Reaction path modelling used to explore the relationship between secondary mineral precipitation and low Si content in the meltwaters of a polythermal surge-type glacier

Abstract:

The subglacial chemical weathering environment is characterized by low temperatures and the hydrolysis and carbonation of freshly comminuted mineral surfaces. Such conditions motivate the hypothesis that relatively low silica fluxes should be found in glacierized basins. Additionally, it is often assumed that glacier meltwaters are far from saturation and that the water chemistry is controlled solely by the dissolution of primary silicates and trace quantities of sulphide and carbonate minerals. Alternatively, we propose that the formation of secondary minerals and precipitates in the delayed drainage system play an important role in controlling the low silica fluxes observed in subglacial envrionments.

Borehole and proglacial meltwater samples were collected from a polythermal surge-type glacier overlying granodiorite bedrock in the St. Elias Mountains of Yukon, Canada. The meltwater chemistry, along with the mineralogy of the bedrock and suspended sediments indicate the presence of mineral precipitation accompanied by substantial basal freeze-on. This is supported by field evidence of debris rich basal ice at the terminus and at the base of a borehole. The surplus of Cl^- above the supraglacial input is used to calculate the amount of basal freeze-on in the delayed drainage system, and the amount of mixing between the delayed and fast drainage systems. We use Geochemist’s Workbench for reaction path modelling with a focus on the silica composition to simulate the chemical evolution of glacial meltwater from (1) the initial water rock contact, (2) basal freeze on, and (3) mixing and post mixing reactions. Unless there is a substantial degree of non-stoichiometric dissolution, we find that the observed proglacial water chemistry at the terminus is largely controlled by the hydrochemistry of water in the delayed drainage system. Lastly, we use this model to explore the relationship between the proglacial water chemistry and the daily glacier surface velocities for the period of July, 2013, with an eye to identifying a relationship between water chemistry and short-term glacier dynamics.