Kristin Haas (TU Darmstadt)
Geosciences
The research area
The city of Bad Waldsee in south-eastern Baden-Württemberg is located between Lake Stadtsee and Lake Schlosssee and was formed at the end of the last ice age. The old town is directly situated on the south-western shore of Lake Stadtsee. Today, 57% of its catchment area of approx. 1,975 ha is used for agriculture; its surface area is approx. 14 ha with a maximum depth of 10.7 m. Both lakes are connected by the Pfaffenbach, which flows along the edge of the old town. Lake Stadtsee is mixed each spring and autumn and forms a stable stratification during the summer months. In the stratification phase, the lowest water layer (hypolimnion) up to about one metre above the bottom is completely oxygen-free from July to November.
In 2006, sediment cores were taken from the deepest part of the city lake. This sediment core, about 8 m long, covers the last approx. 3,300 years and is seasonally laminated with the exception of the Iron Age phase. The individual sediment layers are approx. 3-10 mm thick and have a maximum thickness in the Middle Ages. The period of investigation planned for the project, from 1200 to 1800 AD, has a thickness of approximately 3.5 m. The sediment cores of Lake Schlosssee are also predominantly laminated, but the individual layers are less clearly formed. Lake Schlosssee will therefore be used for comparative purposes.
Seasonally Laminated Lake Sediments - Warves
Varves are seasonally laminated lake sediments. Varved sediments, such as those deposited in Lake Stadtsee, offer a special feature. The seasonal layers can be counted similar to tree rings. This enables a high-resolution chronology (varve chronology) on calendrical time scales. There are different types of varves with different formation processes. A distinction is made between clastic-physical and biochemical varves. Clastic-physical varves occur preferentially in regions of high latitude and in mountains. In high mountain lakes, external sediment input predominates, which is controlled by summer snow or glacier melt. The sediments consist of a coarser summer layer and a fine-grained winter layer. In the mid-latitudes, which include the region around Bad Waldsee, biochemical varves predominate, consisting of contrasting light and dark layers. The light spring and summer layers consist of calcite crystals and simultaneously deposited diatoms. The deposits of the autumn and winter months are characterised by organic material and clastic particles, which are responsible for the dark colouring of the winter layer.
Working Programme
The sediment cores of the Lake Stadtsee will be sampled for the planned investigations in approx. 5 annual sections. If possible, special events recorded in historical written sources will be resolved to the exact year.
The working concept of the project follows a so-called multi-proxy approach, i.e. sedimentological, biochemical and biological parameters of the lake sediments are used as indirect indicators (proxies) for past changes and analysed together in the sense of a comprehensive environmental reconstruction.
Precise dating is necessary to enable a high-resolution reconstruction of environmental conditions. Lake sediments are usually dated using the radiocarbon method on large plant remains. This serves as the basis for creating an age model of the sediments. In addition, the period from the Middle Ages to the Early Modern Period is recorded in detail in order to obtain an exact varve chronology. The varve counting carried out here makes it possible to date the sediment section exactly. Since laminated sedimentary sections are embedded in unlaminated older and younger sequences, the varve chronology is referred to as floating. By linking it to historical events and its high chronological accuracy, the floating varve chronology can be transformed into an absolute (varve) chronology.
In addition to natural processes, anthropogenic environmental changes are archived in the lake. Since Lake Stadtsee is very closely linked to the city of Bad Waldsee and the surrounding area, information about local and regional effects of climate changes in the direct living environment of humans as well as about the development of the city and the surrounding countryside is stored in the sediments. In Bad Waldsee, the sediments of Lake Stadtsee illustrate the special situation of increased anthropogenic influence on the water body from the Middle Ages at the latest.
The development of the city is expected to be indirectly reflected in the lake sediments through wastewater discharges (pollutants), soot inputs and hydrological interventions.
Inputs of persistent pollutants such as heavy metals and organic pollutants to the lake are determined using various geochemical methods. Polycyclic aromatic hydrocarbons (PAHs) from combustion processes and urban fires are deposited as thin layers of soot or ash in the sediment. Specific PAH
patterns can be used to determine the sources of origin. The analyses are carried out by means of gas chromatography (GC-MS).
Heavy metals can enter the lake through medieval metal processing, textile production as well as dyeing and tanning. By means of inorganic geochemical investigations (ICP-MS, XRF) and sequential extraction, the metal species and bioavailability can be determined. The aim is to verify whether extensive environmental pollution with pollutants was already caused by anthropogenic activity in the Middle Ages and which activities were associated with it.
Weather conditions and climatic trends are also archived by the lake sediments. Extreme events change the sediment facies and are determined using thin sections from representative sections. Weather events such as floods lead to the formation of coarse-grained clastic layers in the sediment, which are determined by microscopic examination of the thin sections. Flood frequencies over several centuries can be derived from the frequency of these layers.
The mineral composition of the sediment is derived from μXRF scans of the sediment cores. Increased contents of elements such as silicon, titanium and aluminium provide evidence of increased clastic external material input into the lake. In general, this is due to increased soil erosion in the catchment area caused by forest clearing, urban construction activity or intensification of agriculture or land use change. With the help of further information from the historical written archives, possible effects and causes of increased soil erosion can be deduced.