Ocean changes in the North Atlantic over the Late Holocene: A multi-proxy approach

In today's North Atlantic, warm and salty surface waters are transported northwards from the subtropics, releasing their heat and eventually sinking to form a deep southward-flowing water mass. This process in the meridional-vertical plane is termed the Atlantic Meridional Overturning Circulation (AMOC). Changes in the strength and structure of the AMOC play a critical role in the Earth’s climate system and have been thought to be implicated in the climate variability of the Late Holocene. In this thesis, two sediment cores RAPiD-35-25B and RAPiD-17-5P recovered from the Eirik Drift (South of Greenland) and the Iceland Basin are used to produce Late Holocene palaeoceanographic reconstructions of some of the main constituents of the AMOC at sub-decadal to multidecadal resolution. Upper water column reconstructions from the Eastern Labrador Sea based on multi-species planktonic foraminiferal δ18O, Mg/Ca and faunal assemblages indicate millennial to centennial variability in the influence of polar waters reaching the Labrador Sea which likely led to reductions in deep water convection in the Labrador Sea. Inferred shifts of Labrador Sea Water formation appear concomitant with climatic anomalies recorded in the North Atlantic region. It is suggested that these climatic oscillations may have resulted from a coupled ocean-atmosphere response to reductions in solar irradiance. The paired δ18O and Mg/Ca composition of the thermocline-dweller G. inflata, shows multi-centennial shifts in the temperature and salinity of the North Atlantic Current during the last millennium with a potential link to solar forcing. The recorded hydrographic variability is explained in terms of the effects of atmospheric blocking events on subpolar gyre dynamics. Sortable silt mean grain size measurements show millennial-scale trends towards slower and faster vigour of the overflows east and west of Iceland during the Late Holocene. Potential upstream and downstream mechanisms are investigated and it is concluded that the millennial-scale variability in the strength of the overflow likely resulted from insolation driven Neoglacial changes in the freshwater budget and atmospheric circulation in the Nordic Seas. Furthermore, an antiphasing relationship between the strength of the two overflows is found, supporting previous predictions from modelling studies