Jökull - 01.01.2009, Side 77

Deglacial and Holocene sediment distribution in Hestvatn, South Iceland
foraminifera and molluscs from the 1995 Hestvatn
cores, with the age of the Vedde Ash, and confirmed
a reservoir age of 400 years at this location; low ! 13C
values of the foraminifera and molluscs indicate that
the site was greatly influenced by freshwater during
the Younger Dryas chronozone.
The uppermost part of the record is based on di-
agnostic tephra layers correlated between sediment
cores from 3 lakes in Iceland (Table 2; Jóhanns-
dóttir, 2007). Comparison of the humic acid dis-
solved organic carbon (HA-DOC) 14C dates with
the tephrochronology of core HST03-2A reveals that
most radiocarbon dates are too old, and they are there-
fore not considered for chronological control (Hann-
esdóttir, 2006).
Seismic unit I is only found in the north basin and
has acoustically chaotic appearance with variable re-
lief and irregular, faint or no internal reflectors (Fig-
ure 3). Its thickness varies from 1 to a maximum of
10 m and forms in places discontinuous hummocks
as seen in line N3 (Figure 3). Core HST03-2A sam-
pled the top of this seismic unit, revealing an over-
consolidated diamicton with silty matrix and pebbles.
Difficulties were experienced penetrating these sedi-
ments during coring.
Seismic unit II (both a and b) is 16 m thick on av-
erage, with a maximum thickness of >25m and is con-
siderably thicker in the south basin than in the north-
ern basin (Figures 3 and 4 and Table 3). Seismic sub-
unit IIa forms the basal seismic unit in the south basin
(Figure 4) and shows strong multiple reflectors, which
grade into layers of strong internal reflectors interbed-
ded with more chaotic layers in sub-unit IIb. This sub-
unit IIa fills in the uneven basal topography of the un-
derlying seismic unit I in the north basin, especially in
seismic lines N1, N3 and N6, thereby smoothing out
the basin sediment surface. Cores HST03-1A and 2A
did not capture any of the sediment forming sub-unit
IIa. Compared to the lithology of the sediment cores,
some of the stronger reflectors within seismic sub-unit
IIb correspond to sandy-gravelly lenses (colored blue
in Figure 3), whereas more chaotic layers represent
gray clayey sediments with little or no structure, but
ice rafted debris and shell fragments (Hannesdóttir,
2006). The boundary between seismic sub-units IIb
and IIIa is channelized and reflects an erosional sur-
face.
Seismic sub-unit IIIa is acoustically more dis-
tinctly stratified than the underlying seismic unit II. It
can be traced throughout both basins although it has
variable thickness. It is over 4 m thick at core site
HST03-2A in the north basin (Table 3), whereas at
core site HST03-1A in the south basin it is only about
1 m thick. This unit is thickest towards the southern
part of seismic lines in the north basin, and in the mid-
dle part of seismic lines in the south basin (Figures 3
and 4). A comparison with the lithology of the two
cores (1A and 2A) shows multiple graded sequences
interbedded with finely laminated sediments.
Table 3. Thickness and volume of seismic units
from the two sub-basins. Sedimentation rate for the
various seismic units is calculated as far back as
the chronological control reaches from the sediment
cores. Since seismic unit I is only found scattered in
the north basin, its thickness is included in thickness
and volume numbers of seismic unit II. – Þykkt
og rúmmál seteininga úr báðum dældum vatnsins.
Setmyndunarhraði ólíkra eininga er reiknaður
eins langt aftur og aldursgreiningar leyfa. Þar eð
eining I er einungis óreglulega dreifð í norðurdæld
vatnsins er hún reiknuð sem hluti af einingu II hér.
Mean Volume Sed. rate
thickness (km3x10!3) cm/yr
(m) mean max
North basin (0.52 km2)
seismic unit IIIb 7.6±0.7 10.1 4.0 0.08
seismic unit IIIa 3.1±0.7 4.9 1.6 0.37
seismic unit II (a+b) 15.5±3.1 25.0 8.1 1.45
South basin (0.28 km2)
seismic unit IIIb 6.4±1.2 9.1 1.8 0.06
seismic unit IIIa 2.1±0.6 3.6 0.6 0.21
seismic unit II (a+b) 19.2±5.9 34.7 5.4 0.45
The uppermost seismic sub-unit IIIb displays in
most seismic profiles numerous regular, internal hori-
zontal reflectors, typically more reflective than the un-
derlying units, which compared to the core lithofacies
represent abundant tephra layers and finely laminated,
organic rich sediment. This unit is on average 11 m
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