EVIDENCE FOR LARGE PREINDUSTRIAL PERTURBATIONS OF THE BLACK-SEA CHEMOCLINE

RECENT studies1-8 have documented significant short-term vertical fluc tuations in the position of the oxic-anoxic interface (chemocline) in the waters of the Black Sea, the world's largest anoxic basin. Natural 5,9 and anthropogenic3,4,8,10 influences have been invoked as possible causes of the observed fluctuations, but it has been difficult to est ablish the relative importance of these two forcings. One reason is th at observations of the magnitude of chemocline displacement have not e xtended sufficiently far in the past to eliminate the possibility of a nthropogenic changes in freshwater input. Here we present chemical ana lyses of shelf sediments, collected from the Bosporus region of the Bl ack Sea, which contain a record of past water column chemistry. We fin d that the chemocline in this region rose by at least 40-50 m greater- than-or-equal-to 250-300 years ago, precluding anthropogenic forcing a s a viable cause. Although our results do not rule out an anthropogeni c cause for the recent variations, they do show that natural perturbat ions more than twice as large as the more recent changes have occurred in the past.