Gerald H. Haug (Max Planck Institute for Chemistry and ETH Zürich), Anja Studer (MaxPlanck Institute for Chemistry), Samuel L. Jaccard (Bern University), Alfredo Martinez-Garcia (Max Planck Institute for Chemistry), Ralf Tiedemann (Alfred Wegener Institute), and Daniel M. Sigman (Princeton University)
We argue for a pervasive link between cold climates and polar ocean stratification. In boththe Subarctic North Pacific and the AntarcticZone of the Southern Ocean, ice ages weremarked by low productivity. The accumulated evidence from sediment cores points to anincrease in density stratification that reducedthe supply of nutrients from the ocean interiorinto the sunlit surface in both of these regions. The last ice age was associated withstratification of the Antarctic and the subarctic North Pacific and it can be argued that thewell-known glacial decrease in North Atlantic Deep Water indicates a similar stratificationof the North Atlantic. This link also applies to longer timescales, including the onset ofextensive northern hemisphere glaciation 2.7 million years ago, which was concurrent withstratification of the Subarctic North Pacific and the Southern Ocean.
The generality of thecooling/stratification connection calls for ageneral mechanism. Such a mechanism isprovided by the non-linearrelationship between the temperature of seawaterand its density:cooling of the ocean will decrease the role thattemperature plays in the density structure ofthe polar water column, allowing the freshwater capthat is always present in polar regions tocause greater density stratification, allowing the freshwater cap tointensify further. Nutrient-rich polar ocean regions such as the Antarcticand the Subarctic Pacific represent a “leak” inthe biological pump, allowing deeply sequestered carbon dioxide to escape back into theatmosphere, and stratification of these regionslargely stops that leak. Thus, the link betweenclimate cooling and the stratification of nutrient-rich polar regions represents a positivefeedback in the climate system, raising atmospheric carbon dioxide during warm periods andreducing it during cold periods.