of the southern Labrador Sea. Ran Ridge and older mid-Labrador Sea crustal basement are well-defined. The West Thulean Rise (pre-60 Ma BP), counterpart to the Thulean Rise of the Northeast Atlantic, extends between the Charlie Fracture Zone and Ran Ridge {anomaly 24). Topographic and basement structural relief along a fault (herein called the Leifur Eiriksson Fracture Zone), offsetting the Ran Ridge crest (60–45 Ma BP) at 57 °N, 45 °W, is correlated with magnetic anomaly trends. The pre-Thulean and Thulean time structures of the mid-ocean ridge and magnetic data isochrons define the initial spreading orientations of the Ran/Reykjanes Triple Junction. Seismic data extend the known area of the transparent, kilometre-thick sedimentary body in the southern central Labrador Sea, and a circular trend of elongate crests surrounding a vortex region lend definition to the proposed name, Gloria Drift. Dunal morphology is characteristic of the surface of the entire Gloria Drift, and is also revealed in the mid-layer (35 Ma BP) extending throughout the entire sediment blanket. Terrigenous clay deposits sculpted by the Northeast Atlantic Deep Water form the elongate drift crests. The crests mark the paths or perimeters of the moving and still water masses of the bottom water gyre and probable anticlockwise paleocirculation that developed the Gloria Drift. The Imarssuak Mid-Ocean Channel is controlled by mountains and lies within the presumed relic rift valley of Ran Ridge. The Deep Sea Drilling Project at site 113 on the flood plain of Imarsuak Channel recovered cores consisting of turbidite silts and sands, mixed with volcanogenic (Iceland?) materials, and heavy minerals and clay. At this site, the commencement of transport in the channel is correlated with the onset of glaciation in early Pliocene. North of the junction with Imarssuak Channel, the Northwest Atlantic Mid-Ocean Channel lacks the high reflectivity indicative of recent (Pleistocene) turbidite deposition; hence, we conclude that the main source for highly reflective turbidite silts and sands in the lower Northwest Atlantic channel is the Imarssuak Channel. Meanders in the Northwest Atlantic Channel between 52 °N and 56 °N compare almost equally in proportions of channel widths and meander wavelengths, to the northeast Atlantic's Maury Channel at the same latitude.

The overflow of Norwegian sea deep water across the wyville-thomson Ridge

Accumulating evidence has pointed to a source of Northeast Atlantic Deep Water in the northern Rockall Trough and prompted a re-examination of geological and hydrographic data. The first showed that the sedimentary Feni Ridge could be traced along the length of the westernRockall Through to the foot of the Wyville-Thomson Ridge, and the second that dense water frequently existed at the lowest point of the ridge crest. Collected soundings and new data suggested a path by which this might enter the Rockall Trough, and overflow of Norweigian Sea Deep Water was observed taking place from the ridge crest in a narrow layer in July 1972. The flow is probably intermittent and appears to entrain Atlantic Water from intermediate depths.

On the water masses of the Northwest Atlantic Ocean

The water masses of the Northwest Atlantic Ocean are examined by means of potential temperature-salinity diagrams. It is shown that in general below 750-m depth there are three water masses; Labrador Sea Water overlies Northeast Atlantic Deep Water, derived from the overflow of the Scotland-Iceland Ridge, and this in turn overlies Northwest Atlantic Bottom Water. The latter is derived from the mixing of water overflowing the Iceland-Greenland Ridge with Atlantic water and modified Labrador Sea Water in the vicinity of the ridge. Mixing of Northeast Atlantic Deep Water with Northwest Atlantic Bottom Water is considered to give rise to the well-known North Atlantic Deep Water.