Climatic evolution during the Middle Pleistocene warm period of Bilshausen, Germany, compared to the Holocene

Abstract: Middle Pleistocene warm periods are particularly useful for comparison with our present warm period. One of them, Marine Isotope Stage (MIS) 11, has been considered a potential analogue for the Holocene, whereas older interglacials, e.g. MIS 13, are distinctly different. However, terrestrial Middle Pleistocene sites, other than the Holsteinian, and quantitative climate reconstructions for this period are very rare in Europe. A unique terrestrial site in North Central Europe is known from Bilshausen, Germany. The sediments are varved and indicate that this interglacial lasted ∼25,000 years, more than twice as long as the Holocene so far. The warm period probably correlates to MIS 11, or possibly to MIS 13. A new palynological investigation with large pollen counts is presented which forms the basis for quantitative temperature reconstructions based on the probabilistic indicator taxa approach (the so-called pdf-method). The range of tree species that formed the vegetation during the Bilshausen warm period is reminiscent of the Holocene, but the vegetational as well as the reconstructed climatic development shows distinct differences. No pronounced initial successional phase is recorded in the pollen stratigraphy from Bilshausen. In addition, a mesocratic or optimum phase of forest development as in the Holocene is unclear in the Bilshausen sequence. Reconstructed temperature values are several °C lower than Holocene temperatures for most of the Bilshausen interglacial. Only for several millennia in the late phase of the Bilshausen interglacial were July temperatures higher than the Holocene July temperatures. Seasonality was much stronger during the Bilshausen interglacial and the temperature trends differ between the two interglacials. Likely reasons for the pronounced differences are, on one hand, global temperature, insolation changes and atmospheric circulation. On the other hand, more regional factors such as the configuration of the North Sea basin may also be responsible for the colder winter temperatures and stronger seasonality during the Bilshausen interglacial compared to the Holocene. Reliable absolute dating of the Bilshausen interglacial is needed, which would allow a more detailed evaluation of the forcing factors that determined the climate during the Bilshausen interglacial. [Copyright &y& Elsevier]