Marginal basins are effective amplifiers and sensitive recorders of paleo-environmental events of global significance. All ocean basins go through a marginal basin phase as part of their evolution, and there are a number of marginal basins in the modern world ocean. In response to sea level changes, the link between a marginal basin and the ocean can be episodically restricted or even severed. The Black Sea is a present-day example of a marginal basin where the connection changes dramatically with sea level. Extreme changes in environmental conditions are typical of this kind of threshold-regulated behavior; most of these changes occur abruptly and responses have dramatic consequences on the basin's hydrologic, sedimentary, geochemical, and ecological systems. For example, temporal variations in water column density structure related to changes in circulation could influence the oxygenation of a marginal basin, drastically affecting the geochemistry of the water column and sediments.