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 Vatnajokull glacier, Iceland. The volcanic fissure erupted for some 14 days and it extended between two known subglacial central volcanoes. Most of the melt water drained to the south into the Grimsvotn caldera from where it escaped 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 divide beneath the glacier and into the river basin of Jokulsa a Fjollum, flowing to the north. A few days later, signs of melt water from the volcanic site were detected in the glacial river Jokulsa a Fjollum. Distinct changes in the chemical composition of the water were observed. Both discharge and turbidity of the river were somewhat higher than normal for the season, but there was no extreme flood (jokulhlaup). Total dissolved solids (TDS) and conductivity of the river water, as well as bicarbonate, were found to be higher than previously observed. Traces of sulphide and mercury were detected, which are never recorded at normal conditions. The stable isotope ratios, δD and δ¹⁸O, the ¹⁴C apparent age and the δ¹³C value were also found to be anomalous. In a few weeks the chemistry was back to normal. The chemical changes were most likely caused by a direct flow of melt water from the northernmost part of the main volcanic fissure or from small cauldrons created at the rim of the Bardarbunga caldera. The flow 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 clearly that simultaneous monitoring of chemical changes and flow rate in glacial rivers can deliver valuable data for following subglacial volcanic activity in space and time and may be used to give warning before a major catastrophic flood. Chemical study of glacial rivers can also be used to find out if a jokulhlaup is connected with a simultaneous volcanic eruption or not.