Carbon tetrachloride and chlorofluorocarbons in the South Atlantic Ocean, 19°S

Exploratory measurements of a suite of anthropogenic halocarbon compounds (CCl4, CCl2FCClF2 (CFC-113), CH3CCl3, CCl3F (CFC-11)) were made using a new analytical technique on RV Meteor cruise 15 along 19°S (World Ocean Circulation Experiment (WOCE) Line A9)) in the Atlantic Ocean during February–March 1991. A separate analytical system was used to determine CCl2F2 (CFC-12) and CCl3F (CFC-11). A limited number of CFC-113 profiles indicated that it was undetectable below 400–500 m. The CCl4 data indicate that the entire Brazil Basin contains readily measurable levels of CCl4 (>0.05 pmol kg−1), whereas the deep Angola Basin contains very low levels (≤0.02 pmol kg−1). Slightly higher levels were found close to the bottom in the deep Angola Basin: possibly an anthropogenic signature. In contrast, most of the deep Brazil Basin and all of the deep Angola Basin (>1000 m) had undetectable levels of CFC-11, CFC-12, and CFC-113. Preindustrial levels of CCl4 in the atmosphere were therefore negligible (atmospheric mixing ratio <0.1 pptv). CCl4/CFC-11 ratios are used to estimate apparent ages and dilution factors for the North Atlantic Deep Water and Antarctic Bottom Water. Whereas CCl4/CFC-11/CFC-12 levels are internally consistent in deep waters, suggesting near-conservative behavior, there is evidence for very rapid removal of CCl4 in the thermocline. Removal rates suggest that in addition to neutral hydrolysis, some other loss pathway must be involved

Moessbauer-Spektroskopie und Spinrelaxation des 6S-Zustandes von Fe(III) in LiNbO3:Fe

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Tracing the flow of North Atlantic Deep Water using chlorofluorocarbons

Chlorofluorocarbon (CFC) and hydrographic data collected in the North Atlantic in the late 1980s and early 1990s are used to confirm and add to earlier work on the large‐scale circulation pathways and timescales for the spreading of North Atlantic Deep Water (NADW) components and how these components relate to the hydrographic structure. Throughout the western North Atlantic, high CFC concentrations are coincident with newly formed NADW components of Upper Labrador Sea Water (ULSW), Classical Labrador Sea Water (CLSW), and Overflow Waters (OW). ULSW is marked by a CFC maximum throughout the western subtropical and tropical Atlantic, and CLSW is marked by a CFC maximum north of 38°N in data collected in 1990–1992. Iceland‐Scotland Overflow Water (ISOW) splits into two branches in the eastern basin, with one branch entering the western basin where it mixes with Denmark Strait Overflow Water (DSOW) and the densest branch flows southward along the bottom in the eastern basin. DSOW contributes the largest portion of the CFC signal in OW. It is estimated that these NADW components are at 60–75% equilibrium with the CFC concentration in the atmosphere at the time of formation. The large‐scale data set confirms that NADW spreads southward by complex pathways involving advection in the Deep Western Boundary Current (DWBC), recirculation in deep gyres, and mixing. Maps of the CFC distribution show that properties within the gyres are relatively homogeneous, particularly for OW, and there is a profound change at the gyre boundaries. The density of the core of ULSW increases in the equatorward direction because of entrainment by overlying northward flowing Upper Circumpolar Water and at the equator, ULSW has the same density as CLSW in the subtropics but is warmer and saltier. The density of OW decreases between the subpolar region and the subtropics. This is caused by the least dense part of OW exiting the subpolar region in the DWBC, while the densest component recirculates in the subpolar basins. Some variability is observed in OW density in the subtropics and tropics because of variability in mixing with Antarctic Bottom Water and changes in the subtropics that are probably related to the transport of different vintages of DSOW. Ages derived from CFC ratios show that the NADW components of northern origin spread throughout the western North Atlantic within 25–30 years. This corresponds to a spreading rate of 1–2 cm s−1 and is comparable to the time a climate anomaly introduced into the subpolar North Atlantic will take to penetrate the entire western North Atlantic Ocean

Flow and mixing in the rift valley of the Mid-Atlantic Ridge

High levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation. In topographically confined areas, such as the deep median valleys of slow-spreading ridges, these fluxes strongly influence the local hydrography and dynamics. Data from a segment-scale hydrographic survey of the rift valley of the Mid-Atlantic Ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes. Comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment-scale gradients. The rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional along-segment flow into a cul-de-sac. Therefore, most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity >10−5 m s−1. The observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude, in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column. Hydrographic budgets in the rift valley yield diffusivity values of order 5 × 10−3 m2 s−1, consistent with estimates derived from statically unstable overturns, the largest of which were observed downstream of topographic obstacles in the path of the along-segment flow. This suggests vertical shear associated with cross-sill flows as the dominant contributor to the mechanical mixing in the rift valley

Report and preliminary results of Poseidon Cruise 237/2, Vigo - Las Palmas, 18.3.-31.3.1998

Coccolithophore sampling; Deployment of DOMEST moorings; Field tests of the acoustic moorings; Hydrography and collection of water samples; Nutrients, oxygen and dissolved aluminium; Plankton biomass; Phytoplankton production rate; Carbon dioxide in sea-water and atmosphere; Tracers; Particle flux measurements with drifting particle traps; Particle flux measurements with moored particle traps. (orig.)Available from TIB Hannover: RO 7630(121) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman