Late-Glacial and Early Holocene environmental changes affecting the shallow lake basin of La Narce du Béage (Ardèche, Massif Central, France)

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Late-Glacial and Early Holocene environmental changes affecting the shallow lake basin of La Narce du Béage (Ardèche, Massif Central, France) André-Marie Dendievela,b,c,∗, Karen Serieyssola,∗∗, Benjamin Dietrec,d, Hervé Cubizollea, Amélie Quiquereze, Jean Nicolas Haasb a

University of Lyon, ENTPE, UMR CNRS 5023 LEHNA-IPE, 3 Rue Maurice Audin, 69518, Vaulx-en-Velin Cedex, France University of Lyon, UMR CNRS 5600 EVS-ISTHME, Université Jean Monnet, 6 Rue Basse des Rives, 42023, Saint-Etienne Cedex 02, France c University of Innsbruck, Institute of Botany, Sternwartestrasse 15, 6020, Innsbruck, Austria d University Bourgogne-Franche-Comté, UMR CNRS 6249 Chrono-Environnement, 16 Route de Gray, 25030, Besançon Cedex, France e University Bourgogne-Franche-Comté, UMR CNRS 6298 ARTeHIS, 6 Boulevard Gabriel, 21000, Dijon, France b

ARTICLE INFO

ABSTRACT

Keywords: Diatoms Macrofossils Palynology Shallow mountain lake Climatic change Ecosystem acidification

The sedimentary sequence of La Narce du Béage Basin is one of the few natural archives covering the last 18 ka cal. BP in the French Massif Central. This paper focuses on the palaeoecological reconstruction of environmental and climatic changes affecting this shallow periglacial lake from the Late-Glacial to the Early Holocene. After field surveys and geophysical mapping, two cores (cores A and D) were extracted and dated. Plant macrofossils, pollen, non-pollen palynomorphs (NPPs) and diatom assemblages were compared within the cores to study vegetation, temperature, pH, water level and ice cover changes through time. During the Oldest Dryas (18–14.75 ka cal. BP), climatic conditions were the coldest with a short ice-free season allowing the development of periphytic diatoms only (ecologically comparable to Arctic ones). A cold and dry steppe landscape – exposed to severe erosion – comprised an herbaceous and shrub flora (e.g. Artemisia, Chenopodiaceae, Helianthemum, Ephedra, Hippophaë, Juniperus). Animal presence is suggested by coprophilous fungi. Periphytic diatoms and allochtonous pollen (Cedrus) underlines windy conditions, and the possible erosion of neo-formed soils. Then, during the Bølling-Allerød interstadial (14.75–12.7 ka cal. BP), the ice-free season increased and some trees/ shrubs (Salix, Betula and Juniperus) established locally within open grasslands. During the Younger Dryas (12.7–11.7 ka cal. BP) cooler conditions favoured steppe taxa again (Helianthemum, Achillea, Artemisia, Caryophyllaceae, Ranunculaceae) and increased erosion. The ice cover on the lake prevailed even if the conditions were not as cold as during the Oldest Dryas. During the Preboreal (11.7–10 ka cal. BP) Isoëtes echinospora and I. lacustris developed in the eulittoral zone of the lake, which was surrounded by a swamp forest (with Betula nana and B. pubescens). Pinus sylvestris, Betula pendula, Corylus avellana and trees from the Quercetum mixtum rapidly established in the uplands. Occurrences of Gaeumannomyces and Xylomyces fungi – both phytopathogens on broadleaved trees – and decreasing herb values indirectly hint at a close forest canopy. At the beginning of the Boreal (10 ka cal. BP), aquatic and semi-aquatic taxa developed on open water due to longer ice-free seasons (diatoms, micro-algae, Alisma plantago-aquatica, Potamogeton, Typha). The final terrestrialization towards a mire occurred as a consequence of the Holocene warming and related to water chemistry changes (acidification, eutrophication).

1. Introduction Small lakes, known to be “sentinels” during rapid climatic shifts, may be directly affected by physical, chemical and biological changes

(Adrian et al., 2009; Vincent, 2009; Havens and Jeppesen, 2018). Their lacustrine sediments contain “natural archives” such as diatoms, plant macrofossils and pollen recording local-to-regional ecosystem trajectories and environmental changes over time (Smol, 2016). In order to

Corresponding author. University of Lyon, ENTPE, UMR CNRS 5023 LEHNA-IPE, 3 Rue Maurice Audin, 69518, Vaulx-en-Velin Cedex, France. Corresponding author. E-mail addresses: [email protected], [email protected] (A.-M. Dendievel), [email protected] (K. Serieyssol), [email protected], [email protected] (B. Dietre), [email protected], [email protected] (H. Cubizolle), [email protected] (A. Quiquerez), [email protected] (J.N. Haas). ∗

∗∗

https://doi.org/10.1016/j.quaint.2019.09.014 Received 9 February 2019; Received in revised form 18 July 2019; Accepted 16 September 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: André-Marie Dendievel, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2019.09.014

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understand the evolution of lake ecosystems since the last deglaciation, multi-proxy stratigraphic approaches were widely performed, mostly in Arctic regions, where these changes were abrupt and rapid (Wolfe, 1996; Birks et al., 2000; Smol et al., 2001; Briner et al., 2006; Aarnes et al., 2012). Contrary to high latitude areas, the South-Eastern Massif Central (SEMC) was not covered by a massive ice cap during the Last Glacial Maximum and offered only small glaciers – including rockglaciers – and periglacial conditions (Etlicher, 2005; Defive et al., 2011). The environmental history of the Late-Glacial and the Early Holocene (18–8.2 ka cal. BP) was studied in the SEMC through palynological and sediment magnetism studies performed at medium to low resolution on lake sediments (Reille and de Beaulieu, 1988; Thouveny et al., 1994; Sifeddine et al., 1996; Reille et al., 2000). These studies allowed a good overview on the temporal evolution of the vegetation cover at a regional scale, but lacked accurate and systemic radiocarbon dates, or showed stratigraphic gaps overlapping the Late-Glacial/Holocene transition at the Lakes Issarlès and St. Front for instance (Coûteaux, 1984; Andrieu-Ponel et al., 1995). More recent approaches were also performed at La Narce du Béage Basin in the Mézenc Massif (Ardèche Uplands, SEMC, France). A first sedimentological analysis of a core (core D) extracted from the outlet of the basin (magnetic susceptibility, organic matter content, grain size) was published to reconstruct depositional settings during the last 18 ka cal. BP (Dendievel et al., 2015). This work highlighted the presence of a lacustrine ecosystem during the Late-Glacial and then, its gradual infilling and eutrophication, directly linked to the post-glacial warming. A terrestrial mire developed during the Holocene and its peaty sediments recorded several phases of human impact since the Neolithic as demonstrated by Dendievel et al. (2019). In the present paper, we propose to achieve a stratigraphic and palaeoecological study (geophysics, diatoms, macrofossils and palynology) of the bottom sediments from La Narce du Béage Basin. We aim at reconstructing environmental changes affecting this small mountain ecosystem (vegetation, water level, pH, temperature and trophy) during the climatic shift from the Late-Glacial to the Early Holocene (i.e. Greenlandian stage according to Cohen et al., 2018), from 18 to 9 ka cal. BP. A particular focus will be devoted on the origin of the topographical depression and its initial ecological functioning thanks to a cross comparison of different cores. We also compared our results with other multi-proxy sequences of the Northern Hemisphere.

area (‘Natura 2000’) of about 0.9 ha. It hosts successive vegetation belts of hygrophilous tall herbs from the edges to the centre where Molinia caerulea (moor grass) dominates (see Dendievel et al., 2019 for details). This wetland is supplied by groundwater and its overflow goes to the Veyradeyre River (a tributary of the Loire River which is the longest River of France). The wetland outlet presents a high biodiversity with Comarum palustre (marsh cinquefoil), Carex vesicaria (inflated sedge), Equisetum palustre (marsh horsetail) and Sphagnum (Dendievel, 2017; Dendievel et al., 2019). 3. Materials and methods 3.1. Coring, sampling and dating The La Narce du Béage Basin was surveyed with electrical resistivity tomography (ERT). This geophysical method is widely employed to reconstruct the geometry of depositional environments, evaluate the thickness of the sediment layers, and define drill locations (Slater and Reeve, 2002; Laigre et al., 2012; Hausmann et al., 2013; Comas et al., 2015). A 141-m long longitudinal ERT profile, crossing the wetland from the SE to the NW (Fig. 1-A), was acquired by using a multimode resistivity imaging system (Syscal Junior Switch, Iris Instruments©). A 3-m electrode spacing and a Wenner-Schlumberger array were adopted to maintain a high horizontal resolution and to restrict sensitivity to vertical variations (Fig. 1-A: 15 m in-depth; see also Fig. S1 in supplementary material, with an ancillary ERT profile reaching the granitic substratum at 35 m in-depth). The ERT investigation was completed by penetrating tests with graduated rods at the same spacing and by coring, to confirm the stratigraphy and the bottom-limit of the sediment layers (Fig. 1-B). Then, two cores were taken in the thickest zones, by using a Russian peat corer (Jowsey, 1966): the core D (3.56 m long) was extracted near the outlet, whereas the core A (5.5 m long) was extracted in a deeper part of the basin, relatively close to the shores (Fig. 1). After coring, sediment samples were extracted in the laboratory. Diatoms were absent in the upper peaty part of the cores but they were successfully extracted every 16 cm, from 1.4 m to the bottom on core D, and every 4 cm, from 1.8 m to the bottom on core A. In addition, macrofossil samples were taken from core A (from 1.8 m to the bottom) at a mean resolution of 13 cm, and with a highest resolution (every 4 cm) between 2.77 and 2.16 m (Younger Dryas and Early Holocene). Pollen samples were extracted from the same core every 8 cm (from 5.5 to 3 m), and every 4 cm from 3 to 1.82 m. Due to the omnipresence of minerogenic lacustrine clays, it was very complex and time-consuming to find enough organic elements from terrestrial plants for radiocarbon dating. For that reason, macrofossils were extracted only from core A, and all other radiocarbon dates were performed on bulk peat, gyttja or organic clays (Table 1). The calibration was achieved using the CALIB 7.0 program with the “IntCal.13” curve (Stuiver and Reimer, 1993; Reimer et al., 2013). We also added three palynostratigraphic dates based on major pollen changes to improve the age control (Table 1). For each core, an age-depth model was calculated (see below) by using the clam package in R (Blaauw, 2010; R Core Team, 2016).

2. Regional and local settings The La Narce du Béage Basin (1220 m a.s.l.) is located on the Mézenc Mountains (Ardèche Uplands, SEMC, France; Fig. 1). This region has an oceanic climate with a strong mountain effect. The mean annual temperature is ca. 5 °C, with cold winters (Januarymean: -6 °C) and rather warm summers (Julymean: 20 °C) (Defive and Vidal, 1997). The annual rainfall varies from 900 to 1200 mm/year, reaching a maximum in spring and in autumn, often due to Mediterranean (“Cevenol”) floods. Nowadays, snow also frequently occurs from October to April on the Plateau and the toposoil of the mire, established on the former lake, is frozen for three to four months a year (Dendievel, 2013). From a geological point of view, this region is characterised by a granite-migmatitic basement (Defive et al., 2011). Basaltic outflows and phonolitic protrusions affected this region during the Upper Miocene, between 11 and 7.5 Ma (Mergoil and Boivin, 1993). Late-Pleistocene volcanic events consisted of maar explosions and Strombolian-type eruptions (Nomade et al., 2014). Finally, regolith and soils evolved under periglacial conditions during the Last Glacial (Valadas, 1984; Petit-Maire, 1999; Defive et al., 2011). All these formations are present around La Narce du Béage which is surrounded by the Cherchemuse Strombolian cone and by two Miocene basaltic lava flows (Fig. 1-A). Fallen boulders surrounded the mire and most likely composed its basement (Fig. 1-A; see also Fig. S1, supplementary material). The current wetland, occupying a former lake basin, is a protected

3.2. Diatom analysis Diatom samples were processed following Serieyssol et al., 2010 by using 30% H2O2 for the extraction of organic matters, and 33% HCl if needed to remove soluble salts. For most of the samples, HCl was not used. The samples were heated to 25 °C maximum. After cleaning, the samples were rinsed several times, depending on the amount of clays. A known volume of liquid was deposited on the coverslip. After drying in a closed environment, diatoms were mounted in Naphrax®. The diatoms were counted using a Zeiss Axioskop at ×100 magnification and identified using literature references (Germain, 1981; Krammer and Lange-Bertalot, 1986, 1988; Krammer and Lange-Bertalot, 1991, Krammer and Lange-Bertalot, 1991; Bey et al., 2013). Diatom ecology 2

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Fig. 1. A) Location of the La Narce du Béage mire in the South-Eastern Massif Central (SEMC, France) with the studied cores A and D and local geological settings. B) Electrical Resistivity Tomography (ERT) cross profile (Wenner-Schlumberger array, electrode spacing = 3 m). Interpretation of rocks and sediment resistivities are given after inversion following Dendievel (2017). Abbreviations: AS = anthropogenic soils (former agrarian terraces and plots), BF = beech (Fagus sylvatica) forest.

was based on Germain (1981), Denys (1991), Van Dam et al. (1994) and Bey et al. (2013). Modern generic epithets and names were used for species that have undergone revision since these texts were written (according to https://pubs.usgs.gov/ds/ds329/data/Algal_Attributes_ Nomenclature_v9.txt). Between 300 and 400 diatoms valves were counted per sample, except for 5.16 and 5.48 m (core A) where the whole slide was counted with respectively 73 and 75 valves. For zonation and principal component analysis (PCA) only species with values greater than 1% were used. Valve counts were log10-transformed before analyses using PCA (McCune and Mefford, 2017) and Psimpoll 4.10 programs (Bennett, 2002). PCAs were performed using the variance/ covariance (centred) with a distance-based biplot for cores A and D together, while Psimpoll was used for data transformation and constrained clustering – CONISS (Grimm, 1987). Finally, the number of zones was based on the broken stick model (Bennett, 1996).

3.3. Macrofossil analysis Macrofossil samples extracted from core A (mean sediment volume = 15 ml) were sieved under a trickle of water (meshes: 1 mm, 500, 250 and 125 μm). Placed in Petri boxes with distilled water, macrofossils of the two first meshes (1 mm and 500 μm) were identified with a Leica MZ-6 stereomicroscope (×0.75 to ×57.7 magnification) using seed and fruit collections of the Institute of Botany of the University of Innsbruck, and determination keys (Katz et al., 1965; Berggren, 1969, 1981; Lévesque et al., 1988; Smith, 2004; Cappers et al., 2006). All macrofossils were quantified, but the main peat components (Sphagnum stems, Cyperaceae vegetative parts and Isoëtes macrospores) were counted on a quarter of a Petri box only, and numerically extrapolated to the whole sample. To highlight ecological changes on the core, we used multivariate regression trees – MRT (Birks, 2014). This clustering was achieved by using mvpart (v.1.6–2; Therneau et al., 2014) and MVPARTwrap packages (v.0.9–1; Ouellette 3

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Table 1 Details of radiocarbon dates performed on sediment and plant macrofossils from La Narce du Béage (Massif Central, France) for the cores A and D. The date in italics was rejected. The calibration was achieved using the CALIB 7.0 program with the “IntCal.13” curve (Stuiver and Reimer, 1993; Reimer et al., 2013). Abbreviations: BS = budscales, L = Leaf fragments, S = seeds.

Core A

Core D

Sample

Depth (cm)

Details

Laboratory code

Age uncal. (1 σ)

Calibrated Age (2 σ; cal. BP)

δ13C (‰)

ANB ANB ANB ANB ANB ANB ANB

181.5–182.5 236.5–237.5 274.5–278.5 303–307 358–362 450–456 547–549

Betula sect. Alba (S) Betula sp. (L, BS, S) Wood fragments Palynostratigraphic Age Palynostratigraphic Age Palynostratigraphic Age Bulk Organic clay

ETH-50455 Lyon-12777(GrA) Lyon-12778 (GrA)

Lyon-14407 (SacA50649)

9628 ± 44 BP 9025 ± 45 BP 10690 ± 60 BP 13010 ± 80 BP

9684–9530 10255–9948 12723–12558 12800–12600 14100–13900 14750–14550 15841–15277

−24.3 −15.5

146–148 178–180 196–198 228–230 274–276 284–286 324–326 354–356

Bulk Peat Bulk Peat Bulk Peat Bulk Gyttja Bulk Gyttja Bulk Gyttja Bulk organic clay Bulk organic clay

Lyon-10654 (OxA) Lyon-10653 (OxA) Lyon-10652 (OxA) Lyon-10651 (OxA) Lyon-10650 (OxA) Lyon-10649 (OxA) Lyon-10648 (OxA) Lyon-10647 (OxA)

8140 ± 40 BP 8700 ± 40 BP 9360 ± 40 BP 10790 ± 45 BP 12365 ± 50 BP 12350 ± 50 BP 13285 ± 50 BP 14645 ± 60 BP

9247–9000 9881–9544 10696–10442 12752–12656 14729–14124 14701-14111 16164-15773 18007-17636

−27.6 −27.9 −25.2 −26.5 −15.1 −17.4 −20.1 −22.3

182 237 276 305 360 453 548

D8 D7 D6 D5 D4 D3 D2 D1

and Legendre, 2013) in R (R Core Team, 2016).

freshwater deposits (OM ca. 20%). This layer accumulated from 15 to 12.7 ka cal. BP in core A (410–272 cm) and from 14.5 to 11.1 ka cal. BP in core D (314–205 cm). Finally, a transition to a terrestrial environment developed above with almost 2 m of relatively resistive peat sediments (OM ranged from 30% to 99%; ρ from 120 to 300 Ω m).

3.4. Palynological analysis The palynological samples from core A were stirred in HCl (10%) to dissolve carbonates, and a known quantity of Lycopodium annotinum spores was added as marker. Samples were sieved at 150 and 7 μm, and the in-between fractions were treated by chlorination (0.5 ml of concentrated NaClO3 and HCl, 1 min at 95 °C) followed by an acetolysis (9 vol of acetic anhydride and 1 volume of concentrated sulfuric acid, 1 min at 95 °C). The samples were mounted on slides in glycerine and stained with fuchsine. Pollen grains, spores, and non-pollen palynomorphs (NPPs) were observed with an Olympus BX50 light microscope at ×400 magnification with phase contrast. NNPs were identified according to the reference collection of the Institute of Botany of the University of Innsbruck and to relevant literature (Moe, 1974; Moore et al., 1991; Fægri et al., 1993; Beug, 2004; Miola, 2012). All microremains were quantified with PolyCounter (Nakagawa, 2012) until a minimum of 500 tree pollen were identified. Pollen zones were defined according to the broken stick model, run on a hierarchical CONISS clustering of the square-root transformed counts of terrestrial pollen taxa using the package rioja (Bennett, 1996; Juggins, 2013) and R (R Core Team, 2016).

4.2. Diatoms Diatom taxa found in cores A and D are presented on Figs. 3 and 4. Principle component analysis (PCA) of the different species gathered 61.8% of the total dataset variance on the first two axes (1st axis – 43.7%, 2nd axis −18.1%, Fig. 5). PCA sample scores were plotted for the first two axes (Fig. 5). For the first axis, we found Aulacoseira ambigua and A. laevissima (0.83 and 0.20, respectively) as the main factors influencing the PCA scores in the positive direction, while Staurosirella pinnata (= Fragilaria pinnata), Staurosira venter (= F. construens v. venter), Gyrosigma acuminatum, Pseudostaurosira brevistriata (= F. brevistriata) and P. parasitica (= F. parasitica) influenced in a negative direction (−0.39, −0.27, −0.13, −0.09 and −0.07 respectively). Aulacoseira is considered as a planktonic species which rarely lives outside of water bodies, but can also be regarded as tychoplanktonic often found among macrophytes in ponds (Buczkó et al., 2010). Most Fragilarioid species (such as Pseudostaurosira brevistriata, Fragilaria capucina, Fragilariforma constricta, F. virescens, Staurosira construens v. gracile and v. subsalina, S. venter, Staurosirella lapponica, and Pseudostaurosira parasitica) are periphytic but several of these species (Fragilaria capucina, S. construens v. gracile and v. subsaline, S. venter, Staurosirella lapponica, and Fragiliforma virescens) can occur in the plankton, and are considered as tychoplanktonic, deriving from other habitats (Denys, 1991). Axis 1 is associated with changes from periphytic (negative half) to planktonic assemblages (positive half). For axis 2, Gyrosigma acuminatum and Staurosirella pinnata influenced in a positive direction (species scores 0.61 and 0.33, respectively), while S. venter, P. brevistriata, and Aulacoseira valida tended towards the negative direction (−0.60, −0.28 and −0.18, respectively). Gyrosigma acuminatum grows on epiphytes, stones and mosses in the littoral zone according to Germain (1981). G. acuminatum and S. pinnata also occurs in less oxygenated conditions with fairly high to moderate saturation (> 50% and > 75% saturation, respectively), while both Staurosira venter and P. brevistriata live in highly oxygenated waters (about 100% and > 75% saturation, respectively) (Van Dam et al., 1994). G. acuminatum can sometimes live in wet places, but usually in water bodies, while Staurosirella pinnata supports regularly wet and moist conditions. Staurosira venter prefers aquatic conditions while P. brevistriata lives in aquatic or wet conditions. Aulacoseira valida is found in Nordic-alpine

4. Results 4.1. Stratigraphy and chronology The stratigraphies and age-depth model supporting our results are presented on the Fig. 2. Briefly, three major sedimentary facies were present in the Narce du Béage Basin as validated by field observations and geophysical measurements (Fig. 1): (i) silty-clayey lacustrine sediments, (ii) gyttja, and (iii) peat. In the two cores extracted from the site, basal layers were composed by lacustrine sandy clays (Fig. 2). These lake sediments were present in the major part of the current wetland, directly established on the block-stream basement (Fig. 1). According to electrical resistivity imaging, lake deposits offered less conductivity than the block basement, meaning the presence of water inside. Resistivity values (ρ) of the lake sediments ranged from ca. 60–120 Ω m (Fig. 1). According to radiocarbon ages, this first layer of silty-clayey lacustrine sediments accumulated from 18 to 15/14.5 ka cal. BP before present, depending on core location (central core A: from the bottom to 410 cm; outlet core D: from the bottom to 314 cm). The next layer was a lake gyttja, representing a true aquatic environment with organic-rich 4

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Fig. 2. Radiocarbon age-depth models for the cores A and D from the “La Narce du Béage” (SEMC, France). Age-depth models were calculated based on radiocarbon dates (Table 1) and plotted by using a linear regression within the CLAM package (Blaauw, 2010) in R (R Core Team, 2016). Three palynostratigraphical dates were added for the age-depth model of core A. Stratigraphy: 1) Lacustrine sandy clays, 2) gyttja, 3) peat layers. Abbreviation: YD = Younger Dryas.

lakes and ponds (Houk, 2003). The axis 2 represents an evolution from less oxygenated periphytic zones with wet and moist place (upper negative quadrant) to more oxygenated and aquatic conditions (lower negative quadrant). Three groups were determined: Group 1 contained the samples from the deepest parts of the cores (diatom zone 1) with periphytic species that support less oxygenated conditions. Group 2 was found in the middle part of the cores with wetter periphytic, and more oxygenated conditions: it matches with diatom zones 2, 3 and 4. Group 3 contained samples from the zone 5 and was dominated by two Aulacoseira taxa, which indicates more open water surface. In addition, five main ecological zones (Figs. 3 and 4) were determined for both cores and had similar species changes. Zone 1 was dominated by Gyrosigma acuminatum and Staurosirella pinnata while, in zone 2, G. acuminatum disappeared and other Fragilarioid species became more important. Zones 1 and 2 corresponded to the Oldest Dryas. Pseudostaurosira brevistriata increased in zone 3 while, in the upper part, A. valida appeared. This zone corresponded to the Bølling and Allerød chronozones. Zone 4 marked the almost disappearance of S. pinnata, a decrease in P. brevistriata and an increase in Pinnularia subcapitata, a benthic-epontic species which lives regularly in wet or moist areas. Maximum amounts of A. valida were registered. Within zone 5, Aulacoseira ambiqua became the dominant species (> 50%). Above, the bog became too acid for the preservation of diatom frustules, and/or silica recycling caused diatom dissolution.

(incl. Carex riparia/vesicaria), Ranunculus cf. hederaceus (aquatic buttercups) and Nitella (Charophyte algae) colonised the lake. Isoëtes echinospora and I. lacustris (Quillworts) were also present. Hygrophilous and aquatic insects (Trichoptera and Coleoptera) also occurred frequently after 12.7 ka cal. BP. Zone 2 is dominated by Isoëtes suggesting persistent oligotrophic conditions from 11.8 to 10.5 ka cal. BP (262–240 cm). An increase in SL indicated the proximity of woods. Among the semi-aquatic (temporarily submerged) or aquatic fauna, Acarian mites were frequent. Zone 3 (10.5–9.5 ka cal. BP, 240–180 cm) revealed high amounts of Potamogeton, Alisma, Sparganium, Typha, or Cyperaceae, and Byrophyta (mosses; i.e. Calliergonella cuspidata, Meesia longiseta, Sphagnum) were also abundant (Fig. 6). The high amounts of emergent plants, sedges and mosses supported the development of an Early Holocene fen, also hosting a swamp forest with Betula nana and B. pubescens (dwarf and moor birches), Salix (willows) and Alnus glutinosa (black alder), whereas Pinus, and Betula pendula (silver birches) were present locally during the Preboreal (for Holocene vegetation, see also Dendievel et al., 2019). 4.4. Palynology Five different local pollen assemblage zones (LPAZ) were determined according to the CONISS clustering (Fig. 7). Zone 1 (15.6–14.75 ka cal. BP; 550–462 cm) was characterised by very important amounts of Artemisia (sagebrush), Chenopodiaceae (chenopods) and Poaceae (grasses), which may represent an open steppe landscape with some animals around as indicated by spores from coprophilous fungi such as Sporormiella. Arboreal pollen mainly originating from Pinus was accompanied by shrub pollen from Juniperus (juniper), Helianthemum (rockroses), Ephedra distachya and E. fragilis (ephedras) or Hippophaë rhamnoides (sea buckthorn). Pollen from cedar (Cedrus) was also regularly found, as well as important but decreasing quantities of microscopic charcoals. At the beginning of zone 2 (14.75–12.75 ka cal. BP; 462–301 cm), Pinus was less frequent whereas important amounts of Salix, Betula and Juniperus were observed (around 14.5 ka cal. BP). Botryococcus and Pediastrum indicated additional micro-algal populations other than diatoms in the lake. Concomitantly, pollen of Rumex acetosa-type (sorrel), Cichorioideae (chicories), Calluna vulgaris (heather) and Rumex scutatus-type (French sorrel) became significant, together with Ranunculaceae, Plantago maritima-type (goose tongue), Sanguisorba minor (small burnet), Filipendula (meadowsweet), as well as spores of Filicales (ferns). Cedrus, Ephedra distachya-type and

4.3. Macrofossils The MRT partitioning highlighted three main local macrofossil assemblage zones called LMAZ (Fig. 6). LMAZ 1 covered almost 2.9 m of the stratigraphy and can be divided into three subzones. Subzone 1.1 (15.8–14.7 ka cal. BP; 550 to 455 cm) was mainly characterised by minerogenic elements linked with the local erosion within the watershed. There were very few organic remains. Subzone 1.2 (14.7–12.7 ka cal. BP; 455 to 292 cm) recorded a reduced number of minerogenic particles. A seed of Linaria (toadflax) and Substantia Lignosa – SL (wood remains) may suggest some first vegetation during this period. Potamogeton (pondweeds) and Cyperaceae (sedges) very likely developed on or at the interface of the former Late-Glacial lake. Subzone 1.3 was related to the Younger Dryas cold episode (12.7–11.7 ka cal. BP; 292–262 cm) and offered a strong amount of minerogenic particles. Asteraceae (cf. Taraxacum officinale – dandelion) and Pinus sylvestris (pine) were present on the uplands. During this subzone, Cyperaceae 5

Fig. 3. Selected diatom taxa which accounted at least once for more than 1% of the assemblage in core A (La Narce du Béage, SEMC, France). Zones were determined using log 10 (x+1) transformed data. Taxa discussed in the text have bold names. Stratigraphy: 1) Lacustrine sandy clays, 2) gyttja, 3) peat. Abbreviations: YD = Younger Dryas, Prebor. = Preboreal.

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Fig. 4. Selected diatom taxa which accounted at least once for more than 1% of the assemblage in core D (La Narce du Béage, SEMC, France). Zones were determined using log 10 (x+1) transformed data. Taxa discussed in the text have bold names. Stratigraphy: 1) Lacustrine sandy clays, 2) gyttja, 3) peat. Abbreviations: Prebor. = Preboreal.

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Fig. 5. Principal Component Analysis of both cores A and D from La Narce du Béage (SEMC, France) using species which accounted at least once for more than 1% of the assemblage. Data was log 10 (x+1) transformed. Solid triangle = core A, open triangle = core D. Abbreviations: AAMB = Aulacoseira ambigua, ALAE = A. laevissima, AVAL = A. valida, FBRE = Pseudostaurosira brevistriata (= Fragilaria brevistriata), FCVE = Staurosira venter (= F. construens v. venter), FPAR = P. parasitica (= F. parasitica), FPIN = Staurosirella pinnata (= F. pinnata), GTRU = Gomphonema truncatum, GYRO = Gyrosigma acuminatum.

Hall (2002) found S. pinnata only in sites above the tree-line, i.e. in very open environments. In forty Austrian Alpine lakes, Schmidt et al. (2004) demonstrated the importance of pH, conductivity, Ca2+ and Mg2+ but also of July water temperature which significantly influenced the total diatom dataset. Fragilariaceae were significantly affected by the ice cover duration. F. capucina responded to mean water temperature in July, occurring during summertime (temp. range 1.7–12.4 °C), and with a short ice cover duration (163–277 days). Small Fragilaria sensu lato are usually located along or at the cold end of the temperature gradient (Pienitz et al., 1995; Lotter et al., 1997) and associated with periphytic taxa (Voigt et al., 2008). Fragilaria s.l. are known to be pioneer species and appear first in newly formed ponds and lakes (Saulnier-Talbot and Pienitz, 2001; Saulnier-Talbot et al., 2015). Interestingly, a survey of studies on periphytic diatoms from High Arctic ponds (Beyens, 1989; Hamilton et al., 1994; Douglas and Smol, 1995) noted only four species: Staurosirella pinnata (= Fragilaria pinnata), S. lapponica (= F. lapponica), F. capucina v. gracilis and F. capucina v. vaucheriae. These Fragilarioid species seem to support unstable and harsher environmental conditions, with lower temperatures, shorter ice-free periods, and/or increased windiness. In Victoria Island (Canadian Arctic), only two occurrences were noted for Staurosira construens (= F. construens) while Staurosirella pinnata occurred in 22 samples and F. capucina in 46 samples (Michelutti et al., 2003). Laing and Smol (2000) found that S. pinnata had higher abundance in tundra areas while Staurosira venter was more abundant at lower latitudes in the boreal forest. By inference, S. venter seems to prefer warmer conditions (with trees) than Staurosirella pinnata which likes tundra conditions. Gandouin et al. (2016) used the ratio of Staurosirella pinnata to Straurosira venter to distinguish colder phases. According to this ratio (Fig. 8) the coldest conditions were registered in both cores in the diatom zone 1 at La Narce de Béage, i.e. during the Oldest Dryas. However, the end of the Oldest Dryas saw warmer conditions which continued in the Bølling/Allerød (Fig. 8). The persistence of

Chenopodiaceae almost disappeared. By the third zone (12.75–11.7 ka cal. BP; 301–262 cm) a massive reduction in arboreal taxa was recorded, as well as for Botryococcus and charcoal particles. A rise in open environment taxa (Helianthemum, Artemisia, Achillea-type, Caryophyllaceae, Poaceae) and of some aquatics such as Myriophyllum (water milfoil), aquatic Neorhabdocœla worms or Ranunculaceae delimited the Younger Dryas chronozone. Then, during the fourth zone (11.7–10.5 ka cal. BP; 262–242 cm), Pinus and Betula recovered, rapidly accompanied by Corylus avellana, Ulmus (elm) and Quercus (oak), while herb taxa were generally less frequent. During this period, very high values of aquatic taxa such as Isoëtes microspores and Botryococcus were recorded, as well as increasing values of Myriophyllum alterniflorum and Neorhabdocœla. For the last and 5th zone (242–182 cm; 10.5–9.6 ka cal. BP) Corylus avellana was the main tree taxa, accompanied by Betula, Ulmus, and Quercus. Rising values of fungi such as Gaeumannomyces and Xylomyces – both phytopathogens on broadleaved trees – indirectly reflected a closed tree canopy around La Narce du Béage. Pollen from mire taxa such as from Sparganium-type (bur-reed) and Potamogeton/ Triglochin-type (pondweed/arrow-grass) were also regularly found. 5. Discussion 5.1. Local lake evolution: key diatoms insights into the lake geomorphology and climatic evolution Diatom analysis provided major insights into past lake conditions, in particular regarding the evolution of the hydrological settings such as water level, temperature, trophy and pH. At La Narce du Béage, Staurosirella pinnata was one of the main species at the bottom of the cores A and D (Figs. 3 and 4). This taxon seems to be typical of Arctic areas as shown by Lim et al. (2001) in the Canadian High Arctic where S. pinnata was found in three substrata: sediments, mosses and rocks (descending number of occurrences in each microhabitat). Bigler and 8

Fig. 6. Selected macrofossil taxa (n/15 ml) from the La Narce du Béage core A (SEMC, France). Local macrofossil assemblage zones (LMAZ) were defined according to the MRT partitioning. Abbreviations: Abd = abdomen fragments, Br = bracts, BS = bud-scales, Cht = chitin, F = fruits, L = leaves, MS = macrospores, N = needles, OO = oospores, P = Particles, S = seeds, SL = Substantia Lignosa (wood), St = stems, VR = vegetative plant remains. Stratigraphy: 1) Lacustrine sandy clays, 2) gyttja, 3) peat. Abbreviations: Bøl. = Bølling, YD = Younger Dryas, Bor. = Boreal.

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Fig. 7. Selected pollen, spores, and non-pollen palynomorphs (NPPs), as well as micro-charcoal record from the La Narce du Béage core A (SE Massif Central, France). Pollen and spores are expressed as percentages of the total pollen sum (excluding mire plants, cryptogams and non-pollen palynomorphs), and exaggerated curves (×10) are plotted in pale colours. Local Pollen Assemblage Zones were defined according to the CONISS clustering. Chronozones: OD = Oldest Dryas, BØ = Bølling, AL = Allerød, YD = Younger Dryas, PB = Preboreal, BO = Boreal. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 8. Ratio of Staurosirella pinnata/Staurosira venter in cores A and D from La Narce du Béage (SEMC, France). High ratio phases represent the coldest period – i.e. Diatom zone 1 – and correspond to the Oldest Dryas chronozone recognized in the two cores.

Fragilarioid diatoms during the Allerød can be compared to artic conditions in newly formed lakes (Birks et al., 2000). There are also some differences between the two studied cores from the La Narce de Béage and one may cite Staurosirella pinnata's curves (Figs. 3 and 4). Indeed, this species was present longer in core D and at stronger values than in core A. Pseudostaurosira brevistriata also differed from core D to core A with a more continued presence in the latter (Figs. 3 and 4). P. brevistriata became an important component of the two cores during Bølling and Allerød periods. Indeed, most Fragilarioid species (F. capucina complex, Straurosira venter, S. construens v. graciles and v. subsalina) are tychoplanktonic (Denys, 1991): they derived very likely from other habitats and were usually put in suspension by wind action. Their presence is linked to turbid conditions and could also derive from the physical erosion of immature soils. Schmidt et al. (2004) noted that F. capucina occurred when increased summer water temperatures stayed for a longer period, and confirmed its distribution in sites below the subarctic tree-line. Rhoicosphenia abbreviata, a rheophilous taxa, living in flowing water, was found only in the outlet core D during the Oldest Dryas (Diatom zone 1) and the Younger Dryas (Diatom zone 4). Pinnularia borealis was found at La Narce de Béage only in the core A (Oldest Dryas, Diatom zone 1) whereas Hantzschia amphioxys (Diatom zone 1) occurred in greater amounts in core A than in core D. Hantzschia amphioxys and P. borealis are found in Arctic mosses (Beyens, 1989; Douglas and Smol, 1995; Lim et al., 2001; Michelutti et al., 2003). The latter – P. borealis – is both rheophilous and aerophilous; its presence suggests strong erosion (Voigt et al., 2008). Stauroneis phoenicenteron, a typical taxa from shallow freshwater lakes (Pienitz and Smol, 1993), was also more frequent in core A (Diatom

zones 1 and 2). Tabelleria flocculosa, found in the Older and Younger Dryas is a tychoplanktonic taxa attached to plants and stones (Denys, 1991). This diatom mosaic suggested a pioneer flora established on stony surfaces and in shallow waters during the Oldest Dryas at least, and very likely until the early Younger Dryas. Field and geophysical surveys revealing the accumulation of lake sediments directly on boulders (Fig. 1) also assumed these geomorphological settings. According to our hypotheses, these differences in diatom stratigraphy are mainly related to the position of each core within the basin: the core A was located next to the shoreline and surrounded by steep slopes, while core D was near the outlet and offered aquatic water conditions for a longer period. Diatoms are also very indicative to reconstruct the extent, the timing and the occurrence of ice and snow cover, directly linked to climatic changes, as noted by Smol (1988) and by Douglas and Smol (2010). During colder periods, ice cover is more extensive and, very often, only a small zone around the edge of a pond will melt. This leads to a lower overall diatom production and “taxa characteristic of very shallow littoral and semi-terrestrial environments tend to be relatively more common” (Douglas and Smol, 2010). During warmer years, ice cover shrinks and overall production increases the number of diatom taxa from deeper water substrates and planktonic habitats. Therefore, with longer ice-free seasons, new substrates may become available and could support complex and diverse aquatic communities (Douglas and Smol, 2010). Smol (1988) suggested that the periphytic/planktonic diatom ratio is closely related to the duration and the extent of the ice cover: a high variety of habitats and planktonic species could indicate longer ice-free seasons. However, other variables also effect this 11

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Fig. 9. Comparison of pH gradient (from acidobiontic to alkaliphilous species), water level conditions (from aquatic to semi-aerophilous species) and trophy (oligotrophic to eutrophic state) changes inferred from the diatom species in cores A and D from La Narce du Béage (SEMC, France).

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Fig. 10. Synthesis of geomorphological, climatic, wind, pH and nutrient information extracted from the multi-proxy study of the La Narce du Béage sedimentary sequences (SEMC, France). Abbreviations: Bø = Bølling, diat. = diatoms, Eutroph. = eutrophication, IFS= Ice free season, MF = macrofossils, YD = Younger Dryas.

relationship, such as water depth, nutrients, and or sediment in-wash. Schmidt et al. (2004) went further and linked the duration of ice cover with July summer water temperatures. At La Narce du Béage, the Oldest Dryas registered the coldest period with 100% of periphytic taxa and/or the coldest July mean temperatures (Fig. 8). During the Bølling and the Allerød, the appearance of planktonic Aulacoseira valida suggested a warming trend with increased mean July temperatures. Based on periphytic taxa, the Younger Dryas (YD) cooling is clearly seen in the core A (only a very small increase occurred in core D) and marked lower July mean temperatures and greater ice cover. Pinnularia subcapitata, which was previously observed in the Oldest Dryas, reappeared during this zone (Figs. 3 and 4). This taxon is classically found in high numbers in highland headwaters (Cantonati et al., 2017). During the YD, acidophilous taxa increased to the levels of the Oldest Dryas (Figs. 9 and 10). As proposed by Douglas and Smol (2010), less CO2 can escape from lakes with an increase in ice cover and so the pH decreases. A strong correlation between pH and mean air temperatures over the past 200 years were found by Sommaruga-Wögrath et al. (1997): cooling would cause a decline in pH, while warming would increase the pH. Warming periods would also increase in-lake alkalinity and offer longer water retention (Sommaruga-Wögrath et al., 1997; Koinig et al., 1998). Thus, alkaliphilous taxa (high pH) increased during the Bølling and the Allerød at La Narce de Béage, while acidophilous taxa (low pH) augmented during YD and Preboreal periods. Species living in regularly moist conditions (Fig. 9-B) also increased, showing longer ice cover in winter. The Oldest Dryas had the highest amount of these species. A high trophic status (Fig. 9-C) was recorded for the Bølling and Allerød, with longer ice-free seasons and increasing runoff and nutrient input. Finally, during the Boreal, major changes occurred in both cores and sedimentation shifted towards peat as well demonstrated by the macrofossil record (Fig. 6). Aulacoseira valida, revealed a July optimum mean temperature of ca. 12 °C according to Bigler and Hall (2002). It lives in dystrophic conditions and mire environment as indicated by Krammer and Lange-Bertalot, 1991 and appeared in the site with the development of fibric peat. With the Boreal, Aulacoseira species dominated the diatom association (Figs. 3 and 4). Aulacoseira ambiqua, for instance, an aquatic species, occurred in open water zones of the newly formed Holocene mire. The increasing number of eutrophic species is

also typical of peat areas (Fig. 9-C). Moreover, a large mix of species occurred throughout the year, as water conditions changed from higher water levels (after snow melt and spring rains) to lower water levels (dry summers or winters) (Fig. 9-C). Finally, above the Boreal zone, no diatom was preserved because of the development of an acidic mire with Bryophyta (including Sphagnum) mats and possible heath development supporting Betula nana – dwarf birches (Dendievel et al., 2019). Sphagnum influenced significantly the diatom development compared to other types of substrata and limited diatom distribution (Bertrand et al., 2015). In combination with Boreal warmer temperatures, decreasing pH due to rock weathering and to the establishment of an acidophilous flora, could have increased diatom dissolution and might explain the loss of diatoms in the mire. Some studies suggested a threshold between pH 5.1 to 3.5 for many species to maintain a population or a lowering by 30% than during pre-acidification years (e.g. DeNicola, 2000). 5.2. Evolution of local aquatic and regional terrestrial flora and vegetation Local to regional flora and vegetation can be described on the basis of palynological and macrofossil data (Figs. 6, 7 and 10) which reflect the vegetation cover at different scales and furnish complementary information (Birks, 2014). It also constitutes an essential framework to better interpret diatom results representative of the lacustrine flora. Macrofossils are also widely used to describe the aquatic flora in shallow lake environments (Zhao et al., 2006). The Oldest Dryas (bottom of the core A) was characterised by a steppic vegetation, dominated by Juniperus, Ephedra, Artemisia, Helianthemum, Chenopodiaceae and Poaceae. High pollen percentages of Pinus (20–40%) suggested the presence of pine woods, but could also come from regional to long-distance pollen transport. These results coincide with the wellknown continental sequences collected on the Bouchet Lake and the Limagne marsh (Pons et al., 1987; Reille and de Beaulieu, 1988). Locally, at La Narce du Béage, Ephedra distachya and E. fragilis, as well as Hippophaë rhamnoides were growing on very sandy and acidophilous soils. Such assemblages can be characterised as a part of a cold periglacial pioneer flora. This assemblage was found in the Late-Glacial open landscapes of the Jura Mountains and of the Western Alps as well 13

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(David et al., 2006; Cupillard et al., 2015). This picture fits with important minerogenic inputs into the lake, coming from rock alteration – basalt and granite – in the catchment. This pioneer stage was indeed very open and subject to windy conditions and erosion, as demonstrated by small percentages of allochthonous cedar pollen (Cedrus) very likely coming from the Mediterranean area (de Beaulieu and Reille, 1975; Magri and Parra, 2002). The presence of spores of Sporormiella and their decline during the Older Dryas may eventually be linked to megafaunal presence and its following extinction, similarly to what was found in North American lake sediments (Davis and Shafer, 2006). Then, from 14.75 to 12.7 ka cal. BP (Bølling/Allerød interstadial), Betula, Salix, Juniperus, Calluna vulgaris and Sanguisorba surrounded the lake according to the palynological data (Fig. 7). This tree/shrub belt explained well the increasing importance of wood macro-remains in the sediment (Fig. 6). This forest development is certainly due to the climate warming just after 14.75 ka cal. BP. It is also consistent with sites in Greenland and Switzerland where this warming happened ca. 14.7–14.6 ka cal. BP (Van Raden et al., 2013). According to Dendievel et al. (2015), the organic matter content, linked to the lake internal production and to nutrient inputs from the watershed (erosion of soils), increased from the Bølling (ca. 8%) to the Allerød (ca. 23%). Nonetheless, open grasslands and shrubs still developed within the watershed, composed of Artemisia, Poaceae, Linaria and Salix (Figs. 6 and 7), and suggested high tolerance to cold and dry periods with a prolonged snow cover (Aarnes et al., 2012). Ranunculaceae appeared abruptly during this period (Fig. 7), most certainly linked to aquatic communities (i.e. Ranunculus sect. Batrachium) occupying the lake as the water level rose after snow and ice melting, as in similar lakes during the Late-Glacial (Reille and de Beaulieu, 1988). Annual average precipitations reconstructed by pollen studies in the Massif Central and in the Jura Mountains also supported this hypothesis with a rainfall increase from ca. 220 mm/yr during the Oldest Dryas to ca. 800 mm/yr during the Bølling/Allerød interstadial (Peyron et al., 1998; Allen et al., 2008). However, a part of these Ranunculaceae pollen may also derive from Late Glacial grassland communities (Godwin, 1975). The Younger Dryas (YD; 12.7–11.7 ka cal. BP) was mainly characterised by pollen from species growing in cold and open environments (> 70%), in particular Artemisia, Achillea-type, Helianthemum, Caryophyllaceae and Poaceae. Asteraceae (cf. Taraxacum officinale) and Pinus sylvestris macrofossils reflected this steppe-like landscape as well. The global YD cooling resulted, at a local scale, in the severe erosion of the watershed, providing numerous minerogenic particles to the coring location (Fig. 6). As regards lake communities, this period promoted an increase in local biodiversity with (1) Cyperaceae, probably growing on the lake edges and (2) Ranunculaceae, Charophytes (Nitella) and Isoëtes (I. echinospora and I. lacustris) colonising the freshwater lake. Isoëtes are key aquatic species from the YD to the Preboreal according to both macrofossils and palynological data (Figs. 6 and 7). The domination of Isoëtes populations indicated shallow conditions (< 2 m of water) and the persistence of local oligotrophic conditions (Hannon and Gaillard, 1997; Dendievel et al., 2019). A similar trend was also recorded at the nearby Bouchet, Lake ca. 1000 m a.s.l. (Reille and de Beaulieu, 1988). In terms of nature conservation and heritage, this record is also very important, as Isoëtes lacustris is highly threatened in numerous French lakes today (https://inpn.mnhn.fr/espece/cd_nom/103843/tab/statut). Generally, macrofossils became more frequent at La Narce du Béage after the YD end and during the Early Holocene (i.e. Greenlandian stage according to Cohen et al., 2018), as the post-glacial warming occurred. A gradual increase in temperature was underlined both by pollen and macrofossils, following three main steps. Step 1, from 11.7 to 10.5 ka cal. BP, was characterised by a persistence of Late-Glacial species (Artemisia in particular). Pine and birch forests developed very quickly together with hazel (since 11 ka cal. BP) at a regional-to-local scale (Fig. 7). Step 2 established as Isoëtes populations decreased from 10.5 to 10.1 ka cal. BP in the lake. Concomitant occurrences of Potamogeton,

Sparganium and Alisma plantago-aquatica (water-plantain) suggested a July mean temperature between 10 and 13 °C according to Gaillard and Birks (2007). A coeval gradual warming was also exemplified by pollen occurrences from deciduous forests of Corylus avellana, Betula, Ulmus and Quercus. In step 3 (since 10.1 ka cal. BP) the lake became eutrophic to dystrophic with a substantial development of Potamogeton and green algae (Botryococcus). Peat mats (including Sphagnum) were spreading from the shores. Such an environment was very acidic, with species supporting low pH conditions such as Betula nana, B. pubescens, Alnus glutinosa and Salix. This continuous turnover of species during the Early Holocene is typical of post-glacial landscapes (Matthews, 1992). This pioneer tree phase occurred ca. 1.3 ka cal. BP after the YD end on the Béage Plateau and seemed a bit delayed, in particular for birches, compared to European data in general (Giesecke et al., 2017). However, the local birch optimum fitted well to regional birch histories compiled by de Beaulieu et al. (1988). Local to regional increase of birches and willows after ca. 10.1 ka cal. BP, combined with the decrease in Poaceae, revealed the substitution of open wetland margins by swamp forests. Local oligotrophic key species also disappeared (Isoëtes) linked to the lake infilling process. The increase of mean July temperature can be estimated towards 12–15 °C according to this macrofossil assemblage. It seems to be consistent with other Western European continental records at a similar elevation and showing a warmest month temperature of ca. 15–18 °C (Peyron et al., 2005; Gandouin et al., 2016). The disappearance of aquatic insects and the accumulation of peat layers, established the end of the terrestrialization process at ca. 9.7 ka cal. BP on the whole site (Fig. 6). 6. Conclusions The Late-Glacial sequence of La Narce du Béage is one of the major records on which multi-disciplinary analyses were intensively conducted in the Eastern Velay (S-E Massif Central, France). Based on diatoms, macrofossils and palynological data, this study provided the reconstruction of key hydro-ecological changes affecting a small and shallow periglacial mountain lake during the rapid warming of the last deglaciation (Fig. 10). We demonstrated that the lake evolution was not only related to climate but was also dependant on local dominant vegetation and on geomorphological settings: substratum, in-wash of neoformed soils, and establishment of the mire with acidophilous species (Fig. 10). The beginning of the Late-Glacial (from 18 to 16 ka cal. BP) registered the coldest periods with a short ice-free season. High concentrations of epiphytic and subaerial diatom taxa suggested that the lake was very shallow, with diatoms growing directly on rocks. Palaeoecological data characterized a steppe environment exposed to severe erosion, with abundant minerogenic particles and an open vegetation growing on sandy soils (Juniperus, Artemisia, Chenopodiaceae, Poaceae, Thalictrum, Helianthemum, Hippophaë rhamnoides, Ephedra distachya and E. fragilis). After 16 ka cal. BP, tychoplanktonic diatoms and pollen inputs from Mediterranean areas (Cedrus trees) likely indicated windy conditions. The lake level gradually rose as the snow/ice cover smelted due to shorter winter seasons, and this changed the trophy (meso-to oligotrophic state) and the aquatic flora (benthic to planktonic diatoms). The Bølling/Allerød interstadial (14.75–12.7 ka cal. BP) showed a warming trend with a longer ice-free season suggested by diatom ratios. A boreal forest dominated by Pinus occurred around the mire. During the Younger Dryas and the Early Preboreal, the climate remained relatively dry with a long ice season and low lake levels at first (Fig. 9). At the end of the Preboreal (11 ka cal. BP) warmer and more mesotrophic conditions occurred. The lake level rose with some delay and the water tended to acidify. Peat mats expanded from the shores to the middle of the water, leading to the lake terrestrialization, and the full development of a mire (Dendievel et al., 2019). As demonstrated by Jones and Yu (2010) and by Ruppel et al. (2013), the Early Holocene warming was a major phase for lake terrestrialization in the Northern Hemisphere. In this context, our multiproxy study 14

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provided a case study for the evolution of small periglacial lakes in midmountain areas in the course of past (or even present) rapid climatic changes.

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Funding This study was supported by the AGES program (2013–2016) cofounded by the European Union. Material and facilities were provided by the Research Unit UMR CNRS 5600 EVS-ISTHME at the University Jean Monnet of Saint-Etienne (France) and by the Institute of Botany of the University of Innsbruck (Austria). Acknowledgements We want to thank all of the colleagues implied in the field work around the La Narce du Béage wetland, in particular Ardito-Christopher Bastone, Marie-Charlotte Bres, Margot Mathey and Mathieu Prieux for geophysical field measurements. Thanks also to Emmanuelle Defive and Arnaud Tourman for geomorphological discussions and advices, as well as to Werner Kofler for the preparation of the palynological samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.quaint.2019.09.014. References Aarnes, I., Bjune, A.E., Birks, H.H., Balascio, N.L., Bakke, J., Blaauw, M., 2012. Vegetation responses to rapid climatic changes during the last deglaciation 13,500–8,000 years ago on southwest Andøya, arctic Norway. Veg. Hist. Archaeobotany 21, 17–35. https://doi.org/10.1007/s00334-011-0320-4. Adrian, R., O'Reilly, C.M., Zagarese, H., Baines, S.B., Hessen, D.O., Keller, W., Livingstone, D.M., Sommaruga, R., Straile, D., Van Donk, E., Weyhenmeyer, G.A., Winder, M., 2009. Lakes as sentinels of climate change. Limnol. Oceanogr. 54, 2283–2297. https://doi.org/10.4319/lo.2009.54.6_part_2.2283. Allen, R., Siegert, M.J., Payne, A.J., 2008. Reconstructing glacier-based climates of LGM Europe and Russia – Part 2: a dataset of LGM Precipitation/temperature relations derived from degree-day modelling of palaeo glaciers. Clim. Past 4, 249–263. Andrieu-Ponel, V., Bonifay, E., Reille, M., Rhoujjati, A., Thouveny, N., 1995. Stop 29: lac de St-front. In: Schirmer, W. (Ed.), Quaternary Field Trips in Central Europe. Pfeil, Munich, pp. 1515–1518. Bennett, K.D., 1996. Determination of the number of zones in a biostratigraphic sequence. New Phytol. 132, 155–170. Bennett, K.D., 2002. Psimpoll. Belfast. Version 4.10. Available at: http://www.chrono. qub.ac.uk/psimpoll/psimpoll.html. Berggren, G., 1969. Atlas of Seeds and Small Fruits of Northwest-European Plant Species (Sweden, Norway, Denmark, East Fennoscandia and Iceland) with Morphological Descriptions. Part 2, Cyperaceae. Swedish Museum Natural History, Stockholm. Berggren, G., 1981. Atlas of Seeds and Small Fruits of Northwest-European Plant Species (Sweden, Norway, Denmark, East Fennoscandia and Iceland) with Morphological Descriptions. Part 3. Salicaceae – Cruciferae. Swedish Museum Natural History, Stockholm. Bertrand, J., Serieyssol, K., Ector, L., 2015. The influence of land use and the nature of the substrate on the diatom association from ponds found in two regions of France. Cryptogam. Algol. 36 (3), 1–18. https://doi.org/10.7872/crya/v36.iss3.2015.305. Beug, H.-J., 2004. Leitfaden der Pollenbestimmung für Mitteleuropa und angrenzende Gebiete. Pfeil, Munich. Bey, M.-Y., Ector, L., Chavaux, R., Béranger, P., 2013. Atlas des diatomées des cours d’eau de la région Rhône-Alpes. Direction Régionale de l'Environnement de l’Aménagement et du Logement Rhône-Alpes, Lyon. Beyens, L., 1989. Moss dwelling diatom assemblages from Edgeøya (Svalbard). Polar Biol. 9, 423–430. https://doi.org/10.1007/BF00443228. Bigler, C., Hall, R.I., 2002. Diatoms as indicators of climatic and limnological change in Swedish Lapland: a 100-lake calibration set and its validation for paleoecological reconstructions. J. Paleolimnol. 27, 97–115. Birks, H.H., Battarbee, R.W., Birks, H.J.B., 2000. The development of the aquatic ecosystem at Kråkenes Lake, western Norway, during the late glacial and early Holocene - a synthesis. J. Paleolimnol. 23, 91–114. https://doi.org/10.1023/ A:1008079725596. Birks, H.J.B., 2014. Challenges in the presentation and analysis of plant-macrofossil stratigraphical data. Veg. Hist. Archaeobotany 23, 309–330. https://doi.org/10. 1007/s00334-013-0430-2. Blaauw, M., 2010. Methods and code for ‘classical’ age-modelling of radiocarbon sequences. Quat. Geochronol. 5, 512–518. Briner, J.P., Michelutti, N., Francis, D.R., Miller, G.H., Axford, Y., Wooller, M.J., Wolfe, A.P., 2006. A multi-proxy lacustrine record of Holocene climate change on

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