Sea-ice cover, sea-surface salinity and halo-/thermocline structure of the northwest North Atlantic: modern versus full glacial conditions

Seasonal sea-ice develops along the eastern continental margins in the northern North Atlantic, where freshwater and/or meltwater outflow are responsible for relatively low salinity in surface waters and very pronounced water mass stratification. Sea-ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea floor. Organic-walled dinoflagellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern North Atlantic, and used to develop transfer functions (best analogue method) for the reconstruction of the seasonal spreading and duration of sea-ice cover, in addition to salinity and temperature of the warmest month of the year. Application of the best analogue approach to cores from the Labrador Sea reveals large variations in sea-ice cover and sea-surface conditions throughout the last glacial stage and during the early Holocene. Isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub-surface water masses. Specific study of the last glacial maximum LGM time slice (16-20 ka on a C-14 time scale) in the northwestern North Atlantic shows much more extensive sea-ice than at present, with perennial sea-ice lying along the continental margins of eastern Canada. Seasonal spreading of the cover of sea-ice offshore was accompanied by large seasonal contrasts in temperature, with very cold winters but relatively warm summers, a pattern linked to strong stratification between a buoyant low saline surface layer having a low thermal inertia, and the underlying intermediate oceanic waters. (C) 1999 Elsevier Science Ltd. All rights reserved.