Jökull - 01.01.2015, Side 84

Jónsdóttir et al.
and not strong enough to explain the rather abrupt
fluctuations that began in the 13th century CE and
the multi-century climate variations of the past 2000
years. Decadal scale fluctuations have been linked
with solar activity and volcanic eruptions, or induced
by internal variability. The longer-term fluctuations
were additionally driven by changes in ocean circu-
lation (Broecker, 2000; Denton and Broecker, 2008).
Changes in climate variability over the North At-
lantic region have been explained for example with
fluctuations of Atlantic Water in the northern North
Atlantic (Hald et al., 2011), shutdown of deep wa-
ter convection, sustained by sea ice formation, in the
Nordic Seas in response to negative solar insolation
anomalies (Berner et al., 2011), repeated explosive
volcanism with centuries of lower-than-modern solar
irradiance (Miller et al., 2012), changes in solar ac-
tivity (Jiang et al., 2015) and shifts in the NAO from
positive mode to negative (Mann et al., 2009; Trouet
et al., 2009). Miller et al. (2012) also suggested that
repeated explosive volcanism alongside low summer
insolation across the Northern Hemisphere acted as a
climate trigger resulting in increasing sea ice expan-
sion and a sea-ice/ocean feedback then maintained
suppressed summer air temperature, long after the
volcanic aerosols were removed from the atmosphere.
Changes in the foraminifera fauna in Arnarfjörður
indicate a clear sign of changes in the oceanic cur-
rents around Iceland, with intervals more influenced
from the warmer Irminger Current to times with more
influence from colder currents. Changes in the NAO
also seem to play a role in the environment in Arn-
arfjörður, with warmer and wetter conditions during
the MWP compared to the LIA. A comparison of
the BWT record from Arnarfjörður and other proxy
records from Iceland with both reconstructed solar
irradiation (Steinhilber et al., 2009) and indices of
volcanic activities (Sigl et al., 2015) is shown in Fig-
ure 7. Cold intervals in the Arnarfjörður record fall
within both low solar irradiance and increased vol-
canic activity between 1200 and 1300 CE, and again
between 1450 and 1550 CE, when both sea ice north
of Iceland increased as well as when a drop in tem-
perature from the lake records occurred. The coldest
time period in Arnarfjörður took place between 1650
and 1750 CE, during the time of the Maunder min-
ima (1645–1715 CE) when the volcanic indices also
indicate several large volcanic eruptions (Figure 7).
It may thus be difficult to disentangle the different
forcing mechanism during this interval but the Arnar-
fjörður record is showing very similar environmental
and temperature changes as has been recorded in and
around Iceland during the last 2000 years.
CONCLUSIONS
The proxy records from Arnarfjörður show several
environmental changes occurring over the time pe-
riod ca. 350–1850 years CE. Cibicides lobatulus is
the most dominant species at the bottom part of
the core but the Arctic species C. reniforme and
E. excavatum increase upwards. The core was divided
into three time periods based on Cluster Analyses on
the foraminiferal assemblages. The oldest part of the
core (350 to 800 CE) shows a cooling but fairly sta-
ble condition (DACP). Between 800 and 1200 CE
(MWP) the bottom water temperature increases and
many of the Arctic species diminish. The MWP was
wetter and warmer, under the influence of positive
NAO and coarser grained material was carried out to
the fjord, possibly due to higher precipitation, higher
discharge from land to sea and stronger currents in the
fjord eroding the coastline of Arnarfjörður. Around
1100 CE the bottom water temperature is ca. 4.5◦C,
which is similar to the mean annual BWT in Arn-
arfjörður today. The MWP-LIA transition occurred
around 1200 CE with a significant change in the fau-
nal assemblage when Arctic species increases, indi-
cating colder conditions in the fjord. The proxies in
the core from Arnarfjörður suggest that the LIA was
very unstable and shifted from colder to less colder
times. From ca. 1200 to 1400 CE the climate was
cooling, but after 1400 CE the conditions started to
get harsher and there was a clear change in the fau-
nal assemblage with C. reniforme and E. excavatum
becoming the dominant species. The coldest time pe-
riod in Arnarfjörður was between 1650 and 1750 CE
(note: the core is missing the upper most 150 yr) when
there was a clear drop in BWT, TC and carbonate and
increase in density, MS and coarse grained material.
The climate and environmental variability in Arnar-
84 JÖKULL No. 65, 2015