Department of Physical Geography

Evidence for birch forests
and a highly productive
environment near the marginof the Fennoscandian ice sheet
in the Värriötunturit area,
northeastern Finland

Fredrik Bogren

Master’s thesis
Physical Geography and Quaternary Geology, 60 Credits

NKA 2302019PrefaceThis Master’s thesis is Fredrik Bogren’s degree project in Physical Geography and
Quaternary Geology at the Department of Physical Geography, Stockholm University. The
Master’s thesis comprises 60 credits (two terms of full-time studies).
Cooperation with University of Helsinki.
Supervisor has been Karin Helmens at the Department of Physical Geography, Stockholm
University. Co-supervisors have been Minna Väliranta, Department of Environmental
Sciences, University of Helsinki and Benedict Reinardy, Department of Physical Geography,
Stockholm University. Examiner has been Stefan Wastegård at the Department of PhysicalGeography, Stockholm University.
The author is responsible for the contents of this thesis.

Stockholm, 11 July 2019

Björn Gunnarson
Vice Director of studies

Abstract

High-resolution records of early Holocene deposits are rare, and as a consequence reconstruction of terrestrial environments very soon after the deglaciation has often been difficult. In this study the palaeoenvironmental conditions of early Holocene (c. 10600-7500 cal. yr BP) are reconstructed in the Värriötunturit
area of northeastern Finland, using evidence from plant macrofossils and pollen preserved in a lake sediment sequence retrieved from the small lake Kuutsjärvi. Special emphasis is put on the environment
immediately following the deglaciation as the base of the sediment sequence is rich in minerogenic material interpreted to have been deposited by meltwater pulses from the retreating ice sheet. The abundance
and variety of fossil remains in these early meltwater deposits provide evidence for a very productive
ice-marginal environment in the area between the lake and the ice sheet, and the presence of tree-type
Betula macro remains as well as high percentage values of tree-type Betula pollen suggests that a subarctic birch forest established just a few years after the deglaciation. In the following centuries the birch
forest around the lake became rich in an under growth of ferns, and at c. 9400 cal. yr BP a transition intoa mixed pine and birch forest took place. Due to absence of indicator plant taxa in the sediment it was notpossible to reconstruct temperature conditions for any parts of the sequence in this study. However, therapid colonisation of birch forests suggests that the climate was warm already during deglaciation, whichis also in accordance with climatic conditions reconstructed for the early Holocene in the nearby Sokli
area just 10 km away, as well as in other parts of Fennoscandia and Russia.

Keywords:

Early Holocene, productive ice-marginal environment, subarctic birch forests, plant macrofossils, pollen,
palaeoenvironmental reconstructions, palaeoclimate reconstructions, Värriötunturit, Kuutsjärvi, northeastern Finland, northern Fennoscandia.

Table of contents
1

Introduction
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Climatic variations during the Holocene . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 The method of macrofossil analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Aim of the study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5
5
5
6
7

2

Site description
2.1 Regional setting and study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Glacial history of the region and study area . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Kuutsjärvi and its immediate surroundings . . . . . . . . . . . . . . . . . . . . . . . . .

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7
9
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3

Material and methods
3.1 Field work and subsampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Macrofossil preparation and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Loss on ignition, radiocarbon dating and other proxies . . . . . . . . . . . . . . . . . . .
3.4 DEM analysis of geomorphology and topography . . . . . . . . . . . . . . . . . . . . .

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4

Results and interpretation
4.1 Lithostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 The minerogenic section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 The gyttja section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Deglaciation of the Värriötunturit area . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Chronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Proxy interpretation and environmental reconstruction . . . . . . . . . . . . . . . . . . .
4.4.1 Zone I, 562.5 - 541 cm (c. 10600 cal. yr BP) . . . . . . . . . . . . . . . . . . .
4.4.1.1 Overflow sediments (Subzone I-b and Subzone I-d) . . . . . . . . . .
4.4.1.2 Gyttja sediments (Subzone I-a and Subzone I-c) . . . . . . . . . . . .
4.4.2 Zone II, 541 - 530 cm (10600 - 10400 cal. yr BP) . . . . . . . . . . . . . . . . .
4.4.3 Zone III, 530 - 460 cm (10400 - 9400 cal. yr BP) . . . . . . . . . . . . . . . . .
4.4.4 Zone IV, 460 - 340 cm (9400 - 7500 cal. yr BP) . . . . . . . . . . . . . . . . . .
4.5 Temperature reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 Contemporary surface sediment analysis . . . . . . . . . . . . . . . . . . . . . . . . . .

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5

Discussion
5.1 The ice-marginal environment during the early Holocene . . . . . . . . . . . . . . . . .
5.2 Migration of trees into the study area following the deglaciation . . . . . . . . . . . . .
5.3 Climatic conditions during the early Holocene . . . . . . . . . . . . . . . . . . . . . . .
5.4 The absense of high-temperature indicator taxa in the sediment . . . . . . . . . . . . . .
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Future research in the area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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30

Acknowledgements

31

References

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1

Introduction

1.1

BackgroundThe global climate is projected to change considerably due to human induced warming in the coming
decades and centuries, and particularly in the arctic and subarctic regions temperatures are expected toincrease multiple times the global average, which is attributed to the so called polar amplification mechanism (IPCC, 2013). This means the climate and environment in Fennoscandia will be significantly
different in the near future, with changes in species richness in the boreal region (Saetersdal et al., 1998)
as well as a latitudinal shift in the treeline further to the north (MacDonald, 2010). To understand thecomplexities of these imminent future shifts, and to be able to refine and evaluate climate models, dataon climate and environmental changes of the past are crucial (Shuman, 2014). Furthermore, all anthropogenic influence on the climate will inevitably be interconnected with future natural variations, which is
yet another reason why it is important to have knowledge about causes and effects of the past variations(Bradley, 2008).
In contrast to the currently ongoing anthropogenic warming, climate change during the majority of
the Holocene was however caused predominantly by external forcing factors such as orbital variations
and changes in insolation (Bradley, 2008; Borzenkova et al., 2015). Instrumental climate data tracking
these changes does in general only exist for the last 150 years or so (or in some local regions at best for
up to 300 years), and thus other sources, i.e. natural archives, are needed to track changes further backin time (Bradley, 2008). One such type of natural archive especially abundant in Fennoscandia are lake
sediments, which can contain a number of different biological and chemical proxies that can be used
to reconstruct past climate and environments, either through direct qualitative interpretation or throughquantitative statistical measures (Shuman, 2014).
In this study palaeoenvironmental reconstructions are made based on macrofossil analysis on a lake
sediment sequence. Macrofossils are preserved remains of organic material, such as seeds, leaves andbark, and the method is unique as it can confirm the presence of plant or animal taxa in a direct way.
In other words, if a taxon is found in the sediment, it is very strong evidence for a local presence ofthat specific taxon in or near the natural archive at some point in time (Birks, 2013). By confirming thepast distribution of species in this way (and with the assumption that the age of the fossil remain canbe determined properly), the environment and climate can then be reconstructed by interpretation of theecology as well as by using modern analogues in species distribution (Shuman, 2014).
1.2

Climatic variations during the Holocene

After an abrupt warming following the Younger Dryas, oxygen isotope (δ 18 O) data from the Greenland
ice sheet suggests that the global temperatures throughout the Holocene remained relatively stable compared to the fluctuations seen during the last glaciation (Seierstad et al., 2014; Lowe and Walker, 2015).
However, palaeoenvironmental reconstructions using a variety of proxies have shown that the climate
variability in northern Europe during the Holocene has been rather complex, with proxies pointing in different directions regarding reconstructed temperatures. In sea surface temperature (SST) records basedon marine foraminifera from the Barents sea, the summer temperatures were at its highest in the early
Holocene after which they decreased and remained stable throughout the rest of the epoch (Hald et al.,2007). Terrestrial pollen records from Fennoscandia (even from the northern parts close to Barents sea)
are on the other hand often showing lower than present summer temperatures during the early Holocene,
and then a period during mid-Holocene (roughly 8000 to 5000 cal. yr BP) with warmer than present summer temperatures (e.g. Seppä and Birks, 2001; Seppä et al., 2009). This period is commonly called theHolocene Thermal Maximum (HTM) and is also represented in e.g. direct temperature measurementsfrom boreholes in the Greenland ice sheet (Dahl-Jensen, 1998), as well as in a weaker pattern in the global
temperature trends from the Greenland oxygen isotope data (Vinther et al., 2006; Seierstad et al., 2014).
However, in the last few decades studies on plant macrofossils and chironomids (Chironomidae, lakeflies) have, similar to the aforementioned SST records, suggested that the climate in Fennoscandia dur5

ing early Holocene could have been warmer than previously thought. Macrofossil inferred temperaturesfrom indicator plant taxa in northern Fennoscandia indicate that the very beginning of Holocene (between
11000 and 10000 cal. yr BP) had several degrees warmer summer temperatures than present (e.g. Väliranta et al., 2015; Shala et al., 2017), which is also in agreement with what Luoto et al. (2014) foundin chironomid-based records in northern Finland. This corresponds to the peak in summer insolation
at northern latitudes, during a time when the winter insolation also was lower than present. The combination indicates that the seasonality was greater, with warmer summers and colder winters, and thusthat the climate in Fennoscandia was more continental during the early Holocene (Engels et al., 2014).
Furthermore, findings of macro- and megafossils (very large wood pieces) from boreal trees (Kullman,
1999; Paus, 2010; Paus et al., 2011; Paus and Haugland, 2017) as well as broad-leaved deciduous trees(Kullman, 1998a,b) in the southern Scandes and central Norway dated back to at most c. 9700-9500 cal.
yr BP suggest these trees were growing in the area shortly following the deglaciation, which could alsoinfer high temperatures during the early Holocene.
Most proxies indicate that a long-term cooling trend started in Fennoscandia during the late Holocene(from c. 5000 cal. yr BP to the present), likely related to the decreased insolation in northern Europe(Borzenkova et al., 2015). This triggered readvance and reformation of old glaciers in the Scandes(Karlén, 1988; Nesje and Kvamme, 1991), as well as an altitudinal shift in the tree line (Kullman, 1995).
Looking at higher resolution climatic shifts during the entire epoch, some irregularities in the generallong-term trends appear. One of the most severe cold period in Fennoscandia during the entire Holocene
was probably the Little Ice Age (LIA) between c. 500 and 100 cal. yr BP (Lowe and Walker, 2015), andin the early Holocene, three distinct shorter cold events are recorded around 8200, 9300 and 11400 cal.
yr BP in oxygen isotopes from the Greenland ice cores (Rasmussen et al., 2007; Thomas et al., 2007).
The cold event at 8200 cal. yr BP is commonly referred to as the 8.2k event and is also detected in somestudies using pollen and geochemical data in northern Fennoscandia (e.g. Allen et al., 2007; Seppä et al.,2007). The event is widely believed to have been caused by huge fresh water input into the Atlantic fromthe collapsing glacial lake Agassiz in North America (Barber et al., 1999; Clarke et al., 2004), and theduration of the event is suggested to have been a mere 160 years (Thomas et al., 2007).
1.3

The method of macrofossil analysis

Macrofossils are, as already briefly mentioned above, remains of organic material found in anoxic environments favouring preservation, e.g. in peat or lake sediment. For remains to be considered macrofossils
as opposed to a microfossils (e.g. pollen), they should typically be of at least 0.5 mm in size and should bestudiable using a stereo microscope of fairly low magnification, often 10x-40x. Most remains are usuallyof plant origin in the form of seeds, leaves, roots, bark, etc, but animal macrofossils such as ephippium(resting stage) of water fleas (Cladocera) or statoblasts (resting spores) of moss animals (Bryozoa) are often found in lake sediment and can also be identified and utilised in palaeoenvironmental reconstructions
to a certain extent. In lake sediment the remains can be deposited by wind, water streams, slope wash,
animals or by falling from plants near the littoral zone, in combination reflecting the local environment
inside and in close proximity to the lake. This is in contrast to mires, where the plants are deposited insitu, and thus in mire environments it is much more rare to find macro remains from the surrounding local
area (Birks, 2013).
The biggest advantage of macrofossil analysis is that it can confirm that a taxon was locally present in
the area. Pollen, on the other hand, generally gives a more regional picture of the vegetation due to longer
dispersal distances of the pollen grains, and therefore cannot be used to definitely confirm occurrence of
taxa locally (Birks, 2001, 2013). Especially certain conifer pollen can be dispersed over very long distances (e.g. Picea and Pinus) due to their air sacks attached to the pollen (Szczepanek et al., 2017), and in
arctic or tundra environments this can skew the result (Birks, 2013). Moreover, with macrofossil analysis
it is also often easier to identify a taxon into species level, whereas with pollen this is sometimes muchharder (Bennett and Willis, 2001; Birks, 2001). Apart from giving an indication of the local vegetationand environment, macrofossils can also provide information on climate by the use of indicator speciesof certain taxa (often aquatic or wetland species) that is known to be controlled primarily by tempera6

ture. The contemporary distribution of these species combined with modern-day mean July temperature
measurements is used to infer a minimum required July temperature for the taxa found in the macrofossil
assemblage, and as a result the palaeoclimatic conditions of the studied area (Iversen, 1954; Isarin andBohncke, 1999; Väliranta et al., 2015).
Despite these clear advantages of macrofossil analysis, it is however important to consider that even
if a taxon is not found in the macrofossil assemblage, it does not automatically mean that the taxon was
absent in the local vegetation. This is because macrofossils are inherently rare in comparison with e.g.
pollen, and different rates in decomposition (Mauquoy and van Geel, 2007) as well as other taphonomic
factors such as distance to shore (Shala et al., 2014b) can mask the true abundance of certain taxa. As
such, different methods can provide different information of the palaeoenvironment and therefore many
proxies are increasingly used in combination in so called multiproxy studies to get a more complete picture
and to enhance the interpretation of the environment (Birks, 2013).
1.4

Aim of the study

Presented herein is a high-resolution macrofossil analysis of a sediment sequence from the small lake Kuutsjärvi in northeastern Finland. The sequence is covering the early Holocene, and initial investigations
have provided evidence for well-preserved macrofossils in a bottom minerogenic rich section overlain by
gyttja. This section is thought to have been deposited by glacial meltwater very soon after the deglaciation, thus providing an excellent glance on the vegetation and climate in the area just at the transition
between the last glaciation and the current interglacial period. Furthermore, radiocarbon dating indicatesthat the time-resolution of the sediment sequence is on the decadal scale, with a resolution of 1 cm sediment equalling c. 15 years. Studies on records covering the early years after deglaciation are still rare in
Fennoscandia, and especially studies on sediment sequences of this resolution. This study constitute onepart of a larger multiproxy collaboration on the sediment sequence, with the aim to reconstruct the environment and climate in the area throughout the Holocene. The specific aim of this part is to contributeto the knowledge of early Holocene environments and climate in northern Fennoscandia, based on themacrofossil analysis method. The following four main objectives were specified:a) To reconstruct the deglaciation phase of the study area, and establish the process that deposited theminerogenic rich sediment in the bottom of the sequence.
b) To reconstruct the ice-marginal environment during early Holocene, and the subsequent vegetation
development in the area during the first few thousand years of the Holocene.
c) To evaluate on the climate in the area during the early Holocene, and determine the minimum July
temperature based on presence of indicator plant taxa identified during the macrofossil analysis.
d) To evaluate on the migration pattern of tree species into the area following the deglaciation, as wellas the possibility of plant refugia nearby.

2

Site description

2.1

Regional setting and study area

This study was conducted using sediment from the small lake Kuutsjärvi, located above the arctic circle
within the Värriö Strict Nature reserve in northeastern Finland (67◦ 44’50” N 29◦ 36’37” E, approximately
341 m a.s.l.), just next to Värriö Subarctic Research Station (operated by the Institute for AtmosphericResearch at University of Helsinki). Due to the very small size of the lake (6715 m2 ), as well as theapparent influence the surrounding area have had on its formation (which is described in more detail in
section 2.3 and 4.2), a study area incorporating the entire northern part of the Värriötunturit fell chain
is outlined in Fig. 1. The regional setting and local characteristics of the area will first be be described
below, while the immediate area surrounding Kuutsjärvi is described in detail in section 2.3.
7

Figure 1. Detailed zoom-in map of the study area in the northern part of the Värriötunturit fell chain. The small orientation map
at the top shows the location of the study area in a Fennoscandian setting, and the underlying topographical map shows the study
area in a regional setting in northeastern Finland. As Kuutsjärvi is a very small lake situated just south of Kotovaara it is notdepicted in much detail on this map, instead a selection of photos of the lake can be seen in Fig. 2 and a close up map view of thelake can be seen in Fig. 3. Note that the catchment of Kuutsjärvi was calculated in Esri ArcGIS using elevation data of 2 meter
resolution, and may therefore not be a perfect reflection of the true catchment area. The black dot north of Kuutsjärvi marks the
position of Värriö Subarctic Research Station. This map contains data from the Elevation Model 2 m, Elevation Model 10 m
and Topographic 1:100 000 database provided by the National Land Survey of Finland under CC BY 4.0 license (downloaded
2019/03).

With its highest summit at 552 m a.s.l., the Värriötunturit fell chain is a sequence of rounded peaks
stretching approximately 10 km in a N-S direction in the northeastern parts of Finland, close to the Russian
border. To its east and south the topography is substantially mountainous in the form of similar roundedpeaks, while to the north and west the topography is much gentler. Geologically the whole region isquite homogeneous with a bedrock consisting mainly of granitoids and paragneiss, alongside scatteredsmaller patches of ultramafic and volcanic rock, all of which are of Precambrian age and belonging to
the Fennoscandian shield (Geological Survey of Finland, 2019a). However, just some 10 km northwestof the study area there is an anomaly in the bedrock; the so called Sokli Carbonatite Massif consisting of8

an igneous intrusion of carbonatite dating back to the Devonian, around 365 Ma (O’Brien and Hyvönen,2015). The Sokli area has been subject to several palaeoenviromental studies due to its unique conditions
for sediment preservation in the deeply weathered bedrock (Shala, 2014), harbouring records for the entire
Weichselian (MIS 2-5d) and stretching as far back as to the Eemian interglacial (MIS 5e) and into the
Saalian glaciation (MIS 6; see e.g. Helmens et al., 2012; Helmens, 2014; Plikk et al., 2016; Helmens
et al., 2018).
Superficial deposits in the region are dominated by till and peat, but block fields and exposed bedrock
outcrops cover extensive areas on and near the mountain peaks (Geological Survey of Finland, 2019b).
Additionally, systems of glaciofluvial deposits are stretching in the form of esker chains in a NW-SE
direction throughout the region, reflecting the general retreat pattern of the ice sheet. One such esker
chain is outlined in the southern parts of the study area, crossing the Värriötunturit fell chain. Vegetationwise the region is within the mixed conifer forest zone (Väliranta et al., 2009) with tree vegetation mainly
consisting of spruce (Picea abies), pine (Pinus sylvestris) as well as birch (Betula pubescens and Betula
pendula). The overall majority of the region are also well below the altitudinal tree line, except for thehighest elevated mountain peaks, e.g. the peaks of the Värriötunturit fell chain. The latitudinal limit ofspruce is situated c. 100 km to the north, whereas the latitudinal limit of pine is situated an additional c. 50
km further north from the spruce limit. Beyond the pine limit there are pure birch forest almost all the way
to the Barents sea coast, before the landscape is turning into a dwarf-shrub tundra (Helmens et al., 2012).
Present-day climate conditions in the region are subarctic and classified as Dfc according to the Köppen
system (Beck et al., 2018). Closest meteorological station to the study area is conveniently located at theVärriö Subarctic Research Station (station name: Salla Värriötunturit, 67◦ 44’55” N 29◦ 36’41” E), andclimate data measured between 1975-2018 reveal a mean annual temperature at -0.4 ◦ C, mean January
temperature at -11.7 ◦ C, mean July temperature at 13.3 ◦ C and mean annual precipitation at 589 mm(Finnish Meteorological Institute, 2019).
The study area (Fig. 1) comprises of the three peaks Ykkönen, Kakkonen and parts of Kolmonen of
the Värriötunturit fell chain, as well as the slightly smaller peak Kämppälaki east of Ykkönen, and the two
smaller peaks north of Kuutsjärvi, Jyppyrävaara and Kotovaara. In between Kakkonen and Kolmonen the
deep gorge Syväkuru is located, with a depth of as much as 60 m measuring in relation to the surrounding
peaks (Johansson, 1995). Another similar deep gorge, Pirunkuru, is found northwest of Kuutsjärvi in between the peaks Jyppyrävaara and Kotovaara. These two gorges are very straight and are likely formed in
weakness zones in the bedrock. The depression in which Kuutsjärvi is situated (Kuutsjärvenkuru) does
also share similarities with these gorges, but is not as deep and steep. Overall the area is extensively
forested, but Ykkönen, Kakkonen and Kolmonen reach above the forest line into tundra environment,
only sustaining open fell heaths with lone scattered trees in the form of krummholz on top of their peaks.
Patches of wetlands are spread out across the area, mainly within valleys and along streams. The dominating wetland type is with forest growth, but smaller treeless mires are also found, especially west of theVärriötunturit fell chain.
2.2

Glacial history of the region and study area

The region of northern Finland where the study area is situated has probably been regularly glaciatedthroughout the Quaternary, including during some of the smaller mountain type ice sheets associated withthe 40 ka cyclicity in the beginning of the period (see Kleman et al., 2008). However, direct evidence forearlier glaciations than the Saalian (MIS 6) are uncommon and only one occurrence of till attributed topre-Holstein cold stages (prior to MIS 9 or MIS 11) has been found in Finland (Johansson et al., 2011).
From the Weichselian glaciation (MIS 5d - MIS 2) there is more evidence and unlike the southern parts of
Fennoscandia that was not fully glaciated until MIS 4 c. 70 ka and the last glacial maximum (LGM, MIS
2) c. 20 ka, the northern parts underwent at least three phases of stadials and interstadials throughout theWeichselian glaciation (Johansson et al., 2011, and references therein). The ice sheet history in northern
Finland has, however, proven to be quite dynamic as recent data from Sokli suggests that the area was
unglaciated until MIS 5b (Alexanderson et al., 2008; Helmens et al., 2012) and that it even was ice freeduring parts of MIS 3 (Helmens, 2014).
9

In the subsequent extensive cold stage, the LGM (MIS 2), the ice stretched far beyond the Fennoscandian
peninsula connecting with the ice over Barents sea (The Barents-Kara ice sheet) to the north and the study
area was at this period underneath the interior parts of the Fennoscandian ice sheet (Hughes et al., 2016).
During the Younger Dryas the ice front remained outside of the study area, in the Kola peninsula to theeast of the study area, meaning that the study site was not deglaciated until relatively late, during early
Holocene. Data from models made by Hughes et al. (2016), Stroeven et al. (2016) and Patton et al. (2017)
are providing evidence for a very rapid deglaciation of the remaining ice sheet after the end of Younger
Dryas. Radiocarbon datings from lake Loitsana in the nearby afformentioned Sokli Carbonatite Massif
time the deglaciation to around 10700 cal. yr BP (Shala et al., 2014a). At this phase of the deglaciation
the ice divide was cutting through just north of the study area, stretching in a W-E direction, and theice sheet retreated in a northwestern direction as evident by the esker systems in the region (Johansson,
1995).
In the Värriötunturit area glaciofluvially eroded gorges are prevalent (Johansson, 1995), with the mostprominent ones being Syväkuru and Pirunkuru. These gorges were most likely carved out over multiple
deglaciations, which is evident by terraces of deposited boulders at the mouth of Syväkuru and Pirunkuru where cosmogenic nuclide dating has yielded ages older than the last deglaciation (Karin Helmens,
personal communication). The gorges were probably partly eroded by subglacial meltwater prior to theice retreat, as the hydrostatic pressure enables water flowing independently of topography (Johansson,
1995). However, an esker chain is stretching on top of Kolmonen instead of following the valley throughSyväkuru, showing that at least during parts of deglaciation the subglacial channels diverted into otherroutes. Erosion also took place after the deglaciation by proglacial meltwater, which is noticeable by thelack of material left inside the gorges, as well as the very clear channels cut into the sediment in a fan
shape east of Syväkuru.
2.3

Kuutsjärvi and its immediate surroundings

As mentioned in section 2.1, Kuutsjärvi itself is situated in a gorge-like valley between Ykkönen andKotovaara (Fig. 1), with steep sides both to the north and south of the lake (see right-hand photo in Fig.
2). Although the depression is not as deep and distinct as the other gorges described above, it is clear thatthe lake must have been carved out by glacial meltwater in a similar fashion as Syväkuru and Pirunkuru.
This is also strengthened by the bathymetry of Kuutsjärvi, presenting a very deep profile with a sediment
depth of more than 6 meter and an overlying water depth reaching up to 8 meters in the majority of thelake, with only a thin strip on the eastern part, near the outlet, with shallow water. On the basis of thesecharacteristics, the lake can be described as a plunge lake (see e.g. Meinsen et al., 2011) which musthave been created by especially heavy flows of meltwater during the last deglaciation, or more likely, last
several deglaciations. Today the lake has an inlet situated at the western side and an outlet (Kuntasjoki)
at its eastern side (Fig. 3). The inlet and outlet are very small streams in comparison to the wide channel
they are flowing through, thus making it unlikely these streams have been responsible for any significant
preglacial or postglacial erosion of the valley.
The lake is oligotrophic (Tallberg et al., 2015) and is (except for the research station) surrounded byforests (Fig. 2). At its southern side, i.e. on the northexposed steep slope, the adjacent vegetation is lushand primarily covered in spruce (Picea abies), birch (Betula pubescens/Betula pendula) and aspen (Populus tremula) with undergrowth of blueberry (Vaccinium myrtillus), lingonberry (Vaccinium vitis-idaea)
and ferns. At the northern side however, the soil is dryer and the vegetation is primarily composed of pine
(Pinus sylvestris) with little undervegetation present. Both the inlet and outlet are densely vegetated withtall herbs and willow (Salix spp.), and the beginning of the outlet also forms a small wetland. Furthermore,
the lake is fringed by a narrow strip of aquatic, shore and wetland vegetation consisting of shrubs (e.g.
Betula nana, Salix spp.), dwarf shrubs (e.g. Calluna vulgaris, Empetrum nigrum, Ledum palustre, Vaccinium spp.), herbs (e.g. Carex spp., Menyanthes trifolata, Cornus suecica, Chamerion angustifolium),
ferns (e.g. Equisetum spp.) and mosses (e.g. Sphagnum spp.). The catchment area of the lake (drawn in
Fig. 1 based on digital elevation model data) is relatively small and mainly comprises of areas south ofthe lake, up on the hills of Ykkönen and Kämppälaki.
10Figure 2. Photos of Kuutsjärvi photographed during the summer of 2018 (left-hand side of figure, looking towards the southeastern part of the lake) and spring of 2019 (right-hand side of figure, looking towards the southwestern part of the lake; photographer: Karin Helmens). In both photos it can be seen that the forest reach near the shoreline of the lake, and that the shore
vegetation zone is very narrow. In the winter environment of the right-hand photo the steep rising slope on the southern side ofthe lake is especially visible.

3

Material and methods

3.1

Field work and subsampling

The coring of Kuutsjärvi was conducted prior to this study during the winter in the beginning of 2016. Two
sediment cores of approximately 5.6 m each (subsequently named KJ1 and KJ2, and hereafter referred
to as such) were retrieved in 2 m-sections from the middle part of the lake using a heavy PP-piston
corer (Putkinen and Saarelainen, 1998) with a diameter of 6 cm. The approximate distance between
the boreholes were circa 1 m (Karin Helmens, personal communication) and while GPS coordinates were
never documented, the approximate position of the coring was noted on a map. This approximate position
is marked out in Fig. 3.
Immediately after the coring of these two main cores, one of them, KJ1, was subsampled at theVärriö Subarctic Research Station next to the lake (see proximity of the station in Fig. 3). Two distinct
sediment sections were noted in the core; organic bearing sediment (predominantly gyttja or diatom richgyttja) and minerogenic rich sediment interlayered with gyttja. Subsampling of the gyttja (1-541 cm) was
done in 1 cm samples without considering different lithological units, while the bottom more minerogenic
part of the core (541-562,5 cm) was subsampled after lithology (see further detailed description of thelithostratigraphy in section 4.1). The other core, KJ2, was kept intact for ITRAX-scanning (to obtaingeochemical data) and ancient DNA analysis (neither included in this study). Both the sample bags from
KJ1 and the intact KJ2 core were after fieldwork brought to the University of Helsinki where they werestored in a cold room at the Department of Environmental Sciences. All of the above was done before
this study started. However, due to the lack of available sediment left for macrofossil analysis in some of
the KJ1 sample bags from the bottom minerogenic section of the core, the same bottom section of KJ2
also had to be subsampled. To make it easier to match the two cores with each other, this subsampling
was done in exactly the same lithological units as for KJ1, and during this subsampling a more detailed
sediment log was also drawn (see section 4.1 and Fig. 4).
In addition to the field work in 2016 prior to this study, another field work was conducted in the study
area in June 2018, as a part of this study. During this fieldwork the present environmental setting andgeomorphology of the area was investigated, which included aerial photography using a drone. To geta picture of the present-day macrofossils preserved in the sediment, surface sediment samples were also
retrieved from Kuutsjärvi. These samples were collected from a small boat using a Limnos sedimentcorer (a description of it is available in Kansanen et al., 1991) and only the top few cm of the sediment
was transferred into a small plastic bag on site. Several samples were taken, but only three of them were11

Figure 3. Close up map of Kuutsjärvi showing its inlet and outlet (Kuntasjoki), as well as its proximity to the Värriö Subarctic
Research Station. The green circle mark out where the coring of KJ1 and KJ2 was undertaken in beginning of 2016, and the red
triangles mark out where the surface sediment samples were taken during the summer of 2018.

later analysed, one near the inlet, one in the deep part of the lake, and one close to the outlet. These threesample spots are noted in Fig. 3. After arriving back in Stockholm the sample bags were stored in a fridge
for seven months until analysis in January 2019.
3.2

Macrofossil preparation and analysis

When radiocarbon dateable macrofossils were collected from the KJ1 sample bags by Minna Väliranta(prior to the start of this study), preliminary analyses showed that the upper part of the sediment corecontained only scarce macrofossil remains. Accordingly, in this study it was decided early on not toanalyse the whole core, but rather focus on the minerogenic part and the lower part of the gyttja, as theearly Holocene is of most interest to the multi-proxy project on Kuutsjärvi. Subsequently, only sample
bags from the depths 340-541 cm were included, as well as the sample bags from the minerogenic part(hereafter referred to as ’A-samples’, see further description in section 4.1). All of the A-samples wereanalysed, while the gyttja samples were analysed in a 4 cm interval. On a few occasions when a sample
bag was missing from the gyttja sequence, or when there was very little material left in a particular bag, adeviation from the 4 cm interval was allowed. Specifically this was the case between the depths 369 and375, resulting in an unanalysed gap of six cm; between 413 and 418 cm, a gap of five cm; and between
461 and 466 cm, also a gap of five cm. Consequently nearby samples close to these deviating depths arealso instead analysed in a denser interval.
The material left for macrofossil analysis following other analysed proxies varied slightly in the sample
bags, but were for the most part around 20 ml within the gyttja samples (with two sample bags containing
only around 15 ml and a few ones slightly more than 20 ml), and between 15-50 ml within the A-samples.
The greater variance in sample volume within the minerogenic section is due to the varying thicknessof each unit (see description of the lithostratigraphy in 4.1), but regardless all available sediment left inthe each bag were analysed. For the contemporary surface samples, approximately 50 ml were analysedfrom each sample bag. Note that the macrofossil count in the resulting diagram was not adjusted for the12different volumes, because of differences in sedimentation rate between the gyttja and minerogenic richsediment.
Before analysis the sediment was sieved using a 125 µm mesh sieve to get rid of fine minerogenic material but still ensure the retainment of even the smallest seeds. The analysis of the remaining sediment
was then carried out by putting small amounts of material at a time in a gridded petri dish, whereafter
it was scanned through using a stereo microscope at 10x-40x magnification. In those cases when it wasnecessary to distinguish a specific pattern in e.g. a bark piece or a seed wing, a high-power light microscope with up to 100x magnification was used as well. All identifiable macrofossils were identified into
lowest taxonomic level possible and counted in absolute numbers, while unidentifiable macrofossils insome cases were estimated in relative abundance (subjective scale). This was for example the case with’unidentifiable moss remains’ (see Fig. 8), where it was possible to determine that the remains were ofmoss origin, but due to decomposition and missing diagnostic parts of the remains, not any further details
than that.
As reference literature for the most common macrofossils found in northern Europe, Birks (2013)
and Mauquoy and van Geel (2007) was used. Identification of more specific macrofossil details are
based on the following literature: Eurola et al. (1992), Hallingbäck and Holmåsen (1995) and Laineet al. (2016) for mosses; Berggren and Anderberg (1981), Anderberg (1994) and Cappers et al. (2006)
for seeds; Sweeney (2004) for needles; Tomlinson (1985) for buds and budscales; and Kats et al. (1977)
for bark. For information on ecology of the identified species, Mossberg et al. (1992) and Anderberg andAnderberg (2019) was primarily used. The stratigraphic taxa diagram (Fig. 8) was compiled and plottedusing the program C2 version 1.7.7 (Juggins, 2016). Zones in the diagram was determined by visualinspection of the data.
3.3

Loss on ignition, radiocarbon dating and other proxies

While the focus in this study is solely on macrofossil analysis, other proxies such as pollen, diatoms,
chironomides, as well as loss on ignition (LOI, to determine organic carbon amount), have also been
analysed using the same sediment by researchers affiliated with University of Helsinki and Stockholm
University. As these proxies are not within the scope of this thesis, and have not been analysed as apart of it, they will not be included as separate entities. However, when it comes to loss on ignition
values, radiocarbon ages and some selected pollen taxa, these will be presented along with the macrofossil
analysis data presented in Fig. 8 (see section 4.4) to enhance interpretation. Therefore a brief descriptionof their methods will follow here:
Loss on ignition: The loss on ignition analysis was carried out by Sanna Piilo at the Department ofEnvironmental Sciences at University of Helsinki during the autumn of 2016, using material from theKJ1 samples bags. General LOI methodology was used, with sediment first drying at 105◦ C and then
combusting at 550◦ C, thus avoiding combustion of inorganic carbonates. See e.g. Heiri et al. (2001) fora more detailed methodology description.
Radiocarbon dating: Material for radiocarbon dating was collected by Minna Väliranta from KJ1, using
two sample bags of the minerogenic section (A6 and A13, see section 4.1 for further description of thesediment) and 17 sample bags of the gyttja section. Of these six radiocarbon ages from the gyttja is
included herein, as only lake sediment between 340-541 cm was used in this study (a radiocarbon sample
from 332 cm is included as it is very close to 340 cm). The material collected for dating were terrestrial
plant remains, such as seeds, leafs or needles, this to avoid dating uncertainties associated with nonterrestrial material, such as incorporation of old carbon (Hua, 2009). The radiocarbon samples were taken
at different occasions, one in mid 2017 that was sent to Poznan Radiocarbon Laboratory and another one
in late 2018 that was sent to the Helsinki Accelerator Laboratory. The resulting radiocarbon ages (Table
1) were calibrated (in this study) with Clam 2.2 (Blaauw, 2010) in R 3.6 (R Foundation for StatisticalComputing, 2018) using the IntCal 13.14 calibration curve (Reimer et al., 2013). An age-depth modelusing smooth spline (0.5 smooth setting) was created for the six samples in the gyttja section, whereof one
13

Table 1: AMS radiocarbon ages, calibrated ages and weighted average ages (from the age-depth model) of macrofossils collectedfrom the KJ1 sample bags. The first letters in the sample code indicate which laboratory that analysed the material; Poz =Poznan Radiocarbon Laboratory and Hela = Helsinki Accelerator Laboratory. When calibrated results occurred as two or moreprobability intervals, all intervals with a probability of 5% or more are included as separate rows. The number in brackets are
the probability for each interval. All ages are in yr BP.

Sample code

Depth (cm)

14 C age

Calibrated age (2σ)

Weighted avg.

Hela-4215
Hela-4216
Poz-91405
Hela-4217*
Hela-4218

332
371
417
462
506

6408 ± 53
7044 ± 62
8000 ± 407534 ± 72
8962 ± 37

7259 - 7426
7731 - 7976
8722 - 9007
8186 - 8446
9926 - 9998 (22.9)
10001 - 10064 (16.3)
10119 - 10225 (55.7)
10372 - 10609 (84.3)
10612 - 10660 (6.9)
9602 - 10157
10248 - 10589 (93.7)

7338
7979
8731

Poz-91406

540

9310 ± 50

Hela-4220*
Hela-4219*

550.5 (A6)
561.9 (A13)8801 ± 769259 ± 73

*

10087
10580
*
*

* Sample excluded from the age-depth model, see text for more information(Hela-4217) was rejected as an outlier. The two samples from the minerogenic section (Hela-4220 andHela-4219) were also excluded from the age-depth model as the sedimentation is expected to have been
instantaneous deposition events (see section 4.1). These samples were instead calibrated individually,
also with Clam 2.2 in R 3.6. See Table 1 for details about the samples, radiocarbon ages, calibrated ages
and weighted average ages (based on the age-depth model).
Pollen analysis: The pollen samples were prepared and counted by Sakari Salonen at the Department ofGeosciences and Geography at University of Helsinki. The same methodology described in Salonen et al.
(2018) was used. Note that as this study focuses on macrofossils, only a small selected taxa consisting oftree-type Betula and Pinus pollen is included in the plant taxa diagram (Fig. 8).
3.4

DEM analysis of geomorphology and topography

A part of this study has been to try to reconstruct the deglaciation pattern of the area, in order to explain thesequences of minerogenic sediment in the bottom of the Kuutsjärvi sediment core (as described briefly
in section 3.1, and in more detail in 4.1). This reconstruction has been done visually by inspection ofgeomorphology and topography, using a high-resolution (2 m) digital elevation model (DEM) based onairborne LIDAR scanning. The data was downloaded as a DEM image (georeferenced .tiff file) available
from the National Land Survey of Finland, and then processed further using Esri ArcGIS 10.6 (Esri,2018). In the program the Hillshade tool was used to get a 3D shading of the surface, and thus bring out
the geomorphology from the DEM. Different settings were elaborated with to make sure that all details in
the geomorphology was considered. In addition to this, in order to enhance topographical understandingof the area, a colour ramp was added to the underlying DEM and the Contour tool was also used create
isolines over the area.

4

Results and interpretation

4.1

Lithostratigraphy

Both KJ1 and KJ2 retrieved from Kuutsjärvi demonstrate the same general pattern in the lithostratigraphy. The sediment consist predominantly of organic rich sediment, in the form of gyttja or more or less14

Figure 4. Sediment log drawn from KJ1 (gyttja part) and KJ2 (minerogenic rich part), showing: A. full depiction of the sedimentused in this study, between 340-562.5 cm; and B. magnification of the bottom minerogenic rich part (541-562.5 cm) displaying
the two distinct overflow events. The colours of the sediment in this depiction represent near original sediment colour, and does
not indicate any other significant aspect of the sediment. Blue numbers are radiocarbon ages (non-calibrated, yr BP). Note thehiatus in B between approx. 560.5-561.5 cm.

mineral rich gyttja, overlaying a lower section containing minerogenic material interlayered with gyttja.
A depiction of the sediment can be seen in Fig. 4, wherein KJ1 was used to draw the gyttja part and KJ2
to draw the minerogenic rich part.
4.1.1

The minerogenic section

The base of the core, between 541 and 562.5 cm, is dominated by minerogenic material and will hereafter
be referred to as the minerogenic section. As this section only comprises of approximately 20 cm in total
15

while at the same time displaying a great deal of variance, it is magnified into a separate log in Fig. 4B.
In the figure the A-sample codes for each subsample unit are noted, and these will for the most part beused instead of the depth when referring to different lithological parts in the text below.
Two main types of sediment are present in the section: silty gyttja in the very bottom between c.
561.5-562.5 cm (A13) and between c. 550-552 cm (A7 and A6); and organic bearing minerogenic sediment in the rest of the section (between A12-A8 and between A5-A1), in which dark organic laminations
and occasionally clearly visible plant fragments are incorporated. The sequences of silty gyttja show
some shifts in colour but are generally lacking other characteristics, such as visible organic material. The
sequences of minerogenic sediment on the other hand are normal graded from gravel and sand into finesilt and clay, and the interpretation is that these represent glaciofluvial sediment from so called ’overflow
events’, or pulses of glacial meltwater periodically transporting minerogenic material into the lake. Key
characteristics for this interpretation is the clear pattern of very coarse to very fine material (i.e. normal
grading sequences), with the inclusion of fine silt and clay (glacial flour) at the top of the sequences.
Furthermore are the fact that the coarsest material, especially the gravel seen in Overflow Event I, would
require such high energy it is unlikely it was transported into the lake by any other means. The only
reasonable alternative deposition process would in that case be through mass movements in the sedimenton the lake side (due to the steep sides of Kuutsjärvi). However, this does not explain the very high abundance and variety of macrofossils in the record (see section 4.4), as well as the inclusion of water animal
remains such as Daphnia ephippium and Plumatella statoblasts throughout the events (also described in
section 4.4), showing they were clearly more gradual in contrast to abrupt mass movements.
During these overflow events, the initial influx of coarse material probably took place in late springand early summer, while the subsequent finer sand and silt is thought to have been deposited throughout
the rest of the summer and autumn, in conjunction with more organic material (hence the darker colourof this sediment). Lastly, the overlying clay most likely stayed in suspension until the freezing of thelake during winter, explaining the lack of plant remains in the clay units. Consequently this suggests theoverflow events occurred during a single year each, although not necessarily annually. The sediment insample unit A12 is notable as it is either a precursor to Overflow Event I deposited during the same year,
or a separate overflow event entirely. However due to its lack of fine silt and clay at the top, which areclear characteristics of the the other two events, a separate overflow event can most likely be ruled out.
Moreover, the sand layer in A12 was observed to be much thinner in KJ1 (noted before subsampling), in
contrast to the rather thick layer seen in KJ2 (depicted in Fig. 4), suggesting it may have been a more
localised influx linked to Overflow Event I. The hiatus seen just below the A12 sand layer in KJ2 wascaused by sampling for ancient DNA from the gyttja below, prior to this study (Karin Helmens, personalcommunication).
It should also be noted that these two particular cores (KJ1 and KJ2) did not reach the bottom of thesediment in Kuutsjärvi. Below the two overflow events seen in the minerogenic section of this sediment
sequence, a series of additional thinner overflow sediment sequences were found in previous cores retrieved in 2014. These earlier overflow events does not contain as much coarse material, but still havethe distinct grading from sand into fine silt and clay, similar to the events in Fig. 4B. In the lowermost
part just above the bedrock, a thick silty-clayey unit is found, and this is interpreted to have settled in the
Kuutsjärvi depression right after the first catastrophic meltwater discharge (see further description of thisin section 4.2).
4.1.2

The gyttja section

In the upper section (340-541 cm), which is dominated by gyttja and hereafter referred to as the gyttja
section, layers of yellow bands are spread unevenly throughout the core. In Fig. 4A the clearest and mostdensely packed bands are marked out, even though it should be noted that the entire gyttja section was
rather yellowish in colour. Investigation into the yellow bands prior to this study revealed that they weremade up of mostly silica, with the interpretation that they represent biogenic silica from diatom blooms(Karin Helmens, personal communication). This gyttja can therefore be described as diatomaceous gyttja.
Generally the gyttja section does not seem to present any large shifts, but the grain size varies slightly
16

Figure 5. Schematic illustration of the deglaciation pattern in the study area, showing how glacial meltwater from the retreating
ice sheet was flowing over different cols into several gorges during separate episodes of the retreat. See section 4.2 for further
explanation. During Episode B when meltwater was running through Kuutsjärvenkuru the distance between the coring site andthe ice sheet was c. 300 m. Note that the yellow labels indicate elevation at the thresholds in present day m a.s.l. This map
contains data from the Elevation Model 2 m database provided by the National Land Survey of Finland under CC BY 4.0 license(downloaded 2018/10).

within the silt spectrum ranging from silty clay to clayey silt or silt. The laminations in the sediment also
tend to be denser in the bottom part (between approximately 445 and 541 cm) than at the top (between
approximately 340 and 445 cm). Two thin but distinct sand layers are found at c. 535 cm and c. 490 cm,
each of them interpreted as to have been deposited during a single year, probably during spring snowmelt
or heavy summer/autumn precipitation.
4.2

Deglaciation of the Värriötunturit area

Based on what was found in the composition of the bottom minerogenic section of KJ1 and KJ2, together
with geomorphological evidence found in the high-resolution (2 m) digital elevation model over the area, aprobable deglaciation pattern is here proposed (Fig. 5). Johansson (1995) did a similar reconstruction over17

the Värriötunturit area, but in much coarser time slices and involving a slightly larger area compared tothis study. That particular reconstruction does however not fully explain the coarse minerogenic material
seen in the bottom sediment of Kuutsjärvi, and some aspects of it does not seem to match with evidence
now seen in the high-resolution DEM (which was not available in the 1990’s). Therefore, the deglaciation
pattern presented herein is primarily meant to use available geomorphological and topographical evidence
in order to explain how it is possible to have overflow sediments interlayered with gyttja in the early phasesof Kuutsjärvi’s lake evolution. The four episode time series on the deglaciation presented in Fig. 5 isdescribed in detail as follows:
Episode A: Initially only the peaks of the Värriötunturit fell chain and the low-lying areas to the east were
ice free, while the ice remained thick enough to be able to discharge its meltwater through the Syväkuru
gorge situated between Kakkonen and Kolmonen (395 m a.s.l). Exactly how much of the northeasternpart of the area that was ice-covered at this stage is difficult to estimate based on this data alone. However,it remains clear that the area directly east of Syväkuru must have been ice free, as the water flow evidently
carved out distinct channels in the sediment (as seen on the DEM of Fig. 6). These channels also suggest
the water torrent must have been very heavy, as the meltwater flow spread out in a fan shape and depositedboulders outside of the main channel. The water deviating northwards appears to eventually came to ahalt, as the traces in the sediment suddenly disappears (Fig. 6). This can be interpreted as the remaining ice
sheet blocking the meltwater from flowing further, and thus the ice sheet extent to the northeast presented
in this reconstruction (Fig. 5A) would seem reasonable. The southern stretch of the subglacial channels
were based on the clear esker chain left behind. In the deglaciation reconstruction made by Johansson(1995) an additional subglacial channel was inserted north of Ykkönen, but there does not not seem to be
any direct evidence for that in the DEM, neither depositional nor erosional, and therefore it was left out
in this reconstruction for simplicity.
Episode B: As soon as the ice retreated below the col west of Syväkuru (395 m a.s.l.), the gorge driedup and meltwater started to accumulate between the ice sheet and the Värriötunturit fell chain. Due to the
topography of the region (see Fig. 1 for a wider perspective), in combination with the northwestern ice
sheet retreat, the only drainage path possible was alongside Kakkonen and Ykkönen towards the north.
The accumulated meltwater was therefore released first after the ice sheet had retreated enough for the
opening of the Kuutsjärvenkuru depression (354 m a.s.l.). Clear ice-marginal channels to the west of
the Värriötunturit fell chain, as well as the deeply eroded Kuutsjärvenkuru depression in itself (Fig. 5
and Fig. 6), indicate that this first phase of meltwater discharge was very heavy. However, following
this initial phase of erosion it is likely that the meltwater only spilled over periodically, depositing the
aforementioned overflow sediments (section 4.1) into Kuutsjärvi. This becomes evident when taking into
account that the initial flood-like erosion likely lowered the elevation of the col, and thus allowed the ice
to discharge meltwater through Kuutsjärvenkuru during a longer period of time. The same mechanism
is mentioned by Mannerfelt (1945) in relation to the drainage of the Drommen ice lake in Jämtland,
northwestern Sweden. Each overflow event is interpreted to have taken place just during a single year,
which suggests the time period in which the ice was capable of discharging overflow sediments into
Kuutsjärvi was likely fairly short, presumably merely during approximately 10 years time (see section 4.3
for more details about this time estimation). During this time period the ice sheet was standing only c.
300 m northwest of Kuutsjärvi (Fig. 5B).
Episode C and D: As soon as the ice retreated further to the northwest, the water could drain throughPirunkuru (334 m a.s.l.) instead of the higher elevated Kuutsjärvenkuru (354 m a.s.l., Fig. 5C). This
marks the end of the overflow events stage of Kuutsjärvi, and the start of the continuous lake phasecorresponding with the depth 541 cm in the sediment record (see section 4.1 and the sediment log in Fig.
4). When the ice retreated into even lower topography (approximately 320 m a.s.l.), the water could nolonger drain through Pirunkuru, but instead took the path nortwards west of Jyppyrävaara (Fig. 5D). Thisfurther separated the ice sheet and its meltwater from influencing the environment in the immediate areaaround Kuutsjärvi.
18

Figure 6. Raw hillshade map created from the DEM showing the meltwater channels to the west of Ykkönen and Kakkonen
flowing into Kuutsjärvenkuru, as well as the deep channels to the east of Syväkuru. The blue polygon is Kuutsjärvi. This map
contains data from the Elevation Model 2 m database provided by the National Land Survey of Finland under CC BY 4.0 license(downloaded 2018/10).

4.3

Chronology

Eight radiocarbon samples were dated in the sediment sequence interval used in this study (including
one sample from 332 cm, see section 3.3 and Table 1). For the gyttja section, an age-depth model was
created (Fig. 7) which positioned the uppermost depth (340 cm) at c. 7500 cal. yr BP and the lowermost(541 cm) at c. 10600 cal. yr BP. One sample (Hela-4217, see Table 1) was excluded from the model andconsidered an outlier, since it was not in line with the other samples. The age-depth model shows thatthe sedimentation rate is relatively linear throughout the gyttja section, with each cm corresponding to c.
14.5-16 years, suggesting a high temporal resolution of the sediment sequence. The sedimentation rate isslightly higher in the lowermost part and is decreasing towards the top. Two radiocarbon samples fromthe minerogenic section were calibrated individually, and the lowermost sample (A13) was in line withthe basal gyttja age, while the mid sample (A6) appears to be too young at c. 9600-10150 cal. yr BP. As
mentioned previously, the interpretation is that each overflow event took place during a single year each,
19

Figure 7. Age-depth model created using the six radiocarbon ages from the gyttja section, whereof one was rejected (marked inred). Note that the uppermost sample is from 332 cm, i.e. slightly outside the sediment interval used in this study.

because of the clear grading profile from coarse to fine material in the sediment record. The gyttja in
between could however potentially have been deposited during several years, as the overflow events were
not necessarily annual events, but the time period for the whole overflow event sequence, including thefive smaller events not present in the KJ1/KJ2 sediment record (see section 4.1), is not likely to be longer
than 10 years. Based on this interpretation, the suggested radiocarbon age in A6 is unrealistically young,and it is therefore considered an outlier.
The deglaciation age of c. 10600 cal. yr BP correspond quite well with the nearby lake Loitsana in
Sokli, where the bottom of the sequence (the deglaciation) was dated to c. 10700 cal. yr BP (weighted
average age from age-depth model, see Shala et al., 2014a,b). While this is slightly older than the weighted
average age in this study, the 2σ probability interval of the calibrated ages are within the same scope(Table 1), even though the interval in Shala et al. (2014b,a) is larger, which is likely related to a larger
radiocarbon age error interval. According to Johansson (1995, 2007) the Sokli area deglaciated lessthan 100 years after the Värriötunturit area, thus it is expected these two areas would yield roughly thesame deglaciation age, or with a slightly older age for Kuutsjärvi. This rapid ice retreat rate also further
strengthen the interpretation that the overflow events were deposited over a single year, and that the whole
overflow event sequence was deposited during a very short time period.
4.4

Proxy interpretation and environmental reconstruction

The macrofossil taxa is summarised in Fig. 8 together with loss on ignition (LOI) values and the selected
pollen taxa of tree-type Betula and Pinus provided by Sakari Salonen. From the results, four main zones
could be distinguised, whereof Zone I was further subdivided into four subzones. Zonation was basedon visual inspection of the plant composition in combination with sedimentological changes (see Fig. 4).
The remains found were in general very well-preseved, especially those in the overflow event units of thelowermost zone, which would rule out the possibility of redeposition of old interglacial material following
the deglaciation. This is nevertheless also shown by the radiocarbon dating of the remains (Table 1). An
assortment of the macrofossils found during the analysis are shown in Fig. 9.
20

4.4.1

Zone I, 562.5 - 541 cm (c. 10600 cal. yr BP)

Zone I represents the ’overflow event stage’ of Kuutsjärvi, where the gyttja sedimentation was interrupted
by large amounts of minerogenic influx, transported by glacial meltwater from the retreating ice sheet (seesection 4.1 and 4.2). In order to better describe the different parts of this stage, the zone was further divided
into four subzones, in which Zone I-b and Zone I-d represent the two separate overflow events, while
Zone I-a and Zone I-c represent the in-between gyttja sedimentation (Fig. 8). In the pollen record Pinus
pollen percentages stay below 1% throughout the zone, while tree-type Betula pollen is rising throughout,starting from approximately 30% in the lowermost part of the zone and reaching above 80% in the upper
part, which in combination with the tree-type Betula macrofossil remains indicate the establishment ofan open sub-arctic birch forest in the area. Characteristic for the whole zone is the generally high amountof well-preserved remains in contrast to successive zones (even though some seeds among the herbs were
only possible to identify into genus level), both in terms of absolute number and variety of taxa found.
More specifically the highest variety and abundance of macrofossil remains are found in the overflow
sediments, and there is a distinct difference in amount and variety between the gyttja and overflow events.
The details of the subzones are described under each corresponding section below.
4.4.1.1

Overflow sediments (Subzone I-b and Subzone I-d)

In the overflow event subzones the organic content drops below 5% (Fig. 8) and the sediment changes toa sequence of gravel or coarse sand grading into silt and clay (Overflow Event I and Overflow Event II,
see Fig. 4B). Well-preserved plant remains of tree-type Betula and dwarf shrubs (Betula nana, Salix sp.
and Empetrum nigrum) dominate the macrofossil assemblage in the overflow sediments, together withdifferent kind of seeds from herbs, where Chamerion angustifolium, Cerastium sp. and Juncus spp. areespecially prevalent. Leaves of mosses (Bryum sp., Polytrichum strictum and Polytrichum jensenii) arealso present, as well as possibly a single remain of a Pinus budscale in Subzone I-b (Fig. 8). This myriadof remains, which is a strong contrast to the lack of remains in the over- and underlying gyttja units, wastransported with the meltwater and must have originated from plants growing in-between the ice-margin
and the lake, i.e. in an ice-marginal environment since the ice sheet supposedly was standing only a few
hundred meters away from the lake (see Fig. 5B, section 4.2). This in turn suggests the area became
vegetated very quickly after the deglaciation.
While seeds or leaves from light demanding tundra herbs such as Dryas octopetala, Saxifraga cernua (cf.) and Cerastium sp. all indicate a rather open landscape (Mossberg et al., 1992; Anderberg andAnderberg, 2019), the high abundances of tree-type Betula macrofossils also suggests trees were alreadygrowing in the area at this stage. Although no pollen samples were analysed in the overflow sediments, thegyttja underlying Subzone I-b contains tree-type Betula pollen percentage values of c. 30%, whereas theoverlying gyttja contains c. 70% (Fig. 8). Modern pollen spectras suggest that a Betula pollen percentage
of 50-60% would indicate the presence of birch forest (Prentice, 1978), and it is thus likely that the area
already very soon after deglaciation was covered in pioneer birch forests. Since the hills of Ykkönen andthe other Värriötunturit peaks were deglaciated even earlier (see Fig. 5), it is also possible that some local
forests were thriving in pockets of favourable conditions already before the deglaciation of the low-lying
areas. Most likely, the birch forest was still rather open and incorporated a mix of trees and herbs, as e.g.
the highly abundant Chamerion angustifolium are common not only in entirely open environment, but also
in those kind of scattered birch forests (Mossberg et al., 1992; Anderberg and Anderberg, 2019). Many of
the other herbs prefer dry or sandy soil, such as Vaccinium vitis-idaea, Campanula rotundifolia and Dryas
octopetala, and these were likely also growing in the forest. The remaining herbs and shrubs, Epilobium
palustre, Epilobium dawuricum, Carex spp., Juncus spp., Rorippa islandica, Betula nana, Empetrum nigrum and Salix sp., often prefer wetter habitats (Mossberg et al., 1992; Anderberg and Anderberg, 2019)
and were likely growing near the stream channel, or close to the shore around Kuutsjärvi.
In general the macrofossil assemblages are very similar between the two overflow events, but thelithostratigraphy shows that Overflow Event II lacks coarser sediment in the form of the gravel presentin Overflow Event I (Fig. 4B), suggesting it was probably less energetic. Although this could possibly
21

22

Figure 8. Macrofossil and pollen taxa diagram for trees, shrubs, herbs, mosses, ferns and animals from the KJ1/KJ2 sediment sequence retrieved from Kuutsjärvi (KJ2 was only used for sample A6
and A13). Macrofossil taxa remains are expressed in absolute numbers and drawn as histograms, or expressed as + (present) or ++ (highly abundent). Pollen taxa are expressed as percentages of
total sum of terrestrial pollen and spores (not shown in this diagram). The diagram is divided into four main zones, in which Zone I is further divided into four subzones. The calibrated age axis
are based on the age-depth model in the gyttja section (Zone II-IV) and single calibration intervals (2σ) in the minerogenic section (Zone I, square icons indicate the depth where the radiocarbon
sample was collected), see Table 1 for details. Note the change in scale at approximately 530 cm.

explain the decrease in variety of herb species seen in Subzone I-d, the assemblage is nevertheless very
similar to Subzone I-b in terms of amount of plant remains. The only significant difference is that ferns
start to appear in Subzone I-d, and that it contains a lot more remains of mosses (Polytrichum jensenii,
Polytrichum strictum and Bryum sp.) and Salix sp. or undifferentiated Betula leaf fragments than Subzone
I-b (Fig. 8). All three moss taxa are related to wet habitats, whereof Polytrichum strictum commonly grows
on fens and bogs, while Polytrichum jensenii and Bryum sp. are often growing near stream channels or lakeshores (Hallingbäck and Holmåsen, 1995) where seasonal flooding might occur (Ulvinen et al., 2002).
This shows that the second weaker overflow event probably was confined to the stream channel, while
the first overflow event washed a larger area and thus also incorporated more plant remains from thesurrounding environment outside the channel.
4.4.1.2

Gyttja sediments (Subzone I-a and Subzone I-c)

Both the gyttja subzones consist of low organic (around 10%) silty gyttja containing few terrestrial plantremains compared to the minerogenic overflow sediments (Fig. 8). In the lowermost gyttja sequence(Subzone I-a) the only identifiable remains found were single seeds of Betula nana, undifferentiated Orchidaceae sp. and Poaceae sp., as well as two leaves of Polytrichum jensenii, thus providing no goodbasis for interpretation of the environment. The combination of few remains and tree-type Betula pollen
percentage at c. 30%, could however possibly suggest that the area around the lake was more scarcely
vegetated at this time, although it is not expected that any major changes would occur in the environment
between the two gyttja units since the time period is very short (see section 4.3). In Subzone I-c plantremains are recorded in greater numbers compared to Subzone I-a, and the general assemblage seems tofollow the overflow sediments (but with fewer herb and moss remains). This confirms that the taxa previously found in Subzone I-b (Overflow Event I) are also present in the environment close to the lake, since
the regular inlet stream likely was not transporting remains from as far away as the glacial meltwater.
In contrast to the small number of plant remains, both subzones do contain a high abundance ofaquatic animal remains in the form of Daphnia ephippium (water flea eggs; Ebert, 2005) and Plumatella
statoblasts (asexual dormant stage; Wood et al., 2005). However, both these animals are very common andcan tolerate many different lake environments, and therefore this might not indicate anything in particular.
The species of Plumatella was not determined, but the most common species in Norway (Plumatellafruticosa) prefer colourless and slightly acidic water conditions (Økland and Økland, 2002).
4.4.2

Zone II, 541 - 530 cm (10600 - 10400 cal. yr BP)

Following the last overflow event a shift into gyttja once again occurs, also around 10600 cal. yr BP(roughly the same as the deglaciation age, see section 4.3). Zone II represents sediment with more minerogenic material and less organic matter than following zones of the gyttja section, as evident by the lower
loss on ignition values. Throughout the zone the LOI is increasing, which can be an indication of soil
formation and further catchment stabilisation in the area surrounding the lake. Compared to the previous
gyttja unit (Subzone I-c), even less terrestrial remains are found, and although in general the same kindof tree, shrub and herb species are recorded as before, virtually all of them decrease in numbers (Fig. 8).
At the same time pollen percentages shows tree-type Betula almost reaching 90%, while Pinus pollen isstarting to increase only in the uppermost parts of the zone. This is probably reflecting pine becoming
more common in scattered occurrences in the area, even if birch are still the prevailing tree type of theforests.
The shrubs and herbs found in this zone (Betula nana, Salix sp., Chamerion angustifolium, Rorippa
islandica, Poaceae sp.) are likely part of the shore vegetation or the immediate forested area aroundKuutsjärvi, but many of the plant remains that were present in the overflow sediments and absent in thiszone were likely growing further away and thus required stronger water energy for transport into thelake. This was probably the case with the identifiable moss remains and possibly also fern sporangia
which disappeared entirely in this zone (Fig. 8). Interestingly, Plumatella statoblasts are also completely
absent, which is harder to explain as this taxa is, as mentioned previously, tolerable to many different
23

Figure 9. Photo examples of plant macrofossils found during the analysis (the depth or A-sample code where the depicted
remain was found is noted in square brackets next to the name): (A) Tree-type Betula seed [533 cm]; (B) Undiff. Betula seed(possibly hybrid) [A12]; (C) Betula nana seed [A8]; (D) Polytrichum jensenii leaf [A3]; (E) Salix sp. leaf [A2]; (F) Chamerion
angustifolium seeds [A7]; (G) Pinus sylvestris needle [421 cm]; (H) Rorippa islandica seed [A8]; (K) Orchidaceae sp. seed
[A9]. Black scale lines next to some macrofossils are 1 mm (size was not measured for all remains).

lake conditions. Possibly the lake water changed into some of the conditions Økland and Økland (2002)
lists as avoided by Plumatella fruticosa. However, it also cannot be ruled out that the reason why somemacrofossils suddenly disappears are due to factors related to decomposition, as the sediment in this zone
(as well as certain other parts of the gyttja section) was noted as particularly decomposed during analysis.
4.4.3

Zone III, 530 - 460 cm (10400 - 9400 cal. yr BP)

A slight increase in grain size can be seen in the beginning of Zone III simultaneous with a LOI stabilisation at c. 30%. In the macrofossil assemblage, the zone mark a shift with a sharp increase in fern
sporangia at the same time as Plumatella reappear in the record, at around 10400 cal. yr BP. Tree-type24

Betula start to appear less frequent in the samples as well, but remain at a high percentages (around 80%)in the pollen record (Fig. 8), suggesting Betula forests were still covering the surrounding area. The ferns
are probably a distinct element of the shore vegetation at this point, but it is possible that the birch forests
adjacent to the lake were also marshy and covered with an undergrowth of ferns, similar to what is seen in
the present-day forest on the southern lush side of the lake. This has also been reported in early Holocene
birch forests in western Norway (Karlsen, 2009). Strangely not a lot of other shore taxa were found in
the sediment, except for occasional single seeds of e.g. Rorippa islandica, Betula nana and Poaceae sp.
found scarcely throughout the zone. However, in the pollen record spores of Dryopteris (a fern genus)
were also found at high percentages around this time (Sakari Salonen, in preparation; not included in Fig.
8), further strengthening the interpretation that ferns were very abundant in the local environment.
In the upper part of the zone, high abundences of Betula or Salix leaf remains are found in two samples.
This sudden increase in single samples is hard to definitely explain, but could maybe be related to eventsof very high water input into the lake, such as heavy rain during summer or autumn, or a severe snowmelt
season. All these deviations may also show that the taphonomic factors varied quite a lot within the zone,
which further stresses that the absence of macrofossils is no proof for the disappearance of the taxa fromthe surrounding environment.
4.4.4

Zone IV, 460 - 340 cm (9400 - 7500 cal. yr BP)

The last zone represents a transition in the environment from a forest dominated by birch into a mixedbirch and pine forest at around 9400 cal. yr BP. This is evident in the pollen record (Fig. 8), where asimultaneous decrease in tree-type Betula pollen is coupled with a sharp increase in Pinus pollen, inlevels commonly associated with pine-birch forests (c. 60% Pinus pollen, see diagram in Prentice, 1978).
In the macrofossil taxa the same pattern is observed, where the remains of tree-type Betula are slightly
reduced while Pinus remains appear in the form of needles and bark. Despite the fact these Pinus remains
were not detected until c. 440 cm., seemingly lagging behind the shift seen in the pollen record, it canalways just be a coincidence that the remains were not deposited at the coring spot. In fact, not a lotof macrofossils are found in this zone, thus making it hard to draw any definite conclusions from theassemblage. For example, the total absence of Betula nana and the decline in fern sporangia may notindicate anything in particular as both these taxa are present by the shore of the lake today, but were notfound in the surface samples (see section 4.6). It is however clear that the fern decline follows the increasein pine, and possibly this is a reflection of the forest in the area surrounding the lake becoming less dense
and wet at this stage.
A sharp decline in Daphnia ephippium and Plumatella statoblasts is recorded as well, at the sametime as Cristatella statoblasts are becoming more abundant (Fig. 8). Cristatella is usually associatedwith more coloured lake water, but also with warmer water temperature since it ideally avoid lakes withsummer water temperatures below 16 ◦ C (Økland and Økland, 2000). Apart from the change in taxa, ashift in the sediment can be seen (see sediment log, Fig. 4A) with the introduction of more frequent yellow
bands, indicating diatom blooms in the lake. Moreoever, the laminations in the sediment are becoming
less dense and the gyttja is more clayey in the beginning of the zone. This in combination with the increaseof the LOI to levels of 30-35% (Fig. 8) suggests that the catchment stabilised further due to soil formation
at this stage.
4.5

Temperature reconstruction

One objective of this study was to try to determine the minimum July temperature (Tjul ) in the early
Holocene based on presence of indicator plant taxa. However, it is evident from the macrofossil analysis
that none of the taxa commonly used as indicator species (see e.g. Isarin and Bohncke, 1999; Helmens
et al., 2012; Väliranta et al., 2015; Shala et al., 2017), for example many Potamogeton species, Najas
spp., and Typha spp. that are known to be controlled especially by temperature (cf. Väliranta et al., 2015),
were found in this study. Tree taxa such as Betula and Pinus are however sometimes used as indicatorspecies and are often attributed minimum July temperatures of 10 ◦ C and 12 ◦ C respectively (see e.g.
25

Table 2: Macrofossil analysis results of three surface sediment samples from Kuutsjärvi (see sampling position for each sample
in Fig. 3). Numbers represent pieces found in absolute numbers in the sample, while + and ++ indicate the abundance as presencein the sample (+ = present, ++ = highly abundant). The volume of the samples were approximately 50 ml each and all of thematerial in each sample was analysed.

Macrofossil

Sample 1

Sample 2

Sample 3

Sarmentypnum s.l. (on lake bottom)
Unidentifiable moss remains
Undifferentiated bark/wood
Betula undiff./Salix leaf remains
Tree-type Betula seed or catkin
Empetrum nigrum seed/leaf
Pinus needle
Picea needle
Juniperus needle
Carex sp. seed
Daphnia ephippium
Cristatella statoblast

++
+
++

++
++
+

7

4

++
+
+
10
4
4

1
9
1

1
1
1
12
1

4

2

Helmens et al., 2012), but this probably more accurately depicts the tree line of these species and not
single occurrences of trees growing in favourable spots. Temperatures from trees should also ideally only
be inferred if evidence for sexual reproduction is detected (e.g. seeds), not based solely on e.g. needle orbark findings (Minna Väliranta, personal communication). As such, it is hard to conclusively say anything
about the temperature conditions of the area in this study, but this will be discussed further in section 5.
4.6

Contemporary surface sediment analysis

In order to have a modern reference, the present-day macrofossil assemblage were determined from contemporary surface sediment samples sampled from Kuutsjärvi in June 2018. The results of the analysisof this sediment are presented in Table 2. The sampling position for each sample referred to in the text
below are plotted in Fig. 3.
The plant remain assemblage found in this uppermost modern sediment record proved to be quite
similar to the fossil taxa in the gyttja section from Kuutsjärvi. Tree-type Betula seeds, leaves and catkinscales, as well as Empetrum nigrum seeds and leaves were found, in similar amounts as in the fossil
record, while the high abundances of conifer needles found here on the other hand was not detected in
the early fossil samples. Another species not found at all in the fossil samples were the Sarmentypnum(s.l.) moss, which at present seems to grow on the bottom of the lake as it was entangled in the coring
equipment in all samples. Moreover, fern sporangia is not found at all in these modern samples (despite
the fact that ferns were observed in field on the lush southern side of the lake), in contrast to their very
high abundance in the gyttja section of early Holocene. Otherwise interestingly is that the macrofossils
found depended a lot on where the sample was taken, indicating transportation by the inlet stream must
play a big role for which kind of remains that end up being preserved in the lake sediment. Sample 2,
retrieved from the middle pelagic part of the lake (see Fig. 3), contains more Daphnia ephippium than theother two samples, and presented a lot less plants remains. Sample 1, cored near the inlet, on the other
hand, provided a lot of needles, seeds and catkin scales, but fewer animal remains. Sample 3, retrieved
near the outlet, contained a lot of leaf and seed remains of Betula, together with a few Empetrum nigrum
seeds and Juniperus needles, but was lot less variable in terms of species diversity compared to the othersamples. Overall the material was fairly degraded, very similar to what was found in parts of the fossil
gyttja section of KJ1, with lots of remains rendered indistinguishable.

26

5Discussion

5.1

The ice-marginal environment during the early Holocene

The unique circumstances surrounding the deglaciation of the Värriötunturit area enabled a rare opportunity to reconstruct the climatic and environmental conditions not only in the early Holocene, but also
in an environment just at the margin of the retreating ice sheet immediately following the deglaciation.
The macrofossils found in the minerogenic section were in general very well-preserved (see Fig. 9) andradiocarbon dating of the remains yielded a calibrated age interval between c. 10300-10600 cal. yr BP,
both in the bottommost sample (A13) and the sample just above the minerogenic section (540 cm; see
Table 1 for details). This, which is in accordance with ice retreat reconstructions for Fennoscandia (e.g.
Stroeven et al., 2016; Hughes et al., 2016), is confirming a Holocene age for the sequence, and it canthus be concluded with no uncertainty that the macrofossils deposited were contemporary and not redeposited from previous interstadial or interglacial sediments. While no taxa that could be used as clear
temperature indicators were identified (more on this in section 5.4), the plant taxa assemblage in itself
suggests that the area surrounding Kuutsjärvi must have been productive and full of vegetation alreadyduring the overflow events, i.e. very shortly after deglaciation when the ice-margin was situated within
300 m from the lake. Pollen percentages and macrofossil assemblage together point towards the presenceof an open subarctic birch forest in the area at this stage, which is in stark contrast to the common view of
ice-marginal environments of the retreating ice sheet. Perhaps the sheltered depression of the meltwater
stream channel were a favourable environment for these pioneer birch forests.
One particularly interesting site to compare this study with is lake Loitsana in the Sokli Carbonatite
Massif area (67◦ 48’17” N 29◦ 16’46” E, 214 m a.s.l.), situated only c. 10 km to the northwest of Kuutsjärvi(see Fig. 1). The lake is unique as it harbours an unusually long sediment record incorporating not only
sediment deposited during the Holocene, but also sediment from the Weichselian interstadials (MIS 5d,
MIS 5c and MIS 3; Helmens et al., 2012, 2018) as well as sediment from the Eemian interglacial (MIS
5e; Helmens, 2000; Plikk et al., 2016). Immediately following the deglaciation of the Loitsana area
the large Sokli ice lake was dammed between the retreating ice sheet and the higher elevated areas to thesoutheast. The lake persisted for less than 100 years and partial drainage in three stages eventually createdan extensive shallow water environment. In the sediment from this shallow lake stage remains of treetype Betula as well as many high-temperature indicator plant taxa such as narrow-leaved Potamogeton
spp., Typha spp. and Nymphaea spp. was found, suggesting warmer than present summer temperatures
and a productive early Holocene environment (Shala et al., 2017; Helmens et al., 2018), similar to thereconstructed environment around Kuutsjärvi in this study.
However, during this productive shallow water stage the ice margin was situated more than 3 km fromthe coring spot, and during the larger lake stages when the ice-margin were closer, only low percentagesof Betula pollen and occasional macrofossil remains of tree-type Betula were preserved in the sediment(Helmens et al., 2018). This implies there might have been a difference between these two sites in termsof ice-marginal environment immediately following the deglaciation. However, the initial size of theice lake and distance from the shore in the first stages may likely have contributed to fewer macrofossil
remains preserved, as also suggested by Helmens et al. (2018). Furthermore the authors argue that a large
percentage of grasses in the pollen sum may have skewed the pollen-based reconstructions, hamperinginterpretation of the terrestrial vegetation. Large amount of Poaceae pollen (c. 26%) is only present in thevery beginning of the Kuutsjärvi sediment record (Subzone I-a), but then rapidly decreases to c. 7% in
Subzone I-c (Sakari Salonen, in preparation; Poaceae pollen is not shown in Fig. 8). There is a possibility
that grasses occurred in high percentages during the earlier overflow events not included in this study (seesection 4.1). Additionally, as briefly touched upon in section 4.4, even within Zone I there might havebeen a succession from a slightly more open into a denser birch forest near the lake, as evident by fewer
plant remains and lower pollen percentages of tree-type Betula in Subzone I-a compared to Subzone I-c(Fig. 8). In any case this succession must have been extremely rapid, as the overflow event stage likely
only comprise of 10-15 years at most (see section 4.1 and 4.3).

27

5.2

Migration of trees into the study area following the deglaciation

In many other studies in Fennoscandia birch forests are reconstructed during the earliest parts of Holocene
using pollen and macrofossil evidence retrieved from lake sediment records. In different locations on
the Norwegian west coast birch forest were found to have established c. 11000-10800 cal. yr BP (e.g.
Birks and Birks, 2008; Karlsen, 2009; Krüger et al., 2011), whereas in the central Norwegian mountains(above 1000 m a.s.l) the birch forest initiation is dated to c. 10300 cal. yr BP (Paus, 2010; Paus et al.,2011). In northwestern Norway, Bjune et al. (2004) reconstructed the establishment of birch forest in the
Dalmutladdo area (close to the Finnish border) soon after the deglaciation around 10200 cal. yr BP, which
is similar in time to the colonisation of birch forests in nearby Andøya at c. 10400 cal. yr BP (Vorren
et al., 2009) as well as on Kvaløya in northernmost Norway (Birks et al., 2012), both of which are locatedoutside the Younger Dryas ice extent (Stroeven et al., 2016). However, further to the east near the Varanger
peninsula, tree birch forests are suggested to have established as early as 11000 cal. yr BP (Seppä, 1996).
In Aareavaara in northeastern Sweden birch were recorded close to the retreating ice sheet (Möller et al.,2013), and in Kipojärvi in northernmost Finland birch forests were recorded in early Holocene at c. 9200
cal. yr BP, although this date is suggested by the authors to be a little too young (Väliranta et al., 2011b).
All in all this is consistent with the results in this study and suggests a very rapid migration of tree-type
Betula closely along the ice margin during the final retreat of the ice sheet, following the Younger Dryas.
A probable migration path into northern Finland and the northernmost parts of Norway are from refugia
in Russia to the east, rather than from the southern parts of Fennoscandia. In northeastern European
Russia evidence for scattered occurrences of tree-type Betula has been found during the early Holocene(Väliranta et al., 2006; Salonen et al., 2011; Väliranta et al., 2011a), together with evidence of lateglacial
refugia in the area (Väliranta et al., 2011a). Furthermore, lateglacial establishments of birch forests dated
to 14000-13000 cal. yr BP were found in the Rostov-Jaroslavl region of Russia southeast of Finland, andtree-type Betula were found to be pioneers in the recently deglaciated area of Russian Karelia (close to thesoutheastern Finnish border) c. 11500 cal. yr BP (Wohlfarth et al., 2007). Other studies in Fennoscandia
also suggests lateglacial refugia for trees, most notable in the central Norwegian Scandes where stomata
of Pinus and Picea was found on lateglacial nunataks (Paus et al., 2011), as well as in the central Swedish
Scandes where Betula pubescens, Pinus sylvestris and Picea abies megafossils where found on a similarlateglacial nunatak (Kullman, 2002).
A minor plant fragment suspected to be a Pinus budscale was found in Subzone I-b in this study, andalthough uncertain this could indicate that scattered occurrences of pine existed in the area already before
the transition into a mixed pine and birch forest at c. 9400 cal. yr BP. Due to the gentle low peaks of thestudy area any lateglacial survival of tree taxa is however unlikely, but the peaks of the Värriötunturit fell
chain were nevertheless deglaciated some time before the low-lying areas (see Fig. 5). The occurrence of
Betula trees and possibly Pinus on the Värriötunturit prior to the final deglaciation is therefore possible,
and with the case of tree-type Betula it might explain the apparent small discrepancy seen in the extremely
rapid migration of birch forests around Kuutsjärvi in comparison to the possible slower colonisation near
the Sokli ice lake. Although the peaks of Ykkönen and Kakkonen at present are above the forest linewith only scattered occurrences of trees (see Fig. 1), the peaks were due to glacial isostasy lower elevatedin early Holocene. The current absolute uplift per year is 6 mm in the region (Kakkuri, 2012), whichmeans the area was at least 64 meter lower elevated at the deglaciation. However, the uplift rate was
likely much higher in early Holocene, so a greater difference can be expected, and considering that thearea are just above the contemporary forest line at present-time, it is not unreasonable to assume the peaks
were forested during the early Holocene. In Abisko in the northern Swedish Scandes the tree-type Betulatree line is suggested to have been at an altitude of 300-400 m higher than present in the early Holocene(Barnekow, 1999; Barnekow and Sandgren, 2001).
5.3

Climatic conditions during the early Holocene

Some studies recording birch forests during the early Holocene are also suggesting higher than present
temperatures during the same time, reconstructed from pollen, indicator plant taxa or chironomids. This28

is consistent in e.g. central Norway (e.g. Paus, 2010; Paus et al., 2011), in northernmost Finland (Väliranta
et al., 2011b) and as previously mentioned in Loitsana, 10 km from the Värriötunturit area (Shala et al.,2014b; Helmens et al., 2018). Furthermore, Väliranta et al. (2015) recorded higher than present temperatures in Russia, and this same pattern has also been observed in other proxies such as the SST record in the
Barents sea (Hald et al., 2007). Altogether this also correspond with the increased summer insolation andreduced winter insolation at high latitudes during the early Holocene (Shala et al., 2014b, and references
therein), indicating greater seasonality and a more continental climate in northern Fennoscandia.
It is not possible to define any temperature interval for the Värriötunturit area due to absence of plant
indicator taxa, and possible reasons for this will be further discussed in the next section (5.4). However,based on evidence in the other studies in the nearby region it is likely the deglaciation took place in an
already warm climate in disequilibrium with the ice sheet, that created favourable conditions for rapid
birch migration and thus made it possible for trees to grow in very close proximity to the margin of theretreating ice sheet. In other words, it can be argued that the ice-marginal birch forests alone indicateswarm summer temperatures during the early Holocene.
5.4

The absense of high-temperature indicator taxa in the sediment

Despite this study providing strong evidence for birch forests covering the area around Kuutsjärvi alreadyat deglaciation age, no high-temperature indicator plant taxa were identified in the sediment record. Thisis somewhat unexpected considering the fact that the nearby aforementioned lake Loitsana site presented
warmer than present temperatures reconstructed from many different aquatic and semi-aquatic plants(Shala et al., 2014b; Helmens et al., 2018). However, there can be several explanations for this and someof those will be discussed below.
First, there is a possibility that these indicator taxa in fact were present in the environment adjacentto the lake but for some reason were not preserved in the sediment, or at least not in the middle part ofthe lake. While the overflow sediments (Subzone I-b and Subzone I-d) contained a variety of plants, it
seems like the shore vegetation was poorly represented in the gyttja section. It can be assumed that ferns
made up a major part of the shore vegetation in Zone III, but the question is why an abundance of ferns
are represented while other shore taxa are not. The middle and deepest part of the lake is usually not thebest coring place for macrofossils (Birks, 2001; Väliranta, 2006), although this usually more accurately
applies to larger lakes, and the surface area of Kuutsjärvi is a very small. At the same time the steep
sides near the lake, as well as the steep bathymetry of the lake itself, is expected to increase depositionand preservation potential in terms of easier transportation of remains to the middle part. Shala (2014)
found that there were no significant differences in amount of macrofossils depending on distance to theshore of lake Loitsana, and correlated this to steep slopes next to the lake. As shown by the contemporary
surface samples retrieved in this study, this does not appear to be the case in Kuutsjärvi despite its muchsmaller size. The type and quantity of plant remains varied a lot depending on which part of the lake thatwas sampled, and the deep middle part seem to contain fewer plant remains in general (Table 2). It istherefore evident that the transportation by water must play a big role for taphonomy in Kuustjärvi, and it
is possible that the slow moving flow of water input in the inlet simply did not provide enough energy totransport remains into the deepest part of the lake. It cannot be ruled out that slightly different macrofossil
assemblage will be found if the coring is instead conducted closer to the inlet.
A second possibility for the differences noted between Kuutsjärvi and Loitsana could be the slightmean July temperature difference between the sites. The present-day mean July temperature near Kuustjärvi reported in this study seems to match well with mean July temperatures reported by e.g. Shala(2014) for lake Loitsana (only a 0.1 ◦ C difference). However, the weather data used in this study include
the years 1975-2018, and if the mean July temperature for Savukoski (the station used in the Loitsana
study, located at 67◦ 17’ N 28◦ 10’ E, 193 m a.s.l.) is calculated based on this interval, the present-day
value there would instead be 14.4 ◦ C (Finnish Meteorological Institute, 2019). While the Savukoski
weather station is much further from Loitsana than the Värriötunturit weather station, the elevation ismore similar to lake Loitsana at 214 m a.s.l than the Värriötunturit station is at 341 m a.s.l, and it istherefore more representative to use this station for Loitsana. Nevertheless it can be concluded that there29

is likely a present-day mean July temperature difference between these two sites, which would be expected based on the elevation difference (c. 0.6 ◦ C per 100 m, see e.g. Vedin, 2004). An explanation
could be that this relative difference in altitude between the sites in early Holocene was enough to hamper
high-temperature indicator taxa like Typha spp., Glyceria lithuanica or Potamogeton spp. to thrive near
Kuutsjärvi, but not enough to slow down the rapid colonisation rate of birch forests in the area. However, another possibility could be that the absence of the high-temperature indicator taxa is not related
to temperature at all, but rather to the local environmental factors of the lake catchment. Kuutsjärvi is avery small and deep lake with forested steep slopes close to the shore. This means that the littoral zone isnarrow, and might not have provided a good habitat for aquatic and shoreline taxa such as Typha spp. or
Potamogeton spp. to thrive. It is on the other hand not surprising that these taxa were found in Loitsana,as the lake has a water depth of only 1-2 m and is surrounded by wetlands (Shala, 2014), thus providing avery suitable habitat for these kind of plants in contrast to Kuutsjärvi (Anderberg and Anderberg, 2019).
5.5

Conclusions

The deglaciation phase of the Värriötunturit area is unique as it provided the right conditions for preservation of plants remains from plants growing in the ice-marginal area immediately following the deglaciation, a time period that is rarely covered in lake sediment sequences. The findings of this study therefore
constitute an important contribution to the growing knowledge on early Holocene climate and environment in northern Fennoscandia, and a few main conclusions can be drawn based of the results:i) Subarctic birch forests were present in the ice-marginal environment immediately after deglaciation, and might also have been present on the ice-free Värriötunturit peaks before the final deglaciation of the low-lying areas. When the tree-type Betula pollen and macrofossil remains were deposited in the sediment during the overflow event stage (Zone I, Fig. 8) it is estimated that the ice
sheet was situated only 300 m from Kuutsjärvi.
ii) The very rapid establishment of birch forest in the area in itself probably indicate a warm climate
during early Holocene, which would also be in accordance with previous evidence from lake Loitsana in the nearby Sokli Carbonatite Massif area (Shala et al., 2017; Helmens et al., 2018), as wellas other studies. However, no definite temperature conclusions can be made based on the result inthis study as no applicable indicator plant taxa were found in the sediment sequence.
iii) Scattered occurrences of pine trees were possibly growing in the area already at deglaciation, but
this remains uncertain based on the minor evidences in this study. A transition into a mixed birch
and pine forest occurred around 9400 cal. yr BP, which is noted to be similar or slightly earlier thanlake Loitsana only 10 km to the northwest (Shala, 2014) and a couple of hundred years earlier than
Kipojärvi in northernmost Finland (Väliranta et al., 2011b). Just like birch, pine probably migrated
into northern Finland from refugia in the east following the deglaciation.
5.6

Future research in the area

This thesis was focusing solely on macrofossil analysis but was part of a larger multi-proxy project on
the lake sediment sequence from Kuutsjärvi, where also pollen, diatoms, chironomids and geochemical
proxies have been, or will soon be, analysed. While this study provides a great deal of details about the
local environment during the early Holocene, it is evident from the environmental interpretations that it
would have been more challenging to make conclusions about the environment exclusively with regard to
the macrofossil remains, especially in the gyttja zones. This is because remains were very scarce in thegyttja section, and thus it was difficult to infer with certainty if the taxa assemblage found were entirely
representative for the vegetation in the area.
As it turns out, the supplemented pollen taxa provided excellent confirmation of the more regional
distribution of birch and pine throughout the sequence, and in combination with the macrofossil remains
it was possible to more accurately reconstruct the terrestrial environment. Without the pollen it would30

have been harder to e.g. pinpoint the shift from birch forest into mixed birch and pine forest, and also todefinitely be able to conclude that complete birch forests were growing in the area following the deglaciation. On the other hand, while tree-type Betula pollen in itself suggest the near presence of birch in thearea, based on modern pollen spectras (Prentice, 1978), the exact location of the birch forests in the icemarginal environment would have been uncertain if not for the abundant tree-type Betula macrofossil
remains present. In that sense macrofossils were essential to definitely confirm the trees, while the pollen
provided the general picture. This proves that a multi-proxy approach is very beneficial and are important
to consider in palaeoenvironmental studies, especially when using natural archives such as lake sediment,
where the material is not preserved in situ as in mires.
The future multi-proxy study of Kuutsjärvi will likely be able to give an even more detailed reconstruction of the environment in the area, and also hopefully a temperature indication via the use of chironomids.
A new core from Kuutsjärvi closer to the inlet might also be of interest for future studies, as it was notedin this study that more macrofossil remains appeared near the inlet. Additionally, other lakes in the area
might be of interest, because just as Kuutsjärvi received overflow sediments from the retreating ice sheet,
it is possible that there are similar overflow sediment deposits containing plant remains in the lakes in thetwo gorges Syväkuru and Pirunkuru. Although these lakes are less accessible for coring during winter,they might be interesting alternatives for future research as there is a possibility that they, unlike Kuutsjärvi, may contain indicator plant taxa that can be used to create an even more detailed reconstructionof the climatic conditions of the area in early Holocene.

Acknowledgements

I want to thank my supervisor Karin Helmens for introducing me to this multi-proxy study on Kuutsjärvi,
and for her support, guidance and encouragement during all the different steps of my work. I also want tothank my co-supervisor Minna Väliranta for introducing me to and guiding me through the macrofossil
analysis process, as well as later helping me with identification of the remains I was not able to identify
myself. Furthermore, I want to thank my second co-supervisor Benedict Reinardy for helping me out withthe lithostratigraphy of the sediment sequence, Sakari Salonen for providing a selection of pollen data forme to complement the macrofossil data with, and Antti Ojala for helping me with questions regarding
LIDAR and geological data for the study area. Last but not least I would like to thank Christos Katrantsiotis and Erik Schytt Holmlund for patiently helping me out with the surface sampling of Kuutsjärvi in thesummer of 2018, despite our inital set-back when we lost the coring equipment in the lake! For helping
with funding of this thesis, I would like to thank:
Gerard de Geers foundation for Quaternary Research for providing the means that made it possible
for me to do field work in Värriö during the summer of 2018, as well as to make an educational trip to
Minna Väliranta at the Department of Environmental Science at University of Helsinki during the autumn
of 2018.
Climate Science and Quaternary Geology Research Unit at the Department of Physical Geography
at Stockholm University for funding a well-needed second trip to Minna in Helsinki in February 2019.
31

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