Late Weichselian and Holocene palaeoenvironmental changes in

Late Weichselian and Holocene palaeoenvironmental changes innorthern Poland based on the Lake Skrzynka record

KARINA APOLINARSKA, MICHAŁ WOSZCZYK AND MILENA OBREMSKA

Apolinarska, K., Woszczyk, M. & Obremska, M. 2012 (April): Late Weichselian and Holocene palaeoenviron-mental changes in northern Poland based on the Lake Skrzynka record. Boreas, Vol. 41, pp. 292–307. 10.1111/j.1502-3885.2011.00235.x. ISSN 0300-9483.

This paper presents a comprehensive palaeoenvironmental data set from Lake Skrzynka, northern Poland. Asediment core from the lake was investigated to reconstruct Lateglacial and Holocene environmental changes innorthern Poland using a combination of palynology and stable carbon and oxygen isotope studies of carbonatesand sediment geochemistry. The undisturbed sedimentation in Lake Skrzynka continues from the Allerød to thepresent. Our data suggest the persistence of dead ice in the Lake Skrzynka basin up to the Allerød. Thesedimentary record of the lake reflects a considerable difference between the Lateglacial/early Holocene andthe middle/late Holocene in terms of environmental conditions. The Lateglacial was characterized by rapidenvironmental changes, while climatic conditions throughout the Holocene were relatively stable. The trophicstate of the lake was strictly dependent on climatically controlled vegetation changes and erosion tendencies in thevicinity of Lake Skrzynka. During the Lateglacial and early Holocene, as a result of predominantly open plantcommunities and enhanced nutrient export from the watershed, Lake Skrzynka experienced an increased trophy.The stabilization of forest cover and reduced input of nutrients resulted in the establishment of oligotrophicconditions in the lake in the early Boreal. During the late Subatlantic, the lake became eutrophic as a result ofhuman disturbance of the local hydrological balance. The postglacial history of Lake Skrzynka can be regardedas representative of small, alkaline, through-flow lakes in temperate climates.

Karina Apolinarska (e-mail: [email protected]), Institute of Geology, Faculty of Geographical and GeologicalSciences, Adam Mickiewicz University, Maków Polnych 16, PL-61-606 Poznan, Poland; Michał Woszczyk (e-mail:[email protected]), Institute of Geoecology and Geoinformation, Faculty of Geographical and GeologicalSciences, Adam Mickiewicz University, Dziegielowa 27, PL-61-680 Poznan, Poland; Milena Obremska (e-mail:[email protected]), Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Warsaw, Twarda51/55, PL-00-818 Warsaw, Poland; received 26th October 2010, accepted 5th October 2011.

The chemical and biological processes occurring inlakes are controlled mainly by regional and local cli-matic and hydrological factors. However, the relation-ships among these factors are not alwaysstraightforward. Nevertheless, it is well established thatchanges in water and nutrient supplies from the water-shed result in lake-level fluctuations, alter water chem-istry and trophic states and, consequently, affect thelithology and chemical composition of sediments.Therefore, lake deposits act as excellent archives ofnatural environmental changes in different climaticzones and at different time scales.

The sediments of hard-water calcareous lakes appearto be very promising for palaeoenvironmental analysesas they create the possibility of studying stable C and Oisotopes. These isotopes are very sensitive indicators ofpast changes in parameters such as air temperature,precipitation and bioproductivity that are crucial to anunderstanding of the complexity of the evolution oflakes. According to Jonsson et al. (2009), however,stable isotope signatures can be site-specific and dependon both climate and local hydrological balances. Inmid-central Europe, within the transitional zone ofmarine/continental climates, stable isotope recordsfrom lakes and mires are still scarce and unevenly dis-tributed (Rózanski 1987; Goslar et al. 1999; Žák et al.2002; Ralska-Jasiewiczowa et al. 2003; Makhnach

et al. 2004; Apolinarska & Hammarlund 2009; Lauter-bach et al. 2010; Rózanski et al. 2010). Therefore, thereis a need for new well-documented sites depicting localand subregional/regional features of climatic and envi-ronmental changes. Records of Lateglacial andHolocene climatic shifts and the trophic evolution oflakes still require meticulous attention.

Palaeoenvironmental information can be derived bycombining stable isotope records with geochemical andpalaeoecological data in sediment cores from lakes thatreact readily to local and regional climatic forcings.Owing to the interference between climatic signals andanthropogenic impacts, lakes in which human distur-bance is particularly low are especially valuable forpalaeoenvironmental studies.

In this paper we present a multi-proxy record ofLateglacial and Holocene environmental changes fromLake Skrzynka, northern Poland. We discuss the physi-cal, chemical and biological factors controlling tempo-ral changes in the C and O stable isotope composition,as well as the geochemistry of lake sediments in relationto regional vegetation changes and regional climatictendencies and human impact. Use of a combination ofproxies and a comparison of the Lake Skrzynka proxydata with other well-documented records from theterritory of the Polish Lowlands allowed us to establishthe scheme of Lateglacial/Holocene environmental

DOI 10.1111/j.1502-3885.2011.00235.x © 2011 The AuthorsBoreas © 2011 The Boreas Collegium

changes and to reveal significant local variations in theevolution of the lake ecosystems in northern Poland.

Study area

Lake Skrzynka is located in the lower course of theSeven Lakes Stream (Bory Tucholskie National Park,NW Poland), occupying the northwestern part of theBrda outwash plain, which was formed during thePomeranian stage of the Weichselian glaciation c.16 200 a BP (Kozarski 1995). The Brda outwash plainconsists of vari-grained sands and gravels underlain byglacial tills and/or Neogene deposits (Poznan silts) thatcrop out in morainic plateaus and along river valleys(Galon 1953).

The name Seven Lakes Stream is somewhat mislead-ing because it is actually a system of eight intercon-nected lakes (Fig. 1), with Lake Skrzynka and LakeMielnica often regarded as one lake. Lake Skrzynkareceives most of its inflow from the stream entering thelake in the NE, but it is also fed by groundwater(Marszelewski 2006). Numerous springs are locatedalong the eastern shore of the lake (Gałka 2007). Itssurface outflow feeds Lake Mielnica, which then dis-charges into Lake Charzykowskie (Fig. 1).

Lake Skrzynka (121 m a.s.l.) is 868 m long and540 m wide, thus covering an area of approximately

0.21 km2 (Marszelewski 2006). It is divided into asmaller eastern part and a wider bay in the western partof the lake (Fig. 1). It receives a mean annual precipi-tation of about 600 mm. The mean annual air tempera-ture is 7°C and the monthly averages range between-3°C in January and 17°C in July (Wos 1999). In theTuchola Forests area, westerly and southwesterly airmasses prevail (Wos 1999). Owing to its shallow depth(mean and maximum depths of 1.8 and 4.3 m, respec-tively), Lake Skrzynka is a polymictic lake with a nearlyisothermal water column (Marszelewski 2006). Themean residence time of water in the lake is approxi-mately 25 days (Bajkiewicz-Grabowska 2004).However, the water renewal time may differ signifi-cantly between the two main basins of the lake(Fig. 1C).

Methods

Fieldwork

Two parallel sediment cores, each 5 cm in diameter,were collected in October 2008 from a floating platformusing a Wieckowski piston corer (for technical details,see Wieckowski 1989). The final sediment profile wascomposed of overlapping segments of the two parallelcores that were sampled and correlated in the labora-

Fig. 1. Location of the investigated site in Poland (A) and within Bory Tucholskie National Park (B). The drilling site, S1, is marked in (C).

Late Weichselian and Holocene palaeoenvironmental changes, N Poland 293BOREAS

tory. The investigated sediments were recovered fromthe central, deepest part (4.3 m) of Lake Skrzynka(Fig. 1; 53°48′53″N, 17°31′17″E). After the sedimentswere described, 1.1-m-long core segments werewrapped in plastic foil, transported to the Institute ofGeology, Poznan, and stored at ~4°C until subsam-pling in the laboratory.

Laboratory work

Palynology. – Palynological samples (1 cm3 each),subsampled at 10-cm intervals, were prepared and ana-lysed using standard methods (Berglund & Ralska-Jasiewiczowa 1986). A minimum of 500 arboreal pollengrains (trees and shrubs, AP) were counted in eachsample. Pollen grains of all herbaceous species (exceptfor the local aquatic and telmatic plants) were countedas NAP (non-arboreal pollen), and the sum of AP andNAP was considered to be 100%.

Geochemistry. – For geochemical analyses, everysecond 5-cm-thick sediment slice was taken. Sedimentsamples were dried at 105°C and homogenized in anagate mill Pulverizette2. The organic matter and car-bonate content were determined by loss on ignitionfollowing the protocol described by Heiri et al. (2001).Carbonate content was calculated as Carb =1.36 LOI925, where LOI925 = CO2 evolved from thesample, as defined by Heiri et al. (2001). The terrig-enous silica (SiO2ter) content was obtained using thefollowing two-step extraction: (1) acid-soluble speciesand organic matter were removed from the sample bydigestion in aqua regia at 100°C in a water bath andsubsequent combustion at 550°C for 4 h, and theresidue of this extraction was assumed to represent thetotal silica (SiO2ter + SiO2biog); (2) biogenic silica (opal)was dissolved with 0.5 n NaOH in a water bath at100°C for 2 h. The residue of step 2 represented terrig-enous silica (SiO2ter). Biogenic silica (SiO2biog) was cal-culated as the weight difference between the total silicaand the terrigenous silica. The total C, N and S con-tents were analysed in a Vario Max CNS elementalanalyser (Elementar Analysensysteme, GmbH,Germany) using Sulfadiazine, Agromat Compost(CP1) and Metals in sewage sludge (SQC001S) as ref-erence materials. The accuracies of the C, N and Sdeterminations were better than 3% for total C andtotal N and between 3 and 10% for total S. The weightpercentage of the total inorganic carbon (TIC) wasderived from the following formula: TIC [%] =0.27 LOI925. The total organic carbon (TOC) was cal-culated by subtracting the TIC from the total C. Thecontents of Fe, Mn, Na, K and Mg in the sedimentswere measured as soluble concentrations (35–38% HCl+ 65% HNO3 3:1 v/v digestion) using an atomic absorp-

tion spectrophotometer (Jena Analytik novAA 300)with analytical accuracy better than 10%. CP1 andSQC001S reference materials were used to control thequality of the results.

Stable isotopes. – Samples for isotope analysis (1 cm3

each) were taken at 10-cm intervals. Samples weredried at 50°C, sieved through a 0.5-mm sieve to elimi-nate sparse mollusc shells, and ground to a finepowder in an agate mortar. The d13C and d18O signa-tures of the carbonates were measured using a GasBench II hooked up to a Finnigan MAT 253 gassource mass spectrometer (both Thermofischer) at theInstitute of Geoscience at J.W. Goethe University inFrankfurt am Main, Germany. Details concerning theanalytical setup are given in Spötl & Vennemann(2003). For a single analysis, 50–120 mg of carbonatewas loaded into Labco exetainer vials. Carrara marblewas analysed with the samples, and its isotopic com-position was calibrated against NBS 19 (Fiebig et al.2005). The results are expressed as per mil (‰) devia-tions from the PDB carbonate standard and have ananalytical precision of �0.06‰ for carbon and�0.08‰ for oxygen.

XRD analysis. – The mineral composition of selectedsamples was determined by means of X-ray diffractionusing a URD-6 diffractometer. The diffractogramswere recorded using the reflection method using CuK�

radiation. The analyses were performed at the Instituteof Geology, Adam Mickiewicz University, Poznan.

SEM analysis. – The graphite-coated surfaces of chosensamples were examined using an S-3700N Hitachi scan-ning electron microscope at the Faculty of Geographi-cal and Geological Sciences, Adam MickiewiczUniversity, Poznan, Poland.

AMS radiocarbon dating. – The dating was performedon eight samples of terrestrial plant macrofossils at thePoznan Radiocarbon Laboratory (Table 1). Conven-tional radiocarbon ages were calibrated using theOxCal v3.1 program (Bronk Ramsey 2005) with theIntCal04 calibration data set (Reimer et al. 2004). Allof the ages mentioned in the text are calibrated unlessstated otherwise.

Results

Lithology

The bottom section of the S1 core (from 9.74 to 8.96 m)was dominated by clastic sediments, mainly sands,

294 Karina Apolinarska et al. BOREAS

interrupted by a 26-cm-thick intercalation of carbonatemud between 9.66 and 9.40 m (Fig. 2). Three ~1-cm-thick layers of fine-grained sand were observed at 9.17,9.10 and 9.06 m. Between 9.02 and 8.98 m, the sedi-ments displayed distinct millimetre-scale lamination.The central part of the profile (8.96–0.8 m) was repre-sented by homogenous beige carbonate mud overlainby light olive organic-carbonate mud within the top0.8-m section of the sequence.

Chronology

Eight AMS 14C dates for terrestrial plant macrofossils(six samples of Pinus periderm, one sample of a smallpiece of wood and one sample of organic detritus) wereobtained from the Lake Skrzynka sediments (Fig. 2;

Table 1). The calibrated ages of all the samples are instratigraphical order. However, the AMS 14C date Poz31651 (9.42 m b.l.f. (below lake floor), Table 1), whichis only 8 years younger than sample Poz 31652 (9.66–9.70 m b.l.f.) (Table 1), was rejected because the sampleappears to be too old owing to probable reworking. Inaddition, the 14C age of sample Poz 31651 disagreeswith the pollen-based biostratigraphy, as the midpointage of 12 982 cal. 14C a BP appears too old to corre-spond to the mid-Younger Dryas (Fig. 3). An age–depth model based on calibrated AMS 14C dates wasconstructed by linear interpolation between midpointsof the individual 2� ranges (Fig. 2). The surface of thelake bottom was assumed to represent the present.

Palynological analysis allowed 10 local pollen zones(LPAZ) to be distinguished (Fig. 3), in accordance withthe pattern of vegetation development in the Tuchola

Table 1. AMS 14C dates obtained on terrestrial macrofossils from a Lake Skrzynka sediment core at Poznan Radiocarbon Laboratory.Conventional 14C ages were calibrated using OxCal v3.1 (Bronk Ramsey 2005) and the IntCal04 calibration data set (Reimer et al. 2004). Theitalicized sample was not considered in the age model (see explanation in the text).

Depth (m b.l.f.) Dated material Lab. No. AMS 14C a BP Cal. a BP (2� range) Cal. a BP usedin age model

1.89 Pinus periderm Poz 31643 380�30 505–318 4122.80 Pinus periderm Poz 31644 1260�30 1281–1088 11864.12 Pinus periderm Poz 31645 2220�30 2331–2153 22425.05 Pinus periderm Poz 31647 2620�35 2840–2622 27317.28 Pinus periderm Poz 31648 4670�40 5577–5313 54458.91 Pinus periderm Poz 31649 8840�50 10 160–9706 99339.42 Wood Poz 31651 10 920�60 12 965–12 818 12 8929.66–9.70 Organic detritus Poz 31652 10 930�60 12 979–12 820 12 900

Fig. 2. Depth–age model based oncalibrated AMS 14C dates. Error bars forindividual AMS 14C dates indicatecalibrated 2� ranges. The grey shadingrepresents the 2� probability range. Theage model was constructed by linearinterpolation between midpoints of theindividual 2� ranges. The surfacesediments were assumed to represent thepresent. AMS 14C date Poz 31651, markedin the figure, was not considered for theage model, as the wood sample datedappears to be too old and disagrees withthe biostratigraphy. Owing to thediscrepancy between interpolated datesbetween 12 900 and 9933 cal. 14C a BP andthe palynological record, the bottom unitof the sequence is based on awell-recognized pollen biostratigraphy,with 12 700 cal. a BP and 11 600 cal. a BPfixed as proper dates. Figures next to thegraph characterize the mean sedimentationrate (mm a-1) between particular levels ofknown age. Next to the age model,lithological characteristics are presented.


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Forests (Milecka 2005). Detailed characteristics of theLPAZ are provided in Table 2.

The sparse radiocarbon dating within the bottomsection of the profile resulted in a discrepancy betweeninterpolated dates between 12 900 and 9 933 cal. 14C aBP (Table 1) and biostratigraphy based on palynology(Fig. 3). The Allerød/Younger Dryas transition deter-mined on the basis of pollen is consistent with the inter-polated date 12 700 cal. a BP and with dates obtainedin the other sites (Goslar et al. 1998). However, theYounger Dryas/Holocene transition indicated bychanges in palynological record was c. 600 yearsyounger in comparison with the interpolated dateobtained, namely c. 10 900 cal. a BP. The biostratigra-phy based on pollen is consistent with other investiga-tions in the Tuchola Forests (e.g. Milecka 2005;Milecka & Szeroczynska 2005). Thus, the interpolateddates obtained for the Lateglacial and early Holocenemay not be regarded as reliable. Hence, the chronologypresented for the Lateglacial and early Holocene sedi-ments from Lake Skrzynka was based on pollen bios-tratigraphy. The Younger Dryas/Holocene transitionwas fixed at 11 600 cal. a BP, in agreement with numer-ous studies (Goslar et al. 1998). The discrepanciesdescribed above indicate variable sedimentation ratesduring the period in question.

As deduced from the age–depth model based onradiocarbon dates for sediments younger than 9933 cal.14C a BP and pollen biostratigraphy for sediments olderthan 9933 cal. 14C a BP, the mean sedimentation ratebetween particular levels of known age ranged between0.26 and 4.59 mm a-1 (Fig. 2). The increasing upwardmean sedimentation rate is regarded as resulting fromlower compaction of the sediment. In accordance withthe sedimentation rate obtained, the 5-cm intervalsbetween individual sediment samples represent 10.9,42.5, 40, 26.2, 61, 137.7, 333, 120 and 250 years, listed inchronological order.

Geochemistry

The bottom section of the sedimentary sequence, c.13 100–10 500 cal. a BP, was mainly composed ofSiO2ter. Between c. 13 100 and 12 800 cal. a BP, thecontent of SiO2ter sharply decreased from 92 to 9.5%,being replaced by increasing contents of carbonates aswell as of TOC, N, S, Mg, K and Fe (Fig. 4). From c.12 800 to 12 100 cal. a BP, the deposits were dominatedby carbonates. A significant enrichment of SiO2ter

accompanied by increased contents of K were notedbetween c. 12 100 and 10 600 cal. a BP. Sharp peaks of

Table 2. Local pollen zones (LPAZ) distinguished in the Lake Skrzynka sediment sequence.

Number of LPAZ Name of LPAZ Depth (m b.l.f.) Description

Sk 10 Pinus-NAP 0.1–2.1 Rise in Pinus and NAP. Share of deciduous trees decreasing. Juniperus pollenappears. High share of human indicators. Increase in percentage value of greenalgae Pediastrum and Tetraedron.

Sk 9 Carpinus 2.1–4.2 The highest percentage of Carpinus, with two maxima (8.4 and 13.7%). In thetop part of LPAZ, the beginning of the continuous curve of cereals. Upperborder limited by decrease of Carpinus and increase in NAP.

Sk 8 Pinus-Carpinus 4.2–5.1 Increase in Pinus. Decrease of all curves of main deciduous trees except Alnus.Continuous curve of Carpinus. Upper border marked by decline of Pinus andincreasing Carpinus.

Sk 7 Quercus-Carpinus 5.1–6.5 Decrease in Corylus. Rise in Carpinus and Fagus curve. Upper border indicatedby an increase in Pinus and decline of Tilia.

Sk 6 Quercus-Corylus 6.5–7.2 Quercus and Corylus share increasing. The first pollen grains of Carpinus andFagus. Upper border limited by an increase in Carpinus.

Sk 5 Ulmus 7.2–8.7 All main deciduous trees, i.e. Corylus, Quercus, Tilia, Ulmus, Fraxinus, Alnus,present. Pollen grains of Hedera helix present. Decreasing share of Tetraedtron(to null). Upper border marked by Quercus increase.

Sk 4 Corylus 8.7–9.03 The highest content of Corylus (up to 18.9%). The beginning of curves of allmain deciduous trees. The highest content of Tetraedron. After initial increase,decrease in Pinus. Upper border indicated by a decline in Corylus.

Sk 3 Betula 9.03–9.16 Increase in Betula share (up to 70.3%). The beginning of Ulmus curve. Decreasein NAP content. Juniperus decreasing to null. Pediastrum share decreasing.Upper border marked by an increase in Pinus and Corylus.

Sk 2 Juniperus-NAP 9.16–9.62 Decreasing share of Pinus. Maximum of Juniperus (10.9%). High percentage ofNAP (up to 30.5%). High content of green algae with maximum of Pediastrum.Upper border indicated by a decline of Juniperus and an increase in Betula.

Sk 1 Pinus-Poaceae 9.62–9.74 The highest percentage value of Pinus (up to 80.6%). Maximum of Salix.Poaceae dominated by herb plants. Upper border marked by an increase inNAP and Juniperus.


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S, Fe, K and Mg could be distinguished at c. 11 500 cal.a BP.

In the interval from c. 10 500 to 200 cal. a BP, car-bonates (mostly calcites as verified by XRD analysis)became the most abundant sedimentary components inthe S1 deposits, with only minor amounts of terrig-enous silica (11.2 to 2.2%), biogenic silica (13.0% totrace) and TOC (24.8 to 8.3%) (Fig. 4). The carbonatecontent varied between 31 and 49% and displayed threedepressions corresponding to distinct enrichments inSiO2biog and, to a lesser extent, to enhanced accumula-tion of organic matter. The contents of Na, K, Mg andFe were low and relatively invariant between c. 10 600and 200 cal. a BP, but Mn varied significantly.

The sediments in the top part of the profile, which areyounger than c. 200 cal. a BP, were characterized byelevated contents of TOC and SiO2biog and a decreasedcontribution of carbonates (Fig. 4). Towards the top,the sediments became progressively enriched in N, S,K, Na and Fe. At the top of the core, a distinct peak ofMg was observed.

From c. 13 100 to 11 000 cal. a BP, the molar TOC/Nratio was high, namely 39, although it decreasedupwards to ~13. Between c. 11 000 and 2200 cal. a BP,the TOC/N ratio showed a gradual decrease from 23 to11, followed by a slight increase. Between 2200 and500 cal. a BP, TOC/N fluctuated from 11 to 22. From c.500 cal. a BP to the present, the ratio showed a decreas-ing trend to 6–10 at the top of the core.

The Fe/Mn ratio, which is an indicator of palaeo-redox conditions, was high between c. 13 100 and10 600 cal. a BP, reaching a maximum of 250–350 atthe end of this period. Between c. 10 300 and 300 cal.a BP, within the carbonate-rich sediments, the Fe/Mnratio was constant and very low (4–21). From c.300 cal. a BP to the present, Fe/Mn values wereslightly higher than in the underlying carbonate sedi-ments, and a sharp maximum of 108 was located at c.300 cal. a BP, which is the bottom of the organic-carbonate muds.

Stable C and O isotopes

In the bottommost part of the profile, c. 12 600–12 300 cal. a BP, a 1.7‰ drop in the carbon isotopesignature was observed, and the value of d13C in car-bonates varied between 1.34 and -0.34‰. The drop isfollowed by 1.9‰ enrichment in heavy carbon isotope(13C) between c. 12 300 and 12 000 cal. a BP. Between c.11 500 and 10 600 cal. a BP, the d13C values exhibitedan increase of 1.7‰, from -1.64 to 0.06‰. Subse-quently, a sharp drop was observed in d13C from 0.06‰to -4.87‰ between c. 10 600 and 9100 cal. a BP. Begin-ning from c. 9 100 cal. a BP, d13C displayed a gradualpositive shift moving upwards in the core up to 1.23‰at c. 300 cal. a BP. In the last 200 years, a significant

depletion of 13C is noted in carbonates, and d13C hasvaried over a broad range from -5.53 to -1.23‰.

Between c. 12 600 and 12 000 cal. a BP, d18O in bulkcarbonates showed an overall positive covariance withd13C. After an initial decrease from -8.84 to -11.08‰for c. 12 600 to 12 300 cal. a BP, the d18O valuesincreased to -9.67‰ at c. 12 000 cal. a BP. Enrichmentin 18O to -6.57‰ was noted at c. 11 500 cal. a BP;however, at c. 9900 cal. a BP, d18O decreased to-8.2‰. Between c. 9900 and 1500 cal. a BP, the d18Ovalues stabilized at approximately -8‰. A decrease to-8.7‰ was noted from c. 1500 to 400 cal. a BP. Thisdecrease was followed by 18O-enrichment (up to-7.6‰) between c. 400 and 150 cal. a BP. In thetopmost deposits, the d18O record displayed rapiddepletion in 18O and changed from -13.2 to -8.7‰.This change in stable oxygen signatures was parallel tothat of d13C.

Discussion

Lateglacial and early Holocene (9.74–9.04 m b.l.f.)

Allerød and Younger Dryas (9.74–9.16 m b.l.f.). – Theoldest layers of the Lake Skrzynka succession weredeposited during the Allerød chronozone when vegeta-tion in the vicinity of the lake was dominated by Pinusand Salix, with open plant communities composedmainly of grasses (LPAZ Sk1, Fig. 3). The oldest sedi-ments are composed of terrigenous silica with anadmixture of organic matter (Fig. 4). The sedimentscontain remnants of Carex and Sphagnum, which wereassociated with wetlands, while pollen from aquaticspecies is absent (Fig. 3). These features argue for anon-lacustrine character of the sedimentation duringthe Allerød, possibly in a shallow swampy depressionthat formed above progressively melting dead ice. Theonset of sediment deposition within the Lake Skrzynkabasin is consistent with observations at other sites in theTuchola Forests (Lake Ostrowite (Fig. 1B); Milecka2005) and in central Poland (Lake Gosciaz (Fig. 1A);Pazdur et al. 1995), NE Poland (Lake Hancza(Fig. 1A); Lauterbach et al. 2010) and Belarus(Makhnach et al. 2004).

Along with a rapid change in the sediment composi-tion from clastic deposits with organic matter to car-bonate sediments enriched in clastic components in theuppermost Allerød (Fig. 4), indicators of a lacustrineenvironment (i.e. Pediastrum, Tetraedron, Botryococ-cus) appear in the palynological record (Fig. 3). Asimilar change in sedimentation patterns has beenreported from other regions of Poland (Ralska-Jasiewiczowa et al. 2003) and from Belarus (Makhnachet al. 2004). The enrichment in CaCO3 in the uppermostAllerød and lower Younger Dryas sediments can beattributed to biochemical calcite precipitation owing to


the activity of autotrophic organisms in the lake(Fig. 3), inputs of detrital carbonates and leaching ofcarbonate-rich raw soils in the lake’s catchment. Thereare a few lines of evidence supporting the contributionof these mechanisms to carbonate sedimentation inLake Skrzynka during the Lateglacial.

Carbonate leaching in the lake’s catchment is indi-cated by the coincidence of increased sedimentary car-bonates and the establishment of conifer-dominatedforests in the vicinity of the lake. According to Dean &Schwalb (2000), Loizeau et al. (2001) and Andersonet al. (2008), in such circumstances, the enhancedmobility of carbonates is caused by an acidic pH ofsoils.

A detrital origin of the carbonates present in theLateglacial sediments of Lake Skrzynka is supportedboth by the partial dissolution of calcite crystals(Fig. 5A), which is often found in soils, and by stable Cisotope signatures.

d13C values between -0.3 and +1.5‰ measured in thesamples from Lake Skrzynka are close to the d13Cvalues of marine limestones, namely +0.56‰ (Keith &Weber 1964), and bulk carbonates, representing amixture of Palaeozoic and Cretaceous–Tertiary lime-stones in southern Swedish lake sediments, characte-rized by d13C values of +1‰ (Hammarlund et al. 1999).However, the d18O values of the Lateglacial (about -11to -8.8‰) are 18O-depleted relative to the d18O values ofmarine limestones (about -5.25 to -4‰; Keith &Weber 1964; Hammarlund et al. 1999), which suggestsan autochthonous origin of the carbonates present.This conclusion is further corroborated by the fact thatboth d18O and d13C clearly record the cooling of theYounger Dryas, which would not be expected in thecase of detrital carbonates. The 18O depletion in authi-genic Lateglacial calcite is a result of the low d18O ofmeteoric waters in equilibrium with cold climaticconditions.

The calcite crystals occurring in the sediments ofAllerød age are relatively large (mostly between 5 and25 mm; Fig. 5A) compared with those present in theHolocene sequence (ranging between 1 and 8 mm;Fig. 5C). Provided that the calcite crystals in the sedi-ments of Lake Skrzynka were at least partially precipi-tated from the water column, the size differencesbetween the Late Glacial and Holocene portions of thesequence can be explained in terms of the degree ofsaturation of the lake waters with respect to CaCO3. Asstated by Kelts & Hsü (1978), high supersaturation of asolution promotes the formation of micrite. Conse-quently, relatively large calcites occurring during theYounger Dryas would indicate lower levels of carbon-ate saturation in the lake.

Nevertheless, owing to the ambiguity regarding theinterpretation of the origin of carbonates in the Late-glacial deposits, isotope data may not be used forclimate and environment reconstructions.

Fig. 5. SEM images of the Lake Skrzynka sediments. A. Calcitecrystals with signs of dissolution, 9.66 m, Allerød. B. Calcite crystalaggregates, diatom frustrules, 9.50 m, Younger Dryas. C. Diatomfrustrules, chrysophyceae cysts (note the euhedral calcite crystals),7.90 m, middle Atlantic.


A significant drop in the CaCO3 content during thelate Younger Dryas correlates with a distinct drop inPinus in favour of Betula. The change in vegetationcould result in an increase in soil pH (Figs 3, 4) and,thus, cause carbonate removal from the watershed tocease. Moreover, the increased deforestation during thelate Younger Dryas, which is suggested by an elevatedherb pollen concentration (Fig. 3), resulted in enhancederosion and increased supply of SiO2ter to the lake(Fig. 4). As indicated by the high values of TOC/N(i.e. >20), the organic matter also mainly came fromterrestrial sources (Meyers & Teranes 2001).

The Younger Dryas chronozone (LPAZ Sk2), whichbegan with a decrease in the pine share, was character-ized by the dominance of open plant communities withnumerous light-demanding species, namely Juniperus,Artemisia, Helianthemum and Chenopodiaceae (Fig. 3).Similar changes in vegetation, reflecting climate deterio-ration, have been observed throughout most of westernand central Europe, for example in Ireland (O’Connellet al. 1999), NE Poland (Kupryjanowicz 2007; Lauter-bach et al. 2010), Lithuania (Stancikaite et al. 2002),Belarus (Makhnach et al. 2004) and the Czech Republic(Pokorny 2002).

During the Younger Dryas, numerous colonies ofgreen algae were present within the lake (Fig. 3). Pedi-astrum was accompanied by Botryococcus, Tetraedronand Scenedesmus (Fig. 3), which is indicative of watermesotrophy (Jankovska & Komarek 2000). Pedias-trum peaks during the Younger Dryas interval havebeen documented in some North American (Björck1985; Yu 2000) and European (Lotter & Holzer 1994;Goslar et al. 1999; Milecka & Szeroczynska 2005)lakes and have been attributed to intense erosion ofthe watershed under conditions of poorly developedvegetation cover. This process resulted in a largesupply of nutrients to the lake, which led to increasedtrophy.

Preboreal (9.16–9.04 m b.l.f.). – During the Preborealchronozone (LPAZ Sk3), a low-density Betula–Pinusforest developed in the area previously covered by openplant communities, and species characteristic of coldclimates, namely Juniperus and Helianthemum, dis-appeared (Fig. 3), indicating a change to warmer con-ditions. During the Preboreal period, the population ofgreen algae within the lake became dominated byTetraedron, while the abundance of Pediastrumdecreased and Scenedesmus disappeared completely(Fig. 3). The change in the composition of phytoplank-ton corresponds with the increase in the molar TOC/Nratio in the sediments, indicating a smaller contributionof lake phytoplankton to the production of sedimen-tary organic matter.

These changes are a reflection of a broader tendencytowards a decline of the trophic status in lake ecosys-

tems during the early Holocene throughout the PolishLowlands (Ralska-Jasiewiczowa et al. 1998; Goslaret al. 1999; Milecka & Szeroczynska 2005).

The decreasing trophic state of the lake has two pos-sible explanations. Early Holocene climate ameliora-tion favoured the development of denser vegetation(scattered forests) and, thus, suppressed both soilerosion and the leaching of nutrients in the vicinity ofthe lake. In addition, changes in the biota at theLateglacial/Holocene boundary might be linked toenhanced phosphorus fixation in the sediments. In oxicconditions and in alkaline waters, PO4

3- is sorbed byprecipitating Fe/Mn oxides and carbonates and there-fore becomes unavailable for lake phytoplankton(Wetzel 1983; Engstrom & Wright 1984).

The upper Younger Dryas and Preboreal sedimentsare composed mainly of terrigenous silica. However,they exhibit a significant enrichment in Fe and S(Fig. 4). High concentrations of Fe in the Lateglacial/early Holocene deposits are related to the developmentof soils with thick organic horizons. Engstrom &Wright (1984) have pointed out that in organic matter-enriched soils, the mobility of Fe is high owing to thepresence of humic acids. The increased sulphur con-tents are attributed to the dissolution of gypsum in thewatershed (Dean 1993). In the area of the Brdaoutwash plain, a possible source of CaSO4·2H2O maybe the Poznan silts. Early Holocene Fe and/or S peakshave been observed in numerous European lakes, forexample in Poland (Pawlikowski et al. 1982; Ralska-Jasiewiczowa et al. 1998; Lauterbach et al. 2010),Germany (Eusterhues et al. 2005) and Sweden (Digger-feld 1972). Distinct peaks in Fe and S and an Fe/S ratioof ~0.70–0.90 in Lake Skrzynka sediments indicate thepresence of pyrite (Engstrom & Wright 1984; Euster-hues et al. 2005), which, together with the high Fe/Mnratio, indicates strongly reducing conditions in the lake.The rapid decline in the oxygenation of the lake can beexplained by an increase in the water level andmeromixis in the water column (Engstrom & Wright1984). This interpretation is consistent with theincreased lake water level during the Preboreal periodpostulated by Milecka (2005) for Lake Ostrowite(Fig. 1C), which is the source lake of the Seven LakesStream, and by Nowaczyk (1994) for LakeCharzykowskie (Fig. 1C), the lake where the SevenLakes Stream discharges.

Three ~1-cm-thick intercalations of fine sand occurwithin the lacustrine sediments at the Younger Dryas/Preboreal transition and during the Preboreal chrono-zone. The discrete character of these intercalationsindicates activated erosion episodes in the watershed.Although Nowaczyk (1986) noted intensification ofaeolian activity in Tuchola Forests during the YoungerDryas and Preboreal, wind-driven deposition wouldresult in a more diffuse sediment supply over a longerperiod.


The isotope signatures during the Preboreal periodare significantly different from the Lateglacial values(Fig. 4). However, the d13C signatures between -1.5 and0‰ and d18O values of approximately -7‰ are close tothe isotope values of marine limestones (discussedabove). Thus, an admixture of detrital carbonates inthe Preboreal sediments is expected.

Middle and late Holocene (9.04–0 m b.l.f.)

Boreal (9.04–8.70 m b.l.f.). – During the Boreal period,the climate became progressively warmer, which isreflected in the palynological record (Fig. 3; Table 2).LPAZ Sk4 is characterized by a significant decline inNAP in favour of tree pollen. Dense woodland com-munities including all of the main species of deciduoustrees developed.

A 4-cm-thick, finely laminated sediment sequencedeposited at ~9 m b.l.f. indicates that the deep waterconditions noted in the Preboreal period might havecontinued in the early Boreal. Following the middleBoreal (c. 10 300 cal. a BP), Lake Skrzynka became awell-oxygenated hard-water lake. Highly oxidizing con-ditions are demonstrated by the very low Fe/Mn ratiothroughout most of the sedimentary sequence (Fig. 4),and the hard-water character of the lake is manifested inthe very high concentrations of carbonates. The pres-ence of carbonate-rich sediments argues for intense bio-logical activity in the lake. However, the low frequenciesof phytoplankton (Fig. 3) contradict this assumption.Nevertheless, from the Boreal to the late Subboreal, theTOC/N ratio constantly decreased (Fig. 4), indicatingthat phytoplankton significantly contributed to the pro-duction of organic matter in the lake. Within the lakepelagial, cells of autotrophic picoplankton, the smallestphytoplankton, tend to act as nucleation centres forprecipitating calcite, especially in low-trophy hard-water lakes (Dittrich & Obst 2004; Dittrich et al. 2004).Although this was not confirmed by the pollen record,we postulate that picoplankton was responsible for car-bonate precipitation in Lake Skrzynka. An increase inthe abundance of picoplankton may be accompanied bya decrease in nanoplankton, diatoms and Chlorophyta,eg. Pediastrum and Botryococcus (Dittrich & Obst 2004;Dittrich et al. 2004), which can be identified in thepalynological record (Fig. 3).

The gradual decrease in d13C and d18O values duringthe Boreal (Fig. 4) probably resulted from a diminishedcontribution of detrital carbonates and the predomi-nance of authigenic CaCO3 in the lake sediments. Thedecreasing contribution from catchment-derived min-erals in the Lake Skrzynka bottom deposits is sup-ported by the coincidence of a sharp increase in thecarbonate content and a decrease in SiO2ter in the earlyBoreal. Consequently, from the middle Boreal onwards(c. 10 100 cal. a BP, 8.95 m), the sedimentary carbon-

ates can be regarded as predominantly autochthonous,and, hence, the carbon and oxygen isotope data can beinterpreted in terms of environmental changes.

The development of dense woodland communitieswith the onset of the Boreal period (Fig. 3), when theclimate became progressively warmer, resulted in asupply of isotopically light HCO3

- from catchmentsoils. As a consequence, DIC became 13C-depleted,which resulted in isotopically light d13C values ofcarbonates precipitated within the lake (Fig. 4). Asignificant depletion of 13C in autochthonous lakecarbonates is frequently observed in the early stages ofthe Holocene (e.g. Hammarlund et al. 1997, 2003;Moscariello et al. 1998; Ralska-Jasiewiczowa et al.1998).

Atlantic, Subboreal and early and middle Subatlantic(8.70–0.8 m b.l.f.). – During the Atlantic period(LPAZ Sk5; Fig. 3; Table 2), mixed deciduous forestwas the predominant vegetation type in the surround-ings of Lake Skrzynka. All of the tree species charac-teristic of mesocratic forest were present, namelyUlmus, Quercus, Tilia, Fraxinus and Corylus. In addi-tion, pollen grains of Hedera helix appeared, indicatinga mild climate with a high mean annual temperature(Birks 1981). Considerable changes were noted in theforest composition during the Subboreal chronozoneencompassing LPAZ Sk6 and Sk7 (Fig. 3; Table 2).The content of Ulmus declined, and the share ofQuercus and Corylus increased. The importance of newelements, namely Carpinus and Fagus, rose gradually.During the early Subatlantic (LPAZ Sk8; Fig. 3;Table 2), a successive development of deciduous forestwith a considerable share of hornbeam (Carpinus) isnoted. The forest composition changed. Mesocratictrees were replaced by pine (Pinus). The subsequentdevelopment of oak–hornbeam (Quercus–Carpinus)forests is noted during the middle Subatlantic chrono-zone (LPAZ Sk9). However, within LPAZ Sk9, asignificant decrease in the Carpinus share was accom-panied by increased NAP (including Calluna) associ-ated with the presence of cereal pollen grains.

The number of green algae present in Lake Skrzynkawas low between the Atlantic and Subboreal chrono-zones (LPAZ Sk5–7, Fig. 3), namely c. 9400–2800 cal.a BP. Green algae became more abundant during theearly and middle Subatlantic chronozones (c. 2800–600 cal. a BP).

During the period between 10 300 and 400 cal. a BP(the middle Boreal up to the late Subatlantic chrono-zone), all of the geochemical proxies measured, exceptthe carbon isotopes, exhibit relatively invariable values(Fig. 4), indicating high stability of the conditions inthe lake through most of the Holocene.

The minimum values of the sedimentary TOC/Nratio occurring between c. 3100 and 2000 cal. a BP (late


Subboreal and early Subatlantic) agree with theincreased SiO2biog contents and suggest that diatomstemporarily became more important producers oforganic matter in the lake.

In the middle and late Subatlantic chronozone, c.2100–400 cal. a BP, a steep increase and subsequentfluctuations in the TOC/N ratio from 12.5 to 22.4 implyalternating contributions of terrestrial and planktonicorganic matter (Fig. 4). The changes in the TOC/Ntrend observed in the Lake Skrzynka sediment corecoincide with the early and middle Subatlantic humanimpact in the vicinity of Lake Skrzynka identified in thepollen record based on the increase in the NAP contentand the higher contribution of human indicators, espe-cially the grazing indicators R. acetosa/acetosella andP. lanceolata, cereals and Artemisia (Fig. 3). Thedecline in the AP/NAP ratio (Fig. 4) can be interpretedas a result of near-lake deforestation (Meyers &Teranes 2001; Routh et al. 2007). Intensive forest clear-ance and the ceasing of the expansion of the Quercus–Corylus forests during the middle Subatlanticchronozone (1850–1600 a BP conventional age), whichcorresponds archaeologically to the middle and lateparts of the Roman period, have also been recognizedin Lake Wielkie Gacno (Hjelmroos-Ericsson 1981) andLake Suszek (Berglund et al. 1993). Changes in vegeta-tion cover could be responsible for the increased bio-logical productivity in Lake Skrzynka between c. 2700and 1200 cal. a BP that was recorded in the increasedSiO2biog, the higher frequency of green algae and 13C-enriched carbonates (Figs 3, 4).

The narrow range of oxygen isotope values (between-8.7 and -7.6‰) noted between c. 10 100 and 400 cal. aBP (middle Boreal to late Subatlantic chronozone)(Fig. 4) indicates the stability of the factors influencingthe d18O of the lake water. It is probable that the verystable d18O record indicates the through-flow characterof Lake Skrzynka during most of its history. This char-acteristic is confirmed by a lack of correlation betweenthe d13C and d18O records within most of the Holocenehistory of the lake. The most 18O-enriched values areobserved for the Holocene thermal maximum in theAtlantic chronozone (Fig. 4), which results fromthe high air temperatures during this period of theHolocene. A subsequent gradual decrease in the d18Osignal by ~1‰ between the upper Atlantic and themiddle Subatlantic periods may have resulted from agradual decrease in the surface temperature by 2.5°C(a 0.4‰ decrease for every 1°C decrease in temperature,e.g. Ralska-Jasiewiczowa et al. 2003; Apolinarska &Hammarlund 2009). The decrease refers to a change inthe spring and summer temperatures, which is when themajority of the autochthonous carbonates are precipi-tated within the lake. This interpretation agrees withestimates of a 2–3°C temperature change after thethermal maximum during the Atlantic period (Starkelet al. 1998; Bradley et al. 2003).

d18O values in sedimentary carbonates are at leastpartially affected by pathways related to the chemicaltransformation of CO2 in lake waters. As outlined byUsdowski et al. (1991) and Teranes et al. (1999), inconditions of high autotroph activity and increasedlake water pH levels, hydroxylation of CO2 dominatesover the process of hydration, which results in moredepleted d18O signatures in precipitating calcite. Thismechanism could contribute to the ~0.5‰ decrease ind18O recorded in the Lake Skrzynka sequence duringthe late Subatlantic. The particularly high d13C, TOCand N values as well as the carbonate content in theupper Subatlantic sediments support the existence ofenhanced photosynthetic activity and high lake-waterpH levels during CaCO3 nucleation.

The rate of calcite precipitation from lake watersshould be considered as an alternative or supplemen-tary factor contributing to the fractionation of O iso-topes in lake waters. According to Teranes et al. (1999),the rapid precipitation of calcite from highly supersatu-rated solutions in eutrophic lakes promotes isotopicallylight d18O values in carbonates. However, rapidlyformed crystals tend to present large dimensions of upto 30 mm (Teranes et al. 1999), which is not the case inthe Lake Skrzynka sediments. Therefore, the precipita-tion rate-control of the d18O signatures in the lake canbe neglected.

A gradual upcore shift towards less negative d13Cvalues is observed from the Atlantic to the middle Sub-atlantic chronozones (Fig. 4). As suggested previously,one of the factors influencing the carbon isotope com-position of DIC is the photosynthetic activity of phyto-plankton and macrophytes. Even under conditions of arelatively constant intensity of primary productivitywithin the lake (the weight percentage of TOC in thesediments is relatively stable throughout most of thesequence), the gradual infilling of the basin with sedi-ments causes a stronger impact from the preferentialuptake of 12C by photosynthetic organisms owing to thedecreasing volume of the lake. Moreover, the corestudied was drilled within the western basin of LakeSkrzynka (Fig. 1C), where water renewal is partiallyrestricted, and, thus, the water residence time is longer.The 13C/12C ratio within the entire sequence studied is13C-enriched (between -5 and +2‰; Fig. 4) comparedwith the d13C values noted for groundwater and riverineDIC in isotopic equilibrium with CO2 originating fromthe decomposition of land plants in N Europe (between-15 and -10‰) (Andrews et al. 1997). Thus, processesoccurring both within and outside the lake must haveinfluenced d13Cdic and, hence, the d13Ccarb values in LakeSkrzynka. The water feeding Lake Skrzynka flowsthrough six other lakes before it reaches this lake, and,thus, its isotopic composition is already modified byprocesses (e.g. photosynthesis) occurring in theupstream water bodies. Progressive enrichment of heavyC isotopes in DIC along the course of rivers has been


demonstrated by Andrews (2006), and a similar phe-nomenon is expected to occur in the Seven LakesStream. Because continuous water exchange in the lakehas prevented equilibration between carbon isotopes ofatmospheric and aquatic CO2, productivity must havehad a crucial influence on d13Ccarb.

Significant enrichment in 13C, accompanied bymaxima of the carbonate content in sediments,occurred c. 2 700 cal. a BP (Fig. 4). This increase isinterpreted as a short-lasting peak of intense primaryproductivity. No signs of a longer water residence timewere observed because the d18O values did not change.

An increase in d18O by 0.8–1‰, accompanied byincreased heavy carbon isotope values (Fig. 4), is inter-preted as a shift to a somewhat longer water residenceand lake-level declines during the late Subatlantic, c.400 – 170 cal. a BP. Concomitant declines in the lakewater level have been reported from many sites innorthern Poland (Berglund et al. 1993 and referencestherein). In Lake Suszek, which lies within the TucholaForests, the lowest water levels occurred at the begin-ning of the late Subatlantic (Berglund et al. 1993).

Late Subatlantic (0.8–0 m b.l.f.). – The topmost part ofthe sediment core records significant changes in thewithin-lake conditions since c. 170 cal. a BP (Fig. 4).These changes in Lake Skrzynka were related to exten-sive clear cutting in the lake’s catchment, as indicatedby a reduction in AP (Fig. 3). Decline in AP pre-dateschanges in geochemistry and starts c. 350 cal. a BP(Fig. 3). The highly elevated concentrations of Na andK (Fig. 4) indicate intensified leaching of catchmentsoils (Engstrom & Wright 1984; Dean 1993). A nearlytwofold enrichment in terrigenous silica indicatesenhanced surface erosion around the lake. Thesechanges are probably linked to human impact, which isalso apparent in the palynological record (LPAZ Sk10;Fig. 3). The pine forests that gradually spread overextensive areas were accompanied by open plant com-munities associated with Juniperus, a genus that prefersinsolated sites, and a growing proportion of anthropo-genic indicator plants, including cereals. Intense humanactivity was noted in the later part of the Subatlanticchronozone in the area of Lake Wielkie Gacno(Hjelmroos-Ericsson 1981) and in the surroundings ofLake Suszek (Berglund et al. 1993).

Reduced carbonate contents in the sediments indi-cate diminished alkalinity of the lake waters. High con-tents of SiO2biog, organic matter and nitrogen and lowvalues of TOC/N, typical of algal phytoplankton(Robinson 1994; Meyers & Teranes 2001) (Fig. 4),reflect progressive eutrophication. This interpretationis in agreement with the considerable transformationsobserved in the within-lake primary producers (Fig. 3).Green algae, mainly Pediastrum, became more numer-ous. In the middle part of LPAZ Sk10, Tetraedronreappeared, followed by Coelastrum reticulatum, a

species characteristic of eutrophic lakes (Jankovska &Komarek 2000). Changes in the trophic states of thelakes in the Tuchola Forests during the late Subatlanticare commonly related to human presence in the area(Berglund et al. 1993; Milecka & Szeroczynska 2005).

A twofold increase in the Fe/Mn ratio and a rela-tively high concentration of sulphur indicate decreasingredox conditions in the lake (Fig. 4). Within biogenicdeposits, the Fe/S ratio falls to 0.47, which is close tothe stoichiometric relationship of Fe and S in pyrite(Fe/Spyrite = 0.87). The distinct decline in the Fe/S ratioin the upper part of the core suggests enhanced bacte-rial production of H2S in the organic-rich sediments,which enables the formation of pyrite and, thus, fixa-tion of Fe in sediments. The decreased oxygenation ofthe lake waters was presumably caused by microbialdecomposition of the organic matter.

The significant depletion of carbonates with respectto 13C and 18O within the last c. 170 cal. a BP (Fig. 4)indicates that both d13C and d18O were forced by thesame factor. Oxygen isotope values between -11 and-9.5‰ are close to the d18O values of Holocene ground-water in the area (~-9.5‰) (d’Obryn et al. 1997) and tothe mean d18O (-9.86‰) of precipitation noted inCracow, southern Poland, from 1975 to 2002 (http://www-naweb.iaea.org/napc/ih/IHS_resources_isohis.html). Hence, the decreasing tendency observed in theoxygen isotope data may be a consequence of anincreased flushing rate in the lake. Faster waterexchange in the lake would also limit the extent of the13C-enrichment of DIC by photosynthesis, resulting ina shift to isotopically lighter carbonates.

The considerable change in the residence time ofwater in the lake can be attributed to human distur-bance of the lake’s water balance. Murawski (1968 in:Marszelewski & Jutrowska 1998: p. 54) suggests thatthe Seven Lakes Stream channel, that is, its individualsegments connecting the lakes, is not natural and wasexcavated by humans. A natural connection may haveexisted during most of the history of the lakes becauseall of the lakes are located within one subglacialchannel. However, human-induced widening of theconnection between Lake Skrzynka and the local drain-age system would definitely increase the rate of waterexchange in the basin. Increased through-flow in LakeSkrzynka within the upper part of LPAZ Sk10 is con-firmed by the replacement of Tetraedron by Botryocco-cus, which indicates a possible decrease in the trophicstatus of the water. This, however, is inconsistent withthe eutrophication of the lake indicated by changes inthe geochemistry of the sediments described above. Thepresence of 13C-depleted calcites and sulphides may beexplained by a reduction in SO4 through microbiologi-cal processes. The process of calcite 18O-depletion inresponse to eutrophication (Teranes et al. 1999) isrejected as a primary factor causing the decrease of d18Oin the uppermost part of the Skrzynka profile because


the concomitant depletion of 13C is inconsistent withincreased water trophy. The diversified record of thegeochemical, isotope and palynological data indicatesthe complexity of the processes occurring within LakeSkrzynka during the last c. 170 cal. a BP.

Conclusions

The multiproxy approach to studying lake sedimentcores allowed us to disentangle trophic, climatic andhydrological changes in Lake Skrzynka, a hard-waterlake located in the temperate climate zone of centralEurope. Owing to its small volume, the lake reactedreadily to external factors, thus reflecting environmentalfluctuations in northern Poland within the last 13 000years. The low anthropogenic impact on the lake’scatchment has not overshadowed the record of naturaltransformations, except during the last two centuries.

Our data provide evidence for the relatively longpersistence of dead ice in northwestern Poland, consist-ent with observations at other sites in the TucholaForests, central and NE Poland and Belarus. In the LakeSkrzynka basin, dead ice stagnated until the Allerød.Between the Allerød and the present, Lake Skrzynkaunderwent distinct changes in its trophic status, as indi-cated by stable isotope, geochemistry and pollen recordanalyses. These changes were strictly related to regionalvegetation changes, transformations of soils and cyclingof nutrients within the catchment. Similarly to otherparts of Poland, in the Bory Tucholskie area environ-mental changes in the Lateglacial were more rapid thanduring most of the Holocene. The Lake Skrzynka sedi-ment record confirms the link between climate and thetrophic level of surface waters. During the Lateglacialand early Holocene, extensive export of nutrients fromthe lake’s surroundings under conditions of sparse veg-etation cover resulted in eutrophication of the lake.However, the sedimentation pattern in Lake Skrzynkachanged significantly from clastic allochthonous tochemical autochthonous facies owing to alternatingcold and warm climate phases. Early Holocene climateamelioration was accompanied by a short-lastingmeromixis of the lake water column in the Preboreal.Stabilization of forest cover and reduced input of nutri-ents resulted in the establishment of a very stable phaseof oligotrophic, well-oxygenated and hard-water condi-tions in the lake between the Boreal and late Subboreal.Stable oxygen isotopes recorded slight variations in airtemperatures during most of the Holocene, with a 2–3°Ctemperature decrease after the thermal maximumduring the Atlantic period. This change agrees withsimilar estimates for Poland and Europe.

Most of the changes in the trophic status and the lakelevel of Lake Skrzynka reflect trends observed through-out the Polish lowlands and neighbouring areas.Similar conclusions can be made for geochemistry and

isotopes; for example, early Holocene Fe and/or Speaks have been observed in numerous European lakes.Furthermore, significant depletion of 13C in autoch-thonous lake carbonates is frequently observed in theearly stages of the Holocene.

The vegetation history recorded in Lake Skrzynkasediments is consistent with the pollen biostratigraphyrecorded throughout most of western and centralEurope. This fact indicates that the radiocarbon-basedstratigraphy of Lake Skrzynka sediments is invalid forsediments older than c. 10 000 cal. a BP.

Within the last 170 years, the lake has been influ-enced by cultural eutrophication resulting from defor-estation and increased nutrient delivery to the lake.Consequently, the sedimentation changed to abiogenic/carbonaceous-dominated mode, and lake-water oxygenation significantly decreased despite thehigh buffering capacity of oligotrophic hard-waterlakes. However, the anthropogenic impacts were notrestricted to eutrophication. The water exchange timein Lake Skrzynka has been influenced by human-induced changes in the local drainage system.

Acknowledgements. – The study was financially supported by thePolish Ministry of Science and Higher Education, grant N N307 250433. Special thanks are expressed to Prof. Kazimierz Tobolski forencouragement to investigate the problem described. Jarosław Kor-dowski, Sebiatian Tyszkowski and Michał Słowinski (Department ofGeomorphology and Hydrology of Lowlands, PAN Torun) areacknowledged for drilling the cores investigated. Adam Siejkowski(Faculty of Geographical and Geological Sciences, Poznan) isacknowledged for help in chemical analyses. Jens Fiebig (Institute ofGeoscience, Goethe University, Frankfurt/Main) is thanked forstable isotope analyses. We thank Tomasz Goslar (Poznan Radio-carbon Laboratory) for 14C measurements. Małgorzata Mrozek-Wysocka (Institute of Geology, AMU, Poznan) is acknowledged forSEM analysis. We greatly appreciated the detailed reviews and con-structive comments of three anonymous reviewers, which helped toimprove the final presentation. Thanks are also addressed to Jan A.Piotrowski for his editorial work on this manuscript.

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