Coherency of late Holocene European speleothem δ18O records linked to North Atlantic Ocean circulation

Speleothem delta O-18 records provide valuable information about past continental environmental and climatic conditions, although their interpretation is often not straightforward. Here we evaluate a compilation of late Holocene speleothem delta O-18 records using a Monte Carlo based Principal Component Analysis (MC-PCA) method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each delta O-18 record. The MC-PCA approach permits not only the identification of temporally coherent changes in speleothem delta O-18; it also facilitates their graphical depiction and evaluation of their spatial coherency. The MC-PCA method was applied to 11 Holocene speleothem delta O-18 records that span most of the European continent (apart from the circum-Mediterranean region). We observe a common (shared) mode of speleothem delta O-18 variability that suggests millennial-scale coherency and cyclicity during the last 4.5 ka. These changes are likely caused by variability in atmospheric circulation akin to that associated with the North Atlantic Oscillation, reflecting meridionally shifted westerlies. We argue that these common large-scale variations in European speleothem delta O-18 records are in phase with changes in the North Atlantic Ocean circulation indicated by the vigour of the Iceland Scotland Overflow Water (ISOW), the strength of the subpolar gyre (SPG) and an ocean stacked North Atlantic ice rafted debris (IRD) index. Based on a recent modelling study, we conclude that these changes in the North Atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies. However, the mechanisms that ultimately force the westerlies remain unclear.