Early Holocene environment on BjOrnOya (Svalbard) inferred from multidisciplinary lake sediment studies B A R B A R A WOHLFARTH, GEOFFREY LEMDAHL, SIV OLSSON, THOMAS PERSSON. IAN SNOWBALL. JONAS ISlNG and VIV JONES Wohlfarth. B . , Lemdahl. G . . Olsson, S . , Persson. T., Snowball. 1.. Ising, J . & Jones, V . 1995: Early Holocene environment on Bjorneya (Svalbard) inferred from multidisciplinary lake sediment studies. Polar Research 1 4 ( 2 ) . 253-215. BjOrnOya, a small (178 km2) island situated between the mainland of Norway and southern Spitsbergen, provides the opportunity for the reconstruction of early Holocene terrestrial and limnic palaeoenvironments in the southwestern Barents Sea. The AMS ''C dating technique, geochemical, mineral magnetic, micro- and macrofossil analyses were applied lo sediments recovered from lake Stevatnet and the results are interpreted in terms of palaeoenvironmental conditions between 9800 and 8300 I4C B P . After the dis- appearance of local glaciers before ca 9800 14C BP, the lake productivity increased rapidly at the same time as pioneer plant communities developed on soils which gradually became more stable. Insect data indicates that strong seasonal contrasts with mean July temperatures around 9°C and mean January temperatures around -12°C prevailed between 9500 and 8300 I4C BP. These high summer temperatures, possibly as much as 4-5°C higher than the present, favoured the development of a flora including Dryas and Angelica cf. archangelica. The enhanced freeze/thaw processes led to an increased erosion of minerogenic and organic material. After 8000 ''C BP the temperatures may have gradually declined. The environmental reconstruction derived from our data set supports the conceptual insolation model which proposes maximum Holocene seasonality for the Northern Hemisphere at ca 9000 I4C BP. Barbara Wohlfarth, Geoffrey Lemdahl, Siu Olsson, Thomas Persson, Ian Snowball and Jonas Ising. Department of Quaternary Geology, Tornauiigen 13. S-22363 Lund, Sweden; Viu Jones, Department of Geography, Unicrersiry College London, 26 Bedford Way, Londotl W C l H OAP, U . K . Introduction The island of Bjorncbya is situated in the south- western Barents Sea, between the mainland of Norway and the southern end of Svalbard (Fig. 1A). During the last Glacial maximum (ca 18,000 14C BP), the Barents Sea ice sheet reached west to the continental shelfbreak and a glacier, possibly dry-based, covered B j ~ r n ~ y a (Elverhei & Solheim 1983; Vorren et al. 1988; E l v e r h ~ i et al. 1990; Vorren 1992; Elverhei et al. 1995). The ice receded in two steps, firstly at ca 14,800 14C BP from the outer shelf and the southwestern Barents Sea and secondly at ca 13,000-12,000 14C BP from the inner and shallower shelf (Elverhoi et al. 1995). These separate recessions correspond to meltwater signals recorded in marine cores between 15,000-13,000 14C BP (Hald & Dokken 1995). The timing of the decay of the Barents Sea ice sheet is also indicated by an influx of meltwater from major Siberian rivers into the central Arctic Ocean slightly before 14,500 l4c BP (Stein et al. 1994). At ca 12,500 14C B P the first warming of the ocean surface is recorded in marine sediment cores from the continental margn off western Svalbard and the western Barents Sea (Hald & Dokken 1995). Mollusc dates from the northern Barents Sea, and emergence curves from Kong Karls Land, indicate that the Barents Sea ice sheet had disappeared by ca 10,000 I4C B P (Sal- vigsen 1981; Elverhei et al. 1990; Bondevik et al. 1995; Hjort et al. 1995). This date agrees well with oxygen isotope data from marine cores which show a rapid temperature increase of 2-3°C between 10,000-9500 14C BP (Hald & Dokken 1995) and indicate that warm Atlantic water replaced cold Arctic water in the western and southeastern Barents Sea (Hald & Vorren 1987; Vorren et al. 1988). Bulk sediment 14C dates obtained on basal lake sediments from Bjerneya have been used to date the beginning of the organic production in one of the lakes to between 11,200 -C 500 and 8300 + 1200/-1000 lilt BP (Hyvarinen 1968, 1970; Ols- son 1968). However, since the sediments con- tained large quantities of coal particles derived 19 " 0 0 ' 1 9 " 1 0 ' I I B 1 9 " 0 0 1 9 " 1 0 ' L E G E N D * L ak es c or ed b y H yv er in en ( 1 9 6 8 ) -! - E qu is et ur n ar ve ns e. 4 - L U Z U la a rc ua ta , + D es ch ar np si a al pi na , 0 P oa a lp in a, 0 P hi pp si a al gi da , r t Ph ip ps ta c on ci nn a. * F es tu ca v iv ip ar a, jk Sa lix h er ba ce a, po la ri s, * Sa lix r et ic ul at a. 0 K oe ni gi a is la nd ic a, B k O xy ri a di gy na . + Po ly go nu rn vi vi pa ru rn , 0 S ag in a in te rm ed ia , A C er as ti ur n ar ct ic ur n, v C er as ti ur n re ge lii , c S il en e ac au li s. # R an un cu lu s py gr na eu s, ce rn ua . k S ax if ra ga ni va lis , a S ax if ra ga r iv da ri s, 4 Sa xi fr ag a te nu is Sa lix D ra ba a lp in a. + D ra ba n or ve gi ca , 0 S ax if ra ga Fi g. 1 . A . G eo gr ap hi c po si ti on o f B j~ rn ~ y a . B . T op og ra ph ic m ap o ve r B j~ rn ~ y a . T he n or th er n, r at he r fl at p ar t of t he i sl an d is s it ua te d be lo w t he 4 0 m c on to ur l in e. T he m ou nt ai n ar ea s to t he s ou th a nd s ou th ea st r ea ch c a 50 0m a .s .1 . T he l ak es c or ed b y H yv ar in cn ( 19 68 ) d ur in g th e St oc kh ol m U ni ve rs ity S va lb ar d E xp ed it io n in t he y ea rs 1 96 5- 1Y 66 a rc m ar ke d by a rr ow s. C . T op og ra ph ic m ap o ve r th e su rr ou nd in gs o f S te va tn et . T he c or in g po in ts S T 2 -S T 1 a re m ar ke d w ith f ill ed c ir cl es . T he v eg et at io n ar ou nd t he l ak e is a cc or di ng t o E ng el sk j0 n (1 98 6, m ap s 1- 54 ). Early Holocene environment on Bj@rn@ya (Svalbard) 255 from the bedrock, t h e reliability of the dates was questioned by Olsson (1968). The appreciation of the problems associated with the reliability of the original radiocarbon dates from B j ~ r n ~ y a led to a re-investigation of t h e sediments of some lakes within the frame of the P O N A M project during August 1993. During an earlier Svalbard expedition organised by Stockholm University, Hyvarinen (1968) took sediment cores from 18 lakes situated between 10 and 100m a.s.1. (Fig. 1B). The Holocene veg- etation history was reconstructed on the basis of pollen analysis of three lake sequences and was compared to the development on Svalbard and northern Fennoscandia (Hyvarinen 1968, 1970). Although H o r n & Orvin (1928) had described marine terraces/plains of denudation, which they interpreted as younger than the last deglaciation, diatom analyses conducted o n lakes at different altitudes did not reveal any marine influence on their sediments (Hyvarinen 1968). During the August 1993 field work we obtained sediment cores from three lakes, Vestre Skinkevatnet, @stre Skinkevatnet and Stevatnet, situated in the northern part of the island. Two of these lakes, Vestre Skinkevatnet and Stevatnet had been cored earlier by Hyvarinen (1968) who described fairly long sediment sequences. T h e general litho- stratigraphy recovered in all three lakes compares well to Hyvarinen’s (1968) earlier descriptions. In this paper we present new results, obtained from geochemical, palynological, entomological, mineral magnetic, plant macrofossil and diatom analyses of sediment cores recovered from Ste- vatnet. T h e results, which include new AMS 14C dates, a r e used to reconstruct the Early Holocene environment on B j @ r n ~ y a and are compared to earlier investigations on Bjornoya and in other Arctic regions. Geography, climate, bedrock geology and vegetation T h e island is surrounded by steep cliffs and has a surface area of 178 km’. T h e main geographic units a r e the fairly flat “northern plain”, which extends from southwest to northeast, with alti- tudes rarely above 40 m a.s.l., and the southern/ southeastern part, dominated by hills and moun- tains that rise u p to 500 m a.s.1. Most of the lakes a r e situated in the “northern plain”, which covers a surface area of ca 110 km2 (Fig. I B ) . Many lakes a r e small, extremely shallow and dry out during summer. H o r n & Orvin (1928) observed that the ground thawed t o a depth of ca 0.75 m and perma- frost reached down to 60-70 m below the surface. However, unfrozen ground below lake bottoms was reported by Fleetwood et al. (1974). Perma- frost features such as circles, polygones, nets and stribes are especially common o n the limestone terrain. T h e maritime arctic climate of Bjomoya is governed by the Norwegian Current along the western coast and an arctic stream which comes from the east and the north, causing cold air masses to move southwards around the island. Dense fog frequently occurs during the summer months (July: 24 days, August: 22 days) and is a characteristic phenomenon f o r Bj0rnoya. The average yearly temperatures range around ca - 1 S ” C ; the average monthly air temperatures recorded for the time period 1931-1960 range from between -5.7” and -7.2”C for the coldest months January-March and from between +4.5” and +5.0°C for the warmest months, July and August (Steffensen 1969,1982; E n g e l s k j ~ n 1986). Since average precipitation does not exceed 400 mm/yr, the high humidity of the ground and t h e vegetation cover is maintained by the low temperatures and the heavy cloud and fog cover (Engelskjon 1986). T h e bedrock is mainly composed of Devonian, Carboniferous and Permian sandstones, shales. coal seams. conglomerates and fossiliferous lime- stones (Horn & Orvin 1928; Worsley & Edwards 1976) (Fig. 2A). T h e oldest and youngest rocks, the dolomites of the Hecla Hoek Formation and the Triassic sandstones and shales, are only found in the mountain areas to the south and southeast. The strong relationship between vegetation cover and bedrock o n B j ~ r n ~ y a has been described by Engelskjon (1986), who carried out a detailed survey of the island’s plant communities. Although the vegetation can be compared to other areas adjacent t o the Barents Sea, several differences exist e.g. between t h e Spitsbergen and B j ~ r n ~ y a floras. Compared to Spitsbergen, species adapted to cold and water-logged con- ditions a r e more common on Bjornoya, whereas species demanding higher summer temperatures such as e.g. Salix glauca, Betula nana. Dryas octopetala, Cassiope tetragona and Empetriim her- maphroditum are absent (Engelskjon 1986). Most of the island’s eutrophic species concentrate on t h e lime-rich parts, where weathered material 256 B . Wohlfarrh er id. E Y G, P t Early Holocene environment on Bjgirn@ya (Soalbard) 257 from dolomites. limestones and m a r k yields an alkaline lithosol with a high C a content (Eng- elskjon 1986). T h e bedrock-vegetation-relationship is well exemplified in the surroundings of Stevatnet and demonstrated in Figs. 1 C and 2B. The soil which developed o n the “Spirifer Limestone”, a dark grey fossiliferous limestone north of the lake (Horn & Orvin 1928), clearly supports a more extensive vegetation with species such as Phippsia algida, Salix herbacea, Salix polaris. Salix reti- culata, Oxyria digyna, Sagina intermedia, Ranun- culus pygmaeus, Draba alpina. Saxifraga nioalis and Saxifraga renuis ( E n g e l s k j ~ n 1986). O n the western shore of Stevatnet the so-called “Ambi- gua limestone” series occurs, characterised by massive grey limestones, red and violet mottled limestones and red sandstones (Horn & Orvin 1928). Similar t o the eastern shore of the nearby lake Haussvatnet only a sparse vegetation cover with Salix reticulata is found ( E n g e l s k j ~ n 1986). T h e soils which develop o n the “Fusulina Lime- stone” and the “Ambigua Limestone” are alkaline and yield a high content of C a (Engelskj0n 1986), which explains the more extensive vegetation cover north and east of Stevatnet. The “Red Conglomerate” south of Stevatnet comprises sandstones, calcareous sandstones and conglom- erates ( H o r n & Orvin 1928). It is highly fissile and flakey, and shows a red and yellowish-green colour. T h e “Ursa Sandstone” east of Stevatnet forms large and coarse blockfields. It is a white to grey massive sandstone with flakes of muscovite and biotite and fairly large amounts of pyrite. The lithosol of the “Ursa sandstone” is slightly acid. Horn & Orvin (1928) and Keilhau (1831) described that the intercalated shaley layers in the ‘Wrsa sandstone” cause it to break up into coarse blockfields or stony polygons. Apart from these stone polygons which support scattered mosses and lichen turfs, the sandstone areas are very poor in vegetation cover (Engelskjm 1986). Methods Fieldwork Stevatnet is situated at 46 m a.s.1. It has n o visible inflow, but drains north into the 12 m lower lake Hellevatnet (Fig. 1C). T h e core sites (ST 1-ST 4) were concentrated in the western part of the lake, because the water depth in the central part exceeded 6 m and the lake bottom in the eastern part was covered by stones and boulders of the “Ursa Sandstone”, which made coring very diffi- cult. Coring was carried out from a specially con- structed boat with 1 m long strengthened Russian corers with diameters of 7.5 and 5 cm. T h e overlap between the cores was ca 2 0 c m . T h e litho- stratigraphy of the cores was described both in the field and later in t h e laboratory and revealed a fairly uniform stratigraphy, which made it easy to correlate between different cores, using the transition between layers 7 and 8 as a reference horizon (Fig. 3). T h e detailed lithostratigraphic description of the four analysed cores ( S T 2 , 3 A . 3B, 4) is based o n the field descriptions and given in Tables 1-4. Mineral magnetic measurements Routine mineral magnetic measurements were applied t o correlate cores and t o study the origin of t h e ferrimagnetic minerals present in the sedi- ments. Detrital magnetic minerals can provide information about the erosion history of lake catchments, while t h e identification of post- depositional authigenic magnetic minerals may be used to reconstruct sedimentary redox conditions and trophic status. Samples from all four cores were subsampled into contiguous plastic cubes (2 x 2 x 2 cm). Cores S T 2 and S T 3A & B were subsampled immediately after collection. However, it was noted that severe oxidation of the cores had taken place during the time that had lapsed between core collection and measurement. Sections of the cores with originally quite distinct FeS stains had become bright red in colour. Core ST 4 was subsampled and measured approximately o n e year after collection, although the stratigraphy was still in its original form and the FeS colour of the sediment had remained, only with surficial oxidation indicated. S I R M was induced in a maximum magnetic field of 1 Tesla ( h H = 1 T ) with a Redcliff pulse magnetiser, and the remanent magnetisation measured with a Molspin “Minispin” magnet- ometer. T h e samples from S T 2 and S T 3 A & B were dried at 40°C after measurement to calculate mass specific SI units. In the case of S T 4 magnetic susceptibility ( x ) was measured with a Geofyzica Brno “Kappa” bridge and, in addition to the S I R M , a backfield IRM induced at - 100 m T was measured to calculate the S-ratio (Stober & 258 B . Wohlfarth er a / . S T 3 A 4.00 4.10 4 20 4 30 4 4 0 4 50 4.60 4 7 0 4 80 4 90 5 00 5 10 p) 5 2 0 0 f 5 3 0 ul & 5 4 0 m ' 5 5 0 5 0 5 6 0 5 7 0 L - .- 5 5.80 0 5.90 6.00 6.10 6.20 6.30 6 4 0 6.50 6 6 0 6 7 0 6 80 6 90 7 00 ST 2 i i -1 \ \ \ \ \ Lithology j=/ Clay a G W i a Sdt FeS laminatior Sand a Gravel a Stones . --. ST 4 . . . . . . . . . , I , r a t l g r a p h ~ c corrclafion h c t w e c n cores ST 7 . ST 3.4. ST 3 8 and ST 4 . T h e four AMS dates obtalned on mobsrs leaves (ST 2 ) a r e indicated. as well a\ the samplss studied for macrofossils ( w e Table 6) Early Holocene environment on Bj@rn@ya (Sualbnrd) 259 Table 1. Lithostratigraphy of core ST 2 at a water depth of 4.20m. The lithostratigraphic description is based on the field description, where the different gyttja clay layers were mainly distinguished by colour differences. Unit Layer Depth in m Sediment description 111 20 4.20 -4.27 Orange coloured silty gyttja clay. gLB 19 18 17 15 14 4.27 -4.46 4.46 -4.80 4.80 -5.00 5.00 -5.15 5.15 -5.175 Alternating layers of light-brown and black (FeS-) coloured silty gyttja clay (ca 20 FeS laminae), moss layer between 4.335-4.345 m , gLB Black (FeS-) coloured, slightly silty gyttja clay with thin light brown layers (ca 50 FeS laminae), gLB Light-brown silty gyttja clay with some moss remains and few FeS laminae, gLB Light-brown silty gyttja clay with some moss remains and some FeS spots, gLB Orange-yellow coloured silty gyttja clay with a FeS lamina (0.5cm thick) at the upper and lower boundary, gLB Light-brcwn silty gyttja clay, black FeS lamina at 5.20-5.21, g-sLB 13 12 5.21 -5.25 Brown silty gyttja clay, gLB 5.175-5.2 1 - 11 5.25 -5.725 Black (FeS-) coloured clayey gyttja silt with single brown horizons ( 1 cm thick) and moss remains, g-sLB Black (FeS-) silty gyttja clay with some moss remains. g-sLB I1 9 5.725-5.775 Light-brown silty gyttja clay with thin FeS laminae. rich in mosses, gLB 8 7 5.805-5.85 Reddish clayey silt with some FeS laminae. sLB 6 5 5.775-5.805 - 5.85 -5.97 Alternating layers of grey and reddish coloured silty gravelly clay with some small pebbles at 5.94, gLB I 4-3 5.97 -6.05 Yellow-red-grey coloured gravelly sandy silty clay, compact Tublr 2 . Lithostratigraphy of core ST 3A at a water depth of 4.00 m. The lithostratigraphic description is based on the field description. where the different gyttja clay layers were mainly distinguished by colour difference%. Unit Layer 111 20 1’) 18 17 16 15 14 13 12 Depth in m 4.n0 -4 05 4.05 -4.23 4.23 -4.50 4.50 -4.58 4 58 -4 83 4.83 -4.98 4.98 -4.99 4.99 -5.04 5.04 -5.085 Sediment description ~~ ~ Orange coloured silty gyttja clay, g1.B Alternating layers of black (FeS-) coloured and light-brown silty gyttja clay. ZLB Black (FeS-) coloured silty gyttja clay with thin light-brown layers, gLB Red-brown silty gyttja clay, some FeS laminae, gLB Black (FeS-) coloured silty gyttja clay. higher clay content between 4.65-4.68 111. Reddish-brown silty gyttja clay, with some moss remains and few FeS laminae. slightly more clayey between 4.964.98. gLB Yellowish silty gyttja clay, black FeS lamina at 4.99, some moss remains. sLB Reddish-brown silty gyttja clay with thin FeS laminae. somc moss remains. gLB Yellowish silty gyttja clay, g-sLB gLB 11 10 5 085-5.37 5.37 -5.50 Black (FeS-) coloured. silty gyttja clay with few brown coloured laminae. some moss remains. sLB Brown-yellowish silty gyttja clay with some FeS laminae ( 0 . 5 2 cm thick), some moss remains, sLB Black (FeS-) silty gyttja clay. some moss remains, g-sLB 11 9 5.50 -5.58 Reddish-brown silty gyttja clay, faintly FeS laminated, rich in mosses. sLB 8 I 5.605-5.625 Red-brown silty clay, very sLB 5 5.67 -5.84 Grey silty sandy gravelly clay 5.58 -5.605 - I 6 5.625-5.67 Grey-reddish silty clay, sLB 260 B . Wohljarth et a l . Thompson 1979). These samples were not air dried and a r e currently under further inves- tigation. Certain sediment sections of S T 4 were selected for concentration of the magnetic min- erals in accordance with the methods of Snowball & Thompson (1990). Samples were freeze-dried to prevent oxidation of possible magnetic iron sulphides and the magnetic fractions were stored in acetone. T h e magnetic extracts were dispersed in epoxy resin and the magnetic hysteresis proper- ties of the ferrimagnetic fraction measured with a PMC AGFM-2900-2 magnetometer. T h e vari- ation of magnetisation with temperature was investigated with the aid of a horizontal force translation balance at the University of Edin- burgh. X-ray diffraction analysis (XRD) was used to identify the magnetic minerals recovered in the magnetic concentrates. Powder preparations were prepared of magnetic extracts and X-ray scanned over the region 15-55" 2 8 using a Philips diffractonieter with C u KO-radiation. Grain size and geochrwistry The grain-size distribution of samples from cores ST 3 A & B was performed by sieving pre-weighed samples through a 63 pm sieve. T h e coarse frac- tions were further sieved. dried and weighed, while the fine fractions were analysed on a Sedi- graph 5000 E T (Micromeritics) after defloc- culation in Na,P?O:. The results were re- calculated to bulk samples. Total carbon content was determined on cores S T 3 A & B by stepped heating of the samples in oxygen in a Leco furnace, which measures the COz evolved by I R detection (Leco multiphase carbon analyser RC-412). Besides organic material, the sediments also contain a minero- genic carbon phase (coal), which is thermally stable u p t o temperatures of 450-500". To sep- arate organic carbon (COrg) from bedrock derived carbon present as coal (C,,,) the amount of minerogenic carbon was estimated after oxidation of separate samples with HzOz and H N 0 3 , which preferentially oxidises the organic matter. Both carbon phases a r e reported as weight % C of the dry matter. T h e total sulphur content was determined o n subsamples from cores S T 3A. B & ST 4 after digestion of the samples a t 450°C in a mixture of K N 0 3 and N a N 0 3 (50/50 molar %) followed by dissolution in a reagent solution of K N 0 3 / N a N 0 3 in HCI (McQuaker & Fung 1975). The sulphur content was then measured gravimetrically after hot precipitation of SO4'- as BaSO,. Macro- and microfossil analysis For macrofossil analyses, 5-15 cm thick samples from ST 2 . S T 3A and ST 4 were pre-treated with 10% N a O H and sieved through a 0.25 m m mesh. T h e macrofossils were sorted o u t under a bin- ocular microscope. Identifications were carried out by comparisons with modern reference material. T h e reconstruction of the thermal cli- Table 3 . Lithostratigraphy of core ST 3 8 at a water depth of 4.OOm. The Ilthostratigraphic descnption is based on the field description. where the different gyttja clay layers were mainly distinguished by colour differences. Unit Layer Depth in m Sediment descnption I 4 3 1 7 - 5 07 -5 45 5 4.5 -5 51 5 5 1 -5 52 5.52 -5.56 5 56 -5.65 5 . 6 5 -5.82 5 82 -5 935 5 935-5 97 5 97 4 00 6 0 0 4 0 7 Black (FeS-) coloured silty gyttja clay with brown-yellowish layers and some moss remains, gLB Reddish-brown silty gyttja clay. rich i n mosses, some FeS laminae, sLB Black (FeS-) silty gyttla clay. fairlv rich in mosses. g-sLB Reddish-brown and greenish silty clay. FeS laminae in the upper part, sLB Reddish-grey sandy silty clay. very compact. sLB Grey silty sandy gravelly clay with small angular stones (3 cm 0). clay layer between 5.715-5.735. Yellow-grey silty sandv gravel. with silty clay layers, gLB Reddish. sandy silty clay. compact, gLB Yellow-grey sandy silty gravelly clay. compact Reddish to red-brown. sandy silty clay. compact gLB Early Holocene environment on Bj@rn@ya (Svalbard) 261 mate was made by using the Mutual Climatic Range method (MCR) on fossil Coleoptera. This method is based on the present geographical ranges of the species in relation to modern tem- perature (Atkinson et al. 1986). The palaeo- temperatures were reconstructed by use of the mutual intersection of the climatic ranges of selec- ted stenothermic species in the fossil record. The MCR method reconstructs mean July (warmest month) temperature (TMAX) and mean January (coldest month) temperature (TMIN). One of the great advantages of the MCR method is that it is possible to test its accuracy on modern Coleoptera assemblages in relation to climate. Such studies have shown the need of a calibration of the MCR results to obtain the most probable palae- otemperature (Atkinson et al. 1987). This cali- bration is carried out by using the correction equations: TMAX~,,,,,,te,j) = 1.066 TMAX(me,jian) + 0.0142 TMIN~,o,,e,~,d) = 1.416 TMIN(medi,n) -k 1.904 x No. of species X -2.96 The precision of the most probable palaeo- temperature is ca 4 2°C for TMAX and ca +. 5°C for TMIN. In ST 3A subsamples for pollen (including spores, Pediastrum and Bofryococcus) were spaced every 5 cm in the upper layers (layers 10- 15) and every 2 cm in the basal layers (layers 6 9). The samples (2 cm3) were treated with ZnC12 and prepared according to conventional pro- cedures. Tablets with Lycopodium spores were added to the pollen samples to determine con- centration values. The group of Betula pollen includes both tree and dwarf birch. Subsamples for diatoms in core ST 3A were prepared in accordance with the methods described by Battarbee (1986). A M S 14C measurements To obtain datable material for AMS measure- ments subsamples from cores ST 2, ST 3A and ST 4 were sieved through a 0.5 mm mesh and the macrofossils recovered were determined under a stereomicroscope. Intact leaves of Safix species were immediately dried on aluminium foil (at 50°C over night), stored in sterilised glass bottles and sent to the Tandem Laboratory in Uppsala. A full pre-treatment (including 1% HCI, 6 hrs at 80°C and 0.5% NaOH, 1 hr at 60°C) was applied to the Salix samples. The moss remains were submitted in distilled water and pretreated with 1 % HCI (6 hrs at SOT). T a b k 4. Lithostratigrdphy of core ST 4 at a water depth of 4.25111 The lithostratigraphic descrlption is based on the field description, where the different gyttja clay layers were mainly distinguished by colour differences. ~ Unit Layer Depth in m Sediment descnptton 18 5.20 -5.50 17 5.50 -5.74 16 5.74 -5.81 15/14 5.81 -5.865 13 5.865-5.98 12 5.98 -6.07 11 6.07 -6.49 9/10 6.49 -6.65 8 6.65 -6.70 7 6.70 -6.73 6~ 6.73 -6.17 6b 6.77 -6.785 6a 6.785-6.82 5 6.82 4 . 8 4 3 6.88 4 . 9 3 4 6.84 -6.88 2 6.93 -7.00 Alternating layers of black and beige-brown coloured silty gyttja clay with moss remains, gLB Slightly FeS coloured silty gyttja clay with moss remains; more FeS laminae between 5.50-5.60. 5.60-5.64, 5.6b5.70, gLB Light-brown silty gyttja clay with moss remains and few FeS spots, gLB Light-brown silty gyttja clay, at 5.845-5.85 and 5,865.865 yellow coloured layers. sLB Light brown silty gyttja clay. FeS laminations only between 5.94-5.95, otherwise FeS spots. moss remains, gLB Reddish-brown silty gyttja clay, mosses, FeS spots, gLB Alternating layers of black (1-3 cm) and brown-reddish (0.5 cm) silty gyttja clay, gLB Alternating layers of black, greenish, brown-reddish silty gyttja clay, pebble at 6.565. gLB Black moss-rich silty gyttja clay, gLB Reddish-brown silty gyttja clay, FeS spots and laminae, moss remains, gLB Reddish-brown clayey silt, few FeS spots, g-sLB Greenish clayey silt separated by a brown-reddish clay layer (3 mm), sLB Reddish silty clay, very sharp LB Greenish sandy gravelly clay, unregular LB Reddish clayey sandy gravel. g-sLB Reddish-brown sandy silty clay, pebbles, sLB Greenish. yellow, reddish laminated sandy silty clay, pebbles, soft 262 B. Woldfartli er a1 Results Lithostratigraphy. grain size and geochernistrv The lithostratigraphy of the sediments was described in the field where the fresh cores all showed very distinct black FeS stains and horizons. which (with the exception of core S T 4) were found to have disappeared (due to oxida- tion) when the cores were unwrapped in the laboratory. T h e lithostratigraphy presented in Tables 1-4 mainly follows the field description. but has been verified in the laboratory. T h e dis- tinction in different sediment layers (1-20) is based on sediment colour, grain size and organic content. However. because postdepositional chemical reactions are responsible for the colour differences of the sediment column, the litho- stratigraphic sequence was divided into three units. Unit I comprises layers 1-7. unit I1 layers 8-11 and unit 111 layers 12-20. T h e lowermost niinerogenic sediment layers of unit I were recovered in all four cores, but attain a maximum thickness of 55 cm in ST 3B only. In this core the compact reddish-brown and yellow- grey sediments between 6.07-5.935 m (layers 1- 3) show a gradual transition from sandy silty clay to sandy gravelly silty clay and sandy silty clay (Fig. 3, Table 3 ) . Between 5.935-5.82 m (layers 4-5). the sediments are compo3ed of loose sandy clayey gravel and sandy gravelly clay with small pebbles. A reddish-grek sandy silty clay (layer 6) and a silty clay with some FeS stains (layer 7 ) follow with sharp lower boundaries (Fig. 3. Tables 1-4). The water content is constant (ca 20-30%) and reflects the dominance of the silt and sand fractions ( > 5 5 % ) in these bottom layers (Fig. 4 ) . In the uppermost part of unit I . the water content increases slightly to 357r. the sand fraction decreases to 0% and the silt and clay fractions dominate the grain size distribution. T h e total carbon content (C,,,,) varies between 0.3 and 3.3%. Coal particles (C,,,,”) derived from the bed- rock are the dominant carbon source with between 0.4 and 3.1%. Organic carbon (Cur& contributes between 0.2 and 0.8%. The sulphur content is stable below 0.1% (Fig. 4). The transition from unit I t o unit I1 is charac- terised by a drastic change in colour and lithology from reddish-brown silty clay to black silty gyttja clay (Fig. 3). I n cores S T 2 , 3A and 3B. the silty gyttja clays in the lower part of unit I1 are light- brown. reddish-brown and brown-yellowish, whereas the corresponding layers in S T 4 show alternating black, greenish and brown-reddish laminae. T h e silty gyttja clay in the upper part of unit I1 is characterised in all four cores by alternating black and brown-reddish laminae (Fig. 3, Table 1-4). T h e lower boundaries of layers 9 and 10 appear sharp. Layer 10 is only present in cores ST 3 A and S T 4 and seems to be missing in S T 2 and S T 3B. T h e water content increases rapidly from 40% t o 60% at the tran- sition from unit I t o unit I1 and attains 80% in the upper part of unit 11. T h e grain size distribution is dominated by clay and silt, which attain ca 50% each, apart from two excursions at 5.30-5.22 and at 5 . 1 0 m , where the sand fraction increases t o 10% and 5% respectively (Fig. 4). T h e content in C,,, increases from 1.6% at the transition unit 1/11 to a maximum of 4.3% in the upper part of unit 11. This increase is entirely attributable to organic matter. Parallel with the increase in Corgr the sulphur values rise from 0.2% t o 1.8% and remain constant between 0.8 and 1.3% in unit 11. When comparing the curves for organic carbon and sulphur, it becomes obvious that high carbon values coincide with peaks in the sulphur content. T h e upper half of t h e Stevatnet sequence, unit 111 (layers 12-20) comprises silty gyttja clays with colours varying from yellowish, light-brown, red- dish-brown to black. T h e water content decreases slightly t o between 65-70%. but varies, especially in the upper part of unit I11 between 55-75%. T h e grain size distribution displays a dominance of the silt (ca 40%) and clay 50-60% fraction. In the upper part of unit 111, at 4 . 2 9 m , the clay fraction (<2p) decreases t o 20% and the silt fraction (<60 p) increases to 70%. T h e total car- bon content displays in general a slightly decreas- ing trend from 3.2% in the lower part of unit 111 to 2.5% in the top. Corg remains the major fraction of the total carbon content. whereas minerogenic carbon is nearly constant a t ca 0.4-0.6%. T h e sulphur values show a similar gradual decrease from 0.95% to less than 0.2%. Higher sulphur values a t 5 . 0 2 m ( l . l % ) , 4 . 9 4 m (1.2%) and between 4 . 1 0 4 . 2 0 m (0.4%) coincide with higher Core values. Miiieral magrietics A critical prerequisite for the correct environ- mental interpretation of mineral magnetic inves- tigations is the analysis of fresh samples (in this case prior to oxidation). T h e results obtained from core S T 4 a r e presented first as it was noted 4.00 4.20 4.40 4.60 4.80 E .C 5.00 2 a 5.20 % 5.40 5.60 5.80 6.00 4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40 5.60 5.80 6.00 Early Holocene environment on Bj@rn#ya (Svalbard) 263 STEVATNET ST3 A 8 3 B 0 20 40 60 80 0 50 100 Water Content in % Grain size in % Unit 4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40 5.60 5.80 6.00 4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40 5.60 5.80 6 00 0 1 2 3 4 5 0 1 2 Total, minerogenic 8 organic carbon in % Sulphur in % F i g . 4. Water content, grain sizc, carbon and sulphur content of cores ST 3A and 3B. The lithostratigraphy and the analyhes below 5.62 m are interpolated from core ST 3B. that t h e well-wrapped sediments had not oxidised during storage and that the original FeS stra- tigraphy remained intact (see above). Fig. 5 displays the mineral magnetic properties of bulk samples taken from core S T 4. x and S I R M a r e generally low in unit I , between a depth of 7.00 and 6.68m, and indicate a low concentration of magnetic minerals in the minerogenic sediments, while the high S-ratio indicates that antiferrimagnetic minerals (e.g. goethite/haematite) dominate the mineral mag- netic properties. A distinct change occurs in the magnetic properties at the transition from unit I t o unit 11, at 6.68111, and the concentration of ferrimagnetic minerals (indicated by Xand SIRM) increases dramatically. T h e S-ratio decreases t o -0.8 and indicates that a “soft” magnetic mineral (e.g. magnetite/greigite) dominates the mineral magnetic assemblage u p to the top of the core. A p a r t from the lowermost sample at 6.68 m. only insignificant fluctuations occur in the magnetic parameters u p t o a depth of 5 . 8 0 m . In unit 11, between 5.80 and 5.50 m the concentration par- ameters and the S-ratio fluctuate. although they return t o stable values in unit 111, between 5.50 and 5.23 m . T h e linear relationship between S I R M and x in the sediment above 6.68 m (Fig. 6) indicates that only o n e ferrimagnetic mineral makes a significant contribution to the mineral magnetic assemblages in the sediments above 6.68 m . T h e thermomagnetic behaviour of a magnetic concentrate from a sediment depth of 6 . 6 8 m (with high ferrimagnetic concentration) is shown in Fig. 7 A . T h e almost completely irreversible loss of magnetization between 300 and 400°C is typical of ferrimagnetic greigite when heated in air (Snowball & Thompson 1990). The sharp rise in magnetisation o n heating at 150°C is explained by the additional formation of greigite in the 264 B . Wohlfarth et R I . - - T - - - - - E f a a - a - * +++ 3 :+ ++; ++ ++ ++ ++ Q ++t +$+: + { + 3 3+ 8' : + + + + + + ++ ++ + + $ 3 ++ b u l k susceptibility bulk SIRM S - r a t i o SlRMlX ( 1 0 ' ~ S . I . Units) ( r n A m " ) ( k A m ' ) 0.0 4.0 8.0 0 100 2 0 0 . L O O 0.00 1.00 0 4 0 80 5 00 j I + + 5 50 5 75 6 2 5 ++ 7 0 0 - I I ~ I ++ I ++j++ + I+;+:++ + 1 +$ I?++ + I:+ + + * + + ++++,+ + I ++ + + * + f+ Fig. 5 . Mineral magnetic properties o f core ST J 7*50 1 f L + .L sample (Snowball 1991), produced from a phase transformation of unidentified iron sulphides associated with the ferrimagnetic concentrate. The curve shown is typical of those obtained from magnetic concentrates of sediments above 6.68 m and they demonstrate that the magnetic proper- ties a r e dominated by greigite. T h e X-ray dif- fraction analysis of the magnetic concentrate confirms the identification of the ferrimagnetic phase as greigite (Fig. 7B). A magnetic hysteresis curve. representative of the magnetic concen- trates is shown in Fig. 7C. T h e very open central section is typical of natural greigite samples (Snowball 1991) and the high MRs/Ms ratio (0.56) + 0.00 I T , ' and low (Bo)cR/(Bo)c ratio (1.3) indicate that the 0 50 100 150 200 greigite is of a single-domain magnetic grain size. Greigite of variable morphology (needles. plates, cubes etc.) formed by authigenesis at low tem- peratures and pressures in sediments has normally been found to exhibit single-domain magnetic behaviour. bulk S l R M (rnArn') Fig. 6. SIRM Y S magnetic susceptibility for the sediments above a dcpth of 6.68 m i n core ST 4 . The linear relationship demon- strates that onc mineral contributes to the magnetic assemhlagc. Early Holocene environment on Bj#rn#ya (Svalbard) 265 A 1.75 1.50 1 Stevatnet 4 sample 17 extract B I I 0 100 200 300 400 500 600 700 Temperature ("C) . , . . . . . . 55 5 0 45 4 0 35 30 25 20 - 2 8 C - 0.50 -0.25 -0.50 -0.75 -1.00 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 Field (T) Fig. 7. A . Typical thermomagnetic curve of a magnetic con- centrate. The curve indicates the presence of greigite and the absence of magnetite, B. XRD pattern of magnetic concentrate. Greigite is the only ferrimagnetic mineral present. G = greigite, Q = quartz, SH = sample holder. CuKnradiation was applied. C. The hysteresis loop of a magnetic concentrate. The loop is characteristic of single-domain greigite grains. The more scattered nature of the SIRM and S- ratio curves in unit I11 of core ST 4, between 5.80 and 5.50 m, is linked to the observed stratigraphy and is caused by the precipitation of greigite on the surfaces of relatively large moss remains. The discovery that greigite is responsible for the min- eral magnetic characteristics of the sediment that lies above 6.68 m in core ST 4 aids in the inter- pretation of the SIRM measurements carried out on the other cores. Fig. 8 displays SIRM values for all four cores. Greigite is responsible for high values of SIRM above the lowermost minerogenic sediments. Although it is still possible to delimit the different stratigraphic layers in ST 2, ST 3A and 3B, based upon the mineral magnetic measurements, only the magnetic properties of ST 4 can be considered representative of the fresh sediment. The postdepositional authigenic formation of ferrimagnetic greigite prevents the interpretation of the mineral magnetic results in terms of catchment erosion. However, specific geochemical conditions are required to promote greigite formation. Greigite is a precursor to pyrite and may be preserved in sediment sequences when pyrite formation is limited by sulphur availability (Berner 1981). The occur- rence of greigite in the Early Holocene sediments indicates that anoxic conditions prevailed in the deeper regions of the basin following the onset of the deposition of organic material. The Cor$St ratio is approximately 4 in unit I1 of ST 3A (between 5.6 and 5.0m), but increases to ca 7 above 5.0 m and indicates that sulphur availability became a limiting factor in greigite authigenesis. The lower SIRM values in unit I11 of core ST 3A are due to the absence of greigite authigenesis, which indicates that the sedimentary environment altered between a depth of 5.0 and 4.8 m in core ST 3A. Microfossil analysis (pollen, spores, algae) Pollen. - The pollen diagram (Fig. 9) covers the lower part of core ST 3A. The total pollen sum ranges between 54-130 pollen/sample in the bot- tom part, and increases to between 112-418 pollen/sample further up in the sequence. S P Z 1 : Artemisia-Oxyria/Rurnex-Beth zone (5.62-5.52). This pollen zone, which comprises the two lowermost samples (below and above the transition from unit I to unit 11). is characterised by approximately equal proportions of herb and B e t h pollen. Artemisia values attain 20% in the 266 B. Wohlfarth et al ST 2 ST 3A ST 36 ST 4 SlRM rnAm2kg-l SlRM mArn%g” SlRM mAm%g BULK IRM,~MAJ 0 0 0 1 0 0 0 2 0 0 0 0 0 0 1000 2 0 0 0 0 0 0 1 0 0 0 2000 0 0 0 1 0 0 0 0 2 0 0 0 0 - - 4.80 5.20 5.40 5.60 5.80 6.00 6.20 6.40 6.60 6.80 7.00 + + I++:; 3: + + ~ tf + + $ + + 3 :+ ++ + + + ++ + * + +,t: + + + + + + + + 3 +++ 3 + + + + + + !+I?++ 4 A 3 + + + + Erg k T h e S I R M of S r 2 ST 3A B B and ST 4 Greigite I S responsible for the hqh values of SIRM in all of the cores bottom sample, but decrease to 10% in the upper sample, whereas 0xvrialRutne.x frequencies increase to 30%. but show a slow decrease in the upper part of this zone (10%). S P Z 2 : Oxyria/Rirtne.x-Betula-Piniis-zone (5.52-4.95 m). During this pollen zone. herb pol- len values decrease gradually from 45 to 1055, while frequencies o f tree pollen increase from 5 5 t o 90%. Oxyria/Rime.x values decrease gradually from 30 to 5 % . Salix and Arternisia pollen fre- quencies remain at ca 5 % . but decrease in the upper part t o ca 2.5%. Jittiiperirs values increase aliphtly during the uppermost 2 0 c m . but do not attain more than 5 % . T h e lowermost local pollen zone SPZ 1 com- pares well with Hyviirinen‘s (1968. 1970) zone I . where he describes abundant non-arboreal pollen ( N A P ) with a dominance of Betula among the tree pollen. T h e second local pollen zone SPZ 2 may be compared with zone IIa in Hyvarinen‘s (1968. 1970) diagrams, where lower NAP values and a rise in A l n u s are reported. There is, however, a major difference between our diagram and the diagrams presented by Hyvarinen (1968, 1970): Pinus pollen values attain rarely more than 20% in o u r diagrams, but increase in Hyvarinen’s (1968, 1970) zone I1 to more than 50%. This might be explained by the fact that we could have lost a major part of the pine pollen during the pollen preparation, where we applied a ZnC1, treatment to separate the pollen grains. O u r pollen spectrum is clearly dominated by long-distance transported pollen grains. All tree pollen (Betula, Pinus. Alnus) and most of the non-arboreal pollen (Arteniisia, Chenopo- diaceae. Filipetidula, Urtica. Jitniperus, Caryo- phyllaceae) must be regarded as exotic. Salix, Gramineae. Cyperaceae, Oxyria/Rume.r. Rosa- ceae might represent a mixture of local and long- distance transported pollen. Ranunculaceae, N d .. -~ . ........................ ro a OD n N n 268 B . Wohlfarih et al. Ericaceae and Sarifraga which occur in the recent flora ( E n g e l s k j ~ n 1986) may have grown o n the island. T h e dominance of long-distance trans- ported tree pollen in t h e pollen spectrum is a typical feature in Holocene pollen spectra from e.g. Spitsbergen (Hyvarinen 1970) or northern Fennoscandia (e.g. Hjelmroos & FranzCn 1994). Possible sources include areas adjacent to the Barents Sea but may b e as far south as South Europe/North Africa (Hjelmroos & Franzkn 1994). Spores und algae. - Spores of Polypodiaceae (undiff.) are present throughout the whole sequence with ca 5%. Lasrrea dryopteris appears from ca 5.51 m upwards (Fig. 9). It attains values of 1 0 8 at 5 . 1 S m . but decreases below 5% between 4.99-4.95 m . All spores are regarded as long transported. Pediasrritm values increase very rapidly from less than 57c at 5 . 6 2 m to 10% at 5.60 m and to 5 0 8 at 5.56 m . In the upper part of the sequence Pediastrum values a r e around 50- 70%. Boiryococcus decreasesfrom 1 0 7 ~ at 5.62 m to less than 5% at 5.55 m. It is not present between 5.52-5.40 m. but increases again from 5.40 m upwards. Diafoms. - Diatoms are extremely rare in most of the analysed samples of core S T 3 A (Table 5 ) . In unit 1 and in the lowermost part of unit I 1 only few, quite heavily corroded diatoms were found. However. higher u p in unit 11, the preservation was slightly better. T h e diatom assemblage is dominated by Brachysira species and includes Staiironeis anceps, Fragilaria pinnata. Amphora libyca. A m p h o r a cf. ueneta, which indicate fresh- water conditions and a probably higher p H than Table 5 . Diatom taxa recorded in core ST 3A that found in the top sample. Between 5.50- 4 . 5 0 m (units I1 and 111) the diatoms are again very badly preserved. Although the species recovered at 4 . 5 0 m are also poorly preserved and dissolved, robust forms, such as Fragilaria pinnata, Frustulia sp. and Pinnularia can be recog- nised. Fragilaria pinnata is fairly common in soft waters. Only in the t o p sediment at 4.00 m , where Fragilaria virescens var. exiguam becomes t h e dominating form, are the diatoms abundant and well preserved. All the species recorded a t 4.50 m are benthic forms and indicative of slightly acid (possibly around p H 5-6) and shallow waters. Shallow water could explain the absence of centric forms. Macrofossil analysis Macroscopic remains of plants and animals were sorted o u t from cores S T 2, S T 3A and S T 4 (Fig. 3). T h e results of the identification are shown in Table 6. T h e following remarks o n the veg- etational and faunal record is based o n the mod- e r n biology and geographical distribution of the mentioned taxa (Palm 1948; R ~ n n i n g 1979; Eng- e l s k j ~ n 1986; Fitter & Manuel 1986; Hulten & Fries 1986; Nilsson & Persson 1989; Mossberg et al. 1992). In general, mosses and midges dominate the fossil assemblages and are present from the lower- most analysed samples in unit I t o the uppermost sample in unit 111. Mosses a r e especially frequent in the lower part of unit 11. Leaf fragments of dwarf willows such as Salix reticulata, S. herbaceae and S . polaris a r e recorded in units I1 and I11 (Fig. 3 & Table 6 ) . These species are today dominant or characteristic in plant communities on well- Depth ( m j 4 CKJ 4.51) 5.00 5.37 5.54 5 59 5.61 5 6 2 5.66 Unit 111 111 111 11 11 11 I I I ~~ Diatom taxa (listed according to thcir frequency) Fragrlaria uirescerrr var uxrgua. Fragilarra consrrriens var ~ e n r e r . Naoicula cocconeiformis. C y f n b c l l a guumanii. Achrianrhes flexellu. N a ~ r c r i l a iirgirulro. Pinrrlaria sp., Frrisrulina rhornboides var saxonica. Achnanrhes scotica Fragilaria pinnara. Frusrulia s p . . Prnnulana sp. fe\r/no diatoms fewjno diatoms Stouroneis anceps. Fragilaria pinnara. A m p h o r a libyca. A m p h o r a cf. oeneru. Bruchysira sp. diatoms are extremelv rare diatoms are extremely rare diatoms are extremely rare diatoms are extremely rare Early Holocene environment on B j ~ r n f l y a (Svalbard) 269 drained and exposed ground in the inland of Bj@m@ya. In two samples from the upper part of unit I1 (layer l l ) , rather corroded leaf remains of Dryas sp. were found. This plant grows on calcareous soils in Spitsbergen and Scandinavia, but is not found on BjornQya. This species is probably absent because it requires higher sum- mer temperatures than those presently encoun- tered on the island (EngelskjGn 1986). One fruit of Angelica cf. archangelica is recorded in sample 30 of unit I1 (layer 11). Today this species has a more southern distribution with its northern limit in northernmost Norway. It grows on moist sandy or peaty soils. in willow shrubs, meadows and on river shores. Three beetle species were identified among the insect remains (Table 6). The diving beetle Agabus bipustulatus is widespread in Europe. In arctic and alpine regions its main habitat is shallow lakes with stony or silty bottoms. The rove beetles Olophrum boreale and Eucnecosum tenue are dis- tributed in northernmost Europe, where they are found in leaf litter of S a l k and Betula. Caddis fly larvae (Trichoptera) of the family Limnephilidae occur in all sorts of habitats, ranging from fast- running streams to large lakes. Midge larvae ( D i p - tera, Chironomidae) mainly live in the bottom sediments of lakes or ponds. The jaw of the notostracan crustacean Lepidurus (tadpole shrimp) found in unit I1 (layer 11, sample 31) could not be identified to species level. However. Lepidurus articus Pallas is a common species today north of the Arctic Circle, where it lives either in temporary pools or at the edges of lakes (Taylor & Coope 1985). Chronostratigraphy Serious problems concerned with the radiocarbon dating of arctic sediments have been reported in previous studies (e.g. Olsson 1986; Bjorck et al. 1994a, b; Abbott & Stafford 1995; Miller et al. Table 6 . Macroscopic remains of plants and animals from cores ST 2 , 3 A and 4 . L = leaves, Bs = budscales. F = fruit, Ms = Meta/ Mesosternum. L E = left elytron. RE = right elytron, Th = thorax, H = head, Mnd = jaw. Relative abundance of remains are indicated as + = 1-10, + + = 11-100, + + + > 100. See Fig. 3 for the location of the samples. TAXON SAMPLES SPERMATOPHYTA Salicaceae Salix reticulata L. Salix herbacea L. Salix polaris Wahlenb. Salix spp. Dryas sp. Angelica cf. archangelica L. Rosaceae Apiaceae BRYOPHYTA indet. INSECTA Coleoptera Dytiscidae Staphylinidae Agabus bipusfulatirs (L.) Oiophrrrm boreale (Payk.) Eucnecosurn lenue (LeC.) Gen. indet. Trichoptera Limnephilidae indet. Hymenoptera Fam. indet. Diptera Chironomidae indet. NOTOSTRACA Lepidurus sp. 11 (Ms) 8 (RE), 10, 13 (LE), 33 (Th) 38 (RE) 30 (Ms) 23, 27, 38 (Th) 10. 31 ( H ) 4 , 6 , 8 - 1 3 , 1 5 - 1 7 ( + I , 1 8 2 0 (++), 21 (+). 22-25 (++). 26, 27 (+), 2 8 , ~ ’ ) (++). 30 (+I. 31 (++). 32-40 (+) 31 (Mnd) 270 B . U’ohlfnrfh e f a / . 1995). T h e major problem in coal-bearing regions is that bulk sediment dates give erroneous ages d u e to “old carbon” derived from bedrock (Ols- son 1968; Bjiirck et al. 1994a). Another source of error may be I4C-depleted organic carbon. which is stored in tundra environments and incorporated in the sediments at a later date ( A b b o t & Stafford 1995). To obtain correct radiocarbon dates, which exclude these sources of error. we only chose carefully selected macrofossils for dating. T h e selected samples contained >2 rng dried material. T w o samples were composed of moss remains and t w o samples of leaves of Snli.1- sp. (Table 7 ) . T h e four A M S measurements obtained enable us to date the beginning of the organic production in the lake at the transition from unit I to unit I1 and the uppermost part of unit I1 (Fig. 3). To be able to compare o u r AMS dates to the bulk radio- carbon dates obtained earlier on Bj0m0ya (Hy- varinen 1968.1070; Olsson 1968). i t was necessary to transform them into a bulk sediment age. If the organic carbon content and the age of the macrofossils in the sediment are known. as in this case ( s e e Table 7 and Fig. 4 ) . the influence of old bedrock carbon on bulk sediments can be calculated with the formula: L J where A,,,, = the macrofossil Abuli. = the bulk “C age A M S “C measurement x = the fraction C,,, of C,,, If this formul,i is applied to our dates (Table 7 ) . the AMS “C ages of the samples would result in bulk “C ages of between 16.955 and 10.130 B P In this example the contribution of 60-20% ot ”old carbon” causes age differences of between 7200 and 1800 radiocarbon years. The gradual decrease in the amount of minerogenic carbon in the younger sediments decreases the difference between the “true age” and the bulk age. T h r e e dates on sediment samples from Vestre Skinkevatnet (northern part of B j s r n ~ y a ) , obtained on the humic fraction (extracted with 1 % N a O H ) were published by Hyvarinen (1968, 1970) and discussed by Olsson (1968). The lower- most date. Ua-2031 with an age of 11,200 ? 500 I4C B P was obtained ca 10 cm below Hyvarinen’s (1968) pollen zone transition 1/11. Ua-2050 (in pollen zone IIb) gave an age of 4600 + 120U/ - 1oCKl ’.‘C BP. However, Ua-2049, ca 5 cm below the pollen zone transition I I / I I I , gave an older age of 6000 + 3100/-2200 I4c BP. In the case of sample Ua-2031 additional wet combustion with H 2 0 1 and H N 0 3 resulted in an age of 8300 + 1400/-1200 “C B P (Ua-2042). T h e non- dissolved remains were dated at 8900 + 1200/ - 1000 “C BP (Ua-2064) (Hyvarinen 1968; Olsson 1968). Olsson (1468) suggested that the dis- crepancies between t h e different fractions of the bulk sediment were caused by coal particles. The large errors and the age inversion of the two younger samples (Ua-2050, Ua-2049) indicate that too little humus could have been extracted for a measurement (Olsson 1968) which makes a comparison t o our A M S dates impossible. Based on the assumption that our local pollen zone SPZ 1 (5.62-5.52 m ) in core S T 3 A (Fig. 9 ) might be comparable to Hyvarinen’s (1968) zone I. then o u r A M S ages of between ca 9800-9500 “C BP should correspond approximately t o Hyva- rinen‘s bulk age of 11,200 2 500 I4c BP (Ua-2031). If this correlation holds true, one might draw the conclusion that the amount of “old carbon” present in sample Ua-2031 is ca 15-2096 which Tuhlr 7 . A M S ”(’ mca\uremcnts on plant m a c r o f o s d s from corcs ST 2 and ST 3 A Thc ilniount of hcdrock carhon (C,,,,) has bcen calculated from the carhon \slur\ ( i f the correapondms I d y m For the calsulatlons of thc bulh ”C age B P , see formula in thc 1 C Y I Sample usight L.ih no Corc hcforc. ’nflrr p r c - AMS “C C,,,,, Bulh “C h!er nt). Depth ( n i l T a x o n treatment ( m e ) y r \ B P ( ‘ T ) ape Br l x l n l S u h ( L c a \ e \ l 2 58,’l 84 8340 z xs ‘0 10.130 47YY S o h (Lea\ e , 1 2.71; 1 . E 8.140 t_ I 2 5 20 10.130 41.21 Aryophyro indet. - ,?J.27 Y565 2 95 30 41-w 51 ? A 8 5 5 % E r v o p h u a indrt - 18 3 97Y5 ? Y O 61 Early Holocene environment on Bj@rn@yvn (Sualbard) 271 would result in a “true age” of ca 9650 l4C BP. Such a n age would then correspond to o u r age attribution for SPZ 1 . However, this approach is highly hypothetical, because (1) the dates were performed o n material from two different lakes and (2) we d o not know the minerogenic carbon content of the old samples. Palaeoenvironmental synthesis Before 9800 ‘‘C B P The highly minerogenic bottom layers (unit I) possess alternating coarse- and fine-grained sedi- ments with a high concentration of redeposited coal particles. The lithology of these diamictons reflects t h e erosion of the surrounding bedrock. T h e compactness and the low water content, especially of t h e lower part of unit I (layers 1-4), implies over-consolidation. W e suggest that these sediments were originally deposited as tills by a locally active glacier during the ice retreat (see also Salvigsen & Slettemark 1995). whereas the overlying sediments (layers 5-6) may originate from glaciofluvial/fluvial deposition. A s confirmed by the mineral magnetic analyses, the reddish-brown colour of t h e lowermost sedi- ment is attributable to ferric oxides/oxyhy- droxides, such as haematite and goethite, which a r e stable only in oxic environments (Berner 1971, 1981). T h e fact that this sediment was almost devoid of easily metabolised organic matter prob- ably explains why it remained oxic after deposi- tion, and why the ferric oxides survived. Few diatoms a r e found in this sediment which was probably deposited from a turbid water column, a n inhospitable environment for living organisms. Oxic conditions after deposition may also have enhanced the corrosion and dissolution of t h e diatom valves. T h e decreasing grain size, as well as the decrease in minerogenic carbon in the upper part of unit I. indicate that the erosion slowly ceased, possibly d u e to gradually more stable soils, which allowed deposition of a silty clay (Figs. 3 and 4). However. the transitions between the layers, and especially between layers 6 and 7, are sharp and may indicate active sediment resuspension and the formation of a subsequent hiatus. The pollen sample from layer 7 includes mainly long-trans- ported pollen grains, but also suggests that Salix was growing in the surroundings. Algae of the type Botryococcus, mosses and midges found in the sediments indicate that the biological pro- duction in the lake had gradually started. Oxic conditions may also have prevailed during the deposition of the upper part of unit I, which is corroborated by the mineral magnetic analysis and explains why only so few (corroded) diatoms a r e present in the sediment. 9800-9500 l4C BP T h e transition from the minerogenic sediments in unit I to the organic sediments in unit I1 involves a drastic increase in the sulphur content and coincides with an abrupt change of the colour from reddish-grey to black (Figs. 3 and 4). The black sediment colour is derived from metastable sulphides of iron. in particular greigite (Fe,S,), the formation of which represents an arrested stage in pyrite diagenesis (Berner 1971, 1981). T h e microbial reduction of sulphate is a strictly anaerobic process, and the formation of greigite in sediments is intimately linked t o the diagenetic processes associated with organic matter decomposition and the formation of H2S. The bedrock of Bj0rn0ya and the Stevatnet catchment is rich in iron sulphides (e.g. pyrite) and iron oxides and is potentially a major source of both iron and sulphur. Another important source of sulphate must b e sea spray, which is transported by winds over the island and deposited in the lakes. It seems therefore reasonable to assume that the availability of easily metabolised organic matter, rather than of sulphur or iron, was a limiting factor in the sulphide diagenesis. As a result, conditions which supported oxygen deple- tion and associated sulphide formation did not develop until reactive organic compounds were added to the sediment through the onset of bio- logical production. This development coincides with a n increase in the number of midges, moss remains and in Pediastruin in the sediment. Once the prerequisites were given, restricted exchange of oxygen d u e t o long seasons of ice-cover may have further enhanced the oxygen depletion in the basin. Relatively harsh conditions and/or a high amount of clay in the water column may be t h e reason why diatoms are still rare in the sediments from this time period. The pollen record is dominated by long-trans- ported tree pollen. Salix, Ranunculaceae, Eri- caceae and Saxifraga pollen may be regarded as local. 272 B. Wohlfcirtir et 01. 95004300 IJC B P The gradual increase in organic matter in the sediment during this period is probably d u e to a combination of enhanced production in the lake. development of pioneer plant communities and increased inwash of organic material from the surrounding catchment (see below). Apart from midges. remains of Coleoprerci. Tri- clzoptera and Hyrnerioptern are found together with algae and mosses. The few diatoms present in the lower part of unit I1 indicate freshwater conditions and a slightly higher p H value than that which the top sample indicates. The low concentration of diatoms at 5.37 m could be related t o a high clay content in the water column. The pollen include long-distance transported tree and non-arboreal pollen. However. S&x. Rosaceae . Ericaceae , Ranunculaceae and S o x i - fragri may be regarded as local pollen producer\. The vegetation i n the close surroundings of the lake may have been composed of. among others. willow shrubs ( S a l i x reticirlatn. Sci1i.Y polnris) on well-drained grounds. of Dryas plants o n cal- careous soils and Angelica cf. archangelicn. which g r o w 5 on moist sandy or peaty soils or in willow scrub areas. The climatic reconstruction using the MCR method is based on two beetle species. ,4gahir.s hipiistiilcrtiis and Olopkritrn horeale recorded i n sample\ 1 0 and 11 of unit 11 (Table 6 ) . The results indicate a probablc mean July temperature (TMAX,,,,,) of 9.3 -t 2°C and a mean January temperature (TMIN,,,,,) o f -12.3 t 5°C. A warmer climate during this time period than today is also suggested by the presence of the 5,ascular plants I h y c i s sp. and Arrgelica cf. arcliangelico. which require higher summer temperatures than those prevailing on the island at present (Engels- k j s n 19%). T h e MCK results from Stevatnet sug- gest t h u b 1-5°C higher mean July temperatures than present. The winters seem to have been slightl! colder than present on B j s r n s y a . with mean January temperatures 5 6 ° C lower than today A strong seaaonality. which implies more con- tinental conditions during this period. may esplaiii some o f the changes in the lakeicatchmcnt ecosystem that are recorded in the lake sediments. The formation of the distinctly sulphide laminated sediment in the upper part of unit I1 is connected to cyclic. possibly seasonal alterations of the redox conditions. The fact that the laminations arc best preserved in core ST4, which was collected from the deepest part of t h e lake, implies that bottom anoxia developed during seasonal stratification of the lake or during periods of long ice cover. Furthermore, the preservation of these laminae suggests that bottom anoxia was severe enough to eliminate benthic animals, whose grazing and burrowing activities normally obliterate sediment structures. T h e increased summer temperatures must have enhanced thaw processes in the upper- most soil layers which explains the episodes of increased supply of allochthonous material, both minerogenic and organic. 8.300 ’“C B P lo present T h e r e a r e no A M S “C dates available which would allow an age assignment for the silty gyttja clays in unit 111. We can only assume that the sediments were deposited between 8300 “C B P and the present. which indicates that the sedi- mentation rate in general decreased considerably compared to the previous periods. During the deposition of the lower part of unit 111 rather calm conditions may have prevailed in the lake basin. followed by sporadic inwash of minero- genic material. In the upper part of unit 111, a sudden drastic increase in grain size is accompanied by an increase in minerogenic and organic carbon and i n the sulphur values. Increased in-wash of material from the sur- rounding slopes by soil processes/active weathering/freeze-thaw processes might be an explanation for this phenomenon. Chironomidae and Limnephilidae are present among the limnic fauna, and shrubs of Sdix refi- citlrctci and Salix herbacea grew in the lakes‘sur- roundings. These plants are a preferred habitat for the two rove beetles Olophrurn horeale and E i ~ c n e r o ~ i m tenue. Long-transported tree pollen increase gradually and non-arboreal pollen show a slight decrease. T h e benthic diatoms which are abundant in the t o p sample indicate that the lake has become slightly acid (possibly around p H 5- 6 ) . Conclusions The interpretation of o u r data set is based on lithology. grain size. mineral magnetic measure- mentb. geochemistry. micro- and macrofossil analyses and AMS “C dates on plant macro- fossils. This multi-disciplinary approach allows us Early Holocene environment on B j l r n l y a (Svalbard) 273 4~ years BP 8000 - 8500 - Fig. 10. A palaeo- environmental synthesis of the earlv Holocene palaeoenvironment on BjmnBya based o n the Stevatnet sequence. LAKE Relabvely calm condibons Increased inwash of minerogenic and organic material from the catchment Seasonal anoxia in deeper parts High organic production organic production Beginning of the organic production DEGLA 10.000 CATCHMENT Increased freeze/lhaw processeslincreased erosion Pioneer vegetaeon with willow shrubs, Dryas and Angelica archangelica Establishment of a pioneer vegetation and gradual stabilisation of the soils " IATION TEMPERATURE Gradual temperature decrease Increased seasonality mean July temperatures ca. 9'C and mean January temperatures ca. - 1 2 ' ~ Gradual temperature increase ............. " COVERED BY LOCAL GLACIERS t o suggest a possible environmental scenario for the early Holocene on Bjornaya, summarised in Fig. 10. Immediately after large scale deglaciation and prior to ca 9800 "C BP, glacial sediments were deposited in the lake basin. T h e time period between 9800 and 9500 I4C BP is characterised by gradually more stable soils, avery sudden increase in limnic production, although long seasons of snow and ice cover may have restricted an exten- sive development of the biological communities. The increase in limnic production coincides with the abrupt sea surface temperature rise of 2- 3°C between ca 10,000 and 9500 I4C BP (Hald & Dokken 1995) which was caused by the replace- ment of cold Arctic waters with warm Atlantic water in the western and southeastern Barents Sea (Hald & Vorren 1987; Vorren et al. 1988). Between ca 9500 and 8300 "C BP a more con- tinental climate. with strong seasonal contrasts prevailed o n B j ~ r n o y a . The marked temperature increase, with mean July temperatures around +9"C, is possibly as much as 4-5°C higher than present and the mean January temperatures around - 12°C may have been 5 6 ° C lower than present. T h e plant communities on Svalbard show that high summer temperatures affect plant growth, mostly through the length of the snow- free growing season (Ronning 1969; Engelskjon 1986; Skye 1989). High summer temperatures on BjOrnoya between 9500 and 8300 14C BP were responsible for the development of a pioneer community including Dryas and Angelica archnn- gelica. As a consequence of the strong seasonality, freeze/thaw erosional processes in the sur- rounding soils must have been enhanced. This, in combination with the more extensive vegetation cover caused an increase in the deposition of allochthonous organic material in the lake. This increase, coupled with high autochthonous pro- ductivity in the lake during the summer months. may have caused oxygen deficiency, particularly severe during seasonal stratification and periods of ice cover. Rapid snow melt and a n influx of a large quantity of water into the lake during the spring months would have caused temporary de- stratification and oxygenation of the sediments. This may explain why the lake experienced alter- nating oxic and anoxic conditions, indicated by the FeS laminated sediments and the precipitation of authigenic greigite (Hilton 1990). Between 8300 14C BP and the present the sedi- mentation rate decreased considerably and apart from two periods which display an increased in- wash of minerogenic and organic material. the lake basin experienced fairly calm sedimentation. Anoxic conditions coincided with the increase in total carbon content. T h e lack of ''C dates makes it, however, impossible t o discuss these phases in more detail. The diatoms in the top sediment indicate that the lake has become more acid. O u r temperature estimates for the time period 274 B. Wohlfarth et a1 between 9500-8300 “C BP compare well with other studies from around the Kara. Laptev and East Siberian Seas. where a climatic optimum is described between 10.000-8500 “C BP (Verkulich et al. 1995) when arctic hunters seem to have migrated into these high altitudes (Pitulko RC Makeyev 1 Y Y 1 ) . Higher temperatures than present for the early Holocene are also recorded from several studies on Svalbard. Thermophilous marine m o l l u x s flourished e.g. between ca 9500- 1000 “C B P (Salvigsen et al. 1990. 1992) and the Linnebreen glacier melted completely during this period (Mangerud & Svendsen 1990). Tem- perature estimates based on plant macrofossil studieh in w’est Spitsbergen indicate that between ca 80011 and 4000 ’‘C BP the mean July tem- perature was cil 2°C higher than today (Birks 1991). which is slightly later than our “tem- perature optimum”. Compared t o our data this may suggest that temperature had already slightly decreased after 8000 ‘“C B P . and that the warmest period during the early Holocene occurred around 9500 and 8300 “C BP. as recorded in the sediments recovered from Stevatnet. This coincides well with calculations of the solar radi- ation for the Northern Hemisphere which suggest that maximum Holocene seasonality was reached at around YO00 “C BP. with summer insolation higher and winter insolation lower than today (Kutzhach & Webb 1993). This pattern matches our environmental reconstruction convincingly well. ~ ~ k t i i ~ i ~ l e i l ~ e ~ n e i i r ~ ~ O u r t h a n k go to the Swedish Polar Secretariat a d the S o r u c g i a n Polar In,titute for logistic and economic >upport of the held work. to the N o r w g i a n Coastal G u a r d . \ \ h o arranged rhc tramport t o and from the island. to the stat1 at Radio Bloriioya. \\hose great hospitalit! com- pcnsated tor all the loge! 2nd rain! da!5 during the field work. to the helicopter crew5 of the Soru-egian “Redningstjenc\ten“ f o r their hclp with the transport 01 thc fieldwork equipment and the Epectacular flight5 w e r the i\land, to X Bao ( L u n d ) tor the pollen prcparations and r h e sulphur analyses o n core ST 3 8 . to C . P e t c n ( E d i n h u r r h l for mcasuring the thcrmomagnetic prupertic\. t i ) ,LI -J C i ; i i l l . d ILund) and 1 1 . Il!\arincn ( t l c l - sinki) f o r their help in intcrpreting the pollen diagram :ind to S . Bjorck ~C‘openhageii) .ind 0 Bennike (Copenhagen) s h o criticall! read the nianu\cript and suggccted man! \aluahle imprix c‘incntc. References Ahhott. \ I B . & Stattord. T \5 1495: Radiocarbon reservoir .ice\ and carhon q c l i n g i n arctic ,ind high-elevation Iakc ,)>tern. Absrrucr. S e c o n d Annual P A L E Research Mc>ering. L i t o m i l l e . Washington. 4-6 February 1945. Atkinson. T. C . . Briffa. K . R . . Coope, G . R . . Joachim. J . M. gi Perry. D W . 1986: Climatic calibration of coleopteran data. P p . 851-858 in Berglund. B. E . (ed.): Handbook of Holocene Palaeoecology and Paliirohvdrology . Wiley , Chi- Chester. Atkinson. T. C.. Briffa. K . K. & Coope. G. R. 1987: Seasonal temperatures in Britain during the past 22.000 years. recon- structed using beetle remains. Nnrrcre 3.52. 587-5Y2. Battarhee. R . W . (1986): Diatom analysis. Pp. 527-570 in Berglund. B . E . ( e d . ) : Handbook ofHolocene Palaeoecology and Palaeohydrologv. Wiley, Chichester. Berner. R . 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