Long-term evolution of (millennial-scale) climate variability in the North Atlantic over the last four million years

During the past 4 million years (Ma) global climate has changed dramatically. From a state in which global average temperatures were several degrees higher than today and continental ice sheets in the Northern Hemisphere were absent or small it gradually developed towards the regular paced glacial/interglacial cycles that characterize the last 700 thousand years (ka) and during which large continental ice sheets episodically covered large parts of Europe and North America. This PhD thesis examines climate-induced changes in ocean surface characteristics, ice-rafting events, and aeolian input to the North Atlantic during the past 4 Ma using sediments from IODP Expedition 306 Site U1313. The results in Chapter 5 demonstrate that during the intensification of the NHG the ocean circulation in the North Atlantic changed significantly. This could be concluded from a decrease in SSTs and increase in marine productivity starting at Site U1313 around 3.1 Ma, which indicate a weakened influence of the warm surface waters from the North Atlantic Current (NAC) and increased influence of the high productivity area associated to the arctic front (AF). Chapter 6 provides a reconstruction of changes in mass accumulation rates of lipids derived from terrestrial higher plants waxes (long-chain n-alkanes and n-alkan-1-ols) at IODP Site U1313 for the past 3.4 Ma to reconstruct changes in aeolian input to the North Atlantic. The results show that together with a change in ocean circulation, the intensification of NHG was associated with a drastic increase in the aeolian input of terrestrial material to the mid-latitude North Atlantic. This increase is likely related to a strengthening of the North American sources in the late Pliocene due to the appearance of continental ice sheets and associated glacial outwash plains. In Chapter 7 a high-resolution record of millennial-scale climate variability, SSTs and ice-rafted debris (IRD) characteristics based on organic geochemical and mineralogical proxies, for the period between 960 and 320 ka is used to gain more understanding of abrupt climate change during periods with different boundary conditions. During marine isotope stage (MIS) 16 (~ 643 ka) both the organic geochemical and mineralogical characteristics of IRD indicate the first occurrence of Hudson Strait (HS) Heinrich(-like) events. As SSTs during MIS 16 were higher compared to previous glacials, the occurrence of HS Heinrich events indicates enhanced ice discharge from the LIS at this time, not simply the survivability of icebergs due to cold conditions in the North Atlantic.