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Environmental inferences and chironomid-based temperature reconstructionsfrom fragmentary records of the Weichselian Early Glacial and Pleniglacial

periods in the Niederlausitz area (eastern Germany)

S. Engels a,⁎, S.J.P. Bohncke a, J.A.A. Bos a, O. Heiri b, J. Vandenberghe a, J. Wallinga c

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 4,3584 CD Utrecht, The Netherlands

c Netherlands Centre for Luminescence Dating, Delft University of Technology, Faculty of Applied Sciences, 2629 JB Delft, The Netherlands

Received 19 July 2007; received in revised form 26 November 2007; accepted 8 December 2007

Abstract

We inferred past climate conditions from lacustrine sediments intercalated in Weichselian Early Glacial and Early Pleniglacial fluvial andaeolian sediments, exposed in two opencast lignite mines from the Niederlausitz area (eastern Germany). A chronology was established usingradiocarbon and luminescence dating methods. Both lithology and chironomid fauna indicate that the former shallow lakes were situated on afloodplain. Palaeotemperature estimates calculated from the fossil chironomid-assemblages of the Early Glacial lacustrine deposit indicate meanJuly air temperatures of ca. 15 °C, which is in line with results derived in earlier studies from the Niederlausitz area and from northwestern Europe.The Early Pleniglacial lacustrine deposits consist of an organic-rich gyttja, intercalated with sand and silt lenses. The chironomid-assemblagesshow that a shallow meso- to eutrophic lake was present at the study site, and chironomid-inferred palaeotemperature estimates indicate an abruptdecline in July air temperatures from 15–16 °C to ca. 13 °C. In combination with other proxies from the same record, this suggests a Dansgaard/Oeschger like climate event.© 2008 Elsevier B.V. All rights reserved.

Keywords: Weichselian; Chironomids; Pollen; Plant macrofossils; Climate reconstruction; Lake sediments; Floodplain; Germany

1. Introduction

TheWeichselian period has been a research topic of interest forseveral decades as it is a period of dynamic climate evolution thatwas not substantially influenced by human activity. The numberof lacustrine records covering (parts of) the Weichselian on theEuropean continent is limited, and high-resolution quantitativereconstructions of past climate change are scarce. With thedevelopment of the so-called transfer function approach (e.g.Birks, 1998), new tools have become available to quantitativelyinfer past changes in climate from fossil assemblages of differentgroups of organisms such as diatoms, pollen or chironomids.

Chironomids are sensitive indicators of past changes inwater depth, nutrient availability and summer temperatures(e.g. Walker and Cwynar, 2006; Brooks, 2006) and well-preserved head capsules of the larvae are usually abundant inlake sediments. In recent years the development of chirono-mid-based transfer functions has greatly improved theusefulness of chironomids for palaeoenvironmental recon-struction. Using transfer functions, we are able to providequantitative estimates of past environmental conditions basedon fossil chironomid-assemblages (e.g. Walker et al., 1997;Brooks and Birks, 2001; Heiri and Lotter, 2005). Initialapplications of these transfer functions have focussed on theLate-Glacial period (e.g. Walker et al., 1991; Brooks andBirks, 2000) and the Holocene (e.g. Heiri et al., 2003; Velleet al., 2005) and were aimed at reconstructing past July air

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⁎ Corresponding author. Tel.: +31 20 598 7265; fax: +31 20 598 9941.E-mail address: stefan.engels@falw.vu.nl (S. Engels).

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temperatures. Chironomid-based studies on lacustrine sedi-ments from Europe predating the Late-Glacial are scarce, andinclude Becker et al. (2006), Gandouin et al. (2007) and Engelset al. (2007a). The latter study provides (to our knowledge) thefirst chironomid-inferred palaeotemperature estimates for theMiddle Weichselian.

The opencast lignite mines of eastern Germany provide largeexposures of Weichselian sediments that are mostly of fluvialorigin, and have been the subject of extensive studies in the past(e.g. Wolf and Alexowsky, 1994; Mol, 1997a; Bos et al., 2001;Kasse et al., 2003; Hiller et al., 2004; Bohncke et al., 2008).Lacustrine deposits, intercalated in these fluvial and aeoliansediments, are suitable archives for palaeoenvironmentalreconstructions based on the multitude of proxies available toinfer past changes in vegetation, environment and climate fromlake sediments (e.g. Smol et al., 2001a,b).

Lacustrine sediments encountered in the opencast mines ofeastern Germany all originate from former lakes situated onriver floodplains. Hence, they potentially may have beenaffected by flooding by the nearby river. Modern training setsnormally do not include lakes that are prone to flooding byrivers, which creates a non-analogue situation between ourfossil floodplain lakes and the existing modern training sets. Toour knowledge, no quantitative palaeoenvironmental recon-structions on chironomid remains from floodplain lakes hasbeen attempted so far.

In a recent study of 33 lakes in Finland, Engels et al. (2007b)have shown that the variability of chironomid-based reconstruc-tions and prediction residuals from floodplain lakes is similar tolakes unaffected by riverine influence. This suggests thatfloodplain lakes are suitable alternatives for reconstructingsummer temperatures in situations where no lakes isolated fromriverine influence are available, as is the case in the Niederlausitzarea of eastern Germany.

In this paper we show the results of a detailed analysis of thesedimentary succession of deposits, exposed in two opencastlignite mines in the Niederlaustiz area: Nochten and Reichwalde(Fig. 1). Two lacustrine deposits, dated to the Early Glacial andthe Early Pleniglacial, are analysed for a range of proxies and inthis paper we aim to reconstruct mean July air temperatures byapplying chironomids as a proxy for these two time-windowsfor which quantitative data on past climate conditions is scarce.

2. Geological setting

The Lausitzer ice-marginal valley, situated in the southernpart of eastern Germany, was formed during the Saalianglaciation and has an E–W orientation. The former valley ischaracterised by a number of large opencast lignite mines,including the Reichwalde and Nochten mines (Fig. 1). Activerecovery of Miocene browncoal, situated approximately 100 mbelow the current surface, exposes the overlying sediments thatare mainly of Saalian (130–200 ka) and Weichselian (12–110 ka) age. Previous studies in the Nochten mine includestudies of the fluvial succession (Mol, 1997a,b; Kasse et al.,2003) and of the vegetation and climate development (Boset al., 2001). The Reichwalde mine was previously visited byBohncke et al. (2008) who qualitatively reconstructed environ-mental and climatic conditions from a short lacustrine recordand provided a first chironomid record for the sequence.

3. Methodology

3.1. Data acquisition

In June 2004 we studied a 1000 m long N–S orientedexposure in the Nochten mine. As scree was present on theslopes, no continuous sections could be analysed but instead,

Fig. 1. Location map of the Nochten and Reichwalde mines in the Niederlausitz area (after Mol, 1997a). The locations of the SR-X1 and LM8 sample boxes areindicated by arrows.

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many vertical logs were investigated. Special attention wasgiven to lithology, sedimentary structures and periglacialphenomena. Samples for optically stimulated luminescence(OSL) dating were taken from the aeolian and fluvial deposits,while the organic material was collected using sample boxes,and transported to the Vrije Universiteit Amsterdam (TheNetherlands) for detailed botanical (pollen, macro-remains) andzoological (chironomids) analysis. To obtain a better under-standing of the chronology, additional macro-remains weretaken from the sample boxes for radiocarbon dating. Duringfieldwork in 1999, a large N–S oriented exposure in theReichwalde mine was analysed and the aeolian and fluvialdeposits were studied in a fashion similar to the Nochtensediments.

3.2. Chronology

Using both radiocarbon and OSL dating techniques, achronology was established for the Weichselian sediments ofthe Nochten mine (Table 1; Fig. 2).

We applied quartz OSL dating to determine the time ofdeposition of the fluvial sediments (Wallinga, 2002). The OSLsignal of sand-sized quartz grains is set to zero by lightexposure prior to burial, and builds up after burial due toionizing radiation from surrounding sediments (with a smallcontribution from cosmic rays). The age of a sample isobtained from measurements of the dose absorbed by thegrains since the last light exposure (equivalent dose; De,expressed in Gy) and measurement of the radiation flux thegrains were exposed to since burial (dose rate; expressed inGy/ka). Quartz OSL dating is usually applicable to sedimentsup to 100–150 ka of age.

For equivalent dose determination we used the single-aliquotregenerative dose (SAR) procedure (Murray and Wintle, 2003).Dose recovery tests (Roberts et al., 1999; Wallinga et al., 2000)indicated that a laboratory dose could be accurately measuredusing this procedure (average dose recovery ratio of 1.00±0.01;in excellent agreement with unity). For dose rate estimation weemployed gamma-ray spectrometry.

3.3. Chironomid analysis

The organic lacustrine deposits of SR-X1 (Nochten mine,Fig. 2) and LM8 (Reichwalde mine, Fig. 2) were sampled everyother centimeter, and were analysed for chironomid remains and arange of palaeobotanical proxies. For SR-X1, a total of 10 wetsediment samples with aweight range of 3.8–11.1 gwere used forchironomid analysis. The LM8 sequence includes 18 sedimentsamples with a weight range of 2.0–16.7 g. The samples weretreated with cold KOH for at least 4 h and rinsed on a 100 µmsieve before hand-sorting the chironomid head capsules (hcs)under a dissecting microscope (35× magnification). Thechironomid hcs were identified following Wiederholm (1983),Moller Pillot (1984), Schmid (1993), Rieradevall and Brooks(2001) and Heiri et al. (2004). A chironomid percentage diagramwas constructed using the computer programs TILIA and TG.VIEW (Grimm, 1991–2004).

Of the available chironomid–temperature transfer functionsand training sets, the Swiss training set was selected as it isclosest to our site and it includes the majority of the taxaencountered in our fossil sediments. Although our sampling siteis not situated within the region where the training set wasdeveloped, Heiri et al. (2007) and Ilyashuk et al. (2005) showedthat transfer functions can be applied outside their region oforigin provided that the results are evaluated carefully.

The Swiss chironomid–July air temperature transfer functionis based on a 2-component weighted-averaging partial leastsquares regression (WA-PLS) calculated on square root trans-formed percentage abundances of chironomid taxa identified insurface sediment assemblages from 115 lakes in the Alpine region(Heiri and Lotter, 2005; Bigler at al., 2006). The model wasscreened for outliers and a total of 14 lakes were deleted from themodel based on ecological criteria. These outliers included lakesaffected by unusual hydrological conditions, exceptionally deeplakes, lakes with a strong running-water influence on chironomid-assemblages and lakes strongly affected by local topography (seeVon Gunten et al. (2007) for details). The screened inferencemodel covered a July air temperature range of 5–18.4 °C, had aleave-one-out cross-validated Root Mean Square Error of

Table 1OSL ages and uncalibrated radiocarbon dates from the Weichselian sedimentary records of Nochten and Reichwalde

OSL sample Unit Material Equivalent dose Dose rate OSL age

Gy S.E. Gy/ka S.E. ka S.E.

NCL-6605051 N4 Fluvio-aeolian sand 25.5 1.4 1.19 0.05 21.4 1.5NCL-6605052 N4 Fluvio-aeolian sand 31.0 1.5 1.32 0.05 23.6 1.4NCL-6605053 N2 Shallow fluvial sand 88.8 4.3 1.10 0.05 81 5NCL-6605054 N2 Shallow fluvial sand 81.0 3.4 1.26 0.05 64 4NCL-6605055 N1 Shallow fluvial sand 69.1 3.1 0.93 0.04 74 5NCL-6605056 N1 Shallow fluvial sand 107.1 5.3 1.25 0.05 86 5NCL-6605057 N1 Shallow fluvial sand 46.3 2.2 0.57 0.04 82 6

14C Sample Unit Material 14C age

ka BP S.E.

GrA-30631 N1 Carex fruits, Salix twig fragments N45GrA-30632 N1 Carex fruits, Salix twig fragments 43.0 ±0.9/0.7GrA-22168 RW3 Potamogeton fruits N47GrA-22169 RW3 Potamogeton fruits 45.8 ±3.8/2.6

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Prediction (RMSEP) of 1.4 °C, a coefficient of determination (r2)of 0.88 and a maximum bias of 1.28 °C. Sample specificprediction errors were calculated using 999 bootstrap cycles.Following Birks et al. (1990) and Heiri et al. (2003), the chi-square distance between the individual fossil samples and themost similar sample in the modern calibration data set wasdetermined, and distances larger than the 2nd and 5th percentile ofall chi-squared distances in the modern data were identified asfossil samples with “no close” and “no good” analogues,respectively. A canonical correspondence analysis (CCA) withmean July air temperature as the sole constraining variable wasperformed, where the fossil samples were added as passivesamples. Cut-off levels of the 95th and 90th percentiles of all theresidual distances to the first canonical axis in the modern trainingset was used as an estimate of the fit of the fossil samples totemperature (“very poor” and “poor” respectively; Birks et al.,1990). The percentage of rare taxa was calculated for each fossil

sample, where a rare taxon was defined as having a Hill's N2(Hill, 1973) below 5 in the calibration data set (e.g. Heiri et al.,2003). WA-PLS, Hill's N2 and chi-square distances werecalculated using computer program C2 version 1.4.2 (Juggins,2003), CCA was performed using CANOCO version 4.51(ter Braak and Šmilauer, 1998).

3.4. Botanical analysis

Plant macrofossils were hand-sorted from large sedimentsamples that were derived from the same depth intervals as thechironomid-samples.

Samples with varying weight were treated with cold KOH(5%) for at least 4 h and washed over a 100 μm sieve, afterwhich macrofossils were picked from the sieve residue.

Palaeotemperature estimatesweremade based on the plant taxaby using the climate indicator plant speciesmethod (sensu Iversen,

Fig. 2. Composite sedimentological logs of the Nochten and Reichwalde mines, with cryogenic features, chronology and sedimentary environments. The hatched linesindicate a possible correlation between the different sedimentary units.

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1954; Kolstrup, 1980). The relationship between the moderngeographical limit of plant distribution and temperature can beused to reconstruct past minimum temperatures based on fossilrecords of plant remains, as the individual plants require certainminimum mean summer temperatures to flower and reproduce.

4. Lithology and geochronology

The independent OSL-based chronology derived from theNochten sedimentary record suggests a rather constant accumula-tion of sediments during the Weichselian, and is compared toexisting chronological sequences from the Nochten (Bos et al.,2001; Kasse et al., 2003) and Scheibe mine (Mol, 1997a,b).

The Reichwalde mine does not have an independentchronology (except for the two radiocarbon dates that are atthe range limit of the radiocarbon dating technique). Thegeneral sedimentological sequence is, however, similar to thatof the Nochten mine, although the Weichselian sedimentcolumn is 2 m thicker. Combining the assumed simultaneousoccurrence of cryogenic features, deflation surfaces and theformation of the Usselo soil, the sedimentological sequenceswere correlated as indicated in Fig. 2 (hatched lines). Due to thelack of independent determination of the elevation (in m asl) forthe Reichwalde record, the relative positioning of that columnhas an uncertainty of a few meters.

Several studies have shown that sediments from the previousinterglacial (the Eemian) are absent from the Niederlausitz area(e.g. Wolf and Alexowsky, 1994; Mol, 1997a). Kasse et al.(2003) identified a complex of organogenic deposits (their Unit1a) intercalated by fluvial sands as Early Glacial sediments in anE–W exposure in the Nochten mine. These sediments laydirectly on top of the Saalian glaciofluvial sediments. In thisstudy, we dated Unit N1 to approximately 80 ka (3 OSL dates,Fig. 2), which indicates an age near the end of the EarlyWeichselian Glacial. That these Early Glacial deposits areprobably formed over a large area in the Niederlaustiz region, isfurther demonstrated by Mol (1997a) and Hiller et al. (2004),who also recognised a complex of organic-rich Early Glacialdeposits discordantly overlying Saalian fluvio-glacial sedimentsin the Scheibe mine, and by the complex of organic sedimentsobserved in the Reichwalde mine that are correlated to the EarlyWeichselian Glacial as well (Units RW1 and RW2).

The 6 m thick clastic deposit of Unit N4, characterised byparallel laminations and frost cracks, overlies the Early Glacialsediments. The OSL dates estimate an age of 60–70 ka for thissediment interval, roughly equivalent to the Early Pleniglacialand thus indicate that there is no (large) hiatus between thesediments of Units N1 and N2. Bos et al. (2001) and Kasse et al.(2003) document a similar 4 m thick sediment body consistingof sheet-flood or crevasse splay deposits. They report threeluminescence dates in his interval, indicating an age of 40–50 ka. In addition, they show three radiocarbon dates that arenear the limit of the method (~40 14C ka BP). Based on this agecontrol Kasse et al. (2003) reconstruct a major hiatus spanningat least the Early Pleniglacial and possibly also the earlierparts of the Middle Pleniglacial. This interpretation is in sharpcontrast to our OSL chronology.

The luminescence ages reported by Bos et al. (2001) andKasse et al. (2003) were obtained through a multiple aliquotinfrared stimulated luminescence (IRSL) procedure on fine-grain feldspar (M. Frechen, personal communication 2007). Thefeldspar IRSL signal has been shown to be unstable ongeological timescales (e.g. Huntley and Lian, 2006), and willtend to underestimate the burial age unless the ages arecorrected for anomalous fading. We note that an anomalousfading rate of approximately 6% per decade would explain thediscrepancy between quartz OSL and feldspar IRSL ages. Suchfading rates are in line with those reported by Huntley and Lian(2006). However, we cannot be certain that anomalous fadingresulted in age underestimation for the Bos et al. (2001) andKasse et al. (2003) samples, since fading measurements werenot carried out.

We also need to consider the possibility that the quartz OSLsignal was not completely reset in all grains prior to deposition.This would result in a remnant dose, causing overestimation ofthe true burial dose and thus of the age (e.g. Wallinga, 2002).However, the equivalent dose distributions do not indicate theoccurrence of heterogeneous bleaching. Hence we regard itunlikely that our OSL ages grossly overestimate the burial age.

In the light of the uncertainties with regard to the previouslyreported ages (both 14C and IRSL) and convincing demonstra-tions of the accuracy of quartz OSL ages obtained using theSAR procedure (Murray and Olley, 2002; Wallinga, 2002), weconclude that our new ages are more robust than the previousage estimates and that they provide a more reliable chronologyfor the site.

It is interesting to note that multiple-aliquot OSL dates onquartz grains from a fluvial flood deposit in the Scheibe mine(also located some 10 m below the surface) by Mol (1997a)yielded ages of approximately 70 ka, in accordance with thedates we derived for Unit N2. The uncertainties on the dates byMol (1997a) were large and the authors interpreted their OSL-ages as overestimations of the true age of the sediments basedon a priori expectations of the age of the sediments based onpalynological results from underlying sediments. Our resultssuggest however that the OSL ages of Mol (1997a) may havebeen correct.

The coarse-grained sediments of Unit N3 were not dated asinsufficient bleaching of the material was expected from fastsedimentation in the high-energetic braided river system. Thesediments overlying the coarse-grained sediments of Unit N3are well-dated using OSL-techniques to ~22 ka (LatePleniglacial) and therefore the sediments of Unit N3 mighthave been deposited in the Late Pleniglacial, but possibly alsomuch earlier (Middle to Early Pleniglacial). Kasse et al. (2003)dated the same deposits in the Nochten mine throughradiocarbon dating to ~36 14C ka BP, and there the deposit iswedged between two sedimentary units that are IRSL dated to~20 ka and ~45 ka. Caution is however needed withinterpretation of this chronology, as radiocarbon ages in thisrange may underestimate the true age (e.g. Briant et al., 2005)and the IRSL ages may be affected by anomalous fading.

Unit N4 is dated to the Late Pleniglacial (2 OSL-dates: 20–23 ka), which is in good accordance with 2 radiocarbon and 3

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IRSL dates from Kasse et al. (2003) from the Nochten mine, andalso with age estimates of 18–20 ka (OSL-dates) of a largefluvio-aeolian deposit encountered in the Scheibe mine by Mol(1997a,b). The large syngenetic ice-wedge casts that wereencountered in the Nochten mine could therefore have beenformed during the Last Glacial Maximum (LGM), and are alsopresent in Unit RW5, which is correlated to Unit N4. Thesediments from Units N4 and RW5 probably belong to asedimentary facies that is widespread in the region, which wasexpected due to its (partly) aeolian origin.

The gravel string encountered in the upper part of unit N2 islikely to be the eastern German equivalent of the BeuningenGravel Bed from the Netherlands (Van der Hammen et al.,1967; Zagwijn and Paepe, 1968). The soil horizon that is found50 cm above the gravel string would than be the equivalent ofthe German Finow Soil or the Dutch Usselo Soil, which are bothdated to the Allerød period (Van der Hammen et al., 1967; VanGeel et al., 1989; Kaiser et al., 2006). The aeolian deposits ofunit N5 therefore were probably formed during the YoungerDryas Interstadial.

4.1. Lithology of sample box SR-X1

Sample box SR-X1 (114.5 m asl) makes up the lower part ofsedimentary unit N1 and is OSL dated to approximately 80 ka.The lowermost 14 cm of the core consist of a silty gyttja. Asandy layer (~4 cm thickness), completely barren of organicmatter, is present on top of the silty gyttja, after which a secondphase of deposition of organic-rich sediments occurred. As the

lower part of Unit N1 was not heavily cryoturbated at thelocation of SR-X1, continuous horizontal laminae consisting ofsand- and silt lenses were still visible in the organic-rich deposit.They suggest a regular input of clastic material from a nearbyriver system. The upwards decreasing organic content of thecore (Fig. 3) suggests that the influence of the river increased intime. The exposure showed that the gyttja deposits of SR-X1extend laterally for several tens of meters, after which theywedge out and are replaced by soils and peat deposits.

4.2. Lithology of sample box LM8

The total thickness of the whole fluvial cycle fromwhich LM8is retrieved is ca. 2 m (unit RW3), and consists of low-energeticanastomosing-river deposits. Below sample box LM8, sandy andsilty floodplain sediments are present that have been interpreted asa distal floodplain facies of the river that was active at that time.The diminishing grain size and the eventual deposition of a lightgrey loam (core depth 50–37 cm) show the decreasing influenceof the river. Subsequently, cryoturbation structures developed in awater-saturated environment (Bohncke et al., 2008). Water isfinally released as a result of progressive melting of thesegregation ice at the top of the permafrost.

Following the formation of the lake, horizontally bedded red-brown gyttja was deposited (38–16.5 cm core depth). Thisdeposit has the highest organic content, increasing upward from~10% to ~30% (Fig. 4). At 16 cm core depth a sharp decline inorganic matter is initiated and organic matter values drop to~5%. Between 16.5 and 6 cm core depth, a layer of dark grey-

Fig. 3. Chironomid abundance diagram of selected taxa from the SR-X1 sample box, and the organic content of the sediment (estimated through loss-on-ignition).Chironomid taxa are classified based on the habitats they are predominantly found in, although some taxa (e.g. Limnophyes, Cricotopus-type) can occur in a range ofhabitats types. Chironomid-inferred mean July air temperature estimates with sample specific error bars as determined by bootstrap cross-validation are plotted to theright.

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brown clay was deposited, and towards the top of the sequencethe number of silty and sandy layers increases. These inputs ofcoarse clastic sediment are interpreted as moments of fluvialinput to the basin, possibly through flooding of the lake. Thereare no clayey or silty deposits above 6 cm core depth; the fluvialsands suggest that the basin was completely filled during adramatic flood event.

5. Chironomid records and their palaeoclimatic interpretations

5.1. Chironomid record of SR-X1 (Weichselian Early Glacial)

The concentration of chironomid hcs per gram of wetsediment is generally low in the SR-X1 record, and follows thetrend of the percentage of organic matter content of thesediments (Fig. 3). Count sums are between 53 and 165 hcs upto 21 cm core depth. Near the top of SR-X1, there is a sharpdecrease in chironomid concentrations, and the uppermostsample only yielded 7 hcs.

A total of 47 taxa were identified in the SR-X1 record, ofwhich a selection is plotted in Fig. 3. Chironomid taxa areclassified according to ecological information presented byMoog(1995). The encountered morphotypes ofMetriocnemus spp. andCricotopus-type have been amalgamated in this figure to enhancereadability. The majority of taxa in the SR-X1 record indicate thatthe lake was probably very shallow. Although most taxa aretypically found in standing water habitats, several of the mostabundant taxa, with occurrences over 10% of the assemblagethroughout the record (e.g. Georthocladius), are often associatedwith terrestrial or semi-terrestrial habitats such as wet soils.Limnophyes is usually associated with very shallow water in the

littoral of lakes and with streams, but can also occur in (semi-)terrestrial habitats (Brooks et al., 2007). Other taxa (plotted to theright of Fig. 3) are known to occur in flowing water habitats andare uncommon in lake sediments (Rheotanytarsus) or can befound in both flowing and standing water habitats (e.g. Cricoto-pus-type). Both the chironomid taxa associated with flowingwater habitats, as well as those associated with (semi-) terrestrialhabitats, could have been transported to the lake through frequentfloodings of a nearby river. This implies that the former lake wasmost likely situated in a floodplain. Although there are someshifts, especially in the abundances of Microtendipes pedellus-type, no clear trend can be identified in the chironomid record ofcore SR-X1. The genus Microtendipes is reported to be anindicator of intermediate temperatures in northern Europe(Brooks and Birks, 2001), and it is most common in the littoraland sublittoral zones of lakes with coarse sediments (Brodersenand Lindegaard, 1999; Brooks et al., 2001).

5.2. Chironomid record of LM8 (Early Pleniglacial)

A total of 54 chironomid taxa are identified in the 18 samplesfrom LM8, and selected taxa are plotted in Fig. 4 (after Bohnckeet al., 2008). The count sum of the individual samples is 56–161 hcs, with the exception of the uppermost sample that onlyyielded 14 hcs. The chironomid-assemblages shown in Fig. 4suggest that a shallow lake existed at the study site during theperiod of infilling and the lake was probably meso- to eutrophic,as indicated by several taxa such as Ablabesmyia or Chirono-mus plumosus-type (e.g. Brodersen and Quinlan, 2006). Most ofthe abundant fossil taxa currently occur in low altitude ortemperate lakes in NW and Central Europe.

Fig. 4. Chironomid abundance diagram of selected taxa from the LM8 sample box (after Bohncke et al., 2008), and the organic content of the sediment (estimatedthrough loss-on-ignition). Chironomid taxa are classified based on the habitats they are predominantly found in, although some taxa (e.g. Limnophyes, Cricotopus-type) can occur in a range of habitats types. Chironomid-inferred mean July air temperature estimates with sample specific error bars as determined by bootstrap cross-validation are plotted to the right.

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In the lower part of the record Chironomus anthracinus-typeshows abundances over 15%. After this initial peak, the valuesof C. anthracinus-type decline in favor of M. pedellus-type,which has a high abundance throughout the remainder of therecord (Fig. 4). Dicrotendipes and Glyptotendipes, present inthe lower part of LM8, are often associated with the presence ofmacrophytes (Moller Pillot, 1984) and are most common inmeso- to eutrophic lakes (Brodin, 1986).

At 12.5 cm core depth M. pedellus-type, Cladopelmalateralis-type and Polypedilum nubeculosum-type valuesdecrease in favor of Tanytarsus lugens-type, Procladius, andC. anthracinus-type. The chironomid concentration decreasesfrom 50 to 10 hc/g.

T. lugens-type is one of the taxa that dominates in the upperpart of the record, and is often considered as an indicator ofcool, oligotrophic conditions (e.g. Brodin, 1986; Porinchu andMacDonald, 2003). The high values of C. anthracinus-type,Procladius and T. lugens-type could indicate an increase inlake-depth, although this is unlikely considering the geomor-phological situation of the lake. These three taxa are alsoindicative for lower temperatures than the taxa that were moreabundant in the lower part of the record.

5.3. Quantification of July air temperatures

The chironomid-inferred July air temperature record of SR-X1 shows stable reconstructed temperatures around 14.5 °C(Fig. 3). All sample specific prediction errors are between 1.5and 1.6 °C. The sample just below the clastic interval that isbarren of chironomids (26.5 cm core depth) shows a lowerreconstructed temperature of 12.9 °C. Above the clastic interval,the reconstructed temperature again is 14.8 °C. The uppermostsample is not plotted in this figure, as its count sum is muchlower than 50, the minimum count sum recommended for

numerical analysis (e.g. Heiri and Lotter, 2001; Quinlan andSmol, 2001). Although most chironomid taxa identified in theSR-X1 record are well represented in the Alpine calibration dataset, the absence of Georthocladius in the modern data set resultsin high cumulative abundances of taxa included in the fossildata but absent in the modern training set (average: 14.0%;range: 4.6–35.0%). When Georthocladius is excluded from theanalyses, the sum of rare taxa (Hill's N2b5) is 0.0–0.4% for allfossil samples from SR-X1. Furthermore, even when excludingGeorthocladius, the nearest-modern-analogue (as chi-squareddistance) and goodness-of-fit (as squared residual distance)calculations resulted in ‘no good’ analogues for all fossilsamples (except for sample 26.5 cm, which has ‘no close’analogues) and in ‘poor fit’ to ‘very poor fit’ scores for all fossilsamples. This result is unexpected, as all the dominant taxa(except for Georthocladius) are well represented in the Swisscalibration data set. Probably, the combination of taxaencountered in the fossil assemblages of SR-X1 is not foundin the calibration data set, perhaps as a result of the influence ofriver inundations on the fossil lake ecosystem of SR-X1.Previous studies by Heiri et al. (2003) and Engels et al. (2007a)have shown that chironomid-assemblages from early Holoceneor even older deposits can have poor analogue statistics, eventhough the abundant taxa in their records were well representedin the modern calibration data sets.

The reconstructed mean July air temperatures for the LM8record sequence (Fig. 4) are relatively high (15 to 16 °C) for thelowermost part of the record, but steadily decline above 17 cmcore depth to values around 13.0 °C. The sample specific errorestimates are between 1.4 and 1.5 °C for all samples. On average92.4%of the identified fossil chironomids of the LM8 recordwerepresent in the modern training set and used to obtainpalaeotemperature estimates (range: 85.6–100%). There are norare taxa present in the fossil chironomid-assemblages, and only 1

Fig. 5. Selected pollen-taxa (expressed as relative abundances (%)), plant macrofossils (expressed as number of remains per sample), and the number of statoblasts ofthe bryozoan Cristatella mucedo in the SR-X1 record.

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sample (12.5 cm core depth) shows a ‘poor fit’ to temperature.Again, all samples have ‘no good’ analogues in the calibrationdata set (with the exception of the sample at 11.0 cm core depthwhich has ‘no close’ analogues). Although there are no goodanalogues for both the samples of the SR-X1 record and the LM8record, Birks (1998) states that WA-PLS can perform relativelywell in poor analogue situations. Heiri et al. (2007) for instanceobtained reliable mean July air temperature reconstructions forLake Hijkermeer, even during intervals were the fossil chirono-mid-assemblages showed ‘no good’ analogue conditions.

6. Palaeoclimatic reconstructions from other proxy records

6.1. SR-X1 (Weichselian Early Glacial)

Palaeobotanical analyses indicate that treeless vegetationexisted at the Nochten-site during at least part of the EarlyGlacial (Fig. 5). The lower part of the record shows values ofPinus of approximately 10%, increasing to ca. 25% in theupper part of the record. Lotter et al. (1992) suggest thatabundances of Pinus pollen below the 20% cut-off most oftenreflect long distance transported pollen, and do not indicate thelocal presence of pine stands. However, tree pollen values mayhave been suppressed in our record due to the local presence ofCyperaceae and Poaceae at the study site. Other tree taxa thatmight indicate long-distance transported pollen (e.g. Alnus) alsoshow increasing abundances towards the top of the record.There is however no macro-fossil evidence for the presence oftrees at our study site. The local vegetation probably consistedof a low-shrub tundra dominated by Betula and Salix shrubs.

Macro-remains of aquatic plants suggest that the site was ashallow lake surrounded by a sedge swamp, which is consistentwith the chironomid fauna. Leaf spines of Ceratophyllumdemersum, found in low numbers throughout the record,indicate a minimum mean summer temperature (MMST) of15 °C (Litt, 1994). Other abundant temperature-indicatorspecies like Ranunculus subgen. Batrachium or Cristatellamucedo indicate a MMST of at least 10 °C (respectivelyLacourt, 1968; Brinkkemper et al., 1987). Although the 15 °CMMST reconstructed using plant–climate indicator species isunexpectedly high when compared to other climate reconstruc-tions from the Niederlausitz region (see below), it does concurwith our high chironomid-based palaeotemperature estimates of15 °C.

6.2. Other climate reconstructions from the Niederlausitzregion for the Weichselian Early Glacial

Bos et al. (2001) studied a sequence of Early Glacial deposits,and reconstructed vegetation that is initially dominated by pine(Pinus sylvestris). Many pine wood-fragments were found, aswell as macro-fossils of other conifers (Abies, Picea, Larix). Pi-nus pollen also reached an abundance of 40–60% during theinterval that the authors correlated with the Brørup Interstadial,whereas Betula only reached values between 8 and 18%. Thesepollen assemblages resemble pollen diagrams derived from otherlocations in the Lausitz area that date back to both the Odderade

and Brørup Interstadial (Bos et al., 2001). Findings of macro-remains of Typha latifolia, T. angustifolia and P. sylvestrissuggest that the minimum mean summer temperature during theEarly Glacial was N13 °C.

In the upper part of their Early Glacial sequence (correlatedwith the Rederstall stadial/early Odderade Interstadial), Boset al. (2001) record high values of NAP (reaching 60%) withmuch lower percentages of Pinus pollen (below 20%).However, in the record of Bos et al. (2001), Betula reacheshigh values up to 30% whereas it only reaches abundances of10% in our record. Bos et al. (2001) find local indicators of areed swamp, including the massive presence of Typha seeds thatindicate a MMST of 13 °C. Within the youngest Early Glacialdeposits, Bos et al. (2001) found poor microfossil andmacrofossil assemblages. A minimum mean summer tempera-ture around 8–10 °C, is reconstructed, and there is no evidencefor the presence of trees.

A detailed comparison between our record and the differentrecords of Bos et al. (2001) is hampered because of the highlyvariable character of the Early Glacial pollen-assemblages. Wereconstruct a treeless vegetation with an increasing influence oflong-distance transported pollen as well as reworked pollen (bothincluding Pinus), and it might be possible that our recordcorrelates best with the deposits described by Bos et al. (2001) asbelonging to the earlier parts of the Odderade Interstadial. TheMMSTs reconstructed by Bos et al. (2001) are in the order of13 °C, whereas our chironomid-based palaeotemperature esti-mates are 2 °C higher. Using chironomid-remains, we recon-structed mean July air temperatures instead of minimum meansummer temperatures as derived from plant-indicator species. Itwas therefore also expected that the chironomid-based palaeo-temperature estimates are slightly higher than the estimatesderived from plant–climate indicator species. Furthermore, Boset al. (2001) used a core taken from a deposit formed in a reedswamp, a habitat not suitable for many aquatic plants that are usedin the climate-indicator species approach. This might haveprevented the local occurrence of such plants, and mightpotentially have caused an underestimation of the MMST.

6.3. LM8 (Early Pleniglacial)

Bohncke et al. (2008) show that aquatic plant remains andpollen-assemblages from the LM8 record suggest a highminimum mean July air temperature of ca. 12–14 °C shortlyafter the formation of the lake and during the initial period ofdeposition of sediments. They reconstruct treeless vegetation thatis initially dominated byBetula shrubs and other tundra-elements.At 12.5 cm core depth the pollen record shows an increase in thepine pollen values, and Pinus becomes the dominant taxon. Theauthors interpret this change as an indication of a relative increasein long-distance transport as a result of amore patchy shrub tundravegetation and a decreasing pollen production at the study site.Bohncke et al. (2008) furthermore demonstrate a largerabundance of Cenococcum geophilum sclerotia, as well as anincreased presence of reworked palynomorphs, which theyinterpret as indicating a larger influence of surface erosion. Weshow in Fig. 4 that at the same core depth, the chironomid

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concentration declines sharply, and that there is a shift indominant chironomid taxa. The organic content of the sedimentdrops considerably to values of ~5% and the chironomid-inferredpalaeotemperature estimates (Fig. 4) show a decrease byapproximately 4 °C. Bohncke et al. (2008) interpret theirqualitative inference of cold climate conditions before theformation of the lake, high temperatures during the infilling ofthe lake and the abrupt transition towards cold-climate conditionsnear the top of the record as evidence for a Dansgaard/Oeschger(D/O)-like climate evolution phase (e.g. Dansgaard et al., 1993).Our quantitative chironomid-inferred temperature reconstructionsupports the initial interpretation by Bohncke et al. (2008).

7. Comparison with palaeoclimatic reconstructions fromNW Europe

7.1. WeichselianEarlyGlacial summer temperature reconstructions

Aalbersberg and Litt (1998) suggested a minimummean Julyair temperature around 14 °C for northwestern Europe for theOdderade and Brørup Interstadials, with local temperatures incentral and northern Germany that reach 15 °C to 16 °C. For theNiederlausitz area, the Odderade Interstadial showed climaticconditions that were similar to that of the Brørup Interstadial, withmean summer temperatures estimates reaching a maximum of15 °C. Both the Brørup and the Odderade Interstadial temperatureestimates fit with our chironomid-inferred temperature estimates.

7.2. Weichselian Pleniglacial summer temperature reconstructions

Although our chronology suggests that the sediments ofLM8 were deposited during the Early Pleniglacial, otherchronologies from the Niederlausitz area indicate an alternativeearly Middle Pleniglacial age (i.e. ~55 ka; see above). Very fewterrestrial sites offer quantitative temperature estimates for theEarly Pleniglacial in northwestern and central Europe. UsingColeopteran (beetle) based temperature estimates, Coope et al.(1997) reconstruct temperatures of the warmest month of 10 °Cto 13 °C for central London and between 7 °C and 11 °C nearOxford, England. To the south, mean July temperatures of 15–16 °C are reconstructed using coleopteran-assemblages from LaGrande Pile, France (Ponel, 1994).

Fossil coleopteran (beetle) assemblages from several sites inEngland show that the climate during the Middle Weichselianwas probably temperate and oceanic with mean monthly Julytemperatures reaching levels at least as warm as those of thepresent day (Coope, 2002). Analysis of fossil coleopteranassemblages from the Oerel-site (northern Germany) suggesteda mean July temperature of 11 °C at this location (Behre et al.,2005). This cooler temperature is in contrast with the hightemperatures reconstructed in England.

Using 29 sites from north-western Europe, Huijzer andVandenberghe (1998) estimate a minimum mean July tempera-ture of ca 7 °C in Poland and 11 °C in western Europe usingpalaeobotanical data. However, the number of indicator specieswas low in the Polish records, which are therefore considered tobe less reliable by the authors.

The temperature estimates based on Coleoptera data fromwestern Europe show that there have been intervals during theEarly and Middle Pleniglacial during which the temperatures ofthe warmest month reached values that were almost equal tothose of the present-day. The reconstructions based onpalaeobotanical data represent minimum mean summer tem-peratures, and therefore are lower than the palaeotemperatureestimates derived from coleopteran remains.

Our data from eastern Germany, based on chironomid-assemblages, indicates that former temperatures during summermonths were slightly lower than those of the present-daysituation, which is in line with the coleopteran-based palaeo-temperature estimates.

8. Conclusions

Chironomid-based temperature reconstructions for theWeichselian Early Glacial average around 15 °C, slightlylower than the present-day temperature at the study site. Thepollen-assemblages from the same record indicate that treelessvegetation was present at the study site, which suggests adelayed vegetation response, as temperatures were sufficientlyhigh for the development of a forest. The high chironomid-based palaeotemperature estimates are in accordance withaquatic plant-indicator macro-fossils found in the samesediment core. Climatic reconstructions for northwesternEurope based on Coleopteran remains also indicated relativelyhigh summer temperatures for the Weichselian Early Glacial.

The chironomid-inferred July air temperatures for the EarlyPleniglacial are around 15–16 °C for a large part of the record, buta sharp decrease to 13 °C is reconstructed near the top of thesediment sequence. The organic content of the sediment, thechironomid concentration and the local vegetation also showabrupt changes at this core depth, and suggest a D/O-like climateevent. Although a direct comparison between different sites ishampered because of the uncertain chronology, there are severalsites in northwestern and central Europe that also yielded highpalaeotemperature estimates for the Early Glacial and Plenigla-cial, which are as high or nearly as high as at present.

This is the first study that uses lacustrine floodplain depositsin order to obtain quantitative, chironomid-based mean July airtemperature reconstructions, and the results show that thesesedimentary environments have a high potential for palaeocli-matic studies even considering the complex processes occurringon floodplains. The combination of chironomid-based tempera-ture inference with other means for quantitative temperaturereconstruction, such as the analysis macrofossils of aquaticmacrophytes, seems a useful way forward in developingfloodplain lake sediments as palaeoclimate archives.

Acknowledgements

The Laubag Mining Company, and especially Mr. and Ms.Domko, are sincerely thanked for granting us access to theNochten and Reichwalde mines and also for providingassistance. The aid of L. Jonker and R. Nagtegaal during thefield work is appreciated. We thank C. Johns (NCL) for OSL

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sample preparation, measurement and analysis, and M. Frechen(Leibniz Institute for Applied Geosciences, Germany) forproviding additional information on luminescence datingmethods employed in earlier studies of the Nochten site. C.Kasse (Vrije Universiteit) is thanked for the valuable discus-sions on the geochronology. We thank two anonymousreviewers for their useful suggestions. The research project ofS.E. is supported by the Council of Earth and Life Sciences ofthe Netherlands Organization for Scientific Research (grant-no813.02.004). This is Netherlands Research School of Sedimen-tary Geology publication 20071031.

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