Late Quaternary stratigraphy and paleo-oceanography of the Grand Banks continental margin, eastern Canada

Quaternary sediments cored on the continental slope off the Grand Banks and on nearby seamounts and abyssal hills have been correlated back to at least isotopic stage 6. using lithologic and hioslratigraphic markers and warm-cold cyclicity in microfossil assemblages. The sequence is dated using limited oxygen isotope and ^l4C data. The oldest continental slope cores penetrate to a glacial stade with an extrapolated age of 50. 0(H) B. P. The oldest scamount cores probably penetrate to isotopic stage 13. Watcrmass distribution over the area results from interaction of the Labrador Current with locally generated shelf water, and to a lesser extent, the Gulf Stream. Planktic foraminiferal assemblages and sedimentological evidence suggest that arctic Labrador Current core water extends onto the eastern Grand Banks only during full interglacials. During intcrstadials, this arctic core water is absent, probably because its source-was blocked by ice. Cold water during stadials probably results mostly from local cooling on the continental shelf.     

Cloud condensation nuclei spectra derived from size distributions and hygroscopic properties of the aerosol in coastal south-west Portugal during ACE-2

In this work we propose and test a method to calculate cloud condensation nuclei (CCN) spectra based on aerosol number size distributions and hygroscopic growth factors. Sensitivity studies show that this method can be used in a wide variety of conditions except when the aerosol consist mainly of organic compounds. One crucial step in the calculations, estimating soluble ions in an aerosol particle based on hygroscopic growth factors, is tested in an internal hygroscopic consistency study. The results show that during the second Aerosol Characterization Experiment (ACE-2) the number concentration of inorganic ions analyzed in impactor samples could be reproduced from measured growth factors within the measurement uncertainties at the measurement site in Sagres, Portugal.
CCN spectra were calculated based on data from the ACE-2 field experiment at the Sagres site. The calculations overestimate measured CCN spectra on average by approximately 30%, which is comparable to the uncertainties in measurements and calculations at supersaturations below 0.5%. The calculated CCN spectra were averaged over time periods when Sagres received clean air masses and air masses influenced by aged and recent pollution. Pollution outbreaks enhance the CCN concentrations at supersaturations near 0.2% by a factor of 3 (aged pollution) to 5 (recent pollution) compared to the clean marine background concentrations. In polluted air masses, the shape of the CCN spectra changes. The clean spectra can be approximated by a power function, whereas the polluted spectra are better approximated by an error function.     

What status for the Quaternary?

The status of the Quaternary, long regarded as a geological period effectively coincident with the main climatic deterioration of the current Ice Age, has recently been questioned as a formal stratigraphic unit. We argue here that it should be retained as a formal period of geological time. Furthermore, we consider that its beginning should be placed at the Gauss-Matuyama magnetic chron boundary at about 2.6 Ma, rather than at its current position at about 1.8 Ma. The Quaternary would be formally subdivided into the Pleistocene and Holocene epochs. The global chronostratigraphical correlation table proposed is enclosed at the back of this issue.     

Map and GIS database of glacial landforms and features related to the last British Ice Sheet

A review of the academic literature and British Geological Survey mapping is employed to produce a 'Glacial Map', and accompanying geographic information system (GIS) database, of features related to the last (Devensian) British Ice Sheet. The map (1:625 000) is included in a folder and GIS data are freely available by web download (http://www.shef.ac.uk/geography/staff/clark_chris/britice.html). Emphasis is on information that constrains the last ice sheet. The following are included: moraines, eskers, drumlins, meltwater channels, tunnel valleys, trimlines, limit of key glacigenic deposits, glaciolacustrine deposits, ice-dammed lakes, erratic dispersal patterns, shelf-edge fans and the Loch Lomond Readvance limit of the main ice cap. The GIS contains over 20 000 features split into thematic layers (as above). Individual features are attributed such that they can be traced back to their published sources. Given that the published sources of information that underpin this work were derived by piecemeal effort over 150 years, then our main caveat is of data consistency and reliability. It is hoped that this compilation will stimulate greater scrutiny of published data, assist in palaeoglaciological reconstructions and facilitate use of field evidence in numerical ice-sheet modelling. It may also help direct field workers in their future investigations.     

A THEORETICALLY BASED DETERMINATION OF BOWEN-RATIO FETCH REQUIREMENTS

Determination of fetch requirements for accurateBowen-ratio measurements of latent- and sensible-heatfluxes is more involved than for eddy-correlationmeasurements because Bowen-ratio sensors are locatedat two heights, rather than just one. A simplesolution to the diffusion equation is used to derivean expression for Bowen-ratio fetch requirements,downwind of a step change in surface fluxes. Theserequirements are then compared to eddy-correlationfetch requirements based on the same diffusionequation solution. When the eddy-correlation andupper Bowen-ratio sensor heights are equal, and theavailable energy upwind and downwind of the stepchange is constant, the Bowen-ratio method requiresless fetch than does eddy correlation. Differences infetch requirements between the two methods aregreatest over relatively smooth surfaces. Bowen-ratiofetch can be reduced significantly by lowering thelower sensor, as well as the upper sensor. The Bowen-ratio fetchmodel was tested using data from a fieldexperiment where multiple Bowen-ratio systems weredeployed simultaneously at various fetches and heightsabove a field of bermudagrass. Initial comparisonswere poor, but improved greatly when the model wasmodified (and operated numerically) to account for thelarge roughness of the upwind cotton field.     

Physical State and Composition of Polar Stratospheric Clouds Inferred from Airborne Lidar Measurements During SESAME

The airborne lidar LEANDRE was deployed from Kiruna (Sweden) in January1995, during Phase III of the Second European Stratospheric Arctic andMid-latitude Experiment (SESAME). Polar Stratospheric Clouds (PSC) weredetected on several flights. Four lidar retrievals, on two different days, arepresented together with the thermal history of air masses, derived from 5-daybackward trajectories calculated from the TOPCAT model, using ECMWF analyses.The first PSC case was observed at the synoptic scale and at a temperatureclose to the ice frost point, with no significant depolarization, suggestingthat particles were in the liquid phase, probably liquid ternaryH₂SO₄/HNO₃/H₂Osolution forming a Type 1b cloud. Locally, on the same day, a Type II PSC (icecloud) was detected above the Scandinavian Mountains and was related tomesoscale processes. On another day, the third lidar retrieval only showedbackground aerosols signature, whereas the last one was performed at the NATequilibrium temperature and its measured depolarization ratio was consistentwith few solid state Type Ia particles, probably composed of NAT. From thethermal histories, the different behaviour of the two Type I clouds isexplained in terms of cooling rates and compositional hysteresis, driven bytemperature.