Amarine regressionis a geological process occurring when areas of submergedseafloorare exposed during a drop in sea level. The opposite event,marine transgression,occurs when flooding from the sea covers previously-exposed land.[1]
Description
editAccording to one hypothesis, regressions may be linked to a "slowdown in sea-floor spreading, leading to a generalized drop in sea level (as the mid-ocean ridges would take up less space)...."[2]That view considers major marine regressions to be one aspect of a normal variation in rates ofplate tectonicactivity, which leads to major episodes of globalvolcanismlike theSiberian Trapsand theDeccan Traps,which in turn cause largeextinction events.
Evidence of marine regressions and transgressions occurs throughout the fossil record, and the fluctuations are thought to have caused or contributed to severalmass extinctions,such as thePermian–Triassic extinction event(250 million years ago, Ma) andCretaceous–Paleogene extinction event(66Ma). During the Permian-Triassic extinction, the largest extinction event in the Earth's history, the global sea level fell 250 m (820 ft).[3]
A major regression could cause marine organisms in shallow seas to go extinct, but mass extinctions tend to involve both terrestrial and aquatic species, and it is harder to see how a marine regression could cause widespread extinctions of land animals. Regressions are, therefore, seen as correlates or symptoms of major extinctions, rather than primary causes. The Permian regression might have been related to the formation ofPangaea.The accumulation of all major landmasses into one body could have facilitated a regression by providing "a slight enlargement of the ocean basins as the great continents coalesced."[4]However, that cause could not have applied in all or even many of the other cases.
Ice ages
editDuring theice agesof thePleistocene,a clear correlation existed between marine regressions and episodes ofglaciation.As the balance shifts between the globalcryosphereandhydrosphere,more of the planet's water in ice sheets means less in the oceans. At the height of thelast ice age,around 18,000 years ago, the global sea level was 120 to 130 m (390-425 ft) lower than today. A cold spell around 6 million years ago was linked to an advance in glaciation, a marine regression, and the start of theMessinian salinity crisisin theMediterraneanbasin. Some major regressions of the past, however, seem unrelated to glaciation episodes, with the regression that accompanied the mass extinction at the end of theCretaceousbeing one example.
See also
edit- 8.2-kiloyear event– Rapid global cooling about 8,200 years ago
- Marine terrace– Emergent coastal landform
References
edit- ^Monroe, James Stewart, and Reed Wicander.Physical Geology: Exploring the Earth.Fifth edition; Thomson Brooks/Cole, 2005; p. 162.
- ^Courtillot, p. 141.
- ^Courtillot, Vincent.Evolutionary Catastrophes: The Science of Mass Extinction.Cambridge, Cambridge University Press, 1999; p. 89.
- ^Ward, Peter D.Rivers in Time: The Search for Clues to Earth's Mass Extinctions.New York, Columbia University Press, 2000; p. 77.