Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document th e pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccoli th fraction. The observed overall correlation between bottom flow speed, as given by the 'sortable silt' mean size index, and the bulk coccolith conce ntration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of se diment redistribution by bottom current. The down-core variations in coccol ith assemblage structure (species %) indicate that changes in properties of surface waters south of Iceland occurred as two distinct steps at ca. 11.2 and 6 ka. The onset of North Atlantic Drift water influence over the Garda r Drift after 11.2 ka was associated with excess export flux of coccolith c arbonate in the vicinity of an active frontal system. The period from 10 to 6 ka saw the progressive warming of the study area, which culminated betwe en 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated chang es in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established afte r a last cooling step between 3.5 and 2.8 ka. These long-term reorganisatio ns of the surface hydrology are interpreted as the response of the North At lantic to the combined force of the solar insolation and the waning Laurent ide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi. The se successive decreases in the export fluxes of E. huxleyi exhibit a distin ct millennial pacing, in phase with previously recorded Holocene advection of cool, ice-bearing waters from the Greenland-Iceland seas to the British islands. From a clear analogy with the most recent extreme variation in glo bal ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be found in long-term modulations of t he atmospheric processes associated with the North Atlantic Oscillation (NA O). (C) 2000 Elsevier Science B.V. All rights reserved.