Why Did the Deep-Sea Ecosystem in the Eastern Mediterranean Collapse 10,000 Years Ago?
Sediments at the bottom of the ocean reveal information about the overlying water column, such as changes in algae production, the amount of biological material that is consumed and the conditions under which this happens. Sediment cores retrieved from the seabed contain this information for the past. In 1947, Swedish scientists discovered recurring layers of organically rich sediment, known as sapropels, in the deep basins of the eastern Mediterranean during an expedition. Sapropels form when the bottom water becomes very oxygen-poor or even anoxic, wiping out all consuming organisms. Depositions of sapropel are therefore evidence of a dramatic climate change.
The youngest of these sapropel layers is dated between 10,500 and 6,000 years ago. How exactly anoxic conditions developed, which caused the death of deep-sea organisms in the eastern Mediterranean, remained unclear. A new study by scientists at the Max Planck Institute for Meteorology and the University of Hamburg has recently delivered the first sound explanation of the relevant processes.
Changes started already 4,000 years earlier
In climate simulations covering the last 21,000 years, Katharina D. Six, Uwe Mikolajewicz and Gerhard Schmiedl show how a sequence of different processes leads to a gradual reduction in the oxygen concentration in water layers below 1,000 metres. Even though sapropels became visible 10,500 years ago, the relevant changes in the Mediterranean circulation began 4,000 years earlier. Continuous warming of the sea surface during the deglaciation and the steady sea level rise caused increasing stratification of the water column, which prevented oxygen transport towards the sea floor. During the African Humid Period, which began 14,000 years ago and was associated with a greening of the Sahara due to enhanced precipitation, large quantities of nutrients entered the eastern Mediterranean in addition and caused algae growth. The enhanced settling of organic material resulted in an increased supply of plant food at depth, which, in turn, necessitated greater amounts of oxygen for its consumption. However, the circulation did not sufficiently supply oxygen. As a consequence, the deep-sea ecosystem collapsed. The dead organic material accumulated on the sea floor and formed the layers of sapropel found in the sediment core. Katharina Six:
“The uniqueness of our study is that we simulate the entire period from the last glacial maximum to the present day, including significant climate changes such as sea level rise. This allows us to take into account slow but relevant climate changes that are neglected in simulations covering only short time slices.“
The study investigates not only the formation of sapropels, but also their termination and the transition to the physical and biogeochemical conditions known today in the eastern Mediterranean, which are characterized by low biological productivity in the surface and oxygen-rich water in the deep. The good agreement between the model results and observed time series in the sediment underlines the quality of the simulation.
Original publication
Katharina D. Six, Uwe Mikolajewicz & Gerhard Schmiedl (2026): Transient deglacial simulations unravel the causes of Mediterranean sapropel formation. Communications Earth & Environment. DOI: 10.1038/s43247-026-03290-9
Contact
Dr. Katharina Six
Max Planck Institute for Meteorology
katharina.six@mpimet.mpg.de
Uwe Mikolajewicz
Max Planck Institute for Meteorology
uwe.mikolajewicz@mpimet.mpg.de
Prof. Dr. Gerhard Schmiedl
University of Hamburg
gerhard.schmiedl@uni-hamburg.de