Jökull - 01.01.2015, Blaðsíða 82

Jónsdóttir et al.
Around 1450 CE there are changes in the record.
The estimated bottom water temperature drops down
to ca. 2◦C, with a change in the foraminiferal fauna.
Cibicides lobatulus decreases and C. reniforme be-
comes the dominant species in the fjord. This is also
the time when other proxy records in the fjord start to
become more unstable. The MS record shows fluc-
tuating values and the X-rays higher abundance of
coarse grained material in the core. There is a peak
in the coarse grained material at ca. 1450–1500 CE
which lines up with a drop in the bottom water tem-
perature estimations. At approximately 1500 CE the
bottom water temperature increases again, with a de-
crease in the Arctic species E. excavatum. There is no
coarse grained material detected in the core but the
MS shows fluctuating values.
The interval between ca. 1650–1750 CE (Figure
6) shows defined changes towards harsher conditions
in Arnarfjörður with a drop in the bottom water tem-
perature (temperature changes of 1◦C), TC and the
carbonate values and at the same time the Arctic wa-
ter species indicators E. excavatum, Q. stalkeri and
C. reniforme increase. Elphidium excavatum adapts
easily to a wide range of environmental conditions,
but often blooms during extremely cold and unstable
conditions (Hald et al., 1994). In this interval the bot-
tom water current indicator C. lobatulus has its low-
est values and the diversity of the foraminifera fauna
also reaches a low. Colder conditions in the fjord are
also evident by an increase in both density and MS
values as well as increased amount of coarse grained
material. The coarser grained material found in Arn-
arfjörður after 1200 CE is not believed to have a dis-
tal source. Quantitative XRD analysis of the sedi-
ments in Arnarfjörður found no or little quartz (Fig-
ure 6). Icelandic bedrock consists of basalts with
quartz only found in low abundances, whereas sed-
iment from western Greenland contains quartz (An-
drews and Jennings, 2014). The most likely explana-
tion of those coarse grained material peaks are thus
eroded material from the coast of Arnarfjörður.
Comparison to other areas
Comparing the results from Arnarfjörður to other
proxy records from the same region reveals consid-
erable similarities in climate variability.
The oldest part of Arnarfjörður core indicates the
lowest bottom water temperature during the Dark
Ages cooling period at ca. 450 CE and ca. 750 CE
(Figure 7). That is in good agreement with records
from the Malagen fjord in Norway which show tem-
perature minimum around 400 CE and 600–800 CE
(Hald et al., 2011). The warmest interval in the Arn-
arfjörður record is during the MWP. Proxy-based tem-
perature reconstruction from Haukadalsvatn, north-
west Iceland, for the past 2000 years, characterized
medieval times between ca. 900 and 1200 CE as a rel-
atively warm period with higher productivity than be-
fore or after (Geirsdóttir et al., 2009). The DACP-
MWP boundary is not as evident in the lake record
like in Arnarfjörður (Figure 7), but the onset of colder
conditions is similar, around 1200 CE and two of the
most severe temperature drops in Arnarfjörður during
the LIA coincide with similar drops in the lake record.
The second interval of LIA cooling in the lake record,
between 1450 and 1500 CE, lines up with a drop in the
estimated BWT in Arnarfjörður. More unstable con-
ditions in Arnarfjörður after 1450 CE with estimated
bottom water temperature dropping down to ca. 2◦C
coincides with a major environmental shift detected in
records from sediment cores on each side of the Den-
mark Strait based on weight% of calcite and quartz
(Andrews and Jennings, 2014).
In North Iceland a number of high-resolution stud-
ies have been carried out on core MD99-2275 (Eiríks-
son et al., 2006; Knudsen et al., 2012; Massé et al.,
2008; Sicre et al., 2008), located on the shelf northeast
of Iceland. One of these studies used IP25 biomarker
produced by sea ice loving algae to reconstruct sea
ice over the last millennium (Massé et al., 2008). The
sea ice proxy shows no or little sea ice until ca. 1300
CE (MWP-LIA) when there is a small rise which
lines up with a BWT decrease in Arnarfjörður (Figure
7). There is a second increase in IP25 and decrease
in Arnarfjörður BWT around 1500 CE. The biggest
IP25 peak is around 1600–1800 CE, which coincides
with the coldest time in Arnarfjörður for the time pe-
riod ca. 350–1850 years CE. The sea surface temper-
atures (SSTs) for core MD99-2275 was warmer be-
tween 1000 and 1350 CE, suggested to represent en-
hanced heat transport across the Denmark Strait (Sicre
82 JÖKULL No. 65, 2015