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

• Published in: The Holocene
• Main Authors: Kirkbride, Martin P., Dugmore, Andrew J., Brazier, Vanessa
• Format: Article in Journal/Newspaper
• Language: English
• Published: SAGE Publications 2006
• Subjects: Paleontology; Earth-Surface Processes; Ecology; Archeology; Global and Planetary Change; Hekla; Ice cap; Iceland
• Online Access: http://dx.doi.org/10.1191/0959683606hl956rr; http://journals.sagepub.com/doi/pdf/10.1191/0959683606hl956rr

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.