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.

Bristol Glaciology Centre School of Geographical Sciences University Road Bristol BS8 1SS UK

Department of Ecology Charles University Viničná 7 12844 Prague 2 Czech Republic

Earth and Planetary Sciences University of California Santa Cruz CA 95064 USA

Institute of Geochemistry and Petrology ETH Zurich Clausiusstrasse 25 8092 Zürich Switzerland

National Oceanography Centre University of Southampton Waterfront Campus European Way Southampton SO14 3ZH UK

Ocean and Earth Science National Oceanography Centre Southampton University of Southampton Waterfront Campus European Way Southampton SO14 3ZH UK

School of Earth and Environment University of Leeds Leeds LS2 9JT UK

School of Earth and Ocean Sciences Cardiff University Main Building Park Place Cardiff CF10 3AT UK

School of Earth Sciences University of Bristol Bristol BS8 1RJ UK

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Insights into silicon cycling from ice sheet to coastal ocean from isotope geochemistry

Silicon isotopes in Arctic and sub-Arctic glacial meltwaters: the role of subglacial weathering in the silicon cycle