Radiocarbon dating of mid-Holocene megaflood deposits in the Jokulsa a Fjollum, north Iceland

Two megafloods in the canyon of the Jokulsi i Fjóllum, the major northern routeway for glaciovolcanic floods from Vatnajókull, have been closely dated by 14C AMS dates from Betula macrofossils within peat immediately below beds of flood-deposited sand. Ages of c. 4415 and c. 4065 yr BP (5020 and 4610 cal. yr BP) are consistent with the presence of the Hekla 4 tephra (c. 3830 yr BP) resting on the upper surface of the younger flood sand. These sediments are correlated across the Jókulsa a Fjollum canyon with the upper flood sands in a stack recording around 16 flood events. Deposits on both sides of the canyon were trimmed by the last megaflood after the Hekla 3 tephra fall at c. 2900 yr BP, and the highest Holocene flood stages were at the culmination of a series peaking at c. 3500 yr BP. These floods have wider palaeoclimatic significance because they require the formation of large subglacial reservoirs below Vatnajókull. Therefore, the dated floods indicate that a large composite ice cap covered volcanoes in the southeastern highlands through the early and middle Holocene, and that flood routeways largely switched to the south after c. 3500 yr BP.