Mid-Holocene jökulhlaups in Jökulsá á Fjöllum, NE-Iceland, correlated to eruptions in Bárðarbunga volcano 6.3 to 4.1 ka ago

Numerous jökulhlaups have rushed down the river Jökulsá á Fjöllum in NE-Iceland during the Holocene. Some of these fall under the category of catastrophic floods that carved out the present-day Jökulsá canyon, over 100 km north of the present-day Vatnajökull.

Volcanic glass in sedimentary beds deposited by 16 jökulhlaups (glacial floods) in river Jökulsá á Fjöllum, between 6.3 and 4.1 ka ago, correlates the jökulhlaups to three volcanic systems beneath Vatnajökull ice cap. Chemical characteristics of Bárðarbunga volcanic system dominate in 12 sedimentary beds, those of Grímsvötn and Kverkfjöll in one bed each, two remain unsolved.

The characteristics of the Bárðarbunga glass in the jökulhlaup sediments are mostly low TiO₂ and high MgO (TiO₂ <1.6, MgO >7.3 w%). Seventeen basaltic “Low-Ti” tephra layers from Bárdarbunga have been identified in soils in N-Iceland from this same period. Grain characteristics of the tephra indicate phreatomagmatic origin. Dispersal maps confirm source area below northwest Vatnajökull and tephra volume (bulk) of the order of 1 km³ for the largest layers.

The mid-Holocene floods confirm the existence of glaciers on Bárðarbunga, Kverkfjöll, and Grímsvötn 6.3 to4.1 ka ago. The magnitude of these jökulhlaups is not well constrained, but apparent cross sections indicate a peak discharge of order 30,000 -100,000 m³/s and likely total volume of some km³. The source areas of these repeated jökulhlaups 6.3 to 4.1 ka ago were most likely the calderas of the central volcanoes, which may have changed in size and form since the mid-Holocene.

Eruptions within the Bárðarbunga caldera are therefore a possible source for 12 of these floods. Bárðarbunga may have hosted a geothermal area and a subglacial caldera lake similar to present day Grímsvötn, which may explain the repeated, apparently similar-magnitude jökulhlaups over this long period.

With recent unrest at the Bárðarbunga volcanic system, including the 2014-2015 Holuhraun eruption with magma drainage and collapse at Bárðarbunga caldera, jökulhlaups in this category must be considered in preparations for future hazards. On its nearly 180 km long course from Vatnajökull to the bay of Axarfjörður, Jökulsá á Fjöllum traverses several habitated and recreational areas. Keeping in mind significantly thicker ice cover at present, potential jökulhlaups larger than the 6.3-4.1 ka floods should also be considered a possibility.