Abstract
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 (δ30SiDSi) in Crescent Stream, McMurdo Dry Valleys for samples collected between December and February in the 2014-2015, 2015-2016 and 2016-2017 austral seasons. The δ30SiDSi 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 δ30SiDSi compositions, ranging from + 0.90 ‰ to + 1.39 ‰, 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 δ30SiDSi 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 δ30SiDSi 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.
Highlights
McMurdo Dry Valleys (MDV), the largest ice-free area of Antarctica, is covered in continuous permafrost, with low surface temperatures and low rates of precipitation
Sub-glacial meltwaters have low dissolved silicon (DSi) concentrations ranging from 3 to 94 μM and δ30SiDSi compositions ranging from −0.58 to +0.87, attributed to amorphous Si and clay mineral dissolution during sub-glacial water-mineral interaction (Hatton et al, 2019a,b)
Permafrost-dominated large Arctic Rivers have higher DSi concentrations ranging from 25 to 310 μM, and δ30SiDSi compositions ranging from +0.39 to +2.72 (Pokrovsky et al, 2013; Mavromatis et al, 2016; Sun et al, 2018)
Summary
McMurdo Dry Valleys (MDV), the largest ice-free area of Antarctica, is covered in continuous permafrost, with low surface temperatures and low rates of precipitation. A wealth of evidence using dissolved element concentrations and ratios (Lyons et al, 1998; Nezat et al, 2001), Sr and Li isotope analysis (Lyons et al, 2002; Witherow et al, 2010; Dowling et al, 2013) and mineralogical studies (Gooseff et al, 2002; Maurice et al, 2002) shows that Si is supplied to the streams by weathering of stream sediments with silicate mineral weathering rates an order of magnitude greater than rates in lower latitude, warmer locations (Gooseff et al, 2002) These studies demonstrate that chemical weathering of silicates occurs in MDV streams (Lyons et al, 2002), despite the cold temperatures, lack of precipitation and low abundance of complexing organic material (Burkins et al, 2001), and thereby contribute to the atmospheric CO2 draw down over geological timescales (Berner and Berner, 1997). Considering only the net release of solutes from weathering reactions, and not accounting for the formation of secondary weathering products in the near and extended hyporheic zone in MDV streams may contribute to an underestimation of actual weathering rates in this polar region and associated atmospheric CO2 consumption
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