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

The environs of the Glacier de Tsanfleuron, Switzerland, was used as a stud y site to investigate the controls on the relative efficiency of solute gen eration 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 compos ed of chemically reactive pure and impure Mesozoic and Tertiary limestones with accessory pyrite. Spot sampling of ice, snow and meltwaters in the lat e melt season was supplemented by systematic measurements of the main melts tream, including during periods of rainfall, and simple laboratory leaching and weathering experiments. Isotopic parameters were used to investigate water sources. Mast meltwater and glacier ice samples lay close to a meteoric water line (delta D = 8.3 d elta(18)O + 14) defined by waters from small tributary streams. Heavy isoto pic excursions of bulk meltwater chemistry were caused by rainfall events, recovering within days to a delta(18)O baseline around -12 permil. No regul ar diurnal variations in delta(18)O were apparent. The atmosphere is the source of Cl- and most Na+, but the bulk of other sol utes are generated in the environment. Ion loads of up to 1 meq l(-1) are r apidly attained by calcite dissolution. Over periods of weeks to months pyr ite oxidation generates sulphate and acidity that drives further calcite di ssolution. Low water-rock ratio weathering environments have characteristic ally high SO42-, Mg2+, Sr2+, and ratios of these species to calcium. The ch aracteristic cation ratios are influenced by non-congruent calcite dissolut ion. The ratio of sulphate to other species is highest where water-rock con tact times are highest, although this relationship is complicated by spatia l variations in pyrite abundance. Meltstream time series illustrate that 90 per cent of daily ion yields in f ine weather are concentrated in the 12 h time period of higher discharge. I on yields increase downstream mainly by a combination of dissolution of cal careous suspended sediment and input from tributaries and seepage from the till banks. Rainstorms lead to increased solute concentrations and resultin g hourly fluxes can match daily fine weather ion fluxes. Excess nitrate app ears to be largely sourced from the proglacial surface. The capacity of the proglacial environment for yielding significant subsurface water as a resu lt of the storm seems low, unlike non-glacial environments. This implies th at most of the excess solutes mobilized by storms comes from subglacial sou rces. Increased efficiency of yield of solutes from low water/rock ratio su bglacial weathering environments persists after the isotopic signature of t he rainfall event has died away. A simple conceptual model of the sources of water and solutes agrees with c onclusions from attempts at hydrograph separation, that mixing of water res ervoirs of fixed solute composition cannot be used for quantitative descrip tions of the system. Estimated annual solute yields (17 ton per km(2) per m precipitation) are high, but cannot be readily expressed in purely areal t erms 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 infor mation is required on processes in the early melt season to substantiate th is statement. Copyright (C) 1999 John Wiley & Sons, Ltd.