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\begin{document}

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\title{Reconstruction of a coastal raised bog development in the proximal part of the Curonian Spit, Kaliningrad Region, Russia, South-Eastern Baltic}

\author{Maxim G. Napreenko$^{1}$, Tatiana V. Napreenko-Dorokhova$^{1,2}$, Dmitry A. Subetto$^{3}$}

\affil{1}{Shirshov Institute of Oceanology RAS, Moscow,  Russia}
\affil{2}{Immanuel Kant Baltic Federal University, Kaliningrad,  Russia}
\affil{3}{Herzen State Pedagogical University, Saint Petersburg,  Russia}

\abstract{The Holocene development of a peculiar coastal peatland (raised bog Svinoye), southern Curonian Spit, Russia, is reconstructed based on the analysis of taxonomic diversity of plant macrofossils and radiocarbon dating of peat and gyttja deposits. The development of the bog can be referred to the primary mire formation, which was preceded by a period of gyttja accumulation (7500--7000~cal. yr BP) in a shallow water body. The mire vegetation development started from 7000~cal.~yr~BP through the spread of the alder carrs and reed stretches that presumably experienced inundation during transgression events of the forming Baltic over the period 7000--6300~cal.~yr~BP). These habitats were successively replaced by the communities of the rich fen, transition mire and active raised bog. During the whole post-transgression period\linebreak\newpage}\noindent\centerline{\parbox{100mm}{ (6300~cal.~yr~BP -- present time), the vegetation successions were not influenced by the sea-level oscillations but were determined by climatic and anthropogenic factors. The diagrams of the botanical composition of peat enabled to assign and describe 12 formation phases for the Svinoye bog ecosystem. In contrast to the other peatlands in the region, the stage of transition mire persisted here over a long-term period of 1200 years. The formation of a typical raised bog began around 3400~cal.~yr~BP. As recent as 200 years ago, the ecosystem had the characteristics of a maritime climate mire. The mean peat accumulation rate was defined for the period of the last 7000 year as having the highest values in the Late Atlantic and the Late Sub-Atlantic (1.7--2~mm/yr), while the lowest increment (0.8~mm/yr) is recorded for the whole Sub-Boreal.}}


\section{Introduction}

     The raised bog Svinoye is a unique ecosystem within the natural complex of the Curonian
Spit in the Russian south-eastern Baltic coast. Large raised bogs are not found in any other sand
barrier throughout the shoreline of Baltic. Being a specific habitat, the bog Svinoye defines the
peculiar landscape features in the Curonian Spit, especially in its proximal part which has a
different history of origin than the rest territory of the spit.

     The formation of the Curonian Spit was studied and is still under investigation of a number
of researchers
[\itc{Badyukova et al.,} \reflink{Bady06}{2006}, \reflink{Bady07}{2007};
\itc{Berendt,} \reflink{Bere69}{1869};
\itc{Dobrotin,} \reflink{Dobr18}{2018};
\itc{Gams,} \reflink{Gams32}{1932};
\itc{Gaigalas et al.,} \reflink{Gaig91}{1991};
\itc{Gudelis,}  \reflink{Gude90}{1990},   %  {1989-1990},     %%% ???+
\reflink{Gude98}{1998};
\itc{Gudelis et al.,} \reflink{Gude93}{1993};
\itc{Kabailien\'{e},} \reflink{Kaba67}{1967}, \reflink{Kaba95}{1995};
\itc{Kharin et al.,} \reflink{Khar06}{2006}, \reflink{Khar13}{2013};
\itc{Michaliukaite,} \reflink{Mich62}{1962};
\itc{Moe et al.,} \reflink{Moe05}{2005};
\itc{Paul,} \reflink{Paul44}{1944};
\itc{Povilanskas et al.,} \reflink{Povi06}{2006};
\itc{Sergeev,} \reflink{Serg15}{2015};
\itc{Sergeev et al.,} \reflink{Seea15}{2015}, \reflink{Serg16}{2016};
\itc{Wichdorff,} \reflink{Wich19}{1919}; etc.].
Nevertheless, there are
some issues remained concerning the genesis of different parts in this specific geological body.
In this respect, the investigation of peat bed and ist botanical composition in the Svinoye bog can
elucidate both the questions of the proper bog development and some geological,
geomorphological, palaeoclimatic, and palaeoenvironmental issues related to the Curonian Spit
and adjacent areas in the Baltic Sea and the Curonian Lagoon.

\begin{sloppypar}
     The article is aimed at providing new insights into temporal development of the Svinoye
bog ecosystem during the Holocene based on the analysis of plant macrofossils in peat and
outcomes of radiocarbon dating of organogenic mire deposits.
\end{sloppypar}

     The raised bog Svinoye occupies a greater part of the territory in the proximal side of the
Curonian Spit
(\figref{1}),           % (fig. 1),
between the mouth channel of Trostyanka river and the Baltic Sea coast
from which it is separated by 300--400 m wide zone of the black alder wet woodland. The
peatland has a total area of 238 ha, comprising a 50-ha central site of the open {\itshape Sphagnum}
dominated raised bog with scattered low pines and major, heavily drained, peripheral part
covered by the high pine forest with {\itshape Ledum} and {\itshape Vaccinium myrtillus}.

\begin{sidewaysfigure}                      % Fig 1
\figurewidth{140mm}
\setimage{}{}{155mm}{}{2019es000680-f01}
\fcaplabel{Sketch map of the study area (a) and the site of investigation (b,~c). Locations of the boreholes for taking samples are indicated by the triangles.}
\end{sidewaysfigure}

\section{Material and Methods}

\begin{sloppypar}
     The peat bed coring and further sediment core retrieval were performed both in the central
($54\deg 57'51.4''$ N; $20\deg 31'02.8''$ E,  core 1) and the peripheral ($54\deg 57'51.4''$ N; $20\deg 31'02.8''$ E, core 2)
parts of the bog
(\figref{1}c).           %  (fig. 1, c).
Successive 0.5-m long peat monoliths were sampled using the
Russian peat corer (TBG-1 model). The coring was executed according to the standard technique
and methodological guidelines
[\itc{Minkina,} \reflink{Mink39}{1939}].
\end{sloppypar}


     Starting from the horizon 7.5 m in the core 1 and from the horizon 6.0 m in the core 2, the
peat samples were analysed under the microscope for taxonomic identification of plant
macrofossils.  The laboratory treatment of each peat sample included its cleaning, elutriation and
estimation of decomposition degree
[\itc{Piavtchenko,} \reflink{Piav63}{1963}].
The wet sample of peat was cleaned
under running water on a 250-$\mu$m sieve (grid \numero \:025K) to remove textureless humus particles.
Then, the specimen of cleaned peat was examined under the microscope (\mbox{Micromed-3} model) to
determine the taxonomic identity of plant macrofossils and estimate the percentage ratio of
different plant residues in a sample.

     The identification of plant macrofossils in peat was performed using a range of
identification keys and guides
[\itc{Dombrovskaya et al.,} \reflink{Domb59}{1959};
\itc{Katz et al.,} \reflink{Katz77}{1977};
\itc{Korotkina,} \reflink{Koro39}{1939};
\itc{Matyushenko,} \reflink{Maty39a}{1939a}, \reflink{Maty39b}{1939b}].

     The species names in Latin are cited according to the
\itc{Cherepanov}'s check-list [\reflink{Cher95}{1995}]    %%% ???
for
vascular plants, and the ``Check-list of mosses in Eastern Europe and Northern Asia''
[\itc{Ignatov et al.,} \reflink{Igna06}{2006}]
for bryophytes.

     Four samples from the core 1 were dated using radiocarbon scintillation method in the
Radiocarbon Laboratory of the Institute of Geography, Russian Academy of Sciences (Moscow,
Russia), laboratory index ``IGAN''. Obtained radiocarbon dates were calibrated using the
programme CALIB (version 7.1.0 $^{14}$ChronoCentre,\linebreak QueensUniversityBelfast) and the calibration
curve IntCal13
[\itc{Reimer et al.,} \reflink{Reim13}{2013}].


\begin{sidewaystable}                                   % Table 1
\tablewidth{155mm}
\ftablabel{Radiocarbon Dating of Peat and Gyttja Samples in the Raised Bog Svinoye}
\vspace{5pt}
\footnotesize
\begin{tabular}
{@{}l@{\hspace{20pt}}
c@{\hspace{15pt}}
c@{\hspace{15pt}}
c@{\hspace{10pt}}
c@{}}
\hline
\\ [-7pt]
Depth of    & Lab. \numero  &  Radiocarbon age,  &  \multicolumn{2}{c}{Calibrated age interval for  1$\sigma$ cal. yr BP} \\
sampling, m &  (IGAN)       &   $(^{14}$C), BP   &  beginning--end   &      probability                \\
  [7pt]  \hline   \\ [-4pt]
1.25--1.15   &    4801    &      $800 \pm 90$    &  \bfseries{\underline{664--796}}     &   \bfseries{0.933} \\
             &            &                      &      875--892     &              0.067 \\
2.55--2.45   &   4803     &       $1930 \pm 70$  &  \bfseries{\underline{1814--1952}}  &    \bfseries{0.894} \\
             &            &                      &       1958--1987  &             0.106 \\
5.70--5.60   &   4804     &       $4950 \pm 80$  &  \bfseries{\underline{5596--5748}}   &     \bfseries{0.954}  \\
             &            &                      &       5831--5843   &            0.046 \\
8.95--8.85   &   4807     &       $6390 \pm 80$  &  \bfseries{\underline{7264--7341}}   &    \bfseries{0.540} \\
             &            &                      &    7348--7417     &          0.460\\
             [4pt] \hline
\end{tabular}
\end{sidewaystable}


\begin{sloppypar}
     As an integration of the outcomes of the macrofossil analysis
[\itc{Napreenko and Napreenko-Dorokhova,} \reflink{Napr19}{2019}]
     with the estimation of peat decomposition degree and radiocarbon dating
(\tabref{1}),           % (Table 1),
the summary diagrams of vegetation successions
(\figref{2})           % (fig. 2)
have been plotted for each
sediment core using C2 software
(Juggins S., C2 Version 1.7.6. [Electronic resource], 2014.
Mode of access:
\href{https://www.staff.ncl.ac.uk/stephen.juggins/software/C2Home.htm}{https://www.staff.ncl.ac.uk/stephen.juggins/softw are/C2Home.htm}).
The assigned stages of vegetation depict the
dominating plant communities in the mire during different periods of the Holocene over the last
7300 years
(\figref{2}a, \figref{2b}).              % (fig. 2 a,b).
\end{sloppypar}

     The age of the stages was calculated through the interpolation procedure between two
known calibrated radiocarbon dates.\vspace*{-.5pc}


\begin{sidewaysfigure}                      % Fig 2
\figurewidth{140mm}
\setimage{}{}{155mm}{}{2019es000680-f02}
\fcaplabel{Botanical composition of peat and stages (successions) of vegetation on the raised bog Svinoye: a) sediment core~1, b)~sediment core~2.}
\end{sidewaysfigure}

\section{Results}

     Two sediment cores (10.0 and 6.5 m respectively) were retrieved and studied in course of
investigation. The core 1 is represented by various types of peat in the upper part (8.6--0.0 m)
and by a thin layer of gyttja in the basal part (8.95--8.60 m), the lower horizons of which yielded
a radiocarbon age of 7300 cal.~yr~BP
(\tabref{1}).           % (Table 1).
The lowermost segment in the core 1 (10.0--8.95
m) is represented by clay with sand. The core 2 consists entirely of peat deposits.

     The core 1 contains a 3-m thick layer of swamp and fen peat (8.6--5.0 m). The lower part
of it, 8.6--7.0 m, consists of highly decomposed peat with a large number of ligneous residues
(apparently, Alnus). This part of the peat bed was studied primarily in the field without the use of
microscopic methods. The interlayer 7.5--7.0 is heavily water-logged, forming a peat suspension
in water in some places. The upper layers (6.50--5.25 m) contain mainly herbaceous residues of
{\itshape Phragmites}, {\itshape Carex} and {\itshape Scheuchzeria}, and represented by the same types of fen peat.

     In the core 2, the fen peat composes only the basal 50 cm (6.50--6.00 m) and contains a
very small amount of ligneous residues.

     A distinct layer of the transition mire peat is recorded within the horizon 5.25--4.25 m in
the core 1 and 6.00--4.25~m in the core~2. It is referred to as {\itshape Sphagnum} transition mire peat since
the remnants of Sphagna dominate in its composition in both cores.

     The upper part of the peat bed (4.25--0.00 m) is formed by the {\itshape Sphagnum} and {\itshape Sphagnum-Eriopho\-rum} raised bog peat in both cores, wherein there is a clear decline of the decomposition
degree for the {\itshape Sphagnum} peat from the horizon of 2.5--2.0 m. Thus, the upper part of the peat
bed (the raised-bog peat bed) in the Svinoye bog is divided into two layers: the lower one with
the high degree of decomposition (35--40\%)
and the uppermost one with the low degree (15--20\%).

     The uppermost surface layer (0.15--0.00 m) consists of highly decomposed {\itshape Sphagnum} peat,
mineralized as a result of the bog drainage during last century.

     Based on the data of the botanical composition of peat, we assigned 9 stages of vegetation
(local macrofossil assemblage zones, LMAZ) for the sediment core 1 in the central part of the
bog
(\figref{2}a).           % (Fig. 2, a).

     Stage I ({\itshape Phragmites -- Drepanocladus}), 6300--5800 cal.~yr~BP. The zone is marked by a
high content of rhizomes and radices of {\itshape Phragmites} (up to 80\%).
Among the accompanying
residues found in different ratios, there are roots of sedges ({\itshape Carex}~sp.) and different aquatic
plants (not identified), leaves of the hydrophilic mosses ({\itshape Drepanocladus} sp.), and ligneous
remnants of alder ({\itshape Alnus glutinosa}). This stage reflects the vegetation of monodominant reed
stretches with the flooded sites occupied by the hydrophilic communities of mosses and minor
herbaceous. Peat accumulation reaches here a maximum rate of 2~mm/yr.

     Stage II ({\itshape Scheuchzeria + Carex + Menyanthes -- Sphagnum squarrosum}), 5800--5200~cal.~yr BP. This stage is defined by the dominating residues of fen species such as Rannoch-rush
({\itshape Scheuchzeria palustris}), sedges ({\itshape Carex lasiocarpa, C. rostrata, C. limosa}), buckbean
({\itshape Menyanthes trifoliata}), purple marshlocks ({\itshape Comarum palustre}), and eutrophic fen mosses
({\itshape Sphagnum squarrosum, Calliergon sp.}).  The remains of birch bark ({\itshape Betula} sp.) were also found
in a small amount. The stage corresponds to a rich-fen habitat with considerable herb diversity
and patchy vegetation. The peat accumulation rate has more than twofold decreased -- 0.8~mm/yr.

     Stage III ({\itshape Eriophorum + Fruticules -- Sphagnum fuscum}), 5200--4000~cal. yr~BP, is
characterized by the dominance of {\itshape Sphagnum fuscum} residues (up to 60--70\%).  There is a
considerable percentage of cotton grass ({\itshape Eriophorum vaginatum}) and dwarf shrubs (Ericaceae).
The bryophytes of different mire types are distinct: {\itshape Polytrichum strictum}, {\itshape Tomenthypnum nitens},
{\itshape Aulacomnium palustre, Sphagnum palustre}. The stage corresponds to phytocoenoses of the
transition mire which were gradually transforming into a raised bog. The peat accumulation remained at the same rate of 0.8~mm/yr.

     Stage IV ({\itshape Scheuchzeria} -- Sphagna), 4000--3400 cal. yr~BP, is defined by the dominance of
the hydrophilic {\itshape Sphagnum} species -- {\itshape S. cuspidatum} (over 60\%).
The radices of {\itshape Scheuchzeria
palustris} are also in abundance. A distinct component here is {\itshape Sphagnum papillosum}, a species of
the flat and moist hummocks on coastal bogs. The stage reflects the herb- and {\itshape Sphagnum}-dominated
vegetation of wet bog carpets and soaks. The peat accumulation rate is 0.8~mm/yr.

     Stage V ({\itshape Eriophorum} + Fruticules -- Sphagna), 3400--2600 cal.~yr~BP, is characterized by
the dominance of {\itshape Eriophorum vaginatum} and hummock-forming {\itshape Sphagnum} species: {\itshape S. rubellum}
and {\itshape S. capillifolium}. Dwarf shrubs and some species of moist depressions, {\itshape Sphagnum balticum,
S. tenellum, S. cuspidatum}, are present in a small amount. The stage corresponds to an open
raised bog occupied by numerous hummocks of {\itshape Eriophorum} and Sphagna. The peat accumulation rate is 0.8~mm/yr.

     Stage VI ({\itshape  Sphagnum cuspidatum + S. tenellum}), 2600--1900 cal.~yr~BP. There is a distinct
predominance of the hydrophilic hollow species of Sphagna: {\itshape S. cuspidatum, S. tenellum, S. balticum}. The percentage of hummock-forming Sphagnum species and cotton grass decreased
markedly. This composition matches to the vegetation cover of wet depressions on hummock-hollow bog complexes. The peat accumulation rate remains at 0.8~mm/yr, while increases at the end
of the stage.

     Stage VII ({\itshape Eriophorum} -- Sphagna), 1900--850 cal.~yr BP, is similar to the LMAZ~V.
{\itshape Eriophorum  vaginatum, Sphagnum rubellum} and {\itshape S. capillifolium} dominate in different
proportions, while the dwarf shrub percentage is insignificant. The stage corresponds to the
hummock vegetation of the open raised bog. The peat accumulation rate increases up to 1.1~mm/yr.

     Stage VIII ({\itshape Sphagnum rubellum + S. fuscum}), 850--200 cal.~yr~BP. The hummock-forming
species as {\itshape Sphagnum rubellum} and {\itshape S. fuscum} dominate, while the portion of {\itshape S. capillifolium},
cotton grass and dwarf shrubs declines. The stage corresponds to phytocenoses of large and
medium-size hummocks on an open raised bog. The peat accumulation rate reaches 1.7~mm/yr.


\begin{sloppypar}
     Stage IX ({\itshape Baeothryon + Scheuchzeria} -- Sphagna), 200--100 cal.~yr~BP. {\itshape Sphagnum rubellum} continues to dominate. {\itshape S. capillifolium, S. tenellum,
S. balticum, and S. cuspidatum} are recorded in a small amount. A peculiar characteristic of the stage is an appearance of an Atlantic species, the tufted
bulrush ({\itshape Baeothryon cespitosum}). The amount of cotton grass and dwarf shrubs increases. The stage reflects the vegetation of an open raised bog with
low hummocks and {\itshape Sphagnum} carpets natural for the coastal bogs of a maritime climate. The peat increment remains at a high rate of 1.7~mm/yr.
\end{sloppypar}

     During the last 100 years, the formation of raised bog Svinoe is marked by a stronger influence of anthropogenic disturbance resulted in
significant drainage and essential compositional change of vegetation. The present-day bog vegetation stage is to be defined as {\itshape Calluna + Eriophorum}
-- Sphagna for the central part, and {\itshape Pinus -- Ledum + Vaccinium } for the forested edge zone.

     Similar to the first sediment core, there have been assigned 8 vegetation stages (LMAZ) for the core 2 from the peripheral bog zone
(\figref{2}b).           %  (���. 2, b).

     Stage I ({\itshape Menyanthes + Carex -- Sphagnum squarrosum}) is correlated with stage II in the core I from the bog centre. Residues of {\itshape Carex lasiocarpa}
and {\itshape Menyanthes trifoliata} are dominant. A distinct presence of mosses {\itshape Sphagnum squarrosum, S. teres, Tomenthypnum nitens} is also recorded.
Remains of {\itshape Phragmites and Alnus} (bark) are in a small amount. The stage corresponds to a fen with rich biodiversity and patchy vegetation.

     Stage II ({\itshape Eriophorum} + Fruticules -- {\itshape Sphagnum fuscum}) is correlated with stage III in the core I. This local zone is defined by the dominant moss
remains of {\itshape Sphagnum fuscum}
(up 40\%)       %(�� 40\%)  %%% ???+
and abundant residues of {\itshape Eriophorum vaginatum}, ericaceous dwarf shrubs, and some species of different types
of mires ({\itshape Polytrichum strictum, Aulacomnium palustre, Sphagnum imbricatum} s.l.). The stage corresponds to transition mire habitats which were
gradually transforming into a raised bog.

     Stage III ({\itshape Eriophorum -- Sphagnum imbricatum + S. capillifolium}). The dominant plant remains are cotton grass and two species of mesotrophic
hummock mosses: {\itshape Sphagnum imbricatum}  % � +
and {\itshape S. capillifolium} (over 60\%
for both). The green moss remains ({\itshape Aulacomnium palustre Dicranum} sp.) and
dwarf shrubs are recorded in a small amount. The stage reflects the vegetation of transition mire dominated by cotton grass and Sphagna.

     Stage IV ({\itshape Eriophorum -- Sphagnum fuscum}) can be, to a certain degree, correlated with stage V in the core 1. The residues of {\itshape Eriophorum
vaginatum} and {\itshape Sphagnum fuscum} distinctly prevail in this local zone. The remains of dwarf shrubs are present in a small amount. The percentage of other
species is insignificant. The stage represents the vegetation of an open raised bog dominated by hummocks of {\itshape Sphagnum} and {\itshape Eriophorum}.

     Stage V ({\itshape Sphagnum cuspidatum + S. tenellum}) completely matches stage VI in the core 1, is also defined by the prevalence of hydrophilic
hollow species of Sphagna ({\itshape S. cuspidatum} and {\itshape S. tenellum}) with the distinctly fewer percentage of cotton grass remains. The stage corresponds to
water-logged depressions on the raised bogs.

     Stage VI ({\itshape Eriophorum -- Sphagnum rubellum}) is correlated with stage VII in the core 1 and defined by the dominant moss remains of {\itshape Sphagnum
rubellum} accompanied by the residues of {\itshape Eriophorum vaginatum, S. capillifolium} and dwarf shrubs. The stage corresponds to the vegetation of
hummocks on an open raised bog.

     Stage VII ({\itshape Sphagnum balticum + S. tenellum + S. rubellum}) with correlated stage VIII in the core 1. The dominant plant residues are remains of
Sphagnum species representing different microhabitat ecology, both the hollow species -- {\itshape Sphagnum balticum, S. tenellum}, and the hummock-forming
ones -- {\itshape S. rubellum} %  �
and {\itshape S. capillifolium}. The remains of cotton grass are in small amount. The stage represents communities of different microtopography
elements on a patterned raised bog with hollow-hummock complex.

\begin{sloppypar}
Stage VIII ({\itshape Baeothryon + Scheuchzeria} -- Sphagna) is correlated with stage IX in the core 1. The dominant species are the same as in the
previous local zone: {\itshape Sphagnum rubellum, S. tenellum, S. capillifolium}, and {\itshape S. balticum}. There is an appearance of {\itshape Baeothryon cespitosum}. The remains
of {\itshape Eriophorum} and dwarf shrubs are also found in a certain amount. The stage reflects the vegetation of an open coastal patterned bog of a maritime
climate with the low hummocks and {\itshape Sphagnum} carpets.
\end{sloppypar}


\section{Discussion}

\begin{sloppypar}
     As it is resulted from the diagrams, the vegetation stages (LMAZ) are, in general, well-correlating each other in both cores which enables to
derive, based on this fact, more common development phases (periods) for the whole ecosystem of the raised bog Svinoye.
\end{sloppypar}

     The beginning of the Svinoye bog formation is to be related with the mid-Holocene Littorina transgressions in the eastern Baltic sector. Before
the Pre-Littorina time, as it is suggested from the available geological data
[\itc{Badyukova et al.,} \reflink{Bady07}{2007};
\itc{Gams,} \reflink{Gams32}{1932};
\itc{Kharin et al.,} \reflink{Khar13}{2013};
\itc{Sergeev,} \reflink{Serg15}{2015}],
the mire formation area was situated on a residual Pleistocene morainic hill, composed of both glacier and lacustrine clay.  The latter is considered to
have the Ancylus age
[\itc{Gams,} \reflink{Gams32}{1932};
\itc{Kunskas,} \reflink{Kuns70}{1970}].

\begin{sloppypar}
     Before the onset of the Littorina transgressions, the study area was not likely to be exposed to inundation, although it was the region where the
mouths of palaeorivers -- Pra-Deima, Pra-Matrosovka, and Pra-Kurovka
[\itc{Badyukova et al.,} \reflink{Bady07}{2007};
\itc{Kharin et al.,}  \reflink{Khar13}{2013};
\itc{Sergeev,} \reflink{Serg15}{2015}]
-- were located
and, according to palaeomaps
[\itc{Sergeev,} \reflink{Serg15}{2015}],
 have fragmented the morainic plain onto separate upper sites.
 \end{sloppypar}


     The first Littorina transgression in the South-Eastern Baltic region (L1, 8300--8000 cal.~yr~BP) is often considered to have resulted into a fast sea
level rise
[\itc{Badyu\-kova et al.,} \reflink{Bady07}{2007};
\itc{Damu\v{s}yt\'{e},} \reflink{Damu11}{2011}]
and formation of a range of bays along the coast between Sambian Peninsula and Rybachy Plateau
[\itc{Sergeev,} \reflink{Serg15}{2015}].
Nevertheless, these events have not led to flooding the study area, as well as the most territory of today's Curonian Lagoon and
Neman Delta
[\itc{Damu\v{s}yt\'{e},} \reflink{Damu11}{2011}].
This is in agreement with our data -- the very first layers of gyttja are dated 7300 cal.~yr~BP, i.e. they are already
correlated with the period of the second Littorina transgression.

     Thus, before the onset of the second Littorina transgression (7500 cal.~yr~BP), the territory of the study area (today's bog Svinoye and
surrounding sites) was located within the terrestrial environment without traits of mire formation.

\begin{sloppypar}
\subsubsection{The period of a shallow water body with gyttja accumulation \mbox{(7500--7000 cal.~yr~BP)}.} During the second Littorina transgression (L$_2$, 7500--7000~cal.~yr~BP), the sea level rise is likely to have led to the inundation of the vast region covering today's area of Curonian Lagoon and some territories
adjacent to its southern and eastern coast. The palaeomaps of this region
[\itc{Damu\v{s}yt\'{e},} \reflink{Damu11}{2011};
\itc{Sergeev,} \reflink{Serg15}{2015}]
demonstrate the shoreline exceeding 3 to 8~m its present-day position. It implies that the study area (territory of the bog Svinoye) was covered with a shallow water body, where the accumulation
of gyttja deposits occurred during that time (7500--7000 cal.~yr~BP). The character of these deposits was, apparently, affected by the abrasion of the
morainic remnants and relief leveling
[\itc{Badyukova et al.,} \reflink{Bady07}{2007}].
Our samples of sandy gyttja from the depth 8.95--8.6 m in the core 1, which
corresponds to the indicated age, implicitly confirms these theses.
\end{sloppypar}

\begin{sloppypar}
     The layers of sandy gyttja became probably a substrate for primary mire vegetation development on the study area during the regression of the
Littorina basin (7000--6600 cal.~yr~BP). The widespread mire development was recorded during that time on a vast coastal area adjacent to the recently
formed brackish lagoon
[\itc{Kunskas,} \reflink{Kuns70}{1970};
\itc{Bitinas et al.,} \reflink{Biti02}{2002}].
The same conclusion is confirmed by our data concerning the onset of the Bolshoye
Mokhovoye bog formation in the southern part of the Neman Delta
[\itc{Napreenko-Dorokhova and Napreenko,} \reflink{Napr18}{2018}].
\end{sloppypar}

\begin{sloppypar}
\subsubsection{The period of alder carrs (7000--6300 cal.~yr~BP).} The first mire communities on the study area of the Svinye bog were the wet black alder carrs
with the dense herb layer of {\itshape Phragmites}. Having likely to be experienced an inundation of the area during the regular sea-level rise of the Littorina
basin, these communities died, forming a half-meter layer of heavily water-logged ligneous peat. This peat is well defined and visible on a horizon 7.5--7.0 m in the core 1 of the Svinoye bog deposits. Through interpolation, we date this event in the interval 6600--6300 cal.~yr~BP that may have been
related to one other peak of the second Littorina transgression, the exact time frame of which is still under discussion
[\itc{Badyukova et al.,}  \reflink{Bady07}{2007}],
despite
the interval 7500--7000~cal.~yr~BP adopted by many authors.
\end{sloppypar}

\begin{sloppypar}
According to the point of view stated already by
     \itc{Gams} [\reflink{Gams32}{1932}],
     the waterlogging of peat was caused by the isostatic submersion of ligneous and
{\itshape Phragmites} peat layers into underlying gyttja deposits. Nevertheless, we recorded the similar water-logged layers of ligneous peat and ligneous reed
peat, dated the same age, in the lower part of the Bol'shoye Mokhovoye bog, whereas, the deposits of gyttja were not found there.
\end{sloppypar}

     Thus, the formation of the water-logged layers of fen peat, both on the western (Sviniye bog) and the eastern (Bol'shoye Mokhovoye bog) coast
of Pre-Curonian Lagoon, was more likely a consequence of sea-level rise during the Littorina basin transgression, or there has been a small-scale
isostatic submersion of a whole southern coast of the lagoon in this area. The probability of the glacio-isostatic processes in this region is suggested in
some recent investigations
[\itc{Uscinowicz,} \reflink{Usci03}{2003};
\itc{Badyukova et al.,}  \reflink{Bady07}{2007};
\itc{Sergeev,} \reflink{Serg15}{2015}];
the first paper also refers to a possible transgression in the
Neman Delta at 6000~cal.~yr~BP.

\begin{sloppypar}
\subsubsection{The period of reed stretches (6300--5800 cal.~yr~BP).} As evident from the analysis of peat structure for the horizons lying above 7.0~m (6300~cal.~yr~BP), there had been no further inundation on the area of Svinoye bog formation. The areas of the vanished alder carrs were colonised by the
monodominant stretches of the common reed ({\itshape Phragmites}) that had been existing here for 500 years. Among the reed stands, there were more
inundated sites occupied with the hydrophilic riparian herbs and aquatic mosses. The warm and humid climate of the mid-Holocene favoured,
apparently, to the intensive growth of tall herbaceous, while the hydrophilic environment prevented decomposition of the fast accumulating biomass in
these habitats, therefore the peat increment reached here the very high rate of 2~mm/yr. The 1.5-m layer of rapidly formed fen reed peat became a
substrate for the development of new vegetation stages.
\end{sloppypar}

\begin{sloppypar}
\subsubsection{The period of a herb-dominated rich fen (5800--5200 cal.~yr~BP).} The next ecosystem change on a forming mire approximately coincides with
the boundary of the Holocene climatic periods, the Atlantic and the Sub-Boreal (5731  cal.~yr~BP). In the study area, the communities of {\itshape Phragmites}
disappeared, having been replaced by the rich-fens with the polydominant short-grass patchy vegetation. The most widespread (and peat-forming)
species are {\itshape Scheuchzeria palustris, Carex lasiocarpa, C. rostrata, C. limosa, Menyanthes trifoliata, Comarum palustre}. Despite the lowland location of
the territory and the hydrophilic biotope character, the productivity of the mire communities decreased, and the peat accumulation rate had more than
two times declined (0.8~mm/yr).  This change may have been related both to the size of peat-forming plants and overall climate change in the Sub-Boreal towards lower humidity.
\end{sloppypar}


\subsubsection{The period of a Sphagnum-dominated transition mire (5200--4000 cal.~yr~BP).} The stage of the transition mire existed in the study area for a
rather long-term period, 1200 years. Based on the plotted diagrams, during this phase, the whole mire area was covered by the communities of
moderately dry hummocks dominated by {\itshape Sphagnum fuscum}, by {\itshape Polytrichum strictum}, by {\itshape Eriophorum vaginatum} and ericaceous dwarf shrubs. Nevertheless,
these phytocenoses included a range of bryophyte species typical for the mesotrophic habitats: {\itshape Tomenthypnum nitens, Aulacomnium palustre,
Sphagnum palustre, S. imbricatum} s.l. Their occurrence and presence of sedges confirm the transition character of mire during that period.  It is
noteworthy that there was no pine which is usually a characteristic element for similar peatlands. In spite of the wide distribution of Sphagna, the
hummock productivity was quite low on the relatively dry transition mire habitats, thus the peat accumulation rate did not change as compared with the
previous stage (0.8~mm/yr).

\begin{sloppypar}
\subsubsection{The period of Scheuchzeria-Sphagnum lawns in the bog centre and transition mire communities in the edge zone (4000--3400 cal.~yr~BP).}
This ecosystem change also occurred on a boundary of two periods of the Holocene -- the Early and the Late Sub-Boreal (4041 cal.~yr~BP). There had
been a formation of the wet lawns of {\itshape Sphagnum cuspidatum, S. papillosum and Scheuchzeria palustris} in the central part of the bog that may indicate
the common climate humidification. The marginal part of the bog had been still occupied by the transition mire communities which were similar to
those of the previous stage. Such differentiation of the vegetation cover is most likely to be the evidence of the oligotrophic conditions in the mire
centre, which usually leads to a formation of the dome-shaped peat surface of the bog.
\end{sloppypar}

\subsubsection{The period of a raised bog with the emerging micro- and mesotopography (3400--2600 cal. yr~BP).} During this period, there had been the
development of a typical raised bog in the central part of the peatland which involved the formation of the vegetation community complexes and had
led to a microtopography differentiation onto large drier elevations (hummocks) and small wetter depressions (pockets). The dominant elements in
community composition were already the typical species of the coastal bogs, {\itshape Sphagnum rubellum} and {\itshape S. capillifolium}. The marginal and sub-marginal
sites of the bog were dominated by hummocks of {\itshape Eriophorum vaginatum} and {\itshape Sphagnum fuscum}, though the mesotrophic transition mire species has
vanished from their composition. Through this process, the whole mire graded into a raised-bog stage and was likely to become a dome-shaped
peatland. The peat accumulation rate remained the same as in the previous three periods (0.8~mm/yr).

\subsubsection{The period of hollow complexes (2600--1900 cal. yr~BP).} The transition to this phase coincided with the beginning of the last climatic period of
the Holocene, the Sub-Atlantic (2592 cal.~yr~BP), characterised by humid and cool climate. Through the increase of precipitation in the coastal region,
this climatic change has, apparently, led to the formation of large water-logged hollows dominated by hydrophilic Sphagna -- {\itshape S. cuspidatum, S.
tenellum} and {\itshape S. balticum}. The coverage of hummocks and abundance of ecologically-related species of drier habitats have decreased. The hollow
complexes are suggested to occupy the major part of the bog during that phase. The peat accumulation rate began to increase since the second half of
the period.


\subsubsection{The period of an active raised bog (generative complex phase) (1900--850 cal.~yr~BP).} During that period, the formation of the raised-bog
generative complex occurred in the study peatland as a vegetation community complex of vigorously growing {\itshape Sphagnum}-dominated hummocks with
{\itshape Eriophorum vaginatum} and dwarf shrubs. The hollow complexes became, in contrast, much less abundant. The cause of such changes is still unclear.
The major peat-forming species was {\itshape Sphagnum rubellum}, which is peculiar as producing a considerable peat mass increment. This is considered to
have led to an increase of peat accumulation rate (1.1~mm/yr) and more intensive growth of the mire peat bed.


\subsubsection{The period of a raised bog with continental features (850--200 cal.~yr~BP).} The beginning of the period occurred in the Late Sub-Atlantic and
approximately coincided in time with the onset of the Little Ice Age, usually associated with the cooler environment and a certain increase in
continentality. Perhaps, this climatic change explains the secondary widespread distribution of {\itshape Sphagnum fuscum} in the Svinoye bog. Being an
edificator species in the continental regions in Eurasia, it provides the highest peat increment as well as {\itshape S. rubellum}. Both species are dominant during
this phase of mire development, having affected to the very high rate of the peat bed increase. The dominant vegetation community complex consisted
of large and high {\itshape Sphagnum} hummocks. The abundance of cotton grass and dwarf shrubs declined. The peat accumulation rate reached 1.7~mm/yr.


\subsubsection{The period of a maritime climate raised bog (200--100 cal.~yr~BP).} The beginning of the phase corresponds to the end of the Little Ice Age and
some mildening of the climate. The communities of low hummocks and {\itshape Sphagnum} carpets became the most widespread in the Svinoye bog. The
indicator elements in their composition were the Atlantic species typical for the coastal mires in the oceanic and suboceanic regions with a considerable
degree of precipitation. Among them are {\itshape Baeothryon cespitosum, Odontoschisma sphagni, Sphagnum molle, S. imbricatum} s.l. The hydrophilic species
of mosses, {\itshape S. tenellum, S. balticum % �
and S. cuspidatum}, became again very common. The peat accumulation remained at a rather high rate of 1.7 mm/yr.


     Despite the differences in species composition and structural peculiarities of the bog communities during the last four phases, it is required to be
taken into account that different bog microtopography forms can usually alternate each other as mire development occurs. Although the environmental
properties in these sites of the bog are also getting changed, the general structure of the bog ecosystem remains stable. In this respect, the last four
phases of the Svinoye bog development can be combined into one general phase -- the period of a patterned dome-shaped active raised bog.

\subsubsection{The period of a settled forested peatland (100 cal.~yr~BP -- present time).} Having experienced a considerable anthropogenic disturbance during
the last 100 years, the Svinoye mire has lost up to this moment its structure of the dome-shaped active raised bog and graded into a settled, densely
forested peatland. The peat increment has significantly slowed down in the central part of the bog, or completely ceased in the marginal and sub-marginal zone.

     It is noteworthy to underline that, as it is evident from our research, the genesis of the Svinoye bog is not related to succession on any terrestrial
or lacustrine ecosystem existed prior to the onset of the mire formation. Having regard to a specific character of the initial stages of the Svinoye bog
evolution, the way of its development is to be referred to a primary mire formation, which is assigned -- together with terrestrialisation and
paludification -- as a separate type of mire evolution in the coastal areas experiencing isostatic fluctuations and transgression effects
[\itc{Sj\"{o}rs,} \reflink{Sjor83}{1983};
\itc{Succow and Joosten,} \reflink{Succ01}{2001}].

\section{Conclusions}

     Based on the results of macrofossil analysis for two cores taken from organogenic deposits of the Svinoye raised bog and taking into account
radiocarbon dating for a range of samples, the following conclusions were made.

\begin{enumerate}
\item
	The initial stages of the Svinoye raised bog formation -- a coastal mire in the proximal part of the Curonian Spit -- were closely related
with the transgression events in the Pre-Baltic basin (Littorina Sea) during the period 7500--6300 cal.~yr~BP.
\item
During the period 6300--100 cal.~yr~BP, the vegetation development in the mire ecosystem was determined mainly by the climatic
factors and did not experience the impact of marine and lagoon waters.
\item
In the last 100 years, a considerable change in the bog structure was caused by intensive human disturbance.
\item
The development of the whole mire ecosystem included 12 periods (phases):

 -- the period of a shallow water body with the gyttja accumulation as a substrate for the primary mire vegetation (7500--7000 cal.~yr~BP);

 -- the period of alder carrs (7000--6300 cal.~yr~BP);

 -- the period of reed stretches (6300--5800  cal.~yr~BP);

 -- the period of a herb-dominated rich fen (5800--5200 cal.~yr~BP);

 -- the period of a {\itshape Sphagnum}-dominated transition mire (5200--4000 cal.~yr~BP);

 -- the period of {\itshape Scheuchzeria-Sphagnum} lawns in the bog centre and transition mire communities in the edge zone (4000--3400 cal.~yr~BP);

 -- the period of a raised bog with the emerging micro- and mesotopography (3400--2600 cal.~yr~BP);

 -- the period of hollow complexes (2600--1900 cal. yr~BP);

 -- the period of an active raised bog (generative complex phase) (1900--850 cal.~yr~BP);

 -- the period of a raised bog with continental features (850--200 cal.~yr~BP);

 -- the period of a maritime climate raised bog (200--100 cal.~yr~BP);

 -- the period of a settled forested peatland (100 cal.~yr~BP -- present time).

\item
The initial stages of the Svinoye bog evolution are to be referred to a primary mire formation.
\item
The peat accumulation rate was the highest (1.7--2.0 mm/yr) in the Late Atlantic, during the presumable influence of transgression
events, and during the Sub-Atlantic. The least peat increment (0.8 mm/yr) was recorded during the whole Sub-Boreal.
\item
The assigned mire development periods (phases) are in good agreement with the change of climatic periods during the Holocene.

\end{enumerate}

\acknowledgments{The fieldwork, the macrofossil analyses and its interpretation were supported by Russian Science Foundation (project no. 18-77-00030), the
radiocarbon dating, by Russian Foundation for Basic Research (project no. 18-05-80087). The laboratory treatment of peat samples was funded from
the Russian Academic Excellence Project at the Immanuel Kant Baltic Federal University. The data interpretation and review were performed under
the state assignment of the Shirshov Institute of Oceanology (research theme no. 0149-2019-0013).}

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\end{document}
