Evidence for Large Preindustrial Perturbations of the Black-Sea Chemocline

RECENT studies1-8 have documented significant short-term vertical fluctuations in the position of the oxic-anoxic interface (chemocline) in the waters of the Black Sea, the world's largest anoxic basin. Natural5,9 and anthropogenic3,4,8,10 influences have been invoked as possible causes of the observed fluctuations, but it has been difficult to establish the relative importance of these two forcings. One reason is that observations of the magnitude of chemocline displacement have not extended sufficiently far in the past to eliminate the possibility of anthropogenic changes in freshwater input. Here we present chemical analyses of shelf sediments, collected from the Bosporus region of the Black Sea, which contain a record of past water column chemistry. We find 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 as a viable cause. Although our results do not rule out an anthropogenic cause for the recent variations, they do show that natural perturbations more than twice as large as the more recent changes have occurred in the past.