Solute generation and transfer from a chemically reactive alpine glacial-proglacial system

Abstract

The environs of the Glacier de Tsanfleuron, Switzerland, was used as a study site to investigate the controls on the relative efficiency of solute generation and removal from glacial and proglacial environments. Here, a 1500 m wide glacier forefield consists of a karstic limestone plateau flanked to the north by a till-floored valley. Bedrocks and glacial debris are composed of chemically reactive pure and impure Mesozoic and Tertiary limestones with accessory pyrite. Spot sampling of ice, snow and meltwaters in the late melt season was supplemented by systematic measurements of the main meltstream, including during periods of rainfall, and simple laboratory leaching and weathering experiments. Isotopic parameters were used to investigate water sources. Most meltwater and glacier ice samples lay close to a meteoric water line (δ D= 8·3 δ18O + 14) defined by waters from small tributary streams. Heavy isotopic excursions of bulk meltwater chemistry were caused by rainfall events, recovering within days to a δ18O baseline around −12 permil. No regular diurnal variations in δ18 O were apparent. The atmosphere is the source of Cl− and most Na+, but the bulk of other solutes are generated in the environment. Ion loads of up to 1 meq l−1 are rapidly attained by calcite dissolution. Over periods of weeks to months pyrite oxidation generates sulphate and acidity that drives further calcite dissolution. Low water–rock ratio weathering environments have characteristically high SO42−, Mg2+, Sr2+, and ratios of these species to calcium. The characteristic cation ratios are influenced by non-congruent calcite dissolution. The ratio of sulphate to other species is highest where water–rock contact times are highest, although this relationship is complicated by spatial variations in pyrite abundance. Meltstream time series illustrate that 90 per cent of daily ion yields in fine weather are concentrated in the 12 h time period of higher discharge. Ion yields increase downstream mainly by a combination of dissolution of calcareous suspended sediment and input from tributaries and seepage from the till banks. Rainstorms lead to increased solute concentrations and resulting hourly fluxes can match daily fine weather ion fluxes. Excess nitrate appears to be largely sourced from the proglacial surface. The capacity of the proglacial environment for yielding significant subsurface water as a result of the storm seems low, unlike non-glacial environments. This implies that most of the excess solutes mobilized by storms comes from subglacial sources. Increased efficiency of yield of solutes from low water/rock ratio subglacial weathering environments persists after the isotopic signature of the rainfall event has died away. A simple conceptual model of the sources of water and solutes agrees with conclusions from attempts at hydrograph separation, that mixing of water reservoirs of fixed solute composition cannot be used for quantitative descriptions of the system. Estimated annual solute yields (17 ton per km2 per m precipitation) are high, but cannot be readily expressed in purely areal terms because of likely significant losses to the underlying karstic system. A tentative conclusion is that the proglacial environment is overall less efficient at producing solutes than the glacial environment, but more information is required on processes in the early melt season to substantiate this statement. Copyright © 1999 John Wiley & Sons, Ltd.