Clay mineral precipitation and low silica in glacier meltwaters explored through reaction-path modelling

Abstract

ABSTRACT. The 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 Ca 2+ 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 reactionpath 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.

Silica Dissolution and Precipitation in Glaciated Volcanic Environments and Implications for Mars

“…The coatings measured in this study contain laminations and unconformities (Figure 3b), indicating that they were most likely emplaced during successive periods of deposition. These observations support the hypothesis that silica fluxes in glacier outwash waters are lowered by subglacial precipitation of silica-rich secondary phases (Crompton et al, 2015). Supporting these chemical observations, remote sensing of the most mafic glacial catchments also suggests enhanced silica deposits in the proglacial plains ( Figure S3; Scudder et al, 2017) and mineralogical analyses have found evidence for poorly crystalline, silica-rich alteration phases in mafic glacial flour ( Figure S2; Rampe et al, 2017;Smith et al, 2017).…”

Channelized, distributed, and disconnected: subglacial drainage under a valley glacier in the Yukon

“…Direct instrumentation and radar scattering (Wheler and Flowers, 2011;Wilson et al, 2013) reveal a polythermal structure with a basal layer of temperate ice. Exposed bedrock in the valley consists mainly of highly fractured Shield Pluton granodiorite (Dodds and Campbell, 1988;Crompton et al, 2015). Borehole videos have also shown the presence of granodiorite cobbles in the basal ice, and highly turbid water near the bottom of freshly drilled boreholes.…”

“…The limited available stream gauging data suggests typical summer flow around 1-2 m 3 s −1 , with maximum values around 5 m 3 s −1 and minima below the measuring capacity of the gauging station (Crompton et al, 2015). However, the outlet stream was never observed to run dry (Jeffrey Crompton, personal communication, 2018).…”

Excavation of subglacial bedrock channels by seasonal meltwater flow

“…Rates of subglacial weathering remain difficult to quantify (e.g. Crompton et al, 2015) and the role of subglacial weathering in propagating and weakening bedrock fracture poorly understood. Boulders can be readily transported by the computed shear stresses, thus macro-abrasion -the fracturing of bedrock due to impacting particles (Chatanantavet and Parker, 2009;Whipple et al, 2000Whipple et al, , 2013 -is perhaps important.…”