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Quaternary Science Reviews 27 (2008) 1493– 1504

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Quaternary Science Reviews

0277-37

doi:10.1

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journal homepage: www.elsevier.com/locate/quascirev

Paleolimnological response to millennial and centennial scale climatevariability during MIS 3 and 2 as suggested by the diatom record in LesEchets, France

Linda Ampel a,�, Barbara Wohlfarth b, Jan Risberg a, Daniel Veres c

a Department of Physical Geography and Quaternary Geology, Stockholm University, SE-10691 Stockholm, Swedenb Department of Geology and Geochemistry, Stockholm University, SE-10691 Stockholm, Swedenc ‘Emil Racovita’ Speleological Institute, Clinicilor 5, 400006 Cluj, Romania

a r t i c l e i n f o

Article history:

Received 12 November 2007

Received in revised form

21 April 2008

Accepted 24 April 2008

91/$ - see front matter & 2008 Elsevier Ltd. A

016/j.quascirev.2008.04.014

esponding author. Tel.: +46 86747569; fax: +

ail address: linda.ampel@natgeo.su.se (L. Amp

a b s t r a c t

A 27 m long sediment sequence retrieved from the central part of the Les Echets basin in France has

been analysed in sub-centennial resolution for biogenic silica and fossil diatom remains. The sequence

corresponds to the later part of Marine Isotope Stage (MIS) 3 and to most of MIS 2. Distinct changes in

diatom productivity, diversity and taxonomic composition between 36.2 and 31.7 kyr BP appear to relate

to Dansgaard–Oeschger (DO) climate variability. Intervals characterized by low diversity, productivity

and small-sized benthic diatom taxa are most likely a response to colder conditions in relation to DO

stadials. In contrast, higher diversity, productivity and a high abundance of planktonic taxa indicate a

response to warmer temperatures during DO interstadials. The time interval between 30.3 and 15.7 kyr

BP is characterized by continuous low diatom productivity and a benthic dominated community with

intermediate species richness, suggesting a transition to more stable conditions. Three time intervals

with extremely low concentrations of diatom valves (46.1–36.2, 31.7–30.3 and 26.3–23.6 kyr BP) overlap

with ages reported for Heinrich (H) events 4, 3, and 2. We speculate that the lake at Les Echets suffered

from severe ecological stress as a response to H events. This is the first detailed study exemplifying the

response of a lake, based on diatoms, to climate variability during late part of MIS 3 and most of MIS 2 in

Europe.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The Last Glacial was characterized by significant climateinstability. Cold background conditions were punctuated byabrupt warming events with a temperature increase of up to15 1C in only a few decades (Johnsen et al., 1995; Landais et al.,2005). The rapid shifts to interstadial conditions were followed bya more progressive temperature decrease back to stadial settingsbefore another abrupt warming set in. These quasi-cycles, with anaverage duration of approximately 1500 years, were first recog-nized in the Greenland ice core records (d18O) and are referred toas Dansgaard–Oeschger (DO) cycles (Johnsen et al., 1992, 2001;Dansgaard et al., 1993). Moreover, episodes of extremely cold andharsh climate conditions occurred every 7000–10,000 years at theend of a series of DO cycles. These so-called Heinrich (H) eventswere first discovered as distinct horizons of ice-rafted debris inmarine sediment cores from the North Atlantic (Heinrich, 1988;

ll rights reserved.

46 8164818.

el).

Bond et al., 1992; Broecker, 1994). Intense research efforts havebeen directed at reconstructing the nature of these climate shiftswithin ice core and marine sediment sequences, but their impactover continental areas is less well constrained since only a fewdetailed, and chronologically well-constrained terrestrial studiesare available (Voelker et al., 2002).

High-resolution pollen stratigraphical sequences from theMediterranean region reveal for example distinct vegetationchanges in southern Europe, which coincide with the centen-nial-millennial climate variability identified within Greenland andNorth Atlantic records (Allen et al., 1999; Sanchez Goni et al.,2002; Tzedakis et al., 2004). In contrast, paleoenvironmentalreconstructions from central Europe are still fragmentary andoften lack good chronological control (Voelker et al., 2002),although earlier investigations such as for example, long lacus-trine sequences from France illustrate their potential for detailedenvironmental reconstructions (Woillard and Mook, 1982; Beau-lieu and Reille, 1984a, 1992).

The understanding of natural climate variability, of its under-lying causes and of its impact on terrestrial ecosystems requiresspatially and temporally well-constrained paleorecords. In an

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L. Ampel et al. / Quaternary Science Reviews 27 (2008) 1493–15041494

attempt to fill this gap, two new sediment cores were retrievedfrom Les Echets in France with the purpose to perform a high-resolution multiproxy investigation. Earlier studies from LesEchets had revealed a more or less continuous record with severalvegetation oscillations during the Last Glacial period (Beaulieuand Reille, 1984a, b, 1989) suggesting that this site had a goodpotential for detailed studies of abrupt climate change. Indeed,detailed lithostratigraphical, sedimentological, geochemical, andchronological analyses applied on the new sediment sequencesfrom Les Echets (Veres et al., 2007; Wohlfarth et al., 2008) havedemonstrated a clear ecosystem response to centennial-millennialclimate variability between 46 and 15 kyr BP.

Diatoms are good indicators of paleolimnological changebecause they: (i) grow in almost all aquatic environments, (ii)have short (sub-annual) life cycles and therefore provide thepotential to register rapid environmental changes, (iii) depend to alarge extent on the type of habitats available as well as on thephysico-chemical conditions within a lake and its surroundings.Stratigraphical examination of diatom assemblages and theircompositional alterations can therefore reveal important informa-tion on environmental conditions affecting a lake and itscatchment in the past (e.g., Battarbee et al., 2001). Because oftheir great potential as paleoenvironmental indicators we applieddiatom and biogenic silica (BSi) analysis with the purpose tofurther disentangle the impact of DO cycles and H events on thepaleolake and on the terrestrial environment surrounding LesEchets.

2. Regional setting

Les Echets is located in eastern France, ca 15 km northeast ofLyon on the Dombes Plateau (451540N, 41560E), at 267 m a.s.l. Theplateau is covered by glacial deposits, which are assigned to thepenultimate glaciation (Riss). The Alpine Rhone glacier did notoverride the Dombes Plateau during the last glaciation (Wurm)but terminated in the valley ca 15 km to the east of Les Echets.Wurmian moraines occur east of the plateau, while Wurmianglaciofluvial deposits are evident further south (Veres et al.,2007). The lake basin occupies a depression formed by the Rhoneglacier during the penultimate glaciation and is dammed byfrontal moraines at its western rim. During the Rissian deglacia-tion the depression became filled with water, and the lake and itssediments thus record environmental change through the LastInterglacial/Glacial cycle. At its maximum, the lake covered ca1300 ha, but became gradually filled—in between the end of theLast Glacial period and the early Holocene (Beaulieu and Reille,1984a).

3. Materials and methods

In autumn 2001, two long sediment sequences were recoveredfrom the Les Echets basin. The core retrieved from the central part(EC1) of the basin (ca 1100 m from the shore) has a total length of44 m while the core retrieved closer to the shore (EC3) (ca 700 mfrom the shore) has a length of 24 m (see Veres et al., 2007 fordetails on core locations). Analyses on fossil diatom remains andBSi content was made on core EC1 between 30.07 and 3.30 m. Thecontiguous loss-on-ignition (LOI) curve (2 cm sampling incre-ments) presented in Veres et al. (2007) was used as a guide forsub-sampling for diatom and BSi analysis. Where the LOI curveshowed large changes we sampled at ca 5 cm intervals (between27.53 and 24.17 m), where changes were more subtle, sampleswere spaced at ca 10 cm intervals (between 30.07–27.53 m and24.17–13.63 m) and at ca 15 cm intervals (between 13.63 and

3.30 m). Additional samples were later selected, where the diatomstratigraphy revealed taxonomic changes. In total, 295 sampleswere analysed for BSi and 381 for diatoms.

3.1. Age model

Age estimates for core EC1 are based on 48 14C measurementson terrestrial and limnic plant macrofossils, pollen concentratesand bulk sediments and on 22 infrared stimulated luminescence(IRSL) dates between 5.49 and 30.07 m (for details on individualdates and laboratory procedures, see Wohlfarth et al., 2008). 14Cdates, which fall beyond the oldest ages of the internationallyagreed IntCal04 calibration curve (Reimer et al., 2004), werecompared to the updated calibration data set of Hughen et al.(2006) (Fig. 1). The age model was computed using the Bayesiansoftware Bpeat (Blaauw and Christen, 2005) and is described indetail in Wohlfarth et al. (2008).

3.2. Biogenic silica analysis

Samples for BSi analysis (1 cm3) were measured at the NationalEnvironmental Research Institute in Roskilde, Denmark, using awet alkaline extraction technique (DeMaster, 1981). Approxi-mately 30 mg of freeze dried and homogenized sediment sampleswere put in polypropylene vials and 40 ml of 1% Na2CO3 solutionwas added just prior to digestion. Vials were placed in a shakerbath at 85 1C and 100 rpm with caps slightly loosened to vent gas.After 3 and 5 h of digestion, a 1 ml aliquot from each sample wasremoved and put in 9 ml HCl (0.021 N). Aliquots were thenanalysed for dissolved silica (DSi) using the molybdate bluemethodology (Technicon Industrial Method 186-72W-Modified)with ascorbic acid as the reductant on a Technicon AutoAnalyzer IIsystem. Extracted Si concentrations were calculated using thefollowing equation:

Si ðwt%SiO2Þ ¼ðDSi in mMÞ � ð60 g=moleÞ � ð0:04 LÞ � ð10Þ � ð0:1Þ

amount weighed ðmgÞ.

A least square regression analysis was made on the increase in Siextracted versus time and extrapolation to the intercept was usedto estimate BSi concentrations (Conley and Schelske, 2001).

3.3. Diatom analysis

Samples for diatom analysis (0.5–1 cm3) were treated with HCl(10%) to dissolve carbonates and then put in H2O2 (17%) over nightprior to heating at 100 1C for 2–4 h to oxidize organic matter.Subsequently, samples were allowed to settle in distilled water in100 ml beakers for 2 h and then decanted, a process repeated untilthe water was transparent. NH4OH solution was added todeflocculate clay particles and samples were again decanted untiltransparency (Battarbee et al., 2001). The residues were placed oncoverslips and allowed to air-dry before being mounted ontoslides with Naphraxs. Diatoms were identified and countedacross transects under X1260 magnification with oil immersionusing a Zeiss Axiophot light microscope. Taxonomic identifica-tions were made with reference to Cleve-Euler (1951, 1952,1953a, b, 1955) and Krammer and Lange-Bertalot (1997, 1999,2004a, b). At most levels, a minimum of 400 diatom valves wasidentified. At depths where diatom valves were few at least a fullslide, i.e., 25 transects, was scanned. Other siliceous microfossilremains (chrysophyte cysts, phytoliths, and sponge spicules) werealso counted (results not shown). Diatom species abundance isexpressed as a percentage relative to the total number ofidentified valves. The diatom diagram (Fig. 2) shows only themost common species, i.e., those occurring in more than three

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Fig. 1. 14C dates (filled circles) and IRSL (open circles) dates for Les Echets core EC1

(upper panel). The 14C dates were matched to the updated calibration data set of

Hughen et al. (2006), while the IRSL dates are fitted to the dashed line. Computed

age-depth model (Blaauw and Christen, 2005) displaying the chronological

uncertainty (lower panel). Darker colours indicate more likely calendar ages. See

Wohlfarth et al. (2008) for details.

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samples, or if less, with an abundance of 43% in one sample.Samples with a diatom count lower than 60 were excluded fromthe percentage calculations because those samples resulted inover-exaggerated peaks in the relative abundance curves. Thenumbers of identified valves at these levels are instead shown bysymbols. The species have been grouped as planktonic or benthictaxa, where the benthic species are divided into three groupsbased on their distribution patterns. Species have been arrangedin succession order within each of the groups. The diatom diagramin Fig. 2 was constructed using the Tilia programme (Grimm,1991), while the summary diagram presented on an age scale inFig. 3 was created in Grapher.

3.4. Numerical analyses

Stratigraphical diatom assemblage zones (DZ) were deter-mined through optimal partitioning using sum-of-squares criteria(Birks and Gordon, 1985) in the computer program ZONE (Lotterand Juggins, 1991). The number of significant zones wasdetermined by the broken stick approach with the programBSTICK (Bennett, 1996). Taxonomic richness was calculated byrarefaction analysis (Birks and Line, 1992) with a common countsum of 350 using the program Analytic Rarefaction 1.3 (Steven M.Holland; www.uga.edu/�strata/software/). To evaluate the com-positional turnover in the record, the diatom data was analysed byDetrended Correspondence Analysis (DCA) using the programCANOCO (version 4.5) (ter Braak and Smilauer, 2002). Onlyspecies occurring in more than three samples, or if less, with anabundance of 43% in one sample and samples with a count sumof 460 valves were used for the DCA. Axis 1 had a gradient lengthof 2.56 S.D. units and the data was square-root transformed andrare taxa were down-weighted. The results of the numericalanalyses are presented on an age scale together with the diatomsummary diagram in Fig. 3.

4. Results

4.1. Biogenic silica

Results of the BSi analysis are presented on a depth scale inFig. 2 and on a time scale in Fig. 3. BSi concentrations in thesediments are low throughout the analysed sequence(30.07–3.30 m). The interval between 27.47 and 22.52 m showsthe highest variability of BSi concentrations with values rangingfrom 0.2% to 3.5%. The lower part of the sequence, between 30.07and 27.47 m and the interval between 22.52 and 3.30 m bothdemonstrate very low values, ranging from 0.1% to 1.6%, and smallvariability. Because siliceous remains other than diatoms wererarely encountered the BSi content can be considered as a diatompaleoproductivity index.

4.2. Diatoms

Eighteen diatom zones (DZ) were identified as being significant(Figs. 2 and 3). Their individual characteristics and assemblagecomposition are described in Table 1. The result of the rarefactionanalysis (Fig. 3) demonstrates species richness values between ca15 and 40. The most pronounced changes in species richnessoccur between 27.47 and 22.52 m, although the intervals between21.41 and 16.09 m and between 14.93 and 5.50 m also show somevariability. The DCA sample scores give the highest variabilitybetween 27.47 and 22.52 m suggesting that the diatom commu-nity experienced the largest compositional turnovers between36.2 and 31.7 kyr BP. The first axis in the DCA analysis (l1 ¼ 0.32)explains 28.3% of the variance, while the second DCA axis(l2 ¼ 0.09) explains an additional 8.3%.

5. Discussion

5.1. Biogenic silica

The BSi record indicates low diatom paleoproductivitythroughout the analysed sequence (30.07–3.30 m) with slightlyhigher values in DZ-2, 4, 6, and 8. The similar range of BSi valuesfor zones quasi-barren of diatoms (DZ-1, 10, and 15) and for zonesdominated by benthic taxa (DZ-3, 5, 7, 9, 11–14, and 16–18) makesthe BSi analyses somewhat ambiguous, especially when compared

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Fig. 2. The most common diatom species in Les Echets between 30.07 and 3.30 m. For stratigraphical levels where less than 60 valves were identified is the number of

frustules for each species shown as: + ¼ 1–10, ++ ¼ 11–20, +++ ¼ 21–30, ++++ ¼ 31–40, +++++ ¼ 41–50, ++++++ ¼ 51–60 instead of relative abundance in percent. The gray

contours on curves represent an exaggeration factor of 3.

L. Ampel et al. / Quaternary Science Reviews 27 (2008) 1493–15041496

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Fig. 2. (Continued)

L. Ampel et al. / Quaternary Science Reviews 27 (2008) 1493–1504 1497

to the changes in diatom abundance observed during microscopicidentification work (e.g., the diatom sum, Fig. 2). Swann andMackay (2006) describe a similar discrepancy between diatom

concentration (quantified by microspheres) and BSi in sedimentsfrom Lake Baikal, which they interpret as removal of BSi fromdiatom valves due to increased dissolution at the sediment-water

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Fig. 3. Summary diatom diagram and results of numerical analyses between 40 and 15 kyr BP. Erosion (dry density) and lake organic productivity (TOC, TN, LOI, and BSi)

indicators for EC1 as well as the NGRIP d18O record and ages reported for H events are shown for comparison.

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interface. If their interpretation is valid, it would in our case implythat DZ-3, 5, 7, 9, 11–14, and 16–18 represent periods withincreased dissolution, while DZ-1, 10, and 15 would have beenaffected by less dissolution and would therefore result in similarBSi values. No quantitative measures of diatom concentration(e.g., microspheres) were applied during analysis of the Les Echetssediments, which makes it difficult to evaluate such an effect.Nevertheless, no signs of corrosion have been observed on valvesin the benthic dominated communities (DZ-3, 5, 7, 9, 11–14, and16–18) to support this hypothesis. Instead, we find it likely thatthe relative changes in diatom abundance between zones quasi-barren of valves and zones dominated by benthic species, wherereliable counts could still be obtained, is probably beyond thedetection limit of the BSi method. Although BSi extraction is ameasure of the content of siliceous microfossils, sources ofuncertainty exist. Leaching of siliciclastic sediment particlesduring digestion, especially of clayey-silty sediments like in LesEchets, might contribute with up to 1% of the extracted silica(Schettler et al., 2006). Also sources of precipitated silica andsmall fragments of siliceous microfossils (e.g., phytoliths) notdetectable under the microscope, might contribute to the BSi

content. Leaching from sediment particles together with diffusesources of BSi might therefore represent a background value ofaround ca 1%. At the same time, relative changes in diatomabundance between zones quasi-barren of diatoms (DZ-1, 10, and15) and benthic dominated zones (DZ-3, 5, 7, 9, 11–14, and 16–18)are of too low amplitude to be distinguished by the BSi method.

BSi concentrations in zones that contain planktonic species(DZ-2, 4, 6, and 8), on the other hand, illustrate more discerniblerises, which presumably represent more significant increases indiatom abundance. The most valid interpretation would probably beto classify zones quasi-barren of diatoms (DZ-1, 10, and 15) togetherwith zones dominated by benthic taxa (DZ-3, 5, 7, 9, 11–14, and16–18) as characterized by very low diatom productivity, whilezones characterized by the presence of planktonic taxa (DZ-2, 4, 6,and 8) represent periods of slightly increased productivity.

5.2. Diatom stratigraphy

The diatom assemblages in the Les Echets sediments consist oftaxa that are typical for subalpine to alpine regions in northern

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Table 1Summary of the diatom stratigraphy of Les Echets, core EC1

Zone Depth (m) Age (kyr BP) Description

DZ-1 30.07–27.47 46.1–36.2 Diatom valves are rare.

DZ-2 27.47–26.99 36.2–35.7 Diploneis elliptica (Kutzing) Cleve, Navicula radiosa Kutzing, Stauroneis anceps Ehrenberg, Cymbella

leptoceros (Ehrenberg) Kutzing, Navicula vulpina Kutzing, Cocconeis placentula Ehrenberg, Navicula

rhynchocephala Kutzing, and Amphora libyca Ehrenberg increase abruptly at the lower boundary.

Subsequently, the planktonic flora expands markedly and Cyclotella delicatula Hustedt together

with Cyclotella comensis Grunow become the most abundant taxa.

DZ-3 26.99–26.41 35.7–35.3 Fragilaria taxa dominates with species like F. construens f. venter (Ehrenberg) Hustedt, F.

construens f. construens (Ehrenberg) Hustedt, F. pinnata Ehrenberg, F. brevistriata Grunow, and F.

construens f. binodis (Ehrenberg) Hustedt together with Amphora inariensis Krammer, Navicula

jaernefeltii Hustedt, N. jentzschii Grunow, and N. scutelloides W. Smith.

DZ-4 26.41–25.65 35.3–34.7 Cyclotella delicatula and C. comensis are the most common species. Navicula radiosa, N.

rhynchocephala, and Navicula pupula Kutzing are abundant, while Diploneis elliptica, Stauroneis

anceps, Cymbella leptoceros, Navicula vulpina, Cocconeis placentula, Navicula bacillum Ehrenberg,

Navicula striolata (Grunow) Lange-Bertalot, Navicula trivialis Lange-Bertalot, Navicula laevissima

Kutzing Cymbella subaequalis Grunov, Cymbella angustata (W. Smith) Cleve, and Cymbella incerta

Grunow co-occur in smaller amounts.

DZ-5 25.65–25.19 34.7–34.4 Fragilaria construens f. venter, F. construens f. construens, F. pinnata, Amphora inariensis, and

Fragilaria brevistriata are the most abundant species. Additional taxa are Navicula jentzschii, N.

scutelloides and Cyclotella bodanica v. lemanica (O. Muller ex Schroter) Bachmann.

DZ-6 25.19–24.75 34.4–34.2 Planktonic taxa are common, represented by Cyclotella delicatula, C. comensis, Cyclotella ocellata

Pantocsek, and C. bodanica v. lemanica. The benthic community is characterised by Diploneis

elliptica, Navicula radiosa, Stauroneis anceps, Navicula rhynchocephala, N. pupula, N. trivialis, N.

laevissima, and Navicula concentrica Carter.

DZ-7 24.75–24.42 34.2–33.5 Fragilaria taxa are frequent (F. construens f. venter, F. construens f. construens, F. pinnata, and F.

brevistriata) together with Amphora inariensis, Navicula jentzschii, and N. scutelloides.

DZ-8 24.42–23.71 33.5–32.9 Cyclotella delicatula and C. comensis are abundant together with Diploneis elliptica, Navicula

radiosa, N. vulpina, N. rhynchocephala, N. pupula, N. bacillum, N. striolata, N. trivialis, N. laevissima,

Cymbella incerta, and Navicula concentrica.

DZ-9 23.71–22.52 32.9–31.7 Fragilaria construens f. venter, F. construens f. construens, F. pinnata, Amphora inariensis, and

Fragilaria brevistriata are the most common species.

DZ-10 22.52–21.41 31.7–30.3 Diatom valves are rare.

DZ-11 21.41–20.56 30.3–29.6 Fragilaria construens f. venter reach their maximum occurrence throughout the analysed sequence

(63.8%). F. construens f. construens and F. pinnata are also common.

DZ-12 20.56–20.01 29.6–29.2 The dominant taxa are Fragilaria construens f. venter, F. construens f. construens and F. brevistriata.

Diploneis elliptica, Fragilaria pinnata, Amphora inariensis, Navicula jentzschii, N. scutelloides,

Amphora libyca, and A. ovalis (Kutzing) Kutzing are present with a lower abundance.

DZ-13 20.01–19.28 29.2–28.6 Fragilaria construens f. venter is the most dominant species. F. construens f. construens are also

common together with F. brevistriata which attain their maximum abundance in the sequence at

19.80 m (53.4%). Diploneis elliptica, Cocconeis placentula, Fragilaria pinnata, and Amphora inariensis

are present in small numbers.

DZ-14 19.28–16.09 28.6–26.3 Fragilaria construens f. venter, F. construens f. construens, and F. brevistriata are the dominant

species, while most other species are present in small abundances.

DZ-15 16.9–14.93 26.3–23.6 Diatom valves are rare.

DZ-16 14.93–11.85 23.6–19.0 Most abundant species are Fragilaria construens f. venter, F. construens f. construens, F. pinnata,

Amphora inariensis, and Fragilaria brevistriata. In addition are several species from the Amphora,

Cymbella and Navicula genus together with Diploneis elliptica present.

DZ-17 11.85–6.20 19.0–15.7 Fragilaria spp. and Amphora spp. are the most abundant taxa and F. pinnata reach their highest

abundance in the sequence. Cymbella ehrenbergii Kutzing is also significant for this zone.

DZ-18 6.20–3.30 15.7–o15.7 Fragilaria construens f. venter, F. pinnata, Amphora inariensis, Navicula jentzschii, N. scutelloides,

Amphora libyca, A. ovalis, Cymbella ehrenbergii, and Amphora fogediana Krammer are the most

common species. Diatom valves are rare at the end of the zone (3.44–3.31 m).

L. Ampel et al. / Quaternary Science Reviews 27 (2008) 1493–1504 1499

Europe, Asia and Alaska. Most of the species are characteristic ofoligotrophic, oligosaprobic, slightly alkaline, pH-circum neutral toalkaliphilic conditions (Krammer and Lange-Bertalot, 1997, 1999,2004a, b).

The diatom stratigraphy presented in Fig. 3 has three maincharacteristics. First, the most prominent features are the distinctshifts in relative abundance of planktonic and benthic taxa

between 36.2 and 31.7 kyr BP (DZ-2–9), which corresponds tothe later part of Marine Isotope Stage (MIS) 3. These shifts are alsoconcurrent with distinct oscillations in species composition,richness, and BSi. Second, the major part of the stratigraphybetween 30.3 and 26.3 kyr BP (DZ-11–14), and 23.6 and 15.7 kyrBP (DZ-16–18), which corresponds to MIS 2, shows a distinctbenthic dominance, low values of BSi and an intermediate species

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richness. Thirdly, three intervals between 46.1 and 36.2 kyrBP (DZ-1), 31.7 and 30.3 kyr BP (DZ-10), and 26.3 and 23.6 kyrBP (DZ-15) are characterized by a very low abundance of diatoms.

5.2.1. Diatom assemblage shifts between 36.2 and 31.7 kyr BP

The distinct community shifts seen in DZ-2–9 are character-ized by rapid changes between low diversity benthic dominatedassemblages (DZ-3, 5, 7, and 9) and more species-rich planktonicdominated assemblages (DZ-2, 4, 6, and 8). Sediment lithologyand geochemical analyses (Veres et al., 2007; Wohlfarth et al.,2008) indicate a distinct pattern of lake organic productivity (LOP)and catchment erosion changes coinciding with the diatomchanges over DZ-2–9. In addition, preliminary pollen stratigraphicresults on EC1 revealed variations in the relative abundance ofarboreal pollen (AP) versus non-arboreal pollen (NAP) coincidingwith changes in the diatom community (Wohlfarth et al., 2008).Continuous sediment accumulation with alternating layers ofclayey silty gyttja and clayey gyttja silt characterizes this interval.The organic-rich sediments reveal phases of relatively high LOPand AP content, while the more mineral-rich horizons show aconsistent decrease in LOP indicators and increase in NAP. Thequasi-cyclic alterations of the different LOP and catchmenterosion proxies, as well as AP/NAP values, was interpreted aschanges between more moist/warm intervals during whichbiological and AP production was higher and catchment erosionless pronounced, and colder intervals characterized by limitedprimary production and increased erosion and NAP production(Wohlfarth et al., 2008). The timing and recurrence of theseintervals were associated, within the uncertainty of the agemodel, to DO climate variability. As DZ-3, 5, 7, and 9 overlap withlow LOP phases, which have been correlated to DO stadials andDZ-2, 4, 6, and 8 overlap with higher LOP phases which have beenlinked to DO interstadials (Wohlfarth et al., 2008) we concludethat community shifts in DZ-2–9 are also related to DO-climatevariability. The small increase in planktonic taxa at the end ofDZ-9, which coincides with a marked increase in species richness,was not differentiated by ZONE. This attenuated event coincideswith another increase in LOP indicators and AP that have beeninterpreted as a DO climate response by Wohlfarth et al. (2008).

The influence of climatic factors on diatom communities hasbeen discussed extensively (e.g., Fritz, 1996; Kilham et al., 1996;Anderson, 2000). The link between climate and lakes is complexas various weather conditions can have a substantial influence ona range of chemical and physical interactions within a lake (Fritz,1996). Atmospheric temperature, solar radiation, wind and rain-fall represent primary control factors, which successively influ-ence other environmental parameters (Fritz, 1996; Kilham et al.,1996). Atmospheric temperature not only influences watertemperature but also determines the time of ice formation onthe lake and ice break up. Ice cover, solar radiation and windenergies influence mixing regimes and thermal stratificationpatterns, which in turn determine light regimes and nutrientdistribution (Kilham et al., 1996; Anderson, 2000). The balancebetween evaporation and precipitation influences lake levels andnutrient inputs, since direct rainfall and runoff brings morenutrients into the lake system (Fritz, 1996). Climate has also alarge influence on the surrounding catchment in terms ofweathering rates, vegetation growth and soil formation, whichinfluence the nutrient supply and degree of erosion (Anderson,2000).

A striking characteristic for DZ-2, 4, 6, and 8 is the significantincrease of Cyclotella ocellata, C. comensis, and C. delicatula.Fahnenstiel and Glime (1983) describe C. comensis, and C. ocellata

as common during the thermal stratification period in LakeSuperior and suggest that stable light and temperature fields

during stratification were an important factor for their expansion.Scheffler et al. (2005) show that C. comensis is common inoligotrophic to mesotrophic lakes and that stable stratification ofthe water column is the most important factor for populationgrowth. However, the ecological preferences for C. ocellata aresomewhat ambiguous since contrasting physical and chemicalsettings have been reported (Wunsam et al., 1995; Rioual et al.,2007). C. delicatula seems to prefer oligotrophic to mesotrophicconditions (Fritz et al., 1993) and has been shown to occur duringthe stratification period in Lake Superior (Barbiero and Tuchman,2004). Kiss et al. (2007) showed that C. delicatula is common inthe mixed epilimnion in a meromictic, oligotrophic lake in Spainduring spring and restricted to the metalimnion during thesummer stratification period. In association with the expansionof Cyclotella spp., a distinct increase of a large number of benthictaxa can be observed in the Les Echets record. High diversity of thediatom community is suggested by the occurrence of Cymbella

subaequalis, Navicula concentrica, C. angustata, C. incerta, C.

leptoceros, N. bacillum, Cocconeis placentula, N. vulpina, Stauroneis

anceps, Diploneis elliptica, N. laevissima, N. striolata, N. trivialis,N. pupula, N. radiosa, and N. rhyncocephala, and supported byincreased values of the species richness index.

This stands in distinct contrast to the less diverse flora duringDZ-3, 5, 7, and 9, which is dominated by Fragilaria spp., Navicula

scutelloides and N. jentzschii. Interpreting Fragilaria communitiescan be problematic since they are known to have a broadecological spectrum. Their high surface-to-volume ratio and theirhigh specific growth rates make them competitive during nutrientlimiting conditions (Lotter et al., 1999). Additionally, they are alsocommon just after the isolation of coastal lakes (Stabell, 1985),and under highly eutrophic conditions (Bennion et al., 2001), andare often the most abundant pioneer species after deglaciation(Haworth, 1976). Their small size and high reproductive rate, aswell as large ecological amplitude characterize them as species,which are favoured by r-selection (Denys, 1990). When dominant,Fragilaria spp. might thus be a good indicator of high environ-mental stress, physical disturbance, and unstable transientconditions (Denys, 1990; Anderson, 2000). Although a numberof recent studies have documented different temperature pre-ferences among individual Fragilaria species (Podritske andGajewski, 2007). N. scutelloides and N. jentzschii are taxa that arerelatively common within sediments from the Ancylus lake phaseof the Baltic Sea (Tynni, 1975; Risberg et al., 1996; Wastegard etal., 1998; Hedenstrom and Risberg, 1999). The co-occurrence ofFragilaria spp., N. scutelloides and N. jentzschii under overall lowdiatom productivity (as indicated by BSi) might suggest that DZ-3,5, 7, and 9 were episodes dominated by relatively cold andnutrient poor conditions.

Several studies of Canadian, Scandinavian and Russian sub-arctic to arctic (Smol, 1988; Sorvari and Korhola, 1998; Douglasand Smol, 1999; Laing et al., 1999; Ruhland and Smol, 2005;Solovieva et al., 2005) and European subalpine to alpine lakes(Lotter et al., 1999; Lotter and Bigler, 2000) demonstrate a strongrelationship between the abundance of planktonic versus per-iphytic species, particularly Fragilaria spp., and the length of theice cover season. Colder conditions lead to persistent ice cover,and only a narrow moat of ice free water develops in the littoralzone during summer. Restricted habitat areas combined with ashort growing season only permit small, well-adapted benthictaxa to grow and overall diatom productivity and diversity areconsiderably reduced. Warmer conditions with less extensive icecover on the other hand, make deeper substrates and pelagic areasavailable for diatom growth, which increases productivity anddiversity and permits planktonic species to develop (Douglas andSmol, 1999). In addition, nutrient input, pH, conductivity, mixingand stratification patterns change as a consequence of different

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climate/ice cover situations, which further influences communitycomposition and volume (Douglas and Smol, 1999).

Although Les Echets is situated at a lower altitude and latitudeas compared to these studies, we speculate that DO climatevariability might have caused shifts in physico-chemical condi-tions similar to what has been observed in arctic and alpine lakes.The Fragilaria spp.– N. jentzschii– N. scutelloides communities aremost likely an expression of low light conditions and competitionfor resources and habitats. Colder temperatures might havepromoted extensive lake ice cover and shorter growing seasons,which limited the time and possibilities for diatom production.Moreover, a reduction of the catchment vegetation (Wohlfarthet al., 2008) might have increased erosion and sediment input intothe water column thus, limiting light penetration. Colder summerconditions could have limited soil development and decreasednutrient leaching into the lake system. The Cyclotella speciesidentified here seem to be dependent on thermal stratification forgrowth. Even though Les Echets might not have been ice coveredduring stadial summers, we hypothesize that a shorter open waterseason, cold water temperatures, and windier conditions couldhave suppressed thermal stratification, thereby constrainingCyclotella spp. growth. The more diverse communities in DZ-2,4, 6, and 8 suggest that conditions became more favourable inassociation with DO interstadials. Warmer temperatures andlonger summer seasons could have allowed stratification todevelop, which favoured the expansion of Cyclotella spp. (Sorvariet al., 2002; Smol et al., 2005). In addition, a longer growingseason could have had a positive effect on the succession as wellas on secondary and tertiary growth of diatoms. A widertemperature range during the growing season would havepermitted a larger number of taxa to grow at their optimaltemperature. Also, increased growth of submerged vegetationwould have offered more varied habitats for benthic species.Improved soil and vegetation development in the catchment withincreased nutrient leaching and decreased erosion might havebeen other contributing factors. Overall, warmer temperaturesand longer growing seasons would have decreased competitionand increased diversity and productivity.

An alternative explanation for the community shifts in DZ-2–9could be that the former lake at Les Echets experienced dramatichydrological changes, which resulted in marked lake levelvariations. Relative abundance of planktonic and benthic taxahas indeed been described as indicating lake level fluctuations(Wolin, 1996; Stevens et al., 2006; Stone and Fritz, 2006).Planktonic taxa live drifting in open water, while benthic speciesare attached to for example stones and submerged vegetation, orlive within the sediments in the littoral zone. Shifts in the relativeabundance of planktonic and benthic species can thereforeindicate changes in availability and location of habitats as aconsequence of water level rise/fall (Wolin and Duthie, 1999). Lakelevel variations also lead to changes in sediment accumulation.A lower level causes for example increased transport of coarsersediments further out in to the basin (Digerfeldt, 1998). Althoughthe very low abundance of planktonic diatoms in Les Echetscoincides with a slight increase in the sand fraction in thesediments, it is uncertain whether this increase is due to a waterlevel fall or to increased erosion due to soil destabilization in thecatchment. The interval of DZ-2–9 (27.47–22.52 m) in EC1 hasbeen correlated to the sediment sequence between 940 and645 cm in core EC3, which is situated closer to the shore (Veres etal., 2007). If this correlation holds true, the water depth at EC1could not have decreased below ca 17 m between 36.2 and31.7 kyr BP. A water depth of 17 m should be sufficient forplanktonic diatom growth. Lake level changes are thereforeunlikely the main reason for the presence versus near to absenceof planktonic species. Although variations in the water level could

still have occurred, their extent would not have been large enoughto control the presence/absence of planktonic diatoms.

Only few European records have been analysed for diatomscovering the same time period as Les Echets and observationsof possible responses to DO climate variability are thus rare. Thelarge crater lakes in Italy show for example BSi fluxes on DOtimescales during MIS 3 and 2 (Allen et al., 1999; Ramrath et al.,1999). However, detailed studies using diatom species identifica-tion have only been performed in Lago Grande di Monticchio overone DO cycle corresponding to MIS 2 (Nimmergut et al., 1999) andin Lago Albano for the time interval 28–17 kyr BP (Guilizzoni et al.,2000). Although the timing of the studied events cannot becompared to Les Echets, similar physico-chemical parameters(e.g., ice cover, lake stratification, and nutrient supply) seem tohave changed in response to DO climate variability.

5.2.2. Diatom assemblage shifts during MIS 2

The sequences between 30.3 and 26.3 kyr BP (DZ-11–14) andbetween 23.6 and 15.7 kyr BP (DZ-16–18) correspond in time toMIS 2. Two minor increases in LOP proxies and DCA axis 1 scoresat the upper zone boundary of DZ-11 and at the beginning of DZ-14 can be recognized. These suppressed rises might indicate weakresponses to two DO interstadials during MIS 2. Except for thesetwo minor events, DZ-11–14 and DZ-16–18 show generally lowLOP, which coincide with a significant reduction in the abundanceof planktonic diatoms. These values are comparable to those inDZ-3, 5, 7, and 9, which suggest that similar conditions might haveprevailed during MIS 2 and the DO-stadials of MIS 3: cold watertemperatures and short growing seasons, which preventedthermal stratification and LOP. However, an important differenceexists in terms of assemblage composition and diversity betweenthe benthic-dominated communities in DZ-3, 5, 7, and 9 and thosein DZ-11–14 and DZ-16–18. Diatom stratigraphy and speciesrichness values show that species diversity is higher in most partsof zones DZ-11–14 and DZ-16–18, as compared to zones DZ-3, 5, 7,and 9. At some occasions, species richness even reaches valuessimilar to DZ-2, 4, 6, and 8. In DZ-11–14 and DZ-16–18, Fragilaria

spp. are the most abundant taxa (equal to DZ-3, 5, 7, and 9), butco-occur with taxa which were previously more typical for DZ-2,4, 6, and 8, i.e., C. leptoceros, N. bacillum, C. placentula, D. elliptica, N.

laevissima, N. trivialis, N. pupula, N. radiosa, and N. rhynchocephala.Moreover, species that previously occurred in very low numbers,e.g., Fragilaria construens f. binodis, Amphora ovalis, Cymbella

ehrenbergii, and A. fogediana, now become more abundant. DZ-11–14 and DZ-16–18 appear to represent more stable conditionsas indicated by the DCA axis 1 scores and the two intervals couldbe described as intermediary between states of reduced (DZ-3, 5,7, and 9) and increased (DZ-2, 4, 6, and 8) stratification,productivity, and diversity. Thus, low LOP together with a lowpresence of planktonic taxa, indicate that Les Echets experienced aclimatic and environmental set-back in association with MIS 2,while diatom species richness estimates suggests that conditionswere not as severe as during the DO stadials of MIS 3.

The overall trend for the intervals DZ-11–14 and DZ-16–18of more stable conditions than previously recorded in thestratigraphy compare well with the regional climate trend duringMIS 2 as illustrated by the Greenland d18O curve (Svenssonet al., 2006). The weak or absent response of the diatomcommunity to DO events during MIS 2 is most likely explainedby a more local influence on the site during this time period.We suggest that the expansion of alpine glaciers into theforelands of the Swiss Alps shortly after 30 kyr BP (Preusseret al., 2007) would have made the limnic ecosystem in LesEchets less susceptible to the regional influence of DO warmingevents.

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Recent studies have suggested that individual species ofFragilaria sensu lato might have different temperature optima.Podritske and Gajewski (2007) showed for example an inverserelationship between F. construens and F. pinnata coinciding withtemperature changes over the Holocene. Following their inter-pretation, DZ-16–18 might have been colder than DZ-11–14.

5.2.3. Diatom assemblages in DZ-1, 10, and 15

DZ-1, 10, and 15 are more or less barren of diatoms andcorrespond to previously identified sequences characterized bycontinuously low LOP and high values for NAP and catchmenterosion indicators (Wohlfarth et al., 2008). Increased dissolutionof diatom frustules is a possible explanation, but no visible signsof corrosion were recognized on the few valves found. As the agemodel does not indicate any significant changes in accumulationrate during these intervals, dilution of the valves in the sedimentsis not a likely explanation either.

Ages for H events are difficult to constrain and age estimates inthe literature vary (Hemming, 2004). However, DZ-1, 10, and 15,overlap with ages reported for H events 4, 3, and 2 (Fig. 3) (Bardet al., 2000; Thouveny et al., 2000; de Abreu et al., 2003; Roucouxet al., 2005). Therefore, we speculate that harsh climaticconditions associated with H events had a significant impact onthe limnic ecosystem in Les Echets. The precise consequences ofsuch dramatic episodes and which environmental parametersmight have been most critical for such a dramatic reduction indiatom growth are difficult to resolve. A possible scenario couldhave been periods of extremely persistent ice cover for severalyears, sealing off the lake (Doubleday et al., 1995). Furthermore,increased values of catchment erosion indicators suggest adestabilization of the surrounding soils. This could have increasedsurface run-off resulting in a higher amount of suspendedsediment in the water column, which might have limited lighttransparency thus hampering diatom growth.

DZ-10 and 15 are characterized by continuous accumulation offine-grained sediments, while DZ-1 demonstrate silty and sandysediments and a hiatus at 28.00 m covering the time periodbetween 40.3 and 36.3 kyr BP (Wohlfarth et al., 2008). Thesediments suggest that the intervening conditions restrainingdiatom growth might have been different during DZ-1 incomparison to DZ-10 and 15. Because H4 is known as one of themost severe H events (Sanchez Goni et al., 2002; Vautravers andShackleton, 2006) we speculate that extremely cold and aridconditions (Tzedakis et al., 2004; Roucoux et al., 2005) caused adramatic lake level lowering at Les Echets. A low lake level andharsh climate might have resulted in an unstable depositionalsetting, while an open landscape and exposed shores facilitatedincreased input of coarser sediments.

6. Conclusions

The detailed diatom stratigraphy and BSi record of Les Echetssediment core EC1 reveal a distinct response of the former lake toDO cycles and H events during MIS 3 and 2. Between 36.2 and31.7 kyr BP several shifts in diatom productivity, composition andspecies richness are recognized. Episodes of low diatom produc-tivity and diversity alternate with periods of high productivity anddiversity. The low diversity intervals are characterized by benthic-dominated communities where Fragilaria spp. are the mostabundant taxa. These communities are most likely a reflectionof colder temperatures associated with DO stadials and moreextensive lake ice cover, shorter growing seasons, and/or in-creased catchment erosion. In contrast, high LOP periods arecharacterized by more diverse benthic communities and anexpansion of planktonic Cyclotella spp. We hypothesize that

warmer temperatures during DO interstadials promoted longergrowing seasons, lake-water thermal stratification during sum-mer, and increased nutrient input, which favoured the develop-ment of more complex assemblages.

At 30.3 kyr BP and coincident with the start of MIS 2, Les Echetsseems to have entered a phase of more stable lake conditions.Although diatom productivity and abundance of planktonictaxa decreased, the species richness index suggests a rathercomplex community composition. The absence of obviousdiatom responses to DO events during MIS 2 coincides withthe expansion of alpine glaciers into the lowlands. This repre-sented a change to more local influences at Les Echets andtherefore made the site less receptive to climate influences on aregional scale.

Three distinct horizons between 46.1 and 36.2 kyr BP, 31.7 and30.3 kyr BP, and 26.3 and 23.6 kyr BP, where very few diatomvalves were identified are interpreted as a response to harshenvironmental conditions in relation to H events. Overall itappears that H events had the most severe impact on the limnicecosystem of Les Echets during MIS 3 and 2, while DO climatevariability during MIS 3 resulted in distinct community andproductivity fluctuations.

Acknowledgements

This article is a contribution to the Eurocores on Euroclimateproject RESOLuTION. We acknowledge financial support from theSwedish Research Council. We thank Daniel Conley for help withthe BSi measurements at the National Environmental ResearchInstitute in Roskilde, Denmark and for valuable discussions;Christian Bigler for performing the zonation of the diatomdiagram and fruitful discussions on interpretations and taxon-omy; Sherilyn Fritz and Anna Ludikova for valuable discussionsand suggestions. The comments from two anonymous reviewersgreatly improved the manuscript.

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