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

摘要

The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12 000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction. The observed overall correlation between bottom flow speed, as given by the 'sortable silt' mean size index, and the bulk coccolith concentration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of sediment redistribution by bottom current. The down-core variations in coccolith 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 Gardar Drift after 11.2 ka was associated with excess export flux of coccolith carbonate 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 between 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated changes in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established after a last cooling step between 3.5 and 2.8 ka. These long-term reorganisations of the surface hydrology are interpreted as the response of the North Atlantic to the combined force of the solar insolation and the waning Laurentide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi. These successive decreases in the export fluxes of E. huxleyi exhibit a distinct 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 global ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be