Jökull - 01.01.2004, Síða 37

Reviewed research article
Sediment thickness and Holocene erosion rates derived from a
seismic survey of Hvítárvatn, central Iceland.
Jessica Black1, Gifford Miller1, Áslaug Geirsdóttir2, William Manley1 and Helgi Björnsson2
1Institute of Arctic and Alpine Research and University of Colorado, Boulder, Colorado 80309, USA
2Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík, Iceland
Jessica.L.Black@colorado.edu; gmiller@colorado.edu; age@hi.is; William.Manley@colorado.edu; hb@raunvis.hi.is
Abstract — More than 100 km of seismic reflection profiles of the sediment fill in the glacier-dominated lake
Hvítárvatn, central Iceland, reveal over 65 m of stratified postglacial sediment in the main depositional basin.
Five diagnostic seismostratigraphic units, defined on the basis of acoustic properties tied to lithostratigraphic
breaks in sediment cores from the lake, can be traced throughout the sediment fill. Isopach maps of these
units exhibit different spatial patterns, suggesting significant changes in the primary sediment delivery systems
throughout the Holocene, and implying significant changes in the size and position of Langjökull. The mass of
sediment in the lake is estimated to be between 35x1013 g and 121x1013 g. Suspended sediment loss from the
lake currently averages 5x1010 g of fine sediment each year. The average Holocene bedrock erosion rate across
the catchment is calculated to be between 2 and 5 cm ka−1, although actual erosion rates under Langjökull
are probably much higher, whereas erosion across the non-glaciated portion of the catchment is presumably
significantly less.
INTRODUCTION
Lake sediments are one of the most reliable sources of
proxy data for paleoclimate reconstructions (Bradley,
1999). Glacial lakes in particular often have high sed-
imentation rates, and their sediment fill may be annu-
ally laminated (varved). Varve thickness commonly
reflects climatic controls, especially through the in-
fluence of summer melting on sediment flux to the
lake. However, glacial lakes can also experience large
sediment gravity flows or glacier advances that may
cause major disturbance of the sediments (Francus et
al., 2002). Seismic surveys provide baseline informa-
tion of the distribution, thickness, and nature of lacus-
trine sedimentation that allow an evaluation of the in-
tegrity of the sediment fill. In basins subject to distur-
bances, such as glacial advances, jökulhlaups, or other
extreme events, they provide essential information re-
quired to design a successful sediment-coring pro-
gram. Modern GPS-based seismic surveys are capa-
ble of providing secure estimates of the total volume
of sediment and allow quantitative reconstruction of
sediment distribution within lake basins. Seismic sur-
veys, combined with lacustrine sediment cores, have
been used to provide constraints on the glacial his-
tory of southern Iceland from Hestvatn (Hardardóttir
et al., 2001) and to study changes in sediment deliv-
ery through the Holocene at Lagarfljót, eastern Ice-
land (Gudjonsson and Desloges, 1995).
Hvítárvatn is a glacier-dominated lake on the east-
ern margin of Langjökull, Iceland’s second largest
ice cap (925 km2; Figure 1). It is ideally positioned
to provide a continuous record of Holocene climate
change as: 1) glacial erosion and soft bedrock re-
sult in high lacustrine sedimentation rates, 2) diag-
nostic tephras of known age aid the geochronology,
3) Iceland’s sensitivity to changes in North Atlantic
circulation is expected to produce clear signals in
key environmental proxies preserved in the lacustrine
JÖKULL No. 54 37