A late Holocene jökulhlaup, Markarfljót, Iceland: nature and impacts

Abstract

— Approximately 1200 years ago (c. 1230 14 C yr. BP) a mature birch wood in southern Iceland was devastated by inundation in a catastrophic jökulhlaup flowing westwards from Katla. This flood was the youngest prehistoric flood in a series of Holocene jökulhlaups that covered the Markarfljót-Landeyjar area of southern Iceland. Sedimentological analysis presented here indicates the flood was associated with an eruption of the volcano Katla, similar to historical jökulhlaups along the south coast of Iceland. A new radiocarbon date, tephrochronological studies and observation of the remaining tree stumps combined with interpretation of previously published data shows that the trees were around 60–100 years old when inundated and that much of the lowlands of Landeyjar were probably similarly vegetated at this time. Deposits and scoured bedrock in similar stratigraphic locations upstream may relate to passage of the flow along the main Markarfljót valley from northwest Mýrdalsjökull. From a hazard perspective it is critical to establish that such medium-scale events have occurred in the past and their nature and impact, particularly since they are generally censored from older Holocene sedimentary records by later geomorphic activity.