Rapid climatic events as recorded in Middle Weichselian thermokarst lake sediments

ARTICLE IN PRESS

Quaternary Science Reviews 27 (2008) 162–174

Rapid climatic events as recorded in Middle Weichselian thermokarst lake sediments S.J.P. Bohnckea,, J.A.A. Bosa, S. Engelsa, O. Heirib, C. Kassea a

Department of Paleoclimatology and Geomorphology, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands b Palaeoecology, Laboratory of Palaeobotany and Palynology, Institute of Environmental Biology, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands Received 1 February 2006; accepted 12 September 2007

Abstract From a Middle Weichselian sediment sequence in the opencast brown coal mine of Reichwalde (eastern Germany), a 40 cm thick thermokarst gyttja deposit has been sampled. The AMS 14C dates, although at the limit of detection, indicate an early Middle Weichselian age of the gyttja. Pollen, botanical, and zoological (e.g. chironomids) macroremains have been analyzed. Botanical and chironomid taxa indicate warm climatic conditions in the bottom part of the sequence. For this lower part the botanical data suggest a minimum mean July temperature of 12–14 1C. Following this, a cooling is indicated, coinciding with an increased clastic deposition in the lake. A return to permafrost conditions is reconstructed for the upper part of the sequence. The combined evidence strongly suggests a degradation of permafrost due to increased warming in response to a D/O event as a forcing factor for the thermokarst lake formation. r 2007 Elsevier Ltd. All rights reserved.

1. Introduction The Weichselian Early, Middle, and Late Pleniglacial have been correlated, respectively, with Oxygen Isotope Stages 4, 3, and 2 of the marine d18O record (Martinson et al., 1987; Behre and van der Plicht, 1992, Table 1). Investigations of the Greenland ice cores over this interval revealed many rapid climate oscillations in the oxygen isotope record (Johnsen et al., 1992; Dansgaard et al., 1993), the so-called Dansgaard/Oeschger (D/O) events. A D/O event typically starts with an abrupt warming of Greenland by 5–10 1C over a few decades or less, followed by a gradual cooling over several hundreds to more than a thousand years and often ends with an abrupt final reduction of temperature back to cold (stadial) conditions (Ganopolski and Rahmstorf, 2001). Climate studies based on marine cores from the Atlantic Ocean (Bond et al., 1993) also have shown these short-term climatic fluctuations. The period between the successive D/O events is

Corresponding author.

E-mail address: [email protected] (S.J.P. Bohncke). 0277-3791/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2007.09.017

most often around 1500 years or a multiple thereof (Bond et al., 1997, 1999; Grootes and Stuiver, 1997). However, the majority of these climate oscillations are not reflected in the NW European continental palynological record, except at more southerly locations, where interstadial periods seem to have been registered better in pollen records, e.g., Les Echets, La Grand Pile, and the Velay region (e.g. De Beaulieu and Reille, 1984, 1992; Guiot et al., 1989; Reille and de Beaulieu, 1990; Reille et al., 2000). In the northwestern European terrestrial record only three to five interstadials have been recognized during the Weichselian Pleniglacial (e.g. van der Hammen et al., 1967; van der Hammen, 1971; Zagwijn, 1974; Kolstrup and Wijmstra, 1977; Vandenberghe, 1985; Ran, 1990; Behre and van der Plicht, 1992). Correlation between the ice-core record and the terrestrial botanical record therefore remains unclear. Based on the ice-core records it is likely that there were intervals during the Pleniglacial when climate in NW Europe was suitable for the development of boreal forests. However, pollen and macroremain analyses have shown that such forests were not present (e.g. Kolstrup, 1990; Bos et al., 2001). This absence seems to conflict with the

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174 Table 1 Chronostratigraphy of the Weichselian and comparison with the d18O record of the GRIP ice core (Johnsen et al., 1992), the Oxygen Isotope Stages (Martinson et al., 1987) and the terrestrial Interstadials and Stadials (e.g. Ran and van Huissteden, 1990; Behre and van der Plicht, 1992; Dansgaard et al., 1993). Ages follow Martinson et al. (1987) and Walker et al. (1999)

163

To test whether thermokarst formation is internally or externally driven, the thermokarst infilling can be analyzed for temperature-dependent proxies. If thermokarst initiation was triggered by climate warming, then evidence of such a thermal spike should be present in especially the lower parts of the thermokarst infilling. Given the short duration of the Pleniglacial warm spikes in the ice-core record, one may assume that the bulk of the thermokarst infilling might already represent the waning stage of the thermal spike and the development toward a succeeding cold phase. Our hypothesis is that, analogous and synchronous to the D/O events registered in the ice and marine cores, rapid climate warming occurred over northwestern Europe and that these rapid climate oscillations could have been recorded as thermokarst features. 2. Geological setting

temperature regime as reconstructed from pollen, macroremains, and beetles and often has been the subject of speculations (e.g. Coope, 1975, 2000; Kolstrup and Wijmstra, 1977; Kolstrup, 1990; Ran, 1990; Van Geel, 1996; Bos et al., 2001, 2004). A variety of factors has been suggested to explain the virtual absence of trees in NW Europe during the Pleniglacial: (1) too dry and continental climate, (2) wind stress, (3) heavy grazing pressure from large mammals, (4) highly dynamic soil conditions due to periodic permafrost, (5) the suddenness and intensity of the climatic warming, and (6) the short duration of the warming intervals that left too little time for trees to migrate from their refugia in southern Europe. The latter two arguments suggest that vegetation response was not rapid enough to react to the short-lasting warming events. However, fast migrating biotic proxies such as Chironomidae and aquatic botanical taxa could be able to register even very abrupt warming events. An abrupt climate warming following a prolonged cold (stadial) period could lead to permafrost degradation and the formation of thermokarst lakes (so-called thaw lakes). This suggests that the response to a sudden and relatively short climate warming could be documented in the infill of such a thaw lake. However, not every thermokarst lake is the result of external forcing mechanisms like a climatic warming. The formation of thermokarst lakes can also be due to internal forcing mechanisms, such as natural fires, erosion by river cut banks and inundations (Hopkins and Kidd, 1988).

The opencast lignite mines in the Niederlausitz, eastern Germany, reveal extensive Weichselian sediment series deposited in a fluvial and partly aeolian context (Fig. 1). These sediments are subject to an ongoing research program into the fluvial, palaeoenvironmental, and climatological history of the region (Mol, 1997a, b; Bos et al., 2001; Kasse et al., 2003). The sequence in these quarries consists of local deposits from the rivers Spree and Neibe. The latter flowed in a western direction through the ice marginal valley during part of the Middle Pleniglacial (Fig. 1). Within the Weichselian fluvial and aeolian deposits frequent organic intercalations occur, which can be used for the reconstruction of the Pleniglacial vegetation and climate (Bos et al., 2001). In the opencast lignite mine of Reichwalde some of the organic sediments were interpreted as thermokarst lake deposits. The identification of former thermokarst situations and thermokarst sediments is complicated. Over the past decade we developed several diagnostic criteria by which thermokarst phenomena could be distinguished. These criteria are: (1) strong deformations and ice-wedge casts below the thermokarst deposit ascribed to the degradation of the permafrost; (2) a sharp contact between the deformations and the overlying thermokarst infilling; (3) an abrupt and undeformed organic infilling of the thermokarst depression with gyttja reflecting an immediate presence of a lake after the permafrost degradation. A gradual deepening of the lake and a concomitant drowning of the vegetation in such case should be lacking; and (4) the infill is mostly horizontally laminated. 3. Material and methods During a 2-month field work campaign in the Reichwalde opencast lignite mine (Fig. 1) in June 1999, the sequence in the eastern wall of the mine was logged. Five stacked fluvial and aeolian units were identified, and the

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

164

WELZOW MINE

Germany

WELZOW

N

Poland SPREMBERG BAD MUSKAU Sp

WEIßWASSER

ree

iß

e

NOCHTEN MINE

HOYERSWERDA SCHEIBE MINE

REICHWALDE MINE

5 km

older deposits (mostly till)

Ne

REICHWALDE ice-marginal valley (former Neiße river)

lakes

mines

cities towns

rivers

sample location

Fig. 1. Location map.

LM8

sand thin sand bands clay

gyttja

loam

← loading structure

sand

← frost fissure

Fig. 2. Thaw lake deposit LM8 and position of the sampled box.

intercalated organic horizons were described. One of the organic layers, i.e. LM8, was sampled in a plastic box (20  25  80 cm) in order to secure enough material for pollen, macroremain, chironomid, and loss-on-ignition analyses and for 14C dating on selected seeds/fruits (Fig. 2). The lithostratigraphical position of the gyttja deposits could be tentatively correlated with the Middle Pleniglacial. The lithology of the deposit is displayed in Figs. 2–4. 3.1. Thermographic analysis A LECO TGA-601 was used to determine the loss-onignition of 18 samples of approximately 2 g (dry material). During the first 13 min of the treatment, samples were heated to a maximum of 105 1C. Moisture evaporates from

the sample, and the dry weight of the sample is measured (Wdr (g)). Subsequently, the temperature is raised to 335 1C for a period of 35 min. The atmosphere in the oven consists of 100% oxygen, and all organic carbon is burned during this time interval. In order to determine the ‘‘classic’’ losson-ignition, the temperature in the oven is further raised to 550 1C for an additional period of 23 min, the atmosphere consisting of air again. Afterwards, the residue is weighed (Wgl (g)) and W550 is calculated using: W550=((Wdr Wgl)/ Wdr)*100%. W550 is considered equivalent to the ‘‘classic’’ LOI550. 3.2. Botanical analysis The plastic box was sampled at 1 cm intervals in the laboratory. Macroremain samples, varying between

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

165

De p Lith th (c olo m) gy

Reichwalde, sequence LM8

0 5

Organic content (%) 0 sand 150-210 μm, coarse up to 800 μm

10

20

Chironomid-based relative Tjul (CCA sample scores) 0.5

0

0

0

5

5

10

10

15

15

20

20

25

25

30

30

-0.5

-1.0

continuous thin sand bands

10 15

dark grey brown clay grey clay band

20 25 reddish brown gyttja

30 35

sand lenses, intercalated with loam

35

35

40

light grey loam, undulating base, i.e. loading

40

40

sand

Fig. 3. LM 8 sequence: (a) lithology, (b) TGA results, (c) Chironomid-based relative Tjul.

20–60 g wet weights were treated with cold 5% KOH for 5 min (for organic samples) or sodium pyrophosphate (for clayey samples) and washed over a 200 mm sieve. Macroremains were picked out from the sieve residue. A Wild stereomicroscope (magnification up to 50  ) was used during analysis. All material, including the left over sediment, has been stored in case the samples need to be re-examined in the future. Pollen samples were prepared following Faegri and Iversen (1975) with additional sodium polytungstate heavy liquid separation to remove clastic material. Subsequently, to remove fine particles the material was sieved over a 7–8 mm nylon mesh, mounted in glycerine jelly and sealed with paraffin wax. A Zeiss light microscope with phase contrast (magnification 400 and 630  ) was used during analysis. To calculate the percentage curves of the individual taxa a pollen sum consisting of trees and shrubs, Ericales and dry herb taxa (including Poaceae) was employed. A combined pollen percentage and macroremain diagram (based on absolute values per sample) was constructed using TILIA.GRAPH (Grimm, 1991–2004) and CORELDRAW computer programs. Plant macroremain and pollen identifications were made by comparison with modern reference material from the collection of the Vrije Universiteit, Amsterdam. The nomenclature of pollen types in general refers to the Northwest European Pollen Flora (Punt et al. (1976–2003) or otherwise to Moore et al. (1991), plant macroremains to Berggren (1969, 1981), Anderberg (1994),

Ko¨rber-Grohne (1964, 1991) and for Potamogeton to Cappers (1993). Palaeotemperature estimates were made based on the botanical taxa by using the climate indicator plant species method (sensu Iversen, 1954; Kolstrup, 1980). This method uses the relationship between the geographical limit of plant distribution and temperature, i.e. plants require a minimum mean summer temperature to flower and reproduce. Table 2 shows selected plant taxa and their required summer temperatures. 3.3. Chironomid analysis Eighteen samples of 4–8 cm3 were taken at approximately 2 cm intervals for chironomid analysis. The samples were soaked in 10% KOH for 4 h and subsequently washed through a 100 mm sieve. Remaining fine sand grains were removed through decanting. Head capsules were handpicked at 35  magnification under a Wild stereomicroscope using a Bogorov sorting tray. After dehydration, head capsules were mounted on microscope slides using Euparals mounting medium. Identifications were made at 100  and 400  magnification, and head capsules were identified mainly according to Hofmann (1971), Wiederholm (1983), Schmid (1993), and Moller Pillot (1984a, b). To explore the possible relationships between the fossil samples and different environmental variables we used a subset of the 40 shallowest lakes of the Swiss 81 lakes

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

166

Reichwalde, sequence LM8 Microfossil and macroremain diagram with a selection of regional taxa fr u its Aln Er us i Br cace u Em ckenae in d Bupetruthaliaet. Ac pleu m n on ru igr Pla itu m um m f Ox ntag na alcat typ p Playria o me ellu um t e t d s t ype n Po tag ype ia yp o e t e m Sa nti xif lla ariti ag typ ma a e c t Po yp ea e ac e ea e Ap Lo iace a Ar tus ty e t Coemis pe r Th nus ia Dr alictr sue c Fayas um ica Hebace l a i Cr ant e Coucifehemu Rumporae m Gemex sitae Po um t acet tubu Colygo ype osa liflor n ae Linmpo um Plaum csitaecf. vi v a l n i Ca tag tha igul par Pa ryop o marticu ifloraum Gepavehylla jor m type e Ch ntianr radceae e Sa nopaceaicatu xif od e m Po rag iac lle a a ea ns n e um dro To sa tal ce of a rew ork ed Ce pa lyn no co om cc orp um hs ge op hil um

Bio zo ne 14 C da tes

Tundra/steppe taxa and Poaceae

Ericales

Be tu Pin la n us ana ,

5

Sh ru

Tre e

s

0

Po bs Tu acea n Er dra/e Fraicale step pe Ju ngus tax a Saniperla NA l u Be ix s P tul a

De pth (c Lit ho m) log y

Trees and shrubs

215

10

RWB-4

206

15 > 47,000

217

20

233

RWB-3

235

25

229 179 180 125 163 144 127 158

35

RWB-2 RWB-1

Betula Pinus

40

20 40 60 80 100%

45,850 ± 3750/2550

224

30

20 40% 5

20 40%

20% 20%

20 40%

20

40

Reichwalde, sequence LM8 Microfossil and macroremain diagram with a selection of local taxa m

Aquatic taxa

10

15

25

30

35

40

hiu

Bio

zo

ne

ac

up gro

Ra n Zy uncu gn lu s e Da mata Subg ph c nia eae . Bat r

lis ati

nii

qu

rau

sa

sb cu

ulu

oc

nc

oc

nu Ra

try Bo

m yllu ph rio

+ +

20

My ri My ophy ll r My iophy um s p r Po ioph llum icat tam yllu alt um og m v erni eto er flor Po ta n tici um lla Po mog tum t e Po amog ton tam et cris Po og on pu tam et mu s og on o cro Cr eto bt na ist n us tus at Pe ella m prae ifoliu lo s dia str uced ngus um o, sta tob las

5

My

0

Lit

De

pth (c ho m) log y

Dr ep Sc ano o c Ca rpidi ladus u l Ty liergo m sc spp. p o n Vio ha a sp. rpioi n de Ca la pa gus s t l i l t f Mo ha ust olia r Sp nolepalu is ty arg te stri pe an psil s ty ium ate pe Sp a Sp rgan arg ium Sp an a h Eq agn ium mngus uis um in tifo etu im liu um m m Cy pe rac ea e Ca rex Ca elata re Ca x aq r u Po ex n atilis ig t Fil entill ra ipe a p Ga nd alu u l str Me ium t la is y n Ele tha pe aq o c Ny ha ua ti m r Ch pha is pa ca ara ea lus ce alb tris ae a

ts

Riparian herbs

RWB-4

+ + +

+ + + + + + + ++ + + + + + + + + + + + + + + + +

RWB-3 + + + + + +

RWB-2 RWB-1

10% 10 1010 20% 50%

10 10 10 10

10 10

20

40

20 40% 10 10 10 10% 10 10 10 10 10

20 40 60%

20%

20 40 60%

5

20%10

Fig. 4. (a) Microfossil and macroremain diagram of LM8 sequence with a selection of regional taxa. Microfossils are displayed by curves and given in percentages, macroremains are displayed by amounts as histograms or as presence (+) or abundance (+++). Exaggeration of microfossil curves 5  . (b) Microfossil and macroremain diagram of the LM8 sequence with a selection of local taxa. Microfossils are displayed by curves and given in percentages, macroremains are displayed by amounts as histograms or as presence (+) or abundance (+++). Exaggeration of microfossil curves 5  .

training set developed by Heiri et al. (2003) as a modern analogue. Because of the differences in the general setting between the modern training set (the Swiss Alps) and the

fossil environment (a braided-river floodplain) we decided to use a qualitative rather than a quantitative method to reconstruct environmental variables.

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174 Table 2 Temperature indicator botanical and zoological taxa present in the deposits Species

Minimum mean July temperature (1C)

Reference

Botanical taxa Polygonum viviparum Betula nana

5 7

Papaver Empetrum Filipendula ulmaria

7.4 7.7 8

Caltha palustris Carex rostrata

8 8

Cornus suecica Potentilla palustris

8 8

Kolstrup (1980) Brinkkemper et al. (1987) and Ran (1990) Vorren (1978) Vorren (1978) Brinkkemper et al. (1987) Kolstrup (1980) Brinkkemper et al. (1987) Kolstrup (1980) Brinkkemper et al. (1987)

Juniperus communis

8 10 (taller plants)

Cryptogramma crispa Eleocharis palustris Myriophyllum alterniflorum

9.9 10 10

Ranunculus subgen. Batrachium Myriophyllum spicatum Myriophyllum verticillatum Nymphaea alba Frangula alnus Potamogeton mucronatus Typha angustifolia

10

Zoological taxa Cristatella mucedo

410 410 12 13 13

167

material reflecting atmospheric 14C concentrations such as seeds and fruits from terrestrial plants was absent, fruits of aquatic plants had to be dated. The sample at the base of the organic gyttja (34–36 cm core depth) consisted predominantly of Potamogeton mucronatus Schrad. (van der Meijden, 2005 or P. friesii Rupr. Oberdorfer, 1994) and P. praelongus. This sample provided an age of 45,85073750/ 2550 14C yr BP. The second sample retrieved from core depth 20–22 cm consisted predominantly of P. praelongus. This sample yielded an infinite age of 447,000 14C yr BP. The dates indicate that they are at the limit of the detection by the AMS 14C technique and although an ageing effect due to the uptake of old carbon can be expected in the aquatic macroremains, this effect can be ignored in view of the age indicated and the large standard error on the dates. Both the 14C ages, however, suggest an early Middle Weichselian age (early stage 3) for the infill of the thaw lake. 4. Results

Isarin and Bohncke (1999) and Kolstrup (1980) Vorren (1978) Kolstrup (1980) Kolstrup (1980) and Isarin and Bohncke (1999) Brinkkemper et al. (1987) Kolstrup (1979, 1980) Kolstrup (1979, 1980)

14

Kolstrup (1979, 1980) Kolstrup (1980) Brinkkemper et al. (1987) Kolstrup (1979, 1980)

X10

Lacourt (1968)

To test the relationship between mean July air temperature (Tjul) and the fossil chironomid assemblages, we performed a canonical correspondence analysis (CCA; ter Braak, 1986) using July air temperature as the sole constraining variable, the 40 shallow lakes as active variables and the 18 fossil samples as passive variables. Sample scores on the first ordination axes are plotted against core depth to visualize relative differences in fossil Tjul. CCA analyses are calculated using the program CANOCO (ter Braak and Smilauer, 2002) and downweighted percentage data. 3.4. Chronology Macroremains were selected for AMS 14C dating at two levels: one at the base of the gyttja (core depth 34–36 cm) and one higher up at core depth 20–22 cm. Because organic

4.1. Lithology and thermographic analysis Below the sampled horizon, floodplain sediments occur in which the influence of the river clearly declines. The grain size diminishes and eventually a light gray loam was deposited on top, i.e. core depth 40–38 cm (Fig. 3a). These deposits have been interpreted as a distal facies of the then active river. The presence of a frost fissure suggests that from this moment onwards permafrost established on the site (Fig. 2). Subsequently, cryoturbation or load-cast structures developed in a water-saturated environment. Under these circumstances high density loamy deposits sink down and due to permafrost degradation the low-density sandy sediments get pushed up. Moreover, the lithology demonstrates that the melting of the permafrost was not due to increased inundations of the river floodplain, because this would then have given horizontally bedded clayey overbank deposits (deriving from the inundations) on top of cryoturbation structures. Instead the loamy sediments have been incorporated into the cryogenic structures. Due to a progressive melting of segregation ice, water is further released and what was once the surface, sinks and forms the bottom of a small lake. Wave-erosion can potentially have enlarged the initial pool. Following the formation of the lake, horizontally bedded red-brown gyttja deposits were formed (38–16.5 cm core depth). This phase shows the highest organic content as demonstrated by the TGA results. Up to core depth 16 cm, the organic content steadily increases (Fig. 3b). Between core depth 31 and 34 cm, a small perturbation disturbs the horizontally laminated gyttja and sand. Small clay pebbles are present in this interval. At core depth 16 cm, a sharp decline in organic matter is initiated and organic matter values drop to 5% (Fig. 3b). Between core depth 16.5 and 6 cm, a layer of dark gray-brown clay was deposited, with an

ARTICLE IN PRESS 168

S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

increasing number of silty and sandy layers near the top of the sequence. These inputs of coarse clastic sediment are interpreted as moments of increased fluvial input to the basin. Above 6 cm the basin is filled with sand and the sample location is again within the reach of the floodplain. In these sandy sediments frost fissures are present, which suggest a return to permafrost conditions and mean annual air temperatures between 4 and 8 1C (Huijzer and Vandenberghe, 1998). The total thickness of the whole fluvial cycle is ca 2 m thick (Bontebal and Smit, 2001). Five of these cycles have been registered during the logging of the profile faces. These sequences are interpreted as deposited by lowenergetic anatomosing river systems. 4.2. The palaeobotanical record Consecutive samples were analyzed through the basal part of the gyttja, because if the thermokarst lake was formed due to external forcing by climate warming, then evidence for this Pleniglacial thermal spike should be present in the bottom layers of the infill. The overlying sediment was analyzed every 4 cm. The botanical record of sequence LM8 is divided into four zones (Figs. 4a and b). 4.2.1. Zone RWB-1 (40.5–38 cm core depth) No pollen samples are available for this interval. However, macroremains of Carex spp., Characeae, P. praelongus, Bryozoa, Daphnia and Cenococcum geophilum are recorded. These indicate temporary, shallow water conditions after inundation of the floodplain. Deposition of loam during these flooding events represents the terminal phase of river activity. Findings of the soil fungus C. geophilum may be connected with surface erosion and reworking. 4.2.2. Zone RWB-2 (38–31 cm core depth) Zone RWB-2 is characterized by tree and shrub (Betula, Salix, Pinus, Alnus, etc.) values of ca 50–55%, high nonarboreal pollen (NAP) and Poaceae percentages (together up to 50%), and an increasing Juniperus curve with a maximum of 8% in the central part of this zone. Willow pollen may have been derived from dwarf willows such as Salix herbacea and S. polaris. Birch pollen was recorded with high values up to 35% and was probably derived from Betula nana, a cold-adapted dwarf-birch, which is also indicated by finds of its fruits (Fig. 4a). Modern surface sample studies from the (sub) Arctic, however, suggest that B. nana has the tendency to be overrepresented in the pollen values (e.g. Iversen, 1954; Rymer, 1973). Some of the Betula pollen also may have been derived from tree birches through long-distance transport. Pinus pollen shows percentages in general below 20%, i.e. the rational limit of pine (Lotter et al., 1992; Lang, 1994), and was interpreted as long-distance deposition. Alder pollen may have originated from Alnus incana, a tall, thicket-forming shrub. A. incana is an early succession species and may

have been a pioneer of recently deglaciated areas in Europe (compare S´rodon´, 1980; Litt, 1994; Caspers and Freund, 2001; Dambeck and Bos, 2002; Bos et al., 2005). However, no A. incana macroremains were recorded that may suggest local presence. The relatively high values of long-distance transported pollen (Pinus and possibly some Betula and Alnus), the low amounts of B. nana fruits and low pollen values of Salix therefore indicate that the landscape was rather open and treeless. The absence of trees in north and central Europe during this period was also suggested by other authors (Behre and Lade, 1986; Behre, 1989; Caspers and Freund, 2001; Behre et al., 2005). The combined pollen and macroremain data indicate regional vegetation characterized by heliophilous herbs, grasses, and dwarf shrubs of B. nana and Salix, which can be interpreted as low shrub tundra (sensu Bliss and Richards, 1982). At this time, the lake was shallow and gyttja was being deposited. Aquatic communities developed with both submerged and floating-leaved taxa, e.g., Potamogeton spp., Myriophyllum spp., Nymphaea alba, Ranunculus Subgen. Batrachium, Characeae and algae. The botanical taxa suggest that the water was transparent, carbonate-rich and meso- to eutrophic. At the sample site the water depth was at most 2–3 m (compare Hannon and Gaillard, 1997). Swamp vegetation inclusive Carex spp., Sparganium spp., Equisetum, Viola palustris, Caltha palustris, and Drepanocladus spp. fringed the shores, while Scorpidium scorpioides probably formed floating mats in these shallow, base-rich waters (cf. Dickson, 1973). Pollen of thermophilous species present in this zone, i.e. N. alba (12 1C), Frangula alnus (13 1C, Table 2) and Typha angustifolia (14 1C, Table 2), indicate minimum mean summer temperatures between 12 and 14 1C. However, the most solid evidence for relatively high summer temperatures is provided by the presence of P. mucronatus fruits, a taxon that requires a minimum Tjul of 13 1C (Table 2) and appears later in this zone. Furthermore, thriving juniper shrubs need a minimum mean July temperature that exceeds the 10 1C (Iversen, 1954), while the abundance of both pollen and seeds of Myriophyllum spicatum and M. verticillatum also suggests a minimum Tjul of 410 1C (Table 2). The botanical taxa thus suggest a minimum mean Tjul of 13 1C during this zone. For references to temperature indications see Table 1. 4.2.3. Zone RWB-3 (31–12.5 cm core depth) At 31 cm Sparganium spp. and P. crispus disappear, and later also P. obtusifolius and P. mucronatus, and the values of Equisetum, Myriophyllum and Characeae decline. In the lake, aquatic vegetation was dominated by taxa such as P. praelongus, Ranunculus Subgen. Batrachium and algae (Pediastrum, Botryococcus braunii and Zygnemataceae), and animal taxa such as Bryozoa and Daphnia. The robust plants with large submerged leaves of P. praelongus (Weeda et al., 1991) probably reduced the penetration of light and

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

hence caused the decline in Characeae at ca 30 cm. Swamp vegetation including Equisetum, Carex spp., Eleocharis palustris, Mentha aquatica, Filipendula, Potentilla palustris, Calliergon, and Drepanocladus spp. fringed the shores. The local taxa indicate, besides a decrease in the lake water depth, that the water became less carbonate-rich and less transparent. The latter was probably caused by the hydroseral succession process within the lake as well as by a higher abundance of algae, such as Pediastrum, Botryococcus, and Zygnemataceae. These algae to a large extent caused the increase in organic content (Fig. 3b). The increasing accumulation of organic matter probably also influenced the disappearance of M. alterniflorum (compare Weeda et al., 1987). The presence of Carex aquatilis and higher abundance of C. elata indicates seasonally fluctuating water levels (Weeda et al., 1994). The abundance of Zygnemataceae supports this interpretation, since they preeminently occur in shallow waters and the spores can survive in unfavorable conditions, e.g., drying out of temporary pools (van Geel, 2001). In this zone many thermophilious taxa (Juniperus, Frangula, Myriophyllum spp., P. mucronatus, T. angustifolia, and N. alba) decline or disappear. The remaining taxa, i.e. Ranunculus Subgen. Batrachium, M. alterniflorum, E. palustris and Cristatella mucedo are less warmth requiring and only need a minimum mean Tjul of 410 1C.

169

lakes in the Swiss Alps (Lotter et al. 1997; Bigler et al. 2006) Microtendipes has a high abundance throughout the entire record (Fig. 5). Other genera that occur throughout the fossil record are, e.g., Limnophyes, Dicrotendipes, and Polypedilum, all suggesting a shallow lake environment. At core depth 12.5 cm, there is a shift in dominant taxa and the chironomid record is therefore divided into two zones. 4.3.1. Zone RWCh-1 (36–12.5 cm core depth) In the lower part of this zone Chironomus anthracinustype shows abundances over 15%. After this initial peak, the values of C. anthracinus-type decline in favor of Microtendipes. The presence of genera such as Dicrotendipes and Glyptotendipes suggests that macrophytes were present in the lake (e.g. Moller Pillot, 1990), which was confirmed by the botanical data. The lake was meso- to eutrophic, as indicated by several species such as Ablabesmyia or C. plumosus-type (e.g. Lotter et al., 1998; Brodersen and Quilan, 2006). Most of the abundant fossil taxa currently occur in lowland or temperate regions in NW Europe.

4.3. The chironomid record

4.3.2. Zone RWCh-2 (12.5–8 cm core depth) At 12.5 cm core depth Microtendipes, Cladopelma, Metriocnemus terrester-type and Polypedilum values decrease while taxa such as Procladius, Tanytarsus lugenstype and C. anthracinus type show higher abundances. Chironomid productivity rates drop considerably, a trend that is often interpreted as a decline in either temperature or in trophic state. In view of the return of fluvial activity to the site, a decline in trophic state can be excluded as a forcing factor. T. lugens-type is one of the taxa that dominates in this part of the record, and is often considered an indicator of cool, oligotrophic conditions (e.g. Porinchu and MacDonald, 2003, and references therein). The high values of C. anthracinus on the other hand also could point to an increase in nutrient availability, as they are often reported to be characteristic for high productivity shallow lakes (Brodersen and Lindegaard, 1997). The botanical aquatic taxa, however, suggest no increase in the nutrient availability within the lake. The genus Chironomus is also known to be common in shallow, relatively warm, arctic (or high elevation) ponds (Walker, 1990).

The chironomid assemblages recorded in sequence LM8 confirm that a shallow, meso- to eutrophic pond or lake existed at the sample location during the early Middle Weichselian (Table 1). A total of 54 genera, species or morpho-types have been identified and the most abundant taxa are plotted in Fig. 5. The genus Microtendipes is often interpreted as thermophilous with a limited distribution in arctic and alpine lakes (Olander et al., 1999; Larocque et al., 2001; Porinchu et al., 2003) and with a preference for shallow or littoral habitats. For instance, it currently occurs in temperate to subalpine

4.3.3. Chironomid-based temperature reconstructions Fig. 3c shows the scores of the fossil assemblages on the first ordination axis after the CCA-run with July air temperature as the sole constraining variable. The scores on this ordination axis represent relative temperature values, where a higher (or less negative) value indicates lower temperatures. Temperatures are high and rather stable in the lower parts of the record. At core depth 12.5 cm a decline in temperature is inferred, as already suggested by the transition in the chironomid assemblages (Fig. 5).

4.2.4. Zone RWB-4 (12.5–6 cm core depth) In this zone, the percentages of Pinus, Alnus, Cornus suecica, reworked palynonomorphs and numbers of C. geophilum increase, while the percentages of Betula decrease. This suggests that in the surrounding region rather patchy tundra vegetation developed. The landscape became more open and surface erosion (increase Cenococcum) and long-distance transport (e.g. increase in Pinus and Alnus values) were important processes. At the sample location, sedge swamp vegetation including C. elata, C. aquatilis, Ranunculus Subgen. Batrachium and Characeae was present, indicating fluctuating water levels and a water depth of ca 0.25–1 m (compare Hannon and Gaillard, 1997). Minimum mean July temperatures were probably around 10 1C (indicated by Ranunculus Subgen. Batrachium, M. alterniflorum and C. mucedo).

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

170

Fig. 5. Chironomid diagram of the LM8 sequence. Chironomid abundances are given in percentages of the total Chironomid sum.

5. Discussion Not every thermokarst lake is formed as a result of external forcing mechanisms like a climatic warming, as they can also be formed as the result of an internal forcing mechanism. If thermokarst initiation was triggered by climate warming, then the evidence of such a thermal spike should be present especially in the basal part of the thermokarst infilling. Up to now, no explicit evidence for climatic control governing the process of thermokarst formation has been shown in the sedimentological record of eastern Germany. In the thermokarst infilling of LM8, aquatic plant remains suggest a high minimum mean Tjul of ca 12–14 1C shortly after the formation of the lake and during the initial period of deposition. Aquatic plant species can spread relatively quickly by water bird transportation, are independent of soil formation and thus can react relatively quickly to climate warming (e.g. Iversen, 1954, 1973, p. 27; Figuerola and Green, 2002). Chironomids are another fastmigrating proxy, as the life cycle of these animals is short and the adults are able to fly and thus potentially move a large distance within one generation. During the last cold

stage, the temperate European flora and fauna became restricted to the mountains of southern Europe (i.e. Balkan, Italy, and Iberian Peninsula) and in some cryptic refugia in central and northern Europe (e.g. Bennett et al., 1991; Willis, 1996; Stewart and Lister, 2001; Willis et al., 2001; Svenning, 2003). With respect to the rapid spread of botanical and zoological taxa after deglaciation, especially these northern cryptic refugia, located in areas of sheltered topography, are important (Stewart and Lister, 2001; Hampe et al., 2003). The relatively high Tjul during initial deposition suggests that permafrost degradation and subsequent thermokarst lake formation was climatically induced. Apparently, the lower part of the record was formed during a relatively warm interval. At core depth 12.5 cm several events are witnessed: 1. the organic content of the sediment drops considerably to values of 5%, which can either be due to a decline in primary production or to an increase in clastic material, or a combination of both, 2. the pollen record demonstrates an increase in the pine pollen values and Pinus becomes dominant over Betula, which suggests an increase in long-distance transport as a result of a more patchy shrub tundra vegetation, 3. both the larger

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

abundance of C. geophilum sclerotia and the increased presence of reworked palynomorphs suggest a larger role for surface erosion, 4. chironomid production rates decline, there is a shift in dominant chironomid taxa, and CCA with Tjul as the sole constraining variable suggests a sharp decline in summer temperatures. Together with the return of permafrost conditions as witnessed in the sedimentary sequence (i.e. frost fissures), this suggests a recurrence of stadial conditions. The succeeding cycle starts again with interstadial conditions, the melting of the permafrost and the formation of load-cast structures or cryoturbations. Five of these cycles have been recorded in the Reichwalde mine during the early Middle Weichselian, which can be tentatively correlated with D/O cycles 17–13. Many of these changes, however, also could be a reflection of a lowering in the water level of the lake. The botanical taxa suggest a constant decrease in lake depth, initiated around 31 cm core depth. Since this is a gradual process, it does not explain the abrupt change in temperature at core depth 12 cm. This excludes lake depth as the forcing factor for the major change in the fossil chironomid assemblages at core depth 12 cm. In contrast, the reconstructed temperatures do show a sharp transition at core depth 12.5 cm (Fig. 3c), suggesting palaeotemperature as the forcing factor for the concurrent changes in the lake catchment. The high initial temperatures together with the sharp transition to a colder environment in the upper part of the sequence suggest a climatic evolution that resembles a D/O event. In the absence of a reliable time control on the core, the rapidity of the temperature change at core depth 12.5 cm can only be a rough estimate based on sediment accumulation rates derived from the literature (e.g. Berglund, 1986; Korhola et al., 2002). Sediment accumulation rates of gyttja in lakes come to 0.5–1 mm/year (Berglund, 1986). In subarctic and arctic regions, however, sediment accumulation rates may be lower, 0.1–0.5 mm/ year (e.g. Berglund, 1986; Korhola et al., 2002). Taking into account a compaction of 50% of the original thickness this would than amount to 0.25–0.05 mm/year. The registered temperature change occurred within 3 cm, which would represent a time slot of 120–600 years. Climate oscillations such as the D/O events provide us with a framework in which many more warming and cooling events could occur during which permafrost could develop and subsequently degrade in West and Central Europe. The short duration of the warming events explains why there was little time for arboreal vegetation to react, e.g., by a shift in the vegetation zones over NW Europe. If the deduction that we are dealing with a warming event is correct, several of the numbered interstadials that have been distinguished in the Greenland ice cores (Johnsen et al., 1992) may be correlated with this event. These are interstadials 14, 15, and 16 (Table 1). The radiocarbon age of the deposit may indicate correlation with interstadial 14, i.e. the Glinde interstadial (Behre and van der Plicht, 1992; Dansgaard et al., 1993).

171

6. Conclusions The combined evidence demonstrates that formation of the thermokarst lake was initiated by warm climate conditions. Pollen, botanical macroremains, and chironomid analyses show that the basal infilling of the thaw lake occurred during a period with high mean July air temperatures (12–14 1C). This warm interval was probably too short for boreal forest to react. The return to permafrost conditions higher up in the sequence meets the perception that we are dealing with an early Middle Weichselian D/O event. The rapid warming initiated the degradation of the permafrost and the formation of the thaw lake. The infilling of the thaw lake represents the waning phase of the warming and the subsequent return to cold climate conditions. The available AMS 14C dates, although at the limit of detection, indicate an early Middle Weichselian age of the gyttja and the D/O event registered possibly represent D/O 14, 15, or 16. Acknowledgments The authors would sincerely like to thank Lesley Smit and Wim Wildschut for botanical analyses. Ing. Martin Konert is greatly acknowledged for assistance during the laboratory procedures. Furthermore, Stephen Brooks and an anonymous referee are thanked for useful comments on an earlier draft of this paper. References Anderberg, A.L., 1994. Atlas of seeds and small fruits of NorthwestEuropean plant species with morphological descriptions. Part 4, Resedaceae-Umbelliferae. Swedish Museum of Natural History, Risbergs Tryckeri AB, Uddevalla. Behre, K.-E., 1989. Biostratigraphy of the Last Glacial period in Europe. Quaternary Science Reviews 8, 25–44. Behre, K.-E., Lade, U., 1986. Eine Folge von Eem und WeichselInterstadialen in Oerel/Nieder-sachsen und ihr Vegetationsablauf. Eiszeitalter und Gegenwart 36, 11–36. Behre, K.-E., van der Plicht, J., 1992. Towards an absolute chronology for the Last Glacial period in Europe: radiocarbon dates from Oerel, northern Germany. Vegetation History and Archaeobotany 1, 111–117. Behre, K.-E., Ho¨lzer, A., Lemdahl, G., 2005. Botanical macro-remains and insects from the Eemian and Weichselian site of Oerel (northwest Germany) and their evidence for the history of climate. Vegetation History and Archaeobotany 14, 31–53. Bennett, K.D., Tzedakis, P.C., Willis, K.J., 1991. Quaternary refugia of north European trees. Journal of Biogeography 18, 103–115. Berggren, G., 1969. Atlas of seeds and small fruits of Northwest-European plant species with morphological descriptions. Part 2, Cyperaceae. Swedish Natural Science Research Council, Berlingska Botryckeriet, Lund. Berggren, G., 1981. Atlas of seeds and small fruits of Northwest-European plant species with morphological descriptions. Part 3, SalicaeaeCruciferae. Swedish Museum of Natural History, Berlings Arlo¨v. Berglund, B.E., 1986. Palaeoecological reference areas and reference sites. In: Berglund, B.E. (Ed.), Handbook of Holocene Palaeoecology and Palaeohydrology. Wiley, Chichester, New York, pp. 111–126. Bigler, C., Heiri, O., Krskova, R., Lotter, A.F., Sturm, M., 2006. Distribution of diatoms, chironomids and cladocera in surface

ARTICLE IN PRESS 172

S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

sediments of thirty mountain lakes in south-eastern Switzerland. Aquatic Sciences 68, 154–171. Bliss, L.C., Richards, J.H., 1982. Present-day arctic vegetation and ecosystems as a predictive tool for the artcic-steppe mammoth biome. In: Hopkins, D.M., Matthews, J.V., Schweger, C.E., Young, S.B. (Eds.), Paleoecology of Beringia. Academic Press, New York, pp. 241–257. Bond, G., Broecker, W., Johnsen, S., Mc Manus, J., Labeyrie, L., Jouzel, J., Bonani, G., 1993. Correlations between climate records from North Atlantic sediments and Greenland ice. Nature 365, 143–147. Bond, G.C., Showers, W., Cheseby, M., Lotti, R., Almasi, P., de Menocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G., 1997. A pervasive millennial cycle in the North Atlantic Holocene and Glacial times. Science 278, 1257–1266. Bond, G.C., Showers, W., Elliot, M., Evans, M., Lotti, R., Hajdas, I., Bonani, G., Johnson, S., 1999. The North Atlantic’s 1–2 kyr climate rhythm: relation to heinrich events, Dansgaard/Oeschger cycles and the Little Ice Age. In: Clark, P.U., Webb, R.S., Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales. American Geophysical Union, Washington, DC, pp. 35–58. Bontebal, M., Smit, L., 2001. Rivieractiviteit tijdens het Boven-Pleistoceen in relatie tot klimaatsveranderingen in het Lausitzer Urstromtal, Duitsland. M.Sc. Thesis, Vrije Universiteit Amsterdam, 88pp. Bos, J.A.A., Bohncke, S.J.P., Kasse, C., Vandenberghe, J., 2001. Vegetation and climate during the Weichselian Early Glacial and Pleniglacial in the Niederlausitz, eastern Germany-macrofossil and pollen evidence. Journal of Quaternary Science 16, 269–289. Bos, J.A.A., Dickson, J.H., Coope, G.R., Jardine, W.G., 2004. Flora, fauna and climate of Scotland during the Weichselian Middle Pleniglacial-palynological, macrofossil and coleopteran investigations. Palaeogeography, Palaeoclimatology, Palaeoecology 204, 65–100. Bos, J.A.A., Huisman, J., Kiden, P., Hoek, W.Z., van Geel, B., 2005. Early Holocene environmental change in the Kreekrak area (Zeeland, S-W Netherlands): a multi-proxy analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 227, 259–289. Brinkkemper, O., van Geel, B., Wiegers, J., 1987. Palaeoecological study of a Middle Pleniglacial deposit from Tilligte, The Netherlands. Review of Palaeobotany Palynology 51, 235–269. Brodersen, K.P., Lindegaard, C., 1997. Significance of subfossile chironomid remains in classification of shallow lakes. Hydrobiologia 342/343, 125–132. Brodersen, K.P., Quilan, R., 2006. Midges as palaeoindicators of lake productivity, eutrophication and hypolimnetic oxygen. Quaternary Science Review 25, 1995–2012. Cappers, R.T.J., 1993. The identification of Potamogetonaceae fruits found in The Netherlands. Acta Botanica Neerlandica 42, 447–460. Caspers, G., Freund, H., 2001. Vegetation and climate in the Early- and Pleni-Weichselian in northern central Europe. Journal of Quaternary Science 16, 31–48. Coope, G.R., 1975. Climatic fluctuations in northwest Europe since the Last Interglacial, indicated by fossil assemblages of Coleoptera. In: Wright, A.E., Moseley, F. (Eds.), Ice Ages: Ancient and Modern. Seel House Press, Liverpool, pp. 153–168. Geological Journal 6 (special issue). Coope, G.R., 2000. Middle Devensian (Weichselian) coleopteran assemblages from Earith, Cambridgeshire (UK) and their bearing on the interpretation of ‘full glacial’ floras and faunas. Journal of Quaternary Science 15, 779–788. Dambeck, R., Bos, J.A.A., 2002. Lateglacial and Holocene landscape evolution of the northern upper Rhine river valley, southwestern Germany. Zeitschrift fur Geomorphologie (Suppl. Bd. 128), pp. 101–127. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjo¨rndottir, A.E., Jouzel, J., Bond, G., 1993. Evidence for general instability of past climate from a 250-kyr ice-core record. Nature 364, 218–220.

De Beaulieu, J-L., Reille, M., 1984. A long Upper Pleistocene pollen record from Les Echets, near Lyon, France. Boreas 13, 111–132. De Beaulieu, J.-L., Reille, M., 1992. The last Climatic Cycle at La Grande Pile (Vosges, France) a new pollen profile. Quaternary Science Reviews 11, 431–438. Dickson, J.H., 1973. Bryophytes of the Pleistocene. Cambridge University Press, Cambridge. Faegri, K., Iversen, J., 1975. Textbook of Pollen Analysis, third ed. Munksgaard, Copenhagen. Figuerola, J., Green, A.J., 2002. Dispersal of aquatic organisms by waterbirds: a review of past research and priorities for future studies. Freshwater Biology 47, 483–494. Ganopolski, A., Rahmstorf, S., 2001. Rapid changes of glacial climate simulated in a coupled climate model. Nature 409, 153–158. Grimm, E.C., 1991–2004. TILIA, TILA.GRAPH, and TGView. Illinois State Museum, Research and Collections Center, Springfield, USA. /http://demeter.museum.state.il.us/pub/grimm/S. Grootes, P.M., Stuiver, M., 1997. Oxygen 18/16 variability in Greenland snow and ice with 10 3–105-year time resolution. Journal of Geophysical Research 102, 26455–26470. Guiot, J., Pons, A., de Beaulieu, J-L., Reille, M., 1989. A 140,000-year continental climate reconstruction from two European pollen records. Nature 338, 309–313. Hampe, A., Arroyo, J., Jordano, P., Petit, R.J., 2003. Rangewide phylogeography of a bird-dispersed Eurasian shrub: contrasting Mediterranean and temperate glacial refugia. Molecular Ecology 12, 3415–3426. Hannon, G.E., Gaillard, M.-J., 1997. The plant macrofossil record of past lake-level changes. Journal of Paleolimnology 18, 15–28. Heiri, O., Lotter, A.F., Hausmann, S., Kienast, F., 2003. A chironomidbased Holocene summer air temperature reconstruction from the Swiss Alps. Holocene 13, 477–484. Hofmann, W., 1971. Zur Taxonomie und Palo¨kologie subfossiler Chironomiden (Dipt.) in Seesedimenten. Archiv fur Hydrobiologie 6, 1–50. Hopkins, D.M., Kidd, J.G., 1988. Thaw lake sediments and sedimentary environments. In: Proceedings of the Fifth International Conference on Permafrost, Trondheim, pp. 790–795. Huijzer, B., Vandenberghe, J., 1998. Climatic reconstruction of the Weichselian Pleniglacial in northwestern and central Europe. Journal of Quaternary Science 13, 391–417. Isarin, R.F.B., Bohncke, S.J.P., 1999. Summer temperatures during the Younger Dryas in North-western and central Europe inferred from climate indicator plant species. Quaternary Research 51, 158–173. Iversen, J., 1954. The Late-Glacial flora of Denmark and its relation to climate and soil. Danmarks Geologiske Undersøgelse 2, 88–119. Iversen, J., 1973. The development of Denmark’s nature since the Last Glacial. Danmarks Geologiske Undersøgelse V(7-C), 126pp. Johnsen, S.J., Clausen, H.B., Dansgaard, W., Fuhrer, K., Gundestrup, N., Hammer, C.U., Iversen, P., Jouzel, J., Stauffer, B., Steffensen, J.P., 1992. Irregular glacial interstadials recorded in a new Greenland ice core. Nature 359, 311–313. Kasse, C., Vandenberghe, J., van Huissteden, J., Bohncke, S.J.P., Bos, J.A.A., 2003. Sensitivity of Weichselian fluvial systems to climate change (Nochten mine, eastern Germany). Quaternary Science Reviews 22, 2141–2156. Kolstrup, E., 1979. Herbs as July temperature indicators for parts of the Pleniglacial and the Late-glacial in The Netherlands. Geologie Mijnbouw 59, 377–380. Kolstrup, E., 1980. Climate and stratigraphy in Northwestern Europe between 30,000 BP and 13,000 BP, with special reference to The Netherlands. Mededlingen Rijks Geologische Dienst 32, 181–253. Kolstrup, E., 1990. The puzzle of Weichselian vegetation types poor in trees. Geologie en Mijnbouw 69, 253–262. Kolstrup, E., Wijmstra, T.A., 1977. A palynological investigation of the Moershoofd, Hengelo, and Denekamp interstadials in the Netherlands. Geologie en Mijnbouw 58, 377–380.

ARTICLE IN PRESS S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174 Ko¨rber-Grohne, U., 1964. Bestimmungsschlu¨ssel fu¨r subfossile JuncusSamen und Gramineen-Fru¨chte. Probleme Ku¨stenforschung im Su¨dlichen Nordseegebiet 7. Ko¨rber-Grohne, U., 1991. Bestimmungsschlu¨ssel fu¨r subfossile Gramineen-Fru¨chte. Probleme Ku¨stenforschung im Su¨dlichen Nordseegebiet 18, 169–234. Korhola, A., Sorvari, S., Rautio, M., Appleby, P.G., Dearing, J.A., Hu, Y., Lami, A., Cameron, N.G., 2002. A multi-proxy analysis of climate impacts on the recent development of subarctic Lake Saanaja¨rvi in Finnish Lapland. Journal of Paleolimnology 28, 59–77. Lacourt, A.W., 1968. A monograph of the freshwater Bryozoa— Phylactolaemata. Zoologische Verhandelingen 93, 3–159. Lang, G.H., 1994. Quartaire Vegetationsgeschichte Europas. Gustav Fischer Verlag, Jena. Larocque, I., Hall, R.I., Grahn, E., 2001. Chironomids as indicators of climate change: a 100-lake training set from a subarctic region of northern Sweden (Lapland). Journal of Paleolimnology 26, 307–322. Litt, T., 1994. Pala¨oo¨kologie, Pala¨obotanik und Stratigraphie des Jungquarta¨rs im nordmittel-europa¨ischen Tiefland. Dissertationes Botanicae 227, 1–185. Lotter, A.F., Eicher, U., Birks, H.J.B., Siegenthaler, U., 1992. Late-glacial climatic oscillations as recorded in Swiss lake sediments. Journal of Quaternary Science 7, 187–204. Lotter, A.F., Birks, H.J.B., Hofmann, W., Marchetto, A., 1997. Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of past environmental conditions in the Alps. I. Climate. Journal of Paleolimnology 18, 395–420. Lotter, A.F., Birks, H.J.B., Hofmann, W., Marchetto, A., 1998. Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of past environmental conditions in the Alps. II. Nutrients. Journal of Paleolimnology 19, 443–463. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research 27, 1–29. Mol, J., 1997a. Fluvial response to climate variations. The Last Glaciation in eastern Germany. Ph.D. Thesis, Vrije Universiteit, Amsterdam. Mol, J., 1997b. Fluvial response to Weichselian climate changes in the Niederlausitz (Germany). Journal of Quaternary Science 12, 43–60. Moller Pillot, H.K.M., 1984a. De larven der Nederlandse Chironomidae (Diptera). Inleiding, Tanypodinae en Chironomini. Nederlandse Faunistische Mededlingen 1a. Moller Pillot, H.K.M., 1984b. De larven der Nederlandse Chironomidae (Diptera). Orthocladiinae. Nederlandse Faunistische Mededlingen 1b. Moller Pillot, H.K.M., 1990. De larven der Nederlandse Chironomidae. Autoekologie en verspreiding. Nederlandse Faunistische Mededlingen 1c. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, second ed. Blackwell Scientific Publications, Oxford. Punt, W., et al. (Eds.), 1976–2003. NEPF, The Northwest European Pollen Flora, vols. 1–7. Elsevier, Amsterdam. Oberdorfer, E., 1994. Pflanzensoziologische Exkursionsflora. Ulmer, Stuttgart. Olander, H., Birks, H.J.B., Korhola, A., Blom, T., 1999. An expanded calibration model for inferring lakewater and air temperatures from fossil chironomid assemblages in northern Fennoscandia. Holocene 9, 279–294. Porinchu, D.F., MacDonald, G.M., 2003. The use and application of freshwater midges (Chironomidae: insecta: diptera) in geographical research. Progress in Physical Geography 27, 378–422. Porinchu, D.F., MacDonald, G.M., Bloom, A.M., Moser, K.A., 2003. Late Pleistocene and early Holocene climate and limnological changes in the Sierra Nevada, California, USA inferred from midges (Insecta: Diptera: Chironomidae). Palaeogeography, Palaeoclimatology, Palaeoecology 198, 403–422.

173

Ran, E.T.H., 1990. Dynamics of vegetation and environment during the Middle Pleniglacial in the Dinkel Valley (The Netherlands). Mededlingen Rijks Geologishce Dienst 44 (3), 141–205. Ran, E.T.H., van Huissteden, J., 1990. The Dinkel Valley in the Middle Pleniglacial; dynamics of a tundra river system. Mededlingen Rijks Geologische Dienst 44 (3), 209–220. Reille, M., de Beaulieu, J-L., 1990. Pollen analysis of a long upper Pleistocene continental sequence in a Velay Maar (Massif Central, France). Palaeogeography, Palaeoclimatology, Palaeoecology 80, 35–48. Reille, M., de Beaulieu, J.-L., Svobodova, H., Andrieu-Ponel, V., Goeury, C., 2000. Pollenanalytical biostratigraphy of the last five climatic cycles from a long continental sequence from the Velay region (Massif Central, France). Journal of Quaternary Science 15, 665–685. Rymer, L., 1973. Modern pollen rain studies in Iceland. New Phytologist 72, 1367–1373. Schmid, P.E., 1993. A key to the larval Chironomidae and their instars from Austrian Danube region streams and rivers. Part 1. Diamesinae, Prodiamesinae and Orthocladiinae. Federal Institute for Water Quality of the Ministry of Agriculture and Forestry, Wien, 524pp. S´rodon´, A., 1980. Paleohistoria olszy w Polsce (Palaeohistory of alder in Poland) (in Polish with English summary). In: Bia"obok, S. (Ed.), Olsze—Alnus Mill. Nasze drzewa les´ ne, PWN, Warszawa-Poznan´, pp. 7–33. Stewart, J.R., Lister, A.M., 2001. Cryptic northern refugia and the origins of the modern biota. Trends in Ecology & Evolution 16, 608–613. Svenning, J.-C., 2003. Deterministic Plio-Pleistocene extinctions in the European cool-temperate tree flora. Ecology Letters 6, 646–653. ter Braak, C.J.F., 1986. Canonical Correspondence Analysis: a new eigenvector technique for multivariate direct gradient analysis. Ecology 67, 1167–1179. ter Braak, C.J.F., Smilauer, P., 2002. CANOCO Reference Manual and CanoDraw for Windows User’s Guide. Biometris Wageningen, Wageningen, 500pp. Vandenberghe, J., 1985. Palaeoenvironment and stratigraphy during the Last Glacial in the Belgian–Dutch border region. Quaternary Research 24, 23–38. van der Hammen, Th., 1971. The Denekamp, Hengelo and Moershoofd Interstadials. Mededlingen Rijks Geologische Dienst 22, 81–85. van der Hammen, Th., Maarleveld, G.C., Vogel, J.C., Zagwijn, W., 1967. Stratigraphy, climatic succession and radiocarbon dating of the Last Glacial in the Netherlands. Geologie en Mijnbouw 46, 79–95. van der Meijden, R., 2005. Heukels’ flora van Nederland. WoltersNoordhoff, Groningen. van Geel, B., 1996. Factors influencing changing AP/NAP ratios in NWEurope during the Late-Glacial period. Il Quaternario 9, 599–604. van Geel, B., 2001. Non-pollen palynomorphs. In: Smol, J.P., Birks, H.J.B., Last, W.M. (Eds.), Tracking Environmental Changes using Lake Sediments, Terrestrial, Algal and Silicaceous Indicators, vol. 3. Kluwer Academic Publishers, Dordrecht, pp. 99–119. Vorren, K.-D., 1978. Late and Middle Weichselian stratigraphy of Andøya, North Norway. Boreas 7, 19–38. Walker, I.R., 1990. Modern assemblages of arctic and alpine Chironomidae as analogues for late-glacial communities. Hydrobiologia 214, 223–227. Walker, M.J.C., Bjo¨rck, S., Lowe, J.J., Cwynar, L.C., Johnsen, S., Knudsen, K.-L., Wohlfarth, B., INTIMATE group, 1999. Isotopic ‘events’ in the GRIP ice core: a stratotype for the Late Pleistocene. Quaternary Science Reviews 18, 1143–1150. Weeda, E.J., Westra, J., Westra, Ch., Westra, T., 1987. Nederlandse oecologische flora, wilde planten en hun relaties, vol. 2. IVN, de Lange/Van Leer, Deventer, 304pp. Weeda, E.J., Westra, J., Westra, Ch., Westra, T., 1991. Nederlandse oecologische flora, wilde planten en hun relaties, vol. 4. IVN, Salland/ de Lange, Deventer, 317pp. Weeda, E.J., Westra, J., Westra, Ch., Westra, T., 1994. Nederlandse oecologische flora, wilde planten en hun relaties, vol. 5. IVN, Lecturis BV, Eindhoven, 400pp.

ARTICLE IN PRESS 174

S.J.P. Bohncke et al. / Quaternary Science Reviews 27 (2008) 162–174

Wiederholm, T. (Ed.), 1983. Chironomidae of the Holarctic region. Keys and diagnoses. Part I—Larvae. Entomologica Scandinavica (Suppl. 19). Willis, K.J., 1996. Where did all the flowers go? The fate of temperate European flora during glacial periods. Endeavour 20, 110–114.

Willis, K.J., Rudner, E., Su¨megi, P., 2001. The full glacial forest of Central and Southeast Europe. Quaternary Research 53, 203–213. Zagwijn, W., 1974. Vegetation, climate and radiocarbon datings in the Late Pleistocene of the Netherlands, Part II: Middle Weichselian. Mededlingen Rijks Geologische Dienst N.S. 25, 101–111.