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Quaternary Science Reviews 197 (2018) 246-256 


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An Icelandic terrestrial record of North Atlantic cooling c. 8800-8100 
cal. yr BP 

Check for 

Sigrun Dogg Eddudottir • , Egill Erlendsson , Gudrun Gisladottir a> b 

a Institute of Life and Environmental Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland 
b Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland 


Article history: 

Received 4 May 2018 
Received in revised form 
6 July 2018 
Accepted 9 July 2018 
Available online 25 August 2018 


Climate change 
Vegetation dynamics 

8.2 ka event 
North Atlantic 
Lake sediment 

1. Introduction 

Iceland‘s location in the North Atlantic makes the terrestrial 
environment of the island sensitive to changes in atmospheric and 
oceanic systems in the region. The terrestrial ecosystem is partic¬ 
ularly responsive to changes in ocean currents since the island is 
located at the junction of the relatively warm Irminger Current and 
the colder East Greenland Current (e.g. Stefansson, 1962; Hansen 
and 0sterhus, 2000). Palaeoenvironmental reconstructions from 
Icelandic lake sediments have revealed centennial and millennial 
scale environmental responses to climate change during the Ho¬ 
locene (Hallsdottir, 1995; Rundgren, 1998; Hallsdottir and 
Caseldine, 2005; Geirsdottir et al., 2009, 2013; Larsen et al., 2011, 
2012; Striberger et al., 2012; Blair et al., 2015; Eddudottir et al., 
2015, 2016; Schomacker et al., 2016). A prominent anomaly in 
some Icelandic palaeoenvironmental reconstructions is a cold 
period between c. 8700-7900 cal. yr BP, interrupting a trend of 
warming climate during the early Holocene (Geirsdottir et al., 2009, 

* Corresponding author. 

E-mail address: (S.D. Eddudottir). 

0277-3791 /© 2018 Elsevier Ltd. All rights reserved. 

2013; Larsen et al., 2012; Eddudottir et al., 2015). The cooling during 
this period is evident from proxies sensitive to changes in spring 
and summer temperatures and is reflected in changes in both 
terrestrial vegetation (Eddudottir et al., 2015) and lake productivity 
(Larsen et al., 2012). The cooling observed in Iceland falls within the 
period c. 9000-8000 cal. yr BP, which is characterised by cold 
conditions in the Northern Hemisphere (Mayewski et al., 2004), 
and coincides with a cold anomaly beginning c. 8600 cal.yr BP 
recorded around the North Atlantic (cf. Rohling and Palike, 2005). 
This longer cold period is distinct from the cold anomalies related 
to the 8.2 ka cold event (e.g. Alley et al., 1997; Baldini et al., 2002; 
Alley and Agustsdottir, 2005; Rohling and Palike, 2005; Bamberg 
et al., 2010), which has been linked to the flux of meltwater at 
the final drainage of the glacial lakes Agassiz and Ojibway into the 
North Atlantic c. 8430 ± 300 cal. yr BP (Barber et al., 1999). 

The timing and nature of terrestrial ecosystem changes in Ice¬ 
land can highlight the nature of centennial scale climate fluctua¬ 
tions in the North Atlantic and provide information to complement 
marine records. The aim of this study was to create a detailed re¬ 
cord of environmental changes in northwestern Iceland in response 
to early Holocene climate change. Here we present a palynological 
reconstruction of the period c. 10,100-7000 cal.yr BP. To establish 
whether changes in environmental stability occurred during this 
period the multi-decadal resolution palynological dataset is sup¬ 
plemented by plant macrofossils and measurements of organic and 
physical properties of the sediment, including organic matter (OM), 
total carbon (TC), total nitrogen (TN), carbon-to-nitrogen ratio (C/ 
N), stable isotope ratios of carbon (5 13 C) and nitrogen (6 15 N), 
magnetic susceptibility (MS) and dry bulk density (DBD). 

2. Regional setting 

The site chosen for this study is Kagadarholl palaeolake (KAGA; 
65°35' 16" N, 20°07' 58" W, 114 m a.s.l.) located in northwest Ice¬ 
land, about 10 km south of the town of Blonduos (Fig. 1). The 
palaeolake is today a fen and is about 0.45 km 2 in area, with a 
drainage basin of about 0.6 km 2 . The vegetation is dominated by 
sedges (Cyperaceae spp.), especially Eriophorum angustifolium 
(common cotton grass), the dwarf shrubs Betula nana (dwarf birch), 
Salix phylicifolia (tea-leaved willow) and Salix lanata (woolly 

S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 





65°50 , 0 ,, N- 




▲ Weather st 
★ Coring 
— 1000 
m 400 







Fig. 1 . The Kagadarholl study site, a) Map showing the location of the coring site in northwest Iceland, b) map of Iceland showing the location of Kagadarholl and, c) overview of the 

willow), and the herb Cardamine nymanii (lady's smock). The 
overgrown lake is surrounded by Betula nana -dominated dwarf- 
shrub heath, hayfields and eroded, gravelly hills. The closest 
weather station is Blonduos (Table 1 ), which is located by the coast 
and may therefore represent a more maritime climate than 
Kagadarholl. For more detailed site information see (Eddudottir 
et al., 2015, 2017). 

3. Material and methods 

Sediment cores were retrieved from the centre of the palaeolake 
using a Livingstone piston corer and a Bolivia adaptor fitted with 75 
mm diameter polycarbonate tubes. A series of overlapping cores 
was used to construct a continuous Holocene sequence with 
0 cm at the top of the core (see Eddudottir et al., 2015). The period c. 
10,100-7000 cal. yr BP is covered by three overlapping core seg¬ 
ments connected by changes in magnetic susceptibility (MS) and 
tephra geochemistry (Fig. SI). A description of sediment charac¬ 
teristics according to the Troels-Smith system (Aaby and Berglund, 
1986) is presented in Table SI. 

The pollen dataset consists of 46 samples, covering the period c. 
10,100-7000 cal. yr BP, comprising 31 samples previously analysed 

as part of a dataset published in Eddudottir et al. (2015) and 15 new 
samples. Subsamples for pollen analysis (2 cm 3 ) were collected at 
2-8 cm intervals. Pollen samples were prepared using 10% HC1,10% 
NaOH, acetolysis (Faegri et al., 1989; Moore et al., 1991 ) and dense- 
media separation (Bjorck et al., 1978; Nakagawa et al., 1998) using 
LST Fastfloat (a sodium heteropolytungstate solution, density 1.9 
gem -3 ). A tablet containing spores of Lycopodium clavatum 
(Stockmarr, 1971; batch no. 177745) was added to each sample to 
calculate pollen accumulation rates (PARs; grains cm -2 year -1 ). 
Samples were mounted on glass microscope slides in silicone oil of 
12,500 cSt viscosity. A minimum of 300 indigenous terrestrial 
pollen grains were counted for each sample. Identification of pollen 
grains and spores was based on Moore et al. (1991) and a pollen 
type slide collection at the University of Iceland. Pollen sums and 
categories followed Hallsdottir (1987) and Caseldine et al. (2006). 
Pollen and spore taxonomy followed Bennett (2016) and amend¬ 
ments specific to the Icelandic flora by Erlendsson (2007). The 
sediments were analysed for plant macrofossils at contiguous 5 cm 
intervals and sample volume varied between 35 and 50 mL (for 
further detail see Eddudottir et al. (2015)). Pollen and macrofossil 
diagrams were constructed using TILIA version 1.7.16 (Grimm, 
2011 ). 

Table 1 

Observations from the weather station Blonduos. Unpublished data from the Icelandic Met Office. 

Elevation 8 m a.s.l. 

Period 1961-1990 

Mean July temperature 9.4 ° C 

Mean January temperature -2.5 °C 

Mean tritherm temperature (mean June, July, August temperature) 8.7 °C 

Mean annual precipitation 458 mm 


S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 

Fig. 2. Age-depth model for the Kagadarholl core. 

DBD and OM were measured at 1 cm contiguous intervals. OM 
was measured by loss on ignition (LOI) by combustion for 5 h at 
550 °C in a muffle furnace (Bengtsson and Enell, 1986) and DBD was 
calculated by dividing the dry weight of a sample with the volume 
of the wet sample (Brady and Weil, 1996). Magnetic susceptibility 
(MS) measurements were performed using a Bartington MS2 meter 
and Bartington MS2F probe (Dearing, 1994) at 1 cm intervals on 
split core segments. 

The core was subsampled for measurements of total carbon 
(TC), total nitrogen (TN) and 5 13 C and 5 15 N stable isotope ratios of 
bulk sediments at the same depths as the pollen samples, where 
possible. The samples were dried at 50 °C, homogenized using a 
ball mill and sieved through a 150 pm mesh. A total of 46 samples 
were analysed. The carbon, nitrogen and isotope measurements 
were performed on a Thermo Delta V isotope ratio mass spec¬ 
trometer (IRMS) interfaced to a NC2500 elemental analyser at the 
Cornell University Isotope Laboratory (COIL). 

The chronology for the Kagadarholl core (Fig. 2) was constructed 
using a combination of tephrochronology and radiocarbon dates 
(Eddudottir et al., 2015). The age-depth model for the core (Fig. 2) is 
based on the previously dated tephra layers Hekla 6 (6060 cal. yr 
BP; Gudmundsdottir et al., 2011), Katla S (6600 cal.yr BP; 
Eddudottir et al., 2015), Hekla 5 (7050 cal.yr BP; Thorarinsson, 
1971) and Saksunarvatn (10,300 cal.yr BP; Rasmussen et al., 
2006) and radiocarbon dated macrofossils (Table 2). A smooth 
spline age-depth model (Fig. 2) was constructed in R using the 
package Clam (Blaauw, 2010). Ages are given in calibrated years 
before present (cal. yr BP). According to the 95% confidence of the 
age model the age uncertainty is ±100-120 yrs for most of the 
record; lowest c. 8600-7400 cal.yr BP (<± 100 yr) and highest 

(±120-130 yrs) c. 10,100-9700 cal. yr BP. 

Ordination analyses were performed on the pollen dataset to 
detect patterns in terrestrial vegetation development in the vicinity 
of the lake. Detrended correspondence analysis (DCA) was initially 
used. As the first axis length of 1.3527 suggests a linear response in 
the dataset, a principal component analysis (PCA) was preferred. 
The PCA was performed on a Hellinger transformed dataset that 
included terrestrial pollen taxa with abundances >1%. PCA analysis 
was also performed on a standardised dataset of %TC, %TN, %OM, 
C/N, MS, DBD, 5 13 C and 5 15 N along with the most important pollen 
groups and taxa ( Betula , Juniperus communis , shrubs, herbs and 
aquatics) to determine the relationships between vegetation and 
sediment properties. This analysis includes 42 samples analysed for 
both pollen and organic and physical properties. All ordination 
analyses were made in R using the package vegan (Oksanen et al., 

4. Results 

4.1. Pollen assemblage zones (PAZs) 

The pollen record covers the period c. 10,100-7000 cal.yr BP 
and is divided into five pollen assemblage zones (PAZs) based on 
CONISS performed in TILIA (Fig. 3). 

PAZ 1 (10,100-9600 cal. yr BP) 

Juniperus communis pollen is -15—32% of total land pollen (TLP), 
Betula is -14-32% of TLP and Cyperaceae is -16-24% of TLP. Other 
important pollen types are Empetrum nigrum (-1-10% of TLP), Salix 
(-3-11% of TLP), Galium (-0.3-6% of TLP) and Thalictrum alpinum 
(-3-9% of TLP) (Fig. 3). The Juniperus communis PAR increases to 
>600 grains cm -2 yr -1 c. 9800 cal. yr BP (Fig. 4). Mean Betula pollen 
diameters are -17 ± 1 pm within this PAZ (Fig. 5). Potamogeton 
natans- type pollen (-18-82% of total pollen and spores) is the most 
important aquatic pollen taxon (Fig. 3). Selaginella selaginoides 
megaspores and Potamogeton natans seeds along with Luzula and 
Carex nigra seeds are the most common plant macrofossils recorded 
within this PAZ (Fig. 6). Non-triporate Betula pollen range between 
2 and 9% (Fig. 4). 

PAZ 2 (9600-9200 cal. yr BP) 

Juniperus communis pollen increases to -38-58% of TLP and 
Cyperaceae decreases to -4-13% of TLP. Betula ranges between 14 
and 30% of TLP. Galium and Thalictrum alpinum pollen decrease 
within this PAZ, along with Empetrum nigrum and Potamogeton 
natans- type pollen. Iso'etes echinospora spores appear c. 9500 cal. yr 
BP (Fig. 3). The relative abundance of Juniperus communis pollen 
increases after c. 9600 cal. yr BP and the PAR is -1300-3200 grains 
cnrT 2 yr -1 between c. 9600 and 9200 cal. yr BP. This period delivers 
the highest TLP PAR in the record (-3200-6400 grains cm -2 yr -1 ), 
as well as high TLP concentration (-26,000-50,000 grains cm -3 ; 
Fig. 4). Mean Betula pollen diameters are -18 ± 1 pm within this PAZ 
(Fig. 5). Betula nana seeds and leaves and Selaginella selaginoides 
megaspores are the most common plant macrofossils recorded. 

Table 2 

Radiocarbon dates from the Lake Kagadarholl sediment. 

Lab code 

Depth (cm) 

14 C date ± la 

5 13 C 

Calibrated age range 




7003 ±36 



Potamogeton leaf fragments 






Empetrum nigrum and Betula pubescens various items 



8209 ±38 



Potamogeton leaf fragments 



8695 ±38 



Potamogeton fruits and leaf fragments 



8997 ±39 



Potamogeton fruits and leaf fragments 

S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 


Fig. 3. Pollen percentage diagram for the Kagadarholl core. 

Betula pubescens macro fossils appear at the top of this PAZ (Fig. 6). 
Non-triporate Betula pollen range between 2 and 5% (Fig. 4). 

PAZ 3 (9200-8800 cal.yr BP) 

Juniperus communis pollen decreases within this PAZ to 
-22-35% of TLP and Betula pollen increases to -34-49% of TLP. 
Pollen of Oxyria digyna and Potentilla-type appear within this PAZ 
but remain <1% (Fig. 3). The Juniperus communis PAR decreases to 
<900 grains cm -2 yr -1 c. 9100 cal.yr BP, and the Betula PAR in¬ 
creases to >1000 grains cm -2 yr -1 after c. 9200 cal.yr BP (Fig. 4). 
Mean Betula pollen diameters increase to -20 ± 1 pm (Fig. 5). Betula 
pubescens macrofossils are abundant within this PAZ (Fig. 6). Non- 
triporate Betula pollen range between 2 and 7% (Fig. 4). 

PAZ 4 (8800-8100 cal. yr BP) 

Betula pollen decreases after c. 8700 cal.yr BP to <32% of TLP 
and Juniperus communis pollen continues to decrease to -11-26% of 
TLP. Poaceae pollen increases to >20% of TLP c. 8800 cal.yr BP and 
Cyperaceae to >16% of TLP c. 8700 cal. yr BP (Fig. 3). The Betula PAR 
decreases to <600 grains cm -2 yr -1 after c. 8800 cal.yr BP and 
<200 grains cm -2 yr -1 after c. 8500 cal.yr BP. The TLP PAR de¬ 
creases to <1400 grains cm -2 yr -1 c. 8800 cal.yr BP and <900 
grains cm -2 yr -1 c. 8500 cal.yr BP (Fig. 4). Mean Betula pollen di¬ 
ameters are -20 ± lpm within this PAZ (Fig. 5). Myriophyllum 
alterniflorum pollen increases while Potamogeton natans- type pol¬ 
len decreases (Fig. 3). Myriophyllum alterniflorum seeds are the 
most common plant macrofossils within this PAZ (Fig. 6). Non- 
triporate Betula pollen range between 1 and 10% (Fig. 4). 

PAZ 5 (8100-7000 cal. yr BP) 

Betula pollen increases rapidly between c. 8100 and 8000 cal. yr 
BP from -37% TLP c. 8100 cal. yr BP to -79% TLP c. 8000 cal. yr BP. 
Juniperus communis pollen decreases dramatically within this PAZ 
to -1-9% of TLP. Other pollen taxa such as Poaceae, Cyperaceae, 
Salix, Thalictrum alpinum and Empetrum nigrum decrease. Galium 
pollen disappears along with Selaginella selaginoides spores (Fig. 3). 
The Betula PAR is high from c. 8000 cal.yr BP (-1500—2500 grains 
cm -2 yr -1 ) and the TLP PAR is -1800-3200 grains cm -2 yr -1 
(Fig. 4). Mean Betula pollen diameters are -21 ± 1 pm (Fig. 5). Betula 
pubescens catkin scales and fruits and Myriophyllum alterniflorum 
seeds are the most abundant plant macrofossils in this PAZ (Fig. 6). 
Non-triporate Betula pollen range between 2 and 5% (Fig. 4). 

4.2. Principal component analysis of pollen data 

The first PCA axis accounts for 66.5% of the variance in the 
dataset and the second axis for 12.6%. The terrestrial pollen 
assemblage in PAZ 1 (c. 10,100-9600 cal.yr BP) is influenced by 
Empetrum nigrum, Poaceae, Cyperaceae, Thalictrum alpinum and 
Galium. Juniperus communis is the dominant pollen type in PAZ 2 (c. 
9600-9200 cal. yr BP), however the influence of Betula pollen in¬ 
creases in PAZ 3 (c. 9200-8800 cal.yr BP). Empetrum nigrum, 
Poaceae, Cyperaceae, Thalictrum alpinum and Galium, as well as 
Oxyria digyna show a strong influence on the pollen assemblages 
within PAZ 4 (c. 8800-8100 cal.yr BP). Betula becomes the domi¬ 
nant pollen taxon within PAZ 5 (c. 8100-7000 cal.yr BP) (Fig. 7). 

4.3. Organic matter and physical properties 

MS and DBD values are high (DBD 0.16-0.34 g cnrr 3 , excluding 
tephra layers) between c. 10,100 and 9300 cal. yr BP (PAZ 1 and 2). 
Values decrease after c. 9300 cal. yr BP and the lowest values for the 


5.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 


























/ # 



2000 4000 





500 10001500 






4000 8000 



Fig. 4. Pollen accumulation rates of selected pollen taxa and TLP, concentration of TLP and percentage non-triporate Betula pollen. 

Fig. 5. Boxplot of Betula pollen grain diameters for each PAZ in the Kagadarholl record. 

two proxies are recorded after c. 8500 cal. yr BP (DBD 0.09-0.19 
gem -3 ). Organic matter proxies (%TC, %TN, C/N and %OM) increase 
with time. %TC increases from -10-13% in PAZ 1 to >20% in PAZ 5. 
The C/N ratio increases from -13% in PAZ 1 to -15% in PAZ 5. The 
5 13 C isotope record shows decreased values with time, from — 18%o 
at c. 10,100 cal.yr BP to<— 21%o c. 9100 cal.yr BP. This trend is 
reversed after c. 8700 cal. yr BP and higher values (>— 20%o) are 
recorded between c. 8400 and 8100 cal.yr BP. Values decrease 
to — 21 and -23%o after c. 8100 cal. yr BP (Fig. 8). 

Although dissolved inorganic carbon (DIC) is found in rivers in 
Iceland (Kardjilov et al., 2006), the contribution of inorganic carbon 
toTC in Icelandic lake sediments is considered small due to the lack 
of carbonate bedrock (e.g. Langdon et al., 2010; Larsen et al., 2012; 
Johannesson, 2014; Harning et al., 2018) and changes in TC are 
considered to reflect changes in total organic carbon (TOC). 

4.4. Principal Component Analysis of physical and organic matter 
properties and pollen 

The first PCA axis accounts for 61.8% of the variance in the 
dataset and the second axis for 14.5%. High MS and DBD influence 

the samples in PAZs 1 and 2. Herb and shrub pollen, 5 13 C, as well as 
aquatics are the dominant features in the samples in PAZ 1. In PAZ 2, 
Juniperus communis pollen and 5 15 N are the most important factors 
influencing the samples. PAZ 3 is characterised by increased in¬ 
fluence of Betula pollen and organic proxies (%OM, %TC and %TN), 
while the importance of the C/N ratio increases in PAZ 4. The 
dominant factors influencing the samples in PAZ 5 are Betula pollen 
and organic proxies (%OM, %TC, %TN and C/N) (Fig. 9). 

5. Discussion 

5.1. Early Holocene (c. 10,100-9200 cal. yr BP) 

The pollen assemblage from Kagadarholl suggests the presence 
of Juniperus communis-Betula nana-Salix shrub heath for the first 
four centuries of the record (Fig. 3). This is supported by the 
dominance of pollen of Juniperus communis and Betula within PAZ 1. 
Other important pollen taxa are Empetrum nigrum and Salix, grasses 
(Poaceae), sedges (Cyperaceae), Thalictrum alpinum and Galium 
(Figs. 3 and 7). The presence of Betula nana is evidenced by small 
Betula pollen diameters -17 ± 1 pm (Fig. 5) concurrent with modern 
Betula nana pollen (Makela, 1996), as well as the presence of Betula 
nana macro fossils (Fig. 6). Macro fossils of heath taxa such as 
Empetrum nigrum, Salix spp., Selaginella selaginoides and grami- 
noids (e.g. Carex nigra and Luzula spp.) support the presence of 
heath around the lake (Fig. 6). 

The shrub heath was replaced by Juniperus communis scrub c. 
9600 cal. yr BP, evidenced by a relative increase in Juniperus com¬ 
munis pollen to >38% of TLP (Fig. 3) and PARs of >1300 grains cm -2 
yr -1 (Fig. 4). Juniperus communis is dominant in the TLP assemblage 
in PAZ 2. The high PAR of Juniperus communis pollen in the lake 
during this period indicates the presence of the upright growth 
form of the species around the lake (Eddudottir et al., 2015). Juni¬ 
perus communis produces large quantities of pollen (Huntley and 
Birks, 1983), but low amounts of pollen are found in lakes despite 
strong local presence (Birks and Bjune, 2010). However, high per¬ 
centages and PARs of Juniperus communis can be produced by an 
upright growth form of dense scrub (Birks, 1973). Similar peaks in 
Juniperus communis pollen are also seen in other records from 
northern Iceland during this period (Hallsdottir, 1995; Rundgren, 
1998; Caseldine et al., 2003, 2006; Hallsdottir and Caseldine, 
2005), when climate was relatively warm (Langdon et al., 2010) 

S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 


Trees and shrubs Herbs Pteridophytes Aquatics Sphagnum 

Fig. 6. Plant macrofossil diagram for Kagadarholl. Concentrations in 50 mL of sediment. 

and environmental conditions probably favoured Juniperus com¬ 
munis over Betula pubescens (Eddudottir et al M 2015). A decrease in 
pollen from dwarf shrubs, graminoids and other herbs (Figs. 3, 4 
and 7) also reflects the transition to a new vegetation commu¬ 
nity. This period is characterised by environmental instability, 
indicated by relatively high sedimentation rate (Fig. 2) and large 
minerogenic input into the lake, as suggested by relatively high and 
variable DBD (>0.16 gem -3 ) and MS. Organic matter (<30%), %TC 
(-8-12%) and %TN (~0.8-1.2%) are relatively low and 5 13 C values 
decrease from -18.6 to -20.6%o. Although the lowest C/N ratios in 
the record occur during this period, values of 14.2-15.9 (Fig. 8) 
show that terrestrial organic matter contributed a significant 
component to the sediment (Meyers and Lallier-Verges, 1999). The 
TLP PAR (-3200-6400 grains cm -2 yr -1 ) is the highest during this 
period, and the TLP concentration is also high (-26,000-50,000 
grains cm -3 ) (Fig. 4), suggesting warm conditions favourable for 
pollen production. 

Small mean Betula pollen diameters (-18 ± 1 pm) c. 9600-9200 
cal.yr BP (Fig. 5) indicate that Betula nana was still the dominant 
Betula species, although Betula pubescens macrofossils appear in 
the record c. 9300 cal. yr BP (Fig. 6). The percentage of non-triporate 
Betula pollen ranges between 2 and 9% during this period (Fig. 4) 
and is above 3% in most of the pollen samples. A high proportion of 
non-triporate pollen indicates the presence of hybrid individuals of 
Betula nana and Betula pubescens. Modern individuals of Betula 
nana and Betula pubescens in Iceland produce on average 2.4% and 
0.7% of non-triporate pollen, respectively. In contrast, hybrid in¬ 
dividuals of the two species produce about 12.2% non-triporate 
pollen (Karlsdottir et al., 2008). The relatively high percentages of 
non-triporate Betula pollen indicate some hybridisation between 
the two species during the early Holocene as has been recorded at 
other sites in Iceland (Karlsdottir et al., 2009, 2012; Karlsdottir, 
2014; Eddudottir et al., 2015, 2016). 

5.2. Early birch woodland development (c. 9200-8800 cal.yr BP) 

Birch woodland development is indicated by increased impor¬ 
tance of Betula in the pollen assemblage c. 9200 cal.yr BP (PAZ 3; 
Fig. 7). The Betula PAR increases to >1000 grains cm -2 yr -1 after c. 
9200 cal. yr BP (Fig. 4) and the relative abundance to >40% of TLP c. 
9100 cal.yr BP (Fig. 3). This corresponds to a decrease in Juniperus 
communis pollen to <35% of TLP, probably reflecting competition 
for space with Betula pubescens and the effects of an increasingly 
closed canopy upon the shade intolerant Juniperus communis 
(Thomas et al., 2007). Mean Betula diameters increase to 
-20 ±1 pm (Fig. 5), indicating an increased importance of Betula 
pubescens pollen (Makela, 1996). As woodland developed, envi¬ 
ronmental stability increased, resulting in less minerogenic mate¬ 
rial deposited in the lake, which is reflected in lower values of DBD 
(0.13-0.19 g cm -3 ) and MS. This change in sediment characteristics 
is reflected in the sedimentation rate that decreases c. 9200 cal. yr 
BP (Fig. 2). Organic matter proxies show an increase in %TC 
(14.7-20.9%) and C/N (13.3-14.7), while 5 13 C decreases (-20.3 
to — 23.7%o) (Fig. 8). Terrestrial plants in Icelandic lakes have 5 13 C 
values that are significantly lower (-24.4 to -30.9%o) than those of 
aquatic plants (-11,5 to -15,l%o; Wang and Wooller, 2006). Hence, 
the decrease in 5 13 C, accompanied by relatively high C/N ratios may 


5.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 




o' ^ 

< CM 

1 £ I 

TO o 

•2 w 
a> .g 
QQ co 




O — 

(da ja "i bo) 06v 





§ "co CO 
o c E 

8 s 

1 CO 




Fig. 8. Organic and physical properties of the KagaSarholl sediments compared with 
selected proxies from Greenland ice core records, a) Organic matter (OM), b) total 
carbon (%TC), c) total nitrogen (%TN), d) C/N ratio, e) nitrogen isotope ratio (5 15 N), f) 

Fig. 9. Principal component analysis of organic and physical properties and pollen 
from the KagaSarholl sediments. 

represent an increase in terrestrial input into the lake as woodland 
encroached upon the area. This is supported by an increase in 
Betula pubescens macrofossils in the sediment during this period 
(Fig. 6). This stage of woodland development is associated with a 
period of warm climate in northern Iceland (e.g. Caseldine et al., 
2006; Langdon et al., 2010; Eddudottir et al., 2015). 

5.3. Cooling climate (c. 8800-8100 cal.yr BP) 

The progression from Juniperus communis scrub to Betula 
pubescens woodland was stalled by external factors c. 8800 cal. yr 
BP, prompting a change towards more open landscape for c. 700 
years. A change in the pollen assemblage is interpreted as a change 
in vegetation community c. 8800 cal. yr BP (Fig. 7), characterised by 
an increase in Poaceae and Cyperaceae and a further decrease in 
Juniperus communis pollen (Fig. 3). The Betula PAR drops below 600 
grains cm -2 yr -1 (Fig. 4) and the relative abundance of Betula pollen 
decreases to <45% of TLP c. 8800 cal.yr BP and <32% after c. 8700 
cal.yr BP (Fig. 3). The drop in Betula pollen indicates a decrease in 
pollen deposition from Betula species. Despite a decrease in Betula 
pollen accumulation, mean Betula pollen diameters remain 
~20 ± 1 pm (Fig. 5), suggesting that Betula pubescens remained a 
part of the vegetation community and was not replaced by Betula 
nana. The increase in grass and sedge pollen and decline in Betula 
pollen suggests a more open landscape around the lake as wood¬ 
land retreated (Figs. 3 and 7). These changes in the pollen assem¬ 
blage are also recorded in a previous (lower resolution) 
palynological reconstruction from the same lake (Vasari, 1972; 
Vasari and Vasari, 1990). 

The low concentration and PAR of terrestrial pollen (Figs. 4 and 
8) during this period suggests a decrease in pollen production of 
most terrestrial plants. This is reflected in an increase in the relative 
abundance of Poaceae and Cyperaceae pollen (Fig. 3), while PARs of 
the two pollen taxa do not show significant increases (Fig. 4). The 

carbon isotope ratio (5 13 C), g) Betula PAR, h) total land pollen (TLP) concentration, i) 
Magnetic susceptibility (MS) showing values below 40 for clarity, j) dry bulk density 
(DBD) showing values below 0.4 gem -3 for clarity, k) Volcanic forcing based on the 
GISP2 sulfate record (Kobashi et al., 2017), 1) GISP2 I< + , showing values <2.5 ppb for 
clarity, black line represents 50 year mean values (Mayewski et al., 1997), and m) 
Renland ice-core 5 ls O record, 20 year average, uplift corrected (Vinther et al, 2009). 

S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 


changes in the pollen assemblage may have been caused by lower 
spring and summer temperatures hindering the reproduction of 
Betula pubescens (Pichugina, 1971) and possibly other taxa. Colder 
conditions may have caused a shift towards a period of vegetative 
reproduction by plants. For instance, following damage or stress, 
Betula pubescens can reproduce by sprouting from basal buds or 
regenerate from stems (Kauppi et al, 1987; Verwijst, 1988). Prior to 
c. 8800 cal.yr BP, most of the plant macrofossils found in the 
sediment are Betula pubescens catkin scales and fruits, however 
between c. 8800 and 8100 cal.yr BP very few terrestrial plant 
macrofossils are recorded (Fig. 6). This, along with the decrease in 
pollen deposition, may indicate a drop in seed production by 
terrestrial plants. Fligher 5 13 C values are recorded c. 8700 cal. yr BP, 
with the highest values between c. 8400 and 8100 cal. yr BP (-18.9 
to —20.5%o). The C/N ratio increases slightly during the same period 
from 14.4 to 15.1 (Fig. 8). This coincides with the lowest input of 
land pollen (TLP) into the lake (Fig. 4), and an increase in macro¬ 
fossils of the aquatic taxon Myriophyllum alterniflorum (Fig. 6). The 
increase in aquatic plant macrofossils and decreased deposition of 
terrestrial plant pollen and macrofossils in the sediment would lead 
to higher 5 13 C values in the sediment closer to the 5 13 C of aquatic 
plants (-11.5 to -15.1%o), than terrestrial plants (-24,4 to -30,9%o; 
Wang and Wooller, 2006). 

Environmental stability at Kagadarholl increased from c. 8800 to 
8100 cal. yr BP, despite a response by the vegetation community 
suggesting colder climate. Minerogenic input into the lake 
decreased during the period, indicated by declining DBD (<0.14 
gem -3 ) and MS values, with the exception of a tephra layer 
deposited c. 8550 cal.yr BP (Fig. 8). The data show a progression 
from more minerogenic to more organic sediment with time, 
without a regression to more unstable conditions (Fig. 9). This is 
contrary to several other studies in Iceland that have found evi¬ 
dence of increased soil erosion in the wake of cooling climate c. 
8700 cal.yr BP at highland sites (Larsen et al., 2012; Geirsdottir 
et al., 2013; Gunnarsson, 2016; Harning et al., 2018). The tall 
vegetation cover around the Kagadarholl lake and the position of 
the lake in the lowlands may be the reason for the different 
response. This may indicate that environmental instability 
increased in the highlands, while stable conditions prevailed in the 
lowlands during this period. 

Indications of colder spring/summer temperatures and 
possible glacier advances, corresponding with the changes in 
vegetation at Kagadarholl, are recorded in Lake Hvitarvatn in the 
western highlands of Iceland beginning c. 8700 cal.yr BP, with 
two rapid cooling episodes at c. 8500 and 8200 cal.yr BP (Larsen 
et al., 2012). Chironomid-inferred July temperatures (July CI-T) 
from nearby Trollaskagi peninsula show a gradual cooling be¬ 
tween c. 9000 and 8100 cal. yr BP, with a drop in July CI-T c. 8500 
cal.yr BP. However, cooling is not evident in the pollen record 
from the sites in Trollaskagi. On the contrary, the relative abun¬ 
dance of Betula increases after c. 8400 cal. yr BP (Caseldine et al., 
2006). Changes in pollen assemblages similar to those at 
Kagadarholl are recorded at Vatnskotsvatn in Skagafjordur, east of 
Hunafloi between c. 8300 and 8000 cal. yr BP, where development 
of birch woodland was interrupted by reoccurrence of Juniperus- 
Salix-Betula nana heath (Hallsdottir, 1995). Records that show a 
response to long-term cooling c. 8800-7900 cal.yr BP are 
concentrated in northwestern and western Iceland, although it 
should be noted that most available palaeoecolgical and envi¬ 
ronmental records covering the period are from northern Iceland. 
A short, sharp cooling with a change to cooler winter and spring 
temperatures is observed in the only record available from eastern 
Iceland c. 8200-8000 cal.yr BP (Striberger et al., 2012), which 
shows more similarities to responses observed in northern Europe 
(Rohling and Palike, 2005; Seppa et al., 2007, 2009) than those in 

northwestern and western Iceland. Marine records northwest and 
west of Iceland show a long period of cooling, beginning c. 
8750-8600 cal. yr BP, and intensifying c. 8200 cal. yr BP (Knudsen 
et al., 2004; Olafsdottir et al., 2010). A shorter cold period, 
accompanied by an increase in sea-ice c. 8300-8100 cal.yr BP, 
indicating decreased strength of the Irminger current, is recorded 
north of Iceland (Ran et al., 2006). Other marine records around 
Iceland show a relatively minor cooling associated with the 8.2 ka 
event (e.g. Andersen et al., 2004; Castaneda et al., 2004; Ran et al., 
2006; Jiang et al., 2015). 

5.4. Woodland recovery (c. 8100-7000 cal.yr BP) 

Betula becomes the dominant pollen taxon in the terrestrial 
pollen assemblage after c. 8100 cal.yr BP when Betula pubescens 
woodland begins to recover (Figs. 3, 4 and 7). A sharp increase in 
Betula pollen is recorded after c. 8100 cal. yr BP, as the PAR increases 
from -200 to >2000 grains cm -2 yr -1 between c. 8100-8000 cal. yr 
BP. This change is confirmed by a relative increase of Betula pollen 
from -30 to 80% of TLP, showing relative changes in pollen taxa 
abundance, independent of changes in sedimentation rates. Rela¬ 
tive abundances and PARs of other pollen taxa decrease, including 
Juniperus communis , Salix, Poaceae, Cyperaceae and Thalictrum 
alpinum (Figs. 3 and 4). The change in pollen assemblage occurs 
within less than a century, indicating a rapid response of the 
vegetation community to warming climate. A relatively stable 
woodland community is reflected in the pollen assemblage until 
the end of the record c. 7000 cal. yr BP (Figs. 3, 4 and 7). The most 
stable period in the record is characterised by an increase in %OM to 
>37% and %TC to >19%. The 5 13 C decreases to < —21 %o and the C/N 
ratio is 14.6-16.2, probably indicating a greater input of organic 
terrestrial material into the lake (Figs. 8 and 9). This may be in part 
due to the reappearance of Betula pubescens catkin scales and fruits 
in the sediment (Fig. 6). This period corresponds to the Holocene 
Thermal Maximum (HTM), the most stable period of the Holocene 
in Iceland, lasting until c. 6000-5500 cal. yr BP (Caseldine et al., 
2006; Larsen et al., 2012; Striberger et al., 2012; Geirsdottir et al., 
2013; Blair et al., 2015; Eddudottir et al., 2015, 2016; Eddudottir, 

5.5. Climate anomalies c. 8800-8000 cal yr BP 

The changes to colder springs/summers documented in the 
palaeoecological reconstruction from Kagadarholl occur c. 
8800±110 cal.yr BP (Figs. 3, 4 and 7). Responses to cooling are 
recorded in other lakes in northwestern and western Iceland c. 
8700cal.yr BP (e.g. Larsen et al., 2012; Geirsdottir et al., 2013; 
Harning et al., 2018). The responses manifested in proxies sensitive 
to spring and summer temperatures in Iceland, such as the Betula 
PAR at Kagadarholl (Fig. 8), are similar to proxies that record global 
climate anomalies c. 8600 and 8000 cal. yr BP (e.g. Rohling and 
Palike, 2005; Wanner et al., 2011). A cold temperature anomaly is 
observed in some records from the Greenland Ice Sheet beginning a 
few centuries before the more severe 8.2 ka event (Fig. 8; Vinther 
et al., 2009; Kobashi et al., 2017). The period is characterised by a 
complex pattern of climate change in the North Atlantic region and 
beyond when remnants of large ice sheets were still present in the 
Northern Hemisphere (Mayewski et al., 2004). Rohling and Palike 
(2005) identified a longer period of climate anomalies recorded 
in summer proxies from c. 8600 cal. yr BP, distinct from a shorter 
event affecting winter and early spring temperatures beginning c. 
8300 cal. yr BP. The shorter event is recorded in pollen based 
temperature reconstructions in Northern Europe and indicates 
colder winters and early springs between c. 8300 and 8000 cal. yr 
BP in response to freshwater input into the North Atlantic. 


S.D. Eddudottir et al. / Quaternary Science Reviews 197 (2018) 246-256 

However, pollen records from northern Fennoscandia show little or 
no evidence of colder conditions in May and June (Seppa et al., 
2007, 2009). 

Atmospheric circulation changes are observed in records c. 
8650-8000 cal. yr BP, such as expansion of the polar vortex, 
suggested by records of potassium ions (Fig. 8) and dust in the 
GISP2 core from Greenland (Taylor et al., 1992, 1993; Mayewski 
et al., 1997), weakening of the African, Indian and Asian mon¬ 
soons between c. 8500 and 8000 cal.yr BP (Gasse, 2000; Hong 
et al., 2003; Yuan et al., 2004), and migration of the Intertrop- 
ical Convergence Zone (ITCZ) (Haug et al., 2001). Several drivers 
have been proposed for cooling climate during this period; i.e. 
changes in solar output, meltwater discharge from the Laurentide 
Ice Sheet and volcanism. Although a solar output minima is 
recorded c. 8400 cal. yr BP (Steinhilber et al., 2009) it is not 
considered enough to explain the observed changes in climate 
(Wanner et al., 2011). Episodic cooling of the Irminger Current 
occurred c. 9500—8100 cal. yr BP due to fluxes of freshwater from 
the Laurentide Ice Sheet into the North Atlantic (Jennings et al., 

2011) . Three pulses of freshwater from the Laurentide Ice Sheet 
are recorded, peaking c. 8636, 8565 and 8147 cal.yr BP (Jennings 
et al., 2015). This coincides with three events of relative sea level 
increase recorded in Scotland between c. 8760 to 8218 cal. yr BP, 
each lasting 105-130 years (Lawrence et al., 2016). The first two 
events are linked to the opening of the Tyrrell Sea and the last 
one with the drainage of lakes Agassiz and Ojibway. All events 
are part of a series of freshwater events originating from Hudson 
Bay and Hudson Strait between c. 11,500 and 8000 cal.yr BP 
(Jennings et al., 2015). The period between c. 9000-8000 cal.yr 
BP is also characterised by large volcanic eruptions, with the 
highest occurrence of volcanic activity during the Holocene 
recorded in the Greenland Ice Sheet c. 8600-8000 cal.yr BP 
(Fig. 8; Kobashi et al., 2017). There is evidence of intensified 
volcanic activity in the Kamchatka Peninsula during this period 
(Braitseva et al., 1995, 1997), including the Karymsky caldera 
eruption c. 8770±210 cal.yr BP (7889±67 14 C yr) and Kuril 
Lake-Iliinsky eruption c. 8460±60 cal.yr BP (7666±19 14 C yr; 
Braitseva et al., 1997), an extremely large eruption that produced 
-120-140 km 3 of pyroclastic material (Braitseva et al., 1997). 
Furthermore the largest Holocene lava on Earth, the hjorsarhraun 
lava was formed c. 8590±170 cal.yr BP (7800±60 14 C yr; 
Hjartarson, 1988). The minimum size of the eruptive material 
produced is estimated to be -25 km 3 DRE (dense rock equivalent) 
(Hjartarson, 2011). The recent, much smaller (1.2 km 3 DRE; 
Bonny et al., 2018), Holuhraun eruption originating from the 
same volcanic system, has demonstrated that high amounts of 
S0 2 and aerosol particulate matter are emitted from effusive 
eruptions from the system (Ilyinskaya et al., 2017). Model sim¬ 
ulations of volcanic forcing have suggested that a large volcanic 
eruption can cause climate cooling and sea-ice expansion that 
can last for over a century (Kobashi et al., 2017; Miller et al., 

2012) , resulting in prolonged periods of cold summers (Miller 
et al., 2012). A succession of large eruptions can therefore 
potentially cause centuries-long cold periods (Kobashi et al., 
2017), such as the one observed in the Kagadarholl record. 

The pollen record from Kagadarholl suggests relatively stable 
conditions at the site between c. 8800 and 8100 cal. yr BP, despite 
a change in vegetation community. The early response of the 
palaeoenvironmental proxies in northwestern and western Ice¬ 
land to colder spring and summer temperatures is probably due 
to northwestern Iceland's marginal location and the region's 
sensitivity to ocean currents, showing more similarities to re¬ 
sponses in some marine records (Knudsen et al., 2004; 
Olafsdottir et al., 2010) and Greenland ice core records (Fig. 8; 
Vinther et al., 2009; Kobashi et al., 2017) than terrestrial records 

from Northern Europe (Seppa et al., 2007, 2009). This demon¬ 
strates the ability of Icelandic palaeoenvironmental re¬ 
constructions to capture signals of North Atlantic climate 

6. Conclusions 

The pollen record from Kagadarholl clearly demonstrates a 
change in vegetation community beginning c. 8800 cal.yr BP. 
During the period c. 8800-8100 cal.yr BP, cold climate probably 
interrupted the vegetation succession from Juniperus communis 
scrub to Betula pubescens woodland that had taken place during a 
relatively warm early Holocene c. 10.100—8800 cal. yr BP. The re¬ 
cord shows a significant decrease in pollen production from 
terrestrial plants, especially Betula pubescens c. 8800-8100 cal. yr 
BP, probably reflecting colder spring and/or summer temperatures. 
Despite a colder climate, proxies of organic and physical properties 
of the sediment do not indicate increased instability, i.e. soil erosion 
in the area around the lake, showing the stability of the established 
Betula pubescens woodland and the resilience of the more open 
vegetation community in response to colder climate. The birch 
woodland shows quick recovery with warming climate between c. 
8100 and 8000 cal. yr BP. The results show the potential of 
terrestrial multi-proxy records from Iceland for reconstructions of 
climate variability in the North Atlantic. 


The authors would like to thank Olafur Eggertsson, Fri3t>6r Sofus 
Sigurmundsson and Porsteinn Jonsson for assistance in the field. 
Jessica Lynn Till is thanked for proofreading the manuscript. We 
would like to thank Olga Kolbrun Vilmundardottir and Scott John 
Riddell for discussions on vegetation communities. We would like 
to thank two anonymous reviewers for their valuable comments 
and suggestions. The Blonduvirkjun hydropower plant kindly 
hosted us during fieldwork. The research was funded by the 
Landsvirkjun Energy Research Fund, the University of Iceland 
Eimskip Fund, the University of Iceland Research Fund, and the 
Icelandic Research Fund (grant no. 141842-051). 

Appendix A. Supplementary data 

Supplementary data related to this article can be found at 


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