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An increase in iron supply associated with enhanced dust inputs could be responsible for higher marine phytoplankton production leading to the typically lower glacial atmospheric CO2 concentrations, as suggested by the ïron hypothesis.'' The enhanced dust supply may also have provided the oceans with significant amounts of silica, which would have favored the growth of diatoms over coccolithophores, as suggested by the "silica hypothesis.'' Here we present new data on molecular biomarkers in a sediment core from the midlatitudes of the Southern Hemisphere, which reveal dust-induced changes in the relative contribution of the phytoplankton to total productivity. Our results illustrate a shift in the relative abundance of siliceous over calcareous organisms during glacial times, when terrestrial aeolian input was enhanced. Although we did not detect a significant glacial decrease in coccolithophorid productivity, the decrease in the CaCO3/Corg rain ratio could have still contributed to some extent in lowering atmospheric CO2 levels.