Age model and chemistry of sediment core STK7 from Steisslinger See

Stable isotope records for carbon and oxygen in bulk carbonates, carbon in bulk organic matter, and for total and chromium-reducible sulfur in a lacustrine sediment core from Lake Steisslingen (Southwest Germany) show several distinct and abrupt shifts during the last 15,000 years. Variations in the isotopic composition of authigenic carbonates indicate two major phases in the lake history. In the pre-Holocene, the hydrological budget of the lake was apparently stable. Variations of delta18O values of authigenic carbonates were, therefore, dominantly controlled by temperature changes. A decrease in the delta18Ocarb values of about 2 per mil at the Allerød/Younger Dryas transition is interpreted as a drop in mean annual air temperatures of approximately 5°C. An abrupt temperature increase of a similar magnitude is inferred at the Younger Dryas/Preboreal boundary. Throughout most of the Holocene, the isotopic composition of authigenic carbonates was influenced by marked changes in the hydrological budget of the lake. A major positive excursion in the delta13Ccarb and delta18Ocarb values at the beginning of the Atlantic and a smaller one in the Preboreal were related to evaporation effects, which indicate that dry climatic conditions must have prevailed at that time. A simultaneous increase in delta13C values of bulk organic matter at the beginning of the Atlantic suggests a high level of productivity in the lake. As a consequence, aqueous sulfate became limited as indicated by variations in the delta34S values of total and chromium-reducible sedimentary sulfur. Therefore, we conclude that the beginning of the Atlantic was characterized not only by dry but also by warm climatic conditions, which triggered a higher productivity in the lake. In the Subatlantic sediments, large variations in carbon, oxygen, and sulfur isotope ratios were observed as a result of human activities, causing considerable perturbations in the biogeochemical element cycling of Lake Steisslingen. Results obtained by the study of the continuous 15 ka record of Lake Steisslingen document clearly that isotopic proxy data from lacustrine sediments can provide useful information on environmental and climatic changes of local, regional, and in the case of the Younger Dryas event, of even hemispherical significance.

Holocene precipitation change in different monsoon sub-regions (time-slices and transient data) simulated by different global climate models, with links to model results

The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak, and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to the models, the Indian and North African monsoons are coupled, showing similar rainfall trend and moderate correlation in rainfall variability in all models. The East Asian monsoon changes independently during the Holocene. The dissimilarities in the seasonality of the monsoon sub-systems lead to a stronger response of the North African and Indian monsoon systems to the Holocene insolation forcing than of the East Asian monsoon and affect the seasonal distribution of Holocene rainfall variations. Within the Indian and North African monsoon domain, precipitation solely changes during the summer months, showing a decreasing Holocene precipitation trend. In the East Asian monsoon region, the precipitation signal is determined by an increasing precipitation trend during spring and a decreasing precipitation change during summer, partly balancing each other. A synthesis of reconstructions and the model results do not reveal an impact of the different seasonality on the timing of the Holocene rainfall optimum in the different sub-monsoon systems. They rather indicate locally inhomogeneous rainfall changes and show, that single palaeo-records should not be used to characterise the rainfall change and monsoon evolution for entire monsoon sub-systems.