rapid hydrological changes during the holocene revealed by stable isotope records of lacustrine

Quaternary Science Reviews 22 (2003) 353–370

Rapid hydrological changes during the Holocene revealedby stable isotope records of lacustrine carbonates from

Lake Igelsj .on, southern Sweden

Dan Hammarlunda,*, Svante Bj .orcka, Bj�rn Buchardtb, Carsten Israelsonc,Charlotte T. Thomsenb

aQuaternary Geology, Department of Geology, Lund University, Tornav. 13, S-223 63 Lund, SwedenbGeological Institute, University of Copenhagen, Øster Voldg. 10, DK-1350 Copenhagen K, Denmark

c National Institute of Radiation Hygiene, Knapholm 7, DK-2730 Herlev, Denmark

Received 10 October 2001; accepted 22 July 2002

Abstract

A Holocene sediment sequence from Lake Igelsj .on, south central Sweden, was studied by stable oxygen- and carbon-isotope

analyses of different carbonate components. The deposit, which covers the time-span from ca 11,500 cal BP to the present, was laid

down in a small, kettle-hole lake, the hydrological balance of which is presently dominated by groundwater flow. Isotopic records

obtained on bulk carbonates originating mainly from summer-produced, calcitic algal encrustations exhibit several rapid shifts of

more than 2%, likely reflecting pronounced hydrological variations. Corresponding isotopic data obtained on calcitic gastropod

opercula from parts of the profile show subdued responses to major climatic shifts, probably related to an extended calcification

season. The isotopic records were complemented by studies of modern isotope hydrology, and our interpretations are based on a

simplistic climate-hydrology model in which variations in groundwater generation within the lake catchment produce changes in

groundwater level and related adjustments of lake level and surface/volume ratio of the basin during the ice-free season. Assumed

periods of decreased lake volume in a relatively dry climate (low lake level) are characterised by enrichment in 18O and 13C resulting

from increased evaporation/inflow ratio and atmospheric equilibration, respectively. In clear contrast to this situation, intervals of

more humid climatic conditions give rise to increased lake volume (high lake level), possibly surface over flow, and relatively

depleted isotopic ratios. Relatively humid conditions, which may correlate to a wide-spread cooling event recorded by various

proxies across the North Atlantic region, are indicated by distinct isotopic shifts at ca 8300 and 8000 cal BP, bracketing a period of18O-depletion. The period between ca 8000 and 4000 cal BP was characterised by relatively dry and stable climatic conditions,

whereas the subsequent part of the Holocene experienced a more humid and variable climate following marked and coherent

depletions in 18O and 13C at ca 4000 cal BP.

r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Regional climate variability and sub-Milankovitchclimate variations have become topics of great interest.In this respect, the Holocene is probably the bestcandidate for such studies; the possibilities of obtainingwell-dated records from this period are exceptional andHolocene records may be found in most regional

settings and climate zones. Variations in moistureavailability caused by changing precipitation andevaporation are important aspects of Holocene climatechange, although sometimes overlooked in palaeocli-matic research focussing to a greater extent on pasttemperatures. The hydrological balance of lakes mayrespond sensitively to changes in net precipitation andhumidity. Based on several reconstructions of past lakelevels through detailed lithostratigraphic and vegeta-tional studies of lake sediments, the Holocene palaeo-hydrological development of southern Sweden isrelatively well known (Digerfeldt, 1988, 1997; Harrison

*Corresponding author. Tel.: +46-46-222-79-85; fax: +46-46-222-

48-30.

E-mail address: [email protected] (D. Hammarlund).

0277-3791/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 3 7 9 1 ( 0 2 ) 0 0 0 9 1 - 4

and Digerfeldt, 1993; Almquist-Jacobson, 1995). Inorder to complement these records, which do not allowreconstruction of short-term fluctuations, we haveperformed a study of Holocene climate variations basedon high-resolution records of stable oxygen- andcarbon-isotopes from a carbonate-rich sediment succes-sion deposited in a small lake in south central Sweden.The aim of the study is to detect and date changes in thestable isotope composition of different carbonatecomponents of the sediments, and to analyse these datain the light of modern isotope hydrology of the lake.Thus, temporal changes in water balance and relativeestimates of temperature and seasonality, potentiallyresponsible for the observed trends and isotopic offsetscan be evaluated. We also try to relate and compare ourisotopic records with other types of studies, mainlyglacier fluctuations in the Scandes Mountains, toachieve a more holistic view of the Holocene climatic,and especially hydrological, development of southernScandinavia. The strong climatic influence of the NorthAtlantic Drift on this maritime-continental borderregion makes it a potential tracer region for distur-bances in ocean circulation during the Holocene (cf.Bond et al., 1997), which can place these isotopic data inan even wider context.

Oxygen- and carbon-isotope ratios in lacustrinecarbonates can be used to infer climate changes throughthe influence of several processes within the climatesystem, one of which is the hydrological budget ofthe lake under study. Lakes with short residence times,i.e. low ratios of lake/catchment area maintain theisotopic characteristics of the recharging water (e.g. vonGrafenstein et al., 1996). On the other hand, significantenrichment in the heavy isotopes—2H and 18O in water,13C in dissolved inorganic carbon (DIC)—occurs in lakewaters subject to more extensive and prolonged ex-posure to evaporation and atmospheric exchange,respectively, thus reflecting catchment water balance(e.g. Gibson et al., 1993; von Grafenstein et al., 2000).Temporal changes in the influence of these processesresulting from variations in catchment hydrology can betraced by isotopic analyses of lacustrine carbonates(d13C and d18O), and related palaeohydrological im-plications were reviewed by Talbot (1990) and re-examined by Li and Ku (1997). As the first Holocenestable isotope record of its kind from Scandinavia, thepresent study of a hydrologically sensitive lake is basedon oxygen- and carbon-isotope analyses of two specificcarbonate components, algal encrustations and gastro-pod opercula, representing calcification during differentseasons of the year. This approach, which has previouslybeen applied to Late Weichselian lake sediments insouthern Sweden (Hammarlund and Keen, 1994;Hammarlund and Lemdahl, 1994; Hammarlund et al.,1999), yields additional details of the Holocene climaticdevelopment in the region.

2. Site description

Lake Igelsj .on is situated in the northeastern part ofthe province of V.asterg .otland in south central Sweden(581280N, 131440E; Fig. 1), at an elevation of 111 m a.s.l.The lake is located in an area of predominantlyglaciofluvial deposits within the Middle Swedish end-moraine zone which is associated with a glacial re-advance during the Younger Dryas stadial (Bj .orck et al.,1988). The glacial deposits are underlain by Cambriansandstone whereas the nearby residual mountain,Mount Billingen, consists of Cambro-Ordovician alumshales and limestones intruded by younger dolerites.

The small, roundish lake is ca 70 m long and 50 mwide, and has a gently undulating bottom-surfacetopography with water depths varying between 1.5 and2.5 m (Table 1). The surface catchment area is ca

Fig. 1. Map of southern Scandinavia showing the location of the study

area.

Table 1

Measured or estimated physical and chemical parameters of Lake

Igelsj .on

Measured/estimated parameter Result

Catchment area ca 320,000m2

Lake area ca 2500m2

Maximum water depth ca 2.5m

Lake volume ca 5000m3

Groundwater input (estimated) ca 3.6 l/s

Residence time (estimated) ca 20 days

Maximum water temperature ca 201C

Period of ice cover Late Nov.–March

pH (July 1999) 7.1

D. Hammarlund et al. / Quaternary Science Reviews 22 (2003) 353–370354

0.32 km2 (Fig. 2) but surface run-off to the lake is verylimited. Sub-surface groundwater dominates input tothe lake and the average residence time is estimated to ca20 days based on modern groundwater formation withinthe catchment. A slightly larger lake of the samecharacter and equal elevation ca 500 m to the east isconnected to Lake Igelsj .on through an artificial canalthat was excavated through an intervening peatlandduring the early 20th century. At present-day climaticconditions there is no surface outflow from the lakes andthe threshold of the catchment is situated at ca 114 ma.s.l. The hummocky landscape surrounding the site isoccupied mainly by kame deposits (sands and gravels)laid down during the late Younger Dryas deglaciation(12,000–11,500 cal BP). Further details of local geology

and deglaciation dynamics in the area were given byBj .orck and Digerfeldt (1984).

Modern mean annual precipitation is 745 mm ofwhich about 50% is lost by evapotranspiration, andmean annual air temperature is +5.91C. January andJuly mean air temperatures are �3.51C and +15.81C,respectively. Meteorological data (Fig. 3A) were col-lected during the period of 1961–1990 at Remningstorp,ca 3 km south-west of Lake Igelsj .on (Alexanderssonet al., 1991).

3. Methods

3.1. Fieldwork and subsampling

Sediment cores were collected from the ice in thecentral part of the lake in January 1997 at anapproximate water depth of 2.2 m, using 1m longRussian peat samplers, 7.5 and 10 cm in diameter. Theca 7.3 m thick sediment sequence was described in detailin the field (Table 2). The uppermost part of thesequence, which consists of very loose gyttja, wassampled with a simple gravity corer. After correlationof a sequence of over-lapping core segments in thelaboratory the profile was contiguously subsampled into122 sections, 30–180 mm thick, taking into accountlithostratigraphic boundaries. Minor aliquots were usedfor analyses of total carbon and calcium carbonatecontents and stable isotope analyses of bulk carbonates.In the interval of 7.19–5.06 m below the water surfacesamples of bulk carbonates were collected at higherstratigraphic resolution before subsampling into sec-tions, with samples ranging in thickness from 5 to20 mm. For radiocarbon dating and extraction ofcarbonate macrofossils for additional stable isotopeanalyses the remaining parts of the 122 sections werecarefully washed through a 200 mm sieve and the residuewas examined under a binocular microscope.

3.2. Lithology and carbonate mineralogy

Total carbon content (TC) of the sediments wasdetermined by combustion at 11501C in pure oxygenwith subsequent detection of carbon dioxide by infraredabsorption photometry in an Eltra-Metalyt unit (preci-sion better than 71%). The calcium carbonate content(CaCO3) was measured by sodium hydroxide titrationto neutral pH after dissolution of 0.5 g of sample in0.5 M hydrochloric acid and boiling for 20 min (preci-sion better than 70.5%). The organic carbon content(OC) was calculated as OC=TC�(CaCO3� 12/100).The amount of residual, non-carbon containingclastic material, residue (R), was calculated asR=100�(OC� 2.5)�CaCO3. Dried and homogenised,untreated samples were used for the lithological

Fig. 2. Map of the study area showing the position of Lake Igelsj .on

and its catchment in relation to major topographical features in the

area (25m contour lines). The thick hatched line indicates the surface

catchment of the lake. Arrows mark flow directions of present-day

streams in the area (solid lines) and the outlet stream from Lake

Igelsj .on that may have occurred in the past (hatched line). Stars

indicate positions for sampling of glacial deposits (two samples of

glaciofluvial sand and one till sample) showing no measurable content

of calcium carbonate.

D. Hammarlund et al. / Quaternary Science Reviews 22 (2003) 353–370 355

analyses. The records of OC, CaCO3, and R are given aspercentages of total dry weight in Fig. 4. The miner-alogical composition of carbonates from selected sedi-

ment samples was determined by X-ray diffractionanalysis using Cu-a radiation in a Philips PW 3710diffractometer.

1 2 3 4 5 6 7 8 9 10 11 1201020304050607080

mm

-5

0

5

10

15

20

OC

Mean annual data (1961-1990):Temperature: +5.9°CPrecipitation: 639 mm

(A)

(B)1 2 3 4 5 6 7 8 9 10 11 12

Month

-22-20-18-16-14-12-10-8-6

-22-20-18-16-14-12-10-8-6

δ 18O

(‰

V-S

MO

W)

Fig. 3. (a) Modern temperature and precipitation data (line and bars, respectively) from Remningstorp, ca 3 km south-west of Lake Igelsj .on (1961–

1990; Alexandersson et al., 1991). Precipitation data refer to measured values, corresponding to 639mm/yr, whereas the corrected (real) value is

estimated to 745mm/yr; (b) Composite-monthly values of d18O in precipitation from Tors .o, ca 40 km north of Lake Igelsj .on (Fig. 1; 50m a.s.l.). The

isotopic data were collected during the period of January 1975–December 1979. Black dots refer to individual months (42 values in total, 18 values

missing) and the thick solid line represents monthly mean values. The long-term weighted annual mean for the years 1975–1979 (thin solid line) is

�10.7%. The data were derived from the global network of isotopes in precipitation (GNIP) database maintained by the International Atomic

Energy Agency and the World Meteorological Organisation.

Table 2

Lithostratigraphic description

Unit Depth

(m)

Description OC (%) CaCO3 (%)

12 2.20–3.24 Dark brown, very loose, slightly calcareous gyttja. Lower boundary very

gradual

18.8–29.0 34.7–59.3

11 3.24–4.16 Brown to beige, loose, faintly laminated calcareous gyttja. Lower boundary

rather gradual

3.27–22.0 45.4–90.8

10 4.16–4.36 Dark brown, slightly calcareous algal gyttja. Lower boundary sharp 19.6–38.8 6.10–48.6

9 4.36–5.12 Brown to greenish or pinkish beige, distinctly laminated, algae-rich

calcareous gyttja. Lower boundary sharp

6.09–14.7 50.1–82.4

8 5.12–5.42 Dark brown to reddish brown, laminated, calcareous-rich algal gyttja.

Lower boundary rather sharp

16.9–31.4 22.4–54.8

7 5.42–7.05 Pinkish beige, distinctly laminated algal-rich calcareous gyttja with some

greenish and brownish layers. Lower boundary rather gradual

2.99–17.1 55.5–91.3

6 7.05–7.13 Dark grey, slightly silty, algal-rich calcareous gyttja, Lower boundary rather

gradual

8.87–9.78 51.9–61.1

5 7.13–8.29 Beige to brownish grey, laminated, algal-rich calcareous gyttja. Lower

boundary gradual

0.834–3.85 85.3–97.8

4 8.29–8.40 Grey, silty calcareous gyttja. Lower boundary sharp 0.535 81.7

3 8.40–8.80 Yellowish grey to dark grey, laminated, slightly silty calcareous gyttja.

Lower boundary rather sharp

0.608–2.39 86.6–93.4

2 8.80–9.50 Brownish grey to dark grey, silty calcareous gyttja. Lower boundary rather

sharp

0.538–1.19 73.1–91.2

1 9.50–9.55 Brownish grey, calcareous-rich, clayey silt 0.230 29.2

OC and CaCO3 represent contents of organic carbon and calcium carbonate, respectively. Depths are related to the water surface.


3.3. Radiocarbon dating

Delicate macroscopic remains of terrestrial andemergent aquatic plants, mostly determined to specieslevel (Table 3) were sampled from the sieve residue at 22levels and radiocarbon dated by accelerator massspectrometry (AMS) at the Tandem Laboratory, Upp-sala University (Ua) and at the radiocarbon laboratory,Department of Geology, Lund University (LuA),respectively. Six of the radiocarbon dates (labelled‘‘LuA’’ in Table 3) were obtained from the core collectedin 1997, whereas the remaining 16 dates (labelled ‘‘Ua’’in Table 3) were obtained from a nearly identical corecollected at the same position in 1996. The two coreswere readily correlated on the basis of lithostratigraphiccharacteristics. The numerous radiocarbon dates in theinterval of 4.80–7.30 m were carried out for comparisonwith an independent chronology obtained by thermalionisation mass spectrometry (TIMS) U–Th dating ofthis part of the sediment sequence (Israelson et al.,1997). Apart from reported radiocarbon ages, thechronological terminology in the following text is basedon calendar-year ages before 1950 (cal BP) as derived bycalibration of radiocarbon ages against the IntCal98calibration curve (Stuiver et al., 1998) using the CALIB4.1 software. The calibrated ages reported here thus

differ slightly from the data given by Israelson et al.(1997), which were related to the 1996 datum year basedon the IntCal93 calibration curve (Stuiver and Reimer,1993).

3.4. Stable isotope analyses

For oxygen- and carbon-isotope analyses of bulkcarbonates, sediment samples were freeze-dried andhomogenised. Samples of Chara sp. calcitic encrusta-tions (each consisting of 5–15 mg) and calcitic operculaof Bithynia tentaculata gastropods (each consisting ofgenerally 15–25 opercula) were collected after sieving ofcontiguous stratigraphic sections within core intervals,where these materials occur as macroscopic remains. Allbulk sediment samples and most of the carbonatemacrofossil samples were processed on a preparationline described by Buchardt (1977). The carbon dioxideevolved was analysed for 18O/16O and 13C/12C ratios in aFinnigan-MAT 250 mass-spectrometer at the Geologi-cal Institute, University of Copenhagen. Samples notexceeding 3mg in weight (nine Bithynia tentaculata

samples and one Chara sample) were analysed in a VGPrism Series II mass-spectrometer at the Department ofEarth Sciences, G .oteborg University. The preparationfollowed standard procedures (McCrea, 1950), and the

-12 -11 -10 -9 -8 -7 -6 -5 -4 -3

δ 18O (‰ V-PDB)

0 20 40 60

% OC

0 50 100

% CaCO3

0 20 40 60 80

% Residue (R)

-7 -6 -5 -4 -3 -2 -1 0 1 2 3

δ 13C (‰ V-PDB)

Unit 4 reworked fromlower part of unit 2

2

3

4

5

6

7

8

9

10

0 20 40 60 0 50 100 0 20 40 60 80 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -7 -6 -5 -4 -3 -2 -1 0 1 2 3

IG-1

IG-2

IG-3

IG-4

IG-5

1

2

3

4

5

6

7

2

3

4

5

6

7

8

9

10

De

pth

be

low

wate

rsurf

ace

(m)

8

9

10

11

12

Fig. 4. Elemental analysis data and stable isotope records from Lake Igelsj .on plotted against sediment depth. Organic carbon content (% OC) and

calcium carbonate content (% CaCO3) are expressed as dry weight percentages. Residue refers to an estimation of minerogenic content (non-carbon

containing siliciclastic material) of the sediments based on the following calculation; Residue=100�(% OC� 30/12)�(% CaCO3). The molar weight

of organic material has been assigned the value of 30 on the assumption of an ‘‘average’’ chemical composition of CH2O. Oxygen and carbon isotope

records obtained on carbonates (% V-PDB) are indicated by filled circles (bulk carbonates, predominantly Chara calcite), open circles (calcitic

encrustations precipitated on Chara sp.), and diamonds (calcitic opercula of Bithynia tentaculata). The shaded zone indicates that unit 4 is assumed to

be reworked from older sediments by slumping. The left-hand column refers to lithostratigraphic units (Table 2) and the right-hand column refers to

isotopic zones described in the text.


analytical reproducibility for d13C and d18O is betterthan 70.03% and 70.07%, respectively. The resultsare expressed as d-values (per mil deviations from theV-PDB standard) by calibration against laboratorystandards, thus ensuring compatibility of results ob-tained from different laboratories.

Modern water samples collected from the lake and itsvicinity during the period of 1996–1999 (Table 4) wereanalysed for 18O/16O ratios at the Ice Core Laboratory,Niels Bohr Institute for Astronomy, Geophysics andPhysics, University of Copenhagen, and for 2H/1Hratios at the Geological Institute, University of Copen-hagen using the Zn-reduction method as described byColeman et al. (1982). The results are related to the V-SMOW standard with an analytic reproducibility of70.02% and 73% on the d-scales for d18O and d2H,respectively.

4. Sediment description and chronology

The sediment succession was divided into 12 litho-stratigraphic units (Table 2) with depths (9.55–2.20 m)related to the water surface of the lake. Apart from abasal silt layer (unit 1), the lowermost 1.2 m of theprofile (units 2–4) consists of greyish calcareous gyttjawith a varying proportion of silt. Above ca 8.3 m algal-rich calcareous gyttjas and calcareous algal gyttjas

prevail, characterised by distinct colour alterations andlaminations ranging in thickness from millimetres or lessto several centimetres. The content of minerogenicmaterial (R) is negligible in units 5–8 with the exceptionof a distinct peak at ca 7.1 m (unit 6). In units 9–12slightly higher R values prevail (Fig. 4). Bands ofbrownish algal gyttja with a reduced content ofcalcareous matter occur at ca 7.1, 6.2, 5.2, 5.1, 4.8 m,and at ca 4.4–4.2m. The uppermost 1-m part of thesuccession (unit 12) consists of loose, brownish calcar-eous gyttja. Restricted amounts of macroscopic shells ofmolluscs such as Bithynia tentaculata, Valvata cristata,Valvata piscinalis, and Pisidium sp. occur in units 7–12.Based on X-ray diffraction analysis, carbonates in thebulk sediment samples used for isotopic analyses wereidentified as exclusively low-Mg calcite. No othercarbonate minerals were detected. Abundant mm-sized,tubular encrustations formed on the stems of Chara sp.in units 2–5 indicate that the calcite precipitated mainlyas a consequence of photosynthesis in Chara algae(McConnaughey, 1991). Within units 6–12, however,the calcite is more fine-grained and no samples of Chara

encrustations could be collected.The 22 radiocarbon dates are compiled in Table 3. A

calendar-year age–depth model for the sediment succes-sion based on these dates, which closely agrees with theU/Th chronology reported by Israelson et al. (1997) forthe interval of 4.80–7.30 m, is shown in Fig. 5. Two of

Table 3

Radiocarbon dates

Sample

depth (m)

Lab. no. Material analysed Weight

(mg)

Reported age

(14C yr BP)

Calibrated age

(mid-intercept) (cal BP)

Calibrated age

(2s Interval)

3.42–3.49 LuA-4467 Nym., Bet., Que. 2.8 11807100 1065 925–1293

4.04–4.10 LuA-4468 Nym., Bet., Aln. 5.6 2000790 1936 1724–2294

4.84–4.90 Ua-11219 Nym. 1.0 30707150 3287 2853–3632

4.90–4.95 Ua-11220 Nym., Til., Bet. 1.3 29957170 3182 2757–3625

4.97–5.00 Ua-11190 Nym., Til., Bet., Car. 5.2 34457210 3691 3212–4281

5.04–5.07 Ua-11188 Nym., Aln., Bet. 1.5 31957190 3431 2887–3868

5.18–5.24 Ua-11221 Nym., Aln. 3.0 35807160 3869 3469–4405

5.42–5.48 Ua-11222 Nym., Bet. 3.5 39357140 4412 3932–4827

5.65–5.70 Ua-11189 und. 2.0 47357220 5553 4848–5925

5.85–5.91 Ua-11223 Til., Bet. 1.2 44107180 5020 4453–5579

6.04–6.10 Ua-11224 Nym., Bet. 0.8 54307260 6230 5612–6780

6.22–6.26 Ua-11108 Bet. 1.8 63207240 7252 6662–7661

6.30–6.35 Ua-11193 Nym., Bet., Pin. 19.0 92207270 10,316 9561–11,181

6.62–6.68 Ua-11225 Bet., Aln., und. 1.0 68607220 7676 7321–8151

6.88–6.92 Ua-11226 Bet., Aln. n.d. 73707170 8175 7840–8450

6.97–7.05 Ua-11191 Bet., Pin. 12.0 86707140 9575 9431–10,169

7.13–7.18 Ua-11227 Nym., Bet., Ulm. 1.8 76607230 8412 7977–9027

7.28–7.33 Ua-11228 Bet., Aln., Pin. 2.0 75257190 8352 7957–8696

7.74–7.81 LuA-4469 Bet., Pin. 3.0 88807100 10,063 9560–10,222

8.11–8.20 LuA-4470 Bet., Emp., und. 3.0 95207130 11,029 10,425–11,195

8.85–8.92 LuA-4471 Bet., Emp. ca 2.5 86807210 9586 9155–10,235

9.17–9.35 LuA-4472 Dry. ca 1.0 94907140 10,714 10,287–11,188

Nym.=fruits of Nymphaea alba, Bet.=fruits and/or catkin scales of Betula pubescens, Que.=bud scales of Quercus robur, Aln.=fruits and/or catkin

scales of Alnus glutinosa, Til.=fruits of Tilia cordata, Car.=fruits of Carex sp., Pin.=seeds of Pinus sylvestris, Ulm.=fruits of Ulmus glabra,

Emp.=seeds of Empetrum nigrum, Dry.=leaf fragments of Dryas octopetala, und.=undetermined terrestrial macrofossils, n.d.=weight not

determined.


the radiocarbon dates clearly yielded too high ages inrelation to adjacent dates and the assumed time-depthcurve. These dates (Ua-11193 and -11191) were bothobtained on samples partly consisting of undeterminedwood remains and/or relatively coarse terrestrial macro-fossils that could have sustained in the catchment soilfor some time before being deposited in the lake. Thelowermost date suggests that the sedimentation wasinitiated shortly after the Pleistocene–Holocene transi-tion. However, the presence of abundant seeds ofEmpetrum nigrum and leaf fragments of Dryas octope-

tala in the lower part of unit 2 (Table 3) gives evidenceof a tree-less environment during the deposition ofthe oldest sediments. This type of vegetation can becorrelated to a pollen assemblage zone which was datedto the latest part of the Younger Dryas Chronozone byBj .orck and Digerfeldt (1986). The isotopic recordsobtained from the lower part of the profile (below ca7.5 m) also correlate well with corresponding datapresented by Bj .orck and Digerfeldt (1984). Based onthese inferences and more recent chronological evidence(Bj .orck et al., 1996, 1997), the onset of sedimentation inLake Igelsj .on can be assigned an age of ca 11,500 cal BP,i.e. the Younger Dryas–Preboreal transition. Thus, atleast one of the two lowermost dates (Ua-4471) likelyyielded an erroneously low age, possibly as a result ofcontamination. This age model is in general agreementwith studies of the local ice recession (Bj .orck andDigerfeldt, 1984; Bj .orck et al., 2001), postulatingdeglaciation during the later part of the Younger Dryas

stadial (GS-1) followed by persistence of stagnant ice fora short period of time.

Unit 4 exhibits an anomalously high content ofminerogenic material and a highly divergent carbon-isotope signature (see below) as compared to adjacentstrata (Fig. 4). These observations together with thepresence of macroscopic remains of Dryas octopetala

and Empetrum nigrum, a vegetational assemblage closelyresembling that of the lower part of unit 2, suggests thatunit 4 consists of sediments reworked from olderdeposits by means of slumping.

The average sedimentation rate for the sedimentsuccession was ca 0.64 mm/yr, with values exceeding1.0 mm/yr in the partly minerogenic sediments of units1–3 (Fig. 5). The subsequent decrease in sedimentationrate, with minimum values of ca 0.3 mm/yr in the upperpart of unit 7, may be partly related to post-depositionalcompaction. In the upper part of the sequence,characterised by loose gyttjas, the sedimentation rateincreased again to ca 1.1 mm/yr.

5. Results and interpretations of the stable isotope

analyses

5.1. Modern isotope hydrology

To investigate the modern hydrological balance of thelake and its relation to the Holocene isotopic records,oxygen- and hydrogen-isotope data were obtained on

Table 4

Isotopic data from modern water samples collected in and near Lake Igelsj .on

Site Date d18O d2H d

Groundwater (nearby well) 1 Nov. 1996 �10.20 �73.7 7.90

Groundwater (nearby well) 6 Jan. 1997 �10.21 �72.5 9.18

Groundwater (spring at the lake) 8 Jan. 1997 �10.31 �73.3 9.18

Groundwater (nearby well) 22 Mar. 1999 �10.82 �75.8 10.8

Groundwater (nearby well) 7 Apr. 1999 �10.70 �76.2 9.40

Groundwater (nearby well) 18 May 1999 �10.70 �74.9 10.7

Groundwater (nearby well) 21 June 1999 �10.64 �74.5 10.6

Groundwater (nearby well) 13 July 1999 �10.56 �74.4 10.1

Lake surface 24 Jan. 1996 �10.11 �74.3 6.58

Lake surface 20 May 1996 �9.67 �71.7 5.66

Lake surface 25 July 1996 �8.61 �66.3 2.58

Lake surface 1 Nov. 1996 �9.38 n.d. n.d.

Lake surface 7 Jan. 1997 �10.17 �73.3 8.06

Lake surface 22 Mar. 1999 �11.66 �84.0 9.28

Lake surface 7 Apr. 1999 �10.45 �74.8 8.80

Lake surface 18 May 1999 �9.43 �68.6 6.84

Lake surface 21 June 1999 �8.55 �65.3 3.10

Lake surface (19.01C) 16 July 1999 �8.24 �62.3 3.62

Lake, 0.5m depth (18.61C) 16 July 1999 �8.31 �62.4 4.08





Isotopic units: % (V-SMOW), n.d.=not determined. d=deuterium excess value (d2H�8d18O)


modern water samples from the study area. Ground-water samples collected near the lake show only minorchanges in isotopic composition over the year, with d18Ovalues ranging from �10.8% to �10.2% (Table 4;Fig. 6). The subdued seasonal variations point to arelatively large recharge area and extensive mixing ofgroundwater in the aquifer supplying water to the lake.Close correspondence with the weighted annual meanvalue of precipitation (�10.7%) observed at Tors .o (50 ma.s.l.; ca 40 km north of Lake Igelsj .on (Figs. 1 and 3B))suggests that the isotopic ratio of groundwater isinherited from the average of the seasonal isotopicrange of precipitation that typically occurs in the region.Lake-water samples collected during the winter plotclose to the groundwater samples, which indicates thatregional groundwater dominates input to the lakeduring most of the ice-covered season (December–February). During other parts of the year the lake-water isotopic composition exhibits substantial varia-tions with an d18O amplitude of at least 3.5%. Thesechanges are likely related to input of isotopicallydepleted snowmelt during the spring (March–April)and isotopic enrichment caused by evaporation from the

lake-water surface during the rest of the ice-free season(May–November). On a hydrogen-/oxygen-isotopecross-plot (Fig. 6) all groundwater samples as well aslake-water samples from January, March, and April plotclose to the global meteoric water line (Craig, 1961),whereas lake-water samples from May to July fall alonga local evaporation line with a slope near 5 as expectedfor water bodies affected by evaporation (Craig andGordon, 1965). These results suggest that the waterbody is affected by a well-developed evaporativeisotopic enrichment in spite of the rather short residencetime (ca 20 days or less) as estimated from catchmentsize and meteorological data. Surface run-off from thecatchment and direct precipitation seems to have nomajor impact on the isotopic composition of lake waterduring the ice-free season.

5.2. Stable isotope records of carbonates

Oxygen- and carbon-isotope records have beenobtained from three different types of carbonatesamples. These include bulk carbonates (dSed; 129samples in the interval of 9.55–2.20 m), Chara sp.

0 2000 4000 6000 8000 10000 12000

Calibrated 14C age (cal BP)

2

3

4

5

6

7

8

9

10

Depth

belo

w w

ate

r surf

ace (

m)

2

3

4

5

6

7

8

9

10

0 2000 4000 6000 8000 10000 12000

1

2

3

4

5

6

7

8

9

10

11

12

Fig. 5. Age model based on radiocarbon dates of terrestrial macrofossils as compiled in Table 3. The left-hand column refers to lithostratigraphic

units (Table 2). The shaded zone indicates that unit 4 is assumed to be reworked from older sediments by slumping.


encrustations (dCha; 28 samples in the interval of9.50–7.13 m), and Bithynia tentaculata opercula (dBit;73 samples in the interval of 6.54–2.20 m). Based ondistinct shifts, prevailing trends, and consistent levels inthese parameters the studied sediment sequence has beendivided into five isotopic zones (IG-1–IG-5) as shown inFig. 4. The different isotopic parameters within therespective zones are described and interpreted in detailbelow.

The isotopic records exhibit several rapid andextensive shifts and the major trends are broadly similarfor d18O and d13C, although no strong mutual correla-tions exist across the record as a whole (Fig. 7). Thissuggests that the isotopic composition of carbonates iscontrolled mainly by hydrology rather than lake-watertemperature or changes in the isotopic composition ofinput water (cf. Talbot, 1990). Short-term fluctuationsare likely related to variations in evaporation/inflowratio (d18O) and atmospheric exchange (d13C), respec-tively, through changes in lake volume and residencetime. Lake waters that have undergone evaporationexhibit systematic enrichment in 18O (Craig andGordon, 1965; Gibson et al., 1993), and an increase inevaporation/inflow ratio (prolonged residence time) alsoincreases the extent to which lake-water DIC isisotopically equilibrated with atmospheric carbon diox-

ide, which leads to enrichment in 13C (Turner et al.,1983). These hydrological processes have profoundconsequences for the isotopic signatures of lacustrinecarbonates precipitating in closed-basin lakes with highevaporation/inflow ratios (e.g. Talbot and Kelts, 1990).Thus, we propose a simplistic climate-isotope hydrologymodel to explain the major variations in d18O and d13Cwith time, in which changes in catchment hydrology andlake level are coupled to net precipitation and ground-water level of the local unconfined aquifer (Fig. 8). Inorder to achieve a better understanding of the fairlycomplex relationships between the different variables,the data set has been analysed with multivariatestatistical techniques, performed with the CANOCOprogramme (Ter Braak, 1988). These statistical runsverified the model described below. In Fig. 9 the dSed

records are displayed against a calendar-year age scale.

5.2.1. Zone IG-1: 9.55–8.29 m (ca 11,500–

10,700 cal BP)

This zone represents the initial ca 800-year period ofthe Holocene lake history, which at least in its earliestphase, immediately following the deglaciation, wascharacterised by sparse terrestrial vegetation and un-stable soils highly susceptible to erosion. This isevidenced by the initially high content of minerogenic

-12 -11 -10 -9 -8

δ 18O (‰ V-SMOW)

-90

-80

-70

-60

-50

δ 2 H (

‰ V

-SM

OW

)GMW

L: δ 2 H= 8 δ

18 O+ 10

LMWL: δ 2 H

= 7.37δ18 O

+ 2.95

LEL: δ 2 H = 4.87δ18 O + 23.5

Lake water March (snow-melt)

Ground-water

Lake water November(no δ 2H data)

April

January

May

June -July

Fig. 6. Plot of d18O versus d2H values for samples of Lake water (K) and adjacent groundwater (J) collected during the period of 1996-1999 (Table

4). GMWL is the global meteoric water-line (Craig, 1961), and LEL refers to the local evaporation line defined by lake-water samples collected

during the ice-free season (r=0.98).


material (residue) in the sediments (Fig. 4). Therefore,deposition of detrital carbonates derived from glacialdeposits in the catchment has to be considered at this

stage as a potentially complicating process at theinterpretation of isotopic records obtained onbulk carbonates (Hammarlund and Buchardt, 1996).

-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2

δ18O (‰ V-PDB)

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

δ 13C

(‰

V-P

DB

)

δ Sed (zone IG-1)

δSed (zone IG-2)

δSed (zone IG-3)

δ Sed (zone IG-4)

δ Sed (zone IG-5)

δ Bit (zone IG-4)

δ Bit (zone IG-5)

δ Cha (zone IG-1)

δCha (zone IG-2)

δCha (zone IG-3)

Fig. 7. Plot of d13C versus d18O showing the approximate isotopic composition of Ordovician limestone from the local bedrock (large open square)

as compared to carbonate samples from Lake Igelsj .on (Fig. 4).

δ 18O δ 13CClimate Lake level

Evap./infl.ratio

Atmosph. equilibr.

DRYand/or

WARM

HUMIDand/or

COLD

LOW

HIGH

HIGH

LOW

HIGH

LOW

STRONG

WEAK

HIGH

LOW

Groundwater exchange

Surface outflow ?

c. -7 ‰

c. -9 ‰

c. +1 ‰

c. -3 ‰

Hydrology

Fig. 8. Simplistic climate-isotope hydrology model for Lake Igelsj .on.


However, based on several independent lines of evi-dence, any major influence of this process on the dSed

records can be dismissed. Firstly, samples of unweath-ered glacial deposits from the lake catchment (Fig. 2)exhibit no measurable content of calcium carbonate,consistent with observations by Lundqvist et al. (1931)of glaciofluvial deposits in the Lerdala area withexclusively crystalline clasts or Palaeozoic materialconfined to sandstone and alum shale particles. Sec-ondly, samples of the local Ordovician limestone thatcrops out on the slopes of Mount Billingen exhibit d18Oand d13C values in the ranges of ca �7% to �6% and ca+1% to +2%, respectively (B. Buchardt, unpublisheddata). Samples of bulk carbonates in the lowermost partof the sequence are consistently depleted in 18O by ca5% as compared to the expected oxygen-isotopecomposition of any such detrital carbonates, whichpoints to an endogenic origin (Fig. 7). In addition, theisotopic data obtained on Chara encrustations aregenerally in good agreement with the respective dSed

data even in the most minerogenic-rich, basal part of theprofile (Fig. 4). This consistency, which is valid for theentire part of the profile in which individual Chara

encrustations could be identified and subsampled (units1–5), demonstrates that the records of dSed wereobtained on calcite originating mainly from photosyn-thesis in Chara algae.

Within the studied sequence, minimum values ofd18OSed around –11.5% were recorded in zone IG-1. Theca 2% lower values recorded in zone IG-1 as comparedto surface-sediment samples may be related either tomore 18O-depleted precipitation, lower evaporation/inflow ratio, higher lake-water temperature, or acombination of these factors. In addition, during itsearly history the lake may to some extent have beeninfluenced by 18O-depleted melt-water from bodies ofstagnant glacier ice in the catchment (Bj .orck andDigerfeldt, 1986).

The record of d13CSed exhibits a pronounced decreas-ing trend from ca +2% to �3% which was most likelybrought about by the successive establishment ofterrestrial vegetation and related soil development inthe lake catchment. The rapid post-glacial immigrationof plants as a response to warming at the onset of theHolocene resulted in a succession from herb-dominated,discontinuous vegetation to forest, initially dominated

-12 -11 -10 -9 -8 -7 -6 -5 -4 -3

δ 18OSed (‰ V-PDB)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

Age

(cal B

P)

-4 -3 -2 -1 0 1 2 3

δ 13CSed (‰ V-PDB)

LOW HIGH WEAK STRONG

HIGH LOW

HIGH LOW

South Swedishlake levels

δ 18O-inferred

evaporation/inflow ratio

δ 13C-inferred

atm. equilibration

(Lake Bysjön)

ADVANCE RETREAT

ADVANCE RETREAT0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

South Norwegianmountain glaciers

(Jostedalsbreen)

Fig. 9. Stable oxygen and carbon isotope records obtained on bulk carbonates (predominantly Chara calcite) from Lake Igelsj .on plotted against

calendar age as inferred from the age model shown in Fig. 5. General implications of the isotopic records are compared to the Holocene sediment-

limit reconstruction from Lake Bysj .on, ca 300 km to the south, representing regional lake-level fluctuations (Digerfeldt, 1988), and to a

reconstruction of Holocene glacier fluctuations in the Jostedalsbreen area in western Norway, ca 500 km northwest of Lake Igelsj .on (Nesje et al.,

2001). Shaded zones represent the early Holocene period when the assumed climate-isotope hydrology model (Fig. 8) is likely overrided by local

palaeo-geographic effects (see text).


by birch, in the study area (Bj .orck and Digerfeldt, 1984).As discussed by Hammarlund et al. (1997) on the basisof a similar carbon-isotope record from northernSweden, this development leads to increased release of13C-depleted carbon dioxide from soil respiration andsuccessive depletion with time in 13C of DIC ingroundwater and lakes. As stated previously, unit 4can be assumed to have been reworked from the lowerpart of unit 2, which means that the anomalously highvalues of d13C in the upper part of zone IG-1 are notdirectly related to environmental change.

5.2.2. Zone IG-2: 8.29–7.19 m (ca 10,700–8400 cal BP)

Within this zone a distinct increase in d18OSed wasrecorded, and if this change is explained exclusively bylake-water temperature it would require a temperaturedecrease of more than 101C during the summer (Craig,1965). This is highly unlikely based on other proxyrecords such as regional pollen data which show nosigns of a cooling at this stage (Digerfeldt, 1977). Aslight increase in organic carbon content and persis-tently very high values of carbonate content wouldrather suggest increased aquatic productivity and atleast maintained lake-water temperatures. Slightly high-er values of d13CSed as compared to the upper part ofzone IG-1 also point to increased photosynthetic activityin the lake, giving rise to 13C-enrichment of DIC(McKenzie, 1985). An enrichment in 18O in precipitationand hence in groundwater feeding the lake could havecontributed to the observed increase in d18OSed, as wellas an increase in evaporation/inflow ratio brought aboutby a general decrease in net precipitation. Such ahydrological development (i.e. towards lowered lakelevel; Fig. 8) may also be related to the rather extensiveisostatic rebound at this stage leading to successiveisolation of the local glaciofluvial aquifer from thenearby sea, and thus a general lowering of the ground-water table (Bj .orck and Digerfeldt, 1986).

5.2.3. Zone IG-3: 7.19–6.90 m (ca 8400–8000 cal BP)

This narrow zone is characterised by rapid andextensive isotopic shifts. The pronounced decrease ind18OSed at 7.11 m is assumed to reflect mainly a decreasein evaporation/inflow ratio of the basin related toincreased net precipitation (Fig. 8). The 18O-depletioncoincides with a significant peak in minerogenic residue(unit 6) which is interpreted as an increase in catchmenterosion in response to elevated lake-level and possiblyenhanced run-off. The associated increase in suspensionload probably resulted in decreased light penetrationand deteriorated conditions for Chara photosynthesis asindicated by the decrease in carbonate content anddisappearance of macroscopic Chara encrustations. Theenhanced input of nutrients in the form of siliciclasticmaterial likely also induced an increase in phytoplank-ton production, which explains the distinct increase in

d13CSed accompanied by increased organic content in thelower part of the zone. Subsequent to the increase incatchment erosion and lake productivity the aquaticsystem adapted to the altered hydrological balance, anda substantial decrease in evaporation/inflow ratio can beinferred also from lowered d13CSed values, althoughslightly delayed as compared to the decrease in d18OSed.The 13C-depletion reflects an increase in the supply ofgroundwater with relatively 13C-depleted DIC in rela-tion to DIC in isotopic equilibrium with atmosphericcarbon dioxide (Fig. 8). The absence of an isotopicresponse to input of any potential detrital carbonates isevidenced by the clearly asynchronous changes in theoxygen- and carbon-isotope records in relation to thepeak in minerogenic residue of unit 6 (cf. Zone IG-1above).

5.2.4. Zone IG-4: 6.90–5.22 m (ca 8000–4000 cal BP)

Based on very distinct isotopic shifts to higher levelsin the records of dSed at the IG-3/IG-4 zone boundary, areturn to, and even manifestation of the hydrologicalconditions that prevailed in zone IG-2 can be assumed.The record of d18OSed exhibits rather stable valuesaround �7% throughout the zone which may suggestthat steady-state conditions in terms of evaporativeenrichment in 18O were obtained (cf. Lister et al., 1991),following a short-lasting intervening period of relativelyrapid through-flow and lowered evaporation/inflowratio in zone IG-3. The ca 3% higher values of d13CSed

as compared to the upper part of zone IG-2 probablypartly reflect higher aquatic productivity and increasedphytoplankton production as evidenced by a generalincrease in organic carbon content. However, thegeneral covariance with corresponding trends observedin d18OSed from zone IG-3 and upwards through theprofile suggests that hydrology rather than aquaticproductivity was the main factor controlling d13C ofDIC and limnic carbonates. Further evidence of thisrelationship is given by d13C data obtained on bulkorganic material from parts of the sediment succession(Thomsen, 2000), showing parallel isotopic shifts ascompared to the d13CSed record. Periods of 13C-depletion are generally associated with lithologicalchanges (increased organic carbon content and de-creased carbonate content). Such episodes (e.g. at ca6.2 m) were likely associated with increased nutrientsupply caused mainly by elevated lake level during thesummer, thus reflecting increased humidity (Fig. 8).

The records of dBit; which extend upwards from ca6.5 m (Fig. 4), are broadly parallel with correspondingdSed data, although with consistent offsets (Fig. 7). Assuggested by concentric growth increments, calcificationof Bithynia tentaculata opercula takes place during theentire part of the year when the organisms are active,interrupted by periods of winter hibernation (T. vonProschwitz, personal communication). As precipitation


of Chara encrustations (represented by the d18OSed

record) occurs mainly during the early part of thesummer (cf. M .orner and Wallin, 1977), the higher valuesof d18OBit can be assumed to reflect lower lake-watertemperatures on average across extended active seasonsas compared to Chara photosynthesis. The moreextensive, corresponding offset between the two d13Crecords (5.2–7.0%) is caused partly by kinetic carbon-isotope fractionation related to proton pumping duringassimilation of bicarbonate in Chara algae (McCon-naughey, 1991). This effect has been demonstratedpreviously by Hammarlund et al. (1997, 1999). How-ever, the consistently high values of d13CSed are probablyto a large extent related to isotopic exchange withatmospheric carbon dioxide (Turner et al., 1983) as aresult of the prevailing water balance conditions (i.e.relatively high evaporation/inflow ratios).

5.2.5. Zone IG-5: 5.22–2.20 m (ca 4000 cal BP to

present)

Following a distinct decrease in d18OSed at the IG-4/IG-5 transition, relatively large variations were recordedwithin zone IG-5, indicating considerable changes inevaporation/inflow ratio of the basin. However, super-imposed on these fluctuations is a general depletion in18O with time, which suggests a long-term change fromthe predominant mode of lake contraction that pre-vailed in zone IG-4 to prolonged periods of increasedlake volume and possibly surface outflow (Fig. 8). Ageneral increase in precipitation, likely causing enhancedslope erosion, is also indicated by higher values ofminerogenic residue in the upper part of the profile. Inaddition, the d18OSed record may to a lesser extent havebeen influenced by changes in lake-water temperature,although a decrease in summer lake-water temperatureduring the later part of the Holocene (e.g. M .orner andWallin, 1977) would have counteracted the observedcarbonate 18O-depletion. On the other hand, a generaldepletion in 18O of precipitation, perhaps in combina-tion with a relative increase in winter precipitation,could have contributed to the recorded trend. However,the interpretation in terms of an over-riding hydrology-driven response is supported by the general decrease ind13CSed which indicates increased lake volume, de-creased evaporation/inflow ratio, and successively morehumid climatic conditions.

As opposed to the situation in the preceding zone, therecords of dBit exhibit general trends that have nocounterparts in the corresponding dSed data. This is mostlikely related to differences in isotopic characteristics ofthe lake water between the specific seasons of the yearduring which precipitation of the respective carbonatecomponents occurred. Whereas, relatively high values ofd18OSed in zone IG-4 reflect precipitation of Chara

encrustations in lake water subject to 18O-enrichmentdue to enhanced evaporation/inflow ratios during the

summer, this process did not affect average lake-waterd18O to the same extent during the extended season ofshell formation of Bithynia tentaculata (cf. Fig. 6).Within zone IG-5 a general decrease in evaporation/inflow ratio, influencing mainly lake-water d18O duringthe summer, led to a successive decrease in d18OSed whilethe record of d18OBit was largely unaffected by thishydrological change. A corresponding response relatedto seasonal differences in d13C of DIC is evident fromrather stable values of d13CBit across the IG-4/IG-5transition as compared to the distinct long-termdecrease in d13CSed. These observations suggest thatpronounced lake contraction developed on a seasonalbasis during parts of the lake history (mainly zone IG-4), with minimum lake volume and maximum lake-leveldrawdown during the summer.

Based on comparisons between measured or esti-mated temperatures at the bottom of the lake (Table 4)and corresponding values inferred from surface-sedi-ment d18O of carbonates and modern lake-water d18O(Craig, 1965), possible disequilibrium precipitation canbe assessed for the analysed components (Fronval et al.,1995). Average lake-water d18O for the ice-free seasonduring calcification of Bithynia is probably in the rangeof �10% to �9% V-SMOW (Fig. 6). Although largevariations were recorded in the uppermost part of theprofile, the use of a d18OBit value of –6.5% V-PDBresults in a bottom-water temperature range of ca +51Cto +8.51C. This estimate is in good agreement withlocal mean annual air temperature (+5.91C) andexpected temperature of regional groundwater, whichmay point to equilibrium precipitation as commonlyassumed for molluscs (Fritz and Poplawski, 1974). Asimilar exercise for d18OSed (ca �9% V-PDB) within alake-water d18O-range of �9.5% to �8.5% V-SMOWduring the summer (mainly June) yields temperaturesnear the bottom of the lake during precipitation ofChara calcite of ca +161C to +201C. These relativelyhigh values suggest that bicarbonate assimilation inChara algae may be associated with a slight kineticfractionation effect, giving rise to 18O-enrichment incalcitic encrustations (cf. Hammarlund et al., 1999).

6. Discussion

The major variations in hydrological balance of LakeIgelsj .on inferred from the isotopic records were likelyassociated with changes in groundwater and lake-waterlevels of several metres (Fig. 8). The altitudinal differ-ence between the present-day lake level, representingexclusively groundwater exchange, and the threshold ofthe basin is ca 3m. Although, the available data do notprovide unambiguous evidence for surface outflowacross the threshold at any stage, or other absolutemeasures of lake level, the assumed hydrological


fluctuations must have been extensive and most likelyrelated to regional changes in climate. Therefore, otheravailable records of climatic humidity for the Holoceneshould be considered for comparison. In Fig. 9 theisotopic records are compared with a reconstruction ofHolocene variations in sediment limit from Lake Bysj .on(Digerfeldt, 1988), ca 300 km to the south (see Fig. 1).As shown by Digerfeldt (1988) and Harrison andDigerfeldt (1993) based on a compilation of similarstudies from several other sites, the Lake Bysj .on recordis a representative proxy for long-term, lake-levelvariations in southern Sweden caused by changes innet precipitation. The curve is based on detailedsediment-stratigraphic and pollen-analytical studies oflake sediments along transects across bathymetricalgradients, and the approach exploits the assumptionthat lake levels and sediment limits of small lakessituated in areas of highly permeable glaciofluvialdeposits respond sensitively to climatically inducedvariations in regional groundwater levels (Digerfeldt,1988). Comparison is also made with a glaciation recordfor the Jostedalsbreen area in the Scandes Mountains ofsouthern Norway (Nesje et al., 2001), ca 500 km NW ofLake Igelsj .on (Fig. 9). This record, which is based onvariations in organic content of three proglacial lacus-trine sequences, is assumed to reflect changes in glaciermass-balance as a result of winter precipitation andsummer temperature. Thus, if our simplistic climate-isotope hydrology model is valid for major responses ofcatchment water balance to regional hydrologicalchanges (Fig. 8), mutual trends in these two independentclimate records should be visible in the oxygen- andcarbon-isotope records from Lake Igelsj .on. It should benoted, however, that the coupling of the isotopic recordsto palaeohydrology may be slightly modified by theeffects of varying precipitation isotopic composition andlake-water temperature (d18O) and aquatic productivity(d13C), respectively. Individual periods and specificaspects of the Holocene climatic development in theregion are discussed below.

6.1. The early Holocene (ca 11,500–8000 cal BP)

Lake status records from several sites in southernSweden exhibit a pattern of consistent low-stands duringthe late Preboreal, culminating around 10,500 cal BP(Fig. 9; Harrison and Digerfeldt, 1993). The absence ofany isotopic enrichment prior to ca 9000 cal BP thatwould reflect a corresponding water-level lowering atLake Igelsj .on, may be explained by the proximity of thesite to the Preboreal sea. Following deglaciation the seaprotruded into Lake L(angen, situated only a few kmnorth of Lake Igelsj .on (Fig. 2), and this former marinebay was not isolated from the sea by glacio-isostaticrebound until ca 11,000 cal BP (Bj .orck and Digerfeldt,1986). The coastal setting probably decreased the

regional groundwater gradient, which may have im-peded any lake-level lowering and reduction of lakevolume. Furthermore, the proximity to the sea duringthe early Holocene may also have led to a locally moremaritime climate and relatively low evaporation/inflowratios as suggested by low values of d18OSed at this stage.However, by ca 9000 cal BP the influence of theseenvironmental conditions on the hydrological balanceof the lake had probably ceased.

A correlation of the lithological and isotopic excur-sions in the interval of ca 8300–8000 cal BP (zone IG-3)with the widely recognised, so-called 8200-yr eventseems highly probable although the dating control doesnot allow for a definite coupling to a mutual climaticforcing (cf. Bennett, 2002). This event, originallyidentified as a distinct depletion in 18O of Greenlandice-cores (Johnsen et al., 1992; Grootes et al., 1993;Alley et al., 1997), has been attributed to a major melt-water discharge that retarded the North Atlanticthermo-haline circulation (Barber et al., 1999; Renssenet al., 2001). Its climatic consequences have generallybeen recorded as a 200–400 yr period of cooling, mainlyaffecting summer conditions in the North Atlanticregion. Such a development has been demonstratedindependently based on several different proxies (Klit-gaard-Kristensen et al., 1998; Nesje and Dahl, 2001;Tinner and Lotter, 2001; Snowball et al., 2002),including a record of d18O obtained on limnic ostracodsfrom southern Germany (von Grafenstein et al., 1998).The assumed increase in net precipitation at LakeIgelsj .on, as inferred from lowered values of d18OSed, isthus consistent with a regional lowering of summertemperature and a related decrease in evaporation (cf.Tinner and Lotter, 2001). A short, contemporary periodof cooling is indicated by decreased frequencies ofbroad-leaved trees in the regional pollen record fromLake Flarken, ca 10 km north of Lake Igelsj .on(Digerfeldt, 1977). Indeed, the Lake Bysj .on record(Fig. 9) shows evidence of increased net precipitationat this stage, although the duration of the lake-levelhigh-stand clearly exceeds what would be expected if itreflects the North Atlantic perturbation at ca8200 cal BP. However, it should be noted that themethodology used for sediment-limit reconstructionmay overestimate the chronological extension of lake-level high-stands due to subsequent erosional phases(Digerfeldt, 1988). Additional palaeoclimatic evidenceof the 8200-yr event in Scandinavia is provided by apronounced re-advance of mountain glaciers in southernNorway, the ‘‘Finse event’’, which has been attributedmainly to decreased summer temperature (Dahl andNesje, 1996; Nesje et al., 2001). The lack of anysignificant increase in winter precipitation as inferredfrom the Norwegian glaciolacustrine records (Nesjeet al., 2001) may suggest that the observed depletion in18O of limnic carbonates at Lake Igelsj .on was related


primarily to decreased evaporation/inflow ratios duringthe summer, and not to a change in the seasonaldistribution of precipitation.

6.2. The mid-Holocene (ca 8000–4000 cal BP)

The termination of the Finse event coincides with amajor reduction of glacial activity in the ScandesMountains as a consequence of elevated summertemperature, conditions that prevailed until ca4000 cal BP (Dahl and Nesje, 1996; Nesje et al., 2001).Such a scenario can also be invoked to explain thesubstantial isotopic enrichments at ca 8000 cal BP, whichsuggest that the water balance of Lake Igelsj .onresponded to drier and warmer summer conditions(Fig. 8). This climatic mode, implying a general lake-level lowering, seems to have persisted during a periodof ca 4000 yr, without any major fluctuations in theisotopic records. During this interval the regionalvegetation of southern Sweden was dominated by stableclimax forests with several warmth-demanding species(Digerfeldt, 1977; Lager(as, 1996), which suggests thatthe inferred hydrological status of the lake was causedmainly by relatively high summer temperatures ratherthan low amounts of precipitation, although the twoprocesses likely interacted (cf. Dahl and Nesje, 1996). Abrief period of increased net precipitation, perhapsrelated to lowered summer temperature, is indicated atca 6700 cal BP, although the extent and regionalsignificance of this temporary shift is difficult to assess.A mid-Holocene interval of relatively low net precipita-tion, partly due to high summer temperatures, isconsistent with elevated tree-limits and retraction ofmountain glaciers in the Scandes Mountains (Kullman,1995; Dahl and Nesje, 1996), as well as with predomi-nance of low sediment limits and lake levels in southernSweden (Fig. 9).

6.3. The late Holocene (ca 4000 cal BP–present)

A continuous increase in net precipitation, leading torising groundwater levels, expanding lake volume, anddecreasing evaporation/inflow ratio of the lake, isindicated by general isotopic depletion with time afterca 4000 cal BP. This development, which is particularlywell expressed in the d18OSed record (Figs. 4 and 9), isconsistent with long-term trends towards higher lakelevels in southernmost Sweden at this stage (Digerfeldt,1988; Harrison and Digerfeldt, 1993), and a similarpattern has been recorded at a site ca 150 km north ofLake Igelsj .on (Almquist-Jacobson, 1995). The distinctlowering of the sediment limit and lake level at LakeBysj .on around 1500 cal BP (Fig. 9) is believed to berelated to a large extent to anthropogenic deforestationand increased wind exposure (G. Digerfeldt, personalcommunication), which means that the level of Lake

Bysj .on probably remained relatively high also duringthis period. As discussed by Harrison and Digerfeldt(1993), the general lake-level rise during the later part ofthe Holocene is related mainly to increased netprecipitation in response to decreased summer insola-tion at mid-northern latitudes. Further evidence of thisprocess and its gradational impact on vegetation andhydrology has been provided by various palaeoclimaticrecords from Scandinavia. It is, for example, noteworthythat Granlund already in 1932 concluded that thepossibly most extensive recurrence surface (RY) inSwedish peat bogs was RY5, tentatively dated to ca4000 cal BP by pollen-based correlation to archaeologi-cal chronology. Recurrence surfaces mark distincttransitions from high- to low-humified peat as aresponse to increased net precipitation. Granlund(1932) stated that this was a time of increasedprecipitation, the most distinct Holocene temperaturedrop and extensive waterlogging. These conditions werefavourable for expansion of peatlands (e.g. Svensson,1988) and caused altitudinal retraction of tree-limits andincreased glacial activity in the Scandes Mountains(Kullman, 1995; Dahl and Nesje, 1996; Karl!en andKuylenstierna, 1996; Nesje et al., 2001). According toGranlund (1932), an additional recurrence surface(RY3), dated to ca 2500 cal BP, reflects a secondsignificant increase in humidity as a result of increasedprecipitation in combination with lowered temperature.This climatic shift is also indicated in the isotopicrecords as distinct depletions in 18O and 13C at ca 4.5 m(Figs. 4 and 9). It is noteworthy that several details ofthe Norwegian record of glacier fluctuations subsequentto reformation of the Jostedalsbreen ice cap around6000 cal BP (Nesje et al., 2001) are reproduced bysecond-order variations in the isotope stratigraphies atLake Igelsj .on. This correlation, which is particularlystrong with the d13CSed record (Fig. 9), lends increasedcredibility to these independent data sets as sensitive andregionally significant palaeohydrological proxies.

As indicated by rather dramatic shifts in the isotopicrecords from Lake Igelsj .on, in combination withconsiderably increased variability (Figs. 4 and 9), muchof this change in climate may have occurred relativelyrapidly within a few 100 years shortly after 4000 cal BP.Independent evidence of such a shift between distinctclimate modes, involving also decreased climate stabi-lity, has been presented by Anderson et al. (1998) andSnowball et al. (1999). Both studies postulate an abruptclimatic change at ca 3700 cal BP, identified as decreasedpeat humification in Scotland and increased soil erosionin northern Sweden, respectively. Processes within theclimate system itself, independent of orbital forcing,have to be invoked to explain these observations,although the main climatic parameter responsible forthe inferred increase in net precipitation is unclear(increasing precipitation, decreasing temperature, or a


combination of both). A major change in the atmo-spheric circulation pattern over Northwest Europeseems to have taken place at about this time, perhapsas a result of a weakening of North Atlantic thermo-haline circulation as indicated by declining sea-surfacetemperatures and salinity, (Duplessy et al., 1992; Ko-cand Jansen, 1994). Bond et al. (1997) identified increasedice rafting in the North Atlantic at ca 4200 cal BP (eventno. 3). However, according to INTCAL98 (Stuiver et al.,1998) it is one of the few Holocene Bond events that donot correlate with major shifts towards increased atmo-spheric 14C content, and therefore it is possibly notrelated to neither decreased solar forcing or disturbedocean ventilation. It may, however, be related to ageneral change in circulation patterns caused bydecreased summer insolation at mid- to high northernlatitudes. In fact, only three times during the last 100 kahas insolation been lower than during the last 4000 yr(Berger, 1978), and perhaps a critical climatic thresholdfor the Northern Hemisphere was passed around4000 cal BP.

7. Concluding remarks

The isotopic records from Lake Igelsj .on provideevidence of several rapid changes in net precipitationduring the Holocene, the most extensive of whichoccurred between 8300 and 8000 cal BP and around4000 cal BP, respectively. These climatic shifts, whichaffected large parts of northern and central Europe,were probably related to large-scale rearrangements ofatmospheric circulation patterns. For example, thewidely recorded cooling shortly before 8000 cal BP, withinferred records of lowered growth-season temperaturesdistributed from northern Sweden to Switzerland(Snowball et al., 2002; Tinner and Lotter, 2001), seemsto be associated with an increase in effective moisture ofequally regional significance. This pattern suggests asouthward displacement of the Polar Front across mostof the North Atlantic region, giving rise to coolersummers and drastically altered cyclonic pathways,preferentially during winter seasons. A general coolingaccompanied by decreased snow accumulation inGreenland (Alley et al., 1997) suggests an expansion ofthe Polar high-pressure vortex and an associatedsteepening of the temperature and pressure gradientsin the North Atlantic region (von Grafenstein et al.,1998). This pattern probably induced a strong zonalatmospheric flow with enhanced cyclonic activity acrossthe seaboard of Northwest Europe, perhaps contribut-ing to expansion of mountain glaciers in southernNorway (Dahl and Nesje, 1996) and increased springsnow-melt in northern Sweden (Snowball et al., 2002).Summer conditions may also have been characterised byincreased precipitation, although the increased humidity

was related mainly to lowered temperatures (cf. Tinnerand Lotter, 2001).

As compared to the early Holocene perturbationsdiscussed above, the boundary conditions influencingthe climatic development at 4000–3500 cal BP differedsubstantially in terms of the absence of remnantLaurentian ice-caps and a markedly altered orbitalconfiguration. However, a coupling of the abruptincrease in effective moisture at this stage to someanalogous, but hitherto unresolved, ocean-atmosphereforcing seems highly probable. The shift towards morehumid and variable climatic conditions recorded at LakeIgelsj .on at ca 4000 cal BP correlates well with the onsetof mild and wet winter conditions as inferred fromglaciolacustrine deposits in Norway (Nesje et al., 2001).This change, which was attributed to a prevailingweather regime characterised by a positive NorthAtlantic Oscillation index, thus demonstrates theregional character of the enhanced moisture flux acrossthe Scandinavian Peninsula during the later part of theHolocene.

Our results demonstrate the usefulness of isotopicdata obtained on sediments from hydrologically sensi-tive lakes as proxies for variations in moisture regime,and may thus serve as inspiration for similar studieselsewhere. Further details, such as the relation of arelatively humid climate during the 8200 yr event to aprobable temperature decline, as well as the regionalsignificance of second-order changes in the inferredrecord of net precipitation, will have to be explored byadditional studies based on similar approaches. Com-plementary information on long-term dynamics of theocean-atmosphere circulation during the Holocene maybe gained by reconstruction of the isotopic compositionof ambient precipitation as proposed by Rozanski et al.(1997) based on d18O records from open-basin lakes(e.g. Edwards et al., 1996; von Grafenstein et al., 1998;Hammarlund et al., 2002). This type of information mayto some extent be incorporated in the d18OSed recordsfrom Lake Igelsj .on, although effectively masked byhydrological alteration.

Acknowledgements

T.K. Hellner, Igelsj .o Farm, provided logistic supportand kindly assisted in the water-sampling programme. S.Johnsen, Geophysical Institute, University of Copenha-gen, and B. Schmitz, Department of Earth Sciences,G .oteborg University, supplied some of the isotopicdata. B. Warming skilfully carried out most of theisotopic analyses. T. von Proschwitz, Natural HistoryMuseum of Gothenburg provided valuable informationon gastropod ecology. The GNIP database maintainedby IAEA and WMO was consulted for precipitationisotopic data. Comments by G. Digerfeldt, A. Nesje,


and two anonymous reviewers on earlier versions of themanuscript are greatly appreciated. Financial supportwas provided by the Swedish Natural Science ResearchCouncil (NFR).

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rapid hydrological changes during the holocene revealed by stable isotope records of lacustrine

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