Description: We report, numerically and in graphical form, measured tritium concentrations from five hydrographic stations in the North Atlantic. Fairly homogeneous concentrations are observed in a surface layer typically 400 m deep. In the thermocline, concentrations decrease steadily down to the a σθ = 27.3 density horizon, and are more variable further down. The tritium in the lower part of the thermocline originates from the Subarctic Intermediate Water and the Mediterranean Water. There is a relative tritium maximum associated with the Mediterranean Water on the easternmost station of the section. In the deep water (σθ 〉 27.8), concentrations east of the Midatlantic Ridge are close to the limit of detection down to 2500 m, and undetectable further down, while west of the ridge tritium is found throughout the water column. The deep water tritium is associated with the deep-water advective cores of Arctic origin. The present tritium data can serve as northern boundary values in attempts to use tritium in studies of the North Atlantic main thermocline dynamics. The present data together with data from the literature point to a general division of the North Atlantic main thermocline into two layers separated by an isopycnal surface near σθ = 27.3.

Description: 14C concentrations, as weil as 13C, hydrographic and nutrient data are reported for 5 hydrographic stations that form a transatlantic section near 40° N ("Meteor" cruise no. 23, 1971). Precision (for 14C ± 0.3% or better) and comparability with literature data are specified. A planned intercomparison with the US GEOSECS program within the Newfoundland Basin deep water failed because of variability of water characteristics. The observed 14C values decrease from about Δ 14C = + 80‰ at the surface to -70‰ at 2000 m depth. Deeper down, the values west of the Midatlantic Ridge remain similar, whereas those east of the ridge decrease further, to about -110‰. It is shown that bomb-14C is prominent down to about 1500 m depth. Beyond this depth the bomb 14C component is small and is negligible in the eastern basin below 2800 m. On the basis of the 14C-tritium correlation, the distribution of natural 14C below about 1500 m depth is derived from the observations. In the deep and bottom water east of the ridge the 14C-salinity relationship seemingly is non-linear. Contrary to expectation, the 14C concentration in the bottom water is not lower than found on an US GEOSECS station near 10° N. Apparently, lateral concentration differences in the Northeast Atlantic bottom water as well as nonlinearity of the 14C-salinity relationship at 40°N do not exceed 10‰ in Δ 14C.

Description: Hydrographie data (salinity, temperature, oxygen, silicate, and phosphate) obtained on 5 stations ("Meteor" cruise 23, leg C, 8 to 26 June 1971) on a section from Lisbon, Portugal, to 44° N, 43° W (Newfoundland Basin) by both water sampling and in situ observation by the "Bathysonde" (STD), are summarized. A strong core of Mediterranean water was found at the eastern boundary of the section (38.5° N, 11.5° W). At this station, the core is accompanied by low nutrient concentrations and brings about an extended oxygen minimum (ca. 500 to 1400 m depth). The core quickly weakens towards the west and is, at the Mid-Atlantic Ridge, only apparent in the Bathysonde data. Two salinity maxima are observed within the core of Mediterranean water, the center of which speads along the isopycnal σt = 27.7. Dissolved oxygen shows a rapid concentration increase with depth below the Mediterranean water core; concentration variations with depth below the range of this increase are only small. The mean deep-water oxygen concentration increases from 5.5 ml/kg (below 2500 m) to 6.20 ml/kg (below 1500 m) in an east-west direction on the section. The upper boundary of the deep-water oxygen concentration range thereby rises from 2000 to 1300 m; this boundary marks the upper boundary of the Arctic Intermediate water. Core depths of Arctic Intermediate and of Iceland-Scotland overflow water, are derived from the potential-temperature/salinity diagrams obtained in the western basin, and are extended to the other stations by assuming lateral spreading to occur along isopycnal surfaces. The core depths for the Intermediate water obtained in this manner, are supported also by the potential-temperature to silicate relations. The bottom water of the westernmost station of the section, at 44° N, 43° W, is of Denmark Strait origin, and it produces a distinct reversal in the vertical trends of salinity, silicate, phosphate, and oxygen, at 4300 m depth. The concentration of the nuclear-weapon produced nuclide tritium increases within the Denmark Strait water core towards the bottom. Further tritium concentration peaks appear in the intermediate and deep water at this station. At the next Station east on the section at 43° N 34 ° W, tritium concentrations are essetially zero below 2000 m depth, and are distinctly smaller than on the westernmost station, between 600 m and 2000 m depth. This "Meteor" section was track F of the Atlantic network of the international Geochemical Ocean Sections Program (GEOSECS).

Description: Simulated transient-tracer distributions (tritium, 3H3, freons) on the isopycnal horizons σ0=26.5 and 26.8 kg m−3 are presented for the East Atlantic, 10° −40°N. Tracer transport is modeled by employing a baroclinic flow field based on empirical data in a kinematic isopycnal advection-diffusion numerical model, in which winter convection is taken as the mechanism of communication with the ocean surface layer, and the isopycnal diffusivity is a free parameter. Diapucnic transport is ignored. The simulations employ time-dependent tracer boundary conditions, which are constructed on the basis of available observations. Simulations are compared to data obtained on a meridional section in 1981 (F/S Meteor, cruise 56/5). Best simulations were obtained by means of a subjective optimization procedure. On both levels, the observed distributions and the best simulated distributions agree well. The fact that the surface boundary conditions and interior distributions of the tracers are distinctly different leads us to the conclusion that our model provides a consistent description of upper main-thermocline ventilation and interior transport Surface-water densities in February are found to represent adequately the winter outcrop boundaries with an uncertainty of about ±300 km across. The required isopycnal diffusivity south of 29°N is 1700 m2 s−1, and 2900 m2 s−1 further north (+70/−40%). Interior transport is found to be predominantly advective. Advective ventilation across 30.5°N east of 33°W amounts to only 12% and 40% for the 26.5 and 26.8 horizons of the total ventilation rates reported by Sarmiento. The North Atlantic/South Atlantic Central Water boundary near 15°N is found to be predominantly determined by advection.

Description: We present a new method to obtain samples for the measurement of helium isotopes and neon in water, to replace the classical sampling procedure using clamped-off Cu tubing containers that we have been using so far. The new method saves the gas extraction step prior to admission to the mass spectrometer, which the classical method requires. Water is drawn into evacuated glass ampoules with subsequent flame sealing. Approximately 50% headspace is left, from which admission into the mass spectrometer occurs without further treatment. Extensive testing has shown that, with due care and with small corrections applied, the samples represent the gas concentrations in the water within ±0.07% (95% confidence level; ±0.05% with special handling). Fast evacuation is achieved by pumping on a small charge of water placed in the ampoule. The new method was successfully tested at sea in comparison with Cu-tubing sampling. We found that the ampoule samples were superior in data precision and that a lower percentage of samples were lost prior to measurement. Further measurements revealed agreement between the two methods in helium, 3He and neon within ±0.1%. The new method facilitates the dealing with large sample sets and minimizes the delay between sampling and measurement. The method is applicable also for gases other than helium and neon. Highlights ► We describe a novel method to obtain water samples for the measurement of helium isotopes and neon. ► No sample treatment is required between sampling and measurement. ► The method is highly accurate, mechanically simple and well suited for oceanographic work. ► A comparison with Cu-tubing samples has shown full agreement.

Description: The eastern Mediterranean transient (EMT) was caused by a combination of high‐salinity waters intruding into the Aegean Sea and the two particularly strong winters of 1991–1992 and 1992–1993. The approach in this chapter is to search for specific signatures in the historic hydrographic observations, which date back to 1910. To deal with the problem that up into the 1950s the data not only are of limited precision but also have gaps of about 20 years, it is advantageous to consider the fact that the evolution of the actual EMT is rather well documented over a similar time span. The chapter begins by outlining the characteristics of the current EMT. Thereafter, a selection of suitable hydrographic observations among the available historic data is provided to compare these with signatures expected from the evolution of the actual EMT.

Description: We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation.

Description: We present field measurements of air-sea gas exchange by the radon deficit method that were carried out during JASIN 1978 (NE Atlantic) and FGGE 1979 (Equatorial Atlantic). Both experiments comprised repeated deficit measurements at mixed position over periods of days or longer, using a previously described precise and fast-acquisition, automatic radon measuring system. The deficit time series exhibit variations that only partly reflect the expected changes in gas transfer. By evaluating averages over each time series we deduce the following average gas transfer velocities (average wind velocity and water temperature in parentheses): JASIN phase 1: 1.6 ± 0.8 mid (at -6 mis, 13 °C) JASIN phase 2: 4.3 ± 1.2 mid (at -8 mis, 13 °C) FGGE: 1.2 ± 0.4 mid (at -5 mis, 28 °C) 0.9 ± 0.4 mid (at -7 mis, 28 °C) 1.5 ± 0.4 mid (at -7 mis, 28 °C) The large difference between the JASIN phase 2 and FGGE values despite quite similar average wind velocity becomes even larger when the values are corrected to a common temperature. Both values are, however, fully compatible with the range of gas transfer velocities observed in laboratory experiments and the conclusion is suggested that their difference is caused by the highly different wind variability in JASIN and FGGE. We conclude that in gas exchange parameterization it is not sufficient to consider wind velocity only. A comparison of our observations with laboratory results outlines the range of variation of air-sea gas transfer velocities with wind velocity and sea state. We also reformulate the radon deficit method, in the light of our observed deficit variations, to account explicitely for non-stationarity and horizontal inhomogeneity in the near-surface radon deficit layer (i.e., mixed-layer and upper thermocline). We show that neglection of non-stationarity and horizontal inhomogeneity in previous radon work introduces considerable uncertainty in deduced gas transfer velocities. We furthermore discuss the observational requirements that have to be met for an adequate exploitation of the radon deficit method, of which an observation area of minimum horizontal inhomogeneity and monitoring of the remaining inhomogeneities are thought to be the most stringent ones.