A 6,000-YEAR SEDIMENTARY MOLECULAR RECORD OF CHEMOCLINE EXCURSIONS INTHE BLACK-SEA

THE Black Sea is the world's largest anoxic basin; it is also a contem porary analogue of the environment in which carbonaceous shales and pe troleum source beds formed1. Recently, Repeta et al.2,3 reported that anoxygenic photosynthesis may be an important component of carbon cycl ing in the present Black Sea, owing to a shoaling of the chemocline an d consequent penetration of the photic zone by anaerobic waters in the past few decades4,5. It has been suggested4 that this was due to an a nthropogenic decrease in freshwater input to the Black Sea, although n atural causes were not ruled out. Here we report the distributions of sequestered photosynthetic pigments in eight core samples of sediments from the Black Sea ranging in age from zero to 6,200 years before the present. Our results show that photosynthetic green sulphur bacteria (Clorobiaceae) have been active in the Black Sea for substantial perio ds of time in the past. This finding indicates that the penetration of the photic zone by anaerobic waters is not a recent phenomenon, and s uggests that natural causes for shoaling of the chemocline are more li kely than anthropogenic ones4.