Eemian Lake development, hydrology and climate: a multi-stratigraphic study of the Hollerup site in Denmark

A classic northwest European open section with lacustrine Eemian sediments, Hollerup, has been studied with respect to sedimentology, geochemistry, st able isotopes, diatoms and mineral magnetic analyses, and correlated by geo chemistry and diatoms to a previously pollen analysed section by Andersen ( 1965). This correlation has enabled the section to be related to Muller's ( 1974) tentative Eemian absolute chronology, and shows that the Hollerup Eem ian sediments appear to cover c. 11,000 yr. Our studies show that the onset of the Eemian was characterized by a major lake level rise followed by an almost 3000 yr long period of high, but oscillating lake levels. It is argu ed that the latter part of this period of highly maritime climate can be de fined as the Eemian climatic optimum. This period was interrupted by a few hundred years long phase of low lake level, coinciding with the immigration of spruce, followed by medium-high lake levels. The next c. 3500 yr, coinc iding with the Carpinus pollen zone, seem to have been characterized by fai rly humid and mild conditions, although slightly more arid than during the preceding optimum. The Carpinus period ended with a more than 1000 yr long gradual lake level fall, and this period of lake level change, concurring w ith the transition into the Pinus pollen dominated period, terminated with an extreme low lake level event. This 300-500 yr long arid phase coincides with a distinct peak in pine pollen, and was followed by higher but slightl y oscillating lake levels in a cooler climate. The end of the Eemian seems to have been characterised by a gradual cooling, until almost pure elastic sedimentation and a marked expansion of herb pollen grains mark the onset o f the Weichselian. Based on regional correlations to other European terrest rial and North Atlantic marine proxy records, we argue that the distinct hy drologic and climatic events displayed in the Hollerup record were possibly caused by a combination of sea level changes, changes in the hydrologic cy cle and changing oceanic circulation patterns. The latter two may have been triggered by the opening and closure of the Baltic Sea-White Sea connectio n, the initial build-up of continental ice sheets, and the first major impa cts of meltwater induced freshwater in key areas for thermohaline circulati on. (C) 2000 Elsevier Science Ltd. All rights reserved.