Globally averaged riverine silicon (Si) concentrations and isotope composition (δ³⁰Si) may be affected by the expansion and retreat of large ice sheets during glacial-interglacial cycles. Here we provide evidence of this based on the δ³⁰Si composition of meltwater runoff from a Greenland Ice Sheet catchment. Glacier runoff has the lightest δ³⁰Si measured in running waters (-0.25 ± 0.12‰), significantly lower than nonglacial rivers (1.25 ± 0.68‰), such that the overall decline in glacial runoff since the Last Glacial Maximum (LGM) may explain 0.06-0.17‰ of the observed ocean δ³⁰Si rise (0.5-1.0‰). A marine sediment core proximal to Iceland provides further evidence for transient, low-δ³⁰Si meltwater pulses during glacial termination. Diatom Si uptake during the LGM was likely similar to present day due to an expanded Si inventory, which raises the possibility of a feedback between ice sheet expansion, enhanced Si export to the ocean and reduced CO₂ concentration in the atmosphere, because of the importance of diatoms in the biological carbon pump.