The impact of the 1996 subglacial volcanic eruption in Vatnajokull on the river Jokulsa a Fjollum, North Iceland

A subglacial volcanic eruption took place in October 1996 beneath the Vatna jokull glacier, Iceland. The volcanic fissure erupted for some 14 days and it extended between two known subglacial central volcanoes. Most of the mel t water drained to the south into the Grimsvotn caldera from where it escap ed a month later during a major jokulhlaup (extreme flood) into the glacial rivers flowing to the south from Vatnajokull. At the start of the eruption , the northernmost part of the volcanic fissure extended across the water d ivide beneath the glacier and into the river basin of Jokulsa a Fjollum, fl owing to the north. A few days later, signs of melt water from the volcanic site were detected in the glacial river Jokulsa a Fjallum. Distinct change s in the chemical composition of the water were observed. Both discharge an d turbidity of the river were somewhat higher than normal for the season, b ut there was no extreme flood (jokulhlaup). Total dissolved solids (TDS) an d conductivity of the river water, as well as bicarbonate, were found to be higher than previously observed. Traces of sulphide and mercury were detec ted, which are never recorded at normal conditions. The stable isotope rati os, delta D and delta(18)O, the C-14 apparent age and the delta(13)C value were also found to be anomalous. In a few weeks the chemistry was back to n ormal. The chemical changes were most likely caused by a direct flow of mel t water from the northernmost part of the main volcanic fissure or from sma ll cauldrons created at the rim of the Bardarbunga caldera. The now of melt water to the north from the volcanic centre ebbed soon after or even just before the volcanic eruption in Vatnajokull ended. This experience shows cl early that simultaneous monitoring of chemical changes and flow rate in gla cial rivers can deliver valuable data for following subglacial volcanic act ivity in space and time and may be used to give warning before a major cata strophic flood. Chemical study of glacial rivers can also be used to find o ut if a jokulhlaup is connected with a simultaneous volcanic eruption or no t. (C) 1999 Elsevier Science B.V. All rights reserved.