Jökull - 01.01.2015, Blaðsíða 74

Jónsdóttir et al.
Fjords along the Vestfirðir peninsula capture both
the marine and terrestrial climate signal, and thus pro-
vide a direct link between land and sea. Fjords typ-
ically have a higher sediment accumulation rate than
the deeper ocean, and are therefore useful for high-
resolution palaeoclimatic and palaeoceanographic re-
construction. In fjords, foraminiferal assemblages
predominantly consist of benthic species. Whereas,
planktic species have low percentage (Korsun et al.,
1995) due to fresh input of runoff water from land as
well as strong surface currents.
In this paper a marine sediment core from the
fjord Arnarfjörður in NW-Iceland is studied to re-
construct the marine climate and the environmental
history of the fjord for the time period ca. 350–1850
years CE (Common Era). The reconstruction is based
on physical inspection of the sediment core by using
X-radiographs to identify coarse grained material, ev-
ery 1 cm, and applying X-ray diffraction (XRD) to
gain mineralogical information. Magnetic suscepti-
bility (MS), total carbon (TC), and density measure-
ments were carried out as well in order to estimate
bioactivity and terrestrial input. However, the em-
phasis was on the benthic foraminiferal faunal study
where down-core distributional patterns were used to
describe the oceanographic settings and identify envi-
ronmental changes in the study area.
REGIONAL SETTING AND
OCEANOGRAPHY
Arnarfjörður is the second largest fjord in the Vest-
firðir peninsula, 30 km long and 5–10 km wide, lo-
cated between the fjords of Dýrafjörður in the north
and Tálknafjörður in the south (Figure 1b). Towards
its head it divides into two main tributaries; Borgar-
fjörður and Suðurfirðir separated by Langanes (Figure
1c). The area east of Arnarfjörður, called Gláma, rises
900 m above sea level. Freshwater input to the fjord
comes from numerous rivers along the coastline.
In 2001 and 2002 Marine Research Institute
(MRI) mapped the seabed of Arnarfjörður on the re-
search vessel Árni Friðriksson (www.hafro.is). The
bathymetric map shows the fjord to be a typical
threshold fjord, with the mouth of the fjord being
much shallower than the head (Figure 1c). The av-
erage depth of Arnarfjörður is 90–100 m. Multiple
morainal ridges were identified within the fjord, re-
flecting the advances/retreat of the Icelandic ice sheet
during the last deglaciation in the area. The ridges
indicate a cross-cutting relationship between outlet
glaciers from Borgarfjörður and Suðurfirðir as well as
a couple of morainal ridges towards the head of Borg-
arfjörður.
In June 2005 and October 2006 two temperature
and salinity profiles were made across Arnarfjörður,
one of them very close to the core location (Figure
2). The temperature profile shows bottom water in
October at ca. 5.5◦C and around 7.5◦C closer to the
surface. The salinity in the fjord is around 34.5‰
at the bottom but decreases upwards to about 34.4‰.
The profile for June shows summer stratification in the
fjord system, with a 40 m thick surface layer of 7 to
8◦C and salinity of 34.35 to 34.45‰ and bottom wa-
ters of 3◦C and salinity of 34.5‰. This stratification
of fresher surface water is typical in fjords (Inall and
Gillibrand, 2010). The stratification is caused by the
density differences in the early spring coastal current,
running in a clockwise direction around Iceland, and
the existing cold fjord water. In September or Octo-
ber the surface water temperatures cool sufficiently to
become denser than the underlying water masses and
sinks to the bottom and the water column in the fjord
mixes. Both oxygenated and nutrient-rich waters are
then brought to the ocean floor leading to increased
productivity (Stefánsson and Ólafsson, 1991).
Land-fast ice is fairly common in Icelandic fjords
such as Arnarfjörður where sufficient input of fresh-
water exists. The freshwater floats on top due to den-
sity difference and when it is cold enough it freezes.
When the land-fast ice breaks up it drifts out the fjord
with the currents and eventually melts.
MATERIALS AND METHODS
Core A2010-10-586 (65◦45.25’N; 23◦44.67’W) was
obtained in August 2010 on the research vessel Árni
Friðriksson RE 200. The coring equipment consisted
of Benthos Model 2175 piston corer. The sediment
core is 520 cm long, collected from 98 m depth from
the middle part of the fjord (Figure 1c). The core was
74 JÖKULL No. 65, 2015