Abstract
AbstractThe subglacial chemical weathering environment is largely controlled by low temperatures and the presence of freshly comminuted minerals with a high surface area. These characteristics are believed to promote dissolution processes that give rise to low silica and high Ca2+fluxes emanating from glacierized basins. We test an alternative hypothesis, that mineral precipitation reactions in the subglacial environment play an equally important role in controlling the water chemistry in glacierized basins. We analyze borehole and proglacial water chemistry from a subarctic polythermal glacier, complemented by mineral XRD analysis of suspended sediment, till and bedrock samples. In conjunction with a thermodynamic analysis of the water and mineral chemistry, we use reaction-path modelling to study the chemical enrichment of water through the glacier system. We find that the high pH of the subglacial environment is conducive to secondary mineral precipitation, and that it is not possible to balance the water chemistry using dissolution reactions alone. We show that low silica can be explained by standard weathering reactions without having to invoke mineral-leaching reactions. Our results suggest that subglacial weathering intensity may be significantly underestimated if the production of secondary minerals is not considered.
Highlights
Glacierized basins have a unique chemical weathering environment, characterized by low temperatures, variable access of meltwater and atmospheric gases to the glacier bed, and high amounts of rock comminution, thereby providing fresh and abundant mineral surfaces
To approximate the silica deficiency in the proglacial water, we apply a series of stepwise inverse mass-balance calculations to individual samples by first partitioning all of the Na+ in solution to andesine, based on the incongruent dissolution of andesine to kaolinite as
As an alternative means to explain the proglacial water quality, we have focused our modelling on weathering rates that depend on the saturation state, and mineral precipitation reactions
Summary
Glacierized basins have a unique chemical weathering environment, characterized by low temperatures, variable access of meltwater and atmospheric gases to the glacier bed, and high amounts of rock comminution, thereby providing fresh and abundant mineral surfaces. This has motivated much research into how chemical weathering differs between glacierized and non-glacierized basins (e.g. Anderson and others 1997; Lafreniere and Sharp, 2005; Pogge von Strandmann and others, 2006; Wimpenny and others, 2011), and the relevant impacts on atmospheric CO2 drawdown (e.g. Sharp and others, 1995a; Hodson and others, 2000; Tranter and others, 2002a). Further uncertainty exists in trying to relate the water chemistry at the terminus to chemical reactions in the distributed system, where waters may have residence times that are orders of magnitude longer than in a channelized system
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