Mapping Si and N Isotopes in Southern Ocean Surface Waters in Relation to Fe and Nutrient Concentrations and Diatom Valve Morphology
Final Report Abstract
During this project, we analyzed a suite of samples of surface waters and water column profiles along the Polarstern ANTXXIIl/9 cruise track. This cruise track ran from Punta Arenas, Chile, across the Weddell Sea to the ice edge near Neumayer Station, along the ice edge to Prydz Bay, and then up the Kerguelen Plateau en route to Cape Town, South Africa. The samples were analyzed for their content of trace metals, nutrients, and phytoplankton pigments as well as for their silicon isotopic composition. One accomplishment associated with this research has been to begin mapping the distribution of trace elements in the Atlantic and Indian sectors ofthe Southern Ocean, areas where little previous work had been done in this regard. The 70 surface water samples measured have confirmed that concentrations of metals like iron, nickel, cadmium, zinc, copper, cobalt, and lead are generally quite low in the Southern Ocean, even close to the ice edge. Our results also show variations in metal concentrations associated with frontal systems in the Southern Ocean and, unsurprisingly, with proximity to islands. In terms of the silicon isotopes, the surface water data are some of the first for the Antarctic and Indian sectors of the Southern Ocean and as such, these data are of tremendous use. The silicon isotopic composition of diatom microfossils from sediment cores are a key proxy from which we reconstruct patterns of nutrient use and CO2 removal from surface waters by phytoplankton in the past. The data we have collected from surface waters show that the silicon isotopic composition of the nutrient, dissolved silicon, varies with the degree of nutrient removal, exactly what is needed if silicon isotopes in diatom microfossils are to be utilized as a nutrient use proxy. The relationship between nutrient use and isotopic composition is such, however, that the frequent input of new nutrients into surface waters must be taken into account when doing the reconstructions, rather than using a model that considers only a single input of nutrients at the beginning of the growing season.
