Up to 200 000 year-old sediment core from the Eastern Mediterranean.
Dark layers (sapropels) contain high concentrations of organic substance
and living bacteria
The Mediterranean Sea is generally very poor in nutrients. The sediments contain only low amounts of organic substance. However, depending on the varying distance between earth and sun there are periods of increased sun irridiation about every 20 000 years. This results in dramatic increases of the rainfall and inflow to the Mediterranean. These periods are reflected in the sediment composition. Layers with high content of organic substance are formed which are called sapropels.