A number of recent studies utilized silicon isotopes to trace changes in the biogeochemical cycling of silicon in the paleocean [1, 2]. The main controlling process on this cycle is the uptake of silicon by opal-precipitating phytoplankton. Accounting for about 60% of the oceanic primary production, diatoms link the dissolved silicon (nutrient) pool and oceanic CO (sub 2) uptake from the atmosphere. Since the Southern Ocean plays a key role in ocean circulation and deep-sea ventilation, its underlying opal-rich sediments are particularly suitable for studying the link between atmospheric CO (sub 2) variations and phytoplankton nutrient utilization. The degree of surface silicon utilization is to a first order a function of nutrient supply from below and should be reflected in the diatom silicon isotope composition. In combination with carbon isotope (delta (super 13) C), nitrogen isotope (delta (super 15) N) and micronutrient (esp. Fe) information it is possible to reconstruct the efficiency of the biological pump that determines whether the surface ocean acts as a net source or sink for atmospheric CO (sub 2) . Here, we present down-core (0-170 ka) silicon isotope variations of diatoms from the Atlantic Sector of the Southern Ocean (OPD Site 1093) and examine the impact of glacial-interglacial climate change on the degree of silicon isotope utilization. The results show that the degree of silicon isotope utilization during interglacial periods is different from that of peak glacial periods. During peak glacial periods, silicon utilization is inefficient at a time where an efficient biological pump is assumed to draw down additional atmospheric CO (sub 2) , implying a more complex relationship between silicon in frustules and nutrients contributing to organic tissues. In addition, ambiguity exists as to whether the increased efficiency in the biological pump during glacials was, as commonly believed, a contributor to the low atmospheric CO (sub 2) concentrations or a consequence and therefore "only" a positive feedback.