PRELIMINARY EXAMINATION OF VALVE SIZE OF FRAGILARIOPSIS KERGUELENSIS AS A POTENTIAL INDICATOR OF NUTRIENT CONTENT OF THE SOUTHERN OCEAN ACROSS THE GLACIAL TERMINATION

Cortese and Gersonde (2007) contended that changes in valve size of the diatom (Phylum Bacillariophyta) Fragilariopsis kerguelensis were due to changes in nutrients associated with changes in the position of the Polar Frontal Zone. We hypothesized that wind may serve as another source of nutrients that may affect diatom size.

Using smear slides examined under a differential-interference microscope at 400x magnification, we measured mean valve length and width of F. kerguelensis in 19 sub-samples from a sediment core taken from Site 1094 the Southern Ocean on Leg 177 of the Ocean Drilling Program. Analysis revealed a strong inverse relationship of length of F. kerguelensis valves with the record of natural gamma radiation (NGR), an indicator of terrigenous sediment delivery to the site.

Extensive ice cover of the last glacial period would have increased the continent-ocean temperature gradient and thus winds transporting iron-rich eolian sediments and wind-induced upwelling and delivery of nutrient-rich waters to the sea surface. Thus this outcome differs from our expectation of increasing valve size during the last glacial period. One potential explanation for the unexpected decrease in valve length is that the increase in iron delivery during the last glacial period stimulated an increase in the number of individual diatoms and competition for silica, causing it to become a limiting nutrient. Alternatively, extensive sea-ice coverage may have led to light-limitation.