Variations in the areal extent of the cryosphere appear to exert an important control over glacial‐interglacial changes in the mode of formation and source areas for deepwater and bottom water masses. During interglacials, when ice extents are minimal, these water masses form by mechanisms which are well documented for the present interglacial. Climatically related advances and retreats of sea ice in the northern hemisphere during glacials probably resulted in shifting loci of formation for each of the three types of North Atlantic Deep Water (NADW). Probable glacial advances of grounded ice sheets to the continental shelf margins in Antarctica precluded formation of Antarctic Bottom Water (AABW) at the presently active continental shelf sites. Because these shifts in source areas for AABW and NADW probably involved source waters with salinity, temperature, oxygen content, and nutrient levels that differed from modern values, deep and bottom waters are likely to have had different characteristics in glacials than they do during interglacial
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