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Macrofossil reconstruction of preboreal wetland formed on dead ice block a case study of the Borzechowo mire in East Pomerania, Poland

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In order to reconstruct environmental changes in the Borzechowo mire, a sediment core was subjected to macrofossil and stratigraphic analyses. The mire is located in the eastern part of the Pomeranian Lakeland (Tuchola Forest, northern Poland). It is a limnogenic mire, formed as a result of terrestrialisation of a water body. The time of basal peat accumulation was estimated by radiocarbon dating as 9860±130 14C BP (Gd-12393) and by palynological analysis as Preboreal. The analysis of macrofossils shows that in that period, considerable hydrological changes took place in the study area. These hydrological changes were caused by melting of dead ice blocks that was common place in the Late Glacial and the Early Holocene.
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Studia Quaternaria, vol. 27 (2010): 3–10.
MAC RO FOS SIL RE CON STRUC TION OF PRE BO REAL WET LAND
FORMED ON DEAD ICE BLOCK: A CASE STUDY OF THE
BOR ZECHOWO MIRE IN EAST POM ERA NIA, PO LAND
Mi cha³ S³owi ñski
De part ment of Geo mor phol ogy and Hy drol ogy of Low lands, In sti tute of Ge og ra phy and Spa tial Or gani za tion,
Pol ish Acad emy of Sci ences, ul. Ko per nika 19, 87- 100 To ruñ, Po land, e- mail: mi chal@geo pan.to run.pl
Ab stract
In or der to re con struct en vi ron men tal changes in the Borzechowo mire, a sed i ment core was sub jected to macrofossil
and strati graphic anal y ses. The mire is lo cated in the east ern part of the Pom er a nian Lakeland (Tuchola For est, north -
ern Po land). It is a limnogenic mire, formed as a re sult of terrestrialisation of a wa ter body. The time of basal peat ac -
cu mu la tion was es ti mated by ra dio car bon dat ing as 9860±130 14C BP (Gd-12393) and by palynological anal y sis as
Preboreal. The anal y sis of macrofossils shows that in that pe riod, con sid er able hy dro log i cal changes took place in the
study area. These hy dro log i cal changes were caused by melt ing of dead ice blocks that was com mon place in the Late
Gla cial and the Early Ho lo cene.
Key words: Pre bo real, wet land, mac ro fos sils, ba sal peat, bur ied dead ice, N Po land
IN TRO DUC TION
A peat layer found at the base of limnic de pos its, over ly -
ing min eral bot toms of biogenic sed i men tary bas ins, is gen -
er ally as so ci ated with rapid hy dro log i cal changes at the
be gin ning of lake for ma tion. In the early post-gla cial land -
scape of Po land (Liberacki 1958, Kozarski 1963, Stasiak
1963, 1971, Wiêckowski 1966, 1993, ¯urek 1990, B³aszkie-
wicz, Krzymiñska 1992, Nowaczyk 1994a, 1994b, 2006,
Wojciechowski 2000, B³aszkiewicz 2003, 2005, 2007),
Netherlands (Hoek et al. 1999) and North Amer ica (Kova-
nen, Easterbrook 2001), such fea tures were formed as a re -
sult of melt ing of dead-ice blocks bur ied within gla cial de -
pos its. De pend ing on var i ous con di tions, both re gional and
lo cal, the melt ing of dead-ice blocks lasted from the be gin -
ning of the Late Gla cial till the Preboreal pe riod (B³aszkie-
wicz 2005).
Ac cord ing to the hydrologic and ge netic clas si fi ca tion
by Succow (1988), mires formed on sandy-grav elly de pos its
and fed by wa ter from melt ing dead-ice blocks can be de fined
as paludified. Dur ing their for ma tion due to the wan ing of
dead-ice blocks, the over ly ing de pos its col lapsed and the
telmatic sed i ments were sub merged as a re sult of sub si dence
of ket tle holes, and sub se quently cov ered with limnic de pos -
its of the emerg ing lakes.
Lit er a ture of the sub ject re ports rel a tively large num ber
of sites where peat de pos its de vel oped above bur ied dead-ice
blocks, but in only a few such sites a macrofossil anal y sis was
con ducted (Marek 1994, Wojciechowski 2000, Kowalewski
et al. 2001, Wright, Stefanova 2004, Drzymulska 2006).
Such an anal y sis is es pe cially im por tant when lo cal en vi ron -
men tal con di tions are to be de fined in a ba sin where biogenic
ac cu mu la tion takes place. In the case of fos sil sed i ments,
macrofos sil anal y sis is vi tal to dis tin guish prop erly be tween
depositional en vi ron ments, i.e. limnic (a layer at the bot tom
of the lake) and telmatic (start ing to ac cu mu late around the
lake’s per im e ter) (Eslick 2001). Since there are doubts con -
cern ing proper clas si fi ca tion of basal peat, macrofossil anal -
y sis seems cru cial (Marks 1996, Ga³ka 2007, Tobolski
2007).
STUDY SITE
The Borzechowo mire is lo cated in the east ern part of the
Pom er a nian Lakeland (Pojezierze Pomorskie), at the bor der
be tween the Tuchola For est (Bory Tucholskie) and the Ko-
ciewie Lakeland (Pojezierze Kociewskie), and within the
catch ment of the Wda River (Fig. 1). The mire oc cu pies the
bot tom of a subglacial chan nel formed on the outwash plain
at the di rect forefield of the gla cier in the max i mum ex tent of
the Pom er a nian phase (B³aszkiewicz 2005).
The mire is an ex am ple of a de graded fen. In the 1980s,
the area was drained, which in flu enced sig nif i cantly the lo cal
eco sys tem. In terms of hy drol ogy and or i gin, the Borze-
chowo mire is limnogenic, i.e. formed by the pro cess of
terrestrialisation of a wa ter body (Succow 1988).
The lithofacial and chronostratigraphic char ac ter is tics
of sed i ments fill ing the fos sil lake ba sin near Borzechowo
were de scribed by B³aszkiewicz (2005). Ac cord ing to that
au thor, the base of the biogenic de pos its lies di rectly on the
min eral basal com plex. A con tin u ous layer of the Preboreal
peat of var i ous thick ness (3–10 cm), was found at the depth
of 11.42 m. The above layer un der lies a thick stra tum of cal -
car e ous gyttja, which in the top sec tion is cov ered by peat.
MA TE RIAL AND METH ODS
The sed i ment core was ob tained with the use of 110
cm-long (5 cm di am e ter) Living stone corer in Wiêckowski’s
mod i fi ca tion (Wiêckowski 1956). The sed i ment for macro-
fos sil anal y sis was col lected at the cen tral sec tion of the
subglacial chan nel, at the con tact zone with the Wda River
val ley (Fig. 1). In the cen tral sec tion of the chan nel, for the
pur pose of this study, we took the com plete pro file of bio-
genic sed i ments, whose to tal length was 11.42 m. Li thol ogy
of the sed i ments was de scribed fol low ing West (1977).
Prep a ra tion of sam ples for macrofossil anal y sis fol -
lowed widely ac cepted meth ods (Tobolski 2000). Be fore the
anal y sis the cores were frag mented into 1-cm sec tions. The
bot tom de pos its, highly de com posed, were boiled in the so lu -
tion of po tas sium hy drox ide (KOH) for about 10 min utes.
Next, the sludge was sieved through mesh sizes of 0.1 mm,
0.25 mm and 0.5 mm. Each sam ple was used to pre pare four
mi cro scope slides. De ter mi na tion of both fos sil and subfossil
re mains was based on the lit er a ture (Bertsch 1941, Landwehr
1966, Katz et al. 1977, Grosse-Brauckmann 1972, Nilsson
1972, Beijerinck 1976, Daniels, Eddy 1985, Smith 2004,
Tobolski 2000, Ga³ka 2006, Velichkevich, Zastawniak
2006) and the ref er ence col lec tion of the De part ment of
Biogeography and Paleoecology.
The re sults ob tained from the macrofossil study were
ana lysed and pre sented in the form of macrofossil di a grams
plot ted by the C2 soft ware (Jug gins 2003). The un count able
macrofossils are pre sented in the di a gram on a five-level
scale. Count able items, such as seeds and nee dles, are given
in ab so lute val ues. The lo cal macrofossil as sem blage zones
(LMAZ) were de lim ited.
The dig i tal ter rain model for the study area has been gen -
er ated through the near est neigh bour al go rithm from points
ob tained as a re sult of vectorisation of con tour lines from a
top o graphic map on a scale of 1:10 000 (sheet num ber and
name: 335.113 Borzechowo). The ESRI ArcMap9 soft ware
was used.
4M. S£OWIÑSKI
Fig. 1. Lo ca tion of the study site and the geo log i cal cross-sec tion the Borzechowo mire. 1 – dig i tal ter rain model (DTM) with lo ca tion of a
core as well as the course of the geo log i cal sec tion; 2 – li thol ogy of the pro file; 3 – geo log i cal sec tion A-B across the Borzechowo mire (af ter
B³aszkiewicz 2005).
RE SULTS
Re sults of the macrofossil anal y sis are shown in Fig. 2
and Ta ble 1. The di a gram in Fig. 2 in cludes six cat e go ries
(brown mosses and Sphag num mosses, trees and shrubs,
aquatic plants, telmatic veg e ta tion, in ver te brates and oth ers).
The anal y sis of biogenic de pos its in di cates that a 14-cm basal
peat, sep a rated by a layer of ae olian sands (4.5 cm) is un der -
lain by sands of var i ous grain sizes (depth 11.42 m). The peat
layer is over lain by a grey ish-crÀme cal car e ous gyttja. The
be gin ning of basal peat ac cu mu la tion was dated on 9860±
130 years BP (Gd-12393). On the ba sis of the macrofossil
anal y sis of the core sec tion, two stages of de vel op ment of the
study ob ject can be dis tin guished: telmatic and limnic.
Telmatic phase
Telmatophytes ini ti ated the de vel op ment of the mire.
They in cluded Thelypteris palustris and Equisetum sp. Ac -
cord ing to the eco log i cal in di ca tor val ues (Zarzycki et al.
2002) those spe cies pre fer me dium shad ing and fer tile
(eutrophic) en vi ron ments with neu tral re ac tion (6£pH<7)
and form dense patches (K³osowski S., K³osowski K. 2001).
They were ac com pa nied by Phragmites aus tra lis and Carex
spp. The ex ist ing ba sin with biogenic sed i ments was then
over grown by shrubs of Betula spp. This point of view is sup -
ported by the subfossil find ings linked with this spe cies, such
as fruit and periderm. Betula spp. was ac com pa nied by the
dwarf shrub of Betula nana, whose fruit was found in the bot -
tom de pos its. In the Preboreal pe riod this spe cies was abun -
dant in the veg e ta tion of the Pom er a nian re gion (Lata³owa
2003). The other spe cies pres ent in the area in cluded trees:
Populus tremula and Pinus sylvestris. Dur ing this phase,
fires oc curred in the mire area. This is in di cated by nu mer ous
sharp-edged pieces of char coal of up to 17 mm in di am e ter.
Two peaks of char coal con tent were re corded: the first
one at the depth from 11.38 m to 11.35 m, and the other one at
11.31 m. The fires on that mire, prob a bly in clud ing the neigh -
bour ing ar eas, burnt the veg e ta tion. This ini ti ated short-last -
ing ae olian and de nu da tion pro cesses, lead ing to the accumu-
lation of a min eral layer. The larg est stock of the sandy frac -
tion in the ana lysed sec tion of the pro file is re corded at the
depth of 11.31 m. Due to the fire, the pH value in the sur face
layer in creased, while the con tent of the to tal ni trate de -
creased. This brought about a change in both the soil and veg -
e ta tion cover. The wet land sur face low ered down to the
wa ter ta ble, re sult ing in sec ond ary plant suc ces sion fol lowed
by the paludification pro cess (Kania et al. 2006).
The fire caused edaphic changes within the mire sub stra -
tum, which al tered the veg e ta tion. New spe cies ap peared,
such as Lycopus europaeus, Filipendula ulmaria, Calama-
grostis sp., Scirpus lacustris, Eriophorum angustifolium and
Epipactis sp. More over, the den sity of emer gent veg e ta tion
in creased – it in cluded Thelypteris sp. and Carex sp. It must
be stressed that the larg est stock of seeds of Lycopus euro-
paeus is cor re lated with the wan ing of char coal con tent. This
is a re sult of the fast ad ap ta tion of this spe cies – a pi o neer of
sec ond ary suc ces sion (Kania et al. 2006). Lycopus europa-
eus tol er ates shad ing in the eutrophic en vi ron ment with neu -
tral re ac tion (6£pH<7) (Zarzycki et al. 2002). It is a dif fer en -
tial spe cies of the phytosociological class Phragmitetea
(Podbielkowski, Tomaszewicz 1982). Filipendula ulmaria,
whose seeds were found at a sim i lar depth, also in di cates
such edaphic con di tions.
Mosses are im por tant bioindicators of the nat u ral en vi -
ron ment as well as com po nents of de pos its (Dick son 1986,
Tobolski 2006, Janssens 1990). Brown mosses are ex cel lent
in di ca tors of minerotrophic hab i tats. Meesia triquetra is the
dom i nant brown moss re corded in the ana lysed de posit. This
spe cies is treated as a relic of the rich con ti nen tal fens
(Jannsens 1990, Lamentowicz 2005, Swinehart 1995), in
some pub li ca tions also de scribed as a gla cial relic (Tobolski
2003, Ga³ka 2007). They played an im por tant role in the
peat-form ing pro cesses at the late Pleis to cene and early Ho -
lo cene (Jasnowski 1957a, 1957b, 1959). In this phase, peat
mosses were less abun dant than brown mosses.
Testae amoe bae are sen si tive in di ca tors in stantly re act -
ing to changes in the nat u ral en vi ron ment, such as eutrophi-
cat ion, acid i fi ca tion, drain age or waterlogging (Tolonen
1986, Charman 2002, Lamentowicz 2006, 2007a, 2007b).
The ap pear ance of Centropyxis aculeata in di cates the
change of wa ter con di tions in the mire. It im plies the chang -
ing hy dro log i cal con di tions in the ba sin of biogenic ac cu mu -
la tion, which is proved by the pres ence of the amoeba Arcella
discoides (Lamentowicz 2005, 2008). This spe cies pre fers
wet hab i tats (Warner 1990a), but it tol er ates a wide range of
pH (Lamentowicz 2005). Nearly the en tire sec tion of the core
un der ques tion con tains head cap sules of Chironomidae as
well the re mains of Cladocera and Oribatida. At the end of
MACROFOSSIL RE CON STRUC TION OF PREBOREAL WETLAND 5
Ta ble 1
Char ac ter is tics of the lo cal macrofossil as sem blage zones (LMAZ) from the bot tom de pos its of the Borzechowo mire
Zone Depth (m) Char ac ter is tics
BorzM1
Carex-Lycopus 11.42–11.24
This zone in cludes mire subfossil plant re mains: Carex sp., Phragmites aus tra lis, Thelypteris sp., Equisetum sp.,
Calamagrostis sp., Filipendula ulmaria, Lycopus europaeus, Ranuculus sp., Eriophorum angustifolium, Pinus
sylvestris, Betula sp. and Calluna vulgaris. Other find ings in clude moss (Sphag num spp., Polytrichum com -
mune, Messia triquetra, Drepanocladus) and re mains of Pinus sylvestris, Populus tremula and fruits of Betula
nana. This zone also in cludes wood, char coal and sand. In the fi nal sec tion of this zone, wa ter plant re mains ap -
pear (Ceratophyllum demersum and Nymphaea sp).
BorzM2
Nymphaea-
Ceratophyllum
11.24–11.18
A char ac ter is tic fea ture of this zone is the pres ence of aquatic plants, such as Ceratophyllum demersum,
Nymphaea sp., Najas ma rina and Chara sp. Large amounts of Mollusca ap pear, whose abun dance de creases
when mov ing up. Aquatic fauna: Arcella discoides, Chironomidae and Cladocera.
6M. S£OWIÑSKI
.2 .giF.erim eikswohcezroB eht fo taep lasab eht fo mar g aid lissoforcam tnalP
the telmatic phase, Thelypteris palustris and Scirpus lacus-
tris in creased in abun dance. Their max i mum is re corded at
the depth of 11.26 m. Other plant re mains that be came more
abun dant at that time in clude utricles of Carex sp. (ros-
trata?), nee dles of Pinus sylvestris, fruits of Betula sp. and
epi der mis of Calamagrostis sp.
Limnic phase
The be gin ning of this phase in di cates a rad i cal trans for -
ma tion of the con di tions of biogenic ac cu mu la tion. The
semi-aquatic hab i tat evolves into an aquatic one. The palyno-
log i cal anal y sis made by B. Noryœkiewicz (un pub lished
data) in di cates that those changes took place at the turn from
the Preboreal to the Bo real pe ri ods. The ex ist ing then wa ter
body was sur rounded by pine-birch for ests with a quickly
spread ing ha zel.
The limnic phase doc u ments the first stage of lake de vel -
op ment. Be sides the change of the de posit from a peaty to
limnic one (cal car e ous gyttja), also other changes were re -
corded, e.g. in the subfossil plant re mains and the ap pear ance
of Mollusca. Ceratophyllum demersum was found at the
depth 11.27 m. It is a free-float ing aquatic plant, which pre -
fers fer tile or very fer tile small astatic wa ter bod ies of neu tral
or even al ka line re ac tion (6£pH>7). Ceratophyllum demer-
sum reaches its max i mum abun dance in shal low wa ter bod ies
heated in ten sively in sum mer. En vi ron men tal tol er ance en -
ables this spe cies to grow at the depth from 0.5 to 10 metres
(Hannon, Gaillard 1997). It cre ates dense patches (K³o-
sowski S., K³osowski K. 2001, Zarzycki et al. 2002). The
larg est amount of Mollusca is found in the third centi metre of
the core in the limnic phase, at the depth of 11.24 m. The lake
was oc cu pied by spe cies of the ge nus Nymphaea with
free-float ing leaves. This is proved by its re mains, such as the
epi der mis and idioblasts. Plants of the ge nus Nymphaea pre -
fer meso-eutrophic wa ter bod ies, whose re ac tion ranges from
slightly acidic to al ka line (5£pH>7) (Zarzycki et al. 2002).
They also tol er ate a small depth of wa ter, and rarely live be -
low 3 metres in depth (Hannon, Gaillard 1997). Due to their
fast growth and large pro duc tion of plant bio mass, those
plants con trib ute to shallowing and over grow ing of wa ter
bod ies (Janecki 1999). Nymphaeids were ac com pa nied by
Ceratophyllum demersum, whose nee dle-like leaves were
found at the top of the ana lysed sec tion. The pres ence of oo -
spores in di cates that Charales also grew there. Lake con di -
tions are also con firmed by the pres ence of head cap sules of
Chironomidae and the de creas ing abun dance of Oribatida. In
this en vi ron ment, large com mu ni ties of Cladocera de vel -
oped. The lake shores were over grown by sedge com mu ni -
ties with brown mosses of the ge nus Drepanocladus, as well
as Thelypteris palustris, Phragmites aus tra lis and Scirpus
lacustris. The top sec tion of the core also in cluded the seeds
of Najas ma rina. This macrophyte is an an nual plant and
forms thick un der wa ter mead ows. Najas ma rina pre fers
shal low meso-eutrophic lakes up to 3 m deep (op ti mum
depth is about 1 m) (Hannon, Gaillard 1997), and al ka line re -
ac tion of pH>7 (Zarzycki et al. 2002). Such con di tions are
found in both fresh wa ter and salty wa ter bod ies (sa lin ity up
to 10‰). Najas ma rina was also re corded in de pos its of the
Bal tic Sea (Bennike et al. 2001).
Plant in di ca tors found in the ana lysed sec tion of the core,
such as Ceratophyllum demersum, Lycopus europaeus and
Scirpus lacustris, in di cate that mean min i mum tem per a ture
for July at the base of biogenic ac cu mu la tion ranged be tween
13°C and 16°C (Isarin, Bohncke 1999, Bos et al. 2007). The
en tire sec tion of the core in cluded an ad mix ture of un iden ti -
fied or ganic mat ter, which is plot ted in the di a gram as UOM.
DIS CUS SION
High res o lu tion anal y sis of the bot tom sec tion of the core
en abled re con struc tion of the evo lu tion of the mire, which
had de vel oped on a bur ied block of dead ice. Ac cord ing to
the macrofossil anal y sis for the bot tom sec tion of the core,
the de posit found di rectly on the min eral base is thought to be
fen. It was ac cu mu lated in telmatic con di tions, which are ev i -
denced by subfossil plant re mains. On the ba sis of the pres -
ence and rel a tive abun dance of spe cific plant spe cies, the
so-called in di ca tor plants, it was pos si ble to in fer the type of
fen. It was Limno-Phragmitioni (reed peat) (Tobolski 2000,
To³pa et al. 1967). In in ves ti ga tions on palaeohydrological
changes of wetlands and cor re la tions be tween that peat and
en vi ron men tal fac tors, ¯urek (1990, 1993) proved that peat
was de vel op ing on sandy de pos its in the Late Gla cial pe riod.
Ac cord ing to him, this phe nom e non is linked with melt ing
out of dead ice blocks and de vel op ment of peat de pos its on
top of it. In creas ing rel a tive wa ter ta ble fi nally sub merged the
peat.
Sum ma riz ing all the re search, it may be con cluded that
the ‘basal peat’ came into be ing as a re sult of in ten si fied
paludification dur ing an early phase of the dead ice melt-out.
Wa ter logged basal sed i men tary com plex, found on top of the
melt ing block, con trib uted to the de vel op ment of minerotro-
phic peat-form ing veg e ta tion. Ground wa ter played a cru cial
role in the de vel op ment of the mire. It stim u lated sed i men ta -
tion of biogenic mat ter di rectly on sandy-gravely de pos its.
The rate of peat ac cu mu la tion de pends mostly on cli ma tic
and hy dro log i cal con di tions in the place where biogenic ac -
cu mu la tion oc curs. Those con di tions in flu ence the biogenic
de com po si tion and ac cu mu la tion. Plant com mu ni ties that
take part in peat sed i men ta tion are also of great sig nif i cance.
How ever, be sides cli ma tic and hy dro log i cal con di tions they
are also de pend ent on edaphic con di tions. The macrofossil
anal y sis, es pe cially of the subfossil artefacts re corded at the
bot tom of the Borzechowo mire, made it pos si ble to re con -
struct palaeohydrological con di tions in this lo cal ity. The
basal de pos its that fill up the bed of the subglacial chan nel in -
di cate two phases of de vel op ment: telmatic and limnic. Both
plant and an i mal spe cies found in the de pos its in di cate fre -
quent os cil la tions of the wa ter ta ble. The telmatic phase in di -
cates a ris ing wa ter ta ble. At that time a shal low astatic wa ter
body ex isted. This was the ef fect of sys tem atic melt ing out of
dead ice blocks, which fi nally caused the deep en ing of the
ba sin and in tro duc tion of new el e ments in limnic flora. A sig -
nif i cant wa ter rise, which brought about a change from a
telmatic to a limnic en vi ron ment, is in di cated by the pres ence
of plant macrofossils of Najas ma rina, Nymphaea sp. (Han-
non, Gaillard 1997), al gae Chara sp. (Podbielkowski 1978,
Pe³echaty et al. 2007) and an i mal macrofossils of Tardigrada
(Cro mer 2008), Cladocera and Chironomidae.
MACROFOSSIL RE CON STRUC TION OF PREBOREAL WETLAND 7
The basal peat was rarely ana lysed in re spect of palaeo -
ec ol ogy and macrofossils. A few pa pers dis cuss the floristic
com po si tion and evo lu tion of wet land de vel op ment on bur -
ied blocks of dead ice (Marek 1994, Wojciechowski 2000,
Kowalewski et al. 2001, Wright, Stefanova 2004, Drzy-
mulska 2006). Both floristic and faunistic com po si tion of the
bot tom sec tion of the cores, which in cluded basal peat, in di -
cate palaeohydrological changes in the sed i men tary bas ins.
The thresh old was the first win ter sea son when the wa ter
above the peat bog sur face did not freeze to the bot tom
(B³aszkiewicz 2005). This trig gered a sud den dis in te gra tion
of dead ice, and a quick sub merg ing of the mire. The macro-
fos sil com po si tion in di cates quick changes in the wa ter ta ble
(Hannon, Gaillard 1997). The or i gin of the lake ba sin of the
Borzechowo mire cor re lates with the re corded palaeohydro-
log i cal changes in the early post-gla cial land scape ar eas in
Cen tral Eu rope. On the ba sis of the re search by Niewia-
rowski (1990) and Ralska-Jasiewiczowa (1987), ¯urek
(1990) de lim ited two pe ri ods of low wa ter ta ble. The first one
was con nected with the fi nal stage of the dead ice melt-out,
cor re lated with the palaeo eco logi cal in ves ti ga tions in the
bot tom de pos its of the Borzechowo mire.
The macrofossil anal y sis is a key el e ment in an un am big -
u ous def i ni tion of biogenic sed i ments and of the char ac ter of
peat de pos its (Tobolski 2000, 2006). Along with other
palaeoeco log i cal anal y ses (anal y ses of pol len, tes tate amoe -
bae, cladocerans, di a toms, chi rono mids, etc.) it re flects a
wider spec trum of en vi ron men tal changes and en hances pos -
si bil i ties for their more ac cu rate in ter pre ta tion, as well as the
pro cesses that stim u lated them.
Ac knowl edge ments
I thank Ass. Pro fes sor Miros³aw B³aszkiewicz and Pro fes sor
Tomasz Goslar, Dr Jaros³aw Kordowski for sup port ing our work
and care ful re vi sion of the manu script. I also would like to thank re -
view ers for their valu able re marks on the manu script, Dr Bo¿ena
Noryœkiewicz for shar ing her un pub lished data, and Sebastian
Tyszkowski, MSc, for co op er a tion in the field work. This work is
part of a re search pro ject funded by the Min is try of Sci ence and
Higher Ed u ca tion: Cli mate and en vi ron ment change in the Late
Gla cial and early Ho lo cene in the area Tuchola For est in the light of
high-res o lu tion palaeo eco logi cal anal y sis (no. N N306 085037)
(Prin ci pal In ves ti ga tor Michal S³owiñski).
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10 M. S£OWIÑSKI
... It is a complex river-lake system, which resulted from the connexion of lakes generated during the Late Glacial in dead ice depressions (Koutaniemi and Rachocki, 1981;Błaszkiewicz, 2005;Kaiser et al., 2007;van Loon et al., 2012;Michczyńska et al., 2013). The formation of the TRZ depression is associated with the melting of a buried ice block (Błaszkiewicz, 2005;Słowiński, 2010;. The area, in general, was characterised by a zonal-areal type of ice sheet decay, resulting in diverse reliefs with numerous ice melt-out forms and kames. ...
... An important aspect in the study of river valley development within the Weichselian ice limit is lakes, which at present frequently occur along the river courses and whose deposits are widespread at the valley bottoms. Reconstructions of the history of glacial lakes in this area of the Central European Lowland suggest strong causal relationships between the lake basins and the river valleys, and the impact that the melting of buried dead ice blocks in the substratum exerted on local geomorphology and hydrology (Koutaniemi and Rachocki, 1981;Błaszkiewicz, 2005, Kaiser et al., 2007Słowiński, 2010;Słowiński et al., 2015). A strong influence of glacial landscape on the evolution of river valleys has also been determined in the area of Cordilleran ice sheet in British Columbia (Collins and Montgomery, 2011) and in the area covered by Laurentide ice sheet (Wright, 1972;Phillips and Robert, 2005;Arbogast et al., 2008). ...
... Occurrence of dead ice blocks in subglacial channels (also referred to as tunnel valleys) has been frequently suggested for the area of Weichselian glaciation both in northern central Europe (Więckowski, 1966;Nitz, 1984;Böse, 1995;Schultz and Strahl, 2001;Niewiarowski, 2003;Błaszkiewicz, 2005, Kaiser et al., 2007Cedro, 2007;Słowiński, 2010;Michczyńska et al., 2013;Słowiński et al., 2015) and North America (Florin and Wright, 1969;Porter and Carson, 1971;Last and Vance, 2002;Schwalb and Dean, 2002). This is indicated among others by younger outwash plains found at high elevations around the subglacial channels (Jørgensen and Sandersen, 2006), deformations at the base of lacustrine deposits caused by melting of buried dead ice (Błaszkiewicz, 2011), and the occurrence of a thin layer of basal peat below lacustrine sediments that must have been originally deposited on top of dead-ice blocks (Więckowski, 1966;Florin and Wright, 1969;Niewiarowski, 2003;Błaszkiewicz, 2005Błaszkiewicz, , 2011Kaiser et al., 2007;Słowiński et al., 2015). ...
Thesis
Holocene climate variability is generally characterized by low frequency changes than compared to the last glaciations including the Lateglacial. However, there is vast evidence for decadal to centennial scale oscillations and millennial scale climate trends, which are within and beyond a human lifetime perception, respectively. Within the Baltic realm, a transitional zone between oceanic and continental climate influence, the impact of Holocene and Lateglacial climate and environmental change is currently partly understood. This is mainly attributed to the scarcity of well-dated and high-resolution sediment records and to the lacking continuity of already investigated archives. The aim of this doctoral thesis is to reconstruct Holocene and Late Glacial climate variability on local to (over)regional scales based on varved (annually laminated) sediments from Lake Czechowskie down to annual resolution. This project was carried out within the Virtual Institute for Integrated Climate and Landscape Evolution Analyses (ICLEA) and funded by the Helmholtz Association and the Helmholtz Climate Initiative REKLIM (Regional Climate Change). ICLEA intended to gain a better understanding of climate variability and landscape evolution processes in the Northern Central European lowlands since the last deglaciation. REKLIM Topic 8 “Abrupt climate change derived from proxy data” aims at identifying spatiotemporal patterns of climate variability between e.g. higher and lower latitudes. The main aim of this thesis was (i) to establish a robust chronology based on a multiple dating approach for Lake Czechowskie covering the Late Glacial and Holocene and for the Trzechowskie palaeolake for the Lateglacial, respectively, (ii) to reconstruct past climatic and environmental conditions on centennial to multi-millennial time scales and (iii) to distinguish between local to regional different sediments responses to climate change. Addressing the first aim, the Lake Czechowskie chronology has been established by a multiple dating approach comprising information from varve counting, tephrochronology, AMS 14C dating of terrestrial plant remains, biostratigraphy and 137Cs activity concentration measurements. Those independent age constraints covering the Lateglacial and the entire Holocene and have been further implemented in a Bayesian age model by using OxCal v.4.2. Thus, even within non-varved sediment intervals, robust chronological information has been used for absolute age determination. The identification of five cryptotephras, of which three are used as unambiguous isochrones, is furthermore a significant improvement of the Czechowskie chronology and currently unique for the Holocene within Poland. The first findings of coexisting early Holocene Hässeldalen and Askja-S cryptotephras within a varved sequence even allowed differential dating between both volcanic ashes and stimulated the discussion of revising the absolute ages of the Askja-S tephra. The Trzechowskie palaeolake chronology has been established by a multiple dating approach comprising varve counting, tephrochronology, AMS 14C dating of terrestrial plant remains and biostratigraphy, covers the Lateglacial period (Allerød and Younger Dryas) and has been implemented in OxCal v.4.2. Those age constraints allowed regional correlation to other high-resolution climate archives and identifying leads and lags of proxy responses at the onset of the Younger Dryas. The second aim has been accomplished by detailed micro-facies and geochemical analyses of the Czechowskie sediments for the entire Holocene. Thus, especially micro-facies changes had been linked to enhanced productivity at Lake Czechowskie. Most prominent changes have been recorded at 7.3, 6.5, 4.3 and 2.8 varve kyrs BP and are linked to a stepwise increasing influence of Atlantic air masses. Especially, the mid-Holocene change, which had been widely reported from palaeohydrological records in low latitudes, has been identified and linked to large scale reorganization of atmospheric circulation patterns. Thus, especially long-term changes of climatic and environmental boundary conditions are widely recorded by the Czechowskie sediments. The pronounced response to (multi)millennial scale changes is further corroborated by the lack of clear sediment responses to early Holocene centennial scale climate oscillations (e.g. the Preboreal Oscillation). However, decadal scale changes at Lake Czechowskie during the most recent period (last 140 years) have been investigated in a lake comparison study. To fulfill the third aim of the doctoral thesis, three lakes in close vicinity to each other have been investigated in order to better distinguish how local, site-specific parameters, may superimpose regional climate driven changes. All lakes haven been unambiguously linked by the Askja AD1875 cryptotephra and independent varve chronologies. As a result, climate warming has only been recorded by sedimentation changes at the smallest and best sheltered lake (Głęboczek), whereas the largest lake (Czechowskie) and the shallowest lake (Jelonek) showed attenuated and less clear sediment responses, respectively. The different responses have been linked to morphological lake characteristics (lake size and depth, catchment area). This study highlights the potential of high-resolution lake comparison for robust proxy based climate reconstructions. In summary, the doctoral thesis presents a high-resolution sediment record with an underlying age model, which is prerequisite for unprecedented age control down to annual resolution. Sediment proxy based climate reconstructions demonstrate the importance of the Czechowskie sediments for better understanding climate variability in the southern Baltic realm. Case studies showed the clear response on millennial time scale, while decadal scale fluctuations are either less well expressed or superimposed by local, site-specific parameters. The identification of volcanic ash layers is not only used for unambiguous isochrones, those are key tie lines for local to supra regional archive synchronization and establish the Lake Czechowskie as a key climate archive.
... It is recorded from between the Late Glacial and the Early Holocene at Linje (Kloss 2005(Kloss , 2007. In the Preboreal period it was present at Borzechowo (Słowiński 2010) and in the Wda river valley (Słowiński et al. 2015), as well as in the north-east at Kuźnica (Urban et al. 2011) and Taboły (Drzymulska 2006), in the east at Radzików (Dobrowolski et al. 2012) and to the west at Grodzisko (Gałka et al. 2020). There are only two known places where Meesia triquetra occurred in the Boreal period, Tuchola (Lamentowicz 2005) and Perty (Gałka et al. 2015a), and two with occurrences in the Boreal and/or Atlantic, Taboły and Borki (Drzymulska 2006). ...
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Some glacial relict (mosses) have survived from the Ice Age up to the present time in specific, long-lasting habitats. Mires are one of the most common ecosystems in which they are present. In this paper the past distribution of eight species of such peat-forming mosses in Poland in the past is discussed. The distribution and dates of previously published moss finds in Poland have been mapped. In almost every case the largest number of places where they were found was in northern, or more precisely northeast Poland. A significant difference in the number of known find sites for individual species and many sites of unknown age were found. The rarest moss was definitely Cinclidium stygium and the most frequent was Meesia triquetra. Data on the distribution of mosses in the Late Glacial and Holocene proved to be scarcer than expected. Only a few sites were found where the occurrence of species such as Meesia triquetra, Calliergon giganteum, Tomentypnum nitens, Pseudocalliergon trifarium, Helodium blandowii and Scorpidium scorpioides was fairly continuous for this period. Therefore, the status of these mosses as glacial relicts seems to merit some thought. Certainly, there is a further need for high resolution research on bryophyte macrofossils, combined with accurate dating.
... The YD/PB transition was dynamic not only because of the rather rapidly rising air temperature but also because of the accompanying changes in the natural environment (degradation of permafrost, including the melting of the buried lumps of dead ice, or, for example the migration of vegetation from southern refugia). This contributed to the unstable and nonunidirectional nature of the changes (Płóciennik et al., 2022;Słowiński, 2010). ...
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In this work, we studied the course of climatic fluctuations at the beginning of the Preboreal period using multiproxy analyses (Chironomidae, pollen, diatoms, Cladocera, stable isotopes, macrocharcoal, microlithofacial) combined with varve chronology of sediments. The results showed a decrease in the mean July temperature and stronger continentalism followed by climate wetting at the start of the Preboreal period. This was reflected in the remodeling of the vegetation cover from birch forests to more open spaces with grass vegetation dominance, as well as that of the species composition of Cladocera and diatoms, and the high variability of isotopic composition. In addition, the results showed the impact of local conditions (fires and relict permafrost) on the amplification of environmental responses due to climate cooling. The environmental transformations presented here correspond temporally to the changes that are well documented in other parts of Europe as the Preboreal Oscillation.
... Together with a rapid decrease in cyanobacteria in this phase, we observe isoetid succession with initial dominance by Elatine hydropiper and, later, I. echinospora, which testifies to major changes in the aquatic environment (Birks et al., 2000). With this succession, we see the withdrawal of plants with floating leaves (Potamogeton species) that is also documented at other sites in Central Poland (Słowiński, 2010;Mirosław-Grabowska et al., 2020). Spores of I. echinospora and some I. lacustris found at the end of the Younger Dryas indicate lowering nutrient contents and low water conductivity (Szmeja, 2001a(Szmeja, , 2001b. ...
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... It is situated in the outwash plain of the Wda River, which developed during the Pomeranian phase of the Vistulian glaciation . The Martwe lake was formed by the melting of a buried ice block (Kordowski et al., 2010;Słowiński, 2010;Słowiński et al., 2015). The entire peatland as well as the lake (3.56 ha) is protected at the national level as Nature Reserve. ...
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... BC, there is a visible increase in both floating-leaved and submerged plants, such as Nymphaea alba and Najas marina, which penetrate to water depths of up to a maximum of three metres (Düll and Kutzelnigg, 2005). These species also slow down water dynamics and accelerate the rate of accumulation of biogenic sediments in the place of their occurrence (Słowiński, 2010, van der Valk, 2006. The occurrence of water lily or spiny naiad may thus be associated with a further decrease in the lake level in that period . ...
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A multi-proxy record derived from sediments collected from an infilled lake basin within the area of the Serteya II site, Western Russia, was used to reconstruct the palaeoenvironment development of this site over the past ca. 9000 years. Despite there being archaeological evidence of a human presence in this area spanning several millennia, the results from pollen analysis indicate only a small-scale human impact on vegetation during the prehistory, reinforcing existing ideas that lifeways based upon hunting, fishing and gathering (h-f-g) were maintained over an extended period of time at this location. Human activities remained relatively extensive during the historical periods, although the first evidence of cereal cultivation dates to the Middle Ages. The results of earlier investigation, which suggested that cereal cultivation and animal husbandry may have begun at Serteya in the Late Neolithic, have not been confirmed by the presented dataset (or, at least, cannot be seen in it). The results from macrobotanical analysis reveal a gradual lowering of the lake-level, but with intervening transgression phases ca. 5550-3600 cal. BC and, more briefly, around the time of the 4.2 ka BP cooling event. The Late Neolithic pile dwelling settlement (dated to 2500-2200 cal. BC) that was present in the immediate area, and from which the samples are drawn, was situated in shallow water-the depth of which varied seasonally-and in the close vicinity of the shoreline. The gathering of berries and nuts, as well as starch-rich aquatic plants, seems to have played an important role in subsistence, which was further supported by hunting and fishing. Following a brief phase during which the pile dwelling settlement went into decline, the increased exploitation of aquatic plants appears to have intensified between ca. 2200 and 2000 cal. BC.
... The sedimentation in this lake started before 13,900 yr cal BP. The depression of Lake Kaniewo developed as a result of the buried dead ice, which is a common feature in the young glacial landscape in the Central European Lowlands (Zawisza et al., 2019;Dräger et al., 2016;Dietze et al., 2016;Zawiska et al., 2015;Słowiński et al., 2015;Słowiński, 2010). ...
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This study presents the results and interpretations of biological (floral and faunal) and geochemical (isotopic and chemical element composition) investigations of the Late Glacial and Holocene sediment accumulated in the Kaniewo palaeolake. These data allowed us to determine the indicators of environmental changes in this lake ecosystem, such as zooplankton and vegetation development, trophic state, water temperature, and water level. We documented the three cold periods (Oldest, Older, and Younger Dryas) separated by two warm periods (Bølling, Allerød). Additionally, we found traces of cold oscillations within the Allerød, lasting approximately 150 years, probably the Intra-Allerød-Gerzensee oscillation, recorded in western and southern Europe. Our data show that sediment accumulation began at the end of the Oldest Dryas period before ca. 13,900 yr cal BP. The top of the studied sediments represented the transition from the Late Glacial to the early Holocene (Preboreal period, ca.10,500 yr cal BP). The lithology of the sediments changed from silty sands at the bottom, then calcareous detritus gyttja partially interbedded by lake marl to peat at the top. Organic matter occurring in the deposits reflects algae and terrestrial sources. In the cold periods, small number of both Cladocera individuals and species (e.g. Bosmina (E.) longispina, Chydorus sphaericus, Alona affinis), corroborate the unfavourable conditions for the development of zooplankton. Cladocera species indicated the initial oligotrophic status of the lake and the subsequent increase in trophic status. Plant and animal macrofossils indicate initial shallow lake ecosystem. Chara meadow developed at the bottom of the lake with a belt of vascular plants (Carex sp., Hippuris vulgaris, Scirpus sp.) in the littoral zone of the lake. Rich shallow vegetation was accompanied by animals, such as Mollusca (e.g. Valvata piscinalis, Pisidium obtusale, Bithynia tentaculata) and Oribatidae sp., Cristatella mucedo, and Plumatella type.
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While of higher plant origin, a specific source assignment of sedimentary leaf wax n-alkanes remains difficult. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. In particular, for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we tested to what extent compositional changes in leaf wax n-alkanes can be linked to known vegetation changes by comparison with high-resolution palynological data from the same archive. We analyzed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake (TRZ, northern Poland), covering the Late Glacial to early Holocene (13360–9940yrBP). As an additional source indicator of targeted n-alkanes, compound-specific carbon isotopic data have been generated in lower time resolution. The results indicated rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We found a shift towards higher average chain length (ACL) values at the Allerød–Younger Dryas (YD) transition between 12680 and 12600yrBP, coevaled with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), shrubs, and dwarf shrubs (Juniperus and Salix). The termination of the YD was characterized by a successive increase in n-alkane concentrations coinciding with a sharp decrease in ACL values between 11580 and 11490yrBP, reflecting the expansion of woodland vegetation at the YD–Holocene transition. A gradual reversal to longer chain lengths after 11200yrBP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show that n-alkane distributions reflect vegetation changes and that a fast (i.e., subdecadal) signal transfer occurred. However, our data also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes in ACL need to be discussed in the context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm genera, such as Pinus, produce only a very low amount of n-alkanes and for this reason their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound-specific hydrogen isotope ratios for paleohydrological reconstructions.
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While of higher plant origin, a specific plant source assignment of sedimentary leaf wax n-alkanes remains difficult. Recent compilations of global plant data sets have demonstrated an overlapping and non-systematic production of different chain-length homologues among different classes of terrestrial vegetation. Further, n-alkane distributions can change within the same species due to environmental changes. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. However, in particular for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we aim to identify the major leaf wax contributors to a Central European lacustrine system. Therefore, we tested to what extent leaf wax n-alkane compositional changes (expressed through compound concentration ratios, such as nC27 vs. nC31, average chain length ACL, etc.) can be linked to known vegetation changes, specifically during the Younger Dryas cold period (YD), by comparison with high-resolution palynological data from the same archive. We analysed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake – TRZ – (Northern Poland), covering the Late Glacial to early Holocene (13,360–9,940 yrs BP). As additional source indicator of targeted n-alkanes, compound specific carbon isotopic data have been generated in lower time resolution. The results showed rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We find a shift towards higher ACL values at the Allerød/YD transition between 12,680 and 12,600 yrs BP, coeval with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), as well as shrubs and dwarf shrubs Juniperus and Salix. The termination of the YD was characterized by a successive increase of n-alkane concentrations coinciding with a sharp decrease of ACL values between 11,580–11,490 years BP, reflecting the expansion of woodland vegetation at the YD/Holocene transition. Centennial reversals to longer chain lengths during the Allerød could possibly be linked to Greenland Interstadial 1b (GI-1b). A similar pattern during the early Holocene has more likely been triggered by rapid ecological responses in course of warming, rather than to reflect a local impact of a Preboreal Oscillation or 11.4 yr event. Another gradual increase in ACL values after 11,200 yrs BP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show, that n-alkane distributions reflect vegetation changes and that a fast (i.e. subdecadal) signal transfer occurred. However, our results also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes of ACL need to be discussed in context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm species, such as Pinus, produce only very low amount of n-alkanes and thus their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound specific hydrogen isotope ratios for paleohydrological reconstructions.
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This paper presents a comprehensive palaeoenvironmental data set from a trough between glacial curvilineations (an intra-GCL trough) of the Dobrzyn Lake District, north-central Poland. A sediment core taken from the depression was investigated to reconstruct Late Glacial and Holocene environmental changes in north-central Poland using a combination of palynological, macrofossil, sedimentological and geochemical studies. Four main development phases of the studied intra-GCL trough were distinguished. Accumulation of the analysed sediments occurred from the Late Vistulian (Glacial Phase) through the Late Glacial to present times. Our data suggest the persistence of dead ice in the basin up to the Older Dryas (Lacustrine Phase). The sedimentary record of the depression reflects a considerable difference between the Late Glacial/early Holocene and the middle/late Holocene in terms of environmental conditions. The Late Glacial was characterised by rapid environmental changes, while climatic conditions throughout the Holocene were relatively stable, except for the Subboreal period. A small lake was strictly dependent on climatically controlled vegetation changes and erosion tendencies in the vicinity of the Zb�ojenko site. The lake finally disappeared at the turn of the Younger Dryas and the Preboreal Period. The Preboreal, Boreal and Atlantic Periods encompassed the Early Paludical Phase, during which the water level used to rise and hence the mire could be flooded. The area was covered with dense vegetation and the rate of denudation processes around the site was moderate. The Late Paludical Phase included the Subboreal and most of the Subatlantic. Deciduous forests (with Quercus, Carpinus, Corylus) dominated around the site. Major changes occurred between the late Subboreal and the early Subatlanic as a result of the growing human impact on the environment. The Subatlantic ends with the human impact phase (Middle Age), dominated by human impact, little accumulation of sediments and highly increased denudation.
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The studies of lake sediments in the Gostyninskie Lake District initiated in 1979 include by now eight lakes, four of them forming the "Na Jazach' lake complex. The Lake Wikaryjskie reveals four distinct cycles of the organic matter accumulation in its carbonate gyttjas. The Lake Szczawinskie is submitted to the intensive filling processes during the last decades. Its homogenous carbonate gyttja sediments evidence a very stable sedimentation regime during the whole time of lake existence. The annually laminated sediments of the Lake Gosciaz are sulphur-carbonate gyttjas with a high content of hydrotroilite of intensive black colour. The sediments of other lakes belonging to the "Na Jazach' lake complex - Wirzchon, Brzoska and Mielec are filled with the similar carbonate gyttjas. The bottom sediments of two closed lakes surrounded by forests - Mrokowo amd Swiete - are dystrophic dark-brown algal fine detritus gyttjas without carbonates except for a layer of grey gyttja containing carbonates. -from English summary
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Buried dead ice blocks started to melt out at the very moment when the area was not occupied by an ice sheet. This process could be already active at presence of permafrost. Final melting-out of dead ice blocks occured probably any later than at the beginning of the Typical sequence of false with puts at the very bottom is not an evidence of lake deepening due to gradual melting-out of buried dead ice block. It proves however, a decided change of hydrological regime - and therefore of draining and feeding with water (underground too). Such phenomenon could be due to significant climatic changes, resulting in considerably higher precipitation and/or decreased evaporation but also - to retreat of permafrost and resulting changes in location of a ground water level. Increasing water depth in lake was mused by gradual rise of water level, but not by lowering of the reservoir bottom, due to melting-out of a hypothetical dead ice block.
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The work done among the lake depressions to be found in Eastern Pomerania revealed very marked differentiation in Late Glacial and Early Holocene geomorphological development. In relation to the time at which lake sedimentation came into effect and proceeded in the Late Glacial, it was possible to distinguish the following three main morphogenetic groups among lake basins: 1) lake troughs in which the onset of sedimentation was associated with the Pre-Alleröd period; 2) basins in which lakes begun to form at the time of the Balling-Alleröd warm period; 3) lake basins in which the absolute onset of sedimentation only took place in the Pre-Boreal period. At the same time, detailed study of entire lake sedimentation basins has made it clear that there are numerous gaps in lake sedimentation. The longest documented break in the process lasted for around 3000 years (between the oldest Dryas and the Pre-Boreal period). There are depressions in which the onset of lake sedimentation as such was preceded by glaciolimnic sedimentation. From the point of view of the temporal relations, it tis possible to identify among these types of depression in which the Pre-Alleröd lakes coming into existence were a certain kind of successor to already existing glaciolimnic bodies of water, as well as types in which there was a clear period of hiatus between glaciolimnic and limnic accumualtion, sometimes lasting up to several thousand years. A very interesting example of the relationship between the glaciolimnic and limnic sediments has been noted in the area of the former Gniew ice-dammed lake. The lake depressions there were shaped within the area affected by that lake through the melting of the mass of dead ice sitting within the glaciolimnic sediments. The difference in time between the end of glaciolimnic sedimentation and the onset of that of the limnic kind was of at least 4000 years in this case. There is no doubt that the main cause of the noted fact that lakes are of different ages is the complex character of the process by which masses of dead ice melted. This process proceeded at varying rates from the moment the ice sheet withdrew, lastingin certain depression structures until the very onset of the Prereal period. Indeed, in some depressions it was phasic in nature, taking place at different times in successive morphological levels, thereby being associated with the development of local river networks and constituting a cause of the gaps which occurred in lake sedimentation. The onset of lake sedimentation should be assigned to the Pre-Alleröd period, as small bodies of water came into being in some depressions at that time. In them it was possible for lake sediments (mainly mineral, but also in part biogenic) to accumulate. The lakes in question were relatively shortlived, flowing away as river networks organized themselves through the linking together of separate depressions within valley systems. The main period in which lake arose should nevertheless be identified as the Bölling-Allleröd complex. It was then that the principle phase of melting of masses of dead ice took place, and most of the studied lake basins were fully shaped. However, there are certain depressions in which the dead ice hung on until the Pre-Boreal. Indicative of this are the Pre-Boreal peats occurring at great depths (sometimes even more than 20 m below the level of today's biogenic plains), at the bottom of the lacustrine sediments. The hypsometric relationships between these sediments and fluvial structures point unequivocally to the presence of an ice substratum during the period in which they accumulated. Geomorphological analysis of depressions found to have low-lying Pre-Boreal peats point to a particularly important factor favouring the retention of masses of dead ice into the Pre-Boreal being - not only a mineral cover - but also a constant drainage of the depressions in question. This condition is met most especially by depressions located next to watercourses and subject to incision in he Late Glacial. In turn, in those places where morphological conditions favoured the protracted retention of standing waters, the consequent thermal impact on ice laying below was to bring about its accelerated melting, and hence the full shaping of lake basins as early as at the onset of the Late Glacial.