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International audience; In summer 1996, a tracer release experiment using sulphur hexafluoride (SF 6) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996-2002 period documented by ∼10 shipboard surveys. The tracer marked “Greenland Sea Arctic Intermediate Water” (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002. For the first 18 months, the tracer remained mainly in the Greenland Sea. Vigorous horizontal mixing within the Greenland Sea gyre and a tight circulation of the gyre interacting slowly with the other basins under strong topographic influences were identified. We use the tracer distributions to derive the horizontal shear at the scale of the Greenland Sea gyre, and rates of horizontal mixing at ∼10 and ∼300 km scales. Mixing rates at small scale are high, several times those observed at comparable depths at lower latitudes. Horizontal stirring at the sub-gyre scale is mediated by numerous and vigorous eddies. Evidence obtained during the tracer release suggests that these play an important role in mixing water masses to form the intermediate waters of the central Greenland Sea. By year two, the tracer had entered the surrounding current systems at intermediate depths and small concentrations were in proximity to the overflows into the North Atlantic. After 3 years, the tracer had spread over the Nordic Seas basins. Finally by year six, an intensive large survey provided an overall synoptic documentation of the spreading of the tagged GSAIW in the Nordic Seas. A circulation scheme of the tagged water originating from the centre of the GS is deduced from the horizontal spread of the tracer. We present this circulation and evaluate the transport budgets of the tracer between the GS and the surroundings basins. The overall residence time for the tagged GSAIW in the Greenland Sea was about 2.5 years. We infer an export of intermediate water of GSAIW from the GS of 1 to 1.85 Sv (1 Sv = 10 6 m 3 s -1) for the period from September 1998 to June 2002 based on the evolution of the amount of tracer leaving the GS gyre. There is strong exchange between the Greenland Sea and Arctic Ocean via Fram Strait, but the contribution of the Greenland Sea to the Denmark Strait and Iceland Scotland overflows is modest, probably not exceeding 6% during the period under study.

Measurements of the carbonate system and the transient tracers, CFC-11 and carbon tetrachloride (CCl4), in the Greenland Sea and western Eurasian Basin are used to deduce the sources and magnitude of excess dissolved inorganic carbon in the deep Greenland Sea. The salinity in the deep Greenland Sea has increased during the past 20 years as a result of Arctic Ocean water advecting into the area. From salinity and temperature data the relative amount of advective water below 1500 m is estimated to ∼2% yr−1 between 1982 and 1994. Data from the western Eurasian Basin, collected during the Oden 91 cruise, are used as the advective source waters. A mixing box model is applied to estimate the evolution of CFCs and anthropogenic total dissolved inorganic carbon in the deep waters of the Greenland Sea since preindustrial times. The source functions are the surface water concentrations of CFC-11, CCl4, and total dissolved inorganic carbon CT, which are the result of anthropogenic emission. In order to explain the measured CFC data in the Greenland Sea an annual percent renewal by deep water formation of between 0.5 and 1.2% is needed below 1500 m, corresponding to a mean ventilation of 0.17±0.05 Sv, considering realistic uncertainties in the calculations. This ventilation gives a sequestering of anthropogenic carbon dioxide, which in 1994 equaled 2.4±0.7×1012 g C yr−1.

Hydrographic, nutrient and halocarbon tracer data collected in July-August 1994 in the Norwegian Sea, the Faroe Bank Channel (FBC), the Iceland and Irminger Basins and the Iceland Sea are resented. Special attention was given to the overflow waters over the Iceland-Scotland Ridge ISOW). The Iceland-Scottland overflow water ISOW) was identified along its pathway in the Iceland Basin, and entrainment of overlying water asses was quantified by multivariate analysis (MVA) using principal component analysis (PCA) and Partial Least Square (PLS) calibration. It was concluded that the deeper portion of the ISOW in the FBC was a mixture of about equal parts of Norwegian Sea Deep Water (NSDW) and Norwegian Sea Arctic Intermediate Water (NSAIW). The mixing development of ISOW during its descent in the Iceland Basin was analysed in three sections across the plume. In the southern section at 61˚N, where the ISOW core was observed at 2300 m depth, the fraction of waters originating north of the ridge was assessed to be 54%. MVA assessed the fractional composition of the ISOW to be 21% NSDW, 22% NSAIW, 18% Northeast Atlantic Water (NEAW), 11% Modified East Icelandic Water, 25% Labrador Sea Water (LSW) and 3% North East Atlantic Deep Water. It may be noted that the fraction of NEAW is of the same volume as the NSDW. On its further path around the Reykjanes Ridge, the ISOW mixed mainly with LSW, and at 63˚N in the Irminger Basin, it was warmer and fresher (θ=2.8°C and S=34.92) than at 61°N east of the ridge (θ=2.37°C and S=34.97). The most intensive mixing occurred immediately west of the FBC, probably due to high velocity of the overflow plume through the channel, where annual velocity means exceeded 1.1 msˉ¹. This resulted in shear instabilities towards the overlying Atlantic waters and cross-stream velocities exceeding 0.3 msˉ¹ in the bottom boundary layer. The role of NSAIW as a component of ISOW is increasing. Being largely a product of winter convection in the Greenland Sea when no Greenland Sea Deep Water (GSDW) is formed, it spreads above the older and denser deep water in the Nordic Seas. Little or no GSDW, which earlier was considered to be the principal overflow water, has been formed since 1970. This shows that the Iceland-Scotland overflow may also be maintained with intermediate waters as the principal overflowing component. Decadal variability in ISOW properties has not been insignificant, as since the early 1960s there has been a decrease in salinity and temperature, by 0.06 and up to 0.5°C, respectively. Such a trend applies also to the LSW, particularly in the Irminger Basin, where it was warmer, saltier and less dense in the late 1950s and early 1960s (θ≈3.5˚C, S≈34.9,σ1.5≈34.64 kgmˉ³) than in 1994 (θ≈2.9˚C, S≈34.86,σ1.5≈34.69 kgmˉ³)CFC tracers were used to assign apparent ages of water masses, showing that the NSDW had an apparent age of about 30 years and that the age of Iceland Sea Deep Water exceeded 25 years. NSAIW observed in the southern Norwegian Sea was estimated to be 6-16 years old. An upper age limit of LSW in the Iceland Basin was found to be 18-19 years. It was further concluded that the products of the onset of intense wintertime convection in the Labrador Sea in the late 1980s were not yet observed in the northern central part of the Iceland Basin. The LSW in the Irminger Basin was found to be significantly younger. Two layers were found there. A shallower layer at a depth of 1000-1500 m depth was older than the layer beneath by about 4 years, while the deeper layer at 1500-1800 m depth was assessed at an apparent age ranging between less than 1 (formed during the previous winter) and 4 years.

The biogenic release of halogenated volatile organics from the ocean by natural sources and the biochemical pathways which lead to the production of these substances are not well understood. In this chapter, some of the marine sources are discussed and results are presented and reviewed on the role of vanadium bromoperoxidases from seaweeds in the formation of halomethanes. Further, it will be shown that most of the predominant seaweed species from the North Atlantic, which are also present in the Arctic Ocean, contain bromoperoxidases. In contrast, in members of the Desmarestiales collected in the Weddell Sea (Antarctic) which provide the bulk of the biomass of benthic seaweeds in the Antarctic waters, no bromoperoxidase activity could be detected. The distribu;tion of seaweeds with bromoperoxidase activity is correlated with the seawater concentration of bromoform.

Electron impact ionization mass spectrometric confirmation of organochlorine insecticides and polychlorinated biphenyls (PCBs) in extracts from environmental samples cleaned up for ECD/GC analysis has often been hampered by the presence of petroleum hydrocarbons, usually appearing in concentrations highly above, and interfering with, those of organochlorine compounds. A clean‐up procedure is presented, by which these compounds are isolated from aliphatic as well as polyaromatic (petroleum) hydrocarbons through a combination of gel permeation HPLC and adsorption chromatography on silica microcolumns.

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior1,2. Deep-water formation varies substantially on annual and decadal timescales3,4,5, and responds to regional climate signals such as the North Atlantic Oscillation6,7; its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change8. Here we report direct measurements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. In summer, we found rapid turbulent vertical mixing of about 1.1 cm2 s-1. In the following late winter, part of the water column was mixed more vigorously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing outside the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically transported in convective plumes, which reached from the surface to, at their deepest, 1,200–1,400 m. Convection was limited to a very restricted area, however, and smaller volumes of water were transported to depth than previously estimated9. Our results imply that it may be the rapid year-round turbulent mixing, rather than convection, that dominates vertical mixing in the region as a whole.

The concentrations of carbon tetrachloride, tetrachloroethylene. 1,1,1-trichloroethane, and bromoform have been determined for Arctic seawater in the Svalbard area in August and September 1980 by a pentane extraction of the seawater followed by glass capillary gas chromatographic separation and electron capture detection. The vertical distributions in a transection across Fram Strait show an inflow of 50 tons/year of carbon tetrachloride, 46 tons/year of tetrachloroethylene, and 114 tons/year of 1,1,1-trichloroethane due to contamination of Atlantic water in the industrial latitudes, and the outflow through the same transection 50, 60, and 129 tons/year, respectively. The uncertainties in these figures are estimated to 50%. Bromoform has a biogenic source in the belts of benthic algae growing in the coastal waters of Svalbard. The mean concentrations for open surface waters were 0.83 (s = 0.17) ng/L, 0.68 (s = 0.23) ng/L, 2.5 (s = 1.8) ng/L, and 9.8 (s = 4.0) ng/L for carbon tetrachloride, tetrachloroethylene, 1,1,1-trichloroethane, and bromoform, respectively. The anthropogenic substances, carbon tetrachloride, tetrachloroethylene, and 1,1,1-trichloroethane, show an undersaturation in the surface water compared with atmospheric northern hemisphere concentrations, which is why the Arctic seawater is thought to act as a sink for atmospheric halocarbons from anthropogenic sources. The biogenic production of bromoform causes an oversaturation in open sea surface waters when compared with the atmospheric background, which is why bromoform is supposed to be a source of atmospheric bromine.