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Global Climate Change Digest

A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999



Item #d99apr28

“Sulphidic Mediterranean Surface Waters During Pliocene Sapropel Formation,” H. F. Passier et al.,Nature 397, 146-149 (1999).

Pliocene-era sapropels (organic-matter-rich layers of sediment) in the eastern Mediterranean were found to contain fossils of a substance produced by photosynthetic green sulphur bacteria, indicating that the photic zone of the water column was anoxic and that iron sulfide was formed in the water column. These observations, made at widespread locations in the Mediterranean, led to the conclusions that (1) large areas were euxinic (having restricted circulation and stagnant or anaerobic conditions) for long periods during the Pliocene and (2) the degradation of the organic material strongly influenced the euxinic conditions.

Item #d99apr29

“The Role of Mat-Forming Diatoms in the Formation of Mediterranean Sapropels,” A. E. S. Kemp et al., Nature 398, 57-61 (1999).

Two theories have been offered to explain the formation of sapropels: extreme stratification of the eastern Mediterranean and massive influxes of organic matter. Electron microscopy of diatom-bearing sapropels revealed seasonal deposition of alternating monospecific diatoms and mixed diatom assemblages. The theory is offered that these diatoms produced large mats that floated to the surface during summer stratification and died off and sank in the fall and winter, consuming all the available oxygen with the decomposition of their organic material as they sank. Mass- balance calculations indicate that enough organic material would be left to produce the sapropels when they settled on the bottom. This theory would reconcile the two previous approaches and would open the way for interpreting the sapropel record for paleoclimatic insights.

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