Jökull - 01.01.2001, Blaðsíða 3

Jökulsárlón at Breiðamerkursandur
JÖKULSÁRLÓN: DATA AND
DEVELOPMENT
Breiðamerkurjökull retreated during the 20th century
because the ice flux did not compensate for ice loss
at the terminus by melting and calving into Jökulsár-
lón (Figures 2 and 3). This recession has been docu-
mented by several authors (Kjartansson, 1957; Price,
1969, 1971, 1982; Howarth and Price, 1969; Boulton
et al., 1982, 1989; Víkingsson, 1991; Jóhannesson,
1994, 1995; and Björnsson and Eydal, 1998). Aerial
photographs from different dates are available after
1945 (Figure 4) and satellite images from 1973 and
1978 (Björnsson et al., 1999). The glacier thickness
and the topography of the subglacial depression were
mapped by radio echo sounding in 1991 (Björnsson
et al., 1992; Björnsson, 1996, 1998) and a bathymet-
ric survey of Jökulsárlón was undertaken by Boulton
et al. (1982). The eastern part of Breiðamerkurjökull
extends below sea level 25 km upstream from the lake
(Figure 2c).
Since its maximum extension in the 1890s the ter-
minus of Breiðamerkurjökull has retreated 4 km and
areas covered by 200 m thick ice a century ago are
now free of ice (Figures 2–4). In 1934 the lake Jökul-
sárlón emerged at the glacier terminus and expanded
slowly. In the early 1950s the retreat rate of the calv-
ing front increased suddenly to 150 m yr
(Figure 5).
It remained constant at that values until the late 1960s,
after which it slowed to 30 m yr
on average for the
next 20 years. During the 1990s the retreat rate in-
creased again to 200 m yr
on average, resulting in
an expansion of the lake by 0.5 km

yr
.
Volume changes of the calving terminus of
Breiðamerkurjökull can be separated into contribu-
tions from downglacier ice flow, surface melting and
calving. The calving flux (in units m

s
) at the
glacier front can be described as
 

    
(1)
where


is the ice flow through a vertical cross-
section into a terminus column (at a defined distance
upstream from the calving front),
 
denotes the sur-
face mass balance over the column,

is the vol-
ume change due to changes in the ice thickness and

is the change due to the retreat at the front. In
1997 to 1999 a project was carried out to quantify
the various mass balance components of the glacier
terminus. The ice flux toward Jökulsárlón (


) was
estimated from measurements of glacier surface ve-
locity (  ) at stakes located about 1500 m above the
calving front (Figure 6), and along a central flow-
line toward the lake (Figure 2a). The ice thickness
along the cross section was obtained from Björnsson
et al. (1992). The mean surface velocity through the
cross section (of area 
) was calculated from the ve-
locity distribution, and the mean velocity of the ice
(  ) through the cross section was assumed to be
equal to 90% of the surface velocity (see Nye, 1965).
The ice was transported through a cross section of

! #"%$'&)(*$  m  with an average velocity of 
= 258 m yr
(see later discussion on calving rate),
thus the influx of ice was


=
",+.-&)(*$,/
m

yr
. The
main inflow of ice to Jökulsárlón is through a narrow
tongue considerably smaller than the entire cross sec-
tion used in our calculations (Figure 4). The surface
area ( 0 ) downstream of the cross section is 4.4 km 
over which the mass balance was b = -10 m yr

water-equivalent or -11 m yr
of ice. The surface
mass balance obtained from mass balance measure-
ments was thus
 1324& 0 576#89& (*$,/ m  yr
.
The surface thinning rate :4;
<%:,= (deduced from ex-
isting maps and DGPS measurements) was -5 m yr
, = 0 & :4;
<%:,= = >","?&( $#/ m  yr
. From aerial pho-
tographs the retreat of the glacier front was found to
comprise

=
>@,$?&( $#/
m

yr
of ice. Hence, we
calculate a calving flux of
AB",C,$?&(*$,/
m

yr

through a cross–section of area   D6,6E-&( $  m  ,
yielding a calving rate of 582 m yr
. These values
for the period 1997 to 1999 illustrate typical condi-
tions at the terminus after the lake was formed.
ICE FLOW OF
BREIÐAMERKURJÖKULL AND
CALVING INTO JÖKULSÁRLÓN
Field observations on Breiðamerkurjökull were used
to tune theoretical models of ice flow and to obtain an
empirical relationship for the calving rate at Jökulsár-
lón.
JÖKULL No. 50 3