A peak in , accompanied by fluctuations in other isotope concentrations, is often associated with mass extinctions. Some workers have attempted to explain this event in terms of climate or sea level change – perhaps arising due to a build-up of glaciers; however, such factors alone do not appear to be sufficient to explain the events. An alternative hypothesis is that changes in ocean mixing were responsible. An increase in density is required to make water downwell; the cause of this densification may have changed from hypersalinity (due to ice formation and evaporation) to temperature (due to water cooling).