Impact of Subglacial Geothermal Activity on Meltwater Quality in the Jökulsá Á Sólheimasandi System, Southern Iceland

Abstract

The influence of subglacial geothermal activity on the hydrochemistry of the Jokulsa a Solheimasandi glacial meltwater river, south Iceland, is discussed. A radio echosounding and Global Positioning System survey of south-west Myrdalsjokull, the parent ice-cap of the valley glacier S6lheimajokul1, establishes the geometry and position of a subglacial caldera. A cauldron in the ice-cap surface at the basin head is also defined, signifying one location of geothermally driven ablation processes. Background H2S concentrations for the Jokulsa meltwaters in summer 1989 show that leakage of geothermal fluids into the glacial drainage network takes place throughout the melt season. Chemical geothermometry (Na+/K+ ratio) applied to the bulk meltwaters tentatively suggests that the subglacial geothermal area is a high-temperature field with a reservoir temperature of =289-304"C. A major event of enhanced geothermal fluid injection was also detected. Against a background of an apparently warming geothermal reservoir, the event began on Julian day 205 (24 July) with a burst of subglacial seismic activity. Meltwater hydrochemical perturbations followed on day 209 and peaked on day 213, finally leading to a sudden and significant increase in flow on day 214. The hydrochemical excursions were characterized by strong peaks in meltwater H2S, SO:-and total carbonate concentrations, transient decreases in pH, small increases in Ca2+ and Mg2+ and sustained increases in electrical conductivity. The event may relate to temporary invigoration of the subglacial convective hydrothermal circulation, seismic disturbance of patterns of groundwater flow and geothermal fluid recruitment to the subglacial drainage network, or a cyclic 'sweeping out' of the geothermal zone by the annual wave of descending groundwater. Time lags between seismic events and meltwater electrical conductivity responses suggest mean and maximum intraglacial throughflow velocities of 0.032-0.132 m s-l, respectively, consistent with a distributed drainage system beneath Solheimajokull. Because increases in flow follow hydrochemical perturbations, the potential exists to use meltwater hydrochemistry to forecast geothermally driven flood events in such environments.

Reconstruction of the mean specific mass balance of vatnajökull (iceland) with a seasonal sensitivity characteristic

“…For Sólheimajökull we use the SSC of an outlet glacier of Vatnajökull with similar characteristics, namely Skaftafellsjökull. Mean annual precipitation over the two glaciers is roughly the same: glacial runoff measurements indicate a mean annual precipitation of about 4 to 6 m over Sólheimajökull (Lawler et al 1996), while we find a value of 4 m for Skaftafellsjökull. This correspondence can be explained by the similar proximity to the sea and aspect of the two glaciers (Fig.…”

Seasonal release of anoxic geothermal meltwater from the Katla volcanic system at Sólheimajökull, Iceland

“…The fourth source (oxidation of dissolved hydrogen sulphide, δ 34 S = + 5.8 to + 6.2‰) seems to hold only a weak influence over meltwater sulphate isotopic composition. Despite the fact that glacial meltwaters are previously recorded to contain concentrations of H 2 S up to 3 ppm during the summer season (Sigvaldason, 1963;Lawler et al, 1996), this sulphur source does not seem to dominate the isotopic composition of the sulphate molecule. Optimal conditions promoting the oxidation of aqueous H 2 S require pH 6.8-7.5.…”

“…It supports a maximum ice thickness of 433 m (Mackintosh et al, 2002;Kruger et al, 2010;Russell et al, 2010;Sigurdsson, 2010) and ranges in elevation from 1500 m.a.s.l. down to an over-deepened snout with bed elevation 50 m below sea level (Lawler et al, 1996;Le Heron and Etienne, 2005;Scharrer et al, 2008;Russell et al, 2010). The total area of the glacier is approximately 78 km 2 with the accumulation zone comprising approximately 56% of the total glacier area (Lawler et al, 1996;Roberts et al, 2000;Le Heron and Etienne, 2005).…”

“…down to an over-deepened snout with bed elevation 50 m below sea level (Lawler et al, 1996;Le Heron and Etienne, 2005;Scharrer et al, 2008;Russell et al, 2010). The total area of the glacier is approximately 78 km 2 with the accumulation zone comprising approximately 56% of the total glacier area (Lawler et al, 1996;Roberts et al, 2000;Le Heron and Etienne, 2005). Dynamic advance and retreat cycles are exhibited by the glacier (Schomacker et al, 2012), which are frequently asynchronous with other glaciers along the south coast of Iceland (Dugmore and Sugden, 1991).…”

“…The Sólheimajökull subglacial drainage system conveys volatile species derived from the ice-covered Katla caldera on the Mýrdalsjökull-Katla system, with the frequent smell of hydrogen sulphide emanating from the meltwater streams providing a strong indication of persistent subglacial geothermal activity (Björnsson et al, 2000). Existing research has documented the background chemistry in bulk meltwater outputs from the catchment and tentatively attributed these to variations in the strength of the Katla geothermal field (Sigvaldason, 1963;Lawler et al, 1996). However, the drivers of geothermal activity, and whether any associated chemical signal could be used reliably to understand processes in the subglacial geothermal system at Katla, remain poorly understood.…”