Jökull - 01.01.2015, Blaðsíða 77

Marine climate variability from Arnarfjörður, NW Iceland
temperature estimates (D. Klitgaard-Kristensen, per-
sonal communication 2008). The average sample spe-
cific standard error for the temperature estimates is
±0.7◦C.
Physical proxies
The magnetic susceptibility and density was measured
every 0.5 cm using a Geotek Multi-Sensor Core Log-
ger (MSCL) at the Institute of Earth Sciences, Univer-
sity of Iceland. These physical properties normally in-
dicate the proportion of minerogenic to total biogenic
material in the core. The archive half of the core was
radiographed in Orkuhúsið on a Tosrad radiographic
system. The radiographs were mainly used for identi-
fying shells in the core for the radiocarbon dating and
detection of coarse grained material by counting the
number of clasts >2 mm (Grobe, 1987).
The X-ray diffraction measurements were carried
out at INSTAAR, University of Colorado, Boulder,
on 20 mg samples from a 1 cm slice every 10 cm.
The weight% of quartz has been inferred as a measure
of drift ice transport (Andrews and Jennings, 2014)
because Icelandic sediment is rich in basaltic grains
and low in quartz or other lithics (Andrews and Prin-
cipato, 2002). As carbonate bedrock is not present
in Iceland, the carbonate content of the sediment is
an integrated measure of marine productivity, reflect-
ing changes in carbonate-secreting marine organisms
(Andrews et al., 2001). Therefore the weight% of cal-
cite is a measure of surface water stratification and
nutrient supply (Andrews and Jennings, 2014). Sam-
ples for total carbon analysis were measured with a
carbon Analyser by UIC (CM5014) CO2 Coulometer,
at the University of Iceland. Approximately 20 mg
bulk material was measured every 5–8 cm or total of
75 samples. The total carbon is often used as a mea-
sure of marine productivity.
RESULTS
Age model
The chronology in core A2010-10-586 is based on
four AMS radiocarbon dates and one crypto-tephra
layer (Figure 3, Table 1) with the geochemical compo-
sition of the Katla volcano. A Katla tephra deposited
∼700 CE has been found in marine sediments north
of Iceland (Kristjánsdóttir et al., 2007; Guðmunds-
dóttir et al., 2012) and was included in the age model.
The sediment accumulation rate was calculated by lin-
ear interpolation between the dated levels. The lower
part of the core has the lowest sedimentation rate of
2.5 mm/yr, increasing to 4.0 mm/yr between ca. 700
and 1000 CE and decreasing to 3.2–3.9 mm/yr to-
wards the top (Figure 3), resulting in a time span from
ca. 350 CE to 1850 CE.
Figure 3. Age-depth model for core A2010-10-586,
based on four 14C AMS datings. The depth of the
Katla tephra (red triangle) was incorporated into the
age model. – Aldursmódel fyrir kjarna A2010-10-586,
byggt á fjórum kolefnisaldursgreiningum og einu
gjóskulagi (rauður þríhyrningur).
Foraminifera
A total of 37 benthic foraminifera species were iden-
tified, including one agglutinated species. The most
dominant species from the core are listed in Table 2.
The three most dominant species were Cibicides lo-
batulus, Cassidulina reniforme and Elphidium exca-
vatum (Figure 4). The core was divided into three
units based on a cluster analysis from the foraminifera
results.
JÖKULL No. 65, 2015 77