Soil particles reworking evidences by AMS C dating of charcoal

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés
S1251-8050(00)01485-3/FLA
Géosciences de surface / Surface Geosciences
(Pédologie / Pedology)
Soil particles reworking evidences by AMS 14C dating
of charcoal
Christopher Carcaillet
Department of Forest Vegetation Ecology, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden
Received 9 October 2000; accepted 27 November 2000
Communicated by Patrick Lavelle
Abstract – Soil charcoal dating is a time proxy for soil pedogenesis. I test the stratification
hypothesis by AMS 14 C dating of charcoal fragments from soil profiles between 1 700 and
1 900 m with respect to altitude within the Alps. The charcoal fragments are around 1 mm
in size. There is no age/depth relationship for charcoal particles of the size millimetres. The
results are discussed in light of the role of soil fauna, up-rooting and colluvial processes.
Although biotic pedoturbation is poorly described in mountain and sub-alpine elevation, I
hypothesise that this process is very active and plays a major role on the soil functioning.
 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS
14 C
/ charcoal / soil / bioturbation / soil fauna / up-rooting / Alps / France
Résumé – Brassages particulaires dans des sols mis en évidence à l’aide de datations au 14 C
par AMS. La datation des charbons de bois enfouis dans les sols est un indicateur de la pédogenèse.
L’hypothèse de la stratification des sols par la datation au 14 C par AMS de charbon de bois d’Abies
provenant de sols entre 1 700 et 1 900 m d’altitude dans les Alpes est testée. Les charbons ont
une taille d’environ 1 mm. L’absence de relation entre l’âge et la profondeur d’enfouissement des
particules de charbon de taille millimétrique est mise en évidence. Les résultats sont discutés à
la lumière de nos connaissances sur le rôle de la pédofaune, des déracinements et des processus
colluvionnaires. Bien que les pédoturbations d’origine biotique soient rarement décrites aux altitudes
montagnardes et subalpines, l’hypothèse selon laquelle ces processus sont très actifs et jouent un
rôle majeur dans le fonctionnement des sols est formulée.  2001 Académie des sciences / Éditions
scientifiques et médicales Elsevier SAS
14 C / charbon de bois / sol / bioturbation / pédofaune / déracinement / Alpes / France
Version abrégée
1. Introduction
L’hypothèse selon laquelle les sols sont stratifiés a été
testée. L’étude est basée sur la datation au 14 C par AMS de
charbon de bois de sols entre 1 700 et 1 900 m d’altitude
dans les Alpes. L’accent est mis sur plusieurs dates 14 C par
profil, voire pour le même horizon. Les dates sont mesurées
sur Abies qui abondait au milieu de l’Holocène. Les
fragments d’Abies se trouvent en général dans les horizons
profonds, alors que ceux de surface sont plus riches en pins
et en espèces de lande. S’il existe une stratification, même
très grossière, on peut le montrer en opposant les dates
d’Abies aux espèces qui se sont développées depuis 2 000
à 3 000 ans.
2. Méthodes
Deux profils de sols ont été prélevés à Aussois (45◦15 N ;
et autant à Saint-Michel-de-Maurienne (« SaintMichel » 45◦ 15 N ; 6◦ 30 E), localités situées en Savoie
(France). Le profil AUSSOIS 4 est situé sur un plateau,
alors que AUSSOIS 1, MAUR 6 et 13 sont le long de pentes
6◦ 45 E)
E-mail address: [email protected]
(C. Carcaillet).
21
C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
douces. Les profils ont été creusés jusqu’à la roche mère.
Le sol a été échantillonné en prélevant des blocs le long de
la face verticale du profil. Les blocs ont été échantillonnés
du fond vers la surface afin d’éviter la chute de particules
conduisant à rajeunir les horizons les plus profonds. Un
prélèvement représente de 10 à 15 L de terre fine par niveau
de 20 cm. L’extraction des charbons de plus de 400 µm a
été réalisée par flottation avec un courant ascendant, suivie
d’un tri à sec à la loupe. Les charbons ont été identifiés botaniquement (×200, ×500). L’abondance de charbon est
exprimée en concentration (mgcharbon · kg−1
terre ). Dix datations au 14 C par AMS ont été réalisées sur des fragments
aériens. À quatre reprises, deux ou quatre fragments ont dû
être réunis pour disposer de la masse minimale. Les charbons ont été nettoyés sous loupe binoculaire (× 40), afin
de retirer les fragments de racines, les hyphes de champignons et les particules minérales susceptibles de modifier
l’âge du charbon. Chaque fragment a été traité durant 24 h
à l’aide d’une solution aqueuse de Na4 P2 O7 pour extraire
les composés organiques adsorbés par le charbon.
3. Résultats
Les quatre nouvelles dates des profils AUSSOIS 1 et
4 révèlent une inversion de stratigraphie. Six nouvelles
datations 14 C ont été réalisées dans les profils MAUR 6
et 13, et aucune ne permet de révéler une stratification
particulaire. Afin de comparer la totalité des datations 14 C
de cette étude et des précédentes, les datations ont été
calibrées, puis réunies dans un profil synthétique, un pour
Aussois et un pour Saint-Michel. La synthèse, fondée sur
31 datations 14 C entre 1 700 et 2 050 m d’altitude, présente
une distribution aléatoire en fonction de la profondeur.
4. Discussion
La notion de stratification de sol n’est pas démontrée en
se fondant sur une série de datations 14 C, mesurées chacune d’elles sur un seul charbon, voire un petit groupe
de deux à quatre. Des profils de sol se développent dans
des pentes et peuvent suggérer que la non-stratification
des charbons provienne de processus colluviaux. D’autres
sont sur des replats et montrent les mêmes distributions de
dates, sans relation avec l’âge et la profondeur d’enfouissement. Bien que le colluvionnement ne puisse pas être
totalement écarté, il n’explique pas à lui seul les résultats
observés. La pédoturbation par la faune du sol peut expliquer le remaniement particulaire. Entre 1 700 et 2 000 m,
le groupe des mélangeurs de sols est caractérisé par les coléoptères coprobies fouisseurs et les fourmis. Les vers anéciques sont peu fréquents, bien que présents. Des mammifères (Marmota marmota, Microtus nivalis) peuvent aussi
perturber la stratification particulaire des sols. Cependant,
les vers anéciques n’abondent pas, les coléoptères coprobies et les fourmis ne fouissent pas jusqu’à 1 m de profondeur et les marmottes ne sont pas uniformément réparties.
Enfin, les profils d’Aussois ont été échantillonnés en forêt,
milieu que les marmottes ne fréquentent généralement pas.
La pédoturbation par la faune ne peut expliquer à elle seule
22
la distribution observée des dates de charbon dans les sols.
Tous les profils de sols étudiés sont situés, depuis environ
9 000 ans, dans l’étage forestier. En conditions naturelles,
le chablis est l’une des perturbations les plus importantes
de la canopée, créant des dépressions et des monticules de
sol par déracinement. Ce processus modifie durablement et
en profondeur la structure du sol par un abrupt remaniement particulaire. Bien que le cycle naturel des déracinements soit inconnu dans les forêts des Alpes, il constitue
très probablement un processus clé pour expliquer les remaniements mis en évidence par les datations 14 C de charbon.
Des arguments ont été avancés pour expliquer la distribution des âges des charbons en fonction de la profondeur, qui est telle que la faune du sol remonte vers la surface seulement les fines particules inorganiques (limons,
argiles). Il est difficile d’accepter un tel concept dans lequel les charbons échapperaient à la faune, qui n’ingèrerait que des particules inorganiques. Des discordances similaires dans la distribution de datations 14 C des sols ont
été rapportées en arguant de la pollution de la matière organique (MO) par du 14 C anthropique, rajeunissant d’environ 3500 BP la MO par rapport au charbon. Pourquoi le
charbon, connu pour sa très haute capacité d’adsorption, ne
serait-il pas pollué par du 14 C anthropique ?
L’explication est plus simple, si l’on veut bien prendre
les faits pour ce qu’ils sont, sans chercher à confirmer le
paradigme de la stratification des sols. Le vivant remanie
les charbons aussi bien que d’autres types de MO de dimension millimétrique. Du colluvium discret de particules
fines peut s’opérer, même s’il est très difficile à mettre en
évidence. Alors que la MO et les charbons sont remaniés de
la surface vers la profondeur et réciproquement, les colluviums agradent l’épaisseur du sol, et donc diminuent à long
terme la capacité de la pédofaune à s’enfouir très profondément. Avec le temps, le résultat est une apparente stratification particulaire. La datation d’assemblages de charbon,
méthode employée dans les précédentes études soulignant
une stratification, génère un âge moyen qui peut n’avoir
que très peu de rapport avec l’âge de chacun des charbons
de l’assemblage. L’âge moyen réduit la possibilité de révéler des anomalies de stratification. Alors que la plupart des
études ont utilisé cette stratégie de datation, la présente approche est fondée, au mieux, sur un charbon pour une mesure d’âge. Si l’effet colluvionnaire peut être écarté, l’absence de stratification relève donc seulement de remaniements particulaires.
5. Conclusion
Une stratégie de datation peut être responsable du
constat de stratification dans les sols. Le colluvionnement
peut jouer un rôle clé dans l’enfouissement au-delà de la
profondeur limite d’activité de la pédofaune. Si la pédofaune remanie la matière organique de la surface des sols
vers la profondeur, l’inverse ne peut pas être exclu pour les
charbons peu ou pas détruits par la faune. Aux altitudes
forestières, la pédofaune et les déracinements ont proba-
C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
blement joué un rôle important pour expliquer la distribution aléatoire de particules de taille millimétrique. Après
quelques siècles ou millénaires, le résultat apparent peut
être assimilé au produit du modèle de « biodiffuseurs ».
Puisque les plus larges particules, de taille centimétrique
et plus, ne peuvent pas être remaniées par la pédofaune,
exception faite des marmottes, leur probabilité d’être distribuées en profondeur dans les sols est élevée après quelques
millénaires, résultant des stone lines s’expliquant par un
modèle de « convoyeur de surface ».
1. Introduction
2. Study area
Wood charcoal is frequently used as a time proxy of
pedogenesis [5, 22]. Soil charcoal fragments appear
stratified [3, 5, 32] and would result from the pedoturbation by soil fauna [5, 16]. The assumption of particle stratification closely depends on the type of dated
material, such as phytoliths or charcoal versus organic
soil matter that provide different age/depth models in
the same soil profile [23, 25]. The soil stratification is
unperfected [7, 9, 16, 25, 30] and sometimes totally
irrelevant to the concept of stratification [6, 17]. Although the 14 C dating of charcoal fragments seems to
reject the concept of soil stratification [6], the botanical identifications of charcoal from these same profiles suggest a rough stratification of assemblages [7,
9, 30]. Based on these observations, the concept of
‘wave stratification’ has been proposed, where the
burial depth of charcoal depends on the time since the
fire; the apparent burial speed varies between fragments due to pedoturbation processes that also contribute to charcoal particles reworking upward. The
depth distribution of charcoal particles thus deciphers
a wave of charcoal [7].
Here, I test the hypothesis that soils are stratified
using AMS 14 C dating of wood charcoal buried in
four soils at elevations between 1 700–1 900 m above
see level (a.s.l.) in the Alps. Nine charcoal fragments
from these soils corresponding to different species
(Abies, Pinus Sectio sylvestris, Rhododendron, and
Picea) have been previously dated [6]. However, only
one date per horizon has been carried out, and two or
three dates were obtained for each soil profile at best.
In the present study, I emphasise a higher number of
14
C dates in each profile and on a series of dates at the
same soil horizon. The new dates have been carried
out only on Abies, which have abounded during the
mid-Holocene before man-made deforestation [12–
34]. Fragments of Abies occur generally in the deepest
horizons of soil profiles within the study area, while
the upper horizons are rich in Pinus S. sylvestris
or in heathland species. If stratification exists, even
only roughly, I should be able to demonstrate it
based on 14 C dating of Abies versus taxa, e.g. Pinus
S. sylvestris, Picea, or Ericaceae, having expanded
during the last 2 000–3 000 years.
The two study localities, Aussois (45◦ 15 N; 6◦ 45 E)
and Saint-Michel-de-Maurienne (‘Saint-Michel’;
45◦ 15 N; 6◦ 30 E), are located ∼10 km apart in the
upper Maurienne valley (Savoy’s Alps, France). Climatic data were obtained from weather stations located at 1 360 m a.s.l. in Saint-Michel and at 1 490 m
a.s.l. in Aussois, both with southern exposure. Mean
annual air temperatures are 7.0 and 6.2 ◦ C in SaintMichel and Aussois, respectively. The mean temperature for the coldest (January) and the warmest month
(July) are −0.7 and 15.0 ◦ C at Saint-Michel and −3.2
and 15.2 ◦ C at Aussois. Snow covers the ground
for 3–4 months per year at 1 400 m a.s.l. on southern slopes. Mean monthly precipitation is 79 (± 14)
and 59 (± 10) mm at Saint-Michel and Aussois, respectively. Between 1 700 and 1 900 m a.s.l. on the
southern exposure, the present-day vegetation consists of mixed-coniferous forests dominated by Pinus
sylvestris and Picea abies. On calcic soils, Pinus uncinata abounds with Picea abies. Traditional land use
was mainly devoted to livestock grazing and cultivation, resulting in large areas of meadows and heathlands within these elevations on southern exposures.
3. Methods
Two soil profiles were sampled at Aussois and at
Saint-Michel. Soil sampling was avoided when soils
were obviously disturbed by human activities, located
at the foot of steep or long slopes, or were eroded
or hydromorphic. The profile AUSSOIS 4 is situated
on a plateau, but profiles AUSSOIS 1, MAUR 6 and
MAUR 13 are located along gentle slopes. For these
last three profiles, I cannot rule out eventual particle flow, or erosion processes triggering input or output of soil materials. However, I have not observed
any kind of evidence that could support such eventual processes, or could suggest polygenic or colluvial soils. Profiles were taken in trench dug down
to the bedrock whenever possible [8]. Soil material
was sampled by cutting soil blocks from the face of
a vertical soil profile. Blocks were collected at different depths from the bottom to the surface to avoid
particles falling from the upper horizons resulting in
a risk of assemblage rejuvenation [8]. Block limits
23
C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
Table. Radiocarbon dates of soil wood charcoal fragments (Abies) from Saint-Michel-de-Maurienne (MAUR) and from Aussois (AUSSOIS) in the
Vanoise Massif (northern French Alps). Calibration was performed using the CALIB 3.0 program [29] and reported as intercept midpoint with 2σ
range. Symbols ∗ and # indicate measurements on two and four fragments, respectively.
Tableau. Âges radiocarbones de charbon de bois (Abies) provenant de Saint-Michel-de-Maurienne (MAUR) et d’Aussois dans le massif de la
Vanoise (Alpes françaises du Nord). Les datations 14 C ont été calibrées à l’aide du programme CALIB 3.0 [29] et présentées avec une marge
statistique à 2σ. Les symboles ∗ et # indiquent les mesures effectuées sur deux et quatre fragments, respectivement.
Profiles
MAUR 6 / 20–40 cm
MAUR 6 / 40–60 cm
MAUR 6 / 60–85 cm
MAUR 6 / 60–85 cm
MAUR 13 / 35–40 cm
MAUR 13 / 65–80 cm
AUSSOIS 1 / 15–30 cm
AUSSOIS 4 / 30–50 cm
AUSSOIS 4 / 30–50 cm
AUSSOIS 4 / 30–50 cm
Mass (mg)
5.1
7.1
27.5
8.0
9.5
7.7
6.6
14.8
5.9
5.1
Sample codes
Lyon-865(OxA)
Lyon-866(OxA)
Lyon-867(OxA)
Lyon-868(OxA)
Lyon-1078(OxA)
Lyon-869(OxA)
Lyon-870(OxA)
Lyon-871(OxA)
Lyon-872(OxA)
Lyon-873(OxA)
correspond to natural horizon limits. However, when
the horizon’s height was higher than 20 cm, horizons
were separated in several levels. About 10–15 L of
dry fine material was sampled per levels of ∼ 20 cm
height.
The horizons have been described on the field, as
well as the depth, the skeletal content, the colours and
the structure. The pH was measured at the laboratory
in horizons of soils lying on carbonaceous bedrock
(Aussois). Texture was estimated on the field and at
the laboratory. A synthetic soil description per horizon based on field observation has been previously
published [6]. At Saint-Michel soils have been sampled in meadows on acid bedrock areas. Soils are acid
brunisols with a micro-crumb rhyzogenous structure
and the humus belongs to the eumull. At Aussois, under Pinus uncinata covering carbonate rich moraines,
the humus is a thick dry calcic dysmull produced by
pine needles, ericoides twigs and leaves. Soils are carbonated brunisol in this environment.
Charcoal was extracted from soil by flotation [8].
Dry charcoal fragments have low density, so flotation,
with or without an ascending water flow, together with
water sieving and manual sorting under a binocular
microscope (× 20) allowed the separation of charcoal
from other soil particles. Charcoal particles were
identified with an incident light microscope (× 200,
× 500), and were compared with descriptions of wood
anatomy (e.g. [18]).
Ten 14 C dating measurements were carried out
on plant shoot material (table). Accelerator mass
spectrometry (AMS) 14 C measurements were used
due to small size of soil charcoal fragments. Four
times, two and four fragments were pooled to reach
the minimum required mass for AMS measurements
(table). Prior to the AMS dating, charcoal fragments
24
14 C
years (BP)
# 4040 ± 55
∗ 4605 ± 45
6070 ± 50
∗ 5550 ± 55
4100 ± 50
4455 ± 55
3675 ± 55
4730 ± 45
3990 ± 40
∗ 3745 ± 45
Calibrated years (BC)
2857–2463
3505–3123
5205–4808
4492–4261
4825–4441
3498–3095
2200–1780
3640–3372
2617–2411
2296–1922
were cleaned under binocular microscope (× 40)
to remove small roots, fungal hyphae and mineral
particles that could rejuvenate or age the charcoal.
Each fragment was treated during a minimum of 24 h
with a solution of Na4 P2 O7 to extract the organic
compounds adsorbed in the charcoal porosity. The
solution was changed daily up to the end of release
of organic-compounds by charcoal. Generally, five to
seven days were sufficient to clean chemically the
charcoal fragments.
The soil-charcoal concentration permits the description of the vertical charcoal distribution at different levels in a given soil profile and the comparison of
different profiles. Concentration is expressed as mg of
charcoal per kg of dry soil particles less than 2 cm in
diameter. Only charcoal fragments larger than 400 µm
were quantified because the amount of charcoal fragments smaller than 400 µm in diameter is generally
negligible.
4. Results
A single new date (3675 BP) obtained in the
uppermost soil horizon of AUSSOIS 1 profile (table)
revealed a stratigraphy totally inversed compared
to the previous dates (figure 1). In AUSSOIS 4,
three new dates from the same soil horizons provide
14
C measurements between 4730 and 3745 BP. The
three new dates are located in the mid soil profiles,
while the two previous dates in the uppermost and
deepest horizons both indicate recent dates, 75 and
405 BP, respectively (figure 1). Here, as well as in
AUSSOIS 1, there is no evidence of stratification.
More new 14 C dates have been carried out in profiles MAUR 6 and MAUR 13. In the first profiles, four
C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
Figure 2. Soil profiles from Saint-Michel-de-Maurienne. Same legend as in figure 1.
Figure 1. Soil profiles from Aussois. The black bar corresponds to
the anthracomass concentration per level (mg·kg–1 ). New dates are
in bold in the boxes. Dates are expressed in uncalibrated 14 C years
before present (details on dates are in the table).
Figure 1. Profils de sol d’Aussois. Les traits noirs correspondent aux
concentrations d’anthracomasse par niveau (mg kg–1 ). Les nouvelles
dates sont en gras et entourées. Les datations fournies sont en années
14 C, non étalonnées avant 1950 (des détails sur les dates sont donnés
dans le tableau).
dates have been processed in three different horizons
(table) to complete the previous dates on Abies in the
deepest horizon, and on Pinus S. sylvestris in the middle horizon. The four new dates are Mid-Holocene,
but cover a rather wide phase between ∼6100 and
4000 BP (figure 2). The two previous dates are included in this range of 2000 years. No stratification
can be evidenced and dates deciphered a large distribution of Mid-Holocene dates in an 80 cm thick
soil, only 20 cm below the soil surface. In profile
MAUR 13 three 14 C dates were previously measured
between ∼3600 and 4000 BP. The new dates, 4455 BP
and 4100 BP, extend this range by 500 14 C years (figure 2).
In order to compare the totality of 14 C dates available in the study area, the calibrated dates have been
pooled in the same synthetic profile, one for Aussois,
Figure 2. Profils de sol de Saint-Michel-de-Maurienne. Même
légende que pour la figure 1.
and one for Saint-Michel (figure 3). The synthesis
based on 14 and 17 dates between 1 700–2 050 m a.s.l.
at Aussois and at Saint-Michel, respectively, displays
an apparently random distribution with depth. Furthermore, in Aussois, the oldest dates between ∼4 000
and 6 000 cal. yr BP are in higher positions in the synthetic soil profiles than a group of young dates located
in depth (figure 3).
5. Discussion
Neither in the AUSSOIS nor in the MAUR profiles, charcoal dates have enhanced a so-called stratification, while charcoal botanical assemblages could
suggest a rough stratification, e.g. abundance of P. S.
sylvestris in the uppermost centimetres of soil in
MAUR 6 and presence of Abies alba in the deepest,
or presence of Rhododendron at the soil surface and
abundance of Abies alba deeper in MAUR 13 (figure 1). The notion of soil stratification is obviously
not supported by a series of dates, each carried out on
a single or a small group (2–4) of charcoal fragments.
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C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
Figure 3. Distribution of AMS 14 C dates according to the soil depth
between 1 700–2 050 m a.s.l. at Aussois and at Saint-Michel-deMaurienne (Vanoise Massif, North French Alps). The vertical bar
indicates the thickness of the soil level containing the dated charcoal
fragment, and the horizontal bar the range of the calibrated dates at
2 σ. The thick crosses correspond to the new dates provided by the
present study, and thin crosses are the previous published dates from
the same sites (see [6]).
Figure 3. Distribution des âges 14 C par AMS en fonction de
la profondeur d’enfouissement entre 1 700–2 050 m d’altitude à
Aussois et à Saint-Michel-de-Maurienne (massif de la Vanoise, Alpes
française du Nord). Le trait vertical représente l’épaisseur de sol
contenant le fragment de charbon daté et le trait horizontal correspond
à l’étendue de la date étalonnée avec une incertitude de 2 σ. Les croix
épaisses correspondent aux nouvelles datations de cette étude, et les
croix fines aux dates publiées dans une précédente sur le même site
(voir [6]).
ous that particles belonging to the size-classes of sand
are abundantly transported through soil, from the surface to the bottom and vice versa. Although anecic
earthworms are present, they do not abound in these
elevations. Furthermore, mountain dung beetles and
ants do not dig deeper than 1 m and marmots are not
uniformly distributed. Finally, the soil profiles have
been sampled in forests, an ecosystem that Alpine
marmots do generally not use. Pedoturbation by soil
fauna cannot alone resolve the observed distribution
of dates on soil charcoal (figure 3).
All soil profiles sampled between 1 700–2 050 m
a.s.l. are situated in the forest belt for millennia.
In natural forests, tree-fall gaps resulting from uprooting are one of the more important canopy disturbances and trigger the formation of mounds and hollows [4, 14]. Such processes strongly modify the soil
structure for long time and are then responsible for the
abrupt apparent reworking of soil particles [27, 31].
Although the natural cycle of such soil disturbance is
unknown in Alpine forests, it appears as one of the
most important processes of reworking of soil material, as well as the soil fauna activity [28]. Although it
is not possible to prove the past role of up-rooting, it
is probable that it plays an important role on the particle reworking, as evidenced by radiocarbon dating of
charcoal fragments.
5.1. Causes of non-stratification
Some soil profiles are located along slopes, e.g.
AUSSOIS 1, MAUR 6 and 13 (figure 1) and may
then suggest that the non-stratification of soil particles
resulted from colluvial processes, while others are
located on a plateau (AUSSOIS 2, 3 and 4, MAUR 4,
7 and 8 [6]) and show the same kind of results,
i.e. there is no relationship between age and burying
depth. Although colluvium is not totally ruled out, this
process cannot explain the observed results.
Pedoturbation by soil fauna is used frequently to
explain soil particle reworking [24]. In altitudes between 1 700–2 000 m, the functional group of soil
fauna reworkers is mostly characterised by dung beetles and ants [21]. Anecic earthworms are infrequent
at similar elevations [10], but occur in our soil profiles [6]. Mammals, such as marmots or alpine voles
can also disturb the soil stratification by reworking of
the material. The role of soil fauna has been known
for long time [11, 20] and has been used frequently to
explain burying of artefacts in soil [1, 19, 26]. However, this type of one way process, from the surface to
the soil bottom, may occur for centimetre-sized particles that have a low chance to be reworked by animals
from the bottom to the surface. Charcoal fragments
that have been dated in the present study have generally been collected in the soil fraction of 0.8–2.0 mm,
and most of them have a size around 1 mm. It is obvi-
26
5.2. Interpretations in previous studies
Several arguments have been put forward to explain age/depth distribution of charcoal fragments,
e.g. charcoal is not transported by soil fauna from the
bottom to the surface, but only the fine grain inorganic
material (silt, clay). The deposit of fine grain inorganic particles on the soil surface would contribute
to bury the other particles [5, 8]. It is rather difficult
to accept the idea that charcoal is not selected by soil
fauna, and that biota only ingest inorganic particles
and transport them toward the surface. Why would
charcoal not be transported by earthworms, beetles,
termites, etc., when it has been evidenced that pollen
can be translocated throughout brunisol profiles [13,
33]?
Discrepancies of age/depth distribution of 14 C dates
in soil profiles have been reported between soil
organic matter and wood charcoal [16, 25]. It has
been suggested that the pollution of organic matter
by anthropic 14 C can explain a 3500 BP rejuvenation
compared with the age of charcoal at the same
depth [25]. Why would only soil organic matter be
polluted by anthropic 14 C, while the charcoal presents
the highest adsorption capacity [2]? Further, I do not
evidence rejuvenation of 14 C dates of charcoal from
the uppermost horizons (figure 3).
C. Carcaillet / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 21–28
The explanation is probably simpler than generally
evoked. First, biota reworked charcoal as well as other
kinds of soil material if dimensions are not too large,
i.e. millimetre-sized. Second, a discrete colluvium of
fine grain particles certainly occurs, even if it is extremely difficult to evidence. While organic matter
and charcoal are reworked by biota from the surface
to deeper soil horizons, colluvium contributes to increase the soil thickness and then decrease on longterm the capacity of biota to bury themselves in the
deepest soil horizons. With time, the result is an apparent stratification of particles. The bulk dating of
pooled charcoal fragments, a method applied by many
[3, 5, 16, 25, 32], contributes to provide a mean age
of charcoal assemblages. This mean age limits the
capacity to pin point dating anomalies by reducing
the real age variability between samples. While most
studies have applied such a dating strategy, my approach is based on the dating of generally one fragment to obtain one date. If colluvium cannot occur
due to the profiles’ location on a plateau such as in
AUSSOIS 4 (figure 1) or in AUSSOIS 2 and 3, the absence of stratification supports only particle reworking. Between 1 700–2 050 m elevation in the Alps,
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