Jökull - 01.01.2004, Page 52

Jessica Black et al.
Unit 1. In contrast to Unit 1, the semi-transparent
Unit 2 implies more homogenous sediment, without
episodic sedimentation. This thin unit could reflect
either a shorter time period or a considerably lower
sedimentation rate than in either underlying or over-
lying units. Activation of only a small portion of the
Hvítárnes Delta implies a relatively limited discharge
to the lake (Figure 11). The absence of strong reflec-
tions implies that no thick tephra was deposited dur-
ing this interval and there were no large pulses of sed-
iment.
The series of fine parallel reflections in Unit 3 are
interpreted to represent increasingly episodic delivery
of sediment to the lake. Fróðá, the primary sediment
source for this unit, is currently a glacial stream orig-
inating from a northeastern lobe of Langjökull. The
large volume of sediment deposited in Unit 3 implies
this section of Langjökull was more active than other
regions of the ice cap during this interval (Figure 11);
possibly the ice cap was only covering the northern
portions of its modern perimeter. The strong reflec-
tors interspersed with weaker reflections are corre-
lated with thick tephra deposits found in the sediment
cores.
Unit 4 reflects the most recent conditions in
Hvítárvatn, an interval of rapid sedimentation under
a glacier-dominated sediment erosion and transporta-
tion regime. Sedimentation has been highly variable
in the different depocenters, and is strongly influ-
enced by sediment gravity flows and scour and re-
deposition of lake sediment by active glacial erosion
within the lake basin. The thick sediment deposits
in the Central Deep are likely a reflection of active
erosion by Norðurjökull and Suðurjökull. Sediment
gravity flows in the Central Deep, interpreted from the
seismic records and confirmed in sediment cores, sup-
port the interpretation of direct sediment erosion and
transport by glacial processes. In contrast to condi-
tions during deposition of Unit 2, the entire length of
the Hvítárnes Delta was actively prograding (Figure
11), an interpretation suggested by the seismic pro-
files and confirmed in sediment cores. The delta is
a massive feature, and requires a significant sediment
load and fluvial discharge to support a wide prograd-
ing margin.
Erosion rates
The seismic survey and derived volumetric estimates
of the total sediment fill in Hvítárvatn allow a first-
order estimate of average erosion rates across the
catchment during the Holocene. Sediment volumes
and corresponding sediment mass were calculated for
each of the seismostratigraphic units, and summed to
estimate the total volume and mass of sediment de-
posited in the lake within our seismic survey area. The
largest volume of sediment was deposited in Unit 1,
followed by Units 3 and 4 (Table 1, Figure 12). The
total volume of sediment deposited in the seismic sur-
vey portion of Hvítárvatn since deglaciation ∼10 ka
BP is 32.1x107 m3 with a mass of 54.6 x1013 g (Ta-
ble 1).
Figure 12. Calculated sediment volumes for seismic
units 1 through 4 within the surveyed regions of
Hvítárvatn. – Reiknað rúmmál sets í einingum 1 til
4 í Hvítárvatni.
Our seismic survey only covered about one third
of the lake. Outside the survey region, lake sed-
iments were either too disturbed to reliably survey
(LIA Scour) or too thin and intermittent (Southern
Shallows) to provide continuity to the seismic records.
To calculate the total volume of sediment in Hvítár-
vatn we estimated the sediment volume outside the
survey region. Based on the few seismic lines that
cross the LIA Scour zone, we estimate an average
sediment fill of 30 m remains in this region. For the
Southern Shallows we estimate an average sediment
thickness of 10 m. The estimated volume and mass
of sediment in all of the lake basins is summarized in
Table 1.
52 JÖKULL No. 54