No abstract available

[en] This report summarises the results of preliminary geochemical investigations at three North Atlantic study areas. The two eastern sites, on the Cape Verde abyssal plain (CV2) and east of Great Meteor Seamount (GME) were visited during 1982. The results presented are preliminary. Studies in the western Atlantic, close to the Nares Abyssal Plain study site are more detailed and are presented in a separate paper. The report shows for the first time the relative redox status of the three sites. The differences are unexpectedly large, the most reduced cores being recovered at GME and the most oxidised at CV2. The sporadic nature of Recent sediment accumulation at these sites is also emphasised. In order to place these preliminary results in context their relevance to the production of mathematical system models is discussed in a closing section. The necessity for such models to rest on sound foundations of geochemical understanding is noted. Suggestions on future research priorities are offered for discussion. (author)

[en] The geochemistry of a suite of box and 2m gravity cores from the Nares Abyssal Plain has been characterised by means of pore water analyses, XRF determination of major and trace element concentrations, mineralogy and ²³⁰Thsub(excess) dating. The interstitial fluid environment of those deep-sea clays is mildly reducing, although one site exhibits manganese remobilisation and precipitation. Despite their marked colour differences, there is a similarity in clay mineralogy between the grey silt/clay turbidites and the brown clays found in the area. Sediment accumulation rates of pelagic brown clays range between 0.5 and 1.0 cm/10³ yr. These pelagic brown clays are metal-rich relative to the grey clays, and a model is used to estimate the hydrogenous metal fluxes on the assumption that they are constant over the Plain. This model gives values of approx. 1300 μg/cm²/10³ yr for Mn, approx. 2600 μg/cm²/10³ yr for Fe and Co, Ni, Cu, V and Zn in the range 6 to 26 μg/cm²/10³ yr. An associated model-derived estimate of the detrital contents of the same elements agrees well with the mean values of the grey clays and of average shale. Metal-poor brown clays and assorted minor lithologies are intermediate in composition between these two end-members. (author)

[en] The sediments of the Madeira Abyssal Plain, east of Great Meteor Seamount, are dominated by distal turbidite deposition. While the turbidites exhibit a wide compositional range, individual examples can be correlated over a wide area and are relatively homogeneous. Organic C oxidation, by bottom water oxygen, proceeds from the turbidite tops downwards after emplacement in pelagic conditions, and the progress of this oxidation front is marked by a sharp colour contrast in the sediments. In turbidites with Csub(org) > 0.5%, redistribution of authigenic U occurs to form a concentration peak (4 to 9 ppm U), just below the oxidation front or colour change. Several tens μg U/cm² may be mobilised, and in all examples studied > 60% of the remobilised U is relocated into the peak. Following burial by subsequent turbidites, such U concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 x 10⁵ years. In the case of thin turbidites where labile Csub(org) is almost exhausted, the U peaks may be located in underlying sedimentary units because of their relationship to the oxidation front. A redox mechanism for U peak formation is suggested from these data rather than a complexation with organic matter. (author)

[en] An irregular concentration/depth profile of uranium in deep-sea turbidities, previously elucidated, has been exploited to obtain in-situ effective diffusion coefficients for the long-lived members of the ²³⁸U natural series. The findings are relevant to the assessment of deep-sea sediments as potential repositories for high-level radioactive waste, because waste actinides decay through the same chains of daughter radionuclides as natural actinides. This work was part of the CEC Mirage project-Second phase, Natural analogues research area

[en] This study concerns the surface of a single distal turbidite emplaced on the Madeira Abyssal Plain a few hundred years ago. The vertical and horizontal homogeneity of the unit and the low pelagic sediment accumulation rate enable a ready identification of the post-emplacement signals of near-surface bioturbation. Data have been gathered to define ²¹⁰Pb_excess, ²³⁰Th_excess and pelagic foraminifera profiles with depth. These are used as constant input flux tracers and treated by models conventionally used to determine bioturbation parameters. The mixed layer depths (7-11 cm), bioturbation coefficients (0.08-0.18 cm² y^-1) and inventory data (10-23 dpm cm^-2) derived from the ²¹⁰Pb_excess data in five out of six cores are similar to those previously observed elsewhere with the same model in the deep-sea under steady-state sediment accumulation conditions. Although the mixed layer depths are similar for all three tracers, the ²¹⁰Pb_excess bioturbation intensities are larger by a factor of >3 than those estimated from ²³⁰Th_excess profiles and about an order of magnitude larger than those from the foraminifera profiles. It is suggested that the conventional ²¹⁰Pb_excess model and those for other short-lived tracers probably over-estimate the intensity of biomixing of foraminifera for palaeontological and related geochemical purposes. (orig.)

[en] A method is described for the determination of the natural series isotopes ²³⁰Th, ²²⁶Ra and ²¹⁰Pb in sea water. Large volume in-situ pumps were used first to collect the particulate fraction on 1-μm membrane filters, and second to sorb the dissolved fraction from the filtered stream on to manganese oxyhydroxide-impregnated filter cartridges. The values thus obtained are compared with other data from the same oceanographic area obtained by alternative sampling and analytical methods. The dissolved Th and Pb fractions are essentially quantitatively extracted by the first of the pair of Mn-impregnated cartridges used and good data are obtained. Lower and more variable efficiencies are observed for Ra which cannot be determined so reliably. This behaviour contrasts dramatically with that reported recently in laboratory experiments at near-neutral pH, where Ra was found to behave ideally towards Mn oxyhydroxide sorption while readily hydrolyzed elements such as Th do not. This difference in marine and laboratory behaviour at neutral pH is ascribed to surface availability for hydrolyzed species in homogeneous and heterogeneous systems, respectively

[en] Long-term radionuclide mobility is a primary concern for evaluating options for the disposal of radioactive waste. Information relevant to the long timescales necessary may be obtained from natural radionuclides. Here we examine a characteristic irregular profile of uranium concentration with depth, in deep-sea turbidite units more than 500,000 years old. This is enough time either for secular equilibrium to have been established, or for daughter radionuclides to have achieved steady-state distributions in response to local advection and diffusion. We find that advection is absent and that the ²³⁸U-series radionuclides ²³⁴U and ²³⁰Th are remarkably immobile, with very small effective diffusion coefficients, D < 5 x 10^-13 cm² s^-1. But a fraction of the total ²²⁶Ra (another daughter radionuclide in the ²³⁸U decay series) is mobile, with D = 10^-9 - 10^-10 cm² s^-1. This work suggests that the reducing nature of the sediments at depth is important in maintaining such low D values. The in situ diffusion behaviour observed must be correctly predicted by any model used for evaluating waste disposal options. (author)

[en] The problem of atmospheric-aerosol-induced aureole effects in astronomical observations of the solar corona is considered theoretically and by analysis of sample data obtained at Sacramento Peak Observatory. Both conventional solar-telescope and coronagraph observations are examined in terms of spectral coverage, spatial resolution, and instrument speed; effects due to the angular size of the sun; and the phase effect of forward scattering on the flying aerosol particles. Graphs, sample images, and a calculated atmospheric-transmission profile based on the AFGL standard model are provided, and the implications of the analysis for image-processing methods are briefly indicated. 18 refs

[en] Cores from three different Atlantic localities (equatorial, Cape Verde Rise and Porcupine Abyssal Plain) are shown to have anomalous high U contents (5-8 ppm total) in sediments laid down doring the last glacial stage (12-24 ky BP). Radiocarbon data demonstrate that the sediments hosting the peak U levels were accumulated at rates similar to those immediately above and below. All the cores exhibit maximum Mn levels, characteristic colour changes, and maximum U levels in the same sequence with increasing depth in core. On the evidence of the similarities between the cores, and pore waster U data from a Porcupine Abyssal Plain site, it is proposed that the authigenic U enrichments are syndiagenetic and possibly active. No correlation is observed between sediment authigenic U and C_org contents. The source of enrichment is bottom water U which has diffused downwards into the sediments to be sorbed at a particular redox level, located 10-30 cm below the oxic/post-oxic boundary marked by the colour change. The magnitude of the enrichments is caused by the persistence of this boundary at a particular level as a result of the decrease in mean sediment accumulation rate between the last glacial stage (5.2 up to at least 19.1 cm ky^-1) and the Holocene (2.2-4.1 cm ky^-1). Similar accumulation rate contrasts are expected to be widespread in the Atlantic, and the implications for previous reported work, particularly from the Mid-Atlantic Ridge, are discussed. (orig.)