THE POLAR MARINE DIATOM EUCAMPIA ANTARCTICA: A TEST OF ITS PALEOCEANOGRAPHIC UTILITY

Proxy records based on polar marine diatoms in sediment cores have been critical elements of reconstructing climatic and oceanographic changes of the Southern Ocean. The marine diatom species, Eucampia antarctica, has been used as a proxy for both changes in oceanic circulation and changes in the extent of winter sea ice in the Southern Ocean. This species exhibits morphologic variability, with the asymmetric variety associated with sub-polar waters and the symmetric form associated with more southerly, polar water masses. In addition, theoretically, chain length of colonies can be estimated based on the ratio of terminal to intercalary valves. Colonies are more likely to be longer under the higher light conditions associated with limited seasonal sea ice and shorter under conditions of more extended sea ice cover. Enhanced utility of this proxy is dependent on an improved understanding of the modern environmental constraints on the distribution of the different morphological forms of this species. Accordingly, distributional data were generated from phytoplankton samples collected along five transits between southernmost South America and the Antarctic Peninsula. The transits took place during the summer through fall, between 2004 to 2007. The data demonstrate that under modern conditions, an overwhelming proportion of the cells were symmetric, of the polar variety. Sub-polar forms, which were an extremely minor component of these phytoplankton samples, must occur farther to the north. The ratio of terminal to intercalary valves is only a fair reflection of chain length, as many living chains exhibited no terminal valves. Consequently, the use of this proxy as an indicator of sea extent probably underestimates sea ice extent and should be used with caution.