Jökull - 01.01.2004, Side 50

Jessica Black et al.
Figure 10. Images illustrating the range of tephras found in sediment cores from Hvítárvatn. a: multiple, thin,
fine-grained basaltic tephras (dark), b: thick, coarse-grained, rhyolitic tephra, presumably of Hekla origin, and
c: thick, basaltic tephra. Individual tephra lenses range from a few mm to more than 7 cm in thickness. –
Breytileiki gjósku sem fundist hefur í setkjörnunum úr Hvítárvatni.
Within the Central Deep, acoustic profiles suggest
that several large sediment gravity flows ponded in
the main basin, presumably associated with the Little
Ice Age advance of Suðurjökull into the central basin
and the development of an unstable terminal moraine
(Figure 6). To test this interpretation, we positioned
sediment core 02HVT-06 to intercept a thin lens of
ponded sediment interbedded within normal stratified
deposits (Figure 6). The 5.5 m long core consists
of two finely laminated sections that bracket a 2.2 m
thick unit of massive to heavily disturbed sediment,
consistent with a sediment gravity flow. The mas-
sive unit represents a discrete event that interrupted
the regular deposition of laminated sediment, and sup-
ports the interpretation of the seismic data for the Cen-
tral Deep.
DISCUSSION
Evolution of sediment sources during the Holocene
The physical characteristics of the sediment record
preserved in Hvítárvatn reflect the balance between
glacial and fluvial erosion and transportation pro-
cesses. The lake currently carries a high concentration
of suspended sediment (ca. 30 to 40 mg L−1), domi-
nated by fine-grained glacier flour (fine silt and clay),
some of which settles out in winter, when the lake
is ice-covered. Whenever outlet glaciers are calving
into the lake, sedimentation is dominated by glacier
50 JÖKULL No. 54