The distribution and speciation of mercury in the South and equatorial Atlantic

Mercury concentration and speciation were measured in surface and deep ocean waters of the South and equatorial Atlantic. In the surface waters, total Hg was 2.9±1.7 pM on average, with a significant fraction present as reactive Hg (1.7±1.2 pM). The reactive Hg fraction consisted of elemental Hg (Hg°) as the dominant species (1.2±0.8 pM). Measurements in surface waters also showed that Hg partitioned to the “colloidal” phase (0.33±0.28 pM) and was associated with particulate matter (0.1±0.05 pM). No dimethylmercury (DMHg;<0.01 pM) or monomethylmercury (MMHg; <0.05 pM) was detected in mixed layer samples. The highest DMHg concentrations were found in recently formed deep waters – Antarctic Intermediate Water and Antarctic Bottom Water and in the equatorial sub-thermocline region. Higher concentrations of DMHg coincided with higher values of apparent oxygen utilization, indicative of a link between microbial activity and methylated Hg production. The lowest-deep water DMHg concentrations were found in the core of the North Atlantic Deep Water. Incubation experiments on-board demonstrated that light enhanced the decomposition of DMHg, with MMHg as the major product. In deep waters, Hg° was still an important constituent and is likely formed as a decomposition product from MMHg. These results suggest that methylated Hg production occurs primarily in regions of high biological activity, and that ionic Hg is strongly complexed to organic matter (colloidal material) in open ocean surface waters.     

Hydrography and through-flow in the north-eastern North Atlantic Ocean: the NANSEN project

The circulation and hydrography of the north-eastern North Atlantic has been studied with an emphasis on the upper layers and the deep water types which take part in the thermohaline overturning of the Oceanic Conveyor Belt. Over 900 hydrographic stations were used for this study, mainly from the 1987–1991 period. The hydrographic properties of Subpolar Mode Water in the upper layer, which is transported towards the Norwegian Sea, showed large regional variation. The deep water mass was dominated by the cold inflow of deep water from the Norwegian Sea and by a cyclonic recirculation of Lower Deep Water with a high Antarctic Bottom Water content. At intermediate levels the dominating water type was Labrador Sea Water with only minor influence of Mediterranean Sea Water. In the permanent pycnocline traces of Antarctic Intermediate Water were found.Geostrophic transports have been estimated, and these agreed in order of magnitude with the local heat budget, with current measurements, with data from surface drifters, and with the observed water mass modification. A total of 23 Sv of surface water entered the region, of which 20 Sv originated from the North Atlantic Current, while 3 Sv entered via an eastern boundary current. Of this total, 13 Sv of surface water left the area across the Reykjanes Ridge, and 7 Sv entered the Norwegian Sea, while 3 Sv was entrained by the cold overflow across the Iceland-Scotland Ridge. Approximately 1.4 Sv of Norwegian Sea Deep Water was involved in the overflow into the Iceland Basin, which, with about 1.1 Sv of entrained water and 1.1 Sv recirculating Lower Deep Water, formed a deep northern boundary current in the Iceland Basin. At intermediate depths, where Labrador Sea Water formed the dominant water type, about 2 Sv of entrained surface water contributed to a saline water mass which was transported westwards along the south Icelandic slope.     

A Zonal Pathway for NADW in the South Atlantic

Several large deployments of neutrally buoyant floats took place within the Antarctic Intermediate (AAIW), North Atlantic Deep Water (NADW), and the Antarctic Bottom Water (AABW) of the South Atlantic in the 1990s and a number of hydrographic sections were occupied as well. Here we use the spatially and temporally averaged velocities measured by these floats, combined with the hydrographic section data and various estimates of regional current transports from moored current meter arrays, to determine the circulation of the three major subthermocline water masses in a zonal strip across the South Atlantic between the latitudes of 19°S and 30°S. We concentrate on this region because the historical literature suggests that it is where the Deep Western Boundary Current containing NADW bifurcates. In support of this notion, we find that a net of about 5 Sv. of the 15–20 Sv that crosses 19°S does continue zonally eastward at least as far as the Mid-Atlantic Ridge. Once across the ridge it takes a circuit to the north along the ridge flanks before returning to the south in the eastern half of the Angola Basin. The data suggest that the NADW then continues on into the Indian Ocean. This scheme is discussed in the context of distributions of dissolved oxygen, silicate and salinity. In spite of the many float-years of data that were collected in the region a surprising result is that their impact on the computed solutions is quite modest. Although the focus is on the NADW we also discuss the circulation for the AAIW and AABW layers.     

The 1996 IOC contaminant baseline survey in the Atlantic Ocean from 33°S to 10°N: introduction,sampling protocols,and hydrographic data

The third in a series of cruises designed to establish the present-day concentrations of trace elements and synthetic organic compounds in major water masses of the ocean, the 1996 Intergovernmental Oceanographic Commission Contaminant Baseline Survey occupied six vertical profile stations in the subtropical and tropical Atlantic. Underway surface samples also were acquired in the transects between these stations. This paper uses the temperature, salinity, oxygen, nutrient, and chlorophyll results from the cruise to set the hydrographic background for the other papers in this special volume. Major features sampled during the surface transect include the Brazil Current, the South Equatorial Current, and the offshore Amazon Plume. Utilizing the above parameters to identify water masses, we observed Antarctic Bottom Water (AABW) that ranged from a relatively undiluted form at 33°S (Station 10) to a highly attenuated form at 8°N (Station 6). Similarly, North Atlantic Deep Water (NADW) was obtained in various mixing stages along its flow path, and samples of NADW and AABW exchanging through the Romanche Fracture Zone to the eastern Atlantic basins were also taken. In addition to these deep water masses, representative samples of Antarctic Intermediate Water and Circumpolar Deep Water were acquired. Besides standard hydrography, these data also were used to verify the sampling integrity of the trace metal-clean, Go Flo bottles deployed on a Kevlar hydrographic cable.     

Temperature fluctuations and current shear in Antarctic Bottom Water at the Vema Sill

The Vema Channel acts as a major conduit for the equatorward spreading of Antarctic Bottom Water between the Argentine and Brazil Basins. For almost two years the thermal stratification above its saddle depth (4660 m) – called Vema Sill – was recorded by moored thermistors and current meters. The lowest 490 m of the water column was instrumented to monitor the well-developed benthic boundary layer of Antarctic Bottom Water. The latter can be subdivided into Weddell Sea Deep Water on the sea bed and lower Circumpolar Deep Water above it. The data show fluctuations on various scales including periods, each about 1–2 weeks long, when the abyssal stratification virtually disappeared. Assuming a stable ratio between density and temperature, time series of bulk Richardson numbers are estimated from temperature and current shear data. The results suggest a potential for intermittent episodes of locally generated vertical mixing.     

Reactive mercury on the Scotian Shelf and in the adjacent northwest Atlantic Ocean

Depth profiles of reactive Hg were measured at four stations, two on the Scotian Shelf and two in the adjacent waters of the NW Atlantic. The average concentration of Hg from the Shelf stations (2.2 pM) was similar to the levels found at two deep stations offshore (2.3 pM) and previously in the Sargasso Sea. No major water column features were evident in any of the profiles.A series of surface samples were collected on a transect from the mouth of Halifax harbour to a station 400 km offshore. This transect showed a rapid decrease in Hg with increased distance from shore, with most of the gradient at the inshore end of the transect.     

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 (σ₄=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.     

The oxygen isotope composition of water masses in the northern North Atlantic

The ratio of oxygen-18 to oxygen-16 (expressed as per mille deviations from Vienna Standard Mean Ocean Water, δ¹⁸O) is reported for seawater samples collected from seven full-depth CTD casts in the northern North Atlantic between 20° and 41°W, 52° and 60°N. Water masses in the study region are distinguished by their δ¹⁸O composition, as are the processes involved in their formation. The isotopically heaviest surface waters occur in the eastern region where values of δ¹⁸O and salinity (S) lie on an evaporation–precipitation line with slope of 0.6 in δ¹⁸O–S space. Surface isotopic values become progressively lighter to the west of the region due to the addition of ¹⁸O-depleted precipitation. This appears to be mainly the meteoric water outflow from the Arctic rather than local precipitation. Surface samples near the southwest of the survey area (close to the Charlie Gibbs Fracture Zone) show a deviation in δ¹⁸O–S space from the precipitation mixing line due to the influence of sea ice meltwater. We speculate that this is the effect of the sea ice meltwater efflux from the Labrador Sea. Subpolar Mode Water (SPMW) is modified en route to the Labrador Sea where it forms Labrador Sea Water (LSW). LSW lies to the right (saline) side of the precipitation mixing line, indicating that there is a positive net sea ice formation from its source waters. We estimate that a sea ice deficit of ≈250 km³ is incorporated annually into LSW. This ice forms further north from the Labrador Sea, but its effect is transferred to the Labrador Sea via, e.g. the East Greenland Current. East Greenland Current waters are relatively fresh due to dilution with a large amount of meteoric water, but also contain waters that have had a significant amount of sea ice formed from them. The Northeast Atlantic Deep Water (NEADW, δ¹⁸O=0.22‰) and Northwest Atlantic Bottom Waters (NWABW, δ¹⁸O=0.13‰) are isotopically distinct reflecting different formation and mixing processes. NEADW lies on the North Atlantic precipitation mixing line in δ¹⁸O–salinity space, whereas NWABW lies between NEADW and LSW on δ¹⁸O–salinity plots. The offset of NWABW relative to the North Atlantic precipitation mixing line is partially due to entrainment of LSW by the Denmark Strait overflow water during its overflow of the Denmark Strait sill. In the eastern basin, lower deep water (LDW, modified Antarctic bottom water) is identified as far north as 55°N. This LDW has δ¹⁸O of 0.13‰, making it quite distinct from NEADW. It is also warmer than NWABW, despite having a similar isotopic composition to this latter water mass.     

Roles of Physical Processes in the Carbon Cycle Using a Simplified Physical Model

A simplified physical model is proposed in this article to describe differences among basins in substance distributions which were not well described by previous simplified models. In the proposed model, the global ocean is divided into the Pacific/Indian Ocean (PI), the Atlantic Ocean (AT), the Southern Ocean and the Greenland/Iceland/Norwegian Sea. The model is consisted of five physical parameters, namely the air-sea gas exchange, the thermohaline circulation, the horizontal and vertical diffusions, and the deep convection in the high-latitude regions. Individual values of these parameters are chosen by optimizing model distribution of natural ¹⁴C as a physical tracer. The optimal value for a coefficient of vertical diffusion in the low-latitude region is 7.5 × 10^–5 [m²s^–1]. Vertical transports by the Antarctic Bottom Water and the North Atlantic Deep Water are estimated at 1.0 Sv and 9.0 Sv. Global-mean air-sea gas exchange time is calculated at 9.0 years. Using these optimal values, vertical profiles of dissolved inorganic carbon without biological production in PI and AT are estimated. Oceanic responses to anthropogenic fluctuations in substance concentrations in the atmosphere induced by the industrialization and nuclear bomb are also discribed, i.e., the effects appear significantly in AT while a signal is extremely weak in PI. A time-delay term is effective to make the PI water older near the bottom boundary.     

Deep-water circulation in the Northeast Atlantic and climatic changes during the Late Neogene (DSDP Site 141)

Oxygen and carbon stable isotope data of Pyrgo murrhina and flux rates of calcium carbonate in the bio- and magnetostratigraphically dated sediment sequence at DSDP Site 141 were used for a reconstruction of the deep-water circulation in the Northeast Atlantic during Late Miocene and Pliocene times. A distinct change towards reduced advection of deep water recorded near 5.4 Ma is contemporaneous with the cessation of the outflow of the saline Mediterranean water into the Atlantic. During the Pliocene, between 4.5 and 2.75 Ma and between 2.1 and 1.8 Ma, North Atlantic Deep Water (NADW) circulation was sluggish and Site 141 possibly influenced by Antarctic Bottom Water (AABW). Near 2.75 Ma, the advection of well-oxidized NADW was strongly intensified. This change is related to an onset of major Arctic ice growth and/or a major cooling of NADW.     

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.     

Phytoplankton chlorophyll stocks in the Antarctic Ocean

Phytoplankton chlorophyll stocks in the Indian sector of the Antarctic Ocean were estimated on the basis of published data collected from nine cruises of the Icebreaker,Fuji in 1965–1976, during routine observations of the Japanese Antarctic Research Expedition (JARE). Surface chlorophylla concentration, measured at 631 stations in waters south of 35°S, ranged from 0.01 to 3.01 mg m^–3, At about half of the stations the values were less than 0.24 mg and at only 29 stations were high values more than 1.00 mg m^–3 recorded. The levels of surface chlorophylla stocks were estimated in three groups; (1) data obtained on the southward leg through the eastern Indian sector (middle-late December), (2) those on the northward leg through the western Indian sector (late February–early March) and (3) those on the northward leg through the eastern Atlantic sector (late February–early March). Furthermore, mean values and standard deviations were calculated for each of six different water masses from north to south,i. e., subtropical water between 35°S and the Subtropical Convergence (STC) zone, water within the STC zone, Subantarctic Upper Water, water within the Antarctic Convergence (AC) zone, Antarctic Surface Water between the AC zone and 63°S, and Antarctic Surface Water south of 63°S. Mean values of surface chlorophylla concentrations for each of the six water masses on the three legs ranged from 0.15 to 0.58 mg m^–3 and were comparable to those reported by other workers previously. Seasonal periodicity of phytoplankton chlorophyll stock is discussed. The surface chlorophyll stock in the oceanic water of the Antarctic Ocean does not seem to be so high as previously believed.     

Hydrographic structure and circulation in the central area of the North Eastern Atlantic

On the basis of the hydrographic data observed within the Canary Basin in autumn 1985, temperature-salinity properties, distributions of water masses and barocltne flow field, as well as the volume transports in this area are described more detailly. The analyses indicate that the activity in the waters of the Canary Basin is mainly attributed to the interleaving and mixing between the originated water masses (e. g. Surface Water, North Atlantic Central Water, Mediterranean Water and Deep Water) and the modified water masses (Subpolar Mode Water, Labrador Sea Water and Antarctic Intermediate Water) from the outside of the study area and the variation of themselves. The east recirculation of the Subtropic Gyre in the North Atlantic consists of Azores Current and Canary Current.Azores Current is formed with several flow branches around the Azores Island, while the main flow lies at 35?N south of the Azores Island. It begins to diverge near the 15?W. The return flow found off the Portugal coast may be its     

An estimate of water mass transformation in the southern Weddell Sea

Bottom water formation changes the characteristics of water masses entering the southern part of the Weddell Sea through atmosphere-ice-ocean interaction in which both sea and shelf ice play an important role. Modified water, in particular Weddell Sea Bottom Water, recirculates in the west. By comparing the in- and outflowing water masses we have estimated transformation rates on the basis of a data set obtained during the Winter Weddell Gyre Study from September to October 1989. This consisted of a salinity-temperature-depth (CTD) section carried out by R/V “Polarstern” from the northern tip of the Antarctic Peninsula to Kapp Norvegia and data from three current meter moorings maintained from 1989 to 1990 in the eastern boundary current off Kapp Norvegia. Because of the lack of sufficient direct current measurements in the interior and the western boundary current, it was necessary to derive mass transports on the basis of available data combined with physical and geometrical arguments. At the mooring site barotropic currents were measured. They were extrapolated to the interior under the assumption that wind-driven, baroclinic and barotropic current fields are of similar shape. The location of the gyre centre was determined from drifting buoy tracks and geopoten-tial anomaly. A linear current profile from the eastern boundary current to the centre of the gyre was assumed, and the western outflow was determined according to mass conservation. Different assumptions on the transition from the boundary current to the interior and the location of the centre result in a wide range of transports with most likely values between 20 and 56 Sv. The total mass transport was split into individual water masses. Differences between inflow and outflow result in a transformation rate of 3–4 Sv from Winter and Warm Deep Water to Antarctic and Weddell Sea Bottom Water. The net heat and salt transport across the transect implies heat fluxes from the ocean to the atmosphere of 3–10 W m⁻² and ice formation rates of 0.2–0.35 m year⁻¹.     

Seismostratigraphic analysis of the Transkei Basin: A history of deep sea current controlled sedimentation

The Earth's climate is controlled by various factors, with large scale ocean currents playing a significant role. In particular, the global thermohaline circulation of water masses like the Antarctic Bottom Water (AABW), or the North Atlantic Deep Water (NADW), is a global motor for maintaining the exchange of water masses. The AABW and NADW have met and interacted off South Africa since Oligocene times. Here, the narrow deep Agulhas Passage gateway, located between South Africa and the submarine Agulhas Plateau, constrains bottom water exchange between the southeast Atlantic and the southwest Indian Ocean. A seismostratigraphic analysis of sedimentary structures in the Transkei Basin, which opens up at the eastern end of the Agulhas Passage, was carried out, to reconstruct the palaeocurrents off South Africa. The analysis of newly collected high resolution seismic reflection data showed the effect of large scale current deposition. There are at least 5 major sedimentary phases to observe, some of which seem to be influenced by NADW and AABW. The first stage represents ongoing deep sea sedimentation from middle Cretaceous to middle Tertiary times. Later stages are separated by discordances, which represent the onset of AABW and NADW, among others, triggered by the opening of the Drake Passage gateway ( 35 Ma) and the closure of the Isthmus of Panama ( 3 Ma). We found two large drift bodies located one above the other. Corresponding to their shape and position, the older drift is inferred to have been deposited by currents flowing in a north–southerly direction, whereas the younger drift lies perpendicular to it and seems to be built up by west–east flowing currents.     

Neon distribution in South Atlantic and South Pacific waters

We present and assess the distribution of neon in South Atlantic and South Pacific waters, on the basis of more than 3000 mostly new neon data which were obtained primarily under the hydrographic program of the World Ocean Circulation Experiment (predominantly southern summer to fall cruises). Data precision is better than±0.5%, and the set is internally consistent within±0.3% and partly better, and compatible with reported high-quality neon values. Using suitably averaged data (precision 0.1–0.3%), we find that the total range of neon anomalies relative to a solubility equilibrium with atmospheric neon at the observed potential temperature and salinity (using the solubilities of Weiss, J. Chem. Eng. Data 16 (1971) 235) is approximately 0–4%, and below 2000 m depth, 3–4% only. We consistently observe two types of neon depth profiles, one for the temperate-latitudes ocean, which is characterized by a near-surface maximum and a minimum in Antarctic Intermediate Water, and one for the Southern Ocean that essentially displays a steady increase with depth. The neon distribution reflects the influence of air injected by submerged air bubbles, the areal distribution of atmospheric pressure, seasonal temperature changes in the mixed layer and solar heating below it, and interaction with sea ice and glacial ice, largely in keeping with previous work. However, it appears that interaction with sea ice reduces neon anomalies distinctly less than the literature suggests. The temperate-ocean shallow maxima point to widespread subsurface heating in the course of the summer season by roughly 1 K. Among the major source water masses of the deep waters, the neon anomalies are lowest in Antarctic Intermediate Water (∼1.5%), intermediate in North Atlantic Deep Water (∼3%, confirming previous work) and similarly in Circumpolar Deep Water, and highest in Antarctic Bottom Water (∼3.8%). The anomalies in Southeast Pacific deep waters (>2500 m) are comparatively less (only∼3.3%), as a result of the contribution of Antarctic Intermediate Water. The present study is the first attempt to deal with the oceanic distribution of neon in a systematic fashion. The results can serve to assist assessments of the oceanic distributions of other dissolved gases.     

Facies distribution and dissolution depths of surface sediment components from the Vema channel and the Rio Grande rise (southwest Atlantic Ocean)

Surface sediments from the Vema channel and from the Rio Grande rise (western extremity) contain less calcareous fossils with increasing water depth. The pteropodal (aragonite) lysocline corresponds to the 3200-m isobath. The pteropodal compensation depth is found above the lower boundary of the North Atlantic Deep Water: 3500 m. The planktonic foraminiferal lysocline (4050 m) seems to be very close to the abyssal thermocline and is therefore here the upper limit of the Antarctic Bottom Water. The foraminiferal and the coccolith compensation depths seem to coincide: 4500 m. Distribution and dissolution of calcareous sediment components are the same all around the western extremity of the Rio Grande rise (southern, western and northern flanks).     

Silver in the far North Atlantic Ocean

Total (unfiltered) silver concentrations in higher latitudes of the North Atlantic (52–68°N) are reported for the second Intergovernmental Oceanographic Commission (IOC) Global Investigation of Pollutants in the Marine Environment (GIPME) baseline survey of 1993. These silver concentrations (0.69–7.2 pM) are oceanographically consistent with those (0.24–9.6 pM) previously reported for lower latitudes in the eastern North and South Atlantic (Flegal et al., 1995). However, surface (⩽200 m) water concentrations of silver (0.69–4.6 pM) in the northern North Atlantic waters are, on average, ten-fold larger than those (0.25 pM) considered natural background concentrations in surface waters of the central Atlantic. In contrast, variations in deep far North Atlantic silver concentrations are associated with discrete water masses. Consequently, the cycling of silver in the far North Atlantic appears to be predominantly controlled by external inputs and the advection of distinct water masses, in contrast to the nutrient-like biogeochemical cycling of silver observed in the central Atlantic and Pacific oceans.     

The Namib Col Current

Recent measurements indicate the transatlantic extent of the Namib Col Current at depths of 1300–3000 m near Lat. 22°S in the South Atlantic Ocean. This current forms a continuous circulation structure from the Namib Col on the Walvis Ridge to the western trough, though its characteristic change as deep water with varying properties enters and leaves the current owing to a meridional flow component. Transport estimates from hydrographic sections on the Walvis Ridge and at 15°W near the crest of the Mid-Atlantic Ridge indicate a strength of about 3 × 10⁶ m³ s⁻¹. The current is part of a larger-scale eastward flow at Lon. 25°W; transport estimates across the salinity maximum core there show a similar strength. Associated with this high-salinity high-oxygen current is a basin-wide front in these properties of varying intensity (weaker in the east) marking the transition to deep water whose North Atlantic characteristics have been partly erased by mixing with Circumpolar Deep Water in the southwest South Atlantic. The water which finally crosses the Walvis Ridge is supplied both by the eastward flow of this (diluted) North Atlantic Deep Water and by a general southeastward interior flow from the northern Angola Basin. Evidence suggests that this deep water continues south in the eastern Cape Basin, leaving the South Atlantic near the African continent.     

The distribution and preferential biological uptake of cadmium at 6°W in the Southern Ocean

The spatial and temporal distribution of cadmium (Cd) and phosphate in the Southern Ocean are related to biology and hydrography. During a period of 18 days between transects 5/6 and 11, a phytoplankton spring bloom developed in the Polar Frontal region. Upper water Cd concentrations were not depleted and ranged from 0.2 to 0.8 nM at about 10 m depth. These relatively high Cd concentrations are attributed to upwelling of Upper Circumpolar Deep Water (0.5–1.2 nM in the core) in combination with low biological productivity (0.2 to 0.3 mg m⁻³ chlorophyll-a, 0.3 g C m⁻² d⁻¹). Total particulate Cd concentrations at 40 m depth were between 0.02 and 0.14 nM with the maximum in concentration in the Polar Frontal region. Most of the particulate Cd at this depth (85–94%) was detected in the first phase of a sequential chemical leaching treatment which includes adsorbed Cd as well as Cd incorporated in algae. The Polar Frontal region was characterized by minima in Cd concentration and Cd/phosphate ratio of seawater at both transects; values were the lowest at transect 11 after development of the spring bloom which was dominated by diatoms. This decreasing Cd/phosphate ratio in seawater during spring bloom development was attributed to preferential Cd gross uptake which more than compensated the process of preferential Cd recycling. Within the Upper Circumpolar Deep Water, Cd showed a maximum in concentration similar to that of the major nutrients. Both the Cd concentration and the Cd/phosphate ratio of the deeper water increased in southern direction, from 0.4 to 0.7 nM and from 0.2 to 0.3 nM/μM, respectively. Antarctic Intermediate Water has a Cd concentration of 0.21 nM with a Cd/phosphate ratio of 0.10 nM/μM. In Antarctic Bottom Water, Cd concentrations ranged from 0.60 to 0.82 nM.