Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon inventories

Chlorofluorocarbon (CFC) inventories provide an independent method for calculating the rate of North Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11 inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water (ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland–Scotland Overflow Water (ISOW), and 5.9 million moles for Denmark Strait Overflow Water (DSOW). The inventories directly reflect the input of newly formed water into the deep Atlantic Ocean from the Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990, the formation rates estimated by this method represent an average over this time period. Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970–1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about 17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in the past, it is clearly an important component that is exported out of the North Atlantic with other NADW components.     

Water mass properties and fluxes in the Rockall Trough, 1975–1998

A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.     

High primary productivity and f-ratio in summer in the Ulleung basin of the East/Japan Sea

To better understand the cause of high summer primary productivity in the Ulleung Basin located in the southwest part of the East/Japan Sea, the spatial dynamics of primary, new, and regenerated productivities (PP, NP, and RP) were examined along the path of the Tsushima Warm Current system in summer 2008. We compared hydrographic and chemical parameters in the Ulleung Basin with those of the Kuroshio Current in the Western Pacific Ocean and the East China Sea. In summer, integrated primary productivity (IPP, 0.37–0.96 g C m⁻² d⁻¹) and integrated new productivity (INP, 26–221 mg N m⁻² d⁻¹) within the euphotic zone in the Ulleung Basin were higher than those in the East China Sea and the Western Pacific Ocean (0.17–0.28 g C m⁻² d⁻¹, 2−5 mg N m⁻² d⁻¹, respectively). In contrast, there was no pronounced spatial variation in integrated regenerated productivity (IRP, 43–824 mg N m⁻² d⁻¹). Strong positive correlations between IPP and INP (also the f-ratio), and between nitrate uptake rate in the mixed layer and nitrate upward flux through the top of pycnocline in summer in the Ulleung Basin imply that the high IPP was mainly supported by supply of nitrate from the underlying water in the euphotic zone. Shallowing of the pycnocline depth as the current enters the East/Japan Sea facilitates nitrate supply from the nutrient-replete cold water immediately below the pycnocline through nitrate upward flux. A subsurface maximum in PP at or above the pycnocline and a high f-ratio further support the importance of this source of nitrate for maintaining the high summer PP in the Ulleung Basin. In comparison, the high PP layer was observed at the surface in the following fall and spring in the Ulleung Basin. Our results demonstrate the importance of hydrographic features in enhancing PP in this oligotrophic Tsushima Warm Current system.     

Inorganic carbon in the Labrador Sea: Estimation of the anthropogenic component

The intermediate and deep waters of the Labrador Sea are dominated by recently ventilated water masses (ventilation ages <20 yr). Atmospheric gases such as CO₂ and chlorofluorocarbons are incorporated into these water masses at the time of formation and subsequently transported via boundary currents into the North Atlantic interior. Recent measurements of total carbonate were used in tandem with total alkalinity and oxygen to estimate the levels of anthropogenic carbon dioxide in the Labrador Sea region. Upper water column anthropogenic CO₂ estimated in this manner showed good agreement with levels calculated from CO₂ increase in the atmosphere. In spring 1997, anthropogenic contributions to total carbonate (CTant) were 40±3 μmol/kg in water penetrated by deep convection the previous winter and slightly lower (37±2 μmol/kg) in the deeper convective layer formed in the winters of 1992–1994. Consistent with the concurrent profiles of CFC-11, levels decrease into the older NEADW (North East Atlantic Deep Water) with levels of 30±3 μmol/kg and then increase near bottom within the layer of DSOW (Denmark Strait Overflow Water). The distribution of CTant shows the flow of new LSW southwards with the western boundary current and also eastwards into the Irminger Sea. We estimate that 0.15–0.35 Gt carbon of anthropogenic origin flow through the Labrador Sea within the Western Boundary Undercurrent per year.     

A suspended-particle rosette multi-sampler for discrete biogeochemical sampling in low-particle-density waters

To enable detailed investigations of early stage hydrothermal plume formation and abiotic and biotic plume processes we developed a new oceanographic tool. The Suspended Particulate Rosette sampling system has been designed to collect geochemical and microbial samples from the rising portion of deep-sea hydrothermal plumes. It can be deployed on a remotely operated vehicle for sampling rising plumes, on a wire-deployed water rosette for spatially discrete sampling of non-buoyant hydrothermal plumes, or on a fixed mooring in a hydrothermal vent field for time series sampling. It has performed successfully during both its first mooring deployment at the East Pacific Rise and its first remotely-operated vehicle deployments along the Mid-Atlantic Ridge. It is currently capable of rapidly filtering 24 discrete large-water-volume samples (30–100 L per sample) for suspended particles during a single deployment (e.g. >90 L per sample at 4–7 L per minute through 1 μm pore diameter polycarbonate filters). The Suspended Particulate Rosette sampler has been designed with a long-term goal of seafloor observatory deployments, where it can be used to collect samples in response to tectonic or other events. It is compatible with in situ optical sensors, such as laser Raman or visible reflectance spectroscopy systems, enabling in situ particle analysis immediately after sample collection and before the particles alter or degrade.     

Mesoscale physical variability affects zooplankton production in the Labrador Sea

Surface distribution (0–100 m) of zooplankton biomass and specific aminoacyl-tRNA synthetases (AARS) activity, as a proxy of structural growth, were assessed during winter 2002 and spring 2004 in the Labrador Sea. Two fronts formed by strong boundary currents, several anticyclonic eddies and a cyclonic eddy were studied. The spatial contrasts observed in seawater temperature, salinity and fluorescence, associated with those mesoscale structures, affected the distributions of both zooplankton biomass and specific AARS activity, particularly those of the smaller individuals. Production rates of large organisms (200–1000 μm) were significantly related to microzooplankton biomass (63–200 μm), suggesting a cascade effect from hydrography through microzooplankton to large zooplankton. Water masses defined the biomass distribution of the three dominant species: Calanus glacialis was restricted to cold waters on the shelves while Calanus hyperboreus and Calanus finmarchicus were widespread from Canada to Greenland. Zooplankton production was up to ten-fold higher inside anticyclonic eddies than in the surrounding waters. The recent warming tendency observed in the Labrador Sea will likely generate weaker convection and less energetic mesoscale eddies. This may lead to a decrease in zooplankton growth and production in the Labrador basin.     

Dimethylsulphide and dimethylsulphoniopropionate in Antarctic sea ice and their release during sea ice melting

This study presents concentrations of dimethylsulphide (DMS) and its precursor compound dimethylsulphoniopropionate (DMSP) in a variety of sea ice and seawater habitats in the Antarctic Sea Ice Zone (ASIZ) during spring and summer. Sixty-two sea ice cores of pack and fast ice were collected from twenty-seven sites across an area of the eastern ASIZ (64°E to 110°E; and the Antarctic coastline north to 62°S). Concentrations of DMS in 81 sections of sea ice ranged from < 0.3 to 75 nM, with an average of 12 nM. DMSP in 60 whole sea ice cores ranged from 25 to 796 nM and showed a negative relationship with ice thickness (y = 125x^− 0.8). Extremely high DMSP concentrations were found in 2 cores of rafted sea ice (2910 and 1110 nM). The relationship of DMSP with ice thickness (excluding rafted ice) suggests that the release of large amounts of DMSP during sea ice melting may occur in discrete areas defined by ice thickness distribution, and may produce ‘hot spots’ of elevated seawater DMS concentration of the order of 100 nM. During early summer across a 500 km transect through melting pack ice, elevated DMS concentrations (range 21–37 nM, mean 31 nM, n = 15) were found in surface seawater. This band of elevated DMS concentration appeared to have been associated with the release of sea ice DMS and DMSP rather than in situ production by an ice edge algal bloom, as chlorophyll a concentrations were relatively low (0.09–0.42 μg l^− 1). During fast ice melting in the area of Davis station, Prydz Bay, sea ice DMSP was released mostly as extracellular DMSP, since intracellular DMSP was negligible in both hyposaline brine (5 ppt) and in a melt water lens (4–5 ppt), while extracellular DMSP concentrations were as high as 149 and 54 nM, respectively in these habitats. DMS in a melt water lens was relatively high at 11 nM. During the ice-free summer in the coastal Davis area, DMS concentrations in surface seawater were highest immediately following breakout of the fast ice cover in late December (range 5–14 nM), and then remained at relatively low concentrations through to late February (< 0.3–6 nM). These measurements support the view that the melting of Antarctic sea ice produces elevated seawater DMS due to release of sea ice DMS and DMSP.     

Geochemical constraints on primary productivity in submarine hydrothermal vent plumes

The amount of metabolic energy available for primary production by chemolithoautotrophic microorganisms in a submarine hydrothermal plume is evaluated using geochemical models. Oxidation of elemental sulfur and metal sulfides precipitated in the hydrothermal plume represent the largest potential sources of metabolic energy in the plume (∼600 cal/kg vent fluid from each source). Among dissolved substrates, oxidation of H₂ potentially provides the greatest amount of energy (∼160 cal/kg). Smaller, but still significant, amounts of energy are also available from sulfate reduction (54 cal/kg), methanogenesis (17 cal/kg), and methanotrophy (13 cal/kg). Only negligible amounts of energy are available from oxidation of Fe(II) or Mn(II) compounds or Fe³⁺ reduction (<1 cal/kg vent fluid). The models suggest that most primary production in the plume should occur in the early stages of plume development from sulfur- and H₂-oxidizers entrained in the plume or colonizing the surfaces of minerals settling from the plume. The total primary productivity potential in the plume is estimated to be about 50 mg dry wt biomass/kg vent fluid. This translates to a global annual biomass production in hydrothermal plumes on the order of 10¹² g dry wt/yr, which represents only a small fraction of the total photosynthetic biomass production in the oceans (∼10¹⁷ g dry wt/yr). Nevertheless, biomass generated in hydrothermal plumes may represent a significant fraction of the organic matter in the deep ocean as well as that deposited in sediments in ocean basins.     

Year-long float trajectories in the Labrador Sea Water of the eastern North Atlantic Ocean

Year-long Lagrangian trajectories within the Labrador Sea Water of the eastern North Atlantic Ocean are analysed for basic flow statistics. Root-mean-square velocities at 1750 m depth are about 2 cm/s, except within the North Atlantic Current, where they are twice as large. These values are consistent with previous Eulerian measurements and extend those results to a much larger domain of the eastern basin. Mean flow estimates in boxes large enough to contain about 1 float-year of data indicate that Labrador Sea Water, having crossed the Mid- Atlantic Ridge (not resolved) near 50–55^°N, presumably with the North Atlantic Current, partially recirculates to the north in the subpolar gyre, as well as entering the subtropical gyre and continuing south and west. The circulation of this water mass, as defined by the 1 yr average velocities, is stronger than traditional models of deep circulation would suggest, with an interior flow of roughly 1 cm/s. Mean speeds up to 3 cm/s were observed, with the highest values near the Azores Plateau. North of 45°N–55°N, mean eastward speeds closer to 0.2 cm/s were observed. Wind-generated barotropic fluctuations may be responsible for some part of the transport at this depth.     

Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider

We investigated the 2005 spring phytoplankton bloom in the Labrador Sea using Seaglider, an autonomous underwater vehicle equipped with hydrographic, bio-optical and oxygen sensors. The Labrador Sea blooms in distinct phases, two of which were observed by Seaglider: the north bloom and the central Labrador Sea bloom. The dominant north bloom and subsequent zooplankton growth are enabled by the advection of low-salinity water from West Greenland in the strong and eddy-rich separation of the boundary current. The glider observed high fluorescence and oxygen supersaturation within haline-stratified eddy-like features; higher fluorescence was observed at the edges than centers of the eddies. In the central Labrador Sea, the bloom occurred in thermally stratified water. Two regions with elevated subsurface chlorophyll were also observed: a 5 m thin-layer in the southwest Labrador Current, and in the Labrador shelf-break front. The thin layer observations were consistent with vertical shearing of an initially thicker chlorophyll patch. Observations at the front showed high fluorescence down to 100 m depth and aligned with the isopycnals defining the front. The high-resolution Seaglider sampling across the entire Labrador Sea provides first estimates of the scale dependence of coincident biological and physical variables.     

Spatiotemporal development of physical,chemical, and biological characteristics of stormwater plumes in Santa Monica Bay,California (USA)

This study characterized stormwater plume development and associated phytoplankton dynamics in a coastal marine ecosystem through shipboard monitoring. We focused on plumes within Santa Monica Bay, California (USA), a coastal system that is subject to rapid pulses of untreated runoff from the urbanized watershed of Los Angeles during the winter rainy season. The physical, chemical, and biological signatures of stormwater plumes were tracked over time after each of 4 precipitation events ranging in magnitude from 1.5 cm to 9 cm. Low salinity surface plumes persisted in Santa Monica Bay for at least 2 to 5 days over spatial scales of up to 15 km. This is consistent with a 6-day residence time for surface water plume parcels, which was estimated from a drifter trajectory in the bay. Shipboard sampling and salinity measurements in the surf zone showed that plumes often persisted even longer nearshore. Plume waters were generally characterized by higher concentrations of dissolved nitrogen, colored dissolved organic matter, and higher light attenuation than non-plume waters. The magnitude of the effect of stormwater runoff on phytoplankton dynamics was dependent on the size of each storm and subsequent residence time of runoff within the bay. Rain events led to increases in primary productivity, phytoplankton biomass, and specifically, increases in diatom biomass, as measured by concentrations of biogenic silica.     

Observation and parameterization of small icebergs: Drifting breakwaters in the southern ocean

The variability of small-size iceberg distributions is revealed from a novel analysis of satellite altimeter data. A strong annual cycle is modulated by pulse-like events confined to single ocean basins, with dense iceberg populations in the South Atlantic in 2004–2005, and in the South Pacific in 2008. Anomalies in sea surface temperatures of the order of 1 °C may be related to the iceberg distribution. Icebergs also appear very strongly associated with anomalies in the heights of ocean waves. A preliminary parameterization of wave blocking by icebergs significantly reduces wave model errors in the region south of 45° South, and has a perceptible influence on all the west coasts of the Southern hemisphere.     

Spatial distributions of dimethylsulfide in the South China Sea

The concentration of dimethylsulfide (DMS) and supporting parameters were determined in surface seawater and vertical profiles at 26 stations in the South China Sea. The concentrations of DMS in surface seawater ranged from 61 to 148 ng S/l, with a mean of 82 ng S/l. High concentrations of DMS were found in the productive regions. The vertical profiles of DMS were characterized by a maximum at depths typically between 20 and 75 m. The concentrations of DMS were correlated with the levels of chlorophyll a both in the surface seawater and in the vertical distribution. The concentrations of DMS were higher than expected for this chlorophyll-poor tropical sea, as indicated by markedly high DMS (ng S/l)/chlorophyll a (μg/l) ratios ranging from 315 to 3524 with a mean of 1768 for all the surface seawater samples. DMS concentration was significantly correlated with seawater temperature and dissolved oxygen, but it showed an inverse relationship to nutrients (including nitrate, phosphate and silicate). On the basis of sea surface concentrations of DMS and gas exchange calculations, the mean flux of DMS from the South China Sea to the atmosphere was estimated to be 5.5 μmol m⁻² d⁻¹.     

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March–May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographic data set currently available for this region. Waters above 100–150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s^-1. A simple model that balances across-shelf diffusion of heat and salt from offshore Upper Circumpolar Deep Water with vertical diffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vertical diffusion of 10^-4 m² s^-1 and horizontal diffusion coefficients for heat and salt of 200 and 1200 m² s^-1, respectively. When the effects of double diffusion are included in the model, the required horizontal diffusion coefficients for heat and salt are 200 and 400 m² s^-1, respectively.     

Bathymetric influence on dissolved methane in hydrothermal plumes revealed by concentration and stable carbon isotope measurements at newly discovered venting sites on the Central Indian Ridge (11–13°S)

Methane is a useful tracer for studying hydrothermal discharge, especially where the source fluids are of low temperature and lack metal precipitates. However, the dual origins of deep-sea methane, both chemical and biological, complicate the interpretation of methane observations. Here, we use both the concentration and stable carbon isotopic composition (δ¹³C) of dissolved methane to trace hydrothermal plumes and identify the source and behavior of methane at two sites of newly discovered hydrothermal activity on the Central Indian Ridge (11–13°S). At both sites, methane and optical anomalies between 2500 and 3500 m at all stations indicate active hydrothermal discharge. We compared methane concentrations and δ¹³C at three stations, two (CTIR110136 and CTIR110208) with the most prominent anomalies at each site, and a third (CTIR110140) with near-background methane values. At stations CTIR110136 and CTIR110208, the concentration and δ¹³C of methane in distinct plumes ranged from 3.3 to 42.3 nmol kg⁻¹ and −30.0 to −15.4‰, respectively, compared to deep-water values of 0.5 to 1.2 nmol kg⁻¹ and −35.1 to −28.9‰ at the station with a near-background distal plume (CTIR110140). δ¹³C was highest in the center of the plumes at CTIR110136 (−15.4‰) and CTIR110208 (−17.8‰). From the plume values we estimate that the δ¹³C of methane in the hydrothermal fluids at these stations was approximately −19‰ and thus the methane was most likely derived from magmatic outgassing or the chemical synthesis of inorganic matter. We used the relationship between δ¹³C and methane concentration to examine the behavior of methane at the plume stations. In the CTIR110208 plume, simple physical mixing was likely the major process controlling the methane profile. In the CTIR110136 plume we interpret a more complicated relationship as resulting from microbial oxidation as well as physical mixing. We argue that this difference in methane behavior between the two areas stems from a distinct bathymetric dissimilarity between the two stations. The location of CTIR110208 on the open slope of a ridge allowed rapid plume dispersion and physical mixing, whereas the location of CTIR110136 in a small basin surrounded by wall structures inhibited physical mixing and enhanced microbial oxidation.     

Absolute velocity along the AR7W section in the Labrador Sea

Nearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80 TW)), and much larger than the annual average values (O(10–20 TW)).     

Mixing and advection of a cold water cascade over the Wyville Thomson Ridge

The Wyville Thomson Ridge forms part of the barrier to the meridional circulation across which cold Nordic Sea and Arctic water must traverse to reach the Atlantic Ocean. Overflow rates across the ridge are variable (but can be dramatic at times), and may provide a subtle indicator of significant change in the circulation in response to climate change. In spring 2003, a series of CTD sections were conducted during a large overflow event in which Norwegian Sea Deep Water (NSDW) cascaded down the southern side of the ridge into the Rockall Trough at a rate of between 1 and 2 Sv. The NSDW was partially mixed with overlying North Atlantic Water (NAW), and comprised about 1/3rd of the cascading water. The components of NAW and NSDW in the overflow were sufficiently large that there must have been a significant divergence of the inflow through the Faroe-Shetland Channel, and of the outflow through the Faroe Bank Channel.As the plume descended, its temperature near the sea bed warmed by over 3 °C in about a day. Although the slope was quite steep (0.03), the mean speed of the current (typically 0.36 m s⁻¹) was too slow for significant entrainment of NAW to occur (the bulk Richardson number was of order 5). However, very large overturns (up to 50 m) were evident in some CTD profiles, and it is demonstrated from Thorpe scale estimates that the warming of the bottom waters was due to mixing within the plume. It is likely that some of the NSDW had mixed with NAW before it crossed the ridge. The overflow was trapped in a gully, which caused it to descend to great depth (1700 m) at a faster rate, and with less modification due to entrainment, than other overflows in the North Atlantic. The water that flowed into the northern part of the Rockall Trough had a temperature profile that ranged from about 3 to 8 °C. Water with a temperature of >6 °C probably escaped into the Iceland Basin, between the banks that line the north-western part of the Trough. Colder water (< 6 °C) must have travelled down the eastern side of the Rockall Bank, and may have had a volume flux of up to 1.5 Sv.     

Natural and anthropogenic sources of Hg,Cd, Pb,Cu and Zn in seawater and sediment of Mljet National Park,Croatia

Distributions of Hg, Cd, Pb, Cu and Zn in seawater and sediment from Mljet National Park, Adriatic Sea are presented for the first time. Natural and anthropogenic factors play an important role in determining resultant trace metals' concentrations in the region. We place particular emphasis on the saline “lakes” of Malo Jezero and Veliko Jezero, which have restricted flows of seawater. In Malo Jezero lake, fresh karstic spring water generated by flooding, and weathering of dolomites are the main sources of naturally elevated Cd, Pb and Zn concentrations (20.7 ± 1.6, 289 ± 19, 1260 ± 0.08 ng L^?1, respectively); anthropogenic input is negligible. In Veliko Jezero lake enhanced Cu and Zn contents originate from anthropogenic input (tourism and agriculture). Distributions of the Pb and Zn in the water columns of both lakes are influenced by natural aragonite precipitation and sedimentation. Exceptionally high total Hg concentrations of 24.2 and 33.7 ng L^?1 in the water column of Malo Jezero, sampled during periods of high rainfall associated with strong eastern winds, suggest an airborne input. Total Hg concentrations in waters of both lakes are elevated because of inefficient mixing. Two different metal distribution patterns exist in the sediment columns. First, Hg, Pb, Cu and Zn show elevated concentrations in recent sediments due to anthropogenic input. Second, Cd content increases with depth due to reprecipitation via a downward redox boundary shift.Described natural processes, as well as anthropogenic influence, enhance levels of trace metals. Careful study followed by suitable interpretation based on geochemical data were necessary to distinguish natural from anthropogenic sources.     

Long-term hydrographic changes at 52 and 66°W in the North Atlantic Subtropical Gyre & Caribbean

In July–September 1997 two hydrographic lines were done in the western N. Atlantic along longitudes of 52 and 66°W as part of the WOCE one-time hydrographic survey of the oceans. Each of these two lines approximately repeated earlier ones done during the International Geophysical Year(s) (IGY) and the mid-1980s. Because of this repeated sampling, long-term hydrographic changes in the water masses can be examined. In this report, we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35°N and compare our results to those presented by Bryden et al. (1996), who examined changes along a zonal line at 24°N, most recently occupied in 1992. Since this most recent 24°N section in 1992, substantial changes have occurred in the western part of the subtropical gyre at the depths of the Labrador Sea Water (LSW). In particular, we see clear evidence for colder, fresher Labrador Sea Water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24°N. At shallower depths inhabited by waters that are an admixture of Mediterranean (MW) and Antarctic Intermediate Waters (AAIW), our recent survey shows an increase in salinity, which can only be attributed to changes in water masses on potential temperature or neutral density surfaces. Furthermore, waters above the MW/AAIW layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40-year period spanned by our sections. These latter changes have been dominantly due to a vertical sinking of density surfaces as T/S changes in density surfaces are small, but depths of individual T/S horizons have increased with time. The net change since the IGY shows a mean temperature increase between 800 and 2500 m depth at a rate of 0.57°C/century with a corresponding steric sea level rise of 1 mm/yr, and a net downward heave with small values near the top and bottom, and a maximum rate of −2.7 m/yr at 1800 m depth. Changes in the deep Caribbean indicate a warming since the IGY due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth, but no significant change in the deep salinity.     

Visual observations of the vertical distribution of plankton throughout the water column above Broken Spur vent field,Mid-Atlantic Ridge

Visual observations were made in September 1997 during the 39 cruise of R/V “Akademik Mstislav Keldysh” with 2 deep-sea manned submersibles “Mir” aboard. During 4 dives the following plankton countings were made: 3 vertical throughout the water column during the day, 2 vertical in the upper 1000 m at night, and 1 oblique in the plume area during the day. Biomass profiles are represented for each dive for all abundant animal groups: copepods, euphausiids+decapods+mysids, chaetognaths, medusae, ctenophores, siphonophores, cyclothones, myctophides, radiolarians, and the total zooplankton. Plankton distribution shows 2 aggregations, one within the main pycnocline and the other near the plume; Gelatinous animals and radiolarians dominate in both aggregations by biomass and make a significant contribution to the plankton biomass throughout the water column. Oblique counting indicates the presence of aggregations of animals near the upper and lower borders of the plume and biomass depletion within the plume core.