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Advantages and limits of radiocarbon dating applied to peat inception during the end of the Lateglacial and the Holocene: the example of mires in the Eastern Massif Central (France)

Intérêts et limites de la datation par le radiocarbone pour le calage chronologique de l'apparition des tourbières à la fin du Tardiglaciaire et au début de l'Holocène: l'exemple du démarrage de la turfigenèse dans le Massif Central oriental (France)
Hervé Cubizolle, Pierre Bonnel, Christine Oberlin, Arnaud Tourman et Jérôme Porteret
p. 187‑208

Résumés

L’apparition des tourbières au cours des 12000 dernières années est révélatrice de changements environnementaux importants, tant d’origine climatique que d’origine anthropique. Aussi, est-il fondamental, dans le but d’une reconstitution paléoenvironnementale, de pouvoir caler chronologiquement le démarrage de la turfigenèse sur un grand nombre de sites. La répartition dans le temps des dates obtenues peut ensuite être confrontée aux données géomorphologiques, paléoécologiques et archéologiques locales et régionales pour tenter de retracer l’évolution des paysages et des sociétés humaines qui les ont façonnés. Néanmoins, si la datation par le radiocarbone est l’outil le plus adapté au travail de calage chronologique des couches basales des tourbières, l’entreprise est complexe et pose un certain nombre de problèmes méthodologiques très rarement abordés dans la bibliographie. Une première difficulté est d’identifier le faciès qui, sur la stratigraphie, matérialise le démarrage de la turfigenèse. Il convient ensuite de déterminer le ou les secteurs de la tourbière où la tourbe a commencé à s’accumuler. Cet article propose une méthode d’échantillonnage des couches basales de tourbe fondée sur un travail minutieux de restitution des stratigraphies couplé à des analyses sédimentologiques et à des examens micromorphologiques. Cinq sites pilotes ont été retenus: trois tourbières bombées et deux tourbières basses. Parmi les résultats notables de l’étude, on insistera sur l’apport de l’analyse micromorphologique. Par ailleurs, le grand nombre de datations par le radiocarbone qui ont été réalisées a permis d’apprécier la précision que l’on est en droit d’attendre d’un tel travail de calage chronologique, une information déterminante avant d’entreprendre la confrontation avec les autres données paléoenvironnementales.

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Texte intégral

This study was supported through the CNRS Long Term Ecological Research “Bassin versant de la Loire”, the Loire Conseil Général and the Rhône-Alpes Conseil Régional. We would also like to thank Peter van den Haute of the Soils Sciences Laboratory of the University of Gand in Belgium. He provided us with thin plates of very good quality. Finally, the authors would like express theirs thanks to E. Juvigné and M. Fontugne for the constructive remarks.

1 - Introduction

1In the field of palaeoenvironmental studies, it is the palaeoecological archives that are stored in peat that are exploited, essentially through palynological analysis and tephrostratigraphy. However the study of mires is also of interest when reconstructing the evolution of environments during the Late Glacial and the Holocene. Mire formation requires a shift from a negative hydrological balance to a null or positive hydrological balance, and such an event implies notable changes in the functioning of a site’s watershed. These changes, be they geomorphologic, hydrological, biogeographical, or ecological in nature, may possibly be linked to climate change, to volcanic activity, or to Man’s actions affecting vegetation and/or the circulation of water. Eliminating volcanic activity as there was no significant volcanic phenomena in the granitic eastern Massif Central during the last 15000 years, we operated on the hypothesis that the initiation of peat inception could have either a palaeo-climatic signification, or else be indicative of the impacts of human societies on the ecosystems of this region (Cubizolle et al., 2003; Cubizolle, 2005). This idea has been the foundation of several studies conducted primarily by Anglo-Saxon authors during the second half of the 20th century (Pearsall, 1950; Conway, 1954; Smith, 1970; Moore, 1975, 1993; Korhola, 1995; Payette & Rochefort, 2001; Charman, 2002; Cubizolle et al., 2003; Cubizolle & Argant, 2006).

2This demonstration is based on interdisciplinary research combining geomorphologic analysis, archaeological studies, and palaeoecological analysis for the most part (Cubizolle et al., 2004a; Cubizolle, 2005). The construction of a body of data is based in large part on radiocarbon dating, applied not only to palaeoecological and archaeological findings, but also used to date the initiation of peat accumulation. The approach can be summarized in the following steps:

  • select, following detailed field recognizance, asample of sites so as to be representative of the variety of mires in the given geographical area,

  • reconstruct mire stratigraphy, using topographicalmeasurement with a tacheometer, radar profiles, and coring,

  • identify the basal layer, based on laboratory analy-sis and micromorphology expertise, and sample it for dating,

  • study the temporal distribution of the radiocarbondates obtained,

  • and confront this distribution of dates with theother palaeoenvironmental data available.

3Presenting all these operations and the results obtained by comparing these different methods would be too ambitious for one paper. A detailed synthesis of results was recently compiled in French for the granitic eastern Massif Central (Cubizolle, 2005). Here we focus on one of the most difficult aspects of the approach: determining the date at which peat inception initiated. This essential task raises three main problems:

  • how does one determine, in the field on the stratigraphy of fresh cores, what layer is the basal layer, i.e. the layer corresponding to the first appearance of peat?

  • How does one identify the sector(s) within a sitewhere peat inception first began?

  • How precise can we expect the radiocarbon dates to be? Are they precise enough to establish a chronology upon which the palaeoenvironmental reconstruction of the mire can be based?

4Although radiocarbon dating has been used since the 1970’s, answering these questions is rather difficult because the development of a standardized methodology for sampling basal layers is still at a preliminary stage. Very few authors give detailed explanation of how samples were selected (Charman, 2002). Moreover, the number of dates of basal layers in particular remains quite small (Payette & Rochefort, 2001). Also, there are very few regional synthesises of basal layer dates, even in Great Britain where palaeoecological and archaeological research has produced a great number of dates (Moore, 1975, 1993; Moore et al., 1984; Tallis, 1991). In the French Massif Central no prior research specifically focused on peat inception and the dating of basal layers, althought many mires have been studied by palynologists and tephrochronologists (Etlicher et al., 1987; Beaulieu de et al., 1988; Juvigné, 1992; Juvigné et al., 1996).

5Some other syntheses are also available for Scandinavia (Foster et al., 1988; Foster & Wright, 1990; Foster & Jacobson, 1990; Korhola, 1992, 1995), for Quebec and Labrador (Payette & Rochefort, 2001), for Kalimantan (Morley, 1981; Sieffermann et al., 1988; Rieley et al., 1992) and for Indonesia (Neuzil, 1997).

2 - Usefulness and limitations of radiocarbon dating

6We do not present the method of radiocarbon dating here. Extensive literature is available to which the reader can refer (Evin & Oberlin, 1998; Evin et al., 1999; Evin, 2002; Fontugne, 2002, 2004). Radiocarbon dating is a remarkable, widely used tool that has revolutionised archaeological and palaeoenvironmental studies. It is nonetheless a tool that should be used appropriately and with caution, requiring:

  • a concise scientific question and clear understanding of how dating can help progress on the reasoning pertaining to that question, or as Gaston Bachelard put it, “measurement should be based on thought and not thought on measurement” (Bachelard,1977).Obviously, this also requires sufficient understanding of how radiocarbon dating works.

  • rigorous, methodical sampling: in the present case, this implies determining in the field what stratigraphic layer to sample and how much material to collect. We followed the recommendations of Olsson (1986) who states that one should avoid samples that contain material from more than one facies. This implies that one has to clearly identify limits between one facies and the next in the field. But one should also keep in mind that deposits can be disturbed, i.e. cinders and woody debris may not be of the same age as surrounding materials. Also foreign materials can be introduced in the sample during coring or when extracting samples from cores. One should therefore be very attentive to how equipment and sampled material is handled during sampling (wash and wipe of the corer before each coring, always use a clean knife to separate sections, wrap samples in clean plastic bags and do not reuse them, label samples carefully in the field, and store samples in a cool place). In low mires with comparatively thin peat accumulations, also check that there are no plant roots that are currently growing on the surface of the mire.

  • select the most significant material in relation to sampling goals. For example peat, cinders and woody debris can be mixed together in the sample. One should establish with the carbon dating centre which material should best be dated or how many different dates should be done on the sample.

7Once date has been obtained, its interpretation will be enhanced by taking into account the following points:

  • like any physically based measurement, a 14C dating is automaticallyassociated with a degree of uncertainty as was demonstrated by the comparison between 14C dates and tephrochronological data (Juvigné et al., 1996). This is due to the difficulties in fine-tuning the settings of the apparatuses used in different steps of the dating process, to statistically random error inherent to radioactivity (Evin & Oberlin, 1998), and to the socalled “reservoir effect” by which hollow fibres accumulate fine material from other sources, this is a particular problem in basal peat facies and underlying sandy organic facies (Kilian et al., 1995; Blaauw et al., 2004).

  • the existence of radiocarbon thresholds increases the degree of imprecision for certain periods, namely during the 15th millennium cal. yr. B.P., between 11800 cal. yr. BP and 8900 cal. yr. BP., as well as between 2750 cal. yr. BP and 2100 cal. yr. BP.

  • the small differences in 14C ages obtained for periods prior to 12 800 BP must be interpreted with great prudence as the curve of 14C age corrections is less precise before that date, and becomes very imprecise beyond 25 000 BP.

  • pollution induced by human or animal interventions, or during handling of the sample is always possible.

8Moreover, irregularities in the curves used to calibrate radiocarbon dates induce variations in the probability of the occurrence of the calibrated dates. It is therefore possible to use dates that have higher probabilities of occurring than others as a working hypothesis, but one should keep in mind that this is an artefact of the dating technique and does not correspond to the true probable ages of sampled materials.

3- Definition and sampling of mire basal layers to be dated

9It is unanimously recognized that identifying and characterizing in the field one or more layers corresponding to the beginning of peat inception is a difficult task (Charman, 2002). To date there is no general agreement as to how to identify and describe these layers. Following a synthesis of approaches in the literature, we use five examples to show how this was done in the mires of the granitic eastern Massif Central.

3.1 - What can be retained from the diversity of methodological approaches?

10Charman (2002) recalls that the initiation of peat inception is a major change in pedology, shifting from a mineral soil to an organic one. This process can take place at various rates, but is generally thought to take a century or more (Payette & Rochefort, 2001). The transition from peat layers to sandy organic or silty organic facies and then to the mineral facies underlying them may appear to be quite simple at first, but is often quite subtle. Moreover peat inception is not an irreversible process, particular during its initial phases. A preliminary layer of peat may have become degraded or been overlain by mineral deposits before a new organic layer developed, that initiated the development of the studied mire (Cuikshank & Cruikshank, 1981).

11Charman (2002) concludes that defining the basal layer should be based on three types of information: macroscopic observation of stratigraphy, organic matter content, and the nature of pollen and plant remains. We also include the microscopic observation of basal facies as recommended by Korhola (1995). These are the four criteria we used to study the initiation of peat inception in the granitic eastern Massif Central (Cubizolle, 2005).

12Although most authors seem to agree with these general recommendations, opinions diverge if one attempts to establish more precise criteria. Regarding the description of basal layers, the extreme diversity of facies in the lower strata of mire formations makes it impossible to define standard references for basal layer properties. Sediment colour determined using the Munsell code (1992), and the degree of peat decomposition, estimated by the von Post test (Payette & Rochefort, 2001), are only two criteria that can be used in a relatively objective manner. Characterizing texture is done in the field by touching and tasting the material. Also, description would be more precise if one extracts and analyses several cores. Thus field experience plays a key role in successfully identifying and characterizing basal layers.

13Further work in the laboratory focuses on the composition of the samples: granulometry is measured separately for both the mineral content and the fibre content. This can be complicated due to the presence of mineral sediment in sapric peat, and to the presence of organic matter in the underlying mineral facies. And there are several difficulties when applying the ignition loss method to evaluate mean organic matter content from dry ash. First the presence of mineral elements of various sizes unevenly distributed in basal peat layers can cause great variations in values obtained by ignition loss within a given layer. The presence of woody elements can also perturb measures made on the sediment facies, yielding much higher organic matter content than sandy sapric peat formations. Also, the success of fire-loss measurements depends on many other parameters (Chapman, 1964; Mörnsjö, 1968; Zoltaï & Johnson, 1985) including the nature of the vegetation from which the deposit originated, the degree of decomposition of the organic matter, the quality of the waters flowing into the mire, and to the input of volcanic cinders, as well as mineral and organic particles transported by winds. Hence it is illusory to establish a precise limit for organic matter content separating peat formations from other deposits. Moreover, there is no consensus on a minimum amount of organic matter content that could be used to distinguish peat from other formations: proposed values range from 25% to 70% ! For these reasons, most authors rely solely on field observation of mire stratigraphy to identify the basal layer. Some authors also consider a minimum required level of organic matter content. For example Korhola (1992, 1995), chose a value of 70% in studies on the origins of Finnish mires, based on the work of Kaule (1976) and Andreyko et al. (1983). For our part, we see the estimation of organic matter content from the base upwards along a core sample as a means of verifying our field interpretation as to the amount of change in pedology from mineral to peat formations. In the cores we have studied, this change from sandy or silty organic facies to peat facies occurs when the amount of organic matter content is on the order of 30%.

14It is difficult to do systematic analysis of pollen grains or of plant remains when studying a large number of mires. This type of analysis was conducted on about a dozen sites, whereas dating has been done for 64 sites (Cubizolle, 2005). Our palynological results were presented in a separate publication (Argant & Cubizolle, 2005; Cubizolle et al., 2005).

15Although the analysis of micromorphology is common place for mineral soils (Fedoroff & Courty, 1992), it is much less frequently done on histosol (Castel, 1991; Lusiana et al., 2002). In general, soil micromorphology involves using a microscope to study soil characteristics in order to understand the origins of soils, to retrace their history and to better understand how they function (Courty & Fedoroff, 2002).

16I.I.Y. Castel (1991) did one of the rare studies in which micromorphology was used to analyse the transition from peat formations to wind born deposits in dunes in the northern part of the Netherlands, and also to assess the reliability of 14C dates obtained for the contact between peat formations and sand formations. Micromorphologic analysis is more commonly used to reconstruct local palaeoenvironmental settings, often in conjunction with other methods in palaeoecology (Puffe & Grosse-Brauckman, 1963; Babel, 1975; Lee 1983; Davis & Wickham, 1987; Bouma et al., 1988).

17However there is no single way of describing thin plates. According to Bhiry (2001), the most widelyused approach to describing thin plates is based on genetic description, while recognizing that there is also a morphological approach. Nor is there any reference book that brings together examples or gives interpretations of soil characteristics, so this information must be repeatedly searched for in the literature (Bonnel, 2004, 2005). In our studies, thin plates made by the Soil Sciences Laboratory of the University of Gand in Belgium followed the protocol by Fitzpatrick (1993).

18The lack of chronological precision can also be worsened by the thickness of the sediment sample taken for dating and analysis. This thickness varies considerably from one author to another, and is not always indicated. Because the material at the base of mires has sometimes been compacted considerably, 1 cm can represent tens of years in some cases, but thousands of years in others. Some researchers proposed the solution of dating by AMS. However for Sphagnum peat, which does not present a reservoir effect (Blaauw et al., 2004), the use of AMS is not justified. In composite organic facies with substantial mineral content, using AMS requires sorting types of organic materials (wood, leaf, other organic remains…) to do separate dating on each of them. This is technically difficult and can become very costly when applied to numerous cores. And in the end, one must chose among the many dates obtained at a given level to attribute an age to the initiation of peat inception, a choice that is not necessarily obvious and can even be somewhat arbitrary.

19We chose to do the dating with peat samples ranging from 1 to 3 cm thick, forming a half-cylinder 55 mm diameter, which corresponds to section of Russian hand corer. Thus the volume of our samples ranges from 10 to 25 cm3, i.e. enough to obtain a conventional dating, which is faster and less expensive than AMS. AMS date was used on samples with too little organic matter for conventional dating, or for dating in relation to pollen analysis. Again conventional dating on greater volumes also reduces the risk that any minor pollution corrupts the dating of the sample, while offering sufficient precision given our analysis objectives. Using the same corer, Korhola (1992) selected samples between 6 and 10 cm thick, and took the risk of mixing materials from two neighbouring cores that presented similar stratigraphies, but we never did this.

20Finally, the empirical nature of the methods used to identify the basal layer implies a certain degree of subjectivity. It is impossible to be certain that all researchers truly dated the layers that correspond to the beginning of peat inception at all of the sites they have studied. This makes comparison between studies rather delicate.

3.2 - Development and application of the method used in the granitic Eastern Massif Central mires

21Here three mires from the eastern part of the Massif Central are presented in order to illustrate(fig.1) methodological discussion.

22Dating the initation of peat inception in the Verrerie bog (Monts de la Madeleine, alt. 1115 m) and at Egaux (Monts du Forez, alt. 1405 m)

23At the Verrerie bog, three cores separated by 15 cm were extracted from the basal layers in the portion of the mire where the peat formation was thickest: one corewasforpollenanalysisand14Cdating(C-7019bis1 ), one for ignition loss (C-7019 bis 2), and one for micromorphology analysis (C-7019 bis 3). The stratigraphy was remarkably constant among the three cores.

24Macroscopic description indicates a first change of facies around –435 cm, passing progressively from mesic peat to sapric peat (fig. 2). The latter, becoming sandy at its base, overlies of sand organic layer 8 cm thick constituted primarily by sandy to gravelly material. The bottom of the core made contact with sandyloamy colluvium containing wood cinders. The later two discontinuitiesareverydistinct.In core C-7109bis3 we note within the colluvial formation an oblique contact between an upper gray-brown facies (10YR 3/1) and a lighter coloured lower facies (10YR 4/2) (fig. 3). The evolution of water content and organic matter content in core C-7109 bis 2 clearly confirms the key position of the sandy organic facies in the initiation of peat inception (fig. 2), because in this formation organic matter content jumps suddenly from 12% to 32% (fig. 2). No significant difference was observed between the 2 colluvial microfacies (fig. 2).

25Thin plate analysis of core C-7019 bis 3 confirms the oblique contact within the colluvial formation. It separates a sandy-loamy low layer comprising microfacies 1, 2, and 3 from a more gravelly overlying formation corresponding to microfacies 4 and 5 (fig. 3). A similar contact, but not visible by the eye, was noted between the colluvial formation – microfacies 5 - and the sandy organic formation - microfacies 6 - that becomes progressively richer in organic matter content towards its top - microfacies 7 to 11 - (fig. 3). In contrast, the transition to the sapric peat formation is close to horizontal at its lower limit, situated in the field at - 449 cm, and was placed at the contact between microfacies 10 and 11 (fig. 3). The sandy organic formation was found to be highly mineral in places, microfacies 8 showing greater resemblance with the colluvium in microfacies 5 than with microfacies 6, 7, and 9.

26The micro-stratigraphic diagram shows the specificity of the colluvial microfacies 3 to 5: abundance of wood cinders, scats, phytoliths, scletorium and plant remains (fig. 3). The sandy organic facies also contains many soil fauna scats, but is distinguished from the colluvial facies by a much lower percentage of inorganic content, and the presence of woody debris. At the top of the column in the sapric peat formation, wood is much more abundant whereas the inorganic fraction is substantially reduced and scats are absent. Lastly, worn rounded mineral particles (wearing due to transport) were observed at MPol, becoming more numerous we worked upwards through the stratigraphy, from the colluvium to the sapric peat.

27The results of the micromorphologic analysis agreed well with the macroscopic description, except for certain contacts that seemed abrupt to the naked eye were found to be more progressive, particularly the transition from the colluvium to the sandy organic facies. The transitional nature of this contact is further explained by the presence of scats, indicating an environment that was not water logged, and therefore no peat formation could have occurred, whereas the absence of scats in overlying formations coincides with a rapid increase in organic matter content (> 65% at the contact with the sapric peat formation) (fig. 3). Note also that the presence of scats signifies that bioturbation occurred in these formations, which implies that trying to obtain a precise chronology for these formations using AMS makes no sense. Also, peat inception clearly began with microfacies 11, around –449 cm (fig. 3).

28Based on these observations, dating was done on six samples taken from core C-7109 bis 1 (fig. 2, tab. 1) to date zones of contact. Thus, although the microfacies in core C-7109 bis 1 correspond well to those of the core examined by microscope, samples –455 cm / –456 cm and, to a lesser degree, –463 cm / –464 cm are composed of a mixture of 2 adjacent microfacies. This illustrates the inconvenience of working on more than one core.

29The results in table 1 show good coherency, without any inversion in the chronology. In calibrated ages, the intervals of time proposed range from 540 years to 90 years, this variability being due to 14C thresholds. An erroneous evaluation of the level at which peat inception began, i.e. placed too high or to low on the core, would give older or more recent ages by about 400 to 450 years. Peat inception began around 8060 +/– 50 BP, i.e. 9049-8779 cal. yr. BP. (tab. 1). The top of the sandy organic facies was dated to 8400 +/– 50 BP, i.e. 9509-9289 cal. yr. BP., so we can estimate that it took less than 5 centuries for pedological conditions to change and for a histosol to develop.

30The example of the Egaux mire is not presented in detail as the conclusions are very similar to the case described above. But the case does present a particularity, there were no major difficulties in describing the cores as the contacts between facies are quite evident, but microscopic analysis showed major perturbations in the succession of microfacies (fig. 4). The sandy organic facies is most affected, but the stratigraphy is also perturbed in the sapric peat formation (fig. 4).

31In this core, the limits of microfacies between –350 cm and –365 cm are very difficult to define. In contrast a wedged-shaped form is clearly visible below –355 cm, perforating microfacies 12 to 7 (fig. 4). Thus the passage between the sandy organic formation and the sapric peat was completely disturbed, whereas the contact between the sandy organic formation and the underlying colluvium were sub horizontal and quite rapid.

32In the microstratigraphic diagram (fig. 4), the colluvium with low organic content is clearly distinguished, with only a few fragments of wood in the sandy-loamy deposit. However the other facies were not well differentiated because the structural perturbations made it difficult to distinguish between formations or obtain relevant samples. The diagram shows a sudden drop starting from microfacies 8 in the percentage of both organic and inorganic material. Like at the Verrerie site, there were wood cinders in the colluvial formations.

33The dates obtained indicate that peat started to accumulate around 6185 +/– 50 BP i.e. 7244‑6913 cal. yr. BP., and that about 439 year went by between the last colluvial deposits and the formation of the first layer of peat.

34The microscopic study of the basal layers of the Egaux mire show that substantial perturbations may have affected the different facies that form the zone of contact between peat and the underlying mineral deposits. The naked eye generally perceives these contacts as a transition from one facies to another. This has three consequences regarding the choice of the samples to be dated:

  • because of the perturbations observed at the con-tact between peat and sandy organic facies, it is not possible to obtain precise dates,

  • for these same reasons, it is not pertinent to use or-ganic matter to select levels to be sampled either,

  • an approximate evaluation of the contact betweenformations would displace the sample 2 or 3 cm above or below the exact limit having not affect on the precision of the dating.

35The problem dating the beginning of peat inception at Chaulme fen (Monts du Forez, alt. 1130 m)

36The micromorphologic study of the Chaulme fen core showed perfect concordance with field and laboratory observations (fig. 5). The perturbations in the micro-stratigraphy were very minor, and microfacies appeared as a succession of distinct subhorizontal layers. However we noted very irregular limits between microfacies 2 and 3, corresponding to the contact between colluvial and sandy-organic formations (fig. 5). We also found a great deal of wood in microfacies 9, 10, 11 and 12, which correspond to the upper part of the sapric peat formation for microfacies 9, and to mesic peat for the other three.

37The choice of samples to be dated was simple. However 14C results were surprising (tab. 1 and fig. 5), showing inversions in the succession of dates: the date obtained at the base of the peat formations at –207/ –209.5 cm is the oldest and the date just below it, at the summit of the sandy organic facies, is more recent by 1135 years BP. Below these levels there are other inversions but the differences are quite small.

38Obviously one must take into account the imprecision of 14C calibration for this period of transition between the Late Glacial and the Holocene. However palynological results for these two facies (sandy organic and peat) indicate a cold dry climate that seemingly corresponds to the recent Dryas (J. Argant, pers. comm). But dating the peat at 11350 BP corresponds to the Alleröd, at the beginning of the phase of Betula and Pinus (Beaulieu et al., 1988). We can therefore suspect that the peat formation has been polluted by older organic matter from surrounding slopes or the watershed upstream, as the fen is located in a small alluvial plain. In consequence, we chose the date of the summit of the sandy organic facies to situate the beginning of peat inception for this fen, i.e. 10215 +/– 105 BP (12587-11341 cal. yr. BP.).

39This example shows that the dates obtained for some sites can be relatively imprecise 200 to 400 years in this case often it is impossible to be understand what caused the 14C dates to be overestimated.

Tab. 1: Radiocarbon dates concerning the basal layers of the peat bog of la Verrerie (C-7109bis-1, Monts de la Madeleine, alt. 1015 m), les Egaux peat bog (C-8016, Monts du Forez, alt. 1405 m), la Chaulme fen (C-52018, Monts du Forez, alt. 1130 m). The oldest radiocarbon dates of peat inception are in bold print.

Tab. 1: Radiocarbon dates concerning the basal layers of the peat bog of la Verrerie (C-7109bis-1, Monts de la Madeleine, alt. 1015 m), les Egaux peat bog (C-8016, Monts du Forez, alt. 1405 m), la Chaulme fen (C-52018, Monts du Forez, alt. 1130 m). The oldest radiocarbon dates of peat inception are in bold print.

Fig. 1: Location of mires studied in this paper.

Fig. 1: Location of mires studied in this paper.

Fig. 2: Stratigraphy, loss on ignition, radiocarbon dating of cores C-7109 bis 1 and 2, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.

Fig. 2: Stratigraphy, loss on ignition, radiocarbon dating of cores C-7109 bis 1 and 2, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.

Fig. 3: Stratigraphy and micromorphological diagram of core C-7109 bis 3, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.

Fig. 3: Stratigraphy and micromorphological diagram of core C-7109 bis 3, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.

Fig.4: Stratigraphy and micromorphological diagram of basal layers cored (C‑8016) in the peat bog of les Egaux (Monts du Forez, alt. 1405 m).

Fig.4: Stratigraphy and micromorphological diagram of basal layers cored (C‑8016) in the peat bog of les Egaux (Monts du Forez, alt. 1405 m).

Fig. 5: Stratigraphy and micromorphological diagram of basal layers cored in the fen of la Chaulme (Monts du Forez, alt. 1130 m).

Fig. 5: Stratigraphy and micromorphological diagram of basal layers cored in the fen of la Chaulme (Monts du Forez, alt. 1130 m).

4 - Identifying the oldest parts of mires

40Once the stratigraphic layer to be dated has been identified, one must determine where the oldest part of the mire is, the part where the peat started to form first, and that we want to date to determine when peat inception began.

41The most reliable way to ensure dating the sector(s) where the earliest peat inception began is to increase the number of cores and dates obtained at each mire. However this in not financially feasible when studying more than just a few mires. Indeed, the few studies on an international scale that exploit a large number of sites are often based on data collected over several decades (Zoltai, 1995), because of the effort and expense involved.

42Here the idea was to optimize sampling effort based on a model of mire development. We tested this model at 4 pilot sites.

4.1 - The model of Moore and Bellamy (1974)

43Moore and Bellamy (1974) proposed that we distinguish 3 types of mire systems each of which correspond to a model of the initiation and expansion of peat formations, processes which are in turn conditioned by the climatic, geomorphologic and hydrological contexts of mires.

44The primary mires – “Primary peats and hence primary mire systems” are mires that originated due to both a favourable geomorphic setting and favourable macroclimatic conditions. They are situated in the lowest, most depressed parts of depressions and valleys, in sectors where hydromorphy is likely to occur, i.e. positive or null hydrological balances. That is why this type of mire can even be found in tropical regions with long dry seasons.

45Secondary mires – “Secondary peats and hence secondary mire system” develop under relatively humid climates, which limits their geographical distribution somewhat. In this case, peat formation occurs outside the most depressed sites, thereby increasing the surface of water retention. However these mires do not extend beyond the valley or depression they are located in, and depend on the water table level for the development.

46Tertiary mires –"Tertiary peats and hence tertiary mire systems“ are only found at mid latitudes, not high latitudes. Moore and Bellamy (1974) describe 11 categories for which the common characteristic is that the mires extend beyond the limits of the topographic depression they first appeared in. Under adequate climate conditions, peat accumulations grow, eventually reaching astagewherethemireisnolongerfullydependant on the local water table level. These mires are therefore ombrotrophic and in most cases raised bogs.

47It appears that the three systems described above can be seen as corresponding to successive stages in the development of mires under ideal climatic conditions, although this idea is not explicitly stated by the authors. Ombrogenous mires or bogs are the only case in which peat inception can occur both in depressions and in comparatively higher positions at the same time.

48Thus, according to this model, mires originate in one or more low spots within a depressed topography, and then, depending on climate conditions, they are either restricted to their initial extent, or can undergo 1 or 2 successive stages of expansion. We can therefore conclude that in order to locate the oldest layers of a mire, we must search for the lowest points underlying the peat accumulation. Reconstructing palaeotopography and mire stratigraphy are essential steps in locating these low points, as what may appear today to be a single large mire may in fact result from the expansion and connection of what was initially more than one mire, spreading out from different points, and possibly originating at different periods. Local and site specific climate and hydro‑geomorphological conditions can explain why the initiation of peat inception was not concomitant between A and B, and how differing growth rates lead to a thicker accumulation today in the younger of the two initial mires.

49This model was tested on four sites with differing characteristics.

4.2 - The distribution of radiocarbon ages in the basal layers of the Verrerie bogs

50(Monts de la Madeleine, alt. 1115 m)

51A total of 3.7 km of topographic profiles was collected, and a total of 25 radiocarbon dates were obtained (tab. 2). The thickest part of the peat accumulation spans ESE to WNW in a zone corresponding to the talweg of the small palaeo-valley (fig. 6A). Analysis of the dates showed that the oldest age obtained for the basal layer is 8060 +/– 50 BP. This date is obtained in the deepest part of the mire, under the zone of schlenken (fig. 6B and tab. 2) and was confirmed by pollen analysis of the core (Argant & Cubizolle, 2005). However, we obtained a date of 7660 +/– 60 BP at the same depth, only about 10 m to the west of the first core. More generally, the correlation between the ages obtained for the cores and the depth at which samples were taken is not very convincing (fig. 7). Note however that this situation is very similar to the one described by Korhola for finnish bogs (1992, fig. 48, p. 79).

52Thus in the Verrerie bogs, samples made at depths of around 450 cm yield very different ages, from 8060 +/– 50 BP to 6490 +/–45 BP, i.e. a difference of 1570 years BP. Moreover very similar ages, 7130 +/– 75 BP and 7245 +/– 65 BP were obtained at depths of –343/ –341 cm and –259/–257 cm respectively. Also, like in the Chaulme mire example above, two cores gave radiocarbon ages at the base of the sapric peat formation that were 200 to 400 year older that ages obtained for the underlying sandy organic facies, but no additional evidence was found to provide a satisfactory explanation of these inversions.

4.3 - The distribution of radiocarbon ages in the basal layers of the Egaux bog

53(Monts du Forez, alt. 1405 m)

54Previously we described the particularly disturbed basal layers of the Egaux bog. Here we present the chronology of this perturbed formation initially observed via miocromorphologic analysis.

55Similar to the Verrerie mire, the initiation of peat inception was based on the age obtained for the base of the sapric peat formation, except for core 1071, where it was situated between two dates corresponding respectively to the end of the deposit of the underlying sandy organic formation, and the end of the accumulation of sapric peat in the overlying formation (tab. 3). The oldest age was indeed found in the part of the mire where the peat is thickest, in the schlenken zone. However, the sample taken from a slightly higher level was dated to 6685 +/– 50 BP in core 8000 compared to the date at 6185 +/– 50 BP in core 8016. The graph in figure 8 indicates that the link between depth and age is also inconsistent at this site.

56The dates obtained for cores 8000 and 8016 agree well with conclusions from pollen analysis done by Janssen and van Straten (1982). Although the stratigraphic location of the samples studied is not clearly indicated by these Dutch authors, the depth they give of 3.4 m necessarily situates the sample in the same part of the mire as core 8016. At the base of the column, the palynological diagram shows pollens typical of the transition between lower Atlantic and upper Atlantic, characterised by an increase of Quercus, followed shortly thereafter by Ulmus and Tilia, and final Fraxinus a bit later.

Fig. 6: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m

Fig. 6: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m

A, distribution of vegetation and depth of peat; B, distribution of radiocarbon dates obtained from the basal layers of peat accumulation.

Tab.2: Radiocarbon dates obtained from the basal layers of the raised bog of la Verrerie in les Monts de la Madeleine, alt.1015 m.

Tab.2: Radiocarbon dates obtained from the basal layers of the raised bog of la Verrerie in les Monts de la Madeleine, alt.1015 m.

The oldest radiocarbon dates of peat inception are in bold print.

Fig. 7: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m: relationship between radiocarbon ages BP obtain in the basal layers and the depth of dated samples.

Fig. 7: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m: relationship between radiocarbon ages BP obtain in the basal layers and the depth of dated samples.

Fig. 8: The raised bog of les Egaux in les Monts du Forez, alt. 1405 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.

Fig. 8: The raised bog of les Egaux in les Monts du Forez, alt. 1405 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.

Tab.3: Radio carbondates obtained from the basal layers ofthe raised bog of les Egaux in les Monts du Forez, alt.1405 m.

Tab.3: Radio carbondates obtained from the basal layers ofthe raised bog of les Egaux in les Monts du Forez, alt.1405 m.

The oldest radiocarbon dates of peat inception are in bold print.

4.4 - The distribution of radiocarbon ages in the basal layers of the Digonnière bog

57(Massif du Pilat, alt. 1055 m)

58Ten dates of the basal layer were made for the Digonnière bog in the Pilat mountains (tab. 4) of which eight are from the base of the mound, and two from the base of the bog encroaching on the bog to its north, and north northwest (fig. 9).

59In the graph in figure 10 showing the relationship between these radiocarbon ages and sample depth, we can see that the dates of the bog’s basal layer show less dispersion than in the cases of the Verrerie and Egaux bogs. However a date of 9140 +/– 70 BP, i.e. 1049010188 cal. yr. BP, was obtained for the basal layer of the lagg to SSW of the bog (fig. 9). This is the oldest date obtained at this site, even thought the sample dated was only at a depth of –95/–92 cm. The sample was clearly from a peat formation, as this fine peat contains 85.7% organic matter. However the difference with the date of 9000 +/– 40 BP, i.e. 10220-9909 cal. yr. BP, obtained for the base of the bog is quite small, as the intervals of incertitude in real years overlap. Moreover, these ages coincide with a 14C threshold, so we must consider the two samples as being contemporary to each other. Somehow the circulation of water was modified, impeding the accumulation of peat in what became a lagg, whereas peat inception continued over the rest of the site.

Tab. 4: Radiocarbon dates obtained from the basal layers of the raised bog of la Digonnière in le Massif du Pilat, alt. 1055 m. The oldest radiocarbon dates of peat inception are in bold print.

Tab. 4: Radiocarbon dates obtained from the basal layers of the raised bog of la Digonnière in le Massif du Pilat, alt. 1055 m. The oldest radiocarbon dates of peat inception are in bold print.

Fig. 9: Cross sections of the raised bog of la Digonnière in Massif du Pilat (alt. 1055 m) and the oldest radiocarbon dates obtained from the basal layers.

Fig. 9: Cross sections of the raised bog of la Digonnière in Massif du Pilat (alt. 1055 m) and the oldest radiocarbon dates obtained from the basal layers.

Fig. 10: The raised bog of la Digonniere in le Massif du Pilat, alt. 1055 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.

Fig. 10: The raised bog of la Digonniere in le Massif du Pilat, alt. 1055 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.

4.5 - The distribution of radiocarbon ages in the basal layers of the JAS fen

60(Monts du Forez, alt. 1075 m)

61Here is an example of a fen initiated by human intervention, similar to the many mires that formed following human modifications of the environment in the eastern Massif Central. (Cubizolle et al., 2004b). This fen is situated in a depression of about 1000 m2, dug out by humans (fig. 11). A total of six samples were dated, five were taken every 6 meters along a cross-section, and the sixth located 22 m downstream of the crosssection, at the deepest part of the fen. Resulting dates are given in table 5.

62First we see that the two samples from core JAS-E, of which JAS-E2 is the deepest sample collected at this site, are both modern aged samples, whereas all the other dates correspond to the Middle Ages. These samples appear to have been polluted. In man-made sites, generally very small and situated in an agricultural environment from their very beginning, we can imagine a great number of possible sources for such pollution: attempts to flush out solid materials from artificial reservoirs, but also drainage, tree planting, getting a tractor stuck in the fen, etc. For the other dates we see that regardless of the depth, the ages differ very little. Calibration shows that all dates except JAS-DG fit within the fork between 1170 cal. yr. BP and 1071 cal. yr. BP.

Fig. 11: Longitudinal section and stratigraphy of the fen of le Jas in the Monts du Forez (alt. 1075 m).

Fig. 11: Longitudinal section and stratigraphy of the fen of le Jas in the Monts du Forez (alt. 1075 m).

5- Discussion and conclusion

63Identifying the layer of peat that, once dated, will provide an estimation of the age of a mire is not an easy task, and explanations given in the literature offer little help as they are both rare and incomplete in general.

64Our experience indicates that it is impossible to define a single criteria for selecting samples to date. Even measures of organic matter content are not reliable. One should therefore rely primarily on microscopic description, the only method that can easily be applied to a large number of sites. This approach should be based on a large number of cores per site to identify the facies that corresponds to the beginning of peat inception. Furthermore, the transition between peat layers and underlying mineral formations must be completed by analysis of dry ash content obtained by ignition loss, and by micromorphologic analysis. It is only by using this combination of methods that can one be sure to identify the basal layer with certainty.

65Thus the usefulness of micromorphologic analysis is evident. In general, it provides verification of initial field interpretations of core stratigraphy. It has also shown that considerable disturbance of the transition between peat formations and underlying mineral formations is not so rare, at least in our study area. When such disturbance is observed, it implies that obtaining a precise date of the beginning of peat inception is at best improbable, and more likely impossible. On the other hand the micromorphologic analysis shows that a sample 2 cm thick is a good choice.

66In spite of the great precautions taken, misjudging the contacts between facies and thereby the sample to be dated is always possible. This can induce errors in the dates attributed to basal layer formation on the order of a few years to a few centuries. However our experience has shown that once researchers have obtained in depth knowledge and understanding of the mires in their study area, samples can be collected at a level of precision of about 1 cm. In our work, this implies that, depending on the estimations obtained and the number of samples dated, the range of uncertainty of the ages attributed to the initiation of peat inception is on average about 120 calendar years, the median being 72 years (Cubizolle, 2005).

67Correctly situating the beginning of peat inception requires radiocarbon dating of a good number of samples from different points at each site. Samples must be selected from the basal layer of peat formations, but also from the underlying sandy organic formations, and from overlying peat formations. Indeed the experience obtained from different sites has provided greater understanding of site structure:

  • first, the oldest ages are generally obtained wherethe peat accumulations are thickest, but this in not always the case. Moreover, the relationship between depth and age is not linear. The correlation we observe between age and depth is virtually identical to the relationship observed by Korhola (1992, p. 78-79, fig. 48) for Finnish bogs. Therefore one should ideally obtain as many dates as possible, particular for larger mires as we sometimes found that different locations in larger mires yielded very different ages at comparable depths, or similar ages at very different depths. Regarding the implications of these cases, we agree with the conclusions of Korhola (1992). Undoubtedly, irregularities in the topography of the initial site in which the mire developed explain in large part these variations among the dates at which peat inception initiated within a given mire. For comparatively small mires, on the order of 1000 m2 or less, the number of dates can be much smaller as the underlying topography of smaller mires is generally simpler than for larger mires. In the case of small bogs resulting form human intervention however, the risk of extensive disturbance is much greater as we saw in the Jas example, so such cases require more dates in relation to the total surface to obtain a solid estimation of the age of the peat formations.

  • secondly, we have to admit that it is difficult to de termine where peat inception began simply by examining the surface of a mire. In the Egaux example, inception began in the centre of the mire, but in the Verrerie case the point of initiation is far from the centre. It is interesting to note however that in bogs, the oldest dates are obtained beneath the schlenken zone, when this zone exists (fig. 6).

  • thirdly, regarding the interpretation of the ages obtained, one should keep in mind that inversions can be due to the inclusion of older exogenous materials in upper layers, to the inclusion of more recent materials in older layers,or to pollution duringcoringor the handling of samples. Also, excluding such perturbations, the oldest date obtained corresponds technically to the minimum approximate age of the mire, but perhaps even older ages could have been obtained elsewhere in the mire, and theoretically it is even possible that an older peat accumulation was somehow removed from the site prior to the initiation of the present peat accumulation, be it by erosion or by human intervention (Cubizolle, 2005).

68Thus there are many possible sources of imprecision, linked to both the dating process itself and to sampling. Nonetheless, the precision of the dates of the initiation of peat inception we obtained appear to be satisfactory as the processes leading to the initiation of a mire span one or more centuries (Payette & Rochefort, 2001), implying that an error of a few decades is negligible and an error of a few centuries is tolerable. However one should be prudent when interpreting the ages of peat formations in relation to palaeoenvironmental reconstruction. Comparison with palaeoclimatic, palaeoecological and archaeological results must take into account the relative uncertainty of the dating process.

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Table des illustrations

Titre Tab. 1: Radiocarbon dates concerning the basal layers of the peat bog of la Verrerie (C-7109bis-1, Monts de la Madeleine, alt. 1015 m), les Egaux peat bog (C-8016, Monts du Forez, alt. 1405 m), la Chaulme fen (C-52018, Monts du Forez, alt. 1130 m). The oldest radiocarbon dates of peat inception are in bold print.
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Titre Fig. 1: Location of mires studied in this paper.
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Titre Fig. 2: Stratigraphy, loss on ignition, radiocarbon dating of cores C-7109 bis 1 and 2, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.
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Titre Fig. 3: Stratigraphy and micromorphological diagram of core C-7109 bis 3, peat bog of la Verrerie, Monts de la Madeleine, alt. 1015 m.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-4.jpg
Fichier image/jpeg, 50k
Titre Fig.4: Stratigraphy and micromorphological diagram of basal layers cored (C‑8016) in the peat bog of les Egaux (Monts du Forez, alt. 1405 m).
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-5.jpg
Fichier image/jpeg, 51k
Titre Fig. 5: Stratigraphy and micromorphological diagram of basal layers cored in the fen of la Chaulme (Monts du Forez, alt. 1130 m).
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-6.jpg
Fichier image/jpeg, 53k
Titre Fig. 6: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m
Légende A, distribution of vegetation and depth of peat; B, distribution of radiocarbon dates obtained from the basal layers of peat accumulation.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-7.jpg
Fichier image/jpeg, 42k
Titre Tab.2: Radiocarbon dates obtained from the basal layers of the raised bog of la Verrerie in les Monts de la Madeleine, alt.1015 m.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-8.jpg
Fichier image/jpeg, 80k
Titre Fig. 7: The raised bog of la Verrerie in les Monts de la Madeleine, alt. 1015 m: relationship between radiocarbon ages BP obtain in the basal layers and the depth of dated samples.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-9.jpg
Fichier image/jpeg, 32k
Titre Fig. 8: The raised bog of les Egaux in les Monts du Forez, alt. 1405 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-10.jpg
Fichier image/jpeg, 27k
Titre Tab.3: Radio carbondates obtained from the basal layers ofthe raised bog of les Egaux in les Monts du Forez, alt.1405 m.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-11.jpg
Fichier image/jpeg, 89k
Titre Tab. 4: Radiocarbon dates obtained from the basal layers of the raised bog of la Digonnière in le Massif du Pilat, alt. 1055 m. The oldest radiocarbon dates of peat inception are in bold print.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-12.jpg
Fichier image/jpeg, 70k
Titre Fig. 9: Cross sections of the raised bog of la Digonnière in Massif du Pilat (alt. 1055 m) and the oldest radiocarbon dates obtained from the basal layers.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-13.jpg
Fichier image/jpeg, 31k
Titre Fig. 10: The raised bog of la Digonniere in le Massif du Pilat, alt. 1055 m: relationship between radiocarbon ages BP obtained from the basal layers and the depth of dated samples.
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-14.jpg
Fichier image/jpeg, 23k
Titre Fig. 11: Longitudinal section and stratigraphy of the fen of le Jas in the Monts du Forez (alt. 1075 m).
URL http://journals.openedition.org/quaternaire/docannexe/image/1049/img-15.jpg
Fichier image/jpeg, 53k
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Hervé Cubizolle, Pierre Bonnel, Christine Oberlin, Arnaud Tourman et Jérôme Porteret, « Advantages and limits of radiocarbon dating applied to peat inception during the end of the Lateglacial and the Holocene: the example of mires in the Eastern Massif Central (France) »Quaternaire, vol. 18/2 | 2007, 187‑208.

Référence électronique

Hervé Cubizolle, Pierre Bonnel, Christine Oberlin, Arnaud Tourman et Jérôme Porteret, « Advantages and limits of radiocarbon dating applied to peat inception during the end of the Lateglacial and the Holocene: the example of mires in the Eastern Massif Central (France) »Quaternaire [En ligne], vol. 18/2 | 2007, mis en ligne le 01 juin 2009, consulté le 15 février 2025. URL : http://journals.openedition.org/quaternaire/1049 ; DOI : https://doi.org/10.4000/quaternaire.1049

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Auteurs

Hervé Cubizolle

UMR CNRS 5600, CRENAM, 6 rue Basse des Rives, 42023 Saint Etienne cedex 2. E-mail: herve.cubizolle@univ‑st-etienne.fr

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Pierre Bonnel

Centre de Datation par le Radiocarbone, Université Claude Bernard, Lyon I, 40 boulevard Niels Bohr, 69 622 Villeurbanne Cedex. E-mail: pierre.bonnel@univ-st-etienne.fr

Christine Oberlin

Centre de Datation par le Radiocarbone, Université Claude Bernard, Lyon I, 40 boulevard Niels Bohr, 69 622 Villeurbanne Cedex. E-mail: oberlin@univ-lyon1.fr

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Arnaud Tourman

UMR CNRS 5600, CRENAM, 6 rue Basse des Rives, 42023 Saint Etienne cedex 2. E-mail: arnaud.tourman@univ-st-etienne.fr

Articles du même auteur

Jérôme Porteret

UMR CNRS 5600, CRENAM, 6 rue Basse des Rives, 42023 Saint Etienne cedex 2. E-mail: jerome.porteret@univ-st-etienne.fr

Articles du même auteur

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