Jökull - 01.01.2015, Page 81

Marine climate variability from Arnarfjörður, NW Iceland
DISCUSSION
Clear similarity is apparent when comparing the trans-
fer function inferred temperature estimations with the
factor scores from the PCA loadings (Figure 5). Axis
1 in the PCA shows cold water species on the negative
side but species that are found in environment with
stronger bottom currents on the positive side. The
PCA factor could reflect a variability in the strength
of the Irminger Current which brings warm waters to-
wards Iceland. The mean annual bottom water tem-
perature in Arnarfjörður today is ∼4.5◦C (Figure 2,
Figure 5). The estimated bottom water temperatures
indicate similar temperature values for the bottom part
of the core and around 1100 CE. The values decrease
towards the top of the core.
Climate variability in Arnarfjörður, ca. 350–1850
years CE: proxies interpretations
The timing of the three zones in the Arnarfjörður core
may be correlated with three known time periods:
Dark Ages Cold Period (DACP), Medieval Warm Pe-
riod (MWP) and the Little Ice Age (LIA).
Zone 1 – The Roman Warm Period (ca. 250 BC to CE
450) and DACP (ca. 450 to 950 CE)
The base of the core has the highest estimated tem-
perature (∼5◦C) with C. lobatulus as the dominant
species. High abundance of C. lobatulus often indi-
cates stronger bottom water currents (Table 2). The
overall trend demonstrates a stepwise decrease in
temperature throughout this interval with increasing
abundance of the Arctic species C. reniforme and
E. excavatum. The MS and density values stay fairly
stable throughout this interval. The X-rays show
fine grained sediment with no coarse grained material
present. The total carbon percentage has a decreas-
ing values upwards, which could suggest a decrease
in bioproductivity indicating increasing influence of
colder currents in Arnarfjörður.
Zone 2 – MWP (ca. 950 to 1200 CE)
For this part of the core the most noticeable
changes in the foraminifera is the abrupt decrease
of E. excavatum. Both C. lobatulus and A. gallowayi
show increasing values at the beginning of this in-
terval and stay fairly stable throughout it. The bot-
tom water temperature derived from the foraminifera
indicates increasing values for this interval with the
highest temperature of ca. 4.5◦C around 1100 CE,
which is similar to the mean annual BWT in Arnar-
fjörður today. At around 1000 CE there is an obvi-
ous change in the environment, with a rise in the MS
and density values and coarser grained material is de-
tected more frequently on the X-radiographs. Usu-
ally, the presence of coarse grained material in sedi-
ment cores is linked to colder conditions. However,
in a fjord environment the coarser grained material
can also represent more eroded terrestrial material
from the drainage area. Trouet et al. (2009) show
that the MWP, also known as the Medieval Climate
Anomaly (MCA), was characterized by the NAO in
its positive mode. During a positive NAO phase the
Northern Europe and Iceland receives more precip-
itation (Hurrel, 1995), the influence of the Irminger
Current is stronger on the North Icelandic Shelf and
the northerly winds decrease. With more precipita-
tion over Iceland the runoff from land increases and
the rivers carry more terrestrial material to the sea. In-
creasing strength of currents could also have produced
stronger tidal waves resulting in more material be-
ing eroded from the coast and that way brought more
material into the fjord. Between 1000 and 1200 CE
the carbonate wt% in Arnarfjörður reaches its lowest
point. The lower values in the core might be reflecting
the increasing stratification of the water column, due
to increase precipitation and runoff, which would, re-
duce the nutrient availability.
Zone 3 – LIA (ca. 1200 to 1900)
The bottom water temperature in this interval shows a
significant decrease (of about 1◦C) from the previous
interval. The Arctic water indicators E. excavatum,
Q. stalkeri and Buccella sp. start to increase around
1200 CE. The time period from 1200–1400 CE in-
dicate cooling but fairly stable conditions with de-
creasing MS values and little coarse grained material.
The carbonate records from Arnarfjörður reveal a
short-term increase, which can be linked to less strati-
fication and more food supply. At the same time there
is a short increase in the diversity of the foraminiferal
fauna.
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