Climate oscillations and tephrochronology in eastern middle Sweden during the last glacial–interglacial transition

JOURNAL OF QUATERNARY SCIENCE (1999) 14 (5) 399–410 CCC 0267-8179/99/050399–12$17.50Copyright 1999 John Wiley & Sons, Ltd.

Climate oscillations and tephrochronology ineastern middle Sweden during the lastglacial–interglacial transitionJONAS BJORCK* and STEFAN WASTEGÅRDStockholm University, Department of Quaternary Research, S-106 91 Stockholm, Sweden

Bjorck, J. and Wastegård, S. 1999. Climate oscillations and tephrochronology in eastern middle Sweden during the last glacial–interglacial transition.J. Quarternary Sci., Vol. 14, pp. 399–410. ISSN 0267-8179.

Received 13 August 1998; revised 20 January 1999; accepted 24 January 1999

ABSTRACT: Two sequences spanning the last glacial–interglacial transition in southern Ostergot-land, eastern middle Sweden have been investigated for high-resolution vegetation change andtephrochronology. Organic carbon and pollen analysis indicates that the Younger Dryas–Pre-boreal climatic transition was characterised by at least one well-defined oscillation or possiblytwo shorter climatic oscillations. The Vedde Ash (ca. 12 000 GRIP yr BP or ca. 10 300 14C yrBP) has been identified at both sites, significantly increasing the known distribution of thismarker horizon. In addition, a previously unrecorded rhyolitic tephra of Icelandic origin hasbeen identified at ca. 9000 14C yr BP. The expansion of Corylus into southern Ostergotland isestimated to be ca. 9400 14C yr BP. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: Preboreal oscillation; lacustrine records; Vedde Ash; last glacial–interglacial transition; expan-sion of Corylus.

Introduction

The last glacial–interglacial transition (15–10 k cal. yr BP)was a period characterised by a succession of rapid, extremeclimatic oscillations throughout the North Atlantic. The eventstratigraphy recently proposed by Bjorck et al. (1998b) ident-ifies a period of relatively warm temperature (formerly knownas the Bølling–Allerød) as GI-1 (14.7–12.65 k GRIP yr BP).At least three short-lived cold events are superimposed onan overall cooling trend through GI-1, which culminates inthe Younger Dryas Stadial (now GS-1), spanning the period12.65–11.5 k GRIP yr BP (Johnsen et al., 1992). Rapid warm-ing at 11.5 k GRIP yr BP marks the termination of GS-1 andthe onset of the Holocene Epoch (Dansgaard et al., 1989;Bjorck et al., 1996). Although this pattern has been describedin marine (Bond et al., 1993; Hafliason et al., 1995) andterrestrial sequences (Goslar et al., 1995; Lowe et al., 1995;Bjorck et al., 1996), the timing and extent of these events isstill hotly debated. Possibly the most detailed records havebeen obtained in sequences from shallow lakes, which seemto respond rapidly to temperature oscillations. By usingtemperature-sensitive proxies (chironomids, beetles, oxygenisotopes, etc.) in lake sediments, a number of short-lived

* Correspondence to: J. Bjorck, Stockholm University, Department of Quatern-ary Research, S-106 91 Stockholm, Sweden. Email: Jonas.BjorckKgeo.su.se

Contract grant sponsors: Swedish Natural Science Research Council (NFR)Contract grant sponsors: Swedish Society for Anthropology and Geography(SSAG)

climatic oscillations have been identified during this timespan and quantified from both sides of the North Atlantic(e.g. Levesque et al., 1993; Hammarlund and Lemdahl, 1994;Cwynar and Levesque, 1995; Lowe et al., 1995; Bjorck et al.,1996; Brooks et al., 1997).

The majority of chronologies developed for marine andterrestrial sequences spanning the last glacial–interglacialtransition are based upon radiocarbon dating. There aresignificant problems in the application of this method,because it can be shown that some of the basic assumptionson which the technique relies are frequently undermined(e.g. hard water error, reservoir effect). In addition, a radio-carbon year is not directly equivalent to a sidereal year,with a gradual divergence during the last glacial–interglacialtransition (Bard et al., 1993). Although the dendrocalibrationcurve extends only into the mid-GS-1, laminated 14C-datedsequences are now becoming available throughout the lastglacial–interglacial transition (Hughen et al., 1998; Kitagawaand van der Plicht, 1998). These new data sets includeseveral radiocarbon plateaux that make the calibration of14C ages problematic.

Alternative approaches are now being developed to corre-late sequences. These include the use of stable oxygen andcarbon isotopes (e.g. Ahlberg et al., 1996; Turney et al.,1997a) and the use of time-parallel marker horizons, e.g.tephra. The identification of the Vedde Ash and the Saksunar-vatn Ash in the GRIP ice-core (11 980 6 80 and 10 180 6 60GRIP yr BP respectively; Gronvold et al., 1995) providean independent chronological control and the potential forcorrelation of terrestrial and marine sequences (e.g. Haflia-son et al., 1995; Turney et al., 1997b; Wastegård et al., 1998)with high-resolution ice-core sequences in Greenland.

400 JOURNAL OF QUATERNARY SCIENCE

Here we present two high-resolution lacustrine sequencesspanning the last glacial–interglacial transition from easternmiddle Sweden. Our objectives were twofold. Firstly wewanted to see whether the early Preboreal oscillation (cf.Bjorck et al., 1996) that has been detected in southwesternSweden also could be confirmed in eastern middle Sweden.Secondly, we wanted to date the expansion of Corylus,which is an important early Holocene time marker in south-ern Scandinavia (cf. Digerfeldt, 1977; Gaillard et al., 1996).

Study area and previous work

The location of the area investigated is shown in Fig. 1.The entire bedrock is Precambrian crystalline material. Themajority of Quaternary deposits are varved clay in the valleybottoms, with till covering most of the areas above thehighest shoreline.

The highest shoreline was formed by the Baltic Ice Lakeand is located between ca. 130 m in the south and ca. 140 m

Figure 1 Study area in the county of Ostergotland, eastern middle Sweden. The investigated sites Hogstorpsmossen and Fågelmossen arebogs containing Late-glacial and early Holocene lacustrine sediments. Lake Striern, Lake Ammern and Lake Vån are sites investigated byGoransson (1977). Stockholm, the Baltic Sea, Lake Vattern, Lake Jarnlunden, Lake Åsunden, and the villages of Rimforsa and Kisa areshown for orientation. The highest shoreline is indicated by the dotted line.

Copyright 1999 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 14 (5) 399–410 (1999)

in the north (cf. Fig. 1; Agrell, 1976; Kristiansson, 1986;Bjorck, 1995). The altitudinal differences in different partsof Sweden reflect variations in the timing of deglaciationand isostatic uplift. Both sites are situated above the highestshoreline. Hogstorpsmossen is at an altitude of 220–225 m,whereas Fågelmossen is at 170–175 m.

The pollen record in the lowermost layers at Hogs-torpsmossen and Fågelmossen suggests that the area wasdeglaciated during the late Allerød (Jonas Bjorck, submitted).This also is supported by the clay-varve chronology for thearea (Kristiansson, 1986; Wohlfarth et al., 1998).

Goransson (1977) has previously studied Holocene veg-etational development in southern Ostergotland. The veg-etational history is reflected in the pollen diagrams fromthree lakes (Lake Stiern, Lake Vån and Lake Ammern; Fig. 1)and one bog in northeast Småland (Mabo mosse). Goranssonobtained several conventional 14C dates and determined therational limit of Alnus (A°) to ca. 8560 14C yr BP using peat,and to ca. 8700–8900 14C yr BP using gyttja. However, heobtained only one date on the rational limit of Corylus(10 220 6 105 14C yr BP), which he considered too old asa result of hardwater effects (cf. Olsson, 1991).

401LAST GLACIAL–INTERGLACIAL CLIMATE OSCILLATIONS, SWEDEN

Goransson also detected a short early Preboreal climaticdeterioration in the pollen diagram and in the loss-on-ignition curve from Lake Striern. He correlated the oscillationwith the Piottino-oscillation originally described from Switz-erland by Zoller (1960) and from northwest Germany byBehre (1966).

Methods

The sequences were cored with Russian peat samplers(Jowsey, 1987). All cores were immediately placed in PVCtubes, wrapped in plastic film and stored in a cold roomuntil further treatment. A detailed lithostratigraphy for bothsequences was completed in the laboratory.

Samples of 1 cm3 wet sediment were taken at 1–2-cmintervals in the sediments accumulated at the YoungerDryas–Preboreal boundary and in the gyttja deposited at theonset of the Corylus expansion. A lower sampling resolutionwas used (5–10 cm) in other parts of the cores. Lycopodiumtablets were added to the pollen samples in order to calcu-late absolute pollen concentration (Stockmarr, 1971). Pollenpreparation followed Berglund and Ralska-Jasiewiczowa(1986, fig. 22.1). Identifications and nomenclature of pollenand spores mainly follow Moore et al. (1991). The subdiv-ision of pollen-stratigraphical data into local (LPAZ) andregional (RPAZ) pollen assemblage zones was, in part, madeby stratigraphically constrained cluster analysis (CONISS ofGrimm, 1987). Similar sampling intervals to the pollen analy-ses was used for determination of organic carbon content.The latter was measured in an ELTRA CS 500 Carbon Deter-minator and is calculated as percentage of dry weight ofthe sample.

Levels between 770 and 600 cm in Hogstorpsmossen and610 and 500 cm from Fågelmossen were subsampled con-tiguously in 5-cm blocks for tephra. The samples weretreated by burning for 4 h at a temperature of 650°C, soakedovernight in 10% HCl to reduce the organic and carbonatecontent, and were then sieved through mesh diameters of24 and 80 mm. Thereafter most samples were treated witha separation technique using a heavy liquid, sodium poly-tungstate Na6(H2W12O40)H2O, with relative densities of 2.4and 2.5 g cm−3, which allows rhyolitic Vedde Ash shards tofloat and the bulk of mineral particles (e.g. quartz, feldspar)to sink (Turney, 1998). Samples from the upper parts of thesequences contained only a small amount of mineral par-ticles and were treated without floating.

Subsequently, 1-cm samples with constant volumes of1 cm3 were removed from the 5-cm blocks in which tephrahad been found. The rhyolitic tephra concentration wasquantified by counting all tephra shards in the fraction withinthe appropriate relative density range.

The Laacher See Tephra is known to have a broad densityrange (Ch. van den Bogaard, personal communication, 1997)and different fractions between 2.1 and 2.5 g cm−3 of thesupernatant were chosen to extract this tephra. This rangewas chosen because of a high vesicularity and the phonoliticcomposition of the glass shards. The residue above2.5 g cm−3 was also analysed for possible occurrence oftephra, but the high minerogenic content made any identifi-cation impossible.

The tephra shards identified were analysed geochemicallyon an electron microprobe at the University of Edinburgh.These samples were treated by a combination of acid diges-tion (e.g. Dugmore, 1989) and density separation methods


(Turney, 1998), but were not ashed because this is knownto alter the geochemical composition of the tephra shards(Dugmore et al., 1995). The preparation for microprobeanalysis and subsequent analytical procedures follow Dug-more et al. (1995).

Radiocarbon dating was carried out by using the AMSfacility at the Ångstrom Laboratory, Division of Ion Physics,Uppsala University. Owing to the scarcity of terrestrial mac-rofossils in the sediments studied, bulk samples (1-cmintervals) were dated using the NaOH-soluble fraction (SOL).Even though bulk sediment dates may give ages severalhundred years older than dates based on terrestrial macrofos-sils (Olsson, 1991; Bjorck et al., 1998a) we believe that itis important to obtain an age for the expansion of Corylusand for the previously unknown tephra in the area. It alsohas been demonstrated that comparative dating of terrestrialplant macrofossils and gyttja extracts can, in some cases,give consistent ages over the Younger Dryas–Holocene tran-sition and in the early Holocene (Gulliksen et al., 1998).

Palaeoenvironmental reconstruction andtephrochronology

A short description is given below of the five regional pollenassemblage zones (RPAZ) of Southern Ostergotland (SO-2through to SO-6) identified in this study (Figs 2–5). Theoldest zone, SO-1, which is correlated with the late Allerødinterstadial (GI-1a; Bjorck et al., 1997), is described else-where (J. Bjorck, submitted).

RPAZ SO-2

The total pollen concentration is low. More than half of thepollen spectra is dominated by herbs, with Artemisia as theprincipal taxon. Chenopodiaceae and Oxyria type (mainlyO. digyna: Moore et al., 1991) also are common. Amongshrubs, Salix dominates and Juniperus and Betula nana alsoare present. The composition of the pollen flora suggests atundra vegetation of herbs and dwarf shrubs, indicating astadial climate correlated with the Younger Dryas stadial(GS-1). The organic carbon content displays low and slightlyincreasing values throughout the zone.

A distinct peak in rhyolitic tephra concentration isrecorded in the middle of this zone at both Hogstorpsmossenand Fågelmossen (Figs 2–5). Reworking and inwash into thebasin were therefore of minor importance after the tephrafall-out. The morphology, colour and geochemical signatureof the shards suggests that this tephra horizon can be corre-lated with the rhyolitic component of the Vedde Ash (Figs 6and 7, Tables 1 and 2; e.g. Mangerud et al., 1984; Birkset al. 1996). The maximum concentration is 173 rhyoliticshards cm−3 at Hogstorpsmossen (718–717 cm) and 43 rhyol-itic shards cm−3 at Fågelmossen (581–580 cm).

RPAZ SO-3

Pinus dominates the early part of the zone and Betula thelater part. The lower boundary is characterised by decliningherb pollen frequencies of Artemisia, Chenopodiaceae and

402JO

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Figure 2 Organic carbon content, tephra concentration and pollen percentages of selected species from Hogstorpsmossen covering the last glacial–interglacial transition.

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Figure 3 Organic carbon content, tephra concentration and pollen concentration (grains cm−3) of selected species from Hogstorpsmossen covering the last glacial–interglacial transition. Note the different scales.

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Figure 4 Organic carbon content, tephra concentration and pollen percentages of selected species from Fågelmossen covering the last glacial–interglacial transition.

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Figure 5 Organic carbon content, tephra concentration and pollen concentration of selected species (grains cm−3) from Fågelmossen covering the last glacial–interglacial transition. Note the different scales.

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Figure 6 Plots of FeOtot–K2O and SiO2–CaO ratios for the Vedde Ash at Hogstorpsmossen and Fågelmossen. Average values and 1svariation for various terrestrial and marine sites with Vedde Ash are also shown (Kråkenes from Birks et al., 1996; SU9032 (south ofIceland) from Lacasse et al., 1995; Lake Madtjarn from Wastegård et al., 1998; Whitrig Bog from Turney et al., 1997b).

Figure 7 Tephra shard from the Vedde Ash layer in Fågelmossen,581–582 cm. Length of shard ca. 40mm.

Table 1 Relative concentrations (wt %) of oxides of the nine major elements in glass shards from Hogstorpsmossen as determined by electronmicroprobe. Totals below 95% for the Vedde Ash have been discarded

Hogstorpsmossen SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O Total

Unknown Tephra1 66.59 0.09 11.43 1.55 0.00 0.05 0.58 2.74 3.58 86.612 69.70 0.10 11.75 1.42 0.09 0.09 0.64 3.34 3.58 90.71

Vedde Ash1 68.98 0.28 13.23 3.81 0.17 0.27 1.34 5.27 3.50 96.852 69.59 0.29 13.30 3.90 0.18 0.25 1.32 5.06 3.47 97.363 70.21 0.32 13.08 3.70 0.21 0.27 1.21 4.86 3.51 97.374 69.02 0.21 12.94 3.69 0.13 0.17 1.20 4.84 3.52 95.725 70.05 0.33 13.17 3.93 0.16 0.27 1.37 5.23 3.40 97.916 69.94 0.29 13.21 3.87 0.15 0.25 1.30 5.02 3.50 97.537 70.18 0.31 13.09 3.93 0.21 0.27 1.32 4.40 3.52 97.238 69.76 0.25 13.04 3.37 0.14 0.21 1.17 4.78 3.48 96.20

Mean (Vedde Ash) 69.72 0.28 13.13 3.78 0.17 0.24 1.28 4.94 3.49 97.021s 0.49 0.04 0.12 0.19 0.03 0.04 0.07 0.28 0.04 0.73


Oxyria type. The percentages of Filipendula, Ranunculustype and Epilobium type increase at the boundary. Empetrumstarts to increase at the lower boundary and displays a peakin the upper part. Hippophae occurs at the upper boundary.Betula nana type has a peak in the middle or the lowerpart of the zone, where pollen of some herbs display adouble peak, i.e. Artemisia, Chenopodiaceae and Oxyriatype, and Filipendula decreases. Pollen of aquatic plants andspores of Pteridophytes start to appear in the middle part ofthe zone and increase towards the upper boundary. Thetotal pollen concentration increases at the lower boundarybut decreases again in the middle or upper part. The pollencomposition suggests a climatic amelioration at the lowerzone boundary, and that the vegetation changed from aherb tundra to an open shrubland or park tundra with Betula,Salix, Empetrum and Juniperus.


Table 2 Relative concentrations (wt %) of oxides of the nine major elements in glass shards from Fågelmossen as determined by electronmicroprobe. Totals below 95% have been discarded

Fågelmossen SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O Total

Vedde Ash1 69.75 0.27 13.10 3.56 0.09 0.22 1.39 5.18 3.47 97.032 68.85 0.29 13.11 3.69 0.11 0.22 1.27 4.34 3.50 95.383 69.31 0.29 13.50 3.59 0.08 0.19 1.27 4.57 3.51 96.314 69.01 0.28 12.82 3.64 0.08 0.24 1.24 4.97 3.45 95.735 69.06 0.28 13.21 3.73 0.10 0.21 1.33 4.25 3.24 95.416 69.38 0.25 13.37 3.59 0.07 0.22 1.20 4.21 3.42 95.717 68.95 0.19 13.23 3.78 0.18 0.26 1.23 4.89 3.45 96.168 69.67 0.29 13.48 3.89 0.19 0.29 1.15 4.24 3.29 96.499 70.08 0.26 13.39 3.82 0.16 0.25 1.32 4.59 3.67 97.54

Mean 69.34 0.27 13.25 3.70 0.12 0.23 1.27 4.58 3.44 96.201s 0.42 0.03 0.21 0.11 0.05 0.03 0.07 0.36 0.13 0.74

A rise in the organic carbon content at the SO-2–SO-3boundary corresponds to a lithological change from claygyttja to gyttja. This can be interpreted as an effect of therapid warming at the Younger Dryas–Preboreal boundary(cf. Bjorck et al., 1996).

Fluctuations in the organic carbon content occur in RPAZSO-3, where some herbs display two peaks, i.e. Artemisia,Chenopodiaceae and Oxyria type. The fluctuations in pollenpercentages, concentration values and organic carbon con-tent can be correlated with the early Preboreal oscillation(Bjorck et al., 1996, 1997) in southwestern Sweden. Thefluctuating curves, however, indicate that at least one ormaybe two short-lived early Preboreal climatic oscillations(cf. Whittington et al., 1996) may have occurred in Ostergot-land after the climatic improvement at the Younger Dryas–Preboreal boundary.

RPAZ SO-4

Betula continues to increase at the lower boundary andreaches a maximum in the zone. Betula nanatype, Empetrumand Juniperus display a maximum and Hippophae occursin the zone. Among aquatics Myriophyllum alterniflorumappears, with a continuous curve throughout the zone, andNymphaea starts to appear at the upper boundary. Sporesof Pteridophytes reach a maximum in the zone. The pollenconcentration increases slightly up to the middle part, butdecreases again at the upper zone boundary. Corylus appearswith low percentages (, 1%) in the upper part. The compo-sition of the pollen flora suggests that the vegetation hasbecome more dense and dominated by Betula.

The organic carbon content displays rather stable orslightly increasing values. A small decrease occurs in theupper part of the zone in Hogstorpsmossen.

RPAZ SO-5

Corylus increases at the lower boundary whereas Empetrumand Juniperus decrease. Hippophae occurs at the lowerboundary. Alnus starts to appear with low percentages(, 1%). Among aquatics Myriophyllum alterniflorum isreplaced by Nymphea. Pollen concentration displays a mini-


mum at the lower boundary but increases upwards. Thepollen composition suggests a vegetation change to a borealforest with Pinus, Betula and Corylus.

The organic carbon content increases at Hogstorpsmossenwith a slight decrease in the upper part. A slight decreasealso can be seen in the upper part of the zone at Fågelmoss-en.

The continuous increase in organic carbon content inzones SO-4 and SO-5 is interrupted by a small decrease,which occurs at or slightly after the expansion of Corylus.A climatic deterioration close to the expansion of Corylushas been suggested from southwestern Sweden (Robertssonand Olsson, 1995) and in Scotland (Whittington et al., 1996).This phase, however, is characterised by a rapid drop inlake levels throughout northwestern Europe (e.g. Digerfeldt,1988), and it is possible that the fluctuations in the organiccarbon content may be related to other processes in thelake catchment areas, such as an increase in reworkedminerogenic matter in connection with the lake-level lower-ing.

Rhyolitic tephra of low concentration was found at 624–622 cm in Hogstorpsmossen (Figs 2, 3 and 8, Table 1). Theconcentration is very low, with a maximum of 11shards cm−3 (623–622 cm). Microprobe analyses could becarried out only on two very fine-grained shards, whichresulted in low totals of only ca. 87 and 91%. The resultsof the analyses, however, indicate that the rhyolitic tephrahorizon is of Icelandic origin (e.g. high SiO2 content andmoderately high K2O content) and almost certainly does notconsist of reworked Vedde Ash particles, because the content

Figure 8 Tephra shard of unknown origin from Hogstorpsmossen,623–624 cm. Length of shard ca. 40mm.


of K2O is higher than in the Vedde Ash and FeO and CaOcontents seem to be much lower (Table 1). This tephra is asyet unrecorded in the literature and could occur further westin Scandinavia, where the concentration might be expectedto be higher and the geochemical signature confirmed. Thetephra occurs shortly after the expansion of Corylus butbefore the increase of Alnus in Hogstorpsmossen, and couldprove to be an important marker horizon for the earlyHolocene of northern Europe. No shards were identifiedat Fågelmossen.

RPAZ SO-6

This zone is represented with certainty only in Fågelmossen.However, the transition SO-5–SO-6 probably also occurs atHogstorpsmossen, where Alnus increases in the uppermostsample to 0.7% and concentration values increase fromca. 500 to 3000 grains cm−3. A high concentration of totalpollen persists, with Nymphaea dominating the aquaticassemblage. The overall pollen composition suggests thatAlnus was expanding into the area.

The organic carbon content increases again after thedecline in zone SO-5, with high and stable values around45–50% recorded from the middle part of the zone.

Timing of events

The AMS radiocarbon ages on bulk sediment samples areshown in Table 3 and on the pollen diagrams. The expansionof Corylus has been dated to 9375 6 135 14C yr BP inHogstorpsmossen and to 9710 6 125 14C yr BP in Fågel-mossen. Considering the rather short distance between thesites it is reasonable to believe that the expansion occurredmore or less simultaneously at both sites. Radiocarbon datingon the expansion of Corylus using bulk samples of organicsediments have yielded ages of 9380–9700 14C yr BP insouthern Sweden (Digerfeldt, 1977). In southernmostSweden, AMS dates on terrestrial macrofossils suggest theexpansion of Corylus occurred at 9000–9100 14C yr BP(Gaillard et al., 1996). This suggests that at least the olderof the dates from Ostergotland may be several hundred 14Cyears too old as the spread of Corylus to Sweden was likelyto have been very rapid, although the 9500–9600 yr BPradiocarbon plateau makes radiocarbon dating of the expan-sion of Corylus problematic.

The tephra horizon in RPAZ SO-5 is bracketed by twoAMS radiocarbon dates: 9255 6 145 14C yr BP at 621 cmand 8980 6 125 14C yr BP at 625 cm. The age-to-depthrelationship is reversed, but the two dates overlap within astandard deviation of 2s. As the tephra horizon is situatedcloser in the profile to the expansion of Alnus than Corylus,

Table 3 AMS dates on bulk sediment (SOL fraction) from southern Ostergotland

Depth (cm) Sediment Site 14C age BP (6 1s) AMS sample no.

621 Gyttja Hogstorpsmossen 9255 6 145 Ua-13325625 Gyttja Hogstorpsmossen 8980 6 125 Ua-13326634 Gyttja Hogstorpsmossen 9375 6 135 Ua-13327561 Gyttja Fågelmossen 9710 6 125 Ua-13328


the younger age seems to be more reliable. The increase ofAlnus has been dated in the area to 8700–8900 14C yr BPin gyttja (Goransson, 1977). Thus, the age of the tephra canbe estimated to ca. 9000 14C yr BP.

Discussion and conclusions

It is evident from a number of sites in northwestern Europe,Iceland and Greenland that at least one cold event followedthe general climatic improvement at the Younger Dryas–Preboreal boundary. This has been interpreted as an effectof increased freshwater forcing and decreased thermohalinecirculation in the Nordic Seas (Bjorck et al., 1997).

The results from most investigations indicate a single coldevent during the early Preboreal (Bjorck et al., 1997 andreferences therein). A few records, however, suggest a morecomplex pattern, with several oscillations between cool andwarm conditions during the early part of the Preboreal, asreflected, for example, by stable isotopes (Whittington et al.,1996). At least two early Preboreal oscillations are alsoindicated in variations in organic carbon content at sites inScotland (cf. Turney et al., 1997b). The fact that up to sixseparate early Preboreal marginal moraines were formed inVarmland (Lundqvist, 1988), southwestern Sweden, isanother indication that several short climatic oscillations mayhave affected the retreat of the ice-front. Two early Preborealoscillations also are indicated in the marine stratigraphyin this area, at 9900 and 9700 14C yr BP, respectively(Wastegård, 1998).

The results from the variables analysed show that at leastone and possibly two cooling events followed shortly afterthe general early Preboreal warming. This is reflected in thepollen records by an increase of Artemisia, Chenopodiaceaeand Oxyria-type pollen coinciding with a decrease in thethermophilous Filipendula. The increase in Artemisia andChenopodiaceae in Ostergotland, eastern middle Sweden, ismore pronounced than at sites studied in southwest Sweden,which were situated at a greater distance from the ice-margin during the Preboreal (Bjorck et al., 1997). The earlyPreboreal oscillations also can be detected in the decreaseor fluctuating values in the organic carbon content at bothsites presented here. At Hogstorpsmossen, the early Preborealis characterised by at least one, or possibly two, shortoscillations in the organic carbon and pollen record,although in Fågelmossen the evidence is less distinct.

More stable conditions followed after the early Preboreal,with a denser vegetation cover and increasing lake pro-ductivity. An oscillation is detected in the organic carboncurve close to the expansion of Corylus, but this changecan be explained by an increase in reworked minerogenicmatter as a result of regional lowering of the lake levels inthe middle to late Preboreal (cf. Digerfeldt, 1988). The


Preboreal lake level lowering also could have contributedto regional changes in vegetation (cf. Digerfeldt, 1988).

The analyses of tephra show that the Vedde Ash can befound as far east as southeastern Sweden, more than2300 km from the eruption centre of the Katla volcano onIceland (Lacasse et al., 1995). The concentration of shardsis much lower compared with the site in southwesternSweden, Lake Madtjarn, where the occurrence of this tephrawas first confirmed in Sweden (Wastegård et al., 1998). Thisindicates that the sites investigated in the present paper areprobably situated close to the eastern boundary of the disper-sal plume of the Vedde Ash (cf. Wastegård et al., 1998,fig. 1).

The peaks in rhyolitic Vedde Ash concentration at Hogs-torpsmossen and Fågelmossen are located in the middleof the Younger Dryas, which is in accordance with otherinvestigations of terrestrial sequences in northwestern Europe(e.g. Mangerud et al., 1984; Birks et al., 1996; Turney et al.,1997b; Wastegård et al., 1998). The tephra fall-out is AMSdated to the 10 400–10 300 14C yr BP radiocarbon plateau(e.g. Birks et al., 1996; Wastegård et al., 1998), and thecalendar year age has been estimated to ca. 12 000 yr BP(Wastegård et al., 1998), which accords with the age of11 980 6 80 GRIP yr BP in the GRIP ice-core (Gronvoldet al., 1995).

A search for the Laacher See Tephra was undertaken inthe sediments of supposed Late Allerød age (J. Bjorck,submitted). This tephra layer was deposited in central andnorthern Europe following an eruption of the Laacher SeeVolcano at 11 230 6 40 14C yr BP (Hajdas et al., 1995). Notephra shards were found, which suggests that the northeast-ern plume of the Laacher See Tephra may have been restric-ted to the southeastern part of the Baltic Sea area (cf. Juvigneet al., 1995), or that the sediments analysed were depositedafter the fall-out of tephra. It is also possible that the densityseparation technique is unsuitable for detecting low concen-trations of the Laacher See Tephra, with its wide densityrange.

The discovery of a previously unrecorded early Holocenerhyolitic tephra horizon at Hogstorpsmossen may prove ofgreat importance for correlating early Holocene sequencesin Scandinavia. It is too early, however, to use this tephraas a marker horizon, as the concentration is very low andthe geochemistry not yet fully established. The age of thetephra horizon (ca. 9000 14C yr BP) corresponds in time tothe basaltic Saksunarvatn Ash (e.g. Mangerud et al., 1986;Birks et al., 1996). The possibility that the tephra found atHogstorpsmossen represents an earlier unknown rhyoliticphase of the Saksunarvatn Ash is unlikely. No rhyolitictephra production is known from the Grımsvotn Volcanicsystem (probable source of the Saksunarvatn Ash, e.g. Gron-vold et al., 1995) and rhyolitic tephra has not been reportedat any site in connection with the Saksunarvatn Ash (e.g.Merkt et al., 1993; Birks et al., 1996).

Acknowledgements The investigation has been supported by theSwedish Natural Science Research Council (NFR) and the SwedishSociety for Anthropology and Geography (SSAG). Ann-Marie Rob-ertsson and Jan Lundqvist kindly read an earlier draft of the manu-script and proposed valuable improvements. Chris Turney correctedthe language and also suggested many valuable improvements.Comments by Svante Bjorck and an anonymous reviewer alsoimproved the final quality of the paper significantly. Microprobeanalyses were undertaken at the NERC Electron Microprobe Facility,Department of Geology and Geophysics, University of Edinburgh,with the support of Peter Hill. The map was drawn by LaszloMadarasz. Jonas Bjorck carried out the organic carbon and pollenanalyses and Stefan Wastegård carried out analyses of tephra. Both


authors are responsible for the interpretation of the results and thediscussion. This paper is a contribution to INTIMATE; INTegrationof Ice Core, Marine and Terrestrial records – a core programme ofthe INQUA Palaeoclimate Commission.

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Climate oscillations and tephrochronology in eastern middle Sweden during the last glacial–interglacial transition