Table Of Content1095975
research-article2022 HOL0010.1177/09596836221095975The HoloceneApolinarska et al.
Research Paper
The Holocene
High-resolution record of geochemical, 2022, Vol. 32(8) 764 –779
© The Author(s) 2022
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Central European spring-fed fen: journals.sagepub.com/home/hol
implications for regional paleoclimate
during the early and mid-Holocene
Karina Apolinarska,1 Rafał Kiełczewski,1 Krzysztof Pleskot,2
Magdalena Marzec,3 Liene Aunina4 and Mariusz Gałka5
Abstract
Despite the increasing interest in spring-fed fens’ sediments as a palaeoenvironmental archive, their potential as high-precision climatic records is
rarely used to its full extent. Here, we present a detailed early to mid-Holocene record of environmental changes in the Turtul hanging spring-fed fen,
northeastern Poland, to test whether the well preserved and accurately subsampled alternating peat and tufa sediments allow us to recognise short-
duration local and regional climatic changes. Our reconstructions are based on a loss-on-ignition analysis of the sediment, carbon and oxygen stable
isotope values of tufa, and biotic proxies: plant macrofossil and malacological analyses. A detailed environmental reconstruction was possible due to the
combination of detailed sampling (1 cm intervals) and high sediment accumulation rates. The two sediment sequences collected from the fen revealed
the congruent palaeoenvironmental history; however, the temporal shift associated with the distance of the coring site from the water outflow was
also apparent, stressing the need for careful selection of the coring site in palaeoenvironmental research. Peat deposition at Turtul started at 10,300 cal
yr BP; however, tufa precipitation did not begin before 9250 cal yr BP. During the most active tufa deposition, from 7850 to 6100 cal yr BP, a high
sedimentation rate was observed (0.11–0.25 cm yr−1). We found several abrupt and short-duration declines in carbonate deposition corresponding to
climatic fluctuations recorded in geological archives from the southeastern Baltic region. The carbonate drops at 8200–8130, 7900–7850, 7400–7300,
6600–6500, 5980–5870 and 5400 cal yr BP were associated with climate cooling or drying, whereas those at 10,200–9250, 8450–8340 and 6160–6100 cal
yr BP can be attributed to drying with no cooling. We conclude that the evident sensitivity of the Turtul fen to climatic shifts that emerge from our high-
resolution (one sample every 17 years on average) reconstruction complements the still rare high-resolution palaeoenvironmental records in the area.
Keywords
carbonate fen, continental carbonates, eastern Baltic area, Greenlandian, Meghalayan, palaeoenvironment
Received 4 October 2021; revised manuscript accepted 13 March 2022
Introduction confined groundwater finds its way to the surface through a
hydrogeologic window in impermeable sediments or through a
High-resolution records are of prime importance in palaeoenvi-
tectonic fault (Dobrowolski et al., 2016). Calcareous spring-fed
ronmental studies. Among the terrestrial archives in the interme-
fens are usually small (<1 ha) and restricted to areas in direct
diate climatic zone, laminated lake sediments (Poraj-Górska
proximity to the active groundwater outflow (Dobrowolski,
et al., 2017) and tree rings (Ljungqvist et al., 2020), each with
yearly or even seasonal time controls, are of the greatest interest.
High-resolution records can also be derived from deposits in 1Institute of Geology, Adam Mickiewicz University, Poland
which the annual time control is absent. Here, the high sediment 2 Geohazards Lab, Institute of Geology, Adam Mickiewicz University,
accumulation rate enables detailed palaeoenvironmental recon- Poland
structions. Among such archives are spring-fed fen deposits char- 3Suwalski Landscape Park, Poland
acterised by intensive CaCO deposition (Dobrowolski et al., 4 Laboratory of Geobotany, Institute of Biology, University of Latvia,
3
2016; Gałka et al., 2018). Latvia
Calcareous spring-fed fens are a unique type of marsh ecosys- 5D epartment of Biogeography, Paleoecology and Nature Conservation,
Faculty of Biology and Environmental Protection, University of Lodz,
tem, in which the continuous supply of carbonate-enriched
Poland
groundwater is of prime importance and ensures CaCO precipi-
3
tation and the presence of specific fauna and flora (Grootjans
Corresponding author:
et al., 2015; Succow, 1988). The supply of the groundwater is Karina Apolinarska, Institute of Geology, Adam Mickiewicz University,
either descensive, at the hanging spring-fed fens located at slopes Krygowskiego 12, Poznań 61-680, Poland.
in river valleys (Osadowski et al., 2009) or ascensive, when the Emails: [email protected]; [email protected]
Apolinarska et al. 765
2011). Carbonates and Ca2+ ions supplied at the fens are derived spring-fed fens’ deposits, although covering long time frames, for
from the dissolution of carbonate rocks occurring in the aquifer example, the entire Holocene, are characterised by a low sedi-
bedrock; therefore, the geographic location of the spring-fed fens ment accumulation rate on the order of 60–100 yr cm−1, and the
is widespread across the globe, including different continents resolution decreases to 350–550 years per sample (Dabkowski
(Bedford and Godwin, 2003; Hájková et al., 2012), wherever car- et al., 2019; Hájek et al., 2016; Hájková et al., 2015). The existing
bonate rocks occur. In east-central Europe, spring-fed fens are high-resolution records of the spring-fed fens (10–20 years per
known from different locations, including young and old glacial sediment sample), allowing for the recognition of even short-
areas (Wołejko et al., 2018), karstic regions (Dobrowolski et al., duration climatic shifts, are mostly restricted to the upper Holo-
2016) and mountains (Hájková et al., 2015). Precipitation of cene and cover the period of the last ca. 4000 cal yr BP or less
CaCO, most commonly referred to as tufa (Grootjans et al., (Gałka et al., 2018, 2021; Jamrichová et al., 2014). To date, in
3
2015; Pedley, 2009), is controlled by two major variables at the central Europe, the high-resolution spring-fed fen sediment
fen. First, it is sensitive to largely climatically driven changes in records of the lower Holocene (25–40 years per sediment sample)
the groundwater level and groundwater circulation intensity, as it were studied by Šolcová et al. (2018) in southern Slovakia and
controls the chemical dissolution of carbonate rocks and the sup- Dobrowolski et al. (2019) in northern Poland (Spurgle site).
ply of Ca2+-supersaturated waters in the spring (Capezzuoli Of the two genetic types, in palaeoenvironmental studies,
et al., 2014; Wołejko et al., 2019). Second, if the water supply is cupola spring-fed fens are favoured over the hanging type (Dobro-
sufficient, temperature is also important because the greater the wolski et al., 2019), which is believed to be prone to erosion and
difference between the groundwater and surface air temperature easily changes from peat accumulating to eroding systems
is, the more intensive the loss of groundwater CO and eventually (Wołejko et al., 2019). Additionally, it was observed (Hájková
2
CaCO3 precipitation (Andrews and Brasier, 2005; Pedley, 2009). et al., 2012) that fens located on slopes may undergo CaCO3 dis-
Tufa precipitation is most intensive in conditions of active water solution in their higher located sites and precipitation in the lower
supply at the fen and high temperatures during spring and sum- sites. This may strongly influence the palaeoenvironmental
mer. CaCO deposition is further enhanced by CO assimilation record, thereby causing discontinuity of the sedimentary record
3 2
by micro- and macroflora, which become tufa-coated and com- (Hájková et al., 2015). However, under moderate water flow and
monly preserve macrophyte morphologies (Capezzuoli et al., when the fen does not occur on steep slopes, sedimentation can be
2014; Pedley, 2009). When, for instance, in response to decreased continuous (Wołejko et al., 2019), and the sediment sequence can
effective precipitation, groundwater outflow becomes insufficient be successfully used in palaeoenvironmental research.
to sustain carbonate precipitation, and/or temperature drops sig- This study investigated the environmental history of the Turtul
nificantly, tufa formation ceases, and peat deposits occur in the hanging spring-fed fen (northeastern Poland) by employing high-
centre of the fen (Grootjans et al., 2006). The outcome of the resolution geochemical (loss on ignition and C and O stable iso-
combined influence of changes in the water supply and tempera- topes), plant macrofossil and malacological analyses. We aimed
ture is the formation of alternating layers of peat and carbonate to complement the still-lacking high-resolution palaeoenviron-
deposits, which is characteristic of spring-fed fen sedimentary mental records and test whether the well-preserved and accurately
deposits (Dobrowolski, 2011; Succow, 1988). The sensitive reac- subsampled (1 cm intervals) alternating peat and tufa sediments
tion to climatic changes makes interbedded peat and calcareous allow us to recognise the short-duration local and regional cli-
tufa deposits a valuable archive of environmental changes matic changes. The spatial variability of environmental condi-
throughout the Holocene (Hájek et al., 2016; Limondin-Lozouet tions at the fen is tested by analyses of two sediment cores – from
et al., 2013; Šolcová et al., 2018). In palaeoenvironmental stud- the central part and the slope of the spring-fed fen cupola.
ies, the sedimentological record is supported by the analyses of
biotic (plant macrofossils and molluscs) and abiotic (sediment
Study site
geochemistry, including the records of C and O stable isotopes)
proxies, a combination that assures more detailed palaeoclimatic The Turtul (Tu, 54°13′22.5″ N, 22°48′9.96″ E, 197 m a.s.l.)
and palaeoecological reconstructions (Dabkowski et al., 2019; groundwater-fed fen investigated in the present study is located in
Dobrowolski et al., 2019; Gałka et al., 2021). northeastern Poland in the mosaic landscape of Suwalski Land-
Although fens usually cover small areas, they are character- scape Park in the Turtul settlement (Figure 1). The site is included
ised by the natural spatial variability of the ecological conditions in the network of protected areas Natura 2000.
controlled by the proximity to the water outflow and the topogra- The landscape of the area was formed during the Pomeranian
phy of the fen (Hájek et al., 2006). On the top of the fen mound, Phase (ca. 16–17 10Be/36Cl kyr BP; Dzierżek and Zreda, 2010;
where groundwater emerges at the surface, small ponds com- Rinterknecht et al., 2006) of the Weichselian glaciation (Marks,
monly occur. In contrast, the slopes of the fen are usually swampy 2012). The activity of the ice sheet and its subglacial waters
but with no standing water. This has a profound influence on the resulted in an uneven topography characterised by numerous
ecological conditions and influences the habitat of fauna and flora morainic and kame hills, with maximum heights of 270 m a.s.l.,
and the sedimentary environment, which are eventually recorded separated by depressions (150–170 m a.s.l.) where wetlands and
in the deposits (Wołejko et al., 2019). Therefore, proper selection lakes occur. Glacial sands, gravels and tills are the dominant types
of the coring site is crucial. of sediments in the area (Krzywicki, 1990; Laskowicz, 2012).
Despite the increasing interest in spring-fed fens’ sediments as The Turtul fen is of the hanging groundwater-fed fen type
a palaeoenvironmental archive (Blaus et al., 2020; Dobrowolski developed on the eastern slopes of the Czarna Hańcza River val-
et al., 2019; Jamrichová et al., 2018), their potential as high- ley. Water supply to the fen is either direct, from meteoric waters,
resolution climatic records has rarely been used to their full but mainly indirect, from the outflow of descending groundwaters
extent. In numerous fens, where the high sediment accumulation of the local near-surface aquifer located in the slopes of the
rate, characteristic of the actively precipitating CaCO sites, is Czarna Hańcza River valley cutting Quaternary unconsolidated
3
observed (i.e. 5–10 yr cm−1, Dobrowolski et al., 2016, 2019; deposits (Wołejko et al., 2018), and from the surface runoff from
Gałka et al., 2018, 2021; Limondin-Lozouet et al., 2013), a sam- the slopes of the river valley (Figure 1). The surface of the fen is
pling resolution on the order of a decade or less is achievable. swampy and has small, several cm deep water pools scattered on
However, this potential is lost, and the resolution decreases to the surface.
100 years or more when 5- to 10-cm-thick sediment samples are In the Turtul fen, tall sedge vegetation dominates. The top of
used (Hájková et al., 2012; Jamrichová et al., 2018). Some of the the peat mound, in the spot where the sediment cores
766 The Holocene 32(8)
Figure 1. (a) Location of the Turtul spring-fed fen and sites cited in the text across central Europe: Bobolice, Ogartowo, Spurgle
(Dobrowolski et al., 2019); Komarów (Dobrowolski et al., 2016); Jaczno (Makri et al., 2021); Purwin (Apolinarska and Gałka, 2017); Kojle
(Gałka, 2014; Gałka et al., 2014, 2015); Szurpiły (Kinder et al., 2020); Żabińskie (Zander et al., 2021); Kurajnovas (Heikkilä and Seppä, 2010);
Laihalampi (Heikkilä and Seppä, 2003). (b and c) Digital elevation models based on LiDAR data with location of the Turtul spring-fed fen, and
cores Tu 1 and Tu 2.
were collected, has sparse vegetation on swampy peat. Carex corers. Tu 1 was retrieved at the highest point of the spring-fed
acutiformis, Equisetum palustre, E. fluviatile, Cardamine amara fen (54°13′22.5″ N, 22°48′9.96″ E, 197 m a.s.l.), whereas Tu 2
and Impatiens noli-tangere are the most common plant species about 10 m apart, at the NW slope of the fen. To obtain a continu-
there. In the weakly developed moss layer, brown moss species ous sediment record the sediment core segments were collected
grow, mostly Plagiomnium elatum and Brachythecium rivulare. from two parallel holes, about 20 cm distant, with offset drive
Towards the Czarna Hańcza River, mainly monodominant stands depths. Diameter of the Tu 1 core amounted to 9 cm between the
of Carex acutiformis occur. In the moss layer, only some speci- surface and 3.5 m of depth, and 5 cm between 3.5 and 4 m. The
mens of Plagiomnium elatum, and Climacium dendroides are 5 cm in diameter peat corer was used for drilling the 4 m long Tu
found. The richest vegetation occurs on the northern slope of the 2 sediment sequence. The contact between the highly compacted
fen, where the ground is the softest and most swampy because of decomposed peat present in the lowermost section of the Tu 2
the active spring discharge. Carex paniculata dominates there. sediment sequence and siliciclastic sediments was reached using
Other common plant species include Geranium robertianum, a Dutch auger. The sediment cores’ segments were placed in
Angelica sylvestris, Equisetum palustre and Myosotis palustris. U-shaped PVC gutters, wrapped in the plastic film and stored in
The moss layer is moderately developed and mainly consists of the cold room until further subsampling.
Plagiomnium elatum. Almost the whole surface of the fen is
under the canopy of deciduous trees, including Alnus glutinosa,
Laboratory methods
Betula sp. and Salix sp.
Both Atlantic and continental air masses influence the tem- Core cutting and subsampling of the sediments. In the labora-
perate climate of the study area; however, eastern air masses tory, the cores were sliced into 1-cm thick fragments. Each sedi-
prevail. The area receives mean annual precipitation amounts of ment slice was subsampled for the analysis of plant macrofossils
approximately 650–700 mm, with the maximum and minimum and molluscs (about 2/3 of the total volume, that is, approxi-
precipitation occurring in July and February, respectively. The mately 20 cm3 in Tu 1 core between the surface and 350 cm, and
mean annual air temperature is 6.5°C, and the monthly averages approximately 7 cm3 in Tu 1 core between 350 and 400 cm, and
range between −5°C in January and 16 –17°C in July (Lorenc, the Tu 2 core), and the sample for geochemical and isotopic
2005). analysis (about one-third of the volume, that is, approximately
10 cm3 in Tu 1 core between the surface and 350 cm, and approx-
Material and methods imately 3 cm3 in Tu 1 core between 350 and 400 cm, and the Tu
2 core). Samples for plant macrofossil and malacological analy-
Field methods
sis were washed and sieved under a warm-water spray using a
Coring. Two sediment sequences, Tu 1 and Tu 2, were drilled 0.20-mm mesh sieve. Samples for geochemical and isotope
using two 50 cm long, 9 and 5 cm in diameter, Instorf type peat analyses were dried at 40°C and pulverised in an agate mortar
Apolinarska et al. 767
(Pulverizette 2). The lithology of the sediments was described environmental requirements (Alexandrowicz and Alexandrowicz,
during sediment subsampling. 2013), that is, land, hygrophilous and aquatic species. We applied
stratigraphically constrained cluster analysis (CONISS) to square
Radiocarbon dating. To establish the chronology of the studied root transformed malacological data assessing the number of sig-
sediment sequences, radiocarbon dating of plant macrofossils was nificant zones by the broken stick model (Bennett, 1996). The
performed in the Poznań Radiocarbon Laboratory. Ten samples analysis was performed with rioja package version 0.9–15.1 (Jug-
from the core Tu 1 and seven from the core Tu 2 were used. The gins, 2017) in R ver. 3.5.1. (R Core Team, 2018).
resulting conventional radiocarbon dates were calibrated with
Bchron (Haslett and Parnell, 2008) applying the IntCal20 atmo-
Geochemical analysis
spheric curve (Reimer et al., 2020). The age-depth model was
constructed using the rbacon package version 2.3.9.1 (Blaauw Loss on ignition (LOI). Samples for LOI analysis were taken at
and Christen, 2019) in R ver. 3.5.1. (R Core Team, 2018). The 1-cm intervals in Tu 1 core and 10-cm intervals in Tu 2 core. Pul-
rbacon package performs Bayesian age-depth modelling that verised sediment samples were combusted at 550°C for 4 h and
includes the dating probability distribution. Given the substan- subsequently at 950°C for 2 h to determine the geochemical com-
tially lower accumulation rates of peat comparing to carbonates position of the sediments (Heiri et al., 2001). The weight loss at
on spring-fed fens, we set boundaries in rbacon between the main 550°C is presumed to represent the percentage of organic matter
lithological units to force the software to model sharp changes in (OM). The carbonate (CO2−) content was calculated by multiply-
3
accumulation rates at these depths. A mean value for the sediment ing the mass of CO evolved in the second step of the analysis (the
2
accumulation rate (SAR), expressed as cm year−1, was calculated weight loss at 950°C) by 1.36. Finally, the CaCO content was
3
based on the mean values of the probability distribution of the calculated by multiplying the CO2− content by 1.66. The residue
3
modelled age (μ) for each depth, for which a date was modelled. represented mainly by organic silica, siliciclastic grains and vari-
The applied formula was as follows: SAR = 1 cm/(μ – ous precipitates like iron oxides was calculated by subtracting
depth depth – 0.5 cm
μ depth + 0.5 cm), where μdepth is the mean value of the modelled age CaCO3 and OM contents from the total sample. The results are
for a given depth. For better readability of the following sections, presented as the percentage concentrations of organic matter,
μ values rounded to tens were selected as reflecting the modelled CaCO3 and residue.
age, which was expressed as cal yr BP (Before Present).
Stable isotope analyses of carbonates. Stable carbon (δ13C) and
Plant macrofossils analysis. Plant macrofossils were analysed at oxygen (δ18O) isotopic compositions were measured in the bulk
1-cm intervals totalling 395 samples and 400 samples in Tu 1 and carbonate tufa after the LOI analysis which allowed to determine
Tu 2 cores, respectively. Plant macrofossils were identified under the CaCO share in the sediments. Samples for isotope analysis
3
a Nikon SMZ800 stereoscopic microscope at the magnification of were taken at 2-cm intervals, wherever the CaCO content in the
3
10–200× using the appropriate keys, including Velichkevich and sediments exceeded 2% (agreed with the stable isotope laboratory
Zastawniak (2006, 2008). The plant macrofossils have been sum- as a threshold value of accurate analysis). The δ13C and δ18O val-
marised in the diagrams in absolute numbers. The volume per- ues of the carbonates were measured using a Gas Bench II hooked
centages of the different vegetative remains were estimated to the up to a Finnigan MAT 253 gas source mass spectrometer (both
nearest 5% or presence/absence. Numbers of seeds, fruits and bud Thermo Fischer). Details concerning the analytical setup are
scales were counted separately, employing a stereoscopic micro- given in Spötl and Vennemann (2003). Isotope values are reported
scope. Macroscopic charcoal pieces (>1 mm) were also counted relative to VPDB based on a NBS-19 calibrated Carrara marble
during plant macrofossils analysis, and their presence provides value of +2.02‰ (δ13C) and −1.76‰ (δ18O) (Fiebig et al., 2005).
information on past local fire occurrence (Mooney and Tinner, On average, replicated precision (1SD, multiple measurements of
2011). The ecological requirements of several key plant species the Carrara standard) was better than ±0.06‰ for δ13C and better
(Ellenberg et al., 1991; Zarzycki et al., 2002) were used to distin- than ±0.08‰ for δ18O.
guish wet and dry phases in the peatland’s development. The
nomenclature followed are Tutin et al. (1993) for vascular plants,
Results
Hill et al. (2006) for mosses, and Hájek et al. (2006) for fen
mosses. We applied stratigraphically constrained cluster analysis Lithology, chronology and sediment accumulation
(CONISS) to square root transformed macrofossils data assessing rate
the number of significant zones by the broken stick model (Ben-
A detailed description of the lithology in the Tu 1 and Tu 2 sedi-
nett, 1996). The analysis was performed with rioja package ver-
ment sequences is shown in Table 2, and the organic matter and
sion 0.9–15.1 (Juggins, 2017) in R ver. 3.5.1. (R Core Team,
CaCO contents are shown in Figure 2. Despite some differences
2018). The final zonation of macrofossil data relied on the combi- 3
in the depth where lithostratigraphic shifts occur, the sedimentary
nation of CONISS analysis and visual inspection of macrofossil
sequences share the same type and succession of sediments. The
diagrams.
fine-grained clastic bedrock sediments are reached 480 cm below
the surface in the Tu 2 core; however, the bottom of organic sedi-
Malacological analysis. After selecting the plant macrofossil ments are not reached in the Tu 1 core. The biogenic sediments in
remains, sediment samples from Tu 1 and Tu 2 sediment sequences both sequences start with well-decomposed herbaceous moss peat
were examined for mollusc shells. Complete mollusc shells and (mean organic matter content of 84% in Tu 1). The peat is over-
recognisable shell fragments were hand-picked and identified lain by peat-tufa intercalations with varying proportions between
under a low-power binocular microscope (Zeiss Stemi 2000-C) the two sediment components. The grain size of the tufa changes
using the reference collection of Institute of Geology AMU in from silty to coarse, with frequent fragments exceeding 1 cm in
Poznań as well as keys and atlases (Piechocki and Wawrzyniak- diameter. The significant enrichment in CaCO is noted from 366
3
Wydrowska, 2016; Welter-Schultes, 2012). The mollusc nomen- to 84 cm in Tu 1 and between 378 and 59 cm in Tu 2; however,
clature follows that of Welter-Schultes (2012). The mollusc values fluctuate from 17% to 94%, with a mean of 73%, and from
remains are expressed as absolute numbers. To better illustrate the 22% to 90%, with a mean value of 72%, in Tu 1 and Tu 2, respec-
response of the mollusc population to paleoenvironmental tively. The highest percentage share of CaCO is observed
3
changes, molluscs were divided into groups according to their between 277 and 104 cm in Tu 1 (37–94%, mean of 82%) and
768 The Holocene 32(8)
Figure 2. Age-depth models of the Tu 1 and Tu 2 cores presented against lithology of the sediment sequences. The age-depth models
constructed using the rbacon R package are based on 10 (Tu 1) and 7 (Tu 2) AMS 14C dates. The calibrated ages are shown in transparent blue.
Black stippled lines indicate 95% confidence intervals. The red curve shows a single ‘best’ model based on the weight mean age for each depth.
The horizontal lines at the transition between tufa and peat deposits visible in the models mark the depths of boundaries we set to force the
software to model rapid accumulation rate shifts (see Method section for rationale).
from 288 to 104 cm in Tu 2 (22–90%, mean of 81%). After 79 cm Plant macrofossil analysis
in Tu 1 and 59 cm in Tu 2, the shift to poorly decomposed herba-
Core Tu 1. Five zones in the local vegetation development were
ceous moss peat occurs with only a trace share (usually below 1%
determined based on a visual inspection and CONISS analysis of
and in all samples below 5%) of CaCO and a dominance of
3 macrofossil data of core Tu 1 (Figure 3). In the Tu 1-ma-1 zone
organic matter, which is 79% on average.
(ca. 10,000–9150 cal yr BP; 395–364 cm), herbs, represented
The results of the radiocarbon dating of the Tu 1 and Tu 2
mainly by seeds of Apiaceae, are replaced by wood, accompanied
cores are presented in Table 1, and the age-depth models are
by charcoal pieces (>1 mm), which have the greatest share of the
shown in Figure 2. The ages of all the radiocarbon dates in
sediment sequence. The share of Urtica dioica and sclerotia of
both sediment cores increase with depth, reaching 9760–
fungi that increases concurrently was also noted. The Tu 1-ma-2
10,800 cal yr BP and 10,118–10,251 cal yr BP at the bottoms
zone (9150–8830 cal yr BP; 364–343 cm) is dominated by Carex
of Tu 1 and Tu 2, respectively. Given that no outlier dates were
rostrata and wood pieces and frequent macrofossils of trees: Alnus
found, we used all the samples to construct age-depth models
glutinosa, Pinus sylvestris and Betula sect. albae. Within zone Tu
with rbacon (Blaauw and Christen, 2019). A clear difference in
1-ma-3 (8830–7620 cal yr BP; 343–252 cm), abundant macrofos-
accumulation rates between the peat intervals and the interca-
sils of B. sect. albae and Betula pubescens occur. Stellaria palus-
lating peat-tufa sediments, emerging from the preliminary
tris, which is characterised by the most continuous record, is
inspection of our radiocarbon dates, justifies setting the bound-
accompanied by Rumex acetosella, U. dioica, C. rostrata, Carex
aries in the rbacon models described in the methods section.
paninculata/appropinquata, and Caltha palustris. In the upper
The results of our Bayesian age-depth modelling reveal that in
part of the zone, Tu 1-ma-3 stems of brown mosses and Tomenty-
both sediment cores, the accumulation rate (SAR, Figure 2)
pnum nitens occur for the first time. T. nitens becomes abundant
was low in peat (0.1–0.6 mm year−1) and increased markedly in
and dominates in zone Tu 1-ma-4 (7620–6200 cal yr BP; 252–
the peat-tufa intercalations (0.5–2.5 mm year−1). The highest
122 cm). Abundant seeds of Cladium mariscus occur at ca. 7000 cal
SAR in the Tu 1 core, 1.1–1.4 mm year−1, was observed
yr BP. Zone Tu 1-ma-5 (6200–4620 cal yr BP; 122–67 cm) is dom-
between 189 and 284 cm (ca. 7100–7870 cal yr BP), whereas
inated by herb rootlets and epidermis and is macrofossil-poor. A
in the Tu 2 core, the highest SAR of 1.7–2.5 mm year−1 was
single seed of C. mariscus occurs at ca. 5540 cal yr BP.
observed between 295 and 339 cm (ca. 7680 and 7910 cal yr
BP). According to our models, the accumulation of intercalat-
ing peat-tufa sediments corresponds with the periods of 9180– Core Tu 2. Five zones in the local vegetation development were
5400 cal yr BP and 8350–5300 cal yr BP in the Tu 1 and Tu 2 determined based on a visual inspection and CONISS analysis of
sediment cores, respectively. macrofossil data of core Tu 2 (Figure 4). Sparse plant macrofossils
Considering the high sediment accumulation rate in Turtul in zone Tu 2-ma-1 (8890–8360 cal yr BP; 400–380 cm) are domi-
during the early and middle Holocene (Figure 2), enabling the nated by fruits of U. dioica. Very abundant charcoal pieces occur
high-resolution reconstruction of the environmental conditions here. The Tu 2-ma-2 zone (8360–7860 cal yr BP; 380–330 cm) is
during the deposition of the intercalating peat-tufa sediment, the dominated by C. rostrata and frequent macrofossils of trees: A.
plant macrofossil and malacological data presented here are glutinosa, P. sylvestris and B. sect. albae. S. palustris, C. panincu-
restricted to samples older than 5000 cal yr BP. lata/appropinquata, Menyanthes trifoliata, and C. palustris also
Apolinarska et al. 769
Table 1. Radiocarbon dates of macrofossils from Tu 1 and Tu 2 cores.
Core depth (cm) Material dated Nr. Lab. C14 date (BP) Age (cal yr BP) (95.4%)
Core Tu 1
Tu 1 30–31 Charcoal pieces, Menyanthes trifoliata seed Poz-123272 1750 ± 30 1568–1709
Tu 1 73–74 Tomentypnum nitens stems with leaves, Betula sect. albae fruit, Poz-123271 4495 ± 35 5041–5301
charcoal piece
Tu 1 108–109 Tomentypnum nitens stems with leaves, Betula sect. albae fruits Poz-123270 5260 ± 40 (0.47 mgC) 5930–6119
Tu 1 144–145 Alnus glutinosa twig, Betula sect. albae fruit Poz-123269 5740 ± 40 6441–6644
Tu 1 188–189 Tomentypnum nitens stems with leaves Poz-123268 6290 ± 40 7157–7314
Tu 1 212–213 Tomentypnum nitens stems with leaves Poz-123267 6360 ± 40 7383–7418
Tu 1 251–252 Carex sp. fruits, Tomentypnum nitens stems with leaves, Alnus Poz-123548 6760 ± 40 7570–7674
glutinosa fruit, Betula sect. albae fruit
Tu 1 287–288 Betula pubescens fruit + fruit scales, Carex sp. fruits, Rumex sp. Poz-123547 6850 ± 50 7586–7785
fruits
Tu 1 341–344 Carex rostrata fruits, Pinus sylvestris seeds + fragment of needle Poz-120627 7490 ± 40 8278–8378
Tu 1 391–393 Apiaceae fruits, Urtica dioica fruits, charcoal pieces Poz-120693 8870 ± 50 9760–10,180
Core Tu 2
Tu 2 37–39 Bidens tripartita fruit, Betula pubescens fruit, brown moss stems, Poz-124384 455 ± 30 (0.25 mgC) 476–535
Epilobium sp. seed, Apiaceae fruits, Carex sp. fruit
Tu 2 84–85 Charred brown moss stems, Tomentypnum nitens stems with Poz-123424 4895 ± 30 5585–5711
leaves, Carex rostrata fruit
Tu 2 175–176 Brown moss stems Poz-124383 6120 ± 40 6894–7087
Tu 2 297–298 Brown moss stems, Betula sect. albae fruit and fruit scale Poz-124382 6940 ± 50 (0.15 mgC) 7675–7864
Tu 2 338–340 Betula sect. albae fruits, Betula pubescens fruit scales, Alnus gluti- Poz-124381 7010 ± 40 7737–7934
nosa fruit, Carex sp. fruit, Pinus sylvestris seed + periderm
Tu 2 387–389 Charcoal pieces, Urtica dioica fruits Poz-124379 7900 ± 40 8595–8791
Tu 2 470–480 Well decomposed peat, bulk sample Poz-126408 9020 ± 50 10,118–10,251
Figure 3. Local plant succession in Turtul spring-fed fen, core Tu 1, based on plant macrofossils presented along with sediment lithology. Taxa
with (%) are estimated volume percentages, and all others are counts (with X-axis scale labels; note scale differences).
occur in this zone. Abundant stems of brown mosses characterise plant macrofossils are rare; only occasional increases in brown
zone Tu 2-ma-3 (7860–7550 cal yr BP; 330–277 cm). Single finds mosses, T. nitens, and Equisetum sp., are observed. An abrupt
of T. nitens are also present. Fruits of B. sect. albae are the most increase in B. sect. albae and wood pieces from ca. 5650 to 5600
abundant here. Tu 2-ma-4 (7550–7120 cal yr BP; 277–217 cm) is cal yr BP is concurrent with abundant charcoal pieces. The assem-
barren of plant macrofossils with only a single seed of Typha sp. blage changes with the transition to peat, in which fossils of the
present. In zone Tu 2-ma-5 (7120–5530 cal yr BP; 217–78 cm), Juncus articulatus type are abundant.
770 The Holocene 32(8)
Figure 4. Local plant succession in Turtul spring-fed fen, core Tu 2, based on plant macrofossils presented along with sediment lithology. Taxa
with (%) are estimated volume percentages, and all others are counts (with X-axis scale labels; note scale differences).
Malacological analysis G. truncatula, and among hygrophilous species, E. alderi occurs.
In zone Tu 2-moll-3 (7250–6640 cal yr BP; 235–162 cm), the
Core Tu 1. Five statistically significant mollusc zones were dis-
sparse mollusc assemblage is composed of hygrophilous (C. min-
tinguished in core Tu 1 based on CONISS analysis (Figure 5).
imum and E. alderi) and land (V. pulchella and N. hammonis)
Zone Tu 1-moll-1 (9150–8360 cal yr BP; 364–315 cm) is charac-
snails. Within zone Tu 2-moll-4 (6640–5470 cal yr BP; 162–
terised by a high diversity and abundance of aquatic mollusc spe-
74 cm), hygrophilous and land species gradually decline.
cies, including mainly Anisus leucostoma, Anisus vortex, Valvata
cristata, Radix labiata, Pisidium casertanum and hygrophilous
Succinea putris. In zone Tu 1-moll-2 (8360–7740 cal yr BP; 315– Tufa carbon and oxygen stable isotope values
267 cm), the diversity and abundance of aquatic species declines;
Within the peat-tufa intercalations in the Tu 1 sediment sequence,
however, Galba truncatula becomes common, accompanied by
δ13C values vary between −11.6 and −6.79‰, and a gradual
Bathyomphalus contortus and hygrophilous species: Carychium
enrichment in 13C occurs (Figure 7). The δ18O values change
minimum, Euconulus alderi and S. putris. In zone Tu 1-moll-3
between −10.39 and −8.33‰. Strong fluctuations between the
(7740–7240 cal yr BP; 267–206 cm), the sparse mollusc assem-
samples characterise both isotope records. The correlation
blage is composed of hygrophilous (C. minimum and E. alderi)
between C and O stable isotope records, which can inform about
and land (V. pulchella and N. hammonis) snails. Zone Tu 1-moll-4
the rate of water exchange resulting from the intensity of the
(7240–6850 cal yr BP; 206–169 cm) is dominated by aquatic
water supply at the fen, is positive and strong (r = 0.66) between
Stagnicola palustris. The abundance of hygrophilous C. minimum
ca. 9200 and 8440 cal BP but changes to weak and negative (r =
and E. alderi increases. Within zone Tu 1-moll-5 (6850–5510 cal
−0.24) between 8840 and 7700 cal yr BP with an increased CaCO
yr BP; 169–84 cm), a gradual change from a mollusc assemblage 3
content in the sediments. A strong negative correlation (r =
dominated by hygrophilous species, including the endemic and
−0.54) is observed between ca. 7700 and 5510 cal yr BP during
protected Vertigo genesii/geyeri, to land snails, namely, V. pul-
the major phase of tufa precipitation in Turtul.
chella, N. hammonis, Vitrea crystallina and slugs, is observed.
Protected Vertigo angustior occurs here.
Discussion
Core Tu 2. Four statistically significant mollusc zones were dis-
Spatial variation in the sediment records of Turtul
tinguished in core Tu 2 (Figure 6). Zone Tu 2-moll-1 (8360–7810
fen
cal yr BP; 380–320 cm) is characterised by a high diversity and
abundance of aquatic mollusc species, including mainly A. leu- The Tu 1 and Tu 2 sediment sequences reveal congruent pal-
costoma, G. truncatula, B. contortus, P. casertanum, V. cristata aeoenvironmental histories, common trends in the geochemical
and hygrophilous S. putris. In zone Tu 2-moll-2 (7810–7250 records, and a similar succession of plants, mosses and molluscs
cal yr BP; 320–235 cm), aquatic species are dominated by (Figures 3–7). However, a detailed inspection of the records
Apolinarska et al. 771
Figure 5. Local mollusc succession in Turtul spring-fed fen, core Tu 1, presented along with sediment lithology. Mollusc assemblages divided
into aquatic, hygrophilous and land species. Taxa presented as shell counts (with X-axis scale labels; note scale differences).
Figure 6. Local mollusc succession in Turtul spring-fed fen, core Tu 2, presented along with sediment lithology. Mollusc assemblages divided
into aquatic, hygrophilous and land species. Taxa presented as shell counts (with X-axis scale labels; note scale differences).
772 The Holocene 32(8)
reveals differences in the timing of deposition between the two
cores, including the start and termination of carbonate deposition
(Figure 7). The observed shifts are partially covered by the confi-
dence intervals (2sigma) of the ages derived from the models,
which vary between 165 and 810 years in Tu 1 and from 110 to s.
518 years in Tu 2 (Figure 2). sil
s
o
The apparent differences between the two cores can be attrib- of
r
uted to the well-recognised internal fen variability (cf. Gałka ac
m
et al., 2017, 2018). The delayed start of CaCO3 deposition and nt
concurrent occurrence of molluscs in the Tu 2 sediment sequence a
pl
(8350 cal yr BP) compared to the Tu 1 core (9250 cal yr BP) d
n
(Figure 7) was probably due to the different locations of the two s a
coring sites. Tu 1 was drilled in the highest point of the fen where sc
u
water outflow was located and where CaCO sedimentation oll
3 m
started first. As the water circulation was gradually unblocked, as s,
recorded by the intensification of CaCO precipitation at ca. pe
3 o
8350 cal yr BP in Tu 1 (Figure 7), the spatial reach of the Ca2+- ot
s
enriched spring water was more extensive and allowed for tufa e i
precipitation and the occurrence of aquatic and hygrophilous mol- bl
a
lusc species. The different locations of the two cores are also O st
reflected in the subsequent temporal shifts in sediment geochem- d
n
istry and biotic proxies. However, the general pattern of changes a
C
is very similar in Tu 1 and Tu 2. The discontinuous but frequent OI,
occurrence of Tomentypnum nites in Tu 1 and only sparse occur-
L
rence of this moss in Tu 2 (Figure 7) can indicate the difference in y,
g
o
the substrate between the two spots. According to the habitat pref- ol
h
erences of T. nitens (Nicholson and Gignac, 1995), the top of the Lit
fen must have been less swampy compared to its slopes. A similar s:
e
change in the substrate is apparent today. Additionally, the poor c
n
e
preservation of the moss remains in the Tu 2 core prevent their u
q
accurate taxonomic classification. se
The discrepancy in the termination of CaCO deposition in nt
the two cores, ca. 5500 versus 5300 cal yr BP, in 3Tu 1 and Tu 2 me
di
(Figure 7), respectively, may also be related to changes in the se
chemical composition of the water flowing on the fen. When water u 2
T
emerging at the fen flows down on the fen surface, it loses CO2 d
through outgassing and absorption by autotrophs. As a conse- an
1
quence, the water pH increases, and CaCO3 can precipitate at some Tu
distance from the source (Capezzuoli et al., 2014). Therefore, it is e
h
lcioklelleyc ttehda ft atvhoeu lroecda tthioen t efmropmo rawrhyi pcher sthiset eTnuc e2 o fs ecdairmboennat tceo dreep wosais- a of t
tion, even after it stopped at the centre of the fen location. The at
d
importance of the above mechanism for CaCO precipitation is d
evidenced by the mean sediment accumulation rate 3values (Figure 2). cte
e
In core Tu 2, the highest SAR values of 0.17–0.25 cm yr−1 were sel
observed between ca. 7680 and 7910 cal yr BP (295–339 cm) and of
n
were noticeably higher than those observed between ca. 7100 and o
7870 cal yr BP (189–284 cm) in the Tu 1 core: 0.11–0.14 cm yr−1. pilati
Considering the above discussion, careful selection of the coring m
o
site is crucial in terms of accurate palaeoenvironmental research of C
spring-fed fens. In studies where one sediment sequence is anal- 7.
e
ysed, the coring should be done in the central and in commonly the r
u
highest, part of the fen close to the outflow where water is not g
Fi
subjected to geochemical modifications observed along the
increasing distance from the source.
The early and middle Holocene ecosystem changes in the
Turtul spring-fed fen are a response to regional and local climate was initiated (Figure 7, Table 2). The highly decomposed peat
changes. Given the central location of the Tu 1 core in the underlying the peat-tufa intercalations typically occurs at the bottom
spring-fed fen in Turtul, high-resolution LOI analysis, and C of the spring-fed fen sediments and records accumulation associated
and O stable isotope analysis performed exclusively for this with an active layer of permafrost that thawed seasonally during the
core, the climatic reconstruction is based mainly on the data progressive warming of the early Holocene (Dobrowolski et al.,
from the Tu 1 sediment sequence supplemented with the data 2019). The deposition of peat in Turtul started as a response to a
from the Tu 2 core. major environmental transformation in the region that was forced
by the final decay of the Scandinavian Ice Sheet and change in
the atmospheric circulation in the Baltic region (Lauterbach
Start of peat deposition (480–368 cm; 10,300–9250 cal yr BP) et al., 2011). The replacement of cold and dry air from the north-
Deposition of peat in Turtul started at ca. 10,300 cal yr BP (Tu 2) and east by warm and moist westerlies accelerated the spread of
continued until 9250 cal yr BP (Tu 1), when CaCO precipitation deciduous trees in the region (Gałka et al., 2015), with pioneer
3
Apolinarska et al. 773
Table 2. The lithology of Tu 1 and Tu 2 sediment sequences.
Core Tu 1 Core Tu 2
Depth (cm) Sediment type Depth (cm) Sediment type
0–84 Poorly decomposed herbaceous-moss peat 0–59 Poorly decomposed herbaceous-moss peat
59–84 Peat-tufa intercalations with dominating share of poorly
decomposed peat, fine grained CaCO
3
84–104 Peat-tufa intercalations with dominating share of poorly 84–104 Peat-tufa intercalations with dominating share of poorly
decomposed herbaceous-moss peat, fine grained CaCO decomposed herbaceous-moss peat, and gradually increasing
3
content of fine grained CaCO
3
104–277 Tufa-peat intercalations with dominating share of coarse 104–236 Tufa-peat intercalations with dominating share of coarse
grained CaCO grained CaCO
3 3
277–296 Peat-tufa intercalations with dominating share of well 236–288 Peat-tufa intercalations with dominating share of poorly
decomposed herbaceous-moss peat, fine grained CaCO decomposed herbaceous-moss peat
3
296–310 Tufa-peat intercalations with dominating share of coarse 288–306 Well decomposed herbaceous-moss peat with very little
grained CaCO CaCO
3 3
310–366 Tufa-peat intercalations with dominating share of well 306–320 Peat-tufa intercalations with coarse grained CaCO
3
decomposed herbaceous-moss peat, fine grained CaCO 320–378 Well decomposed herbaceous-moss peat contaminated with
3
CaCO (minor share)
3
366–395 Well decomposed herbaceous-moss peat 378–480 Well decomposed herbaceous-moss peat
>395 Not sampled
>480 Siliciclastic sediment
Betula-Pinus forests noted since ca. 11,250 cal yr BP (Gałka type and a shift to higher productivity in the lake (Makri et al.,
et al., 2014). 2021). The observed environmental changes were related to
The open environment at Turtul that was dominated by herbs prominent warming in the Baltic Sea basin (Borzenkova et al.,
(Apiaceae taxa) persisted until ca. 9600 cal yr BP (380 cm) when 2015; Heikkilä and Seppä, 2010) and central-eastern Europe
trees entered the fen, as indicated by the increase in the wood (Davis et al., 2003) (Figure 8). In the Turtul spring-fed fen, the
pieces (Figure 3). The site became more fertile and favoured the combined increase in humidity and warming enhanced CaCO
3
spread of Urtica dioica. The substantial share of charcoal pieces precipitation (Figure 7) and stimulated the development of a
(>1 mm), which increased as trees entered the fen, indicates drier diverse and abundant mollusc assemblage (Figure 5).
conditions at the site and/or its surroundings, favouring local fire Very wet conditions at the Turtul fen between 9250 and 8450
events. This is supported by the presence of fungal remains (scle- cal yr BP (368–320 cm) are indicated by the highest share of
rotia) and large charcoal pieces (>5 mm), the share of which was aquatic mollusc species (Figure 5), characteristic mainly of small
the highest after 9400 cal yr BP (Figure 8) when the cold and dry and shallow but permanent water basins (Anisus leucostoma, Val-
Erdalen Event, Bond event 6 (Bond et al., 2001), occurred. The vata cristata, Anisus vortex, Radix labiata and Pisidium caserta-
increase in the intensity of fire events in the early Holocene was num). Similarly, plant macrofossils indicate a gradual increase in
recorded at many sites in Central and Eastern Europe (Feurdean the water level (Figure 3). The remains of Alnus glutinosa, U.
et al., 2020). dioica and charcoals (>1 mm) were gradually replaced with
Carex rostrata, which is the plant that usually occurs in a very wet
habitat such as lake shores and hollows in peatlands (Ellenberg
Carbonate spring fed-fen (368–79 cm, 9250–5540
et al., 1991; Zarzycki et al., 2002). The presence of the water
cal yr BP)
pools at the fen is confirmed by a strong positive correlation (r =
The first increase in the CaCO content in the Tu 1 sediments at 0.66) between δ13C and δ18O values, indicating the influence of
3
ca. 9250 cal yr BP (368 cm) indicates the start of enhanced the residence time effects, including evaporation, on the C and O
groundwater circulation, as suggested by Dobrowolski et al. stable isotope records (Andrews, 2006). The share of wood pieces
(2019) for northern Poland. Based on the highly variable CaCO gradually decreased; however, the presence and occurrence of
3
content (Tu 1; Figure 7), it is suggested that at this stage, only macroremains of A. glutinosa, Betula sect. albae and Pinus syl-
shallow water circulation was active in the area and was highly vestris indicate that trees grew on the fen surface.
susceptible to even small shifts in the water supply in the spring. Short drying (8450–8340 cal yr BP; 320–314 cm) within the
The negative δ13C and δ18O values of the carbonates (Figure 7), generally wet stage is recorded as a decrease in CaCO content
3
which are typical of the authigenic CaCO precipitated at the and a decline or retreat of most of the aquatic mollusc species
3
spring-fed fens (Andrews et al., 1997; Gałka et al., 2021), exclude (Figures 5 and 7). However, small water ponds must have
the detrital origin of CaCO, the supply of which was documented occurred at the site, as indicated by the dominance of Galba trun-
3
in the Purwin spring-fed fen (Apolinarska and Gałka, 2017) catula and the presence of Bathyopmphalus contortus (Figure 5).
located 10 km northeast of the Turtul site. Sedimentary and pal- The fen became overgrown by U. dioica, C. rostrata, C. panicu-
aeobotanical records from Kojle-Perty Lake (located in the vicin- lata, Stellaria palustris and Caltha palustris (Figure 3).
ity of Purwin Lake) (Gałka et al., 2015) and the Purwin spring-fed Between 8340 and 7850 cal yr BP (314–281 cm), the conditions
fen (Apolinarska and Gałka, 2017) indicate an abrupt shift to a at the fen remained wet and favoured CaCO precipitation. The
3
wet habitat at ca. 9400 cal yr BP and 9200 cal yr BP (Figure 8), supply of the fen with groundwater was intensified as the correla-
respectively. The nature of the change to humid conditions was tion between δ13C and δ18O records changed to weakly negative
regional, as the timing of the onset of CaCO deposition at Turtul (r = -0.24). A high (up to 91%) CaCO content (Figure 7) can be
3 3
was concurrent with observations at Komarów (Figure 1) and linked with gradual regional warming (Borzenkova et al., 2015;
other cupola spring-fed fens (Dobrowolski et al., 2002, 2005, Davis et al., 2003; Heikkilä and Seppä, 2010). The strongly
2016) located on uplands in eastern Poland. In Lake Jaczno increased CaCO content in Tu 1 was concurrent with the onset of
3
(Figure 1), 9200 cal yr BP marks a change in the sedimentation tufa deposition in the Tu 2 core (Figure 7). Small water ponds at the
