Nathan Stevenard
PhD thesis of Paris-Saclay University– (directrice C. Kissel, co-encadrante: A. Govin) Thèse Phare CEA, Soutenue le 28 Novembre 2023
The aim of this PhD project was to reconstruct, over the last 400,000 years, the variations in the strength of the Iceland-Scotland Overflow Water (ISOW), which is one of the two deeper branches of the AMOC in the North Atlantic. Xray fluorescence, grain size and environmental magnetism analyses were applied to the detrital fractions of three sedimentary archives (MD03-2673, MD03-2679 and MD03-2685) located along the present-daypathway of ISOW (Björn and Gardar drifts) (Fig. 1).
On glacial-interglacial timescales, the ISOW is strong intensity during interglacial periods (s.l.) and weak (but not absent) intensity during glacial periods.
The transitions between these two regimes appear to be triggered by millennial scale events. The state of the intensity regime following these events seems to depend on the Northern Hemisphere ice-sheet volume, particularly when a benthic d18O threshold value of 4 ‰ is crossed.
On millennial timescales, ISOW intensity variations show a strong coupling between surface temperatures of the subpolar gyre, of Greenland, and freshwater discharges in the North Atlantic.
A complex pattern seems to emerge from these couplings, suggesting that variations in ISOW intensity (slowdown/strengthening) act as a consequence of an extension/retraction of the subpolar gyre and the amount of freshwater discharged into the North Atlantic, but also as a cause of cooling/warming in the Northern Hemisphere.
Past climate optima, sometimes warmer than today, show the strongest intensities of the last 400,000 years. They occur at the end of the climatic optima, several thousand years after the surface temperature and greenhouse gas maxima. These late maxima in ISOW intensity appear to be linked to a particular orbital context, influencing regional climate in the Nordic Seas, close to convection areas.
Changes in ISOW intensity appear to be, on all time scales, dominated by density changes between the upstream (Gulf Stream, North Atlantic drift) and downstream (convection areas in the Nordic seas) North Atlantic surface/subsurface hydrological system. Considering the importance of such mechanisms in climate models could help improve and better represent future oceanographic changes.