Jökull - 01.01.2009, Side 67

Reviewed research article
Deglacial and Holocene sediment distribution in Hestvatn,
South Iceland, derived from a seismic and multibeam survey
Hrafnhildur Hannesdóttir1, Áslaug Geirsdóttir1, Gifford H. Miller2,
William Manley2 and Nigel Wattrus3
1Institute of Earth Sciences and Department of Earth Sciences, University of Iceland,
Askja, Sturlugata 7, 101 Reykjavík, Iceland
2Institute of Arctic and Alpine Research and Department of Geological Sciences,
University of Colorado, 1560 30th Street, Boulder, CO 80303
3Large Lakes Observatory and Department of Geological Sciences,
University of Minnesota, Duluth, Duluth, MN 55812, USA
hrafnha@hi.is
Abstract— More than 100 km of seismic reflection profiles together with a new multibeam survey of the sedi-
ment fill in lake Hestvatn, South Iceland, reveal two sub-basins filled with up to 44 m of deglacial and Holocene
sediment. The chronology is constrained by geochemically characterized tephra layers and radiocarbon dates
on marine molluscs. The Vedde Ash and Saksunarvatn Ash provide key chronological control for the deglacial
sediment. Five seismic units are tied to the lithostratigraphy of sediment cores obtained from the two basins.
The seismic units reveal major changes from glacial, to glacial-marine, to lacustrine sedimentary environments.
Isopach maps of the seismic units document changes in the primary sediment depocenters, sedimentation rates
and sediment delivery through time. The Vedde Ash, found only in glacial-marine sediment in the southern
basin, suggests that during the Younger Dryas the northern basin of Hestvatn was occupied by an outlet glacier
from the main Icelandic Ice Sheet that calved into a marine embayment. A high-resolution multibeam bathy-
metric survey reveals multiple shallow ridges between the two sub-basins of Hestvatn, which presumably acted
as a pinning point for the calving glacier. During and after the retreat of the glacier and isolation of the lake
basin, lacustrine sediments were mainly deposited via northern and northwesterly inlets, resulting in higher
sedimentation rates in the north basin compared to the south basin. Early in the lacustrine phase repeated tur-
bidites were deposited, interpreted to originate in glacial outburst floods (jökulhlaups) following a retreating
ice-margin in central Iceland.
INTRODUCTION
Most studies of the deglaciation of Iceland reveal a
dynamic Iceland Ice Sheet that responded rapidly to
changes in solar radiation, ocean currents, and sea
level. Seismic surveys and sediment core studies of
the Iceland Shelf and glacial geological mapping on
land show evidence for a rapid but step-wise deglacia-
tion, associated with changes in relative sea level
(Syvitski et al., 1999; Andrews et al., 2000; Jennings
et al., 2000; Eiriksson et al., 2000; Geirsdóttir et al.,
2002; Andrews and Helgadóttir, 2003; Geirsdóttir et
al., 2009, Ingólfsson and Norðdahl, 1994, 2001). Be-
cause isostatic recovery exceeded sea level rise dur-
ing deglaciation (Norðdahl and Pétursson, 2005) most
JÖKULL No. 59 67