A review of Early Weichselian climate (MIS 5d-a) in Europe

This report addresses the transition from the last interglacial into the last glacial period in Europe, which corresponds to the time interval between approximately 122,000 and 70,000 years before present. Based on state-of-the-art paleoclimatic and paleoenvironmental information from selected terrestrial, marine and ice core records, questions regarding the magnitude, duration, and cyclicity of early glacial stadial and interstadials are discussed. One of the most important aspects in this respect is the timing of climatic/environmental changes seen in terrestrial, marine, ice core and speleothem records, and most importantly, how and on which basis and by which proxy these climatic shifts are defined. Since correlations between archives are made to understand the sequence of events and the response of different systems to a change in climate, timescales are of uttermost importance. Independent chronologies however only exist for a few archives (Greenland ice cores, U/Th dated speleothems, Lago Grande di Monticchio varve record), while the timescales for other records and archives have been obtained through tuning to an independent chronology or to the astronomical time scale. Ice core and speleothem isotopic records basically monitor atmospheric changes, but also contain an important local component. Marine records provide information on sea surface and deep-sea temperature and salinity changes, which vary with location; and terrestrial records (primarily pollen stratigraphies) allow reconstructing changing vegetation patterns. Each of these archives thus has its own multitude of proxies, which respond in different ways to an externally triggered shift in climate, such as changes in incoming solar radiation. Disentangling the response of these proxies in terms of climate is one challenge; another challenge is to obtain a detailed enough correlation between the archives in order to understand what is the trigger, what is the response, and which part adds additional feedbacks. The end of the last interglacial, i.e. the Eemian or MIS5e, and the transition into the last glacial period was initially triggered by a decrease in incoming summer insolation at high northern latitudes. The change in orbital configuration subsequently led to a series of time-transgressive changes (e.g. gradual vegetation replacement; changes in sea surface temperature, salinity, strength of the North Atlantic Current). As Northern Hemisphere ice sheets started to grow and expand these changes became more and more pronounced. The sequence of events leading from the last interglacial into the last glacial has been described using climate models with different complexity and boundary conditions, paleodata series, and comparisons between model out put and paleo data. One possible scenario involves the development of summer sea ice in the northern North Atlantic already during the later part of the Eemian interglacial, as a response to decreasing summer insolation. This would have accelerated a vegetation shift, which in turn would have led to a decrease in albedo. More extensive sea ice could, through brine formation, have increased the Atlantic meridional overturning circulation, which would have supplied more moisture to the cold high northern latitudes. This in turn would have favoured the growth of ice sheets. A response of the vegetation in northern Europe to the decrease in summer insolation and to increased summer sea ice may be seen as early as 122-120 thousand years (ka) ago, but became most distinct around 115 ka, when North Atlantic sea surface temperatures show signs of a first minor cooling. This cold event (labelled sea surface cooling event C26) defines the Marine Isotope Stage (MIS) 5e/5d transition in marine cores and seems to correlate with Greenland stadial GS26. This first distinct cooling event on Greenland is paralleled by cold temperatures in Antarctica, which implies synchronous cooling in both hemispheres. However, while Antarctica remained cold, Greenland started to warm just before 110 ka, suggesting the start of the so-called bipolar see-saw mechanism. The first marked cooling over Greenland at 110108 ka (GS25) was accompanied by a distinct drop in North Atlantic sea surface temperatures (C24 event), by an increase in ice-rafted debris and by marked vegetation changes in southern Europe. This shift in vegetation defines the end of the terrestrial Eemian in southern Europe, which in comparison to marine records, occurred during MIS 5d. Paleo records thus suggest that the response of the vegetation to North Atlantic cooling events was delayed in southern Europe by at least 5 ka as compared to northern Europe