Annual Layer Counting (Varves)
Maar lakes have a very deep basin structure and thus a residence time of years or even decades for the water body. Accordingly, the water in the basin is old and reacts with the sediments of the lake bottom strongly. Under the modern climatological conditions the water warms each summer and stratifies; warm water with oxygen in the surface layer, and cold water without oxygen at depth. This deep water anoxia is thus a summer phenomenon, which is destroyed when the surface cools below 4°C in fall. At this temperature water has its maximum density and thus sinks to depth to fill the deep water with fresh oxygenated water during the winter. It the anoxic summer deep water however has reacted with the bottom sediments, and dissolved iron, manganese and other elements get enriched at depth, then the deep water can reach a density higher than the 4°C cold sinking surface water. Under this circumstance the deep water is incorporated into the annual circulation and remains permanently without oxygen at depth.
Such an oxygen free permanent deep water, called monimolimnium, is not disturbed by feeding fish and is witout any current movement. All sediment deposited into such a basin is sedimented in thin layers, which remain preserved. The contrast in the composition of the settling particle between summer (diatoms and calcite), fall (leafs and organic debris from underwater vegetation), winter (clay and silt under the ice cover) and late winter (sand, gravel, eroded soil particles) caused changes seasonal layering. These seasonal sublayers are preserved if the water is free of oxygen during summer. The preserved seasonal layers are called varves and can be counted under a microscope to achieve an annual resolution time control. The seasonal layers also provide information about the climatic and hydrographic conditions of each season, and present a proxy record of the microfacies analysis.
It is the preservation of these varves, which make a maar lake unique. However, not all lakes develop a monimolimnion, or only temporarily. Accordingly, each of the ELSA cores, even if sediments were deposited at the same time, show a very different lithology and geochemistry.
The cores used for the ELSA-20 time series represent those cores, which preserve the varves in the best possible way. The ELSA-20 time series is, however, not yet presented on a varve counted time scale, but tuned to the established ice core chronology (Sirocko et al., 2021). The next generation of ELSA-23 will present respective time series with counted floating chronologies, wherever possible.