Non-eustatic controls on sea-level change in semi-enclosed basins.

摘要

Marginal basins with shallow connections to the world ocean can become isolated from marine influence when eustatic sea level drops below the dividing sill. During isolation the basins may have higher or lower base-levels than the open ocean. This decoupling from global sea level and seawater allows preservation of a distinct history that constrains water balance and erosion in the surrounding continental masses, providing records of continental climates not preserved in marine records. The Mediterranean and Black Seas both experienced isolation stages and significant draw down of base-level below the dividing sill depth. In the Late Miocene (Messinian) a combination of tectonic compression at the marine gateways and eustatic sea level drop isolated the Mediterranean. A negative water balance then caused a lowering of Mediterranean base-level by at least 1500 m, exposing the continental shelves and slopes, and allowing the emergence of bathymetric highs in the central basins. Subaerial exposure features on the summit and evaporite onlap at the base of the Eratosthenes seamount, a carbonate platform in the Levantine basin of the Mediterranean, show the extent of base-level lowering.; The Black Sea has periodically become isolated during glacio-eustatic low stands of the Pleistocene. Submerged shoreline deposits and beveled strata during the last isolation exposed the continental shelves out to −105 m below modern sea level. Integration of seismic data, sediment cores, radiocarbon dating, and geochemical analyses indicates that the lowest water level occurred not during the glacial maximum but during the early deglaciation. A brief return toward glacial conditions in the Younger Dryas cold period resulted in increased freshwater input to the Black Sea and a rise in base-level above the outside eustatic level. Subsequent base-level fall to −105 m occurred with resumed warming climate. The first marine invasion, which is seen prominently in the 87Sr/86Sr composition of mollusk shells, is associated with the sudden drowning of the shelf up to a depth of −30 m. The timing of this “flood” event is now constrained to be 8360 y (conventional 14C years), nearly 1300 years earlier than had been previously proposed.