APPROACHES TO THE USE OF MARINE DIATOMS IN FINGERPRINTING GLACIAL ICE RETREAT

Diatoms, single-celled algae with a siliceous frustule, are generally well preserved in sediments of the Southern Ocean. Their use in reconstructing the history of glacial ice retreat and advance is based on several lines of analysis. First and most simply, diatoms need sufficient light to conduct photosynthesis; light levels beneath floating ice shelves are too low for this to occur. Consequently, absolute diatom abundance can be used to evaluate changes in overlying ice shelf cover, an approach that has been used in sediments from the Larsen embayments (northwestern Weddell Sea, Antarctica) to reconstruct the contrasting histories of the Larsen A and B ice shelves through the Holocene. A second approach is to assess whether specific diatoms or diatom assemblages are associated with melting glacial ice. Varved sediments around the Antarctic margin with extremely high concentrations of Chaetoceros resting spores, for example, have been observed to characterize the transition from the last glacial to the present interglacial. The role of iron fertilization via glacial ice melt is speculated to favor Chaetoceros, with its rapid reproduction rates. However, Chaetoceros resting spores can typify high nutrient conditions that are not limited to this transitional event. Unusually high abundance of another species, Fragilariopsis vanheurckii, however, appears to be associated with modern phytoplankton assemblages in the Larsen B embayment and in the underlying sediments that characterize the newly open water system. This species also was observed in higher than normal abundance in deglacial sediments from the Palmer Deep, Antarctica, suggesting that F. vanheurckii may be useful for tracking glacial ice melt in surface waters of the Southern Ocean.