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Oceanography

The mystery of the sapropels

Nature volume 398, pages 27–29 (1999)Cite this article

The facts of the case are simply stated. In the eastern Mediterranean, sediments of Plio-Pleistocene age (5 million to 10, 000 years old) contain quasi-periodic laminated intervals whose lithology and geochemistry indicate that they accumulated under conditions of low — or even no — oxygen. Because the bottom waters are generally well oxygenated today, the puzzle has been: what conditions in the past reduced the oxidation state?

In principle, there are two ways to do this. First (option 1), sequester the deep water from atmospheric exchange for long enough that all of the dissolved oxygen is used up and the deep water stagnates through the oxidation of accumulated organic matter. This means turning off the regional vertical circulation, effectively putting a lid on the system. In the salty Mediterranean, the most likely way to do this is by adding a surface layer of relatively fresh water, either from massive melting of northern ice sheets funnelled through the Black Sea3 or from greatly increased run-off of the Nile River4.

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Figure 1: A sapropel.

A. E. S. KEMP

References

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  1. Division of Ocean Sciences, Room 725, National Science Foundation, 4201 Wilson Boulevard, Arlington, 22230, Virginia, USA

    Connie Sancetta

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  1. Connie Sancetta
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Sancetta, C. The mystery of the sapropels. Nature 398, 27–29 (1999). https://doi.org/10.1038/17930

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