CATASTROPHIC EVENTS DURING THE DEGLACIATION AND EARLY HOLOCENE OF ICELAND

Evidence of ice dammed lakes in central Iceland and catastrophic floods towards southern Iceland during the last deglaciation are found in long sediment cores from two large lakes. The lakes, Hestvatn and Hvítárvatn, lie in the same large catchment. Hestvatn in the southern lowlands, is a low-elevation lake that meets the necessary requirements to answer questions on the deglaciation and the early Holocene environmental change in Iceland. It is strategically situated below the highest marine limit in the area and contains both a lower marine and an upper lacustrine phase, providing continuous time-series of key proxies. Hvítárvatn, an extant glacier-dominated lake at the base of Langjökull, a large ice cap in Central Iceland, records evidence for Holocene fluctuations of the Langjökull outlet glaciers. A series of strandlines more than 100 m above Hvitarvatn define higher lake levels and suggest a sequence of catastrophic discharge of the glacial lake during the deglaciation.

The new lake cores offer for the first time an accurate dating on the isolation of the Hestvatn basin in early Holocene. The boundary between the marine and the lacustrine sedimentation in Hestvatn is apparent from both the seismic record and visual inspection of the new cores. Repeated turbidites characterize the boundary and the lowest part of the lacustrine section. Accurate dates on these catastrophic floods are now available and pinpoint these events between 9000 and 10000 14C yrs. Furthermore, a thick basaltic tephra with Saksunarvatn tephra affinities lies between two turbidites within the lower lacustrine levels. A tephra layer with similar geochemistry is also found in the basal part of the Hvitarvatn sediment. We propose that the melt-water pulses found at the boundary between marine and lacustrine sediments in Hestvatn document catastrophic floods that drained Hvítárvatn and mark the establishment of the Hvítá river and Gullfoss gorge.

With the new 14C dates from this part of the Hestvatn core, we have not only been able to identify the isolation of Hestvatn but possibly also the onset of glacier retreat from the Hvitarvatn basin during deglacial times and early Holocene.