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 strongly with the sediments of the lake bottom. 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. In the anoxic summer, deep water however has reacted with the bottom sediments, and dissolved iron, manganese and other elements get enriched at depth. If this process leads to a density higher than that of 4 °C cold sinking surface water, the deep water is no longer involved into the seasonal mixing processes and remains permanently without oxygen at depth; the perfect environment for varve formation and preservation, called monimolimnion.
The cores used for the ELSA-20 time series represent those of our cores, which preserve the varves in the best possible way. The ELSA-20 time series is not yet presented with a varve-counted chronology, but tuned to the established ice core chronology, see above. The next generation ELSA-23 will present respective time series with floating varve chronology, wherever possible.