Sea-ice extent in the Southern Ocean during the Last Glacial Maximum: another approach to the problem

Determining past sea-ice distribution is an important goal of paleoceanographers. Here, we present a possible approach to deter-mining past sea-ice distribution in the Southern Ocean during the Last Glacial Maximum (LGM). Diatoms are the principal opal-forming organisms south of the Antarctic Polar Front; their productivity is partly mediated by the presence/absence of sea ice. We reasoned that there should be good coherence between percentage biogenic opal in surface sediments and percentage annual sea-ice cover. This hypothesis was tested by comparing percentage biogenic opal in surface sediments against modern-day sea-ice cover in surface waters directly above each core site. The chronology for each core was determined by various means (biostratigraphy, C-14 age dating, and carbonate and opal stratigraphy). With the resulting curve we estimate that yearly concentration of sea ice can be determined to within 30%. Using these data, we estimated percentage sea-ice cover during the LGM for a number of sediment sites (50-66 degrees S) from the Southern Ocean. Core sites now beneath 100% open water witnessed some 25-60% sea ice during the LGM while core sites presently beneath sea ice during half of the year witnessed more than 75% sea-ice cover during the LGM.