Jökull - 01.01.2004, Blaðsíða 55

Sediment thickness and erosion rates within Hvítárvatn, central Iceland
bly derived from erosion beneath Langjökull. The ice
cap occupies approximately 35% of the Hvítárvatn
catchment. If we assume that ice cap erosion was
much greater after the onset of Neoglaciation, ca. 5
ka, bedrock erosion rates might have been as high as
10 to 30 cm ka−1 beneath the ice cap during the late
Holocene. ăIf most of the erosion occurred during the
Little Ice Age, erosion rates could have been as high
as 1 m ka−1 beneath the ice cap, and correspondingly
less across the rest of the catchment (<1 cm ka−1).
Development of a secure geochronology for sediment
cores recovered from Hvítárvatn will allow a more
reliable estimate of erosion rates at specific intervals
within the Holocene.
CONCLUSIONS
A seismic reflection survey of the glacial lake Hvítár-
vatn provides essential information from which op-
timal sites to recover sediment cores for paleoenvi-
ronmental analyses can be identified. The survey
also provides a framework with which to evaluate the
changing influence of different sediment delivery sys-
tems to the lake since regional deglaciation. Much of
the sediment fill in Hvítárvatn was severely disturbed
when two outlet glaciers advanced into the lake dur-
ing the Little Ice Age. Disturbance includes sediment
gravity flows, glacial tectonism and unstable moraine
slopes. However, large areas distal to the IA moraines
remained undisturbed and are suitable for sediment
coring. Seismostratigraphic units identified in the pro-
files reflect different patterns of sediment accumula-
tion in the lake basin through the Holocene. Isopach
maps reveal changes in the dominance of sediment
sources to the lake through the Holocene, implying
a shifting ice volume in Langjökull.
Interpretation of seismic reflection data from
Hvítárvatn demonstrates that up to 65 m of sediment
fill the lake. By integrating the seismic data with GIS
technology, the total sediment fill in the lake is es-
timated to be 0.33 km3. Combining the total sedi-
ment fill, with estimates of the sediment lost from the
lake through suspended load in the outflow stream,
Hvítá, measured directly for the past 25 years, al-
lows a maximum estimate of the total mass of sedi-
ment delivered to the lake by its catchment since re-
gional deglaciation (121 x 1013 g). From these fig-
ures, the average Holocene erosion rate across the
Hvítárvatn catchment is calculated to be between 2
to 5 cm ka−1. Average erosion rates for the Hvítár-
vatn catchment are lower than rates derived from stud-
ies of erosion by glaciers in Iceland (3 to 4 mm a−1
[Boulton, 1974; Boulton et al., 1982]; 1 to 3 mm a−1
[Tómasson, 1991]. However, the Hvítárvatn catch-
ment values are similar to denudation rates calculated
from marine sediment cores (Andrews 2004). Av-
erage catchment-wide Holocene erosion rates almost
certainly underestimate late Holocene erosion rates
beneath Langjökull, which may have been as high as
0.5 to 1 m ka−1 during some intervals.
Acknowledgements
The seismic survey of Hvítárvatn was supported by
grants from RANNIS, the Icelandic Research Coun-
cil, and the University of Iceland Research Funds to
ÁG and GHM. We are indebted to Kjartan Thors for
his careful work on the seismic profiles and enthusi-
astic support of this project and Sædís Ólafsdóttir for
assistance with the seismic profiling campaign. Sedi-
ment coring in Hvítárvatn during February and March
2002 was also supported by a RANNIS grant to ÁG
and GHM. We thank Þorsteinn Jónsson and Svein-
björn Steinþórsson for their assistance with the cor-
ing operation and their creative logistical support dur-
ing the coring program. The U.S. National Science
Foundation awards OPP-0138010 and ATM-0318479
to GHM and ÁG, and a Grant of Excellence from
RANNIS to ÁG and GHM supported the data reduc-
tion of the seismic profiles.
ÁGRIP
Rof og setmyndun á nútíma í ljósi endurvarpsmæl-
inga á seti Hvítárvatns
Sumarið 2001 voru gerðar endurvarpsmælingar á
setlögum Hvítárvatns við Langjökul. Heildarlengd
mældra sniða er yfir 100 km. Þessi snið gefa mynd af
setfyllu Hvítárvatns sem er mest um 65 m þykk, lög-
un undirliggjandi berggrunns og lagskiptingu í setinu.
Samhliða þessum mælingum voru teknir setkjarnar úr
seti vatnsins. Með samanburði á niðurstöðum endur-
varpsmælinga og setásýnda í setkjörnum er unnt að
JÖKULL No. 54 55