02966nas a2200205   4500008004100000245011200041210006900153260001200222490000600234520228000240100002102520700002202541700002302563700002602586700002102612700001502633700002402648700001702672856007102689       2020                        eng d00aSilicon isotopes reveal a non-glacial source of silicon to Crescent Stream, McMurdo Dry Valleys, Antarctica0 aSilicon isotopes reveal a nonglacial source of silicon to Cresce  c06/20200 v83 a<p>In high latitude environments, silicon is supplied to river waters by both glacial and non-glacial chemical weathering. The signal of these two end-members is often obscured by biological uptake and/or groundwater input in the river catchment. McMurdo Dry Valleys streams in Antarctica have no deep groundwater input, no connectivity between streams and no surface vegetation cover, and thus provide a simplified system for us to constrain the supply of dissolved silicon (DSi) to rivers from chemical weathering in a glacial environment. Here we report dissolved Si concentrations, germanium/silicon ratios (Ge/Si) and silicon isotope compositions (δ<sup>30</sup>Si<sub>DSi</sub>) in Crescent Stream, McMurdo Dry Valleys for samples collected between December and February in the 2014&minus;2015, 2015&minus;2016, and 2016&minus;2017 austral seasons. The δ<sup>30</sup>Si<sub>DSi</sub> compositions and DSi concentrations are higher than values reported in wet-based glacial meltwaters, and form a narrow cluster within the range of values reported for permafrost dominated Arctic Rivers. High&nbsp;δ<sup>30</sup>Si<sub>DSi</sub>&nbsp;compositions, ranging from +0.90&permil; to +1.39&permil;, are attributed to (i) the precipitation of amorphous silica during freezing of waters in isolated pockets of the hyporheic zone in the winter and the release of Si from unfrozen pockets during meltwater-hyporheic zone exchange in the austral summer, and (ii) additional Si isotope fractionation via long-term Si uptake in clay minerals and seasonal Si uptake into diatoms superimposed on this winter-derived isotope signal. There is no relationship between&nbsp;δ<sup>30</sup>Si<sub>DSi</sub>&nbsp;compositions and DSi concentrations with seasonal and daily discharge, showing that stream waters contain DSi that is in equilibrium with the formation of secondary Si minerals in the hyporheic zone. We show that&nbsp;δ<sup>30</sup>Si<sub>DSi</sub>&nbsp;compositions can be used as tracers of silicate weathering in the hyporheic zone and possible tracers of freeze-thaw conditions in the hyporheic zone. This is important in the context of the ongoing warming in McMurdo Dry Valleys and the supply of more meltwaters to the hyporheic zone of McMurdo Dry Valley streams.</p>1 aHirst, Catherine1 aOpfergelt, Sophie1 aGaspard, François1 aHendry, Katharine, R.1 aHatton, Jade, E.1 aWelch, Sue1 aMcKnight, Diane, M.1 aLyons, Berry  uhttps://www.frontiersin.org/articles/10.3389/feart.2020.00229/full02674nas a2200253   4500008004100000245007700041210006900118260001200187490000600199520187800205100002102083700002602104700002102130700002202151700001902173700002502192700002102217700002202238700001702260700002302277700002502300700002402325856007102349       2020                        eng d00aSilicon isotopic composition of dry and wet-based glaciers in Antarctica0 aSilicon isotopic composition of dry and wetbased glaciers in Ant  c07/20200 v83 a<p>Glaciers and ice sheets export significant amounts of silicon (Si) to downstream ecosystems, impacting local and potentially global biogeochemical cycles. Recent studies have shown Si in Arctic glacial meltwaters to have an isotopically distinct signature when compared to non-glacial rivers. This is likely linked to subglacial weathering processes and mechanochemical reactions. However, there are currently no silicon isotope (δ30Si) data available from meltwater streams in Antarctica, limiting the current inferences on global glacial silicon isotopic composition and its drivers. To address this gap, we present dissolved silicon (DSi), δ30SiDSi, and major ion data from meltwater streams draining a polythermal glacier in the region of the West Antarctic Peninsula (WAP; King George Island) and a cold-based glacier in East Antarctica [Commonwealth Stream, McMurdo Dry Valleys (MDV)]. These data, alongside other global datasets, improve our understanding of how contrasting glacier thermal regime can impact upon Si cycling and therefore the δ30SiDSi composition. We find a similar δ30SiDSi composition between the two sites, with the streams on King George Island varying between -0.23 and +1.23&permil; and the Commonwealth stream varying from -0.40 to +1.14&permil;. However, meltwater streams in King George Island have higher DSi concentrations, and the two glacial systems exhibit opposite DSi &ndash; δ30SiDSi trends. These contrasts likely result from differences in weathering processes, specifically the role of subglacial processes (King George Island) and, supraglacial processes followed by in-stream weathering in hyporheic zones (Commonwealth Stream). These findings are important when considering likely changes in nutrient fluxes from Antarctic glaciers under climatic warming scenarios and consequent shifts in glacial thermal regimes.</p>1 aHatton, Jade, E.1 aHendry, Katharine, R.1 aHirst, Catherine1 aOpfergelt, Sophie1 aHenkel, Susann1 aSilva-Busso, Adrián1 aWelch, Susan, A.1 aWadham, Jemma, L.1 aLyons, Berry1 aBagshaw, Elizabeth1 aStaubwasser, Michael1 aMcKnight, Diane, M.  uhttps://www.frontiersin.org/articles/10.3389/feart.2020.00286/full