Editing Extinction event (section)

====Oceanic overturn====
Oceanic overturn is a disruption of [[thermo-haline circulation]] that lets surface water (which is more saline than deep water because of evaporation) sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms that inhabit the surface and middle depths. It may occur either at the beginning or the end of a [[glaciation]], although an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water.<ref>{{cite journal | vauthors = Wilde P, Berry WB | title=Destabilization of the oceanic density structure and its significance to marine "extinction" events | journal=Palaeogeography, Palaeoclimatology, Palaeoecology | volume=48 | issue=2–4 | pages=143–62 | year=1984 | url=http://www.marscigrp.org/ppp84.html | doi=10.1016/0031-0182(84)90041-5 | bibcode=1984PPP....48..143W }}</ref>

Unlike other oceanic catastrophes such as regressions (sea-level falls) and anoxic events, overturns do not leave easily identified "signatures" in rocks and are theoretical consequences of researchers' conclusions about other climatic and marine events.

It has been suggested that oceanic overturn caused or contributed to the [[Late Devonian extinction|late Devonian]] and [[Permian–Triassic extinction event|Permian–Triassic]] extinctions.