Jökull - 01.01.2009, Side 84

Hannesdóttir et al.
ure 9). This provides a more detailed picture of the
structure of the Younger Dryas glacier in South Ice-
land than previously shown (Geirsdóttir et al., 1997,
2000; Harðardóttir et al., 2001a; Norðdahl and Péturs-
son, 2005) (Figure 9).
The North Atlantic region experienced a series
of abrupt climatic changes during the Pleistocene-
Holocene transition (e.g. Bradley et al., 2002). The
two most prominent being the Younger Dryas dated
to 12.9–11.7 ka (Rasmussen et al., 2006) in the GRIP
ice core record and the Preboreal Oscillation be-
ginning at 11.5 ka (Rasmussen et al., 2007). The
temperature oscillations have been related to vari-
able strength of the thermohaline circulation of the
North Atlantic, influenced by increased freshwater in-
put (e.g. Mercer, 1969; Broecker et al., 1989; Koc
Karpuz and Jansen, 1992; Björck et al., 1996; Clark
et al., 2001; Broecker, 2003). Former ice-marginal
lakes are known from both sides of the North Atlantic
and outbursts of various freshwater sources have been
suggested to cause the Younger Dryas and Preboreal
cooling (e.g. Broecker et al., 1989; Keigwin et al.,
1991; Sarnthein et al., 1995; Hald and Hagen, 1998;
Teller, 2002; Jennings et al., 2006).
The jökulhlaups entered Hestvatn during a 600
year period between 10.6 and 10.0 ka, which is a few
hundred years after the termination of the Pre-Boreal
Oscillation. Did jökulhlaups flow into the paleobay
of the southern lowlands before that time? Jökulhlaup
activity during deglaciation of South Iceland has been
reported from a number of sites (Geirsdóttir et al.,
1997, 2000; Jennings et al., 2000). Lacasse et al.
(1996) find turbidites in marine sediment cores on the
south Iceland shelf, which they assign to jökulhlaup
activity following volcanic or glacial events occurring
in southern Iceland during the last two glaciations and
the early Holocene. As mentioned before, the preser-
vation potential in a marine setting is not as good as
in the lacustrine environment due to several factors.
Jökulhlaups do not form underflows as easily in salty
water, bioturbation results in homogeneous mud, and
jökulhlaup deposits are hard to distinguish from sed-
iments deposited in front of a calving glacier as was
the case in the south basin of Hestvatn. We can there-
fore not rule out the possibility that jökulhlaups en-
tered the Hestvatn site prior to 10.6 ka BP, although
they are not distinguished in the marine sedimentary
record. However, our record in the Hestvatn basin
suggests repeated jökulhlaups during the retreat of the
Iceland ice cap from the central highlands with major
routes towards south. The volume of the jökulhlaups
originating north of Hestvatn probably was too small
to cause significant changes in the thermohaline cir-
culation of the North Atlantic. Due to the proximity
to the formation site of North Atlantic Deep Water,
deglacial jökulhlaups in Iceland might have had a lo-
cal impact on deep-water formation. However, their
volume compared with meltwater released from e.g.
Lake Agassiz during deglaciation (e.g. Teller et al.,
2002; Clarke et al., 2004) is minimal. The turbidite
record of the Hestvatn cores provides us with a more
detailed picture of the deglacial environment in the
southern lowlands of Iceland.
CONCLUSION
The new sediment cores from Hestvatn, re-evaluation
of seismic profiles and a multibeam survey provide
new insight to the deglaciation of the southern low-
lands of Iceland. Interpretation of more than 100 km
of seismic reflection profiles of bottom sediments in
lake Hestvatn, South Iceland, reveals two sub-basins
filled with up to 44 m of deglacial and Holocene sed-
iments. Together with sediment cores retrieved from
both basins, a major change in sedimentary environ-
ments from glacial marine to lacustrine sedimentation
is observed. Implications for Younger Dryas glacier
extent are derived from the surveys and sediment
cores, suggesting that during deglaciation the northern
basin was occupied by an outlet glacier whereas the
southern basin accumulated glacial marine sediments.
Glacial retreat is observed in the marine record, fol-
lowed by isostatic rebound that lead to isolation of the
lake basin around 10.6 ka. This provides important
information on relative sea level change and glacial
rebound. Erosional surfaces are seen at the boundary
of marine and lacustrine sediments, on top of which
sequence of turbidites are deposited, thought to reflect
episodic sedimentation, related to jökulhlaups during
the deglaciation.
84 JÖKULL No. 59