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Sapropels: The dark legacy of the Mediterranean Sea

08.06.2015

How was the eastern Mediterranean Sea pushed into deep-water oxygen deficiency and collapse of deep-sea ecosystems during the early Holocene?

Sediment core from 800 m water depth: during the sapropel formation there were repeated oxygen transfers.
This sediment core from 2500 m water depths in the eastern Mediterranian Sea illustrates the Holocene. The black layer shows the youngest sapropel S1.

The past 13.5 million years of the deep Eastern Mediterranean Sea (EMS) are characterised by recurrent episodes of oxygen deficiency. Under the oxygen deficiency, remineralisation of upper-ocean organic matter fallout is reduced, which leads to the preservation of a dark and organic-rich sediment layer, the sapropel. In a recent study published in Nature Communications, Dr. Rosina Grimm and her co-authors of geologists, biogeochemists, and climate modellers from the Max Planck Institute for Meteorology and various other national and international research facilities, explain the evolution of the most recent episode of deep-water oxygen deficiency in the eastern Mediterranean Sea (EMS).

In general, the mechanisms leading to sapropel formation are either a reduction or shutdown of the thermohaline EMS circulation, which subdues deep oxygenation, an enhanced biological production, which increases respiratory oxygen consumption of sinking organic matter, or both. In contrast to the present-day well-oxygenated and extremely low productive EMS, sapropel formation indicates radical ocean-biogeochemical responses to environmental changes.

In their study, the authors make the first attempt to disentangle the external (climatic) and internal (circulation and biological production) causes for the most resent deposited sapropel S1 by combining sediment core data with results of highly idealised simulations using a prognostic ocean-biogeochemical model including a sediment module.

The model and proxy results agree that S1 deposition requires a long prelude of at least 5500 years of restricted deep-water ventilation (deep-water stagnation) at the low production rates seen in pre-S1 sediment records. From the required duration of the deep-water stagnation the authors infer that the climatic changes associated with the last deglaciation must have essentially preconditioned the S1 deep-water stagnation. These climatic change are the global warming and upper-ocean freshening (freshening of the EMS due to both the enhanced and freshened Atlantic water inflow, which are related to the melting of the major ice-sheets). Both the climate warming and upper-ocean freshening caused an enhanced density gradient between the dense (cold and salty) deep-water emplaced during the late glacial and an increasingly buoyant upper-ocean layer (warm and fresh) during the deglaciation, and thus to the development of the S1 deep-water stagnation.

Therefore, the 5500 years required for complete oxygen consumption cannot be explained by the commonly proposed insolation-driven northward shift of the African rain belt (the last green Sahara period) with the onset of strongly enhanced Nile runoff around 2000 to 4000 years before S1 deposition. Also other sources of enhanced freshwater input such as the reconnection with the Black Sea and its brackish inflow to the EMS came into effect later, and thus cannot have caused the evolution of S1 oxygen deficiency. Nevertheless, these enhanced freshwater sources further reduced the upper-ocean density, and therefore contributed to maintain the deep-water stagnation throughout the long period (3800 years) of S1 deposition

Prof. Gerhard Schmiedl and Katharina Müller-Navarra from the Center for Earth System Research and Sustainability (CEN) as well as Prof. Kay-Christian Emeis from the Helmholtz-Zentrum Geesthacht (HZG) were involved in the study.

 

More information:

Original publication:
Grimm, R., Maier-Reimer, E., Mikolajewicz, U., Schmiedl, G., Müller-Navarra, K., Adloff, F., Grant, K.M., Ziegler, M., Lourens, L.J., Emeis, K.-C. Late glacial initiation of Holocene eastern Mediterranean sapropel formation. Nature Communications. 6:7099. doi: 10.1038/ncomms8099 (2015).

Contact:
Dr. Rosina Grimm
Max Planck Institute for Meteorology
Email: rosina.grimmdummy@mpimet.mpgdummy2.de

Dr. Uwe Mikolajewicz
Max Planck Institute for Meteorology
Phone: +49 (0)40 41173 243
Email: uwe.mikolajewiczdummy@mpimet.mpgdummy2.de