Upper North Atlantic Deep Water Research Articles

The seawater neodymium and lead isotope record of the final stages of Central American Seaway closure

AbstractThe shoaling and final closure of the Central American Seaway (CAS) resulted in a major change of the global ocean circulation and has been suggested as an essential driver for strengthening of Atlantic Meridional Overturning Circulation (AMOC). The exact timing of CAS closure is key to interpreting its importance. Here we present a reconstruction of deep and intermediate water Nd and Pb isotope compositions obtained from fossil fish teeth and the authigenic coatings of planktonic foraminifera in the eastern equatorial Pacific (Ocean Drilling Program (ODP) Site 1241) and the Caribbean (ODP Sites 998, 999, and 1000) covering the final stages of CAS closure between 5.6 and 2.2 Ma. The data for the Pacific site indicate no significant Atlantic/Caribbean influence over this entire period. The Caribbean sites show a continuous trend to less radiogenic Nd isotope compositions during the Pliocene, consistent with an enhancement of Upper North Atlantic Deep Water (UNADW) inflow and a strengthening of the AMOC. Superimposed onto this long‐term trend, shorter‐term changes of intermediate Caribbean Nd isotope signatures approached more UNADW‐like values during intervals when published reconstructions of seawater salinity suggested complete closure of the CAS. The data imply that significant deep water exchange with the Pacific essentially stopped by 7 Ma and that shallow exchange, which still occurred at least periodically until approximately 2.5 Ma, may have been linked to the strength of the AMOC but did not have any direct effect on the intermediate and deep Caribbean Nd isotope signatures through mixing with Pacific waters.

Reconstruction of intermediate water circulation in the tropical North Atlantic during the past 22,000 years

Decades of paleoceanographic studies have reconstructed a well-resolved water mass structure for the deep Atlantic Ocean during the Last Glacial Maximum (LGM). However, the variability of intermediate water circulation in the tropics over the LGM and deglacial abrupt climate events is still largely debated. This study aims to reconstruct intermediate northern- and southern-sourced water circulation in the tropical North Atlantic during the past 22kyr and attempts to confine the boundary between Antarctic Intermediate Water (AAIW) and northern-sourced intermediate water (i.e., upper North Atlantic Deep Water (NADW) or Glacial North Atlantic Intermediate Water) in the past. High-resolution Nd isotopic compositions of fish debris and acid-reductive leachate of bulk sediment in core VM12-107 (1079m depth) from the Southern Caribbean are not in agreement. We suggest that the leachate method does not reliably extract the Nd isotopic compositions of seawater at this location, and that it needs to be tested in more detail in various oceanic settings. The fish debris εNd values display a general decrease from the early deglaciation to the end of the Younger Dryas, followed by a greater drop toward less radiogenic values into the early Holocene. We propose a potentially more radiogenic glacial northern endmember water mass and interpret this pattern as recording a recovery of the upper NADW during the last deglaciation. Comparing our new fish debris Nd isotope data to authigenic Nd isotope studies in the Florida Straits (546 and 751m depth), we propose that both glacial and deglacial AAIW do not penetrate beyond the lower depth limit of modern AAIW in the tropical Atlantic.

Circulation, Heat, and Freshwater Transport at 36°N in the Atlantic

Abstract In May and June 2005, a transatlantic hydrographic section along 36°N was occupied. A velocity field is calculated using inverse methods. The derived 36°N circulation has an overturning transport (maximum in the overturning streamfunction) of 16.6 Sv (1 Sv ≡ 106 m3 s−1) at 1070 m. The heat transport across the section, 1.14 ± 0.12 PW, is partitioned into overturning and horizontal heat transports of 0.75 and 0.39 PW, respectively. The horizontal heat flux is set by variability at the gyre rather than by mesoscale. The freshwater flux across the section is 1.55 ± 0.18 Sv southward based on a 0.8-Sv flow from the Pacific through the Bering Strait at a salinity of 32.5 psu. The oceanic divergence of freshwater implies a net input of freshwater to the ocean of 0.75 Sv over the North Atlantic and Arctic between 36°N and the Bering Strait. Most (85%) of the recently ventilated upper North Atlantic Deep Water (water originating in the Labrador Sea) transport across the section occurs in the deep western boundary current rather than being associated with an interior pathway to the west of the mid-Atlantic ridge.

A major and long-term Pliocene intensification of the Mediterranean outflow, 3.5-3.3 Ma ago

Largely continuous millennial-scale records of benthic delta O-18, Mg/Ca-based temperature, and salinity variations in bottom waters were obtained from Deep Sea Drilling Project (DSDP) Site 548 (East Atlantic continental margin near Ireland, 1250 m water depth) for the period 3.7-3.0 Ma ago. High epsilon(Nd) values of -10.7 to -9 show that this site monitored changes in Mediterranean Outflow Water (MOW) throughout the mid-Pliocene. Bottom water variability at Ocean Drilling Progam (ODP) Site 978 (Alboran Sea, 1930 m water depth) provides a complementary record of MOW composition near its West Mediterranean source. Both sites show a singular and persistent rise in bottom water salinities by 0.7-1.4 psu, and in densities by similar to 1 kg m(-3) from 3.5 to 3.3 Ma ago, which is matched by an similar to 3 degrees C increase in bottom water temperature at Site 548. This event suggests the onset of strongly enhanced deep-water convection in the Mediterranean Sea and a related increase in MOW flow as a result of major aridification in the Mediterranean source region. In harmony with model suggestions, the enhanced MOW flow has possibly intensified Upper North Atlantic Deep Water formation.

The mid-depth circulation of the northwestern tropical Atlantic observed by floats

A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16°N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.

Abrupt climate change and collapse of deep-sea ecosystems

We investigated the deep-sea fossil record of benthic ostracodes during periods of rapid climate and oceanographic change over the past 20,000 years in a core from intermediate depth in the northwestern Atlantic. Results show that deep-sea benthic community "collapses" occur with faunal turnover of up to 50% during major climatically driven oceanographic changes. Species diversity as measured by the Shannon-Wiener index falls from 3 to as low as 1.6 during these events. Major disruptions in the benthic communities commenced with Heinrich Event 1, the Inter-Allerød Cold Period (IACP: 13.1 ka), the Younger Dryas (YD: 12.9-11.5 ka), and several Holocene Bond events when changes in deep-water circulation occurred. The largest collapse is associated with the YD/IACP and is characterized by an abrupt two-step decrease in both the upper North Atlantic Deep Water assemblage and species diversity at 13.1 ka and at 12.2 ka. The ostracode fauna at this site did not fully recover until approximately 8 ka, with the establishment of Labrador Sea Water ventilation. Ecologically opportunistic slope species prospered during this community collapse. Other abrupt community collapses during the past 20 ka generally correspond to millennial climate events. These results indicate that deep-sea ecosystems are not immune to the effects of rapid climate changes occurring over centuries or less.

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The variability of North Atlantic Deep Water (NADW) properties in the western tropical Atlantic between 66 ∘ W / 20 ∘ N and 60 ∘ W / 10 ∘ N is investigated based on repeated hydrographic sections. The water at the western boundary carries a CFC maximum, indicating the direct transport of newly formed NADW within the Deep Western Boundary Current towards the tropics. The offshore extension of this region with young water varies from 50 km at 16 ∘ N to 500 km at 10 ∘ N , depending on the width of the continental slope. The mean properties for the different NADW components reflect the changes in the water mass formation regions, especially the Labrador Sea. The greatest signal can be seen in the density range of the Labrador Sea Water (LSW), where a continuous freshening between 2000 and 2004 is observed. This is attributed to the arrival of low-salinity LSW formed in the Labrador Sea after 1988. In addition to the decreasing salinity in the LSW, the two lower NADW components also reveal a freshening trend, as has been observed in the subtropical and subpolar North Atlantic [Dickson. B., Yashayaev, I., Meincke, J., Turrell, B., Dye, S., Holfort, J., 2002. Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature 416, 832–837]. This tendency is not restricted to the boundary current region, but extends over the whole western tropical Atlantic. The spreading time of NADW from the Labrador Sea to the tropical Atlantic at 16 ∘ N is estimated by the salinity signal to be 10–13 years, corresponding to a mean velocity of 2.0–2.5 cm/s. The dilution factor of NADW determined from the CFC difference between the Labrador Sea and the tropics is 4–5 for all components, assuming an admixture of old, CFC-free waters. The NADW properties at 16 ∘ N on isopycnals show large spatial variability along the section. Profiles with high concentration of CFCs and low salinity in the LSW layer occur not only at the western boundary, but also in the interior, where they alternate with profiles influenced by older, CFC-poor water. At 10 ∘ N , the properties especially in the Upper NADW are horizontally homogenized. This is caused by the change of the shape of the continental slope, which is very steep at 16 ∘ N and gradually declining at 10 ∘ N , leading to a broadening of the upper portion of the boundary current between the two sections. The NADW at 10 ∘ N is a composite of the water from the western boundary at 16 ∘ N and about one third admixture of water from the interior. During the spreading from 16 ∘ N to 10 ∘ N , the uppermost portion of the Upper Labrador Sea Water becomes fresher and poor in CFCs by vertical mixing with the overlying Upper Circumpolar Deep Water. The resulting diffusivity K ρ is of the order of 10 - 4 m 2 / s .

Seasonal variability in the Deep Western Boundary Current around the Eastern tip of Brazil

The spreading of recently ventilated North Atlantic Deep Water (NADW) from the formation region to the equatorial Atlantic occurs mainly in the Deep Western Boundary Current (DWBC). When crossing the equator between 44°W and 35°W, the DWBC is split in two velocity cores through a chain of seamounts around the Atoll das Rocas at 3.5°S. Further eastward the DWBC contributes to the zonally oriented equatorial current system. The circulation of the NADW in the crucial region around the eastern tip of Brazil is examined using 8 CTD and Chlorofluorocarbon (CFC) sections along 35°W and along 5°S, respectively, taken mainly in spring and fall in the years 1990–2002. As expected from the short direct flow path between the two sections, the CFC concentrations in the upper NADW (1400–2200 m) were similar at 35°W and 5°S during boreal autumn. In spring, however, a significant downstream CFC decrease was observed. If one attributes the decrease solely to the older age of water further downstream, the CFC concentration age difference between 35°W and 5°S in May 2002 would be 3–5 years. We interpret the aging to be caused by an eastward detour of the flow with the deep equatorial circulation before reaching 5°S in spring. Another conspicuous anomaly was found in the middle NADW (2200–3400 m) with downstream decreasing salinities in boreal spring, but not in autumn. This variability might also be caused by differences in the deep equatorial circulation, but in contrast to the uNADW, one cannot exclude enhanced mixing with water of South Atlantic origin in spring to be the cause of that variability. No seasonal difference was observed in the hydrography or the CFC concentrations for the lower NADW. The weaker CFC decrease along the equator compared with that in the DWBC downstream of 35°W, and the topographic features along the downstream path, point to a predominantly eastward flow of the deep lNADW core. The lNADW CFC core is no longer observed at 11°S. Repeated observations of CFCs in the DWBC highlight the diverse variability affecting the components of NADW.

Late Holocene variability of upper North Atlantic Deep Water temperature and salinity

Magnesium/calcium ratios in benthic foraminifera (Cibicidoides pachyderma) from a sediment core on the Laurentian Slope (1854 m) exhibit strong millennial‐scale fluctuations during the past 4000 years. We convert these data to seawater paleotemperatures using a new monospecific linear equation. Results suggest that the temperature of upper North Atlantic Deep Water (dominated by Labrador Seawater today) has varied by at least 2°C during the late Holocene. Millennial scale coolings coincide with previously identified periods of increased drift ice and regional glacier advances, including the Little Ice Age. Paired oxygen isotope measurements indicate that salinity and perhaps density were reduced during the cold periods. We discuss possible mechanisms for transmitting this cold, fresh signal from surface waters to intermediate depths. Our reconstructed late Holocene ranges in upper North Atlantic Deep Water properties greatly exceed those of the instrumental record and imply that large changes may be yet to come.

Variability of silt grain size and planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5–2.2 Ma) to Pleistocene (1.6–0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (∼1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ∼3.2 and 2.75 Ma. This is in agreement with the ‘superconveyor model’ which postulates that the highest NADW production took place prior to ∼2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the ‘superconveyor’ at ca. 2.75 Ma. However, our data do not support a ‘superconveyor’ in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ∼3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.

Past extent of sea ice in the northern North Atlantic inferred from foraminiferal paleotemperature estimates

Using 150 core top samples, we developed a conservative but fairly robust new measure to reconstruct past changes in (maximum) seasonal sea ice distribution in the northern North Atlantic, hitherto a major unknown. The proxy is based on Similarity Maximum Modern‐Analog Technique (SIMMAX) estimates of threshold temperatures near the sea surface (SST). Today, almost 100% of all sites with SST >2.5°C during summer, >0.4° for winters 1978–1987, and >0.75°C for Little Ice Age winters lie seaward of the sea ice margin. When applied to >60 sediment records of the Last Glacial Maximum, this proxy shows that peak glacial sea ice was far more restricted than in the “classic”CLIMAP Project Members[1981]reconstruction. During glacial summer, sea ice only covered the Arctic Ocean and western Fram Strait. The northern North Atlantic and Nordic Seas were largely ice‐free and thus formed a high‐latitude moisture source for the continued buildup of continental ice sheets. In contrast, sea ice spread far south across the Iceland Faeroe Ridge during glacial winter, with an inferred patch of sea ice also in the central east Atlantic, near the center of the Azores High. A broad ice‐free channel extended from 50° to 60°N, forming an ideal site for large‐scale convection of glacial upper North Atlantic Deep Water. The extreme seasonality in glacial sea ice formation and melt in the Nordic Seas implies a major consumption of the regional energy income.

A global ocean climatological atlas of the Turner angle: implications for double-diffusion and water-mass structure

The 1994 Levitus climatological atlas is used to calculate the Turner angle (named after J. Stewart Turner) to examine which oceanic water masses are favorable for double-diffusion in the form of diffusive convection or salt-fingering and which are doubly stable. This atlas complements the Levitus climatology. It reveals the major double-diffusive signals associated with large-scale water-mass structure. In total, about 44% of the oceans display double-diffusion, of which 30% is salt-fingering and 14% is diffusive double-diffusion. Results show that various central and deep waters are favorable for salt-fingering. The former is due to positive evaporation minus precipitation, and the latter is due to thermohaline circulation, i.e. the southward spreading of relatively warm, salty North Atlantic Deep Water (NADW) overlying cold, fresh Antarctic Bottom Water. In the northern Indian Ocean and eastern North Atlantic, favorable conditions for salt-fingering are found throughout the water column. The Red Sea (including the Persian Gulf) and Mediterranean Sea are the sources of warm, salty water for the ocean. As consequence, temperature and salinity in these outflow regions both decrease from the sea surface to the bottom. On the other hand, ocean currents are in general sluggish in these regions. In the polar and subpolar regions of Arctic and Antarctic, Okhotsk Sea, Gulf of Alaska, the subpolar gyre of the North Pacific, the Labrador Sea, and the Norwegian Sea, the upper layer water is favorable for diffusive convection because of high latitude surface cooling and ice melting. Weak and shallow diffusive convection is also found throughout tropical regions and the Bay of Bengal. The former is due to excessive precipitation over evaporation and rain cooling, and the latter is due to both precipitation and river runoff. Diffusive convection in the ocean's interior is unique to the South Atlantic between Antarctic Intermediate Water and upper NADW (uNADW). It is the consequence of the intrusive equatorward flow of upper Circumpolar Deep Water, which carries with it the minimum temperature and very low salinity overlying warm, salty uNADW.

Dianeutral exchange between intermediate and deep water in the tropical Atlantic

In the tropical Atlantic the vertical distance between Antarctic Intermediate Water (AAIW) and upper North Atlantic Deep Water (uNADW) cores (defined by their salinity extrema) exceeds 1000 m. In this thick layer, two additional water mass signals, characterized mainly by temperature extrema, are identified in the western boundary region. They are the intrusive flows of upper Circumpolar Deep Water (uCDW) and shallow uNADW (suNADW). The latter is considered a water type of uNADW. Since uCDW is a northward flow as a lower phase of AAIW and suNADW is a southward flow as an upper phase of NADW, a reference of level of no motion (LNM) is defined between uCDW and suNADW at the neutral density surface σN = 27.70 (at about 1150 m). This surface is coincident with a minimum of dianeutral velocity and measured velocity by lowered acoustic Doppler current profiler. Dianeutral motion processes are examined across the five selected neutral surfaces σN = 27.45, 27.60, 27.70, 27.80, and 27.90 that span the depth range between AAIW and uNADW and follow the major water mass signals. Dianeutral transport shows a decreasing tendency toward the middle σN = 27.70 neutral surface and reaches a minimum of 0.6 Sv (1 Sv = 106 m3 s−1), integrated in the tropical region between 10°N and 10°S. Thus the weakening of dianeutral motion and required minimum of epineutral advection at LNM seem to be relevant.

CFC time series in the deep water masses of the western tropical Atlantic, 1990–1999

A unique CFC time series was collected along the 35°W meridian in the tropical Atlantic during eight cruises from 1990 to 1999, in order to investigate the large-scale variability of the circulation of the North Atlantic Deep Water (NADW) through transient tracer distributions. Within the upper NADW (UNADW), it appears that CFC distributions, are highly variable; cores of maximal CFC concentrations are closely associated with salinity and are principally caused by the local dynamics of equatorial and extra-equatorial deep jets within the three particular regimes of the 3–1°S, 1°S–1°N and 1–3°N bands. CFC distribution within the lower NADW (LNADW) is less heterogeneous and linked to oxygen. A double core has been observed for 1999, possibly due to the recirculation of the deep flow constrained by the circulation of the underlying Antarctic Bottom Water (AABW). The variability of the maximal concentrations presumably results from the seasonal or semi-annual variability of the regional deep circulation. For both layers, the temporal evolution of the mean concentrations has been traced throughout the 1990s. At the UNADW level, the temporal increase of the CFC concentration within the 3°S–3°N band is mainly due to the transient signal from the atmosphere. This increase is clearly dominant compared with the local dynamics. Similar behavior has been observed for the LNADW in the 3°S–1°N band, corresponding to the equatorial channel. An attempt has been made to access the transit time for the NADW pathway from the Labrador and Nordic Seas to the tropics. The mean transit time around 25 yr for the UNADW has been compared to other evaluations from hydrographic measurements.

Shell preservation of Limacina inflata (Pteropoda) in surface sediments from the Central and South Atlantic Ocean: a new proxy to determine the aragonite saturation state of water masses

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive “metabolic” aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and δ 13C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO 2 release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.

Role of oceanic circulation on contaminant lead distribution in the South Atlantic

Abstract Both the relatively high lead concentrations and their characteristic anthropogenic isotopic compositions attest to the widespread contamination of industrial lead in the western Equatorial and South Atlantic Ocean. Spatial gradients in those isotopic signatures evidence the conservative lateral transport of lead in oceanic water masses, while the discrete isotopic signatures in deep oceanic waters substantiate the complementary hypothesis that the release of lead from settling particles is relatively small on a decadal time-scale. Specifically, the relatively low radiogenic lead (e.g., 206Pb/207Pb=1.148±0.009) in the Lower-North Atlantic Deep Water (l-NADW) south of 10° North is primarily attributed to US industrial lead emitted in the Northern Hemisphere prior to 1965, and the more radiogenic lead (e.g., 206Pb/207Pb=1.180±0.006) in the Upper-North Atlantic Deep Water (u-NADW) is primarily attributed to subsequent industrial lead emissions in that hemisphere. In contrast, the relatively radiogenic lead (e.g., 206Pb/207Pb=1.186±0.007) in the Antarctic Bottom Water (AABW) seemingly reflects a mixture of natural and anthropogenic lead sources within the Southern Hemisphere; and its isotopic dissimilarity with that (e.g., 206Pb/207Pb=1.159±0.002) of Antarctic Intermediate Water (AAIW) and the AABW may be due to differences in either their aeolian or water-mass inputs.

Upper ocean circulation in the glacial North Atlantic from benthic foraminiferal isotope and trace element fingerprinting

Benthic Cd/Ca and δ13C records from the midlatitude and northern North Atlantic are used to derive nutrient inventories and water mass distribution patterns for the past 50,000 years. Inferred Holocene water column Cd concentrations (CdW′) and δ13C values are 0.17–0.24 nmol kg−1 and 1.0–1.3‰ Peedee belemnite (PDB), which document the dominance of nutrient‐depleted Mediterranean Outflow Water (MOW) and Upper North Atlantic Deep Water (UNADW). Glacial benthic Cd/Ca and δ13C indicate a continued contribution of UNADW to the northern North Atlantic and upper Portuguese margin (CdW′=0.08 nmol kg−1; δ13C=+1.86‰ PDB). At the upper Moroccan margin, glacial CdW′ (0.23 nmol kg−1) is higher, and δ13C (+1.44‰ PDB) is lower. During “Heinrich” events, benthic δ13C decreases by up to 1.3‰, and peak Cd/Ca increases by 0.1–0.14 µmol mol−1; water column phosphorus equivalents are 1.8–2.8 µmol kg−1. The combined Cd/Ca and δ13C pattern indicates that during mean glacial conditions Antarctic Intermediate Water, (AAIW) reached the midlatitude northeast Atlantic (30°N). During Heinrich events, AAIW contribution maximized so that Southern Hemisphere waters filled the North Atlantic basin from bottom water to middepth levels.

North Atlantic Intermediate to Deep Water circulation and chemical stratification during the past 1 Myr

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate‐to‐deep δ13C gradients (commonly reaching >1.2‰) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric δ13C gradients covary with benthic foraminiferal δ18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on δ13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.

Tracer distributions and deep circulation in the western tropical Atlantic during CITHER 1 and ETAMBOT cruises, 1993–1996

This paper presents CFC and nontransient tracer observations in the western equatorial Atlantic Ocean on repeated sections along 7°30′N, the 35°W meridian, and a transect crossing the Ceara Rise. Three World Ocean Circulation Experiment cruises have been carried out in this area, in February‐March 1993 (CITHER 1) and September‐October 1995 and April‐May 1996 (ETAMBOT 1 and 2). Together with the tracer data, the direct current measurements are used to deduce the circulation pathways. The data confirm the principal circulation features of the Upper North Atlantic Deep Water within the area. The Deep Western Boundary Current flows southward along the continental slope, while, adjacent to the DWBC, there is a northward flow corresponding to the DWBC recirculation whose origin appears variable. The largest variability is observed along the 35°W meridional section, where the DWBC bifurcates eastward north of 3°S but the eastward equatorial flow does not appear to be permanent. At the Middle North Atlantic Deep Water level, CFC distributions exhibit the most important contrast between waters of northern and southern origin. The circulation at the Lower North Atlantic Deep Water level, mainly controlled by the topography, looks more permanent because the DWBC is regularly observed during the three cruises, and there is no evidence for a northwestward recirculation along the Mid‐Atlantic Ridge. The transient behavior of CFC distributions, which is superimposed on local circulation effects, can be seen clearly through their temporal evolution within the DWBC. The variability of the deep circulation observed during the 1993–1996 interval rules out a dominant variability in response to semi annual forcing.

Tracing the North Atlantic Deep Water through the Romanche and Chain fracture zones with chlorofluoromethanes

Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core ( σ 1.5=34.70 kg m -3) was observed eastward as far as 5°W. The deep CFM maximum ( σ 4=45.87 kg m -3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water “ages” computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.