Jökull - 01.01.2004, Page 44

Jessica Black et al.
Figure 5. A 1.25 km long seismic profile across Hvítárvatn (see Figure 2 for location), showing the regular fill
east of the Bedrock Ridge, the condensed, but correlative fill on the ridge, the continuation of the main seismic
units west of the ridge and their truncation by the LIA moraine of Norðurjökull. Within the zone of LIA Scour,
west of the moraine, the sediment fill has been eroded and disturbed. Seismic Unit boundaries discussed in
the text are indicated. – Endurvarpslína, 1.25 km að lengd, sem sýnir helstu seteiningarnar í Hvítárvatni, sjá
staðsetningu á 2. mynd. Greina má röskun í setlögunum sem varð til við framrás Norðurjökuls á Litlu ísöldinni.
on their geometry, these flows originated off the ter-
minal moraines of the two outlet glaciers during the
LIA when the glaciers were at their maximum exten-
sion into the lake (Figure 6). Acoustic signatures typ-
ical of ponded sediments are not seen in deeper layers
in the Central Deep, or in any other part of Hvítárvatn.
The sediment fill in the lake thickens (>50 m
thick) toward the Prograding Delta front, indicating
that the delta is actively advancing (Figure 7). The
sediment thickness and small gravity flows identified
in the seismic profiles suggest a much larger lake at
deglaciation, with the shoreline migrating westward
as the delta prograded throughout the Holocene.
The sequence of seismic reflectors on the Bedrock
Ridge contains the same characteristic acoustic fea-
tures as are present in the adjacent basin (Figure 5)
and across the survey area; the only difference is that
the thickness is about half that in the adjacent basin
(12 to 15 m vs. 25 to 30 m). From the differences
in the thickness of the sediment blanket, and its po-
sition as a topographic high in the lake basin, we
conclude that sediment on the Bedrock Ridge is de-
posited dominantly by suspension settling. The height
of the Bedrock Ridge gives protection to sediments
deposited on it from sediment gravity flows originat-
ing from the nearby LIA terminal moraine, and from
the unstable slip face of the prograding delta. We con-
clude that the Bedrock Ridge is an optimal site for
sediment cores designed to evaluate laminae thickness
as possible climate proxies.
Within the last 100 years both Norður- and Suður-
jökull were calving into Hvítárvatn, producing a large
flux of icebergs to the lake. The largest icebergs
scoured the sediment fill in shallow portions of the
lake. An iceberg scour mark is visible on the seismic
line across the relatively shallow Bedrock Ridge (Fig-
44 JÖKULL No. 54