Composite stable isotope records from a Late Weichselian lacustrine sequence at Grrenge, Lolland, Denmark: evidence of Allerod and Younger Dryas environments DAN HAMMARLUND AND BJ0RN BUCHARDT

Hammarlund, D. & Buchardt, B. 1996 (March): Composite stable isotope records from a Late Weichselian Iacustrine sequence at Grznge, Lolland. Denmark: evidence of Allertad and Younger Dryas environments. Boreus, Vol. 25, pp. 8 -22. Oslo. ISSN 0300-9483. Stable oxygen and carbon isotope data from a lacustrine sequence at Grznge, southeast Denmark, revealed distinct cnvironmental changes related to Late Weichselian climatic development in the region. Comparison of isotopic records obtained from sedimentary carbonates and freshwater molluscs enabled reconstruction ol' changes in the lacustrine environment. The degree of thermal and chemical stratification of the lake was evaluated and supported by pollen data from an earlier study of the site. During the Allertad interstadial, dimictic and stagnant conditions characterized the lake, whereas the stratification was disturbed during thc Younger Dryas stadial probably as a result of deforestation and increased wind impact. The origin of sedimentary carbonates was examined by mineralogic and morphologic studies. A considerable input of clastics and detrital carbonates, associated with pronounced enrichment of ''0 and I3C. wds recorded in the Younger Dryas sediments indicating soil degradation and increased erosion. A climatic warming preceding the Pleis- tocene/Holocene boundary is clearly reflected in the different stable isotope records and in the lithostratigraphy. Dun Hunimcirlunct*, Department uf' Qutiternury Geology, Lund Unioersiiy, Tornaoagen 13, S-223 63 Lund. Sweden and Deparfment qf Eirrth Sciences. Uniijersity yf Wiirerloo. Watwloo, Onttrrio, Canada N2/. 3G I ; Hjmn Buchardt, Geologicd Institute, Universiiy of Copenhagen, 0sier Voldgciile 10, DK- 1350 Kahenhnon K , Denmurk; received 2nd May 1995: urcepied 2211d November 1995.

BOREAS

Stable isotope studies of lake sediments have provided important contributions to our current understanding of Late Weichselian and early Holocene climatic de- velopment. Oxygen isotope records from limnic car- bonates exhibit a rapid response to climatic changes comparable in sensitivity to that of vegetation records, such as from pollen and plant macrofossil data (Lotter et ul. 1992; Goslar et (11. 1993; Wohlfarth et al. 1994). Carbon isotope variations are potential - but only partly understood - markers of local environmental changes (Buchardt & Fritz 1980; McKenzie 1985).

The best-known stable isotope records obtained from late Weichselian sediments are those from the alpine regions of Central Europe (e.g. Eicher & Siegenthaler 1976, 1983). These and other records demonstrate a characteristic depletion of ''0 in limnic carbonates deposited during the Younger Dryas sta- dial as compared to the strata below and above. The depletion is interpreted to reflect a significant decrease in "0-concentration of precipitation, and thereby in mean annual air temperature. This hypothesis is sup- ported by close correspondence with vegetation data. Similar changes in oxygen isotope records from inland ice of Greenland (Siegenthaler et al. 1984; Lotter et a1. 1992) and Peru (Thompson et al. 1995) thus indicate a widespread (global?) significance of the Younger Dryas climatic anomaly.

A strongly divergent oxygen isotope record from a Late Weichselian lake at Grrenge in southeast Den- mark was presented by Kolstrup & Buchardt (1982)

to complement a detailed study of the vegetation history of the region. Here, the Younger Dryas stadial is represented by a distinct enrichment of I8O in sedimentary carbonates in direct contradiction to re- sults from Central Europe (e.g. Eicher 1987), Poland (Rozanski 1987; Rozanski et al. 1988; Goslar et al. 1993; Hammarlund 1994), Germany (Pachur & Riiper 1984) and the Baltic Region (Punning et al. 1984). It was suggested that a climatically induced increase in soil erosion with subsequent deposition of al- lochthonous carbonate particles in the lake was the cause of the oxygen isotope anomaly at Graenge, although no evidence of redeposited carbonates was produced.

To clarify the cause of this divergence, a follow-up study was carried out on a new sequence from Grlenge including mineralogic, petrographic and isotope geo- chemical investigations of the limnic carbonates (cf. Kelts & Talbot 1990). Also included in the study were isotopic data from the original sequence at Grlenge (Kolstrup & Buchardt 1982), not formerly published. These include carbon isotope data from sedimen- tary carbonates and carbon and oxygen isotope data from shells of gastropods and pelecypods. Carbonates formed by shell-bearing aquatic organisms are inde- pendent of contamination by detrital components and have been used in several studies to separate and estimate different climatic parameters (e.g. Stuiver 1970; Fritz & Poplawski 1974; Fritz et al. 1975; Lister 1988, 1989; von Grafenstein et al. 1992, 1994). The

10 Dan Hammarlund and Bjmn Buchardt BOREAS 25 (1996)

1957) with elevations of 5-15 m a.s.1. The youngest till unit in the region has a carbonate content in the range of 30 -35% (Binzer 1973). The deglaciation chronology is not known in detail, although the last active ice most probably left the area well before 14 000 BP (Lagerlund & Houmark-Nielsen 1993). However, local dead-ice bodies may have persisted for a considerable time. In the studied area, the Baltic Sea presumably was lower than at present during the entire period since the Late Weichselian deglaciation (Bjorck 1995).

The bedrock in the region consists of Upper Creta- ceous (Maastrichtian) chalk. Depth to the chalk surface is generally less than 25 m (Binzer & Stockmarr 1994). As revealed by local water wells (Storstr~ms Amt unpubl. data), an east-west directed, km-sized, ridge- like chalk feature reaching sea-level altitude coincides with the Grznge depression. Parts of the chalk ridge were encountered c. 2 m below the peat surface during reconnaissance coring at the study site (Fig. 1C; sam- pling point c). This feature may be either a glacially dislocated block or related to deeper structures in the chalk.

The Quaternary stratigraphy of the Graenge basin was first studied by Andersen & M d e r ( 1946). In the central part of the deposit, 1.5-3.3 m of calcareous gyttja was recorded, overlain by c. 3 m of peat. The 2.5 m thick sediment sequence studied by Kolstrup & Buchardt ( 1982; referred to as core A in the present study) consists of 0.02 m of sand at the base of the section, 2.37 m of calcareous gyttja (lake marl) with varying clay and silt content, and 0.12 m of gyttja overlain by peat (Fig. 2). Size and shape of the former lake at Graenge can be assumed to correspond roughly to the present distribu- tion of peat (Fig. lC), which implies a maximum water depth of c. 8 m. Further description of the bog and its exploration history is given by Kolstrup & Buchardt (1982).

Methods Most of the isotopic data presented here ( "C/"C- and '80/'60-ratios of sedimentary carbonates and molluscs) were obtained from the sequence sampled in 1979 (core A) by Kolstrup & Buchardt (1982). To perform addi- tional mineralogic, petrographic and isotopic analyses, a second core was obtained in 1994 (core B). Core B was collected roughly 100 m north of core A, closer to the margin of the peat deposit (Fig. 1C).

Core B was retrieved with a 1 m long Russian peat sampler, 10 cm in diameter, and the sampled sequence was described in the laboratory. Total carbon content was determined by combustion at 1150°C in pure oxygen with subsequent detection of carbon dioxide by infrared absorption photometry in an Eltra-Metalyt unit. Carbonate content was measured by sodium hydroxide titration to neutral pH after dissolution of 0.5 g sample in 0.5 M hydrochloride acid and boihng for

20 minutes. Total organic carbon content was calculated from the difference between total carbon and carbonate carbon content. The organic content of core A (Kolstrup & Buchardt 1982) was determined by loss on ignition at 550°C and is not directly comparable to the organic carbon content of core B (Fig. 2). Pollen analyses were performed at eight levels of core B and c. 300 pollen grains were counted at each level. The mineralogic composition of the sediment samples from Core B was determined by X-ray diffraction using copper-alpha radiation in a Phillips PM 1050 instru- ment. Only semiquantitative evaluations were at- tempted. Scanning electron microscopy (SEM) was applied to investigate the general morphology of the carbonate grains.

For isotopic analysis of sedimentary carbonates, 1 cm thick sediment samples from core B as well as from core A (cf. Kolstrup & Buchardt 1982) were freeze-dried and gently passed through a 150pm sieve. Contiguous sediment sections, 5 cm thick, from core A were washed through a 200 pm sieve and samples of shells and shell fragments of Pisidium sp. and Lymnuea sp. were se- lected. All carbonate samples were analysed for stable carbon and oxygen isotope composition following stan- dard preparation procedures (McCrea 1950). The cryo- genically purified carbon dioxide was analysed in li

Finnigan MAT 250 gas-inlet mass spectrometer at the stable isotope laboratory, University of Copenhagen. The stable carbon and oxygen isotope results are expres- sed as 6-values (per mille deviations from the interna- tional PDB standard; Craig 1957), with an analytical reproducibility of f 0.03%, and k 0.07';/,, respectively.

Sediment description, chronology and correlation The lithostratigraphic subdivision of core B is compiled in Table 1. Lithology, pollen composition and stable isotope data are shown in Fig. 2, in addition to corresponding records from core A (Kolstrup & Buchardt 1982).

Pollen data from core A have been used to establish a chronology of the upper part of the sequence by means of correlation with other pollen records from Denmark, southern Sweden and northern Germany. The climatic cooling of the Younger Dryas stadia1 (c.. 11 000- 10 200 BP; Berglund rt ul. 1994) is evident mainly from decreasing Betulu pollen frequencies and increasing values of Cyperaceae, Poaceue and several other herbs in the lower and middle parts of pollen zone I11 (Fig. 2; Kolstrup & Buchardt 1982). The Allerd-Younger Dryas transition (pollen zone II/III) is encountered between 3.65 and 3.60 m. The Younger Dryas-Prebo- real transition zone (Bjorck 1981) occurs between 3.25 and 3.12 m and is represented by several distinct changes in pollen data, corresponding to the uppermost part of pollen zone I11 (Iversen 1954). Radiocarbon dating of the Younger Dryas-Preboreal transition

BOREAS 25 (1996) Aller0d and Younger Dryas environments, Denmark 1 1

12 Dan Hammarlund and Bjmn Buchardt BOREAS 25 (1996)

Tuhle i. Lithostratigraphic description of core B. ~ ~~ ~

Unit Depth (m) Description

0.00 - 1.95

2.1 1-2.15 2.15-2.22 2.22-2.50 2.50 ~ 2.69

2.69-2.84

2.84-2.92 2.92-3.24 3.24- 3.46

Peat, not investigated

Greyish brown sandy gyttja silt, laminated. Rather gradual lower boundary. Brownish grey silty gyttja sand, laminated. Gradual lower boundary. Brownish grey sandy gyttja silt, laminated. Rather gradual lower boundary. Slightly brownish grey silty calcareous gyttja with occasional mollusc shells. faintly laminated. Rather gradual lower boundary. Grey slightly silty calcareous gyttja with occasional mollusc shells, faintly laminated. Gradual lower boundary. Dark grey slightly silty calcareous gyttja. Rather gradual lower boundary. Whitish grey calcareous gyttja with mollusc shells, faintly laminated. Gradual lower boundary. Grey calcareous gyttja with mollusc shells, faintly laminated. Gradual lower boundary.

10 1.95 2.11 Dark brown gyttja with occasional mollusc shells. Rather sharp lower boundary. 9 8 7 6

5

4 3 2 1 3.46 3.49 Dark grey silty calcareous gyttja.

zone (the upper part of the Younger Dryas Chrono- zone ~ ~ ' Y I S U Mangerud et al. 1974) is complicated by the existence of a I4C plateau around 10 000 BP (Ammann & Lotter 1989; Becker et al. 1991). The radiocarbon date at 3.13-3.10 m of core A (Kolstrup & Buchardt 1982), obtained on carbonate-free gyttja (K-3404; 10 080 k 80 BP), is probably within the I4C plateau.

The chronology of the lower and middle parts of core A is more difficult to assess. The bulk organic dates obtained by Kolstrup & Buchardt (1982) are all too old as a result of incorporation of old carbonate carbon, and the pollen record exhibits no distinct stratigraphic changes. A pollen diagram from a nearby core a t Grznge (Andersen & Maller 1946; Andersen 1980) can be confidently correlated with the record presented by Kolstrup & Buchardt ( 1982), although different chronologic interpretations were made. An- dersen (1980) interpreted the lowermost part of the sequence as deposited during the Balling Chronozone, whereas Kolstrup & Buchardt (1982) claimed that the oldest sediments represent the pollen zone boundary Tc/ll (Older Dryas-Allerad) as defined by Iversen ( 1954). The age of the oldest sediments considered here remains unclear due to erroneous bulk sediment radio- carbon dates (Kolstrup & Buchardt 1982) and the absence of suitable terrestrial macrofossils for dating by accelerator mass spectrometry.

The chronology of core B has been established by means of correlation with core A (carbon content,

pollen stratigraphy, isotopic composition; Fig. 2). The studied sequence has been divided into three oxygen isotope zones (01-0,,1) following Kolstrup & Buchardt (1982). Pollen and oxygen isotope results indicate that units 4-9 (2.92-2.1 1 m; Table 1 ) belong to the lower and middle parts of pollen zone 111 (Younger Dryas stadial; Table 2) and oxygen isotope zones 0,1-0111 respectively (Kolstrup & Buchardt 1982), whereas the high content of organic material and insignificant carbonate content of unit 10 (2.1 I - 1.95 m) suggests it to be correlated with the upper- most part of core A (pollen zone IV). This means that the uppermost part of pollen zone 111, corresponding to oxygen isotope zone 0," (Younger Dryas-Prebo- real transition zone), is represented by a hiatus in core B. This may be caused by a water-level lowering initiated at the end of the Younger Dryas stadial (Berglund & Digerfeldt 1970). Units 2-3 (3.46- 2.92 m) of core B most probably represent the upper- most parts of pollen zone IT (Allerad interstadial; Table 2) and oxygen isotope zone 0, respectively. The lower part of the sequence in core A appears not to be represented in core B.

Carbonate mineralogy and morphology X-ray diffraction indicates that the carbonate phase in the lake sediments is low-magnesian calcite. Neither

7ahle 2. Pollen analysis results expressed as percentages of relevant taxa, total pollen sum including Equiseturn (cf. Kolstrup & Buchardt 1982) and total pollen concentration (10' grains/cm3) from eight levels in core B.

Depth Pinus Rerulu Suhx Juniperus Empetrum Arterni~iu Poaceae Cyperaceae Equisetum Other Sum Couc

2.08 7.8 9.6 2.6 3.1 2.13 27 25 2.9 1.6 2 4 8 16 28 3.0 1.0 2.53 28 26 3.7 0.7 2.68 21 30 1.6 1.0 2.83 22 45 3.1 1.2 2.93 26 53 1.3 0.7 3.38 20 57 3.6 0.7

1.3 0 0 0.1 1.6 0.6 0.3 0 -

0 1.3 1 .0 I .0 1.3 0.6 0.3 0.7

~

2.6

9.1 8.4 6.9 4.0 I .7 I .7

10 6.6

25 33 25 28 16 15 13

63 0.7 0 0 0 0 0 0

~

3.6 6.5 8.0 5.4 7.5 7.4 2.0 3.6

83 I 308 299 299 306 324 300 302

144 20.4 15.0 23.6

36.7 11.4 35.0

I I14

BOREAS 25 (1996) A l l e r ~ ~ d and Younger Dryas environments, Denmurk 13

Fig. 3. Scanning electron micrographs of sediment samples from core B and chalk from point c (Fig. 1C). 0 A. Chalk with coccolith fragmcnts from sampling point c. U B. Carbonate aggregate from sample 3.38 m in core B (Allerad interstadial). D C. Quartz grains and carbonate particles from sample 2.33 m in core B (Younger Dryas stadial). Note coccolith specimen in the centre of the picture. 0 D. Abraded carbonate particles from sample 2.33 m in core B (Younger Dryas stadia]).

aragonite nor siderite was observed. Carbonates from oxygen isotope zone 0, (Allerad interstadial) are com- posed of highly irregular, 10-100 pm large aggregates of euhedral crystals in the range of 1-5 pm (Fig. 3B). Carbonates from oxygen isotope zone 0,,, (Younger Dryas stadial; Figs. 3C and 3D) are to a larger extent composed of regular calcite particles, compared to zone 0, , with a morphology resembling Upper Creta- ceous chalk (Fig. 3A) found at sampling point c (Fig. 1C). Zone 01,, also contains abundant grains of quartz and feldspar. Moreover, well-preserved speci- mens of four different Late Cretaceous coccolith taxa have been identified in the Younger Dryas sediments (Fig. 4), clearly pointing to an allochthonous compo- nent in these deposits.

Interpretation of the oxygen isotope records The oxygen isotope records obtained from core A (Fig. 5) include 45 bulk sediment samples (6”OSed,) varying from -9%, to -5%, (Table 31, 16 samples of Pisidium sp. (6180pis,) ranging from -7.5%, to -5.5%,, and 15 samples of Lymnueu sp. (6’sOL,,,) ranging from -6.5%, to -4%, (Table 4). A record of 25 bulk sediment samples from core B spans from -8.5%, to -4%, (Fig. 2, Table 5) .

The sediment data exhibit a persistent depletion of I8O with time in oxygen isotope zone O,, correspond- ing to the pre-Younger Dryas part of the sequence (pollen zone 11). The depletion of c. 2%, was ascribed to a general decline in mean annual temperature by

14 Dan Hammarlund and Bjmw Buchardt BOREAS 25 ( 1996)

Fig. 4. Scanning electron micrographs of reworked Cretaceous coccoliths from sediments of core B deposited during the Younger Dryas stadial. 11 A. Bisrurum sp. (Cretaceous). B. Arklzangelskielltr sp. (Upper Maastrichtian). U C. Crzbrosphnere//rr ermbrrxii ( Albian to Upper Maastrichtian). 0 D. Kumptnerius mc~gnific~uu (Turonian to IJpper Maastrichtian).

Kolstrup & Buchardt (1982). However, a positive correlation between 6 "0 of limnic carbonates and mean annual air temperature (e.g. Siegenthaler & Eicher 1986) is probably not always valid, given the complex relation between oxygen isotope composition of limnic carbonates and climate change. The two mutually counteracting fractionation processcs involved during atmospheric moisture transport (Dansgaard 1964; Siegenthaler & Oeschger 1980) and carbonate precipita- tion (Epstein rt ul. 1953; Craig 1965) respectively, are related to difierent climatic parameters and may thus operate independently. Furthermore, local hydrological conditions, such as the inflow/evaporation balance, play an important role in determining the l80/I6O ratio of lake water ultimately recorded in lacustrine carbon- ates. 'XO-depletion similar to the sediment data were

recorded in the mollusc data of zone Ol. Together, these records suggest that either a gradual depletion of '*O in lake water or a gradual increase in lake-water temperature, or possibly a combination of these pro- cesses, took place during the Late Weichselian inter- stadial. A decrease in mean annual air temperature as reflected by a successive deplction of '*O in precipnta- tion agrees with insect data from the British Isles (Coope 1977; Atkinson et al. 1987), although it is not supported by the local pollen data (Kolstrup & Buchardt 1982). An increase in lake-water tempera- ture during the warm season of the year (the main period of limnic carbonate precipitation) is contra- dicted by palaeoecological data from southern Sweden (Lemdahl 1988; Hammarlund & Keen 1994; Berglund rt ul. 1994), which indicate a successive summer cool-

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16 Dan Hammarlund and Bjwn Buchardt BOREAS 25 (1996)

Table 3. Stable carbon and oxygen isotope data obtained on sedi- mentary carbonates from core A.

Depth (m) d '3Csed.

3.15 +0.24 3.20 + 1.88 3.25 + 2.22 3.30 +2.20 3.35 +2.13 3.40 + 2.25 3.45 + 2.50 3.50 + 2.67 3.55 + 1.45 3.60 + 0.68 3.65 t 0.49 3.70 +0.35 3.75 +0.11 3.80 t0 .16 3.85 t0 .19 3.90 t 0.48 4.00 +0.1 I 4.05 +0.11 4.10 + 0.90 4.15 + 1.12 4.20 + 1.06 4.25 + 1.53 4.30 + 2.06 4.35 +2.13 4.40 + 1.54 4.45 + 2.00 4.50 + 1.89 4.55 + 1.75 4.60 + 2.26 4.65 + 2.08 4.70 + 2.44 4.15 $3.16 4.80 +2.53 4.85 + 2.83 4.90 + 2.98 4.95 +2.82 5.00 t 2.94 5.05 +2.65 5.10 t 2.84 5.15 t 2.73 5.20 f2.91 5.25 + 2.79 5.30 +2.84 5.35 +3.16 5.40 + 3.42 5.45 + 3.46

fi lxose'l

n.d. -6.98 -6.64 - 5.42 - 5.45 - 5.37 - 5.37 - 5.42 - 6.29 - 7.80 -8.88 - 8.44 -8.74 -8.72 - 8.84 -9.00 -8.63 - 8.67 -8.22 - 8.35 - 8.66 -8.14 - 8.08 - 7.93 -8.34 -8.21 - 8.45 -- 8.10 -7.90 -8.19 -8.09 - 7.59 - 7.54 - 7.48 - 7.40 -7.30 -6.91 -7.37 -7.27 - 7.55 - 7.03 -7.24 - 6.98 -6.90 --- 6.6 I - 6.42

Table 4. Stable carbon and oxygen isotope data obtained on mol- lusc shells from core A.

Depth (m) d13C,,,,

3.10 -3.20 - 1.45 3.20 - 3.30 -0.60 3.30 3.40 - 1.54 3.40-3.50 - 3.32 3.50-3.60 -4.03 3.60-3.70 -4.84 3.70-3.80 -4.64 3.80&3.90 - 3.97 3.90-4.00 - 4.06 4.00-4.10 -4.41 4.10-4.20 - 2.91 4.20 - 4.30 - 3.26 4.30 4.40 n.d. 4.40-4.50 n.d. 4.50 -4.60 -3.43 4.60- 4.70 -2.22 4.70-4.80 - 2.29 4.90 - 1.49 5.00 n.d.

d'XO,,, 6"C,,,

- 6.44 -4.20 -7.19 -2.21 - 7.44 -2.10 - 7.04 n.d. - 6.95 -6.27 - 7.29 n.d. -7.59 - 5.03

-7.17 n.d. -7.01 -5.21 -6.71 -4.49 -6.56 -4.88 n.d. -4.10 n.d. -4.35 - 6.79 -4.20 -6.30 - 3.94 -6.54 -3.08 - 5.65 -3.25 n.d. -2.68

-7.10 -4.15

-4.10 -5.91

n.d.

n.d.

-5.72

-6.25

- 6.42 -6.08 n.d -6.43 -6.06 - 5.83 - 5.68 -5.55 -6.04 -5.91 -4.76 - 5.93 -4.93

Unit: x0 (PDB). Pis. = Pisidium sp., Lym. = Lytnnaea sp., n.d. = no determination.

continental ice-sheets during the Late Weichselian in- terstadial (meltwater peak 1A; Fairbanks 1989).

The difference between 6 lXOSed and corresponding mollusc data (6180,,, and 6180Ly,n.) in zone 0, proba- bly reflects differences in lake-water temperature re- lated to water depth. The shells, which are enriched in

Tublr 5. Stable carbon and oxygen isotope data obtained on scdi- mentary carbonates from core R.

~

Depth (m)

Unit: '& (PDB). n.d. = no determination.

ing during the later part of the Allemd interstadial. A relative decrease in evaporative enrichment of I8O in the lake water during the later part of the Allerard interstadial may have contributed to the recorded changcs in 6 '*0 (Hammarlund & Keen 1994; Ham- marlund & Lemdahl 1994). Such an effect could be linked to decreased wind action due to increased forest cover as indicated in pollen data (Kolstrup & Buchardt 1982; cf. carbon isotope data below). An additional explanation may be a global depletion of '*O in the surface layers of the oceans (the precipita- tion moisture sourcej as a response to rapid melting of "I

2.13 + 1.54 - 4.56 2.155 + 1.89 -4.90 2.18 + I .71 -4.71 2.205 + 1.27 -5.36 2.23 t 1 . 3 1 -4.17 2.255 + 1.62 - 4.66 2.28 + 1.52 - 4.54 2.33 +2.10 - 5.24 2.38 + 1.92 -4.86 2.43 + 1.66 -4.37 2.48 + I 43 -4.00 2.53 +2.22 -4.76 2.58 +2.73 - 5.29 2.63 + 2 86 -5.53 2.68 + 3.01 - 5.13 2.705 + 3.27 - 5.56 2.755 +2.68 - 5.44 2.805 + 1.96 - 5.96 2.88 +0.87 - 7.99 2.93 11 .19 - 8.48 2.98 t 1.72 -8.30 3.08 +1.37 -8.53 3.18 + 1.52 - 8.40 3.28 +2.80 7.91 3.38 + 3.29 -7.54

Unit: %,% (PDB).

18 Dan Hammarlund and Bjern Buchardt BOREAS 25 (1996)

Tubk 6. Stable carbon and oxygen isotope data obtained on Creta- ceous chalk sampled at point c (Fig. I ) .

Depth (m) 6 '3C fi'RO

3.05 + 1.53 - 1.37 3.0s + 1.48 -1.33 3.50 + 1.45 - 1.29 3.90 + 1.55 - 1.09 3.90 + 1.56 - 1.0s

Unit: xo (PDB).

posits, which contain 30-35'1/0 chalk (Binzer 1973). On the assumption that c. 40% of the sediment carbonate content consists of allochthonous material (cf. Kolstrup & Buchardt 1982) with an oxygen isotope composition ranging from -2x0 to - I%,, the endo- genic carbonate component must have remained largely unchanged in terms of 6"0 as compared to late Aller~d levels ( c . -9%,), to produce the observed 6 lXOSed data. A comparison with the mollusc-shell 6 "0 records, representing pure endogenic carbonates (see below), supports this mass-balance estimate. S"OSed data from core B (Fig. 2) are in general agreement with the interpretations made above. The pre-Y ounger Dryas values closely correspond to data from core A, whereas the Younger Dryas I80-enrich- ment is even more pronounced than in core A. This is likely due to a greater proportion of allochthonous vs. endogenic carbonate in core B compared to core A. A relatively higher amount of minerogenic material (Fig. 2) supports this interpretation and is consistent with the more marginal position of core B.

No significant changes in 6 IXOp,, and S1'OLym were recorded across the Aller~d-Younger Dryas boundary (zone O1/Ol1). However, the Younger Dryas cooling, which is clearly reflected in the pollen data (Kolstrup & Buchardt 1982) and indirectly also in diXOScd data, may be concealed in the mollusc records. The effect of a regional depletion of "0 in precipitation caused by a decrease in mean annual air temperaure as indicated in pollen data (Kolstrup & Buchardt 1982), may be offset by an enrichment of "0 in mollusc shells rebated to a decrease in lake- water temperature during the shell growing period of the year. Earlier studies have identified the growth period of Lymnaeu sp. to be restricted to water tem- peratures above a certain threshold (Buchardt & Fritz 1980), and Pisidium sp. probably also restricts its shell formation to the warmer period of the year. The Late Weichselian lake at Graenge was shallow and suscepti- ble to climatically induced changes in lake-water tem- perature (cf. Stuiver 1968). Alternatively, a decrease in inflow/evaporation ratio, perhaps due to drier and/or windier conditions during the Younger Dryas stadial (cf. discussion below), could have resulted in a lake- water "0-enrichment that counteracted a possible "0-depletion of precipitation.

The enrichment of I'O in mollusc shells within the Younger Dryas- Preboreal transition zone (zone 0,) probably reflects "0-enrichment of precipitation fol- lowing climatic warming (Dansgaard 1964) as indi- cated by distinct changes in lithology and pollen composition (Kolstrup & Buchardt 1982). The "0-en- richment may correspond to similar trends in marl and mollusc data recorded at the end of the Younger Dryas stadial in Central Europe (e.g. Eicher 1987; von Grafenstein et al. 1994).

Interpretation of the carbon isotope records The carbon isotope records obtained from core A (Fig. 5) include 46 bulk sediment samples (6 I T S c d ) varying from +O%, to +3.5%, (Table 3), 16 samples of Pisidium sp. (6I3Cp,,) ranging from - 5%" to -0.5%,, and 15 samples of Lymnaea sp. (6 "C, yfn ) ranging from -6.5%, to -2x0 (Table 4). The studied sequence has been divided into six carbon isotope zones (C,-Cvl; Fig. 5). A record of 25 bulk sediment samples from core B spans from + I%, to +3.57{(, (Fig. 2, Table 5).

In core A, 6I3Csed exhibits relatively stable values around + 3%,, in the lower part of the sequence (zone C,), perhaps as a result of a high degree of exchange between atmospheric carbon dioxide (c. -7%,, cf. Deines 1980) and dissolved inorganic carbon (DIC). Dissolution of carbonates in the catchment of the lake may also have contributed. A subsequent depletion of 13C in zones C,,-C,,, to values close to +O%,, may reflect an increased contribution to the DIC pool of

C-depleted carbon dioxide from oxidation of organic material (c. -26%,, cf. Deines 1980). The depletion is initiated at L . 4.80 m, slightly after the first appearancc of mollusc remains. The introduction of molluscs in thc lake probably indicates the establishment of aquatic vegetation (Keen rt al. 1984, 1988; Hammar- lund & Keen 1994) which provides a source of rela- tively "C-depleted DIC. Similar trends were recorded in 6C,,, and 613CLym in zones C,,- C,,,. The 'T-de- pletion of DIC may be a result of increased stagnancy in the lake water due to a more dense vegetation cover around the lake. The pollen record (Kolstrup & Buchardt 1982) suggests the presence of birch in the area, and possibly also pine during the later part of the Al le r~d interstadial (corresponding to zone CII1). Relatively lower values of 6l3CP,, and 6I3CLym, as compared to 613CScd , reflect proximity to decomposing organic material at or close to the bottom of the ldke.

The Younger Dryas stadial (zones Clv-Cv) is re- presented by rapid and significant enrichment of ')C in all three parameters. The 6I3CSed record is partly influenced by redeposited chalk with 6 "C values around + 1.5%, (Fig. 6; Table 6). However, the recorded enrichment exceeds this level, which indicates a considerable enrichment of I3C in the endogenic

13

BOREAS 25 (1996) Allewd and Younger Dryus environments, Denmark 19

carbonate fraction. This hypothesis is supported by strong enrichment of I3C in mollusc shells. This is in all likelihood due to increased incorporation of atmo- spheric carbon dioxide during the Younger Dryas stadial, which may reflect a change from generally stagnant to agitated conditions due to disappearance of the forest vegetation. However, the I3C-enrichment in DIC may to some extent be related to increased dissolution of carbonates. SI3CSed data from the up- permost part of core B (Fig. 2) in the range from + 1%, to +2%,, may reflect a relatively stronger influ- ence of reworked carbonates than in core A (cf. oxygen isotope data above). Within the Younger Dryas-Preboreal transition zone (Fig. 5; zone C,,), pronounced depletion of I3C suggests a return to conditions resembling those recorded in zone CIII. The rapid re-establishment of terrestrial vegetation indi- cated in pollen data (Kolstrup & Buchardt 1982) probably provided an increased supply of I3C-depleted DIC from oxidation of organic material (Hammar- lund 1993). Furthermore, forest vegetation may have created a wind-shield around the relatively narrow lake, which increased the stagnancy of the lake water. A lake-water lowering at this stage, indicated by the hiatus in core B, may also have contributed to this development.

The general changes in carbon isotope composition described above closely correspond to results from the Amose basin in Denmark (Noe-Nygaard 1995), as well as to 6I3C records obtained on bulk organic material from Late Weichselian sequences in south- ernmost Sweden (Hammarlund 1994; Hammarlund & Keen 1994; Hammarlund & Lemdahl 1994), and the British Isles (Harkness & Walker 1991). These similar- ities demonstrate a similar local response of limnic environmental conditions to regional climatic changes in northwest Europe during the Late Weichselian.

Discussion The general implications of the composite isotope record from Grange are that well-documented cli- matic changes in northwest Europe during the Late Weichselian are clearly expressed in the isotope geo- chemistry of the lake. During the Allersd interstadial, relatively favourable climatic conditions prevailed. Sedimentation of calcite-dominated lake marl indi- cates a high limnic productivity. Stable, or even slighl ly increasing, differences between isotopic com- position of sedimentary carbonates and shells (cf. Hammarlund & Lemdahl 1994) imply a certain ther- mal and chemical stratification of the lake. The lake water seems to have been particularly stagnant during the later part of the Allersd interstadial, probably as a result of the establishment of forest vegetation around the lake.

The effects of the rapid Younger Dryas cooling (e.g. Mangerud 1980; Ruddiman & McIntyre 1981; Johnsen et al. 1992; Alley et al. 1993) are not only reflected in the lithology and pollen stratigraphy of the Grlenge lake (Kolstrup & Buchardt 1982), but also as considerable variations in isotope composition. Car- bon and oxygen isotope data obtained from the sedi- mentary carbonates in combination with observations of reworked Upper Cretaceous coccoliths record an increased input of detrital carbonates to the lake. This is interpreted as an indirect effect of climatic change. The Younger Dryas cooling probably induced periglacial activity and increased slope wash within the lake catchment (Berglund et al. 1994). In contrast, the mollusc carbon isotope data provide more direct information on changes of the limnic environment related to the Younger Dryas cooling. The relative enrichment of 13C in mollusc shells probably reflects a change from stagnant to more agitated conditions, where atmospheric carbon dioxide became increas- ingly important as a source of dissolved inorganic carbon in the lake. The most likely cause of this change in the lacustrine environment is deforestation of the area as indicated by pollen data (Kolstrup & Buchardt 1982), resulting in increased wind impact. Increased wind action during the Younger Dryas stadial is also evident from deposits of eolian sand in Denmark and southern Sweden (Kolstrup & Jsrgensen 1979; Berglund & Rapp 1988). Stable iso- tope studies from lakes in southern Germany (von Grafenstein et ul. 1994) suggest reductions in thermal stratification of lakes during the Younger Dryas sta- dial.

The Younger Dryas climatic oscillation thus had a considerable influence on several isotopic and other stratigraphic parameters at Grange. To some extent the changes are metachronous and thereby reveal how different isotopic parameters are coupled to environ- mental changes in and around the lake. The Allersd- Younger Dryas boundary is clearly reflected in pollen data, mainly as a sudden decrease in Betula pollen frequencies at 3.65-3.60 m (Fig. 2). This indicates the disappearance of the forest vegetation that was present during the later part of the Allersd interstadial (Kolstrup & Buchardt 1982), rapidly followed by soil destruction and increased erosive input of minerogenic material and reworked carbonates to the lake. The last-mentioned process is reflected by the enrichment of I80 in sedimentary carbonates, more or less syn- chronous with the change in terrestrial vegetation. However, the lake itself may have responded more slowly as indicated by a slight delay in the enrichment of I3C in the mollusc records.

The climatic development at the end of the Younger Dryas stadial seems to have induced a series of clearly separated environmental changes in the area, indi- cated by the following stratigraphic changes; (1) The decrease in minerogenic content of the sediments at

20 Dan Hummarlund and BjGrn Ruchurdt BOREAS 25 (1996)

3.30-3.20m, accompanied by a depletion of "0 in sedimentary carbonates, reflects a decrease in soil erosion. The first indication of climatic warming and soil stabilization in the pollen record is an increase in the Empetrum pollen frequency at 3.30-3.25 m, fol- lowed at 3.25-3.20 m by distinct increases in Junipe- rus and Filipendula, and decreasing frequencies of Artenzisiu and several other herbs (Kolstrup & Buchardt 1982). These pronounced biostratigraphic changes most probably correspond to the initiation of the Younger Dryas-Preboreai transition zone as iden- tified in southern Sweden by Bjorck (1981). (2) The pronounced depletion of I3C in limnic carbonates at 3.20-3.15 m reflects an increase in organic productiv- ity of the lake, as illustrated by precipitation of pure lake marl, and increasing frequencies of pollen from several aquatic and telmatic plants (Kolstrup & Buchardt 1982). The increased input of I3C-depleted carbon dioxide to the lake water mainly originated from increased decomposition of organic matter in the lake and from soil processes (Hikansson 1985; Ham- marlund 1993, 1994). Roughly coinciding with the onset of pure lake-marl sedimentation (c. 3.20 m), enrichment of '*O in mollusc shells may reflect a general change in the atmospheric circulation pattern giving rise to an enrichment of "0 in precipitation and lake water. This event probably corresponds to similar changes in oxygen isotope records from south- ern Germany (von Grafenstein el al. 1992, 1994). ( 3 ) The change towards pure organic sedimentation at 3.15- 3.10 m marks the response of the lacustrine ecosystem to the onset of warmer conditions charac- terizing the early Holocene. This shift corresponds to the end of pollen zone I11 according to Iversen (1954) and to the end of the Younger Dryas-Preboreal transition zone as defined by Bjorck ( I98 I ) . The suc- cession described above (3.30-3.10 m) presumably took place during the time-span of c. 10200- 10 000 BP (Berglund et al. 1994).

Summary and conclusions Two shallow cores from a Late Weichselian lake deposit at Gramge, southeast Denmark, have been investigated by means of composite geochemical analyses. The study included mineralogic composition, sedimentary texture, carbonate content and organic

matic changes with time. The spatial variations are reflected in carbon and oxygen isotope differences between lake marl and mollusc shells formed at the same time, but at different depths in the lake. These data are related to differences in temperature and bicarbonate 6 ''C composition at different depths and thereby to stratification and stagnancy of the lake. Thus, the composite isotope records in combination with vegetational data reveal distinct differences in the limnic environment during the Allernd interstadial and the Younger Dryas stadial respectively. The tem- poral changes are reflected in most of the mineralogic and geochemical parameters studied. Of these, how- ever, the oxygen isotope composition of sedimentary carbonates appears to respond most rapidly to cli- matic fluctuations during the Younger Dryas stadial. This parameter is complexly related to changes in temperature regime, hydrology and soil stability. As demonstrated by means of mineralogic and morpho- logic studies, a considerable portion of the carbonates deposited during the Younger Dryas stadial were not of endogenic origin. These results emphasize the im- portance of sedimentological investigations as a com- plement to stable isotope records obtained from sedimentary carbonates.

None of the isotopic records provide a basis for calculation of absolute palaeotemperatures. The com- plexity of the hydrological system in the Graenge lake and possible oxygen isotope disequilibrium precipita- tion of calcite precludes this possibility. However, stable isotope studies of lacustrine carbonates provide a precise record of temporal changes in environrncntal parameters. Additionally, detailed studies of isotopic differences between co-precipitated calcareous phases may yield valuable information about local environ- mental conditions of palaeolakes.

Acknowkxfgemenfs. - Our sincere thanks arc extended to J. K . Nielsen and I . Rucka, who assisted in the fieldwork, E. Kolstrup, who provided help in locating old sampling points, E. Thornsen. who kindly identified the Cretaceous coccoliths, S. Bjdrck and R . Wolfe for providing valuable comments on the manuscript, and to B. Warming and J. Fuglsang for technical assistance. Fig. I was prepared by R. Madsen. Suggcstions by the referees, E. Janscn and R . Wohlfarth, improved the quality of the final product. 'This study was partly supported by Grant No. 11-0960 from the Danish National Science Research Council.

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