Jökull - 01.01.2004, Blaðsíða 51

Sediment thickness and erosion rates within Hvítárvatn, central Iceland
Figure 11. Sediment delivery paths for seismic units 1 through 4 derived from the isopach maps of the sediment
thicknesses (Figure 9a-d). – Upprunasvæði og leiðir sets í einingum 1 til 4 í Hvítárvatni, sjá einnig 9. mynd.
flour intermixed with coarser IRD. When Langjökull
is present but outlet glaciers do not reach the lake,
glacier flour is delivered to the lake by meltwater
streams, but the lacustrine sediment would be devoid
of IRD. If Langjökull were absent at some time in the
past, sedimentation would have been dominated by
fluvial erosion and delivery processes, which provide
fewer fine-grained materials and lower sedimentation
rates than under ice-erosional conditions.
The isopach maps of Units 1–4 (Figure 9) demon-
strate a shift of the major sediment pathways to
Hvítárvatn, and by inference changes in the size of
Langjökull. Differences in the spatial distribution of
sediment fill reflect changes in the sources of ero-
sion and the transport of the eroded sediment. These
changes are presumably controlled by the position and
volume of ice in the Hvítárvatn catchment.
The first influx of sediment to the lake basin began
about 10 ka (Unit 1) when the Icelandic Ice Cap re-
ceded from the lake basin and terminated at its south-
eastern margin. It is probable that the ice cap released
large pulses of sediment into Hvítárvatn that are rep-
resented by frequent strong reflectors in the seismic
profiles (Figure 8). Unit 1 sediment filled in the un-
even topography in the deglacial lake basin.
Hvítárvatn must have experienced a fundamental
shift in conditions governing the erosion and trans-
portation of sediment to the lake after deposition of
JÖKULL No. 54 51