Jökull - 01.01.2009, Blaðsíða 71

Deglacial and Holocene sediment distribution in Hestvatn, South Iceland
Table 1. The GLAD4-HST03 cores and their posi-
tion in UTM (Hjörsey datum). – Staðsetning GLAD4-
HST03 kjarnanna í UTM hnitum.
cores UTM N UTM E
GLAD4-HST03-1A 7098130 514226
GLAD4-HST03-1C and 1D 7098122 514204
GLAD4-HST03-2A and 2B 7099341 514063
In 2005, a Reson Seabat 8101 multibeam sonar
system was used to acquire a high resolution bathy-
metric survey on Hestvatn. This system operates at
240 kHz and uses 101 1.5 degree beams to measure
bathymetry. The system’s range resolution is 1.25 cm.
Its lateral resolution is dependent on water depth and
the number of beams retained in the processed data.
The data were acquired on transects spaced at 75 m
intervals ensuring that the survey achieved 100% cov-
erage of the lake floor.
Information on the survey vessel’s motion and po-
sition was obtained from an Applanix POS MV/320
motion sensing and positioning system. This system
measures vessel motion to an accuracy of <0.05 de-
grees and position to <1 m horizontally and <25 cm
vertically. Differential corrections for the positioning
system were sent via a radio link, from a temporary
base station established on the lakeshore.
The speed of sound in water varies with temper-
ature, pressure and salinity. Uncorrected these varia-
tions would produce refraction artifacts in the data. To
correct these, profiles of the speed of sound in the wa-
ter column were collected periodically with a SVPlus
Sound Velocimeter. The collected bathymetry data
were processed using CHARIS HIPS. The process-
ing removes outliers from the data and applies appro-
priate corrections for vessel motion and ray bending
due to sound velocity variations. The processed data
were used to produce a high-resolution bathymetric
map that was gridded at an interval of 2–4% of the
maximum water depth.
RESULTS
Interpretation of seismic survey linked to sediment
core lithostratigraphy
The bathymetric map together with the new multi-
beam bathymetric data reveals two approximately 60
m deep sub-basins, the southern basin and the north-
ern basin (Figure 2). The two sub-basins are con-
nected by a 200 m wide channel at >50 m water
depth, but flanked by two N-S lying elongated ridges
at water depths of less than 10 m and with smaller
N-S lying “drumlin like” ridges varying in height be-
tween 4 and 7 m, occupying the channel floor. The
bathymetry shows steep bedrock walls adjacent to the
two relatively deep and flat sub-basins, and shallow
coves on the southern and northern side of the lake.
The deepest parts of the lake lie approximately 10 m
below present day sea level. The multibeam images
reveal several fan-like structures; in the north as a con-
tinuation of Krákulækur creek; in front of the shallow
cove in the southwest of the lake and into the deep
southern basin, and along the steep bedrock walls on
the eastern side of the lake (Figure 2b).
Locations of seismic reflection lines illustrating
the two sub-basins are shown in Figure 2a. A series
of northwest lying seismic profiles lined up from west
to east within each sub-basin are shown in Figures 3
(north basin) and 4 (south basin). The seismic sur-
vey reveals variable distribution of sediment within
the lake. The two sub-basins act as the main depo-
sitional centers with maximum sediment thickness of
44 m and average thickness of 27±5 m, whereas on
ridges and elsewhere sediment thickness is on aver-
age 5±5 m (Figure 5). Sediment on the sidewalls is
much thinner, in some places absent. Comparison of
the seismostratigraphic units and lithology of the sed-
iment cores allowed identification of three main seis-
mic units (I, II and III), and all but the bottom unit I,
are traceable between the two sub-basins (Figures 3
and 4). The seismic units represent periods of simi-
lar sedimentation environments, but their subdivisions
have no time significance, except for seismic unit III.
Chronological control is poorer for the lower two seis-
mic units in the two sub-basins than for the upper part
of the sedimentary record.
JÖKULL No. 59 71