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EVIDENCE OF JöKULHLAUPS AND SEISMIC INTERPRETATION OF HESTVATN, SOUTHERN ICELAND
HANNESDóTTIR, HRAFNHILDUR Dept. of Geosciences, University of Iceland.
Geirsdóttir, Áslaug Dept. of Geosciences, University of Iceland.
Miller, Gifford H INSTAAR & University of Colorado.
In summer 2003 new long (>20 m) sediment cores were obtained from Lake Hestvatn, South Iceland, using the DOSECC GLAD-200 core rig. A total of 4 cores amounting to 60 m of sediment were collected. Hestvatn 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. It is also situated approx. 60 km in front of the most prominent moraines in Iceland dated from deglacial times.
The new cores offer for the first time an accurate dating on the isolation of the Hestvatn basin in early Holocene as this part of the Hestvatn sediment has not been recovered before. 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 melt-water pulses/turbidites characterize the boundary or the lowest part of the lacustrine section, indicating frequent jökulhlaups at that time. We propose that these jökulhlaups mark the establishment of the Hvítá river that drains Hvítárvatn by Langjökull ice cap (see Black et al., this volume). Below the melt-water pulses, we have found a thick basaltic tephra layer, which marks the boundary between the lacustrine and the marine section of the core. A tephra layer with similar affinities is also found in the basal part of the Hvítárvatn sediment (Black et al., this volume). Just below this tephra layer in the Hestvatn sediment core are several mollusk lenses, which will facilitate the 14C dating of the sediment, the tephra layer at the boundary between sections and the isostatic rebound.
We are using a multiproxy approach to reconstruct environmental change by studying physical characteristics of the sediment, lacustrine primary productivity changes, palynology, and marine micropaleontology. The lake core from the southern basin provides an excellent opportunity to capture the earliest phase of deglaciation in the area and >20 m long core from the northern basin, comprising 10-12 m of lacustrine sedimentation provides a continuous high-resolution Holocene record of environmental change (last 11.5 ka). This sediment will provide the first direct evidence of near-shore marine environments around Iceland during the abrupt climate shifts that occurred between 15 and 10 ka.
A detailed study of seismic profiles from the lake and a comparison with the sediment cores, indicate >25 m of sediment fill, with an average sedimentation rate during the Holocene of >1 m/ka. The seismic profiles have been digitized and by using SURFER 8.0 3-d images of the sediment have been obtained. The aim of this work is to reconstruct the sediment flux into the lake basin during specific time slices.
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