Jökull


Jökull - 01.12.1983, Page 79

Jökull - 01.12.1983, Page 79
maritime precipitation, the chemistry of which is controlled by the amount of ocean spray and the elevation of condensation. The relative contribution of the two sources can be determined if the ion input from the surface rocks and the averaged precipitation composition for the drainage basin can be estimated. To account qualitatively for normal Skeidará discharge an abrasion experiment was performed on basalt of a composition similar to that of the suspended load in the river during its 1972 burst (Table 2). Prelimin- ary results from that experiment indicate that the low Si02 of normal Skeidará discharge (No. 1, Table 3) is reached quite early in the process. The hydrolized ions increase at almost constant ratios during the abrasion until the point ofsaturation for opaline silica is reached at approx. 100 ppm Si02; at lower silica values the silica/alkali ratio of the water can be regarded as constant. The variable composition of precipitation from diíTerent elevation is not well known but the gener- ally accepted compositional trend shows the oceanic ratios of Cl/Na, Na/Ca, and C1/S04 to decrease drastically with elevation because HCl remains preferentially to sulphate in the gas phase. The composition of precipitation in the Skeidará drainage basin is unknown as yet but supposedly it ranges between typical coastal- and high- altitude chemistry. The sodium concentration in the abrasion experiment reached about 4 ppm'at 11.2 ppm SiO0. Normal Skeidará water, containing 11.1 ppm Na, derives accordingly about 40% of its sodium from abrasion. The very low chloride content of the river (3 ppm) indicates that high- altitude precipitation dominates the drainage basin which also is evident from the relatively high sulphate. According to the experiment 47% of the sulphate in Skeidará is dissolved from the rocks, which leaves the computed precipitation contrib- ution with lowCl/Na and C1/S04 ratios. The con- clusion is that the normal Skeidará water derives about half its sodium and sulphate from the precipit- ation, which is mostly ice of high-altitude origin. It is, however, to be noted that thc calculated average sodium of the precipitation feeding the Skeidará drainage basin is four times higher than that mea- sured for the Bárdarbunga ice core (Sci. Inst., unpubl. results). The purpose of the foregoing discussion is to justify the subtraction of solute concentrations in normal Skeidará water to reconstruct the chemistry of the Grímsvötn reservoir from the analyzed jökul- hlaup water. It is suggested here that even if the jökulhlaups from Grímsvötn are immense in their proportions the dissolved load of the water due to abrasion of the sediment load remains approxi- mately the same, and that the observed increase represents an innate characteristic of the ílood water. Therefore the chemical analyses of the hlaup water can be corrected for the ”normal“ component by simple subtraction. SOLUTE CHEMISTRY OF THE JÖKULHLAUPS 1972 AND 1982 A detailed sampling program of Skeidará was undertaken prior to and during the 1972 burst. The first signs of chemical change appeared as a slight but significant increase in chloride and pH value in February 1972, but a more rapid increase in sodium and carbonate on March 2 marks the beginning of the jökulhlaup proper a day after the initiation of hydrogen sulfide odours in the SE lowlands (Thor- arinsson 1974). These chemical indications were apparent a week before detectable increase in the river discharge. The water rapidly assumed a sodium rich (ca. 200 ppm) basic (pH 10) compos- ition with high carbonateand sulphate (No.2, Table 3). This water type made up only a few percent of the burst and was succeeded after March 18 by the burst water proper (No 3, Table 3) the composition of which remained remarkably stable till the end of March, constituting about 95% of the flood volume. Following the course of events to the end the last signs of water in Skeidará disappeared in the last week of April. The main mass of the burst undoubt- edly has a strong geothermal component and in the following we attempt to define its chemical source and to delineate the processes at work producing the two distinct water types of the burst. Fig. 2 shows the 1972-jökulhlaup in terms ofdis- charge per time unit (Rist 1973), with the three water bodies under discussion indicated. The slow initial mixing of sodic carbonate water with the normal Skeidará (Fig. 3) is interesting in two re- spects: firstly, the Iow Mg- and Ca-contents of the discharge indicate that the dissolved load due to abrasion during a 600-fold increase in discharge remains unchanged — otherwise the low alkaline earth composition innate to the sodic water would have been swamped by products of abrasion (compare Nos. 1 and 2, Table 3). Secondly, there are striking chemical indicadons in this water, JÖKULL 33. ÁR 77
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