High-resolution δ13C and δ15N records of diatom-bound organic matter (δ13CD, δ15ND) from ODP site 1094, South of the Polar Front, are combined with reconstructions of summer sea surface temperature and winter sea-ice to establish changes in surface water characteristics associated with the main climatic events of the past 640 ka. The six glacial-interglacial δ13CD cycles show excellent agreement with SPECMAP δ18O and Vostok CO2 content, suggesting that global physical processes rather than local biological factors were the main influences on δ13CD. By contrast, there is weak correspondence between the Vostok dust record and the δ13CD and δ15ND records. The data do not support the hypothesis that glacial iron fertilization of the Southern Ocean was the main factor lowering atmospheric CO2, if dust accumulation represents iron input. The records indicate that sea ice cover, water column stratification and iron addition from deep water mixing are more likely to explain our observations.